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Electric cars with transmissions

Discussion in 'Technical' started by mattjs33, May 9, 2011.

  1. mattjs33

    mattjs33 Member

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    I was wondering if someone could explain to me why electric cars don't come with equipped with traditional gearboxes? I know in simple terms, they don't really NEED them, and I also know that early Teslas had a simple two-speed gearbox that ultimately couldn't be made to work properly.

    But every account I've read about electric cars states that as speed increases, acceleration tapers off. An electric motor can spin an input shaft just as a ICE can, so why not do it and take advantage of the gearing? You'd be able to get that initial low RPM torque hit multiple times. Can you imagine how fast a Roadster would be, with a shorter final drive, hooked to a proper six-speed?!

    Haven't there been private electric conversions where this type of arrangement was used? Is it an issue with drag? Gear whine? Or simply complexity?
     
  2. Tommy

    Tommy Member

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    in simple terms, the electric motor's instant torque, becomes a liability when mated to a multi-speed transmission due to the inability of the transmission to handle the instant torque. Parts "snap" when subjected to this type of stress. It's not worth the trade-off in complexity/cost to make a transmission able to handle the additional stress.
     
  3. TEG

    TEG TMC Moderator

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    Also extra gearbox mechanism can hurt efficiency, and they want to do all they can to optimize range.

    Range at speeds over 100MPH starts to get really short (due to wind resistance), so it is probably a good thing that they limit the top speed.
     
  4. vfx

    vfx Well-Known Member

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    We may see them is specialized racing / high speed record applications, but even that is subject to ever improving motor/controller/battery tech.
     
  5. TEG

    TEG TMC Moderator

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    Probably need to mention that the Roadster's air cooled componentry can be subject to power output governing under high heat situations.
    Even though gearing may allow a Roadster to go 125mph, on a hot day it may actually slow down from that due to heat build up from sustained high power levels.
    So if you added more gears to try to get higher top speed it might not do you much good.
     
  6. clea

    clea Member

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    What would be the issues if they used a continuously variable transmission (CVT)?
     
  7. stopcrazypp

    stopcrazypp Well-Known Member

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    The only electric I'm aware that uses a multi-speed (6 speed electric transmission in this case) transmission is the Brammo Empulse.

    http://www.wired.com/autopia/2011/05/brammo-six-speed-tranmission/

    The plus of a multi-speed transmission is it can give you a good top speed and acceleration. If Brammo succeeds, it may because the 6 speeds allow them to have closer ratios, unlike the failed electric 2-speed Tesla attempted.

    Overall though, the negatives probably outweight the positives. With a multi-speed transmission, it's more complex, less reliable, more expensive, and it might mean lower efficiency. And depending on how you make it, it might make the car less smooth too.
     
  8. bolosky

    bolosky Member

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    This isn't due to gearing (or lack thereof), it's due to power limits in the battery/PEM.

    Basic physics says kinetic energy goes up with the square of velocity, so accelerating 15 MPH/s takes more power at 40 MPH than it does at 10. When you can't supply that power to the motor, you get lower acceleration. This happens in the Roadster at around 40MPH, and no amount of changing the gearing is going to make it better.

    What you CAN get from changing the gearing is two things:
    1) Greater torque at low speed (with lower gearing)
    2) Greater top speed (with higher gearing).

    So, by puttting in a multi speed transmission you could both improve the 0-40 time and get a higher top speed by having the car going faster when the motor redlines. What you're never going to do is improve acceleration over 40 MPH without changing the battery and/or PEM, though.
     
  9. William3

    William3 Member

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    All the CVT cars on the market have very low torque. CVTs tend to be super weak. A Roadster motor (or even a Leaf motor) would just rip it to shreds.
     
  10. Doug_G

    Doug_G Lead Moderator

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    Tesla's mistake was trying to get away with a 2-speed. They thought they could do this just because the motor had the RPM range to do it. Unfortunately when it was time to upshift, the motor had to spool down too far too fast. Either you had to wait a long time for this to happen, or you had to mash it and have the transmission absorb a lot of force. So either shifts took forever or the transmission broke after six months.

    It would have worked a lot better if they used a double clutch configuration and more gears. Sure that means you're not using the full RPM range of the motor, but at least then it would have actually worked. There would be compromises though, including more complexity, weight, lower efficiency, etc.

    On balance going to the simpler one-speed arrangement was almost certainly the correct solution. The gearbox is extremely efficient, lightweight, and reliable... and the downsides are minor. Simpler is almost always better.
     
  11. ovisoftblue

    ovisoftblue New Member

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    A quick explanation

    Here's why electric cars don't need gearboxes. It's because of their complexity and cost. The article has been written by a former Toyota technician. He knows what he's talking about.
     

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