In compression ignition (CI) engines, burning of fuel occurs due to compression of the fuel to very high pressures. At very high pressures the fuel, i.e. diesel, starts burning automatically without the need of any external flame. The cycle of operation of the CI engine is completed in four-strokes: suction, compression, expansion, and exhaust. Each stroke is described in detail below:
1) Suction stroke: At the start of the suction stroke the piston is located at top dead center position. As it moves down, the inlet valve located in the cylinder head opens, while the exhaust valve remains closed. From the inlet valve, air is drawn into the cylinder which continues until the piston reaches bottom dead center or the bottom most position inside the cylinder. At this point the suction stroke completes and the suction or inlet valve closes.
2) Compression stroke: During the compression stroke the piston starts moving in upward and compresses the air in the clearance volume. While in the case of spark ignition (SI) engines, the compression ratio is about 6-10, the CI engine this ratio is about 16-20. This clearly indicates that the compression pressure exerted in the CI engines is much more than in SI engines.
3) Expansion stroke: Towards the end of the compression stroke, the fuel is injected into the clearance volume. Due to excessively high pressures, the fuel starts burning instantly, creating large amounts of thermal energy, which further raises the pressure. Because of this pressure the piston starts moving down. The fuel injection rate is such that the pressure inside the cylinder is maintained constant even though the piston moves down. The expansion stroke ends when the piston reaches the bottom position. During this stroke the inlet and exhaust valves remain closed.
4) Exhaust stroke: After the expansion stroke a number of residual gases are left in the cylinder and need to be cleared from of the cylinder. During the exhaust stroke the exhaust valve opens due to the difference between the atmospheric pressure and the pressure of exhaust gases inside the cylinder. As the piston moves from the bottom to the top position the exhaust gases are swept out of the cylinder. When the piston reaches the topmost position all the exhaust gases are released. As the piston starts moving down, the inlet valves open and fresh air is drawn into the cylinder.
In this way, the cycle of operation of the CI engine keeps on repeating without any hindrance. Since large amounts of pressure are generated inside the CI engines these engines, there is a need to have CI engines be sturdier than SI engines.