octane number

noun
(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.


Origin:
1930–35

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Collins
World English Dictionary
octane number or octane rating
 
n
a measure of the quality of a petrol expressed as the percentage of isooctane in a mixture of isooctane and n-heptane that gives a fuel with the same antiknock qualities as the given petrol
 
octane rating or octane rating
 
n

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
octane number  
A numerical representation of the ability of a fuel to resist knocking when ignited in the cylinder of an internal-combustion engine. The octane number of a given fuel is determined by comparing the amount of knocking that fuel causes when combusted with the amount of knocking caused by two standard reference fuels, isooctane (which resists knocking and has an octane number of 100) and heptane (which causes knocking and has an octane number of 0). The octane number is then assigned as the percentage of isooctane required in a blend with normal heptane to match the knocking behavior of the fuel being tested.
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Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

octane number

measure of the ability of a fuel to resist knocking when ignited in a mixture with air in the cylinder of an internal-combustion engine. The octane number is determined by comparing, under standard conditions, the knock intensity of the fuel with that of blends of two reference fuels: iso-octane, which resists knocking, and heptane, which knocks readily. The octane number is the percentage by volume of iso-octane in the iso-octane-heptane mixture that matches the fuel being tested in a standard test engine. See also knocking

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
To avoid engine knock, the team's engine runs on ethanol, which has a higher
  octane number than gasoline.
In general, a higher octane number means the engine can run more efficiently
  and will have less tendency to knock.
Naphtha isomerization converts the straight chains to branched, significantly
  raising their octane number.
Such a material would have a high octane number and could be a useful additive
  to motor gasoline.
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