Octane rating

Octane rating

Octane rating or octane number is a standard measure of the anti-knock properties (i.e. the performance) of a motor or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating. In broad terms, fuels with a higher octane rating are used in high-compression engines that generally have higher performance.



Octane is a hydrocarbon liquid that is used as a reference standard to describe the tendency of gasoline, petrol, or benzin fuels to self ignite during compression prior to the desired position of the piston in the cylinder as appropriate for valve and ignition timing. The problem of premature ignition is referred to as pre-ignition and also as engine knock, which is a sound that is made when the fuel ignites too early in the compression stroke.

Severe knock causes severe engine damage, such as broken connecting rods, melted pistons, melted or broken valves and other components. The octane rating is a measure of how likely a gasoline or liquid petroleum fuel is to self ignite. The higher the number, the less likely an engine is to pre-ignite and suffer damage.

Isooctane (upper) has an octane rating of 100 whereas n-heptane has an octane rating of 0.

The most typically used engine management systems found in automobiles today monitor the level of knock that is being produced by the fuel being used. In modern computer controlled engines, the timing of the ignition will be automatically altered by the fuel management system to reduce the pre-ignition to an acceptable level.

The octane rating of gasoline is measured in a test engine and is defined by comparison with the mixture of 2,2,4-trimethylpentane (iso-octane) and heptane that 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.[1] A rating of 90 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 the octane number is raised by blending in ethanol, energy content per volume is reduced.

A US gas station pump offering five different (R+M)/2 octane ratings

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 may have octane ratings of 110 or significantly higher. 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.

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.[2][3] 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.[citation needed]

Anti-Knock Index (AKI)

In most countries, including Australia and all of those in 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 Pump Octane Number (PON).

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.

Observed Road Octane Number (RdON)

The final type of octane rating, called Observed Road Octane Number (RdON), is derived from testing gasolines in real world multi-cylinder engines, normally at wide open throttle. It was developed in the 1920s and is still reliable today. The original testing was done in cars on the road but as technology developed the testing was moved to chassis dynamometers with environmental controls to improve consistency.[4]

Examples of octane ratings

The RON/MON values of n-heptane and iso-octane are exactly 0 and 100, respectively, by the definition of octane rating. The following table lists octane ratings for various other fuels.[5][6]

hexadecane < -30
n-octane -10
n-heptane (RON and MON 0 by definition) 0 0 0
diesel fuel 15–25
2-methylheptane 23 23.8
n-hexane 25 26.0 26
1-pentene 34
2-methylhexane 44 46.4
3-methylhexane 55.0
1-heptene 60
n-pentane 62 61.9
requirement for a typical two-stroke outboard engine[7] 69 65 67
Pertamina "Premium" gasoline in Indonesia 88
n-butanol 92 71 83
2,2-dimethylpropane 80.2
"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
n-butane 94[8] 90.1
2-methylbutane 90.3
Pertamina "Pertamax Plus" gasoline in Indonesia 95
Shell "Super Extra" in Indonesia 95
Shell "FuelSave " in Malaysia 95
"EuroSuper" or "EuroPremium" or "Regular unleaded" in Europe, "SP95" in France 95 85–86 90–91
Shell "V-Power 97" in Malaysia 97
"SuperPlus" in Germany, Great Britain, Slovenia and Spain, "SP98" in France 98 89–90 93–94
Shell V-Power in Italy and Germany 100
Eni(or Agip) Blu Super +(or Tech) in Italy 100
IP Plus 100 in Italy 100
Tamoil WR 100 in Italy 100
San Marco Petroli F-101 in Italy(northern Italy only, just a few gas stations) 101
Aral Ultimate 102 in Germany 102
IES 98 Plus in Italy 98
2,2-dimethylbutane 93.4
2,3-dimethylbutane 94.4
ExxonMobil Avgas 100[9] 99.5 (min)
Shell "V-Power Racing" in Australia - discontinued July 2008 [10] 100
"isooctane" (RON and MON 100 by definition) 100 100 100
benzene 101
i-butane 102[11] 97.6
"BP Ultimate 102 - now discontinued"[12] 102 93–94 97–98
t-butanol 103 91 97
2,3,3-trimethylpentane 106.1[13] 99.4[13] 103
ethane 108
2,2,3-trimethylpentane 109.6[13] 99.9[13] 105
toluene 111 95 103
E85 gasoline 94-96[14]
propane 112 97
2,2,3-trimethylbutane 112.1[13] 101.3[13] 106
xylene 117
isopropanol 118 98 108
methanol 108.7[15] 88.6[15] 98.65
ethanol 108.6[15] 89.7[15] 99.15
2,5-Dimethylfuran 119
methane 120 120 120
hydrogen* > 130 very low[16]

*Hydrogen does not fit well into the normal definitions of octane number. It has a very high RON and a low MON,[16] so that it has low knock resistance in practice,[17] 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.[citation needed]

Effects of octane rating

Higher octane ratings correlate to higher activation energies: This being the amount of applied energy required to initiate combustion. Since higher octane fuels have higher activation energy requirements, it is less likely that a given compression will cause uncontrolled ignition, otherwise known as autoignition or detonation.

It might seem odd that fuels with higher octane ratings are used in more powerful engines, since such fuels ignite less easily. However, detonation is undesirable in a spark ignition engine, and is signified by audible "pinging" or in more extreme cases "knock".

A fuel with a higher octane rating can be burnt in an engine with a high compression ratio without causing detonation, as such fuels are less prone to detonation. Compression is directly related to power and to thermodynamic efficiency (see engine tuning), so engines that require a higher octane fuel usually develop more motive power and therefore do more work in relation to the calorific value of the fuel (BTU) being used. Power output is a function of the properties of the fuel used, as well as the design of the engine itself, and is related to octane rating of the fuel. Power is limited by the maximum amount of fuel-air mixture that can be brought into the combustion chamber. When the throttle is partly open, only a small fraction of the total available power is produced because the manifold is operating at pressures far below that of the external atmosphere (depression). In this case, the octane requirement is far lower than when the throttle is opened fully and the manifold pressure increases to almost that of the external atmosphere, or higher in the case of forced induction engines (See supercharged or turbocharged engines).

Many high-performance engines are designed to operate with a high maximum compression, and thus demand fuels of higher octane. A common misconception is that power output or fuel efficiency can be improved by burning fuel of higher octane than that specified by the engine manufacturer. The power output of an engine depends in part on the energy density of the fuel being burnt. Fuels of different octane ratings may have similar densities, but because switching to a higher octane fuel does not add more hydrocarbon content or oxygen, the engine cannot develop more power.

However, burning fuel with a lower octane rating than that for which the engine is designed often results in a reduction of power output and efficiency. Many modern engines are equipped with a knock sensor (a small piezoelectric microphone), which sends a signal to the engine control unit, which in turn retards the ignition timing when detonation is detected. Retarding the ignition timing reduces the tendency of the fuel-air mixture to detonate, but also reduces power output and fuel efficiency. Because of this, under conditions of high load and high temperature, a given engine may have a more consistent power output with a higher octane fuel, as such fuels are less prone to detonation. 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 tested a car using a dynamometer, and found that the power output increased as the AKI was increased up to approximately 96 AKI.

Most fuel filling stations have two storage tanks (even those offering 3 or 4 octane levels): those motorists who purchase intermediate grade fuels are given a mixture of higher and lower octane fuels. "Premium" grade is fuel of higher octane, and the minimum grade sold is fuel of lower octane. Purchasing 91 octane fuel (where offered) simply means that more fuel of higher octane is blended with commensurately less fuel of lower octane, than when purchasing a lower grade. The detergents and other additives in the fuel are often, but not always, identical.

The octane rating was developed by chemist Russell Marker at the Ethyl Corporation in 1926. The selection of n-heptane as the zero point of the scale was due to its availability in high purity. Other isomers of heptane produced from crude oil have greatly different ratings.

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).[18] 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).[19][20]
  • 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.
  • China: 93 and 97 (RON) are commonly offered. In limited areas higher rating such as 99 RON is available. In some rural areas it can be difficult to find fuel with over 93 RON.
  • Chile: 93, 95 and 97 RON are standard at almost all gas stations thorough Chile. The three types are unleaded
  • Egypt: 80 RON is commonly used for all taxis and old cars and is the predominant rating in rural areas. 90 RON and 92 RON are available at almost all gas stations with a negligible price difference between them. 95 RON is becoming more common especially in the big cities and upscale suburbs. All fuels are unleaded.
  • Finland: 95 and 98 (RON), advertised as such, at almost all gas stations. Most cars run on 95, but 98 is available for vehicles that need higher octane fuel. Shell offers V-Power, advertised as "over 99 octane", instead of 98. In the beginning of 2011 95 RON was replaced by 95E10, and 98 RON will be advertised as 98E5. ST1 also offers RE85 on some stations, which is E85 made from waste from which the advertised name "ReFuel" comes from.
  • Germany: "Super E10" 95 RON and "Super Plus E5" 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 only rarely being offered, because lower production amounts make it more expensive than "Super" 95 RON, so it is often not offered any more. Due to a new European Union law, gas stations are being required to offer and sell a minimum rate of the new mixture of "Super" 95 RON with up to 10% Ethanol branded as "Super E10" since early 2011. Due to that process the big suppliers are discontinuing the average "Super E5" 95 RON with <5% Ethanol so cars without the capability for using E10 need to use 98 RON petrol instead.
  • 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.[citation needed]
  • 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. Recently Agip introduced "Blu Super+", a 100 RON gasoline.
  • Israel: 95 RON & 98 RON are normally available at most petrol stations. 96 RON is also available at a large number of gas stations but 95 RON is more preferred because it's cheaper and performance differences aren't very wide and noticeable. "Regular" fuel is 95 RON. All variants are unleaded.
  • Latvia: 95 RON and 98 RON widely available.
  • Lebanon: 95 RON and 98 RON are widely available.
  • Malaysia: Had RON 92 until 2009. Replaced with "regular" unleaded fuel Ron 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 97 RON.)
  • Montenegro: 95 RON is sold as a "regular" fuel. As a "premium" fuel, 98 RON is sold. Both variants are unleaded.
  • 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 neighboring 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.
  • Philippines: A brand of Petron, Petron Blaze is rated at 100 RON (the only brand of gasoline in the Philippines without an ethanol blend). Other "super premium" brands like Petron XCS, Calex Gold, Shell V-Power are rated at 95-97 RON, while Petron Xtra Unleaded, Caltex Silver, and Shell Super Unleaded are rated at 93 RON.
  • Poland: Eurosuper 95 (RON 95) is sold in every gas station. Super Plus 98 (RON 98) is available in most stations, sometimes under brand (Orlen - Verva, BP - Ultimate, Shell - V-Power) and usually containing additives. Shell offers V-Power Racing fuel which is rated RON 100.
  • Russia and CIS countries: 80 RON (76 MON) is the minimum available, the standard is 92 RON and 95 RON. 98 RON is available on some stations but it's usually quite expensive compared to the lower octane rating fuels.
  • Saudi Arabia: Two types of fuel are available at all gas stations in Saudi Arabia. "Premium 91" (RON 91) where the pumps are coloured green, and "Super Premium 95" (RON 95) where the pumps are coloured red. While gas stations in Saudi Arabia are privatized, the prices are regulated by the authorities and have a fixed at S.A.R. 0.45 (U.S. $0.12) and S.A.R. 0.60 (U.S. $0.16) per litre respectively. Prior to 2006, only Super Premium RON 95 was available and the pumps weren't coloured in any specific order. The public didn't know what Octane rating was, therefore big educating campaigns were spread, telling the people to use the "red gas" only for high end cars, and save money on using the "green gas" for regular cars and trucks.
  • South Africa: "regular" unleaded fuel is 95 RON in coastal areas with most fuel stations optionally offering 97 RON. Inland (higher elevation) "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.
  • Sri Lanka: In Ceypetco filling stations, 90 RON is the regular petrol and 95 RON is called 'Super Petrol', which comes at a premium price. In LIOC filling stations, 90 RON remains as regular petrol and 92 RON is available as 'Premium Petrol'. The cost of premium petrol is lower than the cost of super petrol.
  • Taiwan: 92 RON, 95 RON and 98 RON are widely available at gas stations in Taiwan.
  • Thailand: 91 RON and 95 RON are widely available.
  • 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. In April 2006, BP started a public trial of the super-high octane petrol BP Ultimate Unleaded 102, which as the name suggests, has an octane rating of 102 RON.[21] 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 was available throughout the UK in only 10 filling stations, and was priced at about two and half times more than their 97 RON fuel. In March 2010, BP stopped sales of Ultimate Unleaded 102, citing the closure of their specialty fuels manufacturing facility.[22] Shell V-Power is also available, but in a 99 RON octane rating, and Tesco fuel stations also supply the Greenergy produced 99 RON "Tesco 99".
  • United States: in the US octane rating is displayed in AKI. In the Rocky Mountain (high elevation) states, 85 AKI (90 RON) is the minimum octane, and 91 AKI (95 RON) is the maximum octane available in fuel[citation needed]. The reason for this is that in higher-elevation areas, a typical naturally aspirated engine draws in less air mass per cycle because of 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 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 (98 RON) is available.[23] In Colorado as well as parts of the Midwest (primarily Minnesota, Iowa, Illinois and Missouri) ethanol-based E-85 fuel with 105 AKI is available.[24] Often, filling stations near US racing tracks will offer higher octane levels such as 100 AKI[citation needed]. California fuel stations will offer 87, 89, and 91 AKI (91, 93 and 95 RON) octane fuels, and at some stations, 100 AKI or higher octane, sold as racing fuel.
  • Venezuela: 91 RON and 95 RON gasoline is available nationwide, in all PDV gas stations. 95 RON petrol is the most widely used in the country, although most cars in Venezuela would work with 91 RON gasoline. This is because petrol prices are heavily subsided by the government. All gasoline in Venezuela is unleaded.
  • Vietnam: 92 is in every gas station and 95 is in the urban areas.

See also


  1. ^ Kemp, Kenneth W.; Brown, Theodore; Nelson, John D. (2003). Chemistry: the central science. Englewood Cliffs, N.J: Prentice Hall. pp. 992. ISBN 0-13-066997-0. 
  2. ^ http://www.texacoursa.com/glossary/r.html
  3. ^ http://www.texacoursa.com/glossary/m.html
  4. ^ http://www.runyard.org/jr/CFR/OctaneExplanation.htm
  5. ^ Petroleum and Coal, Purdue, http://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/coal.html 
  6. ^ (PDF), IUPAC, http://www.iupac.org/publications/pac/1983/pdf/5502x0199.pdf 
  7. ^ Johnson Operation and Maintenance Manual, 1999
  8. ^ [1]
  9. ^ Exxon Mobil Avgas product description
  10. ^ Shell phases out V-Power Racing fuel (100 RON) – MRT
  11. ^ Energy Citations Database (ECD)
  12. ^ BP Ultimate 102
  13. ^ a b c d e f A. T. Balaban, L. B. Kier, and N. Josh, MATCH (Commun. Math. Chem.) 28 (1992) 13–27.
  14. ^ Changes in Gasoline IV, sponsored by Renewable Fuels Foundation
  15. ^ a b c d 'Impact of alcohol–gasoline fuel blends on the performance and combustion characteristics of an SI engine'. doi:10.1016/j.fuel.2010.01.032. 
  16. ^ a b Ingersoll, John G. (1996). Natural gas vehicles. Lilburn, Ga: Fairmont Press. pp. 327. ISBN 0-88173-218-4. 
  18. ^ http://en.wikipedia.org/wiki/Shell_V-Power
  19. ^ http://www.unitedpetroleum.com.au/index.asp?pgID=65
  20. ^ http://www.unitedpetroleum.com.au/distributor-premium100-locations.asp
  21. ^ http://www.bp.com/liveassets/bp_internet/bp_ultimate/STAGING/brand_assets/downloads_pdfs/pq/bp_ultimate_uk_102_final_press_release.pdf
  22. ^ http://www.bp.com/sectiongenericarticle.do?categoryId=6621&contentId=7060376
  23. ^ Products – Retail Marketing
  24. ^ Growth Energy Market Development


Further reading

  • SAE standard J 1297 Alternative Automotive Fuels, Sept 2002
  • Khoo, Kenny K. Understanding Octane and its Related Components. Yellowknife: Smithsonian Press, 2006.

External links

Octane ratings of some hydrocarbons

Information in general

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