Numerical study of spray combustion and soot emission of gasoline–biodiesel fuel under gasoline compression ignition-relevant conditions

Abstract

A blend of biodiesel and gasoline fuel has been proposed for powering gasoline compression ignition (GCI) engines to improve overall reactivity, which is essential for the auto-ignition quality of GCI engines. Previous studies emphasized the differences in the physicochemical properties of hydrogenated catalytic biodiesel (HCB) and gasoline, which significantly altered the combustion characteristics. This study is a further step in this direction to gain a deeper understanding on the spray combustion and emission behavior of this promising fuel based on computational fluid dynamic simulation in a constant volume chamber. N-Hexadecane and toluene reference fuel were selected as surrogates for HCB and gasoline fuel, respectively. A reduced kinetic mechanism for HCB-gasoline surrogate was developed and combined with a reduced PAH mechanism to simulate HCB-gasoline spray combustion and soot formation. Liquid penetration length, ignition delay, lift-off length, and soot formation for HCB and HCB/gasoline surrogates were validated against available experimental data in the literature. The proposed mechanism along with a multi-step phenomenological soot model could capture the spray characteristics and soot formation of the tested fuels with acceptable accuracy under GCI engine combustion conditions. The numerical results showed larger spray/flame development for HCB fuel as compared with that of HCB/gasoline blend prior to ignition and at ignition stage, which was mainly due to the less fuel spray mixing with the surrounding fresh air. Furthermore, the outer diffusion flames for HCB and HCB/gasoline blend were identical and existed on the stoichiometric region (at ϕ ∼ 1), while the rich pre-mixed core of HCB was located at higher equivalence ratio, which was more favorable for precursors formation. HCB/gasoline fuel had a lower soot mass yield compared with HCB because of its lower C2H2 concentration, which slowed down the surface growth step. In addition, the effects of ambient oxygen level and ambient temperature on flame structure and soot formation were also investigated. The current study provides a useful information about spray combustion and soot formation of HCB–gasoline blend, which has practical applications in GCI engines.