Octane rating

An octane rating, or octane number, is a standard measure of the performance of an engine or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating (igniting). In broad terms, fuels with a higher octane rating are used in high performance gasoline engines that require higher compression ratios. In contrast, fuels with lower octane numbers (but higher cetane numbers) are ideal for diesel engines, because diesel engines (also referred to as compression-ignition engines) do not compress the fuel, but rather compress only air and then inject fuel into the air which was heated by compression. Gasoline engines rely on ignition of air and fuel compressed together as a mixture, which is ignited at the end of the compression stroke using spark plugs. Therefore, high compressibility of the fuel matters mainly for gasoline engines. Use of gasoline with lower octane numbers may lead to the problem of engine knocking.[1]

Principles

The problem: pre-ignition and knocking

Main article: Engine knocking

In a normal spark-ignition engine, the air-fuel mixture is heated due to being compressed and is then triggered to burn rapidly by the spark plug. If it is heated (or compressed) too much, it will self-ignite before the ignition system sparks. This causes much higher pressures than engine components are designed for, and can cause a "knocking" or "pinging" sound. Knocking can cause major engine damage if severe.

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

Isooctane as a reference standard

2,2,4-Trimethylpentane (iso-octane) (upper) has an octane rating of 100 whereas n-heptane has an octane rating of 0.

Octanes are a family of hydrocarbon that are typical components of gasoline. They are colorless 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 or LPG fuels to resist self-ignition.

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, gasoline 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 gasoline contains just iso-octane and heptane in these proportions but that it has the same detonation resistance properties (generally, gasoline sold for common use never consists solely of iso-octane and heptane; it is a mixture of many hydrocarbons and often other additives). Because some fuels are more knock-resistant than pure iso-octane, the definition has been extended to allow for octane numbers greater than 100.

Octane ratings are not indicators of the energy content of fuels. (See Effects below and Heat of combustion). They are 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.

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, 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 tetraethyllead 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

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

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)

Another type of octane rating, called Motor Octane Number (MON), 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 pump gasoline will be about 8 to 12 octane lower than the RON, but there is no direct link between RON and MON. Pump gasoline specifications typically require both a minimum RON and a minimum MON.

Anti-Knock Index (AKI) or (R+M)/2

In most countries, including Australia, New Zealand and all of those in Europe, the "headline" octane rating shown on the pump is the RON, but in Canada, the United States, Brazil, and some other countries, 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 Posted Octane Number (PON).

Difference between RON, MON, and AKI

Because of the 8 to 12 octane number difference between RON and MON noted above, the AKI shown in Canada and the United States is 4 to 6 octane numbers lower than elsewhere in the world for the same fuel. This difference between RON and MON is known as the fuel's Sensitivity,[4] and is not typically published for those countries that use the Anti-Knock Index labelling system.

See the table in the following section for a comparison.

Observed Road Octane Number (RdON)

Another 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.[5]

Octane Index

The evaluation of the octane number by the two laboratory methods requires a standard engine, and the test procedure can be both expensive and time-consuming. The standard engine required for the test may not always be available, especially in out-of-the-way places or in small or mobile laboratories. These and other considerations led to the search for a rapid method for the evaluation of the anti-knock quality of gasoline. Such methods include FTIR, near infrared on-line analyzers (ASTM D-2885) and others. Deriving an equation that can be used for calculating the octane quality would also serve the same purpose with added advantages. The term Octane Index is often used to refer to the calculated octane quality in contradistinction to the (measured) research or motor octane numbers. The octane index can be of great service in the blending of gasoline. Motor gasoline, as marketed, is usually a blend of several types of refinery grades that are derived from different processes such as straight-run gasoline, reformate, cracked gasoline etc. These different grades are considered as one group when blending to meet final product specifications. Most refiners produce and market more than one grade of motor gasoline, differing principally in their anti-knock quality. The ability to predict the octane quality of the blends prior to blending is essential, something for which the calculated octane index is specially suited.[6]

Aviation gasoline octane ratings

Aviation gasoline used in piston aircraft common in general aviation have slightly different methods of measuring the octane of the fuel. Similar to AKI, it has two different ratings, although it is referred to only by the lower of the two. One is referred to as the "aviation lean" rating and is the same as the MON of the fuel up to 100.[7] The second is the "aviation rich" rating and corresponds to the octane rating of a test engine under forced induction operation common in high-performance and military piston aircraft. This utilizes a supercharger, and uses a significantly richer fuel/air ratio for improved detonation resistance.[4]

The most commonly used current fuel, 100LL, has an aviation lean rating of 100 octane, and an aviation rich rating of 130.[8]

Examples

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.[9][10]

Fuel RON MON AKI or (R+M)/2
hexadecane < −30
n-octane −20
n-heptane (RON and MON 0 by definition) 0 0 0
diesel fuel 15–25
2-methylheptane 23 23.8 23
n-hexane 25 26.0 26
1-pentene 34
2-methylhexane 44 46.4 45.2
3-methylhexane 55.0
1-heptene 60
n-pentane 62 61.9 62
requirement for a typical two-stroke outboard motor[11] 69 65 67
Pertamina "Premium" in Indonesia 88 78 83
Pertamina "Pertalite" in Indonesia 90
"Regular gasoline" in Japan (Japanese Industrial Standards) 90
n-butanol 92 71 83
Neopentane (dimethylpropane) 80.2
n-butane 94[12] 90.1 92
Isopentane (methylbutane) 90.3
"Regular Gasoline" in Australia, New Zealand and the US 91–92 82–83 87
Pertamina "Pertamax" in Indonesia 92 82 87
"Shell Super" in Indonesia, "Total Performance 92" in Indonesia 92
2,2-dimethylbutane 93.4
2,3-dimethylbutane 94.4
"YPF Super" in Argentina 95 84 90
Pertamina "Pertamax Plus" gasoline in Indonesia, "Super/Premium" in New Zealand and Australia 95 85 90
"Aral Super 95" in Germany, "Aral Super 95 E10" (10% Ethanol) in Germany 95 85 90
"Shell V-Power" in Indonesia, "Total Performance 95" in Indonesia, "Shell FuelSave " in Malaysia 95
"EuroSuper" or "EuroPremium" or "Regular unleaded" in Europe, "SP95" in France, "Super 95" in Belgium 95 85–86 90–91
"Premium" or "Super unleaded" gasoline in US (10% ethanol blend) 97 87-88 92-93
"Shell V-Power 97" in Malaysia 97
"IES 98 Plus" in Italy, "Aral SuperPlus 98" in Germany, Pertamina "Pertamax Turbo" in Indonesia 98
"YPF Infinia" in Argentina 98 87 93
"Corriente (Regular)" in Colombia 91.5[13] 70 81[14]
"Extra (Super/Plus)" in Colombia 95[15] 79 87[16]
"SuperPlus" in Germany 98 88 93
"Shell V-Power 98", "Caltex Platinum 98 with Techron", "Esso Mobil Synergy 8000" and "SPC LEVO 98" in Singapore, "BP Ultimate 98/Mobil Synergy 8000" in New Zealand, "SP98" in France, "Super 98" in Belgium, Great Britain, Slovenia and Spain 98 89–90 93–94
"Shell V-Power Nitro+ 99" "Tesco Momentum 99" In the United Kingdom 99 87 93
Pertamina "Pertamina Racing Fuel" (bioethanol blend) in Indonesia 100 86 93
"Premium" gasoline in Japan (Japanese Industrial Standards), "IP Plus 100" in Italy, "Tamoil WR 100" in Italy, "Shell V-Power Racing" in Australia - discontinued July 2008 [17] 100
"Shell V-Power" in Italy and Germany 100 88 94
"Eni(or Agip) Blu Super +(or Tech)" in Italy 100 87 94
"isooctane" (RON and MON 100 by definition) 100 100 100
" Petron Blaze 100 Euro 4M " in Malaysia 100
"San Marco Petroli F-101" in Italy (northern Italy only, just a few gas stations) 101
benzene 101
2,5-Dimethylfuran 101.3[18] 88.1[18] 94.7[18]
Petro-Canada "Ultra 94" in Canada [19] 101.5 88 94
Aral Ultimate 102 in Germany 102 88 95
ExxonMobil Avgas 100[20] 99.5 (min)
Petrobras Podium in Brazil 102 88 95
E85 gasoline 102-105 85-87 94-96[21]
i-butane 102[12] 97.6 100
"BP Ultimate 102 - now discontinued"[22] 102 93–94 97–98
t-butanol 103 91 97
2,3,3-trimethylpentane 106.1[23] 99.4[23] 103
ethane 108
ethanol 108.6[24] 89.7[24] 99.15
methanol 108.7[24] 88.6[24] 98.65
2,2,3-trimethylpentane 109.6[23] 99.9[23] 105
propane 112 97 105
2,2,3-trimethylbutane 112.1[23] 101.3[23] 106
xylene 118 115 116.5
isopropanol 118 98 108
1-propanol 118[25] 98 108[25]
toluene 121 107 114
VP C16 Race Fuel 117 118 117.5
methane 120 120 120
hydrogen > 130

Effects

Higher octane ratings correlate to higher activation energies: 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-fuel mix is compressed its temperature rises.

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 heating fuel charge. This will usually destroy an engine quickly if allowed to continue.

There are two types of induction systems on internal combustion engines: naturally aspirated engine (air is sucked in using the engine's 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 high, this translates into a low starting pressure. As the piston travels upward, abnormally high cylinder pressures may result in the mixture auto-igniting or detonating, which is why conservative compression ratios are used in consumer vehicles. 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. 202kPa (29.4Psi) 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

References

  1. 1 2 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. p. 992. ISBN 0-13-066997-0.
  3. ↑ "Leaded Gas Phaseout". U.S. EPA, Region 10. June 1995. Retrieved 2012-06-15.
  4. 1 2 "Octane determination in Piston Engines". vintagebonanza.com.
  5. ↑ "Octane Explanation". runyard.org.
  6. ↑ H. Al-Haj Ibrahim and M. Al-Kassmi, Determination of calculated octane index for motor gasoline, The Arabian Journal for science and engineering, vol. 25, No. 2B, Oct. 2000, pp. 179-186.
  7. ↑ http://www.txideafarm.com/unpublished_blend_paper.pdf
  8. ↑ http://www.aviation-fuel.com/pdfs/avgas100llspecsastmd910_2011.pdf
  9. ↑ Petroleum and Coal, Purdue
  10. ↑ Balaban, Alexandru T. (1983). "Topological Indices based on Topological Distances in Molecular Graphs" (PDF). Pure and Applied Chemistry. IUPAC. 55 (2): 199–206. doi:10.1351/pac198855020199.
  11. ↑ Johnson Operation and Maintenance Manual, 1999
  12. 1 2 Henig, Y. "SciTech Connect: Autoignition of n-butane/isobutane blends in a knock research engine". osti.gov.
  13. 1 2 wpadmin. "Ecopetrol - Energía para el Futuro". ecopetrol.com.co.
  14. 1 2 wpadmin. "Ecopetrol - Energía para el Futuro". ecopetrol.com.co.
  15. 1 2 wpadmin. "Ecopetrol - Energía para el Futuro". ecopetrol.com.co.
  16. 1 2 wpadmin. "Ecopetrol - Energía para el Futuro". ecopetrol.com.co.
  17. ↑ Shell phases out V-Power Racing fuel (100 RON) – MRT
  18. 1 2 3 Daniel, Ritchie (2012). "'Combustion performance of 2,5-dimethylfuran blends using dual-injection compared to direct-injection in an SI engine'". Applied Energy. 98: 59–68. doi:10.1016/j.apenergy.2012.02.073.
  19. ↑ Octane & Other Gasoline Basics
  20. ↑ Exxon Mobil Avgas product description
  21. ↑ Changes in Gasoline IV, sponsored by Renewable Fuels Foundation
  22. 1 2 "BP". bp.com.
  23. 1 2 3 4 5 6 A. T. Balaban, L. B. Kier, and N. Josh, MATCH (Commun. Math. Chem.) 28 (1992) 13–27.
  24. 1 2 3 4 Eyidogan, Muharrem (2010). "'Impact of alcohol–gasoline fuel blends on the performance and combustion characteristics of an SI engine'". Fuel. 89 (10): 2713–2720. doi:10.1016/j.fuel.2010.01.032.
  25. 1 2 "Biofuels - Types of Biofuels - Bioalcohols". biofuel.org.uk.
  26. ↑ Shell V-Power
  27. ↑ http://www.unitedpetroleum.com.au/index.asp?pgID=65
  28. ↑ http://www.unitedpetroleum.com.au/distributor-premium100-locations.asp
  29. ↑ http://www.unitedpetroleum.com.au/united/fuel/ethanol-85
  30. ↑ "China V gasoline and diesel fuel quality standards" (PDF). International Council on Clean Transportation. Retrieved 7 February 2016.
  31. ↑ https://www.recope.go.cr/wp-content/uploads/2013/07/Manual_Productos.pdf
  32. ↑ "INA d.d.". ina.hr.
  33. ↑ "Lukoil Hrvatska". lukoil.hr.
  34. ↑ "Crodux derivati". crodux-derivati.hr.
  35. ↑ http://www.arch.gob.ec/index.php/descargas/leyes-y-reglamentos/doc_download/292-norma-inen-935-2012.html
  36. ↑ "الحكومة المصرية ترفع أسعار الوقود بنسبة تقترب من الضعف". BBC Arabic.
  37. ↑ VTT ja St1 kehittävät uutta vähäpäästöistä liikennepolttoainetta, VTT
  38. ↑ List of E85 pumps in Germany
  39. ↑ http://www.petrolworld.com/asia-headlines/malaysia-to-phase-out-ron-92-petrol-in-2009.html
  40. ↑ http://www.altaposten.no/lokalt/nyheter/article9516487.ece
  41. ↑ "Ceylon Petroleum Corporation". ceypetco.gov.lk.
  42. ↑ "BP" (PDF). bp.com.
  43. ↑ "What's With All the Buttons? Types and Grades of Fuel". Driving Tests.
  44. ↑ http://www.aaa.com/aaa/006/EnCompass/2007/mar/mar_AutoTalk.htm
  45. ↑ http://www.usatoday.com/story/money/cars/2013/05/29/bad-gasoline-low-octane-too-much-ethanol/2369579/
  46. ↑ Sunoco, Inc. "GoSunoco - Partner With Sunoco". sunocoinc.com.
  47. ↑ Growth Energy Market Development

External links

Octane ratings of some hydrocarbons

Information in general

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