Lets get this straight. Your saying that provided with say 1 L of fuel, the engine would run for 1 minute at 2000 rpm, producing a specific power (call it 1 watt for simplicity), and and energy output of 60 J. Then at 4000 rpm, your engine would run the same amount of time (1 minute), produce twice the power (2W) and twice the energy (120 J)?

That makes the engine terribly inefficient at 2000 rpm, harnessing only half the energy it does at 4000 rpm from the same amount of fuel. So are you denying that the engine is inefficient at 2000 rpm? Or are you saying its incredibly efficient (relative to normal engines) at 4000 rpm? Or are we misunderstanding?