Petroleum crude is presently the main source of automotive fuels although alternative fuels like natural gas, LPG are also being used in large numbers of road vehicles in some countries. The renewable fuels like ethyl alcohol and biodiesel too are being used in the form of blends with the petroleum derived gasoline and diesel fuels.  Petroleum crude   contains over 25000 compounds mainly the hydrocarbons composed of hydrogen and carbon elements. A number of these compounds  also contain sulphur in varying amounts and, nitrogen and oxygen in small quantities. The commercial liquid engine fuels are the mixtures of a few hundred different hydrocarbons. 
Typically, the petroleum transport fuels contain about 85 - 86 percent carbon and 14 – 15 percent hydrogen by mass. Sulphur containing compounds are also present in significant amounts.
In this module at first chemical composition and key characteristics of petroleum derived fuels and how they influence engine combustion and emissions are covered. Later, the most promising alternative fuels are also discussed.

Fuel Hydrocarbons and Other Components

Hydrocarbons are broadly grouped into paraffins, cycloparaffins (naphthenes), olefins and aromatics. The olefins are not present in the crude petroleum but are generated during processing and refining of crude when the practical transport fuels are being produced.

Paraffins

The paraffins are saturated hydrocarbons which are also known as alkanes. The carbon atoms have single bond between them. The large alkane molecules are either straight chain or branched chain. The empirical formula for the paraffin family is C_nH_2n+2. Methane (CH₄) is the first member of this family, the higher members being ethane (C₂H₆), propane (C₃H₈), ……n-heptane, n-octane, isooctane (C₈H₁₈), n- hexadecane (n-cetane) and so on. The straight chain paraffins are called normal- (n-) and branched chain are called iso- paraffins. Structures of some paraffins are given below. There are several isomers of isooctane depending upon the position of the branches. The isooctane that is most commonly referred to is 2-2-4 trimethyl pentane having five carbon atoms in straight chain and three methyl groups in 2, 2 and 4 carbon atom positions.