The gasoline engine, used in automobiles and many other types of machinery, is a familiar example of a heat engine. Let’s look at its thermal efficiency. Figure 18-2 shows the operation of one type of gasoline engine. First a mixture of air and gasoline vapor flows into a cylinder through an open intake valve while the piston descends, increasing the volume of the cylinder from a minimum of V (when the piston is all the way up) to a maximum of rV (when it is all the way down). The quantity r is called the compression ratio; for present-day automobile engines its value is typically 8- 10. At the end of this intake stroke, the intake valve closes and the mixture is compressed, approximately adiabatic ally, to volume V during the compression stroke. The mixture is then ignited by the spark plug, and the heated gas expands, approximately adiabatic ally, back to volume rV, pushing on the piston and doing work; this is the power stroke. Finally, the exhaust valve opens, and the combustion products are pushed out (during the exhaust stroke), leaving the cylinder ready for the next intake stroke.