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My textbook states that

"If an engine is to do work on a sustained basis, the working substance must operate in a cycle; ie, the working substance must pass through a closed series of thermodynamic processes, called strokes, returning again and again to each state in every cycle."

I have not understood exactly why it is necessary for the heat engine to operate in a cycle. Why is the operation in a single direction less efficient than that in a cycle?

Please share your insights and help me. Much thanks in advance :-) Regards.

Edit: Although this question may, at first glance, come across as primitive and maybe even a little silly, I request you to please bear in mind that it has not been very long since I began my study of thermodynamics and also, I have only just graduated high school.

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The engine is definitely more efficient if it is not a cycle, cycling back reduces efficiency. But the reason it is necessary is that you want to keep it going, so you must return it to its initial state. Consider the pistons in a car engine, it would be much better to just fire each piston and get a push on the axle and never compress the gas again, because compressing the gas removes energy from the axle. But how could you drive a car down a road if you only fire each piston once? That'd take a lot of pistons!

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The second law of thermodynamics (Kelvin-Planck version) says that a heat engine cannot do work cyclically and isothermally. "Cyclically" is crucial - heat engines that do work isothermally but non-cyclically are trivial (e.g. a gas expanding reversibly).

By the way, heat engines that perform work cyclically and isothermally do exist. In this video one switches the capacitor on and off and the system can repeatedly lift floating weights:

http://www.youtube.com/watch?v=T6KAH1JpdPg "Liquid Dielectric Capacitor"

Switching the capacitor on and off involves no work done on the system so the energy for the work done BY the system (if it repeatedly lifts floating weights) can only come from the environmental heat, in violation of the second law.

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  • $\begingroup$ Switching the capacitor on and off involves no work" is not correct. It requires work to charge a capacitor, and that energy can be recovered as charge flows off. $\endgroup$ Jul 30, 2019 at 15:21

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