Computational study of the pressure dependence of sequential auto-ignition for partial fuel stratification with gasoline

Fuel stratification is a potential strategy for reducing the maximum pressure rise rate in HCCI engines. Simulations of Partial Fuel Stratification (PFS) have been performed using CONVERGE with a 96-species reduced mechanism for a 4-component gasoline surrogate. Comparison is made to experimental data from the Sandia HCCI engine at a compression ratio 14:1 at intake pressures of 1 bar and 2 bar. Analysis of the heat release and temperature in the different equivalence ratio (φ) regions reveals that sequential auto-ignition of the stratified charge occurs in order of increasing φ for 1 bar intake pressure but in order of decreasing φ for 2 bar intake pressure. Increased low- and intermediate-temperature heat release at 2 bar intake pressure compensates for decreased temperatures in higher-φ regions due to evaporative cooling from the liquid fuel spray and decreased compression heating from lower values of the ratio of specific heats. At 1 bar intake pressure, the premixed portion of the charge auto-ignites before the highest-φ regions and the sequential auto-ignition occurs too fast for useful reduction of the maximum pressure rise rate compared to HCCI. Conversely, at 2 bar intake pressure, the premixed portion of the charge auto-ignites last, after the higher-φ regions. More importantly, the sequential auto-ignition occurs over a longer time period than at 1 bar intake pressure such that a sizable reduction in the maximum pressure rise rate compared to HCCI can be achieved.