This bears repeating
Reversibility, is the property of all vapors, that if they expand, they do the same amount of work and consumes the same amount of heat, as when they are compressed. That is compressing them from the same point, and the same factor, restores exactly the original heat, and takes exactly the original amount of work.
Even if you do it an infinite number of times.
Why all vapor's behave identically, and all engines behave identically, with respect to converting heat to and from work.
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Reversibility is irrelevant
If there were an irreversible engine (a new type of engine cycle that only works in one direction for probabilistic reasons, not energy creation or destruction- could happen someday), it would be governed by the same law of Conservation of Momentum, that we now know of more generally as Conservation of Energy.
Right now, we only have reversible engines. They are not impossible, they are unavoidable.
So why, if all engines convert exactly the same amount of work for the same amount of heat, do engines in fact vary in fuel economy?
Because all engines "leak" heat at different rates, and some engines fail to effectively "export" all the work done by the expanding vapor.
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