Editing Octane rating (section)

===Iso-octane as a reference standard===
[[Image:OctaneStds.png|thumb|left|[[2,2,4-Trimethylpentane]] (iso-octane) (upper) by definition is assigned the octane rating of 100, whereas [[n-heptane|''n''-heptane]] (lower) is assigned the octane rating of 0.]]

[[Octane]]s are a family of hydrocarbons that are typical components of gasoline. They are colorless liquids that boil around 125&nbsp;°C (260&nbsp;°F). One member of the octane family, [[2,2,4-Trimethylpentane]] (iso-octane), is used as a reference standard to benchmark the tendency of [[gasoline]] or [[liquefied petroleum gas|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 [[normal heptane]] that would have the same anti-knocking capability 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.<ref>{{cite book |author1=Kemp, Kenneth W. |author2=Brown, Theodore |author3=Nelson, John D. |title=Chemistry: the central science |url=https://archive.org/details/studentlectureno00theo |url-access=registration |publisher=Prentice Hall |location=Englewood Cliffs, N.J |year=2003 |page=992 |isbn=0-13-066997-0 }}</ref> 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).

Octane ratings are not indicators of the energy content of fuels. (See [[#Effects|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.<ref>{{Cite book |url=https://books.google.com/books?id=ygVsCgAAQBAJ&q=Octane+ratings+are+not+indicators+of+the+energy+content+of+fuels.&pg=PT853 |title=Chemistry for the IB Diploma |edition=2nd |last1=Harwood |first1=Richard |last2=Coates |first2=Christopher |last3=Talbot |first3=Christopher |date=2015 |publisher=Hodder Education |isbn=9781471829222 |language=en}}</ref>

Where the octane number is raised by blending in ethanol, energy content per volume is reduced. Ethanol energy density can be compared with gasoline 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 today.  Racing fuels, [[avgas]], LPG and [[alcohol fuel]]s such as [[methanol]] may have octane ratings of 110 or significantly higher. Typical "octane booster" [[gasoline additive]]s include [[Methyl tert-butyl ether|MTBE]], [[ETBE]], [[toluene]] and iso-octane itself. [[Lead]] in the form of [[tetraethyllead]] was once a common additive, but concerns about its toxicity have led to its use for fuels for road vehicles being progressively phased out worldwide beginning in the 1970s.<ref>{{cite web |url=http://yosemite.epa.gov/R10/airpage.nsf/webpage/Leaded+Gas+Phaseout |title=Leaded Gas Phaseout |date=June 1995 |publisher=U.S. EPA, Region 10 |archive-url=https://web.archive.org/web/20080603152013/http://yosemite.epa.gov/R10/airpage.nsf/webpage/Leaded%2BGas%2BPhaseout |archive-date=2008-06-03 |url-status=dead |access-date=2012-06-15 }}</ref>