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Most people don't give car fuel much thought, but it is quite a complex mixture of hydrocarbons

by Stephen K. Ritter
February 21, 2005 | A version of this story appeared in Volume 83, Issue 8


As the price of gasoline reached record levels in the U.S. this past year, some people may have become more curious about what exactly they were pumping into their cars. In a nutshell, gasoline is a mixture of C4 to C12 hydrocarbons specially blended with a few additives to meet the performance needs of automobile engines.

That doesn't sound too complicated, but in actuality gasoline is quite complex, consisting of several hundred compounds. The composition of gasoline can vary widely depending on the blending specifications required for different regions based on climate and environmental regulations. The trick, as one source puts it, is to formulate a gasoline that "does not cause engines to knock apart, does not cause vapor lock in summer but is easy to start in winter, does not form gums and deposits, burns cleanly without forming soot or residues, and does not dissolve or poison the car catalyst or owner."

The raw material for gasoline, at least for now, is crude oil, which can contain as many as 100,000 compounds ranging from methane to those having 85 carbon atoms. In a refinery, some of the major crude oil fractions obtained upon initial distillation are "light ends," such as propane and butane; "straight run" gasoline, which is mostly C5 and C6 alkanes, the higher boiling part of which is sometimes called naphtha; kerosene; diesel fuel; heating oil; and lubricating oils. There also are some nondistillable residues.

Several refinery processes follow the distillation to produce the blending components used to make gasoline. Some of the heavier fractions undergo fluid catalytic cracking to break down larger compounds into smaller compounds, usually branched alkanes. Hydrocracking is a slightly different process that adds hydrogen to unsaturated hydrocarbons as it cracks them. Catalytic desulfurization and denitrogenation use hydrogen to strip out sulfur and nitrogen, usually from aromatic compounds.

Other reactions, typically carried out on the naphtha fraction, include dehydrogenations, dealkylations, cyclizations, and isomerizations. When performed in series, these reactions are collectively known as reforming, and the product, called reformate, is rich in aromatics and branched alkanes.

Alkylation, the opposite of cracking, is a catalytic process that adds an alkane to an olefin, such as isobutane to propylene or butene. The product, called alkylate, is mostly a mix of trimethylpentanes and dimethylhexanes. Polymerization reactions also are used to join propenes and butenes to form pentanes and hexanes.

Gasoline in the U.S. is usually blended from straight run gasoline, reformate, alkylate, and some butane. The approximate composition is 15% C4–C8 straight-chain alkanes, 25 to 40% C4–C10 branched alkanes, 10% cycloalkanes, less than 25% aromatics (benzene less than 1.0%), and 10% straight-chain and cyclic alkenes.

Two important measures for gasoline are the Reid vapor pressure and the octane number. Gasoline has to be volatile enough to vaporize and mix with air to burn, but one problem is that the vapor pressure can go up or down with a change in temperature or with altitude. If the vapor pressure is too high, a vapor lock can occur and prevent the flow of gasoline; if it's too low, the engine might not perform well in cold weather. One way to control vapor pressure is by adding more or less butane.

Octanes are an important component of gasoline because they help provide smoother combustion in the car's cylinders and prevent knocking. Knocking-- the pinging noise sometimes heard from an engine--is caused by irregular pressure waves inside a cylinder that arise from nonuniform combustion. Unchecked, knocking can crack the cylinder heads or pistons and destroy an engine.

In the 1920s, it was discovered that straight-chain alkanes cause more knocking than branched alkanes, and tetraethyllead and other compounds were introduced to help reduce knocking. Tetraethyllead was phased out in the U.S. by 1986, in part because the environmental toxicity of lead was a concern but also because the lead fouled catalytic converters. Alkylate has largely replaced tetraethyllead as an octane enhancer.

Research octane number (RON) and motor octane number (MON) are two measures of octane activity that evolved over time. They are based on how well an engine performs in tests with different ratios of 2,2,4-trimethylpentane (isooctane) to n-heptane; the higher the number, the more the fuel performs like isooctane, which was given an octane rating of 100. Because the ratings are measured under different driving conditions, an average of the RON and MON--known as the antiknock index--is used. In the U.S., the index generally ranges from 87 to 95, which are numbers you see on the gas pump.

Catalytic converters were introduced to cars in the 1970s to help reduce the emissions of unburned fuel, carbon monoxide, and nitrogen oxides. Beginning in 1995, reformulated gasoline containing oxygenates such as methyl tert-butyl ether (MTBE) or ethanol was introduced to aid in more complete combustion to meet national air-quality standards.

The toxicity of MTBE is a concern, so it's in the process of being phased out. Gasoline meeting the standards can be made by increasing the amount of alkylate and not adding any MTBE or ethanol. But federal regulations likely will continue to require oxygenates in some areas.

Finally, a handful of additives are used to improve the performance and stability of gasoline. These include antioxidants, metal deactivators, antirust agents and corrosion inhibitors, anti-icing agents, antiwear lubricants, detergents, and dyes.

All told, gasoline is the most important product coming out of a refinery. In the U.S., it's the heart of a petroleum company's business, and about half of each 42-gal barrel of oil becomes gasoline. By contrast, refineries in Europe produce about half as much gas per barrel because diesel cars are more common.



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