Diesel Engine

Just like an S.I engine, C.I engine also comprises of four strokes for the entire combustion process to occur. Unlike a typical S.I engine where air and fuel are mixed and supplied to the combustion chamber, only air is sent during the suction stroke in C.I engine since air and diesel mixture is a heterogeneous mixture. Diesel is injected at the end of compression stroke. C.I engines don't require spark plugs to ignite the fuel.

SUCTION STROKE:

The inlet valve is open during the suction stroke allowing the air from the atmosphere to enter the combustion chamber. The piston moves from t.d.c to b.d.c, thereby creating suction and sucking more air. The inlet valve closes after the piston reaches b.d.c and the air gets trapped inside the combustion chamber. Due to this piston movement, a very low pressure of about 0.1 bar or less is created inside the cylinder.

COMPRESSION STROKE:

Now both the inlet and exhaust valves remain closed and the piston moves upward from b.d.c to t.d.c. This compresses the air inside the cylinder and increases the pressure and temperature. The pressure achieved is around 30 to 50 bar and the temperature reached is about 600 °C. This pressure and temperature is enough to ignite the fuel that will be injected at the end of compression. The compression ratio of a C.I engine varies from 16:1 to 20:1.

POWER STROKE:

Both the inlet and the exhaust valves remain closed. Diesel is injected into the combustion chamber at the end of compression stroke. The rate of combustion will depend on how fine the diesel is atomized by the fuel injection system. The higher the injection pressure, the finer will be the atomization of diesel. The highly pressurized diesel is injected into the heated air and the heat inside the combustion chamber due to compression is enough to ignite the fuel. The intense heat will vaporize the fuel droplets and burn it releasing heat energy. This energy pushes the piston from t.d.c to b.d.c and the energy is used to drive the crankshaft.

EXHAUST STROKE:

The exhaust valve opens and now the piston moves from b.d.c to t.d.c pushing the exhaust gases out. This completes the four strokes of C.I engine. The exhaust gases are sent to the atmosphere through the exhaust pipe.