Knock propensity and octane number

To determine the octane number, a standard CFR engine is run with the test fuel under specified conditions [9] and the compression ratio is increased until the engine knocks, as indicated by a meter based on an accelerometer attached to the cylinder block. The air/fuel ratio is adjusted for the conditions of maximum knock, which occurs slightly rich of stoichiometric. The test is repeated with a series of reference fuels to find a pair whose critical compression ratios are just above and below that of the test fuel, and the composition of the reference fuel which would exactly match is found by interpolation. 

The reference fuels are mixtures of n-heptane, which knocks easily, and 

Since the early work of Ricardo [7], it has been recognized that some fuels, including many aromatics and ethers, knock less easily than isooctane. This presents no difficulty for an assessment based on the HUCR, or the critical compression ratio (CCR), widely featured in Lovell s comprehensive review of fuel properties in 1948 [10]. However, it does present difficulty for one based on the octane number, because some fuels have octane numbers larger than 100. The octane number scale has been extrapolated in a way which is necessarily arbitrary, and there are secondary standards based on iso-octane containing the anti-knock, lead tetra-ethyl, discussed in Section 7.2.6 [9]. 

Other tests have been used in the past, particularly for aviation gasoline, where it was more important to discriminate accurately between fuels with ON 100. These had the advantage of being more related to real physical phenomena. For example, the performance number [11] was based on a single standard fuel, iso-octane, and the relative indicated mean effective power (imep) (defined in terms of the cylinder pressure [3]), and so is directly related to combustion. The performance number was 100 times the ratio of the knock limited imeps of the fuel and iso-octane. Much of the API 45 project on octane number of mixtures of pure hydrocarbons (see Section 7.2.5) was reported in terms of performance number. This project of the American Petroleum Institute ran from 1938 to 1957, and has provided an invaluable source of basic data. The articles by Lovell [10] and Scott [12] review and interpret these data. Whilst this criterion and these values of CCRs [10,13] are no longer in widespread use for automotive fuels, the data available in the older literature could still be useful in testing chemical models. Because the octane number scale is based on two reference fuels, modelling the octane number of any hydro- 

---