Study On The Fragmentation Of Diesel In-cylinder Aggregates During The Soot Oxidation Course

Improvement of the in-cylinder soot oxidation is one of the most effective solutions to reduce diesel particle emissions.Therefore,it is of great importance to study the in-cylinder soot oxidation mechanism for reducing the soot emissions from diesel engines.Based on an automated total cylinder sampling system and the combustion simulation,the soot samples and the different combustion boundary conditions were obtained.Meanwhile,the oxidation-induced fragmentation of aggregates during the in-cylinder soot oxidation course was analyzed by the engine exhaust emission particle size spectrometer,transmission electron microscopy,micro-raman spectroscopy and X-ray photoelectron spectrometer.Then the factors that affected the fragmentation behavior were analyzed in this work.Furthermore,the evolution of soot physicochemical properties was studied during the fragmentation of soot course.The major work and achievements of this dissertation are listed as follows:The in-cylinder combustion characteristics under two different intake pressures（0.1 MPa,case1 and 0.2 MPa,case2）are analyzed,respectively.According to the evolution of the net mass of soot in the cylinder combustion process,the combustion process can be divided into two phases,i.e.,the soot formation-dominated phase and the soot oxidation-dominated phase.During the soot oxidation-dominated phase,the particle size distributions of in-cylinder soot under both working conditions shows double peaks,and the peak values are 10～30 nm and 80～110 nm respectively.The total particle number concentrations of soot at 12°CA ATDC（case1）and 22°CA ATDC（case2）increase while the mass of soot decrease during the soot oxidation-dominated phase.This phenomenon indicates that the bridges between the primary particles can be attacked by oxidant species so that the structure weakens,i.e.,the aggregates subsequently break up obviously here.The morphology of the in-cylinder soot obtained under two working conditions has no obvious difference,and there is a trend of transformation from chain like to cotton.The fractal dimension of the in-cylinder soot varies in the range of 1.33～1.92.The fractal dimension and the degree of agglomeration have a significant downward trend at the crank angle where the degree of fragmentation is obvious,which provides further evidence for the fragmentation of aggregates.The fragmentation rate of aggregates R_af can be calculated according to the fractal dimension,the degree of particle aggregation and the oxidation rate.As the combustion proceeds in the soot oxidation-dominated course,the fragmentation rate of aggregates descends.In addition,the aggregate fragmentation rate shows a distinctly decline with the decreased in-cylinder O₂ concentration,whereas no significant correlation is found between the aggregate fragmentation rate and the concentration of OH radicals.Meanwhile,the in-cylinder temperature has a certain effect on the fragmentation of aggregates,and a better mixing condition in the cylinder can promote the fragmentation of aggregates.As the combustion proceeds in the soot oxidation-dominated course,the ordered nanostructure degree and the graphitization degree of soot particles increase gradually under both conditions.And it is conducive to the fragmentation of aggregates when the degree of ordered nanostructure and the degree of graphitization are low.Moreover,the relative concentration of C-OH functional groups on diesel soot surface is always higher than that of C=O functional groups,and there is no obvious regularity between the change of relative content and the rate of fragmentation R_af.