Gasoline

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Gasoline

Crude oil in its natural state has very few practical uses. However, when it is separated into its component parts by the process of fractionation, or refining, those parts have an almost unlimited number of applications.

In the first 60 years after the process of petroleum refining was invented, the most important fraction produced was kerosene, widely used as a home heating product. The petroleum fraction slightly lighter than kerosene — gasoline — was regarded as a waste product and discarded. Not until the 1920s, when the automobile became popular in the United States, did manufacturers find any significant use for gasoline. From then on, however, the importance of gasoline has increased with automobile use.

The term gasoline refers to a complex mixture of liquid hydrocarbons that condense in a fractionating tower at temperatures between 100° and 400°F (40° and 205°C). The hydrocarbons in this mixture are primarily single- and double-bonded compounds containing five to 12 carbon atoms.

Gasoline that comes directly from a refining tower, known as naphtha or "straight-run" gasoline, was an adequate fuel for the earliest motor vehicles. But as improvements in internal combustion engines were made, problems began to arise. The most serious problem was "knocking."

If a fuel burns too rapidly in an internal combustion engine, it generates a shock wave that makes a "knocking" or "pinging" sound. The shock wave will, over time, also cause damage to the engine. The hydrocarbons that make up straight-run gasolines proved to burn too rapidly for automotive engines developed after 1920.

Early in the development of automotive fuels, engineers adopted a standard for the amount of knocking caused by a fuel and, hence, for the fuel's efficiency. That standard was known as "octane number." To establish a fuel's octane number, it is compared with a very poor fuel (n-heptane), assigned an octane number of zero, and a very good fuel (isooctane), assigned an octane number of 100. The octane number of straight-run gasoline is anywhere from 50 to 70.

As engineers made more improvements in automotive engines after the 1920s, chemists tried to keep pace by developing better fuels. One approach they used was to subject straight-run gasoline (as well as other crude oil fractions) to various treatments that changed the shape of hydrocarbon molecules in the gasoline mixture. One such method, called cracking, involves the heating of straight-run gasoline or another petroleum fraction to high temperatures. The process results in a better fuel from newly-formed hydrocarbon molecules.

Another method for improving the quality of gasoline is catalytic reforming. In this case, the cracking reaction takes place over a catalyst such as copper , platinum, rhodium, or other "noble" metal, or a form of clay known as zeolite. Again, hydrocarbon molecules formed in the fraction are better fuels than straight-run gasoline. Gasoline produced by catalytic cracking or reforming has an octane number of at least eighty.

A very different approach to improving gasoline quality is the use of additives, chemicals added to gasoline to improve the fuel's efficiency. Automotive engineers learned more than 50 years ago that adding as little as two grams of tetraethyl lead , the best-known additive, to one gallon of gasoline raises its octane number by as much as ten points.

Until the 1970s, most gasolines contained tetraethyl lead . Then, concerns began to grow about the release of lead to the environment during the combustion of gasoline. Lead concentrations in urban air had reached a level five to 10 times that of rural air. Residents of countries with few automobiles, such as Nepal, had only one-fifth the lead in their bodies as did residents of nations such as the United States, with many automotive vehicles.

The toxic effects of lead on the human body have been known for centuries, and risks posed by leaded gasoline became a major concern. In addition, leaded gasoline became a problem because it damaged a car's catalytic converter , which reduced air pollutants in exhaust.

Finally, in 1973, the Environmental Protection Agency (EPA) acted on the problem and set a time-scale for the gradual elimination of leaded fuels. According to this schedule, the amount of lead was to be reduced from 2 to 3 grams per gallon (the 1973 average) to 0.5 g/gal by 1979. Ultimately, the additive was to be totally eliminated from all gasoline.

The elimination of leaded fuels has been made possible by the invention of new and safer additives. One of the most popular is methyl-t-butyl ether (MTBE). By 1988 MTBE had become so popular that it was among the 40 most widely produced chemicals in the United States. In 2001, MTBE was asked to be phased out of production in California by 2003.

Yet another approach to improving fuel efficiency is the mixing of gasoline and ethyl or methanol . This product, known as gasohol , has the advantage of high octane rating , lower cost, and reduced emission of pollutants, compared to normal gasoline.

See also Air pollution; Alternative fuels

[David E. Newton ]