compression

In a four-stroke piston engine, compression is the second stroke, where the intake valve closes and the piston moves upward in the cylinder, reducing the volume of the fuel-air mixture and raising its pressure and temperature. This prepares the charge for ignition on the power stroke. The compression ratio, expressed as a number like 10:1, compares the volume of the cylinder when the piston is at bottom dead center to the volume when it is at top dead center.

Higher compression ratios extract more work from the same amount of fuel because the charge ignites at higher pressure and temperature. Most gasoline engines run compression ratios between 9:1 and 12:1. Diesel engines typically operate at 14:1 to 22:1 because diesel fuel is less volatile and requires higher temperatures to ignite reliably without a spark plug. Racing engines push ratios toward 13:1 or higher, though this demands premium fuel to prevent detonation.

Compression and detonation

The risk of excessive compression is detonation, also called knock or pinging. If pressure and temperature rise too high during the compression stroke, unburned fuel can ignite spontaneously before the spark plug fires, creating shock waves that hammer the piston and damage the engine. This is why high-compression engines require fuel with a higher octane rating. Octane measures a fuel's resistance to detonation under pressure.

Compression is also affected by engine condition. Carbon deposits in the combustion chamber raise the effective compression ratio by reducing clearance volume. Worn piston rings leak gases past the seal, lowering compression pressure and reducing engine power and efficiency. A compression test, which measures cylinder pressure during cranking with all spark plugs removed, diagnoses worn rings, stuck valves, and gasket leaks.

The term "compression" applies equally to two-stroke and four-stroke engines, though the mechanics differ. In a two-stroke, compression occurs while the exhaust port is still open, so scavenging and compression overlap. Superchargers and turbochargers boost compression by forcing more air into the cylinder, raising pressure above what the piston alone can achieve, which increases power output but also increases the risk of detonation.