A comparative study of the physical properties of in-cylinder soot generated from the combustion of n-heptane and toluene/n-heptane in a diesel engine

This study examines the similarities and dissimilarities in the physical properties of in-cylinder soot obtained during combustion of n-heptane and a toluene/n-heptane mixture (TRF20, 20% toluene by volume) in a heavy-duty diesel engine. A total cylinder sampling system was used to sample the in-cylinder soot during different combustion phases. For each sample, the morphology, size, fractal dimension and nanostructure of the soot particles were analyzed using high-resolution transmission electron microscopy and Raman scattering spectrometry. The oxidation reactivity of soot particles was also evaluated in terms of apparent activation energy using thermogravimetric analysis. Throughout the combustion process, soot aggregates from the combustion of TRF20 generally exhibited more primary particles and larger-sized clusters than the n-heptane soot aggregates. Despite the different fuel formulations, both n-heptane and TRF20 showed similar trends during the combustion process in terms of changes in fractal dimension of soot aggregates, mean primary particle size and nanostructure (as characterized by fringe length, separation distance and tortuosity). The presence of toluene, however, led to a decrease in the fractal dimensions of the aggregates and a concurrent increase in the mean size of primary particles. Moreover, in the same combustion stages, the mean fringe length for the TRF20 soot was smaller than that for the n-heptane soot, while the mean separation distance and tortuosity for the TRF20 soot were both larger than those for the n-heptane soot. Relative to the n-heptane soot, the TRF20 soot had less graphitic organization and lower resistance to oxidation.