Study On The Coordinated Control Strategy Of Urea Injection Quantity Of Diesel SCR System

Faced with the increasingly stringent NO_xemission limits of diesel engines,advanced NO_xemission control technologies such as efficient SCR technology,dual SCR technology,tightly coupled SCR technology,SCR electric heating technology,SCRF technology,and solid-state ammonia SSCR technology are effective measures to further achieve ultra-low NO_xemissions within the full operating range of diesel engines,and are of great significance for the development of near zero NO_xemission control throughout the entire life cycle of diesel SCR catalysts in the future.The difficulty of SCR control technology lies in the precise control of urea injection.The urea injection control strategy judges the actual NH₃demand based on the working status of the engine and aftertreatment system,and achieves accurate control of urea injection at appropriate injection times to ensure that the SCR catalyst can achieve high NO_xconversion efficiency and optimal ammonia loading under all operating conditions,ultimately achieving a control effect of near zero NO_xemissions.However,due to the variable operating conditions of the engine,there are also phenomena such as DOC heating state,DPF regeneration,and SCR self aging in the aftertreatment system.Therefore,the urea injection control of the SCR system needs to take into account both engine operating conditions and the working state of the entire aftertreatment system,making the SCR system control a multi time scale coupling control problem.In addition,the inherent characteristics and catalytic reduction chemical reaction process of SCR catalysts are also influenced by various factors,as well as the lateral sensitivity of commercial NO_xsensors to NH₃.Many factors determine that the controlled object of the SCR system is a nonlinear control system coupled with multiple influencing factors and multiple time scales.Therefore,it is necessary to construct an advanced SCR urea control strategy with adaptability to the full life cycle system of SCR catalysts,in order to coordinate and control the precise urea injection of SCR systems with different post-treatment system states and different time scales coupling.Based on the urea injection control requirements of the SCR system,a control oriented urea injection coordination control architecture for the diesel engine SCR system was designed.The main design includes a basic NH₃demand precontrol strategy based on NO_xconversion efficiency,a necessary NH₃coordination control strategy based on the optimal ammonia load,a necessary NH₃feedback correction strategy based on the downstream NO_xemission target value of the SCR,and a urea injection control response mechanism based on system state coordination.The research mainly includes the following aspects:（1）Designed a precontrol strategy architecture based on NO_xconversion efficiency.Based on the precontrol mechanism of basic NH₃demand based on NO_xconversion efficiency,control demand analysis was conducted on the basic NH₃demand,and different mode calculation methods for basic NH₃demand were proposed.This method includes a calculation strategy for basic NH₃demand based on NO_xconversion efficiency model and an online measurement strategy for basic NH₃demand based on real-time engine status.By designing a pattern driven coordinator,it is possible to achieve precontrol and adjustment of basic NH₃demand using different modes in different system states.（2）A necessary NH₃calculation strategy based on the optimal ammonia loading control of SCR catalyst was designed.In response to the calculation of NH₃demand,strategies for calculating the required NH₃adsorption capacity of the SCR catalyst carrier,the speed control mechanism for the required NH₃adsorption capacity,and the necessary NH₃demand calculation strategy for the final SCR system were studied and designed.This design can achieve real-time control of NO_xconversion efficiency and carrier ammonia loading of the SCR catalyst,enabling the SCR system to achieve ultra-low NO_xemissions with minimal NH₃leakage.（3）A NO_xfeedback control reset mechanism based on NO_xemission feedback correction coefficient monitoring was designed to address the robustness issue of the NO_xemission feedback control system.This achieved real-time supervision of the NO_xemission feedback control process,effectively reducing the risk of NH₃leakage in the SCR system and reducing the sticking phenomenon of the NO_xfeedback controller.For the input signal and system control release of NO_xemission feedback control system,corresponding signal filtering strategies and response triggered release coordination strategies were designed to improve the accuracy of system control.In order to eliminate the steady-state error in the calculation process of the feedback correction coefficient for NO_xemissions in the system,a PI regulator based on the target control error of NO_xemissions downstream of SCR was studied and designed.（4）A urea injection coordination state controller has been studied and designed.To address the issue of urea coordinated injection control during the process of SCR system state changing with engine operating conditions,a urea injection control response mechanism based on system state changes can be designed to achieve dynamic opening or closing control of urea injection release with changes in engine operating conditions,thus achieving coordinated control of urea injection under various system states.