Electric cars and gearboxes ?

[J4CKO]

Ok, I understand due to the torque, and its availability from zero rpm, coupled with the high rpm that covers the entire speed range they don't need one but if and electric motor was coupled with a conventional auto box would it be faster or completely pointless ?

[clockworks]

Pretty sure you'd just be wasting power due to mechanical losses in the gearbox, so pointless for a "normal" car.

Might be a case for having a simple gearbox (maybe 2-speed?) in an electric "performance" car, though?

[kambites]

It would put you closer to peak power more of the time, but would also add weight and reduce drive-train efficiency (and hence range). I suspect if you want to make a given electric car go faster, it's probably cheaper and better to fit a more powerful motor than a gearbox.

[thatdude]

I'm sure some of the electric bikes that run in the TT-Zero and the like have gearboxes? Multiplies the torque after all

[kambites]

thatdude said:

I'm sure some of the electric bikes that run in the TT-Zero and the like have gearboxes? Multiplies the torque after all

I dare say some electric racing cars do too.

[Cupramax]

kambites said:

It would put you closer to peak power more of the time.

Please explain how it would do this when an electric motor produces its max torque from 0rpm

[Scuffers]

Power = torque * rpm

Thus 0 rpm = 0 power

And yes adding a gearbox will make it faster, but adds weight and complexity, so most don't bother.

Interesting, tesla originally designed one for the roadster, but ditched it.

[98elise]

No real need in an EV. ICE's suffer from pumping losses so get an efficiency advantage from gear boxes reducing revs. The only benefit for an electric car would be greater touque at lower revs (which is why they are not good for towing).

There is quite a large loss on a gearbox, so if it can be avoided, then its best not to have one.

[kambites]

Cupramax said:

kambites said:

It would put you closer to peak power more of the time.

Please explain how it would do this when an electric motor produces its max torque from 0rpm

Power = torque x engine speed.

[JonnyVTEC]

Would allow better efficiency at a range of speed and better acceleration. 3 speeds would be ideal to be honest.

Still early stage of EV powertain, next step of the volt for example probably will be a range extender designed engine rather than a corsa lump as the EV part now has a degree of volume production and warranty data.

[vx220]

I reckon the gearbox would have to be pretty heavy duty (and so heavy) to withstand the torque

[bearman68]

Struggling to understand why you would need one. It's the limited rev range and the loss in efficiency in an IC engine that requires a gearbox, but that doesn't apply (as I understand it) to an electric motor. The motor has a I think a constant efficiency over a very wide rev range.

[JonnyVTEC]

The current draw and voltage drop are also functions of the power electronics and battery efficiency. Downsizing the motor and inverter for example but having the same acceleration thanks to known technology would do the cost of the parts a favour... For example amount of copper of the motor windings.

[blondini]

J4CKO said:

Ok, I understand due to the torque, and its availability from zero rpm, coupled with the high rpm that covers the entire speed range they don't need one but if and electric motor was coupled with a conventional auto box would it be faster or completely pointless ?

Should accelerate faster in the lower gears as you would be multiplying the torque at the wheels. Trouble is the extra transmission losses would reduce the mileage range per charge.

[Scuffers]

bearman68 said:

Struggling to understand why you would need one. It's the limited rev range and the loss in efficiency in an IC engine that requires a gearbox, but that doesn't apply (as I understand it) to an electric motor. The motor has a I think a constant efficiency over a very wide rev range.

Electric motors do not have an infinite rev range, and most have torque dropping off at the higher limits of rpm.

As said, a 3 speed box would be ideal, and remember, you need some kind of gearing to drive the wheels anyway.

[supersingle]

Scuffers said:

bearman68 said:

Struggling to understand why you would need one. It's the limited rev range and the loss in efficiency in an IC engine that requires a gearbox, but that doesn't apply (as I understand it) to an electric motor. The motor has a I think a constant efficiency over a very wide rev range.

Electric motors do not have an infinite rev range, and most have torque dropping off at the higher limits of rpm.

As said, a 3 speed box would be ideal, and remember, you need some kind of gearing to drive the wheels anyway.

I imagine a CVT transmission would work well. Cheap, simple and lightweight.

[JonnyVTEC]

Even more torque limitations with CVT!

Remember the motor does t drive the wheels direct so extra gears to select wont effect transmission losses! The effect on range would be to increase it not reduce!

[supersingle]

JonnyVTEC said:

Even more torque limitations with CVT!

Not really, some powerful Audis have used CVT, it depends on what it's designed to handle. CVTs have proven unpopular with ICE due to drivers finding the constant engine revs unpleasant. That might not be an issue with an electric motor.

As alluded to in this thread a gearbox allows the designers to spec a smaller motor whilst maintaining low speed drivability. It's swings and roundabouts but an electric motor coupled to a CVT has some advantages over conventional gearbox or straight electric drive.

[Scuffers]

supersingle said:

Not really, some powerful Audis have used CVT, it depends on what it's designed to handle. CVTs have proven unpopular with ICE due to drivers finding the constant engine revs unpleasant. That might not be an issue with an electric motor.

Audi's with cvt are typically detuned versions of the engines

[drdino]

thatdude said:

I'm sure some of the electric bikes that run in the TT-Zero and the like have gearboxes? Multiplies the torque after all

Some of them do, mainly because the characteristics of the drivetrain they are running are not ideal. The top teams (Mugen, Motoczysz, Kingston Uni ) don't have gearboxes. Having said that, I am pretty sure that only the Italian team had a gearbox this year.

One has to keep in mind that the gearbox is mainly there due to the inherent characteristics of the ICE.

I could see a gearbox with 2 ratios if one were to use an EV for two widely different uses (ex. racing/city use).