Post #2772102

> even the waste heat from the compression could be used to achieve more compression No. Waste heat (which is always low-temperature in respect to the device in question) can by definition not be converted to mechanical work. (**Edit:** to uninformed people downvoting this, this is nothing else than [Carnot's law](https://en.wikipedia.org/wiki/Carnot%27s_theorem_(thermodynamics)) in action.) Otherwise, one could build a perpetuum mobile: Convert heat to mechanical work, use that work to generate heat, convert it to work again, and so on. You'd have a machine that generates energy out of nothing, and that's not possible because of the law of energy conservation.