Research On Temperature Field Properties Of Selective Catalytic Reduction In A Diesel Engine

The after-treatment system of urea-selective catalytic reduction(SCR) is a most widely used solution to reduce the discharge of nitrogen oxides(NOx) at present. The temperature of catalytic box is a main factor of influencing the conversion efficiency of NOx, as a result, the strategic of temperature control is not only a premise to control the urea jet accurately, but also the core content of SCR control. In fact, because of the vehicle diesel engine’s working conditions being complex and changeable, and the temperature changing of the catalytic box being inconsistent, the temperature control strategic of catalytic box is very complex and difficult, which has been a key and difficult point to restrict the diesel engine to satisfy a more strict emission standard.To solve the problem of catalytic box’s temperature control strategic, first of all, the author set up a temperature measurement device for this after-treatment system of SCR based on the diesel engine experimental platform. Then, use the experimental data to analysis the catalytic box’s temperature change rule under the condition of different urea injection quantity and different diesel engine’s working condition. And then, set up a mathematic model of the SCR catalytic box temperature, analysis the conversion process carefully. At last, simplify this mathematic model, build a temperature control strategic of catalytic box, and verify this temperature control strategic of catalytic box in the bench and whole vehicle road test.The main research contents of this article are as following:(1) The design of experimental platform for SCR catalytic box temperature measurement. Set up a device to measure the temperature of SCR catalytic box. This device be consists of 4 parts such as SCR catalytic box, urea injection system, the temperature measure system of catalytic box and data collection system. Changing the working condition of diesel engine, the following factors could also be changed such as the initial flow, initial temperature and urea injection quantity in the entrance of the SCR catalytic box, by this way, the tech supports could be supplied to research the temperature changing rule of the catalytic box.(2) Character analysis of temperature field inside of the catalytic box.To analysis the distribution characteristics of temperature field in the SCR after treatment catalytic box, the results are as following: when the diesel engine working in steady working condition, the temperature uniformity of the catalytic box has relationship with the exhaust temperature of the catalytic entrance, but has no relationship with the exhaust flow quantity of the entrance of the catalytic box. For air assist urea injection system, the injection quantity has only a little influence to the temperature of the catalytic box’s entrance.When the diesel engine working in transition condition, the temperature of the catalytic box’s entrance would change following the changing of the diesel engine’s working conditions. The temperature of the catalytic box’s entrance is constantly synchronous along with the vertical direction of the catalytic box, and is constantly different along with the axis of the catalytic box. The time delay of catalytic box temperature along with the vertical direction has important relationship with exhaust quantity, but has a little relationship with the changing step of the catalytic box’s initial temperature.(3) The theoretical analysis of the temperature in the catalytic box.According to the conversation of energy, the author used the sofware of MATLAB/Simulink to set up a detailed catalytic box temperature model, and regarding exhaust temperature in the entrance of the catalytic box, exhaust flow and urea injection quantity as initial condition. Based on this model, the author calculate the temperature in the front of catalyst in the catalyst box, and the temperature in the middle of the catalyst in catalyst box, and the temperature by the end of the catalyst in the catalyst box, and the temperature of the catalyst box’s entrance, and also the thickness of NOx in the downstream of the catalyst box. Verified in the diesel engine bone, the test value is not beyond the tolerance of 5% to the simulation value.After theoretic analysis, the results are as following: In the catalyst box, the changing of the air was caused by the heat transfer medium, so we only have to consider the influence of chemical reaction to the heat of air in the carrier part of the catalyst box. In fact, the energy changing caused by chemical reaction is small than heat conduction by air, so the energy lost caused by chemical reaction could be ignored if the precision request of the catalyst box temperature estimation is not very high.(4) The research of temperature control strategic in the SCR catalyst box.Set up a calculation model for the downstream temperature of the catalyst box, and put forward a real time catalyst box temperature control model. Based on this diesel engine bench, to calculate the accuracy of this temperature control model by using steady state working condition and transition working condition, the result shows the temperature tolerance is in 5% between calculation value and test value. Besides, in the whole vehicle road test period, the temperature tolerance is in 10% between calculation value and test value. As a result, this temperature control strategy of SCR catalyst box could satisfy the requests.