A heat engine converts heat to work. Example: a gasoline car engine.
To be useful, the engine must operate cyclically, with a certain amount of work being done every cycle.
Critical feature of cyclic operation: two temperatures---the higher temperature causes the gas inside a car cylinder system to expand, doing work, and the lower temperature re-sets the engine for another cycle.
In a full cycle of a heat engine, three processes occur:
Efficiency: η ≡ W/Qh
= 1 -(Qc/Qh) < 1
W = useful output
Qh = input energy