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      07-18-2012, 02:38 PM   #23
Drives: 2011 135i - Silver 6MT M-sport
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Originally Posted by Ammonia View Post
I believe something to do with electric motors themselves doesn't require them to have as many, but they usually do.
IC (internal combustion) engines require a gearbox because they only generate power within a (relatively) narrow RPM range. If you look at a dyno chart for a N/A IC engine, you'll see that the torque curve looks like a hill, starting low, peaking, then dropping off. The peak usually occurs within a range of 2-3k RPM. Turbocharged engines expand this significantly, but the range is still 4k RPM on the high side.

Keep in mind, I'm talking ranges, not absolute RPM. So for example, with our hypothetical N/A IC engine, we might see torque rise to within 80% of peak at 3,500 RPM and drop off a cliff at 6,000 RPM. That's a range of 2,500 RPM, and is pretty typical (maybe even aggressive) for your run of the mill N/A IC engine. High performance engines employ technologies to expand that range and push it higher in the RPM range in order to generate more horsepower.

An electric motor has a very different torque curve. Peak torque for an electric motor usually occurs in the hundreds of RPM. It wouldn't be unusual for an electric motor to generate torque peak at 400 RPM, then taper off by a couple of percentage points in the 5k to 6k RPM range, then taper off gradually from there. I found a pretty interesting graph that shows the contrast between the two:

First off, this is an unfair graph. They marginalize the IC engine by comparing a high-performance EV (Tesla Roadster) with an economy oriented IC engine, but even if you were plotting an N55 against the motor in the Tesla, you'd see that the profile of the torque curve on the N55 can't match the one in the Tesla. Scaling the torque plateau of an electric motor upward is simply a matter of engineering. The profile of the curve doesn't change much.

Another down side of IC engines is their link between RPM and efficiency. A gasoline engine must maintain a specific air-fuel ratio in order to sustain combustion. At high RPM, you're moving a lot of air, thus you must supply more fuel. This holds true, even if you restrict air intake with a throttle. You are still turning more RPM and igniting the fuel-air mixture more rapidly, and thus consuming more fuel. This means that if you want to prioritize fuel economy, you need to operate your IC engine at low RPM. If you also want to go fast, you need more gears.

Let's circle back to the original quandary. Why don't electric cars need gearboxes? Two reasons:

1) Electric motors generate usable torque over a very broad RPM range.
2) Electric motors (depending on design) do not suffer a steep efficiency penalty from high-RPM operation
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