Gasoline is a refined product of petroleum consisting of a mixture of hydrocarbons, additives, and blending agents. The composition of gasolines varies widely, depending on the crude oils used, the refinery processes available, the overall balance of product demand, and the product specifications.
The typical composition of gasoline hydrocarbons (% volume) is as follows: 4-8% alkanes; 2-5% alkenes; 25-40% isoalkanes; 3-7% cycloalkanes; l-4% cycloalkenes; and 20-50% total aromatics (0.5-2.5% benzene). Additives and blending agents are added to the hydrocarbon mixture to improve the performance and stability of gasoline. These compounds include anti-knock agents, anti-oxidants, metal deactivators, lead scavengers, anti-rust agents, anti-icing agents, upper-cylinder lubricants, detergents, and dyes. At the end of the production process, finished gasoline typically contains more than 150 separate compounds although as many as 1,000 compounds have been identified in some blends.
What is Gasoline?
Gasoline is an extremely flammable fuel source for automobiles and other vehicles and equipment. A liquid, it can be colorless, pale brown or pale pink.
Gasoline is produced by refining petroleum, and it consists of a complex mixture of over 150 chemicals. The actual make-up of these chemicals varies by petroleum source, manufacturer, and even the time of year. Primary chemicals are benzene, toluene, ethylbenzene, xylenes; and oxygenates, including methyl tert-butyl ether (MTBE).
There are two ways to describe a material, by its chemical composition and by its properties. Both viewpoints can be instructive. Gasoline is a complex mixture of hundreds of hydrocarbons that vary by class, including paraffins, olefins, naphthenes, and aromatics. Within each class, hydrocarbons also vary by size. The mixture of hydrocarbons (and oxygenates) in a gasoline determines its physical properties and engine performance characteristics.
Gasoline is manufactured to meet property limits prescribed by specifications and regulations, not to achieve a specific distribution of hydrocarbons by class and size. To varying degrees, property limits define chemical composition. Consider, for example, gasoline volatility.
Chapter 1, “Gasoline and Driving Performance,” explains that gasoline boils (distills) over a range of temperatures (its distillation profile) and that gasoline specifications limit this range. Each individual hydrocarbon boils at a specific temperature, or boiling point, and in general, the boiling point increases with molecular size. Consequently, requiring a distillation profile is equivalent to requiring a population of hydrocarbons with a range of sizes. The temperature limits of a distillation profile exclude smaller hydrocarbons with lower boiling points and larger hydrocarbons with higher boiling points.
The most common way to characterize the size of a molecule is molecular weight. For a hydrocarbon, an alternate method is by carbon number, that is, the number of carbon atoms in its molecular structure. Butane, for example, has a molecular weight of 58 grams per gram-mole (g/g-mole) and a carbon number of 4 (C4). Benzene has a molecular weight of 78 g/g-mole and a carbon number of 6 (C6).
Figure 3.1 shows the carbon number distributions for typical regular unleaded and premium unleaded gasolines. Note that the range of sizes runs from C4 to C12 with the most prevalent size being C5 for regular and C8 for premium and the average size being C6.9 for regular and C7.1 for premium. The average molecular weight of gasoline ranges from 92 to 95. Figure 3.2 shows, as expected, that gasoline’s cumulative carbon number distribution parallels the distillation profile.
Fig 1. Distillation Profile vs. Cumulative Carbon Number Distribution