REEV concerns

@james the issue in a series hybrid system isn't so much maintaining a speed, which requires relatively little power, but rather acceleration after the battery is "depleted". A Volt genset, if it had to handle the task alone, would have a very hard time passing someone on the highway. This is why they only have 8kWh usable but the overall pack is 16kWh. It gives them a lot of cushion to allow for the battery to add energy for acceleration even after the 8kWh is done.

There could be cicumstances where someone is doing a lot of hard accelerations and passing or maintaining high speeds for long distances where the pack will truly be depleted and the car's performance is significantly reduced.

I consider this a point of advantage for EV over series hybrid, but GM has designed around it so it isn't going to be a common problem. I don't know if fisker is doing something similar. They have spec'd 50 miles range with 20kWh which seems to assume the same Wh/Mile if they are also only using 10kWh as the usable range. Since Wh/Mile is likely to be higher than the volt, they will likely need to use more of the battery to achieve 50 mile range.

But the again they spec'd a very powerful ICE for the Genset so I imagine they can do more under genset power. Downside is poor efficiency.

siry said:

the issue in a series hybrid system isn't so much maintaining a speed, which requires relatively little power, but rather acceleration after the battery is "depleted".

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Not only acceleration on the flats, but also maintaining speed on steep inclines, or into a strong headwind.
(Better not get stuck trying to accelerate uphill into a headwind... it does happen sometimes).

I think this ongoing concern of the Volt not having adequate performance in some hypothetical worst case situation is way overblown. The power output of the ICE in a Prius is about the same as the generator output of the VOLT. The capacity remaining in the battery of Volt at the time it is considered depleted and the generator begins providing power is much greater then the full capacity of the battery in the Prius.

For the Volt to not have acceptable performance you will have to find a stretch of road where you would be able to keep the accelerator of a Prius to the floor for probably considerably more than 15 minutes and it would be under the speed limit for the entire time.

@donauker not a problem for a car like the volt but it was one of my major concerns when we considered REEV for WhiteStar since it was a sports sedan meant to be driven aggressively. Also - the "cost" of not worrying about this issue is a 50% usable range in the battery - that's expensive!

@james - there is a key difference in cell chemistry. because of the cycle life issue you are describing, you have to use iron phospate (like A123) or manganese (like LG Chem) or other less common types in PHEV applications. These types of cells have lower energy density by a factor of about 2.

If you used cobalt oxide cells for a PHEV like the volt, they would not last 2 years (depends on a lot of things, but you get the gist). But in a pure EV like tesla, the pack is so big and contains so much energy that the lower number of cycles covers an awful lot of miles, so you see a longer life like you mention (along with all the temperature and charge balancing and other magic that treats the batteries nicely)

If you used these types of chemistries in an EV you would have a much shorter range (lets say 100-120 miles for the same weight). But a nice benefit would be you would have much better cycle life properties over time.

siry said:

there is a key difference in cell chemistry. because of the cycle life issue you are describing, you have to use iron phospate (like A123)...

If you used these types of chemistries in an EV you would have a much shorter range (lets say 100-120 miles for the same weight). But a nice benefit would be you would have much better cycle life properties over time.

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Yes, and some other wannabe EV makers are also trying to use LiPo because they don't have the capability to create a safety mechanism to keep the cobalt cells happy like Tesla did. (Or even if they did, Tesla's patents could get in their way).

And again, I think we have to consider the PI Prius a whiff because it's got 0 EV-only miles. We can dis GM for it's bad labor deals and parts-bin engineering, but with the Volt, they're really trying. Toyota gets credit for first attempting the complex through-the-drivetrain hybrid they did (I'm looking at you Honda, and that super-cheesy through the road junk you tried). But that was years ago, and now they're just looking like all they want to do is greenwash.

It's 2009. If you can't do at least 20 highway miles EV-only, you're not at the leading edge.

Bloomberg.com: Japan

The new Prius adds software to recommend to drivers more fuel-efficient habits. The 2010 model also offers three different drive settings including “EV” mode that lets the car go as far as 1 mile (1.6 kilometers) using only electricity from its nickel-metal hydride battery packs.

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But the Volt still isn't on the road, and there are over a million Prius so far.
Also the Prius is supposed to be the most aerodynamic car available.

For an around town, urban vehicle, the Volt will probably be more efficient due to PHEV, but if you needed to drive a family of four long distances, the Prius may still be a better way to go.

James, I have been saying the same thing for a long time.
Some had even claimed you could get by with only a 20hp ICE generator in a "REEV/eREV", but only providing steady state flat highway cruising HP doesn't seem nearly enough to me.

Getting from Here to There « Tesla Founders Blog

David Kosowsky said:

The Volt will only charge to 80% and will run only on battery power to 30%. Once at 30%, the generator will kick for “charge sustaining mode”…I’m hoping there is a good engineering reason behind this, but it seems odd to make this decision before deciding on the ICE technology.

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TEG said:

Jan 17, 2008: I made some comments about this design on the Tesla blog a while back. I think the reason is because they want to market the REEVs as a 40+ mile range BEV, and the ICE is supposedly only for “emergency range extension”. If people only ever use the vehicle for a <40 mile commute with a full recharge cycle at the end, then fine. But my concern is what happens when you run the pack down then need sustained power beyond what the ICE generator will provide. My example was a drive from San Jose to Lake Tahoe. You use up your BEV capacity just as you get to the base of the mountains. Now you want to make the run up the hill on fast moving highway 80 and you have a full load of people, gear, and a ski rack on the roof. The wimpy little ICE is going to be hard pressed just to keep you going at full speed and really have little left to recharge the pack then.
I really think this design will only work out if you have a way to force the ICE on early because you know you are going on a longer than usual drive. Fisker seems to offer this with their stealth vs performance paddles. I hope Volt and other REEVs offer a way to override the “default” ICE avoidance behavior.

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TEG said:

...the SF bay area to Tahoe example is tough because many people do that drive in one sitting and the big climb is at the end. It is 87 miles from SF to Sacramento so you almost certainly have drained your batteries by then. Then the climb from Sacramento (at sea level) has to climb to 7000feet to get over Donner Pass I still think a 70hp ICE running a generator through to an eMotor is going to make that hill climb problematic when other cars are holding on to 65-70MPH the whole way.

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Martin said:

I wouldn’t put a lot of stock in public comments about the control algorithms of these various REEVs. They are written by marketing people who massively over-simplify what the engineers are doing. My bet is that none of the REEV players - not GM, not Tesla, not Fisker - are anywhere near done designing and tuning the control algorithms for engine and battery behavior. I also bet that when they are all done, every control algorithms will be so complex that a simple statement like “the engine starts when the state of charge reaches X” will be a meaningless simplification

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TEG said:

Reading through comments about charging spots, something occurred to me. What about when someone brings in a REEV that has run down both the tank and the batteries. Now we have a use for “dual refueling”. They could charge your batteries while they fill your tank. That way you get more EV miles rather than expecting to use some of the fuel to recharge the batteries. Mixing high voltage with gasoline pumps at the same station sounds like a controversial/risky undertaking, but it would make some sense to those who want to “maximize all electric miles” of REEVs.

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REEV Concerns:

...maintaining speed on steep inclines, or into a strong headwind.
(Better not get stuck trying to accelerate uphill into a headwind... it does happen sometimes).

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Martin on Tesla blogs:

Several of you have suggested installing a gasoline (or other fuel) generator in the trunk of a Tesla car as a range extender. Such a feature would convert the Tesla car into a form of plug-in hybrid. ... Note that the gasoline engine for such a hybrid car has to be pretty much as big as a car engine, since it ultimately must power the car by itself when the batteries are exhausted. Don’t think about that 1-cylinder, 1-kW Honda generator you can buy at Home Depot; think Prius engine, at least.

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Teslamotors blogs:

TEG2 said:

• # Jason M. Hendler wrote on December 13th, 2007 at 3:31 pm
  ## I think your estimate of 50 HP to maintain highway speeds is an order of magnitude too high.
• TEG2 wrote on December 13th, 2007 at 4:49 pm
  You really think you could have a 5HP generator in a large, heavy family sedan and make it maintain good highway speed as long as the fuel lasted?
  I mentioned 50HP because I was considering cases where you are driving uphill at 75MPH into a headwind with a full load of luggage strapped to the roof, and maybe even pulling a trailer. If someone wants to take a ski trip from San Jose to lake Tahoe and never have the vehicle restrict your speed potential I think you would need a lot more than 5HP to ensure that they batteries don’t run out of charge and spoil your driving experience. San Jose is to Lake Tahoe is a trip many from Tesla could contemplate. It was in fact one of the “demo” drives that was done to show off the Roadster. In the case of a series hybrid, I would envision that trip would go something like this… Customer drives from San Jose all the way to Sacramento just on battery power (because the system is optimized to try to avoid use of the ICE as long as the batteries have charge). Just as you start to head up highway 80 the computer realizes that the batteries are almost drained and fires up the ICE. Now you are headed up a steep incline with 70MPH+ traffic and you want to get to your cabin before dark (or before the lifts open if you left early in the morning). You better have enough ICE HP to maintain that speed uphill and hopefully even start charging the batteries at the same time because you may need some extra battery power for passing opportunities. Also if the ICE won’t run itself when you park you don’t want to end up in Lake Tahoe with a completely drained set of batteries. I would be concerned about trips like this if it had a whole lot less than 50HP of ICE power available.

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James said:

Then you are in a serious limp mode where you really only have the energy available based on the ICE recharge rate.

How many watts is the on board ICE recharging at?1,500 watts? 2,000 watts? That is enough to allow about 5 to 8 miles of range per hour.

This is just speculation. But I imagine that the battery could get into a charge level where the performance is just pathetic.

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It has been stated that when the Volt reaches the end of its all electric range it will go into "charge sustaining mode" this basically means that the ICE/generator will begin providing the power for the electric drive motor. If more power is temporarily needed by the drive motor than can be provided by the generator it will be drawn from the remaining 30% charge level in the battery and when less is needed the excess will be used to restore the charge level to 30%.

The reason for doing this is that the electricity produced by the ICE is several times as expensive as the electricity available from the receptacle in your garage and it would be a shame to finish charging the battery just as you arrived at your next plug in spot.

There sure does seem to be a lot of misperceptions about the power available from the ICE/generator used in the Volt. We are told here that it is a 53 kW generator. And based on Tesla information here (shown below) that is sufficient power to keep the roadster going at a sustained 103 mph. That is certainly enough to keep the Volt moving along at much greater than safe speed since as you can see below only half that power will keep the roadster moving at over 75 mph.

It takes a lot more power to go up a steep hill than it does to maintain speed on the flats. Those charts don't tell the story of how well it will do loaded up with cargo trying to climb a mountain pass.

Yes, for most driving situations it should be plenty fine, but I still think there will be occasional situations where you find it lacking.

Related reading:
More on How Much Power You Really Need | AutoSpeed Blog

TEG said:

It takes a lot more power to go up a steep hill than it does to maintain speed on the flats. Those charts don't tell the story of how well it will do loaded up with cargo trying to climb a mountain pass.

Yes, for most driving situations it should be plenty fine, but I still think there will be occasional situations where you find it lacking.

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There may well be a place or two in the country where you could find a situation that would be a problem but it would also be a problem to a lot of other vehicles. It would be easy to test it with a loaded Prius. Take the Prius there and push the accelerator to the floor for 20 minutes. If you never reach a speed where you need to lift your foot, the Volt may also slow to the speed of the Prius at that point.

donauker said:

...the Volt may also slow to the speed of the Prius at that point.

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I bet the inverter and eMotor efficiency loses in the Volt will be more than the CVT losses in the Prius' more direct coupling to the drive wheels.


Driving in the mountains - Any problems - PriusChat Forums

I had an driving experience last week with mountain driving. I had four adults in car at ~8000 ft. climbing a steep grade, down to one purple bar (or less) with A/C on, with temperature in 80's. Needless to say, the car didn't have a lot of power... I wouldn't like to drive under these conditions for long periods of time, fortunately it was an unusual situation.

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We returned from a 7,500 mile trip a week ago today. It included crossing the continental divide a few times as well as other mountain climbing. We chose not to just let the engine roar, so we got down to 35 mph at times. However we were fully loaded-- two adults, two teens, two tents, all camping gear, roof rack, etc.... Not maintaining maximum speed on the steeper mountains was fine with us. Be patient and enjoy the beautiful scenery!

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TEG said:

I bet the inverter and eMotor efficiency loses in the Volt will be more than the CVT losses in the Prius' more direct coupling to the drive wheels.

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I assume you are aware that the "CVT" in the Prius is not actually a CVT. At the heart of the Prius "CVT" is a planetary gear set which works as a power split device. The net result is that 72% of the engine torque is directed through reduction gears to the front wheels. The remaining 28% is directed to a motor/generator working as a generator producing electric which is used to either charge the battery or is inverted and used to drive a second M/G which is also connected to the front wheel final drive.

This design is amazing in the way it works but it also means that at least 28% of the power produced by the ICE is subject to double conversion loses.

The point I have tried to make centers on the difference between peak power need and long term continuous needs. That is why I challenge anyone to find a place in the US where you can keep the Prius floored for 20 minutes none stop. It is only after that duration that the Volt may become comparable in performance. Given the number of Priuses on the road I think we can live with comparable in this yet to be found location.