Spectroscopic measurements and 2D digital imaging were used in single cylinder, four-stroke DI diesel engine, optically accessible. It was equipped with a four-valve head and fully flexible electronic controlled ‘Common Rail’ injection system. Three fuel injection strategies, descriptive of the CR diesel engine, were considered. They consisted of a main, a pilot and main and finally pilot, main and post injections. Fuel spray and visible flame propagation were evaluated by digital imaging at high temporal resolution. Autoignition and combustion processes were analysed by broadband ultraviolet-visible flame emission spectroscopy. Radical species such as OH and C2 allowed to characterise the ignition process and pollutant formation. Soot temperature and mass concentration were evaluated by two-colour pyrometry. The presence of C2 and OH radicals strongly characterised CR diesel combustion process during soot formation and evolution. In particular, the high presence of OH concentration for the whole process, from the autoignition to the soot formation and successive phases, contributed to lower the soot levels.

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