Is there a separate electric motor for each wheel in an electric car?
Hi, John - Not at present (2019), but maybe soon.
The video shows why this is the next big idea in electric cars: torque vectoring.
What's torque vectoring, and what's it got to do with electric cars?
As you know, the differential on your average hayburner's axle allows the wheels on either side to rotate at different speeds when you go around a turn. The inner wheel slows down, the outer wheel speeds up, and the average speed is how fast you can take the corner. This averaging can be done mechanically (old school) or it can be done electronically (new school).
But what if you had four different motors, one for each wheel, that could be controlled individually by computer? Unlikely in a hayburner - where are you going to put four combustion engines, and where would all that heat go? - but very practical, at least in theory, for an electric vehicle. With four motors and four controllers, you could have the kind of micromanaging traction control that would allow your EV to climb up the side of a mountain or drag race across a skating rink.
Okay, sounds great, Lynne. So why aren't they everywhere?
That's a good question. The short answer? We can already make a perfectly respectable electric vehicle with just one motor and one controller in a central location, protected from rain, snow, flooding and road damage from flying rocks and whatnot. Why improve on perfection?
(Because we can. After all, if individual motors could do torque vectoring better than a central motor plus an electronically controlled differential, why wouldn't you do it? You could make a faster race car this way, for instance.)
So: if we're going to have four electric motors running your car, where shall we put them? The logical place to put individual motors seems to be in the wheel hub, like we have long done with electric bicycles. Trouble is, that's also the best location for your hard-working motor to get smashed to bits by a flying rock; and in a car, the rocks are flying a lot faster than on a bike, and doing a lot more damage.
That's not good, but maybe we can do something about it, you say. Seal the motor up with something tough, like kevlar. We'll just seal the motor up completely so it's watertight, too.
But: if you seal a motor completely, there's the problem of excess heat building up that's got to go somewhere. How can we release the heat but keep the motor dry at the same time?
These are the engineering puzzles that are the obstacles to wheel motors providing awesome torque vectoring in your LEAF or your Kia Soul EV right now...
...but the landscape seems to be changing.
Protean Electric in the UK builds in-wheel motors for cars and light commercial vehicles, and the Rivian electric truck (coming in 2020) will have one motor for each wheel (the motors are on the axle, not in the wheel) and their advance advertising is ALL about the torque vectoring.