The impact of exhaust gas recirculation (including three levels: 0, 10, and 30) on engine combustion characteristics, gaseous emissions, and particulate properties (i.e., oxidative reactivity, carbonaceous compositions, size distribution, and nanostructure) was studied on a common rail diesel engine operating at low engine load. The results showed that the lack of oxygen with EGR prolongated ignition delay and the premixed portion of combustion started to rise significantly. Higher EC (accumulation mode) with larger particle size could be observed with increasing EGR from 0 to 30, which is attributed to the promotion of soot formation with less available oxygen and the inhibition of soot oxidation with low in-cylinder temperature with increasing EGR. The soot nanostructure observation showed that soot changed from smooth surface under 0 EGR to rugose surface under 10 EGR. Moreover, the amorphous core turned larger with increasing EGR. With increasing EGR to 30, the amorphous core appeared to include the whole primary soot particle. The increase of accessible carbons on the edge sites correlates with the high reactivity with increasing EGR. Through the quantitative analysis of the correlation between the combustion parameters and particle properties, we speculated that in this work, the engine at low load producing very little to no conventional soot or soot-EC coupling with low combustion temperature and short residence time with increasing EGR lead to the soot exhibiting less carbonization level (short fringe length and large fringe curvature) and result in higher reactivity.
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