Influence of operating conditions and fuel properties on nanostructure of soot particles from diesel engine

The influence of engine operating conditions and fuel properties on the graphite crystallite size of soot particles from a small DI diesel engine were experimentally investigated. To examine the effect of aroma content in fuel on the graphite crystallite, both the conventional diesel fuel and the paraffinic hydrocarbon fuel (CN55) with similar ignitability to the diesel fuel but without aroma content were used. The soot particles were extracted from the exhaust gas, and its graphite crystallite size and oxidation reactivity were analyzed by laser Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM) and thermalgravimetric analyzer (TGA). The results showed that there is no significant difference in the shape of rate of heat release and the indicated thermal efficiency with both fuels. However, the graphite crystallite size with the CN55 was smaller than with the diesel fuel under several engine operating conditions, resulting in the higher oxidation reactivity with the CN55. Regardless of the type of fuel and engine operating conditions, a strong positive correlation was observed between graphite crystallite size and high temperature residence time calculated from combustion duration defined as from 10% heat release crank angle timing to 90% heat release crank angle timing and engine speed. These results suggest that the residence time in combustion significantly influences the graphitization of soot particles.