Physics 212 Lecture: "Carnot's Principle and Efficiency of Heat Engines." Jan 30, 1998

A reversible engine is one which does work only via reversible processes: those which leave the engine and its surroundings in the same state as it starts.
Carnot's Principle is Any irreversible engine operating between two heat reservoirs will have a lower efficiency that a reversible engine acting between the same reservoirs.
Reversible (or Carnot) engines have a simple relationship between efficiency and the temperatures of their reservoirs.
One can run a heat engine "backwards" by feeding it energy as an input, and create a temperature difference as the result. We call such "backwards" heat engines "refrigerators."

This lecture discusses material in Chapter 15 of Cutnell and Johnson.