Effect of inter-jet spacing on soot formation in the direct injection diesel combustion process

Last events taking into account the growing enviromental awareness motivated the automotive industry community to further improve internal combustion engines (ICE), which should remain relevant for the foreseeable future. Pollutant emission mechanisms in the last strategies followed to reduce fuel consumption, such as the use of smaller diameters multi-hole injector nozzles, need to be investigated. The effect of jet-spacing in soot formation is yet unclear, this being the goal of the present work. A high-pressure and high-temperature vessel with large optical accesses is employed. Two diesel injectors with six outlet orifices were used. The only difference between them is the geometric distribution of the outlet holes, so 30°, 36°, 45° of jet spacing are compared to an isolated spray. The Diffuse back-illumination (DBI) technique is implemented to compute the soot optical thickness and mass. For that, a novel window was designed together with the injector holder for proper optical access. A wide test matrix varying ambient temperature, density and oxygen concentration as well as injection pressure was selected. The spray configuration with closer sprays (30° and 36°) had higher optical thickness and peak soot mass when compared to the isolated spray probably due to local effects of hot gases re-entrainment. The impact of the inter-jet spacing was smaller under boundary conditions promoting soot formation. Novelty and significance Most of the research studies about the inter-jet spacing have been carried out in engines with limited optical access. In turn, fundamental and focused works, performed in controlled environments inside high-pressure and high-temperature test chambers, have not deeply analyzed combustion emissions such as soot. The main novelty and significance of this work is precisely this analysis. The DBI technique with strong and fast LEDs is applied to assess the temporal evolution of soot formation on the injection event. Light extinction is used to obtain the soot extinction coefficient across the soot cloud of direct injection diesel sprays, which is related to the soot mass produced. Three inter-jet spacing configurations are analyzed and compared to the performance of a single isolated spray. These new results will complete the study of the effect of the jet-to-jet separation on the combustion process available in the literature.