Answer
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Work Step by Step
A Carnot engine (page 422) is an idealized 4-step engine. It follows four steps: an isothermal expansion, an adiabatic expansion, an isothermal compression, and an adiabatic compression.
In a Carnot engine, the process moves so slowly that the process can be considered as a series of equilibrium states. Thus, every step is reversible, and the net entropy change for the whole cycle is zero.
Therefore, a Carnot engine achieves maximum efficiency.
Unfortunately, a Carnot engine is not at all practical to construct. All real engines are less efficient than it, because of losses due to friction and other nonconservative forces.