Impact of fuel formulation on the nanostructure and reactivity of diesel soot

The work focuses on the impact of fuel on soot reactivity and nanostructure. A 2.5 L, 4-cylinder, turbocharged, common rail, direct injection light-duty diesel engine was used in generating soot samples. Three test fuels were used: an ultra low sulfur diesel fuel (BP15), a pure soybean methyl-ester (B100), and a synthetic Fischer-Tropsch fuel (FT) produced in a gas-to-liquid process. A test condition of 2400 rpm and 64 N m (low load), with single and split injection strategies, was chosen for studying the impacts of fuel formulation on the characteristics of diesel soot. The start of injection (SOI) and fuel rail pressures were adjusted such that the three test fuels have similar combustion phasing, thereby facilitating comparisons between soots from the different fuels. The collected soot samples were characterized using thermogravimetric analyzer (TGA), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high resolution transmission electron microscopy (HRTEM). According to TGA, B100 soot exhibits the fastest oxidation on a mass basis followed by BP15 and FT derived soots in order of apparent rate constant. XPS analysis indicates no relation between the surface oxygen content and the soot reactivity. The basal plane diameter obtained from XRD is inversely related to the apparent rate constants for soot oxidation. Quantitative analysis of HRTEM images also suggests soot nanostructure disorder correlates with a faster oxidation rate. XRD and HRTEM analysis results are compared, and they both show excellent correlation of soot nanostructure and reactivity.