Effect of air and exhaust gas dilutions on ultra-fine particulate emissions in different combustion modes

The effect of air and exhaust dilution ratios on the characteristics of ultra-fine particulate matter (PM) and combustion process in different combustion modes at low loads were investigated based on experimental and numerical simulation in this study. The combustion modes included partially premixed compression ignition (PPCI), diffusion combustion (DC), and modulated kinetics combustion (MKC). The air and exhaust gas dilution ratio varied from 0 % to 40 % and 0 % to 55 % respectively. Additionally, the effect of dilutions and combustion modes on PM characteristics is precisely analyzed by computational fluid dynamics. The NOx and Soot emission are reduced in PPCI and DC mode of combustion with the increase of air and exhaust gas dilution rate. However, NOx emission is decreases while Soot is increases in MKC mode. The concentration of particulate number increases in case of both PPCI and DC mode and achieved highest value as 3.3 × 107 and 1.4 × 107 N/cc respectively. Although, concentration of particulate number (PN) in MKC mode first starts climbing and after attaining the highest level of 1.3 × 107 N/cc it falls down. PN concentration in PPCI, MKC, and DC modes decreases as the exhaust gas dilution rate increases. The study of nuclear and accumulate mode of PM was performed separately where the diameter of particles is <1000 nm. Under variable air and exhaust gas dilution ratios, the value of PN is always lies between 40 and 90 % in the nuclear mode PM domain for PPCI and DC modes while for MKC mode, the proportion of PN fall under 13–57 % at exhaust gas ratio above 30 %. The proportion of nuclear mode particulate mass in PPCI mode is >10 % while its values is <1 % for DC and MKC modes.