Gasoline is a transparent; petroleum derived flammable liquid that is used primarily as a fuel in most 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 160-liter (42-U.S.-gallon) barrel of crude oil can yield up to about 72 liters (19 U.S. gallons) of gasoline after processing in an oil refinery, depending on the crude oil assay and on what other refined products are also extracted. 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, which is produced in several grades.