In four stroke compression ignition (CI) engine diesel is used as fuel. It is based on diesel cycle. In diesel cycle heat is added at constant pressure. The four processes in CI engine are as follows.
Suction stroke: During suction stroke the piston moves from Top dead centre (TDC) to Bottom dead centre (BDC). The inlet valve opens and air at atmospheric pressure is drawn inside the engine cylinder. The outlet valve remains closed during the stroke.
Compression stroke: The air is compressed to high temperature and pressure as the piston moves from BDC to TDC. Both the inlet and outlet valves remains closed during the stroke.
Expansion stroke: As the piston starts moving from TDC towards BDC, fuel is injected into the hot compressed air in fine sprays by the fuel injector. The fuel starts burning at constant pressure. The burnt gases drive the piston from TDC to BDC. Both the inlet and outlet valves remain closed during the stroke.
Exhaust stroke: After the expansion stroke the burnt gases are made to escape through exhaust valve to the atmosphere. In this stroke the piston moves from BDC to TDC and the exhaust gases are driven out of the engine cylinder through outlet valve.
The above processes are repeated again and again for continuous working of engine.