Combustion Chamber

The combustion chamber is the space at the top of the cylinder, formed by the cylinder head and the top of the piston, where the compressed air–fuel mixture is ignited by the spark plug. The burning gases expand and push the piston down, producing power.

Its shape, together with the valve and plug positions, affects how quickly and completely the mixture burns, the engine's compression ratio and its resistance to knocking. Common designs include the hemispherical and pent-roof chambers used in most modern four-valve engines; in two-strokes the chamber is formed by the cylinder head dome and squish band.

Carbon deposits in the chamber raise compression and can cause pinking (knock) and hot spots. Using the correct fuel and oil, a healthy air filter and properly working injectors or carb keep the chamber clean.