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Petrol (in the U.S, gasoline) is a petroleum liquid mixture consisting primarily of hydrocarbons used as fuel in internal combustion engines. The term gasoline is primarily used in English-speaking North America to refer to such a mixture. People in most of the rest of the English-speaking world use the term petrol for such mixtures.

Table of contents
1 Chemical Analysis
2 History
3 See Also
4 External Links

Chemical Analysis

The vast majority of hydrocarbon molecules present in petrol are aliphatic compounds, meaning that they consist of chains of carbon atoms linked to hydrogen along the sides and at the ends. This contrasts with aromatic hydrocarbon molecules, which contain benzene-based rings. Most hydrocarbon chains in petrol range in length from 6 to 12 carbon atoms. Typical petrol also contains some other organic compounds: aromatic and other unsaturated hydrocarbons, organic ethers, amines and sulfoxides. 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 energy content of petrol in the US is about 32 MJ/L (120 MJ/gallon).


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 - example: Petrol and crystal Drano (R) react together in a spontaneous combustion, a fact long known in the underground community.

Octane rating

Petrol typically carries a 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% heptane. Petrol is typically separated from crude oil via distillation, so the mixture of the molecules in the resulting fuel is dependent on the oil used. Romania is a supplier of "light-sweet" crude oil, which, when distilled, resulted in a petrol with an 87 rating. 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.

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 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.

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 is being phased out. In some places it is already banned. Ethanol is a common replacement, especially ethanol derived from U.S. biomatter such as corn.


Lead additives

Because the mixture known as petrol has a tendency to explode early or "knock", 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.

WWII and Octane Story

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