Morphological variations of in-flame and exhaust soot particles associated with jet-to-jet variations and jet-jet interactions in a light-duty diesel engine

The variations in soot particle morphology due to jet-to-jet variations and jet-jet interactions have been investigated in a single-cylinder, optically accessible light-duty diesel engine by conducting thermophoretic particle sampling and subsequent transmission electron microscope (TEM) imaging of both in-flame and exhaust soot particles. To this end, the soot sampling experiments were conducted for three different jet configurations including two single-jets (Jet A and Jet B) and a double-jet (Jet A&B) with the latter being investigated at both fixed total fuel mass and fixed injection duration conditions. The results show that the differences in the jet development with respect to the wall cause variations in the soot structures and sizes of aggregates and primary particles in both the in-flame and exhaust samples. However, the jet-to-jet variations are relatively minor compared to the significant impact of jet-jet interactions on soot particles. It is found that the low jet-jet interaction at fixed fuel mass conditions leads to negligible amounts of sampled soot particles; however, the high jet-jet interaction at fixed injection duration conditions causes the increased number of soot particles compared with the single-jet cases, suggesting higher soot formation in the jet-jet interaction region. Both the soot aggregate and primary particle sizes of the double-jet with the fixed injection duration are lower than those of the single-jets, implying that the soot particles in the jet-jet interaction region are in the earlier stage of soot formation. The same trend is observed in the exhaust samples, albeit the differences are much less. It is likely due to the progression in soot formation/oxidation occurring inside the cylinder of the engine before the particles exit through the exhaust. Compared to marked variations in the soot particle sizes, the fractal dimension is largely unchanged due either to jet-to-jet variations or jet-jet interactions in both in-flame and exhaust soot particles.