The Simulation Analysis Of In-Cylinder EGR Stratification And Its Effects On Combustion And Emission

In order to protect the environment,to meet the increasingly stringent emission regulations.In recent years,the development of diesel engines began to pursue high efficiency and low emissions.Although the diesel engine has a higher efficiency than the gasoline engine,but there are poor emissions,NOx and PM are the main emission pollutants in diesel engines.It is a challenge to reduce both emissions at the same time because of the trade-off relationship between its production conditions.Therefore,the method of in-cylinder EGR stratification is proposed,through the active control and regulation of the gas distribution in the cylinder to achieve the appropriate EGR stratification in the chamber.Effectively using the EGR stratification regional gradient effect,to control the spatial distribution of cylinder exhaust gas concentration flexibility,On one hand,through the higher exhaust gas concentration to control the high-temperature area combustion temperature,to reduce NOx emissions;on the other hand,making the generation and oxidation stage of the particulate matter occurs in the relatively oxygen-rich zone,to inhibition of pre-particle formation and promote the later oxidation of the particles,to solve the increased particulate emissions caused by oil and gas mismatch in transient combustion process,local hypoxia and EGR overshoot,and finally achieve the goal of efficient combustion control of the compression engine.This paper based on the high pressure common rail supercharged intercooled diesel engine,the simulation and analysis platform is built by reverse engineering method,and the commercial software AVL-FIRE is used to simulate.In order to clarify the EGR stratification target.Firstly,we study the distribution of the original emission field in the cylinder.Then,the effects of different intake port forms and EGR parameters by single port introduction of exhaust gas with four-valve engine on the EGR stratification were investigated.In order to further optimize the stratification effect,simulate EGR stratification under the structure of exhaust duct and intake port partition.Getting the optimal EGR stratification scheme,compared with the homogeneous EGR engine performance,revealing the influence of EGR stratification on combustion and emission.Research indicates:1.For the four-valve engine used in this study,the initial emissions concentrated in the vicinity of fuel beam,with the fuel beam hit wall,the high-soot area is in the wall and pits,the NO generating region is more biased towards the center of the combustion chamber relative to soot.2.For the single port introduction of exhaust gas scheme,the gas introduced into the tangential port is mainly gathered outside the combustion chamber.The gas introduced by the helical port is mainly concentrated in the middle of the combustion chamber.The use of single port introduction of exhaust gas,intake port rectangular arrangement relative to the diamond arrangement can achieve the greater degree of separation in the cylinder,but with the rectangular arrangement,distribution of gas in the cylinder showed a lateral distribution,the symmetry is relatively poor.EGR pressure has a great influence on the gas distribution area and the airflow movement in cylinder,with the increase of EGR pressure,the EGR rate in the chamber continued to increase;changing the amount of exhaust gas introduced by the single port has no significant effect on the EGR distribution in the chamber,but increasing the EGR rate is beneficial to improve the degree of EGR stratification;changing the temperature of the exhaust gas introduced by the single port has little effect on the EGR stratification in the chamber,there is no significant effect on the temperature distribution in the cylinder,only with the EGR temperature rise,the in-cylinder EGR rate decreased slightly,the in-cylinder temperature increased proportionally.3.For the arrangement of exhaust ducts in helical port,the initial position of the gas entering the cylinder through the duct has a significant effect on the gas distribution in the later stage,if you want to get the distribution of the exhaust gas in the middle of the concentration range corresponding to the original emission concentration field,the exhaust duct side should be as close as possible to the center of the combustion chamber;changing the EGR pressure in the duct can form an ideal EGR stratification,but due to the limitation of the exhaust duct structure,the degree of stratification in cylinder is small.4.In the helical port layout partition,by changing the exhaust gas pressure in the EGR channel,it is possible to form a better EGR stratification pattern corresponding to the original emission concentration field.When the EGR pressure is larger than the air pressure of 12.5 k Pa,the "sandwich" distribution of concentrated in the middle is formed with the 10% of degree of stratification.It is the ideal EGR stratification scheme.5.Comparing the effect of ideal EGR stratification and homogeneous EGR on combustion and emission under 2000 r / min and medium load,during the combustion process,the cylinder pressure of the stratified EGR is slightly lower than that of the homogeneous EGR,while the initial instantaneous heat release peak is higher than the homogeneous EGR,and the ignition time is slightly lagging behind,at the same time,the average temperature of stratified EGR is slightly lower than the temperature of homogeneous EGR,and the ignition delay of stratified EGR are slightly longer,but the overall difference is not significant.In terms of emissions,to ensure that the proportion of the same cylinder exhaust,EGR stratification to a greater extent to play the role of exhaust gas on the inhibition of generation of emissions,NO emissions from the stratified EGR were 7.9% lower than homogeneous EGR and soot emissions by 2.6%.The use of stratified EGR technology,is conducive to breaking the NOx and soot trade-off relationship,to obtain a lower level of emissions...