Gasoline and other petroleum fuels

Gasoline is the most important product of petroleum and is the dominant transportation fuel in the world in the twenty-first century. Other petroleum products with important fuel uses include kerosene (usually refined to jet fuel), diesel oil for railway locomotives and trucks, and heating oils.

Background

Petroleum is the source of nearly all the world’s transportation fuels: gasoline for automobiles, light trucks, and light aircraft; jet fuel for airplanes; and diesel fuel for locomotives, heavy trucks, and agricultural vehicles. Heating oils (also called fuel oils or furnace oils) are used for domestic heating and industrial process heat; they are also used in oil-fired electric generating plants. fuels are a vital component of the energy economies of industrialized nations.

The first step in making all petroleum fuels is distillation of the petroleum or crude oil. Kerosene, diesel oil, and heating oils require comparatively little refining thereafter to be ready for marketing. Considerable effort is put into gasoline production, both to ensure adequate engine performance and to guarantee that sufficient quantities will be available to meet market requirements.

Gasoline

The most important characteristic of gasoline is its combustion performance. When gasoline is ignited in the cylinder, the pressure rises as combustion proceeds. The pressure can, potentially, get so high that the remaining unburned gasoline-air mixture detonates rather than continuing to burn smoothly. The explosion, which can readily be heard, is usually called “engine knock.” Engine knock puts undue mechanical stresses on the engine components, is wasteful of fuel (which the driver will experience as reduced mileage), and reduces engine performance, such as acceleration. Several factors contribute to engine knock. One is the compression ratio of the engine—the ratio of volumes of the cylinder when the piston is at the upward and downward limits of its stroke. Generally, the higher the compression ratio, the more powerful the engine and the greater the acceleration and top speed of the car. A higher compression ratio results in higher pressures inside the cylinder at the start of combustion. If the cylinder pressure is higher to begin with, the engine is more likely to knock.

A second characteristic affecting knocking tendency is the nature of the fuel. The dominant family of chemical components of most gasoline is paraffins. These compounds contain carbon atoms arranged in chains, either straight (the normal paraffins) or with branches (isoparaffins). Normal paraffins have a great tendency to knock, whereas branched paraffins do not. An octane rating scale was established by assigning the normal paraffin heptane the value 0 and the isoparaffin “iso-octane” (2,2,4-trimethylpentane) the value 100. The octane rating of a gasoline is found by comparing its knocking characteristics (in a carefully calibrated and standardized test engine) to the behavior of a heptane/iso-octane blend. The percentage of iso-octane in a blend having the same knocking behavior of the gasoline being tested is the octane number of the gasoline. Gasoline is sold in three grades: regular gasoline with octane number 87, a premium gasoline of about 93 octane, and a medium grade of about 89 octane.

Another important property of gasoline is its ability to vaporize in the engine, measured by the vapor pressure of the gasoline. Gasoline with high vapor pressure contains many components that vaporize easily. This is desirable for wintertime driving in cold climates since easy vaporization helps the engine to start when it is cold. It is not desirable for driving in hot weather, because the gasoline could vaporize in the fuel system before it gets to the engine, leading to the problem of vapor lock, which temporarily shuts down the engine. Oil companies adjust the vapor pressure of their gasoline depending on the region of the country, the local climate, and the season of the year.

Many process streams within a refinery are blended to produce the gasoline that actually appears on the market. Gaseous molecules that would be by-products of refining can be recombined to produce gasoline in processes called alkylation or polymerization. Some gasoline, called straight-run gasoline, comes directly from distillation of the petroleum. Refinery streams of little value can be converted into high-octane gasoline by catalytic cracking. The octane numbers of straight-run gasoline, or a related product called straight-run naphtha, can be enhanced by catalytic reforming. Other refinery operations can also yield small amounts of material boiling in the gasoline range. Various of these streams are blended to make products of desired octane, vapor pressure, and other characteristics.

Environmental concerns about gasoline have centered on the emission of unburned hydrocarbons (including evaporation from fuel tanks), carbon monoxide and nitrogen emissions from combustion, and the presence of aromatic compounds, some of which are suspected carcinogens and contribute to smoke or soot formation. These concerns have led to the development of reformulated gasoline. One aspect of production of reformulated gasoline is increased vapor pressure, which retards evaporation. A second is the removal of aromatic compounds; removal actually complicates formulation because aromatics have desirably high-octane numbers. A third step is the addition of oxygen-containing compounds, oxygenates, which serve several purposes: They reduce the flame temperature, for example, and change the combustion chemistry to reduce formation of carbon monoxide and nitrogen oxides. Oxygenates also have high octane numbers, so they can make up for the loss of aromatics. An example of an oxygenate useful in reformulated gasoline is methyl tertiary-butyl ether.

Jet Fuel

Jet fuel is produced by refining and purifying kerosene. Kerosene is a useful fuel, particularly for some agricultural vehicles, but the most important fuel use of kerosene today is for jet aircraft engines. Because many jet planes fly at high altitudes, where the outside air temperature is well below zero, the flow characteristics of the fuel at very low temperatures are critical. When the fuel is cooled, large molecules of paraffins settle out from the fuel as a waxy deposit. The temperature at which the formation of this wax first begins, noticeable as a cloudy appearance, is called the cloud point. Eventually, fuel can be cooled to an extent where it cannot even flow, not even to pour from an open container. This characteristic temperature is the pour point.

Smoke emissions from jet engines are an environmental concern. The “smoke point” measures an important property of jet fuel combustion. Aromatics are the most likely compounds to produce smoke, while paraffins have the least tendency. A jet fuel with a low smoke point will have a high proportion of paraffins relative to aromatics. The sulfur content of jet fuel can be important, both to limit emissions of sulfur oxide to the and because some sulfur compounds are corrosive. Both sulfur and aromatics contents of jet fuel can be reduced by treating with hydrogen in the presence of catalysts containing cobalt, or nickel, and molybdenum.

Diesel Fuel

A familiar automobile engine operates by igniting the gasoline-air mixture with a spark plug. Diesel engines operate differently: They have no spark plugs but rely on compression heating of the air in the cylinder to ignite the fuel. A diesel engine has a much higher compression ratio than a comparable spark-ignition engine. In a crude sense, a diesel engine actually operates by knocking. The desirable composition for diesel fuel is essentially the inverse of that for gasoline: Normal paraffins are ideal components, while iso-paraffins and aromatics are not. The combustion behavior of a diesel fuel is measured by the cetane number, based on a blend of cetane (hexadecane), assigned a value of one hundred, and alpha-methylnaphthalene, assigned zero, as the test components. A typical diesel fuel for automobile and light truck engines would have a cetane rating of about fifty.

Many of the physical property characteristics of jet fuel are also important for diesel fuel, including the cloud and pour points and the flow characteristics (viscosity) at low temperatures. Sulfur and aromatic compounds are a concern. Aromatics are particularly undesirable because they are the precursors to the formation of soot. As environmental regulations continue to become more stringent, refiners will face additional challenges to reduce the levels of these components in diesel fuels.

Heating Oils

Heating oils, also called furnace oils or fuel oils, are often graded and sold on the basis of viscosity. The grades are based on a numerical classification from number 1 to number 6 (though there is no number 3 oil). As the number increases, so do the pour point, the sulfur content, and the viscosity. Number 1 oil is comparable to kerosene. Number 2 is an oil commonly used for domestic and industrial heating. Both have low pour points and sulfur contents and are produced from the distillation of petroleum. The other oils (numbers 4-6) are obtained by treating the residuum from the distillation process. They are sometimes called bunker oils because they have such high viscosities that they may have to be heated to have them flow up, from the storage tank, or bunker, and into the burners in the combustion equipment.

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