Identifying a critical time for mixing in a direct injection diesel engine through the study of increased in-cylinder mixing and its effect on emissions

Abstract

An experimental study aimed at understanding the effect of in-cylinder mixing on direct injection diesel combustion was conducted in which an auxiliary gas injector was installed on to a single-cylinder diesel engine. The gas injector was used to inject air and nitrogen, thereby enhancing in-cylinder mixing at a specific time in the combustion cycle. The phasing of that gas injection event with respect to the fuel injection event was changed. The results suggest that it is primarily the fuel injected at the tail end of the fuel injection event that has difficulty mixing. Prior to the end of fuel injection, the kinetic energy input during fuel injection is sufficient to mix the fuel with air. However, near the end of fuel injection the kinetic energy is low and the fuel is not followed by any kinetic energy inputs that could help bring about mixing, which causes that fuel to produce large amounts of soot that ultimately have trouble oxidizing. It was also found that in-cylinder turbulence had no effect on NO_x emissions when it occurred after +20° ATDC (after top dead centre), just after the end of fuel injection in these experiments. It was believed that, by that time, the NO_x chemistry was frozen. However, NO_x emissions were affected by early gas injection such that injecting air brought about an increase in NO_x emissions while injecting nitrogen reduced NO_x emissions. This finding suggests that early and prolonged injection of diluents could be used as a soot-friendly form of exhaust gas recirculation, provided that the injection overlaps the end of fuel injection.