IMPACTS OF FUEL FORMULATION AND ENGINE OPERATING PARAMETERS ON THE NANOSTRUCTURE AND REACTIVITY OF DIESEL SOOT
Open Access
- Author:
- Yehliu, Kuen
- Graduate Program:
- Energy and Mineral Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 28, 2010
- Committee Members:
- Andre Louis Boehman, Dissertation Advisor/Co-Advisor
Andre Louis Boehman, Committee Chair/Co-Chair
Randy Lee Vander Wal, Committee Member
Angela Lueking, Committee Member
Thomas Litzinger, Committee Member
Octavio Armas Vergel, Committee Member - Keywords:
- diesel combustion
biodiesel
Fischer-Tropsch fuel
soot properties
soot reactivity
HRTEM image analysis - Abstract:
- This study focuses on the impacts of fuel formulations on the reactivity and nanostructure of diesel soot. A 2.5L, 4-cylinder, turbocharged, common rail, direct injection light-duty diesel engine was used in generating soot samples. The impacts of engine operating modes and the start of combustion on soot reactivity were investigated first. Based on preliminary investigations, a test condition of 2400 rpm and 64 Nm, with single and split injection strategies, was chosen for studying the impacts of fuel formulation on the characteristics of diesel soot. 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. 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. Soot reactivity was investigated by thermogravimetric analysis (TGA). 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. X-ray photoelectron spectroscopy (XPS) indicates no relation between the surface oxygen content and the soot reactivity. Crystalline information for the soot samples was obtained using X-ray diffraction (XRD). The basal plane diameter obtained from XRD was inversely related to the apparent rate constants for soot oxidation. For comparison, high resolution transmission electron microscopy (HRTEM) provided images of the graphene layers. Quantitative image analysis proceeded by a custom algorithm. B100 derived soot possessed the shortest mean fringe length and greatest mean fringe tortuosity. This suggests soot (nano)structural disorder correlates with a faster oxidation rate. Such results are in agreement with the X-ray analysis, as the observed fringe length is a measure of basal plane diameter. Moreover the relation between soot reactivity and structural disorder is consistent with past work by Vander Wal and co-workers, but stands in contrast to past work by Boehman and co-workers which identified surface oxygen content as the primary explanation for increased oxidative reactivity. The characterization results of this study indicate that changing fuel formulation is a potential method to enhance soot reactivity, and thus diesel particulate filter (DPF) regeneration, through decreasing the degree of order in soot nanostructure. All soot samples were partially oxidized to investigate the structural and elemental surface changes during the oxidation process. The HRTEM image analysis of the B100 and BP15 soot at 50% burn-off shows the highly ordered soot nanostructure, coinciding with significant decreases in the apparent rate constants for soot oxidation. In contrast, for FT soot, no significant changes in the soot nanostructure is observed, coinciding with only a slight decrease in apparent rate constant for soot oxidation. The result of HRTEM image analysis and apparent rate constants for soot oxidation show a relationship between the lattice fringe parameters (the median fringe length and mean fringe tortuosity) and the apparent rate constant, coinciding with the trend observed among the initial soot samples generated by different fuels.