Auto-ignition Quality of Gasoline-Like Fuels in HCCI Engines 2003-01-3215

The auto-ignition quality of a fuel of any chemistry at a given engine condition is described by an octane index defined as, OI = (1-K) RON + K MON, where RON and MON are the Research and Motor Octane numbers respectively and K depends only on the engine design and operating conditions. The higher the OI value, the greater is the resistance to auto-ignition. A single cylinder homogeneous charge compression ignition (HCCI) engine has been run at thirty seven different operating conditions using fuels of different chemistries and different known RON and MON values. At each operating condition CA50, the crank angle for 50% of the total heat release, is established for different fuels and from this the value of K is determined. We take T_comp15, the temperature when the pressure reaches 15 bar during the compression stroke, as a generic engine parameter. K is strongly dependent on and increases with T_comp15 and is less strongly dependent on the mixture strength. Surprisingly, there was no significant effect of engine speed on K. A predictive equation for K as a function of T_comp15 and the normalized air/fuel ratio, λ is found. At each operating condition there is an ideal fuel with OI = OI₀ such that heat release occurs at top dead center (TDC). OI₀ increases with increasing P_maxcomp and T_maxcomp the compression pressure and temperature at TDC, and decreases with increasing λ and engine speed. An empirical equation relating OI₀ to these generic engine parameters is found. These predictive equations could also be used to explore control strategies for an HCCI engine running on a fuel of known RON and MON.