Study of the multi-injection combustion process in a transparent direct injection common rail diesel engine by means of optical techniques

A combination of ultraviolet-visible optical diagnostics has been applied in the combustion chamber of a diesel engine in order to study the fuel injection, autoignition, and soot formation processes. Measurements were made in an optically accessible diesel engine equipped with a multivalve cylinder head and a common rail (CR) high-pressure injection system. Several multi-injection strategies, characteristic of new generation CR diesel engines, were tested. They consisted of two (pre+main) and three (pre+main+post) injections per cycle. Fuel injection and visible combustion were studied by imaging the natural flame luminosity, and soot formation and evolution were evaluated by means of the two-colour pyrometry method. The reactions that occur during autoignition and soot formation were investigated by chemiluminescence imaging, while the broadband ultraviolet-visible extinction and scattering spectroscopy (BUVESS) technique was used in order to measure the soot particle size and number concentration. For all the injection strategies investigated, pre injection has contributed to the reduction of the ignition delay of the main injection. Moreover, the present study showed that autoignition can be characterized to some degree by the presence of the OH radical. The effect of post injection on combustion development and soot particle diameter has also been observed and discussed.