In a petrol engine, under certain operating and ambient conditions,knock may occur which can cause damage to the engine. Its occurrence depends as well on engine and fuel properties. A measure of the anti-knock performance of a fuel in a given engine is given by its octane number: the higher the octane number, the higher the resistance to knock. A scale has been devised in which automotive fuels are assigned an octane number, which is based on two hydrocarbons defining the ends of the scale: normal heptane (n-C7H16) which has a value of zero and iso-octane (C8H18 2, 2, 4-trimethylpentane) with an octane number of 100. Blends of these two hydrocarbons define the knock resistance, e.g., a blend of 10% n-heptane and 90% iso-octane by volume has an octane number of 90.
The octane number of a fuel is determined in a standard test engine (single-cylinder, variable compression ratio CFR engine developed under the auspices of the Cooperative Fuel Research Committee in 1931) by means of either the research method (ASTM D-2699; BS 2637) or the motor method (ASTM D-2700; BS 2638). The corresponding research octane number (RON) and motor octane number (MON) are obtained under different test conditions which are summarized in Table 1. [Heywood (1988) and Owen and Coley (1990)]. Typical octane numbers for automotive fuels are given in Table 2. A worldwide summary of octane ratings is published regularly by the Associated Octel Co. Ltd, London.
*Typical octane numbers
Heywood, J. B. (1988) Internal Combustion Engine Fundamentals, McGraw Hill.
Owen, K. and Coley, T. (1990) Automotive Fuels Handbook, Society of Automotive Engineers