The octane number of a fuel is measured in a test engine, and is defined by comparison with the mixture of 2,2,4-trimethylpentane (iso-octane) and heptane which would have the same anti-knocking capacity as the fuel under test: the percentage, by volume, of 2,2,4-trimethylpentane in that mixture is the octane number of the fuel. For example, petrol with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90. This does not mean that the petrol contains just iso-octane and heptane in these proportions, but that it has the same detonation resistance properties. Because some fuels are more knock-resistant than iso-octane, the definition has been extended to allow for octane numbers higher than 100.
Octane rating does not relate to the energy content of the fuel (see heating value). It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. Where octane is raised by blending in ethanol, energy content per volume is reduced.
It is possible for a fuel to have a Research Octane Number (RON) greater than 100, because iso-octane is not the most knock-resistant substance available. Racing fuels, avgas, liquefied petroleum gas (LPG), and alcohol fuels such as methanol or ethanol may have octane ratings of 110 or significantly higher – ethanol's RON is 129 (116 MON, 122 AKI). Typical "octane booster" gasoline additives include MTBE, ETBE, isooctane and toluene. Lead in the form of tetra-ethyl lead was once a common additive, but since the 1970s, its use in most of the industrialised world has been restricted, and its use is currently limited mostly to aviation gasoline.
* 1 Measurement methods
o 1.1 Research Octane Number (RON)
o 1.2 Motor Octane Number (MON)
o 1.3 Anti-Knock Index (AKI)
o 1.4 Difference between RON and AKI
* 2 Examples of octane ratings
* 3 Effects of octane rating
* 4 Regional variations
* 5 See also
* 6 References
* 7 Further reading
* 8 External links
 Measurement methods
 Research Octane Number (RON)
The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.
 Motor Octane Number (MON)
There is another type of octane rating, called Motor Octane Number (MON), or the aviation lean octane rating, which is a better measure of how the fuel behaves when under load, as it is determined at 900 rpm engine speed, instead of the 600 rpm for RON. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON, however there is no direct link between RON and MON. Normally, fuel specifications require both a minimum RON and a minimum MON.
 Anti-Knock Index (AKI)
In most countries, including all of those of Australia and Europe the "headline" octane rating shown on the pump is the RON, but in Canada, the United States and some other countries, like Brazil, the headline number is the average of the RON and the MON, called the Anti-Knock Index (AKI, and often written on pumps as (R+M)/2). It may also sometimes be called the Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2.
 Difference between RON and AKI
Because of the 8 to 10 point difference noted above, the octane rating shown in the United States is 4 to 5 points lower than the rating shown elsewhere in the world for the same fuel. See the table in the following section for a comparison.
 Examples of octane ratings
The RON and MON of n-heptane and iso-octane are exactly 0 and 100, by definition. The following table lists octane ratings for various other fuels.
Fuel RON MON AKI
hexadecane < -30
n-heptane (RON and MON 0 by definition) 0 0 0
diesel fuel 15–25
n-hexane 25 26 26
requirement for a typical two-stroke outboard engine 69 65 67
Pertamina "Premium" gasoline in Indonesia 88
n-butanol 92 71 83
"regular" gasoline in Australia, New Zealand, Canada and the US 91–92 82–83 87
Pertamina "Pertamax" gasoline in Indonesia 92
Shell "Super" in Indonesia 92
Pertamina "Pertamax Plus" gasoline in Indonesia 95
Shell "Super Extra" in Indonesia 95
ExxonMobil Avgas 100 99.5 (min)
"EuroSuper" or "EuroPremium" or "Regular unleaded" in Europe 95 85–86 90–91
"SuperPlus" in Germany, Great Britain, Slovenia and Spain, "SP98" in France 98 89–90 93–94
iso-octane (RON and MON 100 by definition) 100 100 100
"BP Ultimate 102" 102 93–94 97–98
t-butanol 103 91 97
toluene 111 95 103
E85 gasoline 100–5
isopropanol 118 98 108
ethanol 129 116 122
methanol 133 105 119
methane 135 122 129
hydrogen* > 130 very low
*Hydrogen does not fit well into the normal definitions of octane number. It has a very high RON and a low MON, so that it has low knock resistance in practice, due to its low ignition energy (primarily due to its low dissociation energy) and extremely high flame speed. These traits are highly desirable in rocket engines, but undesirable in Otto-cycle engines. However, as a minor blending component (e.g. in a bi-fuel vehicle), hydrogen raises overall knock resistance. Flame speed is limited by the rest of the component species; hydrogen may reduce knock because of its high thermal conductivity.
 Effects of octane rating
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Higher octane ratings correlate to higher activation energies. Activation energy is the amount of energy necessary to start a chemical reaction. Since higher octane fuels have higher activation energies, it is less likely that a given compression will cause autoignition.
It might seem odd that fuels with higher octane ratings are used in more powerful engines, since such fuels ignite less easily. However, an uncontrolled ignition is not desired in a spark ignition engine.
A fuel with a higher octane rating can be run at a higher compression ratio without causing detonation. Compression is directly related to power and to thermodynamic efficiency (see engine tuning), so engines that require higher octane usually deliver more motive power and do more work for a given BTU or calorie of fuel. Engine power is a function of the fuel, as well as the engine design, and is related to octane rating of the fuel. Power is limited by the maximum amount of fuel-air mixture that can be forced into the combustion chamber. When the throttle is partially open, only a small fraction of the total available power is produced because the manifold is operating at pressures far below atmospheric. In this case, the octane requirement is far lower than when the throttle is opened fully and the manifold pressure increases to atmospheric pressure, or higher in the case of supercharged or turbocharged engines.
Many high-performance engines are designed to operate with a high maximum compression, and thus demand high-octane premium gasoline. A common misconception is that power output or fuel mileage can be improved by burning higher octane fuel than a particular engine was designed for. The power output of an engine depends in part on the energy density of its fuel, but similar fuels with different octane ratings have similar density. Since switching to a higher octane fuel does not add any more hydrocarbon content or oxygen, the engine cannot produce more power.
However, burning fuel with a lower octane rating than required by the engine often reduces power output and efficiency one way or another. If the engine begins to detonate, that reduces power and efficiency for the reasons stated above. Many modern car engines feature a knock sensor – a small piezoelectric microphone which detects knock, and then sends a signal to the engine control unit to retard the ignition timing. Retarding the ignition timing reduces the tendency to detonate, but also reduces power output and fuel efficiency. Because of these systems, under certain condtions of high load and high temperature, a given car may produce more power with a higher octane fuel. With a lower octane fuel, these engines systems will be reducing power to control detonation, while with a higher octane fuel, the engine will produce full power. And some modern high performance engines are actually optimized for higher than pump premium (93 AKI in the US). The 2001 - 2007 BMW M3 with the S54 engine is one such car. Car and Driver magazine dyno tested a car and found that the power output increased as the AKI was increased up to approximately 96 AKI. Also, these systems can result in higher fuel mileage for cars designed for the higher octane fuels.
Most fuel stations have two storage tanks (even those offering 3 or 4 octane levels), and you are given a mixture of the higher and lower octane fuel for the intermediate grades. Premium is fuel from the higher octance tank, the minimum grade sold, is sfuel from the lower octane tank. Purchasing 91 (where offered) simply means more fuel from the higher octane tank than purchasing 89; the detergents in the fuel are often the same. But for some producers, the additive package is different between the higher and lower octane rating.
The octane rating was developed by chemist Russell Marker at the Ethyl Corporation c1926. The selection of n-heptane as the zero point of the scale was due to the availability of very high purity n-heptane, not mixed with other isomers of heptane or octane, distilled from the resin of the 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.
 Regional variations
The selection of octane ratings available at the pump can vary greatly from region to region.
* Australia: "regular" unleaded fuel is 91 RON, "premium" unleaded with 95 RON is widely available, and 98 RON fuel is also reasonably common. Shell used to sell 100 RON petrol (5% ethanol content) from a small number of service stations, most of which are located in major cities (stopped in August 2008). United Petroleum sells 100 RON unleaded fuel (10% ethanol content) at a small number of its service stations (originally only two, but it has now expanded to 19 outlets).
* Bahrain: 90 and 95 (RON), standard in all petrol station in the country and advertised as (Jayyid) for Regular or 90 and (Mumtaz) for Premium or 95, prices are BD 0.08 per liter for 90 and BD 0.1 per liter for 95, which corresponded to $0.802 & $1.027 per gallon respectively. The government announced in 2009 that it will raise the prices by 20% and 25% for regular and premium. But then they announced to cancel that plan after a wave of anger in the country.
* Finland: 95 and 98 (RON), advertised as such, at each gas station. Most cars run on 95, but 98 is available for high-performance vehicles.
* Germany: "Normal" 91 RON, "Super" 95 RON and "Super Plus" 98 RON are available practically everywhere. Big suppliers like Shell or Aral offer 100 RON gasoline (Shell V-Power, Aral Ultimate) at almost every fuel station. "Normal" 91 RON is more and more disappearing, because lower production amounts make it more expensive than "Super" 95 RON, so it is often not offered any more.
* Hong Kong: only 98 RON and 99 RON are available in the market. There have been calls to re-introduce 95 RON, but the calls have been rejected by all petrol station chains, citing that 95 RON was phased out because of market forces.
* India: India's Ordinary And Premium Petrols are of 89–91 RON. The premium petrols are generally ordinary fuels with additives, that do not really change the octane value. Two variants, "Speed 93" and "Speed 97", were launched, with RON values of 93 and 97. India's economy-class vehicles usually have compression ratios under 10:1, thus enabling them to use lower-quality petrol without engine knocking.
* Indonesia: Indonesia's "Premium" petrol rated at 88 RON and being subsidized it cost only about US$0.50/liter. Other options are "Pertamax" rated at 92 RON and the "Pertamax Plus" rated at RON 95, which is the highest octane available for automotive gasoline in Indonesia.
* Ireland: 95 RON "unleaded" is the only petrol type available through stations, although E5 (99 RON) is becoming more commonplace.
* Italy: 95 RON is the only compulsory gasoline offered (verde, "green"), only a few fuel stations (Agip, IP, IES, OMV) offer 98 RON as the premium type, many Shell and Tamoil stations close to the cities offer also V-Power Gasoline rated at 100 RON
* Malaysia: the "regular" unleaded fuel is 95 RON, "premium" fuel is rated at 97 RON(but for Shell 97 RON is V-Power), and Shell's V-Power Racing is rated at 99 RON.
* Netherlands: 95 RON "Euro" and 98 RON "Super" are sold at practically every station. Shell V-Power is a 97 RON (labelled as 95 due to the legalities of only using 95 or 98 labelling), some independent test have shown that one year after introduction it was downgraded to 95 RON, whereas in neighbouring Germany Shell V-Power consists of the regular 100 RON fuel.
* New Zealand: 91 RON "Regular" and 95 RON "Premium" are both widely available. 98 RON is available instead of 95 RON at some service stations in larger urban areas.
* Russia and CIS countries: 80 RON (76 MON) is the minimum available, the standard is 92 RON and 95 RON.
* South Africa: "regular" unleaded fuel is 95 RON in coastal areas with most fuel stations optionally offering 97 RON. Inland (higher altitude) "regular" unleaded fuel is 93 RON; once again most fuel stations optionally offer 95 RON.
* Spain: 95 RON "Euro" is sold in every station with 98 RON "Super" being offered in most stations. Many stations around cities and highways offer other high-octane "premium" brands.
* Taiwan: 92 RON, 95 RON and 98 RON are widely available at gas stations in Taiwan.
* Turkey: 95 RON and 98 RON are widely available in gas stations. 92 RON (Regular) has been dropped in 2006.
* Ukraine: the standard gasoline is 95 RON, but 92 RON gasoline is also widely available and popular as a less expensive replacement for 95 RON gasoline. 80 RON gasoline is available for old cars and motorcycles.
* United Kingdom: 'regular' petrol has an octane rating of 95 RON, with 97 RON fuel being widely available as the Super Unleaded. Tesco and Shell both offer 99 RON fuel. BP is currently[when?] trialling the public selling of the super-high octane petrol BP Ultimate Unleaded 102, which as the name suggests, has an octane rating of 102 RON. Although BP Ultimate Unleaded (with an octane rating of 97 RON) and BP Ultimate Diesel are both widely available throughout the UK, BP Ultimate Unleaded 102 is (as of October 2007) only available throughout the UK in 10 filling stations, and is priced at about two and half times more than their 97 RON fuel. Also offered Shell V-Power, but in a 99 RON octane rating, and Tesco fuel stations also supply the Greenergy produced 99 RON "Tesco 99".
* United States: in the Rocky Mountain (high altitude) states, 85 AKI is the minimum octane, and 91 AKI is the maximum octane available in fuel. The reason for this is that in higher-altitude areas, a typical naturally-aspirated engine draws in less air mass per cycle due to the reduced density of the atmosphere. This directly translates to less fuel and reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill up a carbureted car that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but at sea level the fuel may cause damage to the engine. A disadvantage to this strategy is that most turbocharged vehicles are unable to produce full power, even when using the "premium" 91 AKI fuel. In some east coast states, up to 94 AKI is available . In parts of the Midwest (primarily Minnesota, Iowa, Illinois and Missouri) ethanol-based E-85 fuel with 105 AKI is available . Often, filling stations near US racing tracks will offer higher octane levels such as 100 AKI . California fuel stations will offer 87, 89, and 91 AKI octane fuels, and at some stations, 100 AKI or higher octane, sold as racing fuel. Until summer 2001 before the phase-out of methyl tert-butyl ether aka MTBE as an octane enhancer additive, 92 AKI was offered in lieu of 91.
Generally, octane ratings are higher in Europe than they are in North America and most other parts of the world. This is especially true when comparing the lowest available octane level in each country. In many parts of Europe, 95 RON (90–91 AKI) is the minimum available standard, with 97/98 RON being higher specification (being called Super Unleaded). The higher rating seen in Europe is an artifact of a different underlying measuring procedure. In most countries (including all of Europe and Australia) the "headline" octane that would be shown on the pump is the RON, but in Canada, the United States and some other countries the headline number is the average of the RON and the MON, sometimes called the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2. Because of the 8 to 10-point difference noted above, this means that the octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in Canada and the US, would be 91–92 in Europe. However most European pumps deliver 95 (RON) as "regular", equivalent to 90–91 US AKI=(R+M)/2, and deliver 98, 99 or 100 (RON) (93-94 AKI) labeled as Super Unleaded - thus regular petrol sold in much of Europe corresponds to premium sold in the United States.
In other countries "regular" unleaded gasoline, when available, is sometimes as low as 85 RON (still with the more regular fuel, 95, and premium, around 98, available).