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The octane number or octane rating of a fuel indicates its ability to resist knocking or pinging during combustion in a spark-ignition engine. Knocking occurs when the fuel-air mixture in the engine cylinder ignites prematurely due to high pressure and temperature. Fuels with higher octane numbers burn more smoothly, improving engine performance.
The scale of octane numbers is based on two reference hydrocarbons:
Iso-octane (2,2,4-trimethylpentane) – Assigned an octane number of 100 because it has excellent resistance to knocking.
n-heptane (normal heptane) – Assigned an octane number of 0 because it ignites very easily and causes severe knocking.
The octane rating of any fuel is measured by comparing its knocking tendency to mixtures of iso-octane and n-heptane. For example, a fuel with an octane number of 90 behaves the same as a mixture containing 90% iso-octane and 10% n-heptane.
n-heptane, a straight-chain hydrocarbon (C₇H₁₆), has poor combustion control and low resistance to pre-ignition. Its linear structure causes it to auto-ignite quickly under compression, leading to engine knocking. This is why n-heptane is the reference compound with an octane value of 0.
Other options:
Iso-octane is highly resistant to knocking, with a value of 100.
n-hexane is not part of the standard reference scale.
Iso-heptane has better anti-knock properties than n-heptane but is not the 0 reference compound.
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