Studies On The Nanostructure, Surface Fuctional Groups And Oxidation Reactivity Of In-cylinder Particulates From A Diesel Engine Burned With Characteristic Fuels

Combustion conditions, such as initial fuel composition, in-cylinder pressure andtemperature, play an important role on the nanostructure and surface functionalgroups of diesel particulates. The variations in nanostructure and surface functionalgroups can in turn affect the particulate reactivity toward oxidation. Therefore,detailed invesigations of the diesel in-cylinder particulates generared from differentfuels is necessary for better understanding of the diesel particulates evolutionmechanism and the reduction of particulate pollutions. In this dissertation, a totalcylinder sampling system, fueled with n-heptane, n-heptane/toluene, EuroⅢ lowsulfur diesel fuel and F-T fuel respectively, was employed to obtain in-cylinder sootsamples. The evolution mechanism of nanostructure, surface functional groups andoxidation reactivity of in-cylinder soot was subsequently studied using the analyticalmethod such as digital image processing, electron energy-loss spectroscopy, Ramanspectrum, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy,and thermogravimetric analysis. The major work and achievements of this dissertationare listed as follows:1. The in-cylinder particulates generated from the four kinds of fuels all presenttwo sorts of morphology: One is the representative particulate which formed throughagglomeration of small sphere-like primary particles, and the other is amorphousparticulate containing abundant metal and nonmetal elements. The representativeparticles are aggregated in the form of fractal-like geometry. The fractal dimensionsfor the N-heptane, N-heptane blended with20%toluene (v/v), EuroⅢ diesel fuel andF-T fuel particulates are in a range of1.63~1.96、1.48~1.92、1.32~1.84and1.48~1.91respectively. During the combustion process, the fractal dimensions of in-cylinderparticulates first decline at the premixed combustion and early diffusion combustionstages, and then increase at the middle/late diffusion combustion and late combustionstages. The minimum particulate fractal dimension lies within the early diffusioncombustion period. The aromatic and cyclanes compounds in fuels can promote thegeneration of particles with low fractal dimensions.2. The size distributions of primary particles generated from these four fuels aresimilar to Gauss distribution with the peak value of particle diameter in the range of15~25nm. During the combustion process, the mean diameter of primary particlesshows a unimodal distribution with the maximum value locating at the middle/late premixed combustion stage. Meanwhile, the mean fringe separation distance andtortuosity of in-cylinder soot generally decrease, except for a sudden rise between thelate premixed combustion stage and early diffusive combustion stage. In contrast, themean fringe length shows an increasing trend. The aromatic and cyclanes compoundsin fuels can promote the growth of primary particles and bring about an increase inmean particle diameter, fringe separation distance and tortuosity, and a derease inmean fringe length of soot partitulats. Additionally, the ratio of IG/IDin Ramanexperiment and Iπ*/Iδ*in EELS experiment follows a similar trend to that of thefringe length through the combustion history, verifying that the results of the imageprocessing are reasonable.3. The relative content of the aliphatic C–H groups on the in-cylinder soot surface,which was evaluated by the normalized peak height ratio (IC-H/IC=C), generallydecreases during the combustion process, except for a sudden rise between the latepremixed and early diffusive combustion stages. Meanwhile, the concentrations ofboth C–OH and C=O groups present bimodal distributions over the combustionhistory, with the two peaks locating at the middle premixed and late diffusioncombustion stages respectively. Both aromatic and cycloparaffinic hydrocarbons infuels affect the soot nanostructure and degree of graphitization, which has asignificant impact on the relative content of surface aliphatic C–H groups.4. Under the applied engine operating conditions where these four fuels wereused, the apparent activation energies for soot oxidation are in a range of114.57～181.02kJ/mol at various crank angles. Aliphatic C–H groups on the soot surfaceserve as a more important factor governing the soot oxidation reactivity whencompared to the soot nanostructure and oxygenated SFGs.