Research On Influence Of SOF On NO_x Conversion Efficiency In SCR System For Diesel Engine

Diesel commercial vehicles are used for short-haul transportation in city and county areas.Soluble Organic Fraction（SOF）of particulate matter raw emissions accounts for a high proportion of mass.SOF is captured and covered by post-treatment carriers（EGR,Exhaust Gas Recirculation,DOC,Diesel Oxidation Catalyst,DPF,Diesel Particulate Filter,SCR,Selective Catalytic Reduction）directly installed after the turbocharger.For diesel engines directly installed with SCR system,due to the lack of effective SOF removal means,the adsorption and desorption reaction of NH3 on the catalyst surface will be isolated due to SOF coverage,resulting in continuous reduction of NOx conversion efficiency,the NOx emission can exceed regulatory limits.SOF coverage is physical property coverage,no matter whether it is vanadium based,copper based or iron-based,there are similar problems.For future diesel engine China Ⅶ emission,refer to Euro 6 emission regulation,the NOx conversion efficiency will be further greatly reduced on the basis of China VI,i.e.the NOx emission is about 0.12g/kWh.In order to cope with the ultra-low emission of China seven,the main technology route of emission is diesel engine+SCR1+DOC+DPF+SCR2.For SCR1 carriers,SOF coverage will also lead to the reduction of NOx conversion efficiency.Therefore,the research on the influence of SOF on the NOx conversion efficiency of diesel engine SCR system has great significance for the current and future NOx emission control of diesel engines.The SCR carrier of the China V diesel engine under the SCR route and the SCR1 carrier of the China VII emission are similar to the SOF coverage problem.Therefore,the author takes the Weichai China V diesel engine and the SCR system as the object to study the mechanism of SOF coverage.It is found that the average mass fraction of soot in the raw particulate matter（typical steady state）is 24.9%,and the SOF mass ratio is 75.1%.The experimental study on the capture efficiency of PM components（under typical steady state conditions）proves that SCR carrier has high capture efficiency for SOF under low exhaust temperature,and is related to SOF deposition.Soot will be captured by SCR carrier with SOF,and PM capture efficiency is the superposition of both.The SOF deposition test of SCR carrier showed that the SOF deposition amount is about 30g at 24 hours steady state test,and the conversion efficiency of NOx decreased by about 36%.Based on the SOF pyrolysis test,the NOx conversion efficiency can be completely restored,and the captured soot will be released with SOF pyrolysis.At the same time,it is found that the pyrolysis time of SOF is longer than that of Thermal Gravimetric（TG）at the same mass SOF,and the activation energy of the bench pyrolysis is similar to that of TG pyrolysis,natural logarithm of pre exponential factor is only about 36%of TG.The SOF raw emission of diesel engine is the fundamental reason for the SOF coverage of the SCR carrier.This study optimizes and controls SOF raw emission from emission related parts configuration,fuel system control parameters,main injection timing correction and combustion start point control.It is found that the configuration of emission related parts and fuel system control parameters have little effect on the further reduction of SOF.Based on the data of cylinder pressure,a real-time correction algorithm for injection timing consistency is proposed in this study.Experiments show that the algorithm can effectively improve the consistency of injection timing and NOx consistency,but the effect of SOF emission consistency improvement is not obvious.A real-time feedback and control algorithm based on the first derivative of equivalent isentropic exponent of cylinder pressure data is presented in this study.The algorithm can detect the starting point of combustion in real time,control the starting point of combustion to the target value,and maintain the consistency of the main combustion point at different EGR rates.However,the algorithm has less influence on the further reduction of SOF.It is found that DOC technology has little effect on the SOF coverage problem,and can only delay the occurrence time of the problem.The optimization and control of SOF raw emission of diesel engine can not fundamentally solve the problem of SOF coverage of SCR carrier.Therefore,this study establishes a mathematical model for estimating SOF deposition,including SOF raw emission module,SOF capture module by SCR carrier,and SOF pyrolysis module.The SOF raw emission is obtained based on SOF steady state emission and transient correction calculation,and it can also be selected based on gaseous HC conversion calculation.The SOF capture module is based on the Map multiplied by the SOF deposition correction base on SCR upstream temperature and the exhaust gas flow rate.SOF is a complex mixture.In this study,the activation energy of each component is used as the basis for segmentation.SOF pyrolysis is divided into 3 stages,and every stages are modeled respectively.The simulation model of SOF deposition estimation is established by using Matlab/simulink software.In order to improve the accuracy of the estimation model,the hybrid optimization algorithm is used to optimize and identify the transient correction Map for SOF raw emission and 3 stages pyrolysis parameters of SOF.The results show that the average error of the estimation of 4 groups SOF deposition in low-temperature is 2.42%,and the average error of the estimation of 12 groups SOF pyrolysis residual value in high-temperature is 4.03%,which meet the requirements of engineering application.This study carried out the SOF coverage control strategy of SCR carrier.When the SOF deposition is higher than the threshold value and the engine NOx emission fault diagnosis reports the fault of emission exceeding the regulatory limits,the system is switched to SCR regeneration state,and the SCR upstream temperature is enhanced by thermal management to achieve SOF removal.The validation of the platform and vehicle shows that the control strategy can effectively control the SOF coverage of the SCR carrier.Diesel engine NOx emission fault diagnosis is the most important function requirement of OBD（On Board Diagnosis）,and it is also one of the conditions for triggering SCR regeneration mode.In this study,NOx emission fault diagnosis is abstracted as a typical time series classification problem,and one NOx emission fault diagnosis model based on time window and support vector machine（SVM）is proposed.The results of model training and verification show that the fault diagnosis accuracy of real road test data reaches 96.99%when the WHTC cycle test data is used as the.training set,Combining the WHTC cycle with the real road spectrum test data as training set and test set,the accuracy of fault diagnosis can reach over 99%,which shows that the fault diagnosis model has good generalization performance and has high engineering application value.