petrol/Gasoline

Commercial gasoline is a mixture of a large number of different hydrocarbons. Gasoline is produced to meet various engine performance specifications, and many other compositions are possible. Hence, the exact chemical composition of gasoline is undefined. At the refinery, the composition varies according to the crude oils from which it is produced, the type of processing units present at the refinery, how those units are operated, and which hydrocarbon streams (blendstocks) the refinery opts to use when blending the final product.

Gasoline is produced in oil refineries. Roughly 72 liters (19 U.S. gal) of gasoline is derived from a 160-liter (42 U.S. gal)barrel of crude oil.[74] Material separated from crude oil via distillation, called virgin or straight-run gasoline, does not meet specifications for modern engines (particularly the octane rating; see below) but can be pooled into the gasoline blend.

The bulk of a typical gasoline consists of a homogeneous mixture of small, relatively lightweight hydrocarbons with between 4 and 12 carbon atoms per molecule (commonly referred to as C4–C12).[71] It is a mixture of paraffin (alkanes), olefins (alkenes), and cycloalkanes (naphthenes). The terms paraffin and olefin are used in place of the standard chemical nomenclature alkane and alkene, respectively, particularly in the oil industry. The actual ratio of molecules in any gasoline depends upon:

• the oil refinery that makes the gasoline, as not all refineries have the same set of processing units;
• the crude oil feed used by the refinery;
• the grade of gasoline (in particular, the octane rating).

The vrious refinery streams blended to make gasoline have different characteristics.