What does the octane number measure?

The octane number measures the tendency of a fuel to auto-ignite, which is a key property for assessing fuel performance, particularly in internal combustion engines. The octane rating indicates how resistant the fuel is to knocking or pinging during combustion, which occurs when fuel-air mixtures ignite prematurely in the engine cylinder. Fuels with higher octane numbers have greater resistance to auto-ignition, allowing for higher compression ratios and improved engine efficiency without knocking.

In an engine, knocking can lead to inefficient fuel use and potential engine damage, making it essential to use fuels appropriate to the engine's design. The octane number helps consumers and engineers select the right fuel for optimal performance. The other choices do not accurately reflect what the octane number represents, as it specifically focuses on the ignition quality of the fuel rather than combustion heat, energy output, or concentration changes in chemical reactions.