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Why is 2,2,4-trimethylpentane used as a reference fuel for octane ratings?

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structures of common hydrocarbon compounds — hydrocarbon Structures assumed by hydrogen (H) and carbon (C) molecules in four common hydrocarbon compounds.

2,2,4-trimethylpentane, commonly known as iso-octane, is used as a reference fuel for octane ratings because of its exceptional ability to resist knocking during combustion in an internal-combustion engine.12

Experimental studies conducted in the 1920s identified that, among standard fuels available at the time, pure iso-octane produced the least amount of knock, while pure normal heptane produced the most extreme knock. Because of these distinct properties, these two substances were selected as the benchmarks for the octane scale.21

The octane number of a fuel is defined by comparing its performance in a standard test engine to that of a mixture of iso-octane and heptane. For example, if a gasoline sample performs identically to a mixture of 90 percent iso-octane and 10 percent heptane, it is assigned an octane rating of 90. This scale allows for the standardized measurement of a fuel's antiknock quality, which is essential for preventing engine damage and ensuring efficient operation.1234