Soot Formation Analysis by Multiwavelength Spectroscopy in an External Chamber Diesel Engine Equipped with a CR Injection System

Diesel combustion process was studied and characterized by digital imaging and ultraviolet-visible flame emission, extinction and scattering spectroscopy. Optical measurements were applied to a transparent diesel engine, realized by modifying a single cylinder, air-cooled, 4-stroke diesel engine by means of an external combustion chamber on the top of the engine, connected to the main chamber by a tangential passage. Diesel engine was equipped with a fully flexible electronic controlled 'Common Rail' injection system. Measurements were performed at 1000 rpm engine speed for two typical injection strategies. The first one consisted of a main injection in order to compare the results with those ones obtained by conventional injection system operating at low pressure. The other one was based on a pilot and main injection that is typical of current direct injection diesel engines. Air/fuel mixture formation, liquid-vapor distributions, and autoignition process were evaluated, for both engine operating conditions, by combining spectral extinction and flame emission measurements with digital imaging. The presence of OH and CH radicals, detected by chemiluminescence, permitted to characterize and identify both temporally and spatially the first stage of combustion. The temporal evolution of soot particles from carbonaceous precursors to soot oxidation was followed by spectral extinction and scattering measurements in terms of mass concentration and particle size. The results obtained for the fuel injection strategies investigated showed that the diameter of soot particles were smaller than 20nm and, finally, the injection strategies influenced the soot amount but not the particle size.