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Petrol (petroleum spirit) redirects here. For the seabird, see petrel, spelt with an 'e'.

Gasoline, as it is known in North America, or petrol (abbreviated from petroleum spirit), in many Commonwealth countries (sometimes also called motor spirit) is a petroleum-derived liquid mixture consisting primarily of hydrocarbons, used as fuel in internal combustion engines. The term gasoline is commonly used within the oil industry, even within companies that are not American. The word "gasoline" is commonly shortened in colloquial usage to "gas". The term mogas, short for motor gasoline, for use in cars is used to distinguish it from avgas, aviation gasoline used in (light) aircraft. This should be distinguished in usage from genuinely gaseous fuels used in internal combustion engines such as LPG.


Chemical analysis and production

Gasoline is produced in oil refineries. These days, material that is simply separated from crude oil via distillation, called natural gasoline, will not meet the required specifications (in particular octane rating; see below) for modern engines, but these streams will form part of the blend.

The bulk of a typical gasoline consists of hydrocarbons with between 5 and 12 carbon atoms per molecule.

The various refinery streams that are blended together to make gasoline all have different characteristics. Some important streams are:

  • Reformate, produced in a catalytic reformer with a high octane and high aromatics content, and very low olefins (alkenes).
  • Cat Cracked Gasoline or Cat Cracked Naphtha, produced from a catalytic cracker, with a moderate octane, high olefins (alkene) content, and moderate aromatics level. Here, "cat" is short for "catalyst".
  • Hydrocrackate (Heavy, Mid, and Light), produced from a hydrocracker, with medium to low octane and moderate aromatic levels.
  • Natural Gasoline (has very many names), directly from crude oil with low octane, low aromatics (depending on the crude oil), some naphthenes (cycloalkanes) and zero olefins (alkenes).
  • Alkylate, produced in an alkylation unit, with a high octane and which is pure paraffin (alkane), mainly branched chains.
  • Isomerate (various names) which is made by isomerising Natural Gasoline to increase its octane rating and is very low in aromatics and benzene content.

(The terms used here are not always the correct chemical terms. Typically they are old fashioned, but they are the terms normally used in the oil industry. The exact terminology for these streams varies by oil company and by country.)

Overall a typical gasoline is predominantly a mixture of paraffins (alkanes), naphthenes (cycloalkanes), aromatics and olefins (alkenes). The exact ratios can depend on

  • the oil refinery that makes the gasoline, as not all refineries have the same set of processing units.
  • the crude oil used by the refinery on a particular day.
  • the grade of gasoline, in particular the octane.

These days, gasoline in many countries has tight limits on aromatics in general, benzene in particular, and olefins (alkene) content. This is increasing the demand for high octane pure paraffin (alkane) components, such as alkylate, and is forcing refineries to add processing units to reduce the benzene content.

Gasoline can also contain some other organic compounds: such as organic ethers, (deliberately added) plus small levels of contaminants, in particular sulfur compounds such as disulfides and thiophenes. Some contaminants, in particular mercaptans and hydrogen sulfide must be removed because they cause corrosion in engines.


Gasoline is more volatile than diesel or kerosene, not only because of the base constituents, but because of the additives that are put into it. The final control of volatility is often via blending of butane. The desired volatility depends on the ambient temperature: In hotter climates, gasoline components of higher molecular weight and thus lower volatility are used. In Australia the volatility limit changes every month and differs for each main distribution center, but most countries simply have a summer, winter and perhaps intermediate limit.

The maximum volatility of gasoline in many countries has been reduced in recent years to reduce the fugitive emissions during refueling.

Volatility standards may be relaxed (allowing more gasoline components into the atmosphere) during emergency anticipated gasoline shortages. For example, on 31 August 2005 in response to Hurricane Katrina, the United States activated an early switch to "winter gasoline" which has a volatility limit exceeding the usual summertime standard. As mandated by EPA administrator Stephen L. Johnson, this "fuel waiver" was made effective through 15 September 2005 [1]. Though relaxed volatility standards negatively impact ozone and other air quality criteria, higher volatility gasoline (which contains less additives than gasoline whose volatility has been artificially lowered) essentially increases a nation's gasoline supply.

Octane rating

For more details on this topic, see octane rating.

The most important characteristic of gasoline is its octane rating, which is a measure of how resistant gasoline is to premature detonation (knocking). It is measured relative to a mixture of 2,2,4-trimethylpentane (an isomer of octane) and n-heptane. An 87-octane gasoline has the same knock resistance as a mixture of 87% isooctane and 13% n-heptane. The octane rating system was developed by the chemist Russell Marker.


Many of the non-aliphatic hydrocarbons naturally present in gasoline (especially aromatic ones like benzene), as well as many anti-knocking additives, are carcinogenic. Because of this, any large-scale or ongoing leaks of gasoline pose a threat to the public's health and the environment, should the gasoline reach a public supply of drinking water. The chief risks of such leaks come not from vehicles, but from gasoline delivery truck accidents and leaks from storage tanks. Because of this risk, most (underground) storage tanks now have extensive measures in place to detect and prevent any such leaks, such as sacrificial anodes. Gasoline is rather volatile (meaning it readily evaporates), requiring that storage tanks on land and in vehicles be properly sealed. But the high volatility also means that it will easily ignite in cold weather conditions, unlike diesel for example. However, certain measures must be in place to allow appropriate venting to ensure the level of pressure is similar on the inside and outside. Gasoline also reacts dangerously with certain common chemicals; for example, gasoline and crystal Drāno (sodium hydroxide) react together in a spontaneous combustion.

Gasoline is also one of the sources of pollutant gases. Even gasoline which does not contain lead or sulfur compounds produces carbon dioxide, nitrogen oxides, and carbon monoxide in the exhaust of the engine which is running on it.

Through misuse as an inhalant, gasoline also contributes to damage to health. "Petrol sniffing" is a common way of obtaining a high for many people and has become epedemic in many poorer communities such as with Indigenous Australians. In response, Opal fuel has been developed by the BP Kwinana Refinery in Australia, and contains only 5% aromatics (unlike the usual 25%) which inhibits the effects of inhalation.

Energy content

Gasoline contains about 45 megajoules per kilogram (MJ/kg)
Volumetric energy density of some fuels compared to gasoline:

fuel type     MJ/L     BTU/imp gal     BTU/US gal     Research octane
number (RON)
gasoline 29.0 150,000 125,000 91–98
LPG 22.16 114,660 95,475 115
ethanol 19.59 101,360 84,400 129
methanol 14.57 75,420 62,800 123
gasohol (10% ethanol + 90% gasoline) 28.06 145,200 120,900 93/94

A high octane fuel such as LPG has a lower energy content than lower octane gasoline, resulting in an overall lower power output at the regular compression ratio an engine ran at on gasoline. However, with an engine tuned to the use of LPG (ie. via higher compression ratios such as 12:1 instead of 8:1), this lower power output can be overcome. This is because higher-octane fuels allow for a higher compression ratio - this means less space in a cylinder on its combustion stroke, hence a higher cylinder temperature, less wasted hydrocarbons (therefore less pollution and wasted energy), and therefore higher power levels coupled with less pollution overall because of the greater efficiency.

Note that the main reason for the lower energy content (per litre) of LPG in comparison to gasoline is that is has a lower density. Energy content per kilogram is higher than for gasoline (higher hydrogen to carbon ratio). In lay terms, we burn mass, not volume!

As an interesting side note different countries have some variation in what RON is standard for gasoline, or petrol. In the UK, ordinary premium unleaded petrol is always 95 RON whereas super unleaded is usually 97-98 RON. In the US, octane ratings in fuels can vary between 87 AKI (92 RON) for regular, through 90 (95) for mid-grade (European Premium), up to 93/94 for premium unleaded or E10 (Super in Europe)



The mixture known as gasoline when used in high compression internal combustion engines, has a tendency to explode early ( pre-ignition pre-detonation) causing a disturbing "engine knocking" (also called "pinging") noise. Early research into this effect was led by A.H. Gibson and Harry Ricardo in England and Thomas Midgley and Thomas Boyd in the United States. The discovery that lead additives modified this behavior led to the widespread adoption of the practice in the 1920s and hence more powerful higher compression engines. The most popular additive was tetra-ethyl lead. However, with the recognition of the environmental damage caused by the lead, and the incompatibility of lead with catalytic converters, this practice began to wane in the 1980s. Most countries are phasing out leaded fuel; different additives have replaced the lead compounds. The most popular additives include aromatic hydrocarbons, ethers and alcohol (usually ethanol or methanol).

In the U.S., where lead has been blended with gasoline, primarily to boost octane levels, since the early 1920s, standards to phase out leaded gasoline were first implemented in 1973. In 1995, leaded fuel accounted for only 0.6 % of total gasoline sales and less than 2,000 tons of lead per year. Effective January 1, 1996, the Clean Air Act banned the sale of the small amount of leaded fuel that was still available in some parts of the country for use in on-road vehicles. (Fuel containing lead may continue to be sold for off-road uses, including aircraft, racing cars, farm equipment, and marine engines.) The ban on leaded gasoline was presumed to lower levels of lead in people's bloodstream and to thousands of tons of lead being removed from the air.

A side effect of the lead additives was protection of the valve seats from erosion. Many classic car's engines have needed modification to use lead-free fuels since leaded fuels became unavailable.

Gasoline, as delivered at the pump, also contains additives to reduce internal engine carbon buildups, improve combustion, and to allow easier starting in cold climates.


Methylcyclopentadienyl manganese tricarbonyl (MMT) has been used for many years in Canada and recently in Australia to boost octane. It also helps old cars designed for leaded fuel run on unleaded fuel without need for additives to prevent valve problems.

There are currently ongoing debates as to whether or not MMT is harmful to the environment and toxic to humans.

Oxygenate blending

Oxygenate blending adds oxygen to the fuel in oxygen-bearing compounds such as MTBE, ethanol and ETBE, and so reduces the amount of carbon monoxide and unburned fuel in the exhaust gas, thus reducing smog. In many areas throughout the US oxygenate blending is mandatory. For example, in Southern California, fuel must contain 2% oxygen by weight. The resulting fuel is often known as reformulated gasoline (RFG) or oxygenated gasoline.

MTBE use is being phased out due to issues with contamination of ground water. In some places it is already banned. Ethanol and to a lesser extent the ethanol derived ETBE are a common replacements. Especially ethanol derived from biomatter such as corn, sugar cane or grain is frequent, this will often be referred to as bio-ethanol. An ethanol-gasoline mix of 10% ethanol mixed with gasoline is called gasohol. An ethanol-gasoline mix of 85% ethanol mixed with gasoline is called E85. The most extensive use of ethanol takes place in Brazil, where the ethanol is derived from sugarcane. Over 3,400 million US gallons (13,000,000 m³) of ethanol mostly produced from corn was produced in the United States in 2004 for fuel use, and E85 is fast becoming available in many of the United States. The use of bioethanol, either directly or indirectly by conversion of such ethanol to bio-ETBE, is encouraged by the European Union Biofuels Directive.


Long-term U.S. gasoline prices, 1990-2005 (adjusted for inflation using the U.S. CPI).
Long-term U.S. gasoline prices, 1990-2005 (adjusted for inflation using the U.S. CPI).


Before internal combustion engines were invented in the mid-1800s, gasoline was sold in small bottles as a treatment against lice and their eggs. In those early times, the word "Petrol" was a trade name. This treatment method is no longer common, due to the inherent fire hazard and risk of dermatitis and that gasoline is a carcinogen where continued contact might develop cancerous growths.

The word petrol may be derived from Old French pétrole, meaning petroleum: see #Etymology.

Petrol is also abused as a psychoactive inhalant.


The word "gasolene" was coined in 1865 from the word gas and the chemical suffix -ine/-ene. The modern spelling was first used in 1871. The shortened form "gas" was first recorded in American English in 1905.[2] Although, Gasoline originally referred to any liquid offered for sale, sold or used as the fuel for a gasoline-powered engine, but does not include diesel fuel or liquefied gas. Methanol racing fuel would have been be classed as a type of gasoline.[3]

The word "petrol" was first used in reference to the refined substance as early as 1892 (it previously referred to unrefined petroleum), and was registered as a trade name by English wholesaler Carless, Capel & Leonard. [4] [5]

Bertha Benz got petrol for her famous drive from Mannheim to Pforzheim and back from chemists' shops. In Germany petrol is called Benzin but this is not related to her name but to the chemical Benzine.

Recent U.S. gasoline prices, 2003-2005 (not adjusted for inflation).
Recent U.S. gasoline prices, 2003-2005 (not adjusted for inflation).

World War II and octane

One interesting historical issue involving octane rating took place during WWII. Germany received nearly all its oil from Romania, and set up huge distilling plants in Germany to produce gasoline from coal. In the US the oil was not "as good" and the oil industry had to invest heavily in various expensive boosting systems. This turned out to have benefits. The US industry started delivering fuels of ever-increasing octane ratings by adding more of the boosting agents and the infrastructure was in place for post war octane agents additive industry. Good crude oil was no longer a factor during wartime and by war's end, American aviation fuel was commonly 130 to 150 octane. This high octane could easily be used in existing engines to deliver much more power by increasing the compression delivered by the superchargers. The Germans, relying entirely on "good" gasoline, had no such industry, and instead had to rely on ever-larger engines to deliver more power.

However, German aviation engines were of the direct fuel injection type and could use methanol-water injection and nitrous oxide injection, which gave 50% more engine power for five minutes of dogfight. This could be done only five times or after 40 hours run-time and then the engine would have to be rebuilt. Most German aero engines used 87 octane fuel (called B4), while some high-powered engines used 100 octane (C2/C3) fuel.

This historical "issue" is based on a very common misapprehension about wartime fuel octane numbers. There are two octane numbers for each fuel, one for lean mix and one for rich mix, rich being always greater. So, for example, a common British aviation fuel of the later part of the war was 100/125. The misapprehension that German fuels have a lower octane number (and thus a poorer quality) arises because the Germans quoted the lean mix octane number for their fuels while the Allies quoted the rich mix number for their fuels. Standard German high-grade aviation fuel used in the later part of the war (given the designation C3) had lean/rich octane numbers of 100/130. The Germans would list this as a 100 octane fuel while the Allies would list it as 130 octane.

After the war the US Navy sent a Technical Mission to Germany to interview German petrochemists and examine German fuel quality. Their report entitled "Technical Report 145-45 Manufacture of Aviation Gasoline in Germany" chemically analyzed the different fuels and concluded that "Toward the end of the war the quality of fuel being used by the German fighter planes was quite similar to that being used by the Allies".

Current use

The United States uses 360 million US liquid gallons (1.36 billion litres) of gasoline each day. Western countries have among the highest usage rates per head, while eastern developing nations as China typically have the highest usage per square mile/kilometer. Some countries, e.g. in Europe, impose heavy fuel taxes on fuels such as gasoline, leading to greater efficiency and economy in car design.

See also

External links





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