Gasoline, gas, or petrol is a clear 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 42-U.S.-gallon (160-liter) barrel of crude oil can yield up to about 19 U.S. gallons (72 liters) 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 are no longer used in most areas to increase octane rating (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.