How does a four-stroke engine work?
Four piston strokes. Two crankshaft turns. One repeating cycle.
Engines · Crank mechanism
What is a four-stroke engine?
A four-stroke engine completes intake, compression, power and exhaust across four piston strokes and two crankshaft revolutions. This illustration follows a single-cylinder, spark-ignition gasoline engine with a camshaft that coordinates the valves.
Loading…
How it works.
The connecting rod links the piston’s back-and-forth motion to the crankshaft. Combustion supplies energy during the power stroke; the rotating assembly carries motion through the other strokes. The camshaft turns at half crankshaft speed so the valve sequence repeats once every two crankshaft turns.
Intake
The piston moves down while the inlet passage admits the fresh charge. Follow the intake-valve lift and the increasing space above the piston.
Compression
As the piston rises, the trapped charge occupies less space. The spark is timed near the end of compression; this example fires before top dead center.
Power
Burning gas expands and pushes the piston downward. The connecting rod transfers that force to the crankshaft. The colored gas in this diagram represents the phase rather than a calculated temperature.
Exhaust
The exhaust passage opens and the rising piston displaces spent gas. At the end of the second crankshaft revolution, the sequence returns toward intake.
Take this figure with you.
Files are included with full library access.
- Interactive HTML
- Source files
- Readme & license
Why do valve events overlap the four stages?
The four stroke names describe piston travel, not exact valve-switching instants. This model opens and closes valves around the boundaries to illustrate a chosen cam profile. Use the phase label and valve-lift display together; a valve can still be moving as the next stroke begins.
About this model
- Piston, crank and valve motion come from a geometric model; gas flow, flame and exhaust are phase-based illustrations.
- The slow speed is for observation. The diagram does not calculate pressure, temperature, torque, efficiency or fuel consumption.
Adjust the demonstration speed, pause at a valve event and reset to compare the four strokes.
Version 1.0.0
Frequently asked questions
Why does one cycle take 720 degrees?
Each stroke takes about half a crankshaft turn, or 180 degrees. Four strokes therefore span two turns: 4 × 180° = 720°.
Is this a gasoline or diesel engine?
This example uses a spark plug, so it represents a gasoline spark-ignition engine. A four-stroke diesel also has four strokes, but its fuel injection and ignition process differ.