Most internal combustion engines are 4-strokes, powered by either gasoline or diesel fuel.
The 4-stroke cycle is known as the Otto cycle after its inventor Nicolaus Otto who built the first engine in 1876.
4-stroke engines deliver a good balance of power, reliability and efficiency.
The definition of a stroke is an up or down piston motion (reciprocating).
The 4-stroke engine produces 1 power stroke for every 2 crankshaft revolutions.
The 4-stroke engine goes through 4 strokes to achieve 1 power stroke.
The 4-strokes separate each event mechanically using valves, which reduces unburned fuel emissions.
The 4-stroke engine separates the lubricating oil from the fuel, which significantly reduces carbon monoxide emissions.
Intake stroke
Piston moves down the cylinder bore from top dead center (TDC) to bottom dead center (BDC)
Intake valve is open, the exhaust valve is closed
Downward piston motion creates a vacuum (negative air pressure) that draws that air/fuel mixture into the engine via the open intake valve
Compression stroke
Piston moves up the cylinder bore from bottom dead center to top dead center
Both the intake and exhaust valves are closed
Upward piston motion compresses air/fuel mixture in the combustion chamber
Power stroke
At the end of the compression (previous) stroke, the spark plug fires and ignites the compressed air/fuel mixture. This ignition/explosion forces the piston back down the cylinder bore and rotates the crankshaft, propelling the vehicle forward.
Piston moves down the cylinder bore from top dead center to bottom dead center
Both the intake and exhaust valve are closed
Exhaust stroke
Piston moves up the cylinder bore from bottom dead center to top dead center. The momentum caused by the power stroke is what continues the crankshaft movement and the other 3 strokes consecutively.
Intake valve is closed, the exhaust valve is open
This final stroke forces the spent gasses/exhaust out of the cylinder. The cycle in now complete and the piston is ready to begin the intake stroke.