Image 1 Caption: A schematic diagram of a heat engine: heat energy in from hot bath, heat energy rejected to cold bath---there must always be some---and work done---which is the whole point. A heat engine by definition works in a thermodynamic cycle (AKA engine cycle) in which the operations repeat in order.
Features:
W (Q_H-Q_L) Q_L ε = --- = --------- = 1 - --- Q_H Q_H Q_H T_L ≤ 1 - --- = ε_Carnot engine ≤ 1 , T_Hwhere temperatures T_H and T_L are on Kelvin temperature scale and even the Carnot engine CANNOT be 1, unless T_L = 0 K (i.e., absolute zero T = 0 K) which is impossible in practice.
Heat engine efficiency as defined above is only one heat engine desideratum. Another is power: i.e., energy per unit time. The ideal Carnot engine actually has zero power: it runs infinitely slowly. Less-than-ideal Carnot engines have very, very low power, and so have only the aforementioned experimental uses.
But there are practical limits on T_H. You do NOT want your heat engine to melt or meltdown as can happen in nuclear-fission nuclear power plants---which are just heat engines where nuclear fission provides the hot bath.
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