Editing Octane rating (section)

==Measurement methods==<!-- This section is linked from [[Avgas]] -->
[[Image:Gas Station Pump Five Octane Ratings.jpg|thumb|right|A US gasoline station pump offering five different (R+M)/2 octane ratings]]

===Research Octane Number (RON){{anchor|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]] at 600 rpm with a [[variable compression ratio]] under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.<ref>{{Cite book |last1=Steven A. |first1=Treese |title=Handbook of Petroleum Processing |last2=Peter R. |first2=Pujado |last3=David S. J. |first3=Jones |publisher=Springer |year=2015 |isbn=978-3-319-14528-0 |edition=2nd |pages=681, 1796}}</ref> The compression ratio is varied during the test to challenge the fuel's antiknocking tendency, as an increase in the compression ratio will increase the chances of knocking.

===Motor Octane Number (MON){{anchor|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.<ref name=Ullmann/>  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 system|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 lower than the RON,{{Citation needed|date=January 2023}} but there is no direct link between RON and MON.  See the table below.

===Anti-Knock Index (AKI) or (R+M)/2{{anchor|AKI}}===
In Canada, The United States, and Mexico, the advertised octane rating is the average of the RON and the MON, called the '''Anti-Knock Index''' ('''AKI'''). It is often written on pumps as '''(R+M)/2'''. AKI is also sometimes called '''PON''' (Pump Octane Number).

===Difference between RON, MON, and AKI{{anchor|Sensitivity}}===
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,<ref name="vintagebonanza.com">{{cite web|url=http://www.vintagebonanza.com/octane.htm|title=Octane determination in Piston Engines|work=vintagebonanza.com|url-status=live|archive-url=https://web.archive.org/web/20131029190552/http://www.vintagebonanza.com/octane.htm|archive-date=2013-10-29}}</ref> 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 the gasoline in ordinary multi-cylinder engines (rather than in a purpose-built test engine), normally at wide open throttle. This type of test was developed in the 1920s and is still reliable today. The original RdON tests were done in cars on the road, but as technology developed the testing was moved to chassis dynamometers with environmental controls to improve consistency.<ref>{{cite web|url=http://www.runyard.org/jr/CFR/OctaneExplanation.htm|title=Octane Explanation|work=runyard.org|url-status=live|archive-url=https://web.archive.org/web/20110727224512/http://www.runyard.org/jr/CFR/OctaneExplanation.htm|archive-date=2011-07-27}}</ref>

===Octane Index===
The evaluation of the octane number by either of the two laboratory methods requires a special engine built to match the tests' rigid standards, and the 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 substitute methods include FTIR, near infrared on-line analyzers, and others. Deriving an equation that can be used to calculate ratings accurately enough would also serve the same purpose, with added advantages. The term Octane Index is often used to refer to the use of an equation to determine a theoretical rating, in contradistinction to the direct measurements required for research or motor octane numbers. An 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 blended in amounts that will meet final product specifications. Most refiners produce and market more than one grade of motor gasoline, differing principally in their anti-knock quality. Being able to make sufficiently accurate estimates of the octane rating that will result from blending different refinery products is essential, something for which the calculated octane index is specially suited.<ref>{{cite journal |first1=H. |last1=Al-Haj Ibrahim |first2=M. |last2=Al-Kassmi |title=Determination of calculated octane index for motor gasoline |journal=The Arabian Journal for Science and Engineering |volume=25 |issue=2B |date=October 2000 |pages=179–186 }}</ref>

===Aviation gasoline octane ratings===
[[Aviation gasoline]]s used in piston aircraft engines common in general aviation have a slightly different method of measuring the octane of the fuel. Similar to an AKI, it has two different ratings, although it is usually referred to only by the lower of the two. One is referred to as the "aviation lean" rating, which for ratings up to 100 is the same as the MON of the fuel.<ref>{{cite web |url=http://www.txideafarm.com/unpublished_blend_paper.pdf |title=Replacing Leaded Aviation Gasoline |website=Txideafarm.com |access-date=2017-01-06 |url-status=live |archive-url=https://web.archive.org/web/20160304211600/http://www.txideafarm.com/unpublished_blend_paper.pdf |archive-date=2016-03-04 }}</ref> 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.<ref name="vintagebonanza.com"/>{{unreliable source?|date=April 2018}}

The most common currently used fuel, [[100LL]], has an aviation lean rating of 100 octane, and an aviation rich rating of 130.<ref>{{cite web |url=http://www.aviation-fuel.com/pdfs/avgas100llspecsastmd910_2011.pdf |title=Standard Specification for Aviation Gasolines |website=Aviation-fuel.com |access-date=2017-01-06 |url-status=dead |archive-url=https://web.archive.org/web/20160415123705/http://www.aviation-fuel.com/pdfs/avgas100llspecsastmd910_2011.pdf |archive-date=2016-04-15 }}</ref>