Four-Stroke Engine

A four-stroke engine completes a cycle in four strokes of the piston: intake (the piston moves down and draws in the fuel–air mixture), compression (it moves up and compresses the mixture), power (a spark ignites the mixture and the expanding gas pushes the piston down) and exhaust (it moves up and pushes the burnt gas out). The crankshaft turns twice per cycle.

The Otto cycle is the ideal cycle for a petrol engine. It has four processes: adiabatic compression, heat addition at constant volume (the spark and combustion), adiabatic expansion, and heat rejection at constant volume. It is named after Nicolaus Otto, who built the first working four-stroke engine in 1876.

η = 1 − 1/r^(γ−1), where r is the compression ratio and γ is the ratio of specific heats (about 1.4 for air). A compression ratio of 8 gives an ideal efficiency of about 56%; real engines reach about 25–35% because of heat losses and friction.

Compression ratio is the cylinder volume when the piston is at the bottom divided by the volume when it is at the top. Petrol engines use about 8 to 12; diesel engines use about 14 to 22 because they ignite the fuel by compression alone.

A four-stroke engine has one power stroke every two turns of the crankshaft and uses valves. A two-stroke engine has a power stroke every turn and uses ports in the cylinder wall instead of valves. Two-strokes are simpler and lighter but less efficient and more polluting.

Only one stroke in four produces power, so the crankshaft would jerk and slow down in between. The heavy flywheel stores energy during the power stroke and keeps the crankshaft turning smoothly through the intake, compression and exhaust strokes.