Compression ratio
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.
In petrol (gasoline) engines used in passenger cars for the past 20 years, compression ratios have typically been between 8:1 and 12:1.
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.
[5] This can reduce efficiency or damage the engine if knock sensors are not present to modify the ignition timing.
[6] Mazda's Skyactiv-D, the first such commercial engine from 2013, used adaptive fuel injectors among other techniques to ease cold start.
[7] The compression ratio may be higher in engines running exclusively on liquefied petroleum gas (LPG or "propane autogas") or compressed natural gas, due to the higher octane rating of these fuels.
[8] Motorsport engines often run on high-octane petrol and can therefore use higher compression ratios.
Ethanol and methanol can take significantly higher compression ratios than gasoline.
Racing engines burning methanol and ethanol fuel often have a compression ratio of 14:1 to 16:1.
Variable compression engines allow the volume above the piston at top dead centre to be changed.
In most automotive engines, the intake valve closure (which seals the cylinder) takes place during the compression phase (i.e. after bottom dead centre, BDC), which can cause some of the gases to be pushed back out through the intake valve.
On the other hand, intake port tuning and scavenging can cause a greater amount of gas to be trapped in the cylinder than the static volume would suggest.
The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength.
