Is the energy output of an engine constant?

[russ_watters]

... to accelerate from zero to a certain speed assuming the same mass.

I think the qualifier also implied only losses internal to the engine, so it is true at constant speed as well -- the real world application being what I described above.

 

[russ_watters]

So is theoretically possible to have a very powerfull car (a super car for example), that can use less fuel to mantain a given speed than a less powerful car. If that is true, that's an interesting realization. I wonder why doesn't that happen more frequently, if it happens at all.

Some of the losses are a fixed function of size, not a fraction of output. So for identical cars and driving, the smaller engine will essentially always produce better fuel efficiency.

Say, for example, 2 engines are each 30% efficient at peak power, 25% efficient at half power. If one engine can produce 100 hp while the other can produce 200, then at 100 hp, the smaller engine is 30% efficient and the larger engine 25% efficient.