Petrol (commonly known as gasoline in North America) 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. The term mogas, short for motor gasoline, for use in cars is used to distinguish it from avgas, aviation gasoline used in light aircraft. People in the United States use 360 million U.S. liquid gallons (1.36 billion litres) of gasoline each day. The word "gasoline" is very often shortened in colloquial usage to "gas", causing risk of confusion with other fuels (natural gas) and other materials which are genuinely gases.
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 rating; see below) 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 together 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. 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 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 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 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.
Petrol 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: 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.
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 the average of the RON and the MON, sometimes called the "roaD Octane Number" or DON, or (R+M)/2. 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, because 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 lets an engine 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. Some people believe that adding a higher octane fuel to their engine will increase its performance or lessen its fuel consumption; this is false - engines perform best when using fuel with the octane rating they were designed for.
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), requiring 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 Drāno® react together in a spontaneous combustion.
Petrol is also one of the sources of pollutant gases. Even petrol which does not contain lead or sulfur compounds produces carbon dioxide, nitrogen dioxide, and carbon monoxide in the exhaust of the engine which is running on it.
Petrol contains about 45 megajoules per kilogram (MJ/kg)
Volumetric energy density of some fuels compared to petrol:
diesel fuel oil
|residential fuel oil
gasohol (10% ethanol + 90% petrol)
A high octane fuel such as LPG has a lower energy content than lower octane petrol, resulting in an overall lower power output. However, with an engine tuned to the use of LPG this lower power output can be overcome.
Note that the main reason for the lower energy content of LPG is that is has a lower density. Energy content per kilogram is actually higher than for petrol (higher hydrogen to carbon ratio). In lay terms we burn mass, not volume!
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.
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-petrol mix is called gasohol. The most extensive use of ethanol takes place in Brazil, where the ethanol is derived from sugarcane. The use of bioethanol, either directly or indirectly by conversion of such ethanol to bio-ETBE, is encouraged by the European Union Biofuels Directive.
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.
Before internal combustion engines were invented, petrol was sold in chemists' shops in small bottles as a treatment for nits. In those early times the word "petrol" was a tradename. That method of treating nits is NOT recommended for use nowadays.
The mixture known as petrol when used in high compression internal combustion engines, has a tendency to explode early ( pre-ignition pre-detonation) causing a disturbing "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 behaviour 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 1980s. Most countries are phasing out leaded fuel, different additives have replaced the lead compounds. The most popular ones include aromatic hydrocarbons, ethers and alcohol (usually ethanol or methanol).
Petrol as delivered at the pump, also contains additives to reduce internal engine carbon buildups, improve combustion, and to allow easier starting in cold climates.
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 nearly all her oil from Romania, and set up huge distilling plants in Germany to produce petrol from coal. 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.
- However, someone pointed out that: German aviation engines were of the direct fuel injection type and could use emergency methanol-water and nitrous-oxide injection, which gave 50% more engine power for 5 minutes of dogfight. This could be done only five times and then the aero engine went to the scrapyard (or after 40 hours run-time, whichever came first). Most German aero engines used 87 octane fuel (called B4), some high-powered engines used 100 octane (C2/C3)fuel.
- Another pointed out in reply that: 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 analysed the different fuels and concluded "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".