Two-Dimensional In-Cylinder Soot Volume Fractions in Diesel Low Temperature Combustion Mode

Soot Volume Fraction (SVF) measurements were performed in an IFP Energies nouvelles optical single cylinder Diesel engine operated in Low Temperature Combustion (LTC) conditions. The engine was equipped with a sapphire liner, a dedicated flat bowl piston and a six-hole common-rail high pressure injector. The piston design included four quartz windows allowing optical access into the bowl. The aim of this work was to study soot formation and oxidation during the LTC Diesel combustion process and to build a database providing soot formation and oxidation data under a set of engine conditions to help developing and testing Computational Fluid Dynamics (CFD) models. Two complementary optical diagnostic techniques were combined: Planar Laser Induced Incandescence (PLII) and Laser Extinction Method (LEM). The non calibrated two dimensional image of soot distribution obtained by LII was coupled with the line of sight integrated absolute SVF value obtained by LEM to provide two dimensional SVF fields in a plane crossing the center of the chamber, perpendicular to the piston surface. The in-cylinder soot evolution was also compared to exhaust smoke measurements. The influence of injection quantity and timing, injection pressure, dilution rate, intake temperature and fuel nature on SVF was investigated. Three fuels were tested: a two component Diesel surrogate, a standard Diesel and a 5% biodiesel blend. The analysis of the 2D SVF fields showed that soot was systematically observed in two zones located symmetrically on both sides of the common cylinder and injector axis. The engine load and injection pressure, directly related to the local fuel/air ratio, were the main parameters impacting soot levels. The injection timing and the intake temperature were found to significantly affect the in cylinder soot levels but not the exhaust soot emissions. It was proposed that the efficiency of soot post-oxidation is responsible for this result. The increase of the dilution ratio led to a decrease of in-cylinder soot levels but to an increase of exhaust soot emissions. The proposed mechanism for this behavior is a reduction of soot post-oxidation efficiency with increasing dilution. The surrogate Diesel fuel showed earlier and higher in-cylinder soot levels compared to the standard Diesel while lower soot levels were obtained with the 5% biodiesel.