Four-stroke cycle and constant-volume combustion
The Otto cycle powers spark-ignition gasoline engines through intake, compression, power, and exhaust strokes. During intake, the piston draws a fuel-air mixture into the cylinder at constant pressure. Compression raises pressure and temperature isentropically. At top dead center, a spark ignites the mixture, burning it nearly instantaneously at roughly constant volume (the piston is momentarily stationary). Expanding combustion gases push the piston down in the power stroke, producing useful work. Finally, the exhaust valve opens, releasing burned gases at constant pressure.
Pressure-volume analysis and efficiency
On a PV diagram, the Otto cycle appears as a closed loop with sharp pressure rise at constant volume (combustion) followed by isentropic expansion and compression. The enclosed area represents net work per cycle. Higher compression ratios increase the expansion ratio, raising cycle efficiency toward the Carnot limit. However, very high compression ratios cause knocking (uncontrolled combustion), which damages the engine and limits practical ratios to 9:1 to 12:1 in production vehicles.
Engine design and performance optimization
Faster flame propagation and more complete combustion improve thermal efficiency. Fuel octane rating resists knock; higher octane allows higher compression. Variable valve timing and direct fuel injection (spraying fuel directly into the cylinder rather than the intake manifold) increase part-load efficiency by controlling combustion. Turbocharging and supercharging increase pressure before compression, effectively raising the compression ratio and enabling higher power density without mechanical damage.