or petrol, light, volatile mixture of hydrocarbons for use in the internal-combustion engine and as an organic solvent, obtained primarily by fractional distillation and “cracking” of petroleum, but also obtained from natural gas, by destructive distillation of oil shales and coal, and by a process that converts methanol to gasoline using zeolite as a catalyst. Gasoline intended for use in engines is rated by octane number, an index of quality that reflects the ability of the fuel to resist detonation and burn evenly when subjected to high pressures and temperatures inside an engine. Premature detonation produces “knocking” and “pinging”; it wastes fuel and may cause engine damage. The addition of tetramethyl lead and tetraethyl lead to raise the octane number is no longer permitted in the United States because it leads to dangerous emissions containing lead. New formulations of gasoline designed to raise the octane number contain increasing amounts of aromatics and oxygen-containing compounds (oxygenates), such as alcohols, methyl tertiary butyl ether (MTBE), and methylcyclopentadienyl manganese tricarbonyl (MMT). Automobiles are now equipped with catalytic converters that oxidize unreacted gasoline; the cars are designed to run on newly formulated gasolines as well as on gasohol, which contains 10% ethanol or 3% methanol. In addition, since 1998 a number of American automobiles have been equipped to enable them to run on either gasoline or E85, a mixture of 85% ethanol and 15% gasoline. Some racing cars use pure methanol as fuel.
There are five blends of gasoline marketed in the United States. Conventional gasoline, the most widely available, is sold where air quality is satisfactory; since 1992, it has been formulated to evaporate more slowly in hot weather so as to reduce smog, and it now contains detergent additives to reduce engine deposits. Winter oxygenated gasoline, introduced in 1992, is formulated as conventional gasoline with oxygen-rich chemicals added, such as MTBE or ethanol. The oxygen promotes cleaner burning, reducing carbon monoxide, and is generally sold from November to March because cold engines operate less efficiently and produce more carbon monoxide. Reformulated gasoline (RFG), introduced in Jan., 1995, is mandated in areas where toxins in the air are a constant problem; it contains oxygen-rich chemicals in lesser concentrations than the winter oxygenated gasoline and is formulated to reduce certain toxic chemicals found in conventional and winter oxygenated fuels. Oxygenated reformulated gasoline is a wintertime fuel exclusive to the New York City area, where heavy carbon monoxide pollution occurs. California reformulated gasoline, introduced in 1996, has a different formulation and burns cleaner than regular reformulated gasoline. Because MTBE has been implicated as a pollutant, particularly of groundwater, its use is being curtailed. In 1999, California ruled that the MTBE in California reformulated gas must be phased out by Dec. 31, 2002.