A heat engine is a device for transforming heat energy into useful work. Heat engines

• Follow a cyclical process that can be shown on a $pV$ diagram or an energy-transfer diagram.

• Require not only a source of heat but also a source of cooling. These are called the hot reservoir and the cold reservoir.

• Are governed by the first and second laws of thermodynamics.

《 LOOKING BACK Sections 19.2, 19.3, 19.4 and 19.7 Work, heat, the first law of thermodynamics, and the specific heats of gases

A refrigerator is any device—including air conditioners—in which external work is used to “pump energy uphill” from cold to hot. A refrigerator is essentially a heat engine running in reverse. A refrigerator’s efficiency is given by its coefficient of performance.

How good is an engine at transforming heat energy into work? We’ll define an engine’s thermal efficiency as

Conservation of energy—the first law of thermodynamics—says that no engine can have an efficiency greater than one.

Yes. And, surprisingly, it’s set by the second law’s prohibition of spontaneous heat flow from cold to hot. We’ll find that a perfectly reversible heat engine—a Carnot engine—has the maximum efficiency allowed by the laws of thermodynamics. The Carnot efficiency depends only on the temperatures of the hot and cold reservoirs.

《 LOOKING BACK Section 20.7 The second law of thermodynamics

Modern society is powered by devices that transform fuel energy into useful work. Examples include engines for propelling cars and airplanes, power plants for generating electricity, and a wide variety of machines used in industry and manufacturing. These are all heat engines, and all are governed by the laws of thermodynamics. Maximizing efficiency—an important engineering and societal goal—requires a good understanding of the underlying physical principles.