Research On Urea Injection Control Of Diesel Engine SCR System

Compare to gasoline engines,diesel have greater power performance and economy.But diesel has more nitrogen oxide(NOx)emissions than the gasoline engine.Because the emission between Particulate Matter(PM)and nitrogen oxide exist the relation of "Trade-Off",it has been unable to meet the increasingly stringent emission regulations just rely on improving the fuel combustion.Selective Catalyst Reduction has a high nitrogen oxide conversion efficiency,while easy to vehicle modification,it is one of After-treatment technologies that can meet the emission regulations of CHINA V or higher.SCR technology is the main technical means of diesel engine to reduce nitrogen oxides.This paper introduces the composition and working principle of SCR system,the factors influencing the conversion efficiency of the SCR system in the After-Treatment system are analyzed.According to the emission characteristics of diesel engines,determined the main chemical reactions in the SCR system determined,they are Standard SCR reaction,Fast SCR reaction,adsorption and release of ammonia in catalyst,and the oxidation of ammonia.Those reactions are described based on the Arrhenius formula,according to the conservation of energy and the conservation of the quality of each gas component in the catalyst,a one-dimensional finite element model of the catalyst is established.The control model of SCR urea injection was established by simulink platform,the model contains a state machine,Efficiency model,Calculation model of base injection quantity,Feedback control model,DPF coordination module.State machine is key to determine whether the current operating conditions can spray the urea solution.The efficiency model determines the current NOx conversion efficiency based on the bed temperature of the catalyst,the amount of ammonia stored,and also with other factors.The feedback module is used to reflect the difference between the actual conversion efficiency and the efficiency model in real time,to correct the current efficiency.DPF coordination module is a part of control model,When the DPF is in the regeneration process,a delay module designed to prevent leakage of ammonia that stored in catalyst.The catalyst reaction model and the urea injection control model were combined.Based on a diesel engine calibrate its nitrogen oxide emissions MAP,and conversion efficiency MAP of catalyst.Process the Diesel engine ESC and ETC test results,as input into the combined system,Perform simulation verification.The simulation results show that the conversion efficiency of NOx is close to 60%,but also indicates that the control model lacks strong control over between the downstream ammonia leaks and NOx conversion.