Petrol, mixture of the lighter liquid hydrocarbons used chiefly as a fuel for internal-combustion engines. It is produced by the fractional distillation of petroleum oil; by condensation or adsorption from natural gas; by thermal or catalytic decomposition of petroleum or its fractions; by the hydrogenation of producer gas or coal; or by the polymerization of hydrocarbons of lower molecular weight.
Petrol (known in the United States as gasoline) can be produced by the direct distillation of crude petroleum, when it is known as straight-run petrol. It is usually distilled continuously in a bubble tower (see Distillation), which separates the fractions of the oil that will be blended to become petrol from those having higher boiling points, which are blended to form products such as kerosene, fuel oil, lubricating oil, and grease. The range of temperatures in which fractions suitable for petrol boil and are distilled off is roughly between 38° C to 205° C (100° F to 400° F). The yield of petrol from this process varies from about 1 per cent to about 50 per cent, depending on the oil. Straight-run petrol now makes up only a small part of petrol production because of the superior merits of the various cracking processes.
Some natural gas contains a percentage of natural petrol that may be recovered by condensation or adsorption. The most common process for the extraction of this component includes passing the gas as it comes from the well through a series of towers containing a light oil called straw oil. The oil absorbs the petrol, which is then distilled off. Other processes involve adsorption of the petrol on activated alumina, activated carbon, or silica gel.
High-grade petrol can be produced by a process known as hydrofining, that is, the hydrogenation of refined petroleum oils under high pressure in the presence of a catalyst such as molybdenum oxide. Hydrofining not only converts oils of low value into petrol of higher value but at the same time purifies the product chemically by removing undesirable elements such as sulphur. Producer gas, coal, and coal-tar distillates can also be hydrogenated to form petrol.
For use in high-compression engines, it is desirable to produce petrol that will burn evenly and completely in order to prevent knocking, which is the noise and damage caused by premature ignition of a part of the fuel and air charge in the combustion chamber. The antiknock characteristics of a petrol are directly related to its efficiency and are indicated by its octane number. This is a rating that describes the performance of a fuel in comparison with that of a standard fuel containing given percentages of isooctane and heptane. The octane number given to the fuel is the same as the percentage of isooctane in the standard fuel of the same performance. The higher this number, the less likely a fuel is to cause knocking. Cracked petrol has better antiknock characteristics than straight-run petrol, and any petrol can be further improved by the addition of such substances as tetraethyl or tetramethyl lead. Since it was discovered, however, that the emission of lead from such petrols is dangerous to human beings—among other effects, raising blood pressure—research on new ways to reduce the knocking characteristics of petrol was intensified.
Low-lead petrols were introduced in the early 1970s as a result of increased public concern about air pollution, and cars were increasingly equipped with catalytic converters to reduce their emission of pollutants. Because even low-lead petrol “poisons” the catalyst, the proportion of leaded petrol in the United States declined from 73 per cent of the total supply in 1976 to less than 10 per cent by 1990. European countries moved more slowly in this direction, largely by placing extra taxes on leaded petrol. Many environmentalists called for much-increased use of gasohol and cleaner-burning natural gas in the late 1990s. However, there was widespread concern over alternative biofuels' impact on global warming and they came to be considered partly responsible for sharp food price increases experienced worldwide towards the close of the first decade of the 21st century.