Some results of a systematic analysis of important sources of hydrocarbon emissions from a direct injection diesel engine are presented. The following sources are considered and investigated: (1) local over-mixing, (2) poor end of injection, (3) fuel emptying from sac volume.
The analysis uses systematic engine experiments and an existing two-dimensional thick evaporating spray model to determine the contribution of various hydrocarbon sources to the total hydrocarbon emissions in the exhaust.
The results show that at idle and light load conditions, local overmixing is the major source of hydrocarbon emissions. The amount of fuel over-mixed is directly controlled by mixing rate, ignition delay, and the lean limit of combustion. Mixing rate calculations show that the injection rate shape and nozzle geometry are more important than the physical properties of the fuel in determining the amount of fuel overmixed.
The results also show that for a low emission engine using timing advance at light loads for overmixing control, poor end of injection and fuel emptying from the sac volume can be important sources of hydrocarbons. The characterization of these sources and their effect on fuel evaporation and hydrocarbon emissions are discussed.