Study On The Microstructure, Surface Fuctional Groups And Oxidation Reactivity Of Diesel In-cylinder Particles

Diesel particulates are known to be the aggregates of individual primary particles. Within a primary particle, the degree of atomic order as manifested by graphitic layer plane segments and their physical relation to each other are defined as soot microstructure. Soot microstructure can range from amorphous to graphitic depending upon initial fuel identity and synthesis conditions, such as temperature and time. Meanwhile, variations in the microstructure can in turn affect the particulate reactivity toward oxidation. In addition, surface functional groups formed on edge sites which are directly involved in the attack by air are likely to be another factor affecting particulate reactivity. Therefore, based on a novel total cylinder dumpling system, the evolution process and formation mechanisms of microstructure, surface functional groups and oxidation reactivity of diesel in-cylinder soot were investigated using the measurement techniques and analysis method, for example, digital image processing, Raman spectrum, Fourier Transform Infrared spectroscopy, X-ray photoelectron spectroscopy, and simultaneous thermal analysis. The database obtained from this study will be of instructive significance for combustion cycle design and post-treatment processes of a diesel engine. The major research work and achievements of this dissertation are listed as follows:1. The definition of soot microstructure characteristics (i.e. fringe length, fringe separation distance and tortuosity) were introduced according to the nature of diesel particulate TEM images, and the digital image processing algorithm consisted of image normalization, orientation image estimation, Gabor filter, local threshold method, improved OPTA algorithm, et. al., were applied to quantify the microstructure characterization of diesel particulates.2. During combustion process, fringe separation distance and tortuosity decreased sharply in the rapid combustion phase, increased slightly in the mixing-controlled combustion phase, and decreased again afterwards, with a size range of 0.34~0.42nm and 1.22~1.33 respectively. In contrast, fringe length showed a contrary tendency to fringe separation distance and tortuosity, with the size range of 1~2.2nm. An increase in common rail pressure and fuel/ air ratio resulted in larger fringe length and shorter fringe separation distance and tortuosity; on the contrary, an increase in engine speed resulted in larger fringe separation distance and tortuosity and shorter fringe length. The peak intensity ratio of G/D bands in Raman experiment showed the same tendency as fringe length as the combustion proceeded, verifying the accuracy of image processing algorithm used.3. During combustion process, the molar concentration of the carbon-hydrogen groups on the in-cylinder particulate surface, which was indicated by the normalized peak height ratio (IC-H/IC=C), decreased strongly in the rapid combustion phase and then decreased slowly afterwards. The molar percentage of surface oxygen groups (i.e. hydroxyl and carbonyl) of in-cylinder soots presented two peaks through the combustion phases, with the heights of peaks ranging from 11.45 % to 22.37 %. The first peak appeared in the end of the rapid combustion phase, and the second in the late combustion phase. The total surface oxygen groups were significantly influenced by the hydroxyl. The variation tendency of the hydroxyl/ carbonyl ratio was unimodal during combustion process with a maximum value appeared at the beginning of the mixing-controlled combustion phase. In addition, the oxygen/carbon molar ratio had the same tendency as the total surface oxygen groups as the combustion proceeded.4. The onset temperature, maximum rate temperature and burnout temperature of the diesel in-cylinder particulates increased gradually during combustion process, except for a slight decrease in the mixing-controlled combustion phase. The pre-exponential factor and the activation energy of diesel in-cylinder soots were in the range of 7.37×105～1.55×108/s and 123.05～154.53kJ/mol, respectively. The activation energy intensively increased in the rapid combustion phase, declined to some extent in the mixing-controlled combustion phase, and gradually increased again afterward. More graphitic microstructure could lead to higher activation energy, whereas more surface oxygen groups would lessen the activation energy.