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Thread: Comparison between 2 Model X 85kWh cars, with a single and a dual motor: 5 Questions?

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    Member Benz's Avatar
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    Comparison between 2 Model X 85kWh cars, with a single and a dual motor: 5 Questions?

    Suppose we have the following situation.

    There is a 1,000 km long straight road of good quality (specially good for fast driving, like a race track) with no hills, or stoplights or any other obstructions. The weather is fine and there is no traffic at all. And we have two Tesla Model X cars (both have a 100% full charged 85 kWh battery), 1 is red and has 1 motor, and the other is blue and has a dual motor. Both start at the same time and drive (side by side) slowly (say in 40 seconds) up to a speed of 200 km/hr, and the cruise control keeps them at that speed until the battery will have gone fully empty (0%). There are no pit-stops or anything. Both cars have only the driver sitting in them. Both cars are equally equiped, and have the same tyres, etc. These Model X cars are not the Performance or the Signature models, they are just the standard production car with the largest battery pack available (85kWh).

    My questions are:
    1) Does the single motor of the red Tesla Model X have to work "harder", compared to the two separate motors of the blue Tesla Model X? Regarding that both cars are driving at the same speed of 200 km/hr.
    2) Do the two separate motors in the blue Tesla Model X have to work equally "hard"? Or is there a difference between the two separate motors?
    3) Do the two separate motors in the blue Tesla Model X always work together from the start, or will the second motor be turned on only at a certain speed?
    4) Will the total distance of the journey of both cars be the same (when the battery finally is empty), or will 1 of the cars stop earlier than the other?
    5) Can we say something about how long the total distance (separately) of the journey of both cars will be in km?

  2. #2
    These are good questions, and while I don't have the answers, I do remember it being said the dual motors were supposed to make it the "best handling car on the road". Regardless of the validity of that, if it's mainly for handling, I couldn't see the additional motor having to do much work.

    As for the additional motor helping out, maybe that's the case, but then wouldn't you use more energy on the second motor anyway, thus nullifying any efficiency gained by having a "helping hand"? I tend to think the second motor will help in handling and performance, and likely lower efficiency (AWD cars tend to get less MPG than RWD or FWD).

    Someone else with more knowledge can likely enlighten us both, though.

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    Quote Originally Posted by Benz View Post
    Suppose we have the following situation.

    My questions are:
    1) Does the single motor of the red Tesla Model X have to work "harder", compared to the two separate motors of the blue Tesla Model X? Regarding that both cars are driving at the same speed of 200 km/hr.
    Yes. Like any 4-wheel-drive, you have the extra work of spinning the drive shafts/differential/gearbox that are connected to the front wheels on the blue car that are just not present at all on the red car. And the blue car is heavier. Both effects are small, but non-zero. In your artificial case where the road is infinitely long with no stops or gradients, then the transmission loss is more significant.

    So, the total amount of power to be supplied by the two motors in the blue car is (slightly) more than that in the red car.

    2) Do the two separate motors in the blue Tesla Model X have to work equally "hard"? Or is there a difference between the two separate motors?
    Absolutely not. I would speculate that the motors will be of different sizes/power ratings (as the rear will be the same as the Model S, and you don't have enough power from the battery to run two of that size).

    However, even if they two motors were identical, there is no need to deliver the same power to both. The control systems could do anything from a 50:50 power split to leaving one motor spinning undriven and putting all of the power to the other. Under acceleration, this significantly affects the handling of the car and it will be interesting to see what Tesla do - they've got much more freedom than a comparable 4WD ICE car. For steady-state cruise in your example, they will presumably use whatever split uses least energy - and it's hard to say from first principles what that is.

    There is a slight possibility that at some speeds using two motors at half the power is more efficient than one motor working very hard, maybe even enough to offset the losses from the transmission, but I wouldn't bet on it.

    3) Do the two separate motors in the blue Tesla Model X always work together from the start, or will the second motor be turned on only at a certain speed?
    No. If anything, the other way around: at low speeds, performance is limited by traction at the tyres and the maximum current limit of the motor, rather than the available power: this is where two motors are a big benefit.

    4) Will the total distance of the journey of both cars be the same (when the battery finally is empty), or will 1 of the cars stop earlier than the other?
    Almost certainly different, and in your artificial test I would expect the red (RWD) car to go further than the blue (4WD). Probably not a big difference - maybe similar to the difference between 19" wheels/standard tyres and 21" wheels/performance tyres that is seen on Model S.

    But in real use there is another difference: the blue car will be able to regen on all four wheels. This doesn't help at all in your test, and maybe doesn't help that much in real use as the regen is often limited by the battery's ability to accept the charge rather than by traction - but it may allow a more aggressive regen profile than in the RWD car (where the regen control has to be careful not to skid the wheels).

    5) Can we say something about how long the total distance (separately) of the journey of both cars will be in km?

    Not much. At such a high speed, it won't be very large (model X aerodymamics presumably worse than Model S). Maybe a few percent less for the blue car.

    Tesla's graphs for the Model S only go to 80mph/130km/h (for the graph in the blog post with axes) or 115mph/185km/h (for the one on the new calculator page with no axes) so it's hard to extrapolate as far as 200km/h, but for the Model S it must be around 150km. So a bit less than that for the Model X.

    Also in reality, the cruise control won't let you go at full speed to 0 capacity: power limiting and warnings will cut in well before that.

    But an interesting set of questions!
    Last edited by arg; 04-04-2013 at 03:08 AM. Reason: fix quoting, typos

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    Wow, I am glad to have put these questions on paper. Because I sure do like to hear the opinion of somebody who knows a bit (or even a lot) more about automotive than I do. So, thanks for the reply. And I sure hope to get more replies in this thread as time goes by.

    I noticed that I have made a mistake regarding the top speed of 200 km/hr in the situation that I have described. A top speed of 160 km/hr would be more likely and real.

    Has Tesla Motors already unveiled what the top speed is of a Tesla Model X with a 85 kWh battery pack, and with a single (standard) electric motor between the rear wheels? Is it even possible to drive this car at a speed of 200 km/hr? That just might not be the case with this Model X car, I am afraid?

    The top speed of the (standard production) Model S is 200 km/hr.
    The top speed of the Performance Model S is 212 km/hr.

    The Model S really has got a very low drag figure (0.24 I believe). The shape of the Model X is obviously not as aerodynamic as the Model S. That is for sure, so that would mean that the top speed that I have used in the situation that I have described is too optimistic and certainly not correct. That must be the conclusion now. Therefore I shall have to change that. I think that a top speed of 160 km/hr should be very well possible with the standard production Model X with a 85 kWh battery pack, and with a single (standard) electric motor between the rear wheels. Do you agree on this?

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    160km/h won't be a problem I guess, but do keep in mind that speed kills range. That's just the laws of physics kicking in.

    I agree with what has been said above. One motor will probably be more efficient since you have a drive-train less and the car will weigh less.

    The one with dual motors might be at 200km/h (or whatever) in a shorter time, but it will use slightly more.
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    I should have been more clear on the modification of the initial situation.
    What I actually wanted to say is that both of the cars are driven at the same speed (in order to keep the test as simple as possible), which has been decreased to 160 km/hr (to have a more realistic situation).

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    Quote Originally Posted by arg View Post
    But in real use there is another difference: the blue car will be able to regen on all four wheels. This doesn't help at all in your test, and maybe doesn't help that much in real use as the regen is often limited by the battery's ability to accept the charge rather than by traction - but it may allow a more aggressive regen profile than in the RWD car (where the regen control has to be careful not to skid the wheels).
    Now that's an interesting thought!

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    Quote Originally Posted by Benz View Post
    I should have been more clear on the modification of the initial situation.
    What I actually wanted to say is that both of the cars are driven at the same speed (in order to keep the test as simple as possible), which has been decreased to 160 km/hr (to have a more realistic situation).
    That doesn't change the answer.

    I think the RWD drive car will get a slightly better range. This is because there is less mechanical parts turning around in the car.
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    but what if there is a different gear ratio between the front and rear? would that not allow better power consumption at higher speeds if they changed the drive used for cruising for example?
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    If we take a look at the Tesla Motors website.
    On the Model X page is written:

    "Model X is offered with optional Dual Motor All-Wheel Drive. The second motor enables more than all-weather, all-road capabilities: it increases torque by 50%."

    About the Dual Motor: "Are both motors identical and equally powerful (rear and front)?
    If they really would be identical and equally powerful, shouldn't the increase of torque be by 100?
    And if the increase of torque is really 50%, then does it mean that the second motor (front) is half as powerful as the first motor (rear)?
    Does this make sense to anybody?

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