# Spark-ignition engine

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For details of the spark-ignition system, see [Ignition system](/source/Ignition_system).

A **spark-ignition engine** (**SI engine**) is an [internal combustion engine](/source/Internal_combustion_engine), generally a [petrol engine](/source/Petrol_engine), where the combustion process of the air-fuel mixture is ignited by a spark from a [spark plug](/source/Spark_plug). This is in contrast to [compression-ignition engines](/source/Compression-ignition_engine), typically [diesel engines](/source/Diesel_engine), where the heat generated from [compression](/source/Compression_ratio) together with the injection of fuel is enough to initiate the combustion process, without needing any external spark.

## Fuels

Spark-ignition engines are commonly known as petrol engines in most parts of the world, while the term "gas" (shorthand for "gasoline") engines is used primarily in America.[1] Spark-ignition engines can (and increasingly are) run on fuels other than [petrol/gasoline](/source/Gasoline), such as [autogas](/source/Autogas) ([LPG](/source/Liquefied_petroleum_gas)), [methanol](/source/Methanol), [ethanol](/source/Ethanol), [bioethanol](/source/Bioethanol), [compressed natural gas](/source/Compressed_natural_gas) (CNG), [hydrogen](/source/Hydrogen), and (in drag racing) [nitromethane](/source/Nitromethane).[2]

## Working cycle

The working cycle of both spark-ignition and compression-ignition engines may be either [two-stroke](/source/Two-stroke_cycle) or [four-stroke](/source/Four-stroke_cycle).

A [four-stroke](/source/Four-stroke) spark-ignition engine is an [Otto cycle](/source/Otto_cycle) engine. It consists of following four strokes: [suction or intake stroke](/source/Intake_stroke), [compression stroke](/source/Compression_stroke), [expansion or power stroke](/source/Power_stroke_(engine)), [exhaust stroke](/source/Exhaust_stroke). Each stroke consists of 180 degree rotation of crankshaft rotation and hence a [four-stroke](/source/Four-stroke) cycle is completed through 720 degree of crank rotation. Thus for one complete cycle there is only one power stroke while the crankshaft turns by two revolutions.

## See also

- [Multifuel engine](/source/Multifuel)

## References

1. Naijar, Yousef S.H. (2009). "Alternative Fuels for Spark Ignition Engines". *The Open Fuels & Energy Science Journal*. **2** (2): 1–9. [doi:10.2174/1876973X01002010001](https://doi.org/10.2174/1876973X01002010001)

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Adapted from the Wikipedia article [Spark-ignition engine](https://en.wikipedia.org/wiki/Spark-ignition_engine) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Spark-ignition_engine?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.