Gasoline, or petrol, consists almost entirely of low-boiling hydrocarbon molecules, with very small amounts of NSOs. Gasoline used world-wide amounts to ≈2.5 megatonnes per day, about half being consumed by the United States. Virtually all comes from petroleum. A small amount, on a world basis, is made in South Africa from coal, by the process of indirect liquefaction (Chapter 21).

Production of any petroleum product begins with distillation. Petroleum is distilled into fractions (or “cuts”) characterized by a particular boiling-point range. A single distillation cut has components of sufficiently similar properties so that the entire fraction is of reasonably consistent quality. The first liquid to come off the distillation column is gasoline [A]. Its boiling range is from the initial boiling point to 180 °C. That is, whether a petroleum product is or is not gasoline is determined by its boiling range, which, in turn, is determined largely by molecular size. Compounds in gasoline range up to about C12 in size, and consist of straight- and branched-chain alkanes, single-ring cycloalkanes and their alkylated derivatives, and benzene and alkylbenzenes.

Gasoline combustion

The property of gasoline having major importance is its performance as a fuel for spark-ignition internal combustion engines. By far the dominant use of gasoline is as fuel for automobiles and light trucks, and the dominant engine in these vehicles operates on the four-stroke cycle (Figure 14.1) developed by Nikolaus Otto (Figure 14.2) about 125 years ago [B].

In the intake, or induction, stroke, the piston travels downward, and a fuel–air mixture is drawn or injected into the cylinder. In the second stroke, compression, the piston moves upward and, as the name implies, compresses the fuel–air mixture in the cylinder. In the third stroke, called the ignition, working, or power stroke, an electric spark ignites the compressed fuel–air mixture. As the mixture burns, temperature and pressure inside the cylinder rise. Increased pressure pushes the piston downward. This stroke converts the chemical energy in the gasoline molecules into the mechanical work that propels the vehicle. The piston comes back up in the exhaust stroke, sweeping the products of combustion out of the cylinder.