Modeling And Control Strategy Optimization Of SCR System For Heavy-duty Diesel Vehicle

Nitrogen oxides(NOx)emissions from heavy-duty diesel vehicles are one of the main sources of air pollutants.In order to reduce the impact of heavy-duty diesel vehicles on the environment,in recent years,countries around the world have introduced increasingly strict emission regulations for heavy-duty diesel vehicles.Urea selective catalytic reduction(SCR)technology is widely used in heavy-duty diesel vehicles market,because it has high NOx conversion efficiency in the wider range,do not need to modify the original engine,has good durability and suitability.Since China IV,SCR technology has been widely used to meet the requirements of NOx emission regulations.However,in China VI,diesel oxidation catalyst(DOC)and diesel particulate filter(DPF)will be installed before the SCR catalyst,which has a great impact on the performance of SCR catalyst.The problems of model nonlinearity,multi-time scale and the lack of measurement information of physical sensors make it difficult to establish models and develop control strategies for SCR system.Existing models have such problems as large calibration workload,large demand for computing resources and difficulty in integrating into the controller,etc.SCR control system may further reduce NOx emissions through optimization,but there is a lack of research on optimization SCR control model and strategy.In addition,the domestic and foreign technical solutions for the future stricter California ultra-low NOx emission regulations are still in the exploration stage and need to be further studied.Based on this,this paper studied the performance of the aftertreatment system of heavy-duty diesel vehicles through engine bench test,established a control-oriented aftertreatment system model,optimized the urea injection control of SCR system,as well as studied and analyzed the control strategy of realizing ultra-low NOx emission.The factors affecting NOx emission and NO/NOx ratio before SCR inlet were studied by engine bench test,and the correlation analysis was carried out by combining with mutual information(MI)correlation analysis,and a virtual NOx sensor was established to predict NOx emission.The results show that under the steady-state condition,raw NOx emission of the engine is mainly NO,and the average ratio of NO/NOx is about 98%.The catalyst temperature is the main factor affecting the NO/NOx ratio at the SCR inlet.The NO/NOx ratio is affected by the chemical reaction rate and chemical equilibrium of the NO-NO2 reversible reaction,and decreases first and then increases with the increase of the temperature.Based on MI,quantitative analysis the raw NOx emissions and SCR inlet NO concentration and relevance to the influence factors.The results show that the main related parameters with raw NOx emissions include gas pressure,gas flow and fuel injection parameters that influence the engine combustion and inlet and exhaust process of engine,reflecting engine working status of engine operating conditions,and emissions of engine that reflect the burning state of engine.The main parameters related to the NO concentration at the inlet of SCR catalyst are not only those which are more correlated with raw NOx emission,but also the temperature parameters of the aftertreatment system.Based on MI analysis,input parameters were selected from gas pressure parameters,gas flow parameters,fuel injection parameters,engine operating conditions parameters and aftertreatment system parameters,and virtual NOx sensor was established based on back propagation neural network(BPNN).The model after training was verified under the world harmonized transient cycle(WHTC).The verification results showed that the virtual NOx sensor based on MI analysis and BPNN network had better prediction performance for raw NOx emission and NO concentration at SCR inlet.Compared with the prediction based on EGR rate,the root mean square error of BPNN model was reduced by more than 15%.The ammonia(NH3)storage characteristics,NOx emission and NH3 slip characteristics of SCR catalyst were studied on the engine test bench and a control-oriented one-state SCR model was proposed.The results show that SCR catalyst can store a certain NH3,when the urea injection stopped,the stored ammonia gas can still convert NOx to nitrogen.When SCR catalyst temperature is lower than 400?,the moment when NH3 slip reaches 10 ppm is gradually advanced with the increase of temperature and the increase of space velocity.Compared with the effect of space velocity on the moment of NH3 slip,the effect of temperature is more obvious.When the temperature is 450?,there is a significant delay in the moment of NH3 slip.This is mainly because NH3 oxidation reaction will occur at high temperature,which reduces the excess supply of reducing agent and reduces the accumulation rate of NH3 storage in SCR catalyst.When the SCR catalyst has high NH3 storage,the NOx conversion efficiency is higher than 99%in the temperature range of 200?450?.Space velocity has little effect on the saturated NH3 storage in the copper-zeolite SCR catalyst.With the increase of space velocity,the saturated NH3 storage decreases slightly.SCR catalyst temperature has a great influence on the saturated NH3 storage.As the temperature of SCR catalyst increases,the saturated NH3 storage decreases rapidly.The WHTC test results show that NOx emission mainly occurs in the stage of urea shutdown,while NH3 slip mainly occurs at high temperature.Experimental studies on different SCR catalysts sizes have shown that there is an obvious trade-off relationship between WHTC specific NOx emission and NH3 slip.Increasing the volume of SCR catalyst is conducive to reducing both NOx emissions and NH3 slip.Initial NH3 storage has a great impact on emission results.Improving initial NH3 storage can reduce NOx emissions,but will increase NH3 slip.Based on SCR chemical reaction dynamics model,the continuous stirred tank reaction model and the law of conservation of quality,the improved control-oriented one-state SCR model was proposed.Two static equations are used to describe the dynamic process of NOx emissions and NH3 leak,and NH3 coverage ratio is the only dynamic process.It can simplify the SCR model,reduces the computational load and ensure the accuracy.The average absolute error of the calibrated one-state SCR model for NOx emission and NH3 slip is 6.9 ppm and 1.8 ppm,respectively.The one-state SCR model can well predict the NH3 storage in the SCR catalyst,so as to accurately predict NO,emission and NH3 slip.The influence of NH3 coverage ratio on NOx emission and NH3 slip in SCR catalyst was analyzed based on one-state SCR model.NH3 coverage ratio is a key factor affecting SCR catalyst emissions.Increasing NH3 coverage ratio in SCR catalyst can reduce NOx emissions,but will increase NH3 slip.Based on the established control-oriented one-state SCR model and multi-objective genetic algorithm,the NH3 storage control of SCR system is optimized.The factors affecting the steady-state optimal solution are analyzed,the SCR system transient optimization method is proposed,the problem of SCR system transient optimization is solved,and the NH3 coverage ratio zonal control logic is proposed based on the transient optimization results.The optimization results of four typical steady-state operating points in the world harmonized stationary cycle(WHSC)show that there is an obvious trade-off relationship between NOx emission and NH3 slip in the optimal solution set on the Pareto front.On Pareto's front curve,use 10 ppm of NH3 slip as the dividing line.When NH3 coverage ratio is less than 10 ppm,NOx emission devreses rapidly with the increase of NH3 coverage ratio.However,when NH3 coverage ratio is greater than 10 ppm,further increase of NH3 coverage ratio leads to a small reduction in NOx emissions,but a rapid increase in NH3 slip.SCR catalyst temperature is the main factor affecting the target value of optimal steady-state NH3 coverage ratio and optimal solution NOx emission.The target value of steady-state optimal NH3 coverage ratio is basically linear with the change of temperature,and decreases with the increase of temperature,but the NOx conversion efficiency gradually increases with the increase of temperature.The effect of space velocity and inlet NOx concentration on the optimal solution is less than that of temperature.With the increase of space velocity and inlet NOx concentration,the target value of optimal NH3 coverage ratio increases slightly,while the NOx conversion efficiency decreases slightly.The optimal NH3 coverage ratio rate and the optimal solution NOx conversion efficiency fluctuations due to the changes in space velocity and inlet NOx concentration are 5%and 2%,respectively.According to the steady-state optimization results,the SCR system was optimized under transient conditions,that is,the MAP of NH3 coverage ratio target value changing with temperature was optimized.The trade-off relationship between NOx emission and NH3 slip also exists in Pareto front of the transient optimal solution,and the change law is similar to that of the steady-state optimal solution.The core of NOx emission control is to improve NH3 storage at low temperature and reduce NOx emission at low temperature section,especially at the stop injection section.At low temperature,the target value of NH3 coverage ratio in the transient optimal solution is large.At higher temperature,the NH3 coverage ratio rate of the transient optimal solution is lower than that of the steady-state optimal solution.In the urban and rural low-temperature operating conditions,the NH3 coverage ratio change of the transient optimal solution is greater than that of the steady-state optimal solution.However,NH3 coverage ratio change of transient optimal solution is less than that of the steady-state optimal solution in the highway operating conditions.Increasing the change rate of NH3 coverage ratio can increase the NOx conversion efficiency,while reducing the change rate of NH3 coverage ratio can reduce the risk of NH3 slip.By comparing the steady-state and transient optimization results,the zonal control logic is proposed.The MAP of the NH3 coverage ratio target value is divided into six regions,namely the urea stop injection zone,the high NOx emission zone,the low-risk emission zone,the road operating condition transition zone,the high temperature transition zone,and the high risk of NH3 slip zone.The urea stop injection zone and the high NOx emission zone are the main areas of NOx emission.In the high NOx emission zone,when the temperature drops,the NH3 storage should be rapidly increased to avoid the production of a large number of NOx emissions.The temperature range of the low-risk emission zone is located in the high activity range of the SCR catalyst,and the NH3 coverage ratio is easy to control.The temperature in the transition zone of road operating conditions fluctuates sharply,and the NH3 coverage ratio varies greatly in this temperature range.The high temperature transition zone and the high risk of NH3 slip zone are relatively high in temperature,which has a high risk of NH3 slip.It is beneficial to control NH3 slip in the high risk of NH3 slip zone through controlling the high temperature transition zone at a low level.The calculation results show that,based on the zonal control,the WHTC cycle NOx emission is reduced from 8.66 g/kW·h to 0.35 g/kW·h,which is 57.3%lower than the fixed ammonia to NOx ratio(ANR)control,and the average NH3 slip is less than 10 ppm,which meets the requirements of the China VI emission regulations.Based on SCR system modeling and optimization results coupling SIMULINK software,the aftertreatment system simulation platform was built,and the realization of ultra-low emission system and its control strategy were studied.According to the influence law of NH3 storage on NOx conversion efficiency and NH3 slip in SCR catalyst,a urea closed-loop control strategy based on NH3 storage was proposed.According to the target value of NH3 coverage ratio,proportional-integration(PI)control was carried out on NH3 storage in SCR catalyst.The test results met the requirements of China VI emission regulation limits.On the simulation platform,the control strategy requirements of the energy supplement system were analyzed.When the aftertreatment system had no energy supplement system,NOx emissions were mainly generated in the early stage of the federal test procedure(FTP).In the cold start of FTP,97.8%NOx emissions of the entire cycle was generated in the first 700 s.In order to increase the NOx conversion efficiency,energy supplement system can use two strategy that is increasing the maximum working power of the system and raising the target value of SCR catalyst inlet temperature.However,increase the vulcanizing system working power's influence on the NOx emission than raising the target value of SCR catalyst inlet temperature.It is because compared with duration of urea shutdown,the start time of urea injection more influence on the NOx emission,so the main purpose of energy supplement system is shortens the start time of urea injection.In addition,the energy consumption of increasing the maximum power strategy is also less than that of increasing the target temperature.The strategy of increasing the target temperature will also increase the fluctuation of SCR inlet temperature and increase the difficulty of controlling NH3 storage.When the maximum working power of the energy supplement system is 20 kW and the target value of SCR inlet temperature is 300?,NOx emission meets the requirements of California ultra-low NOx emission regulation,and the power consumed by the energy supplement system accounts for about 1.34%of the engine power.The denitration(DeNOx)potential of close-coupled SCR(ccSCR)system was studied.ccSCR system can greatly shorten the starting time of urea injection,and the starting time of urea injection on the FTP test cycle can be reduced from 568 s to 51 s.A two-nozzle joint control strategy for ccSCR system based on NH3 storage is proposed to solve the problem of urea injection control in ccSCR system.This strategy combines the control of NH3 storage in ccSCR catalysts and downstream SCR catalysts.The NOx emission after downstream SCR catalyst meets the requirements of California ultra-low NOx emission regulations.Compared with the energy supplement system,ccSCR system has a higher DeNOx potential.Close-coupled SCR system does not need to introduce additional heat source,and makes full use of exhaust gas heat.Through the heating of engine exhaust,it can quickly light-off to achieve a higher NOx conversion efficiency and reduce system energy consumption.