Study On Control Model For SCR Catalyst Converter Of Heavy-Duty Diesel Engine

With concern for environmental issues,emission regulations become increasingly stringent. So it is very important to research the emission control technology for heavy-duty diesel engines. In Europe, the emission control technology,"combustion optimization + selective satalytic reduction (SCR)", has become the mainstream technical route for heavy-duty diesel engines to meet Euro IV and V emission standard for its advantages of good fuel economy and strong resistance to sulfur poisoning. In our country, with the advancement of the State IV and V emission regulation, the SCR technology has become a hot topic of the research of emission control technology. In this paper, in order to improve the control performance of SCR system, we develop a SCR system which can made the engine meet the State V emission regulation. And based on the current mainstream SCR system, deeply study the four parts: establishing SCR catalytic converter control model, identifying parameters, experimental validation and controller design.According to the feature that three-dimensional modeling of SCR catalytic converter is complex, in this paper, we establish a one-dimensional layered model which is a real-time calculation model for ECU. Based on Eley-Rideal mechanism and combining with the characteristics of lean burn engine, we select four major chemical reactions which can affect the dynamic process of SCR reaction: the adsorption and desorption of ammonia, the selective catalytic reduction and the oxidation of adsorbed ammonia. By the material balance equation, describe the above chemical reaction process. Based on the energy conservation law, derive the equation for calculating temperature in the layered model and then establish a mathematical model of SCR catalytic converter combining with the equations of chemical reaction process. Analyze and compare the selection of the number of cells in the layered model, and recommend the principle of selecting.According to the model features of many parameters and strong nonlinear, propose a model parameter identification method that is based on nonlinear least squares. In order to ensure the convergence of identification algorithms, analyze the sensitivity of each parameter and select the dominant parameters which affect the response characteristics of the model. According to regulatory requirements and requirements of the accuracy of model dynamic performance, achieve the control model parameter identification based on the selected experimental operating conditions.Validate the SCR catalytic converter control model by a bench of heavy-duty diesel engine system. Experiment is divided into steady-state and transient experimental test. In the steady-state experiments, aiming at main engine operating conditions (including all ESC operating conditions), carry out the characteristics of SCR reaction experimental verification; In transient experiments, contrast the SCR reaction characteristics under ETC cycle. The results show that model prediction of temperature of catalytic converters, chemical reaction rate, NH3 leak and other important variables is agreement with experiment, and fully meet the needs of SCR Controller.Based on the above proposed control model of SCR catalytic converter, designed the model-based feedforward controller of SCR system, and do a simulation study by SIMULINK simulation software. The results show that, because the model-based control strategy has considered the dynamic process of reaction in SCR catalytic converter,it can control the system ammonia leak better, and improve the control performance of SCR system at the time of rapid change in working conditions, comparing with the traditional MAP-based control strategy.