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Thread: Could ultracapacitors (or tiny tiny wheels) make Electric Vehicles accelerate faster?

  1. #1
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    Could ultracapacitors (or tiny tiny wheels) make Electric Vehicles accelerate faster?

    Saw this video-

    https://youtu.be/KQ2Eo6wl5r0

    Could the rapid discharge of ultra-capacitors make EV's accelerate faster or is the rate-limiting step the torque that the electric motor can provide?

    As a separate question, would tiny wheels on a car improve the acceleration/ torque??

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    https://www.youtube.com/watch?v=m3dZl3yfGpc

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    Quote Originally Posted by plant View Post
    Saw this video-

    https://youtu.be/KQ2Eo6wl5r0

    Could the rapid discharge of ultra-capacitors make EV's accelerate faster or is the rate-limiting step the torque that the electric motor can provide?

    As a separate question, would tiny wheels on a car improve the acceleration/ torque??

    Thx
    Tiny wheels on a ICE car would make acceleration worse and torque would be unusable unless you built a massive cog into the gearbox. The wheels would spin with any application of power. Wheels are sized as part of the whole power train to be reasonable at acceleration but still capable of high speeds. An ideal wheel would start big and get bigger to achieve good acceleration and really high speeds, but it would need gearing correctly. EVs don't have gears afaik.

    As for the ultra-capacitors, I don't see why there should be a limit to the amount of torque a motor could provide, apart from the mechanical capabilities of the components. I suspect you would need more and heavier windings in the motor to generate a larger magnetic field. More current through a standard motor may just burn it out.
    There is a system used for commercial 3 phase machinery called star delta. This restricts the power input at startup to single phase until the machinery is turning or working slowly, then adds in the extra phases to use more power. Perhaps something similar could be used in EVs.

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    Torque is a constant of the motor. You can gear it down or up to provide more traction or more speed, but the maximum torque is the same.

    Tiny wheels is the equivalent of a lower gear, so you would get better low-speed acceleration. But the maximum speed would be lower.

    In principle ultra-capacitors would provide more power, and if the motors can use it effectively, better acceleration. But I think a Li-Ion battery can discharge very quickly anyhow, so it may be there is no advantage with current motors. EVs are already very good at acceleration, it's the range that is the worry.

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    Quote Originally Posted by kzb View Post

    Tiny wheels is the equivalent of a lower gear, so you would get better low-speed acceleration. But the maximum speed would be lower.
    I don't think that's correct. Look at any bicycle gears on the rear wheel. Lower gears have more teeth, making the sprocket larger. High gears have fewer teeth making the sprocket smaller. Making the wheel smaller is equivalent to having less teeth (less circumference).

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    Quote Originally Posted by headrush View Post
    I don't think that's correct. Look at any bicycle gears on the rear wheel. Lower gears have more teeth, making the sprocket larger. High gears have fewer teeth making the sprocket smaller. Making the wheel smaller is equivalent to having less teeth (less circumference).
    kzb is correct. Making the rear sprocket larger means more torque at the hub, at the expense of reduced speed. Keeping the sprocket smaller and making the wheel smaller has the same effect.

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    Of course, a smaller wheel would likely have a smaller moment of inertia, so it would accelerate faster with a given amount of torque, but smaller wheels are more susceptible to irregularities in a road surface (this is one of the disadvantages of folding bicycles, such as Bromptons). There are other disadvantages to small tire diameters, including that a long, narrow contact patch may be more stable than a short, wide one, and that a small wheel will have greater [apparent] radial accelerations on its outer radius ().
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    Quote Originally Posted by plant View Post
    Could the rapid discharge of ultra-capacitors make EV's accelerate faster or is the rate-limiting step the torque that the electric motor can provide?
    I have mostly seen them discussed in terms of handling surges, which could help with battery longevity and better regenerative braking. My feeling is that the acceleration possible on a Tesla already is about as far as you want to go on a street legal vehicle, though maybe it might matter on a race car. I think maintaining tire traction would be one limiting factor.

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    Lithium batteries have more than enough power density, the limiting factor in acceleration these days is traction. The fastest-accelerating stock vehicle in 2019 was an electric, the Tesla Model S P100D, which can accelerate harder than it can brake.

    Supercapacitors might improve efficiency of regenerative braking, as they can charge faster and then transfer their charge to the batteries at a more efficient charging rate.

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    In the old days electric motors were limited by iron saturation with magnetic field and then permanent magnets demagnetise if the opposing field is too high. These are still limits but modern materials have transformed permanent magnets. Servomotors tend to have no iron so that high peak currents can be used for initial acceleration or jerk. Car motors now are alternating current type with electronics generating the AC from the DC batteries. These rapid switching high efficiency semiconductors are another part of the advance of electric motors. Supercaps handle surges as already reported.
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    The lower inertia of the smaller wheels might allow them to spin up, but the acceleration is limited by the friction, tyre to road. ditto braking. in another question it was pointed out that Tesla might use rocket thrusters but that is another story.
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    I'm reminded that in the days of steam, you could distinguish freight locomotives from passenger one by the wheel diameter. Freight locos had smaller wheels for pulling power, while passenger locos had large ones for high speed.
    Not really seeing a need for that in cars.
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    Quote Originally Posted by Trebuchet View Post
    I'm reminded that in the days of steam, you could distinguish freight locomotives from passenger one by the wheel diameter. Freight locos had smaller wheels for pulling power, while passenger locos had large ones for high speed.
    Not really seeing a need for that in cars.
    The critical factor here was the impossibility of completely balancing the connecting rods on a steam locomotive. That limited the wheel RPM and required large wheels for high speed. With modern diesel-electric or pure electric locomotives the same small wheels serve well at all speeds. For an electric car there will be an optimum size, and a direct-drive motor can be designed accordingly.

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    For the OP: After reading the specs for and seeing some driving demonstrations of modern, production EVs, how much faster do you want them to accelerate? I think we're well within the capability to exceed human comfort. I suppose you might push it for driverless/passengerless vehicles, but really, what's the advantage? Are you envisioning some sort of agility need? Or is this just a mental exercise?

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    Quote Originally Posted by Hornblower View Post
    kzb is correct. Making the rear sprocket larger means more torque at the hub, at the expense of reduced speed. Keeping the sprocket smaller and making the wheel smaller has the same effect.
    In fact, bicycle gearing used to be measured in "inches". That is, the size of penny-farthing wheel that is equivalent to the gear ratio on the bicycle in question:

    https://en.wikipedia.org/wiki/Gear_inches

    BTW, I don't think "more torque at the hub" is strictly correct terminology. The torque is provided by your legs and has a maximum value irrespective of the gearing or wheel size. Torque = Force x Radius.

    So you can have more "force" at the hub by gearing down (at the expense of speed) or more speed (at the expense of "force") by gearing up.

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    Quote Originally Posted by kzb View Post
    In fact, bicycle gearing used to be measured in "inches". That is, the size of penny-farthing wheel that is equivalent to the gear ratio on the bicycle in question:

    https://en.wikipedia.org/wiki/Gear_inches

    BTW, I don't think "more torque at the hub" is strictly correct terminology. The torque is provided by your legs and has a maximum value irrespective of the gearing or wheel size. Torque = Force x Radius.

    So you can have more "force" at the hub by gearing down (at the expense of speed) or more speed (at the expense of "force") by gearing up.
    Torque at the hub does make sense, as you gear up or down you change the torque at high efficiency usually. One advantage of electric drive over IC is a high starting torque from zero revs upwards.
    It reminds me of a bicycle conundrum, with typical gearing, a backward horizontal force at the bottom pedal can drive the bike forwards! The pedal does travel backwards for a short distance. so no "law" is broken,.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
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