[myself6]

Understood.

My problem here is that I don't see how the rotor becomes more difficult to turn.

I guess there could be some properties of electromagnetism that work against the rotation as the field increases do to increased current flow. That would be the ONLY way it COULD get harder to turn.

[TalonDJ]

Yeah, that is exactly what happens. The energy coming out of the wires is CONVERTED from mechanical energy. If it appeared out of thin air then all we would need is a gerbil and some really smooth ball bearings and we could power the whole world.

[Jonx6]

Well my understanding is that it is done in stages. The PMG portion creates enuff amps to create stronger fields that create more amps. It this point, the strength of the next set of coils regulated by the demand place on the alternator. The more demand the stronger the field and the more torque required to turn it. I think the regulator determines the final field strengths.

[null and void]

My problem here is that I don't see how the rotor becomes more difficult to turn.

It's not very obvious, is it? Underlying all the details is a simple fact. Energy can neither be created nor destroyed, only converted in form. (Motion to friction to heat, for example)

In order to get more electrical energy out of a generator/alternator you have to put more mechanical energy in.

An engine takes chemical energy from the fuel and converts it into mechanical energy. More energy out requires more fuel in.

That being said, these kids may have a more efficient way of converting the engine's motion to cooling, and still achieve a fuel savings.