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

Abstract

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 1bar and 2bar. 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 1bar intake pressure but in order of decreasing φ for 2bar intake pressure. Increased low- and intermediate-temperature heat release at 2bar 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 1bar 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 2bar 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 1bar intake pressure such that a sizable reduction in the maximum pressure rise rate compared to HCCI can be achieved.