The Effects of Injection Timing and Diluent Addition on Late-Combustion Soot Burnout in a DI Diesel Engine Based on Simultaneous 2-D Imaging of OH and Soot

The effects of injection timing and diluent addition on the late-combustion soot burnout in a direct-injection (DI) diesel engine have been investigated using simultaneous planar imaging of the OH-radical and soot distributions. Measurements were made in an optically accessible DI diesel engine of the heavy-duty size class at a 1680 rpm, high-load operating condition. A dual-laser, dual-camera system was used to obtain the simultaneous “single-shot” images using planar laser-induced fluorescence (PLIF) and planar laser-induced incandescence (PLII) for the OH and soot, respectively. The two laser beams were combined into overlapping laser sheets before being directed into the combustion chamber, and the optical signal was separated into the two cameras by means of an edge filter.

These simultaneous images show the spatial relationship between the remaining hot reaction zones and the remaining soot in the bulk-gases (i.e. away from the combustion chamber walls) during the final stages of the combustion event. The PLII-soot images also provide an indication of the amount of unoxidized soot remaining in the bulk gases. Injection timing retard and N₂ diluent addition (simulated EGR) were both found to produce substantial changes in the soot burnout at the conditions investigated. These conditions are representative of those that are typically more problematic for production engines in terms of the soot-NO_X tradeoff. The images indicate that incomplete burnout in the bulk gases can result from both insufficient time to complete combustion and extinction of the combustion reactions associated with the remaining soot pockets. Bosch-smoke type engine-out soot measurements were also made, and changes in the engine-out soot with operating conditions were found to correlate with changes in the in-cylinder data. The results strongly suggest that changes in soot burnout in the bulk gases are a major contributor to the increased soot emissions typically observed when timing is retarded and/or diluent is added to reduce NO_X in diesel engines at conditions similar to those investigated.