Spark-ignition engine

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For details of the spark-ignition system, see Ignition system

The term spark-ignition engine is normally used to refer to internal combustion engines where the fuel-air mixture is ignited with a spark. The term contrasts with compression-ignition engines, where the heat from compression alone ignites the mixture. Spark-ignition engines can be either two-stroke or four-stroke, and are commonly referred to as "gasoline engines" in America and "petrol engines" in Britain. However, these terms are not preferred, since spark-ignition engines can (and increasingly are) run on fuels other than gasoline, such as autogas (LPG), methanol, ethanol, compressed natural gas (CNG), hydrogen, and (in drag racing) nitromethane. A four-stroke spark-ignition engine is an Otto cycle engine.

Until recently, a major distinction between spark-ignition and compression-ignition engines has been where the fuel is mixed - spark-ignition engines mix fuel outside the cylinders and compression-ignition engines mix fuel inside the cylinders. However, both two-stroke and four-stroke spark-ignition engines are increasingly being designed with gasoline direct injection (GDi), eliminating this distinction between the two systems.