Physicochemical properties and oxidation reactivity of exhaust soot from a modern diesel engine: Effect of oxyfuel type

Abstract

This paper investigates the physicochemical properties (i.e. nanostructure, degree of graphitization, surface functionalities and carbon chemical state) of soot emitted from a modern compression ignition (CI) engine fueled by diesel (D100) and its blends with 11.5 vol% methanol (M11.5), 8.3 vol% dimethyl carbonate (DMC8.3) and 13 vol% dimethoxymethane (DMM13) with the same oxygen level. Additionally, based on the sensitivity analysis results, the correlation between soot physicochemical properties and its oxidation reactivity is discussed. Results showed that the soot from M11.5 owned the most disordered nanostructure (i.e. shortest fringe length, widest distance and greatest tortuosity) with D100, DMM13, DMC8.3 soot following, while soot graphitization degree by Raman analysis exhibited inconsistent results with nanostructure parameters. On the side of chemical properties, the aliphatic CH content reduced in the ranking of D100>DMC8.3>DMM13>M11.5. As change of fuel formulation, it was found that the O/C ratio presented irregularity under the different engine loads. For the three blended fuels, DMM13 soot possessed the highest surface oxygenated functional groups (SOFGs, including CO, CO and COO) amount, followed by DMC8.3 and M11.5 soot. Moreover, the carbon hybridization ratio (sp3/sp2) presented a trend of M11.5>D100>DMM13>DMC8.3, which verified the analysis results of the nanostructure. The soot oxidation reactivity decreased in the order of M11.5>D100>DMM13>DMC8.3. As the engine load raised, the oxidation reactivity of D100 and DMM13 soot increased, while it reduced for M11.5 and DMC8.3 soot. According to the sensitivity analysis, soot reactivity primarily depended on particles nanostructure, sp3/sp2 ratio and SOFGs, while the impact from aliphatic CH functional groups and O/C ratio were weak. Furthermore, compared to SOFGs, soot reactivity was more sensitivity to particle nanostructure, especially the fringe tortuosity.