Past research has shown that post injections have the potential to reduce Diesel engine exhaust PM concentration without any significant influence in the NOx emissions. In earlier research it was observed that soot reduction due to a post injection is based on three reasons: increased turbulence (1) and heat (2) from the post injection during soot oxidation and lower soot formation due to smaller main injection for similar load conditions (3). The second effect of heat addition during the soot oxidation is debated in the literature.
The experimental investigation presented in the current work provides insight into the underlying mechanisms of soot formation and reduction using post injections under different operating conditions. The experimental data have been obtained using a cylindrical constant volume chamber with high optical access. The soot evolution has been obtained using 2-color-pyrometry. Furthermore, NO and particle mass and size distribution have been captured from the exhaust. In the experiments, the fuel composition, oxygen content and post injection timing has been varied.
All the operating conditions with 21% O2 showed decreased exhaust soot mass for the smallest post injection dwell timing, representing the highest degree of interaction between the two soot clouds. The change in temperature of two merged soot clouds was found to be negligible. Thus, the effect of turbulence induction appears of higher importance than the effect of heat addition. The exhaust soot reduction cannot be observed with reduced O2 background, even with higher degree of interaction. The reason is assumed to be by limited local O2 availability.