Regular gasoline contains predominantly C5 to Q j hydrocarbons; diesel fuel contains mainly hydrocarbons with 9-11 carbon atoms. Generally speaking, the more carbon atoms in the alkane, the higher its boiling point and the lower its vapor pressure—and hence the lesser its tendency to vaporize—at a given temperature. For this reason, gasoline destined for warm summer conditions is formulated with smaller amounts of the smaller, more easily vaporized alkanes such as butanes and pentanes than is that prepared for winters in cold climates. The presence of volatile hydrocarbons in gasoline is vital in cold climates so that automobile engines can be started.

Gasoline that consists primarily of straight-chain alkanes and cycloalka-nes has poor combustion characteristics when burned in internal combustion engines. A mixture of air and vaporized gasoline of this type tends to ignite spontaneously in the engine's cylinder before it is completely compressed and sparked, so the engine "knocks," with a resulting loss of power. Consequently, all gasoline is formulated to contain substances that will prevent knocking.

In contrast to unbranched alkanes, highly branched ones such as the octane isomer 2,2,4-trimethylpentane, "isooctane" (illustrated below), have excellent burning characteristics. Unfortunately, they do not occur naturally in significant amounts in crude oil. The ability of a gasoline to generate power without engine knocking is measured by its octane number. To define the scale, isooctane is given the octane number of 100, and n-heptane is arbitrarily assigned a value of zero.

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