Gasoline
Gasoline, mixture of the
lighter liquid hydrocarbons used chiefly as a fuel for internal-combustion
engines. It is produced by the fractional distillation of petroleum; 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.
See Internal-Combustion Engine.
Gasoline produced by the direct distillation of crude
petroleum is known as straight-run gasoline. It is usually distilled
continuously in a bubble tower (see Distillation), which separates the
gasoline from the other fractions of the oil having higher boiling points, such
as kerosene, fuel oil, lubricating oil, and grease. The range of temperatures in
which gasoline boils and is distilled off is roughly between 38° and 205° C
(100° and 400° F). The yield of gasoline from this process varies from about 1
percent to about 50 percent, depending on the petroleum. Straight-run gasoline
now makes up only a small part of U.S. gasoline production because of the
superior merits of the various cracking processes.
In many parts of the country natural gas contains a
percentage of natural gasoline that may be recovered by condensation or
adsorption. The most common process for the extraction of natural gasoline
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 gasoline, which is
then distilled off. Other processes involve adsorption of the gasoline on
activated alumina, activated carbon, or silica gel.
High-grade gasoline 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 gasoline of higher value but also at the
same time purifies the gasoline chemically by removing undesirable elements such
as sulfur. Producer gas, coal, and coal-tar distillates can also be hydrogenated
to form gasoline. See Hydrogenation.
For use in high-compression engines, it is desirable to
produce gasoline that will burn evenly and completely in order to prevent
knocking, the sound and damage caused by premature ignition of a part of the
fuel and air charge in the combustion chamber of an internal-combustion engine.
The antiknock characteristics of a gasoline are directly related to its
efficiency and are indicated by its octane number. This is a rating that
describes performance of a fuel in comparison with that of a standard fuel
containing given percentages of isooctane and heptane. If the performance of the
rated fuel is the same as that of a standard fuel with a certain percentage of
isooctane, the octane number given the rated fuel is the same as the percentage
of isooctane in the standard fuel. The higher this number, the less likely a
fuel is to cause knocking. Cracked gasoline has better antiknock characteristics
than straight-run gasoline, and any gasoline 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 gasolines combined with such
additives is dangerous to living beings—among other effects, raising blood
pressure—research on new ways to reduce the knocking characteristics of gasoline
was intensified.
Low-lead gasolines were introduced in the early 1970s as a
result of increased public concern about air pollution. After 1975 all new
automobiles in the U.S. were equipped with catalytic converters to reduce the
engine's emission of pollutants. Because even low-lead gasoline “poisons” the
catalyst, the proportion of leaded gasoline in the U.S. declined from 73 percent
of the total supply in 1976 to less than 10 percent in 1990. (European countries
were moving more slowly in this same direction.) The Clean Air Act of 1990
required oil companies to make available cleaner gasoline with a higher oxygen
content in polluted urban areas, beginning in 1992. In the late 1990s many
environmentalists called for the increased use of gasohol and cleaner-burning
natural gas. In addition, in 1999 and 2000 two Japanese automakers introduced
hybrid electric-gas vehicles to the U.S. market that greatly increased fuel
efficiency and lowered emissions of air pollutants. Several U.S. automakers also
began intensive work on vehicles powered by fuel cells that used no gasoline at
all. In 2003 President George W. Bush called for the development of a fuel-cell
car by 2019. See also Electric Car; Fuel Cell.
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