Twinkle Toes Engineering:
Hybrid ? Electric Car Technology
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created 6/09
updated 2/9/12 Leaf vs Volt drive train mini-tutorial
Toyota/Ford/Lexus hybrid block diagram
Batteries
Nickel Metal Hydride (NiMH) batteries
Prius NiMH battery
Battery power out vs load resistance
Lithium ion
Lithium ion comparison to NiMH
Comparison of Volt and Leaf lithium battery thermal management
Battery coolant problems
quasi-electric GM Volt
GM Volt lithium ion battery
GM Volt revealed to be a hybrid!
Toyota Prius plug-in hybrid 2012
quasi-electric Fisker Karma sedan
Tesla Motors lithium ion pure electric cars
Tesla 2 seat roadster
Tesla Model S sedan -- 300 mile range
Comparing electric car motors --- Fisker Karma vs Tesla Model S
Nissan Leaf electric
Leaf 99 mpg EPA sticker
Ford Focus electric
Coda electric
UQM motor + controller supplier
EV-1 specs
Mini-Cooper electric
Mitsubishi MIEV electric car
Electric car batteries and range
Desiging an electric car
Motor
Electrics PM => induction
Motor speed comparisons
Inverter/boost
Why a booster converter?
IGBT transistor modules
Comparing hybrid ? non-hybrid siblings
Toyota hybrid technology
Toyota gen III 2010 Prius specs
Toyota gen III circuit sketch
Ford 2010 Fusion hybrid
Hybrid architecture
Mysteries of the 'power splitter'
Works like a three winding transformer
Toyota's power splitter formula
Local power loop!
Overdrive
New Hyundai hybrid architecture
Chinese BYD F3DM electric
Electric car charge standard --- SAE J1772
Simple built-in charger
Wireless charger
MPGe EPA standard
Wind resistance hp
My personal contributions to hybrid/electric technology
Reference
Wind resistance
13 hp / 9.7 kw @ 55 mph
17 hp / 12.6 kw @ 60 mph
40 hp / 29.6 kw @ 80 mph
57 hp /42.2 kw @ 90 mph (Nissan Leaf top speed)
79 hp /58.5 kw @ 100 mph (GM Volt top speed)
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Leaf vs Volt drive train mini-tutorial (1/5/11)
Here is a technical comparison of the electric drive trains of the GM Volt and Nissan Leaf, both of which have just begun deliveries. They are competing cars with almost the same wheelbase. (Facts about the drive trains are hard to come by and often come from obscure sources, so numbers are subject to change.)
The Leaf is to the conventional car what the jet is to piston planes. Conventional cars (including the Volt) have a lot of complex up/down stuff (pistons, valves), whereas the Leaf drive train is pure rotary. Leaf is about as simple as an electric car can be: series arrangement of battery, (one) inverter, (one) motor hard coupled to front wheels. Inverter/motors are inherently bidirectional in power flow so the same structure acts as a regenerative brake.
Leaf
battery (90 kw, 345V) ?=> inverter (260A) ?=> PM motor (80 kw) ?=> wheels (3,366 lb)
The Volt in contrast has a much more complex drive train than the Leaf. Volt's primary drive train has the same components as the Leaf (somewhat beefed up), but it also has a 2nd half sized motor, a 2nd half sized inverter, a complex hybrid type planetary gear set, three (or four) clutches, and a conventional engine! Its battery is 2/3rd the kwh and weight of the Leaf's, but all this extra stuff makes the Volt 424 lbs heavier than the Leaf, and it makes the car more expensive (41k for Volt vs 32.8k for Leaf).
Volt
battery (165 kw, 345V) ?=> inverter (355A) ?=> Induction motor (110 kw) ?=> planetary gears
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?========> inverter (175A) ?==========> => wheels (3,790 lb)
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engine (1.4L, 66 kw) => Induction? mot/gen (54 kw) => planetary gears
The hybrid Prius, all electric Leaf, and electric/hybrid Volt are all build on a 106 in (approx) wheelbase. The Prius is by far the lightest at 3,042 lbs, 324 lb heavier is the Leaf (3,366 lb) and 424 lb heavier than the Leaf is the Volt (3,790 lbs).
Performance
The Leaf has a single 80 kw motor/inverter to power the car (under all conditions). Probably about 40 kw (of its 80 kw) is needed to overcome wind resistance at 90 mph (top speed) on a flat road.
The Volt has two (and potentially three) sources of torque to accelerate the car: 110 kw main motor, 54 kw mot/gen, and since the engine can be coupled by a clutch to the 54 kw motor, if the gears are rated for it, mechanical torque from the engine could potentially be added too. The result is that even though the Volt is heavier by 424 lbs it accelerates a little quicker (8.8 sec vs 10 sec 0-60 mph for Leaf). Top speed for the Volt is 100 mph vs 90 mph for Leaf.
Leaf has an 80 kw (280 Nm) PM motor and Volt an 110 kw (368 Nm) induction motor. No info on the type of mot/gen in the Volt (my guess is it is also induction). A [Power = Torque x Speed] calculation shows that motor power peaks near 3,000 rpm in both Leaf and Volt.Transmission
The Leaf has no mechanical or electrical gear ratioing. The motor in the Leaf is hard coupled via a fixed step down (x7.94) gearing to the wheels. I figure the PM motor in the Leaf has a speed range of 0 to 9,000 rpm as the car goes from 0 to 90 mph. The motor Torque vs Speed curve has a power peak probably near 30 mph (3,000 rpm), and then rolls off in torque faster than constant hp from 3,000 rpm to 9,000 rpm. This very non-rectangular T vs Speed shape is needed to get acceptable 0 to 60 mph acceleration times.
Volt with its dual (or triple) torque sources has more options. GM apparently implements a variable speed transmission (like Prius) at higher speeds using gen and/or engine torque to supplement the main motor torque at high speed. This allows them to achieve higher vehicle speeds without excessive rpm on their main motor. They appear to also combine torque sources during hard low speed accelerations. When the battery is depleted, the car runs like a conventional hybrid with peak power from the battery supplementing the 66 kw available from the engine.
Inverters
In both Leaf and Volt the 345V battery is hard wired to the inverters providing (for free) a stiff voltage to protect the inverter transistors. Their motors run at 300 VAC (nom) with currents 250A to 350A, so very likely their main inverters use 600V IGBTs. In contrast the Prius power path includes step-up voltage regulator (to compensate for the voltage sag of its small battery), so it probably runs its motor at 600 VAC (nom) and uses 1,200V IGBTs in its inverter.
Battery pack
Leaf battery capacity is 24 kwh (660 lb) of which 19 kwh (80%) is used. Volt battery back is 2/3rd the size (16 kwh, 436 lb) of 10.5 kwh (65%) is used.
In a striking coincidence? both the Leaf and Volt stack 96 lithium ion prismatic modules in series. While the battery chemistry is a little different, the lithium ion cells in both are 3.6V, so both cars have a battery voltage of 345 VDC (open circuit). The Volt uses three cells in parallel in each module (288 cells total), and the leaf uses two (roughly double size) cells in parallel in each of its modules (192 cells total).
The Volt uses liquid heating/cooling to keep the battery temperature within a narrow range (70F +/- 2F). Volt also uses the engine to protect the battery if the battery gets too hot or too cold. In contrast the leaf just air cools the battery pack with a fan. Its battery temperature varies with ambient, there is even a battery temperature gauge on the dash. And of course they have no engine to protect the battery. It has to power the car even if its cold or hot.
Bottom line is that GM, with its higher battery derating and much better battery temp control, is stressing the battery less than the Leaf, so is likely that its loss of capacity over time will be less than in the Leaf. Leaf (or to be specific its customers!) are taking a chance that there may be a substantial loss of battery capacity in a few years, and surprise (!) Nissan specifically excludes battery capacity loss from the car guarantee. Loss of half the battery capacity in a Leaf would render the car near useless without a very expensive repair, whereas a Volt with a reduced capacity battery will still be drivable.
Hidden advantage of electrics' large battery
Electrics with their large battery packs have an advantage over hybrids you never see discussed. This is the issue of battery sag, meaning the sagging of battery voltage under high demand. In a hybrid the battery accelerates the car at lower speeds, where the power demands are modest, and is joined by the engine when power demands increase at higher speeds. My Prius block diagram (below) shows the Prius battery outputting a peak of 27 kw from a 1.3 kwh battery. My analysis shows that the Prius NiMH battery voltage probably sags about 25% under this load. This is a real problem, just when you need to deliver high voltage to the motor, the battery voltage sags. To solve this problem Prius from Gen I to Gen II introduced a (bidirectional) voltage regulator between the battery and inverter. However, this is expensive, because essentially they had to add a 2nd high power inverter in series with the main inverter that drives the motor! (It also raised their bus voltage to about 600V (I think) driving them to 1,200V IGBTs, all of which needs to be handled carefully from a safety viewpoint.)
A Leaf has an 80 kw motor, so Leaf's battery needs to supply 80 kw, x3 higher battery power than in the Prius. But the Leaf's battery is huge compared to the battery in the Prius (660 lbs, 24 kwh vs 80 lb and 1.3 kwh). While lithium ion batteries are not quite as good at supplying high current as NiMH used in hybrids, they are pretty good. Mercedes has gone to lithium ion in their new hybrid. The peak battery voltage sag in the electrics is probably 10% or less. This simplifies the electrical design: the battery can be hard wired to the inverter, no preregulators, no added bus capacitors, and the stiff battery voltage protects the transistors from overvoltage.
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Intro
A primer on the electrical power designs of hybrid cars and electrics by a (former) inverter designer including my Toyota/Ford/Lexus block diagram with gen III Toyota Prius values (below) and a circuit sketch of Prius voltages and currents. I'm not a guy car. I have little real understanding of mechanical components like gears and transmissions, but I do understand the basics of the (famous) Prius 'power splitter'. I don't own a hybrid, so I can contribute nothing from a user viewpoint, but I do know electronics.
So what is a hybrid car?
Here's my Toyota/Ford/Lexus hybrid block diagram with the values of gen III, 2010 Prius, showing currents, voltages, rpm, and power splitter speed constraints (formula ? table). All the blocks in the diagram support bi-directional power flow making the Toyota hybrid architecture very flexible.
an original figure (all rights reserved)
Alex Severinsky hybrid vehicle architecture (circa 1992)
34 (top) are the vehicle wheels
Note has only one motor/inverter
(US Patent 5,343,970, figure 1)
There are key choices to be made in building a hybrid or (quasi) electric car, and one of my goals in researching this essay was to understand how ? why the particular choices have been made. Battery type and hybrid architecture have been discussed ad nauseaum, but there are many other key choices: Motor (? generator)
type PM (+ reluctance) or induction
top speed 6-7 thousand or 12-14 thousand rpm
cooling air or water
Inverter
transistor/IGBT 600V or 1,200V
bus voltage fixed or varied
paralleled no or yes (if yes, how paralleled)
cooling air or water
Battery voltage booster yes/no
Battery
type NiMH or lithium ion
voltage sag low (15 to 20%), moderate (25%), heavy (50%)
oversizing oversize vs lifetime trade
forced air cooling yes/no
ESR at low temp how to live with a x3 to x6 increase in ESR
Engine
speed range fixed or variable
peak hp operation yes/no
Motor
All the hybrids are using (neodymium) PM motors, but the two (quasi) electrics are using induction. My guess is PM motor/generators being smaller integrate best with sun/planet power splitter used in the Toyota/Ford/Lexus hybrids. Induction motors are favored with the motor needs to be larger, need to run hotter, and can stand alone.
Motors are sized by torque, but power is T x speed, so doubling a motor's speed (6,500 => 13,000 rpm) can roughly double its output power. Toyota uses this technique for its 133kw hybrid SUV (Highlander), combining it with a 2:1 step down to allow it to mate with the 6,500 rpm power splitter gears. Tesla Motors uses it too, operating the motor to 13,000 rpm. For PM motors to work well in vehicles a special control technique is needed to add a (nominal) constant hp region to T vs S. This either means field weakening, or a combo PM/reluctance motor as Toyota is using. There is little info available as to the technology used for generators, but it's a good guess that if the motor is PM, then the generator is too. Didn't find much about how motors are cooled.
Inverter, voltage sag, and battery (voltage) booster
There are two standard types of IGBT (transistors): 600V and 1,200V. Battery pack voltages in current vehicles range from 160V to 320V, so if battery pack couples directly to the inverter, then 600V IGBT's are used. This was the technique used in the gen I Priuses, and it is the technique used in the Tesla Motor's Roadster electric car. However, at this low voltage when a lot of inverter power is needed, like in an electric car, the currents get 'wicked' high. Tesla's inverter is rated for 850A and achieves this with massive paralleling (x11).
A problem with direct battery to inverter connection is that the battery voltage sags a lot when it must deliver peak power/current. If you try to pull the absolute max power (100%) the battery pack can deliver, its terminal voltage will sag 50%, meaning the terminal voltage drops in half. This is undesirable for a lot of reasons, so a compromise is called for. I calculate that Toyota limits battery voltage sag to 25%, at which point the battery delivers 75% of its maximum power. In this configuration battery voltage sag rounds off the corner of T vs Speed, reducing the peak output power.
Adding a voltage booster, which is a specialized inverter, between the battery and inverter does a couple of things. First, it improves peak output power because the inverter/motor no longer 'sees' the battery voltage sag. Second, by roughly doubling the voltage to the inverter it halfs the current (for the same power) in the inverter and motor. The higher voltage is handled by using 1,200V IGBT's. Toyota added a booster in 2003 in the transition from gen I to gen II. A draw back is that the booster needs to handle the full power of the battery pack and has to handle bidirectional power flow to allow regenerative braking. A still further complication is that all converters need a large inductance to 'switch against'. In the motor and generator the needed inductance is free, it's built into the motor and generator, but with the voltage booster it must be added in. I have seen zero info about the voltage booster implementation. A further, non trivial, complication with adding a booster is that a (bigger) bus capacitor bank is needed to handle the switching currents from the inverter, because the stabilizing influence of the battery low impedance is no longer visible through the booster.
Toyota appears to regulate (fix) the bus voltage of their inverters using the booster at 650VDC, but some optimization is possible by allowing the bus voltage to vary with vehicle speed and conditions. Ford appears to be modulating the bus voltage up and down a little in the 2010 Fusion. Tesla Motors (might) be doing this too with a cryptic reference that they were (somehow) dropping the bus voltage when the inverter needed to output its maximum 850A during low speed acceleration. The only company that appears to be paralleling is Tesla Motors, apparently they use 66 IGBT's whereas nominally for a motor inverter only 6 are needed, implying they are paralleling x11 to reach 850A. Paralleling is non-standard and can probably be reliable, but it takes careful engineering. Power semiconductor companies are beginning to make high current modules with paralleled die specifically for (electric) vehicles (see below).
800A (550A with 75C water cooling), 600V six-pack inverter
(probably 2 x 400A parallel IGBT die in all six locations)
size: 8.5" x 4", dissipation: 1,500W (max)
Important: mating (six terminal) snubbing capacitor available
Infineon FS800R07A2E3
I found little (hard) info available on inverter cooling, but a poster says Prius uses liquid cooling for the inverter and motor (separate cooling loop from the engine) and air cooling for battery pack. (Prius inverter enclose does appear to show liquid ports.) A new Tesla owner reports he took his 100k new Roadster to an off road amateur (twisty) track and found his inverter over-temperature warning light came on in five minutes followed by the over-temperature warning light on the motor. (It's high performance car, but only for 5 min!) Researching I found out the idiots at Tesla Motors air cooled the inverter. I've worked with both water and air cooling on high power inverters and water cooling is (at least) x5 more effective than forced air. All high power inverters and (PM) motors in vehicles should be water cooled.
Toyota's Atkinson cycle engine
A 2007 UC Davis paper (see reference below) discusses the 1.4 L (gasoline) engine used in the Prius. It says it is the most efficient gasoline engine in production (as of 2007). It uses the Atkinson cycle as opposed to the Otto cycle used by most (or all) car engines. Here's the comparison they make:
Atkinson Otto
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Peak thermal efficiency 37% 25%
Power/Torque output (Atkinson 20-30% less than Otto of same size)
According to this paper the Atkinson cycle has a huge effect on performance. In exchanged for a 20-30% loss in power and torque almost a 50% improvement in fuel efficiency is realized! (Is this reflected in Toyota gas milage numbers?) It is consistent with Ford Fusion, which has very good gas milage numbers. Ford Fusion too uses an Atkinson cycle engine, and it's quite large 2.5L (vs 1.8L is gen III Prius).
Batteries
All hybrids on the market use relatively small NiMH battery packs (100 lbs or so). It turns out that remarkable little energy storage is needed to get big improvements in mileage, especially city mileage. Ford touts their new 2010 hybrid can go 47 mph on battery, but the driving range in all electric mode is only a couple of miles. Batteries at full power discharge in 30 sec or so. The improved gas mileage, which can be double for city driving, is achieved by downsizing and optimizing the engine cycle and speed and capturing a significant fraction of vehicle kinetic energy with regenerative braking.
Available data indicates that NiMH battery lifetime in hybrids has proven to be remarkably good. This indicates that oversizing the battery and small discharge cycles in hybrids (engines quickly recharge the battery after an acceleration) have been very effective at extending battery lifetime. This problem is much more difficult in electric cars where deep discharge of the batteries is required for long range. To improve range higher power density lithium ion batteries will be used, and battery packs will be x4 heavier (400 lbs or so) with x12 energy storage (16 kwh vs 1.3 kwh). (Of course, the battery people are working discharge cycle problem hard and 123 Systems, for example, touts the x10 higher discharge cycles that their battery technology can deliver as a major advance.)
Batteries at peak power can dissipate a lot of power and their chemistries don't like high temperature, so battery cooling is required. If the battery pack is pushed to its limit to deliver peak power ('matched load'), it will dissipated an amount of heat equal to what it delivers. Toyota (I figured out) compromises by restricting the peak load to 75% of battery maximum. This reduces battery heating to 1/3rd of delivered power, which is much better, but it's still a lot of heat. As far as I can tell, everyone air cools their battery pack. A photo of a Toyota SUV battery pack shows three high power axial cooling fans. But I read the GM Volt, who have a much more difficult battery thermal problem with their large lithium ion battery pack, may be using liquid for its temperature control.
NiMH battery data sheet from Panasonic shows a big loss of peak power capability in cold weather (reduces by x3 to x6). Apparently in hybrids this is little noticed, probably because the engine is cut in much sooner. However, it led Audi to cancel a high performance hybrid SUV they were developing when they found the snappy acceleration was just not there in cold weather.
Low temperature data ? peak current discharge on lithium ion batteries for vehicles are (suspiciously) not available. Not only has GM not released its battery spec, but the LG Chem has pulled all the detailed specifications for its (custom) lithium ion batteries. Likewise 123 Systems have not disclosed their vehicle battery spec.
Engine
Marketing literature sometimes gives the impression that the engine in a hybrid or an extended range electric need only run at one speed (or a very narrow speed range). This is partially true, but is somewhat of an exaggeration. The Prius engine runs from 1,000 rpm to 4,500 rpm (5,200 rpm in gen III). The lossey high end range of 4,500 to 6,500 rpm of the conventional engine is gone. While the GM Volt literature claims their engine mearly recharges the battery, it appears that they sometimes need to run the engine at its maximum power point (its highest loss point) so that raw generator current can supplement battery current (increasing it 33% or so) to increase the driving hp, or on long trips or cold temperatures power the vehicle fully.
Introduction
Here is a primer on the battery/inverter technology in hybrid cars from a power/motor control engineer. In the early hybrid years of Toyota ? Honda technical details were hard to come by, even to someone working in the power engineering field. But now a decade later some information about the technology has been published, and with hybrid cars slowing going mainstream and performance improving I decided it was time to take a look at the electrical engineering of hybrid cars. The new 2010 Ford Fusion, which is getting great reviews, is pushing the all electric envelope, touted as able to run all electric up to 40+ mph. It seems hybrid cars are slowing getting more and more electric, in a sense they are evolving toward all electric cars.
I know a lot about inverters, because I used to design them. I've designed little ones and big ones, some as big as those in current hybrids and some bigger. I know a lot about motors too with patents on the control of PM motors and induction motors. I know relatively little about batteries, but I know how to read a data sheet and can often take a few pieces of published data and made reasonable guesses to fill in the gaps. Toyota, of course, was a pioneer in hybrids and has been selling them for more than a decade. They just now introducing their third generation of hybrid technology.
I focused on the major hybrid manufacturer, Toyota, and quickly found the spec of the Panasonic battery used in the Prius. A colleague provided me with a recent paper by a Toyota engineer that describes the development of a higher power (123 kw) inverter and motor for use in a 2005 Toyota SUV. A link in Wikipedia led me to a detailed description of the Prius power splitter by a Prious owner and engineer. I started this essay to record what I recently learned about hybrids, and as a place to put new hybrid info as it becomes available.
The famous GM EV-1 all electric car (1966 - 1999) used lead acid batteries, but no hybrids today use lead acid, except as auxiliary batteries. (EV-1 inverter used IGBT's).Hybrid vs electrics
The batteries in hybrids (like Prius) and electrics are very different. In a hybrid the battery pack is tiny, only a 70 to 100 lbs. It's used for a few seconds to boost acceleration and for low speed driving where power load is low. The engine quickly recharges the battery so the usual discharge is shallow. Batteries like this.
In contrast trying to drive on the highway any significant distance using only electric power is a bitch. The Tesla Roadster battery pack has x43 more energy storage than Prius (56 kwh vs 1.3 kwh), it weighs 1,000 lbs, costs a ton, and still only give 220 mile range in a two seater car.
With current battery technology the electric guys face a nasty trade off: battery life vs depth of discharge. Rechargable power tools fully discharge their lithium ion batteries with the result that the batteries are seriously degraded in three years. GM must be sweating this problem with the Volt. Their current compromise is to 50% discharge the battery. If they 100% discharged, they would double range from 40 miles to 80 miles, but then their batteries would begin to die in three years. The battery manuf are working this problem hard using a fundamentally new class of materials with very high surface area (nano technology) and are claiming x10 improvements in battery life.
Nickel Metal Hydride (NiMH) batteries
Currently (as of summer 2009) all hybrid cars on the market use Nickel Metal Hydride (NiMH) batteries. Typically they use a single stack of 170 to 260 cells in series (D type or equivalent @ 1.2 V/cell), which results in an 'open circuit' voltage in the 200V to 300V range. Amazingly these cells can (each) handle hundreds of amps for a brief time (seconds).
High peak power
In addition to good energy storage, a critical requirement for a vehicle battery pack it must have very low internal resistance so it can deliver large amounts of current and energy quickly to accelerate the vehicle. Low battery internal resistance is also needed to efficiently absorb the regenerated braking energy, which tends arrive in the form of high power/current pulses. It is the capture of the vehicle's kinetic energy via regenerative brakes in a hybrid that makes a hybrid's city mpg higher than highway mpg, exactly the opposite of conventional cars.
Prius NiMH numbers
The internal resistance of the 1.2V Panasonic cell (see below) that Panasonic says is used in the Prius ? some Lexus hybrids is only 1.66 mohm. This gives a short circuit current in the range of 720A = 1.2V/(1.66 x 10^-3 ohm)!! The battery manufacturer rates the 'output power' at its max (of course). A little back of the envelope scribbling shows the battery manufacturer (peak) 'output power' rating is for a matched load, meaning a load resistor equal to the internal resistance of the battery. For the Panasonic Prius battery peak power out is 360 A out (half short circuit current) at half voltage. In round numbers the maximum power that a 200V battery pack can deliver is 100V x 360A = 36kw (48 hp). This number is in the ballpark for electrical hp for current car hybrids (electrical hp numbers are substantially higher in SUVs and trucks), so it is likely that hybrid cars manufacturers do in fact push their batteries pretty hard, either up to or near their rated peak power point.
Realistically NiMH batteries can only deliver high peak power for a few seconds at a time, because at maximum discharge rates a fully charged battery pack is fully discharged in about one minute. Wikipedia says early model Priuses would fully discharge the battery traveling 2,000 feet at 90 mph up a 6 degree slope. This works out to battery discharge in 15 seconds, since at 90 mph the car is traveling 132 ft/sec. (I later found out the explanation for the difference between 1 min and 15 seconds. Toyota restricts the charge/discharge range of the battery pack for battery lifetime reasons, keeping it between 40% (or 60% references differ) to 80% of capacity.)
You can only run most hybrids in all electric mode to about 25 mph. You can't run the useless GM car 'mild' hybrids in all electric mode at all. Customers soon figured out this was pretty useless and didn't buy them, so recently (June 2009) GM announced they are going to stop production of all their hybrid cars (they will still have some hybrid trucks)! The new 2010 Ford Fusion hybrid, which is getting raves, can reportedly go 40+ mph in all electric mode.
To get power from the battery its terminal voltage must be allowed to sag. At maximum power out it will sag to about half of its open circuit voltage. There is no avoiding this. When a hybrid draws hundreds of amps from its battery pack, about half of the open circuit voltage drops across the internal battery resistance (ESR). It's just ohms law. This is a serious complication in the design of the control electronics for the motor in hybrid vehicles, because when high power flows to the motor, the battery voltage may sag from 300V to 150V. There's a way to keep the battery voltage sag from affecting the motor, but it's expensive. It involves adding a (voltage) boost converter between the battery and motor drive electronics. Toyota put in a boost converter in the transition from Gen I to Gen II about 2003.Heat dissipation
Another complication is that when the battery pack is delivering high power it is also dissipating high power. If the Prius battery pack was allowed to deliver its maximum 36kw to the motor, it would be at the same time dissipating an equal amount of power (36kw) internally. In other words at maximum power out half of the cell's stored energy is being lost as heat dissipation within the cell. 36kw is a lot of power, but [energy = power x time], so the energy to be handled thermally is bounded and calculable. Initially the heat energy is absorbed by the thermal mass of the cell chemistry, and over longer times the battery pack must be able to dissipate the average heat lost in the cells. (I have yet to find any info on temperature rises or battery cooling)
After writing above, I found from working the gen III numbers that Prius limits the peak power flow from the battery pack to 75% of its maximum power out capacity (matched load) as rated by the battery manufacturer. This modest (25%) reduction in peak battery output power makes a huge reduction (factor of 4) in the heat energy lost in the battery during peak loads.
Lifetime/charge cycles
Wear out of rechargeable batteries is strongly depended on the size of their charge/discharge cycle. Batteries in portable equipment, which are deeply discharged, have only about a three year lifetime. NASA uses batteries in satellites with solar panels to ride through the satellite night and gets good lifetime by oversizing the batteries so they only discharge about 10%. This is the trick used in hybrid cars. Even though the capacity range is limited to something like 25% (20% to 40%) in most driving the discharges from acceleration are no more than 10% with a quick recharge from the engine. This is how the battery lifetime in hybrids is extended to give an eight years guaranteed lifetime.
But this trick doesn't work (or work well) when the goal is to drive long distances on the battery, like the GM Volt or the Tesla Motors all electric car. GM is stricking a compromise by restricting the usable range of its lithium ion battery to 50% (30% to 80%). An effort to extend battery lifetime probably also explains the unusual cycle they plan to use. When the battery discharges to 30%, the engine turns on but it does not charge the battery. It just provides the average power the car needs while holding the battery charge (nominally) at 30%. Only when the car is plugged in is the battery charged back up to the 80% level. This prevents multiple battery cycles that would otherwise occur on long trips.
Another issue with battery capacity is that charging losses rise (substantially) as the battery approaches its capacity limit, meaning ESR starts to rise as capacity exceeds 80%. For this reason the top 20% of battery capacity is not very useful.
Info in this section is from a guy who makes battery testing equipment and who writes lots of battery articles. His site has a good article on how the lithium battery (with lithium metal) was unstable and dangerous and how it evolved into the lithium ion battery to solve this problem. He also has info on pros/cons of various lithium ion chemistries.
http://www.buchmann.ca/
123 Systems lithium ion cells 'claim to fame' is that their cells with nano technology have nearly solved (or greatly improved) the degradation with load cycle problem. They have data on their site showing cells still good after 300,000 cycles. They argue much less 'over capacity' need be designed into the battery pack with their cells to get the desired lifetime. (Not particularly useful to a vehicle manuf if he wants a 2nd battery source!)
Prius NiMH battery
I was able to get a good handle on the Prius battery pack by finding a data sheet from Panasonic, who makes the NiMH battery cells used in the Prius battery pack. Here's the link:
http://www.peve.jp/e/hevjyusi.html
The curve below shows the internal resistance (ESR) for a module (6 cells in series) is about 10 mohm. This makes the short circuit current an amazing 7.2V/0.01 ohm = 720 amp! This is confirmed by Panasonic's (peak rated) 'output power' of the module at 1,350 watts. The simple voltage source/ESR battery model predicts peak output power at halfopen circuit voltage (3.6V) and half short circuit (360A) current, and 3.6V x 360A = 1,296 watts (within 4% of the 1,350 watts on the data sheet).
Battery type D ?? (or DD??) NiMH (Nickel metal hydride)
(nope, the module is too narrow for D cells)
Prius ? Lexus battery Panasonic Prismatic Module
6 cells series (7.2V = 1.2V x 6)
10 mohm (for whole 6 cell stack)
6.5Ah (for cell and whole battery pack)
28 modules x 6 cells/module = 168 cells
battery pack weight = 28 modules x
1.04 kg/module x 2.2 lb/kg = 64 lb
(additional weight for structure)
open circuit voltage = 168 x 1.2V/cell = 201 V
Kwh for Prius 201V battery pack
201V x 6.5Ah =1.3 Kwh (OK, agrees with published)
7.2V/.01 ohm = 720 A short circuit (calculated)
Max power out (theoretically) with matched 0.01 ohm load
Pout = 3.6 V x ( 7.2V/ .02 ohm)
= 3.6V x 360A
= 1,296 watts (for a module of 6 cells in series)
Rated Power out = 1,350 watts (OK, this is obviously max
with matched load)
The two figures below are the Panasonic data sheet from which I extracted the info above. Note the module contains six cells 1.2 V cells (all NiMH cells are 1.2V) in series giving it an open circuit voltage of 7.2V. At rated peak power out half the 7.2V of the cells drops across the internal ESR (equivalent series resistance) and half across the load. This means the voltage seen by the load at peak power out is 3.6 V (half of 7.2V). So at 1,300 watts out, the current out, and in each cell, must be 1,350W/3.6V = 375 A.
I had read that D cells were the used in the Prius, but that can't be right, because the dimensions of the module are not consistent with D cells, it's too thin. D cells are 1.3 in in dia, but the thickness of this module is only 0.77 inches. Its dimensions are 0.77 in x 4.2 in x 11.2 in and it weighs about 1 kg. (Here's the story from Wikipedia: The original Prius used shrink-wrapped 1.2 volt D cells, and all subsequent THS/HSD vehicles have used custom 7.2 V battery modules mounted in a carrier.)
Panasonic Prius NiMH battery module data sheet
I was surprised to find how light the Prius battery pack is, it's only 80 lbs (other references say 100 lbs and Wikipedia says 118 lbs). Each of 28 modules weighs about a kg which totals to about 64 lbs. People who make advanced battery packs for the Prius give the weight of the standard Prius NiMH battery pack as 80 lbs, which is confirming, 64 lbs of cells + 16 lbs of structure.
Discharge time
At peak power out each NiMH cell is dissipating 216 watts = 0.6V x 360A!! At full current (360A) the battery cell run down time is only 6.5Ah/360A x 60 min/hr = 1.08 min.
I read that Toyota limits severely the usable kwh range of the NiMH battery pack, and this is done to extend the battery pack life (guaranteed for 8 years). One reference said the charge of the batter pack is held within 40% to 80%, Wikipedia says 60% to 80%. The first limit would restrict the usable kwh range to 40% of the nominal 1.3 kwr energy storage and 2nd limit is only half of this or 20% of the nominal energy limit! Note these are are not peak power limits, these are energy limits. The charge of the battery pack must always be held somewhat below 100% to provide a reserve for regenerative braking.
There may be peak power/current limits too, but I have not seen a definitive reference. One reference said the battery current was limited to 80 A, but this seems far too low, compared to the 360A that I calculate the batteries could deliver with a matched load. An 80A limit in the new Priuses with only 28 modules would limit the peak battery power to 14kw = 80A x [201V (open circuit) - 80A x 28 x 0.01 ohm] = 80A x 179V = 14.3 kw (19 hp) (After I wrote above, I figured out that the Toyota limits peak battery power to 75% of maxium (see below), which results in maxiumum battery current of 180 A in gen III Prius.)
Low temperature problem?
Notice the left curve in Panasonic data sheet. It shows a shows a horrendous drop off in peak power with temperature. At -10C (14F) the peak power is down by something like 2/3rd (400 kw/1,300 kw = 0.31). At -25C (-12F) it's halved again! Since the open circuit voltage is fairly temperature insensitive (maybe -10%), the power roll off means the ESR of the cell roughly triples at low temperature (and 14F is not that low). My guess is that this is not too serious a problem for current hybrids, because the control system can compensate by cutting in the engine at lower speeds in cold temperatures. (should see if I can find reference to this in literature).
But in a car like the GM Volt, where I estimate that 3/4 of the peak power comes from the battery pack a big increase in ESR at low temperatures in its lithium cells would substantially reduce the effective 'hp' rating of the car until the batteries warm up (if they ever do). I have been unable to find data on ESR vs temperature for various lithium ion cells.
(update)
I searched for people complaining about poor acceleration in cold weather and found little. Maybe the explanation is this:
Here is some hard battery temperature data from Toyota. A Toyota engineer reporting at an engineering meeting on test of the plug in Toyota (coming early 2012). It confirms major problems with battery capacity at low temperatures. This car uses a lithium ion, air cooled battery giving the car a nominal 13 mile electric range.
I knew from engineering school that maximum power transfer (often) occurs with a 'matched load', so I was not too surprised when I worked out Panasonic's test condition for peak battery power ("Output Power") and found matched load, meaning a load resistance equal to the ESR of the battery. It was not until I wrote out the equation and crunched some numbers did I see how flat is the curve of maximum power out vs load resistance.
The numbers in the table above show the curve is quite flat. Max power out rolls off only 4% at R = 1.5 ESR, 11% at R = 2 ESR, and 25% at R = 3 ESR. Prius specs are 27kw from battery, while my calculated matched load battery power is 36 kw (= 1/4 x 720 A short circuit x 201 V open circuit). Thus Toyota in the Prius has effectively set the load resistor (Rload) = 3 ESR, suffering the 25% reduction in peak power out, but gaining three big advantages, or four if battery life is extended:
* Current out reduced by 2
* Power loss inside battery reduced by 4
* Voltage sag reduced by 2
Looks like a good trade to me.
Hybrid battery pack pictures
Below is the NiMH Prius battery pack (27kW, 202V) with 28 Panasonic six cell modules, which are about 0.8 in wide and 11 inches deep. The much larger battery pack further below is from the Toyota Higherlander SUV hybrid (45kw, 288V). Both packs are air cooled, the three roundish things (right) in the lower photo are probably powerful axial blowers. (Is there a lot of cabin noise from the blowers, since the battery pack is under the rear seat?)
Prius battery pack (27kW, 202V)
source --- http://www.hybridcars.com/gallery/22070/photo
Toyota Highlander SUV NiMH battery pack (45kW, 288V)
right: three axial blowers to cool battery
source --- http://www.hybridcars.com/gallery/22070/photo?page=1
NiMH provides (roughly) twice the energy density of lead-acid, which was used used in 1997 GM EV-1, and lithium ion has (roughly) twice the energy density of NiMH. So future lithium ion hybrids could get twice the distance on same size battery pack as current NiMH hybrids. Well, not really, info on the LG Chem site, a big Korean lithium producer, shows only a 1.56 advantage in weight and 1.5 in volume per KW for their lithium ion battery over NiMH. Lithium cells are 3.7V, so the cell count is reduced by a factor of three.
No standard chemistry
But a big problem currently with Lithium Ion is that unlike NiMH, there is no standard cell chemistry (or size). Every manuf has his own chemistry with different tradeoffs. No 2nd sources, it's all custom. GM 'Volt' manager at a battery conference recently said "convergence" (standardization) of Lithium Ion is a requirement for electric cars.
Lithium ion spec
Neither LG Chem or 123 Systems has the spec of its proposed Lithium ion battery for electric vehicles online. However, 123 Systems does publish the spec of a couple of their high current lithium ion batteries and the best of them has about half the short circuit current of the Panasonic NiMH Prius battery above (not a fair comparison, see below). At low temperature 123 Systems publishes only the A-hr capacity. The short form data sheet does not show ESR (peak power) vs temperature, which is possible red flag that it may rise as in NiMH.
The highest power lithium cell data sheet I can find online is the 123 Systems 26650 (dia..length) cell (below). It's ESR must be non-linear. It's speced 10 mohm typ @ 10A, but on a discharge cycle graph (not shown) it 15 mohm. The lower right curve shows a 0.6V sag (it varies some with # of discharge cycles) at 40A discharge (@ 25C), which is an ESR of 600 mv/40A = 15 mohm consistent with the discharge cycle data. If the 15 mohm applies at still higher currents (not assured), then the short circuit current (Iss) would be 3.3V/0.015 ohm = 220A. This value for Iss is probably pretty close to correct, because the data sheet shows a 10 sec "pulse discharge" of 120A (curiously without specing the voltage).
Just as Panasonic specs their cell for maximum power out using a matched load, it would not surprise me if 123 Systems did likewise. And confirmation that is probably what 123 Systems is doing is that is that their pulse discharge current of 120A is pretty close to the calculated Iss of 220A. While 123 Systems doesn't spec it, the calculated (matched load) peak power out would be 1/4 x Voc x Iss = 1/4 x 3.3V x 220A = 182 watts.
Lithium ion comparison to NiMH
A fair comparison of lithium ion to NiMH can only be done by comparing either an equal weight (or volume) of cells. I compare based on weight. Below is a comparison of the Prius Panasonic NiMH module to lithium ion modules made by two companies that feature nano technology (Altair Nano and 123 Systems). By putting five 7.2V, 1 kg Panasonic NiMH in parallel and three 2.3V, 1.6 kg Altair Nano in parallel they are pretty similar in voltage and weight. For 123 systems I used sixty 70 gram cylindrical cell in a series parallel arrangement. I reduced the 123 Systems cell count by 20% to account for missing structure a module would have.
5,000 A (max) from 10 lbs of batteries -- pretty mind boggling!
Fifty of the cell combos of the figure in series would be a 550 to 600 lb, 330 to 360 V (open circuit) battery pack for an quasi-electric car. Below I show the stored energy (kwh) and peak power out (25C) assuming (like Prius) the load draws half the matched load current. (Note the 10 sec pulse rating of the x30 123 Systems cells (30 x 120A = 3,600A) supports the Prius 1/2 matched current 'rule', but the x1 Altair Nano spec (500A) does not). Panasonic NiMH 11.5 kwh 243 kw
Altair Nano Li ion 17.4 kwh 272 kw
123 Systems 22.8 kwh 619 kw
On an equal weight basis the Altair Nano lithium ion battery has (about) the same peak power capability as a NiMH battery and 50% more energy storage. The 123 Systems peak power is more than twice as high as NiMH and its energy storage is twice as high. There's no published data on ESR vs temperature, but the123 Systems 25C peak power is high enough that ESR could increase x4 at low temperatures and the battery pack could still deliver 150 kw (200 hp).
Altair Nano advertises that it's battery can be recharged in 10 minutes (123 Systems 15 minutes). The power calculations indicate this is reasonable (50 watts for 10 min in a 1.6 kg module). To fully recharge a 17.4 kwh battery pack in 10 minutes would require 6 x 17.4 kw = 100 kw (220 VAC x 500 A).
A123 Systems
A123 Systems high current cylindrical cell
.
ESR (@ 25C) = 600 mv/40A = 15 mohm
A123 Systems 20 kwh battery (Nanophase technology says Fisker spec) is being used in Fisker Karma, which is just starting production (about 200 made). I now find on the A123 site info on a battery pack they make for hybrid cars. The sell the complete pack with thermal management and protection. It's a good bet Fisker with limited engineering buys most of the technology from A123, but maybe not the package, because the form factor in the car is different from what I see in A123 photos.
A123 lists a 23 kwh pack and 22.5 kwh was the original Fisker spec, but in production it has been scaled back to 20 kwh. The core cell is primatic AMP20, 3.3V 20 Ahr (66 wh). The nominal Fisker battery voltage is 330V (in spec), so they have some multiple of 100 of these cells. 20,000 wh/66 wh = 303 (close to what I found speced for Fisker), so clearly the Fisker battery pack is about 300 or so cells arranged in 100 x 3.
Spec is 3C cont (10C 10 sec). C for AMP20 is 20A/cell, so 3C is 60A/cell and three in parallel is 180A. And 180A x 330V = 59kw (79 hp). So when the car is driven electric, the battery pack can deliver about 80 hp max (cont). My rule of thumb says 60 hp is about the wind resistance at 100 mph, but Fisker is a low sleek car, so probably can go faster.
Extrapolating from the smaller cyclinderical cell above resistance of 20Ah cell would be 1.15 mohm. At 3C (60A) this is voltage drop of 69 mv, and 60A x 69mv = 4.1W/cell. This is 4.1W x 300 cells = 1,230 watts dissipation for the whole battery pack when operating at its max continuous output of 59 kw. No problem here.
At peak levels from Fisker spec the battery pack must do 500A (150 kw) or 166A/cell. OK, the peak rating 10 sec is 10C, which is 200A/cell (10 sec).
Conversion of Prius to lithium ion
An independent company (EDrive) sells a lithium ion battery upgrade for the Prius. The 1.3 kwh, 80 lb NiMH Prius battery pack is removed and replaced with an EDrive 7.2 kwh, 200 lb lithium ion battery pack (2 in higher and 3 in longer). The peak power demands on the battery pack are unchanged since it is set by the inverter sizing. Hence each lithium ion cell can have an ESR that is x3 or so higher than NiMH (on an equivalent 1.2V/cell basis) and still provide all the peak power demands of the Prius electronics, which were, of course, designed to match the much smaller NiMH battery pack.
http://www.edrivesystems.com/faq.html
This upgrade costs 12,000 dollars (four hours of labor). The point of the upgrade is to make the Prius into sort of a quasi-electric. It's now plugged in to charge the battery, and since EDrive builds in (?) only a wimpy 1 kw (110/220 VAC) charger, the recharge time is speced at about six hours.
123 Systems conversion of Prius to lithium ion
Turns out that 123 Systems makes a 5 kwh lithium ion battery pack specifically for Prius conversion. Weighs 187 lbs. Conversion is done by HyMotion. Apparently battery is allowed to fully discharge, it is not recharged by Prius, only by plug in. It's not clear how its connected into the Prius architecture, maybe through a diode? Recharge time @ 1 kw from 120 VAC (only) is 5.5 hrs. There no real range spec, just says it "assists" in all electric mode for 30 to 40 miles. There's a link (on 123 site below) to a spec of the battery pack and what do you know the link is dead. Another case of a lithium ion vehicle battery with a hidden spec.
http://www.a123systems.com/hymotion/products/N5_range_extender
Here is interesting data on the Prius with the 5 kwh battery added. It's four EPA city cycles (EPA city cycle is below). Gray shows the engine running. It's a 30 mile, 1.5 hr trip @ 19.5 mph av, which is four EPA city cycles in a row. On the fourth cycle the battery is exhausted, it's a normal Prius, and the engine runs a lot more of the time.
30 mile, 1.5 hr trip @ 19.5 mph av, which is four EPA city cycles in a row
Prius with 5 kwh battery added runs mostly as an electric for 3/4th of time (67 min),
then on last 1/4th it drives like a normal Prius.
The Mercedes S400 Blue is the first production hybrid to use lithium-ion batteries. It's on sale now in Europe and USA in 2010, but when you look in detail at this car, so what? The press trumpets the battery doesn't take up room inside the car; it's under the hood and even fits where the lead acid battery used to go. Do you suppose maybe that's because its only 0. 7 kwh (truly tiny). For god's sake a standard car lead acid battery rated at 75 ah means it can put out [(75/20) A x 20 hr x (> 10.5V)] = > 0.787 kwh. That's right the lithium ion battery has about the same kwh as a standard lead acid battery, no wonder it fits in the same space! For perspective the Prius hybrid has a 1.3 kwh battery. The Mercedes S400 Blue is heavy full size car with a 3.5 liter V6 engine and costs about 88k. This is a 'mild' hybrid. It's can't even go 2 or 3 mph on electric alone says the German engineer. About all the 15 kw motor and battery do is quickly restart the engine.
Technically this does show that the peak current of lithium ion is pretty good, and says there should be no peak power issues with the much larger lithium ion battery packs in electric cars.
(Update 5/2010) Mercedes has a full page in New Yorker for S400 hybrid
"Powered by" a lithium ion battery.... God, marketing guys can spin anything. There's going to be some disappointed Mercedes customers.
"First hybrid sedan powered by a compact lithium ion battery"
"no loss of truck space"
(May 15, 2010 New Yorker full page add)
Note the implications of above for the GM Volt. The Volt is to use only 8 kwh of its 16 kwh battery (for lifetime). Here 5 kwh battery is pretty much depleted in 20 miles, and for maybe 15% of the time the engine is running. The implication is that an 8 kwh (usable) Volt in EPA test cycle driving would yield less than (8 kwh/5 kwh x 22.5 mile) = 36 miles range, whereas GM claims 40 miles. This partly explains the extreme emphasis put on aerodynamics of electric cars, but isn't the Prius after a decade of evolution pretty aerodynamic too? GM claims that aerodynamic changes alone to the production Volt added 6 miles vs the concept Volt (but I'm not sure if the drive cycle was specified).
Really high peak power
In the GM Volt, which is planned to hit the market in Nov 2010, the engine will not be coupled mechanically to the wheels. (Wrong!! This was GM marketing BS) This is very different from all current hybrids, where both the engine and electric motor are mechanically coupled to the wheels. Think about what this means in terms of current and peak power that its lithium ion battery must handle. It's really high!
I don't have hard numbers on the weight (see below) or hp of the Volt vehicle (nor of the battery pack, but for reference I'll compare it with my old mid-size Taurus, which weighed about 3,300 lbs powered by a motor rated at 200 hp (peak). To maximize the peak power to the wheels it's very likely that the Volt's motor's generator current ? power can beadded to the battery current ? power. In round numbers this means that 50 hp of the 200 hp to the wheels comes from the motor with the battery pack delivering the remaining 150 hp. This is probably on the mark, because 150 hp x 0.74kw/hp = 111 kw, and this is the rating of the Volt motor and inverter.
(update) An early report (Aug 2008) out of UK that weight of GM Volt was expected to be 3,500 lbs.
Compare the peak electrical power needed from the battery pack in the Volt to a 'conventional' hybrid, the Prius. Even though the Prius battery cells are running at 180A the electrical power out is only about 36 hp. The Volt battery pack must deliver something like x4 higher peak power than the Prius battery pack.
Lithium ion paralleling
Initially I thought this meant lithium ion cells for vehicles needed to have very low ESR, lower than NiMH. I don't know if such lithium ion cells exists. The lowest lithium ion ESR I have found (123 Systems) is an equivalent basis x2 higher than NiMH (20 mohm for each 1.2V cell eq) But I now see there's a work around. If the lithium cell battery pack has a high energy storage (high kwh), then it will use lots of paralleling (or equivalently the cells with have larger area). This kills two birds with one stone. You gets lots more all electric driving range and the constraints on ESR are loosened.
Very low cell ESR not required with large battery pack
For example, if x6 paralleling is used (as I suspect it is in the Volt) then x3 higher peak current/power can be delivered from the battery pack with each cell have an equivalent ESR twice that of NiMH, which is consistent with ESR of published 123 Systems lithium ion cells. So sort of as a freebie with a large lithium ion battery pack sized for range, enough peak current is available from the battery (perhaps with some supplemental current into the inverter bus from the generator) to allow the design option of driving the wheels only with the motor, dispensing with all mechanical coupling from the engine to the wheels.
We can bound the battery pack volt/current peak options. The following combinations of V and A multiply to 111Kw (150 hp). Most hybrids today have battery pack voltages in the 160 V to 320 V range. Assuming the peak battery power is (like Prius) limited to 75% of the 'matched load' capability, we get:
200V battery pack 150 V x 750 A
300V battery pack 225 V x 490 A
400V battery pack 300 V x 370 A
500V battery pack 375 V x 300 A
Fire risk?
As best as I can tell, what seems to be holding back Prius from going to lithium ion batteries is fire risk. A year or two prior to the introduction of gen III , Toyota said lithium ion was ready for gen III, but when gen III arrived in 2009 it had NiMH. The fire risk assessment come for a few comments of Toyota people. Who knows, maybe it's cost and weight too.
I read lithium ion batteries for cars will not be the same as for laptops, and the reason appears to be fire risk. If something fails in a lithium ion laptop battery, there is a run away oxidation --- translation: fire. Vehicle lithium ion use different chemisty with lower energy density to eliminate the run away condition.
Lithium battery chemistry
Lithium battery chemistry looks complicated and initially I avoided the topic, but a long article (now hugely revised) in Wikipedia gave a good overview. It argues that electric vehicle people were interested in using lithium iron phosphate (LFP) chemistry for their battery (LiFePO4). This is different from most of the small lithium ion batteries used in computers and consumer products. These are mostly lithium cobalt oxide (and some lithium manganese oxide and lithium nickel oxide). The names refer to the material of the cathodes, anodes are generally carbon (graphite). In other words in electric cars iron is to replace cobalt, manganese and nickel.
The iron version of lithium ion batteries has only about 60% of the energy density of the other types, and it has other problems too (like high ESR). So why use it? Well iron is cheaper and more available than cobalt, but the big reason is safety, safety from fire. You can't have cars going up in flames at anything like the same rate as laptop batteries have gone up in flames. Iron also is better for the long lifetime and many cycles car batteries need. Drawbacks (besides lower energy density) is limitation on peak current (high ESR) both charging and discharging. An intrinsic lower peak discharge rate for iron may not be a problem for an electric car, because unlike a hybrid car, there is a lot more battery in an electric car, so each cell is much less stressed when the car accelerates. The Wikipedia article on A123Systems says its chemistry is LFP, which it is working to improve with nano technology.
Wikipedia gives these generic LFP specs:
Gravimetric energy density = >90 Wh/kg (>320 J/g)
Working voltage = 3.0V–3.3 V
Let's check using the GM Volt as a test -- 16 kwh, cell weight of 375 lbs (170 kg)
GM energy density = 16,000 Wh/170 kg
= 94 Wh/kg Ok (agrees nicely with >90 Wh/kg)
GM opts for lithium manganese phosphate
But then I read that LG Chem that GM chose to provide the battery for the GM Volt is not building LFP (lithium iron phosphate) batteries! LG Chem's chemistry is Lithium Manganese Oxide. (This is hard to dig out. LG Chem says almost nothing about their chemistry on there site.) Some sites painted the face off between 123Systems and LG Chem for GM's business as a face off between lithium iron phosphate and lithium manganese phosphate chemistry. Indeed a GM executive said it was all about the chemistry.
One reference summarized the lithium ion battery chemistry this way:
200 Ah LFP (lithium iron phosphate), 7.3 kg
source -- http://www.thunder-sky.com/pdf/2008926101921.pdf
Found some early (2008) data on the lithium battery being developed by Nissan. Nissan formed a joint battery company with NEC called Automotive Energy Supply Corporation (AESC). Their cell is lithium ion with a (spinel) manganese cathode (LiMn2O4). Cell voltage is 3.6V. In 2008 their prototype lithium ion vehicle battery had these specs: (Wow, ESR goes up by almost a factor of ten at -30C)
cell weight 527 grams
voltage 3.6 V
capacity 13 Ah (13Ah x 3.6V = 0.047 kwh)
power (25C, 2.5V) 1,086 watts = (434 A x 2.5V)
ESR (25C, derived) 2.5 mohm = (1.1V/434A)
power (-30C, 1.8V 116 watts = (64 A x 1.8V)
ESR (-30C, derived) 22 mohm = (1.4V?/64A)
With this battery (L3-10) a 24 kwh battery would need 510 cells, which would weigh 592 lbs. The 2008 article quotes a Nissan battery engineer says they are going to increase the capacity to 30 Ahr to reduce the number of cells, then a 24 kwh battery would have 221 cells. This proposed larger cell is in the ballpark for Leaf which has [48 x 4] = 192 cells and a battery pack weight (with structure) 660 lbs.
At 90 kw output each of 192 Leaf cells is outputting 469 watts =(90,000/192 cells), or about 130 A per cell. Scaling the numbers above by (510 cell/192 cell) = 2.66, cell power at -30C is (2.66 x 116 watt) = 308 watts. This indicates that the peak power of the car with battery at -30C is rolled off by (at least) a third. We can also figure the battery sag at 90 kw, and at 25Cm, and it is very small (2.5mohm/2.66 x 130 A = 0.122 V, or a sag from 3.6V to 3.48V. This is a peak battery power dissipation of only (0.122V x 130A x 192 cells = 3.04 Kw), or about 3% of the delivered power.
Comparison of Volt and Leaf lithium battery thermal management (12/15/10)
My nephew asked me what was new with the Volt, so here is my reply covering [Volt = hybrid] and summarizing how Volt and Leaf are handling the thermal issues raised by the lithium ion manganese batteries they are both using.
Toyota Plug-in Prius quasi-electric with 5.2 kwh lithium ion battery is using Leaf like thermal management: three fans (controlled by 42 temperature sensors)Battery coolant problems (update 1/10/2012)
Real world problems of liquid battery cooling my have reared its head. Most new electric cars are liquid cooling the lithium ion battery, the Leaf being the big exception. But what happens if there is a battery coolant leak? Of course, the same question can be asked about coolant leaks in normal gasoline engines, but there is over a century of experience in their design and use.
The known facts are these: Volt has recently recalled all of its 8,000 or so Volts to do something they call strengthening the battery container. Fisker Karma has recently recalled all of its 250 or so cars because of a possible clamp problem allowing battery coolant to leak apparently into the battery container. The Volt recall was prompted by a fire (and 2nd almost fire) in cars sitting weeks after being crash tested. The facts are a little unclear, but the claim is the battery cells were not directly damaged, so the suspicion is that the battery coolant system was damaged.
I suspect the problem may be this. The battery containers are probably a full of little and big circuit boards that make up its monitoring and protection system. And of course the coolant is flowing all through the battery container to get the heat out of every cell. Hence the problem, I am guessing, is that a leak of the coolant is more likely than not to be into the battery container, and (if they are not well sealed) onto those circuit boards. My memory is this was specifically mentioned in the case of the Karma recall, and could explain the otherwise mysterious catching on fire of crash tested Volts that had been sitting in storage for weeks.
quasi-electric GM Volt
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Seeing a Volt (update 11/27/11)
I saw my first Volt today and had a chance to talk to the driver. Volt was plugged in to one of two electric charging stations (installed by US dept of Energy) directly in front of Boston City Hall. Clearly the charging stations had been positioned for publicity as an electric car to be charged needs to be parked in a 'no parking' area on a sweeping curve on a busy downtown Boston main street. I was surprised the sign on the chargers said "free" charging. The driver, who said his company was testing the Volt, said well 'free' means paid for by taxpayers. Also surprised that the cord was part of the charging station. Noticed another weirdness. The Volt charging plug is not conveniently located for on-street charging. With the car parallel parked (as here) the plug is on the drivers side near the front door and near traffic, so the long cord from the charger has to be run around the car to reach the plug.
The Volt is quite a small car and tight inside. The driver commented there was not much head room. He said it had a lot of pep. I was standing next to it as he drove away, and it was nearly silent (no traffic on a Sunday night).
Electric ? quasi-electric car sales (11/28/11)
A total of 9.5 million new vehicles had been sold in the United States this year through October.
* G.M. has sold 5,300 Volts since introducing the car a year ago in late 2010
* Nissan has sold 7,200 all-electric Leafs in the first 10 months of this year
* Tesla has sold about 2,000 electric cars since 2008
Fire risk (11/28/11)
Collision damage to the Volt battery back has shown it can catch on fire. Hard to evaluate this risk at this point. In the inelegant phase of GM spokesperson, “This is a postcrash activity." A crashed Volt sitting in storage recently caught on fire 3 weeks after the crash. A real slow motion problem. Three Volt batteries were then damaged (but how badly is not described) and one caught on fire and one smoked. Fire risk with large lithium ion batteries has long been a serious concern, just look at the recall of laptop batteries for possible fire risk.
GM is sort of covering up the weakness. Look at this statement, "A pressing issue, (GM spokesperson) said, is ensuring that batteries are de-powered by trained service personnel after a collision". In other words GM is sending out people to discharge the batteries after an accident or crash testing. This is equivalent to drain the gas out of the tank of a crashed car, which I suppose is sometimes done.
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Cold weather affects battery (update 2/29/11)
I wrote 'cold weather nightmare' (below) more than a year ago. Now the truth comes out as to why the low temperature characteristics of the lithium ion car batteries have been hidden. Consumer Reports says that the Volt it owns (cost 48k because the dealer slapped on a 5k premium charge!) and is driving in Connecticut this winter (we are not talking northern Minn here!) is only getting 25 to 27 miles electric range. One third of the electric driving range is lost at moderately low temperatures (southern NE winter) Consumer Guide borrowed a Nisson Leaf and reports it "it also gets very short ranges in very cold weather".
Cold weather nightmare?
I'm beginning to suspect that GM must be really sweating about whether the limited temperature range of the lithium ion batteries is so crippling that the GM Volt may not be a practical car, at least in the northern half of the USA. Consider the following tidbits: 1) Many types of Li-ion cell cannot be charged safely below 0C (32 F) (Wikipedia)
2) "Right now we're thinking (below) 0-10?C (32F to 50F) we won't use the battery"
(Jan 2009 article quoting head of GM battery development for Volt)
It appears that first generation (car) lithium-magnesium ion batteries are delicate things, and for lifetime reasons can only be power the car within a fairly narrow temp range (32F to 72F). GM has addressed this problem in three ways: one, well insulating the battery, two adding a battery liquid cooled heater/cooler, and three, using the engine to protect the battery. The Volt has an engine, so if the battery is even a little warm (> 72F) or moderately cold (?32F), the engine comes on to run the car (the battery probably still used in a hybrid mode for accelerations.) So what is the Leaf going to do? With no engine they have to use the battery whether its warm or cold. Do they use a different lithium battery chemistry?
The Volt battery cooling/heating system keeps the battery temperature in a very tight range of 70F +/- 2F. Sure enough if the car is plugged in then the heating/cooling power comes from the grid. If the car is parked (not plugged in) in hot weather, then interestingly battery power is used to run the battery air conditioner, so just parked the battery will be running down. They limit how much the battery discharge in this mode to 75% of capacity. Below this capacity the cells heat up sitting parked in hot weather. They do not run the battery heater when parked in cold weather. The cells just get cold, and then the engine runs when the car is started if cells below 32F. This battery temp control strategy is going to get a good try out, because in 2011 (or 12) the car is to be widely sold in Canada. The problem with high temperatures is battery lifetime. It's 8 years at 70F, but only 5 years at 90F.
If the battery is above 122F or below -13F, then the car simply will not run!! until the battery heater/cooler can bring the battery temperature into line. Posters understanding is that a cold soak forces a plug-in to recover, which sound ridiculous to Canadian posters who think they may come out of work on a cold day and be stranded. One of the key issues which no one has data on is how good is the battery insulation and what is its temperature response time. They are guessing the insulation is good enough that battery temperature will remain below 122F after being parked 8 hours in the Arizona sun.
Summary of GM Volt battery thermal management unit (heater/cooler)
posted by ChrisC at link below 12/12/10
http://gm-volt.com/forum/showthread.php?5243-Volt-thermal-management-system-temperature-band/page12
Winter commute
But it seems to me the crux of the winter problem is this: You buy the car to commute 10 to 15 miles to work and back. With the touted 40 mile all electric range you expect you can run the Volt in all electric mode using only a home charger. But what happens when the car sits outside at work for 10 hours in winter not plugged in? Unless power is bled from the battery to self heat it (which I suppose is a possibility), the battery pack will be at the ambient for the commute home, and according to tidbit 2), if the ambient is below 32F (or maybe even 50F), the battery in your electric car is not just weak, it is totally unusable!
Now you can still get home because the engine/generator can put out 70hp (generator rating), but the car is horribly underpowered. With the battery unusable it's not a hybrid, it's just a heavy 3,500 lb car with a 70 hp available for acceleration!
(update 10/11/10) Finally what looks to be a reasonable detailed spec for the GM Volt has appeared (below).
http://gm-volt.com/full-specifications]
It includes some new info like (finally) the capacity of the gas tank (9.3 gal) and cooling capacities of the battery and electronics and the number of battery cells is finally disclosed (288). Revealing is that the EPA city range is now given as 25 to 50 miles electric, not 40 miles always claimed by marketing. Charging times are given, but are meaningless since the charge kwh is not given. The numbers alos look funny. If 8 kwh is the maximum allowed discharge and since 1.5 kw can be pulled from 120 VAC with a good charger design, then an 8 kwh charge should only take 5 1/3rd hours, not 10 to 12 hours!
Spec
battery (LG Chem) 16 kwh, lithium ion
288 prismatic (10/11/10), 3.5 V? cells (arranged how?)
(220 cells 12/10 Wikipedia )
liquid cooled
436 lbs
built-in charger 3.3 kw (7/10)
usable capacity 10.4 kwh (25% to 90% capacity)
originally 8 kwh (30% to 80% capacity)
(update 10/8/10( GM is now saying 'somewhat more' than 8 kwh
will be used, possibly 10 kwh. Also as the battery ages the
usable fraction of the battery range will be expanded to prevent
milage range from shrinking. Oct 13, 10 PopSci test driver
says 65% of battery (10.3 kwh) is "customer usable" and
Wikipedia now says 10.4 kwh
motor 111 kw (150 hp), PM
Technical SAE paper by GM engineers says Volt motor and generator
as PM (not induction as earlier suspected)
272 ft-lb (368 nm) torque
generator 54 kw (72 hp), PM
max speed 100 mph
0 to 60 mph 8.8 sec
(Leaf at 424 lbs ligher with 80 kw motor is 10.0 sec)
cooling (liquid)
battery pack 7.4 qt
generator 7.7 qt
power electronics 3.1 qt
engine 1.4 L, 4 cyl
(GM Family 0, 1.4 L, 4 cyl, 66 kw, 90 hp @ 5600 rpm)
GM specifies premium gas!
wheelbase 105.6 in (midsize = 108, full size 112)
weight 3,790 lbs (10/11/10), earlier estimated as 3,500 lbs whoops!
(424 lb heavier than 3,366 lb Nissan Leaf)
gas tank 9.3 gal (10/11/10)
9 gal says Consumer Reports (6/28/10 update)
engine mile/gal (implied) 32 mpg = [(340 miles - 40 mi electic)/9.3 gal tank]
Popular Mechanics measures 32 mpg city, 36 mpg highway
charging 110/220 charger built-in (Connects direct to line
via SAE J1772 cable)
10-12 hr @ 120 V (how many kwh?)
4 hr @ 240 V (how many kwh?)
EPA city range (EV mode) 25 to 50 miles electric (varies with terain, driving style, and temp)
Popular Mechanics got 31, 33, and 33 miles EV in test drives
For reference gen III Prius: 106.3" wheelbase, weight 3,042 lbs, 98 hp (1.8L) engine)
Test drive --- Popular Science 10/13/10
Car drives the same in all modes. Transistion between modes almost imperceptable. Engine can barely be heard when it's running slow. In charge depletion mode the car runs as a hybrid, so acceleration is from battery, followed by engine RMP revs increasing as the generator works to not only power the car, but to recharge the battery back to the nominal hold voltage. This decoupling of the engine noise and its lag after acceleration takes some getting used to. Their biggest disappointment is that gas milage in charge depletion mode they measured (just 38 miles) to be 37 mpg. It is a heavy hybrid.
Estimate of peak acceleration power
(update --- The analysis below assumes the Torque vs Speed profile of the car is approx rectangular, making the power vs speed shape triangular. This may or may not be true for the Volt. The peak hp of 141 hp is within the capability of the motor assisted by the geneator (working as a motor), but another way to boost accleration time with limited power capacity is to have a wide contant hp range in the motor. This is what I suspect is done in the Leaf.)
With the weight of the GM Volt speced and 0 to 60 mph time (To) measured, a (rough) esimate of peak (acceleration) power to the wheels can be made. The simplest model is constant torque leading to a linear ramp of velocity with the maximum power point (Po) at 60 mph. This mades the energy delivered to the mass of the car during the acceleration
m = 3,790 lb/2.20 lb/kg = 1,723 kg
v = 60 mph = 26.8 m/sec
To (0 to 60 mph) = 8.8 sec
E car kinetic = 1/2 x m v^2 = 1/2 x Po x To
= (1/2) 1723 x (26.8)^2
= 619 kj
Po = (m v^2)/To
= (1,723 kg x 26.8 ^2 m/sec)/8.8 sec
= 141 kw
Po = 141 kw is a very interesting answer (if on the high side). But it's reasonable since it is within the kw range of the drive train. It requires (as I suspected) that both the main 111 kw motor and the 54 kw mot/gen are being used to accelerate the car. From another view point @ 60 mph the car has acquired 619 kJ of kinetic energy. If the power pulse was rectangular, this power would come in half the 8.8 sec (0 to 60 time) or 4.4 sec, so this makes the peak power (Po) = 619 kJ/4.4 sec = 141 kw.
Let me list the design challenges.
Heavy, expensive battery pack (400 lbs, 700 lbs with structure)
New battery technology, chemisty still being tweaked
Battery lifetime issues due to relatively deep discharge (compared to hybrids)
How much battery protection is needed/tradeoff with energy storage
High power inverter (or smaller parallel inverters like Tesla?)
Battery ESR increase in cold weather limits peak power
(or problems with lithium ion chemistry in cold weather)
Large PM motor (shift from PM to induction to handle heat?)
Motor if PM cannot allowed to get too hot.
Peak power rolls off if hot
Overtemp trips disable car (unlike hybrids
there is no back up with direct engine to wheel)
Engine noise ? vibration not correlated with vehicle speed or torque
Line recharge ports (separate converter with 120 to 240 VAC range?)
(480 VAC not mentioned)
GM Volt at auto show (spring 2009)
Very strange, why isn't the 4 cyl engine exposed as in every other car,
a mechanic will need to go on an Easter egg hunt to find it.
Is that double radiators in front?
Volt has been widely touted by GM as able to go 40 miles without the engine turning on. Now GM says well it's 40 miles (what speed, what terrain?) with all accessories turned off, so it's no heat and no A/C! Notice in the specs the generator hp rating (72 hp) doesn't match the engine hp rating (90 hp), which on its face is odd. This may just be rating conditions, or it could be a real mismatch because the engine is an available engine, whereas the generator would be designed for the Volt. If current from the 72 hp generator can feed directly into the inverter, which I assume it can, then at peak motor/inverter power (150 hp) peak power demand from the battery is 78 hp (58 kw). Electrically for the battery 58 kw (peak) means for x2 paralleling [58,000 = 315V (= 200 cells x 3.5 V x 0.9 sag/2) x 184A] or for no paralleling [630V x 92A].
Tragic flaw?
Some posters to Volt articles think the Volt has a tragic flaw. After its battery runs down, it becomes a heavy car (est 3,500 lbs) with a 1.4 L engine. "It will be very underpowered", said one poster. This thought flashed through my mind too, but it's hard for me to judge from experience, it needs to be calculated. In all non-hybrid cars the engine is sized for the peak power demands, the worst case may be accelerating up a hill.
It is a hybrid (update 7/10, 12/10)
GM recent releases confirm that when running on the extended range engine it works like a hybrid. With such a huge battery there should be plenty of juice for acceleration. The engine (80 hp) has just enough (av) power to overcome air resistance at its 100 mph (rated top speed).
In Oct 2010 the story from GM said only that the engine would be mechanically coupled to the wheels in charge depletion motor above 70 mph. By Dec 2010 the story has changed again. Now Wikipedia says in charge depletion mode engine will sometimes (acceleration) mechanically drive the wheels at speeds of 30 mph to 70 mph, and above 70 mph it drives the wheels all the time.
Volt cost (update Jan 2010)
GM executive speaks the truth about their 40k car ---
GM confirmed that the GM Volt will also have a 12V battery. Clearly 12V is needed to run wiper and window motors and lamps, etc, so the issue is whether to use a 12V battery or a DC/DC on the big battery. There is also the interesting question of how the engine starts. In hybrids it's standard for the generator to run 'backwards' and start the engine. The off-the-shelf engine they are using will normally have a 12V alternator and 12V starter, so I can believe it's just possible that at its introduction the 12V battery will be used to start the Volt engine. Unlike a hybrid where the engine restarts at every traffic light, here the engine will start infrequently. GM of course, as of early 2010, has said nothing about this.Wind resistance power
One poster says continuous highway speeds only require about 47 hp. Since wind resistance rise as speed^2 and power = force x speed, power needed to overcome wind resistance rises as speed^3. Considering only wind resistance it takes 237% more engine power to cruise at at 80 mph than at 60 mph. My scaling of Prius data (confirmed by Wikipedia) is overcoming wind resistance takes about:
13 hp / 9.7 kw @ 55 mph
17 hp / 12.6 kw @ 60 mph
40 hp / 29.6 kw @ 80 mph
57 hp /42.2 kw @ 90 mph
79 hp /58.5 kw @ 100 mph
Conclusion: for a car with a top speed of 100 mph, like GM Volt, the minimum engine size is 80 hp, and surprise, surprise the GM Volt engine at 90 hp is just above the 80 hp wind resistance minimum.
Lithium ion cars hit market?
"G.M. Says Chevy Volt Is Still on Track (delivery in late 2010)" (June 2009) By Oct 2009 they will have 80 built as pre-production test and advertising vehicles. Estimates are it will cost 45k or more, and it looks like a small car!
I read in later 2010 Toyota is planning to sell some lithium ion vehicles to fleet buyers. (Aug 09) GM will also be delivering 125 Volts to electric utilities in Virginia for testing prior to Nov 2010 launch. US dept of energy have given GM 30 million for testing of Volt (125 fleet and 500 customers), which is a subsidy of 46k per vehicle. In other words our government is effectively buying the cars and giving them away!
Volt at low temperature
GM marketing repeatedly claims: "Volt is designed so the first 40 miles of driving are powered by the electric energy stored in the battery." But it now turns out that's not true in cold weather. GM chief engineer has confirmed that at low temperatures the engine will run to supplement the battery. At low temperatures, this car is somewhat like a modern locomotive, where the engine runs a generator and the generator powers the motor.
The lithium ion battteries probably weaken at low temperatures. My guess is the main worry may be large increases in ESR at low temperatures, as in NiMH, dramatically reducing the peak power.
Wikipedia (GM Volt) has the following vague low temp stuff, which is pretty vague but scary enough to say this is California type car!
-- "Right now we're thinking 0-10?C we won't use the battery." (ambiguity #1: does this mean 0 to 10C or does it mean somewhere between 0C and -10C. It reads the former and this is how its quoted in Wikipedia, but my guess is it's more likely to be the latter.)
So here is the GM chief battery guy saying in early 2009 that the battery may not be used if it's temperature gets below some threshold. But this statement is also not clear. Does he mean not used at all, or does it mean not used for range, but can still be used for a few seconds during acceleration like in a hybrid?
The NYT auto writer writes periodically about the Volt. His Nov 24, 09 article says this:
I had read the Volt engine would always run an optimum speed for efficiency, which made sense. Now I read that the Volt engine will be 'tuned' to run at one of several different (fixed) speeds, but there is no explanation from GM as to why. And there is concern its different sounds will be disconcerting as they are not coordinated in the usual way with speed or acceleration. (Wiki -- GM Family 0 has a 1.4 L, 4 cyl rated 90 hp (66 kw) @ 5,600 rpm, which matches well with speced 53 kw generator.) What is going on here? Why is the engine speed varied? Maybe this is the answer (from an article in ESD magazine)
GM Volt on front page as price of 41,000 (base model) is announced. They will begin deliveries in late Nov 2010 as promised, but production levels scheduled for 2011 are very low (10,000) with 2012 production set for 45,000. Going on sale in a few months, but no specs and no mil/gal estimate! They still just say the battery has "more than" 200 cells.
Instrument panel has a 7" touch screen and the car has a 30 Gb hard drive (for audio storage)!! Charge times given as 10 hr @ 120VAC and 4 hr @ 240 VAC, but material on the ChevroletVoltage.com site which supposedly has the specs gives the charge time as 8 hr @ 120VAC and 3 hr @ 240 VAC! Battery is liquid cooled and heated. Comes with a 120 VAC power cord. Four seats (battery pack prevents a bench in rear). 8 yr warrantee not only on battery, but on the whole electrical drive system. The car will initially be sold in seven states, but it includes hot and cold states: Texas, Michigan, NY, NJ, CA, CT and MD.
Range given as 340 miles. Is this [40 miles from battery + 6 gal x 50 miles/gal]? Column in NYT says the Volt is more or less the electric version of the $17,000 Chevrolet Cruze. They frankly admit in the price announcement that you won't get get 40 mile electric range if you use utilities or outside weather is bad, but of course they don't say by how much, zero details!
Usable Kwh?
Specs clearly identify battery as 16 kwh, but nowhere in the recently announce material is the usable kwh given. Previously it was 8 kwh. However video below has a battery gauge (see minute 2:21) which shows the battery being discharged to about 1/4th of capacity, which would be a 12 kwh usage!
http://www.chevroletvoltage.com/index.php/Content/technical-presentations-blogs-other-information.html
Chirping sound
Well the good news is finally an electric car has a built in noise maker to protect people. The bad news is the 'chirping' sound the GM Volt will make. Listen to this horrible horn like sound it makes!!
http://www.chevroletvoltage.com/images/stories/blog2/volt%20chirp.mp3
Premium gas
The GM July 2010 price announcement included the weird fact that the engine will require premium gas. No explanation given and the experts are scratching their heads. One columnist says 80 hp from a 1.4L engine is not bleeding edge. Later someone weaseled this statement from GM press people:
Aerodynamics
Electric cars are focusing on aerodynamics to extend range. According to an Aug 2008 interview with GM designer, the Volt concept car when tested aerodynamically didn't make the 40 mile design range. It made the 40 mile goal after a bunch of aerodynamics tweaks (including lowering the roof to reduce frontal area) that increased the range by 6 miles. This is a 17% driving range increase just from aerodynamics tweaks! I guess electric cars are going to be low cars.
.
Shows Volt plugged in (Jan 2009)
Car male plug is on driver side just in front of the rear view mirror
(Is this plug right? It looks like three pins,but the standard SAE J1772 connector has 5 or 7 pins.)
123 Systems in Watertown (spending 60 million on new factory) and a large Korean company (LG Chem) are the two companies competing to provide the Lithium ion batteries for the GM Volt. In Jan 2009 I read LG Chem has been chosen.
(update) Dec 2009 Michigan Gov with GM officials announced 336 million investment in a plant to make the Volt (Detroit's Hamtramck Assembly Plant).
(update) Jan 9, 2010 Michigan gov showed up at opening of GM's new lithium ion battery plant (for Volt) in Brownstown Township. 106 million of US taxpayer dollars to be invested in this one battery plant! Here is photo (shown with thermal shield) of first Volt battery pack from the plant.
Open and closed views of GM Volt battery pack
Visible stack up looks like 96 modules in series (345V battery voltage)
Each module has three parallel 3.6V lithium ion cells (288 cells total)
Peak power: 330 VDC x 330A = 100 kw (approx)
330A is probably carried by two orange wires (left)
GM Volt lithium ion battery pack (from new Mich factory) Jan 2010
(at right can be seen how the foil insulation is just glued to case)
GM Volt production battery pack (without external foil insulation)
> 200 cells
16 kwh with 8 kwh usage
Even with a production GM Volt battery I can't find the battery voltage. Nor can I find the LG Chem spec for the battery. But some specs can be derived from what is known of the battery pack VAh 72.7 VAh/cell = 16,000 VAh/220 cells
Ah 20 Ah rating @ 3.6V cell = 72.7 VAh/cell/3.6 volt/cell
weight 826 gram/cell = (400 lb x 1 kg/2.2 lb)/ 220 cells
The largest lithium ion battery on the LG Chem site is their E, which is about half the GM Volt cell. This is an old spec (2004).
E1 Ah 10 Ah @ 3.6 V
Weight 245 grams
Dimension 20 cm x 9.4 cm x 0.7 cm
ESR ? 3 mohm = AC impedance @ 1 khz
Max continuous discharge 30 A (P/cell @ 30A = 30^2 x 3 mohm = 2.7 watts)
"short circuit test Z" 50 mohm (I pk = 3.6V/ (50 + 3) mohm = 68 A)
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MIT Technology review 27 min video of GM Battery Plant tour (6/2009)
Some of the best GM Volt battery info I garnered from a long (27 min) video taken by MIT's Technology Review magazine energy editor (Keven Bullis) during a tour June 2009 of GM battery plant. The most interesting speaker, Bill Wallace, Engineering Group Manager), Rechargeable Energy Storage, starts 19:12 into video. (Wallace identifies himself as the engineering group manager for the Volt battery.)
http://www.technologyreview.com/video/?vid=369?channel=business
GM Volt production Li-ion prismatic cell
(screen capture from MIT Technology Reivew GM battery tour video, 6/2009)
-- engineering manager speaking
-- battery is at production stage, 5th iteration of car battery pack design
(100 packs built at time of video, 300 packs to be built by Q3 2009)
-- Volt battery pack is 1/3rd volume and 1/3rd the mass of EV1 battery pack
(EV1 lead acid battery version had roughly same Kwh and peak power spec as Volt)
Ev1(left) vs GM Volt (right) battery pack
(GM Volt 16 Kwh, EV1 lead acid 16 to 18 Kwh)
-- 110 kw "peak power"
-- 400 lbs
-- enclosure is fully sealed against water ? dust (since it's under car)
-- 155 different components inside the battery pack
-- notch in battery pack is for a car structural element to pass though. The battery pack
itself includes some structural elements
-- "over 200" Li-ion cells
(he smiles -- why is the # of cells being hidden, which it clearly is?)
-- we cool and heat every cell in the pack
-- highly insulated, which is key to maintain the batteries' temperature,
"temperature kills batteries"
Questions (to Wallace)
-- Is battery thermal management done with liquid or air? answer liquid
-- What is # of cells. ans, I can't give you an exact number but it's between 200 and 300
when pressed he said closer to 300, but less then 300
-- What temperature are you maintaining inside. "ideal temp for Li-Ion is
room temperature, 25C. Inside temperature is held appox between
10C (50F) and 35C (95F).
-- So what's the thermal strategy, say when you are parked? How much energy will you draw
when parked? Tesla had a problem drawing energy to run a fan when parked. Ans,
basically we're working on it. We have the capability to heat or cool battery in
any mode, even if car is just sitting. Passive insulation is very important.
Once you shut down car you need to be able to maintain (battery) temperature
without using energy. To do this we use a "very expensive" insulation package.
Summary of video GM Volt battery info (6/2009)
Khw 16 Kwh ("usable")
Peak power 110 Kw
# of cells 275 cells +/- 25 cells ("200 to 300, but closer to 300")
288 prismatic cells (10/11/10 spec update)
Battery voltage (oc) 346V (est) based on 288 cells @ 3.6V wired (1/3/11 update)
3 in parallel x 96 in series (based on 2008 article in Pop Sci
on battery pack that showed battery was 300 cells wired 3 in
parallel and x100 in series)
Cell Ah (est) 16 Ah @ 3.6 V = 16,000 Whr/275 cells
Est cell pk current 110,000 watt/990V = 111 A
Cell ESR (est) ? [0.33 (Toyota) x 3.6V/ 111A = 11 mohm]
(scaling from LG Chem spec 3 mohm/1.6 = 2 mohm approx)
Cell weight (est) 384 grams = 16 Ah x 3.6V/150 Wh/kg
Pack cell weight 106 Kg (233 lb) = 384 gram/cell x 275 cells
Pack weight "400 lbs" (unclear if this includes structure, but it seems to)
Thermal 10C to 35C (50F to 95F) operating cell temperature
(uses expensive passive insulation and liquid thermal
heating/cooling system, system is sealed for dust)
Note these numbers hang together well. Each cell looks like it has about x1.6 the capacity of the 2004 LG Chem cell (above), and this agrees with the picture. And for first time real info on the battery pack thermal heating/cooling system and a 10% accurate estimate of number of cells. These Li-ion Khw cell numbers look excellent about x3 higher than NiMH cells by weight.
The Prius NiMH cells are packaged in handleable plastic cases and stacked up. In Volt the photo shows each cell looks like a (delicate) foil with protection provided by a sealed outer structure. In Prius it looks like a bad cell could be popped out and replaced, but not in the GM pack, which is sealed.
Peak voltage sag (10/15/10)
In my GM-Volt block diagram I show the peak power from the batteries could possibly be 165 kw if both the 111 kw main motors and the 54 kw mot/gen are run to their limits. My guess is the 288 cells are arranged as 96 stacks of three in parallel forming a 350 VDC bus. The peak cell current then would be about [1/3 x 165,000/350V = 157 A] @ 165 kw.
Taking the 2 mohm cell impedance estimate above the voltage droop of the bus would be [96 x .002 ohm x 157 A = 30 V] @ 165 kw. This is only a modest 8.5% drop and would have little effect on performance. On the other hand if cell impedance is at the high end of the estimate (11 mohm), or 5.5 times higher than 2 mohm, then the bus sag is something like [5.5 X 30 V = 165 V] (or more), which brings the bus below 200 V and would be intolerable. Conclusion: unless there is a voltage regulator in the GM Volt architecture, then the cell impedance has got to near the low end of the estimated range in the 2 to 4 mohm range
Summary of GM video thermal info
The battery cells are seal into a very well insulated liquid heat/cool structure that will hold the temperature inside between 50F and 95F. Expensive passive insulation has been used to minimize the battery drain when parked in cold or hot weather for temperature control. (No hint as to the energy required for cooling when driving in hot weather. There appear to be no fins on the package that might aid cooling by using the air flow under the car.)
Assuming 1.9 mohm/cell (scaled from 3 mohm LG Chem spec for 16 Ah cell) @ 110 kw
volt drop/cell = 110,000 w/(275 cell x 3.6V/cell) x 1.9 mohm
= 111A x 0.0019 ohm
= 0.21 V (@ 110 Kw peak power)
(minimal sag at peak power 3.6V => 3.4V)
watt dissipated/cell = 0.21 V x 111A
= 23 watt (@ 110 Kw peak power)
watt dissipated/pack = 275 cell x 23w/cell
= 6.4 kw (@ 110 Kw peak power)
efficiency = 110 kw - 6.4 kw/110 kw
= 94% (@ 110 Kw peak power)
ESR losses go as current^2 (power^2), so averaged over driving conditions losses will far lower than the peak power shown above.
The small voltage sag of the large Li-ion battery pack, even at peak power, means that unlike the much smaller battery packs used in hybrids, there is no need to regulate the battery voltage, the battery can be applied directly to the motor inverter. My guess is that they are paralleling x3, resulting in a nominal 330 VDC battery voltage used as the bus for a 600 V, 600A IGBT inverter. Peak transistor current would be 3 x 111A = 333 A.. The 8 X4 inch Infineon six pack shown (far above) would mate nicely with a x3 paralleled version of this battery pack. (EV1variants had battery voltage 312V to 343V).
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Volt battery pack weight estimate
GM volt specs on the car battery pack are 16 Kwh. This is more than an order of magnitude higher than Prius, about x12 = 16 kwh/1.31 kwh times higher. Have not seen a weight spec the Volt battery pack on it, but we can estimate it. If the lithium ion cell has x2 the energy density of NiMH, then the lithium ion cells would weigh 64 lbs (Prius) x 12/2 = 384 lbs. If the energy storage density of lithium ion over NiMH is closer to 1.5, then the weight rises to 512 lbs. Add in 50 to 100 lbs of structure and I get a weight range for the battery pack of 450 to 600 lbs.
(update, July 9, 09)
Prototype battery pack weight reported to be 400 lbs, 700 lbs with structure Wikipedia says 375 lbs.
(update 3/10)
In an article in the New Yorker on lithium by a guy who toured the GM battery plant it says the amount of the element lithium in the 400lb Volt lithium ion battery pack is only 4 lbs.
Peak current requirement for lithium ion cells
The large battery pack ( x12 more kwh's than Prius) planned for the Volt eases the peak current/power demands on the lithium ion cells. I have seen nothing on the Volt battery pack voltage or structure, but it seems to me there must be a lot of paralleling. For example, x12 energy storage could be x2 higher energy density of lithium ion over NiMH with the other factor of x6 coming from paralleling of cells. With x6 paralleling then 150 hp (x3 Prius) electrical could be achieved with each cell putting out half the peak current of NiMH cells. This is in fact consistent with the only spec of a high power lithium cell (from 123 systems) I have found.
Volt plug-in recharge time
GM marketing writes the plug-in full recharge time will be 3 hr from 230 VAC and 6hr @ 115V. Well as a power engineer I decided to check it because it didn't smell right, and sure enough it's (probably) not right.
115 VAC line power limitation
Note GM is claiming the plug-in time using a typical residential 115 VAC line is twice the time it takes when the battery pack is recharged from a higher voltage (higher power) 230 VAC line. The typical 115 VAC line is brought up to it maximum 15A current with a 1,500 watt room heater. The small wire and fuses in 115 VAC lines limit them to delivery of 1.5kw of power.
In six hours a 115 VAC line can only deliver 9Kwh (= 6 hr x 1.5kw). This is a long way from the speced 16Kwh rating. Sure enough when I poked around the LG Chem site, they say the Volt battery pack recharge time, and as manufacturer's of the battery they should know, is 3 hr at 230 VAC and 8 to 9 hr at 115V. I posted this discovery in a comment to NYT article on the Volt. (Recharge time depends on the used capacity of the battery, reportedly it will charge to 80% upper and 30% lower, so 0.5 x 16 = 8kwh, consistent with 6 hr from 120 VAC line. )
NYT driving a Volt prototype (in generator mode)
She asks the question will the car be a slug in generator mode and then never answers the question. Car uses specially designed 'low rolling resistance' tires. A good question is, What is the engine's speed at rated power and peak power. The reviewer found the car's engine/generator cycling disconcertingly from a (barely audiable) low speed to a loud, disconcerting 3,000 or so RPM.
. 
GM Volt with five pin plug (NYT 11/09)
SITTING behind the wheel of a 2011 Chevrolet Volt prototype on Wednesday, I found myself confronting what may be the greatest fear that future owners of electric vehicles will face: a battery-charge indicator showing just a few miles of remaining range. If I were out on a desolate Interstate in a vehicle powered solely by batteries, I’d be praying to the god of electrons for a place to pull off and plug in a charging cord. But my drive is at General Motors’ proving grounds here, and I’m about to experience what the Volt’s vehicle line director (and my passenger), Tony Posawatz, says is the car’s trump card: a gasoline-powered generator under the hood.
Like other reporters, I had already driven Volt prototypes in the battery-powered mode, and they were predictably smooth and silent. But for eventual Volt owners, a crucial — and so far unanswered — question is how the car will perform when the battery’s charge is depleted and all electricity is provided by an onboard generator, driven by a gasoline engine, that has no mechanical connection to the wheels.
Will it be a slug? How annoying will the noise of the generator’s engine be in an otherwise mute car?
G.M. engineers say that a fully charged Volt is capable of 40 miles of purely electric driving before the computer calls for the generator, which has an output of 53 kilowatts (about 71 horsepower), to start and sustain the battery’s minimum charge level — the “extended range” operating mode.
So what is life after 40 like in the Volt?
It takes a few laps of Milford’s twisty, undulating 3.7-mile road course to deplete the remaining eight miles of battery charge. With the dashboard icon signaling my final mile of range, I point the Volt toward a hill and wait for the sound and feel of the generator engine’s four pistons to chime in.
But I completely miss it; the engine’s initial engagement is inaudible and seamless. I’m impressed. G.M. had not previously made test drives of the Volt in its extended-range mode available to reporters, but I can see that in this development car, at least, the engineers got it right.
I push the accelerator and the engine sound does not change; the “gas pedal” controls only the flow of battery power to the electric drive motor. The pedal has no connection to the generator, which is programmed to run at constant, preset speeds. This characteristic will take some getting used to by a public accustomed to vroom-vroom feedback.
A few hundred yards later, as we snake through the track’s infield section, the engine r.p.m. rises sharply. The accompanying mechanical roar reminds me of a missed shift in a manual-transmission car. For a moment the sound is disconcerting; without a tachometer, I guess that it peaked around 3,000 r.p.m.
I asked what was going on. “The system sensed that it’s dipped below its state of charge and is trying to recover quickly,” Mr. Posawatz said. “The charge-sustaining mode is clearly not where we want it to be yet.”
Immediately the engine sound disappeared, although it was still spinning the generator. A few times later in our test, the generator behaved in similar fashion — too loud and too unruly for production — but there is time for the programmers to find solutions. Volt engineers are revising the car’s control software, which will have the effect of “feathering” the transition from the nearly silent all-electric mode to the charge-sustaining mode, when the generator will be operating.
“We’re designing a software set of rules, which will just require more seat time for the engineers to finish,” Mr. Posawatz said. “We have nine months to work this out.” The sound of the generator running at steady highway speeds is something Volt owners, and others who appreciate the flexibility and efficiency of this type of hybrid system, may have to accept.
Unlike many electrics, including the Tesla Roadster, the Volt’s electric drive has no whine. The car feels solid and planted on the road. Clicking the Sport button on the dashboard releases a bit more oomph than when in Normal mode; in terms of efficiency, there isn’t much difference between the two except at peak power.
The Low mode— Chevrolet plans a flashier name for it by next fall — is unique in the electric-car world, and a useful feature. While coasting, it applies electric motor braking, then smoothly blends in the regular brakes.
Even beyond the regenerative function, Low mode offers one-pedal driving in slow speed, stop-and-go, and downhill environments. The regenerative braking, whether applied through the Volt’s foot pedal or by pulling the shift lever down into Low mode, is both progressive and predictable. This is in stark contrast to the harsh, abrupt regenerative braking delivered by BMW’s all-electric Mini-E, for example.
There is minimal body lean in the tight corners. The low-rolling-resistance Goodyear tires created specifically for the Volt provide excellent grip. Throughout my test, the prototype behaves admirably. At its current state of development, the Volt is an extremely refined vehicle.
High speed torque problem? (Sept 2010 update)
There are recent hints (June 27, 2010) that the high speed torque (high speed acceleration) in the GM Volt my be poor and that GM might be planning to radically alter the architecture of the car to fix it! These hints come from an article about the European version of the Volt, the Opel Ampera, which is to be built in the same Michigan factory as the Volt, but goes on sale a year later (end of 2011). This story was on the GM-Volt site (not connected with GM). Here's the link:
http://gm-volt.com/2010/06/27/opel-ampera-journalist-test-drive-questions-high-speed-performance/#comments
The story is that testing of the Volt and Ampera by journalists has been restricted to 50 mph top by a twisty, cone strewn test track they have been allowed to drive on, but that a European journalist got to drive the Ampera on city streets. He reports it accelerates badly above 50 mph. But here's the kicker:
Yikes! The key feature of an electric car, or range extended electric like the Volt, is that only the electric motor drives the wheels. All the torque at all speeds comes from the electric motor. Here there are hints that the GM electric motor drive system is not working well at high speed and that GM has been hiding this fact. This would be an issue of the type of motor and motor control system as reflected in the Torque vs Speed curve. The possible 'fix' is to mechanically add torque from the engine to the wheels through a planetary gear box!! Translation: They are converting their electric car into hybrid? It could be effective. There are 90 hp available from the engine, so why not let the electric motor max out during accelertion using only battery power and add in some (or all) of the 90 hp from the engine mechanically. After all hybrids do work well, and what they have here, they may be thinking, is a hybrid with a big battery.
One very knowledgeable poster (#14) to the above article references a very interesting video (2008?, link below) from GM explaining the architecture of a new Saturn SUV nearing production. The video explains how they plan to use a variation of the Toyota planetary gear arrangement. Instead of a continuously variable transmission, they have a bunch of clutches to switch in four fixed gear ratios in the mechanical path. This reduces the power flow (? rating) in the electrical/motor path because it's just used to smooth out the four fixed mechanical ranges. The two mode Saturn SUV was probably going to share a lot of technology with the Volt, and he quotes GM people saying that. Also it was to have an 8 kwh lithium ion battery (!) to be plugged in and charged from 120 VAC. The poster thinks that the GM Volt has this 2 Mode Saturn transmission in it! He may be right, he makes a very good case. Clearly from a manufacturing point of view the Volt team would want to use a prexisting (or co-developed) transmission if possible. The poster's technical case (Feb 2010) is in link below. This is full of good info including even GM patent on the transmission. He argues that cars with no transmission end up needing motor with very wide speed ranges like 0 - 13,000 rpm, which is what he calculates for EV1 at its top speed of 80 mph. And I see in Tesla and other electric cars. However with the dual mode transmission (below) the two smaller 55kw motors combined together with different gear ratios can be designed to operate over a much smaller speed range (7,500 rpm max) so better efficiency and probably more reliable. I like it, but now in an 'electric' car we a complicated gear box of a hybrid, because basically it is a hybrid, just a hybrid with a big battery and an extended all electric mode. If true, it shows how truly deceptive GM has become! (It also would mean the Volt uses PM motors not induction as earlier reported.)
http://gm-volt.com/forum/showthread.php?t=4181
He points at this UC Davis 2007 paper as background:
http://steps.ucdavis.edu/People/bdjungers/presentations/UCDavis_Spring2007_TechReport.pdf
Transmission for planned GM Saturn SUV 'Dual Mode' hybrid
Note the 'motor' is really two 55 kw PM motors ,
which can be operated separately or lashed together to make a 110 kw motor
(screen capture from link below)
GM Volt revealed to be a hybrid! (10/10/10)
Finally GM comes clean and reveals details of the architecture of the GM Volt. And it is not the simple structure that GM marketing has been saying (or implying) it is [engine => generator => battery => inverter/motor => wheels]. It includes all of this simple structure and below 70 mph this is how the car works. But above 70 mph the 111 kw (main) motor does not have the torque/speed needed for good acceleration and 100 mph max speed.
GM Volt power train showing plantary gears and three clutches (10/10/10)
Two blue/green clutches freeze the ring gear or connect it to the 54 kw motor/gen,
red clutch connects/disconnects the 54 kw mot/gen to the engine
cutaway of GM Volt power train
Here's a sketch I made hi-liting (in blue) the extra components of the Volt drive train that GM has hidden for nearly two years. Independent testers report the car has good acceleration and performance over the whole speed range both in all electric mode and with a depleted battery, but the downside is the cost of adding a complex hybrid type planetary gear set and three clutches. (Details of the architecture are pulled from a patent filed 2007 by GM which is believed to be the architecture of the Volt.) The GM spec has finally disclosed that the battery consists of 288 cells. Here I am guessing that the 288 cells are arranged as 96 stacks of three parallel cells (96 x 3 = 288), because if the lithium cells are 3.6 V or so (exact voltage not disclosed), then the bus voltage comes out about right (350 VDC) for 600 V IGBT's in the inverter.
(best guess) GM Volt architecture
parts in blue (planetary gear set and three clutches) were hidden by GM until Oct 2010
(1/4/10 update) My figuring below that torque is added from the gen/engine path during hard acceleration is probably right.. In a YouTube test drive by an automotive type he says when you step hard on the gas (he is doing 40 mph or so on a country road) you can feel a "kickdown" bringing in extra power. My sketch shows it doesn't matter if the extra torque comes from the generator/battery or engine, in either case the clutches have got to throw and the planetary gears go into action to add in the 2nd torque.
--------------------
This has not been disclosed by GM, but looking at this architecture I find it hard to believe that GM is not using the 54 kw motor/generator to boost acceleration of the car (even from a dead stop). Essentially all the parts are in place to boost both the torque and hp of the car in all electric mode by as much as 50% (!) utilizing the 54 kw generator as a motor and combining torques through the planetary gear set.
However, I can think of one reason why a strong boost via the lower path may not be desirable. With 50% more torque and hp the car in all electric mode would (perversely) have performance that is 'too good'. The engine does not (appear to) have the sustained power capacity the battery has, so in some driving scenarios a noticeable loss of car performance might occur with a depleted battery. I was struck that reviewers noted that the car performed about the same in all electric mode and with battery depleted, so my guess is that this was a design goal, and it's obvious that marketing would like it, because it keeps things simple.
There is a complete description of the GM Volt drive train in Motor Trend magazine and their test drivers got to drive the car for 100 miles in the field. he GM Volt is now revealed to be a hybrid with a three axis planetary gear set like the Prius. Quote: "Markus liked driving the car and he noted he was surprised about the direct mechnical connection."
http://gm-volt.com/2010/10/11/motor-trend-explains-the-volts-powertrain/
Details of the powertrain here:
http://gm-volt.com/2010/10/12/chevrolet-volt-electric-drive-propulsion-system-unveiled
There's even a figure in the article comparing the two planetary gears. The Volt has the engine, wheels and generator hooked connected to different gears than the Prius. There's info about how this functions in the GM patent application (see below).
Motor Trend article on GM Volt geartrain (Oct 2010)
The article description with gas engine on (battery in charge sustain mode) above 70 mph is again a little unclear, but I think this is what is going on. At all speeds includein above 70 mph with depleted battery the engine powers the generator which (coupled to the battery for peak boosts) powers the main motor (150 hp motor). However, it it worked this way above 70 mph the car would be a pig. The generator can't now be used to provide wheel torque as a motor since it is acting as a generator powing the main motor, so in this mode the torque from the engine now couples through the planetary gears directly (mechanically) to the wheels. It is a hybrid, in this mode the wheel are driven by a combination of electric and mechanical torque. GM is quoted as saying: 'The gas engine participates in the motive force' and 'the engine never drives the wheels all by itself', which is correct.
Motor Trend says the performance of the car is good in all modes. It accelerates faster than the Prius (0 to 60 in 8.8 sec (electric), 8.7 sec extended, vs 9.8 sec for Prius.
My comment -- just a hybid with a big battery (10/10/10)
The GM Volt appears to have a sophisticated architecture with good performance. Good.
Not so good, however, is it is not all that radical. It's now coming out (well hidden for years, Why?, Probably because it killed the sex appear) that it's basically just a hybid car with a big battery!! And no wonder it's expensive it has all the planetary gear overhead of hybrid plus higher capacity electronics and a 10k 400 lb battery.
Here's what looks like the GM-volt architecture (from 2007 GM patent)
www.freepatentsonline.com/20090082171.pdf (20090082171, Mar 26, 2009)
A,B,C is planetary gear set
A (sun gear), B (planetary carrier), C (ring gear)
M/G (motor/generator) B is 111 kw main motor
M/G (motor/generator) A is 54 kw main motor
engine power rating: 66 kw/ (89 hp)
US patent office application 20090082171 (filed 2007). GM has confirmed this patent does apply to the GM Volt and that it issued two weeks ago. "Output Split Electrically-Variable Transmission with Electric Propulsion Using One or Two Motors", inventors Brendan M. Conlon, et al. US patent link to application here. -- Key here (I think) is that the engine shaft goes right through the 54 kw generator, so >70 mph with battery depleted the 66kw engine can both power the generator (to power the main motor) and still have some excess torque left over that is coupled mechanically into the planetary get set to the wheels.
-- Two motor mode (> 70 mph), the torques or the two motors are combined for delivery to the Final Drive (wheels), or alternatively, the speed of M/G A determines the gear ratio from M/G B to the Final Drive. This is the Electrically Variable Transmission (EVT).
-- Article commentary on patent says, "GM seems to have stated that the Volt never mechanically couples the Engine to the wheels.", which the patent clearly shows is possible. (If they did say this, and I bet they did, and we now see this is a (marketing) lie. Everyone in forum commentary remembers GM saying the engine would 'never' directly drive the wheels.
Toyota Prius plug-in hybrid 2012 (Aug 2011)
Toyota has announced what they call a plug-in hybrid for sale in early in 2012. By putting a 300 lb, 5 kwh (approx) battery in a Prius they have a direct competitor to the Volt. It's as an electric car for 13 miles, and then runs on its engine as a hybrid (probably like a standard Prius). They are going to start selling in in the north too (Mass, Vermont, NH, Washington). A web site is up with some info, but full specs won't be revealed until Sept auto show.
I think going with a battery 1/3rd the size of the Volt (16 kwh) makes a lot of sense. Instead of an extra 10k for battery it's 3.3k and much of this will be paid for by government subsidies for electric cars. Instead of 1,000 lbs it's 300 lbs and a lot smaller too. Some odd things (not yet sorted out). Read max speed in electric mode is 62 mph, which means its only for city. You can prevent it from going into electric mode, allowing you to drive highway on engine and switch to electric in city. This makes sense and I read Volt will have this too (in Europe). For some reason I don't understand instead of just making the hybrid battery bigger, then have added another battery (maybe even in two segments) for electric mode that is line charged. I can see lots of production reasons why this makes sense, but not sure if functionally it makes sense.
(Oct 2011) Saw Toyota plug-in hybrid (writtten all over the side) on road in Cambridge today. It was marked as a ZipCar, which is apparently cooperating in its testing. Yup, I read in Jan 2011 ZipCar got 8 cars to test for use in three cities and Cambridge MA is one of them.
Battery 5.2 kwh, 288 cells, 3 modules
Battery thermal three cooling fans
Battery weight 350 lb
Charge time 3 hr @ 120 VAC
Motor 60 kw (80 hp)
Fuel tank 11.9 gal
Motor at 60 kw is not large enough to provide the 120-130 kw needed for a fast acceleration, so the engine starts during hard acceleration to provide the additional 60 kw. Engine also turns on when heater or air conditioner is powered on. (The article reads like the car still runs electric on the battery, with the engine only providing electric power to run the heater and air conditioner.)
quasi-electric Fisker Karma sedan
(update 2/10/12)
Fortune magazine in an article on the Fisker Karma says this:
According to the WSJ today the US Energy dept froze further funding to Karma for their follow-on sedan (Nina), since they have missed targets that are part of the 529 million dollar US Energy Dept loan agreement. This was the car planned for production in the old GM factory in Delaware. WSJ says the have accessed 193 million of the 529 million before freeze. With further taxpayer funding frozen Fisker laid off about 26 people working on the sedan at the Delaware plant and 40 in Calif.
A123 Systems
Also today Kevin Bullis of MIT's Technology Review has an article online saying Fisker is a "key customer" for A123 and their recent troubles are bad news for A123. I think A123 System, MA manuf of lithium ion batteries, has made a major mistake in hitching its wagon to Fisker Karma, and its overweight 5,200 lbs 'sub-compact' turkey of a car. That's right its first (and only?) 5,200 quasi-electric car has been ruled a sub-compact by EPA due to its tiny interior volume (all those batteries and the back seat is smaller than a Honda Fit).
Bullis says maybe 30% of its revenue 2011 and 2012 was expected to be from sales to Fisker Karma, but Fisker has "unexpectedly" canceled battery orders. (Could it be their overweight baby is not selling?) Bullis says Karma has now manufactured 1,500 cars and sold hundreds. A123 Systems has built a battery plant to supply Karma and now it has idle capacity. A123 also has a US dept of energy loan (249 million) and for 2011 they are operating at a big loss. Bullis is hinting that if Fisker gets into a lot of trouble, it could drag A123 into bankruptcy, though if they survive 2012 they have a lot of other customers whose orders they expect to grow.
(12/31/11) Well, Fisker got to production (in Finland). In headlines today they are recalling all their cars. Loose coolant clamp could lead to spill into battery compartment which could cause a short (they say). (Good engineering.....)
2012 Fisker Karma production model
(update 8/2011) Fisker Karma is still alive, now branded as a 2012. You actually find a list of retailers and when I checked a
Norwood Ma Jaguar dealer on the list, the dealer site says he will soon be selling the Fisker Karma.
(update 11/2010)
True electric extended range car
Fisker put out a press release that theirs is the only true extended range electric car, which means they are saying that unlike the Volt their engine will not be mechanically coupled to the wheels. This is confirmed in articles as production starts. The engine only runs a generator, it is not mechanically coupled to the wheels as in the Volt.
What Karma doesn't point out is that this super expensive car as a true electric has only moderate performance. If you want your nearly 100k car to give you sport car performance, the engine needs to run. To shave 2 sec off 0 to 60 mph and to increase top speed from 95 mph to 125 mph the you need to (about) double the power to the electric motors using the current from engine/generator (175 kw) added to the 150 kw available from the battery.
When you think about this, it is rather pecular. They have a 20 kwh battery (from A123), and they can't get 300 kw (100A @ 300V say) peak (or sustained) from it? (Nope, see below). Either they didn't with 100A wiring between cells or else the cells can't take the heat of 100A sustained. Maybe with the engine/generator sitting there the temptation was why bother, just run the engine, and keep the battery cool. Problem is this is supposed to be an electric car, and it costs over 100,000!
There is another quasi-electric car in the design stage at a start up company in Detroit ? California. It's the Karma from Fisker Automotive. They allegedly have 200 million in funding (? 500 million in US government loans) and have 175 engineers working on it and are talking about beginning production in early 2010 (slipped to early 2011), earlier than the GM Volt. While designed in US, it would be built by an existing company in Finland, however I read they recently bought for 20 million a closed auto factory in Delaware. The technology is derived from Quantum Technology's ('Q Drive') that they use in (prototype) hybrid vehicles built for the U.S. Army. Basically the Karma is a scaled up GM Volt, much larger, much heavier, much higher performance, and much more expensive. The car is a big (124 in wheelbase) expensive (88k), 4 seater sporty looking sedan that's functionally like the GM Volt. It's a plug-in hybrid with a large lithium ion battery pack for moderate electric range and an onboard engine to provide extended range.
electric range (EPA city) 50 miles
0 to 60 mph 5.9 sec (gas/electic mode)
7.9 sec (electric mode)
top speed 125 mph
engine 2 .0L, 4 cyl, turbo charged, 260 hp (192 kw) (2.2 L original)
(Ecotec engine made by GM)
gas tank capacity 9.0 gal premium (extra 250 miles @ 28 miles/gal)
two 200 hp motors 300 kw = 2 x 150 kw (400 hp total)
(981 ft-lb torque) (1,324 nm) (300 kw corner @ 2,164 rpm)
motors made in China (says Car and Driver)
motor location PM motors in rear wheels, liquid cooled
generator 175 kw (236 hp) (about 150 hp are needed for wind at 125 mph)
inverters three (two for motors, one for gen or charging?)
battery energy 20.1 kwh (from A123 systems) (315 cells) (22.5 kwh original)
(vs 16 kwh for GM Volt) Battery run to 15% capacity
"Nanophosphate technology" Googling this brings up A123 Systems
battery weight 606 lbs (light!)
battery size (in tunnel) 8.1 in w x 14.2 h x 6.14 ft
battery power 160 kw (216 hp) (181 kw says another source)
(Leaf pulls 90kw from their 24 kwh battery)
battery (open circuit) 336V
charge time 6 hr @ 240 VAC, 15A
wheelbase 124 in
weight 5,300 lbs (Karma spec) Yikes!
Beyond 50 mph the car operates as a "normal hybrid vehicle". Car has two modes of operation: stealth mode (all electric mode, 7.5 sec 0 to 60, 95 mph max) and sports mode (electric and engine power, 5.8 sec 0 to 60, 125 mph max). (The two switchable modes makes sense to me, it fits the technology.) There is no transmission. Fisker engineer confirms that the max battery power (200 kw) is not high enough to fully load the motors. Battery power alone is enough to do 7.5 sec 0 to 60 mph, but faster (to meet the 5.8 sec spec) engine power is added to battery power to provide the 300 kw for the motors. Engine is turbo charged to get high hp for given weight, fuel efficiency is secondary. One poster says EPA city favors electric vehicles, in real world driving at higher speed, so says electric range will not be 50 miles but more like 30 miles.
check
wind resistance 27 hp (20 kw) @ 70 mph
battery power 24 kw = 20 kw x 1.20 efficiency + tires
use 1/2 x 22.6 kwh 11.3 kwh = 24 kw x 0.47 hr (28.2 min)
distance @ 70 mph 32.9 miles = 28.2 min x 1.167 m/min
88k (now 95k) Fisker Karma quasi-electric (prototype)
shown at Detroit Auto Show winter 2009
(124 in wheelbase, 4.650 lbs, huge 22 inch wheels)
No way to fast charge this thing. Unlike the Leaf which has a J1772 (240V, 12.5A) plug and a high current DC plug, picture of the charge port shows it has only the J1772 plug. Like the Leaf the battery runs down the spine of the car in a high console, vs Tesla which is going to put a much larger battery flat under the car. For 100k all you get is 120 VAC charge cord! 240 charger and cord is an option!PM Motors
Transmission speed ratio is 4. This thing has huge 22" tires. If 22" is radius (maybe not, tires are weird, a 16" tire appears to have a radius of 12") and 60 mph is 88 ft/sec, I figure motor RPM @ 60 mph to be 1,634 RPM. At 125 mph top speed this is 3,821 rpm (maybe closer to 5,000 rpm if tire radius is only .75 x 22") Much, much slower than the 15,000 rpm of the coming Tesla sedan. This is going to make the motor much heavier. Lets check. Each motor is rated at 650 NM (490 ft-lb) or 1,300 NM for the pair. Calculating the rectangular power corner:
angular speed = Power/ Torque
angular speed = 300,000 kw/1300 NM
= 231 rad/sec (36.7 rev/sec or 2,205 rpm)
Reasonable, combined with my rpm calculation from tire radius and transmission ratio, it puts the rectangular power corner at 2,205/1,634 x 60 mph = 81 mph. Since these are PM motors they are difficult to field weaken, so the rectangular corner needs to be high.
Motor conclusion
Unlike the Tesla which uses a single super high speed (15,000 rpm) motor. This car uses two large, heavy, liquid cooled, low speed (3,800 rpm @ 125 mph), high torque, expensive PM motors. I know something about PM motors, and I know Powertec makes large PM motors, and sure enough I found they have a motor quite close to the Fisker specs.
Matching PM motor
Fisker specs each motor at 200 hp, 500 ft-lb (approx) torque, and I calculate the max speed is 3,800 rpm (@ 125 mph). Below is a page from the Powertec catalog. Check out the E259-E2 motor. It's a 196 hp, 3,600 rpm, 475 ft-lb PM motor. An excellent match. Elsewhere in the spec the size of the E259 frame is given as 11.5" dia and 36" long.
196 hp, 3,600 rpm, 644 Nm (475 ft-lb) PM motor
in Frame E259 (top row), E2 electrical specs (bottom right)
source --- http://www.powertecmotors.com/e250.pdf
Look at the weight of the E259 frame motor, it's 515 lbs! This may explain the mystery of why the Fisker Karma weighs 5,300 lbs. Two of these motors weigh 1,000 lbs, The battery reportedly only weights 600 lbs. Throw in the weight of the controller and the weight of the 260 hp gasoline motor and you've got a 2,000 to 2,500 lb drive train!
-----------------------------------------
There was little technical info on the Fisker site and nothing on the Quantum site, but I found the links below which have some additional Karma info.
http://www.thetruthaboutcars.com/fisker-karma-brochurespecs-stealth-eco-chic-solar-power/
http://auto.howstuffworks.com/fuel-efficiency/hybrid-technology/fisker-karma5.htm
Fisker could have a lot of company. Plug-in-America (below) has a list (with pictures and status) of nearly 50 hybrid and electric cars in the prototype stage.
http://www.pluginamerica.org/plug-in-vehicle-tracker.html
Tesla Motors lithium ion pure electric cars
Tesla 2 seat roadster
There is a lithium ion powered car on the market now, sort of (few hundred on road and production of a few hundred/yr). It's an all electric car, the 100k Tesla Motors 2 seater sports car, 220 mile range, then plug in (at home) to recharge (3hr to 37 hr).
Battery 6,800 lithium-ion cells
Off shelf #118650 (18 mm x 65 mm) cylindrical rechargeable 'protected'
lithium ion battery (w/PTC built in)
375 V (open circuit) (supposition)
battery pack weighs 992 lbs
must be massive paralleling (100 cells in series, 65 in parallel)
(assume 3.6V/cell 65 = 6,800 x 3.6V/375V)
For 375V battery pack voltage => 65 parallel strings of 104 cells/string!
(update, comments from Tesla seem to indicate the battery may be divided
into 11 modules horizontally. Meaning 11 (or maybe 10 used for motor)
36V modules made up of 10 cells and 66 (or 60) in parallel)
3.5 hr recharge (high power 480V charger), 10 hr, 240 VAC,
37 hr 120 VAC
Battery lifetime projection is 500 to 700 deep discharge cycles
battery pack is liquid cooled
Battery kwh 56 kwh (37 hr @ 120 VAC, 1.5kw = 55 kwh est)
(56 kwh looks reasonable for 220 mile range, its x3.5 more kwh than
GM Volt)
Tesla's use of small commodity lithium ion batteries achieves a much
higher energy density (150%) than Volt or Leaf. The Leaf is 24 kwh
at 660 lbs (Volt about 400 lbs for 16 Kwh), but the Tesla roaster's
56 kwh battery at 990 lbs weigh only about 2/3 of what it would
weigh if Volt or Leaf type lithium batteries were used.
Power out 185 kw (250 hp) at 8,000 rpm (T vs Speed peak)
281V x 658A = 185kw (guess --- Voltage sag to 25% @ max pwr, like Prius)
History Tesla had massive redesign in 07/08 to remove unreliable two speed
transmission and forced out CEO. To restore high torque for 4 sec
0 to 60 motor current rating of motor and inverter (at low speed) was
increased from 640A to 850A. This required higher current IGBT's,
terminals, and wiring. (Poster says (bus) voltage was decreased at low
speed, but don't know what this means. Do they reconfigure
battery pack at low speed, or is he just talking about voltage sag.)
Inverter 200 kw
(aka PEM) 72 IGBT's Yikes! for inverter and charger (Tesla marketing)
motor rated at 375V, 185 kw the currents are really high (500 A to 600A)
assume 6 igbt's for inverter that's 11 x 6 = 66
Do they have 11 inverters in parallel (!), each with 150A, 600V IGBT's ?
They might! The battery pack is divided into 11 components.
Redesigned for 850A max (@ low speed)
Motor 375 V induction motor (air cooled)
14,000 rpm ! For 380 Nm motor Pmax speed = 4,650 rpm
(Pmax at 185,000 watt/380 Nm x (1/2pi) x 60 = 4,650 rpm)
4 pole (2 pole pair), 100 lbs ("size of a large watermelon")
(my guess is the custom motor has 11 separate windings, with 33 terminals,
to effectively parallel currents from the 11 inverters)
Powertrain 1.5 (one speed gear box) motor in prototype car runs at 850A
(280 ft-lb/380 Nm) (may 2008)
Redesigned for 850A max (low speed torque, 280 ft-lb/380 Nm)
Tesla induction motor
(In a puff piece in MIT's Technology Review about the lead electronic
designer of Tesla Motors (JB Straubel, age 33) it says Tesla began with
an induction controller they licensed from Alan Cocconi of AC
Propulsion. It was all 'analog' says Straubel, meaning not DSP based, and
gave them a lot of trouble, "no two were alike", it faulted on highway. The
implication being Cocconi's cirucit design was poor. Tesla replaced it
with a DSP based design of their own.)
I can confirm that Cocconi's induction motor controller design was/is
(probably) out of date. I read an induction control patent of his filed
in 1998, and it looked like the kind of discrete logic design I was
doing in 1978 (VCO as slip generator) twenty years earlier and
prior to DSPs.
Gearbox single speed gearbox (one fixed ratio forward, + reverse)
(earlier two speed, but it was unreliable)
Dimensions 92.6 in wheelbase (tiny)
2,720 lbs
2 seat
Performance 0 to 60 in ? 4 sec
Top speed 125 mph
Range 220 miles
Don't know the manuf of the Tesla battery, but its easy to find data sheets for #118650 (18 mm x 65 mm) cylindrical rechargeable 'protected' ithium ion battery (w/PTC built in).
Typ rating : 3.7V, 1.8 to 3.0 Ahr, (0.12 to 0.18) ohm (includes PTC), 46 grams
Energy storage --- 350V (7% sag) x 2.4Ahr x 66 parallel paths = 55 kwh (OK agrees with spec)
Iss = 3.7V/0.18 ohm = 20A. Will deliver 10A peak but this is matched load so sag is bad (50%)
ESR of 0.12 would reduce sag to 30%
Nobody seems to know the cell being used, but a candiate mentioned (I found it too) is
Sony 18650G8
Weight --- 6,800 cells x 46 gm/cell = 700 lbs (OK, about 1,000 lbs with structure)
My summary of Tesla electronics
Found a short technical article that confirmed my inverter guess. Tesla battery and inverter is massively paralleled. There are (likely identical) 11 battery modules eachwith their own inverter. The battery pack(s) voltage is 375 V formed from about 100 (3.6 to 3.75V) cells in series. The battery pack protections are very elaborate (? likely costly) with protections on either individual cells (or groups of six cells) consisting of a series PTC (positive temp coefficient resistor) and fuses. Some of this protection (like PTC) is built into the cell by the cell manuf. There is also extensive electrical and thermal monitoring.
375V battery (maybe 20% sag) => inverter (600 V IGBT's => 375 V motor
reported 850A pk means serious battery voltage sag (to near 200V)
2) Series resistive protection can be used (in form of PTC's and fuses)
The massive paralleling of the inverters leads to advantages too. Robust in the field. If an inverter blows in the field and can be isolated, it would hardly be noticed as car still has 10/11 of it full power and range. Big advantage thermally too. Lots of heat has to be dissipated when 660 A are being switched, and this problem is vastly simplified if the source of the heat (IGBT's) are physically far apart. Components for each design are relatively cheap due to high production. Testing is simplified and vehicle repair is simplified.
The use of 11 (apparently) separate inverters (6 x 11 = 66 IGBT's @ 600V ) makes each converter easier to design and to cool. The peak current in each IGBT is about 60A (figured as each inverter peak power is 200kw/11 = 18kw = 300V (with sag ) x 60A). So the inverter are likely conventional using 100A (or 159A) 600V IGBT's.
but, but .... there is the far from trivial problem of combining the current from 11 inverters. This is actually quite tricky and I have seen no clue as to how they did this. The trick is to do this in a reliable robust way. There are ways to parallel inverter outputs with sharing inductors, but the likelihood that the battery voltage of the modules would be different makes this approach daunting.
My guess is the paralleling is done in the motor with parallel windings, a separate winding for each inverter! They say in their marketing material that the motor is custom. Two parallel motor windings are not unheard of, I worked with (? tested) large PM motors wound this way myself. Eleven windings is pretty extreme, but it would be a robust way to parallel the inverters. The major complication is the wiring coming out of the motor. Instead of the usual 3 (or 9 terminals) there are (at least) 33(= 11 x 3) terminals.
Bottom line --- If I'm right that they use eleven identical 100A, 600V IGBT inverters, physically separate, connected to electrically separate battery packs, driving 11 electrical separate windings in a custom 33 terminal induction motor, then this looks to be a pretty rugged, clean (and of course, expensive) package.
update --- Info from Tesla motors seems to indicate the battery division into eleven modules is not what I guessed, that it apparently is divided horizontally not vertically, meaning the 660 cells of each modules are arranged 10 series and 66 parallel outputting 36V. This makes it much less clear as to how the 11 inverters (if there are 11 inverters) are arranged and paralleled.
Tesla Model S sedan -- 300 miles range
(Update 12/12/11)
For 79k Tesla is licking the range problem in electric cars with a monster 85 kwh battery pack. They claim 300 mile range. The first 1,000 build will have this battery. For marketing purposes they advertise a 20k cheaper version (59k), where they only put in a half size (160 mile range) battery pack.
Model S is coming along, fully speced and priced, with deliveries starting July 2012. Tesla says all 5,000 they can build in 2012 have been pre-sold. From a couple of video I have seen this car looks impressive. The massive battery pack 85 kwh is a flat thin (single) layer of little cylindrical cells standing upright covering the full bottom of the car. This allows the inside of the car to be very open so they have five normal seats and two backward facing child seats. The battery pack is liquid cooled (like Volt). One reviewer said the spacious interior of the Model S is huge compared to the cramped Fisker Karma interior.
Tesla Model S sedan massive 85 khw battery pack
in a single layer of 8,000 cylinderical cells under car
source --- http://www.teslamotors.com/models/features#/safety
Model S sedan toque vs speed
304 kw (410 hp) @ 7,000 rpm corner
source ---Tesla web site
Tesla is now designing an all electric 58k sedan (Model S) (seats 7) with 300 mile range for production in 2011/12. This car is premised on an expected improvment in Li-ion battery capacity of 8% year. Tesla will use the same small lithium ion cells for the Model S that it uses in the Roadster. The Roadster battery pack has 150% higher energy density than the batteries in Leaf or Volt. The Model S will be offered with three different size batter pack: 160, 230, 300 miles.
Motor 9 in !! (liquid cooled, induction)
415 Nm, torque flat to 7,000 rpm (const hp 7k to 15k)
[corner 415 nm x 733 rad/sec = 304 kw (410 hp)]
0 to 16,000 rmp (130 mph)
Battery 8,000 lithium-ion cells (300 mile range option)
Off shelf #18650 (18 mm x 65 mm) cylindrical, 1.6 oz, 3,000 mah, 3.7V (10.8 whr)
rechargeable 'protected' lithium ion battery (w/PTC built in)?, liquid cooled
Packaged in a single layer (upright) that covers the whole bottom of the car
Battery (kwh) 42 kwh ('standard', 160 mile)
65 kwh (230 mile option)
85 kwh (300 mile option)
Battery weight 1,200 lb (300 mile option?)
Recharge 4 hr from 220 VAC
45 min Quick Charge @ 440 VAC (Leaf can be 80% quick charged in 30 min)
Performance 130 mph (top speed)
5.6 sec (0-60 mph)
Transmission single speed (none)
Wheelbase 116.5 in
Weight 3,825 lbs (4,200 lbs with big battery)
Wow, 0 to 120 mph (upgraded to 130 mph) with no gearing. Like the motor in the Tesla Roadster (14,000 rpm) this motor is really going to wind up. (Leaf tops out at 90 mph and I think its motor is then probably at 9,000 rpm)
I see the Model S motor is liquid cooled. I wrote in summer of 2009 that Testa made a mistake in not liquid cooling the motor/inverter in the Roadster as users reported that just a few minutes on a test track produced OT warnings and forced cut backs in performance.
Tesla Model S sedan (left, 116 in wheelbase)
Tesla Roadster (right, 93 in wheelbase)
A Washington Post article discusses the efforts of a CA town to woo Tesla to build the assembly plant for the 2011/12 electric Model S sedan $57,400. It mentions also the state and federal tax dollars that have gone to Tesla.
* City (Downey CA) is waiving 6.9 million in rent on 20 acres (former NASA plant)
* CA is waiving sales tax (7% to 9%) on equipment bought for the new sedan plant
(to keep the plant in CA)
* 465 million in low interest loans from Dept of Energy
@ 1,000 cars => 465,000/car
@ 10,000 cars => 46,500/car
@ 100,000 cars => 4,650/car
* Projected car price of 50K is after a federal tax credit for battery powered cars
---------------------------------------------
Comparing electric car motors --- Fisker Karma vs Tesla Model S (Jan 3, 2012)
A Jan 2012 email to friends (also engineers) comparing the car electric motors including a head to head comparison of the Fisker Karma with the new Tesla Model S sedan.
"So begins the age of the mass market electric vehicles", says a 12/13/10 article announcing the first Leaf delivery in USA to an ordinary individual customer. The first Leaf customer reports on his milage as follows:
A second performance problem (?) is that with only 80 to 90 kw available and a 3,500 lb car I am calculating 0 to 60 acceleration times of 12.6 seconds (ignoring wind resistance)!! GM Volt is a little heavier (3,800 lb), but I think they have something like 141 kw available using combined torque (via planetary gears) from the motor and generator, so are getting 8.8 sec (0 to 60).
But Wikipedia reports 0 to 60 has been tested at 7 sec, so the conclusion has to be the peak power available has to be far higher than 90 kw, more like 160 kw. (Nope, now I think the answer is a T vs Speed profile with a large constant hp range. see next)
(update 12/2/10) Strong Torque vs Speed roll off needed for good acceleration
The assumption of a (simple) triangular power curve for electric cars that peaks at or near its top speed I now think is wrong, because it leads to poor 0 to 60 mph acceleration. When you have a limited power gear train, to get good acceleration the power has got to max out at some moderate speed (like 30 mph). This means either a transmission or a motor T vs Speed profile that rolls way off, giving an approx constant HP range.
More on Leaf acceleration
I read (in 2009 article) that the Leaf will have no transmission, like the Tesla electric. The Volt when running on its two motors effectively does have a (variable) speed transmission (but this may only be at high speed). The Pmax point of the Leaf motor can be calulated from the sped (280 nm, 90 kw) and it comes out at a low speed indicating there is likely a strong T vs Speed roll off over much of the speed range of the car.
90,000 watts/280 Nm = 321 rad/sec = 51.2 rev/sec = 3,070 rpm
So my guess is that the Leaf has numbers something like this:
Linear torque range 0 to 3,000 rpm (0 to 30 mph)
quasi constant hp range (@ 80 kw) 3,000 to 9,000 rmp (30 to 90 mph)
max motor speed (@ 90 mph) 9,000 rpm
Torque will now be constant to 30 mph and slowly roll off (1/speed) as power is held roughly constant from 30 to 90 mph. This wide quasi-hp range implies an induction motor or else a PM motor (like Prius) with high inductance, so it can be field weakened.
(bot) my sketch showing what I think is the Leaf's Torque vs Speed profile (12/2/10)
(top) 'rectangular Torque vs Speed used in servos
Linear range 1/2 x 80 kw x 4.45 sec = 178 kj
Const hp range 80 kw x 5.55 sec = 444 kj
---------------------------
total 622 kj
Combined this is 622 kj, or about 9% higher than 572 kj calculated (below) for the 3,500 lb Leaf (3,366 curb plus driver) at 60 mph, so 9 to 10 sec foor 0-60 mph looks reasonable with the above T vs Speed shape. Or add a passenger and lose a few percent to wind and 10 sec for 0-60 rpm looks about right.
0-30 mph accel time (1/4/11)
But Car and Driver test sheet also shows 0-30 mph at 3.2 sec, which means 30-60 mph took 6.8 sec implying less than half the peak torque meaning a much faster T vs Speed roll off than constant HP. Here is a check of the 0-30 mph. The kinetic energy is 1/4th of 60 mph or [572kj/4 = 143kj]. Doing a triangular accel time with peak power (80 kw) at 30 mph.
0-30 mph 2 x 143kj/80kw (motor) = 3.6 sec
3.6 sec is pretty close to the 3.2 sec Car and Driver reports, so I now suspect the T vs Speed corner at 30 mph is about right, but that torque rolls off faster than my (idealized) curve (above) shows, and this is reasonable when motor losses and wind losses are figured in.
Checking the Leaf motor max rpm from specs (update 6/2011)
Leaf spec: "16 wheels"
Tires: P205/55R16 89H
Single speed gear reducer: x7.94
Max motor speed: 9,800 rpm
Max car speed: 90 mph
I first thought 16" wheel mean a tire dia " tires, but this worked out to be 10,000 rpm @ 60 mph, or 15,000 rpm max @ 90 mph
16" wheel (dia) => 1,261 rev/mile ?=> 1,261 rpm axle @ 60 mph
1,261 rpm x 7.94 = 10,000 rpm motor @ 60 mph (or 15,000 rpm @ 90 mph)
But looking up the tire spec I find from Firestone below:
Size: P205/55R16 89H, Overall Tire Diameter, 24.90. Revolutions per Mile, 837
Ok, more reasonable. 16"/24.9" = 0.643 x 1,261 rev/mile = 810, but the tire spec says 837 rev/mile so something is a little off (see below). I found a max motor speed on a tech car blog of 9,800 rpm. Figuring the axle speed from motor max speed gives 9,800/7.94 = 1,234 rpm axle @ 90 mph. At 60 mph this would be x0.6666 x 1,234 = 822.6 rev/mile. Good this is just about the average of 810 figured from tire dia spec and 837 tire spec. All the numbers are now close, so it looks like
Leaf electric motor top speed (@ 90 mph): 9,800 rpm +/- 1% (figured correctly, using the tire rev/mile spec
the top motor speed is 9,969 rpm (see below))
Straight skinny on tire sizes (update 1/10/12)
Somehow, not unreasonably, I alway thought that a tire referred to, say, as a 16" or 17" tire meant that was its dia or radius, but nope, this is not how tires are sized. Turns out that the reference tire inch number (16", 17", etc) is an inside dia ('dia of the wheel'). To get the tire dia the two side walls must be added (which can be figured from the other tire numbers). But this still does not give the correct rotation rate, because tires when loaded sqush a little. The operating radius is less than the radius of an inflated, unmounted tire, it comes from the measured 'Revolutions per Mile' spec.
For example
Size: P205/55R16 89H, Overall Tire Diameter, 24.90. Revolutions per Mile, 837
Distance per rotation (effective circumference) = 5,280 feet/837
= 6.308 ft (74.699")
Effective dia = 74.699/pi
= 24.096" Ok (3.3% less than 24.90" unmounted)
Refiguring the Leaf top motor speed
At 60 mph one mile is comvered in one minute, so from the tire rotation spec (837 rotations/mile) the tire/axel is making 837 rotations per min @ 60 mph, or 1,255.5 rev/min @ 90 mph (top speed). Multiply this by the spec gearing ratio (x7.94), and we get the max motor speed at 90 mph is [7.94 x 1.255.5 rev/min] = 9,969 rpm
Rule of thumb
The effective radius is just about 75% of the tire inch number (12/16 = .75), so it's a good guess that this ratio probably is pretty close for most car tires, as most car tires seem to have about the same ratio of wheel to side wall.
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(update 3/30/10) Big news, finally an electric car has a price
Nissan announced this week the Leaf will be priced at $32,780 in US. That means when the $7,500 federal tax credit is subtracted, the car's base price would be $25,280. I read that CA offers an additional tax credit of $5,000, which reduces customer price in CA to $20,228. Expected volume for 2011 is 50k vehicles (high).
No price yet from GM for the GM Volt. While the Volt has an engine and generator too, its battery pack is 8 kwh smaller (16 kwh vs 24 kwh). People have been estimating (guestimating) that the lithium ion battery packs manufacturing cost about 1k/kwh. I bet 8k easily pays for GM's engine and generator, which would make GM's drive train costs probably comparable to, or a little less, than the Leaf.
---------------------
Leaf specs
Nissan is making big noises (Summer 09) about their electric car, Leaf, planned for late 2010 about same as GM Volt. Leaf looks pretty similar to Volt (maybe little smaller) but the Leaf is a pure electric car vs the Volt, which is an electric car with a range extender. This may be the first all electric car mass marketed (outside China). The plan is to build it in Japan and a Nissan plant in US. Little technical info released yet. In Jan 2010 US Dept of Energy says it it loaning Nissan 1.4 billion dollars to upgrade their Tenn plant to make the leaf. (US taxpayer with huge loans, grants, and tax credits is really subsidizing the electric car development and early production.)
Battery 24 kwh (lithium ion) Nissan lithium voltage is 3.6V nom
(19 kwh or 80% usable range_
48 modules (w/4 cells/module)
(my guess is 2 cells in parallel for 346V bus = 96 x 3.6V)
360V is nominal battery voltage (1/4/11 update)
DC/DC steps down 360V to 12V for lights, etc.
660 lbs (battery pack, under vehicle)
Range 100 miles (65 to 75 miles realistically, see below)
EPA has measured range at 73 miles (conditions?)
Nissan now says 62 to 138 miles
62 miles --- 14 F (heater on), stop-and-go, traffic jam, 15 mph av
70 miles --- 95 F (A/C on), 55 mph highway (slow highway!)
105 miles --- 72 F (A/C, heater off), commuting, 24 mph av (slow)
(With baseline wind resistance and usable 19 kwh usable capacity
ideal max highway miles vs speed can be calculated)
55 mph x (19 kwh/ 9.7 kw) = 108 miles
60 mph x (19 kwh/ 12.6 kw) = 90 miles
70 mph x (19 kwh/ 20.0 kw) = 67 miles
80 mph x (19 kwh/ 29.6 kw) = 51 miles
90 mph x (19 kwh/ 42.2 kw) = 41 miles (27 min @ top speed)
(Two Tesla customers by driving 55 mph were able to drive
their new Leafs from dealer to home 111 miles. Not really,
article later reveals they panicked and stopped along the way
for a 90 min boost charge (worth 19 miles)
Power out 90 kw (battery)
Motor 80 kw/280 Nm (AC synchronous) (identified as PM motor in one spec)
For 280 Nm motor Pmax speed = 3,080 rpm
(Pmax at 90,000 watt/280 Nm x (1/2pi) x 60 = 3,080 rpm)
(In catalogues 75 hp, 3,600 rpm (induction) motor weighs about 760 lbs)
9,800 rpm @ 90 mph (max) spec (consistent with x7.94 gearing and 24.9" tire dia)
Wheelbase 106.3 inches
Max speed 90 mph
Vehicle weight 3,500 lbs/1,590 kg (Nissan est)
3,366 lbs ('final spec')
Transmission Single speed reducer (1/7.94) (roughly 6,500 rpm motor at 60 mph)
Accel (0 to 60) kinetic energy @ 60 mph (26.8 m/sec) = [(1/2) 1,590 kg (26.8 m/sec)^2] = 572 kj
Rectangular power 572 kj/80 kw = 7.15 sec
Triangular accel time = 14.3 sec
(It can't be this much of pig, can it?) Nov 2011 blog says 0-62 mph is 12 sec
(unofficially tested at 7 sec??)
10.0 sec (tested by Car and Driver magazine)
Onboard charger 3.3 kw built in charger supports
level 1 120 VAC @ 1.4 kw, 20 hr
level 2 240 VAC @ 3.2 kw, 8 hr (Why only 3.2kw?)
(Looks like big mistake here. A 240VAC, 30A 1ph charger
should be able to do 5-6 kw and charge battery in 3-4 hrs, which
is exactly what Ford Focus (w/23 kwh) electric is claiming.)
2nd plug supports external level 3 charger, 480 VAC @ 50 kw,
30 min for 80% charge (causes some additional loss of battery
lifetime if used regularly.) Cost is listed at an absurd 16k!
Battery manuf Nissan (joint venture with NEC)
------------------------
Battery comparison with GM Volt (1/4/11)
New info on battery voltage curiously indicated that GM Volt and Leaf both seriesing the same number of cells [96 (3.6V nom) batteries in series for a 345V bus]. GM has cells three in parallel (total of 288 cells), whereas the Leaf has two cells in parallel (total of 192 cells). Since Leaf has 2/3rd the cells of Volt and 3/2 the kwh, that must mean each Leaf cell is 225% larger than Volt cells. Leaf cells must have 35 Ah rating (= 20,000 kwh/192 cells) vs 15.5 Ahr for Volt cells.
Nissan car manual shows there is a dash display of 'Battery temperature', and say (car) power is reduced to protect battery if battery temperature is 'too low or too high'.
To power the car on of off you need to step on the brake. That's right to power off the car you need to step on the brake! Emergency power off (while driving): hold 'power' button for two seconds, or push rapidly three times.
------------------------
The Nissan battery kwh is 150% that of the GM Volt (24 kwh vs 16 kwh) and like the Volt is mounted under the floor of the car. So how does that square with a claimed milage of 100 miles vs 40 miles (electric) for the Volt. Here's one way to look at it. If 40 miles uses 8 kwh in Volt, then 100 miles in Leaf would need 20 kwh. The Leaf has a 24 kwh battery, so this works out if the Leaf lifetime battery derating is only 17% = [4kwh/24kwh] vs the 50% derating GM is planning.
It just occured to me this may give GM an ace in the hole. If lifetime data on the lithium ion batteries begin to looks good, or battery lifetimes improve in a year or two, then GM with a simple code change can just rerate the Volt for higher electric range (40 => 50, 60 or 70 miles).
Nissan Leaf (2010) pure electric, 100 miles?
(weird bulging headlights are claimed to "split the air" reducing the drag of mirrors)
Unlike the Volt which has an elaborate liquid-cooled heater/cooler for the battery (? uses the engine to avoid using the battery hard when it is hot or too cold), the Leaf battery is air cooled using only a single fan!
The flat battery profile allows the Leaf to have a three seat bench in back. In contrast the hump in the Volt battery divides the back seat in half, so Volt is a four passenger car vs five passengers for Leaf. In an article by a Tesla battery guy he says Nissan experimented with active thermal management for the battery, but it required a hump in the center which would have made it a four seater. He argues that Nissan has under-engineered the battery thermal system, so there is risk it will degrade more than Nissan claims (80% capacity after ten years says Nissan, whereas GM says even with their babying they expect 75-80% capacity after 8 (or 10) years.
Nissan Leaf 24 kwh, 660 lb lithium manganese oxide battery
Air cooled with only a single fan
GM with the Volt has been conservative with the battery, not only thermally controlling the temperature, but is also using the engine to protect during thermal extremes. In addition capacity loss of the battery will only moderately affect it, as all that happens is the engine comes on more quickly. On the other hand Leaf has no motor and a crappy single fan to air cool the battery pack (and apparently no heater to warm it). Nissan is taking a big risk that the Leaf battery pack is not going to prematurely degrade, or maybe it's hoping that the early adopters won't care. If the battery pack lose much capacity, the car will be near useless until its super expensive battery pack is replaced.Leaf 99 mpg EPA sticker
There was a lot of debate about how the EPA would rate the milage of electric cars. What they have done (see sticker below) is equate the electrical battery drain (in kwh or joules) for the car to travel a mile to an amount of gasoline that has the same energy content. I checked standard conversion tables and 33.7 kwh/gal is what I find, so that must mean the EPA standard this is not the energy gasoline releases when burned in a combusion engine, but the total heat energy it released when burned (oxidized) in oxygen at normal 1 atm pressue.
1 gal gasoline ?==> 34 kwh (really 33.7 kwh) (122 Mjoule)
In round numbers a 33 mpg car burns 3% of a gallon of gasoline to go one mile. An electric car like the leaf @ 99 mpg (eq) uses about 0.34 kwh (1.2 Mjoule), which is 1% of the energy in a gallon of gasoline.
Here's the first EPA sticker for an electric car (Nissan Leaf) from I think Nov 2010.
(early version? released by Nissan Nov 2010)
When I look closely at the numbers on the sticker, I see all kinds of problems.
For a starter how can it take 7 hours to recharge the battery at 240 VAC? Standard 120 VAC, 15A circuit can supply 1.5 kw, so 240 VAC @ 30A should be able to supply x4 (twice voltage, twice current) or 6 kwh. It should be possble to recharge fully in 3-4 hours, which is just what Ford Focus electric (23 kwh battery) is saying. What did they do design a half wave charger! (Maybe they did, 7 hr is the Leaf spec. Their built-in charger has only a 3.1kw rating.)
If the car can go 73 miles (presumably at the combined city/highway 99 mpg), then it must have used
(73m/99 mpg) x 34 khw/g = 25 kwh
What? This car has a 24 kwh battery and the 73 miles appears to be a realistic range, and the usable kwh can't be more than 80% (19.2 kwh) of capacity, can it? Or if I figure another way it looks like maybe the math is wrong, that they used 11 cents per kwh, not the 12 cents it says in the footnote.
EV-1 specs
For comparison with the Tesla electric here's info on the first mass produced car (about 1,100 built), the GM EV-1 about a decade ago (manuf 1996 to 1999).
Battery 26 (12V) lead acid (standard)
312 volt (26 x 12 = 312V)
18.7 kwh (60 amp-hour x 312V = 18.7 kwh)
1,310 lbs
55 to 95 mile range
26 NiMH (option)
343 volt (26 x 13.2 = 343V)
26.4 kwh (77 amp-hour x 343V = 26.4 kwh)
1,147 lbs
75 to 130 mile range
Recharge 6 hr @ 220 VAC (6.6 kw)
23 hr @ 110 VAC (1.2 kw)
inductive (Magne Charge) with printed circuit board primary
Motor 102 kw (137 hp) induction motor
150 Nm @ 0 to 7,000 rpm
[(7,000rpm/60) x 2 pi x 150 nm = 110 kw]
Inverter IGBT inverter
Weight 3,086 lb (w/lead acid batteries)
0 to 60 mph 8 sec
Estimate of battery current at peak power
Matched load current = 872A (est) Assume GM at peak power loads battery like Prius (x3 matched load) or 1/2 matched load current. 312 V x 0.75 (sag) x 436A= 102 kw (motor rating). (Confirm: found a 40 Ah 12V lead acid vehicle battery with ESR = 8 mohm, which is a matched load current of 750 to 800 A.)
BMW Mini-Cooper electric proto (3/28/10)
According to NYT the only electric car on USA roads in any numbers (450) besides the Tesla Roadster (> 1,000) is the BMW Mini-Cooper electric (Mini Cooper E). The company is leasing them for one year to 450 customers in New York, New Jersey and greater Los Angeles. Leases started in summer 2009. These are test cars not real production electric cars. They are EV conversions of the Mini Cooper with the battery located where the back seat would normally go. Range 100 to 120 miles (real world).
The monthly rental fee includes a 220 VAC charger than can recharge the batter in 3-4 hours. This is smart. Recharging from 120 VAC as GM is planning is a big mistake. The Times adds, "Charging from a household outlet takes up to 24 hours (others say 21 hours)." (check: 21 hr x 1,500 watt = 31.5 kwh). The implication of charging x6 faster from 220 VAC is x2 voltage and x3 current, in other words 220 VAC @ 45A.
BMW Mini Cooper E electric car
"As for owner complaints, the biggest by far is reduced range in cold weather." (NYT) Oh, yea. At least Cooper (sort of) faced up to the temp problem of the battery by leasing cars in NE not just in the south. However, NY/NJ in winter is not like Minnosota! Range nominally is 100 miles. One customer says he got 128 miles once. "Running the Mini’s battery-powered heater (or air-conditioner) cuts the range as well."
Battery 5,088 lithium-ion cells (grouped into 48 modules, air cooled)
Battery voltage 380 VDC
Range 156 miles ideal (109 city/96 hwy)
Power out 150 kw (motor)
Max speed 95 mph
Wheelbase 97.1 in
Vehicle weight 3,230 lbs
Consumption 0.22 kwh/mile (city)
Recharge 3 hr (240 VAC, 48A rating)
4.5 hr (240 VAC, 32A rating)
26.5 hr (110 VAC, 12A rating)
Note, there is no battery derating in these prototype cars to extend battery life. You can bring the battery charge down to (about) 0%. (In contrast GM Volt is only going to use only half the capacity of its 16 kwh battery.) "To protect the battery" they say they reduce peak torque when the battery charge is less than 10%. (sounds more like a trick to help get you home). One leaser with a 60 mile commute had a 2nd 240 VAC charger installed at work.
From the numbers it does look like BMW just licensed the electric drive train from AC Propulsion. This either means BMW is not really serious about electric cars, or perhaps this was just good engineering. Buy a proven (if unsophisticated) electric drive train from a 3rd party to get a test fleet of electric cars on the road fast. Then you have time to do your own electrical design, convince management about electric cars, and perhaps have an edge over other manufacturers because you will have feedback from a substantial electric fleet in the hands of the public.
Good news for BMW is that the NYT reports that a lot of the first year renters, who have driven it now for nine months or so, like the electric Mini Cooper and want to continue to rent it.
BMW ActiveE (update Feb 2012)
BMW is showing a lot of good engineering in their development of an electric car. The hundreds of small MiniE electrics that BMW leased was on-the-road prototype #1. It has now been replaced by a larger sedan on-the-road prototype #2 (ActiveE) of which BMW will make a 1,1000 (700 to USA) again to be only leased (not sold). While this car is more Leaf like, ActiveE still shows signs of being a partial conversion. NYT says the company plan is for a production electric car in a couple of years and expects to go to a carbon fiber body to cut the weight substantially.
Battery kwh 32 kwh (992 lbs)
Battery 192 cells (liquid heated/cooled)
Range 100 miles ideal
Power out 125 kw (motor)
Vehicle weight 4,000 lbs
Recharge 4 hr (using built in 7.7 kw charger, 240 VAC)
BMW ActiveE (electric prototype #2)
Front seat hits rear seat when pushed back for 6 ft 4 driver!
BMW, like Tesla, as adopted the strategy of fairly heavy regenerative braking when the foot is lifted off the gas pedal, no more coasting. The NYT reviewer, who normally drives a Leaf, says it takes a little getting used to, but once he did he liked it a lot. You learn when to lift off the gas to come to a stop at a traffic light, he says. Essentially you can drive the car with one pedal, do much of your braking without having to move your foot to the brake pedal. BMW speaks favorably of 'one pedal control'. In contrast the Leaf drives more like a normal car, you got to put on the brakes on to stop!
Mitsubishi MIEV electric car (12/25/11)
As of Dec 2011 Mitsubishi is runnng a (short) TV commercial for a pure electric available in USA early 2012. Wikipedia says this car has been on sale in Japan since 2009. It's a tiny car about the size of the Mini-Cooper electric, but with a smaller battery and and less performance. Mitsubishi has got the same size battery as the Volt (16 kwh), but because it's so light (2,580 lbs) its range is 62 vs 35 - 40 miles. While the Mitsubishi is a lot lighter than the Mini-Cooper, it's very odd that the motors in the two care are different by a factor of three (49 kw vs 150 kw)! I suspect the lighter weight of the Mitsubishi may be because it was designed as an electric, whereas the Mini-Cooper spec is for a normal Mini-Cooper converted to electric.
Battery kwh 16 kwh
Battery 88 cells (eq to 3.75V/cell)
Battery voltage 330 VDC
Range 62 miles (EPA)
Power out 49 kw (motor, AC synchronous, PM)
Max speed 80 mph
Wheelbase 100.4 in
Vehicle weight 2,579 lbs
Recharge 50 kw, 80 kw (DC? , Level 3charge) option
7 hr (240 VAC, 15A rating) option
22.5 hr (110 VAC, 8A rating)
Ford Transit electric van (proto) (3/31/10)
Advertized as the Ford's first production electric vehicle in USA will be a Turkish made high profile small van transformed to an electric by Azure Dynamics in Michigan. It's to go on sale at the end of 2010 with sale to commercial customers. Battery pack is on the underside of the vehicle. It appears that Ford is doing something similiar to what BMW did last year, going to a 3rd party electric developer to do a rework get electric vehicles out into the field quickly for feedback.
Battery kwh 28 kwh
Range 80 miles
Max speed 75 mph
Battery voltage 336V
Recharge time 6 to 8 hr @ 240 VAC
Battery weight 600 lbs (reduces van load capacity from 1,600 lbs to 1,000 lbs)
Battery supplier Johnson Controls-Saft
Motor type Induction motor (air cooled)
Motor controller 600V IGBT inverter with DSP based field vector control
Ford Focus electric (update 1/9/11)
(update 11/6/11)
Price of all electric Ford Focus with its liquid cooled 23 kwh batttery is to be $39,995 vs all electric Leaf with its air cooled 24 kwh battery at $36,100 (after a recent price increase). The Focus price is identical (to the dollar) to the GM Volt electric-hybrid.
Ford Focus electric (available 4th quarter 2011) is apparently a direct competitor to the Leaf. Its all electric and the same physical size with almost the same size battery 23 kwh (vs 24 kwh in leaf). The car's top speed is only 84 mph, not much of a highway car. The 'new thing' that Ford marketing is touting is not new. They say the Focus will offer a 6 kw charger (optional) that will be able to recharge the car in 3-6 hours, but I read in the Leaf manual that they have (or will have) a 440 VAC charger that can 80% charge the battery in 30 minues.
The Focus electric will use (like GM) a liquid-cooled active thermal management system for the battery. (Nissan now says it too for "higher end offerings" and for use in Middle East it will go to active battery thermal managment.) A Ford engineer is quoted,
7.5 miles (23 min), mostly 30 mph stop/start (only 2 min above 40 mph), avg speed of 19.6 mph
Nissan is talking about having 20,000 Leaf pre-orders by the time car goes on sale in late 2010. But GM may only make 10,000 Volts in 2011 (from Volt official web site), is not taking pre-orders and has not announced launch locations.
(3/28/10) update)
CEO of Nissan-Renault continues to make big noise about electrics. He claims his company is the only car company investing big bucks in electric car production. The numbers being thrown around are Nissan-Renault spending three billion pounds over the next decade to ramp up to 500,000 electric cars per year. BBC has a video story about a specific Nissan plant in UK that is ramping up to build 50,000 Leafs/yr by 2013. Last line of the video, "The price of which (Leaf) has yet to be revealed!"
Ford Focus electric (3/31/2010) (5/4/2011 update)
So now it's May 2011 and where is the all electric Focus? Well, there's a picture in the new Ford marketing booklet, but it says production begins late 2011. Still a 23 kwh battery. Marketing is featuring 3-4 hr charge time (with 240 VAC) claimed to be twice as fast as Leaf. Car looks nice, and a nice touch is an illuminated ring around the charge port.
May 2011
(Is the large front grill a dummy?)
from Ford Focus video 5/11
(rt is front)
-- Ford’s on-board 6.6-kilowatt charger allows for full charge at home with the 240-volt outlet in three to four hours – charging in half the time as the Nissan Leaf. (Leaf built-in charger is only rated for 3.2 kw) -- Lower price: Based on current plans, the home charging station with standard installation is expected to retail for approximately $1,499, as much as 30 percent less than competitors’ systems
-- Faster charging: With its maximum 32-amp charging capability, Focus Electric owners with the 240-volt home charging station can get a full charge in as little as three to four hours – charging in half the time as the Nissan Leaf. (Note the mini-Cooper charger (for its 35 kwh battery) has both 32A and 48A @ 240VAC rating.)
-- Nonpermanent installation: The charging unit plugs into a 240-volt outlet instead of being hard-wired into the electrical breaker box, making removal and replacement a simple unplug and plug back in operation in the event the owner moves
Ford focus 240 VAC, 32A charger
(designed to plug into a 240VAC outlet)
NYT says Focus will not be compatible with fast DC chargers
-------------------------
Early 2011 Ford will begin selling an all electric version of the Ford Focus. Just like the electric Transient van was outsourced, the electric Ford Fucus design has been outsourced to Magna Internationa, a Canadian company. The Focus, unlike the GM Volt, is an all electric with about 100 mile range. This looks to be a real production car with estimated first year volume of 5k to 10k vehicles.
Battery kwh 23 kwh
Charge time 3-4 hr (using 240 VAC)
Range 100 miles (80 miles in prototyptes)
Max speed 84 mph
Peak power 105 kw (141 hp)
Batteries 98, air-cooled, 60 Ah Lithium-ion
(98 modules x 3.91V x 60 Ah = 23 kwh)
Battery voltage 383 VDC (est from 3.91V x 98 modules = 383 VDC)
Motor 92kw, PM motor (92kw motor from NYT article 11/11, conflicts with peak power of 105 kw)
Max motor rpm 7,500 rpm
Torque 236 lb.-ft. / 320 Nm
Motor controller 600V IGBT s @ peak 365Apk nom (probably)
Transmission single speed gearbox
Wheelbase 104 in
Weight (curb) 3,421 lbs
New electric cars keep popping up out of nowhere. Just beginning production is Coda, a 5 passenger all electric. This is a USA company that Wikipedia says semi-manufacturer a car. They are building an electric version of a gasoline car apparently made in China, but designed in Italy and licensed from Japan! The battery is coming from China. It's privately funded to the tune of 200 million. Recent story says the company at its LA plant has 220 engineers, technicians and corporate staff.
Coda 5 passenger sedan
102.4 in wheelbase, 36 kwh battery, cost 40.8k
This a Ford Focus electric competitor. About same size and power (hair smaller), but with its large battery it is 250 lbs heavier, so its going to be a bit sluggish. The top speed at 85 mph is low.
Battery kwh 36 kwh (728 cells, 104 series x 7 parallel?)
Battery chemistry Lithium iron phosphate (LiFePO4)
Charge time 6 hr (6.6 kw charger on 240 VAC)
Battery thermal Air-cooled Active Thermal Management (translation: fan cooled)
Battery voltage 333 V
Battery location between front and rear wheels
Range 120 miles (manuf claims 150 miles)
Peak power 100 kw (134 horsepower)
Motor PM, 100kw peak, 60 kw cont (UQM® PowerPhase® electric motor)
Max motor rpm
Torque 221 lb-ft (300 nm)
Max speed 85 mph
Motor controller (supplied by UQM)
Transmission single speed (6.54 to 1 gear ratio, 17" wheels)
Wheelbase 102.4 in
Weight (curb) 3,670 lbs
UQM motor + controller supplier
Coda is buying the PM motor + controller from UQM, which manufactures it in Colorado. (40,000 drives annual production capacity). In last quarter revenue was 2M with reported loss of 1.5 M as they geared up to manuf in volume the system below. Looks like this small company is betting big on the Coda. This year they received 4M from DOE for R?D on non-rare earth PM motors for electric cars. Jon F. Lutz is VP for engineering. (motor and drive packaging look very nice)
UQM 100 kw peak PM motor + controller (USA)
model PowerPhase Pro 100 (used in Coda electric 5 passenger sedan)
One of the first lithium-ion battery electric cars expected to reach production status in US (late 2010) is a tiny car from Norway called the THINK city. The THINK company in Norway claims to have been in the electric car business for 17 years and that its latest car is a 5th generation design. They have 1,200 electric cars on the roads in Norway with the factory capability to produce 16,000/yr. However, missing from the THINK company bio is info from NYT which says THINK is emerging from bankruptcy, and this caused them to close their Norwegian plant and shift production to Finland. First cars sold in USA will be imported from Finland. THINK has contracted to reopen an old plant in US (Elkhart Ind) in early 2011, spending 43 million (Dept of Energy loan!) for a facility upgrade, with production capability of several thousand a year.
It was reported in 2008 that THINK was partnering with A123Systems and Ener1 for lithium-ion batteries. Ener1, Think’s biggest shareholder and battery supplier, is headquartered in Indiana. However, a Jan 2010 article in NYT says THINK announced this week that EnerDel, which holds a large stake in Think, will initially become the exclusive supplier of lithium-ion batteries for the car in the United States. (Is EnerDel and Ener1 the same company? Yes, EnerDel is a subsidiary of Ener1.) EnerDel battery model for THINK is E350-25 with prismatic (rectangular) cells. It might be 24 kwh, because this is known to be the size of some EnerDel vehicle battery packs, and it looks about right for the size and mileage. EnerDel got $118 million from US Dept of Energy.
THINK city --- 100 mile two seater Norwegian electric car (available late 2010)
Fast charging
THINK is pushing 'fast charging', which is very interesting. They have contracted with a CA company (AeroVironment) for a roadside 'fast charging' station that (the claim is) provide a 0% to 80% battery charge in 15 minutes. Details not announced, but it is likely it is a charger run from a 440 VAC line, a so-called 'Level 3' charger. (PG?E installed a 440-volt fast-charging station in 2009 off Interstate 80 between San Francisco and Sacramento as a test facility.) Cannot find kwh of the THINK battery.
Tesla's new sedan (as of 2009) was to be designed to 'fast charge' in 45 min from 480 VAC (4 hrs from 220 VAC). Nissan (in 2009) was taking about a 26 min 'fast charge' for a 100 mile electric, probably the Leaf.
Ford Focus (all) Electric (5/09)
In probably a smart move I read that Ford is designing its new European designed Ford Focus to be powered by either a conventional motor or an all electric drive. This will provide economies of scale. The conventional model to go on sale in 2010 followed by the electric version for fleets in 2011. The car design (or more likely just the electrical subsystem) has been contracted out to Canada-based auto parts and assembly supplier Magna International. Well really Magna pitched the electric car to Ford and retains the rights to sell it to other car manufacturers too.
Only preliminary info is available.
Battery 26 kwh lithium ion
Motor 400 volt
Range 100 miles
GM Volt claims 8 Kwh (of 16 kwh battery total) yields a 40 mile electric range. While the Ford Focus may be a little lighter than the Volt, still to claim 100 mile range from a 26 kwh battery means battery manufacturers will have pretty much solved the lifetime problem by 2011 as there is very little excess capacity for lifetime derating.
Electric reliability
Ford escape SUV (300V battery pack) has been used in taxis. A taxi manager at a battery conference recently said some hybrid Escape taxis have now racked up 300,000 miles and the battery pack has proven to be reliable. On the other hand:
"General Motors discovered the batteries already installed in 9,000 2007 model Aura and Vue hybrids were likely to fail prematurely. So a recall was launched in February, and it is still not completed. Batteries for those vehicles had to be replaced before more could be made available for the ‘08 hybrid vehicles" (Jan 2008). (I read the GM batteries were US made)
Electric car 'fuel' mileage
Here is a little exercise comparing 'fuel' costs of electric cars to normal gasoline cars. Obviously there is a lot of assumptions here, but this gives an estimate of incremental cost per mile. I'll figure 'fuel' costs to drive 100 miles.
Gasoline cost $2.70/gal
Gasoline mileage 30 miles/gal
Electricity cost 17.5 cents/kwh (Boston area kwh rate)
For a gasoline car getting 30 miles/gal to go 100 miles the car burns 3.333 gal of gas.
100 miles/(30 miles/gal) x 2.70/gal = $9.00 (@ 100 miles)
GM says its GM Volt will go 40 miles in electric mode using 8 kwh (half the capacity of its 16 kwh battery), hence the electricity cost to recharge the battery 2.5 times is
2.5 x [8 kwh x 0.175/kwh] = $3.50 (@ 100 miles)
electric/gas = $3.50/$9.00
= 39%
Hence the 'fuel' costs for an electric car are about 40% of the fuel costs of a (similar) gasoline car at $2.70/gal (about 25% @ $4.00/gal).
Electric car batteries and range (5/11)
A friend sent me a news article about a professor developing a fast charging battery and asked if this was the solution to electric car range problem. Here's my
reply.
The motors (? generators too probably) of all hybrids on the market are PM synchronous motors using Neodydimium/Iron/Boron magnets, but some (or all?) are not simple PM motors. The Toyota paper below details how they use a combo PM motor and reluctance motor with half to 2/3rd's of the torque coming from reluctance torque. Apparently these motors are used over induction motors because they weigh less.Toyota paper
The only paper I have read on hybrid car motors is a fairly recent paper from Toyota.
'Development of Traction Drive Motors for the Toyota Hybrid System'
by Munehiro Kamiya (of Toyota) (Journal unknown)
This paper describes the development of a high power motor for a Toyota SUV and includes a brief history of Prius hybrids: Gen I and Gen II (Gen III from Toyota is just hitting the market in mid 2009). (A cross-section of the hybrid motor/generator/power splitter integrated structure from this paper is in the Toyota architecture section below.)
Toyota PM/reluctance motor
While the Toyota specs describes the motor type as "Permanent magnet AC synchronous motor", the Toyota paper by Kamiya shows it to be a combo PM/variable reluctance motor. In the 2000 Prius the motor (in terms of torque) is roughly half PM motor (46%) and half variable reluctance motor (54%). In the new motor for the 2005 higher power SUV the variable reluctance is dominant (63%) with PM flux playing a smaller role (37%). The torque vs speed curves show a strong constant hp region, meaning (very likely) that the motor operates above the PM motor base speed.
Toyota hybrid motor(s) ratio of PM torque to reluctance torque
source -- scan from Toyota paper
I am beginning to understand that new electric car companies are really just acting as prime contractors. For example in the case of the Fisker Karma --- It's no secret they buy the gasoline engine from GM. Looking at the web site of their battery supplier, A123, I suspect they are buying the whole battery package from them: cells, package, cooling, thermal manegement. From hints I found I suspect Fisker bought the drive controller from Quantum Technology's ('Q Drive'), and of course they buy the electric motors too (maybe from Powertec). No doubt they buy the regerative brakes and lot of other subsystems too. Of course, the big companies do this too, buying for example the audio systems outside, but they design their own drivetrains.
Electrics PM => induction
Interestingly the one (low volume) electric on the market (Tesla Roadster) and coming quasi-electric (GM Volt) are both using (three phase) induction motors, not the PM/reluctance motors all the hybrids use. (In a 2007 article a CalTech long time electric car designer, Wally Rippel, said all hybrids, with no exceptions, use PM motors.) I have read the hybrids use PM because of its higher energy density, meaning its smaller and lighter. While I originally found this unconvincing, when I look at the Toyota cross-section showing the motor (and generator) tightly integrated with the power splitter I think the smaller argument makes sense. This reason disappears when the power splitter disappears, the motor in an electric is probably stand alone.
The motor for an electric also needs to be x2 to x3 bigger than hybrids, and maybe you could argue (even more) reliable since the engine as a backup source of torque for emergencies is gone. Industrially large PM motors (larger than car motors) do exist, they are rather rare. Induction motors can run to higher temperatures than PM motors, because they are made of only iron and copper, there is no PM to degrade. So the tilt toward induction motors when a bigger motor is needed for electrics is probably because of some combination of thermal and/or cost advantage and less penalty for bigger and heavier.
So what type of generator is being used? I have seen no data, but it's very likely PM in hybrids. In electrics, who knows.
Neodydimium concerns
There are three major concerns with neodydimium (used in PM motors):
* Is there enough supply?
Neodydimium is rare earth elements and as such sources of ore are rare. Most of it is now being used in hybrid car motors and with growth of hybrid cars there concerns supplies will not be adequate in a few years.
* Stability of supply.
China now controls 95% of neodydimium supplies. A company pushing an alternative to PM motors for hybrid cars, Chorus Motors, emphasizes this point.
* High temperature PM degradation
Chorus Motor guys claim at 100C some neodydimium magnets lose half their field strength, but PM motors in hybrids use are better than this. The high temperature characteristics of neodydimium based magnets have been improved by adding 5% of dysoprosium another rare earth. Prius tackles the thermal problem by liquid cooling the motor. The Oak Ridge report (below) says the 2005 Prius motor tested at 21kw (cont) with 35C coolant and 15kw (cont) with 105C coolant, a 29% roll off with temperature. The motor has a high temperature shut down of about 170C to protect the PM from demagnetizing.
Thermal considerations
Toyota says in their spec that both the motor and generator are "water cooled, separate from engine coolant". (I would be surprised if the same water water isn't circulated to cool the inverters and battery pack.) Nope, the battery pack is forced air cooled (? heated).
Surprisingly the Oak Ridge National Laboratory in 2005 put the Prius motor on a dynometer and extensively tested its thermal cooling system, putting on a 50 page (!) report, available online below. This was done to evaluate state of the art in vehicle motors in connection with a fuel cell R?D program that needs a 30 kw motor. (Gen III Prius motor is 'rated' at 60 kw).
http://www.ornl.gov/~webworks/cppr/y2001/rpt/122586.pdf
The high temperature degradation of PM motor is another (never talked about) consideration for the GM Volt. As the Oak Ridge report points out in a conventional hybrid, a roll off in motor performance is compensated for by cutting in the engine sooner. In the GM Volt (? a future fuel cell car) there is no engine to fall back on!
Motor speed
In Priuses 1997 to 2003 the motor had a top speed of 6,000, to 6,700 rpm, which since these are pancake style motor with fairly large diameters, is very fast. For the large SUV the motor size and toque (Power = Torque x speed) were reduced by increasing the top speed to an amazing 12,400 rpm. This is not a prototype motor, this is the top speed of production motors found in vehicles. The paper talks about the special attention that had to be taken in the design of the rotor, so that it could withstand the centrifugal forces tending to make it fly apart! The Oak Ridge report shows the Prius motor to be 4 pole-pair and is commutated with a resolver.
High speed vehicle induction motor
I found some info on high speed induction motors for electric cars at AC Propulsion. Alan Cocconi at AC Propulsion has been working on induction motors and controllers for vehicles for 20 years. He did the prototype for the EV-1 and his technology is licensed by Tesla Motors. He manufacturers a 200 hp (150 kw) high speed induction motor with matching 600V IGBT controller (AC -150) both air cooled, which he sells to people who want to convert their vehicles to electric, and presumably it's used to in prototype vehicles he builds for the military. A pair of them were used in a car that broke the electric land speed record (245 mph). This system directly connects the battery pack to the inverter, meaning the battery voltage sag at peak power must be lived with, it is not regulated out.
I presume this motor is his design since he has patents (one of which I read) on design of low loss rotors for high speed induction motors.
. 
AC Propulson 200 hp, 220 nm, 12,000 rpm, air cooled vehicle induction motor
10 in dia, 14.3 in long, 110 lbs
Efficiency of AC Propulson 200 hp (150 kw), 12,000 rpm vehicle induction motor
with mated 150 kw, 600V IGBT inverter (model AC-150)
Ref: 0.92 x 336V x 485 A (pk) = 150 kw
I am finding there is no agreement on how to size and how fast to run the electric motor in cars. There's a huge spread in top speeds. At the bottom end is Fisker Karma, which I figure has a top speed of about 4,700 rpm (even though this car has a top speed of 125 mph). At the upper end is Tesla, whose torque vs speed curve shows the motor runs to 15,000 (or 16,000) rpm. In between is the Ford Focue electric at 7,500 rpm, GM Volt with an electric motor that goes to 8,000 - 9,000 rpm (check) and Leaf at 9,800 rpm.
All the electric and quasi-electric cars used fixed gearing. A couple of years ago when Tesla began the design of their first car they initially put in a two speed transmission, but it was unreliable, so it was removed for the production car. All the other companies have followed Tesla's lead and use no transmission.
Power = Torque x Speed
The basic physics of motors is [Power = Torque x Speed]. Thus for amount of power, which depends on weight of car, desired acceleration, and wind resistance at top speed, the motor/controller designer has a choice:
High speed/low toque motor or Low speed/high torque motor Motor weight depends on torque (not power)
It appears that Tesla has chosen the former and Fisker Karma the latter. This is a basic choice since the weight and size of a motor is pretty linear with torque. In other words when you buy a motor pretty much you are buying torque. If you can balance it well and figure out how to run it x2, x3, x4 times faster, you get (for little cost) x2, x3, x4 more power for your money and weight. Because the Fisker motors run at such low speeds, the gearing is low, so they need a high torque rating. This makes them large, heavy and expensive. I think each of the two motors is 11.5" in dia and about 3 ft long. In contrast the motor for the new Tesla sedan (which weighs maybe 20% less and a somewhat lower top speed) from pictures is only one motor 9" in dia and about 1 ft long. A huge difference in weight, size, and undoubtedly cost.
There are two other important, related considerations: car maximum speed and PM vs induction motor. With no gearing the motors go from zero speed when the car is at zero speed to top speed when the car is at top speed. Leaf top speed is 90 mph vs 125 mph for the sports car like Fisker Karma. If running the motor fast saves so much weight and cost, why doesn't everyone do it? Well for one thing running 10,000 rpm and faster is high tech. I don't understand the mechanically all the tradeoffs, but it clearly takes a lot of engineering to pull this off reliably in a production vehicle. One issue clearly is balance, another is bearing wear and heating, another issue is heating of the motor iron/magnetics due to high frequency current. Tesla uses the same super high speed strategy with their first car the roadster, so they have a history with it and apparently it has proven reliable for them.
High speed torque vs speed
Another important consideration is control of the motors at high speed and getting good torque at higher speeds. The low speed PM motor is by far the easiest to control. This is the low tech, safe, but expensive choice that Fisker Karma has made. And it's probably a huge mistake, reflecting a basic weakness they have in motor control. Everyone else is running medium high to super high speeds. Trying to run a PM motor above its 'corner frequency' is tricky business. It can (or could) only be reliably done to a limit extent, like getting an extra 30% more speed.
Historically the induction motor was tricky to control, but with modern digital controllers implementing 'vector control' the control problem is pretty much licked, though I suspect the controller still has some looseness over the motor temperature range. A big advantage of an induction motor for high speed operation is that its magnetic field is controlled by the controller, not built into the motor as it is with a PM (permanent magnet) motor. This allows a high speed technique called 'field weakening', where the motor torque rolls off inversely with speed resulting in a 'constant hp region'. This can easily extend the top speed in an induction motor by a factor of 2 whereas a PM motor has trouble achieving 1.3.
Tesla footnote --- Tesla Roadster motor is also induction
GM Volt footnote -- GM motor has usually been identified as induction,
but 2011 SAE paper (below) by GM engineers says clearly both machines in the Volt are PM
http://papers.sae.org/2011-01-0355 Inverter/boost
Inverter
Inverter is the (generic) name of the power electronics box that drives the motor. In goes DC voltage and out comes AC voltage for the motor. It's called an inverter because it (crudely speaking) inverts the DC power from the battery (or capacitors) to AC power for the motor. The AC that's created is quite complex with a variable frequency and amplitude that depends on the motor speed, motor torque, and even the type of motor. In modern inverters the position of the motor rotor is continually tracked, and this information combined with the torque command is used by a specialized computer to drive the six large power transistors (IGBT's) that chop up the DC to make AC. In early Priuses 1997 to 2000 the battery pack voltage was fed directly into the inverter. The inverter DC bus voltage would be the battery voltage, which was 274V at light load, but as power demand on a battery increases its voltage begins to sag. If fully loaded, the battery voltage would sag from 274V to near 140V. This poses a problem in motor control, because there is high voltage available (to drive the motor) at low speed and low voltage available at high speed, which is exactly the opposite of what the motor requires. As the car accelerates up (at constant toque), the motor voltage rises approx linearly with speed (starting from near zero) and the battery voltage sags (starting from 274V), and when the two voltages meet, the maximum motor torque rapidly falls off with increasing speed, or in simple terms, the motor dies.
In Gen II Toyota starts adding a boost converter between the battery pack and the inverter. This is a circuit that boosts the battery voltage (likely in the 200V to 300V range) and regulates it to 500V (in 2003 Prius) and 650 V in the 2005 SUV for the transistors in the inverter. A boost converter is a substantial complication to the power electronics. For one thing the power from the battery is now converted twice before reaching the motor. Not only is this expensive, but adding a boost converter roughly doubles the heat put out by the inverter electronics. However, a big advantage is that the boost converter 'shields' the inverter from the voltage sag of the battery. As long as the battery pack can deliver the required power, it doesn't matter (much) to the inverter how much the battery voltage sags, because the inverter sees a relatively constant voltage at its input. In other words the boost inverter decouples the battery pack from the inverter allowing better optimization. The performance advantage is that the vehicle can accelerate electrically to higher speeds.
Bus capacitors
For another thing the low impedance of the batteries can no longer protect the transistors from overvoltage. Now large, expensive, and not fully reliable bus capacitors need to the output of the boost circuit to provide a stiff, high current bus for the inverter. Traditionally aluminium bus capacitors wear out, so I would like to know how Toyota has addressed the issue of bus capacitors and reliability.
Why a booster converter?
So why boost battery voltage? Two reasons, one it improves performance. The output voltage of the boost can be regulated. This means the sag of battery voltage during peak power demands now does not affect (directly) how the motor runs. The second reason is that as hp in hybrids rise, the raw battery current gets too high to handle easily in the transistors or motor.
Say at high power the battery voltage sags to 150V, but is boosted to 650V at the inverter bus. While the boost transistor(s) have to handle the high battery current, the downstream components, inverter IGBT's and motor, can operate at current about x4 times lower (650V/150V = 4.33). A x4 boost in bus voltage above the battery, reduces the current downstream in the IGBT transistors and the motor by x4. In round numbers for the 123kw SUV this mean IGBT and motor currents are reduced from 800A range, which is very high and difficult to handle, to a much more reasonable 200A range.
IGBT transistor modules
The high power switching transistors that drive the motor are called IGBT (Integrated gate bipolar transistors). They have dominated high power switching for the last 10-20 years or so and are pretty well standardized, with family's of different currents at 600V and 1,200V. Of course the auto market is so large that it's possible that special voltage rated parts are used, but looking at the nominal bus voltages in the Toyota power they are consistent with standard 600V and 1,200V IGBT's.
600V => 1200 V IGBT's
Very likely Toyota's gen I verhicles with 274 VDC nominal buses used 600V IGBT's, and when the voltage booster was added in Gen II to raise the bus voltage to 500V (to 650V in gen III), the IGBT's were changed to 1,200V modules. The motor inverter (peak) current in a gen III Prius (60 kw) is about 92A and in the Toyota SUV (electric system is rated at 123kW) it would be about 190 A peak (190 A x 650V = 123kw). The later requires (at least) a 300A dual IGBT 1,200V modules, which are fairly standard items, which I used myself in designs. (It's a somewhat difficult jump to 600A single IGBT's because it's difficult to mount the required protection capacitors.)
Fusion => 600V IGBT's
The new (2010) Ford Fusion hybrid, on the other hand, although it has a somewhat more powerful motor than the Prius (78 kw vs 60 kw) and a voltage booster, has a bus voltage of only 275 V (maybe modulated up and down somewhat), so it's very likely they are using 600V IGBT's. The 30% higher motor power combined with the low bus voltage, less than half of Prius' 650 VDC, combine to push the peak currents in the Fusion 600V IGBT's to about 280 A. (To first order a 600V IGBT @ 300A with dissipate the same heat as a 1,200V IGBT @ 150A.)
Bus capacitors
Here's a picture of the Toyota gen III inverter opened up I found on Wikipedia. The three big cans on the right are the bus capacitors. Big bus capacitors are needed to stabilize the bus voltage as the chopped up motor current, which can be 200 to 300 A, are switched into and out of it by the transistors. In inverter design sizing these capacitors is tricky, because they are large, relatively expensive, and unlike most electronic components they have a finite lifetime, because they slowly dry out, and the hotter they run, the faster they decay.
(Old) gen I Toyota inverter (bus wired to 274 VDC battery)
Wikipedia identifies this as a Prius inverter (2000 to 2003) (NHW11)
The rating on the bus capacitors can be read off the side: 2,700 uf, 450 VDC, 85C. (85C is the lowest temp grade of bus capacitors) Three 450 VDC caps seems like an odd number. Don't know what the bus voltage is in gen III, but it was 650 VDC in gen II, so it's likely to be the same or higher. Therefore it takes a minimum of two 450 VDC capacitors in series to handle this voltage. With three 450 VDC capacitors in series the voltage rating is 1,350V (1,500 V surge). On first glance this seems like excessive voltage margin, and with all the caps in series they run hotter since they all have to carry the full AC current, but it might be necessary to protect the system in the event of a fault at high motor speed.
(update)
I think the explanation of the 450 VDC bus capacitors (and look of the inverter) is it that this is a gen I inverter (identified in Wikipedia article 'Toyota Prius'). In gen I there was no boost converter, the battery voltage and the inverter bus voltage were the same at 274 V (open circuit). This means the three 450 VDC capacitors now makes sense. They are wired in parallel and just provide local absorption of the PWM (switching) current.
Toyota Prius gen II (left) and gen III (right) inverter enclosures
Pipes shown may be for liquid cooling
(only some of the input/output wiring is shown) Field weakening
Toyota uses field weakening in their partial PM motor, a technique I was involved in developing in the 1980's. This is a control technique that allows the motor to run at reduced torque to speeds higher than is normal, speeds where the internal voltage of the spinning magnets can be considerably higher than the normal terminal voltage (650 V). What's tricky about this is that if the drive 'faults' (shuts down or reboots) while the motor is running at high speed the IGBT's and bus capacitors can potentially be exposed to much higher voltages than normal. It might be that three 450 VDC capacitors are used in series to prevent a failure of the bus capacitor from overvoltage in the event of a high speed fault.
Patents
An article in WSJ (7/1/09) says Toyota has filed for 2,000 patents on its hybrid technology, 1,000 of them related to gen III, which went on sale in May 2009. They don't reveal how much licence revenue they earn from their patents. Ford claims, "Our hybrids are 100% Ford-developed and engineered, (our) execution and architecture are different", says a Ford spokeswoman. Ford recently cross-licensed 20 or so hybrid patents with Toyota, and Ford says no money changed hands. This would tend to confirm that Ford probably has a considerable hybrid patent library, which is pretty surprising since they started work in the field many years behind Toyota.
Comparing hybrid ? non-hybrid siblings
So how much does hybrid technology improve gas milage? A way to get a handle on this is to compare the same car in its hybrid and non-hybrid form. Prius is only a hybrid, but Toyota makes midsize size car in both forms and so does Ford.
standard hybrid
Toyota Camry (109.3 wheelbase) ------------- ------------
engine 2.4 L 169 hp 2.4 L 147 hp
weight 3,307 lbs 3,680 lbs
EPA mileage 22 city/32 highway 33 city/34 highway
sticker 22,650 26,900
standard hybrid
Ford Fusion (107.3 wheelbase) ------------ ------------
engine 2.4 L 175 hp 2.4 L 156 hp
weight 3,342 lbs 3,720 lbs
EPA mileage 22 city/31 highway 41 city/36 highway
sticker 24,700 28,000
The hybrids are 373 lbs (Camry) and 378 lbs (Fusion ) heavier than its non-hybrid sibilings. Non-hybrids are both 22 mpg city and 32/31 highway. The hybrid Camry pushes city to 33 mpg and hybrid Fusion to an amazing 41 mpg, hybrid highway mileage is up too, 4 mpg Camry and 5 mpg Fusion. The hybrids probably drive about the same as the non-hybrids or maybe a little more peppy, they weigh 11% more, but have the equivalent of a 6 cyl vs a 4 cyl.
Toyota hybrid technology
The newest Prius (weight 3,042 lbs) gets 51 mph city, 48 mpg highway. 275 thousand Priuses were sold (worldwide) in the last year. Toyota describes their hybrid technology over the last 12 years as following into three generation. The electrical power (called 'traction battery power' or power available with engine off) is from a Wikipedia number. These numbers are a little lower than I derived from the battery specs, indicating either than there are losses and/or reserves or maybe they don't' pull maximum current from the battery to coverup some of the roll off in current at low temperature.
gen I Prius (2000 -2003)
THS (Toyota Hybrid System) I no boost convert (battery => inverter)
600 V IGBT's (273V bus)
44 hp (33 kw) electrical, 1.5 L 4 cyl engine
273 V, 1.77 kwh battery (228 cells, 6.5Ah)
gen II Prius (2004 - 2008)
THS (Toyota Hybrid System) II boost convert (battery => boost => inverter)
1,200 V IGBT's (500 V bus)
28 hp (21 kw) electrical, 1.5 L 4 cyl engine
201V, 1.31 kwh battery (168 cells, 6.5Ah)
gen III Prius (2009 -
HSD (Hybrid Synergy Drive) advanced version of THS II
1,200 V IGBT's (650 V bus)
36 hp (27 kw) electrical, 1.8 L 4 cyl engine
201V, 1.31 kwh battery (168 cells, 6.5Ah)
Toyota gen III 2010 Prius specs
I read that in 2008 outsiders were speculating Toyota would change to lithium ion batteries for gen III, a Toyota engineer was quoted as saying lithium batteries were ready. The speculation in the press is that gen III has come out with standard NiMH batteries because of fire risk with lithium ion.
motor (electric)
type Permanent magnet AC synchronous motor
power output 60 kw (80 hp)
torque 207 nm
voltage 650 V max (indicates bus voltage is 650 VDC)
Battery
type sealed NiMH
voltage 201.6V (201.6/1.2V = 168 cells or 28 (6 cell) modules
power output 27 kw (36 hp)
The peak electrical power spec is a little difficult to interpret. Note the motor can, and apparently does, have a higher (peak) kw rating than the battery pack. This is because (almost for sure) the generator and motor buses are common. The current into the inverter driving the motor can therefore come from both from out of the battery and (backward) out of the generator inverter. Wikipedia (Toyota Prius) lists the kw rating of the motor as 60 kw. The Toyota spec (above) shows the battery pack output rated at 27 kw.
Peak power ratings
I suspect the 27 kw (peak) from the 28 module battery pack is right, because the numbers fit together and look reasonable. When the motor is putting out its peak 60kw, about half (27 kw) comes from the battery pack (via boost converter) and about half (33 kw) come in to the common inverter bus from the generator inverter. So we have a 60 kw motor/inverter and a 33 kw generator/inverter. This is consistent with the Toyota cross-section diagrams which show the generator is about half the size of the motor.
The numbers on the battery pack also look much more reasonable. As I detailed above, if the battery pack is loaded to it absolute max peak rating (matched load), the battery could deliver 36kw = (360 A x 0.5 x 201.V), but at the same time it is dissipating (as heat) 36 kw internally. By restricting the peak power out of the battery pack to 27 kw, or 75% of its maximum power capacity, the numbers are much more reasonable. At 180 A out the battery terminal voltage sags from 201.6 V to 151 V (180 A x 0.28 ohm = 50.4V). Power out is 3/4th of max capacity (180A x 151V = 27.2 kw), but the internal (heat) losses are reduced by a factor of four to 9 kw (180A x 50 V = 9kw). Now 3/4th's (27 kw/36 kw) of the battery's energy is making it to the wheels vs 50% (36 kw/72 kw) at peak capacity. Also the peak temperature rises within the cells are reduced by a factor of four!
Toyota gen III circuit sketch
I've sketched up (what I think is) the peak power conditions for the gen III 2010 Prius electrical subsystem. About half (33 kw) the peak power for the motor inverter (60 kw) comes in (from the engine) to the common bus capacitors via the generator inverter without going through the battery. The other half (27 kw) of the motor inverter peak power is pulled from the battery pack through the boost converter, which boosts and regulates the voltage on the bus capacitors to 650 VDC.
Bus voltage of 650 VDC in gen III Prius was cranked up from 500 VDC in gen II (evidently the inverter in the in the 2005 Toyota SUV with its 650 VDC bus must have proven reliable!) The motor inverter current (0 to pk) is figured by dividing the peak instantaneous power out (60,000 watts) by the bus voltage (60,000W/650V = 92.3A). The 0.28 ohm battery pack ESR is figured as (10 mohm/module x 28 modules = 0.28 ohm).
Whoops, I just realized something. The boost converter needs to be able to handle bidirectional power flow, which makes it more complex. Power flowing to the battery from the generator and regenerative braking (via motor and motor inverter) needs to be down converted from 650 VDC bus to the 200 V or so battery voltage.
Lexus sedan hybrids
It's maddeningly difficult to get specs on the Lexus hybrids. As of 2010 they will have three hybrid sedans: 4 cyl (not out yet), V6 and V8. One of the Lexus hybrids (crossover) has a 288V battery, which looks like it may be three 288V strings in parallel, which if the cells are the same as Prius would give it a peak kw rating of 115 kw.
288/V/202V x 3 parallel strings x 27 kw (Prius rating) = 115 kw
Hyundai Sonata hybrid has a totally different architecture than Toyota/Ford
(has no power splitter, a clutch, and a six speed transmission) Lexus LS 600h is described as having a "dual stage" variable speed transmission. With two motors (front and rear) it sounds like it probably has two sets of power splitter and motor/generators. Bottom line: The Lexus hybrid sedans are probably pretty much like the Prius except with powertrain components (mechanical and electrical) are scaled up by x2 to x4.
Prius programming bug -- 'no brakes!' (2/8/2010)
It's easy to forget that brakes in a hybrid depend in a complex way on a computer program within the car. Braking is a hybrid is a combination of retarding force from the motor (regenerative brakes) plus hydraulic (electronic?) standard brakes, which dissipates braking energy as heat in brake pads. The program tries to use the motor/regenertive braking as much as possible (obviously, to improve mileage) and standard brakes only a little. I presume there must also must be a failsafe mode with hydraulic brakes able to stop the car in a reasonable distance if the electronics have shut down or faulted (I have seen no spec on this).
In Gen III Prius brake programing was complicated the by adding a third braking mode: anitlock brakes.
Here's my interpretation of the data (below) that I was able to scrounge up on the Fusion. 2010 Ford Fusion hybrid uses a scaled up Prius architecture, higher power and more elaborate controls, which appear to be very effective because Fusion gas mileage is great, 8 mph better than Toyota Camry in city. Fusion battery pack, motor, and 'power splitter' are all very similar to Toyota Prius.
Ford has a voltage booster (between battery and inverters), but it does not boost to a 650 V bus like Prius. It's used (mostly) to remove voltage sag from the 275 V battery pack holding the bus voltage for the motor at 275 V under heavy load. The bus voltage appears to be modulated, possibly lowered at low speeds (to improve inverter efficiency) and allowed to rise to 350 V or so if more generator power is needed? Prius uses (I think) 1,200 V IGBT (@ 92 A pk), but Ford Fusion is probably using 600 V IGBT's operated @ 283A pk = (78kw/60kw x 650 Vbus/275V bus x 92 A).
----------------------------
Ford's Fusion hybrid marketing phrases and news articles:
78 kw, 6,500 rpm, 275 V (60 kw motor in Prius)
PM AC synchronous motor
73 ? kw generator (33 kw generator in Prius)
variable-voltage converter (probably means bus voltage is modulated
with speed ? power)
2.5 L engine, 156 hp @ 6,000 rpm (1.8 L, 134 hp @ 4,500 rpm in gen III Prius)
Atkinson, 4 cyl
1.3 kwh, 275V NiMH battery (Sanyo) (1.3 kwh, 201V NiMH battery Prius (Panasonic)
EPA mileage: 41 city/ 36 highway, all electric: 47 mph, 2 miles distance
power-split device
Electronic Continuously Variable (e-CVT) transmission
-- The Toyota Prius, Ford Escape Hybrid use a gasoline engine and two electric motor-generators (MG1 and MG2) connected to a planetary gear set called the "Power Split Device" to deliver power to the transaxle final drive planetary gear set. (Generator and motor rating on Escape are same as Prius (65 kw motor, 28 kw generator, same 'power splitter'. The Escape hybrid technology was obtained from Toyota. Escape is apparently gen I Ford hybrid. Ford Fusion hybrid is referred to as Ford gen II hybrid.)
-- Series-parallel hybrid transaxle sourced from Aisin. Aisin site shows they make "Hybrid Electric Planetary" transmission for Ford Escape hybrid and Lexus GS450h hybrid.
-- engine’s valve timing, fuel delivery, and spark timing to match the power delivered through the electric motor, permitting very aggressive fuel shutdown under light loads
-- Variable voltage controller (VVC) to the Fusion hybrid that allows the voltage from the battery to be stepped up on demand. During most driving conditions when comparatively little power is needed, the lower voltage increases the efficiency of the electric drive system, while the VVC allows even greater output than the Escape when it's needed for acceleration or heavy regenerative braking."
-- Fancy blue slide says, 'Hybrid Transaxle - eCVT' lists the improvement of the Variable Voltage Converter as 130% motor, 160% generator.
NYT auto reviewer compares two mid-sized 2012 hybrids -- Lincoln MKZ and Lexus HS (7/22/11)
The NYT reviewer after a 1,000 mile drive in both cars finds the Ford hybrid technology does much better in real world fuel economy than the Toyota hybrid technology. Not only does Ford have a big EPA city 41 vs 36, but NYT finds on highway Ford remains near its 36 mpg EPA highway rating while Toyota falls off its 34 mpg highway rating to 28-29 in reality!
Lincoln MKZ hybrid 107.4 wheelbase 3,752 lbs (curb weight) EPA 41 city, 36 hw
Ford Fusion hybrid 107.4 wheelbase 3,720 lbs (curb weight)
Lexus HS hybrid 106.3 wheelbase 3,682 lbs (curb weight) EPA 35 city, 34 hw
Lexus hybrid
From Lexus marketing material and random technical tidbids it's clear the Lexus hybrid has the same hybrid architecture as Toyota and Ford (same sun/planet continuous variable transmission). Lexus, however, mates much larger engines, a 6 cyl and even an 8 cyl, with a modest size electric system. Consider this Lexus marketing claim for their big 8 cyl hybrid:
They use a PM motor (? generator). Their NiMH battery voltage is only 158V = 132 cells x 1.2V/cell. The batteries are made by Panasonic and Sanyo. Usable capacity is 5 kwh, 100A peak discharge. Est 12kw = 0.75 (sag) x 158V x 100A.
Ford Fusion for 2013 --- hybrid shifts to more electric power (1/15/12 update)
Ford has revealed features of its 2013 model Fusion (available late 2012), and it shows the evolution of the hybrid and blending with electric cars. The NYT story on the Jan 2012 Detroit auto show, where the 2013 Fusion was revealed says the consensus of the industry is that hybrids are the route to the electric car.
The 2013 Fusion will be available in three versions: gasoline engine, (upgraded) hybrid, and plug-in hybrid. The 'upgrading' of the hybrid is a shift of the power balance toward the electric drive train. Engine displacement has been reduced 20% (2.5L to 2.0L) and the hybrid battery changed over to a lithium-ion. No numbers but very likely both the hybrid battery kwh and electric motor hp are larger. This hybrid can go up to 62 mph on electric alone vs 47 mph in existing Fusion hybrids. Like the Prius it will be available with a larger (or added) lithium-ion battery charged by plug-in to give it more electric range (again no hard numbers available). Prel Ford spec shows it to have a PM motor and a continuously variable transmission, which probably means it is still using the basic Prius architecture.
Hybrid architecture
I have not compared the Toyota/Ford/Lexus architecture other hybrid architectures (like Honda), but as I have come to understand the Toyota architecture, I like it a lot, it's very sweet and very flexible. I don't own a hybrid, and am unlikely to do so for some time since I have a new car, so I can contribute nothing from a user viewpoint. I have no real understanding of mechanical components like gears, transmissions and power spliters, but I do now understand the basics of the sun/planet gear set, which is central to the Toyota/Ford/Lexus hybrid, and have written it up in detail below.
You can't have an essay on hybrid cars without a (high level) description of the hybrid architecture, so I have dug up some good reference material. The best introductory description of the Toyota architecture I found is the first ref below. It has clear diagrams which appear to be consistent with the cross-section diagram that appears in a Toyota technical paper, so is probably correct. Another good reference is Wikiedia (Toyota Prius).
http://www.cleangreencar.co.nz/page/prius-technical-info
http://en.wikipedia.org/wiki/Toyota_Prius
Below shows the motor connected to the wheel/axle through a fixed (step down) gear ratio. According to a Slate article the Prius tops out at about 100 mph and according to a technical Toyota paper the motor speed tops out at about 6,000 rpm. Thus we can estimate the step down ratio as about 3:1 figured as follows: assume 15" dia tire (3.925 ft/rev) and 100 mph = 147 ft/sec, so wheel rpm @ 100 mph = (147 ft/sec)/(3.925 ft/rev) = 37 rev/sec (2,240 rpm). (Other references confirm: 3:1 fixed step down from motor to differential/wheels)
source -- http://www.cleangreencar.co.nz/page/prius-technical-info
Cross-section of (high power) Toyota gen II for SUV
Engine connects top right thru generator to power split device
3:1 step down to standard differential (then to wheels) (in Prius cars) connects at motor shaft
motor 'Reduction Device' (2:1) is not used in Prius cars (motor 12,000 rpm, generator 6,000 rpm)
source -- scan from 'Development of Traction Drive Motors for the Toyota Hybrid System'
by Munehiro Kamiya, Toyota (technical paper)
you can see in the cutaway that the generator has roughly half the power of the motor
source -- http://www.cleangreencar.co.nz/page/prius-technical-info
Three axes of Prius sun/planets/ring 'power splitter' in exploded view
sun (right) --- generator
planet carrier (middle) --- engine
ring (left) --- motor/wheels
planets are five small gears (partly visible) on center carrier
source --- http://www.cleangreencar.co.nz/page/prius-transmission
center (sun) generator (MG1)
middle (planet carrier) engine (ICE)
outer (ring) motor/wheels (MG2
source -- http://www.cleangreencar.co.nz/page/prius-technical-info
As shown in figure above, the shaft that couples to the differential/wheels is connected to the motor on one end and the 'power splitter' at the other end. The net torque seen by the shaft is the sum of the motor magnetic torque and the power splitter gear torque, and since it's a rigid bar, summation of torques is summation of power. The power than comes in via the motor is the fraction of the engine power that was bled off into the generator and may include power pulled from the battery pack.
Mysteries of the 'power splitter'
A key element in the clever Toyota/Ford/Lexus hybrid architecture is the 'power splitter'. It's a sun/planet/ring gear set that connects together the shafts of the engine, generator and motor. Rotation of the engine goes in and it comes out as some combination of the speeds of the motor (wheels) and generator according to this formula (Toyota gen I and II)
engine speed = 0.72 motor speeed + 0.28 generator speed
or
(3.6 engine speed = 2.6 motor speeed + gen speed)
'Continuously Variable Transmission' (CVT)
If you think of the power splitter gear set as a 'transmission' directing power from the engine to the motor/wheels, then it's a transmission with an auxiliary output (generator speed). Causing current to flow in the generator can put a drag force on the engine such that the generator spins up and steals some of the engine power. The speed of the generator can be controlled over a wide range by electrically varying its drag torque, and in this way different engine speeds are possible at given motor/vehicle speed.
The trick in the Prius hybrid architecture is that the power bled off the engine by the generator is not wasted, it's fed right back into the motor/wheels because the generator and motor inverters share a common bus. The whole thing functions as an Electrically controlled Continuously Variable Transmission (eCVT). (As explained below, if the engine speed is in a sense 'too high' for the motor/wheel speed the power flow in the electrical path reverses, causing a local power loop.)
Toyota marketing shows the action of the parallel motor path acting as variable speed transmission below. Is this accurate? yes ? no Where it's not accurate is high speed highway driving. Here the variable speed transmission operates in 'overdrive' allowing the engine to run slower and more efficiently. The motor instead of adding torque to the engine path acts as a drag and actually reduces torque. (In the diagram this mean the little arrows in the motor path reverse.)
Oversimplified, in high speed 'overdrive' the little arrows (left) reverse
It's actually pretty simple (though it took a while to realize this). The torque ratios between the three input axes are fixed by the design of the power splitter (diameters, # of teeth, etc). Any gear set is like a transformer in that the golded rule is [power out = power in]. With [power = torque x speed] and torque ratios fixed the [power in = power out] rule leads to a constraint that the sum of the motor and generator speeds (properly scaled) be equal to engine speed. A 'power splitter' forces a speed (not power!) relationship among its three shafts, as shown below.
engine power = motor power + generator power
Tengine x engine speed = Tmotor x motor speed + Tgen x gen speed
engine speed = (Tmotor/Tengine) x motor speed + (Tgen/Tengine) x gen speed
Another way to think about the 'power splitter' is that the interlocking teeth force a relationship among the three three shaft angles. You can write an equation that the three angles (properly scaled) must sum to zero, which when you take the derivative leads to the speed relationship.
Works like a three winding transformer
A three axis sun/planet power splitter is the mechanical analog of a three winding transformer. A transformer sums (scaled) currents, a power splitter sums (scaled) shaft speeds.
In a transformer the three voltages are ratioed by the design of the transformer, by the turns ratio. Power is [V x I], so the [power in = power out] rule leads to a constraint that the sum of output currents (properly scaled) be equal the input current.
n1 x i1 = n2 x i2 + n3 x i3
or
i1 = (n2/n1) x i2 + (n3/n1) x i3
Note the voltage ratio between (output) windings 2 and 3 is just the ratio of their scaling factors in the equation above, i.e. [(n2/n1)/(n3/n1) = n2/n3]. In the same way the torque ratio between the power splitter motor shaft and generator shaft is just the ratio of their scaling factors.
Toyota's power splitter formula
Here's the 'power splitter' speed formula with Toyota's ratios.
engine speed = 0.72 x motor speed + 0.28 x gen speed (Toyota)
To better understand what this means in practice for a hybrids I worked out values over a range of vehicle speeds and put them into a table. Obviously in practice min/max rpm limits must be respected to. For the Prius the min/max rpm limits are
engine 1,000 rpm (start) to 4,500 rpm (rated torque)
generator +/- 6,800 rpm
motor/wheels -1,500 rpm (reverse) to 6,000 rpm (102 mph)
(wheel speed = 1/3rd motor speed fixed ratio)
From the Prius speed equation we can 'read off' the design ratio of output axis' torques (motor and generator) built into the 'power splitter' by its design (gear ratios, etc).
(generator axis torque/motor axis torque) = 0.28/0.72 = 1/2.6 = 0.385
or
generator axis torque = 0.385 motor axis torque
(motor axis torque = 2.6 generator axis torque)
From above we can find the fraction of total power flowing through the 'power splitter' that flows in the generator axis.
Pgen/Ptotal = gen toque x gen speed/[gen toque x gen speed + mot torque x mot speed]
Ptotal (via power in = power out equation above) is also the engine power.
Pgen/Peng = 1/[1 + (mot torque x mot speed/gen toque x gen speed)]
= 1/[1 + (2.6 gen torque x mot speed/gen toque x gen speed)]
= 1/[1 + (2.6 mot speed/gen speed)]
or
= gen speed/[gen speed + 2.6 mot speed]
a) When generator speed is low (relative to 2.6 motor speed), Pgen/Pen = [gen speed/2.6 mot speed],
so generator power flow is proportional to generator speed. Note, when generator speed goes negative, generator power flow goes negative, meaning the generator is acting as a motor (looping power).
b) When motor speed is zero (vehicle stopped), Pgen/Peng = 1, meaning all the engine power flows out the generator axis.
c) When (generator speed = motor speed), Pgen/Peng = [1 (1 + 2.6)] = 0.28, or 28% of engine power is flowing out the generator through the parallel electrical path to the wheels.
Here's the table above with an added Pgen/Ptotal column.
Surprisingly power can flow in a local loop around the generator, inverters, motor, power splitter path. Note in the table at high speeds there are entries for zero generator speed. At these combinations of motor and engine speed, no power is flowing in the generator path since power flow requires speed (P = T x speed), 100% of the engine power flows mechanically through the power splitter to the wheels.
What happens as engine speed (at a given motor speed) increases or decreases a little from these null generator points? Clearly the generator begins to turn, positive one way and negative the other, but what about power flow? Graham Davies on his Prius page has a long write up about this.
http://prius.ecrostech.com/original/Understanding/WhatsGoingOnAsIDrive.htm
Davies explains some power loops (he doesn't use this word) when the engine rpm is low causing generator speed in the table (above) to be negative. This is the opposite of what I found in Wikipedia (in a somewhat confused article - Hybrid Synergy Drive), but Davies is probably right. Turns out this is normal driving mode. At moderate speeds on flat terrain only low power is needed to overcome wind resistance, so the engine is run at low speed for low noise and high efficiency. Under these conditions generator (MG1) rotates backwards and acts as motor. The only source of power for it in steady state is motor (MG2) acting as a generator.
As the table above shows the looping power can be considerable. For example, at 68mph reducing engine speed from 2,888 rpm to 2,000 rpm will cause 44% of the engine power to loop backwards in the electrical path with (motor (MG2) is acting as a generator and generator (MG1) acting as motor. The 44% loop back power comes from a drag load the motor (acting as a generator) imposes on the power splitter motor axis. The result is that power splitter motor axis has to put out 144% of engine power with 100% going to the wheels and 44% pulled off and circulated back into the power splitter generator axis via the parallel electrical path. Large amounts of loop back power increases electrical losses and means the teeth in the power splitter must be designed to handle the extra torque load. Very interesting.
Overdrive
Davies and others did a lot of measurements on their Priuses and modeling, so he's probably right. So is this looping power useful or just an annoyance? It's useful, actually it's overdrive. Davies' argues this way.
'Power splitter' doesn't force a power split
Proof that the power splitter doesn't directly force power relationships is this. Consider all electric mode with engine off and not rotating. Even though the battery is driving the wheels directly using the motor the equation says there must be generator speed. And there is, the generator free wheels, it has speed but no power flow. With (engine speed =0) the speed equation is
0 = 0.72 x motor speed + 0.28 x gen speed
gen speed = - 2.6 motor speed
Max generator speed is 6,800 rpm, so above 2,600 rpm ( = 0.39 x 6,800) motor speed, which is 44 mph, the engine has to run (or at least the shaft has to rotate)..
Cool power splitter animations
Center (sun) is generator, middle (planet carrier) is engine, outer (ring) is motor/wheels
. 
. 
a) Engine crank, vehicle stopped b) Drive on battery, engine stopped c) Cruising with power from engine
directly and via generator
created by Graham Davies
source --http://prius.ecrostech.com/original/Understanding/WhatsGoingOnAsIDrive.htm
If one shaft is not rotating, then the other two shafts are connected via a fixed ratio. This is the mode during engine start with the vehicle not moving, and it's the case in all electric mode with the engine off.
a) Generator cranks engine, vehicle stopped (3.6:1 fixed)
3,600 rpm gen = 3.6 x 1,000 rpm engine
b) Motor drives wheels, engine stopped, generator freewheels (-2.6:1 fixed)
-6,800 rpm gen = -2.6 x 2,600 rpm motor (eq to 44 mph)
(all electric speed range 0 to 44 mph)
Three shafts rotating
c) Driving with engine power (optionally battery assist)
2,000 engine rpm = 0.72 x 2,000 motor rpm + 0.28 x 2,000 gen rpm
3,000 engine rpm = 0.72 x 4,000 motor rpm + 0.28 x 400 gen rpm
4,333 engine rpm = 0.72 x 6,000 motor rpm + 0.28 x 0 gen rpm
Same axis
All three shafts in the power splitter rotate around the "same axis". The motor (ring) come in from one side, and (apparently) as shown in the diagram above the engine and generator are concentric, one shaft inside the other. The sun, planets, and ring are all geared together via teeth. No slipping. This 'no slip' is shown below in the graphics by the gray bars always lining up. The rotation rates of the planets themselves is unimportant since they are not connected outside. All the matters with the planets is the rotational motion the planets couple to their 'planet carrier' (blue middle ring).
Power splitter references
I first figured out how the power splitter works from the excellent Prius page of engineer Graham Davies. He created the excellent .gif graphics above.
http://prius.ecrostech.com/original/Understanding/WhatsGoingOnAsIDrive.htm
Flash animation (below) of the Prius power splitter. I got the min/max rpm limits of gen II Prius from this animation. The numbers the animation generates match the speed equation above and my table of speeds. It appears that the speed equation of the 'power splitter' did not change between Toyota gen I and gen II. I have assmed it has remained the same for gen III, but that is not confirmed.
ring/motor (wheel) 0 to 6,500 forward (0 to -1,500 reverse)
(2,000 rpm ring = 34 mph, 6,000 rpm = 102 mph)
engine 1,000 rpm (start) to 4,500 rpm (rated torque)
generator -6,800 rpm to + 6,800 rpm
http://homepage.mac.com/inachan/prius/planet_e.html
Regerative brakes
The motor is the key element in the regenerative brakes. When 'run backwards', it applies a reverse torque on the wheels that brakes the vehicle and regenerates power back to the battery. There is nothing exotic about running a motor 'backwards' to convert it into a generator (or vice versa). Inverter controlled motors act naturally as both motors and generators, no change of control or wiring is needed.
The higher power Toyota SUV (not the Prius car) contains a 2:1 step down coupled to the motor. P = torque x speed and a motor is sized basically by its torque, so the power out of a given size motor can be (ideally) doubled if it runs twice as fast. The SUV hybrid needs over twice the electrical power of the Prius car. This explains why Toyota has pushed the top speed of the SUV motor 12,000 rpm and then coupled it to fixed 2:1 step down (Reduction device) to match the 6,000 rpm rating of the power splitter.
Accleration from stop
-- The electric motor always pulls the car away from a stand still and the petrol engine starts to come in around the same time a non-hybrid car would change to 2nd gear. (In other words the motor (with power only? from the battery pack) provides all the torque at low vehicles speeds (eq to 1st gear)).
Possible replacement planetary gear power splitter (8/22/11)
Stumbled across a 2010 paper by two Hong Kong researchers proposing a replacement for a key element in hybrids: the planetary gear power splitter. In general terms a planetary gear set is three port device with fixed torque ratios that allows variable power flows by varying shaft speeds since [P = Torque x Speed]. The new approach is to combine the motor and generator into a single complex machine with two concentric rotors and a rotating 'magnetic gear' in between. In total three concentric shafts!
M/G1 is generator and M/G2 is x2 higher power motor
Two PM motor/generators combined with a magnetic gear
"Magnetically geared electronic continuously variable transmission (for hybrid vehicles)"
Authors: L. Jian and K. T. Chau, EE Dept, Univ of Hong Kong, 2010
(source -- http://www.jpier.org/PIER/pier107/04.10062806.pdf)
Here is their Prius type sketch (same as mine, but less detail) and to the right a hybrid architecture with their new machine.
. 
(left: Prius (same as my sketch) and (right) the Jian and Chau three shaft machine
New Hyundai hybrid architecture (1/31/11)
A decade or so after Toyota introduced its power-splitter hybrid architecture, Hyundai (Korean) will in 2011 begin selling (in USA) its first hybrid, a mid-sized car with a new hybrid architecture, which the company describes as an "original proprietary hybrid architecture to reduce weight and to improve highway fuel economy". The car is designed to compete with the Toyota Camry hybrid and Ford Fusion hybrid. It sells for 26k only 3k more than the non-hybrid version and gets 40 mph highway and 36 mpg city. It is unusual for a (full) hybrid in that its highway mileage is better than its city mileage.
The architecture looks simple and clean (see my sketch), so why hasn't it been used before? The explanation (from Hyundai engineers) is that the clutches until now were not good enough. The clutch here needs to repeatedly engage quickly and smoothly with the engine and motor running at high speed. Toyota in the early hybrid days tried this architecture and abandoned it because of the clutch. The key to making it work is that Hyundai has designed a new advanced electronically controlled clutch.
First look
I made the sketch (below) from the few facts contained in a (1/30/11) NYT review article.
engine + motor hp 209 hp (vs 187 hp Camry hybrid, 191 hp Fusion hybrid)
motor hp 40 hp
engine hp 169 hp
weight 3,457 lb (263 lb lighter than Fusion hybrid)
0 to 60 mph 9.2 sec
motor hp 41 hp (30 kw, 34?)
torque 151 ft-lb (205 Nm) (rect power peak at 1,400 rpm)
type PM synchronous
engine + motor torque 195 ft-lb (low speed)
battery kwh 1.4 kwh (72 cells x 3.75V/cell @ 5.3 Ahr in series)
weight 96 lb
voltage 270 VDC
drag coefficient 0.25
Combining torques
Note in this architecture the 'combining' of the engine and motor torque (power) is trivial. No planetary gear set here. All that's needed is to wrap the electric motor around the engine/clutch drive shaft. Or from another point of view it's an electric motor with a through shaft, the engine/clutch connected to one side and the transmission to the other side. With no current in the motor windings the motor torque is zero, the motor is effectively 'not there', there is a direct connection of the clutch to the transmission. Put current in the motor winding and the motor just adds its torque to the engine torque. Simple.
The car normally starts only with the electric motor (clutch open) and the car can run to 62 mph (vs 50 mph on other hybrids) using the electric motor alone. This of course is easily achieved because the motor speed can remain low since it is on the input side of the transmission and (probably) runs at the same speed as the engine. Though the small 1.3 kwh battery only has enough power for a few minutes of all electric power at high speeds, it probably allows Hyundai to so some tricks as this hybrid boast improved highway mileage.
The 40 hp motor must be used to replace the 'missing' power and low speed torque lost from converting the engine from otto to atkinson cycle. It's clear from the sketch that anytime the engine is running and has some excess capacity that the inverter can be started up to bleed off some of the engine power to recharge the battery.
Atkinson cycle
-- An atkinson cycle engine adjust valve timing so that the compression ratio is smaller than the expansion ratio, or a shorter compression duration to allow the expansion (power stroke driving the crankshaft) more time to fully run. The goal of the modern Atkinson cycle is to allow the pressure in the combustion chamber at the end of the power stroke to be equal to atmospheric pressure. This converts more of the fuel energy value into mechanical work at the expense of lower the maximum power. Due to the shorter compression stroke the engine does not take in as much air, so for a given weight and size it puts out less hp than an equivalent size otto cycle engine.
According to Wikpedia ('atkinson cycle') a lot of (maybe most) hybrids have their engines tuned to the atkinson cycle (besides the Hyundai Sonata) including Toyota Prius and Camry, Ford Fusion, Lexus, Mercedes and Infiniti hybrids. Use of the atkinson cycle apparently been kept secret with a 2008 article saying Toyota has just announced that the Prius uses this cycle. Supposedly the atkinson cycle (sometime called modifed otto cycle) can improv efficiency by 12-14%.
Hyundai advanced clutch
-- Hyundai engineers suggest that the bigger advancement is its use of a clutch to couple and decouple the engine from the transmission. According to Yang, it's an idea that was discarded in the early years of hybrid development, because of difficulties of engaging and disengaging the clutch fast enough to smoothly blend engine and motor outputs.
-- "The most difficult thing to overcome was the clutch engagement system, to engage without any delay or any shocks. Ten or 15 years ago, the technology was not good enough to accurately control the engine and motor with a clutch. At that time it was not possible. That's why Toyota got away from that idea. By using new advanced electronic systems, we made it possible to engage and disengage very quickly at very high engine speeds. We revisited the old idea with new technology to make it possible." The electronics helping to reduce clutch engagement shock take into account fuel flow, spark advance and throttle position, among other things.
Chinese BYD F3DM hybrid/electric (2/11)
A Chinese hybrid/electric car (BYD F3DM) is scheduled to be sold in USA early 2012 and a few details are beginning to be available. Architecturally it appears to be (basically) a Chinese Volt, but it's smaller than the Volt with only 75 kw (total) of motor/gen compared to 165 kw (total) in the Volt, and its engine is tiny (1 L, 3 cylinder). It began production (sort of) in Dec 2008 almost two years before the Volt, but Wikipedia says as of Nov 2010 only 300 cars have been sold, so it's just barely 'in production'. It plugs in and runs all electric for 30 miles, then its (little) engine starts and it runs as a hybrid. The engine is mechanically coupled to the wheels (through a planetary gear set?) like in the Volt. During a hard acceleration both motors and the engine can be used providing 168 hp (125 kw).
battery kwh 16 kwh (same as GM Volt)
battery type 100, 3.3.V lithium iron phosphate cells (330VDC bus)
active bat thermal management no
motor 50 kw (PM)
gen/motor 25 kw (PM)
engine 1 L, 3 cylinder, 68 hp (50 kw)
wheelbase 102.4 in
weight 3,439 lbs
A follow up NYT story indicates why this car did not sell in China (few sold were mostly bought by government agencies). The 2008 car is not finished, it's really a rough prototype, and needs a lot of work. One big problem is the transition from electric to hybrid. It runs differently in the two modes with a crude transition, and the hybrid mode is a horror. The NYT says when the 3 cycle engine starts "it screeches like a banshee, the steering wheel vibrates, the dashboard hums, you feel the vibration in your molars." As of now, BYD has zero dealerships in USA.
Electric car charge standard --- SAE J1772
7 pin J1772 connector?, 400 VAC, 63A
(Nope, don't think so, cars have no hole for center pin)
Nissan Leaf with 24 khw battery has two connectors side by side:
a) J1772 connector for Level 1, 120 VAC and Level 2, 240 VAC (1 ph?) charging using
built in 3.3 kw charger (about 12.5A)
b) JARI DC connector designed by TEPCO for Level 3 480 VAC 50 kw (nom)
for 30 min 80% charge (about 100A). Charger is external and costs 16k!
(picture shows two large pins, so input here is DC, probably direct to battery
or through diode to prevent external discharge of battery)
Japanese call the TEPCO, CHAdeMO (a pun), and it has been adopted by all the Japanese car
makers (Toyota, Nissan, Mitsubishi and Subaru) for 480 VAC (Level 3) fast charging
Nissan Leaf (24 kwh production car) dual charge ports
right: J1772 for 120/240 VAC, Level 1/2 (about 3.2 kw, 12.5A)
left: JARI DC connector designed by TEPCO for 480 VAC, Level 3 (about 50 kw, 100A)
(6/11 update) Good bet that these will be the standard electric car plugs
.
GM volt (16 kwh production car) J1772 charge port Ford Focus electric illuminated charge port (5/11)
(update 6/11/11)
Defacto standard connectors
From the photos it's pretty clear Leaf, Volt, and Focus are all using the same 120/240 VAC charge port: five pins (three large, two small) with an insertion notch opposite the stand alone large pin. This is the US standard 120/240 VAC plug.
According to a Dec 2010 Leaf user site all the Japanese auto makers have all adopted the two pin (TEPCO) connector used in the Leaf for Level 3, 480 VAC, high power charging, and they think the US Energy dept is about to install the same connector in some charging fast charging stations in US. If US adopts it too, this would make it the defacto world standard for 480 VAC charging.
The 480 VAC TEPCO (CHAdeMO) connector, and associated high current wiring to the battery, apparently also exist now as an option in a tiny new all electric city car from Mitsubishi (i-MiEV). A NYT reviwer test drove the car for a week and took it to be fast charged at the only high power DC charger run in Northern CA (run by Pacific Gas and Electric). He reports delivery of 10 Kwh of charge (to its 16 kwh battery) in 15 min, which is 40 kw transfer rate! No info on losses and no info on battery voltage, but it might be 600V since 40 kw @ 300V is an awfully high 133A [40,000/300 = 133A], which reduces to 67A @ 600V.
Below is a spec for a high power 480 VAC charging 'pump' from a European company (Anker Wade, Dutch). It also uses the Tepco DC connector of the Leaf. The pump specs are:
Input 480 VAC, 3 phase, 70A
Output power 50 kw (max)
Output voltage 240 to 500 VDC
Output current 120A @ 400 VDC (48 kw)
http://akerwade.indigofiles.com/AkerWade_DC_Fast_Charge_stations_nov_8_10r.pdf
----------------------
Ford is featuring that an optional 240 VAC charger will charge their 23 kwh (how discharged?) Focus battery is 3-4 hours. This is possible with 240 VAC, 30A, 1 ph, which compared to a standard 1.5 kw 120 VAC, 15A circuit would have x2 voltage x 2x current for 6 kw capability.
Competing (older) standard?
IEC 309, IEC 92196-1 22 kw, 400Vrms LL, three phase (230Vrms LN), 32A, five pin
3 x 32A rms x 230V rms = 22 kw (connector manuf spec)
Apparently under this standard a 250A version also exists.
Simple built-in charger
To charge the battery in a plug-in electric or quasi-electric from the line a charger is needed. While theoretically the charger could be external to the car, realistically it needs to be built-in so the vehicle can be plugged into an available 120 or 240 VAC line. There's now a recognized standard for plugging in a vehicle, specifying the cable and plug.
But what if the electronics and motor/generator already in the vehicle could be reconfigured to work as a charger, after all they're not doing anything when the car is being charged. If this could be done, there would be three significant gains: cheaper, lighter, more reliable. Alan Cocconi figured out and patented a couple of ways of doing this in the early 1990's. The first patent (1992) assigned to GM and the second (1994) assigned to Cocconi's own company AC Propulsion.
Alan Cocconi
Turns out Cocconi is something of a legend in electric vehicles. A Caltech graduate, he's been designing electric vehicle inverters and motors for 20 years. He did the prototype for the GM famous EV-1, the first mass produced electric vehicle. Two of his inverters and motors powered 'While Lightning' which in 1999 set the speed record for the fastest electric vehicle (245 mph). His company with more than 100 employees has been selling a 200 hp inverter and mating high speed (13,000 rpm) induction motor for 15 years, and it owns a plant in China that can manufacturer 2,000 per year. His patented charging scheme and other technology were licensed by Tesla Motors.
scan from patent # 5,341,075, Combined Motor Drive and Battery Recharge System, Issued 8/23/94
Inventor: Alan Cocconi, Assigned: AC Propulsion
I was interested in how he did it, so I did a patent search. His simplest charger is shown in the figure above from the 1994 patent. Everything to the left is already needed to run the vehicle, only switches K1, K2 and the filter on the right need be added to make the charger. The basic idea is simple. Motor control engineers have long understood how to control the currents in an inverter (inverter is the six IGBT's drawn in simplified from as switches S1 to S6) to regenerate power back from a motor to a battery. The trick here is to open the connection to one motor winding (with switch K1) and insert in series the (single phase) line voltage (with switches K2). When seen from the inverter, the line voltage now looks (pretty much) like the back EMF of a motor, so the normal regen mode of the inverter will now pump power from AC line voltage (120 VAC or 240 VAC, it doesn't much matter) and squirt it out as DC power into the battery at the battery voltage. Cost for the charger is pretty much just the cost of the added two high current contactors (K1, K2). No additional power electronics required!
AC Propulsion AC-150 200 hp motor configured as battery charger
The patent shows the recharging with an induction motor and notes it would work with a PM motor too. Well maybe, but it's not obvious this can be made to work properly. The issue is torque. Motors are designed to make torque when current flows. Here the opposite is required, no torque. Clearly the charging current can't be allowed to move the car, and it's also undesirable to have a strong 60 hz (or 120 hz) torque that would cause the vehicle to hum or sing while being recharged, though a creative marketing man might be able to make this little flaw into a positive! (Curiously the issue of torque is never mentioned in the above patent.)
Torque (in vector terms) = [current cross flux]. Induction motors will allow current flow without torque if they are controlled to keep the flux near zero. However, I'm not sure what the efficiency penalty is, because no flux is achieved by allowing cancelling currents to flow in the rotor. But in a PM motor the flux is fixed by the magnet, so figuring out how to inject currents in a PM motor (at any rotor angle) without getting a vibrating torque is not trivial and maybe not possible. If so, this might be a reason induction motors are favored over PM motors for electric cars.
In a quasi-electric there's more options, because there's also a generator. Application of this patent to the generator and its inverter to make a charger would obviate the concern about vehicle motion, and the generator would be free to rotate a little to an angle where current makes no torque.
GM Volt charger
From little news article below it appears that the GM charger is probably not intgrated with the inverter:
Wireless charger (update 6/2011)
This section is more or less an EE aside discussing wireless power transfer. However, this technology is being proposed by Delphi to wirelessly charge the battery of electric cars, able to deliver 3.3 kw through an 8" gap to a resonant coil mounted under the car. My interest in this was piqued when a friend send me a link to a wireless battery charger for an electric car to be sold by Delphi. You mount this 18" or so rectangular thing on the floor of your garage and attaching a thinner rectangular thing about the same size to the underside of the car, which (presumably) is hard wired into the battery charge port, so it sits pretty much above it when parked. The interesting thing to me as an EE is this product can deliver high power (3.3 kw) though a large gap. The gap is speced at 20 cm (about 8") "or more" and apparently does not need critical axial alignment to work, making parking practical.
Prof Soljacic, WiTricity
Turns out this product is based on work done by a small team at MIT starting in 2005, led by Prof Soljacic, who demonstrated the feasibility of resonant wireless power transfer (with reasonable efficiency) over distances of six feet or more. I vaguely remembered reading about this when it hit the front page of the newspaper five years ago, but I had heard nothing about it since. Soljacic formed a company, WiTricity, to commercialize the technology, and as far as I can tell, this is the first actual product to come out of this work. I was always interested in how they had done this, so when I found they had patents, I dug them out and read them (75 pages worth of patents!) #7,741,734 and #7,825,543. Soljacic is a physicist (two BS's from MIT in physics and EE) working at the Research Laboratory of Electronics and seems to work mostly on non-linear optics.
Included in the links below are a couple of papers with much of the same info as in the patents. One paper has two worked out example where 10 watts is delivered to a lap top with a D/r =5. The efficiency for what they call a dielectric disk (not sure what this is or looks like) is 51% and for a capacitively tuned coil its 61%. I suspect the dielectric disk runs at a higher frequency because most of the ten watts of so of loss is 8.3 watts of radiated power. The losses in the higher efficiency (61%) capacitively loaded coil on the other hand are mostly in the coils, with radiated power very low at (1/2 w). So coils have less radiated power and higher efficiency, but things run hotter.
http://www.witricity.com/pages/invention.html
http://www.mit.edu/~soljacic/wireless-power_AoP.pdf
Tuned resonant coils
The key to getting power to transfer over large distances, and without critical axial alignment, is to use two resonant (high Q) coils tuned to the same frequency. When two coils are far apart, the overlap of their magnetic fields is quite small, even with coils of the same diameter on axis, no more than a few percent or maybe less than 1%. When transformers are built, the materials and geometry are chosen to overlap as much of the fields of the coils as possible, and usually 90 to 95% coupling is achieved.
So the obvious question is how can power transfer work across such large gaps? The trick it turns out is to use resonant coils tuned to the same frequency. Even though there may be only, say, 1% flux coupling between the coils the analysis of the MIT team showed that when power is extracted from the receiving coil it is replaced (next cycle?) by power from the source soil.
High vs low impedance coils
A parallel LC coil will yield a high impedance, hence high voltage (low current) at resonance, so its near field is dominated by the electric field. Turns out this is not so good for humans nearby to be exposed to as our watery bodies absorb a lot of this energy (as heat). If the coil is built as a series LC, then it will have a low impedance at resonance, hence high current (low voltage) at resonance, so its near field is dominated by the magnetic field. The human body is nearly immune to the magnetic field, so this is the preferred way of doing the coupling. Unfortunately, it turns out that a switch from electric to magnetic coupling with coils of the same diameter cuts the efficiency about in half, from something like 40% to 20% according to their initial studies and demo in 2006.
But what frequencies are they using. Microwave? No it turns out the frequencies are quite low, 17 Mhz to 380 Mhz for coils 1 to 30 cm in radius, the lower frequencies used with the bigger coils, it pretty much scales linearly. Their demo transmitted 60 watts over 2 meters with 30 cm (radius) coils powered at 17 Mhz (sinewave). The wavelength of 17 Mhz is 17.6 meters = [3 x 10^8/1.7 x 10^7]. In other words both the coil radius and transmit distance are much smaller than one wavelength. In the demo the ratio of wavelength to coil radius is about 60:1. The patent analysis finds high Q's can be obtained with transmit distance (D) to coil radius (r) ratios of 3 to 10. D/r ratio in the demo is about to 6.7.
Demo
To get 60 watts into a 60 watt light bulb the input power to their (colpits) oscillator was about 400 watts. They pretended not to know the efficiency of the oscillator! The overall efficiency was about 15% to transmit power 2m with a theoretical efficiency for the wireless transfer of 40 - 50%. Unfortunately the 40% efficiency was for a thick open coil (high Z) whereas a more practical thinner wound coil (low Z) gave about half the efficiency (20%). They built and tested both types of coils (same 30 cm radius), but the news stories of course featured the less practical deep coil. If you look closely at photos, you can see a smaller light bulb (30 watt?) was used with the thin coil.
MIT's 2007 demo powering 60 watt light bulb (Prof Soljacic is in 2nd row, striped shirt)
(2 meter gap, 30 cm radius 'thick' transmit and receive coils, freq = ??)
These are self resonance air coils, meaning the capacitance is between turns
(source -- http://www.mit.edu/~soljacic/MIT_WiTricity_Press_Release.pdf)
MIT wound 'thin' coil demo
(2 meter gap, slightly smaller radius coil, 25 cm)
It is likely this is tuned with a capacitor, but it is not visible.
No info on power delivered, but this looks like a smaller light bulb
(source -- http://www.sciencemag.org/content/suppl/2007/06/08/1143254.DC1/Kurs.SOM.pdf)
The $64 question on the wireless 3.3 kw electric car charger is its efficiency. Curiously, suspiciously, it is not given. But looking at it, it does not look capable of dissipating a 1 kw, so I suspect the efficiency must be pretty high (> 70-80%) to make it practical. It is operating with a gap (8") that is equal to or less than the radius, and this case was not discussed in the early papers and patents where lower end D/r was usually 3.
(source -- http://wheels.blogs.nytimes.com/2010/11/02/delphi-and-witricity-developing-wireless-electric-car-charger/)
NYT quotes lots of people saying "who needs it?", but the first time someone drives off with his car still plugged in (lock out?) his opinion will change. Long term something like this may make sense, but it has got to be efficient and not too expensive.
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MPGe EPA standard
EPA has adopted for electric range a milage standard called MPGe, miles per gallon equivalent. Here is the EPA definition:
The E.P.A. offers this definition, which seems clear and reasonable to me.
vs 93 MPGe EPA rating (something is off)
To explain the difference it is necessary to reduce the Volt's electric range to 29 miles for 10.6 kwh usage.
Wind resistance hp
"As the Prius reaches motorway speeds, the Prius will normally be driving solely on the petrol engine." (The engine must initially put out addtional power to recharge the battery, replacing the charge used to accelerate.) The engine is rated 98 hp (gen III) and 76 hp (gen II), so obviously less hp than this is required to overcome air resistance at highway speeds. Wind resistance goes as speed squared and power goes as speed x load, so the power to overcome wind resistance goes as the cube of speed, or 338% more power at 90 mph than 60 mph. The Prius max speed is reportedly about 100 mph, so working backward I get that it must only take about 16 hp to overcome wind resistance at 60 mph. (76 hp gen II/(100 mph/60 mhp)^3 = 76 hp/4.6 = 16.4 hp).
Check --- Wikipedia says car wind resistance runs 10 hp @ 50 mph, 80 hp @ 100 mph, which calculates to 17 hp at 60 mph, almost exactly the 16 hp I estimated from Toyota numbers.
Misleading block diagrams?
Many of the simple block diagrams I have found of hybrids are too simplified to be useful, and some seem just wrong. For example, I often see the generator drawn to the side of the power splitter (below), whereas the cross-section diagram from Toyota shows it clearly in line between the engine and the power splitter.
Misleading? Notice this show the generator to the side of power splitter,
but cross-section diagrams show generator inline betwen the engine and power splitter
source -- http://www.global-greenhouse-warming.com/hybrid-electric-vehicle.html
Well, having tried to sketch it myself, I see the difficulty. Physically the generator is inline between the engine and the power splitter as shown in the figure (way above) showing concentric shafts. But if you start with the physical configuration its difficult to show functionality. Thus showing the power splitter with a side (non-axial) output, while physically misleading, I now think is not really wrong because it does correctly show functionality and is much easier to draw and read.
Power doubly converted?
Some published diagrams show the engine sometimes powers the wheels partially via the generator/motor path. I initially suspected this was wrong. I mean, why convert engine power to electrical power and then back again to mechanical power, but the Prius specs do seem to show this. My own circuit diagram of gen III Prius shows 33 kw of 60 kw of the motor power comes into the motor inverter bus from the generator.
I don't really understand why 33 kw of power would be doubly converted. Why not pass it directly from the engine to the wheels through the power splitter? Maybe it's for flexibility of control or maybe reductions in cost/size of the power splitter. Power flow though the generator/motor path is very easily and quickly controlled electrically, so maybe this is the reason. The efficiency penalty for doubly converting 33 kw is not too large since inverter/motor efficiencies generally run in the 90% to 95% range.
(update)
Obviously when I wrote above paragraphs I had no clue as to how the architecture of the Prius hybrid really worked. I've left it in just as a guidepost as to how I come to understand things. I now understand that:
I worked as a power/motor control engineer for 30 years. Mostly I was a working engineer designing commercial motor drives, which entails doing circuit design, laying out breadboards, BOM, etc. But in the 70's and early 80's I worked in an R?D envioronment and worked on new techniques to control motors. Some of the motor control techniques I personally helped develop are now (it appears) used in hybrid car and electric cars. There are two in particular: field weaking for PM motors and converting induction motors into high performance servo motors.Field weakened PM (permanent magnet) motor technology
In the early 1980's PM motors were the standard motor for servos, as they are today, because they are easy to control, very efficient, and have a convenient rectangular T vs Speed curve. A servo PM motor can be run at any speed up to 'base speed', defined as the speed where the back EMF (from the spinning magnet) equaled the inverter bus voltage, but above that speed the motor torque rapidly dropped away to zero.
For servo applications the 'base speed' speed limit of a PM motor was not a problem, but there are other applications, like vehicles, where a wider speed range with reduced torque at higher speed is needed. The goal in high speed torque is a 'constant hp' curve where torque falls off slowly (as 1/speed). You can do this in induction motors by lowering the component of current that sets the magnet flux level in the motor. but in PM motors the flux level is fixed by the magnet.
Operating the PM motor above base speed was thought at the time to be impossible. PM motors just refused to accept much current above base speed, because (net) voltage needed to ramp up current in the motors' inductance gets squeezed to zero between the back EMF and bus voltage, so the torque dies. There appeared to be little that could be done about this because voltage from a changing magnetic field (like a spinning magnet) is a law of nature, it's built into Maxwell's equations, and a motor's inductance is basic to it creating torque.
But there is a way to run a PM motor x2 (maybe x3) times higher than it's base speed and my boss (Bill Curtiss) and I figured it out.
Our idea was to control PM motor currents in such a way that the back EMF is partially cancelled using the motor inductance. We proposed adding a (non torque producing) quadrature current term in PM motors. Since voltage leads current by 90 degrees in an inductor, this creates a voltage across the motor inductance that is 180 degrees out of phase with the back EMF. It's a cancelling term that makes the motor's back EMF, as viewed from the outside, look smaller than it really is. Above base speed the internal motor back EMF voltage from the spinning magnet can be much higher than the bus voltage, yet with proper control law for motor currents the voltage at the motor terminals, which is the sum of back EMF and inductive voltages (plus IR voltage), can be held to less than the bus voltage allowing the motor to be well controlled.
Fly in ointment?
There is a fly in the ointment though. A nagging concern has always been what happens at high speed if the inverter has a fault, i.e. it suddenly shuts down for some reason. There is still high voltage inside the rapidly spinning motor, but the cancelling voltage now disappears, so there is risk of inverter damage due to over voltage. While this is speculation, this robustness concern may explain why the Toyota's hybrid motors, which are marketed as PM synchronous motors, are revealed in the technical literature to be combo PM/reluctance motors, the reluctance torque allowing the PM flux levels to be substantially reduced.
High performance induction motor technology
In my essay on Tesla's induction motor (below) I describe in some detail my contributions to induction motor control theory.
http://twinkle_toes_engineering.home.comcast.net/induction_motor.htm
Brief summary of my contribution to high performance induction motor control
For several years in the late 1970's all attempts by researchers at getting servo-like performance from the induction motor failed. A step of torque (really a fast ramp) would be requested, but the motor, if it made the torque step at all, would then 'ring' the torque for a half second or so. No one could figure out how to stop the ringing.
Battery cost
Big buck have been poured into battery development for vehicles in USA for 20 years. Progess shall we say has been slow...
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Misc
In a Prius forum someone asked how the engine can be started there's not enough charge in the main NiMH battery. The answer was that the 12 V lead acid battery through MG1 (generator) starts the engine. Is this right? Is it a backup mode? Seems to be to make this work would require an extra step up converter because the generator has high voltage winding.
Here is a good detailed tutorial that explains the main NiMH battery is used to start the motor.
http://prius.ecrostech.com/original/Understanding/WhatsGoingOnAsIDrive.htm
Tesla misc
Owner takes his Tesla Roadster to a 2.5 mile road track. In only 3 to 4 miles(!) of hard driving the power of his inverter is dialed back as its hot warning light comes on, then his motor hot warning light comes on too.
from 240 VAC 17kw is about 68A. Guy at test track recharged at @ 40A from 240 VAC RV outlet.
Tesla has run their induction motor on the dyno to 18,000 - 20,000 rpm. Car limits is 13,000 to 14,000 rpm.
Tesla patent applications
20070284953 motor rotor
20070009787 cooling batteries
20070218353 battery fire suppresion
20070188147 fuse links for batteries
Tesla Motors' Stealth Bloodbath - After raising over $100M, Tesla Motors Inc. has fired over thirty people -- nearly the entire team. Nagging transmission problems were partly to blame for the meltdown. (TeslaFounders; Jan. 10, 2008)
Powertrain 1.5 redesign (to eliminate 2 speed transmission)
"The revised PEM has new transistors (higher I rating) that help improve the overall efficiency, allowing more power with less heat generation."
"The more substantial changes are in the motor. The terminal connectors have been redesigned and the high voltage cables that connect the motor to the PEM have been switched from copper clad aluminum to pure copper. The locations with the highest resistance were attacked directly allowing more current flow without increasing temperatures. The result is 33 percent more torque at the thermal limit than the existing motors."
"I can't keep the Tesla drive train versions straight. Anyone have a program? As best as I can remember it, first it was a two-speed transmission from vendor 1. Failed. Then a two-speed from vendor 2. Failed. Two-speed from vendors 3 and 4. Failed. Single-speed design, let's call that 5. Failed. Now we have yet another design."
Just to make the record clear it goes like this:
1. Two speed from vendor 1: failed
2. Two speed from vendor 2: failed
3. Planned to try a two speed from vendor 3 ? 4.
4. Found better plan during development in Whitestar: go with a single speed while increasing power, so plans for two-speed dropped.
5. Production cars 2-40 gets/will get 1 speed (AKA the "temporary transmission") which works in every way on target (durability and etc), except 0-60 is 5.7 seconds instead of 4 seconds. (Production car #1, AKA Elon Musk's car, has a 2 speed)
6. Production cars starting with #41 is getting the single speed "drive train 1.5" described in this article, which does get 0-60 in 4 seconds.
** "PEM which can now do a 640A current limit on its input as well as a new and higher 850 amp limit on its output are all good incremental upgrades. We now multiply that by the latest motor current increase, 850A/640A (May 2008)
** "Ver. 1.5 adds a current boosting voltage reducing circuit for low speed operations to improve low speed torque." Forum post (not sure what this means.)
One possibility is that at low speed they are using contactors to reconfigure the battery. Putting two five sections in parallel would half the voltage and double the available battery current. It would also make the IGBT's happier by cutting switching loss in half when they conduct maxium current. The negative to this is there's a 'switch point' where torque is lost as the contacts throw and battery reconfigures. Is there an effective gear switch in their one speed transmission? Or, maybe he's just talking about voltage sag
"Eberhard (former CEO of Tesla) wrote a well publicized and needless (a fix was already in progress) negative blog piece regarding the battery coolant pump soon after receiving the car (production car #3).
'via Blog this'
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created 6/09
updated 2/9/12
Toyota/Ford/Lexus hybrid block diagram
Batteries
Nickel Metal Hydride (NiMH) batteries
Prius NiMH battery
Battery power out vs load resistance
Lithium ion
Lithium ion comparison to NiMH
Comparison of Volt and Leaf lithium battery thermal management
Battery coolant problems
quasi-electric GM Volt
GM Volt lithium ion battery
GM Volt revealed to be a hybrid!
Toyota Prius plug-in hybrid 2012
quasi-electric Fisker Karma sedan
Tesla Motors lithium ion pure electric cars
Tesla 2 seat roadster
Tesla Model S sedan -- 300 mile range
Comparing electric car motors --- Fisker Karma vs Tesla Model S
Nissan Leaf electric
Leaf 99 mpg EPA sticker
Ford Focus electric
Coda electric
UQM motor + controller supplier
EV-1 specs
Mini-Cooper electric
Mitsubishi MIEV electric car
Electric car batteries and range
Desiging an electric car
Motor
Electrics PM => induction
Motor speed comparisons
Inverter/boost
Why a booster converter?
IGBT transistor modules
Comparing hybrid ? non-hybrid siblings
Toyota hybrid technology
Toyota gen III 2010 Prius specs
Toyota gen III circuit sketch
Ford 2010 Fusion hybrid
Hybrid architecture
Mysteries of the 'power splitter'
Works like a three winding transformer
Toyota's power splitter formula
Local power loop!
Overdrive
New Hyundai hybrid architecture
Chinese BYD F3DM electric
Electric car charge standard --- SAE J1772
Simple built-in charger
Wireless charger
MPGe EPA standard
Wind resistance hp
My personal contributions to hybrid/electric technology
Reference
Wind resistance
13 hp / 9.7 kw @ 55 mph
17 hp / 12.6 kw @ 60 mph
40 hp / 29.6 kw @ 80 mph
57 hp /42.2 kw @ 90 mph (Nissan Leaf top speed)
79 hp /58.5 kw @ 100 mph (GM Volt top speed)
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Leaf vs Volt drive train mini-tutorial (1/5/11)
Here is a technical comparison of the electric drive trains of the GM Volt and Nissan Leaf, both of which have just begun deliveries. They are competing cars with almost the same wheelbase. (Facts about the drive trains are hard to come by and often come from obscure sources, so numbers are subject to change.)
The Leaf is to the conventional car what the jet is to piston planes. Conventional cars (including the Volt) have a lot of complex up/down stuff (pistons, valves), whereas the Leaf drive train is pure rotary. Leaf is about as simple as an electric car can be: series arrangement of battery, (one) inverter, (one) motor hard coupled to front wheels. Inverter/motors are inherently bidirectional in power flow so the same structure acts as a regenerative brake.
Leaf
battery (90 kw, 345V) ?=> inverter (260A) ?=> PM motor (80 kw) ?=> wheels (3,366 lb)
The Volt in contrast has a much more complex drive train than the Leaf. Volt's primary drive train has the same components as the Leaf (somewhat beefed up), but it also has a 2nd half sized motor, a 2nd half sized inverter, a complex hybrid type planetary gear set, three (or four) clutches, and a conventional engine! Its battery is 2/3rd the kwh and weight of the Leaf's, but all this extra stuff makes the Volt 424 lbs heavier than the Leaf, and it makes the car more expensive (41k for Volt vs 32.8k for Leaf).
Volt
battery (165 kw, 345V) ?=> inverter (355A) ?=> Induction motor (110 kw) ?=> planetary gears
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?========> inverter (175A) ?==========> => wheels (3,790 lb)
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engine (1.4L, 66 kw) => Induction? mot/gen (54 kw) => planetary gears
The hybrid Prius, all electric Leaf, and electric/hybrid Volt are all build on a 106 in (approx) wheelbase. The Prius is by far the lightest at 3,042 lbs, 324 lb heavier is the Leaf (3,366 lb) and 424 lb heavier than the Leaf is the Volt (3,790 lbs).
Performance
The Leaf has a single 80 kw motor/inverter to power the car (under all conditions). Probably about 40 kw (of its 80 kw) is needed to overcome wind resistance at 90 mph (top speed) on a flat road.
The Volt has two (and potentially three) sources of torque to accelerate the car: 110 kw main motor, 54 kw mot/gen, and since the engine can be coupled by a clutch to the 54 kw motor, if the gears are rated for it, mechanical torque from the engine could potentially be added too. The result is that even though the Volt is heavier by 424 lbs it accelerates a little quicker (8.8 sec vs 10 sec 0-60 mph for Leaf). Top speed for the Volt is 100 mph vs 90 mph for Leaf.
Confirmation from Volt manual (update 6/12/11)Motor
The Volt manual (p330) says if the car runs out of gas, then the "vehicle will have less responsive acceleration". Looking at the sketch above, the battery should be able to power both motors (110 kw motor and 54 kw mot/gen) to acclerate the car, unless the battery is peak current limited, which is unlikely, so if acceleration suffers when the engine can't run, this is a pretty good indication that the engine is mechanically aiding accleration.
Leaf has an 80 kw (280 Nm) PM motor and Volt an 110 kw (368 Nm) induction motor. No info on the type of mot/gen in the Volt (my guess is it is also induction). A [Power = Torque x Speed] calculation shows that motor power peaks near 3,000 rpm in both Leaf and Volt.Transmission
The Leaf has no mechanical or electrical gear ratioing. The motor in the Leaf is hard coupled via a fixed step down (x7.94) gearing to the wheels. I figure the PM motor in the Leaf has a speed range of 0 to 9,000 rpm as the car goes from 0 to 90 mph. The motor Torque vs Speed curve has a power peak probably near 30 mph (3,000 rpm), and then rolls off in torque faster than constant hp from 3,000 rpm to 9,000 rpm. This very non-rectangular T vs Speed shape is needed to get acceptable 0 to 60 mph acceleration times.
Volt with its dual (or triple) torque sources has more options. GM apparently implements a variable speed transmission (like Prius) at higher speeds using gen and/or engine torque to supplement the main motor torque at high speed. This allows them to achieve higher vehicle speeds without excessive rpm on their main motor. They appear to also combine torque sources during hard low speed accelerations. When the battery is depleted, the car runs like a conventional hybrid with peak power from the battery supplementing the 66 kw available from the engine.
Inverters
In both Leaf and Volt the 345V battery is hard wired to the inverters providing (for free) a stiff voltage to protect the inverter transistors. Their motors run at 300 VAC (nom) with currents 250A to 350A, so very likely their main inverters use 600V IGBTs. In contrast the Prius power path includes step-up voltage regulator (to compensate for the voltage sag of its small battery), so it probably runs its motor at 600 VAC (nom) and uses 1,200V IGBTs in its inverter.
Battery pack
Leaf battery capacity is 24 kwh (660 lb) of which 19 kwh (80%) is used. Volt battery back is 2/3rd the size (16 kwh, 436 lb) of 10.5 kwh (65%) is used.
In a striking coincidence? both the Leaf and Volt stack 96 lithium ion prismatic modules in series. While the battery chemistry is a little different, the lithium ion cells in both are 3.6V, so both cars have a battery voltage of 345 VDC (open circuit). The Volt uses three cells in parallel in each module (288 cells total), and the leaf uses two (roughly double size) cells in parallel in each of its modules (192 cells total).
The Volt uses liquid heating/cooling to keep the battery temperature within a narrow range (70F +/- 2F). Volt also uses the engine to protect the battery if the battery gets too hot or too cold. In contrast the leaf just air cools the battery pack with a fan. Its battery temperature varies with ambient, there is even a battery temperature gauge on the dash. And of course they have no engine to protect the battery. It has to power the car even if its cold or hot.
Bottom line is that GM, with its higher battery derating and much better battery temp control, is stressing the battery less than the Leaf, so is likely that its loss of capacity over time will be less than in the Leaf. Leaf (or to be specific its customers!) are taking a chance that there may be a substantial loss of battery capacity in a few years, and surprise (!) Nissan specifically excludes battery capacity loss from the car guarantee. Loss of half the battery capacity in a Leaf would render the car near useless without a very expensive repair, whereas a Volt with a reduced capacity battery will still be drivable.
Hidden advantage of electrics' large battery
Electrics with their large battery packs have an advantage over hybrids you never see discussed. This is the issue of battery sag, meaning the sagging of battery voltage under high demand. In a hybrid the battery accelerates the car at lower speeds, where the power demands are modest, and is joined by the engine when power demands increase at higher speeds. My Prius block diagram (below) shows the Prius battery outputting a peak of 27 kw from a 1.3 kwh battery. My analysis shows that the Prius NiMH battery voltage probably sags about 25% under this load. This is a real problem, just when you need to deliver high voltage to the motor, the battery voltage sags. To solve this problem Prius from Gen I to Gen II introduced a (bidirectional) voltage regulator between the battery and inverter. However, this is expensive, because essentially they had to add a 2nd high power inverter in series with the main inverter that drives the motor! (It also raised their bus voltage to about 600V (I think) driving them to 1,200V IGBTs, all of which needs to be handled carefully from a safety viewpoint.)
A Leaf has an 80 kw motor, so Leaf's battery needs to supply 80 kw, x3 higher battery power than in the Prius. But the Leaf's battery is huge compared to the battery in the Prius (660 lbs, 24 kwh vs 80 lb and 1.3 kwh). While lithium ion batteries are not quite as good at supplying high current as NiMH used in hybrids, they are pretty good. Mercedes has gone to lithium ion in their new hybrid. The peak battery voltage sag in the electrics is probably 10% or less. This simplifies the electrical design: the battery can be hard wired to the inverter, no preregulators, no added bus capacitors, and the stiff battery voltage protects the transistors from overvoltage.
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Intro
A primer on the electrical power designs of hybrid cars and electrics by a (former) inverter designer including my Toyota/Ford/Lexus block diagram with gen III Toyota Prius values (below) and a circuit sketch of Prius voltages and currents. I'm not a guy car. I have little real understanding of mechanical components like gears and transmissions, but I do understand the basics of the (famous) Prius 'power splitter'. I don't own a hybrid, so I can contribute nothing from a user viewpoint, but I do know electronics.
So what is a hybrid car?
Hybrid cars can be viewed as gasoline based cars that obtain superior efficiency by employing a very sophisticated electrical/mechanical transmission. The transmission is coupled to a small rechargeable battery sized to to smooth out the peak power demands of driving, allowing the engine to be downsized. Additional efficiency improvements in city driving are obtained by employing regenerative braking that recovers the car's kinetic energy when stopping and wasting of fuel when the car is stopped in traffic is prevented by turning off the engine.Toyota/Ford/Lexus hybrid block diagram
Here's my Toyota/Ford/Lexus hybrid block diagram with the values of gen III, 2010 Prius, showing currents, voltages, rpm, and power splitter speed constraints (formula ? table). All the blocks in the diagram support bi-directional power flow making the Toyota hybrid architecture very flexible.
an original figure (all rights reserved)
Severinsky 'Hybrid Electric Vehicle' patent, US Patent 5,343,970 (filed 1992, issued 9/6/94)
Alex Severinsky, a US engineering professor, entrepreneur and immigrant from Russia, won a hybrid (architecture) patent infringement case against Toyota. After many years of legal battles both Ford and Toyota settled with him in 2010. He claims to be the inventor of the Toyota architecture. Is he? Here is his page one figure from the patent in question (US Patent 5,343,970).
Alex Severinsky hybrid vehicle architecture (circa 1992)
34 (top) are the vehicle wheels
Note has only one motor/inverter
(US Patent 5,343,970, figure 1)
It does feature a three shaft torque sharing device and no transmission. His torque sharing device is not described as a planetary gear structure, nor did I see the words 'infinitely variable transmission' used in the text. Note that the computer is shown directly controlling the torque sharer. While his architecture does have some resemblance to the Toyota architecture, it is certainly not the same. For one thing he has only a single inverter (connected to the motor) and no generator. When highway driving, the computer controls the torque sharer (via locks and pins) such that some of the torque is split off to run the motor/inverter backwards to charge the battery. Severinsky has the motor and engine on two shafts of the torque sharer with the wheels connected to the 3rd shaft. This is not how Toyota does it.Summary overview -- (or what I learned researching ? writing this essay)
Toyota architecture is less obvious and much more subtle. The wheels and motor share one shaft of the power splitter and the generator/inverter are on a 2nd shaft. By coupling the DC links of the two inverters together a dual power path from engine to wheels is created that functions as an infinitely variable transmission (including overdrive). The Toyota power splitter (I am pretty sure) is a purely mechanical device. All the torque and power sharing control is provided by drive currents of the two inverters. There is nothing like this in Severinsky patent.
So does Severinsky deserve credit as the 'inventor' of Toyota power splitter hybrid architecture? Not in my book. Maybe he gets credit for an early simplified version. As an outsider it's hard to know what was 'in the air' at the time. Once you have a motor and engine you need some way to combine their torque outputs. The simplest is just clutches which switch the wheels from one to the other. But it seems kind of obvious that someone is going to draw a box that combines (adds) the torques of the motor and engine using two shafts of a three shaft gearing device with the wheels on the 3rd shaft, which is just what Severinsky has done. I presume three shaft (torque adding) gear structures like this have long been known (at least in theory). The Toyota architecture is a big step ahead of what we see in this Severinsky patent.
There are key choices to be made in building a hybrid or (quasi) electric car, and one of my goals in researching this essay was to understand how ? why the particular choices have been made. Battery type and hybrid architecture have been discussed ad nauseaum, but there are many other key choices: Motor (? generator)
type PM (+ reluctance) or induction
top speed 6-7 thousand or 12-14 thousand rpm
cooling air or water
Inverter
transistor/IGBT 600V or 1,200V
bus voltage fixed or varied
paralleled no or yes (if yes, how paralleled)
cooling air or water
Battery voltage booster yes/no
Battery
type NiMH or lithium ion
voltage sag low (15 to 20%), moderate (25%), heavy (50%)
oversizing oversize vs lifetime trade
forced air cooling yes/no
ESR at low temp how to live with a x3 to x6 increase in ESR
Engine
speed range fixed or variable
peak hp operation yes/no
Motor
All the hybrids are using (neodymium) PM motors, but the two (quasi) electrics are using induction. My guess is PM motor/generators being smaller integrate best with sun/planet power splitter used in the Toyota/Ford/Lexus hybrids. Induction motors are favored with the motor needs to be larger, need to run hotter, and can stand alone.
Motors are sized by torque, but power is T x speed, so doubling a motor's speed (6,500 => 13,000 rpm) can roughly double its output power. Toyota uses this technique for its 133kw hybrid SUV (Highlander), combining it with a 2:1 step down to allow it to mate with the 6,500 rpm power splitter gears. Tesla Motors uses it too, operating the motor to 13,000 rpm. For PM motors to work well in vehicles a special control technique is needed to add a (nominal) constant hp region to T vs S. This either means field weakening, or a combo PM/reluctance motor as Toyota is using. There is little info available as to the technology used for generators, but it's a good guess that if the motor is PM, then the generator is too. Didn't find much about how motors are cooled.
Inverter, voltage sag, and battery (voltage) booster
There are two standard types of IGBT (transistors): 600V and 1,200V. Battery pack voltages in current vehicles range from 160V to 320V, so if battery pack couples directly to the inverter, then 600V IGBT's are used. This was the technique used in the gen I Priuses, and it is the technique used in the Tesla Motor's Roadster electric car. However, at this low voltage when a lot of inverter power is needed, like in an electric car, the currents get 'wicked' high. Tesla's inverter is rated for 850A and achieves this with massive paralleling (x11).
A problem with direct battery to inverter connection is that the battery voltage sags a lot when it must deliver peak power/current. If you try to pull the absolute max power (100%) the battery pack can deliver, its terminal voltage will sag 50%, meaning the terminal voltage drops in half. This is undesirable for a lot of reasons, so a compromise is called for. I calculate that Toyota limits battery voltage sag to 25%, at which point the battery delivers 75% of its maximum power. In this configuration battery voltage sag rounds off the corner of T vs Speed, reducing the peak output power.
Adding a voltage booster, which is a specialized inverter, between the battery and inverter does a couple of things. First, it improves peak output power because the inverter/motor no longer 'sees' the battery voltage sag. Second, by roughly doubling the voltage to the inverter it halfs the current (for the same power) in the inverter and motor. The higher voltage is handled by using 1,200V IGBT's. Toyota added a booster in 2003 in the transition from gen I to gen II. A draw back is that the booster needs to handle the full power of the battery pack and has to handle bidirectional power flow to allow regenerative braking. A still further complication is that all converters need a large inductance to 'switch against'. In the motor and generator the needed inductance is free, it's built into the motor and generator, but with the voltage booster it must be added in. I have seen zero info about the voltage booster implementation. A further, non trivial, complication with adding a booster is that a (bigger) bus capacitor bank is needed to handle the switching currents from the inverter, because the stabilizing influence of the battery low impedance is no longer visible through the booster.
Toyota appears to regulate (fix) the bus voltage of their inverters using the booster at 650VDC, but some optimization is possible by allowing the bus voltage to vary with vehicle speed and conditions. Ford appears to be modulating the bus voltage up and down a little in the 2010 Fusion. Tesla Motors (might) be doing this too with a cryptic reference that they were (somehow) dropping the bus voltage when the inverter needed to output its maximum 850A during low speed acceleration. The only company that appears to be paralleling is Tesla Motors, apparently they use 66 IGBT's whereas nominally for a motor inverter only 6 are needed, implying they are paralleling x11 to reach 850A. Paralleling is non-standard and can probably be reliable, but it takes careful engineering. Power semiconductor companies are beginning to make high current modules with paralleled die specifically for (electric) vehicles (see below).
800A (550A with 75C water cooling), 600V six-pack inverter
(probably 2 x 400A parallel IGBT die in all six locations)
size: 8.5" x 4", dissipation: 1,500W (max)
Important: mating (six terminal) snubbing capacitor available
Infineon FS800R07A2E3
I found little (hard) info available on inverter cooling, but a poster says Prius uses liquid cooling for the inverter and motor (separate cooling loop from the engine) and air cooling for battery pack. (Prius inverter enclose does appear to show liquid ports.) A new Tesla owner reports he took his 100k new Roadster to an off road amateur (twisty) track and found his inverter over-temperature warning light came on in five minutes followed by the over-temperature warning light on the motor. (It's high performance car, but only for 5 min!) Researching I found out the idiots at Tesla Motors air cooled the inverter. I've worked with both water and air cooling on high power inverters and water cooling is (at least) x5 more effective than forced air. All high power inverters and (PM) motors in vehicles should be water cooled.
Toyota's Atkinson cycle engine
A 2007 UC Davis paper (see reference below) discusses the 1.4 L (gasoline) engine used in the Prius. It says it is the most efficient gasoline engine in production (as of 2007). It uses the Atkinson cycle as opposed to the Otto cycle used by most (or all) car engines. Here's the comparison they make:
Atkinson Otto
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Peak thermal efficiency 37% 25%
Power/Torque output (Atkinson 20-30% less than Otto of same size)
According to this paper the Atkinson cycle has a huge effect on performance. In exchanged for a 20-30% loss in power and torque almost a 50% improvement in fuel efficiency is realized! (Is this reflected in Toyota gas milage numbers?) It is consistent with Ford Fusion, which has very good gas milage numbers. Ford Fusion too uses an Atkinson cycle engine, and it's quite large 2.5L (vs 1.8L is gen III Prius).
Batteries
All hybrids on the market use relatively small NiMH battery packs (100 lbs or so). It turns out that remarkable little energy storage is needed to get big improvements in mileage, especially city mileage. Ford touts their new 2010 hybrid can go 47 mph on battery, but the driving range in all electric mode is only a couple of miles. Batteries at full power discharge in 30 sec or so. The improved gas mileage, which can be double for city driving, is achieved by downsizing and optimizing the engine cycle and speed and capturing a significant fraction of vehicle kinetic energy with regenerative braking.
Available data indicates that NiMH battery lifetime in hybrids has proven to be remarkably good. This indicates that oversizing the battery and small discharge cycles in hybrids (engines quickly recharge the battery after an acceleration) have been very effective at extending battery lifetime. This problem is much more difficult in electric cars where deep discharge of the batteries is required for long range. To improve range higher power density lithium ion batteries will be used, and battery packs will be x4 heavier (400 lbs or so) with x12 energy storage (16 kwh vs 1.3 kwh). (Of course, the battery people are working discharge cycle problem hard and 123 Systems, for example, touts the x10 higher discharge cycles that their battery technology can deliver as a major advance.)
Batteries at peak power can dissipate a lot of power and their chemistries don't like high temperature, so battery cooling is required. If the battery pack is pushed to its limit to deliver peak power ('matched load'), it will dissipated an amount of heat equal to what it delivers. Toyota (I figured out) compromises by restricting the peak load to 75% of battery maximum. This reduces battery heating to 1/3rd of delivered power, which is much better, but it's still a lot of heat. As far as I can tell, everyone air cools their battery pack. A photo of a Toyota SUV battery pack shows three high power axial cooling fans. But I read the GM Volt, who have a much more difficult battery thermal problem with their large lithium ion battery pack, may be using liquid for its temperature control.
NiMH battery data sheet from Panasonic shows a big loss of peak power capability in cold weather (reduces by x3 to x6). Apparently in hybrids this is little noticed, probably because the engine is cut in much sooner. However, it led Audi to cancel a high performance hybrid SUV they were developing when they found the snappy acceleration was just not there in cold weather.
Low temperature data ? peak current discharge on lithium ion batteries for vehicles are (suspiciously) not available. Not only has GM not released its battery spec, but the LG Chem has pulled all the detailed specifications for its (custom) lithium ion batteries. Likewise 123 Systems have not disclosed their vehicle battery spec.
Engine
Marketing literature sometimes gives the impression that the engine in a hybrid or an extended range electric need only run at one speed (or a very narrow speed range). This is partially true, but is somewhat of an exaggeration. The Prius engine runs from 1,000 rpm to 4,500 rpm (5,200 rpm in gen III). The lossey high end range of 4,500 to 6,500 rpm of the conventional engine is gone. While the GM Volt literature claims their engine mearly recharges the battery, it appears that they sometimes need to run the engine at its maximum power point (its highest loss point) so that raw generator current can supplement battery current (increasing it 33% or so) to increase the driving hp, or on long trips or cold temperatures power the vehicle fully.
Introduction
Here is a primer on the battery/inverter technology in hybrid cars from a power/motor control engineer. In the early hybrid years of Toyota ? Honda technical details were hard to come by, even to someone working in the power engineering field. But now a decade later some information about the technology has been published, and with hybrid cars slowing going mainstream and performance improving I decided it was time to take a look at the electrical engineering of hybrid cars. The new 2010 Ford Fusion, which is getting great reviews, is pushing the all electric envelope, touted as able to run all electric up to 40+ mph. It seems hybrid cars are slowing getting more and more electric, in a sense they are evolving toward all electric cars.
I know a lot about inverters, because I used to design them. I've designed little ones and big ones, some as big as those in current hybrids and some bigger. I know a lot about motors too with patents on the control of PM motors and induction motors. I know relatively little about batteries, but I know how to read a data sheet and can often take a few pieces of published data and made reasonable guesses to fill in the gaps. Toyota, of course, was a pioneer in hybrids and has been selling them for more than a decade. They just now introducing their third generation of hybrid technology.
I focused on the major hybrid manufacturer, Toyota, and quickly found the spec of the Panasonic battery used in the Prius. A colleague provided me with a recent paper by a Toyota engineer that describes the development of a higher power (123 kw) inverter and motor for use in a 2005 Toyota SUV. A link in Wikipedia led me to a detailed description of the Prius power splitter by a Prious owner and engineer. I started this essay to record what I recently learned about hybrids, and as a place to put new hybrid info as it becomes available.
I've gained a good understanding of the Prius/Ford/Lexus architecture. By feeding the engine power to the wheels through two parallel paths: mechanical (sun/planetary gear set) and electrical (two motor/generators + inverters coupled together), two birds are killed with one stone: It works as a continuously variable transmission, and it provides a DC node where the battery (or boosted battery) can be connected. Since every element in the structure (inverters, motor/generators, booster, battery, even the engine) allows bidirectional power flow, the system is very flexible and can be operated in five or six modes. To an engineer it's very sweet.Batteries
The famous GM EV-1 all electric car (1966 - 1999) used lead acid batteries, but no hybrids today use lead acid, except as auxiliary batteries. (EV-1 inverter used IGBT's).Hybrid vs electrics
The batteries in hybrids (like Prius) and electrics are very different. In a hybrid the battery pack is tiny, only a 70 to 100 lbs. It's used for a few seconds to boost acceleration and for low speed driving where power load is low. The engine quickly recharges the battery so the usual discharge is shallow. Batteries like this.
In contrast trying to drive on the highway any significant distance using only electric power is a bitch. The Tesla Roadster battery pack has x43 more energy storage than Prius (56 kwh vs 1.3 kwh), it weighs 1,000 lbs, costs a ton, and still only give 220 mile range in a two seater car.
With current battery technology the electric guys face a nasty trade off: battery life vs depth of discharge. Rechargable power tools fully discharge their lithium ion batteries with the result that the batteries are seriously degraded in three years. GM must be sweating this problem with the Volt. Their current compromise is to 50% discharge the battery. If they 100% discharged, they would double range from 40 miles to 80 miles, but then their batteries would begin to die in three years. The battery manuf are working this problem hard using a fundamentally new class of materials with very high surface area (nano technology) and are claiming x10 improvements in battery life.
Nickel Metal Hydride (NiMH) batteries
Currently (as of summer 2009) all hybrid cars on the market use Nickel Metal Hydride (NiMH) batteries. Typically they use a single stack of 170 to 260 cells in series (D type or equivalent @ 1.2 V/cell), which results in an 'open circuit' voltage in the 200V to 300V range. Amazingly these cells can (each) handle hundreds of amps for a brief time (seconds).
High peak power
In addition to good energy storage, a critical requirement for a vehicle battery pack it must have very low internal resistance so it can deliver large amounts of current and energy quickly to accelerate the vehicle. Low battery internal resistance is also needed to efficiently absorb the regenerated braking energy, which tends arrive in the form of high power/current pulses. It is the capture of the vehicle's kinetic energy via regenerative brakes in a hybrid that makes a hybrid's city mpg higher than highway mpg, exactly the opposite of conventional cars.
Prius NiMH numbers
The internal resistance of the 1.2V Panasonic cell (see below) that Panasonic says is used in the Prius ? some Lexus hybrids is only 1.66 mohm. This gives a short circuit current in the range of 720A = 1.2V/(1.66 x 10^-3 ohm)!! The battery manufacturer rates the 'output power' at its max (of course). A little back of the envelope scribbling shows the battery manufacturer (peak) 'output power' rating is for a matched load, meaning a load resistor equal to the internal resistance of the battery. For the Panasonic Prius battery peak power out is 360 A out (half short circuit current) at half voltage. In round numbers the maximum power that a 200V battery pack can deliver is 100V x 360A = 36kw (48 hp). This number is in the ballpark for electrical hp for current car hybrids (electrical hp numbers are substantially higher in SUVs and trucks), so it is likely that hybrid cars manufacturers do in fact push their batteries pretty hard, either up to or near their rated peak power point.
Realistically NiMH batteries can only deliver high peak power for a few seconds at a time, because at maximum discharge rates a fully charged battery pack is fully discharged in about one minute. Wikipedia says early model Priuses would fully discharge the battery traveling 2,000 feet at 90 mph up a 6 degree slope. This works out to battery discharge in 15 seconds, since at 90 mph the car is traveling 132 ft/sec. (I later found out the explanation for the difference between 1 min and 15 seconds. Toyota restricts the charge/discharge range of the battery pack for battery lifetime reasons, keeping it between 40% (or 60% references differ) to 80% of capacity.)
You can only run most hybrids in all electric mode to about 25 mph. You can't run the useless GM car 'mild' hybrids in all electric mode at all. Customers soon figured out this was pretty useless and didn't buy them, so recently (June 2009) GM announced they are going to stop production of all their hybrid cars (they will still have some hybrid trucks)! The new 2010 Ford Fusion hybrid, which is getting raves, can reportedly go 40+ mph in all electric mode.
Load conditionsVoltage sag
A simple, useful model of a battery cell is a voltage source (ideal battery) in series with a resistor, typically called the 'ESR' or 'Equivalent Series Resistance'. The ESR in the model causes the battery terminal voltage to sag when current is delivered to the load, just like in a real battery. Electrical engineers routinely characterize systems like this under two load conditions: 'no load' and 'short circuit'. At 'no load' the voltage (V) is measured with no current drawn from the battery. 'Short circuit' is a measurement of current (Iss) that flows (through the short) when the battery terminals are shorted, it is the no load voltage divided by ESR (Iss = V/ESR). In both of these cases the useful output power, meaning power to the load, is zero. Why? Because P = V x I, and in the former case I is zero, and in the latter V is zero.Matched load
If you do the math, you find the maximum power the battery can deliver occurs when the voltage and current are half way to their maximums. This occurs when the battery 'sees' a load that has a resistance equal to its ESR, this is called a 'matched' load. Under these conditions the battery terminal voltage sags to half the open circuit voltage (Voc/2), because the intrinsic cell voltage divides equally between the two ESR's, one inside and one outside the battery. The current that flows is half the short circuit current because the total resistance 'seen' by the battery is doubled (I = Voc/(2 ESR) = 0.5 Iss). The formula for maximum power out is:
Pmax = 0.5 Voc x 0.5 Iss = 0.25 Voc x Iss
where
Pmax = peak power out (watt)
Voc = open circuit voltage (volt)
Iss = short circuit current (amp)
To get power from the battery its terminal voltage must be allowed to sag. At maximum power out it will sag to about half of its open circuit voltage. There is no avoiding this. When a hybrid draws hundreds of amps from its battery pack, about half of the open circuit voltage drops across the internal battery resistance (ESR). It's just ohms law. This is a serious complication in the design of the control electronics for the motor in hybrid vehicles, because when high power flows to the motor, the battery voltage may sag from 300V to 150V. There's a way to keep the battery voltage sag from affecting the motor, but it's expensive. It involves adding a (voltage) boost converter between the battery and motor drive electronics. Toyota put in a boost converter in the transition from Gen I to Gen II about 2003.Heat dissipation
Another complication is that when the battery pack is delivering high power it is also dissipating high power. If the Prius battery pack was allowed to deliver its maximum 36kw to the motor, it would be at the same time dissipating an equal amount of power (36kw) internally. In other words at maximum power out half of the cell's stored energy is being lost as heat dissipation within the cell. 36kw is a lot of power, but [energy = power x time], so the energy to be handled thermally is bounded and calculable. Initially the heat energy is absorbed by the thermal mass of the cell chemistry, and over longer times the battery pack must be able to dissipate the average heat lost in the cells. (I have yet to find any info on temperature rises or battery cooling)
After writing above, I found from working the gen III numbers that Prius limits the peak power flow from the battery pack to 75% of its maximum power out capacity (matched load) as rated by the battery manufacturer. This modest (25%) reduction in peak battery output power makes a huge reduction (factor of 4) in the heat energy lost in the battery during peak loads.
Lifetime/charge cycles
Wear out of rechargeable batteries is strongly depended on the size of their charge/discharge cycle. Batteries in portable equipment, which are deeply discharged, have only about a three year lifetime. NASA uses batteries in satellites with solar panels to ride through the satellite night and gets good lifetime by oversizing the batteries so they only discharge about 10%. This is the trick used in hybrid cars. Even though the capacity range is limited to something like 25% (20% to 40%) in most driving the discharges from acceleration are no more than 10% with a quick recharge from the engine. This is how the battery lifetime in hybrids is extended to give an eight years guaranteed lifetime.
But this trick doesn't work (or work well) when the goal is to drive long distances on the battery, like the GM Volt or the Tesla Motors all electric car. GM is stricking a compromise by restricting the usable range of its lithium ion battery to 50% (30% to 80%). An effort to extend battery lifetime probably also explains the unusual cycle they plan to use. When the battery discharges to 30%, the engine turns on but it does not charge the battery. It just provides the average power the car needs while holding the battery charge (nominally) at 30%. Only when the car is plugged in is the battery charged back up to the 80% level. This prevents multiple battery cycles that would otherwise occur on long trips.
Another issue with battery capacity is that charging losses rise (substantially) as the battery approaches its capacity limit, meaning ESR starts to rise as capacity exceeds 80%. For this reason the top 20% of battery capacity is not very useful.
Info in this section is from a guy who makes battery testing equipment and who writes lots of battery articles. His site has a good article on how the lithium battery (with lithium metal) was unstable and dangerous and how it evolved into the lithium ion battery to solve this problem. He also has info on pros/cons of various lithium ion chemistries.
http://www.buchmann.ca/
123 Systems lithium ion cells 'claim to fame' is that their cells with nano technology have nearly solved (or greatly improved) the degradation with load cycle problem. They have data on their site showing cells still good after 300,000 cycles. They argue much less 'over capacity' need be designed into the battery pack with their cells to get the desired lifetime. (Not particularly useful to a vehicle manuf if he wants a 2nd battery source!)
Prius NiMH battery
I was able to get a good handle on the Prius battery pack by finding a data sheet from Panasonic, who makes the NiMH battery cells used in the Prius battery pack. Here's the link:
http://www.peve.jp/e/hevjyusi.html
The curve below shows the internal resistance (ESR) for a module (6 cells in series) is about 10 mohm. This makes the short circuit current an amazing 7.2V/0.01 ohm = 720 amp! This is confirmed by Panasonic's (peak rated) 'output power' of the module at 1,350 watts. The simple voltage source/ESR battery model predicts peak output power at halfopen circuit voltage (3.6V) and half short circuit (360A) current, and 3.6V x 360A = 1,296 watts (within 4% of the 1,350 watts on the data sheet).
Battery type D ?? (or DD??) NiMH (Nickel metal hydride)
(nope, the module is too narrow for D cells)
Prius ? Lexus battery Panasonic Prismatic Module
6 cells series (7.2V = 1.2V x 6)
10 mohm (for whole 6 cell stack)
6.5Ah (for cell and whole battery pack)
28 modules x 6 cells/module = 168 cells
battery pack weight = 28 modules x
1.04 kg/module x 2.2 lb/kg = 64 lb
(additional weight for structure)
open circuit voltage = 168 x 1.2V/cell = 201 V
Kwh for Prius 201V battery pack
201V x 6.5Ah =1.3 Kwh (OK, agrees with published)
7.2V/.01 ohm = 720 A short circuit (calculated)
Max power out (theoretically) with matched 0.01 ohm load
Pout = 3.6 V x ( 7.2V/ .02 ohm)
= 3.6V x 360A
= 1,296 watts (for a module of 6 cells in series)
Rated Power out = 1,350 watts (OK, this is obviously max
with matched load)
The two figures below are the Panasonic data sheet from which I extracted the info above. Note the module contains six cells 1.2 V cells (all NiMH cells are 1.2V) in series giving it an open circuit voltage of 7.2V. At rated peak power out half the 7.2V of the cells drops across the internal ESR (equivalent series resistance) and half across the load. This means the voltage seen by the load at peak power out is 3.6 V (half of 7.2V). So at 1,300 watts out, the current out, and in each cell, must be 1,350W/3.6V = 375 A.
I had read that D cells were the used in the Prius, but that can't be right, because the dimensions of the module are not consistent with D cells, it's too thin. D cells are 1.3 in in dia, but the thickness of this module is only 0.77 inches. Its dimensions are 0.77 in x 4.2 in x 11.2 in and it weighs about 1 kg. (Here's the story from Wikipedia: The original Prius used shrink-wrapped 1.2 volt D cells, and all subsequent THS/HSD vehicles have used custom 7.2 V battery modules mounted in a carrier.)
Panasonic Prius NiMH battery module data sheet
I was surprised to find how light the Prius battery pack is, it's only 80 lbs (other references say 100 lbs and Wikipedia says 118 lbs). Each of 28 modules weighs about a kg which totals to about 64 lbs. People who make advanced battery packs for the Prius give the weight of the standard Prius NiMH battery pack as 80 lbs, which is confirming, 64 lbs of cells + 16 lbs of structure.
Discharge time
At peak power out each NiMH cell is dissipating 216 watts = 0.6V x 360A!! At full current (360A) the battery cell run down time is only 6.5Ah/360A x 60 min/hr = 1.08 min.
Here's the discharge calculation from a power point of view. The Prius battery pack has 168 cells in 28 modules, which at 1.2V/cell gives an open circuit voltage of 201V. Each cell has a current rating of 6.5 Ah, which at 1.2V/cell is a watt storage rating of 6.5 Ah x 1.2V = 7.8 watt-hour. The 168 cell battery pack thus has energy storage of 168 x 7.8 watt-hour = 1.31 kwh, which agrees with what I see quoted for the Prius battery. At the maximum discharge rate 36kw is delivered to the load and 36 kw dissipated as heat in the cells, so the discharge time of a fully charged battery pack is in the range of 1.31 kwh/(72 kw) x 60 min/hr = 1.08 min.Repeat, a fully charged Prius 1.3 Kwh battery pack can only put out 36 kw (48 hp) for about 1 min before it is fully discharged! The fact that such a battery can effectively assist the car with power in/out peaks, shows that in real world driving power peaks must be relatively brief and separated in time. (Actually the Prius battery maximum load discharge time is probably closer to 30 sec than 1 min. Prius limits the peak discharge rate to 27 kw, which doubles potential discharge time, but for battery lifetime reasons only a small fraction of the battery capacity is used.)Discharge limits
I read that Toyota limits severely the usable kwh range of the NiMH battery pack, and this is done to extend the battery pack life (guaranteed for 8 years). One reference said the charge of the batter pack is held within 40% to 80%, Wikipedia says 60% to 80%. The first limit would restrict the usable kwh range to 40% of the nominal 1.3 kwr energy storage and 2nd limit is only half of this or 20% of the nominal energy limit! Note these are are not peak power limits, these are energy limits. The charge of the battery pack must always be held somewhat below 100% to provide a reserve for regenerative braking.
There may be peak power/current limits too, but I have not seen a definitive reference. One reference said the battery current was limited to 80 A, but this seems far too low, compared to the 360A that I calculate the batteries could deliver with a matched load. An 80A limit in the new Priuses with only 28 modules would limit the peak battery power to 14kw = 80A x [201V (open circuit) - 80A x 28 x 0.01 ohm] = 80A x 179V = 14.3 kw (19 hp) (After I wrote above, I figured out that the Toyota limits peak battery power to 75% of maxium (see below), which results in maxiumum battery current of 180 A in gen III Prius.)
Low temperature problem?
Notice the left curve in Panasonic data sheet. It shows a shows a horrendous drop off in peak power with temperature. At -10C (14F) the peak power is down by something like 2/3rd (400 kw/1,300 kw = 0.31). At -25C (-12F) it's halved again! Since the open circuit voltage is fairly temperature insensitive (maybe -10%), the power roll off means the ESR of the cell roughly triples at low temperature (and 14F is not that low). My guess is that this is not too serious a problem for current hybrids, because the control system can compensate by cutting in the engine at lower speeds in cold temperatures. (should see if I can find reference to this in literature).
But in a car like the GM Volt, where I estimate that 3/4 of the peak power comes from the battery pack a big increase in ESR at low temperatures in its lithium cells would substantially reduce the effective 'hp' rating of the car until the batteries warm up (if they ever do). I have been unable to find data on ESR vs temperature for various lithium ion cells.
(update)
I searched for people complaining about poor acceleration in cold weather and found little. Maybe the explanation is this:
"When you start the Prius with a cold engine, its top priority is to warm up the engine and catalytic converter to get the emission control systems going. The engine will run for several minutes until this happens (how long depends on the actual engine and converter temperature). Unfortunately, it is not possible to prevent the ICE (engine) from starting when you turn on the car, even if all you want to do is move the car onto the driveway to wash it."If the engine is routinely started in cold weather, then the inability of the cold NiMH battery to deliver high current is masked. People on forums did complain of poorer mileage in cold weather, but there are lots of reasons mileage falls off in cold weather.(update) I was right to be concerned about the large increase in ESR at low temperataures. A 2008 story say Audi cancelled a planned (high performance) hybrid saying (essentially) that the NiMH batteries stink at cold temperatures:
"Audi explained that they were not satisfied with the poor cold weather performance and limited capacity of NiMH batteries. When the batteries are low, the vehicle loses a significant amount of performance and Audi engineers wanted to ensure that the performance was consistent. They don't want a driver to pull out for a passing maneuver and have less acceleration than expected due to a cold or flat battery. VW and Porsche are proceeding with their nickel metal hydride battery hybrid plans."(update 10/2/2011)
Here is some hard battery temperature data from Toyota. A Toyota engineer reporting at an engineering meeting on test of the plug in Toyota (coming early 2012). It confirms major problems with battery capacity at low temperatures. This car uses a lithium ion, air cooled battery giving the car a nominal 13 mile electric range.
"At 32? F (0? C), range is cut in half; above 77? F (25? C), when the air conditioning kicks in, the range sinks again. But in addition to the effects of weather on the draw of electricity, Toyota also found a “traffic jam” effect. In good weather on the same route, when the drives averaged 19.4 mph, the EV range was about 4 miles more than when speeds were averaging 13.7 mph." (4 miles/13 miles 'traffic jam' is a 30% effect. No explanation as to why, but my guess is much of the low speed stop/start kinetic energy is not captured by the regerative braking.)At 32F the Toyota lithium ion battery has lost half its capacity! I bet it loses another half at 0F.Battery power out vs load resistance
I knew from engineering school that maximum power transfer (often) occurs with a 'matched load', so I was not too surprised when I worked out Panasonic's test condition for peak battery power ("Output Power") and found matched load, meaning a load resistance equal to the ESR of the battery. It was not until I wrote out the equation and crunched some numbers did I see how flat is the curve of maximum power out vs load resistance.
Power out = i^2 x R and i = V/(R + ESR). Combining and massaging we get, Power out = [V^2/ESR x k/(1+k)^2] where k = R/ESR. Solving for matched load (R = ESR) we get Power out = (1/4) x V^2/ESR, or 1/4th open circuit voltage x short circuit current, consistent with before. The shape of the Power out vs R curve is set by the k/(1+k)^2 term, where k is R scaled by ESR. Here's a table of k/(1+k)^2 multiplied by 4 to normalize it to the peak.
= k | = 4k/(1+k)^2 | |
max battery power out | ||
The numbers in the table above show the curve is quite flat. Max power out rolls off only 4% at R = 1.5 ESR, 11% at R = 2 ESR, and 25% at R = 3 ESR. Prius specs are 27kw from battery, while my calculated matched load battery power is 36 kw (= 1/4 x 720 A short circuit x 201 V open circuit). Thus Toyota in the Prius has effectively set the load resistor (Rload) = 3 ESR, suffering the 25% reduction in peak power out, but gaining three big advantages, or four if battery life is extended:
* Current out reduced by 2
* Power loss inside battery reduced by 4
* Voltage sag reduced by 2
Looks like a good trade to me.
Hybrid battery pack pictures
Below is the NiMH Prius battery pack (27kW, 202V) with 28 Panasonic six cell modules, which are about 0.8 in wide and 11 inches deep. The much larger battery pack further below is from the Toyota Higherlander SUV hybrid (45kw, 288V). Both packs are air cooled, the three roundish things (right) in the lower photo are probably powerful axial blowers. (Is there a lot of cabin noise from the blowers, since the battery pack is under the rear seat?)
Prius battery pack (27kW, 202V)
source --- http://www.hybridcars.com/gallery/22070/photo
Toyota Highlander SUV NiMH battery pack (45kW, 288V)
right: three axial blowers to cool battery
source --- http://www.hybridcars.com/gallery/22070/photo?page=1
NiMH provides (roughly) twice the energy density of lead-acid, which was used used in 1997 GM EV-1, and lithium ion has (roughly) twice the energy density of NiMH. So future lithium ion hybrids could get twice the distance on same size battery pack as current NiMH hybrids. Well, not really, info on the LG Chem site, a big Korean lithium producer, shows only a 1.56 advantage in weight and 1.5 in volume per KW for their lithium ion battery over NiMH. Lithium cells are 3.7V, so the cell count is reduced by a factor of three.
No standard chemistry
But a big problem currently with Lithium Ion is that unlike NiMH, there is no standard cell chemistry (or size). Every manuf has his own chemistry with different tradeoffs. No 2nd sources, it's all custom. GM 'Volt' manager at a battery conference recently said "convergence" (standardization) of Lithium Ion is a requirement for electric cars.
Lithium ion spec
Neither LG Chem or 123 Systems has the spec of its proposed Lithium ion battery for electric vehicles online. However, 123 Systems does publish the spec of a couple of their high current lithium ion batteries and the best of them has about half the short circuit current of the Panasonic NiMH Prius battery above (not a fair comparison, see below). At low temperature 123 Systems publishes only the A-hr capacity. The short form data sheet does not show ESR (peak power) vs temperature, which is possible red flag that it may rise as in NiMH.
The highest power lithium cell data sheet I can find online is the 123 Systems 26650 (dia..length) cell (below). It's ESR must be non-linear. It's speced 10 mohm typ @ 10A, but on a discharge cycle graph (not shown) it 15 mohm. The lower right curve shows a 0.6V sag (it varies some with # of discharge cycles) at 40A discharge (@ 25C), which is an ESR of 600 mv/40A = 15 mohm consistent with the discharge cycle data. If the 15 mohm applies at still higher currents (not assured), then the short circuit current (Iss) would be 3.3V/0.015 ohm = 220A. This value for Iss is probably pretty close to correct, because the data sheet shows a 10 sec "pulse discharge" of 120A (curiously without specing the voltage).
Just as Panasonic specs their cell for maximum power out using a matched load, it would not surprise me if 123 Systems did likewise. And confirmation that is probably what 123 Systems is doing is that is that their pulse discharge current of 120A is pretty close to the calculated Iss of 220A. While 123 Systems doesn't spec it, the calculated (matched load) peak power out would be 1/4 x Voc x Iss = 1/4 x 3.3V x 220A = 182 watts.
Lithium ion comparison to NiMH
A fair comparison of lithium ion to NiMH can only be done by comparing either an equal weight (or volume) of cells. I compare based on weight. Below is a comparison of the Prius Panasonic NiMH module to lithium ion modules made by two companies that feature nano technology (Altair Nano and 123 Systems). By putting five 7.2V, 1 kg Panasonic NiMH in parallel and three 2.3V, 1.6 kg Altair Nano in parallel they are pretty similar in voltage and weight. For 123 systems I used sixty 70 gram cylindrical cell in a series parallel arrangement. I reduced the 123 Systems cell count by 20% to account for missing structure a module would have.
5,000 A (max) from 10 lbs of batteries -- pretty mind boggling!
Paralleling batteriesx50
The biggest numbers above come from paralleling batteries, thirty in the case of the 123 Systems 5,000 A. Note while batteries can probably be paralleled, it's not obvious that this is allowed. The concern would be extra heat and wasted energy due to circulating currents if the internal cell voltages are not well matched. Voltage differences could arise from chemistry differences, temperature differences, aging differences, cycling differences. Very likely batteries to be paralleled should come out of the same lot (or be tested to insure their voltages match) and then be kept together so aging and cycling would be the same.
Fifty of the cell combos of the figure in series would be a 550 to 600 lb, 330 to 360 V (open circuit) battery pack for an quasi-electric car. Below I show the stored energy (kwh) and peak power out (25C) assuming (like Prius) the load draws half the matched load current. (Note the 10 sec pulse rating of the x30 123 Systems cells (30 x 120A = 3,600A) supports the Prius 1/2 matched current 'rule', but the x1 Altair Nano spec (500A) does not). Panasonic NiMH 11.5 kwh 243 kw
Altair Nano Li ion 17.4 kwh 272 kw
123 Systems 22.8 kwh 619 kw
On an equal weight basis the Altair Nano lithium ion battery has (about) the same peak power capability as a NiMH battery and 50% more energy storage. The 123 Systems peak power is more than twice as high as NiMH and its energy storage is twice as high. There's no published data on ESR vs temperature, but the123 Systems 25C peak power is high enough that ESR could increase x4 at low temperatures and the battery pack could still deliver 150 kw (200 hp).
Altair Nano advertises that it's battery can be recharged in 10 minutes (123 Systems 15 minutes). The power calculations indicate this is reasonable (50 watts for 10 min in a 1.6 kg module). To fully recharge a 17.4 kwh battery pack in 10 minutes would require 6 x 17.4 kw = 100 kw (220 VAC x 500 A).
Whoops! 500A is far too high from 220 VAC. With the 70A or so limit (set by new cable standard for plug-in electric cars) a 10 min potential recharge time is extended to [500A/70A x 10 min] = 71 min, or 36 min if the 70A is pulled from 440 VAC. In other words although the battery can potentially be recharged quickly current limitations in the charging cable and also in high power/voltage distribution boxes result (back of the envelope) in full recharge times of more like an hour (220 VAC) to a half hour (440 VAC).The minimum operating temperature of both lithium ion cells (@ unspeced ESR) is pretty low (-40C Altair Nano and -30C 123 Systems).
A123 Systems
A123 Systems high current cylindrical cell
.
ESR (@ 25C) = 600 mv/40A = 15 mohm
A123 Systems 20 kwh battery (Nanophase technology says Fisker spec) is being used in Fisker Karma, which is just starting production (about 200 made). I now find on the A123 site info on a battery pack they make for hybrid cars. The sell the complete pack with thermal management and protection. It's a good bet Fisker with limited engineering buys most of the technology from A123, but maybe not the package, because the form factor in the car is different from what I see in A123 photos.
A123 lists a 23 kwh pack and 22.5 kwh was the original Fisker spec, but in production it has been scaled back to 20 kwh. The core cell is primatic AMP20, 3.3V 20 Ahr (66 wh). The nominal Fisker battery voltage is 330V (in spec), so they have some multiple of 100 of these cells. 20,000 wh/66 wh = 303 (close to what I found speced for Fisker), so clearly the Fisker battery pack is about 300 or so cells arranged in 100 x 3.
Spec is 3C cont (10C 10 sec). C for AMP20 is 20A/cell, so 3C is 60A/cell and three in parallel is 180A. And 180A x 330V = 59kw (79 hp). So when the car is driven electric, the battery pack can deliver about 80 hp max (cont). My rule of thumb says 60 hp is about the wind resistance at 100 mph, but Fisker is a low sleek car, so probably can go faster.
Extrapolating from the smaller cyclinderical cell above resistance of 20Ah cell would be 1.15 mohm. At 3C (60A) this is voltage drop of 69 mv, and 60A x 69mv = 4.1W/cell. This is 4.1W x 300 cells = 1,230 watts dissipation for the whole battery pack when operating at its max continuous output of 59 kw. No problem here.
At peak levels from Fisker spec the battery pack must do 500A (150 kw) or 166A/cell. OK, the peak rating 10 sec is 10C, which is 200A/cell (10 sec).
Conversion of Prius to lithium ion
An independent company (EDrive) sells a lithium ion battery upgrade for the Prius. The 1.3 kwh, 80 lb NiMH Prius battery pack is removed and replaced with an EDrive 7.2 kwh, 200 lb lithium ion battery pack (2 in higher and 3 in longer). The peak power demands on the battery pack are unchanged since it is set by the inverter sizing. Hence each lithium ion cell can have an ESR that is x3 or so higher than NiMH (on an equivalent 1.2V/cell basis) and still provide all the peak power demands of the Prius electronics, which were, of course, designed to match the much smaller NiMH battery pack.
http://www.edrivesystems.com/faq.html
This upgrade costs 12,000 dollars (four hours of labor). The point of the upgrade is to make the Prius into sort of a quasi-electric. It's now plugged in to charge the battery, and since EDrive builds in (?) only a wimpy 1 kw (110/220 VAC) charger, the recharge time is speced at about six hours.
123 Systems conversion of Prius to lithium ion
Turns out that 123 Systems makes a 5 kwh lithium ion battery pack specifically for Prius conversion. Weighs 187 lbs. Conversion is done by HyMotion. Apparently battery is allowed to fully discharge, it is not recharged by Prius, only by plug in. It's not clear how its connected into the Prius architecture, maybe through a diode? Recharge time @ 1 kw from 120 VAC (only) is 5.5 hrs. There no real range spec, just says it "assists" in all electric mode for 30 to 40 miles. There's a link (on 123 site below) to a spec of the battery pack and what do you know the link is dead. Another case of a lithium ion vehicle battery with a hidden spec.
http://www.a123systems.com/hymotion/products/N5_range_extender
Here is interesting data on the Prius with the 5 kwh battery added. It's four EPA city cycles (EPA city cycle is below). Gray shows the engine running. It's a 30 mile, 1.5 hr trip @ 19.5 mph av, which is four EPA city cycles in a row. On the fourth cycle the battery is exhausted, it's a normal Prius, and the engine runs a lot more of the time.
30 mile, 1.5 hr trip @ 19.5 mph av, which is four EPA city cycles in a row
Prius with 5 kwh battery added runs mostly as an electric for 3/4th of time (67 min),
then on last 1/4th it drives like a normal Prius.
The Mercedes S400 Blue is the first production hybrid to use lithium-ion batteries. It's on sale now in Europe and USA in 2010, but when you look in detail at this car, so what? The press trumpets the battery doesn't take up room inside the car; it's under the hood and even fits where the lead acid battery used to go. Do you suppose maybe that's because its only 0. 7 kwh (truly tiny). For god's sake a standard car lead acid battery rated at 75 ah means it can put out [(75/20) A x 20 hr x (> 10.5V)] = > 0.787 kwh. That's right the lithium ion battery has about the same kwh as a standard lead acid battery, no wonder it fits in the same space! For perspective the Prius hybrid has a 1.3 kwh battery. The Mercedes S400 Blue is heavy full size car with a 3.5 liter V6 engine and costs about 88k. This is a 'mild' hybrid. It's can't even go 2 or 3 mph on electric alone says the German engineer. About all the 15 kw motor and battery do is quickly restart the engine.
Technically this does show that the peak current of lithium ion is pretty good, and says there should be no peak power issues with the much larger lithium ion battery packs in electric cars.
(Update 5/2010) Mercedes has a full page in New Yorker for S400 hybrid
"Powered by" a lithium ion battery.... God, marketing guys can spin anything. There's going to be some disappointed Mercedes customers.
"First hybrid sedan powered by a compact lithium ion battery"
"no loss of truck space"
(May 15, 2010 New Yorker full page add)
Note the implications of above for the GM Volt. The Volt is to use only 8 kwh of its 16 kwh battery (for lifetime). Here 5 kwh battery is pretty much depleted in 20 miles, and for maybe 15% of the time the engine is running. The implication is that an 8 kwh (usable) Volt in EPA test cycle driving would yield less than (8 kwh/5 kwh x 22.5 mile) = 36 miles range, whereas GM claims 40 miles. This partly explains the extreme emphasis put on aerodynamics of electric cars, but isn't the Prius after a decade of evolution pretty aerodynamic too? GM claims that aerodynamic changes alone to the production Volt added 6 miles vs the concept Volt (but I'm not sure if the drive cycle was specified).
Really high peak power
In the GM Volt, which is planned to hit the market in Nov 2010, the engine will not be coupled mechanically to the wheels. (Wrong!! This was GM marketing BS) This is very different from all current hybrids, where both the engine and electric motor are mechanically coupled to the wheels. Think about what this means in terms of current and peak power that its lithium ion battery must handle. It's really high!
I don't have hard numbers on the weight (see below) or hp of the Volt vehicle (nor of the battery pack, but for reference I'll compare it with my old mid-size Taurus, which weighed about 3,300 lbs powered by a motor rated at 200 hp (peak). To maximize the peak power to the wheels it's very likely that the Volt's motor's generator current ? power can beadded to the battery current ? power. In round numbers this means that 50 hp of the 200 hp to the wheels comes from the motor with the battery pack delivering the remaining 150 hp. This is probably on the mark, because 150 hp x 0.74kw/hp = 111 kw, and this is the rating of the Volt motor and inverter.
(update) An early report (Aug 2008) out of UK that weight of GM Volt was expected to be 3,500 lbs.
Compare the peak electrical power needed from the battery pack in the Volt to a 'conventional' hybrid, the Prius. Even though the Prius battery cells are running at 180A the electrical power out is only about 36 hp. The Volt battery pack must deliver something like x4 higher peak power than the Prius battery pack.
Lithium ion paralleling
Initially I thought this meant lithium ion cells for vehicles needed to have very low ESR, lower than NiMH. I don't know if such lithium ion cells exists. The lowest lithium ion ESR I have found (123 Systems) is an equivalent basis x2 higher than NiMH (20 mohm for each 1.2V cell eq) But I now see there's a work around. If the lithium cell battery pack has a high energy storage (high kwh), then it will use lots of paralleling (or equivalently the cells with have larger area). This kills two birds with one stone. You gets lots more all electric driving range and the constraints on ESR are loosened.
Very low cell ESR not required with large battery pack
For example, if x6 paralleling is used (as I suspect it is in the Volt) then x3 higher peak current/power can be delivered from the battery pack with each cell have an equivalent ESR twice that of NiMH, which is consistent with ESR of published 123 Systems lithium ion cells. So sort of as a freebie with a large lithium ion battery pack sized for range, enough peak current is available from the battery (perhaps with some supplemental current into the inverter bus from the generator) to allow the design option of driving the wheels only with the motor, dispensing with all mechanical coupling from the engine to the wheels.
We can bound the battery pack volt/current peak options. The following combinations of V and A multiply to 111Kw (150 hp). Most hybrids today have battery pack voltages in the 160 V to 320 V range. Assuming the peak battery power is (like Prius) limited to 75% of the 'matched load' capability, we get:
200V battery pack 150 V x 750 A
300V battery pack 225 V x 490 A
400V battery pack 300 V x 370 A
500V battery pack 375 V x 300 A
Fire risk?
As best as I can tell, what seems to be holding back Prius from going to lithium ion batteries is fire risk. A year or two prior to the introduction of gen III , Toyota said lithium ion was ready for gen III, but when gen III arrived in 2009 it had NiMH. The fire risk assessment come for a few comments of Toyota people. Who knows, maybe it's cost and weight too.
I read lithium ion batteries for cars will not be the same as for laptops, and the reason appears to be fire risk. If something fails in a lithium ion laptop battery, there is a run away oxidation --- translation: fire. Vehicle lithium ion use different chemisty with lower energy density to eliminate the run away condition.
Lithium battery chemistry
Lithium battery chemistry looks complicated and initially I avoided the topic, but a long article (now hugely revised) in Wikipedia gave a good overview. It argues that electric vehicle people were interested in using lithium iron phosphate (LFP) chemistry for their battery (LiFePO4). This is different from most of the small lithium ion batteries used in computers and consumer products. These are mostly lithium cobalt oxide (and some lithium manganese oxide and lithium nickel oxide). The names refer to the material of the cathodes, anodes are generally carbon (graphite). In other words in electric cars iron is to replace cobalt, manganese and nickel.
The iron version of lithium ion batteries has only about 60% of the energy density of the other types, and it has other problems too (like high ESR). So why use it? Well iron is cheaper and more available than cobalt, but the big reason is safety, safety from fire. You can't have cars going up in flames at anything like the same rate as laptop batteries have gone up in flames. Iron also is better for the long lifetime and many cycles car batteries need. Drawbacks (besides lower energy density) is limitation on peak current (high ESR) both charging and discharging. An intrinsic lower peak discharge rate for iron may not be a problem for an electric car, because unlike a hybrid car, there is a lot more battery in an electric car, so each cell is much less stressed when the car accelerates. The Wikipedia article on A123Systems says its chemistry is LFP, which it is working to improve with nano technology.
Wikipedia gives these generic LFP specs:
Gravimetric energy density = >90 Wh/kg (>320 J/g)
Working voltage = 3.0V–3.3 V
Let's check using the GM Volt as a test -- 16 kwh, cell weight of 375 lbs (170 kg)
GM energy density = 16,000 Wh/170 kg
= 94 Wh/kg Ok (agrees nicely with >90 Wh/kg)
GM opts for lithium manganese phosphate
But then I read that LG Chem that GM chose to provide the battery for the GM Volt is not building LFP (lithium iron phosphate) batteries! LG Chem's chemistry is Lithium Manganese Oxide. (This is hard to dig out. LG Chem says almost nothing about their chemistry on there site.) Some sites painted the face off between 123Systems and LG Chem for GM's business as a face off between lithium iron phosphate and lithium manganese phosphate chemistry. Indeed a GM executive said it was all about the chemistry.
One reference summarized the lithium ion battery chemistry this way:
There are four phosphate formulations from which to choose for a cathode material: iron, cobalt, nickel and manganese. Manganese is essentially a voltage compromise between the other three.It now (12/10) appears that Nissan has also chosen a similar battery (lithium manganese oxide) for its Leaf. Apparently there is voltage signature to the chemistry. Lithium iron operates the lowest at about 3.2 to 3.3V whereas lithium manganese operate s little higher (3.5 to 3.6V). Below shows the discharge voltage of LFP (lithium iron phosphate).
200 Ah LFP (lithium iron phosphate), 7.3 kg
source -- http://www.thunder-sky.com/pdf/2008926101921.pdf
Found some early (2008) data on the lithium battery being developed by Nissan. Nissan formed a joint battery company with NEC called Automotive Energy Supply Corporation (AESC). Their cell is lithium ion with a (spinel) manganese cathode (LiMn2O4). Cell voltage is 3.6V. In 2008 their prototype lithium ion vehicle battery had these specs: (Wow, ESR goes up by almost a factor of ten at -30C)
cell weight 527 grams
voltage 3.6 V
capacity 13 Ah (13Ah x 3.6V = 0.047 kwh)
power (25C, 2.5V) 1,086 watts = (434 A x 2.5V)
ESR (25C, derived) 2.5 mohm = (1.1V/434A)
power (-30C, 1.8V 116 watts = (64 A x 1.8V)
ESR (-30C, derived) 22 mohm = (1.4V?/64A)
With this battery (L3-10) a 24 kwh battery would need 510 cells, which would weigh 592 lbs. The 2008 article quotes a Nissan battery engineer says they are going to increase the capacity to 30 Ahr to reduce the number of cells, then a 24 kwh battery would have 221 cells. This proposed larger cell is in the ballpark for Leaf which has [48 x 4] = 192 cells and a battery pack weight (with structure) 660 lbs.
At 90 kw output each of 192 Leaf cells is outputting 469 watts =(90,000/192 cells), or about 130 A per cell. Scaling the numbers above by (510 cell/192 cell) = 2.66, cell power at -30C is (2.66 x 116 watt) = 308 watts. This indicates that the peak power of the car with battery at -30C is rolled off by (at least) a third. We can also figure the battery sag at 90 kw, and at 25Cm, and it is very small (2.5mohm/2.66 x 130 A = 0.122 V, or a sag from 3.6V to 3.48V. This is a peak battery power dissipation of only (0.122V x 130A x 192 cells = 3.04 Kw), or about 3% of the delivered power.
Comparison of Volt and Leaf lithium battery thermal management (12/15/10)
My nephew asked me what was new with the Volt, so here is my reply covering [Volt = hybrid] and summarizing how Volt and Leaf are handling the thermal issues raised by the lithium ion manganese batteries they are both using.
The Volt and Leaf are on the threshold of deliveries (1st Leaf has been received by first individual customer in USA), yet much of the detail of the drive trains are still hidden, no info on battery voltage or type of motor. But some details are dribbling out.
The biggest news revealed a couple of months ago is that GM has been lying, and the Volt is not really an electric car. It is a hybrid. It has the same three shaft planetary gears as a regular hybrid, which allows the engine and the motor torques to be combined to drive the car, and they do this in some modes like at high speed when the battery is flat. This has lots of performance advantages, but adds the cost of a big gear set and a whole bunch of pins and clutches to allow them to switch the engine in and out mechanically.
Another interesting fact I just found out this week. The lithium battery the Volt and Leaf are both using is a delicate thing. If you run it just a little warm (> 72F) or cold (?32F) then you degrade the battery lifetime, which is a major concern of both GM and Nissan, because the battery is so expensive its capacity has got to hold up for the life of the car. They are allowing for a 20-25% capacity loss after 8 to 10 years.
GM Volt has a real edge here. Volt has an engine, so if the battery is hot or cold, the engine comes on and the car runs as a hybrid, not as an electric, even though the battery may be charged. Virtually no one in the automotive world seems to have figured this out yet! GM also has well insulated the battery and uses an expensive liquid heating/cooling system to hold the battery temperature when running in a very tight range (70F +/- 2F). Good for battery, but it eats away at electric range, because if your car is parked in hot sun, the battery air conditioner will come on to keep the battery cool and the power to run the air conditioner is being drained from the battery!
Nissan with the Leaf is sticking their neck out that the battery will hold up, or people won't care if it degrades in a few years. They have no engine to protect the battery. When you run the car, the battery has to be used regardless of hot or cold it got when it was parked. Also they have no fancy heating/cooling system for the battery. All they have is one miserable fan to air cool the battery. Keeps the car cheap, but time will tell if this was a smart or dumb decision. Some people think they have under engineered the battery pack. Ford is going to use a Volt like active temperature system and even Nissan is now saying in 'higher end' electrics or in cars for the Middle East, they will use it to.(update Oct 2011)
Toyota Plug-in Prius quasi-electric with 5.2 kwh lithium ion battery is using Leaf like thermal management: three fans (controlled by 42 temperature sensors)Battery coolant problems (update 1/10/2012)
Real world problems of liquid battery cooling my have reared its head. Most new electric cars are liquid cooling the lithium ion battery, the Leaf being the big exception. But what happens if there is a battery coolant leak? Of course, the same question can be asked about coolant leaks in normal gasoline engines, but there is over a century of experience in their design and use.
The known facts are these: Volt has recently recalled all of its 8,000 or so Volts to do something they call strengthening the battery container. Fisker Karma has recently recalled all of its 250 or so cars because of a possible clamp problem allowing battery coolant to leak apparently into the battery container. The Volt recall was prompted by a fire (and 2nd almost fire) in cars sitting weeks after being crash tested. The facts are a little unclear, but the claim is the battery cells were not directly damaged, so the suspicion is that the battery coolant system was damaged.
I suspect the problem may be this. The battery containers are probably a full of little and big circuit boards that make up its monitoring and protection system. And of course the coolant is flowing all through the battery container to get the heat out of every cell. Hence the problem, I am guessing, is that a leak of the coolant is more likely than not to be into the battery container, and (if they are not well sealed) onto those circuit boards. My memory is this was specifically mentioned in the case of the Karma recall, and could explain the otherwise mysterious catching on fire of crash tested Volts that had been sitting in storage for weeks.
quasi-electric GM Volt
----------------
Seeing a Volt (update 11/27/11)
I saw my first Volt today and had a chance to talk to the driver. Volt was plugged in to one of two electric charging stations (installed by US dept of Energy) directly in front of Boston City Hall. Clearly the charging stations had been positioned for publicity as an electric car to be charged needs to be parked in a 'no parking' area on a sweeping curve on a busy downtown Boston main street. I was surprised the sign on the chargers said "free" charging. The driver, who said his company was testing the Volt, said well 'free' means paid for by taxpayers. Also surprised that the cord was part of the charging station. Noticed another weirdness. The Volt charging plug is not conveniently located for on-street charging. With the car parallel parked (as here) the plug is on the drivers side near the front door and near traffic, so the long cord from the charger has to be run around the car to reach the plug.
The Volt is quite a small car and tight inside. The driver commented there was not much head room. He said it had a lot of pep. I was standing next to it as he drove away, and it was nearly silent (no traffic on a Sunday night).
Electric ? quasi-electric car sales (11/28/11)
A total of 9.5 million new vehicles had been sold in the United States this year through October.
* G.M. has sold 5,300 Volts since introducing the car a year ago in late 2010
* Nissan has sold 7,200 all-electric Leafs in the first 10 months of this year
* Tesla has sold about 2,000 electric cars since 2008
Fire risk (11/28/11)
Collision damage to the Volt battery back has shown it can catch on fire. Hard to evaluate this risk at this point. In the inelegant phase of GM spokesperson, “This is a postcrash activity." A crashed Volt sitting in storage recently caught on fire 3 weeks after the crash. A real slow motion problem. Three Volt batteries were then damaged (but how badly is not described) and one caught on fire and one smoked. Fire risk with large lithium ion batteries has long been a serious concern, just look at the recall of laptop batteries for possible fire risk.
GM is sort of covering up the weakness. Look at this statement, "A pressing issue, (GM spokesperson) said, is ensuring that batteries are de-powered by trained service personnel after a collision". In other words GM is sending out people to discharge the batteries after an accident or crash testing. This is equivalent to drain the gas out of the tank of a crashed car, which I suppose is sometimes done.
It might seem that electric cars could benefit from a discharge resistor built that would be activated by a crash, but the problem is the heat and time it would take. A 1.5 kw resistor running at room heater power levels would take about 10 hours to discharge a Volt 16 kwh battery. An external 10 kw load bank (33A at 330V), which emergency personnel could provide, could do the job in about an hour and a half.The real answer is to robust the protection devices (fuses, etc) within the battery pack and/or change to a lithium-ion battery chemistry more tolerant of damage. I can guarantee you that GM is now working on this. There has long been a 'nail damage' test and spec for small lithium-ion cells.
----------------
Cold weather affects battery (update 2/29/11)
I wrote 'cold weather nightmare' (below) more than a year ago. Now the truth comes out as to why the low temperature characteristics of the lithium ion car batteries have been hidden. Consumer Reports says that the Volt it owns (cost 48k because the dealer slapped on a 5k premium charge!) and is driving in Connecticut this winter (we are not talking northern Minn here!) is only getting 25 to 27 miles electric range. One third of the electric driving range is lost at moderately low temperatures (southern NE winter) Consumer Guide borrowed a Nisson Leaf and reports it "it also gets very short ranges in very cold weather".
(update 2/22/12)Consumer Guide also points out another tradeoff with electric cars: the heater. Heat for the passenger compartment is basically free in a conventional car, but is a really battery drainer in an electric car. The answer of the Volt engineers was to focus on heated seats. Consumer Reports doesn't like it, saying while it keeps your body warm your hands and your feet freeze. The Leaf heater is powerful, but you pay for it in lost range. Consumer Guide (as others) are finding Leaf projection of available miles to be worrying and somewhat flaky. It's probably (as implemented) a bad idea. What Nissan is probably doing is extrapolating range from the incremental power use. This would lead the to the range jumping all over the place (as below).
NYT article about 2nd BMW all electric on road prototype (ActiveE) says test drivers found as much as 40% range loss in cold weather with their first on road prototype (MiniE). Consequently BMW went to liquid heating/cooling of the battery, though I'm not sure how much this helps since the power to heat the battery has to come from the battery!
"On one commute, his range in a Leaf was at 43 miles when he turned onto an eight-mile stretch of highway, but it fell from 43 to 16 miles after eight miles at 70 mph. If it keeps on going down at this rate, will I get to work? Champion said." (from Consumer Reports)-----------------------
Cold weather nightmare?
I'm beginning to suspect that GM must be really sweating about whether the limited temperature range of the lithium ion batteries is so crippling that the GM Volt may not be a practical car, at least in the northern half of the USA. Consider the following tidbits: 1) Many types of Li-ion cell cannot be charged safely below 0C (32 F) (Wikipedia)
2) "Right now we're thinking (below) 0-10?C (32F to 50F) we won't use the battery"
(Jan 2009 article quoting head of GM battery development for Volt)
What! It's an electric car with a range extender engine and at low temperatures "we won't use the battery". Give me a break --- And this is supposed to a practical car?(update12/10 --- If car is unplugged for a long time in cold weather and battery gets cold, then Wikipedia says the car runs on the engine (no hybrid peaks?) until the battery warms up. So what will the Leaf do!!GM liquid cools the battery to keep it from getting too hot, but what about winter? GM battery people say if the car is plugged in then the heat from the charger will keep it warm. Really? I suspect strongly the battery controller pulls line power for heating. (Yup, see below) This has got to be a lot of power, the battery weighs 400 lbs and is spreadout in big T structure under the car. Of course, no mention of how much power this is. The spec on the line cord standard used is 70A, 240 VAC. The math shows this would fully recharge the battery in 1 hr (GM says 3 hr). So you plug it in at 6:00 pm and its fully charged by 9:00 pm, it has enough residual heat to keep it warm until 8:00 am next morning. I don't think so. I'm extra suspicious because GM doesn't frankly admit the battery needs to be heated in winter, they use the euphemism "conditioning" the battery.
"Another of the weaknesses of electro-chemical batteries is degraded performance when they are very cold. GM engineers have devised battery conditioning algorithms to help overcome this." ('Conditioning will be the key to battery performance and durability says GM', article Jan 29, 2009) Asked about battery performance in cold temperatures like North Dakota, GM battery guy notes it's a "self correcting problem", ESR goes up at low temperatures, he says, so the battery generates more heat internally. Well that's one way to look at it.Volt battery thermal management system (update 12/14/10)
It appears that first generation (car) lithium-magnesium ion batteries are delicate things, and for lifetime reasons can only be power the car within a fairly narrow temp range (32F to 72F). GM has addressed this problem in three ways: one, well insulating the battery, two adding a battery liquid cooled heater/cooler, and three, using the engine to protect the battery. The Volt has an engine, so if the battery is even a little warm (> 72F) or moderately cold (?32F), the engine comes on to run the car (the battery probably still used in a hybrid mode for accelerations.) So what is the Leaf going to do? With no engine they have to use the battery whether its warm or cold. Do they use a different lithium battery chemistry?
The Volt battery cooling/heating system keeps the battery temperature in a very tight range of 70F +/- 2F. Sure enough if the car is plugged in then the heating/cooling power comes from the grid. If the car is parked (not plugged in) in hot weather, then interestingly battery power is used to run the battery air conditioner, so just parked the battery will be running down. They limit how much the battery discharge in this mode to 75% of capacity. Below this capacity the cells heat up sitting parked in hot weather. They do not run the battery heater when parked in cold weather. The cells just get cold, and then the engine runs when the car is started if cells below 32F. This battery temp control strategy is going to get a good try out, because in 2011 (or 12) the car is to be widely sold in Canada. The problem with high temperatures is battery lifetime. It's 8 years at 70F, but only 5 years at 90F.
If the battery is above 122F or below -13F, then the car simply will not run!! until the battery heater/cooler can bring the battery temperature into line. Posters understanding is that a cold soak forces a plug-in to recover, which sound ridiculous to Canadian posters who think they may come out of work on a cold day and be stranded. One of the key issues which no one has data on is how good is the battery insulation and what is its temperature response time. They are guessing the insulation is good enough that battery temperature will remain below 122F after being parked 8 hours in the Arizona sun.
Summary of GM Volt battery thermal management unit (heater/cooler)
posted by ChrisC at link below 12/12/10
http://gm-volt.com/forum/showthread.php?5243-Volt-thermal-management-system-temperature-band/page12
Winter commute
But it seems to me the crux of the winter problem is this: You buy the car to commute 10 to 15 miles to work and back. With the touted 40 mile all electric range you expect you can run the Volt in all electric mode using only a home charger. But what happens when the car sits outside at work for 10 hours in winter not plugged in? Unless power is bled from the battery to self heat it (which I suppose is a possibility), the battery pack will be at the ambient for the commute home, and according to tidbit 2), if the ambient is below 32F (or maybe even 50F), the battery in your electric car is not just weak, it is totally unusable!
Now you can still get home because the engine/generator can put out 70hp (generator rating), but the car is horribly underpowered. With the battery unusable it's not a hybrid, it's just a heavy 3,500 lb car with a 70 hp available for acceleration!
GM confirms cold temp problem (update Jan 2010)Both LG Chem and GM are withholding the battery specification. Why? (I don't understand GM's comments above about not using battery below 0C (or 10C). Is it related to charging? LG Chem lithium ion batteries have a discharge spec at -10C and 123 Systems gives their min operating temperature at -30C.)GM Volt design overview
After writing above, GM answered a question about cold battery at work at the opening of their new battery plant. Their (crazy?) answer is that the car will run on the engine until the battery warms up! So the GM bullshit is if you drive less than 40 miles a day, meaning you live within 20 miles of where you work, then the Volt will operate as an electric car. Now we find out that's not true in winter!. It's a half electric, returning home your not driving an electric car. The crazy part of the GM answer was the engine runs 'until the battery warms up'. Well just how is it that the battery warms up? The only efficient way for the battery to warm up is if they are running the hot engine exhaust gases through the battery enclosure. I supposed it's possible, but I bet this is a nightmare in terms of corrosion, etc. So my guess is that the battery is being heated electrically. Either the battery is help powering the car and is heated by ESR loss, or more likely, current from the generator is used run a heater in the battery compartment. And just how many watts is this and how long does it take for the battery to come up to temperature? Surprise, of course, GM doesn't say even though it damn well knows because the battery is now (supposedly) in production.
(update 10/11/10) Finally what looks to be a reasonable detailed spec for the GM Volt has appeared (below).
http://gm-volt.com/full-specifications]
It includes some new info like (finally) the capacity of the gas tank (9.3 gal) and cooling capacities of the battery and electronics and the number of battery cells is finally disclosed (288). Revealing is that the EPA city range is now given as 25 to 50 miles electric, not 40 miles always claimed by marketing. Charging times are given, but are meaningless since the charge kwh is not given. The numbers alos look funny. If 8 kwh is the maximum allowed discharge and since 1.5 kw can be pulled from 120 VAC with a good charger design, then an 8 kwh charge should only take 5 1/3rd hours, not 10 to 12 hours!
Spec
battery (LG Chem) 16 kwh, lithium ion
288 prismatic (10/11/10), 3.5 V? cells (arranged how?)
(220 cells 12/10 Wikipedia )
liquid cooled
436 lbs
built-in charger 3.3 kw (7/10)
usable capacity 10.4 kwh (25% to 90% capacity)
originally 8 kwh (30% to 80% capacity)
(update 10/8/10( GM is now saying 'somewhat more' than 8 kwh
will be used, possibly 10 kwh. Also as the battery ages the
usable fraction of the battery range will be expanded to prevent
milage range from shrinking. Oct 13, 10 PopSci test driver
says 65% of battery (10.3 kwh) is "customer usable" and
Wikipedia now says 10.4 kwh
motor 111 kw (150 hp), PM
Technical SAE paper by GM engineers says Volt motor and generator
as PM (not induction as earlier suspected)
272 ft-lb (368 nm) torque
generator 54 kw (72 hp), PM
max speed 100 mph
0 to 60 mph 8.8 sec
(Leaf at 424 lbs ligher with 80 kw motor is 10.0 sec)
cooling (liquid)
battery pack 7.4 qt
generator 7.7 qt
power electronics 3.1 qt
engine 1.4 L, 4 cyl
(GM Family 0, 1.4 L, 4 cyl, 66 kw, 90 hp @ 5600 rpm)
GM specifies premium gas!
wheelbase 105.6 in (midsize = 108, full size 112)
weight 3,790 lbs (10/11/10), earlier estimated as 3,500 lbs whoops!
(424 lb heavier than 3,366 lb Nissan Leaf)
gas tank 9.3 gal (10/11/10)
9 gal says Consumer Reports (6/28/10 update)
engine mile/gal (implied) 32 mpg = [(340 miles - 40 mi electic)/9.3 gal tank]
Popular Mechanics measures 32 mpg city, 36 mpg highway
charging 110/220 charger built-in (Connects direct to line
via SAE J1772 cable)
10-12 hr @ 120 V (how many kwh?)
4 hr @ 240 V (how many kwh?)
EPA city range (EV mode) 25 to 50 miles electric (varies with terain, driving style, and temp)
Popular Mechanics got 31, 33, and 33 miles EV in test drives
For reference gen III Prius: 106.3" wheelbase, weight 3,042 lbs, 98 hp (1.8L) engine)
Test drive --- Popular Science 10/13/10
Car drives the same in all modes. Transistion between modes almost imperceptable. Engine can barely be heard when it's running slow. In charge depletion mode the car runs as a hybrid, so acceleration is from battery, followed by engine RMP revs increasing as the generator works to not only power the car, but to recharge the battery back to the nominal hold voltage. This decoupling of the engine noise and its lag after acceleration takes some getting used to. Their biggest disappointment is that gas milage in charge depletion mode they measured (just 38 miles) to be 37 mpg. It is a heavy hybrid.
Estimate of peak acceleration power
(update --- The analysis below assumes the Torque vs Speed profile of the car is approx rectangular, making the power vs speed shape triangular. This may or may not be true for the Volt. The peak hp of 141 hp is within the capability of the motor assisted by the geneator (working as a motor), but another way to boost accleration time with limited power capacity is to have a wide contant hp range in the motor. This is what I suspect is done in the Leaf.)
With the weight of the GM Volt speced and 0 to 60 mph time (To) measured, a (rough) esimate of peak (acceleration) power to the wheels can be made. The simplest model is constant torque leading to a linear ramp of velocity with the maximum power point (Po) at 60 mph. This mades the energy delivered to the mass of the car during the acceleration
m = 3,790 lb/2.20 lb/kg = 1,723 kg
v = 60 mph = 26.8 m/sec
To (0 to 60 mph) = 8.8 sec
E car kinetic = 1/2 x m v^2 = 1/2 x Po x To
= (1/2) 1723 x (26.8)^2
= 619 kj
Po = (m v^2)/To
= (1,723 kg x 26.8 ^2 m/sec)/8.8 sec
= 141 kw
Po = 141 kw is a very interesting answer (if on the high side). But it's reasonable since it is within the kw range of the drive train. It requires (as I suspected) that both the main 111 kw motor and the 54 kw mot/gen are being used to accelerate the car. From another view point @ 60 mph the car has acquired 619 kJ of kinetic energy. If the power pulse was rectangular, this power would come in half the 8.8 sec (0 to 60 time) or 4.4 sec, so this makes the peak power (Po) = 619 kJ/4.4 sec = 141 kw.
Let me list the design challenges.
Heavy, expensive battery pack (400 lbs, 700 lbs with structure)
New battery technology, chemisty still being tweaked
Battery lifetime issues due to relatively deep discharge (compared to hybrids)
How much battery protection is needed/tradeoff with energy storage
High power inverter (or smaller parallel inverters like Tesla?)
Battery ESR increase in cold weather limits peak power
(or problems with lithium ion chemistry in cold weather)
Large PM motor (shift from PM to induction to handle heat?)
Motor if PM cannot allowed to get too hot.
Peak power rolls off if hot
Overtemp trips disable car (unlike hybrids
there is no back up with direct engine to wheel)
Engine noise ? vibration not correlated with vehicle speed or torque
Line recharge ports (separate converter with 120 to 240 VAC range?)
(480 VAC not mentioned)
GM Volt at auto show (spring 2009)
Very strange, why isn't the 4 cyl engine exposed as in every other car,
a mechanic will need to go on an Easter egg hunt to find it.
Is that double radiators in front?
Volt has been widely touted by GM as able to go 40 miles without the engine turning on. Now GM says well it's 40 miles (what speed, what terrain?) with all accessories turned off, so it's no heat and no A/C! Notice in the specs the generator hp rating (72 hp) doesn't match the engine hp rating (90 hp), which on its face is odd. This may just be rating conditions, or it could be a real mismatch because the engine is an available engine, whereas the generator would be designed for the Volt. If current from the 72 hp generator can feed directly into the inverter, which I assume it can, then at peak motor/inverter power (150 hp) peak power demand from the battery is 78 hp (58 kw). Electrically for the battery 58 kw (peak) means for x2 paralleling [58,000 = 315V (= 200 cells x 3.5 V x 0.9 sag/2) x 184A] or for no paralleling [630V x 92A].
Tragic flaw?
Some posters to Volt articles think the Volt has a tragic flaw. After its battery runs down, it becomes a heavy car (est 3,500 lbs) with a 1.4 L engine. "It will be very underpowered", said one poster. This thought flashed through my mind too, but it's hard for me to judge from experience, it needs to be calculated. In all non-hybrid cars the engine is sized for the peak power demands, the worst case may be accelerating up a hill.
Infinitely variable transmissionHowever, in terms of engine hp the Volt is not very different from the Prius (or most hybrids). These cars have small battery packs, so on the highway they run entirely on engine power. The Prius gen I and II had a 1.5 L (70 to 76 hp) engine, but it's been increased to 1.8 L (98 hp) in gen III. On the other hand the 2010 Ford fusion hybrid has a 2.5 L engine. The Prius is a pretty small car with a light battery pack (100 lbs or so), so 1.4L for the Volt does look pretty small. but, but ... GM is saying the control strategy with the engine is to hold the battery capacity at 30% on extended drives. I suspect they will allow a small variation around 30% to get extra power for accelerations to mock hybrids. It wouldn't take much in the way of capacity change (few percent) since their battery pack has more than x10 the size of a typical hybrid.
However, what probably is not understood by the poster and most people is that this car has, in effect, a infinitely variable transmission. The inverter provides all the current to the motor which in turn provides all the torque to the wheels. At low to moderate speeds the inverter can always fully torque the motor even if only limited power is available from the generator/battery, because power = [torque x speed]. So in cold weather even if the car has to run on engine power alone (probably unlikely), the car will still accelerate from a stop normally up a highway ramp only beginning to die when the car gets to higher speeds where it needs to merge, and just a few seconds of hybrid peaking from the battery would solve this problem.
It is a hybrid (update 7/10, 12/10)
GM recent releases confirm that when running on the extended range engine it works like a hybrid. With such a huge battery there should be plenty of juice for acceleration. The engine (80 hp) has just enough (av) power to overcome air resistance at its 100 mph (rated top speed).
In Oct 2010 the story from GM said only that the engine would be mechanically coupled to the wheels in charge depletion motor above 70 mph. By Dec 2010 the story has changed again. Now Wikipedia says in charge depletion mode engine will sometimes (acceleration) mechanically drive the wheels at speeds of 30 mph to 70 mph, and above 70 mph it drives the wheels all the time.
Volt cost (update Jan 2010)
GM executive speaks the truth about their 40k car ---
“It’s going to take us three generations of range-extended electric vehicles to get any anywhere near reasonable costs,” said Thomas Stephens, G.M.’s vice chairman for product development. “But if we’re going to have to be ready for the demand in 2020, we have to be out there by 2010 with the first generation.”2nd battery in GM Volt
GM confirmed that the GM Volt will also have a 12V battery. Clearly 12V is needed to run wiper and window motors and lamps, etc, so the issue is whether to use a 12V battery or a DC/DC on the big battery. There is also the interesting question of how the engine starts. In hybrids it's standard for the generator to run 'backwards' and start the engine. The off-the-shelf engine they are using will normally have a 12V alternator and 12V starter, so I can believe it's just possible that at its introduction the 12V battery will be used to start the Volt engine. Unlike a hybrid where the engine restarts at every traffic light, here the engine will start infrequently. GM of course, as of early 2010, has said nothing about this.Wind resistance power
One poster says continuous highway speeds only require about 47 hp. Since wind resistance rise as speed^2 and power = force x speed, power needed to overcome wind resistance rises as speed^3. Considering only wind resistance it takes 237% more engine power to cruise at at 80 mph than at 60 mph. My scaling of Prius data (confirmed by Wikipedia) is overcoming wind resistance takes about:
13 hp / 9.7 kw @ 55 mph
17 hp / 12.6 kw @ 60 mph
40 hp / 29.6 kw @ 80 mph
57 hp /42.2 kw @ 90 mph
79 hp /58.5 kw @ 100 mph
Conclusion: for a car with a top speed of 100 mph, like GM Volt, the minimum engine size is 80 hp, and surprise, surprise the GM Volt engine at 90 hp is just above the 80 hp wind resistance minimum.
Lithium ion cars hit market?
"G.M. Says Chevy Volt Is Still on Track (delivery in late 2010)" (June 2009) By Oct 2009 they will have 80 built as pre-production test and advertising vehicles. Estimates are it will cost 45k or more, and it looks like a small car!
I read in later 2010 Toyota is planning to sell some lithium ion vehicles to fleet buyers. (Aug 09) GM will also be delivering 125 Volts to electric utilities in Virginia for testing prior to Nov 2010 launch. US dept of energy have given GM 30 million for testing of Volt (125 fleet and 500 customers), which is a subsidy of 46k per vehicle. In other words our government is effectively buying the cars and giving them away!
Volt at low temperature
GM marketing repeatedly claims: "Volt is designed so the first 40 miles of driving are powered by the electric energy stored in the battery." But it now turns out that's not true in cold weather. GM chief engineer has confirmed that at low temperatures the engine will run to supplement the battery. At low temperatures, this car is somewhat like a modern locomotive, where the engine runs a generator and the generator powers the motor.
The lithium ion battteries probably weaken at low temperatures. My guess is the main worry may be large increases in ESR at low temperatures, as in NiMH, dramatically reducing the peak power.
Wikipedia (GM Volt) has the following vague low temp stuff, which is pretty vague but scary enough to say this is California type car!
"The battery needs a minimum temperature of between 0?C to 10?C (32?F to 50?F) (well thats' precise) to be used and when the Volt is plugged in the battery will be kept warm enough so that it can be used immediately when the Volt is unplugged. ** If the Volt is kept unplugged, and the temperature of the battery is below the minimum temperature the gasoline engine will run until the battery warms up. This temperature regulation is done since electro-chemical batteries have degraded performance when they are very cold."The Wikipedia footnotes a Jan 2009 interview with GM battery engineer, which says the following:. -- "While the vehicle is plugged in to charge, the battery inherently heats up and the cooling system will keep it at optimal temperature. If the ambient temperature is too cold, the battery will be pre-warmed in order to allow the car to operate on electricity as soon as it is unplugged." (Whoa! How much power does it take to keep a long 400 lb T shaped battery pack warm?)
-- "Right now we're thinking 0-10?C we won't use the battery." (ambiguity #1: does this mean 0 to 10C or does it mean somewhere between 0C and -10C. It reads the former and this is how its quoted in Wikipedia, but my guess is it's more likely to be the latter.)
So here is the GM chief battery guy saying in early 2009 that the battery may not be used if it's temperature gets below some threshold. But this statement is also not clear. Does he mean not used at all, or does it mean not used for range, but can still be used for a few seconds during acceleration like in a hybrid?
Not used at all?Charging battery with onboard generator (update 11/24/09)
If the battery is not to be used at all in cold weather then 0 to 60 mph time is probably going to be pretty bad. The engine/generator can only put out 70 hp the generator rating (though the generator might be able to pass the 90 hp from the engine for a few seconds). At low speed the torque will be OK because full motor torque is available, but as speed picks up the 70 hp limit will cut in and acceleration is going fade. Operated this way the car would likely be a real pig. Another way to look at it is this. The maximum power from the battery pack in all electric mode must be 150 hp because that's the rating of the motor. The power from the engine/generator is only 70 hp, so the power for acceleration is halved if the battery is not used at all.
So I doubt this is going to happen, performance reasons would force GM to operate the car in cold weather more like a hybrid. Even with high ESR in the batteries in cold weather, my guess is substantial peak battery power should be available to assist the engine during acceleration because the battery pack is so large (more than x10 a hybrid).
The NYT auto writer writes periodically about the Volt. His Nov 24, 09 article says this:
"The onboard generator will not charge the Volt’s battery. Its function is to maintain the battery charge at a predetermined level so that the car can keep moving after the battery has run down at about 40 miles. This is a deliberate choice on G.M.’s part — charging from a plug-in connection is far less expensive."Why is this forbidden? Sure you probably can't recharge at x35 the 110 VAC rate because the ESR losses in the battery would be almost a thousand times higher, but surely you should be able to recharge faster than line charging . This might be neat way to both warm and recharge the battery in the morning while eating breakfast. One lithium-ion battery manuf claims his batteries can be recharged in ten minutes. To forbid this seems incredibly stupid to me. Here is my posting to this NYT article covering low temp problem, the onboard charging issue and performance when the battery is depleted.post 43
GM is not being honest about the battery low temperature performance. Look at the euphemism (conditioning) GM uses for heating the battery. Denise Gray of the GM battery development team was quoted in early 2009 saying, “right now we’re thinking 0-10?C we won’t use the battery.” It’s got to take a lot of energy (and time) to heat 700 lbs of battery. Will the line power used to heat the battery be included when the cost to run the car is figured? Don’t count on it.
Seems to me absurd that the user is not going to be allowed to charge the battery using the onboard generator and engine. The generator is rated at 53 kw. This is x35 more power (!) than the 1.5 kw the 110V AC line can deliver.
I wish Mr. Brooke had explained how good performance was maintained when running on generator. My guess is that when charge drops to 30% the car converts in effect into a hybrid. Its large battery at 30% charge would have much more peak power reserves than a conventional hybrid car battery at 90% charge. — Donald E. Fultonand next day I posted this:post 49
On recharging the battery on a cold morning with the on-board generator:
All batteries have some series resistance (ESR) and in cold weather in addition to the battery capacity dropping the ESR will increase substantially. Heat dissipation in ESR (I^2R) varies as the square of the current. The onboard 53 kw generator can supply a ton of current, so on a cold morning it should be able to both recharge and warm the battery in not much more time than it takes to eat breakfast.
This is pretty handy if either you forgot to plug in, or there was no place to plug in. So why is GM going to prevent this? — Donald E. FultonVolt engine speed
I had read the Volt engine would always run an optimum speed for efficiency, which made sense. Now I read that the Volt engine will be 'tuned' to run at one of several different (fixed) speeds, but there is no explanation from GM as to why. And there is concern its different sounds will be disconcerting as they are not coordinated in the usual way with speed or acceleration. (Wiki -- GM Family 0 has a 1.4 L, 4 cyl rated 90 hp (66 kw) @ 5,600 rpm, which matches well with speced 53 kw generator.) What is going on here? Why is the engine speed varied? Maybe this is the answer (from an article in ESD magazine)
" The peak efficiency of the internal-combustion engine, however, is only 30%, and the average efficiency is about 12% at high revolution-per-minute rates."So if the battery pack has lots of charge, maybe the engine is run slow where it's most efficient. But if the battery runs down or its cold, then you need hp from the engine ? efficiency is secondary. It's then necessary to crank up engine speed to increase the hp.GM Volt on front page (update 7/29/10)
GM Volt on front page as price of 41,000 (base model) is announced. They will begin deliveries in late Nov 2010 as promised, but production levels scheduled for 2011 are very low (10,000) with 2012 production set for 45,000. Going on sale in a few months, but no specs and no mil/gal estimate! They still just say the battery has "more than" 200 cells.
Instrument panel has a 7" touch screen and the car has a 30 Gb hard drive (for audio storage)!! Charge times given as 10 hr @ 120VAC and 4 hr @ 240 VAC, but material on the ChevroletVoltage.com site which supposedly has the specs gives the charge time as 8 hr @ 120VAC and 3 hr @ 240 VAC! Battery is liquid cooled and heated. Comes with a 120 VAC power cord. Four seats (battery pack prevents a bench in rear). 8 yr warrantee not only on battery, but on the whole electrical drive system. The car will initially be sold in seven states, but it includes hot and cold states: Texas, Michigan, NY, NJ, CA, CT and MD.
Range given as 340 miles. Is this [40 miles from battery + 6 gal x 50 miles/gal]? Column in NYT says the Volt is more or less the electric version of the $17,000 Chevrolet Cruze. They frankly admit in the price announcement that you won't get get 40 mile electric range if you use utilities or outside weather is bad, but of course they don't say by how much, zero details!
Usable Kwh?
Specs clearly identify battery as 16 kwh, but nowhere in the recently announce material is the usable kwh given. Previously it was 8 kwh. However video below has a battery gauge (see minute 2:21) which shows the battery being discharged to about 1/4th of capacity, which would be a 12 kwh usage!
http://www.chevroletvoltage.com/index.php/Content/technical-presentations-blogs-other-information.html
Chirping sound
Well the good news is finally an electric car has a built in noise maker to protect people. The bad news is the 'chirping' sound the GM Volt will make. Listen to this horrible horn like sound it makes!!
http://www.chevroletvoltage.com/images/stories/blog2/volt%20chirp.mp3
Premium gas
The GM July 2010 price announcement included the weird fact that the engine will require premium gas. No explanation given and the experts are scratching their heads. One columnist says 80 hp from a 1.4L engine is not bleeding edge. Later someone weaseled this statement from GM press people:
'The Volt's unique architecture causes the on-board engine to act more like a generator. As such, premium fuel is required to maximize fuel efficiency. The use of premium fuel in the Volt increases fuel efficiency by five percent or greater over the use of regular fuel. Simply put, premium fuel optimizes this engine's characteristics. Basically, with reduced fuel consumption a key objective, premium fuel is the right solution for the Volt."In other words it's a range ? mpg issue. This is consistent with a small 6 gal gas tank. If premium really does add an extra 5% to milage (47.5 mpg => 50 mpg?, Is this possible?) it would mean pushing range up from 325 miles to (claimed) 340 miles. (There's a weird comment on a GM video about 'gas gone bad'. The implication is that there is a problem with leaving gas too long in a tank.) The marketing guys (July 2010) claim the Volt specs are here: ChevroletVoltage.com, but the spec there are a labeled 'premiminary' and are basically a joke. Picture show the engine, motor and inverter are all under the hood (up front). Top speed: 100 mph (80 hp engine can just about do this), 0 to 60 mph in 9 sec.
Aerodynamics
Electric cars are focusing on aerodynamics to extend range. According to an Aug 2008 interview with GM designer, the Volt concept car when tested aerodynamically didn't make the 40 mile design range. It made the 40 mile goal after a bunch of aerodynamics tweaks (including lowering the roof to reduce frontal area) that increased the range by 6 miles. This is a 17% driving range increase just from aerodynamics tweaks! I guess electric cars are going to be low cars.
.Shows Volt plugged in (Jan 2009)
Car male plug is on driver side just in front of the rear view mirror
(Is this plug right? It looks like three pins,but the standard SAE J1772 connector has 5 or 7 pins.)
123 Systems in Watertown (spending 60 million on new factory) and a large Korean company (LG Chem) are the two companies competing to provide the Lithium ion batteries for the GM Volt. In Jan 2009 I read LG Chem has been chosen.
"A123 is still sort of a startup, they're still ramping up, and A123 has been specializing mostly in ...cylindrical cells, which are good with power tools and stuff. What we need here is prismatic, which is flat cells. And LG Chem is just farther along," GM's Lutz said. (Jan 09)July 9 09 Prototype battery reported to be 200, 3.6V cells. (no info on pack voltage). It's probably either 360 V with double paralleling, or 720 V all in series.(update) A July 3, 2009 report has GM comitting to buy lithium ion batteries from Hitachi for 100,000 cars. However, they are apparently not for the Volt, they are for other GM hybrids. LG Chem is still being reported as the battery supplier for the Volt.
(update) Dec 2009 Michigan Gov with GM officials announced 336 million investment in a plant to make the Volt (Detroit's Hamtramck Assembly Plant).
(update) Jan 9, 2010 Michigan gov showed up at opening of GM's new lithium ion battery plant (for Volt) in Brownstown Township. 106 million of US taxpayer dollars to be invested in this one battery plant! Here is photo (shown with thermal shield) of first Volt battery pack from the plant.
Open and closed views of GM Volt battery pack
Visible stack up looks like 96 modules in series (345V battery voltage)
Each module has three parallel 3.6V lithium ion cells (288 cells total)
Peak power: 330 VDC x 330A = 100 kw (approx)
330A is probably carried by two orange wires (left)
GM Volt lithium ion battery pack (from new Mich factory) Jan 2010
(at right can be seen how the foil insulation is just glued to case)
GM Volt production battery pack (without external foil insulation)
> 200 cells
16 kwh with 8 kwh usage
-- The Volt’s T-shaped battery packs are composed of >200 (Wikipedia says 220 cells) individual 3.6V lithium-ion cells, each of which is a thin sheet about the size of half a piece of paper and about as thick as a CD case. Total weight for the whole pack is about 400 lbs which, when taking into account structural modifications, adds 700 lbs. to the total vehicle weight.Derived Spec of LG Chem cell for GM Volt (see below for update based on GM battery video)
Even with a production GM Volt battery I can't find the battery voltage. Nor can I find the LG Chem spec for the battery. But some specs can be derived from what is known of the battery pack VAh 72.7 VAh/cell = 16,000 VAh/220 cells
Ah 20 Ah rating @ 3.6V cell = 72.7 VAh/cell/3.6 volt/cell
weight 826 gram/cell = (400 lb x 1 kg/2.2 lb)/ 220 cells
The largest lithium ion battery on the LG Chem site is their E, which is about half the GM Volt cell. This is an old spec (2004).
E1 Ah 10 Ah @ 3.6 V
Weight 245 grams
Dimension 20 cm x 9.4 cm x 0.7 cm
ESR ? 3 mohm = AC impedance @ 1 khz
Max continuous discharge 30 A (P/cell @ 30A = 30^2 x 3 mohm = 2.7 watts)
"short circuit test Z" 50 mohm (I pk = 3.6V/ (50 + 3) mohm = 68 A)
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MIT Technology review 27 min video of GM Battery Plant tour (6/2009)
Some of the best GM Volt battery info I garnered from a long (27 min) video taken by MIT's Technology Review magazine energy editor (Keven Bullis) during a tour June 2009 of GM battery plant. The most interesting speaker, Bill Wallace, Engineering Group Manager), Rechargeable Energy Storage, starts 19:12 into video. (Wallace identifies himself as the engineering group manager for the Volt battery.)
http://www.technologyreview.com/video/?vid=369?channel=business
GM Volt production Li-ion prismatic cell
(screen capture from MIT Technology Reivew GM battery tour video, 6/2009)
-- engineering manager speaking
-- battery is at production stage, 5th iteration of car battery pack design
(100 packs built at time of video, 300 packs to be built by Q3 2009)
-- Volt battery pack is 1/3rd volume and 1/3rd the mass of EV1 battery pack
(EV1 lead acid battery version had roughly same Kwh and peak power spec as Volt)
Ev1(left) vs GM Volt (right) battery pack
(GM Volt 16 Kwh, EV1 lead acid 16 to 18 Kwh)
-- 110 kw "peak power"
-- 400 lbs
-- enclosure is fully sealed against water ? dust (since it's under car)
-- 155 different components inside the battery pack
-- notch in battery pack is for a car structural element to pass though. The battery pack
itself includes some structural elements
-- "over 200" Li-ion cells
(he smiles -- why is the # of cells being hidden, which it clearly is?)
-- we cool and heat every cell in the pack
-- highly insulated, which is key to maintain the batteries' temperature,
"temperature kills batteries"
Questions (to Wallace)
-- Is battery thermal management done with liquid or air? answer liquid
-- What is # of cells. ans, I can't give you an exact number but it's between 200 and 300
when pressed he said closer to 300, but less then 300
-- What temperature are you maintaining inside. "ideal temp for Li-Ion is
room temperature, 25C. Inside temperature is held appox between
10C (50F) and 35C (95F).
-- So what's the thermal strategy, say when you are parked? How much energy will you draw
when parked? Tesla had a problem drawing energy to run a fan when parked. Ans,
basically we're working on it. We have the capability to heat or cool battery in
any mode, even if car is just sitting. Passive insulation is very important.
Once you shut down car you need to be able to maintain (battery) temperature
without using energy. To do this we use a "very expensive" insulation package.
Summary of video GM Volt battery info (6/2009)
Khw 16 Kwh ("usable")
Peak power 110 Kw
# of cells 275 cells +/- 25 cells ("200 to 300, but closer to 300")
288 prismatic cells (10/11/10 spec update)
Battery voltage (oc) 346V (est) based on 288 cells @ 3.6V wired (1/3/11 update)
3 in parallel x 96 in series (based on 2008 article in Pop Sci
on battery pack that showed battery was 300 cells wired 3 in
parallel and x100 in series)
Cell Ah (est) 16 Ah @ 3.6 V = 16,000 Whr/275 cells
Est cell pk current 110,000 watt/990V = 111 A
Cell ESR (est) ? [0.33 (Toyota) x 3.6V/ 111A = 11 mohm]
(scaling from LG Chem spec 3 mohm/1.6 = 2 mohm approx)
Cell weight (est) 384 grams = 16 Ah x 3.6V/150 Wh/kg
Pack cell weight 106 Kg (233 lb) = 384 gram/cell x 275 cells
Pack weight "400 lbs" (unclear if this includes structure, but it seems to)
Thermal 10C to 35C (50F to 95F) operating cell temperature
(uses expensive passive insulation and liquid thermal
heating/cooling system, system is sealed for dust)
Note these numbers hang together well. Each cell looks like it has about x1.6 the capacity of the 2004 LG Chem cell (above), and this agrees with the picture. And for first time real info on the battery pack thermal heating/cooling system and a 10% accurate estimate of number of cells. These Li-ion Khw cell numbers look excellent about x3 higher than NiMH cells by weight.
The Prius NiMH cells are packaged in handleable plastic cases and stacked up. In Volt the photo shows each cell looks like a (delicate) foil with protection provided by a sealed outer structure. In Prius it looks like a bad cell could be popped out and replaced, but not in the GM pack, which is sealed.
Peak voltage sag (10/15/10)
In my GM-Volt block diagram I show the peak power from the batteries could possibly be 165 kw if both the 111 kw main motors and the 54 kw mot/gen are run to their limits. My guess is the 288 cells are arranged as 96 stacks of three in parallel forming a 350 VDC bus. The peak cell current then would be about [1/3 x 165,000/350V = 157 A] @ 165 kw.
Taking the 2 mohm cell impedance estimate above the voltage droop of the bus would be [96 x .002 ohm x 157 A = 30 V] @ 165 kw. This is only a modest 8.5% drop and would have little effect on performance. On the other hand if cell impedance is at the high end of the estimate (11 mohm), or 5.5 times higher than 2 mohm, then the bus sag is something like [5.5 X 30 V = 165 V] (or more), which brings the bus below 200 V and would be intolerable. Conclusion: unless there is a voltage regulator in the GM Volt architecture, then the cell impedance has got to near the low end of the estimated range in the 2 to 4 mohm range
Summary of GM video thermal info
The battery cells are seal into a very well insulated liquid heat/cool structure that will hold the temperature inside between 50F and 95F. Expensive passive insulation has been used to minimize the battery drain when parked in cold or hot weather for temperature control. (No hint as to the energy required for cooling when driving in hot weather. There appear to be no fins on the package that might aid cooling by using the air flow under the car.)
Assuming 1.9 mohm/cell (scaled from 3 mohm LG Chem spec for 16 Ah cell) @ 110 kw
volt drop/cell = 110,000 w/(275 cell x 3.6V/cell) x 1.9 mohm
= 111A x 0.0019 ohm
= 0.21 V (@ 110 Kw peak power)
(minimal sag at peak power 3.6V => 3.4V)
watt dissipated/cell = 0.21 V x 111A
= 23 watt (@ 110 Kw peak power)
watt dissipated/pack = 275 cell x 23w/cell
= 6.4 kw (@ 110 Kw peak power)
efficiency = 110 kw - 6.4 kw/110 kw
= 94% (@ 110 Kw peak power)
ESR losses go as current^2 (power^2), so averaged over driving conditions losses will far lower than the peak power shown above.
The small voltage sag of the large Li-ion battery pack, even at peak power, means that unlike the much smaller battery packs used in hybrids, there is no need to regulate the battery voltage, the battery can be applied directly to the motor inverter. My guess is that they are paralleling x3, resulting in a nominal 330 VDC battery voltage used as the bus for a 600 V, 600A IGBT inverter. Peak transistor current would be 3 x 111A = 333 A.. The 8 X4 inch Infineon six pack shown (far above) would mate nicely with a x3 paralleled version of this battery pack. (EV1variants had battery voltage 312V to 343V).
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Volt battery pack weight estimate
GM volt specs on the car battery pack are 16 Kwh. This is more than an order of magnitude higher than Prius, about x12 = 16 kwh/1.31 kwh times higher. Have not seen a weight spec the Volt battery pack on it, but we can estimate it. If the lithium ion cell has x2 the energy density of NiMH, then the lithium ion cells would weigh 64 lbs (Prius) x 12/2 = 384 lbs. If the energy storage density of lithium ion over NiMH is closer to 1.5, then the weight rises to 512 lbs. Add in 50 to 100 lbs of structure and I get a weight range for the battery pack of 450 to 600 lbs.
(update, July 9, 09)
Prototype battery pack weight reported to be 400 lbs, 700 lbs with structure Wikipedia says 375 lbs.
(update 3/10)
In an article in the New Yorker on lithium by a guy who toured the GM battery plant it says the amount of the element lithium in the 400lb Volt lithium ion battery pack is only 4 lbs.
Peak current requirement for lithium ion cells
The large battery pack ( x12 more kwh's than Prius) planned for the Volt eases the peak current/power demands on the lithium ion cells. I have seen nothing on the Volt battery pack voltage or structure, but it seems to me there must be a lot of paralleling. For example, x12 energy storage could be x2 higher energy density of lithium ion over NiMH with the other factor of x6 coming from paralleling of cells. With x6 paralleling then 150 hp (x3 Prius) electrical could be achieved with each cell putting out half the peak current of NiMH cells. This is in fact consistent with the only spec of a high power lithium cell (from 123 systems) I have found.
Volt plug-in recharge time
GM marketing writes the plug-in full recharge time will be 3 hr from 230 VAC and 6hr @ 115V. Well as a power engineer I decided to check it because it didn't smell right, and sure enough it's (probably) not right.
115 VAC line power limitation
Note GM is claiming the plug-in time using a typical residential 115 VAC line is twice the time it takes when the battery pack is recharged from a higher voltage (higher power) 230 VAC line. The typical 115 VAC line is brought up to it maximum 15A current with a 1,500 watt room heater. The small wire and fuses in 115 VAC lines limit them to delivery of 1.5kw of power.
In six hours a 115 VAC line can only deliver 9Kwh (= 6 hr x 1.5kw). This is a long way from the speced 16Kwh rating. Sure enough when I poked around the LG Chem site, they say the Volt battery pack recharge time, and as manufacturer's of the battery they should know, is 3 hr at 230 VAC and 8 to 9 hr at 115V. I posted this discovery in a comment to NYT article on the Volt. (Recharge time depends on the used capacity of the battery, reportedly it will charge to 80% upper and 30% lower, so 0.5 x 16 = 8kwh, consistent with 6 hr from 120 VAC line. )
NYT driving a Volt prototype (in generator mode)
She asks the question will the car be a slug in generator mode and then never answers the question. Car uses specially designed 'low rolling resistance' tires. A good question is, What is the engine's speed at rated power and peak power. The reviewer found the car's engine/generator cycling disconcertingly from a (barely audiable) low speed to a loud, disconcerting 3,000 or so RPM.
. GM Volt with five pin plug (NYT 11/09)
SITTING behind the wheel of a 2011 Chevrolet Volt prototype on Wednesday, I found myself confronting what may be the greatest fear that future owners of electric vehicles will face: a battery-charge indicator showing just a few miles of remaining range. If I were out on a desolate Interstate in a vehicle powered solely by batteries, I’d be praying to the god of electrons for a place to pull off and plug in a charging cord. But my drive is at General Motors’ proving grounds here, and I’m about to experience what the Volt’s vehicle line director (and my passenger), Tony Posawatz, says is the car’s trump card: a gasoline-powered generator under the hood.
Like other reporters, I had already driven Volt prototypes in the battery-powered mode, and they were predictably smooth and silent. But for eventual Volt owners, a crucial — and so far unanswered — question is how the car will perform when the battery’s charge is depleted and all electricity is provided by an onboard generator, driven by a gasoline engine, that has no mechanical connection to the wheels.
Will it be a slug? How annoying will the noise of the generator’s engine be in an otherwise mute car?
G.M. engineers say that a fully charged Volt is capable of 40 miles of purely electric driving before the computer calls for the generator, which has an output of 53 kilowatts (about 71 horsepower), to start and sustain the battery’s minimum charge level — the “extended range” operating mode.
So what is life after 40 like in the Volt?
It takes a few laps of Milford’s twisty, undulating 3.7-mile road course to deplete the remaining eight miles of battery charge. With the dashboard icon signaling my final mile of range, I point the Volt toward a hill and wait for the sound and feel of the generator engine’s four pistons to chime in.
But I completely miss it; the engine’s initial engagement is inaudible and seamless. I’m impressed. G.M. had not previously made test drives of the Volt in its extended-range mode available to reporters, but I can see that in this development car, at least, the engineers got it right.
I push the accelerator and the engine sound does not change; the “gas pedal” controls only the flow of battery power to the electric drive motor. The pedal has no connection to the generator, which is programmed to run at constant, preset speeds. This characteristic will take some getting used to by a public accustomed to vroom-vroom feedback.
A few hundred yards later, as we snake through the track’s infield section, the engine r.p.m. rises sharply. The accompanying mechanical roar reminds me of a missed shift in a manual-transmission car. For a moment the sound is disconcerting; without a tachometer, I guess that it peaked around 3,000 r.p.m.
I asked what was going on. “The system sensed that it’s dipped below its state of charge and is trying to recover quickly,” Mr. Posawatz said. “The charge-sustaining mode is clearly not where we want it to be yet.”
Immediately the engine sound disappeared, although it was still spinning the generator. A few times later in our test, the generator behaved in similar fashion — too loud and too unruly for production — but there is time for the programmers to find solutions. Volt engineers are revising the car’s control software, which will have the effect of “feathering” the transition from the nearly silent all-electric mode to the charge-sustaining mode, when the generator will be operating.
“We’re designing a software set of rules, which will just require more seat time for the engineers to finish,” Mr. Posawatz said. “We have nine months to work this out.” The sound of the generator running at steady highway speeds is something Volt owners, and others who appreciate the flexibility and efficiency of this type of hybrid system, may have to accept.
Unlike many electrics, including the Tesla Roadster, the Volt’s electric drive has no whine. The car feels solid and planted on the road. Clicking the Sport button on the dashboard releases a bit more oomph than when in Normal mode; in terms of efficiency, there isn’t much difference between the two except at peak power.
The Low mode— Chevrolet plans a flashier name for it by next fall — is unique in the electric-car world, and a useful feature. While coasting, it applies electric motor braking, then smoothly blends in the regular brakes.
Even beyond the regenerative function, Low mode offers one-pedal driving in slow speed, stop-and-go, and downhill environments. The regenerative braking, whether applied through the Volt’s foot pedal or by pulling the shift lever down into Low mode, is both progressive and predictable. This is in stark contrast to the harsh, abrupt regenerative braking delivered by BMW’s all-electric Mini-E, for example.
There is minimal body lean in the tight corners. The low-rolling-resistance Goodyear tires created specifically for the Volt provide excellent grip. Throughout my test, the prototype behaves admirably. At its current state of development, the Volt is an extremely refined vehicle.
High speed torque problem? (Sept 2010 update)
There are recent hints (June 27, 2010) that the high speed torque (high speed acceleration) in the GM Volt my be poor and that GM might be planning to radically alter the architecture of the car to fix it! These hints come from an article about the European version of the Volt, the Opel Ampera, which is to be built in the same Michigan factory as the Volt, but goes on sale a year later (end of 2011). This story was on the GM-Volt site (not connected with GM). Here's the link:
http://gm-volt.com/2010/06/27/opel-ampera-journalist-test-drive-questions-high-speed-performance/#comments
The story is that testing of the Volt and Ampera by journalists has been restricted to 50 mph top by a twisty, cone strewn test track they have been allowed to drive on, but that a European journalist got to drive the Ampera on city streets. He reports it accelerates badly above 50 mph. But here's the kicker:
General Motors is working on the problem and this autumn plans to unveil a mechanical direct-drive from the engine to the front wheels through the existing twin-clutch planetary gearbox. This would reduce the energy losses of turning petrol power into electricity to drive the car at high speeds, and would also give the Ampera more spritely overtaking performance.Is Volt really a hybrid?
Yikes! The key feature of an electric car, or range extended electric like the Volt, is that only the electric motor drives the wheels. All the torque at all speeds comes from the electric motor. Here there are hints that the GM electric motor drive system is not working well at high speed and that GM has been hiding this fact. This would be an issue of the type of motor and motor control system as reflected in the Torque vs Speed curve. The possible 'fix' is to mechanically add torque from the engine to the wheels through a planetary gear box!! Translation: They are converting their electric car into hybrid? It could be effective. There are 90 hp available from the engine, so why not let the electric motor max out during accelertion using only battery power and add in some (or all) of the 90 hp from the engine mechanically. After all hybrids do work well, and what they have here, they may be thinking, is a hybrid with a big battery.
But something about this story smells a little. The planetary gear box is the heart of a Toyota like hybrid. How come there happens to be a planetary gear box 'handy' in the Volt/Ampera that they can just tap into. An electric car would be expected to have no gearbox. (Tesla started off with a two speed gear box, partly because the boss thought customers driving a high performance car would like to shift, but took it out when it kept failing.)Hybrid transmission in Volt? (Sept 2010)
One very knowledgeable poster (#14) to the above article references a very interesting video (2008?, link below) from GM explaining the architecture of a new Saturn SUV nearing production. The video explains how they plan to use a variation of the Toyota planetary gear arrangement. Instead of a continuously variable transmission, they have a bunch of clutches to switch in four fixed gear ratios in the mechanical path. This reduces the power flow (? rating) in the electrical/motor path because it's just used to smooth out the four fixed mechanical ranges. The two mode Saturn SUV was probably going to share a lot of technology with the Volt, and he quotes GM people saying that. Also it was to have an 8 kwh lithium ion battery (!) to be plugged in and charged from 120 VAC. The poster thinks that the GM Volt has this 2 Mode Saturn transmission in it! He may be right, he makes a very good case. Clearly from a manufacturing point of view the Volt team would want to use a prexisting (or co-developed) transmission if possible. The poster's technical case (Feb 2010) is in link below. This is full of good info including even GM patent on the transmission. He argues that cars with no transmission end up needing motor with very wide speed ranges like 0 - 13,000 rpm, which is what he calculates for EV1 at its top speed of 80 mph. And I see in Tesla and other electric cars. However with the dual mode transmission (below) the two smaller 55kw motors combined together with different gear ratios can be designed to operate over a much smaller speed range (7,500 rpm max) so better efficiency and probably more reliable. I like it, but now in an 'electric' car we a complicated gear box of a hybrid, because basically it is a hybrid, just a hybrid with a big battery and an extended all electric mode. If true, it shows how truly deceptive GM has become! (It also would mean the Volt uses PM motors not induction as earlier reported.)
http://gm-volt.com/forum/showthread.php?t=4181
He points at this UC Davis 2007 paper as background:
http://steps.ucdavis.edu/People/bdjungers/presentations/UCDavis_Spring2007_TechReport.pdf
Transmission for planned GM Saturn SUV 'Dual Mode' hybrid
Note the 'motor' is really two 55 kw PM motors ,
which can be operated separately or lashed together to make a 110 kw motor
(screen capture from link below)
GM Volt revealed to be a hybrid! (10/10/10)
Finally GM comes clean and reveals details of the architecture of the GM Volt. And it is not the simple structure that GM marketing has been saying (or implying) it is [engine => generator => battery => inverter/motor => wheels]. It includes all of this simple structure and below 70 mph this is how the car works. But above 70 mph the 111 kw (main) motor does not have the torque/speed needed for good acceleration and 100 mph max speed.
(update 8/22/11) GM marketing continues to obscure the true nature of this car, meaning that it is not a hybrid. For example, in a technology video currently on the Volt website a GM manager says, "The Volt uses electricity all the time to run the car." He continues, "It has a an electric motor with 111 kw of peak power and that is alwayswhat drives the car". Well, maybe not outright lies, but certainly deceptive.So for operation above 70 mph GM has added to the architecture a planetary gear set (like Prius) and three clutches. This added path provides a mechanical path for some of the engine power to get to the wheels. So above 70 mph, the GM Volt is basically a hybrid with a big battery; but unlike the standard hybrid here the electrical path is dominant and the mechanical path provides supplementary power as well as a continuous variable transmission. The planetary gears act as a 'power combiner' (where in the Prius the planetary gears act more as a 'power splitter') combining the power from the main (111 kw) motor and the generator (55 kw) run backwards as a motor, or if the battery is depleted, mechanical power from the engine run through the continuous shaft of the generator.
GM Volt power train showing plantary gears and three clutches (10/10/10)
Two blue/green clutches freeze the ring gear or connect it to the 54 kw motor/gen,
red clutch connects/disconnects the 54 kw mot/gen to the engine
cutaway of GM Volt power train
Here's a sketch I made hi-liting (in blue) the extra components of the Volt drive train that GM has hidden for nearly two years. Independent testers report the car has good acceleration and performance over the whole speed range both in all electric mode and with a depleted battery, but the downside is the cost of adding a complex hybrid type planetary gear set and three clutches. (Details of the architecture are pulled from a patent filed 2007 by GM which is believed to be the architecture of the Volt.) The GM spec has finally disclosed that the battery consists of 288 cells. Here I am guessing that the 288 cells are arranged as 96 stacks of three parallel cells (96 x 3 = 288), because if the lithium cells are 3.6 V or so (exact voltage not disclosed), then the bus voltage comes out about right (350 VDC) for 600 V IGBT's in the inverter.
(best guess) GM Volt architecture
parts in blue (planetary gear set and three clutches) were hidden by GM until Oct 2010
(1/4/10 update) My figuring below that torque is added from the gen/engine path during hard acceleration is probably right.. In a YouTube test drive by an automotive type he says when you step hard on the gas (he is doing 40 mph or so on a country road) you can feel a "kickdown" bringing in extra power. My sketch shows it doesn't matter if the extra torque comes from the generator/battery or engine, in either case the clutches have got to throw and the planetary gears go into action to add in the 2nd torque.
--------------------
This has not been disclosed by GM, but looking at this architecture I find it hard to believe that GM is not using the 54 kw motor/generator to boost acceleration of the car (even from a dead stop). Essentially all the parts are in place to boost both the torque and hp of the car in all electric mode by as much as 50% (!) utilizing the 54 kw generator as a motor and combining torques through the planetary gear set.
However, I can think of one reason why a strong boost via the lower path may not be desirable. With 50% more torque and hp the car in all electric mode would (perversely) have performance that is 'too good'. The engine does not (appear to) have the sustained power capacity the battery has, so in some driving scenarios a noticeable loss of car performance might occur with a depleted battery. I was struck that reviewers noted that the car performed about the same in all electric mode and with battery depleted, so my guess is that this was a design goal, and it's obvious that marketing would like it, because it keeps things simple.
There is a complete description of the GM Volt drive train in Motor Trend magazine and their test drivers got to drive the car for 100 miles in the field. he GM Volt is now revealed to be a hybrid with a three axis planetary gear set like the Prius. Quote: "Markus liked driving the car and he noted he was surprised about the direct mechnical connection."
http://gm-volt.com/2010/10/11/motor-trend-explains-the-volts-powertrain/
Details of the powertrain here:
http://gm-volt.com/2010/10/12/chevrolet-volt-electric-drive-propulsion-system-unveiled
There's even a figure in the article comparing the two planetary gears. The Volt has the engine, wheels and generator hooked connected to different gears than the Prius. There's info about how this functions in the GM patent application (see below).
Motor Trend article on GM Volt geartrain (Oct 2010)
The article description with gas engine on (battery in charge sustain mode) above 70 mph is again a little unclear, but I think this is what is going on. At all speeds includein above 70 mph with depleted battery the engine powers the generator which (coupled to the battery for peak boosts) powers the main motor (150 hp motor). However, it it worked this way above 70 mph the car would be a pig. The generator can't now be used to provide wheel torque as a motor since it is acting as a generator powing the main motor, so in this mode the torque from the engine now couples through the planetary gears directly (mechanically) to the wheels. It is a hybrid, in this mode the wheel are driven by a combination of electric and mechanical torque. GM is quoted as saying: 'The gas engine participates in the motive force' and 'the engine never drives the wheels all by itself', which is correct.
Motor Trend says the performance of the car is good in all modes. It accelerates faster than the Prius (0 to 60 in 8.8 sec (electric), 8.7 sec extended, vs 9.8 sec for Prius.
My comment -- just a hybid with a big battery (10/10/10)
The GM Volt appears to have a sophisticated architecture with good performance. Good.
Not so good, however, is it is not all that radical. It's now coming out (well hidden for years, Why?, Probably because it killed the sex appear) that it's basically just a hybid car with a big battery!! And no wonder it's expensive it has all the planetary gear overhead of hybrid plus higher capacity electronics and a 10k 400 lb battery.
Here's what looks like the GM-volt architecture (from 2007 GM patent)
www.freepatentsonline.com/20090082171.pdf (20090082171, Mar 26, 2009)
A,B,C is planetary gear set
A (sun gear), B (planetary carrier), C (ring gear)
M/G (motor/generator) B is 111 kw main motor
M/G (motor/generator) A is 54 kw main motor
engine power rating: 66 kw/ (89 hp)
US patent office application 20090082171 (filed 2007). GM has confirmed this patent does apply to the GM Volt and that it issued two weeks ago. "Output Split Electrically-Variable Transmission with Electric Propulsion Using One or Two Motors", inventors Brendan M. Conlon, et al. US patent link to application here. -- Key here (I think) is that the engine shaft goes right through the 54 kw generator, so >70 mph with battery depleted the 66kw engine can both power the generator (to power the main motor) and still have some excess torque left over that is coupled mechanically into the planetary get set to the wheels.
-- Two motor mode (> 70 mph), the torques or the two motors are combined for delivery to the Final Drive (wheels), or alternatively, the speed of M/G A determines the gear ratio from M/G B to the Final Drive. This is the Electrically Variable Transmission (EVT).
-- Article commentary on patent says, "GM seems to have stated that the Volt never mechanically couples the Engine to the wheels.", which the patent clearly shows is possible. (If they did say this, and I bet they did, and we now see this is a (marketing) lie. Everyone in forum commentary remembers GM saying the engine would 'never' directly drive the wheels.
Toyota Prius plug-in hybrid 2012 (Aug 2011)
Toyota has announced what they call a plug-in hybrid for sale in early in 2012. By putting a 300 lb, 5 kwh (approx) battery in a Prius they have a direct competitor to the Volt. It's as an electric car for 13 miles, and then runs on its engine as a hybrid (probably like a standard Prius). They are going to start selling in in the north too (Mass, Vermont, NH, Washington). A web site is up with some info, but full specs won't be revealed until Sept auto show.
I think going with a battery 1/3rd the size of the Volt (16 kwh) makes a lot of sense. Instead of an extra 10k for battery it's 3.3k and much of this will be paid for by government subsidies for electric cars. Instead of 1,000 lbs it's 300 lbs and a lot smaller too. Some odd things (not yet sorted out). Read max speed in electric mode is 62 mph, which means its only for city. You can prevent it from going into electric mode, allowing you to drive highway on engine and switch to electric in city. This makes sense and I read Volt will have this too (in Europe). For some reason I don't understand instead of just making the hybrid battery bigger, then have added another battery (maybe even in two segments) for electric mode that is line charged. I can see lots of production reasons why this makes sense, but not sure if functionally it makes sense.
(Oct 2011) Saw Toyota plug-in hybrid (writtten all over the side) on road in Cambridge today. It was marked as a ZipCar, which is apparently cooperating in its testing. Yup, I read in Jan 2011 ZipCar got 8 cars to test for use in three cities and Cambridge MA is one of them.
Battery 5.2 kwh, 288 cells, 3 modules
Battery thermal three cooling fans
Battery weight 350 lb
Charge time 3 hr @ 120 VAC
Motor 60 kw (80 hp)
Fuel tank 11.9 gal
Motor at 60 kw is not large enough to provide the 120-130 kw needed for a fast acceleration, so the engine starts during hard acceleration to provide the additional 60 kw. Engine also turns on when heater or air conditioner is powered on. (The article reads like the car still runs electric on the battery, with the engine only providing electric power to run the heater and air conditioner.)
quasi-electric Fisker Karma sedan
(update 2/10/12)
Fortune magazine in an article on the Fisker Karma says this:
"More perceptive reviewers find the car overweight, inefficient, and capable of only mediocre performance (maybe because it has a 2,500 lb drive train!) that falls well short of its eco-friendly goals." (whatever the hell that means)Car and Drives says they found the electric range to only be 24 to 28 miles. Fisker claims 50 miles.(update 2/9/12)
According to the WSJ today the US Energy dept froze further funding to Karma for their follow-on sedan (Nina), since they have missed targets that are part of the 529 million dollar US Energy Dept loan agreement. This was the car planned for production in the old GM factory in Delaware. WSJ says the have accessed 193 million of the 529 million before freeze. With further taxpayer funding frozen Fisker laid off about 26 people working on the sedan at the Delaware plant and 40 in Calif.
A123 Systems
Also today Kevin Bullis of MIT's Technology Review has an article online saying Fisker is a "key customer" for A123 and their recent troubles are bad news for A123. I think A123 System, MA manuf of lithium ion batteries, has made a major mistake in hitching its wagon to Fisker Karma, and its overweight 5,200 lbs 'sub-compact' turkey of a car. That's right its first (and only?) 5,200 quasi-electric car has been ruled a sub-compact by EPA due to its tiny interior volume (all those batteries and the back seat is smaller than a Honda Fit).
Bullis says maybe 30% of its revenue 2011 and 2012 was expected to be from sales to Fisker Karma, but Fisker has "unexpectedly" canceled battery orders. (Could it be their overweight baby is not selling?) Bullis says Karma has now manufactured 1,500 cars and sold hundreds. A123 Systems has built a battery plant to supply Karma and now it has idle capacity. A123 also has a US dept of energy loan (249 million) and for 2011 they are operating at a big loss. Bullis is hinting that if Fisker gets into a lot of trouble, it could drag A123 into bankruptcy, though if they survive 2012 they have a lot of other customers whose orders they expect to grow.
(12/31/11) Well, Fisker got to production (in Finland). In headlines today they are recalling all their cars. Loose coolant clamp could lead to spill into battery compartment which could cause a short (they say). (Good engineering.....)
"Fisker Automotive is recalling all 239 of its 2012 Karma luxury plug-in hybrid cars of which 50 are in hands of consumers. Prices on the 2012 model start at $103,000."Drudge headlines they got 529 million dollar US loan to build cars in Finland. In 2009 when they get loan plan is to build cars in old GM plant in Del. Picture shows Biden sitting behind Fisker Karma at press conference. But later they announce:
"There is no contract manufacturer in the U.S. that could actually produce our vehicle," the car company's founder and namesake told ABC News. "They don't exist here." Really, they couldn't find a contracting company in US prepared to build a complete car. What a surprise the must have been!
2012 Fisker Karma production model
(update 8/2011) Fisker Karma is still alive, now branded as a 2012. You actually find a list of retailers and when I checked a
Norwood Ma Jaguar dealer on the list, the dealer site says he will soon be selling the Fisker Karma.
(update 11/2010)
True electric extended range car
Fisker put out a press release that theirs is the only true extended range electric car, which means they are saying that unlike the Volt their engine will not be mechanically coupled to the wheels. This is confirmed in articles as production starts. The engine only runs a generator, it is not mechanically coupled to the wheels as in the Volt.
What Karma doesn't point out is that this super expensive car as a true electric has only moderate performance. If you want your nearly 100k car to give you sport car performance, the engine needs to run. To shave 2 sec off 0 to 60 mph and to increase top speed from 95 mph to 125 mph the you need to (about) double the power to the electric motors using the current from engine/generator (175 kw) added to the 150 kw available from the battery.
When you think about this, it is rather pecular. They have a 20 kwh battery (from A123), and they can't get 300 kw (100A @ 300V say) peak (or sustained) from it? (Nope, see below). Either they didn't with 100A wiring between cells or else the cells can't take the heat of 100A sustained. Maybe with the engine/generator sitting there the temptation was why bother, just run the engine, and keep the battery cool. Problem is this is supposed to be an electric car, and it costs over 100,000!
Whoops-------------------------------
Lost a factor of 10! This car requires a lot of power to drive its two 200 hp motors driving it. 300 kw is 1,000A at 300V. So the choice was getting 500A vs 1,000A from the battery. Now the decision to split it up with 500A from battery and 500A from generator makes more sense. Power goes as current squared, so using the generator for half the motor current cuts battery I^2R losses by a factor of 4. However, this just goes to show the problem in building a high power electric car.
There is another quasi-electric car in the design stage at a start up company in Detroit ? California. It's the Karma from Fisker Automotive. They allegedly have 200 million in funding (? 500 million in US government loans) and have 175 engineers working on it and are talking about beginning production in early 2010 (slipped to early 2011), earlier than the GM Volt. While designed in US, it would be built by an existing company in Finland, however I read they recently bought for 20 million a closed auto factory in Delaware. The technology is derived from Quantum Technology's ('Q Drive') that they use in (prototype) hybrid vehicles built for the U.S. Army. Basically the Karma is a scaled up GM Volt, much larger, much heavier, much higher performance, and much more expensive. The car is a big (124 in wheelbase) expensive (88k), 4 seater sporty looking sedan that's functionally like the GM Volt. It's a plug-in hybrid with a large lithium ion battery pack for moderate electric range and an onboard engine to provide extended range.
electric range (EPA city) 50 miles
0 to 60 mph 5.9 sec (gas/electic mode)
7.9 sec (electric mode)
top speed 125 mph
engine 2 .0L, 4 cyl, turbo charged, 260 hp (192 kw) (2.2 L original)
(Ecotec engine made by GM)
gas tank capacity 9.0 gal premium (extra 250 miles @ 28 miles/gal)
two 200 hp motors 300 kw = 2 x 150 kw (400 hp total)
(981 ft-lb torque) (1,324 nm) (300 kw corner @ 2,164 rpm)
motors made in China (says Car and Driver)
motor location PM motors in rear wheels, liquid cooled
generator 175 kw (236 hp) (about 150 hp are needed for wind at 125 mph)
inverters three (two for motors, one for gen or charging?)
battery energy 20.1 kwh (from A123 systems) (315 cells) (22.5 kwh original)
(vs 16 kwh for GM Volt) Battery run to 15% capacity
"Nanophosphate technology" Googling this brings up A123 Systems
battery weight 606 lbs (light!)
battery size (in tunnel) 8.1 in w x 14.2 h x 6.14 ft
battery power 160 kw (216 hp) (181 kw says another source)
(Leaf pulls 90kw from their 24 kwh battery)
battery (open circuit) 336V
charge time 6 hr @ 240 VAC, 15A
wheelbase 124 in
weight 5,300 lbs (Karma spec) Yikes!
Beyond 50 mph the car operates as a "normal hybrid vehicle". Car has two modes of operation: stealth mode (all electric mode, 7.5 sec 0 to 60, 95 mph max) and sports mode (electric and engine power, 5.8 sec 0 to 60, 125 mph max). (The two switchable modes makes sense to me, it fits the technology.) There is no transmission. Fisker engineer confirms that the max battery power (200 kw) is not high enough to fully load the motors. Battery power alone is enough to do 7.5 sec 0 to 60 mph, but faster (to meet the 5.8 sec spec) engine power is added to battery power to provide the 300 kw for the motors. Engine is turbo charged to get high hp for given weight, fuel efficiency is secondary. One poster says EPA city favors electric vehicles, in real world driving at higher speed, so says electric range will not be 50 miles but more like 30 miles.
check
wind resistance 27 hp (20 kw) @ 70 mph
battery power 24 kw = 20 kw x 1.20 efficiency + tires
use 1/2 x 22.6 kwh 11.3 kwh = 24 kw x 0.47 hr (28.2 min)
distance @ 70 mph 32.9 miles = 28.2 min x 1.167 m/min
88k (now 95k) Fisker Karma quasi-electric (prototype)
shown at Detroit Auto Show winter 2009
(124 in wheelbase, 4.650 lbs, huge 22 inch wheels)
New electric car companiesSlow charging
When the reviewer talked to Fisker engineers about the noise from the engine exhaust, he says "they just rolled their eyes". This smack of a new startup company not having the time, talent and/or money to build an adequate number of prototypes. Even though they have 400 engineers the task of engineering a new car I suspect is staggering. The number of parts to be designed and made is huge and unlike established car companies it's not just a matter of the old hands doing a tweak on a part they have reliably built for years.
No way to fast charge this thing. Unlike the Leaf which has a J1772 (240V, 12.5A) plug and a high current DC plug, picture of the charge port shows it has only the J1772 plug. Like the Leaf the battery runs down the spine of the car in a high console, vs Tesla which is going to put a much larger battery flat under the car. For 100k all you get is 120 VAC charge cord! 240 charger and cord is an option!PM Motors
Transmission speed ratio is 4. This thing has huge 22" tires. If 22" is radius (maybe not, tires are weird, a 16" tire appears to have a radius of 12") and 60 mph is 88 ft/sec, I figure motor RPM @ 60 mph to be 1,634 RPM. At 125 mph top speed this is 3,821 rpm (maybe closer to 5,000 rpm if tire radius is only .75 x 22") Much, much slower than the 15,000 rpm of the coming Tesla sedan. This is going to make the motor much heavier. Lets check. Each motor is rated at 650 NM (490 ft-lb) or 1,300 NM for the pair. Calculating the rectangular power corner:
angular speed = Power/ Torque
angular speed = 300,000 kw/1300 NM
= 231 rad/sec (36.7 rev/sec or 2,205 rpm)
Reasonable, combined with my rpm calculation from tire radius and transmission ratio, it puts the rectangular power corner at 2,205/1,634 x 60 mph = 81 mph. Since these are PM motors they are difficult to field weaken, so the rectangular corner needs to be high.
Motor conclusion
Unlike the Tesla which uses a single super high speed (15,000 rpm) motor. This car uses two large, heavy, liquid cooled, low speed (3,800 rpm @ 125 mph), high torque, expensive PM motors. I know something about PM motors, and I know Powertec makes large PM motors, and sure enough I found they have a motor quite close to the Fisker specs.
Matching PM motor
Fisker specs each motor at 200 hp, 500 ft-lb (approx) torque, and I calculate the max speed is 3,800 rpm (@ 125 mph). Below is a page from the Powertec catalog. Check out the E259-E2 motor. It's a 196 hp, 3,600 rpm, 475 ft-lb PM motor. An excellent match. Elsewhere in the spec the size of the E259 frame is given as 11.5" dia and 36" long.
196 hp, 3,600 rpm, 644 Nm (475 ft-lb) PM motor
in Frame E259 (top row), E2 electrical specs (bottom right)
source --- http://www.powertecmotors.com/e250.pdf
Look at the weight of the E259 frame motor, it's 515 lbs! This may explain the mystery of why the Fisker Karma weighs 5,300 lbs. Two of these motors weigh 1,000 lbs, The battery reportedly only weights 600 lbs. Throw in the weight of the controller and the weight of the 260 hp gasoline motor and you've got a 2,000 to 2,500 lb drive train!
-----------------------------------------
There was little technical info on the Fisker site and nothing on the Quantum site, but I found the links below which have some additional Karma info.
http://www.thetruthaboutcars.com/fisker-karma-brochurespecs-stealth-eco-chic-solar-power/
http://auto.howstuffworks.com/fuel-efficiency/hybrid-technology/fisker-karma5.htm
Fisker could have a lot of company. Plug-in-America (below) has a list (with pictures and status) of nearly 50 hybrid and electric cars in the prototype stage.
http://www.pluginamerica.org/plug-in-vehicle-tracker.html
Tesla Motors lithium ion pure electric cars
Tesla 2 seat roadster
There is a lithium ion powered car on the market now, sort of (few hundred on road and production of a few hundred/yr). It's an all electric car, the 100k Tesla Motors 2 seater sports car, 220 mile range, then plug in (at home) to recharge (3hr to 37 hr).
Battery 6,800 lithium-ion cells
Off shelf #118650 (18 mm x 65 mm) cylindrical rechargeable 'protected'
lithium ion battery (w/PTC built in)
375 V (open circuit) (supposition)
battery pack weighs 992 lbs
must be massive paralleling (100 cells in series, 65 in parallel)
(assume 3.6V/cell 65 = 6,800 x 3.6V/375V)
For 375V battery pack voltage => 65 parallel strings of 104 cells/string!
(update, comments from Tesla seem to indicate the battery may be divided
into 11 modules horizontally. Meaning 11 (or maybe 10 used for motor)
36V modules made up of 10 cells and 66 (or 60) in parallel)
3.5 hr recharge (high power 480V charger), 10 hr, 240 VAC,
37 hr 120 VAC
Battery lifetime projection is 500 to 700 deep discharge cycles
battery pack is liquid cooled
Battery kwh 56 kwh (37 hr @ 120 VAC, 1.5kw = 55 kwh est)
(56 kwh looks reasonable for 220 mile range, its x3.5 more kwh than
GM Volt)
Tesla's use of small commodity lithium ion batteries achieves a much
higher energy density (150%) than Volt or Leaf. The Leaf is 24 kwh
at 660 lbs (Volt about 400 lbs for 16 Kwh), but the Tesla roaster's
56 kwh battery at 990 lbs weigh only about 2/3 of what it would
weigh if Volt or Leaf type lithium batteries were used.
Power out 185 kw (250 hp) at 8,000 rpm (T vs Speed peak)
281V x 658A = 185kw (guess --- Voltage sag to 25% @ max pwr, like Prius)
History Tesla had massive redesign in 07/08 to remove unreliable two speed
transmission and forced out CEO. To restore high torque for 4 sec
0 to 60 motor current rating of motor and inverter (at low speed) was
increased from 640A to 850A. This required higher current IGBT's,
terminals, and wiring. (Poster says (bus) voltage was decreased at low
speed, but don't know what this means. Do they reconfigure
battery pack at low speed, or is he just talking about voltage sag.)
Inverter 200 kw
(aka PEM) 72 IGBT's Yikes! for inverter and charger (Tesla marketing)
motor rated at 375V, 185 kw the currents are really high (500 A to 600A)
assume 6 igbt's for inverter that's 11 x 6 = 66
Do they have 11 inverters in parallel (!), each with 150A, 600V IGBT's ?
They might! The battery pack is divided into 11 components.
Redesigned for 850A max (@ low speed)
Motor 375 V induction motor (air cooled)
14,000 rpm ! For 380 Nm motor Pmax speed = 4,650 rpm
(Pmax at 185,000 watt/380 Nm x (1/2pi) x 60 = 4,650 rpm)
4 pole (2 pole pair), 100 lbs ("size of a large watermelon")
(my guess is the custom motor has 11 separate windings, with 33 terminals,
to effectively parallel currents from the 11 inverters)
Powertrain 1.5 (one speed gear box) motor in prototype car runs at 850A
(280 ft-lb/380 Nm) (may 2008)
Redesigned for 850A max (low speed torque, 280 ft-lb/380 Nm)
Tesla induction motor
(In a puff piece in MIT's Technology Review about the lead electronic
designer of Tesla Motors (JB Straubel, age 33) it says Tesla began with
an induction controller they licensed from Alan Cocconi of AC
Propulsion. It was all 'analog' says Straubel, meaning not DSP based, and
gave them a lot of trouble, "no two were alike", it faulted on highway. The
implication being Cocconi's cirucit design was poor. Tesla replaced it
with a DSP based design of their own.)
I can confirm that Cocconi's induction motor controller design was/is
(probably) out of date. I read an induction control patent of his filed
in 1998, and it looked like the kind of discrete logic design I was
doing in 1978 (VCO as slip generator) twenty years earlier and
prior to DSPs.
Gearbox single speed gearbox (one fixed ratio forward, + reverse)
(earlier two speed, but it was unreliable)
Dimensions 92.6 in wheelbase (tiny)
2,720 lbs
2 seat
Performance 0 to 60 in ? 4 sec
Top speed 125 mph
Range 220 miles
Don't know the manuf of the Tesla battery, but its easy to find data sheets for #118650 (18 mm x 65 mm) cylindrical rechargeable 'protected' ithium ion battery (w/PTC built in).
Typ rating : 3.7V, 1.8 to 3.0 Ahr, (0.12 to 0.18) ohm (includes PTC), 46 grams
Energy storage --- 350V (7% sag) x 2.4Ahr x 66 parallel paths = 55 kwh (OK agrees with spec)
Iss = 3.7V/0.18 ohm = 20A. Will deliver 10A peak but this is matched load so sag is bad (50%)
ESR of 0.12 would reduce sag to 30%
Nobody seems to know the cell being used, but a candiate mentioned (I found it too) is
Sony 18650G8
Weight --- 6,800 cells x 46 gm/cell = 700 lbs (OK, about 1,000 lbs with structure)
My summary of Tesla electronics
Found a short technical article that confirmed my inverter guess. Tesla battery and inverter is massively paralleled. There are (likely identical) 11 battery modules eachwith their own inverter. The battery pack(s) voltage is 375 V formed from about 100 (3.6 to 3.75V) cells in series. The battery pack protections are very elaborate (? likely costly) with protections on either individual cells (or groups of six cells) consisting of a series PTC (positive temp coefficient resistor) and fuses. Some of this protection (like PTC) is built into the cell by the cell manuf. There is also extensive electrical and thermal monitoring.
"The commodity cells Tesla uses in its pack all have an internal positive temperature coefficient (PTC) current limiting device, which limits short circuit current on an individual cell level. The device is completely passive. The cells also have an internal Current Interrupt Device (CID) that will break and electrically disconnect the cell in the event of excessive internal pressure caused by over-temperature or other failures resulting in over-temperature." (Green Car Congress 22 May 2007)With something like 66 parallel paths (66 = 11 modules x 6 parallel cell/module) only about 10A peak is need from each cell to supply 200 kw = [375V x 0.8 sag (guess) x 66 paths x 10A peak]. Compare this to the 180A peak the Prius needs from its single series stack of cells. The massive paralleling of the battery pack leads to three advantages. 1) No boost converter (battery sag acceptable)
375V battery (maybe 20% sag) => inverter (600 V IGBT's => 375 V motor
reported 850A pk means serious battery voltage sag (to near 200V)
2) Series resistive protection can be used (in form of PTC's and fuses)
The massive paralleling of the inverters leads to advantages too. Robust in the field. If an inverter blows in the field and can be isolated, it would hardly be noticed as car still has 10/11 of it full power and range. Big advantage thermally too. Lots of heat has to be dissipated when 660 A are being switched, and this problem is vastly simplified if the source of the heat (IGBT's) are physically far apart. Components for each design are relatively cheap due to high production. Testing is simplified and vehicle repair is simplified.
The use of 11 (apparently) separate inverters (6 x 11 = 66 IGBT's @ 600V ) makes each converter easier to design and to cool. The peak current in each IGBT is about 60A (figured as each inverter peak power is 200kw/11 = 18kw = 300V (with sag ) x 60A). So the inverter are likely conventional using 100A (or 159A) 600V IGBT's.
but, but .... there is the far from trivial problem of combining the current from 11 inverters. This is actually quite tricky and I have seen no clue as to how they did this. The trick is to do this in a reliable robust way. There are ways to parallel inverter outputs with sharing inductors, but the likelihood that the battery voltage of the modules would be different makes this approach daunting.
My guess is the paralleling is done in the motor with parallel windings, a separate winding for each inverter! They say in their marketing material that the motor is custom. Two parallel motor windings are not unheard of, I worked with (? tested) large PM motors wound this way myself. Eleven windings is pretty extreme, but it would be a robust way to parallel the inverters. The major complication is the wiring coming out of the motor. Instead of the usual 3 (or 9 terminals) there are (at least) 33(= 11 x 3) terminals.
Bottom line --- If I'm right that they use eleven identical 100A, 600V IGBT inverters, physically separate, connected to electrically separate battery packs, driving 11 electrical separate windings in a custom 33 terminal induction motor, then this looks to be a pretty rugged, clean (and of course, expensive) package.
update --- Info from Tesla motors seems to indicate the battery division into eleven modules is not what I guessed, that it apparently is divided horizontally not vertically, meaning the 660 cells of each modules are arranged 10 series and 66 parallel outputting 36V. This makes it much less clear as to how the 11 inverters (if there are 11 inverters) are arranged and paralleled.
Tesla Model S sedan -- 300 miles range
(Update 12/12/11)
For 79k Tesla is licking the range problem in electric cars with a monster 85 kwh battery pack. They claim 300 mile range. The first 1,000 build will have this battery. For marketing purposes they advertise a 20k cheaper version (59k), where they only put in a half size (160 mile range) battery pack.
Model S is coming along, fully speced and priced, with deliveries starting July 2012. Tesla says all 5,000 they can build in 2012 have been pre-sold. From a couple of video I have seen this car looks impressive. The massive battery pack 85 kwh is a flat thin (single) layer of little cylindrical cells standing upright covering the full bottom of the car. This allows the inside of the car to be very open so they have five normal seats and two backward facing child seats. The battery pack is liquid cooled (like Volt). One reviewer said the spacious interior of the Model S is huge compared to the cramped Fisker Karma interior.
Tesla Model S sedan massive 85 khw battery pack
in a single layer of 8,000 cylinderical cells under car
source --- http://www.teslamotors.com/models/features#/safety
Model S sedan toque vs speed
304 kw (410 hp) @ 7,000 rpm corner
source ---Tesla web site
Tesla is now designing an all electric 58k sedan (Model S) (seats 7) with 300 mile range for production in 2011/12. This car is premised on an expected improvment in Li-ion battery capacity of 8% year. Tesla will use the same small lithium ion cells for the Model S that it uses in the Roadster. The Roadster battery pack has 150% higher energy density than the batteries in Leaf or Volt. The Model S will be offered with three different size batter pack: 160, 230, 300 miles.
Motor 9 in !! (liquid cooled, induction)
415 Nm, torque flat to 7,000 rpm (const hp 7k to 15k)
[corner 415 nm x 733 rad/sec = 304 kw (410 hp)]
0 to 16,000 rmp (130 mph)
Battery 8,000 lithium-ion cells (300 mile range option)
Off shelf #18650 (18 mm x 65 mm) cylindrical, 1.6 oz, 3,000 mah, 3.7V (10.8 whr)
rechargeable 'protected' lithium ion battery (w/PTC built in)?, liquid cooled
Packaged in a single layer (upright) that covers the whole bottom of the car
Battery (kwh) 42 kwh ('standard', 160 mile)
65 kwh (230 mile option)
85 kwh (300 mile option)
Battery weight 1,200 lb (300 mile option?)
Recharge 4 hr from 220 VAC
45 min Quick Charge @ 440 VAC (Leaf can be 80% quick charged in 30 min)
Performance 130 mph (top speed)
5.6 sec (0-60 mph)
Transmission single speed (none)
Wheelbase 116.5 in
Weight 3,825 lbs (4,200 lbs with big battery)
Wow, 0 to 120 mph (upgraded to 130 mph) with no gearing. Like the motor in the Tesla Roadster (14,000 rpm) this motor is really going to wind up. (Leaf tops out at 90 mph and I think its motor is then probably at 9,000 rpm)
I see the Model S motor is liquid cooled. I wrote in summer of 2009 that Testa made a mistake in not liquid cooling the motor/inverter in the Roadster as users reported that just a few minutes on a test track produced OT warnings and forced cut backs in performance.
Tesla Model S sedan (left, 116 in wheelbase)
Tesla Roadster (right, 93 in wheelbase)
A Washington Post article discusses the efforts of a CA town to woo Tesla to build the assembly plant for the 2011/12 electric Model S sedan $57,400. It mentions also the state and federal tax dollars that have gone to Tesla.
* City (Downey CA) is waiving 6.9 million in rent on 20 acres (former NASA plant)
* CA is waiving sales tax (7% to 9%) on equipment bought for the new sedan plant
(to keep the plant in CA)
* 465 million in low interest loans from Dept of Energy
@ 1,000 cars => 465,000/car
@ 10,000 cars => 46,500/car
@ 100,000 cars => 4,650/car
* Projected car price of 50K is after a federal tax credit for battery powered cars
---------------------------------------------
Comparing electric car motors --- Fisker Karma vs Tesla Model S (Jan 3, 2012)
A Jan 2012 email to friends (also engineers) comparing the car electric motors including a head to head comparison of the Fisker Karma with the new Tesla Model S sedan.
I have been following electric cars closely and there are now quite a few models either in production or quite close. Nearly everyone is using PM motors: Nissan Leaf, Ford focus electric, Coda electric, Fisker Karma electric, and even the GM volt hybrid. It was suspected GM was using induction, but in a technical paper about the car by their engineers both the motor and generator (also used as a motor) are revealed to be PM. The exception is Tesla. Tesla went induction in the existing roadster electric and will go induction in their new model S sedan. The most interesting difference is between the two high speed (125 mph), high power (268 to 300 kw) models: Fisker Karma (designed in US, made in Finland, just started production) and the Tesla Model S sedan (production summer 2012).Don
Tesla's Model S induction motor goes up to 15,000 rpm, wide constant hp range, single motor about 9" dia, foot long. In contrast Fisker PM motor(s) only go to about 4,700 rpm (with maybe a little field weakening). Since the motor speed and gearing is so low, they need huge motors to get good low speed torque. Their motor is 200 hp PM. Powertec makes a 200 hp, 3,600 rpm PM motor, and it weighs 500 lbs (11" dia and about 30" long).
Fisker has two 200 hp motors! That's a 1,000 lbs of motors, plus 600 lb battery, plus a 260 hp engine (bought from GM), plus a 150 kw generator, plus two 500A controller (bus voltage is only 300V for a 300 kw plant). The power train is probably 2,500 lbs, no wonder this low car weighs 5,300 lbs, and its acceleration is not good even with all this power. The other electric cars run their PM motors to intermediate speeds (7,500 to 10,000 rpm) and a single motor is enough for their modest top speeds and acceleration.
Fisker, like the Volt, runs the battery down the center console of the car making the inside of the car cramped. Their battery (from A123 Systems in MA) can only deliver 500A peak, so when power between 150 kw and 300 kw is needed the engine/generator has to run too. Reviewers are appalled that when the engine starts the car goes from quiet to incredibly noisy.
In contrast Tesla has figured out how to package a massive 85 kwh (Leaf is 24 kwh) as a single thin layer under car (see below). The result is 300 mile electric range and spacious interior, inside is so open they put two jump seats in back and call it a 7 passenger car.
Tesla with an experienced design team in place and field data from the roadster has done almost everything right. In contrast Fisker assembling a design team from scratch with limited time and money for prototypes has done nearly everything wrong. US taxpayer has pretty much paid for the development of both cars. Fisker has a 529 million US Energy loan, and I predict a good chance of being the next Solyndra.
http://www.teslamotors.com/modelsNissan Leaf electric (update 11/22/10)
http://www.fiskerautomotive.com/en-us
"So begins the age of the mass market electric vehicles", says a 12/13/10 article announcing the first Leaf delivery in USA to an ordinary individual customer. The first Leaf customer reports on his milage as follows:
-- He said the car's mileage varies wildly. Chalouhi said he can get 100 miles per charge in slow city driving, but only 50 or 60 miles at 75 mph on the freeway. (This is in temperate San Francisco. 50 miles at 75 mph means a full battery charge is depleted in 40 minutes.) -- An experienced electric car owner says the Leaf dashboard has a huge flaw: it does not report(11/22/10) Leaf is a month away from sales and they are playing the same game as Volt, withholding a bunch of specs. EPA has assigned the dollar cost to fuel the car at an assumed electric cost of 12 cent/kwh. At 0.12/kwh it takes 2.30 worth of electricity to charge the battery [2.30 = 0.12/kwh x 80% x 24kwh battery]. Nissan released several detailed milage scenarios with ranges from 62 miles (cold day with heater on, low speed stop and go) to 138 miles. Nissan released range figures only up to 55 mph. At real highways speed the range numbers are going to badly roll off. My baseline wind resistnace drain is about 10 kw @ 55 mph, but this doubles to 20 kw at 70 mph, and triples to 30 kw at 80 mph. There is only 19 kwh available, so range @ 80 mph could be half @ 55 mph. This is a slow speed, right lane car!
the remaining charge in the battery! Instead Leaf trys to estimate the remaining miles, but this
number jumps erratically, dropping way down when going up a hill and then popping up again when going down the other side. He says fully charged it first reports 93 miles, then switching to econ
mode it says 102 miles. Within the first 15 miles he sees 102, 84, 66, 82 and 78.
-- Reviewer for Slate reports the Leaf drives very well and feels much more like 'real' car than the Prius. Inside it is very quiet.
A second performance problem (?) is that with only 80 to 90 kw available and a 3,500 lb car I am calculating 0 to 60 acceleration times of 12.6 seconds (ignoring wind resistance)!! GM Volt is a little heavier (3,800 lb), but I think they have something like 141 kw available using combined torque (via planetary gears) from the motor and generator, so are getting 8.8 sec (0 to 60).
But Wikipedia reports 0 to 60 has been tested at 7 sec, so the conclusion has to be the peak power available has to be far higher than 90 kw, more like 160 kw. (Nope, now I think the answer is a T vs Speed profile with a large constant hp range. see next)
(update 12/2/10) Strong Torque vs Speed roll off needed for good acceleration
The assumption of a (simple) triangular power curve for electric cars that peaks at or near its top speed I now think is wrong, because it leads to poor 0 to 60 mph acceleration. When you have a limited power gear train, to get good acceleration the power has got to max out at some moderate speed (like 30 mph). This means either a transmission or a motor T vs Speed profile that rolls way off, giving an approx constant HP range.
More on Leaf acceleration
I read (in 2009 article) that the Leaf will have no transmission, like the Tesla electric. The Volt when running on its two motors effectively does have a (variable) speed transmission (but this may only be at high speed). The Pmax point of the Leaf motor can be calulated from the sped (280 nm, 90 kw) and it comes out at a low speed indicating there is likely a strong T vs Speed roll off over much of the speed range of the car.
90,000 watts/280 Nm = 321 rad/sec = 51.2 rev/sec = 3,070 rpm
So my guess is that the Leaf has numbers something like this:
Linear torque range 0 to 3,000 rpm (0 to 30 mph)
quasi constant hp range (@ 80 kw) 3,000 to 9,000 rmp (30 to 90 mph)
max motor speed (@ 90 mph) 9,000 rpm
Torque will now be constant to 30 mph and slowly roll off (1/speed) as power is held roughly constant from 30 to 90 mph. This wide quasi-hp range implies an induction motor or else a PM motor (like Prius) with high inductance, so it can be field weakened.
(bot) my sketch showing what I think is the Leaf's Torque vs Speed profile (12/2/10)
(top) 'rectangular Torque vs Speed used in servos
Linear range 1/2 x 80 kw x 4.45 sec = 178 kj
Const hp range 80 kw x 5.55 sec = 444 kj
---------------------------
total 622 kj
Combined this is 622 kj, or about 9% higher than 572 kj calculated (below) for the 3,500 lb Leaf (3,366 curb plus driver) at 60 mph, so 9 to 10 sec foor 0-60 mph looks reasonable with the above T vs Speed shape. Or add a passenger and lose a few percent to wind and 10 sec for 0-60 rpm looks about right.
0-30 mph accel time (1/4/11)
But Car and Driver test sheet also shows 0-30 mph at 3.2 sec, which means 30-60 mph took 6.8 sec implying less than half the peak torque meaning a much faster T vs Speed roll off than constant HP. Here is a check of the 0-30 mph. The kinetic energy is 1/4th of 60 mph or [572kj/4 = 143kj]. Doing a triangular accel time with peak power (80 kw) at 30 mph.
0-30 mph 2 x 143kj/80kw (motor) = 3.6 sec
3.6 sec is pretty close to the 3.2 sec Car and Driver reports, so I now suspect the T vs Speed corner at 30 mph is about right, but that torque rolls off faster than my (idealized) curve (above) shows, and this is reasonable when motor losses and wind losses are figured in.
Checking the Leaf motor max rpm from specs (update 6/2011)
Leaf spec: "16 wheels"
Tires: P205/55R16 89H
Single speed gear reducer: x7.94
Max motor speed: 9,800 rpm
Max car speed: 90 mph
I first thought 16" wheel mean a tire dia " tires, but this worked out to be 10,000 rpm @ 60 mph, or 15,000 rpm max @ 90 mph
16" wheel (dia) => 1,261 rev/mile ?=> 1,261 rpm axle @ 60 mph
1,261 rpm x 7.94 = 10,000 rpm motor @ 60 mph (or 15,000 rpm @ 90 mph)
But looking up the tire spec I find from Firestone below:
Size: P205/55R16 89H, Overall Tire Diameter, 24.90. Revolutions per Mile, 837
Ok, more reasonable. 16"/24.9" = 0.643 x 1,261 rev/mile = 810, but the tire spec says 837 rev/mile so something is a little off (see below). I found a max motor speed on a tech car blog of 9,800 rpm. Figuring the axle speed from motor max speed gives 9,800/7.94 = 1,234 rpm axle @ 90 mph. At 60 mph this would be x0.6666 x 1,234 = 822.6 rev/mile. Good this is just about the average of 810 figured from tire dia spec and 837 tire spec. All the numbers are now close, so it looks like
Leaf electric motor top speed (@ 90 mph): 9,800 rpm +/- 1% (figured correctly, using the tire rev/mile spec
the top motor speed is 9,969 rpm (see below))
Straight skinny on tire sizes (update 1/10/12)
Somehow, not unreasonably, I alway thought that a tire referred to, say, as a 16" or 17" tire meant that was its dia or radius, but nope, this is not how tires are sized. Turns out that the reference tire inch number (16", 17", etc) is an inside dia ('dia of the wheel'). To get the tire dia the two side walls must be added (which can be figured from the other tire numbers). But this still does not give the correct rotation rate, because tires when loaded sqush a little. The operating radius is less than the radius of an inflated, unmounted tire, it comes from the measured 'Revolutions per Mile' spec.
For example
Size: P205/55R16 89H, Overall Tire Diameter, 24.90. Revolutions per Mile, 837
Distance per rotation (effective circumference) = 5,280 feet/837
= 6.308 ft (74.699")
Effective dia = 74.699/pi
= 24.096" Ok (3.3% less than 24.90" unmounted)
Refiguring the Leaf top motor speed
At 60 mph one mile is comvered in one minute, so from the tire rotation spec (837 rotations/mile) the tire/axel is making 837 rotations per min @ 60 mph, or 1,255.5 rev/min @ 90 mph (top speed). Multiply this by the spec gearing ratio (x7.94), and we get the max motor speed at 90 mph is [7.94 x 1.255.5 rev/min] = 9,969 rpm
Rule of thumb
The effective radius is just about 75% of the tire inch number (12/16 = .75), so it's a good guess that this ratio probably is pretty close for most car tires, as most car tires seem to have about the same ratio of wheel to side wall.
--------------------------------------
(update 3/30/10) Big news, finally an electric car has a price
Nissan announced this week the Leaf will be priced at $32,780 in US. That means when the $7,500 federal tax credit is subtracted, the car's base price would be $25,280. I read that CA offers an additional tax credit of $5,000, which reduces customer price in CA to $20,228. Expected volume for 2011 is 50k vehicles (high).
No price yet from GM for the GM Volt. While the Volt has an engine and generator too, its battery pack is 8 kwh smaller (16 kwh vs 24 kwh). People have been estimating (guestimating) that the lithium ion battery packs manufacturing cost about 1k/kwh. I bet 8k easily pays for GM's engine and generator, which would make GM's drive train costs probably comparable to, or a little less, than the Leaf.
---------------------
Leaf specs
Nissan is making big noises (Summer 09) about their electric car, Leaf, planned for late 2010 about same as GM Volt. Leaf looks pretty similar to Volt (maybe little smaller) but the Leaf is a pure electric car vs the Volt, which is an electric car with a range extender. This may be the first all electric car mass marketed (outside China). The plan is to build it in Japan and a Nissan plant in US. Little technical info released yet. In Jan 2010 US Dept of Energy says it it loaning Nissan 1.4 billion dollars to upgrade their Tenn plant to make the leaf. (US taxpayer with huge loans, grants, and tax credits is really subsidizing the electric car development and early production.)
Battery 24 kwh (lithium ion) Nissan lithium voltage is 3.6V nom
(19 kwh or 80% usable range_
48 modules (w/4 cells/module)
(my guess is 2 cells in parallel for 346V bus = 96 x 3.6V)
360V is nominal battery voltage (1/4/11 update)
DC/DC steps down 360V to 12V for lights, etc.
660 lbs (battery pack, under vehicle)
Range 100 miles (65 to 75 miles realistically, see below)
EPA has measured range at 73 miles (conditions?)
Nissan now says 62 to 138 miles
62 miles --- 14 F (heater on), stop-and-go, traffic jam, 15 mph av
70 miles --- 95 F (A/C on), 55 mph highway (slow highway!)
105 miles --- 72 F (A/C, heater off), commuting, 24 mph av (slow)
(With baseline wind resistance and usable 19 kwh usable capacity
ideal max highway miles vs speed can be calculated)
55 mph x (19 kwh/ 9.7 kw) = 108 miles
60 mph x (19 kwh/ 12.6 kw) = 90 miles
70 mph x (19 kwh/ 20.0 kw) = 67 miles
80 mph x (19 kwh/ 29.6 kw) = 51 miles
90 mph x (19 kwh/ 42.2 kw) = 41 miles (27 min @ top speed)
(Two Tesla customers by driving 55 mph were able to drive
their new Leafs from dealer to home 111 miles. Not really,
article later reveals they panicked and stopped along the way
for a 90 min boost charge (worth 19 miles)
Power out 90 kw (battery)
Motor 80 kw/280 Nm (AC synchronous) (identified as PM motor in one spec)
For 280 Nm motor Pmax speed = 3,080 rpm
(Pmax at 90,000 watt/280 Nm x (1/2pi) x 60 = 3,080 rpm)
(In catalogues 75 hp, 3,600 rpm (induction) motor weighs about 760 lbs)
9,800 rpm @ 90 mph (max) spec (consistent with x7.94 gearing and 24.9" tire dia)
Wheelbase 106.3 inches
Max speed 90 mph
Vehicle weight 3,500 lbs/1,590 kg (Nissan est)
3,366 lbs ('final spec')
Transmission Single speed reducer (1/7.94) (roughly 6,500 rpm motor at 60 mph)
Accel (0 to 60) kinetic energy @ 60 mph (26.8 m/sec) = [(1/2) 1,590 kg (26.8 m/sec)^2] = 572 kj
Rectangular power 572 kj/80 kw = 7.15 sec
Triangular accel time = 14.3 sec
(It can't be this much of pig, can it?) Nov 2011 blog says 0-62 mph is 12 sec
(unofficially tested at 7 sec??)
10.0 sec (tested by Car and Driver magazine)
Onboard charger 3.3 kw built in charger supports
level 1 120 VAC @ 1.4 kw, 20 hr
level 2 240 VAC @ 3.2 kw, 8 hr (Why only 3.2kw?)
(Looks like big mistake here. A 240VAC, 30A 1ph charger
should be able to do 5-6 kw and charge battery in 3-4 hrs, which
is exactly what Ford Focus (w/23 kwh) electric is claiming.)
2nd plug supports external level 3 charger, 480 VAC @ 50 kw,
30 min for 80% charge (causes some additional loss of battery
lifetime if used regularly.) Cost is listed at an absurd 16k!
Battery manuf Nissan (joint venture with NEC)
------------------------
Battery comparison with GM Volt (1/4/11)
New info on battery voltage curiously indicated that GM Volt and Leaf both seriesing the same number of cells [96 (3.6V nom) batteries in series for a 345V bus]. GM has cells three in parallel (total of 288 cells), whereas the Leaf has two cells in parallel (total of 192 cells). Since Leaf has 2/3rd the cells of Volt and 3/2 the kwh, that must mean each Leaf cell is 225% larger than Volt cells. Leaf cells must have 35 Ah rating (= 20,000 kwh/192 cells) vs 15.5 Ahr for Volt cells.
Nissan car manual shows there is a dash display of 'Battery temperature', and say (car) power is reduced to protect battery if battery temperature is 'too low or too high'.
To power the car on of off you need to step on the brake. That's right to power off the car you need to step on the brake! Emergency power off (while driving): hold 'power' button for two seconds, or push rapidly three times.
------------------------
The Nissan battery kwh is 150% that of the GM Volt (24 kwh vs 16 kwh) and like the Volt is mounted under the floor of the car. So how does that square with a claimed milage of 100 miles vs 40 miles (electric) for the Volt. Here's one way to look at it. If 40 miles uses 8 kwh in Volt, then 100 miles in Leaf would need 20 kwh. The Leaf has a 24 kwh battery, so this works out if the Leaf lifetime battery derating is only 17% = [4kwh/24kwh] vs the 50% derating GM is planning.
It just occured to me this may give GM an ace in the hole. If lifetime data on the lithium ion batteries begin to looks good, or battery lifetimes improve in a year or two, then GM with a simple code change can just rerate the Volt for higher electric range (40 => 50, 60 or 70 miles).
Nissan Leaf (2010) pure electric, 100 miles?
(weird bulging headlights are claimed to "split the air" reducing the drag of mirrors)
Unlike the Volt which has an elaborate liquid-cooled heater/cooler for the battery (? uses the engine to avoid using the battery hard when it is hot or too cold), the Leaf battery is air cooled using only a single fan!
The flat battery profile allows the Leaf to have a three seat bench in back. In contrast the hump in the Volt battery divides the back seat in half, so Volt is a four passenger car vs five passengers for Leaf. In an article by a Tesla battery guy he says Nissan experimented with active thermal management for the battery, but it required a hump in the center which would have made it a four seater. He argues that Nissan has under-engineered the battery thermal system, so there is risk it will degrade more than Nissan claims (80% capacity after ten years says Nissan, whereas GM says even with their babying they expect 75-80% capacity after 8 (or 10) years.
CEO of Tesla (Musk) on Leaf battery
-- Tesla uses what Musk described as active liquid thermal control, while the LEAF pack uses an air cooling system. As a result, the LEAF pack will have temperatures “all over the place,” causing it to suffer “huge degradation” in cold environments and basically “shut off” in hot environments, claimed Musk.
Nissan Leaf 24 kwh, 660 lb lithium manganese oxide battery
Air cooled with only a single fan
GM with the Volt has been conservative with the battery, not only thermally controlling the temperature, but is also using the engine to protect during thermal extremes. In addition capacity loss of the battery will only moderately affect it, as all that happens is the engine comes on more quickly. On the other hand Leaf has no motor and a crappy single fan to air cool the battery pack (and apparently no heater to warm it). Nissan is taking a big risk that the Leaf battery pack is not going to prematurely degrade, or maybe it's hoping that the early adopters won't care. If the battery pack lose much capacity, the car will be near useless until its super expensive battery pack is replaced.Leaf 99 mpg EPA sticker
There was a lot of debate about how the EPA would rate the milage of electric cars. What they have done (see sticker below) is equate the electrical battery drain (in kwh or joules) for the car to travel a mile to an amount of gasoline that has the same energy content. I checked standard conversion tables and 33.7 kwh/gal is what I find, so that must mean the EPA standard this is not the energy gasoline releases when burned in a combusion engine, but the total heat energy it released when burned (oxidized) in oxygen at normal 1 atm pressue.
1 gal gasoline ?==> 34 kwh (really 33.7 kwh) (122 Mjoule)
In round numbers a 33 mpg car burns 3% of a gallon of gasoline to go one mile. An electric car like the leaf @ 99 mpg (eq) uses about 0.34 kwh (1.2 Mjoule), which is 1% of the energy in a gallon of gasoline.
Here's the first EPA sticker for an electric car (Nissan Leaf) from I think Nov 2010.
(early version? released by Nissan Nov 2010)
When I look closely at the numbers on the sticker, I see all kinds of problems.
For a starter how can it take 7 hours to recharge the battery at 240 VAC? Standard 120 VAC, 15A circuit can supply 1.5 kw, so 240 VAC @ 30A should be able to supply x4 (twice voltage, twice current) or 6 kwh. It should be possble to recharge fully in 3-4 hours, which is just what Ford Focus electric (23 kwh battery) is saying. What did they do design a half wave charger! (Maybe they did, 7 hr is the Leaf spec. Their built-in charger has only a 3.1kw rating.)
If the car can go 73 miles (presumably at the combined city/highway 99 mpg), then it must have used
(73m/99 mpg) x 34 khw/g = 25 kwh
What? This car has a 24 kwh battery and the 73 miles appears to be a realistic range, and the usable kwh can't be more than 80% (19.2 kwh) of capacity, can it? Or if I figure another way it looks like maybe the math is wrong, that they used 11 cents per kwh, not the 12 cents it says in the footnote.
EV-1 specs
For comparison with the Tesla electric here's info on the first mass produced car (about 1,100 built), the GM EV-1 about a decade ago (manuf 1996 to 1999).
Battery 26 (12V) lead acid (standard)
312 volt (26 x 12 = 312V)
18.7 kwh (60 amp-hour x 312V = 18.7 kwh)
1,310 lbs
55 to 95 mile range
26 NiMH (option)
343 volt (26 x 13.2 = 343V)
26.4 kwh (77 amp-hour x 343V = 26.4 kwh)
1,147 lbs
75 to 130 mile range
Recharge 6 hr @ 220 VAC (6.6 kw)
23 hr @ 110 VAC (1.2 kw)
inductive (Magne Charge) with printed circuit board primary
Motor 102 kw (137 hp) induction motor
150 Nm @ 0 to 7,000 rpm
[(7,000rpm/60) x 2 pi x 150 nm = 110 kw]
Inverter IGBT inverter
Weight 3,086 lb (w/lead acid batteries)
0 to 60 mph 8 sec
Estimate of battery current at peak power
Matched load current = 872A (est) Assume GM at peak power loads battery like Prius (x3 matched load) or 1/2 matched load current. 312 V x 0.75 (sag) x 436A= 102 kw (motor rating). (Confirm: found a 40 Ah 12V lead acid vehicle battery with ESR = 8 mohm, which is a matched load current of 750 to 800 A.)
BMW Mini-Cooper electric proto (3/28/10)
According to NYT the only electric car on USA roads in any numbers (450) besides the Tesla Roadster (> 1,000) is the BMW Mini-Cooper electric (Mini Cooper E). The company is leasing them for one year to 450 customers in New York, New Jersey and greater Los Angeles. Leases started in summer 2009. These are test cars not real production electric cars. They are EV conversions of the Mini Cooper with the battery located where the back seat would normally go. Range 100 to 120 miles (real world).
The monthly rental fee includes a 220 VAC charger than can recharge the batter in 3-4 hours. This is smart. Recharging from 120 VAC as GM is planning is a big mistake. The Times adds, "Charging from a household outlet takes up to 24 hours (others say 21 hours)." (check: 21 hr x 1,500 watt = 31.5 kwh). The implication of charging x6 faster from 220 VAC is x2 voltage and x3 current, in other words 220 VAC @ 45A.
"Several companies, including BMW and Nissan, are developing quick chargers that can fill batteries to 80 percent capacity in a half-hour or less."
BMW Mini Cooper E electric car
"As for owner complaints, the biggest by far is reduced range in cold weather." (NYT) Oh, yea. At least Cooper (sort of) faced up to the temp problem of the battery by leasing cars in NE not just in the south. However, NY/NJ in winter is not like Minnosota! Range nominally is 100 miles. One customer says he got 128 miles once. "Running the Mini’s battery-powered heater (or air-conditioner) cuts the range as well."
“Towed! After only 87.8 miles — sheesh!” he wrote in a blog post at myemini.wordpress.com. Another lessee blogged on G.M.’s Volt site that his Mini’s power gauge fell to zero after just 55 miles on a 23-degree day. BMW acknowledges that range can drop as much as 30 percent in frigid weather." (No mention of cold weater derating or problems on Mini E web site.)Data from Mini E web site and early 2008 BMW press release. A 2008 article notes "AC Propulsion eBox electric vehicles which also sport a 5,088 lithium-ion cell battery with 35 kwhr of capacity" and asks if conversion is really by AC Propulsion. BMW claims the mini E handles as nimbly as a gas powered Mini, but one leaser who has both types of Mini's says not true, the battery version is not as nimble. Battery kwh 35 kwh
Battery 5,088 lithium-ion cells (grouped into 48 modules, air cooled)
Battery voltage 380 VDC
Range 156 miles ideal (109 city/96 hwy)
Power out 150 kw (motor)
Max speed 95 mph
Wheelbase 97.1 in
Vehicle weight 3,230 lbs
Consumption 0.22 kwh/mile (city)
Recharge 3 hr (240 VAC, 48A rating)
4.5 hr (240 VAC, 32A rating)
26.5 hr (110 VAC, 12A rating)
Note, there is no battery derating in these prototype cars to extend battery life. You can bring the battery charge down to (about) 0%. (In contrast GM Volt is only going to use only half the capacity of its 16 kwh battery.) "To protect the battery" they say they reduce peak torque when the battery charge is less than 10%. (sounds more like a trick to help get you home). One leaser with a 60 mile commute had a 2nd 240 VAC charger installed at work.
From the numbers it does look like BMW just licensed the electric drive train from AC Propulsion. This either means BMW is not really serious about electric cars, or perhaps this was just good engineering. Buy a proven (if unsophisticated) electric drive train from a 3rd party to get a test fleet of electric cars on the road fast. Then you have time to do your own electrical design, convince management about electric cars, and perhaps have an edge over other manufacturers because you will have feedback from a substantial electric fleet in the hands of the public.
Good news for BMW is that the NYT reports that a lot of the first year renters, who have driven it now for nine months or so, like the electric Mini Cooper and want to continue to rent it.
BMW ActiveE (update Feb 2012)
BMW is showing a lot of good engineering in their development of an electric car. The hundreds of small MiniE electrics that BMW leased was on-the-road prototype #1. It has now been replaced by a larger sedan on-the-road prototype #2 (ActiveE) of which BMW will make a 1,1000 (700 to USA) again to be only leased (not sold). While this car is more Leaf like, ActiveE still shows signs of being a partial conversion. NYT says the company plan is for a production electric car in a couple of years and expects to go to a carbon fiber body to cut the weight substantially.
Battery kwh 32 kwh (992 lbs)
Battery 192 cells (liquid heated/cooled)
Range 100 miles ideal
Power out 125 kw (motor)
Vehicle weight 4,000 lbs
Recharge 4 hr (using built in 7.7 kw charger, 240 VAC)
BMW ActiveE (electric prototype #2)
Front seat hits rear seat when pushed back for 6 ft 4 driver!
BMW, like Tesla, as adopted the strategy of fairly heavy regenerative braking when the foot is lifted off the gas pedal, no more coasting. The NYT reviewer, who normally drives a Leaf, says it takes a little getting used to, but once he did he liked it a lot. You learn when to lift off the gas to come to a stop at a traffic light, he says. Essentially you can drive the car with one pedal, do much of your braking without having to move your foot to the brake pedal. BMW speaks favorably of 'one pedal control'. In contrast the Leaf drives more like a normal car, you got to put on the brakes on to stop!
Mitsubishi MIEV electric car (12/25/11)
As of Dec 2011 Mitsubishi is runnng a (short) TV commercial for a pure electric available in USA early 2012. Wikipedia says this car has been on sale in Japan since 2009. It's a tiny car about the size of the Mini-Cooper electric, but with a smaller battery and and less performance. Mitsubishi has got the same size battery as the Volt (16 kwh), but because it's so light (2,580 lbs) its range is 62 vs 35 - 40 miles. While the Mitsubishi is a lot lighter than the Mini-Cooper, it's very odd that the motors in the two care are different by a factor of three (49 kw vs 150 kw)! I suspect the lighter weight of the Mitsubishi may be because it was designed as an electric, whereas the Mini-Cooper spec is for a normal Mini-Cooper converted to electric.
Battery kwh 16 kwh
Battery 88 cells (eq to 3.75V/cell)
Battery voltage 330 VDC
Range 62 miles (EPA)
Power out 49 kw (motor, AC synchronous, PM)
Max speed 80 mph
Wheelbase 100.4 in
Vehicle weight 2,579 lbs
Recharge 50 kw, 80 kw (DC? , Level 3charge) option
7 hr (240 VAC, 15A rating) option
22.5 hr (110 VAC, 8A rating)
Ford Transit electric van (proto) (3/31/10)
Advertized as the Ford's first production electric vehicle in USA will be a Turkish made high profile small van transformed to an electric by Azure Dynamics in Michigan. It's to go on sale at the end of 2010 with sale to commercial customers. Battery pack is on the underside of the vehicle. It appears that Ford is doing something similiar to what BMW did last year, going to a 3rd party electric developer to do a rework get electric vehicles out into the field quickly for feedback.
Battery kwh 28 kwh
Range 80 miles
Max speed 75 mph
Battery voltage 336V
Recharge time 6 to 8 hr @ 240 VAC
Battery weight 600 lbs (reduces van load capacity from 1,600 lbs to 1,000 lbs)
Battery supplier Johnson Controls-Saft
Motor type Induction motor (air cooled)
Motor controller 600V IGBT inverter with DSP based field vector control
Ford Focus electric (update 1/9/11)
(update 11/6/11)
Price of all electric Ford Focus with its liquid cooled 23 kwh batttery is to be $39,995 vs all electric Leaf with its air cooled 24 kwh battery at $36,100 (after a recent price increase). The Focus price is identical (to the dollar) to the GM Volt electric-hybrid.
Ford Focus electric (available 4th quarter 2011) is apparently a direct competitor to the Leaf. Its all electric and the same physical size with almost the same size battery 23 kwh (vs 24 kwh in leaf). The car's top speed is only 84 mph, not much of a highway car. The 'new thing' that Ford marketing is touting is not new. They say the Focus will offer a 6 kw charger (optional) that will be able to recharge the car in 3-6 hours, but I read in the Leaf manual that they have (or will have) a 440 VAC charger that can 80% charge the battery in 30 minues.
The Focus electric will use (like GM) a liquid-cooled active thermal management system for the battery. (Nissan now says it too for "higher end offerings" and for use in Middle East it will go to active battery thermal managment.) A Ford engineer is quoted,
“All-electric vehicles do not have a conventional engine on board, so it is critical we maximize the performance of the battery under various operating temperatures.”And this is interesting
The second cost-cutting measure is Ford’s use (in the Focus) of generic pre-made electric car technology. In other words, Ford is using a system already developed by Magna International, a major global auto supply and technology firm, which has been looking for a carmaker to use its new pre-packaged electric car architecture, including motor, transmission, motor controller, lithium ion battery system and chargerOne automotive writer says Nissan is misleadingly giving only the EPA city number (LA4 cycle), which is low speed driving (20 mph av). A Chinese electric, Coda, quotes 120 miles city and 90 miles, highway (US06 cycle) with a 33.8 kwh battery. Scaling this would say the Nissan range realistically is more like 65 to 75 miles, which looks to be scarily low, since in a pure electric if run out of juice your stuck. Here is the EPA city drive cycle:
7.5 miles (23 min), mostly 30 mph stop/start (only 2 min above 40 mph), avg speed of 19.6 mph
Nissan is talking about having 20,000 Leaf pre-orders by the time car goes on sale in late 2010. But GM may only make 10,000 Volts in 2011 (from Volt official web site), is not taking pre-orders and has not announced launch locations.
(3/28/10) update)
CEO of Nissan-Renault continues to make big noise about electrics. He claims his company is the only car company investing big bucks in electric car production. The numbers being thrown around are Nissan-Renault spending three billion pounds over the next decade to ramp up to 500,000 electric cars per year. BBC has a video story about a specific Nissan plant in UK that is ramping up to build 50,000 Leafs/yr by 2013. Last line of the video, "The price of which (Leaf) has yet to be revealed!"
Ford Focus electric (3/31/2010) (5/4/2011 update)
So now it's May 2011 and where is the all electric Focus? Well, there's a picture in the new Ford marketing booklet, but it says production begins late 2011. Still a 23 kwh battery. Marketing is featuring 3-4 hr charge time (with 240 VAC) claimed to be twice as fast as Leaf. Car looks nice, and a nice touch is an illuminated ring around the charge port.
The Focus charge ring is pretty sexy. When you plug in it rotates a couple of times to indicate charging has begun. It then indicates the state of charge of the battery, flashing when a quadrant is charging and on steady when that quadrant is charged.Technically I am curious to see if Ford is going to go the route of Volt with elaborate temperature and derating protection for the battery, or like the Leaf with a barebones approach, the battery either lasts or it doesn't. A 1st generation electric car is going to become obsolete pretty fast, its effective life is short, so worrying about extending battery life to 8-10 years, like in the Volt, while it will later be important, may not be so important with these 1st generation cars. I am beginning to think Nissan is taking a good risk.
Looking on the web site in the photo I found the battery cooling answer ---- "It uses an advanced active liquid cooling and heating process to regulate battery temperature and help maximize battery life." From above it looks like the battery is under the rear seat and trunk. The thing in the center, away from the heat of the engine and battery, is probably the inverter.
May 2011
(Is the large front grill a dummy?)
from Ford Focus video 5/11
(rt is front)
-- Ford’s on-board 6.6-kilowatt charger allows for full charge at home with the 240-volt outlet in three to four hours – charging in half the time as the Nissan Leaf. (Leaf built-in charger is only rated for 3.2 kw) -- Lower price: Based on current plans, the home charging station with standard installation is expected to retail for approximately $1,499, as much as 30 percent less than competitors’ systems
-- Faster charging: With its maximum 32-amp charging capability, Focus Electric owners with the 240-volt home charging station can get a full charge in as little as three to four hours – charging in half the time as the Nissan Leaf. (Note the mini-Cooper charger (for its 35 kwh battery) has both 32A and 48A @ 240VAC rating.)
-- Nonpermanent installation: The charging unit plugs into a 240-volt outlet instead of being hard-wired into the electrical breaker box, making removal and replacement a simple unplug and plug back in operation in the event the owner moves
Ford focus 240 VAC, 32A charger
(designed to plug into a 240VAC outlet)
NYT says Focus will not be compatible with fast DC chargers
-------------------------
Early 2011 Ford will begin selling an all electric version of the Ford Focus. Just like the electric Transient van was outsourced, the electric Ford Fucus design has been outsourced to Magna Internationa, a Canadian company. The Focus, unlike the GM Volt, is an all electric with about 100 mile range. This looks to be a real production car with estimated first year volume of 5k to 10k vehicles.
Battery kwh 23 kwh
Charge time 3-4 hr (using 240 VAC)
Range 100 miles (80 miles in prototyptes)
Max speed 84 mph
Peak power 105 kw (141 hp)
Batteries 98, air-cooled, 60 Ah Lithium-ion
(98 modules x 3.91V x 60 Ah = 23 kwh)
Battery voltage 383 VDC (est from 3.91V x 98 modules = 383 VDC)
Motor 92kw, PM motor (92kw motor from NYT article 11/11, conflicts with peak power of 105 kw)
Max motor rpm 7,500 rpm
Torque 236 lb.-ft. / 320 Nm
Motor controller 600V IGBT s @ peak 365Apk nom (probably)
Transmission single speed gearbox
Wheelbase 104 in
Weight (curb) 3,421 lbs
New electric cars keep popping up out of nowhere. Just beginning production is Coda, a 5 passenger all electric. This is a USA company that Wikipedia says semi-manufacturer a car. They are building an electric version of a gasoline car apparently made in China, but designed in Italy and licensed from Japan! The battery is coming from China. It's privately funded to the tune of 200 million. Recent story says the company at its LA plant has 220 engineers, technicians and corporate staff.
Coda 5 passenger sedan
102.4 in wheelbase, 36 kwh battery, cost 40.8k
This a Ford Focus electric competitor. About same size and power (hair smaller), but with its large battery it is 250 lbs heavier, so its going to be a bit sluggish. The top speed at 85 mph is low.
Battery kwh 36 kwh (728 cells, 104 series x 7 parallel?)
Battery chemistry Lithium iron phosphate (LiFePO4)
Charge time 6 hr (6.6 kw charger on 240 VAC)
Battery thermal Air-cooled Active Thermal Management (translation: fan cooled)
Battery voltage 333 V
Battery location between front and rear wheels
Range 120 miles (manuf claims 150 miles)
Peak power 100 kw (134 horsepower)
Motor PM, 100kw peak, 60 kw cont (UQM® PowerPhase® electric motor)
Max motor rpm
Torque 221 lb-ft (300 nm)
Max speed 85 mph
Motor controller (supplied by UQM)
Transmission single speed (6.54 to 1 gear ratio, 17" wheels)
Wheelbase 102.4 in
Weight (curb) 3,670 lbs
UQM motor + controller supplier
Coda is buying the PM motor + controller from UQM, which manufactures it in Colorado. (40,000 drives annual production capacity). In last quarter revenue was 2M with reported loss of 1.5 M as they geared up to manuf in volume the system below. Looks like this small company is betting big on the Coda. This year they received 4M from DOE for R?D on non-rare earth PM motors for electric cars. Jon F. Lutz is VP for engineering. (motor and drive packaging look very nice)
UQM 100 kw peak PM motor + controller (USA)
model PowerPhase Pro 100 (used in Coda electric 5 passenger sedan)
One of the first lithium-ion battery electric cars expected to reach production status in US (late 2010) is a tiny car from Norway called the THINK city. The THINK company in Norway claims to have been in the electric car business for 17 years and that its latest car is a 5th generation design. They have 1,200 electric cars on the roads in Norway with the factory capability to produce 16,000/yr. However, missing from the THINK company bio is info from NYT which says THINK is emerging from bankruptcy, and this caused them to close their Norwegian plant and shift production to Finland. First cars sold in USA will be imported from Finland. THINK has contracted to reopen an old plant in US (Elkhart Ind) in early 2011, spending 43 million (Dept of Energy loan!) for a facility upgrade, with production capability of several thousand a year.
It was reported in 2008 that THINK was partnering with A123Systems and Ener1 for lithium-ion batteries. Ener1, Think’s biggest shareholder and battery supplier, is headquartered in Indiana. However, a Jan 2010 article in NYT says THINK announced this week that EnerDel, which holds a large stake in Think, will initially become the exclusive supplier of lithium-ion batteries for the car in the United States. (Is EnerDel and Ener1 the same company? Yes, EnerDel is a subsidiary of Ener1.) EnerDel battery model for THINK is E350-25 with prismatic (rectangular) cells. It might be 24 kwh, because this is known to be the size of some EnerDel vehicle battery packs, and it looks about right for the size and mileage. EnerDel got $118 million from US Dept of Energy.
THINK city --- 100 mile two seater Norwegian electric car (available late 2010)
Fast charging
THINK is pushing 'fast charging', which is very interesting. They have contracted with a CA company (AeroVironment) for a roadside 'fast charging' station that (the claim is) provide a 0% to 80% battery charge in 15 minutes. Details not announced, but it is likely it is a charger run from a 440 VAC line, a so-called 'Level 3' charger. (PG?E installed a 440-volt fast-charging station in 2009 off Interstate 80 between San Francisco and Sacramento as a test facility.) Cannot find kwh of the THINK battery.
Tesla's new sedan (as of 2009) was to be designed to 'fast charge' in 45 min from 480 VAC (4 hrs from 220 VAC). Nissan (in 2009) was taking about a 26 min 'fast charge' for a 100 mile electric, probably the Leaf.
Ford Focus (all) Electric (5/09)
In probably a smart move I read that Ford is designing its new European designed Ford Focus to be powered by either a conventional motor or an all electric drive. This will provide economies of scale. The conventional model to go on sale in 2010 followed by the electric version for fleets in 2011. The car design (or more likely just the electrical subsystem) has been contracted out to Canada-based auto parts and assembly supplier Magna International. Well really Magna pitched the electric car to Ford and retains the rights to sell it to other car manufacturers too.
Only preliminary info is available.
Battery 26 kwh lithium ion
Motor 400 volt
Range 100 miles
GM Volt claims 8 Kwh (of 16 kwh battery total) yields a 40 mile electric range. While the Ford Focus may be a little lighter than the Volt, still to claim 100 mile range from a 26 kwh battery means battery manufacturers will have pretty much solved the lifetime problem by 2011 as there is very little excess capacity for lifetime derating.
Electric reliability
Ford escape SUV (300V battery pack) has been used in taxis. A taxi manager at a battery conference recently said some hybrid Escape taxis have now racked up 300,000 miles and the battery pack has proven to be reliable. On the other hand:
"General Motors discovered the batteries already installed in 9,000 2007 model Aura and Vue hybrids were likely to fail prematurely. So a recall was launched in February, and it is still not completed. Batteries for those vehicles had to be replaced before more could be made available for the ‘08 hybrid vehicles" (Jan 2008). (I read the GM batteries were US made)
Electric car 'fuel' mileage
Here is a little exercise comparing 'fuel' costs of electric cars to normal gasoline cars. Obviously there is a lot of assumptions here, but this gives an estimate of incremental cost per mile. I'll figure 'fuel' costs to drive 100 miles.
Gasoline cost $2.70/gal
Gasoline mileage 30 miles/gal
Electricity cost 17.5 cents/kwh (Boston area kwh rate)
For a gasoline car getting 30 miles/gal to go 100 miles the car burns 3.333 gal of gas.
100 miles/(30 miles/gal) x 2.70/gal = $9.00 (@ 100 miles)
GM says its GM Volt will go 40 miles in electric mode using 8 kwh (half the capacity of its 16 kwh battery), hence the electricity cost to recharge the battery 2.5 times is
2.5 x [8 kwh x 0.175/kwh] = $3.50 (@ 100 miles)
electric/gas = $3.50/$9.00
= 39%
Hence the 'fuel' costs for an electric car are about 40% of the fuel costs of a (similar) gasoline car at $2.70/gal (about 25% @ $4.00/gal).
Electric car batteries and range (5/11)
A friend sent me a news article about a professor developing a fast charging battery and asked if this was the solution to electric car range problem. Here's my
reply.
The Ecocomist magazine had a similar story about a fast charging battery a few months ago. I think I posted to it because fast charging is mostly hype. Most of these batteries don't exist, but let's ignore that and assume they do. If the battery ESR can be reduced, it's mostly a good thing because the battery is more efficient and runs a little cooler. (However a poster did point out that a big battery with a very low ESR could be considered a bomb, and he may have a point.) In fact some pretty fast charging large lithium ion batteries, 10 min recharge (!) say their manuf, have been on the market for quite a while, but when you look at products you see much longer recharge times. Why? Comes down to basic physics and cost. 1) Current/WiringSo what's the solution he replied, just better batteries? I continued:
Your 'basic' electric car has 24 kwh battery and a battery voltage in round numbers of 300VDC. To recharge this battery fully in one hour I = 24,000 watt/300V = 80A. To
recharge it 1/10th time (6 min) makes the current 800A. 6 min recharge means all the wiring in the charger, battery, car wiring and its plug would have to be rated for 800A! (The
standard plug adopted by all the electric cars has about a 70A rating, though Nissan has also included in the Leaf a 100A connector for 480 VAC charging)
2) Power source
For a 6 min charge the power is 24kwh/0.1hr = 240kw (1/4 Mwatt). Either a charging station pulls 1/4 Mw/car in real time from the grid or it needs local storage, say the equivalent to x10 cars, or 240 kwh. If the cost of a car battery is 10k, then obviously for 100k you could assemble a 240 khw battery stack. Large sodium sulfur batteries at 2 Mwh each are in production by a Japanese company for load balancing. It takes a small building to hold a 2 Mwh battery. All this cost might (might!) make sense for 'electric gas stations', but it's not something you can afford to do at home.
3) Electric gas stations
A poster to the Economist article pointed out that the concept of 'electric gas stations' may not make any sense. You can't get gasoline at home, you have to go to a gas station. But in an electric car, if you can get home, you can charge up virtually for free (at least in terms of your time). So who is going to stop at an electric gas station, only those people on trips of more than 80 miles? When you think about it, the whole idea of high cost, slow, electric charging stations may not make any economic sense. And if/when batteries get better and car range expands, electric gas stations make less and less sense.
Ford focus electric (23 kwh battery, direct competitor to Nissan leaf) begins production in a few months. They are featuring a 240 VAC (1 ph I think) recharge that will take 3-4 hr. By the way I was curious as to how Ford would handle the battery derating issue. Go the Volt way with fancy temp control, or the risky/cheap Leaf way with just a fan? Answer is in, Ford will be conservative with the battery and will have liquid temp control.
The high current problem is absolute fundamental to power distribution. It's the reason why we have AC power today, why Tesla/Westinghouse AC power using transformers was able to overtake Edison DC power. Fast charging means high power, and the only way to to deliver high power without thick wires is increase the voltage. But high voltage in cars leads to two really bad problems. 1st safety --- Do you want 3,000V battery to get 6 min charging? 2nd chemistry ---- A 'standard' car battery now has about 100 @ 3V cells in series. I know enough chemistry to say it is impossible to raise the cell voltage more than a few tens of a volt, lithium ion cell voltages now are close to the theoretical maximum. So a 3,000 V battery means 1,000 smaller cells @ 3V in series. Theoretically possible I suppose, but I have never heard of anybody working toward a goal like this. And without some revolution in terms of cell construction it's going to be too expensive because it has too much stuff: every cell needs its own anode, cathode, separator, electrolyte, etc.Motor
The only other approach anyone has thought of is to swap battery packs in/out and some companies are pursuing this. This might conceivably work for large standardized fleets, like taxi or truck fleets, but for ordinary users it has two big problems. One, it requires the car be designed with a standardize battery pack (for which no specs exist). The battery is large, heavy, a key design element of the car, so this is a long way off. And two, it brings us back, in spades, to the 'Electric gas station' problem.
So long range means better and or bigger batteries or a range extender. A cute idea from one of the early electric car pioneers was a small charging trailer (he built one) that you attached and towed for long trips! It's the Volt approach with the engine, generator and gas supply external to the car.
The motors (? generators too probably) of all hybrids on the market are PM synchronous motors using Neodydimium/Iron/Boron magnets, but some (or all?) are not simple PM motors. The Toyota paper below details how they use a combo PM motor and reluctance motor with half to 2/3rd's of the torque coming from reluctance torque. Apparently these motors are used over induction motors because they weigh less.Toyota paper
The only paper I have read on hybrid car motors is a fairly recent paper from Toyota.
'Development of Traction Drive Motors for the Toyota Hybrid System'
by Munehiro Kamiya (of Toyota) (Journal unknown)
This paper describes the development of a high power motor for a Toyota SUV and includes a brief history of Prius hybrids: Gen I and Gen II (Gen III from Toyota is just hitting the market in mid 2009). (A cross-section of the hybrid motor/generator/power splitter integrated structure from this paper is in the Toyota architecture section below.)
Toyota PM/reluctance motor
While the Toyota specs describes the motor type as "Permanent magnet AC synchronous motor", the Toyota paper by Kamiya shows it to be a combo PM/variable reluctance motor. In the 2000 Prius the motor (in terms of torque) is roughly half PM motor (46%) and half variable reluctance motor (54%). In the new motor for the 2005 higher power SUV the variable reluctance is dominant (63%) with PM flux playing a smaller role (37%). The torque vs speed curves show a strong constant hp region, meaning (very likely) that the motor operates above the PM motor base speed.
Toyota hybrid motor(s) ratio of PM torque to reluctance torque
source -- scan from Toyota paper
I am beginning to understand that new electric car companies are really just acting as prime contractors. For example in the case of the Fisker Karma --- It's no secret they buy the gasoline engine from GM. Looking at the web site of their battery supplier, A123, I suspect they are buying the whole battery package from them: cells, package, cooling, thermal manegement. From hints I found I suspect Fisker bought the drive controller from Quantum Technology's ('Q Drive'), and of course they buy the electric motors too (maybe from Powertec). No doubt they buy the regerative brakes and lot of other subsystems too. Of course, the big companies do this too, buying for example the audio systems outside, but they design their own drivetrains.
Electrics PM => induction
Interestingly the one (low volume) electric on the market (Tesla Roadster) and coming quasi-electric (GM Volt) are both using (three phase) induction motors, not the PM/reluctance motors all the hybrids use. (In a 2007 article a CalTech long time electric car designer, Wally Rippel, said all hybrids, with no exceptions, use PM motors.) I have read the hybrids use PM because of its higher energy density, meaning its smaller and lighter. While I originally found this unconvincing, when I look at the Toyota cross-section showing the motor (and generator) tightly integrated with the power splitter I think the smaller argument makes sense. This reason disappears when the power splitter disappears, the motor in an electric is probably stand alone.
The motor for an electric also needs to be x2 to x3 bigger than hybrids, and maybe you could argue (even more) reliable since the engine as a backup source of torque for emergencies is gone. Industrially large PM motors (larger than car motors) do exist, they are rather rare. Induction motors can run to higher temperatures than PM motors, because they are made of only iron and copper, there is no PM to degrade. So the tilt toward induction motors when a bigger motor is needed for electrics is probably because of some combination of thermal and/or cost advantage and less penalty for bigger and heavier.
So what type of generator is being used? I have seen no data, but it's very likely PM in hybrids. In electrics, who knows.
Neodydimium concerns
There are three major concerns with neodydimium (used in PM motors):
* Is there enough supply?
Neodydimium is rare earth elements and as such sources of ore are rare. Most of it is now being used in hybrid car motors and with growth of hybrid cars there concerns supplies will not be adequate in a few years.
* Stability of supply.
China now controls 95% of neodydimium supplies. A company pushing an alternative to PM motors for hybrid cars, Chorus Motors, emphasizes this point.
* High temperature PM degradation
Chorus Motor guys claim at 100C some neodydimium magnets lose half their field strength, but PM motors in hybrids use are better than this. The high temperature characteristics of neodydimium based magnets have been improved by adding 5% of dysoprosium another rare earth. Prius tackles the thermal problem by liquid cooling the motor. The Oak Ridge report (below) says the 2005 Prius motor tested at 21kw (cont) with 35C coolant and 15kw (cont) with 105C coolant, a 29% roll off with temperature. The motor has a high temperature shut down of about 170C to protect the PM from demagnetizing.
Thermal considerations
Toyota says in their spec that both the motor and generator are "water cooled, separate from engine coolant". (I would be surprised if the same water water isn't circulated to cool the inverters and battery pack.) Nope, the battery pack is forced air cooled (? heated).
Surprisingly the Oak Ridge National Laboratory in 2005 put the Prius motor on a dynometer and extensively tested its thermal cooling system, putting on a 50 page (!) report, available online below. This was done to evaluate state of the art in vehicle motors in connection with a fuel cell R?D program that needs a 30 kw motor. (Gen III Prius motor is 'rated' at 60 kw).
http://www.ornl.gov/~webworks/cppr/y2001/rpt/122586.pdf
The high temperature degradation of PM motor is another (never talked about) consideration for the GM Volt. As the Oak Ridge report points out in a conventional hybrid, a roll off in motor performance is compensated for by cutting in the engine sooner. In the GM Volt (? a future fuel cell car) there is no engine to fall back on!
Motor speed
In Priuses 1997 to 2003 the motor had a top speed of 6,000, to 6,700 rpm, which since these are pancake style motor with fairly large diameters, is very fast. For the large SUV the motor size and toque (Power = Torque x speed) were reduced by increasing the top speed to an amazing 12,400 rpm. This is not a prototype motor, this is the top speed of production motors found in vehicles. The paper talks about the special attention that had to be taken in the design of the rotor, so that it could withstand the centrifugal forces tending to make it fly apart! The Oak Ridge report shows the Prius motor to be 4 pole-pair and is commutated with a resolver.
High speed vehicle induction motor
I found some info on high speed induction motors for electric cars at AC Propulsion. Alan Cocconi at AC Propulsion has been working on induction motors and controllers for vehicles for 20 years. He did the prototype for the EV-1 and his technology is licensed by Tesla Motors. He manufacturers a 200 hp (150 kw) high speed induction motor with matching 600V IGBT controller (AC -150) both air cooled, which he sells to people who want to convert their vehicles to electric, and presumably it's used to in prototype vehicles he builds for the military. A pair of them were used in a car that broke the electric land speed record (245 mph). This system directly connects the battery pack to the inverter, meaning the battery voltage sag at peak power must be lived with, it is not regulated out.
I presume this motor is his design since he has patents (one of which I read) on design of low loss rotors for high speed induction motors.
. AC Propulson 200 hp, 220 nm, 12,000 rpm, air cooled vehicle induction motor
10 in dia, 14.3 in long, 110 lbs
Efficiency of AC Propulson 200 hp (150 kw), 12,000 rpm vehicle induction motor
with mated 150 kw, 600V IGBT inverter (model AC-150)
Ref: 0.92 x 336V x 485 A (pk) = 150 kw
I am finding there is no agreement on how to size and how fast to run the electric motor in cars. There's a huge spread in top speeds. At the bottom end is Fisker Karma, which I figure has a top speed of about 4,700 rpm (even though this car has a top speed of 125 mph). At the upper end is Tesla, whose torque vs speed curve shows the motor runs to 15,000 (or 16,000) rpm. In between is the Ford Focue electric at 7,500 rpm, GM Volt with an electric motor that goes to 8,000 - 9,000 rpm (check) and Leaf at 9,800 rpm.
All the electric and quasi-electric cars used fixed gearing. A couple of years ago when Tesla began the design of their first car they initially put in a two speed transmission, but it was unreliable, so it was removed for the production car. All the other companies have followed Tesla's lead and use no transmission.
Power = Torque x Speed
The basic physics of motors is [Power = Torque x Speed]. Thus for amount of power, which depends on weight of car, desired acceleration, and wind resistance at top speed, the motor/controller designer has a choice:
It appears that Tesla has chosen the former and Fisker Karma the latter. This is a basic choice since the weight and size of a motor is pretty linear with torque. In other words when you buy a motor pretty much you are buying torque. If you can balance it well and figure out how to run it x2, x3, x4 times faster, you get (for little cost) x2, x3, x4 more power for your money and weight. Because the Fisker motors run at such low speeds, the gearing is low, so they need a high torque rating. This makes them large, heavy and expensive. I think each of the two motors is 11.5" in dia and about 3 ft long. In contrast the motor for the new Tesla sedan (which weighs maybe 20% less and a somewhat lower top speed) from pictures is only one motor 9" in dia and about 1 ft long. A huge difference in weight, size, and undoubtedly cost.
Gearing multiplies torqueCar maximum speed ? PM vs induction motor
The reason the smaller, relatively low toque Tesla motor can apply a lot of torque to the wheels (for good acceleration) is that its transmission ratio is high. If you can run a motor, say, twice as fast you can double the gear ratio. This allow you to cut in half the motor torque (and motor weight) without affecting the torque delivered to the wheels.
There are two other important, related considerations: car maximum speed and PM vs induction motor. With no gearing the motors go from zero speed when the car is at zero speed to top speed when the car is at top speed. Leaf top speed is 90 mph vs 125 mph for the sports car like Fisker Karma. If running the motor fast saves so much weight and cost, why doesn't everyone do it? Well for one thing running 10,000 rpm and faster is high tech. I don't understand the mechanically all the tradeoffs, but it clearly takes a lot of engineering to pull this off reliably in a production vehicle. One issue clearly is balance, another is bearing wear and heating, another issue is heating of the motor iron/magnetics due to high frequency current. Tesla uses the same super high speed strategy with their first car the roadster, so they have a history with it and apparently it has proven reliable for them.
High speed torque vs speed
Another important consideration is control of the motors at high speed and getting good torque at higher speeds. The low speed PM motor is by far the easiest to control. This is the low tech, safe, but expensive choice that Fisker Karma has made. And it's probably a huge mistake, reflecting a basic weakness they have in motor control. Everyone else is running medium high to super high speeds. Trying to run a PM motor above its 'corner frequency' is tricky business. It can (or could) only be reliably done to a limit extent, like getting an extra 30% more speed.
Historically the induction motor was tricky to control, but with modern digital controllers implementing 'vector control' the control problem is pretty much licked, though I suspect the controller still has some looseness over the motor temperature range. A big advantage of an induction motor for high speed operation is that its magnetic field is controlled by the controller, not built into the motor as it is with a PM (permanent magnet) motor. This allows a high speed technique called 'field weakening', where the motor torque rolls off inversely with speed resulting in a 'constant hp region'. This can easily extend the top speed in an induction motor by a factor of 2 whereas a PM motor has trouble achieving 1.3.
speed mph | |||||
(55 kw 'generator' also used as 2nd motor) | PM | 55 | - | ||
range electric (2 motors total) |
GM Volt footnote -- GM motor has usually been identified as induction,
but 2011 SAE paper (below) by GM engineers says clearly both machines in the Volt are PM
http://papers.sae.org/2011-01-0355
Inverter
Inverter is the (generic) name of the power electronics box that drives the motor. In goes DC voltage and out comes AC voltage for the motor. It's called an inverter because it (crudely speaking) inverts the DC power from the battery (or capacitors) to AC power for the motor. The AC that's created is quite complex with a variable frequency and amplitude that depends on the motor speed, motor torque, and even the type of motor. In modern inverters the position of the motor rotor is continually tracked, and this information combined with the torque command is used by a specialized computer to drive the six large power transistors (IGBT's) that chop up the DC to make AC. In early Priuses 1997 to 2000 the battery pack voltage was fed directly into the inverter. The inverter DC bus voltage would be the battery voltage, which was 274V at light load, but as power demand on a battery increases its voltage begins to sag. If fully loaded, the battery voltage would sag from 274V to near 140V. This poses a problem in motor control, because there is high voltage available (to drive the motor) at low speed and low voltage available at high speed, which is exactly the opposite of what the motor requires. As the car accelerates up (at constant toque), the motor voltage rises approx linearly with speed (starting from near zero) and the battery voltage sags (starting from 274V), and when the two voltages meet, the maximum motor torque rapidly falls off with increasing speed, or in simple terms, the motor dies.
Acceleration ? powerBoost converter
Here's the basic physics of power: Power is power, but in the electrical world it's volts x amps and in the mechanical worlds it's torque x speed. P = voltage x current electrical
P = torque x speed mechanical
During vehicle acceleration the torque applied to the wheels is held relatively constant, so the power rises linearly with speed. Translated into electrical terms during acceleration the motor current is constant and the motor voltage rises with speed. The inverter using the 'magic' of switching electronics is able to interchange volts ?=> amps. At the inverter input, which in early Priuses was connected directly to the battery, vehicle acceleration is seen as current increasing linearly with speed and voltage relatively constant, at least at lower speeds.
In Gen II Toyota starts adding a boost converter between the battery pack and the inverter. This is a circuit that boosts the battery voltage (likely in the 200V to 300V range) and regulates it to 500V (in 2003 Prius) and 650 V in the 2005 SUV for the transistors in the inverter. A boost converter is a substantial complication to the power electronics. For one thing the power from the battery is now converted twice before reaching the motor. Not only is this expensive, but adding a boost converter roughly doubles the heat put out by the inverter electronics. However, a big advantage is that the boost converter 'shields' the inverter from the voltage sag of the battery. As long as the battery pack can deliver the required power, it doesn't matter (much) to the inverter how much the battery voltage sags, because the inverter sees a relatively constant voltage at its input. In other words the boost inverter decouples the battery pack from the inverter allowing better optimization. The performance advantage is that the vehicle can accelerate electrically to higher speeds.
Bus capacitors
For another thing the low impedance of the batteries can no longer protect the transistors from overvoltage. Now large, expensive, and not fully reliable bus capacitors need to the output of the boost circuit to provide a stiff, high current bus for the inverter. Traditionally aluminium bus capacitors wear out, so I would like to know how Toyota has addressed the issue of bus capacitors and reliability.
Why a booster converter?
So why boost battery voltage? Two reasons, one it improves performance. The output voltage of the boost can be regulated. This means the sag of battery voltage during peak power demands now does not affect (directly) how the motor runs. The second reason is that as hp in hybrids rise, the raw battery current gets too high to handle easily in the transistors or motor.
Say at high power the battery voltage sags to 150V, but is boosted to 650V at the inverter bus. While the boost transistor(s) have to handle the high battery current, the downstream components, inverter IGBT's and motor, can operate at current about x4 times lower (650V/150V = 4.33). A x4 boost in bus voltage above the battery, reduces the current downstream in the IGBT transistors and the motor by x4. In round numbers for the 123kw SUV this mean IGBT and motor currents are reduced from 800A range, which is very high and difficult to handle, to a much more reasonable 200A range.
IGBT transistor modules
The high power switching transistors that drive the motor are called IGBT (Integrated gate bipolar transistors). They have dominated high power switching for the last 10-20 years or so and are pretty well standardized, with family's of different currents at 600V and 1,200V. Of course the auto market is so large that it's possible that special voltage rated parts are used, but looking at the nominal bus voltages in the Toyota power they are consistent with standard 600V and 1,200V IGBT's.
600V => 1200 V IGBT's
Very likely Toyota's gen I verhicles with 274 VDC nominal buses used 600V IGBT's, and when the voltage booster was added in Gen II to raise the bus voltage to 500V (to 650V in gen III), the IGBT's were changed to 1,200V modules. The motor inverter (peak) current in a gen III Prius (60 kw) is about 92A and in the Toyota SUV (electric system is rated at 123kW) it would be about 190 A peak (190 A x 650V = 123kw). The later requires (at least) a 300A dual IGBT 1,200V modules, which are fairly standard items, which I used myself in designs. (It's a somewhat difficult jump to 600A single IGBT's because it's difficult to mount the required protection capacitors.)
Fusion => 600V IGBT's
The new (2010) Ford Fusion hybrid, on the other hand, although it has a somewhat more powerful motor than the Prius (78 kw vs 60 kw) and a voltage booster, has a bus voltage of only 275 V (maybe modulated up and down somewhat), so it's very likely they are using 600V IGBT's. The 30% higher motor power combined with the low bus voltage, less than half of Prius' 650 VDC, combine to push the peak currents in the Fusion 600V IGBT's to about 280 A. (To first order a 600V IGBT @ 300A with dissipate the same heat as a 1,200V IGBT @ 150A.)
Bus capacitors
Here's a picture of the Toyota gen III inverter opened up I found on Wikipedia. The three big cans on the right are the bus capacitors. Big bus capacitors are needed to stabilize the bus voltage as the chopped up motor current, which can be 200 to 300 A, are switched into and out of it by the transistors. In inverter design sizing these capacitors is tricky, because they are large, relatively expensive, and unlike most electronic components they have a finite lifetime, because they slowly dry out, and the hotter they run, the faster they decay.
(Old) gen I Toyota inverter (bus wired to 274 VDC battery)
Wikipedia identifies this as a Prius inverter (2000 to 2003) (NHW11)
The rating on the bus capacitors can be read off the side: 2,700 uf, 450 VDC, 85C. (85C is the lowest temp grade of bus capacitors) Three 450 VDC caps seems like an odd number. Don't know what the bus voltage is in gen III, but it was 650 VDC in gen II, so it's likely to be the same or higher. Therefore it takes a minimum of two 450 VDC capacitors in series to handle this voltage. With three 450 VDC capacitors in series the voltage rating is 1,350V (1,500 V surge). On first glance this seems like excessive voltage margin, and with all the caps in series they run hotter since they all have to carry the full AC current, but it might be necessary to protect the system in the event of a fault at high motor speed.
(update)
I think the explanation of the 450 VDC bus capacitors (and look of the inverter) is it that this is a gen I inverter (identified in Wikipedia article 'Toyota Prius'). In gen I there was no boost converter, the battery voltage and the inverter bus voltage were the same at 274 V (open circuit). This means the three 450 VDC capacitors now makes sense. They are wired in parallel and just provide local absorption of the PWM (switching) current.
Pipes shown may be for liquid cooling
(only some of the input/output wiring is shown)
Toyota uses field weakening in their partial PM motor, a technique I was involved in developing in the 1980's. This is a control technique that allows the motor to run at reduced torque to speeds higher than is normal, speeds where the internal voltage of the spinning magnets can be considerably higher than the normal terminal voltage (650 V). What's tricky about this is that if the drive 'faults' (shuts down or reboots) while the motor is running at high speed the IGBT's and bus capacitors can potentially be exposed to much higher voltages than normal. It might be that three 450 VDC capacitors are used in series to prevent a failure of the bus capacitor from overvoltage in the event of a high speed fault.
Patents
An article in WSJ (7/1/09) says Toyota has filed for 2,000 patents on its hybrid technology, 1,000 of them related to gen III, which went on sale in May 2009. They don't reveal how much licence revenue they earn from their patents. Ford claims, "Our hybrids are 100% Ford-developed and engineered, (our) execution and architecture are different", says a Ford spokeswoman. Ford recently cross-licensed 20 or so hybrid patents with Toyota, and Ford says no money changed hands. This would tend to confirm that Ford probably has a considerable hybrid patent library, which is pretty surprising since they started work in the field many years behind Toyota.
Comparing hybrid ? non-hybrid siblings
So how much does hybrid technology improve gas milage? A way to get a handle on this is to compare the same car in its hybrid and non-hybrid form. Prius is only a hybrid, but Toyota makes midsize size car in both forms and so does Ford.
standard hybrid
Toyota Camry (109.3 wheelbase) ------------- ------------
engine 2.4 L 169 hp 2.4 L 147 hp
weight 3,307 lbs 3,680 lbs
EPA mileage 22 city/32 highway 33 city/34 highway
sticker 22,650 26,900
standard hybrid
Ford Fusion (107.3 wheelbase) ------------ ------------
engine 2.4 L 175 hp 2.4 L 156 hp
weight 3,342 lbs 3,720 lbs
EPA mileage 22 city/31 highway 41 city/36 highway
sticker 24,700 28,000
The hybrids are 373 lbs (Camry) and 378 lbs (Fusion ) heavier than its non-hybrid sibilings. Non-hybrids are both 22 mpg city and 32/31 highway. The hybrid Camry pushes city to 33 mpg and hybrid Fusion to an amazing 41 mpg, hybrid highway mileage is up too, 4 mpg Camry and 5 mpg Fusion. The hybrids probably drive about the same as the non-hybrids or maybe a little more peppy, they weigh 11% more, but have the equivalent of a 6 cyl vs a 4 cyl.
Toyota hybrid technology
The newest Prius (weight 3,042 lbs) gets 51 mph city, 48 mpg highway. 275 thousand Priuses were sold (worldwide) in the last year. Toyota describes their hybrid technology over the last 12 years as following into three generation. The electrical power (called 'traction battery power' or power available with engine off) is from a Wikipedia number. These numbers are a little lower than I derived from the battery specs, indicating either than there are losses and/or reserves or maybe they don't' pull maximum current from the battery to coverup some of the roll off in current at low temperature.
gen I Prius (2000 -2003)
THS (Toyota Hybrid System) I no boost convert (battery => inverter)
600 V IGBT's (273V bus)
44 hp (33 kw) electrical, 1.5 L 4 cyl engine
273 V, 1.77 kwh battery (228 cells, 6.5Ah)
gen II Prius (2004 - 2008)
THS (Toyota Hybrid System) II boost convert (battery => boost => inverter)
1,200 V IGBT's (500 V bus)
28 hp (21 kw) electrical, 1.5 L 4 cyl engine
201V, 1.31 kwh battery (168 cells, 6.5Ah)
gen III Prius (2009 -
HSD (Hybrid Synergy Drive) advanced version of THS II
1,200 V IGBT's (650 V bus)
36 hp (27 kw) electrical, 1.8 L 4 cyl engine
201V, 1.31 kwh battery (168 cells, 6.5Ah)
Toyota gen III 2010 Prius specs
I read that in 2008 outsiders were speculating Toyota would change to lithium ion batteries for gen III, a Toyota engineer was quoted as saying lithium batteries were ready. The speculation in the press is that gen III has come out with standard NiMH batteries because of fire risk with lithium ion.
motor (electric)
type Permanent magnet AC synchronous motor
power output 60 kw (80 hp)
torque 207 nm
voltage 650 V max (indicates bus voltage is 650 VDC)
Battery
type sealed NiMH
voltage 201.6V (201.6/1.2V = 168 cells or 28 (6 cell) modules
power output 27 kw (36 hp)
The peak electrical power spec is a little difficult to interpret. Note the motor can, and apparently does, have a higher (peak) kw rating than the battery pack. This is because (almost for sure) the generator and motor buses are common. The current into the inverter driving the motor can therefore come from both from out of the battery and (backward) out of the generator inverter. Wikipedia (Toyota Prius) lists the kw rating of the motor as 60 kw. The Toyota spec (above) shows the battery pack output rated at 27 kw.
Peak power ratings
I suspect the 27 kw (peak) from the 28 module battery pack is right, because the numbers fit together and look reasonable. When the motor is putting out its peak 60kw, about half (27 kw) comes from the battery pack (via boost converter) and about half (33 kw) come in to the common inverter bus from the generator inverter. So we have a 60 kw motor/inverter and a 33 kw generator/inverter. This is consistent with the Toyota cross-section diagrams which show the generator is about half the size of the motor.
The numbers on the battery pack also look much more reasonable. As I detailed above, if the battery pack is loaded to it absolute max peak rating (matched load), the battery could deliver 36kw = (360 A x 0.5 x 201.V), but at the same time it is dissipating (as heat) 36 kw internally. By restricting the peak power out of the battery pack to 27 kw, or 75% of its maximum power capacity, the numbers are much more reasonable. At 180 A out the battery terminal voltage sags from 201.6 V to 151 V (180 A x 0.28 ohm = 50.4V). Power out is 3/4th of max capacity (180A x 151V = 27.2 kw), but the internal (heat) losses are reduced by a factor of four to 9 kw (180A x 50 V = 9kw). Now 3/4th's (27 kw/36 kw) of the battery's energy is making it to the wheels vs 50% (36 kw/72 kw) at peak capacity. Also the peak temperature rises within the cells are reduced by a factor of four!
Toyota gen III circuit sketch
I've sketched up (what I think is) the peak power conditions for the gen III 2010 Prius electrical subsystem. About half (33 kw) the peak power for the motor inverter (60 kw) comes in (from the engine) to the common bus capacitors via the generator inverter without going through the battery. The other half (27 kw) of the motor inverter peak power is pulled from the battery pack through the boost converter, which boosts and regulates the voltage on the bus capacitors to 650 VDC.
Bus voltage of 650 VDC in gen III Prius was cranked up from 500 VDC in gen II (evidently the inverter in the in the 2005 Toyota SUV with its 650 VDC bus must have proven reliable!) The motor inverter current (0 to pk) is figured by dividing the peak instantaneous power out (60,000 watts) by the bus voltage (60,000W/650V = 92.3A). The 0.28 ohm battery pack ESR is figured as (10 mohm/module x 28 modules = 0.28 ohm).
Whoops, I just realized something. The boost converter needs to be able to handle bidirectional power flow, which makes it more complex. Power flowing to the battery from the generator and regenerative braking (via motor and motor inverter) needs to be down converted from 650 VDC bus to the 200 V or so battery voltage.
Lexus sedan hybrids
It's maddeningly difficult to get specs on the Lexus hybrids. As of 2010 they will have three hybrid sedans: 4 cyl (not out yet), V6 and V8. One of the Lexus hybrids (crossover) has a 288V battery, which looks like it may be three 288V strings in parallel, which if the cells are the same as Prius would give it a peak kw rating of 115 kw.
288/V/202V x 3 parallel strings x 27 kw (Prius rating) = 115 kw
| Prius | Ford Fusion hybrid | Hyundai Sonata hybrid (2011) | Lexus HS 250h (like Camry hybrid) | Lexus GS 450h | Lexus LS 600h | |
98 hp | 156 hp | 2.4 L 4 cyl | 147 hp | 292 hp | 398 hp | |
27 kw | (23 kw) | 30 kw | ||||
is gen) | ||||||
(has no power splitter, a clutch, and a six speed transmission)
Prius programming bug -- 'no brakes!' (2/8/2010)
It's easy to forget that brakes in a hybrid depend in a complex way on a computer program within the car. Braking is a hybrid is a combination of retarding force from the motor (regenerative brakes) plus hydraulic (electronic?) standard brakes, which dissipates braking energy as heat in brake pads. The program tries to use the motor/regenertive braking as much as possible (obviously, to improve mileage) and standard brakes only a little. I presume there must also must be a failsafe mode with hydraulic brakes able to stop the car in a reasonable distance if the electronics have shut down or faulted (I have seen no spec on this).
In Gen III Prius brake programing was complicated the by adding a third braking mode: anitlock brakes.
"Prius drivers have complained that the car momentarily loses the ability to brake when driven over a pothole or other uneven surface." (NYT) The car went on sale in summer, but many of the 'no brake' complaints didn't come in until customers were driving on ice in Jan. Toyota is recalling all Prius gen III hybrids to fix this.Apparently the brakes software 'bug' pops up with the antilock brake feature is being disabled and normal braking modes reengage. The scary thing is that the gen III Prius was in production for over six months (300k cars built) before this bug was found and fixed. Ford Fusion hybrid also has some sort of braking problem to be fixed by a 'software update'. Also this interesting comment attached to NYT Prius brake article:
After our Generation 2 (2009) Prius experienced a low-speed brake failure resulting in $18,000 damage, we checked the National Highway Transportation Safety Board's database for Gen 2 Prius brake complaints. We found hundreds of them. Compared to the same years of Toyota Corollas, which have traditional brakes, we found that the Prius brake failure complaint rate was over 30 times higher. For Toyota to claim, as it did yesterday, that it has had no complaints about Prius Gen 2 brakes is a huge lie. (comment 19, 2/4/2010)One poster speculated that maybe the anti-lock brakes are at fault. What anti-lock brakes do is momentarily release the brake of a wheel to keep it from breaking away and losing traction. On ice this helps stop the car. But if a road is rough and wheels start bouncing off the ground anti-lock braking engaged can interfere with your ability to stop the car.Ford 2010 Fusion hybrid
Here's my interpretation of the data (below) that I was able to scrounge up on the Fusion. 2010 Ford Fusion hybrid uses a scaled up Prius architecture, higher power and more elaborate controls, which appear to be very effective because Fusion gas mileage is great, 8 mph better than Toyota Camry in city. Fusion battery pack, motor, and 'power splitter' are all very similar to Toyota Prius.
Ford has a voltage booster (between battery and inverters), but it does not boost to a 650 V bus like Prius. It's used (mostly) to remove voltage sag from the 275 V battery pack holding the bus voltage for the motor at 275 V under heavy load. The bus voltage appears to be modulated, possibly lowered at low speeds (to improve inverter efficiency) and allowed to rise to 350 V or so if more generator power is needed? Prius uses (I think) 1,200 V IGBT (@ 92 A pk), but Ford Fusion is probably using 600 V IGBT's operated @ 283A pk = (78kw/60kw x 650 Vbus/275V bus x 92 A).
----------------------------
Ford's Fusion hybrid marketing phrases and news articles:
78 kw, 6,500 rpm, 275 V (60 kw motor in Prius)
PM AC synchronous motor
73 ? kw generator (33 kw generator in Prius)
variable-voltage converter (probably means bus voltage is modulated
with speed ? power)
2.5 L engine, 156 hp @ 6,000 rpm (1.8 L, 134 hp @ 4,500 rpm in gen III Prius)
Atkinson, 4 cyl
1.3 kwh, 275V NiMH battery (Sanyo) (1.3 kwh, 201V NiMH battery Prius (Panasonic)
EPA mileage: 41 city/ 36 highway, all electric: 47 mph, 2 miles distance
power-split device
Electronic Continuously Variable (e-CVT) transmission
-- The Toyota Prius, Ford Escape Hybrid use a gasoline engine and two electric motor-generators (MG1 and MG2) connected to a planetary gear set called the "Power Split Device" to deliver power to the transaxle final drive planetary gear set. (Generator and motor rating on Escape are same as Prius (65 kw motor, 28 kw generator, same 'power splitter'. The Escape hybrid technology was obtained from Toyota. Escape is apparently gen I Ford hybrid. Ford Fusion hybrid is referred to as Ford gen II hybrid.)
-- Series-parallel hybrid transaxle sourced from Aisin. Aisin site shows they make "Hybrid Electric Planetary" transmission for Ford Escape hybrid and Lexus GS450h hybrid.
-- engine’s valve timing, fuel delivery, and spark timing to match the power delivered through the electric motor, permitting very aggressive fuel shutdown under light loads
-- Variable voltage controller (VVC) to the Fusion hybrid that allows the voltage from the battery to be stepped up on demand. During most driving conditions when comparatively little power is needed, the lower voltage increases the efficiency of the electric drive system, while the VVC allows even greater output than the Escape when it's needed for acceleration or heavy regenerative braking."
-- Fancy blue slide says, 'Hybrid Transaxle - eCVT' lists the improvement of the Variable Voltage Converter as 130% motor, 160% generator.
NYT auto reviewer compares two mid-sized 2012 hybrids -- Lincoln MKZ and Lexus HS (7/22/11)
The NYT reviewer after a 1,000 mile drive in both cars finds the Ford hybrid technology does much better in real world fuel economy than the Toyota hybrid technology. Not only does Ford have a big EPA city 41 vs 36, but NYT finds on highway Ford remains near its 36 mpg EPA highway rating while Toyota falls off its 34 mpg highway rating to 28-29 in reality!
Lincoln MKZ hybrid 107.4 wheelbase 3,752 lbs (curb weight) EPA 41 city, 36 hw
Ford Fusion hybrid 107.4 wheelbase 3,720 lbs (curb weight)
Lexus HS hybrid 106.3 wheelbase 3,682 lbs (curb weight) EPA 35 city, 34 hw
"A plus for all the Ford hybrids that I’ve driven, including the MKZ, is that you can drive them fairly fast (especially on the highway), and fuel economy doesn’t seem to suffer greatly. The Lincoln’s fuel economy remained at or near the advertised 36 m.p.g. The fuel economy of the HS, true to my experience with Toyota-branded hybrids, suffered in spirited driving: the mid-30s E.P.A. promise dropped to a high-20s reality.
Certainly, Lexus’s Hybrid Synergy Drive responds well to conservative techniques to save fuel — gradual acceleration, coasting and highway speeds limited to less than the posted limits — but so does the Ford system. Play that game with the HS and you can expect mileage in the mid-40s. When I tried the same techniques with the MKZ and Fusion Hybrids, I got as much as 52 m.p.g." (NYT 7/22/11)The MKZ Hybrid, like the Fusion Hybrid, gets power from the combination of a 2.5-liter 4, a 79-kilowatt electric motor and a nickel-metal-hydride battery pack. Total output is 191 horsepower. It also has a C.V.T. and front-wheel drive. The Lexus HS is propelled by a 2.4-liter 4-cylinder gasoline engine, a pair of 30-kilowatt electric motor-generators and a nickel-metal-hydride battery pack; total horsepower is 187. A continuously variable automatic transmission, or C.V.T., sends power to the front wheels. The Lincoln MKZ can go about 1 mile on electricity, electric only to 25 mph or 47 mph depending on drive mode. Lincoln MKZ is really a Ford Fusion with a Lincoln nameplate. The NYT reviewer makes a big point that the 2012 hybrid and non-hybrid MKZ are the same price, but the MKZ hybrid is an overpriced Fusion hybrid available for 10k less!
Lexus hybrid
From Lexus marketing material and random technical tidbids it's clear the Lexus hybrid has the same hybrid architecture as Toyota and Ford (same sun/planet continuous variable transmission). Lexus, however, mates much larger engines, a 6 cyl and even an 8 cyl, with a modest size electric system. Consider this Lexus marketing claim for their big 8 cyl hybrid:
"The new (Lexus) 389 hp 5 L V8 when paired with the hybrid system's electrical motor, produces a combined output of 438 'horses'."This sure sounds like the V8 Lexus electrical subsytem (49 hp, 36kw) is pretty much the same as in Prius and Fusion. Gas milage is secondary to performance with Lexus, their 8 cyl hybrid has ratings of only 20 city/ 22 highway. According to Wikipedia the Lexus GS450h uses two clutches with a four shaft power splitter (ravigneux-type planetary gear) to achieve a wider speed range, up to 150 mph.Honda hybrids
They use a PM motor (? generator). Their NiMH battery voltage is only 158V = 132 cells x 1.2V/cell. The batteries are made by Panasonic and Sanyo. Usable capacity is 5 kwh, 100A peak discharge. Est 12kw = 0.75 (sag) x 158V x 100A.
Ford Fusion for 2013 --- hybrid shifts to more electric power (1/15/12 update)
Ford has revealed features of its 2013 model Fusion (available late 2012), and it shows the evolution of the hybrid and blending with electric cars. The NYT story on the Jan 2012 Detroit auto show, where the 2013 Fusion was revealed says the consensus of the industry is that hybrids are the route to the electric car.
The 2013 Fusion will be available in three versions: gasoline engine, (upgraded) hybrid, and plug-in hybrid. The 'upgrading' of the hybrid is a shift of the power balance toward the electric drive train. Engine displacement has been reduced 20% (2.5L to 2.0L) and the hybrid battery changed over to a lithium-ion. No numbers but very likely both the hybrid battery kwh and electric motor hp are larger. This hybrid can go up to 62 mph on electric alone vs 47 mph in existing Fusion hybrids. Like the Prius it will be available with a larger (or added) lithium-ion battery charged by plug-in to give it more electric range (again no hard numbers available). Prel Ford spec shows it to have a PM motor and a continuously variable transmission, which probably means it is still using the basic Prius architecture.
Hybrid architecture
I have not compared the Toyota/Ford/Lexus architecture other hybrid architectures (like Honda), but as I have come to understand the Toyota architecture, I like it a lot, it's very sweet and very flexible. I don't own a hybrid, and am unlikely to do so for some time since I have a new car, so I can contribute nothing from a user viewpoint. I have no real understanding of mechanical components like gears, transmissions and power spliters, but I do now understand the basics of the sun/planet gear set, which is central to the Toyota/Ford/Lexus hybrid, and have written it up in detail below.
You can't have an essay on hybrid cars without a (high level) description of the hybrid architecture, so I have dug up some good reference material. The best introductory description of the Toyota architecture I found is the first ref below. It has clear diagrams which appear to be consistent with the cross-section diagram that appears in a Toyota technical paper, so is probably correct. Another good reference is Wikiedia (Toyota Prius).
http://www.cleangreencar.co.nz/page/prius-technical-info
http://en.wikipedia.org/wiki/Toyota_Prius
Below shows the motor connected to the wheel/axle through a fixed (step down) gear ratio. According to a Slate article the Prius tops out at about 100 mph and according to a technical Toyota paper the motor speed tops out at about 6,000 rpm. Thus we can estimate the step down ratio as about 3:1 figured as follows: assume 15" dia tire (3.925 ft/rev) and 100 mph = 147 ft/sec, so wheel rpm @ 100 mph = (147 ft/sec)/(3.925 ft/rev) = 37 rev/sec (2,240 rpm). (Other references confirm: 3:1 fixed step down from motor to differential/wheels)
source -- http://www.cleangreencar.co.nz/page/prius-technical-info
Cross-section of (high power) Toyota gen II for SUV
Engine connects top right thru generator to power split device
3:1 step down to standard differential (then to wheels) (in Prius cars) connects at motor shaft
motor 'Reduction Device' (2:1) is not used in Prius cars (motor 12,000 rpm, generator 6,000 rpm)
source -- scan from 'Development of Traction Drive Motors for the Toyota Hybrid System'
by Munehiro Kamiya, Toyota (technical paper)
you can see in the cutaway that the generator has roughly half the power of the motor
source -- http://www.cleangreencar.co.nz/page/prius-technical-info
Three axes of Prius sun/planets/ring 'power splitter' in exploded view
sun (right) --- generator
planet carrier (middle) --- engine
ring (left) --- motor/wheels
planets are five small gears (partly visible) on center carrier
source --- http://www.cleangreencar.co.nz/page/prius-transmission
center (sun) generator (MG1)
middle (planet carrier) engine (ICE)
outer (ring) motor/wheels (MG2
source -- http://www.cleangreencar.co.nz/page/prius-technical-info
As shown in figure above, the shaft that couples to the differential/wheels is connected to the motor on one end and the 'power splitter' at the other end. The net torque seen by the shaft is the sum of the motor magnetic torque and the power splitter gear torque, and since it's a rigid bar, summation of torques is summation of power. The power than comes in via the motor is the fraction of the engine power that was bled off into the generator and may include power pulled from the battery pack.
Mysteries of the 'power splitter'
A key element in the clever Toyota/Ford/Lexus hybrid architecture is the 'power splitter'. It's a sun/planet/ring gear set that connects together the shafts of the engine, generator and motor. Rotation of the engine goes in and it comes out as some combination of the speeds of the motor (wheels) and generator according to this formula (Toyota gen I and II)
engine speed = 0.72 motor speeed + 0.28 generator speed
or
(3.6 engine speed = 2.6 motor speeed + gen speed)
'Continuously Variable Transmission' (CVT)
If you think of the power splitter gear set as a 'transmission' directing power from the engine to the motor/wheels, then it's a transmission with an auxiliary output (generator speed). Causing current to flow in the generator can put a drag force on the engine such that the generator spins up and steals some of the engine power. The speed of the generator can be controlled over a wide range by electrically varying its drag torque, and in this way different engine speeds are possible at given motor/vehicle speed.
The trick in the Prius hybrid architecture is that the power bled off the engine by the generator is not wasted, it's fed right back into the motor/wheels because the generator and motor inverters share a common bus. The whole thing functions as an Electrically controlled Continuously Variable Transmission (eCVT). (As explained below, if the engine speed is in a sense 'too high' for the motor/wheel speed the power flow in the electrical path reverses, causing a local power loop.)
Toyota marketing shows the action of the parallel motor path acting as variable speed transmission below. Is this accurate? yes ? no Where it's not accurate is high speed highway driving. Here the variable speed transmission operates in 'overdrive' allowing the engine to run slower and more efficiently. The motor instead of adding torque to the engine path acts as a drag and actually reduces torque. (In the diagram this mean the little arrows in the motor path reverse.)
Oversimplified, in high speed 'overdrive' the little arrows (left) reverse
It's actually pretty simple (though it took a while to realize this). The torque ratios between the three input axes are fixed by the design of the power splitter (diameters, # of teeth, etc). Any gear set is like a transformer in that the golded rule is [power out = power in]. With [power = torque x speed] and torque ratios fixed the [power in = power out] rule leads to a constraint that the sum of the motor and generator speeds (properly scaled) be equal to engine speed. A 'power splitter' forces a speed (not power!) relationship among its three shafts, as shown below.
engine power = motor power + generator power
Tengine x engine speed = Tmotor x motor speed + Tgen x gen speed
engine speed = (Tmotor/Tengine) x motor speed + (Tgen/Tengine) x gen speed
Another way to think about the 'power splitter' is that the interlocking teeth force a relationship among the three three shaft angles. You can write an equation that the three angles (properly scaled) must sum to zero, which when you take the derivative leads to the speed relationship.
Works like a three winding transformer
A three axis sun/planet power splitter is the mechanical analog of a three winding transformer. A transformer sums (scaled) currents, a power splitter sums (scaled) shaft speeds.
In a transformer the three voltages are ratioed by the design of the transformer, by the turns ratio. Power is [V x I], so the [power in = power out] rule leads to a constraint that the sum of output currents (properly scaled) be equal the input current.
n1 x i1 = n2 x i2 + n3 x i3
or
i1 = (n2/n1) x i2 + (n3/n1) x i3
Note the voltage ratio between (output) windings 2 and 3 is just the ratio of their scaling factors in the equation above, i.e. [(n2/n1)/(n3/n1) = n2/n3]. In the same way the torque ratio between the power splitter motor shaft and generator shaft is just the ratio of their scaling factors.
Toyota's power splitter formula
Here's the 'power splitter' speed formula with Toyota's ratios.
engine speed = 0.72 x motor speed + 0.28 x gen speed (Toyota)
To better understand what this means in practice for a hybrids I worked out values over a range of vehicle speeds and put them into a table. Obviously in practice min/max rpm limits must be respected to. For the Prius the min/max rpm limits are
engine 1,000 rpm (start) to 4,500 rpm (rated torque)
generator +/- 6,800 rpm
motor/wheels -1,500 rpm (reverse) to 6,000 rpm (102 mph)
(wheel speed = 1/3rd motor speed fixed ratio)
| Generator rpm | Engine rpm | |
(0 mph) | 6,800 (10,800) | 1,889 (3,000) |
(34 mph) | 0 2,000 6,800 | 1,444 2,000 3,333 |
(68 mph) | 0 4,000 | 2,888 4,000 |
(102 mph) | 0 600 | 4,333 4,500 |
From the Prius speed equation we can 'read off' the design ratio of output axis' torques (motor and generator) built into the 'power splitter' by its design (gear ratios, etc).
(generator axis torque/motor axis torque) = 0.28/0.72 = 1/2.6 = 0.385
or
generator axis torque = 0.385 motor axis torque
(motor axis torque = 2.6 generator axis torque)
From above we can find the fraction of total power flowing through the 'power splitter' that flows in the generator axis.
Pgen/Ptotal = gen toque x gen speed/[gen toque x gen speed + mot torque x mot speed]
Ptotal (via power in = power out equation above) is also the engine power.
Pgen/Peng = 1/[1 + (mot torque x mot speed/gen toque x gen speed)]
= 1/[1 + (2.6 gen torque x mot speed/gen toque x gen speed)]
= 1/[1 + (2.6 mot speed/gen speed)]
or
= gen speed/[gen speed + 2.6 mot speed]
a) When generator speed is low (relative to 2.6 motor speed), Pgen/Pen = [gen speed/2.6 mot speed],
so generator power flow is proportional to generator speed. Note, when generator speed goes negative, generator power flow goes negative, meaning the generator is acting as a motor (looping power).
b) When motor speed is zero (vehicle stopped), Pgen/Peng = 1, meaning all the engine power flows out the generator axis.
c) When (generator speed = motor speed), Pgen/Peng = [1 (1 + 2.6)] = 0.28, or 28% of engine power is flowing out the generator through the parallel electrical path to the wheels.
Here's the table above with an added Pgen/Ptotal column.
| Generator rpm | Engine rpm | Pgen/Peng | |
(0 mph) | 6,800 (10,800) | 1,889 (3,000) | 1.00 -- |
(34 mph) | 0 2,000 6,800 | 1,444 2,000 3,333 | 0.00 0.28 0.57 |
(68 mph) | 0 4,000 | 2,888 4,000 | 0.00 0.28 |
(102 mph) | 0 600 | 4,333 4,500 | 0.00 0.04 |
Surprisingly power can flow in a local loop around the generator, inverters, motor, power splitter path. Note in the table at high speeds there are entries for zero generator speed. At these combinations of motor and engine speed, no power is flowing in the generator path since power flow requires speed (P = T x speed), 100% of the engine power flows mechanically through the power splitter to the wheels.
What happens as engine speed (at a given motor speed) increases or decreases a little from these null generator points? Clearly the generator begins to turn, positive one way and negative the other, but what about power flow? Graham Davies on his Prius page has a long write up about this.
http://prius.ecrostech.com/original/Understanding/WhatsGoingOnAsIDrive.htm
Davies explains some power loops (he doesn't use this word) when the engine rpm is low causing generator speed in the table (above) to be negative. This is the opposite of what I found in Wikipedia (in a somewhat confused article - Hybrid Synergy Drive), but Davies is probably right. Turns out this is normal driving mode. At moderate speeds on flat terrain only low power is needed to overcome wind resistance, so the engine is run at low speed for low noise and high efficiency. Under these conditions generator (MG1) rotates backwards and acts as motor. The only source of power for it in steady state is motor (MG2) acting as a generator.
As the table above shows the looping power can be considerable. For example, at 68mph reducing engine speed from 2,888 rpm to 2,000 rpm will cause 44% of the engine power to loop backwards in the electrical path with (motor (MG2) is acting as a generator and generator (MG1) acting as motor. The 44% loop back power comes from a drag load the motor (acting as a generator) imposes on the power splitter motor axis. The result is that power splitter motor axis has to put out 144% of engine power with 100% going to the wheels and 44% pulled off and circulated back into the power splitter generator axis via the parallel electrical path. Large amounts of loop back power increases electrical losses and means the teeth in the power splitter must be designed to handle the extra torque load. Very interesting.
Overdrive
Davies and others did a lot of measurements on their Priuses and modeling, so he's probably right. So is this looping power useful or just an annoyance? It's useful, actually it's overdrive. Davies' argues this way.
Applying a drag force to MG1 (operating it as a generator) raises the engine speed (for a given vehicle speed) giving the car lower gear(s). The faster the generator spins positive (see table), the lower the gear ratio, the lower the engine speed, the higher the fraction of engine power flowing through the electrical path. At vehicle start (a little) power through MG1 provides the I^2R losses for high current in MG2 so it can torque the wheels hard (even without using the battery). .The table shows that the Prius will go to top speed (102 mph) with the engine nearly maxing out its rpm with the generator speed equal zero. But there's excess engine power at top vehicle speed, so it would be nice if we had overdrive, then we could lower engine speed (lowering noise and increasing efficiency). As the table shows, this is what running the generator speed negative does. Operating MG1 as a motor gives us circulating power, but it lowers the engine speed at a given vehicle speed. With the engine running slower at the same vehicle speed, of course, it's torque load rises since nearly 100% of the engine power still goes to the wheels. Now we can run the motor at (say) 3,000 rpm at 102 mph instead of 4,333 rpm with the penalty of some extra losses due to 44% power looping. We have overdrive.http://prius.ecrostech.com/original/SideBars/Overdrive.htm It's actually rather elegant. The more power flowing forward in the parallel electrical path, the lower the gear ratio. The more power flowing backwardin the parallel electrical path (looping) the higher the gear ratio.
'Power splitter' doesn't force a power split
Proof that the power splitter doesn't directly force power relationships is this. Consider all electric mode with engine off and not rotating. Even though the battery is driving the wheels directly using the motor the equation says there must be generator speed. And there is, the generator free wheels, it has speed but no power flow. With (engine speed =0) the speed equation is
0 = 0.72 x motor speed + 0.28 x gen speed
gen speed = - 2.6 motor speed
Max generator speed is 6,800 rpm, so above 2,600 rpm ( = 0.39 x 6,800) motor speed, which is 44 mph, the engine has to run (or at least the shaft has to rotate)..
Cool power splitter animations
Center (sun) is generator, middle (planet carrier) is engine, outer (ring) is motor/wheels
. . a) Engine crank, vehicle stopped b) Drive on battery, engine stopped c) Cruising with power from engine
directly and via generator
created by Graham Davies
source --http://prius.ecrostech.com/original/Understanding/WhatsGoingOnAsIDrive.htm
If one shaft is not rotating, then the other two shafts are connected via a fixed ratio. This is the mode during engine start with the vehicle not moving, and it's the case in all electric mode with the engine off.
a) Generator cranks engine, vehicle stopped (3.6:1 fixed)
3,600 rpm gen = 3.6 x 1,000 rpm engine
b) Motor drives wheels, engine stopped, generator freewheels (-2.6:1 fixed)
-6,800 rpm gen = -2.6 x 2,600 rpm motor (eq to 44 mph)
(all electric speed range 0 to 44 mph)
Three shafts rotating
c) Driving with engine power (optionally battery assist)
2,000 engine rpm = 0.72 x 2,000 motor rpm + 0.28 x 2,000 gen rpm
3,000 engine rpm = 0.72 x 4,000 motor rpm + 0.28 x 400 gen rpm
4,333 engine rpm = 0.72 x 6,000 motor rpm + 0.28 x 0 gen rpm
Same axis
All three shafts in the power splitter rotate around the "same axis". The motor (ring) come in from one side, and (apparently) as shown in the diagram above the engine and generator are concentric, one shaft inside the other. The sun, planets, and ring are all geared together via teeth. No slipping. This 'no slip' is shown below in the graphics by the gray bars always lining up. The rotation rates of the planets themselves is unimportant since they are not connected outside. All the matters with the planets is the rotational motion the planets couple to their 'planet carrier' (blue middle ring).
Power splitter references
I first figured out how the power splitter works from the excellent Prius page of engineer Graham Davies. He created the excellent .gif graphics above.
http://prius.ecrostech.com/original/Understanding/WhatsGoingOnAsIDrive.htm
Flash animation (below) of the Prius power splitter. I got the min/max rpm limits of gen II Prius from this animation. The numbers the animation generates match the speed equation above and my table of speeds. It appears that the speed equation of the 'power splitter' did not change between Toyota gen I and gen II. I have assmed it has remained the same for gen III, but that is not confirmed.
ring/motor (wheel) 0 to 6,500 forward (0 to -1,500 reverse)
(2,000 rpm ring = 34 mph, 6,000 rpm = 102 mph)
engine 1,000 rpm (start) to 4,500 rpm (rated torque)
generator -6,800 rpm to + 6,800 rpm
http://homepage.mac.com/inachan/prius/planet_e.html
Regerative brakes
The motor is the key element in the regenerative brakes. When 'run backwards', it applies a reverse torque on the wheels that brakes the vehicle and regenerates power back to the battery. There is nothing exotic about running a motor 'backwards' to convert it into a generator (or vice versa). Inverter controlled motors act naturally as both motors and generators, no change of control or wiring is needed.
Motor or generatorHybrid NiMH battery packs have low ESR, so just like they can put out lots of energy quickly they can absorb lots of energy quickly. But the electrical system's hp is only a fraction of the vehicle's hp, so clearly if the car is stopped too fast the energy flow (power) recovered from the vehicles kinetic energy cannot all be absorbed. My guess, never having driven a hybrid, is that decelerations rates faster than 1/2 to a 1/3 of the vehicle's maxiumum acceleration rates will max out the electrical system, causing the excess to be dissipated as heat in the normal brakes.Higher power for SUV
If the commanded (positive) current is injected into positive EMF, which is voltage generated by rotating magnetic flux within the motor, then power flows from the battery to the wheels. It's a motor. If the commanded current is reversed, meaning (positive) current is injected into negative EMF, then power flows from the wheels to the battery. It's a generator. Simple. The same six transistors handle power flow both ways, no extra parts are needed, and torque reversals are nearly instantaneous (about a msec).
The higher power Toyota SUV (not the Prius car) contains a 2:1 step down coupled to the motor. P = torque x speed and a motor is sized basically by its torque, so the power out of a given size motor can be (ideally) doubled if it runs twice as fast. The SUV hybrid needs over twice the electrical power of the Prius car. This explains why Toyota has pushed the top speed of the SUV motor 12,000 rpm and then coupled it to fixed 2:1 step down (Reduction device) to match the 6,000 rpm rating of the power splitter.
Accleration from stop
-- The electric motor always pulls the car away from a stand still and the petrol engine starts to come in around the same time a non-hybrid car would change to 2nd gear. (In other words the motor (with power only? from the battery pack) provides all the torque at low vehicles speeds (eq to 1st gear)).
Possible replacement planetary gear power splitter (8/22/11)
Stumbled across a 2010 paper by two Hong Kong researchers proposing a replacement for a key element in hybrids: the planetary gear power splitter. In general terms a planetary gear set is three port device with fixed torque ratios that allows variable power flows by varying shaft speeds since [P = Torque x Speed]. The new approach is to combine the motor and generator into a single complex machine with two concentric rotors and a rotating 'magnetic gear' in between. In total three concentric shafts!
M/G1 is generator and M/G2 is x2 higher power motor
Two PM motor/generators combined with a magnetic gear
"Magnetically geared electronic continuously variable transmission (for hybrid vehicles)"
Authors: L. Jian and K. T. Chau, EE Dept, Univ of Hong Kong, 2010
(source -- http://www.jpier.org/PIER/pier107/04.10062806.pdf)
Here is their Prius type sketch (same as mine, but less detail) and to the right a hybrid architecture with their new machine.
. (left: Prius (same as my sketch) and (right) the Jian and Chau three shaft machine
New Hyundai hybrid architecture (1/31/11)
A decade or so after Toyota introduced its power-splitter hybrid architecture, Hyundai (Korean) will in 2011 begin selling (in USA) its first hybrid, a mid-sized car with a new hybrid architecture, which the company describes as an "original proprietary hybrid architecture to reduce weight and to improve highway fuel economy". The car is designed to compete with the Toyota Camry hybrid and Ford Fusion hybrid. It sells for 26k only 3k more than the non-hybrid version and gets 40 mph highway and 36 mpg city. It is unusual for a (full) hybrid in that its highway mileage is better than its city mileage.
The architecture looks simple and clean (see my sketch), so why hasn't it been used before? The explanation (from Hyundai engineers) is that the clutches until now were not good enough. The clutch here needs to repeatedly engage quickly and smoothly with the engine and motor running at high speed. Toyota in the early hybrid days tried this architecture and abandoned it because of the clutch. The key to making it work is that Hyundai has designed a new advanced electronically controlled clutch.
First look
I made the sketch (below) from the few facts contained in a (1/30/11) NYT review article.
engine + motor hp 209 hp (vs 187 hp Camry hybrid, 191 hp Fusion hybrid)
motor hp 40 hp
engine hp 169 hp
weight 3,457 lb (263 lb lighter than Fusion hybrid)
0 to 60 mph 9.2 sec
motor hp 41 hp (30 kw, 34?)
torque 151 ft-lb (205 Nm) (rect power peak at 1,400 rpm)
type PM synchronous
engine + motor torque 195 ft-lb (low speed)
battery kwh 1.4 kwh (72 cells x 3.75V/cell @ 5.3 Ahr in series)
weight 96 lb
voltage 270 VDC
drag coefficient 0.25
Combining torques
Note in this architecture the 'combining' of the engine and motor torque (power) is trivial. No planetary gear set here. All that's needed is to wrap the electric motor around the engine/clutch drive shaft. Or from another point of view it's an electric motor with a through shaft, the engine/clutch connected to one side and the transmission to the other side. With no current in the motor windings the motor torque is zero, the motor is effectively 'not there', there is a direct connection of the clutch to the transmission. Put current in the motor winding and the motor just adds its torque to the engine torque. Simple.
The car normally starts only with the electric motor (clutch open) and the car can run to 62 mph (vs 50 mph on other hybrids) using the electric motor alone. This of course is easily achieved because the motor speed can remain low since it is on the input side of the transmission and (probably) runs at the same speed as the engine. Though the small 1.3 kwh battery only has enough power for a few minutes of all electric power at high speeds, it probably allows Hyundai to so some tricks as this hybrid boast improved highway mileage.
The 40 hp motor must be used to replace the 'missing' power and low speed torque lost from converting the engine from otto to atkinson cycle. It's clear from the sketch that anytime the engine is running and has some excess capacity that the inverter can be started up to bleed off some of the engine power to recharge the battery.
Atkinson cycle
-- An atkinson cycle engine adjust valve timing so that the compression ratio is smaller than the expansion ratio, or a shorter compression duration to allow the expansion (power stroke driving the crankshaft) more time to fully run. The goal of the modern Atkinson cycle is to allow the pressure in the combustion chamber at the end of the power stroke to be equal to atmospheric pressure. This converts more of the fuel energy value into mechanical work at the expense of lower the maximum power. Due to the shorter compression stroke the engine does not take in as much air, so for a given weight and size it puts out less hp than an equivalent size otto cycle engine.
According to Wikpedia ('atkinson cycle') a lot of (maybe most) hybrids have their engines tuned to the atkinson cycle (besides the Hyundai Sonata) including Toyota Prius and Camry, Ford Fusion, Lexus, Mercedes and Infiniti hybrids. Use of the atkinson cycle apparently been kept secret with a 2008 article saying Toyota has just announced that the Prius uses this cycle. Supposedly the atkinson cycle (sometime called modifed otto cycle) can improv efficiency by 12-14%.
Hyundai advanced clutch
-- Hyundai engineers suggest that the bigger advancement is its use of a clutch to couple and decouple the engine from the transmission. According to Yang, it's an idea that was discarded in the early years of hybrid development, because of difficulties of engaging and disengaging the clutch fast enough to smoothly blend engine and motor outputs.
-- "The most difficult thing to overcome was the clutch engagement system, to engage without any delay or any shocks. Ten or 15 years ago, the technology was not good enough to accurately control the engine and motor with a clutch. At that time it was not possible. That's why Toyota got away from that idea. By using new advanced electronic systems, we made it possible to engage and disengage very quickly at very high engine speeds. We revisited the old idea with new technology to make it possible." The electronics helping to reduce clutch engagement shock take into account fuel flow, spark advance and throttle position, among other things.
Chinese BYD F3DM hybrid/electric (2/11)
A Chinese hybrid/electric car (BYD F3DM) is scheduled to be sold in USA early 2012 and a few details are beginning to be available. Architecturally it appears to be (basically) a Chinese Volt, but it's smaller than the Volt with only 75 kw (total) of motor/gen compared to 165 kw (total) in the Volt, and its engine is tiny (1 L, 3 cylinder). It began production (sort of) in Dec 2008 almost two years before the Volt, but Wikipedia says as of Nov 2010 only 300 cars have been sold, so it's just barely 'in production'. It plugs in and runs all electric for 30 miles, then its (little) engine starts and it runs as a hybrid. The engine is mechanically coupled to the wheels (through a planetary gear set?) like in the Volt. During a hard acceleration both motors and the engine can be used providing 168 hp (125 kw).
battery kwh 16 kwh (same as GM Volt)
battery type 100, 3.3.V lithium iron phosphate cells (330VDC bus)
active bat thermal management no
motor 50 kw (PM)
gen/motor 25 kw (PM)
engine 1 L, 3 cylinder, 68 hp (50 kw)
wheelbase 102.4 in
weight 3,439 lbs
A follow up NYT story indicates why this car did not sell in China (few sold were mostly bought by government agencies). The 2008 car is not finished, it's really a rough prototype, and needs a lot of work. One big problem is the transition from electric to hybrid. It runs differently in the two modes with a crude transition, and the hybrid mode is a horror. The NYT says when the 3 cycle engine starts "it screeches like a banshee, the steering wheel vibrates, the dashboard hums, you feel the vibration in your molars." As of now, BYD has zero dealerships in USA.
Electric car charge standard --- SAE J1772
I see conflicting numbers re: SAE J1772. Wikipedia says it is a 5 pin connector, rated 70A and 240 VAC. Others say it is 7 pin, 400 VAC, 63A. The German connector company, Mennekes, shows both for electric cars. Not sure which goes with J1772. An older IEC standard uses a five pin connector at 32A (max) 400 VAC (230 Vrms LN).Apparently the SAE J1772 (Society of Automotive Engineers) standard for plug in electric cars (SAE Electric Vehicle Conductive Charge Coupler) is still in development, though I read it has been informally agreed by a lot of (potential) electric car manufacturers. The connector is apparently (from a German connector company, Mennekes, brochure) capable of 400 VAC (230 LN) and 63A. Cables would come in different diameters with rating of 63A, 32A and 16A (smaller may use a five pin connector). If powered from a 400 VAC line (three phase), the cable could potentially deliver 43.5 kw (3 x 230Vrms (LN) x 63Arms = 43.5 kw). Asssume pf = 1. If powered from a 230 VAC line @ 63A rms power drops to 230/400 x 43.5= 25kw.
7 pin J1772 connector?, 400 VAC, 63A
(Nope, don't think so, cars have no hole for center pin)
Nissan Leaf with 24 khw battery has two connectors side by side:
a) J1772 connector for Level 1, 120 VAC and Level 2, 240 VAC (1 ph?) charging using
built in 3.3 kw charger (about 12.5A)
b) JARI DC connector designed by TEPCO for Level 3 480 VAC 50 kw (nom)
for 30 min 80% charge (about 100A). Charger is external and costs 16k!
(picture shows two large pins, so input here is DC, probably direct to battery
or through diode to prevent external discharge of battery)
Japanese call the TEPCO, CHAdeMO (a pun), and it has been adopted by all the Japanese car
makers (Toyota, Nissan, Mitsubishi and Subaru) for 480 VAC (Level 3) fast charging
Nissan Leaf (24 kwh production car) dual charge ports
right: J1772 for 120/240 VAC, Level 1/2 (about 3.2 kw, 12.5A)
left: JARI DC connector designed by TEPCO for 480 VAC, Level 3 (about 50 kw, 100A)
(6/11 update) Good bet that these will be the standard electric car plugs
.GM volt (16 kwh production car) J1772 charge port Ford Focus electric illuminated charge port (5/11)
(update 6/11/11)
Defacto standard connectors
From the photos it's pretty clear Leaf, Volt, and Focus are all using the same 120/240 VAC charge port: five pins (three large, two small) with an insertion notch opposite the stand alone large pin. This is the US standard 120/240 VAC plug.
According to a Dec 2010 Leaf user site all the Japanese auto makers have all adopted the two pin (TEPCO) connector used in the Leaf for Level 3, 480 VAC, high power charging, and they think the US Energy dept is about to install the same connector in some charging fast charging stations in US. If US adopts it too, this would make it the defacto world standard for 480 VAC charging.
The 480 VAC TEPCO (CHAdeMO) connector, and associated high current wiring to the battery, apparently also exist now as an option in a tiny new all electric city car from Mitsubishi (i-MiEV). A NYT reviwer test drove the car for a week and took it to be fast charged at the only high power DC charger run in Northern CA (run by Pacific Gas and Electric). He reports delivery of 10 Kwh of charge (to its 16 kwh battery) in 15 min, which is 40 kw transfer rate! No info on losses and no info on battery voltage, but it might be 600V since 40 kw @ 300V is an awfully high 133A [40,000/300 = 133A], which reduces to 67A @ 600V.
Below is a spec for a high power 480 VAC charging 'pump' from a European company (Anker Wade, Dutch). It also uses the Tepco DC connector of the Leaf. The pump specs are:
Input 480 VAC, 3 phase, 70A
Output power 50 kw (max)
Output voltage 240 to 500 VDC
Output current 120A @ 400 VDC (48 kw)
http://akerwade.indigofiles.com/AkerWade_DC_Fast_Charge_stations_nov_8_10r.pdf
----------------------
Ford is featuring that an optional 240 VAC charger will charge their 23 kwh (how discharged?) Focus battery is 3-4 hours. This is possible with 240 VAC, 30A, 1 ph, which compared to a standard 1.5 kw 120 VAC, 15A circuit would have x2 voltage x 2x current for 6 kw capability.
Competing (older) standard?
IEC 309, IEC 92196-1 22 kw, 400Vrms LL, three phase (230Vrms LN), 32A, five pin
3 x 32A rms x 230V rms = 22 kw (connector manuf spec)
Apparently under this standard a 250A version also exists.
Simple built-in charger
To charge the battery in a plug-in electric or quasi-electric from the line a charger is needed. While theoretically the charger could be external to the car, realistically it needs to be built-in so the vehicle can be plugged into an available 120 or 240 VAC line. There's now a recognized standard for plugging in a vehicle, specifying the cable and plug.
But what if the electronics and motor/generator already in the vehicle could be reconfigured to work as a charger, after all they're not doing anything when the car is being charged. If this could be done, there would be three significant gains: cheaper, lighter, more reliable. Alan Cocconi figured out and patented a couple of ways of doing this in the early 1990's. The first patent (1992) assigned to GM and the second (1994) assigned to Cocconi's own company AC Propulsion.
Alan Cocconi
Turns out Cocconi is something of a legend in electric vehicles. A Caltech graduate, he's been designing electric vehicle inverters and motors for 20 years. He did the prototype for the GM famous EV-1, the first mass produced electric vehicle. Two of his inverters and motors powered 'While Lightning' which in 1999 set the speed record for the fastest electric vehicle (245 mph). His company with more than 100 employees has been selling a 200 hp inverter and mating high speed (13,000 rpm) induction motor for 15 years, and it owns a plant in China that can manufacturer 2,000 per year. His patented charging scheme and other technology were licensed by Tesla Motors.
scan from patent # 5,341,075, Combined Motor Drive and Battery Recharge System, Issued 8/23/94
Inventor: Alan Cocconi, Assigned: AC Propulsion
I was interested in how he did it, so I did a patent search. His simplest charger is shown in the figure above from the 1994 patent. Everything to the left is already needed to run the vehicle, only switches K1, K2 and the filter on the right need be added to make the charger. The basic idea is simple. Motor control engineers have long understood how to control the currents in an inverter (inverter is the six IGBT's drawn in simplified from as switches S1 to S6) to regenerate power back from a motor to a battery. The trick here is to open the connection to one motor winding (with switch K1) and insert in series the (single phase) line voltage (with switches K2). When seen from the inverter, the line voltage now looks (pretty much) like the back EMF of a motor, so the normal regen mode of the inverter will now pump power from AC line voltage (120 VAC or 240 VAC, it doesn't much matter) and squirt it out as DC power into the battery at the battery voltage. Cost for the charger is pretty much just the cost of the added two high current contactors (K1, K2). No additional power electronics required!
While this scheme is neat, this is the kind of patent that early researchers get. I would argue that most good motor control engineer would come up with this basic idea in short order (motor back EMF is often modeled like the three phase line), though working out the details and robusting it would take some time.The patent shows the concept can be made to work with a three phase line too. All that's needed is to bring the neutral wires of the motor out, and the contactors insert the three phase line in the neutral connection. Note a requirement to make both of these schemes work is that the battery and all the electronics 'float' from the chassis of the car, but I think this is routinely usually done in vehicles with large battery packs for safety reasons. Here is the efficiency AC Propulsion achieve with their 200 hp induction motor reconfigured as a charger (very likely using their patented method above). This covers the range of the standard vehicle plug-in cord and plug which maxes out at about 16 kw (240 VAC, 70A).
AC Propulsion AC-150 200 hp motor configured as battery charger
The patent shows the recharging with an induction motor and notes it would work with a PM motor too. Well maybe, but it's not obvious this can be made to work properly. The issue is torque. Motors are designed to make torque when current flows. Here the opposite is required, no torque. Clearly the charging current can't be allowed to move the car, and it's also undesirable to have a strong 60 hz (or 120 hz) torque that would cause the vehicle to hum or sing while being recharged, though a creative marketing man might be able to make this little flaw into a positive! (Curiously the issue of torque is never mentioned in the above patent.)
Torque (in vector terms) = [current cross flux]. Induction motors will allow current flow without torque if they are controlled to keep the flux near zero. However, I'm not sure what the efficiency penalty is, because no flux is achieved by allowing cancelling currents to flow in the rotor. But in a PM motor the flux is fixed by the magnet, so figuring out how to inject currents in a PM motor (at any rotor angle) without getting a vibrating torque is not trivial and maybe not possible. If so, this might be a reason induction motors are favored over PM motors for electric cars.
In a quasi-electric there's more options, because there's also a generator. Application of this patent to the generator and its inverter to make a charger would obviate the concern about vehicle motion, and the generator would be free to rotate a little to an angle where current makes no torque.
GM Volt charger
From little news article below it appears that the GM charger is probably not intgrated with the inverter:
"One example (of high cost items) cited by Tony Posawatz, the director of the Chevrolet Volt program, is the on-board charger. Posawatz says the charger cost GM two or three times what it could have, because no company mass-produces a large enough charger for the Volt’s 16-kwh battery pack--one capable of charging from both 110- and 220- volt outlets, lasting for 10 to 15 years in harsh environment including extreme hot and cold temperatures, and enduring constant vibration." (11/2/09)-----------------------------------------------------------------------------
Wireless charger (update 6/2011)
This section is more or less an EE aside discussing wireless power transfer. However, this technology is being proposed by Delphi to wirelessly charge the battery of electric cars, able to deliver 3.3 kw through an 8" gap to a resonant coil mounted under the car. My interest in this was piqued when a friend send me a link to a wireless battery charger for an electric car to be sold by Delphi. You mount this 18" or so rectangular thing on the floor of your garage and attaching a thinner rectangular thing about the same size to the underside of the car, which (presumably) is hard wired into the battery charge port, so it sits pretty much above it when parked. The interesting thing to me as an EE is this product can deliver high power (3.3 kw) though a large gap. The gap is speced at 20 cm (about 8") "or more" and apparently does not need critical axial alignment to work, making parking practical.
Prof Soljacic, WiTricity
Turns out this product is based on work done by a small team at MIT starting in 2005, led by Prof Soljacic, who demonstrated the feasibility of resonant wireless power transfer (with reasonable efficiency) over distances of six feet or more. I vaguely remembered reading about this when it hit the front page of the newspaper five years ago, but I had heard nothing about it since. Soljacic formed a company, WiTricity, to commercialize the technology, and as far as I can tell, this is the first actual product to come out of this work. I was always interested in how they had done this, so when I found they had patents, I dug them out and read them (75 pages worth of patents!) #7,741,734 and #7,825,543. Soljacic is a physicist (two BS's from MIT in physics and EE) working at the Research Laboratory of Electronics and seems to work mostly on non-linear optics.
Included in the links below are a couple of papers with much of the same info as in the patents. One paper has two worked out example where 10 watts is delivered to a lap top with a D/r =5. The efficiency for what they call a dielectric disk (not sure what this is or looks like) is 51% and for a capacitively tuned coil its 61%. I suspect the dielectric disk runs at a higher frequency because most of the ten watts of so of loss is 8.3 watts of radiated power. The losses in the higher efficiency (61%) capacitively loaded coil on the other hand are mostly in the coils, with radiated power very low at (1/2 w). So coils have less radiated power and higher efficiency, but things run hotter.
http://www.witricity.com/pages/invention.html
http://www.mit.edu/~soljacic/wireless-power_AoP.pdf
Tuned resonant coils
The key to getting power to transfer over large distances, and without critical axial alignment, is to use two resonant (high Q) coils tuned to the same frequency. When two coils are far apart, the overlap of their magnetic fields is quite small, even with coils of the same diameter on axis, no more than a few percent or maybe less than 1%. When transformers are built, the materials and geometry are chosen to overlap as much of the fields of the coils as possible, and usually 90 to 95% coupling is achieved.
So the obvious question is how can power transfer work across such large gaps? The trick it turns out is to use resonant coils tuned to the same frequency. Even though there may be only, say, 1% flux coupling between the coils the analysis of the MIT team showed that when power is extracted from the receiving coil it is replaced (next cycle?) by power from the source soil.
My picture/circuit modelThe key is to making the power transfer practical is to keep down the losses, hence high Q, which means not only low ohmic and dielectric losses in the coils and adjacent materials, but also low radiation losses. In other words don't build an antenna. At this point I had no clue as to what frequency range they operated at. All I found in the press material and online was that that the coupling was via the 'near' field, but the patents provided the answers. Turns out that resonant coupling between coils has long been known to occur, and it had in years past occasionally been used when low power was needed remotely, like to power something in the human body or human ear, but apparently no one had really studied it seriously, until the MIT people, to see if it had the potential to wireless charge mobile devices. In engineering school the near field is shown in the text books, but is otherwise ignored. Transformers operate with very tight coupling, and antennas work in the far field. The far field is a radiated field, and here the laws of electromagnetics require that the local electrical and magnetic energy in space be equal. But this is not the case in the near field, the electric field can be much larger than the magnetic field or vice versa. It all depends on how the coils are build and tuned.
They provide no 'picture' of how this happens, but my guess is this: The near magnetic field probably where all the inductive (non-disappative) energy of the coil is stored. Each cycle it builds up and then winds down as the energy shifts to the electric field in the capacitor. Since this near field is in air (unguided by high u magnetic materials) it spreads out, and the receiving coil in its far reaches intercepts only a tiny fraction of it. With the receiving coil in place absorbing power (it will create a current that tends locally to cancel the incoming field) the voltage in the driving coil is slightly affected, because the 'back EMF, i.e. the d(flux)/dt, is slightly affected. So in circuit terms a resistor will appear in the primary circuit. And even though the reactive energy may be x100 the resistive energy lost per cycle, if the Q can be kept high (say 500 -1000) then most (or much) of the real power flowing into the primary coil ends up in the receiving coil.
High vs low impedance coils
A parallel LC coil will yield a high impedance, hence high voltage (low current) at resonance, so its near field is dominated by the electric field. Turns out this is not so good for humans nearby to be exposed to as our watery bodies absorb a lot of this energy (as heat). If the coil is built as a series LC, then it will have a low impedance at resonance, hence high current (low voltage) at resonance, so its near field is dominated by the magnetic field. The human body is nearly immune to the magnetic field, so this is the preferred way of doing the coupling. Unfortunately, it turns out that a switch from electric to magnetic coupling with coils of the same diameter cuts the efficiency about in half, from something like 40% to 20% according to their initial studies and demo in 2006.
Note, I find the efficiency claims in their papers confusing. High numbers (> 50%), apparently the result of theoretical studies, will be followed later in the paper by low numbers. The measured efficiency in their initial 2 meter, 60 watt demo was only 15%.With a capacitively tuned wire coil they say the electric field is mostly inside the capacitor and the near field is mostly magnetic. To deliver a few watts of power they say the near field magnetic strength is about 1 gauss (magnetic field of earth is about half gauss).What frequency/wavelength?
But what frequencies are they using. Microwave? No it turns out the frequencies are quite low, 17 Mhz to 380 Mhz for coils 1 to 30 cm in radius, the lower frequencies used with the bigger coils, it pretty much scales linearly. Their demo transmitted 60 watts over 2 meters with 30 cm (radius) coils powered at 17 Mhz (sinewave). The wavelength of 17 Mhz is 17.6 meters = [3 x 10^8/1.7 x 10^7]. In other words both the coil radius and transmit distance are much smaller than one wavelength. In the demo the ratio of wavelength to coil radius is about 60:1. The patent analysis finds high Q's can be obtained with transmit distance (D) to coil radius (r) ratios of 3 to 10. D/r ratio in the demo is about to 6.7.
Demo
To get 60 watts into a 60 watt light bulb the input power to their (colpits) oscillator was about 400 watts. They pretended not to know the efficiency of the oscillator! The overall efficiency was about 15% to transmit power 2m with a theoretical efficiency for the wireless transfer of 40 - 50%. Unfortunately the 40% efficiency was for a thick open coil (high Z) whereas a more practical thinner wound coil (low Z) gave about half the efficiency (20%). They built and tested both types of coils (same 30 cm radius), but the news stories of course featured the less practical deep coil. If you look closely at photos, you can see a smaller light bulb (30 watt?) was used with the thin coil.
MIT's 2007 demo powering 60 watt light bulb (Prof Soljacic is in 2nd row, striped shirt)
(2 meter gap, 30 cm radius 'thick' transmit and receive coils, freq = ??)
These are self resonance air coils, meaning the capacitance is between turns
(source -- http://www.mit.edu/~soljacic/MIT_WiTricity_Press_Release.pdf)
MIT wound 'thin' coil demo
(2 meter gap, slightly smaller radius coil, 25 cm)
It is likely this is tuned with a capacitor, but it is not visible.
No info on power delivered, but this looks like a smaller light bulb
(source -- http://www.sciencemag.org/content/suppl/2007/06/08/1143254.DC1/Kurs.SOM.pdf)
The $64 question on the wireless 3.3 kw electric car charger is its efficiency. Curiously, suspiciously, it is not given. But looking at it, it does not look capable of dissipating a 1 kw, so I suspect the efficiency must be pretty high (> 70-80%) to make it practical. It is operating with a gap (8") that is equal to or less than the radius, and this case was not discussed in the early papers and patents where lower end D/r was usually 3.
(source -- http://wheels.blogs.nytimes.com/2010/11/02/delphi-and-witricity-developing-wireless-electric-car-charger/)
NYT quotes lots of people saying "who needs it?", but the first time someone drives off with his car still plugged in (lock out?) his opinion will change. Long term something like this may make sense, but it has got to be efficient and not too expensive.
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MPGe EPA standard
EPA has adopted for electric range a milage standard called MPGe, miles per gallon equivalent. Here is the EPA definition:
The E.P.A. offers this definition, which seems clear and reasonable to me.
MPGe explanationQuasi-electric Volt has a MPGe rating of 93 and the much heavier Fisker Karma 52. Check: Volt can go about 35 miles electric says EPA using (I presume) 10.6 (?) kwh of its 16 kwh battery. So the MPGe would be (33.7 kwh gasoline eq/10.6 kwh used) x 35 miles = 111 MPGe
The mpg-equivalent metric expresses the energy consumption of a nongasoline vehicle in terms of how many miles the vehicle could go on gasoline if it used the same amount of gasoline energy as it used of the nongasoline fuel energy. For example, a gallon of gasoline has the energy equivalent of 33.7 kilowatt-hours of electricity. An E.V. (electric vehicle) that uses 33.7 kilowatt-hours to drive 100 miles will use the energy equivalent of one gallon of gasoline, and therefore would have an MPGe of 100 miles per gallon of gasoline equivalent.”
vs 93 MPGe EPA rating (something is off)
To explain the difference it is necessary to reduce the Volt's electric range to 29 miles for 10.6 kwh usage.
Wind resistance hp
"As the Prius reaches motorway speeds, the Prius will normally be driving solely on the petrol engine." (The engine must initially put out addtional power to recharge the battery, replacing the charge used to accelerate.) The engine is rated 98 hp (gen III) and 76 hp (gen II), so obviously less hp than this is required to overcome air resistance at highway speeds. Wind resistance goes as speed squared and power goes as speed x load, so the power to overcome wind resistance goes as the cube of speed, or 338% more power at 90 mph than 60 mph. The Prius max speed is reportedly about 100 mph, so working backward I get that it must only take about 16 hp to overcome wind resistance at 60 mph. (76 hp gen II/(100 mph/60 mhp)^3 = 76 hp/4.6 = 16.4 hp).
Check --- Wikipedia says car wind resistance runs 10 hp @ 50 mph, 80 hp @ 100 mph, which calculates to 17 hp at 60 mph, almost exactly the 16 hp I estimated from Toyota numbers.
Misleading block diagrams?
Many of the simple block diagrams I have found of hybrids are too simplified to be useful, and some seem just wrong. For example, I often see the generator drawn to the side of the power splitter (below), whereas the cross-section diagram from Toyota shows it clearly in line between the engine and the power splitter.
Misleading? Notice this show the generator to the side of power splitter,
but cross-section diagrams show generator inline betwen the engine and power splitter
source -- http://www.global-greenhouse-warming.com/hybrid-electric-vehicle.html
Well, having tried to sketch it myself, I see the difficulty. Physically the generator is inline between the engine and the power splitter as shown in the figure (way above) showing concentric shafts. But if you start with the physical configuration its difficult to show functionality. Thus showing the power splitter with a side (non-axial) output, while physically misleading, I now think is not really wrong because it does correctly show functionality and is much easier to draw and read.
Power doubly converted?
Some published diagrams show the engine sometimes powers the wheels partially via the generator/motor path. I initially suspected this was wrong. I mean, why convert engine power to electrical power and then back again to mechanical power, but the Prius specs do seem to show this. My own circuit diagram of gen III Prius shows 33 kw of 60 kw of the motor power comes into the motor inverter bus from the generator.
I don't really understand why 33 kw of power would be doubly converted. Why not pass it directly from the engine to the wheels through the power splitter? Maybe it's for flexibility of control or maybe reductions in cost/size of the power splitter. Power flow though the generator/motor path is very easily and quickly controlled electrically, so maybe this is the reason. The efficiency penalty for doubly converting 33 kw is not too large since inverter/motor efficiencies generally run in the 90% to 95% range.
(update)
Obviously when I wrote above paragraphs I had no clue as to how the architecture of the Prius hybrid really worked. I've left it in just as a guidepost as to how I come to understand things. I now understand that:
The two parallel power paths from the engine to the wheel provided by the three axis sun/planet/ring gear set ('power splitter') and two coupled inverters (for motor and generator) functions as a continuously variable transmission between engine and wheels controlled by adjusting current flow in the two inverters. The DC bus of the two inverters is connected direct to the battery or via a battery voltage booster. This provides bidirectional power flow from battery to the wheels (via the motor), providing all electric driving and regenerative braking, and bidirectional power flow from battery to the engine (via the generator), providing engine start and battery recharge. And since all the inter-connections are in place all the time and all the elements in the system (battery, motor ? gen, inverters, sun/planet gears, engine) allow bidirectional energy flow, the system is very flexible. The battery can be charged or the battery can assist driving the wheels while at the same time the engine drives the wheels though the variable speed transmission.My personal contributions to hybrid/electric technology
I worked as a power/motor control engineer for 30 years. Mostly I was a working engineer designing commercial motor drives, which entails doing circuit design, laying out breadboards, BOM, etc. But in the 70's and early 80's I worked in an R?D envioronment and worked on new techniques to control motors. Some of the motor control techniques I personally helped develop are now (it appears) used in hybrid car and electric cars. There are two in particular: field weaking for PM motors and converting induction motors into high performance servo motors.Field weakened PM (permanent magnet) motor technology
In the early 1980's PM motors were the standard motor for servos, as they are today, because they are easy to control, very efficient, and have a convenient rectangular T vs Speed curve. A servo PM motor can be run at any speed up to 'base speed', defined as the speed where the back EMF (from the spinning magnet) equaled the inverter bus voltage, but above that speed the motor torque rapidly dropped away to zero.
For servo applications the 'base speed' speed limit of a PM motor was not a problem, but there are other applications, like vehicles, where a wider speed range with reduced torque at higher speed is needed. The goal in high speed torque is a 'constant hp' curve where torque falls off slowly (as 1/speed). You can do this in induction motors by lowering the component of current that sets the magnet flux level in the motor. but in PM motors the flux level is fixed by the magnet.
Operating the PM motor above base speed was thought at the time to be impossible. PM motors just refused to accept much current above base speed, because (net) voltage needed to ramp up current in the motors' inductance gets squeezed to zero between the back EMF and bus voltage, so the torque dies. There appeared to be little that could be done about this because voltage from a changing magnetic field (like a spinning magnet) is a law of nature, it's built into Maxwell's equations, and a motor's inductance is basic to it creating torque.
But there is a way to run a PM motor x2 (maybe x3) times higher than it's base speed and my boss (Bill Curtiss) and I figured it out.
IMEC company lore
As R?D engineers for a motor control consulting company (IMEC), we flew out to Wisconsin in 1981 (or 1982) to describe our new PM control idea to the large electrial company that was funding us (Allen Bradley). I was up at the black board drawing out the vector diagrams of how to do it (the concept was really pretty simple), but the Phd motor control expert that we were trying to convince (Dr. Nondal) was skeptical. "What about the motor's sub-transient impedance", he asked us. Well neither I or my boss had ever heard of the 'sub-transient impedance' and so we sheepishly had to ask ask what it was and still didn't have a clue after hearing Dr. Nondal's explanation.
Sub-transient impedanceNeedless to say we didn't convince the experts that day that the concept was sound, but it was sound. We tried it, and it worked. This concept is now well understood for PM motor control and it's probably being used, at least to some extent, in PM motors in hybrid vehicles, since a wide speed range is needed.Our PM control idea
It turns out, I found out later, that large conventional line operated synchronous motors, which are a somewhat rare and exotic type of motor that had been around for nearly a century, have built into them shorted windings to provide damping, because otherwise these motors oscillate around the phase of the line as they rotate. The 'sub-transient' impedance referred to the fact that these shorted coils could be viewed as causing the impedance of the motor to be time varying. Of course, the motor we were discussing was a servo type PM motor, which has no such shorted coils, so the 'sub-transient impedance- question was out of left field and not relevant to what we were proposing.
Our idea was to control PM motor currents in such a way that the back EMF is partially cancelled using the motor inductance. We proposed adding a (non torque producing) quadrature current term in PM motors. Since voltage leads current by 90 degrees in an inductor, this creates a voltage across the motor inductance that is 180 degrees out of phase with the back EMF. It's a cancelling term that makes the motor's back EMF, as viewed from the outside, look smaller than it really is. Above base speed the internal motor back EMF voltage from the spinning magnet can be much higher than the bus voltage, yet with proper control law for motor currents the voltage at the motor terminals, which is the sum of back EMF and inductive voltages (plus IR voltage), can be held to less than the bus voltage allowing the motor to be well controlled.
Fly in ointment?
There is a fly in the ointment though. A nagging concern has always been what happens at high speed if the inverter has a fault, i.e. it suddenly shuts down for some reason. There is still high voltage inside the rapidly spinning motor, but the cancelling voltage now disappears, so there is risk of inverter damage due to over voltage. While this is speculation, this robustness concern may explain why the Toyota's hybrid motors, which are marketed as PM synchronous motors, are revealed in the technical literature to be combo PM/reluctance motors, the reluctance torque allowing the PM flux levels to be substantially reduced.
High performance induction motor technology
In my essay on Tesla's induction motor (below) I describe in some detail my contributions to induction motor control theory.
http://twinkle_toes_engineering.home.comcast.net/induction_motor.htm
Brief summary of my contribution to high performance induction motor control
For several years in the late 1970's all attempts by researchers at getting servo-like performance from the induction motor failed. A step of torque (really a fast ramp) would be requested, but the motor, if it made the torque step at all, would then 'ring' the torque for a half second or so. No one could figure out how to stop the ringing.
The failure to tame the rotor time constant was indicative of the fact that no one at the time, not even the experts, 'really' understood how the induction motor worked. At least they didn't understand it dynamically. The induction motor, while complicated, had long been written up in the motor text books, and the text books were accurate as long as things didn't change too fast. Think how remarkable this is. The world's most common motor and almost a century since its invention, and no one really understands how it works!Here is where my work comes in.... I solved the ring problem. Yes, I did, the patent record confirms it. I contributed to improved understanding of the induction motor dynamically, and my contribution laid the groundwork for the standard ('vector') induction motor controller of today. I worked on induction motors first at Draper Laboratory (formerly MIT Instrumentation Laboratory), which was a large, mostly government funded research and development laboratory in Cambridge MA. They filed for a patent on my induction motor breakthrough in 1980 (Title: Induction Motor Controller with Rapid Torque Response) with me as sole inventor, and in 1982 the patent was granted. (US patent 4,348,627).
I remember how the break through happened. I was in the quiet Draper library reading a colleague's draft MIT's Master Thesis on another aspect of induction motor control, when it suddenly occurred to me that maybe the phase of the motor currents were not being properly controlled, i.e. there was a missing phase term in the equations. I sketched what I thought the phase term should be (surprise, it is related to the rotor time constant) and we plugged it into a computer induction motor model which we already had that displayed the torque ringing. When the phase term was added, the ringing completely disappeared. So within an hour it was pretty clear that I had guessed right, we had found the key to getting servo like performance from an induction motor! In turns out (in hindsight) that a key stumbling block to understanding the induction motor had been that the textbooks for nearly a century had said induction motors were 'asynchronous' motors (often the chapter title), meaning motors that are phase insensitive. That's wrong! For times less than the rotor time constant (few tens of a second) induction motors are synchronous motors and the phase of the currents (relative to the rotor angle) needs to be controlled.Two years later I left Draper Lab with three other Draper people to form a new start up company (IMEC Corp) to work on induction motor controls and other cutting edge work in motor control. There Robert Comstock and I used our recently won understanding of induction motors to make one of the first induction motor controllers. We developed, prototyped, and patented a complete high performance induction servo controller in 1982. (US Patent 4,484,126, filed 1982, issued 1984). I presented a paper at an engineering conference showing my new model of the induction motor, and Machine Design, a well known engineering magazine, picked up the conference paper and republished it in their magazine as a feature article. These induction motor control concepts are now built into modern induction control theory, often called vector control or field control, which will be used in any (electric) vehicle powered by induction motor(s), like the Tesla Motors Roadster and the coming GM Volt.
Battery cost
Big buck have been poured into battery development for vehicles in USA for 20 years. Progess shall we say has been slow...
"In 1991 the Advanced Battery Consortium was founded and set a near-term target for developing a battery that would cost $150 per kilowatt-hour of storage. (A kilowatt-hour sells for about a dime and will move a car three or four miles.) Eighteen years later, prices are in the range of $750 to $1,000". (NYT 7/27/09) "G.M. would not disclose the price of the (GM Volt) battery pack" (NYT 7/27/09)Oh yea! Government is now planning to spend 2 billon!! on battery development beginning summer 09. Every company that makes batteries is going to be swimming in money, numbers being thrown around are 150 million per company!
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Misc
In a Prius forum someone asked how the engine can be started there's not enough charge in the main NiMH battery. The answer was that the 12 V lead acid battery through MG1 (generator) starts the engine. Is this right? Is it a backup mode? Seems to be to make this work would require an extra step up converter because the generator has high voltage winding.
Here is a good detailed tutorial that explains the main NiMH battery is used to start the motor.
http://prius.ecrostech.com/original/Understanding/WhatsGoingOnAsIDrive.htm
Tesla misc
Owner takes his Tesla Roadster to a 2.5 mile road track. In only 3 to 4 miles(!) of hard driving the power of his inverter is dialed back as its hot warning light comes on, then his motor hot warning light comes on too.
"Typically, the PEM (Power Electronic Modules, aka, inverter) hot warning would come on in 1 to 1.5 laps, then the motor hot warning would come on in several more laps with associated power reductions. As an example, on the highway straight away, with a fresh cool car, I could get 110 MPH, but with reduced power, all I could get was 90 to 95 MPH."Forum guys says PEM is air cooled. Big mistake! They will be moving to water cooling in future cars. Tesla has a 17 kw charger. This gives a charging time of 3.5hr (for about 50kwh) if 17kw is available
from 240 VAC 17kw is about 68A. Guy at test track recharged at @ 40A from 240 VAC RV outlet.
Tesla has run their induction motor on the dyno to 18,000 - 20,000 rpm. Car limits is 13,000 to 14,000 rpm.
Tesla patent applications
20070284953 motor rotor
20070009787 cooling batteries
20070218353 battery fire suppresion
20070188147 fuse links for batteries
Tesla Motors' Stealth Bloodbath - After raising over $100M, Tesla Motors Inc. has fired over thirty people -- nearly the entire team. Nagging transmission problems were partly to blame for the meltdown. (TeslaFounders; Jan. 10, 2008)
Powertrain 1.5 redesign (to eliminate 2 speed transmission)
"The revised PEM has new transistors (higher I rating) that help improve the overall efficiency, allowing more power with less heat generation."
"The more substantial changes are in the motor. The terminal connectors have been redesigned and the high voltage cables that connect the motor to the PEM have been switched from copper clad aluminum to pure copper. The locations with the highest resistance were attacked directly allowing more current flow without increasing temperatures. The result is 33 percent more torque at the thermal limit than the existing motors."
"I can't keep the Tesla drive train versions straight. Anyone have a program? As best as I can remember it, first it was a two-speed transmission from vendor 1. Failed. Then a two-speed from vendor 2. Failed. Two-speed from vendors 3 and 4. Failed. Single-speed design, let's call that 5. Failed. Now we have yet another design."
Just to make the record clear it goes like this:
1. Two speed from vendor 1: failed
2. Two speed from vendor 2: failed
3. Planned to try a two speed from vendor 3 ? 4.
4. Found better plan during development in Whitestar: go with a single speed while increasing power, so plans for two-speed dropped.
5. Production cars 2-40 gets/will get 1 speed (AKA the "temporary transmission") which works in every way on target (durability and etc), except 0-60 is 5.7 seconds instead of 4 seconds. (Production car #1, AKA Elon Musk's car, has a 2 speed)
6. Production cars starting with #41 is getting the single speed "drive train 1.5" described in this article, which does get 0-60 in 4 seconds.
** "PEM which can now do a 640A current limit on its input as well as a new and higher 850 amp limit on its output are all good incremental upgrades. We now multiply that by the latest motor current increase, 850A/640A (May 2008)
** "Ver. 1.5 adds a current boosting voltage reducing circuit for low speed operations to improve low speed torque." Forum post (not sure what this means.)
One possibility is that at low speed they are using contactors to reconfigure the battery. Putting two five sections in parallel would half the voltage and double the available battery current. It would also make the IGBT's happier by cutting switching loss in half when they conduct maxium current. The negative to this is there's a 'switch point' where torque is lost as the contacts throw and battery reconfigures. Is there an effective gear switch in their one speed transmission? Or, maybe he's just talking about voltage sag
"Eberhard (former CEO of Tesla) wrote a well publicized and needless (a fix was already in progress) negative blog piece regarding the battery coolant pump soon after receiving the car (production car #3).
'via Blog this'
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