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The AWD (All Wheel Drive) option

Discussion in 'Model S' started by dpeilow, Mar 28, 2009.

  1. dpeilow

    dpeilow Moderator

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    Regarding the announced AWD option, I'd be interested to know if this will have a driveshaft running up the length of the car or a second motor at the front. If it's the second motor, will these be half-power motors or will the car have a total of 804bhp?!?
     
  2. stopcrazypp

    stopcrazypp Well-Known Member

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    I'm quite sure the AWD option will be in the Sports version just by common sense (it'll cost more & it can add more power so it's perfect for the Sports version). It doesn't make much sense to have a driveshaft when you can put an electric motor right near the wheels. I'm thinking it'll be lower power motors since the battery (160, 230 mile ones at least) probably won't be able to provide 804bhp anyways. You'll probably lose some of the trunk space in the front though.
     
  3. WarpedOne

    WarpedOne Supreme Premier

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    I remember reading or hearing somewhere that there is space in front for additonal motors for AWD. I'm sure they won't go with driveshaft as there is no space for it - central tunnel is full of batteries.

    Will it be one or two I don't know but it certainly won't have 300kW. I doubt the batteries are capable of 600kW of output.

    Two motors would need two PEMs. Martin said somewhere that PEM is probably going to be the least reliable part of an EV. Three PEMs "threesome unreliable". I'd guess at least from the start it would be one motor and differential and then somewhere in the future maybe two inwheel motors. If they are only 20kW each that would be enough already.
     
  4. TEG

    TEG TMC Moderator

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    Yeah, that would be my guess as well. An aux smaller motor just for the front wheels. It would probably sacrifice some of that front trunk space. Going to AWD doesn't necessarily help performance much as you add weight and frequently the RWD version isn't traction limited in most circumstances. AWD is most beneficial for winter driving.
     
  5. WarpedOne

    WarpedOne Supreme Premier

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    One big advantage of ADW is a possibility of a much stronger max regen.

    In RWD system there is only so much regen power that can be applied onto rear wheels while cornering before car looses stability and violently over-steers.
    Remember that pic of new Roadsters dash? It's going to show current power and regen goes only up to 50kW, the max is probably lower around 40kW.

    With AWD, regen would happen on all wheels at the same time - no sudden loss of stability. Other thing is that rear tires could be somewhat narrower with lower drag.
     
  6. shark2k

    shark2k Member

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    Thinking of what I want to ask before typing it, it might seem stupid, but I'm gonna ask anyway.

    Now, I realize that if they go the 2nd motor route for the AWD that it would add a little more weight (at least more than a drive shaft probably?) My question is do you think it would be possible to include an option to basically not use the 2nd motor until you want AWD (for instance during the winter like TEG mentioned)?

    My dad's truck is 2WD and he has the option to put it into 4WD when he needs to. That is why I was thinking about this. Of course, I realize AWD is different than 4WD so my idea could possibly not work/be feasible.

    -Shark2k
     
  7. WarpedOne

    WarpedOne Supreme Premier

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    2 meters long driveshaft with bearings and gearings, housing and mounts can easily weigh over 100 pounds (50 kg). Smaller motor would probably weigh around 60 - 80 pounds. Problem is, there is no space for driveshaft in that body, that space is going to be occupied by batteries.

    Main reason why in a 4WD cars you can disengage FWD or RWD is efficiency. Turning that driveshaft, gears and clutches eats up power and fuel.

    An AWD EV with two motors won't have clutches so both motors will always spin. I don't see how driving only one motor and letting other to freewheel would save energy. The one would have to work harder, produce more heat, need more cooling etc. Its easier (needs less energy to do it) to cool two smaller motors than one bigger. But I could be wrong here.
     
  8. shark2k

    shark2k Member

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    Actually, reading what you had to say WarpedOne, I don't think you are necessarily wrong. What you said makes sense, about the efficiency and being able to engage/disengage 4WD. I also wasn't sure about the weight of a drive shaft vs the weight of an additional electric motor. And of course I didn't even think about the "main" motor needing to do more work to propel the car forward.

    Of course continuing with that last sentence, wouldn't it be similar to just the regular RWD model? As long as the 2nd motor can be "turned off" and the front wheels can move freely (like they would in the RWD model) wouldn't it be the same?

    I just through the question out because I was thinking about it after reading about the possibility of an AWD version. Of course my problem is that I don't think these types of things through, i.e. does what I asked decrease efficiency.

    -Shark2k
     
  9. Tdave

    Tdave Member

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    #9 Tdave, Mar 28, 2009
    Last edited: Mar 28, 2009
    The ultimate is 4 motors, one for each wheel, with direct connect drive, no reduction gears, and completely computer controlled traction and stability control. You get the ultimate in performance cornering control with neutral handling at all times. This would be huge!

    What you also get is the ability to accelerate with the same performance as braking, assuming powerful enough motors, which should be easy if that's the objective. Can the car stop 60 to 0 in 120 feet? Then it can also accelerate 0 to 60 in 120 feet.

    Then add antilock regen braking at each wheel. Conventional brakes only there as a backup but never used in normal operation. Four wheel regen for ALL braking will increase range considerably for mixed, around town driving. That range efficiency allows either greater range or smaller lighter batteries, increasing efficiency even more with the reduced weight.

    I'd like to see us get to this point within 5 years.

    It would also allow the holy grail performance sports car EV.
     
  10. SteveF

    SteveF Member

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    Are you talking about in-wheel motors? I remember Martin specifically stayed away from in-wheel motors for the Roadster, due to the added "unsprung weight" they bring, which would, as he said, "make the car drive like a truck". Maybe for a sedan that isn't as much of an issue, but it was felt NOT to be the way to go for the Roadster. It will be interesting to see how Tesla addresses the 4WD issue for the Model S.
     
  11. mike

    mike EVangelist

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    Is there a down side to AWD that I am not aware of?

    My thinking would be that they could put a smaller AC motor/transaxle up front (a Bluestar usable drivetrain?) and have a CAN-bus connection to a box that contained only the IGBTs up front and have the PEM in the back control it for regen and acceleration (as well as active stability control / hill ascent / decent). In theory a EV should only need one PEM no matter how many electric motors it has, the biggest limiting factor is likely the heat that is generated by IGBTs under heavy acceleration. If a fault is detected with anything for the front drive the car could just throw a check engine light and have the rear drivetrain function in a limp mode or the other way around.

    It would seem to me that the extra costs in doing this would make the car significantly more expensive.
     
  12. TEG

    TEG TMC Moderator

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    Besides extra weight, cost and complexity?
     
  13. Tdave

    Tdave Member

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    No, not in wheel motors, because of the unsprung weight issue. I'm talking about a motor per wheel with a short drive shaft from it to the wheel. The motor would be much smaller because it would require one quarter the power as compared to a single motor to drive the entire car as in the Roadster.

    A motor per wheel is mechanically simpler, although there are indeed four of them. But you eliminate the need for a differential or transaxle. The issue of each wheel rotating at different speeds when cornering is all handled through software, rather than mechanically. Then put the tasks of traction control and stability control and ABS also in software, effectuated through each individual motor (both when accelerating and during regen braking). The response of the motors during those tasks is near instantaneous, done through software. It's ultimate control with instant response. All the complexity is in software rather than in the mechanicals. Isn't that what EVs are all about when compared with an ICE vehicle? Let's take this to an extreme and get to the holy grail sooner rather than later.
     
  14. Tdave

    Tdave Member

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    More: The big payoff is with the stability control system. Software would have the ability to have one wheel in regen with other wheels accelerating, which it may want to do at times during max cornering, in order to keep the car perfectly balanced with neutral handling at all times.
     
  15. WarpedOne

    WarpedOne Supreme Premier

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    #15 WarpedOne, Mar 29, 2009
    Last edited: Mar 29, 2009
    There are many engineering challenges with inwheel motors that are not solved yet, unsprung weight being only one of them. The others are cooling, reliability, durability, required torque etc.

    Much of these problems can be avoided by moving the motors inboard and adding driveshafts. But then you can easily also add one differential and only use one motor per axis. Differentials are supposedly a solved problem - very reliable, durable and efficient. So it becomes more prudent to go with a single motor and differential for front axes and a second motor and differential for rear axis. I'm starting to think this is going to be and EV AWD solution of choice for some time to come (10 years at least).

    Edit: Dave, I've missed your above posts above. Yes, I agree with your points about ultimate control etc. Problem is, each motor needs its own PEM. Such a setup would require 4 of them. I am not sure current automotive electronics is reliable enough yet for a manufacturer to go this route. It has to work for at least 5 years without failures. Automotive environments are much much harsher than indoor environments that we are used to.

    Modern traction control setups are already doing one-wheel braking while the others are pushing. Their problem is they don't have that much control on the amount of braking and they are less responsive than an EV could have and be.
     
  16. TEG

    TEG TMC Moderator

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    Toyota does electric (hyrbid) AWD on Highlander and Lexus RX.
    They use one motor (through a diff) for the front wheels, and another electric motor (through another diff) for the rear wheels.

    If 4 independent motors was a huge improvement over 2 with diffs, I would think someone would have done it by now.

    (Not that I believe the automotive status quo is representative of the ideal, though)
     
  17. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    In the Toyota Highlander and Lexus SUV Hybrids, the front wheels are driven by the Toyota "Hybrid Synergy" system that uses an ICE, two motors, a "power split device" (really a very sweet use of a planetary gear), a battery pack, and the power electronics to implement the hybrid system. This is the same system as in the Prius. To implement AWD, they then have an electric only connection to a third motor driving a diff to the rear wheels. My girlfriend has one of the Lexus models and it works quite well. I think that you can still spin one rear and one front wheel and get stuck, but I have not pushed it that hard.

    The U.S. Army experimented with a 4-motor Hybrid H1 Humvee <http://defense-update.com/features/du-3-05/feature-HED-humvee.htm>. The 4-motor Humvee was considered by many to be the ultimate off-road system. It offered great independent power to each wheel. As a tactical advantage, running in EV mode, it was very quiet.
     
  18. TEG

    TEG TMC Moderator

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    Yeah - thanks - I know it has "power split" and hybrid ICE.
    But at low speeds it can drive all 4 wheels electrically, and my point is it uses electric motors to do so without a drive-shaft between front and rear.
     
  19. TEG

    TEG TMC Moderator

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    They announced AWD as an option (perhaps not in the first year).

    I imagine the mix of AWD will be on the small side, mostly going to people where it snows a lot in winter. Is there anyone here who will be waiting until the AWD model S shows up before they get one?
     
  20. donauker

    donauker Member

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    At this point I am planning on waiting for the AWD model. Hope to get a clue on the time frame tomorrow evening at the DC event.
     

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