Petrol engine

A Petrol engine or Gasoline engine is an internal combustion engine with spark-ignition designed to run on petrol (gasoline) and similar volatile fuels. It differs from a diesel engine in the method of mixing the fuel and air, and in the fact that it uses spark plugs. In a diesel engine, merely the air is compressed, and the fuel is injected at the end of the compression stroke. In a petrol engine, the fuel and air are pre-mixed before compression injection. Pre-mixing of fuel and air allows a petrol engine to run at a much higher speed than a diesel, but severely limits their compression, and thus efficiency .


Petrol engines have many applications, including:


Working cycles

Petrol engines may run on the four-stroke cycle or the two-stroke cycle. For details of working cycles see:

Cylinder arrangement

Common cylinder arrangements are from 1 to 6 cylinders in-line or from 2 to 16 cylinders in V-formation. Alternatives include Rotary and Radial Engines the latter typically have 7 or 9 cylinders in a single ring, or 10 or 14 cylinders in two rings.


Petrol engines may be air-cooled, by fins on the cylinders, or liquid-cooled, by a water jacket and radiator. The coolant was formerly water but is now usually a mixture of water and ethylene glycol. This mixture has a lower freezing-point and a higher boiling-point than pure water. In addition, the cooling system is usually slightly pressurized to minimise evaporation of coolant.

Compression ratio

The compression ratio is the ratio between the cylinder volumes at the beginning and end of the compression stroke. Broadly speaking, the higher the compression ratio, the higher the efficiency of the engine. However, compression ratio has to be limited to avoid pre-ignition of the fuel-air mixture which would cause engine knocking and damage to the engine. Modern motor-car engine generally have compression ratios of between 9:1 and 10:1, but this can go up to 11 or 12:1 for high-performance engines that run on, say, 98 R0N (93 AKI, US Premium- or European Super-grade) petrol. In the 1950s, with low-octane fuel and less well-designed cylinder heads, compression ratios were between 6.5:1 and 7:1. Old tractor engines running on tractor vaporising oil might have compression ratios as low as 4.5:1 but modern tractors have diesel engines.


main article Ignition system
Petrol engines use spark ignition and high voltage current for the spark may be provided by a magneto or an ignition coil. In modern car engines the ignition timing is managed by an electronic Engine Control Unit.


Concerns about global warming and air pollution have put a question mark over the future of the petrol engine. Much has been done to improve its fuel efficiency and reduce emissions and this has bought it more time.


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