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In previous work it has been shown that the autoignition quality of a fuel at a given operating condition can be described by its Octane Index, OI = (1-K)RON - KMON; the larger the OI, the more the resistance to autoignition. Here RON and MON are, respectively, the Research and Motor Octane numbers of the fuel and K is a constant depending only on the pressure and temperature history of the fuel / air mixture in the engine prior to autoignition. The value of K is empirically established at a given operating condition by ranking fuels of different RON and MON and of different chemical composition for their ease of autoignition. Another important parameter at a given operating condition is OI0, the Octane Index of the fuel for which heat release is centred at TDC. In previous work K and OI0 were measured at different operating conditions and were related empirically to pressure and temperature of the mixture before autoignition and to engine speed and mixture strength.