| In order to solve problems of the traditional diesel engine that the mixing time is short and the air fuel mixture is heterogeneous and that it is difficult to reduce NOx and PM emissions simultaneously, premixed charge compression ignition combustion mode as a significant mean to achieve the high efficiency and clean combustion in the future is concerned by the researchers in domestic and overseas.It is an effective way for the internal combustion engine in the future to explore the unified combustion mode of the formation of gasoline premixed mixture gas and the compression ignition of diesel by using wide distillation range fuel,which physical and chemical properties between gasoline and diesel, which can simultaneously give full play to the strengths of gasoline in emission and diesel engine in economy. The optimal control idea of the combustion boundary conditions coupled with wide distillation range fuel in internal combustion engine combustion process was proposed. The optimal method and technology of wide distillation range fuel, EGR, the fuel injection parameters, the combination of them and the control parameters of internal combustion engine combustion process was explored by experiment and simulation. Influence and effect mechanism of wide distillation range fuel, the injection parameters, active conditions in-cylinder, and temperature on compression ignition engine combustion process is revealed. The optimal control strategies of wide distillation range fuel coupled with the combustion boundary condition is studied. The combustion control method of reducing NOx and PM emissions simultaneously and promoting the combustion efficiency and constant volume combustion performances well as the original exhaust emission was explored.At first, a control system platform for study combustion and emission characteristics of wide distillation rang fuel based on a CI engine was established. And the numerical model was established using three-dimensional computational fluid dynamics coupled with chemical kinetics. The results show that the fuel injection timing determined directly the thermo-atmosphere in the process of fuel atomization and combustion and the energy conversion efficiency and that the fuel injection timing has significant effects on combustion and emission. Constant volume combustion was influenced directly by CA50, but the real parameters which influence indicated thermal efficiency are constant volume combustion, combustion efficiency and wall heat transfer losses. The best ISFC depended on a balance between the roles of degree of constant volume, combustion efficiency and heat losses to wall. It is beneficial to extend the ignition delay period and improve the process of fuel and air mixing in consider of better volatility and lower ignitability of wide distillation range fuel. Homogeneous mixture is larger in the ignition delay period and the premixed charge compression ignition combustion is more easily achieved. In addition, the more the proportion of gasoline added, the lower the aromatic hydrocarbon in blend fuels accordingly. It is conducive to the reduction of smoke emissions using the wide distillation range fuel under the same operating condition. However, the mixture is too lean and more lean burn zone is formed when using wide distillation range fuel, resulting in higher CO and HC emissions and lower indicated thermal efficiency compared with diesel. Further numerical results reveal that the heterogeneity of gasoline is lower than that of diesel in the ignition delay period. The highly volatile gasoline promotes the evaporation of the low volatile diesel and inhibits the low-temperature chemical reaction process of high activity components at the same time. The ignition of wide distillation range fuel is mainly triggered by the high activity diesel.Secondly, based on optimizing the distillation range and the cetane number of wide distillation rang fuel, coupled with controlling the in-cylinder active atmosphere and thermo-atmosphere conditions, the study for scheme of control and optimization of combustion processes in order to simultaneous reduction of PM and NOx emissions was carried out, and the effect mechanism of the fuel chemistry and in-cylinder active thermo-atmosphere on combustion and emission was revealed. The results show that as EGR rate increased,concentration of oxygen is reduced and combustion temperature in cylinder is controlled effectively. The NOx emission of G0 is reduced,but its smoke emission has an obvious increasing trend. The trade-off between NOx and soot emissions is remarkable. However, the NOx emission of wide distillation range fuel decreased significantly with the increase of EGR rate while smoke emissions can be maintained at a low level. At low load conditions, incomplete combustion significantly increase, resulting in lower combustion efficiency when the EGR rate exceeded 20%. For the current study of medium load and low load cases, the best control strategies of balancing NOx emissions, Soot emissions and fuel economy was found in light of analysis of experimental and simulation results. And the optimization of the fuel properties and the in-cylinder active atmosphere and thermo-atmosphere conditions was achieved.Fuel injection boundary conditions are important factor that affected the mixture formation, combustion, emissions and performance. Based on the optimization of the fuel properties and the in-cylinder active atmosphere and thermo-atmosphere conditions, further studies for improving the performance of combustion and emissions in CI engine were carried out by controlling the injection parameters coupled with wide distillation range fuel. The results indicate that the optimum fuel injection boundary conditions on wide distillation rang fuel have some differences with conventional diesel fuel, and wide distillation rang fuel less sensitivity for injection parameters adjusted than conventional diesel fuel. For the current study of medium load, the higher injection pressure almost has nothing to do with the soot emission improvement of wide distillation range fuel and NOx emission is increased. Excessive mixing areas are observed at the same time and the reaction chain is disrupted and the products of incomplete combustion are increased. Different pre-injection intervals with 6% pre-injection quantity have little effect on combustion and emission in main injection phase. With an increase in pre-injection quantity the pre-injection of G0 leads to two-step exothermic reaction composed of the cool-flame and hot-flame obviously and the pre-injection of wide distillation range fuel leads to single-stage exothermic reaction composed of cold flame when the pre-injection interval is 30°CA, pre-injection quantity is 10%, 20%, 30% respectively. The NOx emission is reduced with the increase of pre-injection quantity. The smoke emission of G0 is increased with the increase of pre-injection quantity while the smoke emission of wide distillation range fuel is increased slightly but no significant change in general is observed. The spray tip has hit the top surface of piston when pre-injection quantity is reached to 30%.At last, based on the scheme of control and optimization of combustion processes by coupling with combustion boundary conditions and fuel-chemical kinetics in clean and high-efficiency premixed combustion, and further integration optimization method based on CFD, Taguchi method and ANOVA Technique, and on the basis of combustion theory and optimization theory, the research for seeking the best compromise between the performance of NOx emissions, Soot emissions and fuel consumption rate was carried out. The results show that for a medium load case at the optimal combination of parameters and levels, as compared to the reference baseline, NOx emissions decreased slightly, and the decrease was less than 2%, soot emissions and indicated specific fuel consumption have different degree of reduction. For a low load case at the optimal combination of parameters and levels, the reduction of NOx emissions and soot emissions is obvious as compared to the reference baseline, while not indicating the deterioration of fuel consumption rate, so that the best compromise is realized between the performance of NOx emissions, soot emissions and fuel economy. |
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