Definition
A specific liquid hydrocarbon (chemical formula C8H18) used as the reference standard for measuring a fuel's resistance to detonation. Pure iso-octane is assigned an octane rating of 100, meaning it resists knocking very well in a piston engine. Aviation gasoline is rated by comparing how it burns in a test engine against blends of iso-octane and heptane.
Plain English
A particular type of fuel used as the benchmark for how well a gasoline resists engine knock. The closer a fuel behaves to iso-octane, the higher its octane number.
Context Anchor
Seen in aviation gasoline, fuel rating, and piston-engine combustion discussions.
Derivation
From 'iso-' (Greek isos, meaning 'equal' or 'same'), used in chemistry to mean a branched version of a molecule, plus 'octane,' a hydrocarbon with eight carbon atoms (Greek okto, 'eight'). So iso-octane is a branched eight-carbon fuel molecule. Knowing the name describes a chemical structure, not a brand or grade, helps explain why it works as a fixed reference point.
Why Pilots Care
The octane rating derived from iso-octane tells whether the fuel will prevent damaging detonation in high-compression aircraft engines.
Analogy
Think of iso-octane as the 100 mark on a measuring scale. Other fuels are compared to that mark to describe how resistant they are to knock.
Intuition Check
Iso-octane is not just a casual way of saying “high-octane fuel.” It is a specific reference chemical used to define the octane scale.
Example Sentence 1
An aviation gasoline rated 100 octane resists detonation about as well as pure iso-octane under the same test conditions.
Example Sentence 2
Mechanics confirm the engine runs smoothly on fuel blended with sufficient iso-octane for the compression ratio.