Two-Color Combustion Visualization of Single and Split Injections in a Single-Cylinder Heavy-Duty D.I. Diesel Engine Using an Endoscope-Based Imaging System

An experimental study of luminous combustion in a modern diesel engine was performed to investigate the effect of injection parameters on NO_X and soot formation via flame temperature and soot KL factor measurements. The two-color technique was applied to 2-D soot luminosity images and area-averaged soot radiation signals to obtain spatially and temporally resolved flame temperature and soot KL factor. The imaging system used for this study was based on a wide-angle endoscope that was mounted in the cylinder head and allowed different views of the combustion chamber. The experiments were carried out on a single-cylinder 2.4 liter D.I. diesel engine equipped with an electronically controlled common-rail injection system. Operating conditions were 1600 rpm and 75% load.

The two-color results confirm that retarding the injection timing causes lower flame temperatures and NO_X emissions but increased soot formation, independent of injection strategy. Split injections have been shown to be a powerful tool to simultaneously reduce soot and NO_X emissions when the injection timing is optimized. Injection termination and resumption prevents not only fuel-rich combustion zones but also causes more complete combustion during the injection dwell due to better air utilization. However, split injections can also result in worse soot emissions when either the injection timing is too retarded, leading to poor ignition, or the injection dwell is not optimized.

“Stereo” views of two-color images imply that soot formation occurs most likely in the fuel-rich core of the spray plumes near its leading edge. Higher soot KL factors were found along the bottom and the wall of the piston bowl. Peak local flame temperatures for the single injections reached up to 2800 K and above. The calculated KL factors were between 0.1 (thin soot) and 3.0 (thick soot) for all injection cases.