Effects of Fischer-Tropsch diesel fuel on soot formation processes in a diesel spray flame

In order to examine the mechanism by which diesel soot emission is reduced when using Fischer-Tropsch diesel (FTD) fuel compared with conventional diesel fuel (JIS#2) under exhaust gas recirculation conditions, the soot formation processes in a diesel spray flame of the two different fuels (FTD and JIS#2) under different ambient oxygen concentrations (21 to 10%) were investigated via excitation-emission matrix analysis of polycyclic aromatic hydrocarbons (PAHs) and high-speed laser shadowgraphy of soot particles. The experiments were conducted using an optically accessible constant-volume combustion vessel under a diesel-like condition (T(a) = 940 K and P(a) = 2.5 MPa). In the FTD-fuelled diesel spray flame, the timing and region for the first appearance of PAH laser-induced fluorescence (LIF) and soot particles in the flame were delayed and shifted downstream compared with JIS#2. For JIS#2, the LIF appeared first in the shorter wavelength region (350 to 400 nm) and then shifted to the longer wavelength region (above 400 nm), while in the case of FTD, the LIF was observed not in the shorter wavelength but only in the longer wavelength region. The production of soot in the flame was increased by lowering the ambient oxygen concentration from 21 to 1.5% for both fuels, while the timing and region for the first appearance of soot and PAHs in the flame were delayed and shifted downstream. By lowering the oxygen concentration further down to 10%, the timing and region for the first appearance of PAHs and soot were further delayed and shifted downstream and the production of soot was decreased.