Crude oil contains hundreds of different hydrocarbons. Yet U.S. refineries convert half of all crude into gasoline--a blend of fuel stocks, particularly 2,2,4-trimethylpentane (eight carbon atoms chained together) and heptane (seven carbon atoms). The more complex the chain, the more the molecule can be compressed before it ignites spontaneously, allowing an engine to operate at a higher compression ratio--greater power output. The test mixture by which a gasoline's octane rating is judged combines 2,2,4-trimethylpentane and heptane (87 to 13 percent for "87 octane").
Refiners have tried additives over time to boost octane rating. Tetraethyl lead worked in "leaded" gasoline but was phased out because it spoiled catalytic converters. Producers switched to methyl tertiary butyl ether (MTBE), but it has been implicated in contaminating groundwater, and state governments are banning it. An alternative increasingly being used is ethanol, which has an octane rating of 108 or 110; gasoline with 10 percent ethanol is marketed as gas-o-hol.