Outlet temperature characteristics for NO_x catalyst container of diesel engine SCR system and its calculation model

The selective catalytic reduction(SCR) technology is considered as a highly efficient and promising after treatment technology for deducing the diesel engine NO_x emission.The temperature inside an SCR catalyst container isimportant for NO_x conversion efficiency and SCR control precision.Accurate prediction for the temperature of the SCRcatalyst under the condition of transient loads has important influence on the NO_x conversion efficiency.In order to gainthe detail of the temperature distribution inside the SCR catalyst container, this work designs 5 test plates withthermocouples which are mounted inside the SCR catalyst container in different places.These 5 test plates contain a serialof thermocouples located in different position to test the temperature distribution inside the SCR catalyst container.These5 plates are orderly located from the inlet to the outlet of the container, and in each plate, there are totally 17 test pointssuccessively along 2 diameters which are respectively the horizontal and vertical directions.During the experiment, thediesel engine with the SCR system is tested under the varied operation engine speeds and loads to supply the SCR catalystcontainer with exhaust gases of different temperatures and mass flow rates.In addition, a mathematic model is proposed topredict the catalyst temperature at the outlet based on the tested inlet temperature data of the SCR catalyst container.Themathematic model is solved based on a program which is compiled by Matlab/Simulink codes.The experimental resultsshow that under the steady conditions, the temperature inside the SCR catalyst container decreases from the inlet to theoutlet of the container along the axial direction.The reason is that as the exhaust gas flows through the SCR catalystcontainer, the thermal energy of the exhaust gas decreases due to the thermal energy exchanges such as heat convection,heat conduction between the exhaust gas and the catalyst carrier, the container wall, as well as the environmentalatmosphere.The experimental results also show that the outlet temperature is lower than the inlet temperature, and themaximum deviation between the tested inlet temperature and outlet temperature is 10.1%.However, the model predicting outlet temperature has a better deviation of 6.2% with the tested outlet temperature.Under the conditions of transientoperation, the downstream temperature of the catalyst container shows a delay of temperature increasing.As the enginespeeds and loads change, firstly the inlet temperature rises, but the downstream temperature doesn′t increase together withthe inlet temperature, and it begins to rise after 40 s.The downstream temperature reaches its 95% of the maximum after nearly 380 s.The reason of this temperature increasing delay is that the thermal energy transfer from the exhaust gas to theSCR catalyst especially under the transient condition takes much time, and the narrow catalyst holes cause flow resistancesto the exhaust gas.This temperature increasing delay is also demonstrated by the mathematic model.In addition, the modelpredicts that the outlet temperature under the transient condition has a deviation of less than 6% which is better than the deviation between the tested inlet temperature and the outlet temperature(32%).Therefore, the inlet temperature of the SCRcatalyst container is not suitable to be simply adopted as the outlet one for the purpose of the SCR system control strategy,because there is great deviation between the inlet temperature and outlet temperature under both steady and transientoperation conditions.In this work, the proposed mathematic model shows good calculation precision when predicting theoutlet temperature under both steady and transient operation conditions.Compared with the simple method of adopting theinlet temperature as the outlet one, this model for predicting outlet temperature has better control precision especiallyunder transient conditions, which is good for SCR system control strategy.The control algorithm is applied to the SCRsystem of a heavy duty diesel vehicle, and the vehicle is tested based on the ESC(European steady state cycle) and ETC(European transient cycle) test standards.The tests show that NO_x emission is lower than the limit of the China IVemission standard, which demonstrates the proposed algorithm meets the control requirements of the vehicle.