Am I correct in assuming that no matter how powerful an engine or it’s configuration it can never put out more energy than the fuel contains. If you had 100 gallons of gasoline and a 5HP lawn mower and a 2500HP aircraft engine both engines will put out and same energy per the 100 gallons. Is this correct?
Well, different engines can have different efficiencies. A jet engine probably uses fuel more efficiently than a lawnmower, and can get more energy out of it. But, as you state, you can never get more energy out of fuel than it took to make it.
I was guessing that the OP was talking about a piston aircraft engine. It runs on gasoline and is rated in HP, so it’s probably not a jet.
Anyway, if you assume the same efficiencies in the lawnmower and aircraft engines, then yes, they will output the same amount of energy when they burn through all the gasoline. What you’re confusing is power and energy. The aircraft engine will convert all that gasoline’s energy into motion and heat at a MUCH faster rate than the lawnmower. In fact, based on your example, it will burn that fuel 500 times faster.
Power is energy per unit time. So the more power then engine produces, the faster it turns a given amount of energy into work. But even though each engine works at a different rate, they will have done just as much work in the end.
Ditto friedo and aerodave.
These engines are heat engines - that is, they function by getting something hot (the gasses inside them) and letting the working substance - the gasses inside - do mechanical work to pistons or other parts inside. There is a broader statement you can make about heat engines. If the lowest absolute temperature the working substance reaches is X times as high as the highest absolute temperature it reaches, then the highest possible mechanical energy the engine can produce will be X times the total heat energy that the whole system gets in the first place.
clearly you are counting waste heat. So the answer is mostly yes.
I suspect that the burning efficiency of the two types of engines are different-I don’t think either engine actually burns all the fuel injected into the engine. So if this difference is measurable, the answer would be no.
Well, technically it can. Since the fuel combines with oxygen when it is burned, you have to include the energy in the oxygen, too. (But that is usually so outweighed by the inefficiencies of any such engine that ignoring it doesn’t matter.)
>Well, technically it can. Since the fuel combines with oxygen when it is burned, you have to include the energy in the oxygen, too.
Interesting point, but we didn’t carefully define what was originally meant when we referred to the energy in the fuel. If you look up the energy content of a fossil fuel, what the tables and references will tell you is how much energy is released by reacting the fuel with atmospheric oxygen, so the “energy in the oxygen” is already included. There are other forms of energy in the fuel, like its heat content and its gravitational potential energy and the energy you’d get by converting its mass directly, and so forth. I think anybody approaching this as an engineering question would presume from the start that the energy implied here is what you get by combining all the fuel with the oxygen we all assume is just available.
Napier, I think you’re right! 
Damn, I’d swear there’s some kind of echo here. Weird…