C. Douglas Weir wrote GM about the Chevy plug-in electric – gas hybrid Volt’s extended range feature. GM wrote back. First, Doug…
My confusion is the issue of what, exactly, happens as the range is exhausted for battery-only propulsion. I keep seeing the phrase: “a small engine-generator creates additional electricity to extend the range of the Volt several hundred additional miles” or something to that effect.
So, what exactly does this mean? Is the performance of this small engine-generator adequate for cruising down interstate highways and up long steep grades? Does the Volt become underpowered with this out-of-battery charge scenario? Say you were driving a couple hundred miles to the mother-in-law’s and you own a Volt and a G-6. Would the Volt be left at home because it would be unpleasant to drive in extended range mode? Or would it drive just fine on the small motor-generator with performance similar to a standard small four cylinder midsize car?
Also, the wording implies that the drive train would remain electric, with the motor-generator producing enough current to power the electric motor(s) through or around the battery stack. So is it correct that there is no direct drive from the engine to the wheels…rather it is a serial hybrid like a diesel locomotive when the battery is exhausted?
I hope you can clear this up for me. Any added information would be much appreciated.
Answer after the jump.
Hi Douglas. Thanks for the inquiry. Let me explain in simple terms. For more detail, check out media.gm.com/volt and look in the Technology tab. Lots of info there.
The Volt has two modes of operation – EV and Extended-Range. The engine generator seamlessly operates when the energy level in the battery is “depleted”. Although the driver will see the battery as empty, energy in the battery is really sustained or held in what we call a “buffer zone”. Think of it like a home generator when your power goes out. It starts to create additional electricity to power the car hundreds of miles. The electricity is fed to an electric drive unit, which powers the car. There is no mechanical connection between the engine generator and the wheels. As long as you stop and refuel the generator (like in any conventional car today), you can drive from coast-to-coast until you can stop and plug-in to recharge the battery. As far as performance, there is no compromise when in Extended-Range mode. The battery and engine generator work together when peak performance is needed most, such as driving up inclines.
Hope this helps.
Dave
Dave Darovitz
Manager, Chevrolet Volt and E-REV Communications
So the batteries have limited output to the degree they can’t push the Volt up a hill without help from the generator?
So ask one question, get another that’s even better.
It does sound like the engine might come on even before the battery is depleted if the load requires it. This is the case in the Fisker–for peak acceleration the engine must be running. According to Fisker, the batteries cannot discharge energy quickly enough. Then again, even on just the batteries the Fisker will supposedly hit 60 in 7.5.
As far as performance, there is no compromise when in Extended-Range mode. The battery and engine generator work together when peak performance is needed most, such as driving up inclines.
Now this doesn’t make sense. The battery and engine generator CAN”T work together when peak performance is needed most, if the battery is allready depleted
So to get up that step hill after the battery is drained, the VOLT will either limp along only on the generator, or the battery will be pulled down below what is safe for it causing excessive wear.
Is this going to be a problem in real world? Maybe not, the target group for this car more likely to be concerned with the mileage and the environment rather than performance. But pretending that this car will perform without compromise in every situation, when clearly it can’t seems dishonest.
Imagine that dishonest car salesman!
Good question, toxicroach. My first response was that GM’s answer implies that this is the case. But a closer reading leads to–unclear. The engine is clearly insufficient by itself. It’s not clear if the battery is also insufficient by itself.
In the Fisker, the battery is insufficient by itself–in battery-only mode the car takes 7.5 seconds to hit 60, vs. 5.8 using both power sources together. Might be the same way with the Volt.
I would be curious to see a comparison to the Prius plug in hybrid that’s coming up.
MotorScoops.com
t-truck: as Dave D said, the battery is never really “depleted.” Instead, the engine kicks in when it reaches a certain level, and bumps it up to a higher level (which is still far from full). However, if the grade is long enough then eventually the battery will get to the point where it cannot contribute. The question then: how long must the grade be to reach this point?
The emperor has no clothes, but GM doesn’t want consumers to know it.
I think the “small gas engine” will provide terrible performance after the battery is depleted.
This is a car which will have two 0-60 times: one for full batteries, one for empty batteries. That could make for potentially dangerous driving situations where you won’t know ‘which Volt’ you’re driving as you merge into traffic – the powerful one, or the weakling.
t-truck: as Dave D said, the battery is never really “depleted.” Instead, the engine kicks in when it reaches a certain level, and bumps it up to a higher level (which is still far from full). However, if the grade is long enough then eventually the battery will get to the point where it cannot contribute. The question then: how long must the grade be to reach this point?
Sure the engine does come on before the battery is at 0%, but for a reason, very high drain on the batteries is not healthy for their longevity.
So if the batteries are designed not to go below some number say, 25% of full charge, what happens if they are right at that point at a bottom of a step hill?
There the options are excessive wear on the batteries and/or limping along only on the generator.
Not necessarily a problem but something that should be acknowledged. Not every car needs to be everything for every user.
Q: What kind of a towing capacity does the Corvette have?
A: Very limited the Corvette is not a designed to be a tow truck
GM should not be trying to sell the VOLT as performance vehicle and should not foster unrealistic expectations that it is going to be one.
I think we’re splitting a few too many hairs here.
Of course GM has over-promised. That’s true of almost anything they’ve ever introduced.
I reality, I imagine the car will behave exactly like a Prius except with the option of short-range electric-only operation. I also suspect the car will end up with more ICE power than is currently advertised, or at least more electric power available from the genset. The car’s performance, in both modes, will be modest at best, like the Prius. That is – slow. This is NOT a Tesla or Fisker, nor is it advertised to be.
Nonetheless, there will be vast numbers of early buyers timing the amount of ICE usage, whining about it online and claiming fraud and conspiracy. If I were GM, I’d do a better job of managing expectations.
Here we go again: When the Volt is running on its generator, it’s performance will be “adequate”. The only situation where performance will be diminished is on very long, extended upgrades, think the I-70 grade over the continental divide west of Denver. In that situation, the Volt’s performance will be like a compact with about 100 or so horsepower. The Prius suffers from the same problem.
The generator will not come on to augment the batteries until they reach their designated state of depletion. The batteries have plenty of power to maximize the Volt’s electric motor.
This makes sense to me. Let’s assume that once the battery is ’empty’ and the engine switches on, the battery still has enough charge for 5 minutes of continuous full power, before dropping to an unsafe / high wear level of charge. In practise, that means 5 minutes of full throttle. Please find me a public road where you would need full throttle for 5 minutes. As soon as you lift off a little, that time will be extended, or the ICE will continue to operate at full power and extend that time ready for your next full throttle burn.
Does GM market the Volt as a performance vehicle? I do not get that impression.
I suspect there will be times when the safest and most effective thing to do with a Volt will be to pull over to the side of the road and charge the batteries. Much like some gasoline cars used to overheat going up hills. Don’t see that much anymore, but it used to be a big problem. The Volt will probably have a lot of problems like that as the technology will need to mature.
I recently read an article in the Economist…stay with me….on why honeybee colonies always make good decisions on where to move the colony when it is time to split the hive population. Here is why: scouts are sent out and each one does independent research to determine the best choice. When they return to the hive, majority rules. It works because every scout does their job and actually goes to check out the possiblities themselves. With enough scouts reporting back experience and wisdom prevail.
Humans? Lazy. One human “scout” speaks, others jump on the wagon. Emotions rule. Few do independent data gathering. It is too easy just to choose sides among the few who speak out. Bad decisions result. See Congress.
How many of you “scouts” actually read the referenced information about the Volt extended range powertrain before “voting” in your posts above? And did any of you see the SAE paper on how the EPA tests relate to the average driver’s rate of acceleration and deceleration inputs (badly)? Did you like the part that explained why a full performance electric powertrain is the best solution, vs. a two-mode or assist hybrid?
I thought so……
Even if GM can get the Volt built it will probably fail because it will be beyond most consumer’s ability to make the effort to truly grasp its design and intent.
Sure doesn’t help when “Communications Dave” at GM obviously doesn’t understand how the thing works, or apparently cannot explain it in plain English. This is how myths start.
The media.gm.com/volt link explains it all properly. The question remains about what a dog the car will be with a depleted battery at the beginning of a steep mountain grade.
If and when it becomes available, the Volt will be a big surprise to Car and Driver staff. They still think the engine will recharge the battery like a hybrid. Dumbasses. Feb 09 p33
According to gm-volt.com, there is enough spare energy kept in the battery to aid in short hill climbing or acceleration. The battery will then be recharged up to a certain point when energy is available (braking, traveling downhill) In the cases of long hill climbs, you will be limited to 53kW of power.
I have read that it takes about 20 HP (~15kW)to keep a Prius going 60 (overcoming wind and rolling resistance) Judging it’s drag similar to the Volt, that leaves 38kW to lift the Volt. If it weighs about 1600kg, the Volt could maintain 60 mph on a grade at an angle of up to 5 degrees. It would be less if the Volt is heavier, or traveling about 60 mph.
This dosen’t sound like much of a problem to me. I guess it really depends on the maximum charge/discharge rate on the battery pack and the engine/generator’s flexibility in meeting those capabilities.
If we’re talking about an ICE that only operates within a narrow (probably most fuel-efficient) rev. range at all times then it is likely there will be issues. I’m no electrical engineer though, it could be that a rev. limit would be instituted because above a certain level the generator is delivering seriously diminished returns on energy investment. Still, in that instance I imagine the solution is just more gas.
As much as I like the idea of a plug in, extended-range hybrid, I think having a separate generator is a bad idea.
The only advantages I can think of are that it’ll let you design the engine to run at a specific, constant rpm, and it lets you be more flexible with packaging.
Disadvantages are the inefficiency of going from mechanical to electrical back to mechanical power. It’d be better to run it like a Prius and have the IC motor drive the wheels directly, while excess torque goes to charge the battery. Besides having fewer drive line losses, the full power of the IC engine and electric motor can be brought to bear simultaneously for greatly increased power. The biggest advantage, however, is that it would be far CHEAPER than having a separate generator. Those aren’t cheap, and the electric motor can ALREADY function as a generator when it isn’t being used. The architecture just doesn’t make much sense.
A “through-the-road” hybrid would be a very simple way to implement. Have a Volkswagen-esque rear engine/rear drive layout, with an electric drivetrain powering the front wheels. You wouldn’t be able to charge the batteries unless you’re moving, but it would be a very cost-effective design.
Figuring out the optimization of the Volt’s control algorithms has got to be keeping some engineers working pretty hard. When to kick the engine-generator on, how hard to push it and so on. I would think that once the engine-generator starts up it would make sense to run it at its peak efficiency point, which is probably generating in excess of the electrical power required to maintain straight and level cruise at legal speeds. If so, then once it kicks on, the batteries should start building up a charge. But, the software doesn’t know what the driver intend to do next. Is the generator kicking on just two miles from home and that long suckle at the electric plug, or is it kicking on at mile 41 of a 500 mile journey?
Will the Volt be smart enough to get information about the driver’s intentions? If there is an integrated sat-nav system and the driver puts in a flight plan, then the car would know exactly what to expect. This would be ideal, as upcoming elevation changes and driving conditions could be used to optimize the algorithms. Yep, that would be the way to go. Standard integrated sat-nav system and tell the driver that the vehicle will deliver the best economy and performance if you put your destination information in it. Heck, you could in theory even download your plans for the next several days into the car and let it figure out how to optimize fuel burn vs. electric plug-in times.
@John Horner; all your speculations and questions have been hashed out and rehashed endlessly and in great detail at http://www.gm-volt.com
Poor answer from GM but, at this point, it doesn’t matter; you can’t buy this car anyway. Why bother with detailed questions about a theoretical product?
Michael Karesh: “The engine is clearly insufficient by itself.”
I don’t know that to be the case. The Volt, since it’s carrying a 400lb battery, is rather a lard-ass but should come in at 3500 or so lbs and the engine is a 1.4L which, I think, will produce about 110hp. This would be marginal but not insufficient.
I mean, I’ve owned a couple cars with significantly worse power-to-weight ratios and they weren’t the end of the world (although you could see it from the driver’s seat of either one).
Even if the engine is rated at 110 hp the question remains how much of that power will reach the wheels considering significant electrical losses in the Volt’s unique drivetrain.
Horsepower wouldn’t even be the right measure of the engine, would it? I mean its not really an engine anymore, its a generator.
carve: “A “through-the-road” hybrid would be a very simple way to implement. Have a Volkswagen-esque rear engine/rear drive layout, with an electric drivetrain powering the front wheels. You wouldn’t be able to charge the batteries unless you’re moving, but it would be a very cost-effective design.”
It took a moment to realize what you suggest. Ingenious. Your own idea?
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rpn453,
Well, yes. You have nailed it.
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The charge-depletion/charge-sustaining/charge-replenishing problem seems to me to become really interesting only when you add in hills, especially the big ones. GPS and route-planning that’s altitude-aware could provide a solution. The software would want to add as much charge as necessary (or possible) before attacking a hill and would optimize the use of the engine and battery to reach the top with a battery at minimum state-of-charge (because you can “fill up” easily on the way down).
Or – add a way for the driver to give the system advice on how to use the battery. Use natural intelligence (such as it is) to augment the system.
toxicroach: Horsepower and Kilowatts are different units for the same thing – power. You can describe fuel- or electric-powered engines/motors in either HP or KW, but with cars HP is commonly used with internal combustion engines and KW is commonly used with electric motors. This is confusing for a vehicle that uses both.
However, in industry, electric motors are commonly described in HP, with the KW being used to describe the input requirements to the motor. Then you have to deduct efficiency losses to calculate the actual output.
The conversion is this: 1 HP = 0.746 KW, or 1 KW = 1.340 HP.
So here’s an example:
If a 100 HP gasoline motor (at the output shaft) is connected to a generator that is 90% efficient, the generator will produce 67 KW of electricity: 100 x .746 x .90. This energy is then converted again to operate the drive motor for the vehicle. If the drive motor is 90% efficient, the wheels will receive 81 HP. So you can see that efficient energy conversion is vital for the Volt. However, when operating on batteries, the conversion between the gas engine and the generator does not matter, and you deliver the battery energy directly to the drive motor at its efficiency alone.
The math is much more complicated than this, since thermal issues play a large role, as does the electrical management between the batteries, the drive motor, and the rest of the vehicle.
The efficiency losses are in the form of heat, which is why electric motors get warm while running. If they were 100% efficient, all the electricity going into them would be converted to work output.
To be fair, a conventional vehicle faces the same challenges. A transmission has efficiency losses, and the Volt eliminates this problem by not having one. Any variable-speed motor or engine also suffers from varying efficiency, so being able to limit the operating range (RPM range) of the Volt engine will help improve its efficiency. This is not really possible with a conventional vehicle.
Unless the motor is built directly into the wheel, there is some type of rudimentary transmission.
GM have already written a paper that answers the original question, and renders much of the commentary above redundant or worse.
http://media.gm.com/volt/eflex/docs/paper.pdf
I want to see an uphill 1/4 mile dragrace between a Volt with depleted batteries running on the generator, and a 70’s era Mercedes 240D. ;)
Castles made of sand slip into the sea…eventually.
I have been thinking on a different line. The ICE is only there to charge the battery. Power is supplied to the drive train from the battery. The ICE comes on the ensure sufficient charge. My question is “will it charge the battery to full even while driving?” Like the electric fan on small cars, will the ICE continue to run to keep the battery topped up?
OH, and this is false:
Q: What kind of a towing capacity does the Corvette have?
A: Very limited the Corvette is not a designed to be a tow truck
Corvette is a capable tow vehicle. It has plenty of towing power. It is just sprung wrong.
No, the paper I linked to explains that once the car starts driving the battery state of charge will generally fall, until it reaches the minimum level, roughly 30% capacity. From there on the engine will maintain the charge at around that 30% level, while using the battery as a buffer to provide acceleration and absorb regen. The engine runs at a few fixed points, the battery provides or absorbs the difference in power between that power and the demand power for the car.
The only way to restore the full charge in the battery would be to push the car down a very long hill (joke) or plug it back into the grid.
Incidentally the PR droid’s response was accurate, and he did advise the original enquirer to read further on their site. Not too sure why that exchange got such a negative reaction.
DougW hit the nail on the head.
How many of you guys have any experience with electric powered vehicles (golf carts, electric scooters, LSEV’s, etc)? I’ll bet its under 5%. For those that do already know that a electric vehicles that operated “adequately” on full charged batteries on relatively flat and level ground with a median load perform abysmally when low on charge (<35% is typical for LiPoly) OR on a grade/incline OR when loaded past the standard load (3 passengers rather than two, for instance) OR when the ambient temperature falls below a certain point OR at high speed/drain rate. Combine any of these five, and you can easily have a stationary seat instead of a motive vehicle.
The Volt is, at best, going to have to be babied, pure and simple, and driven like a cautious granny with osteoporosis and a bad hip. With no direct interlink between the ICE and the drive wheels (see GM response), remaining battery power is the sole determiner of acceleration rate, climbing power, and payload.
Further, unless the ICE generator is enormous (hint: It isn’t, I’ve read the GM paper and as an MSME I’m shocked by the engineering handwaving going on), it will not even be able to keep the battery at a given charge level, much less provide motive force for the vehicle or recharge the battery. GM expects the 16kW-hr battery pack to provide 100% of the vehicles motive HP until down to a certain point, where after a meager ICE will kick in to supplement what little remains.
Assuming you blow the 40 mile range in an hour of smooth, easygoing flat level driving with the average payload on an average day, the internal ICE would not only now need to power the vehicle entirely (through the electrical system), i.e. supply another 16kW-hr from then on. That’s a buttload of electrical power, as in 220V x 145 Amps per hour.
Now, in reality imagine a GM alternator hooked up to a 12HP lawnmower engine. Without battery maH, its not even going to move, much less move while charging keeping the battery at a reduced equilibrium level. Dead battery = dead Volt.
By contrast, the current generation Prius relies on the battery pack for a mere 28HP, the rest coming directly from the ICE.
Ponder that a bit, and you now know why this is going to be a huge disappointment to anyone suckered into buying one.