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REEV concerns

Discussion in 'Technical' started by Palpatine, Jan 13, 2009.

  1. Palpatine

    Palpatine Banned

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    #1 Palpatine, Jan 13, 2009
    Last edited by a moderator: Jan 14, 2009
    I have my doubts on that type of series hybrid ever being successful. I saw a presentation that walked through the energy flow of that system. The gas engine is only being used to recharge the batteries, not drive the wheels. So after the 40 miles of driving from the fully charged battery, that gas engine needs to generate a lot of electricity to maintain a normal speed vehicle. Many have speculated that the gas engine that GM is using cannot manage a normal driving speed after 40 miles. It would likely be in "limp" mode because that size gas engine cannot generate enough electricity to propel the weight of the Volt.

    When the GM Volt is actually in the hands of independent reviews and car magazines, the weakness of the design will become obvious.

    Toyota is going the right way with their plug-in design.

    Just my opinion.
     
  2. siry

    siry Member

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    @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.
     
  3. Palpatine

    Palpatine Banned

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    Siry, that is a good point. I had not considered that they might invade their reserve to maintain normal acceleration.

    Regardless, I am sticking with Toyota (PHEV) and Tesla (EV) having better designs.
     
  4. TEG

    TEG TMC Moderator

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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).
     
  5. donauker

    donauker Member

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    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.
     
  6. siry

    siry Member

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    @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!
     
  7. Palpatine

    Palpatine Banned

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    Siry, as I understand it, all of those mini-cycles are really hard on the battery. If I recall (I think I read this comment from you in an article) doing the Tesla battery system as a pure EV is far more simple and reliable for a long term life expectancy of 100,000 miles. Is that accurate?

    I think the logic is based on this set of circumstances comparing an EV to a PHEV.

    With a Tesla Roadster having a 244 mile range, most people will use around 10% to 40% of the battery pack each given day that the car is driven. So the battery will spend most of it's life between 50% to 90% charged up. This is actually a very good usage profile to maintain the long term quality of the battery pack.

    With a GM Volt having a 40 mile range, most people will be using the full approved range of the battery each day. So that battery pack will be taken down to 30% charge on a regular everyday basis. I am assuming that the max charge is 80% and minimum reserve is 30%. So that 8 kwh of the 16 kwh is used. This type of regular deep cycling is fairly hard on the batteries over time.

    Is that an accurate description of the situation?
     
  8. siry

    siry Member

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    @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.
     
  9. TEG

    TEG TMC Moderator

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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).
     
  10. SByer

    SByer '08 #383

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    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.
     
  11. TEG

    TEG TMC Moderator

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    Bloomberg.com: Japan
    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.
     
  12. Palpatine

    Palpatine Banned

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    It seems that the GM Volt design will have a key weakness during certain scenarios.

    Since the ICE is only recharging the battery, and it does not recharge fast enough to maintain high acceleration/fast driving, this creates the potential for really horrible performance.

    If you have been driving 40+ miles already and your VOLT battery is drained of your electric grid charge, then it is likely has a state of charge around 30% to 40% when the ICE kicks on to recharge.

    So now you have to do a hard acceleration to deal with a hill or some other high energy consumption driving scenario. Supposedly the GM Volt will allow you to invade the battery reserve for a while. But what if you do this to the point where the state of charge gets really low.

    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.
     
  13. TEG

    TEG TMC Moderator

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    #13 TEG, Mar 6, 2009
    Last edited: Mar 6, 2009
    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
    REEV Concerns:
    Martin on Tesla blogs:
     
  14. TEG

    TEG TMC Moderator

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    Teslamotors blogs:

     
  15. donauker

    donauker Member

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    #15 donauker, Mar 6, 2009
    Last edited: Mar 6, 2009
    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.

    [​IMG]
     
  16. TEG

    TEG TMC Moderator

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    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.
     
  17. TEG

    TEG TMC Moderator

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  18. donauker

    donauker Member

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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.
     
  19. TEG

    TEG TMC Moderator

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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
     
  20. donauker

    donauker Member

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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.
     

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