Petrol (gasoline in the United States and Canada) is a petroleum-derived liquid mixture consisting primarily of hydrocarbons, used as fuel in internal combustion engines. The term gasoline is the common usage within the oil industry, even within companies that are not American. Often the term mogas (short for motor gasoline, for use in cars) is used to distinguish it from avgas, used in light aircraft.

Chemical analysis and production

Petrol 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) for modern engines, but these streams will form part of the blend.

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

The various refinery streams that are blended togther to make petrol 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.
  • 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 a 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.

(Note that the terms used here are not always the correct chemical terms, typically they are old fashioned, but they are the terms normally used within the industry. Also note that the exact terminology for these streams varies by oil company and by country.)

Overall a typical petrol 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 petrol, 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 petrol 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.

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


Petrol is a more volatile fuel than diesel or kerosene. The reason for this is not only the base constituents, but 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: the hotter the weather, the lower the volatility. In Australia the volatility limit changes every month and differs for each main distribution centre, but most countries simply have a summer, winter and perhaps intermediate limit.

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

Octane rating

The most important characteristic of petrol is its Research Octane Number (RON) or octane rating, which is a measure of how resistant petrol is to premature detonation (knocking). It is measured relative to a mixture of isooctane (2,2,4-trimethylpentane) and n-heptane. So an 87-octane petrol has the same knock resistance as a mixture of 87% isooctane and 13% n-heptane.

There is another type of Octane, called "Motor Octane Number" (MON), which is a better measure of how the fuel behaves when under load. Its definition is also based on the mixture of isooctane and n-heptane that has the same performance. Depending on the composition of the fuel, the MON of a modern petrol will be about 10 points lower than the RON. Normally fuel specifications require both a minimum RON and a minimum MON.

In most countries (including all of Europe and Australia) the 'headline' octane that would be shown on the pump is the RON: but in the [United States] and some other countries the headline number is in fact the average of the RON and the MON, sometimes called the "Road Octane Number" or DON. Because of the 10 point difference noted above this means that the octane in the [United States] will be about 5 points lower than the same fuel elsewhere: 87 octane fuel in the [United States] would be 92 in Europe

It is possible for a fuel to have a RON greater than 100. This reflects the fact that isooctane is not the most knock-resistant substance available. Racing fuels, Avgas and LPG typically have octane ratings of 110 or significantly higher.

It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. Using a fuel with a higher octane allows an engine to be run at a higher compression ratio without having problems with knock. Compression is directly related to power, so engines that require higher octane usually deliver more power. Some high-performance engines are designed to operate with a compression ratio associated with high octane numbers, and thus demand high-octane petrol. It should be noted that the power output of an engine also depends on the energy content of its fuel, which bears no simple relationship to the octane rating.

The octane rating was developed by the chemist Russell Marker. The selection of n-heptane as the zero point of the scale was due to the availability of very high purity n-heptane, unmixed with other isomers of heptane or octane, distilled from the resin of Jeffrey Pine. Other sources of heptane produced from crude oil contain a mixture of different isomers with greatly differing ratings, which would not give a precise zero point.


Many of the non-aliphatic hydrocarbons naturally present in petrol (especially aromatic ones like benzene), as well as many anti-knocking additives, are carcinogenic. Because of this, any large-scale or ongoing leaks of petrol pose a threat to the public's health should the petrol reach a public supply of drinking water. The chief risks of such leaks come not from vehicles, but from petrol delivery truck accidents and leaks from underground 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. Petrol is rather volatile (meaning it readily evaporates), meaning that storage tanks on land and in vehicles must 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. Petrol also reacts dangerously with certain common chemicals; for example, petrol and crystal Drano® react together in a spontaneous combustion.

Energy content

Energy content of some fuels compared to petrol:

fuel type BTU/gallon RON
petrol 125000 87-98
LPG 95475 110
diesel fuel oil 138690  
residential fuel oil 149690  
ethanol 84400  
methanol 62800  
gasohol (10% ethanol + 90% petrol) 120900  

A high octane fuel such as LPG actually has a lower energy content than lower octane petrol, resulting in an over all lower power output. However, with an engine tuned to the use of LPG this lower power output can be overcome.


Oxygenate blending

Oxygenate blending adds oxygen to the fuel in the form of oxygen bearing compounds such as MTBE or ethanol. This oxygen in the fuel reduces the amount of carbon monoxide and unburned fuel in the exhaust gas, thus reducing smog. In many areas throughout the U.S. oxygenate blending is mandatory. For example in Southern California, fuel must contain 2% oxygen by weight.

MTBE use is being phased out due to issues with contamination of ground water. In some places it is already banned. Ethanol is a common replacement, especially ethanol derived from biomatter such as corn. An ethanol petrol mix is called gasohol. The most extensive use of ethanol takes place in Brazil, where the ethanol is derived from sugar cane.


Methylcyclopentadienyl manganese tricarbonyl (MMT) has been used for a long time 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 stem problems.


Lead additives

Because the mixture known as petrol has a tendency to explode early (causing a disturbing "knocking" noise in internal combustion engines), lead additives were first blended with fuel in the 1920s. This practice continued through the 1980s. The most popular one was tetra-ethyl lead. However, with the recognition of the environmental damage caused by the lead, and the incompatibility of lead with catalytic converters, most countries are in the process of phasing out the sale of leaded fuel, and different additives to reduce knocking are now used. Among the most popular ones are aromatics, ethers and alcohol (usually ethanol or methanol).

There are also additives to reduce internal engine carbon buildups, to improve combustion, and to allow easier starting in cold climates.

Petrol pumps in Germany

Octane rating

Romania is a supplier of "light-sweet" crude oil, which, when distilled, resulted in a petrol with an 87 rating (DON). 87 octane was the general benchmark for much of the world, and is the current standard rating for "normal" petrol in the US and Canada.

World War II and octane

One interesting historical issue involving octane rating took place during WWII. Germany received the vast majority of her oil from Romania, and set up huge distilling plants in Germany to produce petrol from it. In the US the oil was not "as good" and the oil industry instead had to invest heavily in various expensive boosting systems. This turned out to be a huge blessing in disguise. US industry was soon delivering fuels of ever-increasing octane ratings by adding more of the boosting agents, with cost no longer a factor during wartime. By war's end American aviation fuel was commonly 130 to 150 octane, which could easily be put to use in existing engines to deliver much more power by increasing the compression delivered by the superchargers. The Germans, relying entirely on "good" petrol, had no such industry, and instead had to rely on ever-larger engines to deliver more power. The result is that British and US engines consistently outperformed German ones during the war, playing no small part in the defeat of the Luftwaffe.

See also

External links

da:Benzin de:Benzin eo:Benzino es:Gasolina nl:Benzine ja:ガソリン pl:Benzyna sv:Bensin zh:汽油