| Although the mass of the nanoparticles in the air accounts for only about10%of total emissions, the number of the nanoparticles which has representative diffusivity accounts for about90%of total number concentration. The inhalable nanoparticles with high number concentration are suspended in the air for long periods and great harmful to human’s health, and the diesel engine is the major source of particle pollution. Therefore, the research of the diesel engine soot emissions characteristics and evolution mechanism of the particle size distribution is of great theoretical and realistic significance for the deep understanding of the formation process of the soot particles and the emission controlling of diesel engines.First, in the PhD thesis a detailed soot model was proposed, including a detailed reaction mechanism and a phenomenological semi-empirical soot model.A reduced diesel surrogate fuel chemical reaction mechanism of n-heptane/toluene was used as the gas phase chemical kinetics part of the detailed soot model, and the particle kinetic part considered the effect of particles nucleation, surface reaction, coagulation, oxidation and PAH deposition on the formation process of soot particles and the particle size distribution. The effectiveness and accuracy of the detailed soot model was validated through the comparison between the prediction value and experiment results, and the research and analysis of soot formation characteristics in diesel fuel combustion process was performed. The results show that the number concentration and the diameters of soot particles dramatically increase at the starting of the combustion and rapid burning duration, and are tending to stable afterwards. In the initial combustion stage, a large number of the small soot particles are produced by the pyrolysis of hydrocarbon fuel.In the middle stage of combustion process, a number of large soot particles are produced in the cylinder. In the end of combustion process, the range of particle size in the cylinder is tending to stable, and the range of the maximum particles size is between5nm and20nm.Secondly, based on the theoretical approaches of the particle population balance modeling, the particle population balance equations describing the evolution process of particle size distribution function in the diesel fuel combustion process were built, and the existing detailed chemistry kinetic mechanism was coupled with the soot model based on the particle population balance theory. A computation platform of the direct simulation Monte Carlo method was built to solve the particle population balance equations, to obtain the detail evolution process information of the particle size distribution in the diesel fuel combustion process. The result shows that at the starting of the combustion process the particle nucleation begins to occur, and the reaction rate of the nucleation has showed first increasing then slowly decreasing as the reaction time increased, and the condensation rates of C2H2adsorption and pyrene also increase first and then slowly decrease. The large adsorption reaction rate of C2H2promotes the particle growth and the large particle formation. The OH oxidation rate increases and then decreases.Thirdly, a high temperature high pressure constant volume combustion bomb test platform was built. The planar distribution images of diesel spray combustion flame and soot formation were measured and analyzed by using high speed photograph technique and laser-induced incandescence (LⅡ). The measuring method for the soot formation process in the high temperature high pressure constant volume combustion bomb was given by using the high speed photograph technique and LⅡ. The effects of combustion flame and fuel gas mixing characteristics on soot formation and concentration distribution was studied, under different ambient conditions in constant volume combustion bomb and injection pressure. The result indicates that the LⅡ signal which is proportional to the soot volume fraction increases with the increase of the ambient pressure and temperature. Under the conditions of the same ambient pressure and temperature, the LⅡ signal decreases with the decrease of the fuel injection pressure, resulting in the smaller soot volume fraction in the fuel srpay. The soot particles are primarily produced in the relative fuel-rich region, which is encompassed by the flame surface front at the downstream of the diesel spray.A turbocharged, common rail heavy-duty diesel engine was used in this study. The effect of multiple injection strategies on diesel fuel combustion process, heat release rate, emission and economy of diesel engine was studied. The multiple injection strategies included different EGR level, pilot injection timing, pilot injection mass and post injection timing. Based on endoscope technology, the two-color method was applied to take the flame images in the engine cylinder and obtain soot concentration distribution. The results demonstrate that when EGR level is increased, the intensity of the premixed combustion becomes lower, and the ignition timing of the main combustion process is delayed, and the soot emissions increase. With the advance of pilot injection timing, the peak in-cylinder pressure becomes lower, the ignition delay of the main combustion is shortened, the soot emissions are reduced, and the pilot combustion flame is non-luminous. With the increase of pilot injection fuel mass, the obvious pilot combustion flame can be observed in the cylinder. Under the conditions of different pilot injection fuel masses, a number of the soot particles are encompassed by the flame surface front at the downstream of the diesel srpay, which is the relative fuel-rich region. The soot concentration distribution develops from the edge of the combustion chamber bowl to the bottom, and the soot concentration is tending to decrease. |
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