Octane Number

A rating of the 'knock' characteristics of motor fuels. The octane number is equal to the numerical value of the percentage by volume of iso-octane, C8H18, in a mixture of iso-octane and heptane, C7H16, having the same knock characteristics as the fuel being tested.

The higher the octane number or rating, the greater are the antiknock qualities of the petrol. A rating of how much fuel can be compressed before it ignites spontaneously.

Research Octane Number (RON)
Heptane 0
Diesel Fuel 15 to 25
ISO-Octane 100
Benzene 101
Methanol 108.7
Methane 120

Methyl Tertiary Butyl Ether

A fuel oxygenate used as an additive to gasoline to increase octane and reduce engine knock.

Tetraethyl Lead

One gram of Tetraethyl Lead increases the octane of one gallon of gasoline about 6 numbers. The EPA has phased down the use of lead in gasoline as it has been determined to be a health hazard. Lead has been prohibited in highway vehicle gasoline since January 1, 1996.


Derived from petroleum and used to increase octane. Xylene is highly photochemically reactive and, as a constituent of tailpipe emissions, is a contributor to smog formation.

See also: Cetane Number, Engine Knock, Methyl Tertiary Butyl Ether, Tetraethyl Lead, Xylene.

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Subjects: Chemistry Mechanical Engineering