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Efficiency claim of the Dual Drive Train

Discussion in 'The UK and Ireland' started by Rluner, Oct 13, 2014.

  1. Rluner

    Rluner Member

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    Elon Musk / Tesla are doing a great job forwarding EV technology, however I just heard Elon claim that the D version of the model S can actually get
    more range, and it was implied it was because of the two drivetrains. (which he admits weights more than a single drivetrain). Why would his advisors allow him to claim this?

    Adding more weight to a car can never make it more efficient:- F = Force (the energy the battery needs to use) = mass of the car x the acceleration.

    Newton's second law of motion : F = ma . The vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration vector a of the object.
    Newton's laws of motion - Wikipedia, the free encyclopedia

    And before anyone says its because the second drive train can regenerate in addition to the first drive train, it still can never make up for the extra weight of the second drive train or you would be breaking the conservation of energy:-

    "In physics, the law of conservation of energy states that the total energy of an isolated system cannot change—it is said to be conserved over time. Energy can be neither created nor destroyed, but can change form, for instance chemical energy can be converted to kinetic energy in the explosion of a stick of dynamite.

    A consequence of the law of conservation of energy is that a perpetual motion machine of the first kind cannot exist. That is to say, no system without an external energy supply can deliver an unlimited amount of energy to its surroundings"
    Conservation of energy - Wikipedia, the free encyclopedia

    Think of it this way, if Elon Musk was correct then why not build a 100 drive train Lorry; that would have to be super efficient reneging all that power, even more than the force of moving all the mass of them 100 drive trains? Common sense tells you this cant be right.

    This was a difficult thread for me to word, I hope you get at least the point of the thread even if its not accurately worded?

    Maybe someone can explain my thoughts better than I can?

    No need to flame me, just state your point sensibly.
     
  2. scaesare

    scaesare Active Member

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    A few thoughts:

    - Tesla has already discussed gearing the secondary motor differently so it can run within a more efficient RPM range at highway speeds, and thus the system can vector more power to the appropriate motor at any given speed/load requirement (this, rather than regen, is always what I have seen billed as the chief contributor to additional range)

    - Tesla had already shaved a couple of hundred pounds off The S prior to the announcement of the D cars. While the new models may ultimately weigh more than the old, the additional weight savings of the new motors may be offset somewhat by additional weight trimming elsewhere.

    - All systems are lossy... eliminating some loss (i.e.- increasing efficiency) within the chain allows for more energy to be converted to locomotove force without violating Newton's law

    - There may be some additional regen as well
     
  3. arg

    arg Member

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    This has been discussed here in the past (and mentioned by Tesla in the context of the Model X on a number of occasions).

    The main win from dual motors is that they can be geared differently. Although electric motors have a much flatter speed/efficiency curve than ICEs, the efficiency is not constant. With two motors, they can have one geared for acceleration and one geared for high-speed cruising.

    As a starting point, you might imagine a Model S with the existing RWD motor and unchanged gearing, plus a smaller added motor at the front with much higher gearing. For the same acceleration from rest, the rear motor could be just used as before and the added motor idled. For high speed cruising, the rear motor can idle and the front motor provide the (small) cruising power more efficiently. For traction in snow etc. both can be used.

    In practice, Tesla have tweaked things further, with the front motor providing extra power at low speeds and very likely different gear ratios to the RWD Model S.

    The possibility of extra regen is a much smaller effect, but can't be entirely discounted. Your point is that if the car is slowed entirely by regen, there is a fixed amount of energy available regardless of whether you recover it through one pair of wheels or two - and that is absolutely true. However, in real life the total amount of regen is limited by a number of factors, and often the car has to be slowed with the brakes too - wasting energy that could have been captured by regen. The major limit on regen is the ability of the battery to accept the charge current, and that isn't going to be helped by extra motors. But another limit on regen is that it has to be applied in a very conservative way to avoid loss of traction, particularly when regen is on the rear wheels and the deceleration causes a weight transfer to the front: the extra traction from using both pairs of wheels may allow more regen to be used in some circumstances. Then on top of that, the regen efficiency will also be affected by the gear ratio just as the acceleration was, so potentially allowing the same amount of regen but more of the energy ending up in the battery and less in waste heat in the motor/electronics.

    We know about the gear ratios from various Tesla public statements; nothing has been said (so far as I know) about Regen one way or the other, though the difference would be expected to be small.

    The only surprising thing about all this is that the gain is so large - it would have been easy to imagine that most of the efficiency gain was eaten up by the extra mass of the motor, but evidently not so in the case of the P85D. In the case of the ordinary S85D/S60D, the extra mass is much less (since one large motor has been swapped for two small ones - only the extra mass of the gearboxes/driveshafts etc.to really account for), partly explaining the higher efficiency gain for those models - though the gearing of P85D will be different and may account for some of the difference too.
     
  4. mgboyes

    mgboyes Member

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    Agree with everything that @arg and @scaesare have said - the different gear ratios, motor sizes, and drive inverter sizes will all contribute to different efficiency curves for the two motors, so that the car can choose which motor to use for max efficiency across a wide range of speeds. In the real world greater regen would make for higher efficiency on those rare occasions in everyday driving where you need to brake hard. But I doubt it will affect rated range - I doubt the test cycles used in range calculation involve the need for hard braking from high speeds.

    However we need to take people's inferences about the size of the gains with a pinch of salt. Elon has said that D is more efficient than non-D, in general. But he hasn't said by how much, and the figures being quoted aren't comparable. The D models haven't been EPA or NEDC rated, so their ranges are being quoted as "at 65mph" speeds. In fact if you look at the US design studio the figures appear to suggest the D has a longer range (because EPA ratings are low), but in the UK the design studio suggests they have a shorter range (because NEDC ratings are high).

    Also his comments about greater efficiency were about replacing one large motor with two small ones. I doubt very much that the P85D will have a longer range than the P85+ since it's gone from one large to one large plus one small.

    Ultimately though I'm not sure any of us care, do we? If I were asked for a list of 20 improvements I'd like made to my Model S I don't think "more range" would be one of them.
     
  5. RDoc

    RDoc S85D

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    While it's certainly true that the energy needed to accelerate a car is proportional to the weight of the car, in normal usage, that's not a significant part of the total energy used and the increased weight of the second motor system is pretty small compared to the rest of the car. Once the car gets above around 45 mph or so the main energy use is air resistance which remains constant with weight. The other losses are rolling resistance which goes up very slowly with weight, the battery and conversion losses, and the motor efficiency, none of which are affected by weight. It sounds like they found some energy saving in those last components.

    Regen is a different thing entirely and I suspect is a significant factor in the increase mileage since the EPA tests do include braking. The question isn't if regen can get back more power than the motors generate, rather it's if the new regen can get back a significantly larger fraction than the old system. The S has large brakes for a reason. They dissipate a lot of energy as heat, so if splitting the regen results in more of that wasted energy going back into the battery, the range goes up even though the kinetic energy of the car is slightly higher due to it's increased weight.
     
  6. arg

    arg Member

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    The one other data point we have is Elon/JB's statement from a year ago that the gains from dual motor were enough to overcome the aerodynamic disadvantage of the Model X over Model S.

    So on that basis I would expect all the D models to have some more range than corresponding non-D models. Exactly how much is, as you say, very unclear.
     
  7. Saghost

    Saghost Active Member

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    I suggest you read some of the technical documents about GM's Voltec drivetrain. GM gets a 4% improvement in efficiency on the highway by clutching the second motor in and using it to drop the speed of the main drive motor by 50% or so. It's not exactly the same as what Tesla is doing here, but it works for the same reasons, and is very well documented with multiple SAE papers on the subject.

    Basically, electric motors become less efficient the faster they turn. By transferring power to a motor that is spinning slower, a greater percentage of that power actually reaches the wheels - and this is what extends the range.

    The same amount of energy at the wheels is needed to cover the distance (actually, slightly more energy is needed to compensate for the extra weight,) but the losses further upstream are smaller and so the total energy coming out of the battery is lower.
    Walter
     
  8. yobigd20

    yobigd20 Well-Known Member

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    while true, it's not a matter of weight == efficiency, but rather optimization of the power between forward and rear wheel driving. AWD weight distribution under acceleration is also more evenly distributed vs a RWD drive. more evenly distributed torque means less power required to accelerate. (very heavy RWD torque would require more power to accelerate the same vs smaller torque distributed evenly across 2 motors). while you're cruising it can optimize power distribution towards the smaller motor in the front for increased efficiency. etc etc. i dont know the exact details, but is definitely all about optimization.
     
  9. Vger

    Vger Active Member

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    And recall that Tesla is only claiming a 10 mile improvement, which, like the Voltec is only 4%! It is enough to mention, but not a huge thing. The big deal is the reversal of sign of the difference. All mechanical AWD systems are less efficient than their RWD or FWD counterparts on the same car.

    Also, I agree that while the increased region will help the safety and stability of the car, it does not enter into this increased efficiency as Tesla has stated it. They are only citing the increase at a steady 65 mph, not for a full driving cycle like the EPA test.
     
  10. Rluner

    Rluner Member

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    #10 Rluner, Oct 13, 2014
    Last edited: Oct 13, 2014
    Thanks for some excellent responses, maybe I needed to explain myself better. I understand efficiency in motors can be improved.

    However it appeared to me that he implied the range increase was due to two drive trains and not just one.

    I had presumed in my original post that he puts in the most efficient motor he can in either a single or dual configuration.

    Possible the gearing of a second drive train could account for some extra range as described in some responses above, but I'm not sure. Taking this idea to its limit why not have the second motor so high geared it hardly helps the car to start but once cruising speed is reached it kicks in and takes over from the starting drivetrain (which then stops providing power). This would then give as much range as possible. But I don't believe this is the case, so maybe the assumption is not correct of how the second drivetrain acts at a higher gear than the first.
     
  11. mgboyes

    mgboyes Member

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    Yes it can, and most likely does.

    In addition the smaller motors and less powerful drive inverters may just be inherently more efficient than the larger ones.

    If maximum efficiency across a wide speed range was their only design goal this is exactly what they'd do. But it's not, they want a functioning AWD system (so the front motor needs to be able to contribute at 0mph), and at least with the P85D they want performance (so the front motor needs to be able to contribute a lot at 0mph).

    What we're getting is a balanced setup with slight improvements in efficiency, considerable improvements in roadholding, traction and driveability, and in the case of the P85D enormous improvements in performance.
     
  12. ElectricAvenue

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    Rluner,
    Most of increase in range comes from the gearing. Even if they took it to the limit as you suggest they would get more range but the performance would not be as good. So what they did was to balance the performance and range increases.
    Why does your ICE have 5 gears? Because it the manufacturer knows the gas engine has a non-linear torque curve and wants to try to keep the torque has high as possible for all speeds. Tesla had a flat curve with 1 motor, now they have 2 they have 2 curves they can work with and electronically permit each motor to apply more power in the curve they deem best at any point in time.
     
  13. scottf200

    scottf200 Active Member

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    Saw these numbers in an insideEVs article and put in a little spreadsheet to easily see the diffs. P85D for 0-60 and $ is unbelievable!

    EPA Range 10 more for P85D vs 85
    EPA Range 30 more for 85D vs 85

    y9Hsiyv.png
     
  14. arg

    arg Member

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    Maybe this is the source of Rluner's confusion. An electric motor's torque and efficiency curves are so much more flat than those of an ICE that we are inclined to refer to them as 'flat'. However, that's an exaggeration - efficiency varies significantly with speed, just not nearly as much as an ICE.
     
  15. charliestyr

    charliestyr EVangelist

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    Those figures are not EPA they are tesla guide figures at a steady 65mph. A theoretical rather than real range. Perhaps we will see an improvement in EPA figures when tested but I severely doubt that kind of improvement and the P85D should be less range in theory than outgoing P85 afaik.
     
  16. mgboyes

    mgboyes Member

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    Agreed - the quoted ranges for D and non-D are not comparable. Elon has said the range is better in general e.g. for 85 to 85D, but I doubt it will turn out to be as much as 30 miles. And I expect P85D to have worse range than P85+. Still want one though :)
     
  17. arg

    arg Member

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    #17 arg, Oct 14, 2014
    Last edited: Oct 14, 2014
    I expect it to have more range for the case we mostly care about - cruising down the motorway at continuous high-ish speed. For lower speeds, the driver generally expires before the battery does.

    The only reasons for P85D to do worse than S85D are the extra weight of the rear motor (not an issue at constant speed), and the possibility that the front motor is geared higher than the one on S85D to achive the 155mph top speed - though unclear whether that would be a benefit or a loss at 70mph.

    This does however assume you can restrain your right foot - I can feel a new accessory coming on: Extra 20% range for P85D! Only $50! (product is a stiff spring to insert behind the accelerator pedal).
     
  18. scottf200

    scottf200 Active Member

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    I assumed that range is different because the P85D has different motors?!!?

    5TLZkXF.png
     
  19. TEG

    TEG TMC Moderator

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    It looks like both the P85D and 85D advertise 155MPH top speed. (I wonder how long they can maintain that speed though.)

    What is "S85D" ?
     
  20. ThosEM

    ThosEM Space Weatherman

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    So, it seems that a similar efficiency gain could be achieved with one motor if it had a shiftable transmission with two gears. Tesla tried that in the original roadster, but had breakage problems and decided to simplify at some point. Perhaps that should be reconsidered.

    The extra weight is pretty much irrelevant with regeneration in place of friction braking. That's why hybrids can afford to lug around extra system mass. Regardless of mass, the same energy used to accelerate or climb a hill is returned upon deceleration or descent, less some inefficiency of course, but for the most part, it comes back. Air friction and rolling losses or can never be recovered, but they don't depend much on the weight of the vehicle if the tires are fully inflated. Friction braking losses do depend on weight, since that destroys the energy gained accelerating or climbing, both of which are linear in mass.
     

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