sensitivity while the olefins and aromatics have high octane sensitivity. The commercial gasoline have fuel sensitivity around 7 to 10 units. The test engine operates at low speeds and has an old design of combustion chamber.  In the production high speed engines, under actual operation at varying load, speed and ambient conditions the antiknock performance of commercial motor gasoline may match neither to research method nor to motor method under all operating conditions. In Europe therefore, both the RON and MON for a gasoline are specified in the standards.. Many national gasoline specifications like those in the USA and India use average of the research and motor octane numbers to specify the antiknock quality of motor gasoline. The average of RON and MON is termed as antiknock index (AKI):

Earlier, tetra-ethyl lead (TEL), an anti-knock additive was used to produce high octane number fuels at a low cost. However, in 1975 when the gasoline vehicles for the first time employed catalytic converters for emission control, lead was required to be phased out of gasoline. Lead by itself is hazardous to human health. Now, gasoline is totally lead-free in most of the countries. The lead-free gasoline is blended with high-octane fuel components like aromatics, isoparaffins, alcohols and methyl tertiary butyl ether (MTBE) to improve its anti-knock quality.

Octane number has no direct effect on engine emissions. Severe knocking increases combustion temperatures and may result in higher NO formation. The fuel octane number may affect emissions through dependence of engine compression ratio on the octane number. For lead free gasoline, hydrocarbon composition depends on its octane number. High ON lead free gasoline may contain more of aromatics and olefins and hence unburned hydrocarbons also are likely to have higher concentration of aromatics and olefins,  which are more photo-chemical reactive.