The combustion of alkanes generates carbon dioxide and water on complete oxidation. The energy liberated (known as heat of combustion) for this complete oxidation is an important factor determining the efficiency of a hydrocarbon as fuel. The reaction is exothermic in nature but requires a very high temperature for its initiation. Heats of combustion of some hydrocarbons are given in Table 3.

Table 3.

Most automobiles use petrol or diesel as a fuel. The efficiency of an internal combustion engine depends upon the compression ratio of the fuel-air mixture in the engine. However, increasing this ratio leads to “knocking”( knocking ,   in an internal-combustion engine, sharp sounds caused by premature combustion of part of the compressed air-fuel mixture in the cylinder ) where the power of the engine is greatly reduced. The problem of knocking can be met with in two different ways: (a) proper selection of hydrocarbons in fuel and (b) addition of additives. The second approach employed the addition of tetraethyl lead (TEL) as an additive but recent concern about environmental pollution has seen the decline of its use. The first approach is still used. It employs addition of specific hydrocarbons having high octane number ( octane number is measure of the ability of a fuel to resist knocking when ignited in a mixture with air in the cylinder of an internal-combustion engine).