The Influence Of Egr On The Performance Of Two-stage Turbocharged Diesel Engine And SCR

With the rapid development of my country’s economy,the demand for logistics and transportation has surged.Diesel engines have become the mainstream power equipment in the logistics and transportation industry due to their large torque and good economy.However,its NOx and PM emissions have become the main air pollutants today.With the full implementation of national VI emission regulations,the research and development of diesel vehicles with ultra-low emissions and high economic efficiency has become an important scientific research content for scientific and technological workers.EGR and SCR are currently effective technical means to deal with NOx emissions.Exploring the comprehensive application of EGR coupled with SCR and developing a diesel engine exhaust treatment system with good stability and high economy has very high practical application value.In this paper,the two-stage turbocharged high-pressure common rail diesel engine widely used in freight vehicles is the test object.Based on different working conditions,the influence of EGR rate and fuel injection pressure on diesel engine performance and emissions is studied.Research shows that:(1)The increase of EGR rate will lead to increase of Soot,fuel consumption rate,exhaust temperature,and decrease of NOx and intake air flow.(2)The change of fuel injection pressure has little effect on intake air flow and boost ratio;under the same EGR rate,the higher the fuel injection pressure,the lower the temperature before the vortex;the higher the fuel injection pressure is when the load is high.Conducive to reducing Soot emissions,and smaller injection pressure is conducive to reducing NOx emissions.(3)The exhaust back pressure of diesel engine gradually decreases with the increase of EGR rate.The higher the speed load,the more affected the exhaust back pressure by the change of EGR rate.Based on the structure of the SCR system,the three-dimensional model was constructed,and four different mixer schemes were designed to study the effect of the mixer on the exhaust flow characteristics,exhaust temperature and pressure loss of the SCR system under the same working conditions and different EGR rates.Studies have shown that:(1)The angle and structure of the mixer blades will affect the uniformity of airflow distribution at the front end of the SCR inlet,and with the increase of exhaust flow,the degree of influence will be higher.(2)The four mixer schemes have relatively small differences in temperature distribution uniformity and pressure distribution,but the gas flow disturbance formed after the mixer of scheme 2 is stronger and the turbulent flow energy is greater,which is beneficial to the exhaust gas and the urea aqueous solution.Mixing can improve the overall work efficiency of SCR,and the comprehensive comparison scheme 2 is the best.(3)Under the same working conditions,the higher the EGR rate,the smaller the exhaust flow rate,and the turbulent flow energy after the SCR mixer is reduced.The difference in airflow distribution of different mixer schemes becomes smaller,and the SCR inlet pressure decreases and the temperature increases.On the basis of diesel engine performance and emission test,based on different working conditions,the influence of EGR rate and ammonia-nitrogen ratio on the NOx conversion efficiency of SCR system is studied,and the influence of ammonia-nitrogen ratio and temperature on SCR efficiency is studied and analyzed through one-dimensional simulation software.Studies have shown that:(1)The higher the EGR rate,the lower the NOx emissions at the SCR inlet.When the urea aqueous solution is injected,the NOx emissions at the SCR outlet gradually decrease with the increase in the ratio of ammonia to nitrogen.(2)The NOx conversion efficiency increases faster when the ammonia-nitrogen ratio is 0.8 to 1.1,and increases slowly between 1.1 and1.2.(3)Simulation calculations show that the conversion efficiency of NOx first increases and then decreases with temperature;when the temperature is lower than300 ℃,the conversion efficiency of NOx is lower;when the temperature is310～490 ℃,the conversion efficiency of NOx can be higher;when SCR After the inlet temperature exceeds 500 ℃,as the temperature further increases,the NOx conversion efficiency decreases rapidly.