The Four stroke Engine

How does the four stroke engine work?

The four stroke engine relies on four strokes of the piston to operate. Those four strokes are known as intake, compression, power, and exhaust.
Intake: When the intake stroke starts, the piston is at top dead centre, and as the crankshaft starts to turn, the piston starts to go down and the intake valve(s) open. Because the piston is going down, there is a low pressure zone inside the combustion chamber and because the intake valve is open, and is atmospheric pressure outside of the combustion chamber, this causes air and fuel to be drawn into the combustion chamber. As the piston approaches bottom dead centre, the intake valve begins to close, and when the piston reaches BDC, the valve has fully closed.


        Down          Up           Down         Up          



Compression: On the compression stroke, all valves remain closed as the piston moves up the cylinder. This causes the air/fuel mixture to be compressed. When the piston gets to the top of the cylinder, TDC, the spark plug fires and ignites the air/fuel mixture. This is known as ignition.

Power: After the air/fuel mixture has been ignited, the expanding gases force the piston back down the cylinder.

Exhaust: When the piston gets back down to BDC, after the power stroke, the exhaust valve(s) begin to open and as the piston moves back up the cylinder, the exhaust gases get forced out through the exhaust valve. As the piston approaches TDC, the exhaust valve begins to close and by the time the piston gets to TDC, the valve has completely closed and then it is back to the intake stroke.