Question about electric cars

"Jake - Has Bad Luck So You Don't Have To" (murdersofa)
03/24/2015 at 15:28 • Filed to: None

Why do electric cars all seem to be direct-drive from the motor to the wheel? I mean, there might be gears in there, but there isn't really any shifting going on. Wouldn't the car be faster/more thrifty if you could shift into a high gear for cruising and run the motor slower? Perhaps promote motor longevity? And yes a motor has instant torque but different gear ratios still CHANGE the amount of torque being sent to the wheels, right?

I'm imagining an electric motor from a Tesla in a manual Miata hooked up to the stock transmission. It's giving me happy feelings. Imagine the clutch slippage.

DISCUSSION (25)

Twingo Tamer - About to descend into project car hell. > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:33

The cost of adding a gearbox isn't worth the gain most likely. Plus, on bigger stuff like the Model S especially, you'd need a pretty beefy gearbox to survive. The added weight and complexity wouldn't be great either.

JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:34

Kinja'd!!!

long story short: running at lower RPM is not necessarily better for the efficiency of an electric motor, plus any gains that MIGHT be had by operating the motor in the "ideal" range would be sucked up by the friction losses in the gear-train. Note that the difference between minimum efficiency (77%) and maximum efficiency (93%) is only 16%, about what average drive-line loss is for a modern multi-speed transmission. Some may be better than that, but remember you also add weight and complexity with a transmission. also, 77% and 93% are both OUTLIERS and extreme cases, in about 80% of the area of this efficiency plot the variance is between 84% and 90%, 6% losses would be optimistic even for a a highly-refined transmission.

Aaron M - MasoFiST > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:34

Short answer is not really, an electric motor doesn't have a torque curve per se. You start to lose power on the top end, but if the motor is wound well enough that's mostly from overheating. Ultimately, the engineering required for either a clutch or clutch packs that can take 800 ft-lbs of torque from a dead stop isn't worth the tradeoff of reduced top end, especially when the car already tops out above legal speed limits.

Party-vi > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:35

I think the motor is going to have to generate the same amount of equivalent power to either directly drive the wheels or drive a gearbox that drives the wheels, as-in pull less current to run direct drive or pull a greater equivalent amount of current to run a gearbox to the wheels.

I'm hoping someone can correct the answer I pulled out of my ass.

4muddyfeet - bare knuckle with an EZ30 > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:35

I think because the lack of gearbox tends to go part way to offsetting the weight of a battery bank, especially in smaller cars. Everyone is desperate for longer range and that inevitably means more cells or more dense cells until the battery technology changes.

Jake - Has Bad Luck So You Don't Have To > JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t
03/24/2015 at 15:35

But what about a gearbox to keep the motor as close to that 92+ area as possible?

dogisbadob > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:36

Well the only reason for the gear set in regular engines is to make use of a narrow power band in a certain rpm range.

But electric motors provide all the torque from zero rpm.

Actually, Tesla initially tried a two-speed transmission on the older roadsters, but recalled it due to reliability issues, so now they're all direct drive.

That's the best I can do. Forgive me if this is wrong.

Hahayoustupidludditeshutupandgohandcrankyourmodeltalready > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:36

Isn't necessary.

The torque eats transmissions.

Cé hé sin > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:37

Ideally you'd have a multi ratio gearbox and Tesla originally planned to do that with the Roadster before giving up on the idea - I think the torque involved was an issue. They'll come some time though.

HammerheadFistpunch > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:37

http://oppositelock.jalopnik.com/why-doesnt-thi…

The moral of the story is that the usable torque map is broad enough that the efficiency gains of modulating rpm is offset by the drag of the transmission.

JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:38

edited initial reply to respond to this.

Twingo Tamer - About to descend into project car hell. > Party-vi
03/24/2015 at 15:38

Are you basically suggesting that there'd be no change in efficiency like there would be with running an IC engine in a higher gear?

ranwhenparked > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:38

The tough thing is that electric motors produce nearly 100% of their torque from rest, so it isn't so much the lack of gears being tough on the motor, but the motor being tough on the transmission.

Remember when Tesla had to scrap the two speed transmission in the Roadster and just go to a single gear, because the torque was basically ripping it apart?

Captain of the Enterprise > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:38

The instant torque also has the ability to break most transmissions you would be able to hook up to it anyway, it's very rare for anyone to use transmissions on electric cars because of this. Torque converters and clutches can easily break if you use to much throttle when they try to hook up.

RamblinRover Luxury-Yacht > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:39

Electric motors don't change efficiency per operating at various speeds much at all, and the amount of power they use is proportional to the amount of heat they generate (or rather, vice versa). Heat is far more of a risk than bearing wear/whatever, so speed is not an issue for longevity in any meaningful way. It makes more sense to operate a motor fast and let it cool itself than more slowly under greater load - very unlike a gas engine.

RallyWrench > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:41

No frictional losses of a geartrain to cut range, and you can vary motor output by regulating current. Much simpler than increasing weight and complexity. The range benefits of dropping motor RPM, if any, are probably comparable to the losses that a gearbox would introduce.

1.21 JIGGA WATTS!!! > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:42

Best bet would either be a CVT (continuously variable transmission) or IVT (infinitely variable transmission) in order to do what you are asking. According to Wikipedia (not the best source, I know), CVT's top out at about 88% efficiency for belt driven models. Then you would have to transfer that power to the wheels (aka gearbox/driveshaft) which adds even lower efficiency numbers.

In short, direct drive is probably about as efficient as it gets, unless you somehow found a way to stuff a miniature gearbox between the output shaft of an electric motor and the drive wheels (which would be damn cool) that would keep the motor running at peak efficiency.

JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t > Captain of the Enterprise
03/24/2015 at 15:50

A true, and oft-overlooked bit of information!

Imirrelephant > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:55

I don't have anything to add, but the Q&A going on here is good Oppo!

BATC42 > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 15:56

I think the other have already explained evrything pretty well. And you have to think that you can add a frenquency converter before the electric motor, which would give you different torque/rpm curves depending on the frequency it's set at.

Party-vi > Twingo Tamer - About to descend into project car hell.
03/24/2015 at 16:09

From my limited understanding, yes.

deekster_caddy > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 16:18

Gearboxes create more friction and have their own losses, plus added expense and complexity. Part of the advantage to electric drive is the simplicity of it all. The less moving parts the better.

MarquetteLa > Jake - Has Bad Luck So You Don't Have To
03/24/2015 at 16:42

Funny that you posted this today. A guy I know just bought this NA-cum-EV Miata (with its original 5-speed) this morning: http://www.evalbum.com/2969

Jake - Has Bad Luck So You Don't Have To > MarquetteLa
03/24/2015 at 18:45

Wow, that's a low-horsepower motor. How quick is it with that little power?

MarquetteLa > Jake - Has Bad Luck So You Don't Have To
03/25/2015 at 10:55

Not entirely sure, but they added nearly 1,000 pounds of weight during the conversion, so I'd imagine it's not exactly quick.