The Influence Of In-Cylinder Oxygen Concentration Distribution On Generation Of Emissions For Compression Ignition Engine

Energy crisis and emission regulations pose serious challenges to traditional internal combustion engines.It is imperative to achieve efficient and clean combustion of internal combustion engines.The“trade off”relationship between NO_x and soot has always been a difficult issue in emission control technologies and limits the further application of the diesel engine.Based on the difference of spatial distribution area of NO_x and soot emissions,this project had proposed to use the EGR stratification to control the distribution of oxygen in the cylinder.The target is to concentrate the exhaust gas in the area where the NO_x is generated and maintain a rich oxygen atmosphere in soot generation area,aiming at lowering NO_x and soot emissions simultaneously and achieving efficient and clean combustion of the internal combustion engine.In this thesis,the numerical model of a four-valve electronically controlled high-pressure common rail diesel engine was established by using the professional fluid simulation software-AVL-FIRE.Based on the original distribution of NO_x and soot emissions,two target oxygen pattern of"lower concentration in upper and middle area"and"lower concentration in peripheral and bottom area"were proposed.The influence of the ideal oxygen distribution pattern on generation of emissions was studied.The results showed that the oxygen and exhaust gas stratification can achieve lower emission levels.At the same NO emission level,the demand for EGR rate of the exhaust gas stratification can be reduced by about 7%,reducing the dependence of diesel emission control on a large proportion of EGR rates.With the guidance of the target oxygen concentration distribution pattern and combined with the double inlet structure of the rectangular arrangement of the engine,three exhaust gas delamination schemes for single-duct introduction of exhaust gas,single-air-duct addition of exhaust gas introducing pipe and exhaust gas introducing plate were designed to achieve the target oxygen pattern.The results showed that the introduction of exhaust gas from a single airway made the oxygen concentration laterally distributed in the cylinder.The exhaust gas introducing pipe scheme is conducive to obtaining an ideal oxygen concentration distribution,but the effective introduction diameter of the introducing tube limits the proportion of exhaust gas.The degree of oxygen concentration stratification was too low.Adding an exhaust gas introducing plate with the same introducing effect is beneficial to raising the proportion of exhaust gas in the cylinder and increasing the stratified degree of the oxygen concentration,which can obtain an ideal oxygen concentration distribution.The influence of engine speed and exhaust gas parameters on the distribution of oxygen concentration in cylinder was studied.The results showed that increasing the proportion of exhaust gas introduced can promote the stratified degree of oxygen concentration in the cylinder.However,due to the different characteristics of different intake ducts,the effect of exhaust gas introduction pressure on the distribution of oxygen concentration in cylinde is different.And engine speed has little influence on the distribution of oxygen concentration.The effects of different oxygen concentration profiles on combustion and emission characteristics were calculated and analyzed.The results showed that the distribution of oxygen in the form of"lower concentration in peripheral and bottom area"is more conducive to obtaining a lower emission level and has little influence on combustion.The results also showed that the effect of oxygen concentration stratification on NO_x and soot is more obvious when the proportion of exhaust gas in the cylinder is higher and the oxygen concentration stratification degree is larger.When the overall oxygen concentration in the cylinder is 21.4%and the oxygen concentration stratified degree is 4.5%,the NO level in the stratified form is 4.5%lower than the homogeneous form and soot is reduced by 2.7%.When the overall oxygen concentration in the cylinder is 20.2%,the oxygen concentration stratified degree is 6.8%,the NO is decreased by8.5%,soot is decreased by 4.9%.Engine operating conditions and intake boundary conditions have a significant effect on the combustion of oxygen concentration stratification.The results showed that the effect of oxygen concentration stratification on emissions is better at low to medium speed and the effect of oxygen concentration stratification on emission is better at low and medium load.However,the effect is weaker at high load because of the prolongation of the fuel injection duration and combustion duration.The intake pressure has a great effect on the combustion of oxygen concentration stratification.Moderately increasing the intake pressure is beneficial to improve the effect of oxygen concentration stratification on soot and has little effect on NO emission.At medium load,when the intake pressure rises from 126kPa to 133kPa,the improvement ratio of soot increases from 4.9%to 6.8%.However,excessively increasing intake pressure will shorten the ignition delay and reduce the proportion of premixed combustion,which will reduce the effect of oxygen concentration stratification on combustion and emissions.