petrol engine

gasoline engine

Most widely used form of internal-combustion engine, found in most automobiles and many other vehicles. Gasoline engines vary significantly in size, weight per unit of power generated, and arrangement of components. The principal type is the reciprocating-piston engine. In four-stroke engines, each cycle requires four strokes of the piston—intake, compression, power (expansion), and exhaust—and two revolutions of the crankshaft. In a two-stroke cycle, the compression and power strokes of the four-stroke cycle are carried out without the inlet and exhaust strokes, in one upstroke and one downstroke of the piston and one revolution of the crankshaft. The size, weight, and cost of the engine per horsepower are therefore less, and two-stroke-cycle engines are used in smaller motorcycles, most marine motors, and many handheld landscaping tools (e.g., hedge trimmers and chain saws). Seealso compression ratio; piston and cylinder; rotary engine.

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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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