Gasoline (American English) or petrol (British English) is a transparent petroleum-derived liquid that is used primarily as a fuel in spark-ignited internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. On average, a 42-U.S.-gallon (160-liter) barrel of crude oil yields about 19 U.S. gallons (72 liters) of gasoline after processing in an oil refinery, though this varies based on the crude oil assay.
The characteristic of a particular gasoline blend to resist igniting too early (which causes knocking and reduces efficiency in reciprocating engines) is measured by its octane rating. Gasoline is produced in several grades of octane rating. Tetraethyllead and other lead compounds are no longer used in most areas to increase octane rating. Other chemicals are frequently added to gasoline to improve chemical stability and performance characteristics, control corrosiveness and provide fuel system cleaning. Gasoline may contain oxygen-containing chemicals such as ethanol, MTBE or ETBE to improve combustion.
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