octane number


octane number
   or octane rating
   a measure used to express the ability of gasoline to reduce engine knocking. Gasoline is a complex mixture of hydrocarbons: compounds containing hydrogen and carbon. Beginning chemistry students learn that "octane" is the name of a hydrocarbon whose molecules contain 8 carbon atoms and 18 hydrogen atoms, the 8 carbons being arranged in a long chain. The compound cars need to prevent knocking is not that octane but a different compound of 8 carbon atoms and 18 hydrogen atoms called iso-octane or, in the more precise language of chemical nomenclature, 2,2,4-trimethylpentane. (In an iso-octane molecule there are only 5 carbons in the chain. Carbons 6 and 7 are attached to the sides of the chain at the \#2 position, and the last carbon is hooked onto the \#4 position; chemists call this a branched hydrocarbon.) To determine the octane rating of gasoline, a sample of the gasoline is compared to a laboratory mixture of iso-octane and another hydrocarbon called heptane (heptane has 7 carbons and 16 hydrogens, with the 7 carbons in a chain). The mixture is adjusted until it has the same anti-knocking characteristics as the gasoline being tested. The octane rating is the percentage of iso-octane required in the laboratory mixture to produce this equality of knocking behavior. In fact, it's even more complicated: there are two ways to do the test, producing two ratings called the research octane number (RON) and the motor octane number (MON). The MON is typically 8 or 10 points lower than the RON for the same batch of fuel. In the U.S., the number posted on the gas pump is the average of the RON and MON; this average is called the pump octane number (PON). There is a similar unit, cetane number, used for rating diesel fuel. The octane rating is often misunderstood as a measure of the energy content of the fuel, but what it actually measures is the tendency of the fuel to burn rather than explode.

Dictionary of units of measurement. 2015.

Look at other dictionaries:

  • octane number — ☆ octane number or octane rating n. a number representing the antiknock properties of a gasoline, fuel mixture, etc., determined by the percentage of isooctane that must be mixed with normal heptane to produce the knocking quality of the fuel… …   English World dictionary

  • octane number — Every brand of gasoline has an octane rating or number which indicates its ability to resist knocking. When the numbers were first developed, the researchers found that normal heptane (a hydrocarbon) had almost no ability to resist knocking so… …   Dictionary of automotive terms

  • octane number — (of gasoline) a designation of antiknock quality, numerically equal to the percentage of isooctane by volume in a mixture of isooctane and normal heptane that matches the given gasoline in antiknock characteristics. Also called octane rating.… …   Universalium

  • octane number — 1. A measure of petrol s resistance to pre ignition or pinking , which is the sound made by an engine when pre ignition occurs, particularly at low revs and high load. Two measures are used, for low speed and high speed pre ignition. ULSP in the… …   Petroleum refining glossary

  • octane number — oktaninis skaičius statusas T sritis Standartizacija ir metrologija apibrėžtis Benzino antidetonacinių savybių rodiklis. atitikmenys: angl. octane number vok. Oktanzahl, f rus. октановое число, n pranc. indice d’octane, m; nombre d’octane, m …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

  • octane number — oktaninis skaičius statusas T sritis Standartizacija ir metrologija apibrėžtis Sutartinės skalės skaičius, reiškiantis degalų, skirtų benzininiams varikliams, atsparumą detonacijai. atitikmenys: angl. octane number vok. Oktanzahl, f rus.… …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

  • octane number — noun a measure of the antiknock properties of an automobile fuel; isooctane having an octane number of 100, a fuel with the same antiknock properties as a mixture of 90% isooctane and 10% heptane having an octane number of 90 …   Wiktionary

  • octane number — oktaninis skaičius statusas T sritis Energetika apibrėžtis Skaičius, apibūdinantis benzino ar žibalo detonacinį atsparumą. Pastarasis lyginamas su etaloninio mišinio, sudaryto iš izooktano C₈H₁₈ ir heksano C₇H₁₆, detonaciniu atsparumu. Izooktanas …   Aiškinamasis šiluminės ir branduolinės technikos terminų žodynas

  • octane number — oktaninis skaičius statusas T sritis chemija apibrėžtis Benzino antidetonacinių savybių rodiklis. atitikmenys: angl. octane number rus. октановое число …   Chemijos terminų aiškinamasis žodynas

  • octane number — noun Date: 1931 a number that is used to measure the antiknock properties of a liquid motor fuel (as gasoline) with a higher number indicating a smaller likelihood of knocking called also octane rating compare cetane number …   New Collegiate Dictionary


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