The four, stroke C.I. engine operates in a diesel cycle which is also an air standard cycle.
The four stages are explained below. (Ref. Fig. 10.6 (a))
Stage 1 Suction Stroke (Process A-1)
Suppose the piston is at TDC. Now it starts moving downward creating vacuum above it. The air from air filter enters through inlet manifold to the cylinder. The inlet valve remains open till the piston reaches BDC. The outlet valve remain closed during this suction stroke.
(Ref. Fig. 10.6 (b))
Stage 2 (Compression Stroke) (Process 1-2)
Both the valves remain closed; the piston moves from BDC to TDC. The air is compressed to a compression ratio of 12 to 25 generally. The air temperature reaches auto-ignite state. Now the fuel is injected from injector as soon as the piston reaches at TDC. (Ref. Fig. 10.6 (c))
Stage 3 Expansion Stroke (Process 3- 4)
The fuel starts burning at constant pressure (Process 2- 3). At point 3 fuel supply is cut off. The hot gases of the cylinder expands creating power stroke. Thus power is obtained by piston and flywheel during this expansion stroke. Both the valves remain closed during this process. The expansion is reversible adiabatic. At the end of this stroke the heat is rejected at constant volume condition (Process 4-1). (Ref. Fig. 10.6 (d))
Stage 4 Exhaust Stroke (Process 1-A)
At this stroke the piston moves from BDC to TDC. The exhaust valve opens and inlet valve remain closed. The upward movement of the piston expels out the burnt gases (Process 1- A). (Ref. Fig. 10.6 (e))