Final answer:
In a diesel engine, the four main stages are intake of air, compression of air, ignition of fuel, and exhaust of gases. The step not involved in a diesel engine's cycle is the intake of an air-fuel mixture, as diesel engines only intake air during the intake stage.
Step-by-step explanation:
The cycle of a diesel engine is different from a gasoline internal combustion engine in the way the fuel is ignited. In a diesel engine, no spark plug is used; instead, the air is compressed to a point where its temperature is high enough to ignite the diesel fuel on contact. The four main stages of a diesel engine cycle are: intake of air (where the air is drawn into the combustion chamber), compression of air (where the air is compressed adiabatically, raising its temperature), ignition of fuel (where fuel is added to the hot, compressed air, causing it to ignite), and exhaust of gases (where the combusted gases are expelled). Therefore, the step that is not part of the diesel engine process is the intake of an air-fuel mixture because, unlike gasoline engines, diesel engines intake air only, without fuel, during this stage.