Octane rating

Octane rating measures a gasoline’s resistance to knock in spark-ignition engines by comparing it with reference mixtures of iso-octane and n-heptane.

Engine knocking: Abnormal combustion in a spark-ignition engine produces pressure waves and a characteristic metallic sound. Octane rating measures a fuel’s resistance to this damaging form of combustion.
2,2,4-Trimethylpentane: 2,2,4-Trimethylpentane, commonly called iso-octane, is a branched alkane assigned an octane number of 100. It defines the high-resistance endpoint in the classic reference-fuel scale.
Anti-knock agent: An anti-knock agent is a fuel additive that suppresses knock or raises a fuel’s effective octane rating. Additives can help gasoline meet the knock resistance required by engines.
Cetane number: Cetane number measures how readily diesel fuel ignites under compression, chiefly through ignition delay. It concerns compression-ignition engines, the opposite ignition strategy from octane’s spark-ignition context.
Autoignition: Autoignition is spontaneous ignition of a fuel–air mixture without a spark initiating combustion. High-octane fuels resist autoignition of the unburned mixture ahead of the flame.
n-Heptane: n-Heptane is a straight-chain alkane assigned an octane number of zero on the classic scale. It defines the low-resistance endpoint of the reference mixtures.
Tetraethyllead: Tetraethyllead is a lead-containing compound once widely added to gasoline to increase knock resistance. It raised octane effectively before health and environmental harms drove its phaseout.
Energy density: Energy density is the amount of energy stored per unit volume or mass of a substance. A higher octane rating does not mean gasoline contains more energy.
Research octane number: Research octane number is a standardized measure of gasoline knock resistance under relatively mild test conditions. It is one of the two ratings used to report pump gasoline octane in many countries.
Knock rating engine: A knock-rating engine is a standardized variable-compression-ratio engine used to measure fuel knock resistance. Octane ratings are determined by matching test-fuel knock to reference-fuel blends in these engines.

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