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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.
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 form rings of atoms. Most hydrocarbon chains in gasoline range in length from 6 to 12 carbon atoms. Typical
gasoline also contains some other organic compounds: aromatic and
other unsaturated hydrocarbons, organic ethers, amines and sulfoxides. Gasoline 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.
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 should the gasoline 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. Petrol also readily evaporates, meaning that storage tanks on land and
in vehicles must be properly sealed. However, certain measures must be in place to allow appropriate venting to prevent the
buildup of pressure inside or outside.
The energy content of petrol in the US is about 32 MJ/L (120 MJ/gallon).
Octane rating
Petrol typically carries an 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 was 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 an octane rating 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:
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 componds such as MTBE or Ethanol. This oxygen in the
fuel reduces the amount of carbon monoxide and unburnt 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.
History
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.
Petrol pumps in Germany
WWII and Octane Story
One interesting historical issue involving octane rating took place during WWII.
Germany received the vast majority of their 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 their 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
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