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Transmissions in EVs

Discussion in 'Electric Vehicles' started by kmistry1986, Sep 25, 2017.

  1. kmistry1986

    kmistry1986 Member

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    I got into an interesting conversation with some friends this weekend. I know electric motors are quite different from gas powered engines, but wouldn't an electric motor benefit from having a transmission?

    For example, a gas engine that revs at 2,000 RPMs is more efficient than the engine revving at 5,000 RPMs.

    The range of our cars increases as we drive slower, which means the motors are running at a lower RPM (right terminology?) and drawing less power. So if you had a two speed transmission, once you hit a cruise speed of 70 MPH, it would switch to the second gear which is optimized to have the motor rev (again, right terminology?) at a lower rate but keep the speed at 70.

    Thoughts?

    Sorry in advance if this is a dumb question..I'm not an engineer or anything close to being that! Thanks for the feedback!
     
  2. mongo

    mongo Member

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    Higher power usage at higher speed is mainly due to increased friction (aerodynamic, tire, and drive train rotational).
    Regardless of PRM, overall motor power stays mostly the same to match the force needed to maintain speed.

    ICE's have a peak efficiency band, so the gearing is used to place cruising speed in that band for economy. Peak power usually occurs at a higher RPM, but is less efficient.
    Electrical resistance losses go up with the square of the current, so a higher motor speed (higher voltage) lower current operating point can be more efficient than a lower RPM, higher current operating point.
     
  3. phigment

    phigment Member

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    I may be off here, but torque in the EV drivetrain is higher at lower RPMs. So putting in a transmission would allow for more acceleration at higher speeds (and higher speeds in general).

    I don't believe range will increase because at low speeds you wouldn't really make use of a second gear, and at higher speeds, your efficiency is lost mainly due to wind resistance.
     
  4. widodh

    widodh Model S 85kWh

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    In general: No

    But this video explains it very well:
     
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  5. _jal_

    _jal_ Member

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    Good video ^^

    It seems reasonable, but the dynamics of the electric drivetrain really don't make it advantageous. There's a lot of reading you can do on the topic, but here are a couple of topics:

    - the drives in Model S/X are induction motors. The only friction anywhere is in the bearings for the drive shaft. There are no brushes like you have in DC motors on power tools for instance. It's a really cool bit of science that is worth reading about. The bottom line is that the motor produces all the torque by manipulating electric waveforms to create coupled magnetic fields in the motor that "chase" each other around the drive shaft. There is no mechanical limitation to speed caused by inertia the way there is in an ICE. For instance, in an ICE you have the pistons that go up and down in the cylinder. The piston rod connects to the piston via some kind of mechanical pin. That pin is responsible for starting and stopping the piston, and eventually the piston will get going fast enough that the piston will have so much inertia that the pin will be insufficient to overcome and will cause the pin to break. There are a lot of other kinds of these situations that require the ICE to limit its speed that the induction motor simply doesn't have.

    - it's true that induction motors create the most torque at low RPMs, but that's because the torque generated by the difference in speeds of the field generated in the stator and rotor is greatest when the difference in rotational speeds is greatest. Once the speeds are matched and the rotator and stator are basically coupled, then there is very little torque generated. The presence of more torque, however, does not indicate that the motor is more efficient at that state.

    - the biggest reason ICE motors need to have transmissions is that they have a very narrow band of optimum output. It's a pretty audacious feat of engineering actually. For each cycle they have the same amount of time to do the following four things with the same amount of gas 1) draw in fuel enriched air, 2) compress that "air" so that the fuel can ignite and explode such as to burn all the fuel, 3) expand to actually capture the energy released by the combustion of the fuel, and finally 4) exhaust the cylinder of the gas after combustion. All four have to happen in the same amount of time and in the same amount of space. So if your car's engine is going 1,000,000 RPMs, there simply won't be enough time for the combustion of the gas to complete before it is exhausted for instance. It doesn't matter that the mechanical components will fly away - assuming you could keep the thing together, combustion takes time to propagate through a gas and it just won't be fast enough to be complete. There is a band for engines that is optimum, and the transmission in ICE cars is primarily there to make sure the motor stays in that band during operation. Those constraints just simply aren't there with an induction drive at the same level.
     
  6. gregd

    gregd Active Member

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    The original Tesla Roadster (the "1.0") actually did have a 2-speed transmission. At the time, they needed two gears in order to achieve the goal of a sub-4 second 0-60 time, and also have a good top-end speed. The problem was that the electric motor was so powerful that every transmission broke in one way or another. All of them. Could the mechanical problems be fixed? Probably. But by then the car was late to market, and a new higher power IGBT (transistor) had just come to market which allowed them to use a simple single ratio gearbox and achieve their performance goals by simply pushing more current into the electric motor. Problem solved, and the Roadster "1.5" went to market. 0-60 in 3.9 seconds; "redline" at 14,000 RPM put the top speed at 125 mph. That was nearly a decade ago.

    So, is it possible? Yes, in some specific scenarios, a transmission may help. But the very wide performance range inherent in an electric motor and its controlling electronics, and the inherent weight, added friction, and difficulty in making a good transmission, generally makes it not worth adding one.
     
  7. miimura

    miimura Active Member

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    Both the BMW i8 and BMW 225xe use a GKN eAxle driveline that includes 2-speed transmission for the axle that is not driven by the ICE. The 225xe is quoted as having a 65kW motor on the rear axle. I did not find the kW rating of the motor-generator connected to the ICE in my quick search. Actually, now I cannot find a reference that says the 225xe is two-speed, so it may have the version that disconnects the rear motor at high speed for reduced losses and to prevent over revving the e-motor. Definitely the i8 is two-speed.
     
  8. kmistry1986

    kmistry1986 Member

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    I think I understand and it seems to make sense as to why they don't have them. Thanks for the answers everyone!
     
  9. igotzzoom

    igotzzoom Supporting Member

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    A lot of EV conversions to ICE cars retain a manual multi-speed gearbox. But you can start off from a stop in any of the gears. A lot of them seem to just keep it in third gear as the "default" speed. Cool from a gearhead novelty standpoint, but as stated, not really necessary.
     

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