A visualization and combustion study of interacting fuel sprays in a research diesel engine

An optically-accessible single-cylinder research diesel engine equipped with a dual fuel injection system was employed to study the effects of split injection strategies on fuel vapor development and oxides of nitrogen (NOx) and smoke emissions. The unique fuel injection system allowed for independent control of leading (pilot) and trailing (main) sprays through single-orifice injector nozzles positioned to permit interaction between the two spray plumes.; The evolution of the sprays was studied in the plane of their center lines via the laser-induced fluorescence (Exciplex) technique which permits simultaneous visualization of their liquid and vapor regions in an unfired engine environment. Time-resolved images of the evaporating sprays were analyzed to determine the extent of the vapor, liquid and liquid-free (premixed) vapor zones. The effects of single vs. dual injection at various operating conditions were examined with particular attention to the development of the premixed vapor zones. The findings of the visualization work indicate that dual injection promotes formation of large regions of premixed vapor which are most prominent at high speed operation and large time separations between the leading and trailing injections.; In a complementary investigation the combustion and emission characteristics of this engine were examined in fired-engine tests in which exhaust NOx concentrations and smoke opacity levels were measured. This study revealed that dual injection improves the tradeoff between NOx and smoke emission that is observed when the timing of a single injection is varied. Greatest benefit occurs under conditions of poor spray atomization when the time separation of the two injections is small and permits mutual spray impingement. Here the tradeoff improvement is due to improvements in smoke emission attributed to enhancement of the spray breakup and atomization process by mutual spray impingement. At large spray separations smoke is elevated but NOx emission is suppressed, although visualization findings indicate the probable existence of large premixed zones which would be expected to promote NOx. The dissipation of these zones during the ignition delay period explains the low NOx levels despite the significant premixed regions formed in the early spray history.