Working principle of CI engine:

1. Suction stroke: During this stroke, inlet valve is open and exhaust valve is closed.Only air is sucked into cylinder during this stroke. The piston moves from TDC to BDC and crank shaft rotates through 180˚.

2. Compression Stroke: The air inducted in the cylinder is compressed to the clearance volume. Both the valves are closed during this stroke. The piston moves from BDC to TDC and crank shaft rotates through 360˚.

3. Power stroke or Working stroke: At the end of the compression stroke the fuel (diesel) is injected into the hot compressed air. The rate of injection is such a that pressure remains constant instead of change in piston position. After injection of the fuel is complete the hot gases expand. The piston moves from TDC to BDC position and crank shaft rotates through 540˚.

4. Exhaust Stroke: The inlet valve remains closed and the exhaust valve opens. The piston move from BDC to TDC position which pushes the burnt gases outside the combustion chamber. Crankshaft rotates by two complete revolutions through 720˚.