EV transmissions and Popular Science

As posted from Richard230 on the electricmotorcycleforum.

According to a the following statement on page 22 of the March issue of Popular Science, electric vehicles (not necessarily motorcycles) may have a transmission in their future. Here is the statement:

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Single-speed EV transmissions will give way to two-speed units within 5 to 6 years, according to German supplier ZF. As with gas powered cars, adding more gears to electric motors will make them more efficient, extending their range by up to 20 percent.

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lol.. i'm sure this debate that will start again will require popcorn!! :lol:

http://electricmotorcycleforum.com/boards/index.php?topic=4411.0

Doc

Hogwash.

liveforphysics said:

Hogwash.

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:lol:

The tractor weight showroom in town said the same thing about tractor weights.

Doctorbass said:

German supplier ZF.

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Is that a suppler of...transmissions?

amberwolf said:

Doctorbass said:

German supplier ZF.

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Is that a suppler of...transmissions?

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With the industry seeing the massive fuel economy gains on the current gen of Honda Accord Hybrids by dumping the tranny, ZF is desperately searching out somewhere to be relevant. Dear ZF, switch your company to one that makes direct-drive motors that work without the losses, failure and wear modes, pointlessly complexity and cost, noise, and volume mass on an EV. This is your only option for continued vehicle relevancy, Honda has recently and throughly proved this to the world.

As EV's mature and move away from ICE related models for attaining power densities, and instead embrace the geometries possible to simple create the desired torque over the desired speed range in a single stage, while using a bit less active material (iron/copper) mass than needed than a smaller high RPM motor if designed correctly.

The most beautiful thing about embracing electricity as a propulsion source is the freedom to actually implement the real first principles optimizations of energy conversion in traction drives. EV performance's greatest potential lays in the potential for electricity to be fluidly converted into power at the desired wheel torque/speed in a single stage with no additional serial loss stages inherent to transmissions required (even the clever ZF 2spd freewheeling ones add freewheel drag, though agreed it's very low, but still added mass and complexity and cost and failure modes for no reason if your motor wasn't designed to require a crutch to work in your application.)

Luke,

Thanks for the heads-up on the Honda Accord hybrid. I haven't paid attention to developments in hybrids since full EVs became available. This appears to work very similar, with the substitution of a hydraulic coupler for the Honda's mechanical clutch.

https://www.youtube.com/watch?v=naRY3knqFYU

If we get a doubling of battery efficiency, as many expect in the next decade, hybrids will be only interesting history.

Is there any commercial EV or Hybrid that does not run at least a single stage reduction transmission ?

Other than a hub motor, any electric motor, or ICE engine is going to need some reduction.

Why is that ?
Why don't they make more EV traction motors which function at low rpm like hub motors ?
Then you could eliminate all transmission losses, weight, cost, noise, maintenance, and improve reliability.??

Hillhater said:

Why is that ?
Why don't they make more EV traction motors which function at low rpm like hub motors ?
Then you could eliminate all transmission losses, weight, cost, noise, maintenance, and improve reliability.??

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By running the motor at significantly higher speed than the wheel, they are able to use much smaller motors. The difference in motor size/mass/cost more than offsets the single ratio gearbox. The loss in the gears isn't that high and the automotive industry is pretty good at making reliable gears. Overall it is a preferred value solution to run the motor fast with a reducer versus a motor at wheel speed. I think all the OEMs do it this way.

So a transmission is necessary to enable sufficient torque at low rpm and a practical weight motor ?
But, single reduction will be a compromise between sufficient low speed starting torque, and efficiency at higher speeds. ( for optimum range)

Hillhater said:

So a transmission is necessary to enable sufficient torque at low rpm and a practical weight motor ?

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I like to call it a gear reduction. Most consider a transmission to have multiple ratios and shift on demand. Multiple ratios are not needed in battery electric cars. Not one OEM uses a transmission. The all use gear reduction from the motor to wheel. You may call it a compromise, but is looks like a very reasonable solution to me.

They only use the reduction now because they haven't matured motor geometry sufficiently yet. There is nothing gears can do that a magnetic geometry can't be setup to do, and then you skip the frictional loss and cost and failure modes of the gearing stage.

I could see a transmission and 2 gears useful for specific specialty applications.

Such as, granny gearing for an off road vehicle. But a town vehicle wouldn't need it.

liveforphysics said:

They only use the reduction now because they haven't matured motor geometry sufficiently yet.

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We've only been designing, making and using electric motors for what? 130 years? How much longer do you feel it will take to mature the geometry? Days? Years? Century? So in the mean time, gears serve a useful purpose.

major said:

liveforphysics said:

They only use the reduction now because they haven't matured motor geometry sufficiently yet.

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We've only been designing, making and using electric motors for what? 130 years? How much longer do you feel it will take to mature the geometry? Days? Years? Century? So in the mean time, gears serve a useful purpose.

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Brushless hubmotors are quite new to the scene and the design is still in it's infancy, since the Chinese, the primary manufacturers and users, have been too willing to accept what works instead of optimizing them. The overwhelming majority don't even do much of anything wrt heat dissipation, which is a major limitation. This is clearly evidenced by smooth side covers inside and out instead of easily doubling the heat transfer at a given temperature by increasing the surface area.

A typical EV reduction transmission, (Tesla ). is around 10:1 ratio.
That gives a tenfold increase in motor torque......hence the much revered acceleration.
What motor and system changes would/could be made to give that same 10X torque at the motor shaft ?

The quick answer is a motor with 10 times the air gap area.

wow ! ,10 times the diameter in effect ?
that would be one big motor
?..which would make it impractical..until some one comes up with a technology leap.

major said:

The quick answer is a motor with 10 times the air gap area.

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You only need ~3.16x (10^0.5) greater air gap area if you create your increased area through growing motor radius. This large radius motor can use proportionately smaller teeth for no additional active material weight or cost for a given power level (and theoretically even less active material for a given power level due to the greater surface area and reduced Rth path it has for cooling).

Torque increases with the square of the airgap radius, so an increase of 3.16x radius (or diameter) should yield 10x the torque, if I have added up properly...

Well I did say it was a quick answer But there may be constraint on diameter increase available such as for direct coupling to the wheel. So you could end up with 4 to 5 times the air gap area. Then what? Divide by 4 and stick one at each corner. Four motors to replace one, each with about the same air gap area or a bit more. And they say: Look at all those gears and shafts we eliminate. I've been hearing that for 30 years and not one on the road today. If the brushless hub motor is the answer to matured motor geometry, I'm disappointed in the vision and expect to wait another 30 years to see it actually implemented.

Punx0r said:

Torque increases with the square of the airgap radius, so an increase of 3.16x radius (or diameter) should yield 10x the torque, if I have added up properly...

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..with no change to the electrical load , amps needed , kW etc ??

major said:

Well I did say it was a quick answer But there may be constraint on diameter increase available such as for direct coupling to the wheel. So you could end up with 4 to 5 times the air gap area. Then what? Divide by 4 and stick one at each corner. Four motors to replace one, each with about the same air gap area or a bit more. And they say: Look at all those gears and shafts we eliminate. I've been hearing that for 30 years and not one on the road today. If the brushless hub motor is the answer to matured motor geometry, I'm disappointed in the vision and expect to wait another 30 years to see it actually implemented.

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You only need 2 motors if you're not trying to have AWD.

There are also higher torque density motor types, like transverse flux (because it has a >1 winding factor) if the application needs to keep the motor size small.

It's never going to be optimized to be spinning additional non-active material (like gearing), it's friction, it's failure modes, it's cost, it's efficiency etc. Designs today do it because they are still immature. Fortunately they are evolving faster than ever at this point in time, and soon enough the designs will mature to embrace the inherent ability to directly convert electricity into the torque and speed desired rather than needing lossy wear parts like gear reductions for crutches.