Research On Key Technology Of High Metering Accuracy Air-assisted Urea Pump

At present,the technology for reducing Nitrogen Oxides(NO_x)emissions is considered as one of the core technologies of automobiles by the world's automobile powers.Selective Catalytic Reduction(SCR)technology using Urea Water Solution(UWS)as reducing agent has become the mainstream technological method for reducing NO_x emissions.As one of the most important components of the SCR system,the urea pump is mainly used for conveying and metering UWS.If the UWS flow pumped by the urea pump is less than the required value,the NO_x will not be completely converted,and the unconverted NO_x will be directly discharged into the air.If the UWS flow pumped by the urea pump is greater than the required value,the unreacted NH₃ will be directly discharged to the air,which will cause secondary pollution of the air.Therefore,the output flow of the urea pump is required to be highly matched with the UWS flow required by the system.Here,the urea pump is not only the power source for pumping the UWS,but also has the flow metering function.The metering accuracy is generally used to indicate the flow accuracy of the urea pump,including flow steady-state accuracy,flow repetitive accuracy,and flow linearity.The higher the metering accuracy,the better the output flow of the urea pump matches the UWS flow required by the system,the higher the NO_x conversion rate,the better the effect of exhaust gas purification.Therefore,in order to adapt higher emission standards,it is urgent to develop urea pumps with high metering accuracy to meet the urgent needs of market applications.Diaphragm pump is the most commonly used urea pump structure type because of its good sealing performance,high reliability and long service life.In the paper,the dynamic characteristics and metering accuracy of the diaphragm pump were studied systematically by combining theoretical analysis with experimental research,and the influences of the key components structural parameters such as diaphragm,port valves,and inlet and outlet pipelines on its dynamic characteristics were analyzed in detail.And the urea pump prototype was developed to test the theoretical research results.The main research contents and results are as follows:The overall structure and working principle of the urea pump were introduced,and the key components such as diaphragm pump and mixing chamber were designed.Through theoretical analysis and experimental research,the flow pulsation rate of the diaphragm pump,the lag characteristics of the port valve and the inertia effect of the pipeline were systematically studied.It is found that the diaphragm pump has large flow pulsation,the port valve lag angle has a great influence on the volumetric efficiency,and the pump occurs overfeeding phenomena at no-load,which will make the development of high metering accuracy urea pumps face serious technical challenges.The three-dimensional transient bi-directional fluid structure interaction(FSI)model of the diaphragm pump based on moving mesh was established.The dynamic characteristics of the diaphragm pump,the deformation movement of the diaphragm and the opening and closing characteristics of the port valves were simulated and analyzed under different working conditions.The results show that the instantaneous flow characteristics of the pump inlet and outlet are approximately the same regardless of the diaphragm pump inlet and outlet piping systems.The volumetric efficiency of the pump decreases with increasing back pressure,and the flow pulsation rate of the pump increases as the back pressure increases.The pump speed has little effect on the volumetric efficiency and flow pulsation rate of the pump.The dynamic characteristics of the suction valve and discharge valve are basically the same.The displacement of the valve ball has different degrees of vibration during the opening phase,and its amplitude increases with increasing the back pressure and pump speed.The movement of the diaphragm interacts with the movement of the hard core.The deflection of the diaphragm increases with increasing the back pressure,and the pump speed has little effect on the deflection of the diaphragm.Based on the FSI model of the diaphragm pump,the influences of the diaphragm and the port valve structural parameters on the dynamic characteristics of the diaphragm pump were simulated and analyzed.The results show that the thickness-diameter ratio of the braid,the diameter ratio of the diaphragm,the valve ball mass and the spring preload have a great influence on the dynamic characteristics of the diaphragm pump.The thickness-diameter of the diaphragm and the spring stiffness have little effect on the dynamic characteristics of the diaphragm pump.By optimizing the structural parameters of the diaphragm and the port valves synchronously,the volumetric efficiency of the diaphragm pump is increased by 27.1%,the flow pulsation rate is decreased by 45.8%,and the performance of the diaphragm pump is improved greatly.Based on AMESim^?,a urea pump piping system model was established,and the influences of piping system structural parameters on the flow characteristics of the urea pump were simulated and analyzed.The results show that the inertia effect of the pipeline system has a great influence on the flow characteristics of the diaphragm pump.Excessive inertia of the inlet pipeline will cause overfeeding of the pump at no-load.The volumetric efficiency is still abnormally increased by increasing the outlet back pressure.It should minimize the length of the pipeline and increase the diameter of the pipeline properly.The unreasonable pipeline structure design is easy to form bubbles in the pipeline.The pipeline diameter should be consistent and smooth transition as much as possible to improve the flow characteristics of the urea pump.Three urea pump prototypes and urea pump comprehensive performance test rig were developed,and the comprehensive performance comparison test of the urea pump prototypes was carried out before and after the improvement.The results show that the average metering accuracy of the urea pump prototype is±4.2%before the improvement.After using the theoretical research results to improve the prototypes,the average metering accuracy is±2.0%,which is equivalent to the international high metering accuracy urea pump level.The research methods and results of this paper lay a certain technical foundation for the development of high metering accuracy urea pump,and can provide some reference for the development of diaphragm pump with similar structure.