We will focus on heat engines that use a gas as the working substance. The gasoline or diesel engine in your car is an engine that alternately compresses and expands a gaseous fuel-air mixture. A discussion of engines such as steam generators that rely on phase changes will be deferred to more advanced courses.

A gas heat engine can be represented by a closed-cycle trajectory in the pV diagram, such as the one shown in FIGURE 21.12a. This observation leads to an important geometric interpretation of the work done by the system during one full cycle. You learned in Section 21.1 that the work done by the system is the area under the curve of a pV trajectory. As FIGURE 21.12b shows, the net work done during a full cycle is

FIGURE 21.12 The work W_out done by the system during one full cycle is the area enclosed within the curve.

You can see that the net work done by a gas heat engine during one full cycle is the area enclosed by the pV curve for the cycle. A thermodynamic cycle with a larger enclosed area does more work than one with a smaller enclosed area. Notice that the gas must go around the pV trajectory in a clockwise direction for $W_{\text{out}}$ to be positive. We’ll see later that a refrigerator uses a counterclockwise (ccw) cycle.