The Effect of Fuel-Vapor Concentration on the Process of Initial Combustion and Soot Formation in a DI Diesel Engine Using LII and LIEF

A phenomenological or empirical model based on experimental results obtained from various optical measurements is critical for the understanding of DI diesel combustion phenomena as well as for the improvement of its emission characteristics. Such a model could be realized by the application of advanced optical measurement, which is able to isolate a particular phenomenon amongst complicated physical and chemical interactions, to a DI diesel combustion field. The authors have conducted experimental studies to clarify the combustion characteristics of unsteady turbulent diffusion flames in relation to the soot formation and oxidation process in a small-sized DI diesel engine. In the present study, the effect of fuel vapor concentration on the process of early combustion and soot formation has been investigated using several optical measurements. Laser-induced incandescence (LII), which is the most effective way to visualize soot particles, has been used to obtain the 2-D distribution of soot particles in a sooting field. Furthermore, planar laser-induced exciplex fluorescence (LIEF), which is based on application of the TMPD/naphthalene excited-complex system, has been applied to simultaneous visualization of the liquid and vapor phases of the fuel jet in a DI diesel engine. The experimental results present the effect of fuel vapor distribution on the initial soot formation process subsequent to autoignition in addition to its spatial dependence. The detailed experimental results obtained are presented and discussed in this paper.