Four strokes, two turns
The same cylinder is shown frozen at four moments, one in each stroke. Each cycle takes four strokes of the piston, which is two full turns of the crankshaft.
1. Intake
The piston moves down with the intake valve open, drawing in a fresh charge of fuel and air. The exhaust valve stays shut.
2. Compression
Both valves close and the piston rises, squeezing the mixture into a small space at the top. Compressed mixture burns far more powerfully.
3. Power
Just before the top, the spark plug fires. The burning gas expands and drives the piston down, and the rod pushes the crank pin around. This is the only stroke that makes power.
4. Exhaust
The exhaust valve opens and the rising piston pushes the burnt gas out. Then the intake valve opens and the cycle starts again.
Rod and crank
The connecting rod is hinged to the piston and clamped around the offset crank pin, so it tilts left and right as the pin circles. A camshaft turning at half crank speed opens each valve once per cycle, and the crank's momentum carries it through the three strokes that make no power.