Impact of flash boiling spray on soot generation of a rich fuel–air mixture under various ambient pressures

Abstract

Particulate emission is one of the critical challenges for the modern gasoine direct injection (GDI) engines. Especially under cold start conditions, the global equivalence ratio inside the cylinder is often set quite rich, leading to the generation of a large amount of soot. The fuel distribution state, such as homogeneous mixing and heterogeneous mixing with local rich pockets prior to the combustion is the key factor affecting soot generation under cold start conditions in GDI engines. Flash boiling injection, as a method to promote fuel air mixing, could be a potential solution to sooting problems of rich mixture combustion under cold start conditions. However, the unclear influences of flash boiling spray to fuel–air mixing state and subsequent soot generation pose a challenge to soot reduction in GDI engines. This work aims to study the effect of various flash boiling degrees on fuel–air mixture formation and subsequent soot generation utilizing a customized constant volume combustion chamber (CVCC) incorporated with the gasoline direct injection scheme under moderately cold start ambient temperature of 30 °C. A global high equivalence ratio of 1.7 representing extremely rich combustions under cold start conditions is selected for strong sooting propensity. By setting different fuel temperatures (Tf= 30 °C and 120 °C) and various ambient pressures (Pa= 40 kPa, 100 kPa and 160 kPa), various spray flash boiling states (subcooled, transitional flash boiling and flare flash boiling sprays) are achieved, and finally various fuel distribution states of rich mixtures prior to combustion are obtained. High speed color camera and soot sampling TEM grids were used to capture and evaluate the characteristics of spray atomization, combustion and soot aggregate structures. Based on this investigation, the effect of flash boiling degree on fuel–air mixing and subsequent soot generation is analyzed. The results show that flash boiling spray enhances fuel atomization and produces much more homogeneous rich mixture with smaller local rich pockets and richer global premixed gaseous portion. Over-rich global premixed mixture combustion is prone to nuclei inception and agglomeration, producing soot aggregates with small primary particles. While the local rich pocket combustion is prone to surface growth, producing larger primary particles. Flash boiling spray could reduce the size of soot particles under cold start conditions by generating smaller local rich pockets.