Another important valve is the type discussed and shown in Chapter 9 for gasoline and diesel engines. This type of valve has a shape that matches the shape and size of a passage or opening and can be moved in and out of the opening. In the case of car engines, one set of these valves opens to let a gasoline-air mixture into the cylinders to be ignited by the spark plugs, and another set opens to let the exhaust gases out of the cylinders