Octane

Octane has two closely related meanings in chemistry and petroleum science. It can refer to the eight-carbon alkane C8H18, or more commonly to a blend of eight‑carbon hydrocarbons used in gasoline. In chemical terms, there are 18 structural isomers of octane, with n-octane (the straight‑chain form) serving as the reference component.

Octane rating is a measure of a fuel’s resistance to knocking in spark-ignition engines. Knocking occurs when fuel‑air mixtures detonate prematurely, reducing efficiency and potentially damaging engines. The rating is defined relative to iso-octane (2,2,4-trimethylpentane), assigned a value of 100, while n-heptane is assigned 0. Two standard scales exist: the Research Octane Number (RON) and the Motor Octane Number (MON). Many fuels are labeled by the average of these two values, known as the anti-knock index (AKI). For example, common gasoline might be advertised as 87 AKI (often expressed as 87 octane). A higher octane number indicates greater resistance to knocking and generally allows higher compression ratios in engines.

Octane rating reflects combustion quality rather than energy content. Higher octane fuels do not inherently contain more energy per liter; energy content depends on overall hydrocarbon composition and density. Engine designers specify minimum octane requirements to prevent knocking under intended operating conditions.

Historically, octane ratings helped standardize fuel performance in the early gas-engine era. Additives such as tetraethyl lead were used to raise octane but have been phased out in many regions due to toxicity, with alternatives like certain ethers or ethanol blends now common. Octane remains a key concept in fuel engineering, influencing engine design, performance, and fuel formulation.