Diagrams that show the mechanical cycles of operation in gasoline engines are some-what similar to those described in this chapter for diesel engines, except that there would be one less event taking place during the gasoline engine cycle. Since air and fuel are admitted to the cylinder of a gasoline engine as a mixture during the intake event, the injection event does not apply.

COMBUSTION CYCLES

Up to this point, we have given greater consideration to the stroke of a piston and the related events taking place during a cycle of operation than we have to the heat process involved in the cycle. However, we cannot discuss the mechanics of engine operation without dealing with heat. Such terms as ignition, combustion, and expansion of gases indicate that heat is essential to a cycle of engine operation.

So far, the only difference we have pointed out between diesel and gasoline engines is the number of events that occur during the cycle of operation. We have told you that either the 2- or the 4-stroke cycle may apply to both a diesel and a gasoline engine. Then, one of the principal differences between these types of engines must involve the heat processes that produce the forces that make the engine operate. The heat processes are sometimes called COMBUSTION or HEAT CYCLES. The two most common combustion cycles associated with reciprocating internal-combus-tion engines are the OTTO cycle (gasoline engines) and the DIESEL cycle (diesel engines). Each of these combustion cycles will be discussed.

In talking about combustion cycles, we must bring up another important difference between gasoline and diesel enginesthe difference in COMPRESSION PRESSURE. Compression pressure is directly related to the combustion process in an engine. Diesel engines have a much higher compression pressure than gasoline engines. The higher compression pressure in diesels explains the difference in the methods of ignition used in gasoline and diesel engines.