It differs from a diesel engine in the method of mixing the fuel and air, and in the fact that it uses spark plugs to initiate the combustion process. In a diesel engine, only air is compressed (and therefore heated), and the fuel is injected into the now very hot air at the end of the compression stroke, and self-ignites. In a petrol engine, the fuel and air are usually pre-mixed before compression (although some modern petrol engines now use cylinder-direct petrol injection).
The pre-mixing was formerly done in a carburetor, but now (except in the smallest engines) it is done by electronically controlled fuel injection. Petrol engines run at higher speeds than Diesels partially due to their lighter pistons, conrods & crankshaft (as a result of lower compression ratios) & due to petrol burning faster than diesel. However the lower compression ratios of a petrol engine gives a lower efficiency than a diesel engine.
Petrol engines have many applications, including:
- Motor cars
- Small machines, such as lawn mowers, chainsaws and portable engine-generators
Petrol engines may run on the four-stroke cycle or the two-stroke cycle. For details of working cycles see:
Common cylinder arrangements are from 1 to 6 cylinders in-line or from 2 to 16 cylinders in V-formation. Flat engines -- like a V design flattened out-- are common in small airplanes and motorcycles and were a hallmark of Volkswagen automobiles into the 1990s. Flat 6s are still used in many modern Porsches, as well as Subarus. Many flat engines are air-cooled. Less common, but notable in vehicles designed for high speeds is the W formation, similar to having 2 V engines side by side. 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, with fins (to increase the surface area on the cylinders) and cylinder head; or liquid-cooled, by a water jacket and radiator. The coolant was formerly water, but is now usually a mixture of water and either ethylene glycol or propylene glycol. These mixtures have lower freezing-points and a higher boiling-points than pure water and also prevent corrosion, with modern antifreezes also containing lubricants and other additives to protect water pump seals and bearings. The cooling system is usually slightly pressurized to further raise the boiling point of the coolant.
The compression ratio is the ratio between the total volumes of the cylinder AND the combustion chambers - 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 higher octane fuel.
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.