Gasoline

Gasoline—called petrol in most Commonwealth countries—is a transparent, flammable petrochemical liquid widely used as a fuel for spark-ignited internal combustion engines. It is obtained by refining crude oil and further enhanced with additives to improve performance, stability, and engine compatibility. As one of the highest-volume products of modern refineries, gasoline is central to global transportation systems and industrial economies.
Gasoline consists mainly of organic compounds produced through fractional distillation, chemical conversion, and blending operations within oil refineries. Its quality and behaviour in engines depend on its chemical composition, volatility, and octane rating, a measure of resistance to premature ignition or “knocking”.

History and Etymology

The term gasoline appears in nineteenth-century English and is thought to derive from gas combined with chemical suffixes such as -ol or -ine. The term petrol originates from Medieval Latin petroleum (“rock oil”).
Interest in gasoline emerged alongside the development of early internal combustion engines. Late nineteenth-century Otto-cycle engines initially used highly volatile hydrocarbons derived from coal gas. Advances such as the spray-nozzle carburettor allowed the use of less volatile fuels, but early engines suffered from knocking at high compression ratios.
A turning point came in 1891 with the introduction of the Shukhov cracking process, the first commercial method for breaking heavy oil molecules into lighter fractions suitable for transportation fuels. Throughout the early twentieth century, further refinement technologies and the growth of the automotive industry increased demand for specialised gasoline blends.
For decades, tetraethyl lead was widely added to gasoline to boost octane ratings, but it has been phased out for road use due to the dangers of lead poisoning. Leaded grades still appear in limited aviation, racing, and off-road applications.

Chemical Composition and Refinery Production

Commercial gasoline is a complex mixture of hydrocarbons generally containing between four and twelve carbon atoms per molecule (C4–C12). It is composed of:

• Paraffins (alkanes)
• Olefins (alkenes)
• Naphthenes (cycloalkanes)
• Aromatics

The exact composition varies depending on crude oil quality, refinery configuration, seasonal requirements, and desired octane rating. Modern refineries combine multiple processing streams to create finished gasoline:

• Straight-run gasoline (naphtha): distilled directly from crude oil; low octane; partly reformed or isomerised before blending.
• Reformate: high-octane aromatic-rich stream from catalytic reforming; major source of BTX (benzene, toluene, xylene).
• Catalytic cracked gasoline: produced from fluid catalytic cracking; moderate octane with high olefin content.
• Hydrocrackate: derived from hydrocracking; moderate aromatic levels and mid-range octane.
• Alkylate: high-quality blending component produced from isobutane and light olefins; has high motor octane and no aromatics.
• Isomerate: produced by isomerising low-octane naphtha; contains isoparaffins with moderate octane values.
• Butane: added in controlled quantities to meet vapour-pressure specifications.

Oxygenated compounds—including ethanol, MTBE, and ETBE—are commonly added to improve combustion efficiency and reduce emissions. Ethanol blending is widespread in both the United States and Europe, while MTBE use has declined significantly due to environmental concerns.
Regulations increasingly limit aromatic and olefin content, particularly benzene, which in the European Union must not exceed one per cent by volume. This has raised demand for high-octane paraffinic components such as alkylate and isomerate.

Physical Properties and Stability

Gasoline’s specific gravity ranges from about 0.71 to 0.77, with higher values indicating greater aromatic content. Because it is less dense than water, gasoline floats, and this affects both environmental behaviour and fire-fighting methods; water cannot extinguish gasoline fires unless delivered as a fine mist.
Shelf-life depends on storage conditions. Fresh gasoline typically remains stable for about six months if properly stored. After a year, fuel degradation can affect engine performance. Old fuel is often usable when diluted with new gasoline, but prolonged deterioration can lead to misfiring, injector malfunction, or damage to engine components.

Octane Rating and Engine Performance

Octane rating measures a fuel’s capacity to resist knocking—premature combustion caused by pressure rather than spark ignition. Higher-octane fuels allow engines to operate at higher compression ratios, improving efficiency and performance. Modern formulations achieve octane targets primarily through refining processes and blending oxygenates rather than through lead additives.
Two common octane metrics are:

• Research Octane Number (RON)
• Motor Octane Number (MON)

Both values help determine the suitability of gasoline for specific engines and driving conditions.

Additives and Environmental Considerations

Gasoline blends contain a wide range of additives such as stabilisers, corrosion inhibitors, detergents, and anti-icing agents. These improve fuel system cleanliness, prevent oxidation, and enhance long-term storage performance.
Environmental concerns arise from both production and use. Gasoline vapours contribute to air pollution and greenhouse gas emissions, while refinery operations and transportation systems create additional environmental burdens. Regulations on evaporative emissions, sulphur content, and aromatic levels aim to lessen these impacts.

Modern Refining Output

A typical United States refinery yields about:

• 19–20 gallons of gasoline
• 11–13 gallons of diesel
• 3–4 gallons of jet fuel