In a four-stroke engine the cycle of the operation of engine is completed by four strokes of the piston inside the cylinder. During these four strokes fuel is once injected and burnt inside the engine and two revolutions of the crankshaft are obtained. In a four-stroke spark ignition (SI) engine the burning of the fuel occurs by the spark generated from the spark plug.
Here are the four strokes that occur inside the SI engine during its operation:
Suction stroke: At the beginning of this stroke the piston is at the top dead center or near the cylinder head and is about to move down. At this instance the inlet valve fitted in the cylinder head is opened and the exhaust valve remains closed due to the pressure difference. As the piston moves down the suction pressure is created inside the cylinder, drawing an air-fuel mixture into the cylinder. When the piston reaches the bottom most position or bottom dead center, the suction stroke ends and the inlet valve is closed.
Compression stroke: During this stroke the piston starts moving from bottom dead center to top dead center. As the piston moves up, the air-fuel mixture gets compressed into the clearance volume of the cylinder. At the end of the stroke the spark is generated by the spark plug, which causes the burning of the fuel and the release of large amounts of thermal energy. Due to this heat, high pressures are generated.
Expansion or power stroke: The large amount of pressure generated at the end of the compression stroke pushes the piston towards the bottom dead center. It is during this stroke that the actual power is produced by the engine, hence this stroke is called the power stroke and since the expansion of gases occurs during this process, it is also called the expansion stroke. During this stroke, both the inlet and exhaust valves remain closed.
Exhaust stroke: Towards the end of the expansion stroke the inlet valve remains closed while the exhaust valve opens due to the internal and external pressure difference. The piston starts moving in an upward direction and all the residual gases that are left after the expansion stroke are swept outside the cylinder and escape through the exhaust chamber. At the end of the exhaust stroke, the piston reaches top dead center position and then starts moving in the downward direction to suck the air-fuel mixture and complete the suction stroke.
In this way the cycle of operations of a four-stroke engines keeps repeating until your vehicle is running. During these four-stroke, or two revolutions of the crankshaft, the fuel is injected only once. For the maximum efficiency of the vehicle it should produce maximum power during the power stroke and produce minimum exhaust during the exhaust stroke.