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Efficiency of Geared vs. Single Speed transmission.

Discussion in 'Technical' started by Gerasimental, Feb 8, 2015.

  1. Gerasimental

    Gerasimental Member

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    I have a job interview tomorrow at a company called Drive System Design who have a fairly strong focus on EV and HEV power trains.
    One of their products (well, developed by a sister company) is an EV transmission system claiming to achieve 'up to a 15% reduction in energy consumption on the New European Driving Cycle (NEDC) compared to a single speed EV powertrain and offers a 40% weight saving to the power train.'

    I was always under the impression that EVs didn't NEED multiple gears because their AC induction motors have an almost flat torque/speed characteristic and reach max efficiency at around 20% load and then stay there. Tesla's choice of a single-speed transmission seems to support that. Plus any additional gearing and mechanical components increase losses to friction.

    So what's the situation here? How can a multi-speed transmission increase efficiency, and more suprisngly, save weight? If this is the case, what is Tesla's reason for sticking with single-speed transmission? I'm interested in talking about this at my interview tomorrow but it would be good to have some context or understanding of this before I go.
     
  2. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    As far as I know, the main reason Tesla uses a single speed reduction gear set, is that an electric motor has so much torque it snapped all the multi-speed transmissions. You could have a multi-speed transmission if the motor was very low power, but then you'd have other problems (like selling the car).
     
  3. sandpiper

    sandpiper Active Member

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    Hmmm.. I've never heard of an electric car with a multispeed gearbox, but that's generally because with the high motor efficiency and wide power band, it's an unnecessary complication, and for what benefit? I suppose it would allow Tesla to keep pace with the high powered ICE engines at higher speeds, but I don't see that providing enough benefit to justify the added cost & complication. The torque may be very high in automotive terms, but certainly not in terms of general geared machinery.

    As an aside, the AWD Teslas effectively have a dual speed gearbox. They just run the motor that's in the best location on the efficiency/power curve.
     
  4. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    The first Roadsters had a multi-speed transmission. However, the motor's torque kept breaking them and all were converted to single speed.
     
  5. Saghost

    Saghost Active Member

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    There's a lot of potential benefit, mostly in either low end torque or ultimate high speeds. It's true that an electric motor has a much broader range and can start from 0 rpm, but there are definitely areas where it is more efficient than others. If I remember right, the efficiency of the Volt's drive motor drops almost 10% from the mid rpm middle power peak to the limits of the normal operating range - and by doing the 2 motor EV mode and dropping the main motor rpm by half, they pick up 4% overall system efficiency on the highway.

    Have a look at this chart WarpedOne published in a prior discussion on the subject:
    The real top speed of the Model S - Page 6

    Speeds over ~70 mph start cutting greatly into the efficiency and power output of the Tesla drive motor (this was before the D announcement - I'd be interested to see a P85D and an S85D plotted on the same chart) - limiting the car's top speed to something well below it would otherwise have. Having a taller gear could change all of that (though those speeds are still very inefficient due to simple aerodynamics, and dangerous on most roads.)

    The question is always about the tradeoffs - weight, cost, space. Potentially some of the mass of the transimssion can come out of making a smaller, higher RPM motor, but the total package still has more weight and cost, and more chances to fail. Most OEMs believe that on the balance the tradeoff isn't worth it. They certainly have more data to decide with than I do. :)
    Walter
     
  6. Gerasimental

    Gerasimental Member

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    Hybrid and electric transmission - DSD

    Here is the company and the transmission I mentioned. They are a well respected company with many large OEMs as customers.

    I asked this question at the interview today and they gave pretty much the exact explanation that you gave Saghost.
     
  7. bxr140

    bxr140 Member

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    Formula E uses a 5 speed sequential Hewland gearbox, but they also max out at 200kw.
     
  8. stopcrazypp

    stopcrazypp Well-Known Member

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    That chart shows that a multi-speed transmission would allow the Tesla to stay with-in its peak powerband, however it says nothing about efficiency, as it's not a plot of efficiency.

    And the article in this thread claims the main improvement will be on the NEDC cycle which is low speed (not the speeds mentioned in WarpOne's charts). I imagine also the type of motor (induction, PM, or switched reluctance) will play a big role also.
     
  9. Saghost

    Saghost Active Member

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    Agreed. From what I've read Back-EMF should be eating a lot of efficiency in those rpm ranges, but the chart doesn't cover or discuss that.
     
  10. miimura

    miimura Active Member

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    The BMW i8 uses a two-speed gearbox on the front axle in order to provide both low speed torque and high speed ability.
     

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