Detailed diesel particulates morphology, nanostructures, fractal geometry, and nitrogen oxides (NOx) emissions were analyzed for five different test fuels in a 1.7-L, common-rail direct-injection diesel engine. The accurately formulated fuels permit the effects of sulfur, paraffins, aromatics, and naphthene concentrations to be determined. A novel thermophoretic sampling technique was used to collect particulates immediately after the exhaust valves. The morphology and nanostructures of particulate samples were examined, imaged with a high-resolution transmission electron microscope (HRTEM), and quantitatively analyzed with customized digital image processing/data acquisition systems.
The results show that the particle sizes and the total mass of particulate matter (PM) emissions correlate most strongly with the fuels' aromatics and sulfur content. At constant cetane number (CN) and volatility, fuels with low aromatics and naphthene levels (e.g., iso-paraffin-enhanced fuel) generate lower PM and NOx emissions. Fractal analyses reveal that diesel particulates from the test fuels are similar in shape but smaller than those from a commercial number-2 diesel (D2). Higher levels of aromatics correlate with increased NOx emissions, which is consistent with the current understanding of aromatics’ impact on NOx emissions. The HRTEM examination of nanostructures, along with aggregate size distributions, reveals that the combustion of iso-paraffin-enriched fuel leads to a greater number of soot nuclei on individual primary particles, in contrast to the soot nanostructures of the aromatics-enriched fuel, where only a single nucleus exists on each primary particle. The results indicate that the difference in nanostructure is caused by the different degree of graphitization in soot formation.