The Available and Required Autoignition Quality of Gasoline - Like Fuels in HCCI Engines at High Temperatures

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.

In this work K and OI₀ have been measured in a different engine at different conditions that include much higher intake temperatures and engine speeds than considered before. The focus is also on wide-boiling-range complex fuels rather than simple fuel systems. K is again seen to depend primarily on the compression temperature at 15 bar, T_comp15. However in the higher temperature range, K increases much more sharply with T_comp15 than found previously. There is also an overlap between the new data and the old data, confirming that the variation of K is not engine dependent. The data for OI₀ for the two engines can also be described by a combined model. OI₀ increases with increasing P_maxcomp and T_maxcomp, the compression pressure and temperature at TDC, and decreases with increasing λ and engine speed.