Research On Key Technologies Of Scr Exhaust Aftertreatment System For Diesel Engine Based On Urea Hydrolysis

With the rapid growth of motor vehicle ownership,the air in some cities has begun to show the characteristics of combined pollution of soot and motor vehicle exhaust.Motor vehicle exhaust pollution has become an important source of air pollution,which seriously endangers the atmospheric environment and human health.However,under the conditions of cold start,low speed and light load of diesel vehicle,the existing SCR system is inefficient and easy to generate sediment due to insufficient exhaust temperature.As well as the resulting catalyst failure,exhaust pipe blockage,fuel consumption increase,OBD alarm and torque limit and other issues.In view of the above problems,the research on SCR exhaust aftertreatment technology of diesel engine based on urea hydrolysis is carried out in this paper.Compared with the urea pyrolysis reaction,the temperature required for the urea hydrolysis reaction is lower,which is suitable for the use of diesel vehicles under various working conditions.In addition,and the urea can complete a series of physical and chemical changes such as heating and hydrolysis in the hydrolysis tank.The system can inject NH3 into the exhaust pipe directly,which avoids the sediment problem caused by insufficient thermal decomposition of urea.The main contents of this paper are as follows:1.After analyzing the basic characteristics of urea hydrolysis reaction and the exhaust emission characteristics of diesel engine,the working pressure of the system is determined and the maximum urea liquid supply required for the system is calculated.The core design idea and key points of the system are analyzed,and the scheme of SCR exhaust aftertreatment system based on urea hydrolysis is designed.The composition and working principle of the system are introduced,and the design requirements of the three major components of the system are proposed.2.According to the requirement of heat transfer performance of urea hydrolysis system,the basic structure of urea hydrolysis tank is designed.The material selection of the tank is analyzed,and the strength of the hydrolysis tank is verified by calculation and ANSYS strength analysis.The heat absorption of urea hydrolysis reaction,the heat release from exhaust,and the heat exchange between hydrolysis tank and urea solution are calculated in detail,and the heat exchange performance of SCR system is simulated by FLUENT.3.According to the demand of urea liquid supply in urea hydrolysis system,a high precision diaphragm metering pump is developed.The simulation model of diaphragm metering pump is established by AMESim,and its flow characteristics and the movement characteristics of valve core are simulated.By mechanical limiting the opening of the one-way valve core at the inlet of the diaphragm metering pump,the closing time of the valve core and the influence of urea liquid overflow on the flow rate are reduced.4.According to the pressure and injection demand of urea hydrolysis system,a pressure control nozzle is designed,and its pressure regulation characteristics and anti blocking characteristics are analyzed.In addition,the injection characteristics of the nozzle are simulated by FLUENT,and the effects of nozzle core opening x and taper angle alpha ?z on nozzle orifice section velocity and nozzle orifice mass flow rate are explored.5.The injection accuracy test of diaphragm metering pump,the hydraulic safety and sealing performance test of the system and the performance test of pressure control nozzle are carried out respectively.On the basis of the above test meeting the system requirements,the performance test of urea hydrolysis system is carried out.The test results show that the strength of diaphragm metering pump,hydrolysis tank,pipeline and joint meets the requirements of the system,and the sealing performance of the system is good.The injection accuracy of diaphragm metering pump is ±3%,the injection pressure is 2 MPa,the single displacement is 0.09 m L,and the flow rate of urea can be adjusted in the range of 13.5-40 m L/min.The pressure control nozzle has good opening characteristics and good stabilization effect,the spray droplet size distribution of the nozzle is uniform,and the atomization effect is good.Increasing the heating temperature of the system can improve the hydrolysis rate of urea solution and the working efficiency of SCR system.By changing the volume of urea solution in hydrolysis tank,the output of NH3 can be effectively adjusted.