Gasoline: the basics
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Gasoline is widely used as a fuel for cars. Gasoline (also called Petrol in Europe) is a complex mixture of a number of hydrocarbon fractions produced from the refining of crude oil which has to fulfil a large number of specifications. In most cases, additives are added to improve the quality of the gasoline and to ensure that gasoline specifications are met.
The production process of gasoline involves four main stages:
- Fractionating or distilling the crude oil by use of heat
- Chemical conversion either by cracking larger molecules or combining smaller ones
- Treatment to remove undesirable material, for example, sulphur
- Blending of the various components into finished products
The diagram below shows a typical product yield from a refinery processing a barrel of crude oil.
The fraction that is separated from crude oil via distillation, called virgin or straight-run gasoline or naphtha, has an octane number of around 70 and thus requires the addition of other materials in order to be suitable for use in modern engines. The blending together of the various refinery streams to achieve the desired characteristics is a highly complex process involving sophisticated computer programmes. A modern gasoline is often a blend of 9 or more components.
Finished Gasoline is predominantly a mixture of paraffins (alkanes), naphthenes (cycloalkanes) and olefins (alkenes). The exact ratio varies because:
• Not all refineries have the same processing units
• The crude oils they use come from different sources
• Different grades of gasoline have different ratios
Octane rating of gasoline
An important characteristic of gasoline is its octane rating, which is a measure of how resistant gasoline is to the highly destructive combustion phenomenon known as pre-detonation or knock. The rating is measured in a special engine using isooctane (rating = 100) and heptane (rating= 0) as standards. Thus a gasoline with an octane rating of 95 behaves in the engine in the same way as a blend of 95% isooctane and 5% heptane.
Additives, such as lead, were found to modify this behaviour and increase the gasoline’s octane rating. However, with the discovery of the environmental and health impacts of lead, in combination with its incompatibility with the catalytic converters needed to meet air quality standards, the practise of lead addition was gradually phased out. In its place Oxygenates, such as MTBE, ETBE and ethanol, have been introduced; not only for their ability to increase octane ratings but also to enable more complete combustion, through the introduction of oxygen, which leads to reduced emissions.