Does the carnot heat engine efficiency depends on the working fluid used in the engine? which working fluid used in the engine would be more efficient, air or water?

No

Explanation:

Carnot heat engine:

 Carnot heat engine is an ideal heat engine for all working heat engine.It have all processes are reversible and have four processes in which twom are constant temperature process and other two constant entropy processes.

As we know that efficiency of Carnot cycle given as follows

Where

So from above expression we can say that efficiency of heat engine does not depends on the working fluids.

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