Gasoline (//) or petrol (//) (see the etymology for naming differences) 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. Tetraethyl lead and other lead compounds, once widely used to increase octane rating, are no longer used in most areas (they are still used in aviation and auto-racing). 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.
Gasoline can enter the environment uncombusted, both as liquid and as vapor, from leakage and handling during production, transport, and delivery (e.g., from storage tanks, from spills, etc.). As an example of efforts to control such leakage, many underground storage tanks are required to have extensive measures in place to detect and prevent such leaks. Gasoline contains benzene and other known carcinogens and causes over 100 thousand premature deaths every year.