在不增加出口温度传感器的条件下,为准确预测 SCR(selective catalytic reduction)催化箱温度,该文在柴油发动机台摘架上开展 SCR系统催化箱的进出口温度特性测试。并基于 Mtalab/Simulink模块对 SCR催化箱开展仿真计算,提出了出口温度的计算模型。结果表明:稳态工况及过渡工况下,催化箱进口温度与出口温度变化并不相同；过渡工况时,催化箱出口温度改变过程滞后于入口温度改变过程；提出的计算模型在稳态和过渡工况下均对出口排气温度有良好的预测性能,可在不增加出口排温传感器前提下,获得催化箱出口温度,从而提升了 SCR温度控制精度。同时,该文将提出的算法应用到实际整车试验中,在 ESC(European steady state cycle)和 ETC(European transient cycle)标准测试中, NO_x的排放量均低于国IV排放法规限值,试验测试表明该文提出的算法满足了整车的控制需求。

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.

TK421.5

北京市教委促进人才培养综合改革基金项目(JZ005011201401)