Engine Cycles

An event (a part of the engine cycle) that occurs in the engine cylinder during one piston travel is called a stroke. The engine which completes its working cycle in four strokes of the piston is known as the four-stroke engine.

WORKING CYCLE OF THE FOUR-STROKE DIESEL ENGINE.

1st STROKE-INTAKE. The piston is moved by the crankshaft and connecting rod downwards and, acting like a pump piston, it produces a vacuum in the cylinder. Fresh air is taken into the cylinder through the open intake valve, as a result of the pressure differential between the cylinder and atmospheric air. The exhaust valve is closed. At the end of the intake stroke, the intake valve closes. The pressure in the cylinder at the end of the intake stroke comes to an average of 0.08 to 0.095 MPa and the air temperature, 30 to 50°C.

2nd STROKE-COMPRESSION. The crankshaft continues to rotate, so the piston, after passing BDC at the end of the intake stroke, starts moving upwards. Since both the intake and the exhaust valves are closed, the piston compresses the air in the cylinder. As the air is compressed, its temperature goes up. The compression ratio of the diesel engine being fairly high, the pressure in the cylinder rises up to 4 MPa and the air gets heated up to 600°C. As the piston nears TDC at the end of the compression stroke, a charge of finely atomized diesel fuel is injected into the cylinder through the fuel injector.

3rd STROKE-POWER, or expansion. On coming into contact with the hot compressed air in the cylinder, fine fuel particles are self-ignited. The injection and burning of fuel go on for some time after the piston has passed TDC at the beginning of the power stroke. Because of a delay in the self-ignition of fuel, it burns mainly during this piston stroke. During the power stroke, both valves are closed. The temperature of combustion gases reaches 2000° С and the pressure in the cylinder increases up to 8 MPa. The high pressure of expanding gases pushes the piston downward. The piston transmits this push through the connecting rod to the crankshaft, making the latter perform mechanical work.

4th STROKE-EXHAUST. As the piston reaches BDC on the power stroke, the exhaust valve opens to release the burnt gases under an excessive pressure. The piston then moves upwards again and pushes all of the exhaust gas from the cylinder. Finally, as the piston reaches TDC on the exhaust stroke, the exhaust valve closes and the intake valve opens. The piston then moves downwards once more on another intake stroke, and the cycle of events in the cylinder is then repeated.