Applied Research Of SCR System On High Pressure Common Rail Light-duty Vehicle Diesel Engine

In recent years,diesel engine has been widely used in the automotive field because of its strong power,good economy and high thermal efficiency.But with the deterioration of the environment,automobile emission regulations are becoming more rigid.The main emissions of diesel engines are NO_x and PM,which develop a reciprocal relationship.Researchers across the world try to study how to reduce the main pollutants in diesel engines,Which is of crucial implication for the development of China’s own diesel engine technology brand.This paper analyzes the emission control technologies of the diesel engine mainly about the pre-treatment,in-machine purification and after-teratment.Then it introduces the harm and formation mechanism of main pollutants,and focus on the analysis of the exhaust after-treatment techniques called the selective catalytic reduction technology（SCR）.The situation of SCR system at domestic and abroad are compared.The composition and working principle and recation mechanism of the SCR system are introduced.The test bench was established to analyze the SCR urea decomposition process under different urea decomposition conditions,such as different exhaust temperature and different exhaust flow,different injection rate and different urea urea injection distance of urea decomposition.It was found that improving the exhaust temperature is conducive to urea decomposition through the study.And the exhaust flow is too large to improve the reaction of urea solution.Urea nozzles produce different urea injection rate at different pressure.As the injection rate increases,the efficiency of the reaction is higher.The influence of the arrangement distance of the urea nozzle on the decomposition of the urea solution is comprehensive.Appropriate increase in the distance of the urea nozzle to the reactor is advantageous for the exhaust gas to be fully mixed with the urea and closer to the exhaust pipe of the diesel engine can increase the reaction temperature to promote the reduction reaction.Aiming at the problem of urea crystallization in the process of diesel engine application,the urea crystallization of urea and SCR exhaust pipe was analyzed by thermal decomposition（TGA/DSC）,and the direction of controlling the crystallization of urea was analyzed.It is proved that the reaction temperature of the catalytic reactor can be effectively reduced.In this paper,the SCR system were simulated by AVL BOOST simulation software.The basic equation modeling,combustion model,emissions model and chemical reaction model were analyzed.The boundary conditions and initial conditions and parameters were set,The SCR model and the diesel egnine machine model were established and validate.The reaction influencing factors such as the space velocity,temperature,ammonia/nitrogen ratio,NO₂/NO_x and structural factors such as carrier volume,length,wall thickness and coating thickness were analyzed.The results show that the efficiency of NO_x conversion is most affected by temperature,and the best conversion efficiency is between 375℃and 425℃.The best conversion efficiency is up to 67%,which indicates that the effect of temperature on conversion efficiency is very obvious.When the temperature is too high,the conversion efficiency will decline.While the temperature is too low,the conversion efficiency is not optimal.SCR reaction can not be carried out efficiently.The size of airspeed is also a factor affecting the efficiency of NO_x conversion.When the airspeed increase and exceed a certain value conversion efficiency will drop significantly.The largest decline is up to 41%.At the same time,as the ratio of ammonia to nitrogen increase,the efficiency of NO_x conversion increase until the ammonia concentration could not affect the conversion efficiency.When NO₂/NO_x is 50%,the efficiency of SCR conversion system is the highest,and the rapid reaction is carried out.Increasing the volume of the carrier can improve the NO_x conversion efficiency of the SCR catalyst,and the increase rate is 70% at low temperature.The carrier length has little effect on the SCR conversion efficiency of the SCR catalyst.The increase of the number of carriers and the thickness of the carrier coating will result in a slight increase in NO_x conversion efficiency.Carrier wall thickness is an important factor affecting the strength of carrier structure,wall thickness is too large will lead to NO_x conversion efficiency declining.Finally analyze the effect of SCR catalyst on performance of Diesel Engine.Finally,simulation is verified on the bench test and the measurement results of NO_x emissions from diesel engines were 1.629g/（kW·h）in the thirteen working conditions,and the CO and HC results of the diesel engine were 0.007g/（kW·h）,0.005g/（kW·h）.it can be concluded that the SCR system enables high pressure common rail light vehicle diesel engine to meet the national V emission standards.