Carnot's theorem : When working between the two same temperature limits no engine can be more efficient than a reversible one.
I'm discussing the corollaries below.
An important corollary which immediately follows is that :
All reversible engines working between the same two temperature limits must have the same thermal efficiency.
The most important conclusion we draw is that, the nature of the working substance has no effect on the efficiency of the heat engine. Any substance may be the working substance. The efficiency is determined only by the source and the sink and by nothing else.