Which is why we only have 2 gas engines… Oh, wait. We don’t.
True, there is a high degree of standardization to electric motors. Unfortunately, it’s this kind of standardization:
Individual motors might be a standardized part class, for easy switching out… but there’s a lot of different organizations running around with their own needs and goals, so even within highly-specific categories of operation, there’s still a lot of differences.
The wikipedia page notes 14 different categories of electric motors, which themselves come in different sizes, depending on the specific device they are intended to power. The electric motor in a toothbrush is different from that in a blender, despite the fact that you might call both ‘tiny electric motors’ since they both have fairly minimal power and can’t move you around. Similarly, the motor of of a lawnmower is different from that of a scooter, since the former needs an awful lot more torque; and so on. The size of the motor makes for a good general distinction, but there’s certainly room to subdivide the motors down further to any degree of granularity you like.
I’m personally up for a large variety of engines. Both in size and fuel. Gas, Electric, Alcohol, hydrogen, wood gas, etc… If they want some reference materials about the alcohol engines I’ll dig out my old Twilight 2000 rule book. As for the electric motors… What he just said makes sense to me. Now I have some people say that the Liters listed in gas engines is how much fuel it holds, which makes very little sense to me as liquid fuel is never put into an engine (it’s mixed with air and basically in gas form when it explodes in the engine) and others say it’s the liquid displacement of the engine itself, which makes sense to me personally. Now if anyone wants to do some research to see which is true… it’s up to you. Either way. We could still use a Liter measurement system because the game doesn’t have to mirror reality at all points. 
I’d argue that it makes the most sense to differentiate electric motors by power rather than size, since design and materials used have a much more significant impact on output than mere size.
I agree. I actually like keeping an eye open for a better engine and changing them whenever I can. After all, we got a bajillions guns, why not a bajillions engine? I guess each engine could have a weight, size and power.
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I agree that materials make more power than size, but without size the best materials is the world would not work. So size and materials are directly proportional, so the greater the size, the greater the materials, and therefore the greater the power.
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We don’t need too many arbitrary statistics; extra info that’s not relevant to gameplay isn’t all that important.
Really, in terms of actual gameplay use, I can’t see a purpose for more than class and power. Class is distinctions like ‘Inline-4’ or ‘V8’ for gasoline; ‘small motor’ or ‘large motor’ for electric. For gasoline engines, power is listed in terms of displacement, for electrical engines, power should be listed in terms of (kilo)watts.