| Due to the shortage of energy and the deterioration of environment, Premixed Charge Compression Ignition engines are being paid much attention and world-widely investigated due to their potential of high thermal efficiency and very low emissions of NOx and particulate matter (PM). However, it is difficult to prepare mixture for diesel fuel because of its low volatility, high viscosity and low resistance to self-ignition. Therefore, improving the pre-ignition mixing rate is the key approach to acquire PCCI. Based on the fuel injection strategy control and in-cylinder condition (temperature, pressure) control in the dissertation, PCCI and the multi-stage combustion are further studied including the effects of physical and chemical factors by the means of experiment and numerical simulation.Through controlling the key parameters such as pulse dwell time, pulse fuel mass and their proportion, pulse number and injection timing, the experimental results of diesel stratified PCCI combustion modulated by multi-pulse injection strategy can deduce the cylinder wall wetting and bring on different the stratification of mixture temperature and concentration. In order to understand premixed charge compression ignition combustion, a new combustion model of kinetic-and-turbulent characteristic-time has been developed. A ununiformity function H (φ)was presented by analysis of the effect of fuel/air distributions on the role of turbulent timescale in the combustion model, and then an analytical turbulent timescale coefficient f was deduced, which was proved to be able to correlate the fuel ununiformity with the turbulent timescale in the combustion model. The simulation results agreed with the experimental data well. The ignition process of a stratified PCCI combustion organized by multi-pulse injection was a separated volume autoignition process, which was strongly influenced by the condition of fuel stratification. The combustion temperature of richer zones were drawn down by mass and heat transfer into surrounding lean zones, so that lower combustion temperature and lower formation rate of NO were observed in richer area. Moreover, a"V"type distribution of equivalence ratio was found to be beneficial to retardence of high temperature reaction.In order to maintain the low emissions'advantage of stratified PCCI and expand to higher load, the multi-stage combustion is organized by a sequential combustion processes including the stage of premixed charge compression ignition, the stage of mixing-controlled combustion generated by main injection and the stage by post injection. A combustion model combining simplified chemical mechanism and characteristic-time combustion model was employed to explore the multi-stage combustion process and emissions in a heavy duty diesel engine. The simulation results agreed with the experimental data well. It was found in this study when the mixture with high equivalence ratio fell down to 0.2 before the ignition organized by multi-pulse injection, the ignition happened later and the regions of low combustion temperature below 1400K increased with producing more CO and no NOx, which could limit too fast pressure rising during stratified PCCI stage. Because the mixture has longer mixing time under lower average in-cylinder temperature after later MSOI, the relative more mixture of T≤2000K andφ≤2 and less mixture of T>2600 are produced that can lead to less NOx; meanwhile, more enough mixing of fuel and oxygen keeps the fast heat release rate and thus the high thermal efficiency. The fuel quantity of one time main injection could be lowered by two injections in order to reduce NOx emission obviously,simultaneously with the approximate soot emissions to the one only with one time main injection and 54.4% thermal efficiency.
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