The four-stroke engine is the most common type of internal combustion engine used in various automobiles, trucks, diesel trains, light aircraft, and motorcycles. It is an internal combustion engine in which the piston completes four separate strokes while turning the crankshaft. The four separate strokes are:

1. Intake Stroke: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke, the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing a partial vacuum (negative pressure) in the cylinder through its downward motion.

2. Compression Stroke: The intake valve is closed, and the piston moves up the chamber to the top. This compresses the fuel-air mixture. At the end of this stroke, a spark plug provides the compressed fuel with the activation energy required to begin combustion.

3. Power Stroke: As the fuel reaches the end of its combustion, the heat released from combusting hydrocarbons increases the pressure, which causes the gas to push down on the piston and create the power output.

4. Exhaust Stroke: The exhaust valve opens, and the piston moves up the cylinder bore, forcing the spent combustion gases through the exhaust valve and out of the exhaust systems. As the piston returns to top dead center, the exhaust valve closes, and the intake valve opens, and the 4-stroke engine process repeats.

Each repetition of the cycle requires two full revolutions (720°) of the crankshaft.