I'm reading Feynman's discussion of Reversible engines (Vol I 44.3) in The Feynman Lectures on Physics. His discussion is very abstract, and leaves a lot of practical questions unanswered. Rather than attempting to ask several questions which come to mind, it seems reasonable to consider a real mechanism which comes "as close as possible" to an ideal heat engine.
Has anybody created such a device? That is, one which requires very little additional energy in order to get the machine to return to its initial state after "doing work one the Universe" by transferring the work from the Universe back to the engine?