A petrol engine (known as a gasoline engine in North America) 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 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 utilise 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. 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.[citation needed] The first fast-running petrol engine was invented by the German automobile pioneer Gottlieb Daimler.

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