I think we just get used to pronouncing things a certain way.
NOBODY I know of EVER referrs to an electric motor as anything but a motor, but COULD you? Sure. Would everybody know what you mean? Sure. Would they correct you and say "NO NO, electric MOTOR, not electric ENGINE!" In about 2 second flat!
The internal combustion device located under the hood of your car (bonnet for you English guys), can be interchangeably called an engine OR a motor. The engineers at MB actually give them MOTOR numbers (M103 for example), but we can call them engines no problem, right? The same could work for any other type of energy producing device, but it isn't as widely accepted as an internal combustion engine. Does anybody correct anyone else if they refer to the internal combustion device in their car as an ENGINE or a MOTOR? I don't think so. We've become accustomed to using both terms in that case.
In reference to either solid or liquid fuel, how would that apply to what we call an electric motor? At one time, my thought was that a motor uses an already existing energy source, such as electricity, to produce motion, and an engine uses a FUEL to produce motion. But after enough thought about it, I think it is just what we get used to saying in regards to a particular type of device.

Gilly