Octane rating

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Octane rating or octane number is a standard measure of 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. In contrast, fuels with low octane numbers (but high cetane numbers) are ideal for diesel engines. Use of gasolines with low octane numbers may lead to the problem of engine knocking.[1]



Octanes are a family of hydrocarbon that are typical components of gasoline. They are colourless liquids that boil around 125 °C (260 °F). One member of the octane family, isooctane, is used as a reference standard to benchmark the tendency of gasoline, petrol, or benzin fuels to resist self-igniting. Self-ignition leads to inefficiencies (or even engine damage) if it occurs 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.[2] 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 ratings are not indicators of the energy content of fuels. (See section 4 of this page and 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. Ethanol BTUs can be compared with gasoline BTUs in Heat of combustion tables.

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) more 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 its use for fuels for road vehicles has been progressively phased-out worldwide, beginning in the 1970s.[3]

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.[1] 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 Canada and 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-heptane (RON and MON 0 by definition)000
diesel fuel15–25
requirement for a typical two-stroke outboard engine[7]696567
Pertamina "Premium" gasoline in Indonesia88
"Regular" gasoline in Japan (Japanese Industrial Standards)90
Neopentane (dimethylpropane)80.2
"regular" gasoline in Australia, New Zealand, Canada and the US91–9282–8387
Pertamina "Pertamax" gasoline in Indonesia92
Shell "Super" in Indonesia92
Isopentane (methylbutane)90.3
Pertamina "Pertamax Plus" gasoline in Indonesia95
Shell "Super Extra" in Indonesia95
Shell "FuelSave " in Malaysia95
"EuroSuper" or "EuroPremium" or "Regular unleaded" in Europe, "SP95" in France9585–8690–91
"Premium" or "Super unleaded" gasoline in US (10% ethanol blend)9787-8892-93
Shell "V-Power 97" in Malaysia97
Shell "V-Power 98", Caltex "Platinum 98 with Techron", Esso Mobil "Synergy 8000" and SPC "LEVO 98" in Singapore9889–9093–94
Great Britain, Slovenia and Spain, "SP98" in France9889–9093–94
"SuperPlus" in Germany9888
Tesco "Momentum^99"[9] in UK9987
"Premium" gasoline in Japan (Japanese Industrial Standards)100
Pertamina "Pertamax Racing" in Indonesia100
Shell V-Power in Italy and Germany10088
Eni(or Agip) Blu Super +(or Tech) in Italy1008794
IP Plus 100 in Italy100
Tamoil WR 100 in Italy100
San Marco Petroli F-101 in Italy(northern Italy only, just a few gas stations)101
Petro-Canada "Ultra 94" in Canada [10]101.58894
Aral Super 95 in Germany9585
Aral Super 95 E10 (10% Ethanol) in Germany9585
Aral SuperPlus 98 in Germany9888
Aral Ultimate 102 in Germany10288
IES 98 Plus in Italy98
ExxonMobil Avgas 100[11]99.5 (min)
Shell "V-Power Racing" in Australia - discontinued July 2008 [12]100
"isooctane" (RON and MON 100 by definition)100100100
"BP Ultimate 102 - now discontinued"[14]10293–9497–98
E85 gasoline102-10585-8794-96[16]
hydrogen> 130

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.

The compression ratio is directly related to power and to thermodynamic efficiency of an internal combustion engine (see Otto-cycle). Engines with higher compression ratios develop more area under the Otto-Cycle curve, thus they extract more energy from a given quantity of fuel.

During the compression stroke of an internal combustion engine, as the air / fuels mix is compressed its temperature rises (PV=nRT).

A fuel with a higher octane rating is less prone to auto-ignition and can withstand a greater rise in temperature during the compression stroke of an internal combustion engine without auto-igniting, thus allowing more power to be extracted from the Otto-Cycle.

If during the compression stroke the air / fuel mix reaches a temperature greater than the auto-ignition temperature of the fuel, the fuel self or auto-ignites. When auto-ignition occurs (before the piston reaches the top of its travel) the up-rising piston is then attempting to squeeze the rapidly expanding (exploding) fuel charge. This will usually destroy an engine quickly if allowed to continue.

There are two types of induction systems on internal combustion engines. Normally aspirated engine (air is sucked in using the engines pistons. Or, forced induction engines (See supercharged or turbocharged engines).

In the case of the normally aspirated engine, at the start of the compression stroke the cylinder air / fuel volume is very low, this translates into a low starting pressure. As the piston travels upward, a compression ratio of 10:1 in a normally aspirated engine will most likely not start auto-ignition. But 11:1 may. In a forced induction engine where at the start of the compression stroke the cylinder pressure is already raised (having a greater volume of air / fuel) Exp. 2 Bar (14.7Psi), the starting pressure or air / fuel volume would be 2 times that of the normally aspirated engine. This would translate into an effective compression ratio of 20:1 vs. 10:1 for the normally aspirated. This is why many forced induction engines have compression ratios in the 8:1 range.

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.

See also


  1. ^ a b Werner Dabelstein, Arno Reglitzky, Andrea Schütze and Klaus Reders "Automotive Fuels" in Ullmann's Encyclopedia of Industrial Chemistry 2007, Wiley-VCH, Weinheim.doi:10.1002/14356007.a16_719.pub2
  2. ^ 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. 
  3. ^ "Leaded Gas Phaseout". U.S. EPA, Region 10. 6 1995. http://yosemite.epa.gov/R10/airpage.nsf/webpage/Leaded+Gas+Phaseout. Retrieved 2012-06-15. 
  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. ^ http://www.tesco.com/Momentum99/productBenefits-specs.asp
  10. ^ Octane & Other Gasoline Basics
  11. ^ Exxon Mobil Avgas product description
  12. ^ Shell phases out V-Power Racing fuel (100 RON) – MRT
  13. ^ Energy Citations Database (ECD)
  14. ^ BP Ultimate 102
  15. ^ a b c d e f A. T. Balaban, L. B. Kier, and N. Josh, MATCH (Commun. Math. Chem.) 28 (1992) 13–27.
  16. ^ Changes in Gasoline IV, sponsored by Renewable Fuels Foundation
  17. ^ 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. 
  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.arch.gob.ec/index.php/descargas/leyes-y-reglamentos/doc_download/292-norma-inen-935-2012.html
  22. ^ VTT ja St1 kehittävät uutta vähäpäästöistä liikennepolttoainetta, VTT
  23. ^ http://www.petrolworld.com/asia-headlines/malaysia-to-phase-out-ron-92-petrol-in-2009.html
  24. ^ http://www.bp.com/liveassets/bp_internet/bp_ultimate/STAGING/brand_assets/downloads_pdfs/pq/bp_ultimate_uk_102_final_press_release.pdf
  25. ^ http://www.bp.com/sectiongenericarticle.do?categoryId=6621&contentId=7060376
  26. ^ Products – Retail Marketing
  27. ^ Growth Energy Market Development

Further reading

External links

Octane ratings of some hydrocarbons

Information in general