Research On Control-oriented SCR Prediction Model For Diesel Engines

In order to reduce NO_x emissions from diesel engines,Selective Catalytic Reduction（SCR）has been widely used in diesel engine aftertreatment.The increasingly stringent emission regulations have put forward higher and higher requirements for the performance of SCR.SCR requires controlling NH₃ leakage within limits while ensuring high NO_x conversion efficiency,which poses a significant challenge to urea injection control in SCR.In order to achieve the optimal amount of urea injection and improve SCR performance,a control-oriented SCR prediction model was established based on the requirements of urea injection control strategy,providing feedforward input for the urea injection control strategy.The main research contents and work are as follows:（1）Based on understanding the working principle of the SCR system and combining with the SCR urea injection control strategy,the function of the SCR prediction model was analyzed and determined to calculate the SCR ammonia storage and predict the SCR conversion efficiency.Based on functional requirements,the architecture of the SCR prediction model was determined.（2）A prediction model for SCR ammonia storage was established using Simulink.According to the law of mass conservation,the ammonia storage mass of the SCR catalyst was calculated using the ammonia mass flow rate corresponding to the urea injection rate as input.The ammonia storage prediction model consists of three sub models:urea decomposition model,ammonia consumption calculation model,and ammonia storage calculation model.In the urea decomposition model,the pipe wall temperature before SCR was calculated,and the NH₃ loss caused by the formation of urea deposits was calculated based on the impact of pipe wall temperature and other factors on the formation of urea deposits.In the ammonia consumption calculation model,the ammonia consumption of the denitrification reaction is calculated based on the NO_xemission of the diesel engine and the NO_x conversion efficiency of the SCR.The ammonia oxidation consumption is calculated based on the factors that affect the ammonia oxidation reaction.The ammonia storage rate can be obtained by subtracting the ammonia mass flow corresponding to the urea injection rate from the NH₃ mass flow that generates sediment and consumes the reaction.The ammonia storage calculation model uses an integrator to calculate the ammonia storage mass based on the ammonia storage rate,and considers ammonia storage reset.（3）According to the factors that affect SCR conversion efficiency,relevant calculation models for influencing factors have been established using Simulink,including:diesel engine NO_x emission calculation model,NO₂/NO_x model,SCR temperature model,and SCR aging model.On this basis,a prediction model for SCR conversion efficiency has been established.The NO_x emission calculation model of a diesel engine calculates the NO_x emission based on the operating parameters of the engine,and uses it to compensate and correct the NO_x emission measured by the sensor.The NO₂/NO_x model calculates the proportion of NO₂ in SCR inlet NO_x based on the relevant state parameters of DOC and DPF and the exhaust flow rate.The SCR temperature model calculates the temperature upstream of the SCR based on the operating conditions of the engine.Based on the temperature upstream of the SCR,the temperature downstream of the SCR is calculated using thermodynamic principles.Based on the temperature sensor status signals upstream and downstream of the SCR,the temperature upstream and downstream of the SCR output by the model will be selected between the sensor measurement value and the model calculation value.Based on the temperature upstream and downstream of the SCR,the temperature of the SCR carrier is weighted and averaged.The SCR aging model calculates the mass of HC adsorbed by the catalyst to characterize the HC poisoning of the SCR catalyst,and calculates the SCR aging coefficient based on the average temperature of the SCR carrier and the mileage used on the vehicle.On this basis,the SCR conversion efficiency and the SCR conversion efficiency based on ammonia storage precontrol are predicted according to the factors that affect the SCR conversion efficiency.Based on practical application considerations,the target SCR conversion efficiency for diesel engines is calculated based on the operating parameters of diesel engines.（4）The WHSC experiment verifies that the established SCR prediction model has a good prediction effect.