A push becomes a turn.
Follow one cylinder through intake, compression, power and exhaust. Two crankshaft revolutions. Four different jobs.
Intake
The piston moves down while the intake valve is open, drawing fresh charge into the cylinder.
Try thisAdvance through all four strokes. The piston returns to the top twice—but only one return is followed by the power stroke.
How does a four-stroke engine work?
A four-stroke engine draws in fresh charge, compresses it, extracts work as combustion gases expand, and expels exhaust. A connecting rod links the piston to the crankshaft; one cycle takes two crankshaft revolutions.
The mechanism,
in plain words.
01 · Intake
The piston travels down, drawing fresh charge through the open intake valve. Depending on the engine, fuel may be added in the intake or directly in the cylinder.
02 · Compression
Both valves close and the piston travels upward, compressing the charge. In this simplified petrol-engine model, a spark ignites it near the end of compression.
03 · Power
Combustion raises cylinder pressure. Expanding gases push the piston down; the connecting rod applies force to the offset crank pin, producing torque.
04 · Exhaust
The exhaust valve opens and the piston travels upward, pushing burned gases out. The crankshaft has now made two revolutions, or 720 degrees.
Slider–crank geometry Model & sources
Stroke = 2 × crank radius. Rod length stays constant as its angle changes. The four strokes each span 180° in this idealized model.
NASA: four-stroke engine stages ↗What this model simplifies. Idealized spark-ignition engine. Valve events are aligned with stroke boundaries; real engines use valve lead, lag and overlap. The spark marker is illustrative. Colors indicate the stroke, not simulated pressure or temperature. Animation speed is slowed for teaching.