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Tesla with transmission will solve range and heating issues?

Discussion in 'Model S' started by ggnykk, Feb 9, 2016.

  1. ggnykk

    ggnykk Active Member

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    As an engineer, I have always wondered why Tesla don't go with multi-gear transmission, just like ICE cars. The advantage is that the electric motor only needs to output very low RPM in order for the car to achieve highway speed. That means less electrical power is needed overall, and the range can be extended greatly even if the battery capacity is unchanged at 70 or 90 kwh. The heating issue of battery and electric motor is also greatly reduced, which means Tesla can do aggressive driving for much longer. On top of all that, the top speed of Tesla will probably reach 190 mph instead of the current 155 mph limit. You can pretty much solve multiple major issues of electric car by adding multi-gear transmission.

    Any idea on why Tesla cars are still stuck with single-gear transmission?
     
  2. DougH

    DougH Active Member

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    This is going to be a 300 page thread :smile:
     
  3. Chickenlittle

    Chickenlittle Active Member

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    1. Simplicity
    2. Cost
    3. Decrease in potential for repairs
    4. No need for anyone to go 190 mph in any setting but formula racing

    historically transmission was planned for roadster but company that designed it failed to produce a working one and tesla had to sue
     
  4. Soolim

    Soolim Member

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    I don't agree with your analysis. Gears only introduce more losses. Energy consumption is not meaningfully reduced by running the motor at lower RPM. Gears are used to multiply torque, but horsepower/kW is the same.
     
  5. Jaff

    Jaff Active Member

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    One of the prime reasons the Roadster was delivered late was because it kept "blowing up" the two speed transmissions...
     
  6. Twiglett

    Twiglett Single pedal driver

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    This has been done to death multiple times in multiple threads.
    Its not just Tesla, its pretty much every single BEV manufacturer.

    Why not?
    Complexity not needed for most motoring
    Slowing a motor from 25000 rpm down to 10000 to change gears
    gearbox beefy enough to cope with gobs of torque is expensive
    all of which means is costs way more money than any regular manufacturer wants to commit - given the tiny benefit.
     
  7. bhzmark

    bhzmark Active Member

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    No need for transmission. No need for catalytic converter either. Those are for ice.
     
  8. Twiglett

    Twiglett Single pedal driver

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    ^THIS^
     
  9. ggnykk

    ggnykk Active Member

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    I found it hard to believe that the energy consumption of an electrical motor turning at 2000 rpm isn't meaningfully less than turning at 8000 rpm. Anyone got data or graph based on real test?
     
  10. LoL Rick

    LoL Rick Like Buttah

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    #10 LoL Rick, Feb 9, 2016
    Last edited by a moderator: Mar 13, 2016
    Invest an hour watching this. Learn much, you will.

     
  11. mikeash

    mikeash Active Member

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    Isn't the drivetrain already something like 90% efficient? That leaves you a small amount of room for any efficiency improvements. Certainly range is not going to be "extended greatly" even if the transmission magically makes the motor 100% efficient. Range limits aren't because the motor is at a suboptimal RPM, range limits are because battery energy density is limited and you lose energy to aerodynamic drag and road friction.
     
  12. ggr

    ggr Roadster R80 537, SigS P85 29

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    You claim to be an engineer. Think about it. ICEs generate more power because at higher RPM they support more fuel flow. Is that true for an electric motor?

    Hint: the graph you are looking for is boring. For power, it starts out linear, then goes horizontal when the current tops out.
     
  13. Soolim

    Soolim Member

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    #13 Soolim, Feb 9, 2016
    Last edited: Feb 9, 2016
    Regardless of 2000 rpm or 8000 rpm, the motor needs to output the same shaft HP to drive the car for the same load condition (ignoring gear losses for now). So the energy consumption we are talking about is related to motor efficiency.
    For a given HP motor, the efficiency is determined by the losses of the motor, I trust that you know what they are. Do you think the difference in losses will be significant for the same motor running at 2000 rpm versus 8000 rpm, by the application of VFD?
     
  14. R3D-83

    R3D-83 Member

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    When we speak about electric motor efficiency it basically means PEAK efficiency. But it's not available in the entire rpm and torque range, and the induction motors are actually worse in this sense. You can see the differences here (slide 16). As the graph shows the induction motors only work at their highest efficiency in a very narrow rpm range. On both low and high rpm the efficiency decreases significantly (and the motor producing more heat). The gearboxes has some loss, but it's more true to the complex (4+ speed) gearboxes. The Model S/X has already have a single speed gearset, so adding 1 more speed wouldn't make a big difference. It would have more advantages (in terms of efficiency) than disadvantages. But as we know the TM has some difficulties to design a durable single speed reduction gearbox (multiple drivetrain issues even after a few thousand miles), it was even worse with a two speed. As it was mentioned earlier, they have tried with the early Roadsters, but it ended in disaster. So the conclusion: it definitely would be better (this is why they tried), but it's a big engineering challenge to handle this huge, instant torque.
     
  15. derekt75

    derekt75 Member

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    The motor isn't that much less efficient at 8000 RPM..
    See this graph: First Dyno Video - Page 6

    You could conceivably get more torque to the wheels with a transmission, but the tires are already near the grip limits and the car's low speed acceleration is generally satisfactory. :)
    and, yes, you'd need a transmission to get to 190 mph with the current low speed torque.
    To do that, you'd need to add another piece to the drive train which would add cost and reduce your efficiency&performance from 40mph to 80mph.
    That speed range is a range where I do most of my driving, is the place where I care most about efficiency&performance, and is a place where Model S is already only meh compared to other ICE supercars (e.g., BMW M5).
    Also, one of the real joys of driving Model S is how connected the car's acceleration is to the accelerator pedal. If you're cruising along behind some slow driver at 50mph in an M5, and then you decide to stomp the pedal and pass the other car, the M5 downshifts. It does this ever so much faster than your father's automatic or Infiniti M45, but it still takes a noticeable fraction of a second. In the fixed gear Model S, when you stomp the pedal the car GOES.
    So for the headaches of a transmission (more cost, more maintenance, less responsiveness, less efficiency at cruising speeds) I can get a car that goes over 150mph and is more likely to break the tires loose out of a traffic light.
    Personally, I'll pass. Autobahn drivers might make a different choice, though.
     
  16. Electricfan

    Electricfan Member

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    #16 Electricfan, Feb 9, 2016
    Last edited by a moderator: Mar 13, 2016
    Thanks, I enjoyed that.

    Amazing how even a genius like Marc Tarpenning can be DEAD WRONG on such an important assumption though. Check out what he says at 50:14 in the video.
     
  17. R3D-83

    R3D-83 Member

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    There are only power and torque curves. The efficiency is not event mentioned there since it cannot be depicted with a single line. It depends on both torque and rpm.
     
  18. derekt75

    derekt75 Member

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    Well, if you assume that the max power is limited by the battery, then the difference between power to the wheels and max power tells you a lot about efficiency.
    The published curves would then indicate that the car's efficiency is constant from 40 mph to 70 mph, and that it only drops off after that.
    The measured dyno curve indicates that there's a bit of roll off from 40 mph to 70 mph, but more after that. Either way, the 40 mph power to wheels is pretty similar to the 70 mph power to wheels.
     
  19. Soolim

    Soolim Member

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    Since peak efficiency is hard to achieve unless one deploys a high multi-ratio gear with its corresponding losses, let us settle for the practical aspect of keeping the motor efficiency at 4-5% off peak. With that assumption, I see the difference in motor efficiency from low rpm to high rpm in the range of +/- 4% in normal driving. Hence I don't think the multi-gear transmission with the added complexity is going to significantly improve the range of the BEV. I already have the milling noise, I don't need more gears with additional wear and tear, and maintenance.
     
  20. ecarfan

    ecarfan Well-Known Member

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    I would not describe any of the "issues" you list as "major".
    Tesla's drivetrain design is a well thought through compromise for a passenger car.
    It is obviously is not a track-ready car for "aggressive" driving. It's a 4,600 lb family sedan. So it doesn't need expensive cooling systems and an expensive multi-speed transmission so you can drive for extended periods at 190mph. The market for such an EV is minuscule.
    Could Tesla build such a car? I'm sure they could. It would cost a lot more and they would sell a handful of them. And such a car would do nothing to further Tesla's mission.
     

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