The Four-Stroke Combustion Cycle
A gasoline engine converts chemical energy into rotational mechanical energy through the four-stroke Otto cycle, repeated hundreds of times per minute in each cylinder. Stroke 1 — Intake: the intake valve opens as the piston descends, drawing an air-fuel mixture into the cylinder. Stroke 2 — Compression: both valves close and the piston rises, compressing the mixture to approximately 1/10th of its original volume (a 10:1 compression ratio is typical). Higher compression increases thermal efficiency but requires higher octane fuel to prevent pre-ignition (knock). Stroke 3 — Power (Combustion): near top dead center (TDC), the spark plug fires, igniting the compressed mixture. Rapidly expanding combustion gases push the piston forcefully downward, turning the crankshaft and generating the engine's power. Spark timing is critical — ignite too early and the expanding gases work against the rising piston (knock); too late and energy is wasted. Modern engines use electronic engine control units (ECUs) to vary timing by 10–30 degrees of crankshaft rotation depending on load, speed, and knock sensor feedback. Stroke 4 — Exhaust: the exhaust valve opens as the piston rises again, pushing spent gases out through the exhaust manifold, catalytic converter, and muffler. The cycle repeats continuously at 600–7,000 RPM depending on engine load.