The Visualization of Soot Late in the Diesel Combustion Process by Laser Induced Incandescence with a Vertical Laser Sheet

Although soot-formation processes in diesel engines have been well characterized during the mixing-controlled burn, little is known about the distribution of soot throughout the combustion chamber after the end of appreciable heat release during the expansion and exhaust strokes. Hence, the laser-induced incandescence (LII) diagnostic was developed to visualize the distribution of soot within an optically accessible single-cylinder direct-injection diesel engine during this period. The developed LII diagnostic is semi-quantitative; i.e., if certain conditions (listed in the Appendix) are true, it accurately captures spatial and temporal trends in the in-cylinder soot field. The diagnostic features a vertically oriented and vertically propagating laser sheet that can be translated across the combustion chamber, where “vertical” refers to a direction parallel to the axis of the cylinder bore. The diagnostic allows soot visualization in almost the entire region above the piston bowl late in the cycle (until the piston descends below the imaged field of view). It also enables estimation of the total in-cylinder soot mass as a function of crank angle. These attributes of the diagnostic allow it to provide unique insights into the production, oxidation, and distribution of soot within the combustion chamber. This manuscript reports on the development of the diagnostic and results from its initial application, in which 21-, 18-, and 16-mol% intake-oxygen conditions were examined.