Compression ratio

The compression ratio is the ratio between the maximum and minimum volume during the compression stage of the power cycle in a piston or Wankel engine.

A fundamental specification for such engines, it can be measured in two different ways. The simpler way is the static compression ratio: in a reciprocating engine, this is the ratio of the volume of the cylinder when the piston is at the bottom of its stroke to that volume when the piston is at the top of its stroke. The dynamic compression ratio is a more advanced calculation which also takes into account gases entering and exiting the cylinder during the compression phase.

A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air–fuel mixture due to its higher thermal efficiency. This occurs because internal combustion engines are heat engines, and higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature.

However, several engineering constraints limit the practical implementation of very high compression ratios. Higher compression ratios increase peak cylinder pressures and temperatures, requiring stronger engine components and more robust materials to withstand the additional mechanical and thermal stresses. Additionally, high compression ratios make engines more susceptible to knock and detonation, particularly when using lower-octane fuels, which can damage engine components and reduce efficiency. The thermal efficiency gains from increasing compression ratio also diminish beyond approximately 10:1, as increased friction and heat losses begin to offset the thermodynamic benefits.

In petrol (gasoline) engines used in passenger cars for the past 20 years, compression ratios have typically been between 8:1 and 12:1. Several production engines have used higher compression ratios, including:

When forced induction (e.g. a turbocharger or supercharger) is used, the compression ratio is often lower than naturally aspirated engines. This is due to the turbocharger or supercharger already having compressed the air before it enters the cylinders. Engines using port fuel-injection typically run lower boost pressures and/or compression ratios than direct injected engines because port fuel injection causes the air–fuel mixture to be heated together, leading to detonation. Conversely, directly injected engines can run higher boost because heated air will not detonate without a fuel being present.

Higher compression ratios can make gasoline (petrol) engines subject to engine knocking (also known as "detonation", "pre-ignition", or "pinging") if lower octane-rated fuel is used. This can reduce efficiency or damage the engine if knock sensors are not present to modify the ignition timing.

Diesel engines use higher compression ratios than petrol engines, because the lack of a spark plug means that the compression ratio must increase the temperature of the air in the cylinder sufficiently to ignite the diesel using compression ignition. Compression ratios are often between 14:1 and 23:1 for direct injection diesel engines, and between 18:1 and 23:1 for indirect injection diesel engines.