Study On Spray And Urea Deposition Characteristic Of Vehicle SCR System

To solve the problem of worse and worse living environment, the government has published the severe emissions regulations. Meanwhile, the limited value of NOx for diesel engine becomes lower and lower. To meet the challenge, China has used SCR system as post-processing technique. But SCR has suffered from two main problems of low NOx conversion efficiency in low temperature of exhaust gas and urea crystallization. To solve these problems, this paper focuses on SCR spray characteristic and urea deposition characteristic.A reasonable SCR catalyst model based on experiment data was used to investigate and get the influence rule of ANR(NH3/NOx), temperature of exhaust gas, GSHV(gas hourly space velocity) for the NOx conversion efficiency. Simulation results show increasing ANR can improve NOx conversion efficiency, but when ANR is nearly 1, NOx conversion efficiency is improved less. In addition, a leakage of NH3 is still increasing; exhaust gas temperature can have an important role in NOx conversion efficiency; GSHV at low temperature of exhaust gas can influence NOx conversion efficiency more than at mid-high temperature of exhaust gas.To solve NOx conversion efficiency in low temperature of exhaust gas and urea crystallization better, a 3D SCR spray model based on specific working condition was used to investigate the influence of the number of injector hole, the injector height, injection angle, wall temperature, mass flow rate of exhaust gas and different structure for SCR spray characteristic and urea deposition. Simulation results show: the number of injector hole influences urea deposition more than uniformity and generation of NH3;when the injector height is near the axis of the pipe, the phenomenon of urea deposition and uniformity and generation of NH3 become better; spray characteristic, urea deposition, uniformity and generation of NH3 can be influenced by injection angle obviously; if it takes urea deposition and uniformity of NH3 into account,it needs a reasonable flow rate of exhaust gas; reducing heat loss of wall can relieve urea crystallization; The SCR without hybrid tube can relieve urea crystallization and have good NH3 uniformity and low generation rate of NH3, but NO conversion efficiency is not bad; while SCR with hybrid tube has low NH3 uniformity and high generation rate of NH3 and good NO conversion efficiency. It shows NH3 uniformity and generation of NH3 influence NO conversion efficiency both, but generation rate of NH3 can influence it more. Research results can be used to optimize SCR system.