Fossil Fuels Hydrocarbons Combustion  Elmhurst College
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 What is Gasoline?
Gasoline is a complex mixture of over 500 hydrocarbons that may have between 5 to 12 carbons. Smaller amounts of alkane cyclic and aromatic compounds are present. Virtually no alkenes or alkynes are present in gasoline. Compounds in Gasoline.

Gasoline is most often produced by the fractional distillation of crude oil. The crude oil is separated into fractions according to different boiling points of hydrocarbons of varying chain lengths. This fractional distillation process yields approximately 25% of straight-run gasoline from each barrel of crude oil. See Distillation Oil Refining.

The yield of gasoline may be doubled by converting higher or lower boiling point fractions into hydrocarbons in the gasoline range. See Conversion Refining.

Typical Composition of Gasoline

 General Name


Aliphatic - straight chain


Aliphatic - branched

Aliphatic - cyclic



 ethyl benzene


What is the octane number?

Fuel octane requirements for gasoline engines vary with the compression ratio of the engine. Engine compression ratio is the relative volume of a cylinder from the bottom most position of the piston's stroke to the top most position of the piston's stroke. The higher an engine's compression ratio, the greater the amount of heat generated in the cylinder during the compression stroke.

If fuel octane is too low for a given compression ratio, the fuel prematurely and spontaneously ignites too early and the fuel charge EXPLODES rather than BURNS resulting in incomplete combustion. The net effect is a loss in power, possible engine damage, and an audible "knock" or "ping", referred to as detonation.

 The octane number of gasoline is a measure of its resistance to knock. The octane number is determined by comparing the characteristics of a gasoline to isooctane (2,2,4-trimethylpentane) and heptane. Isooctane is assigned an octane number of 100. It is a highly branched compound that burns smoothly, with little knock. On the other hand, heptane, a straight chain, unbranched molecule is given an octane rating of zero because of its bad knocking properties.

Straight-run gasoline (directly from the refinery distillation column) has an octane number of about 70. In other words, straight-run gasoline has the same knocking properties as a mixture of 70% isooctane and 30% heptane. Many of these compounds are straight chain alkanes. Cracking, isomerization, and other refining processes can be used to increase the octane rating of gasoline to about 90. Anti-knock agents may be added to further increase the octane rating.

Octane rating versus type of compound:

Octane ratings decrease with increasing carbon chain length.
Octane ratings increase with carbon chain branching.
Octane ratings increase in aromatics with same number of carbons.

 What octane gasoline should you purchase?

For most automobiles, use the lowest grade of 87 octane, unless they specifically say to use a higher octane gasoline. Using higher octane grades does not provide any extra power or extra mileage.