Crude oil can be separated into different fractions for diverse uses via fractional distillation. In general, gasoline represents the fraction with a boiling range of -1 to 180 °C; it may include about 500 compounds in the C3-C12 range. The gasoline fraction mainly contains saturated hydrocarbons or alkanes, unsaturated hydrocarbons, naphthene or cyclic hydrocarbons, aromatics, oxygenates, and other heteroatom-containing compounds. While saturated hydrocarbons are thermally and chemically stable, alkenes or unsaturated hydrocarbons are unstable, toxic, and have high octane ratings. Aromatic hydrocarbons are still more toxic and possess the highest octane numbers. Small polycyclic aromatic hydrocarbons (PAHs) such as naphthalene are at relatively low levels in gasoline. No large, highly toxic, multiringed PAHs are present in gasoline at significant concentrations. Oxygenates, which do not contribute energy content, provide reasonable antiknock value. Therefore, oxygenates are a reasonably good substitute for the aromatics. Several other additives are also used to prevent knocking and improve octane number. The components of gasoline depend on location and season which alter evaporative emissions and derivability. According to altitude and temperatures, the volatility can be adjusted.
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