Injection Control Strategy Applied In Urea-SCR System Of Diesel Engine

According to statistics,the total amount of four major diesel pollutants in China have accounted for more than 80%that of motor vehicles.For controlling the emissions,China IV diesel emission regulations has been officially implemented since January 01,2015.Urea selective catalytic reduction?Urea-SCR?device is currently one of the mainstream technology to meet diesel engine emission regulations,but it is limited by the difficult to localization of the key parts and the technical bottleneck of core control strategy.There are high cost,huge structure,low transformation efficiency,poor low temperature performance,common phenomenon of urea crystallization and other issues still existing in Urea-SCR aftertreatment system.In order to overcome this technical bottleneck and promote the process of the industrialization of Urea-SCR aftertreatment system in our country.Besides of optimizing the structure of the Urea-SCR system,it is necessary to further study the urea dosing control strategy and injection method.How to improve the accuracy of the Urea-SCR system state parameters under the transient operating conditions is the difficult point in the study.Closed loop control of Urea-SCR system based on single NOx sensor becomes the key research content in China.The paper takes a heavy duty diesel engine provide by domestic mainstream diesel engine manufacturers as the emission control object,and studies the periodic fluctuation injection control strategy.The main research method is to study the performance of the Urea-SCR system under the condition of the test platform,by combining the independently developed Urea-SCR system control unit.Periodic fluctuation of injection control strategy is studied based on different test effects of urea dosing control strategy.The major research work are as follows:?1?Study the parameters variation of Urea-SCR system.Aimed at the performance index of Urea-SCR system,the change of different parameters in Urea-SCR system and its influence on the performance of the system are analyzed in depth from the theoretical level.The parameters that affect the performance of Urea-SCR system are classified from the control point of view.The variation rang and extent of the dynamic control parameters those have a decisive impact on the performance of the system are further determined by test,which laid the foundation for periodic fluctuation of injection control strategy research;?2?Investigate the effect of periodic injection on the conversation efficiency.The results obtained from the periodic injection test carried out under different operating conditions show that:the phenomenon of the sudden increase of the conversion efficiency can occur in most of the operating conditions,but not all of the injection quantity.This experimental phenomenon has laid a foundation for improving the conversion efficiency in the practical control.Through the depth study on the gain of the conversion efficiency of vanadium based SCR in the periodic injection condition,the control regulation of the parameters is found,which create conditions for obtaining higher conversion efficiency and reducing agent energy efficiency ratio;?3?Develop a model for predicting the"efficiency temperature"of the catalyst.Using heat transfer and engineering thermodynamics theory,the thermodynamic process between the catalytic converter and the exhaust gas is studied,and the thermodynamic model is developed.On the basis of the above,through the upstream and downstream standard temperature difference MAP,the further oxidation of incompletely combustion products and the heat release of the SCR reaction are incorporated into the model,establishing the catalyst temperature prediction model,which can truly reflect the temperature state of the catalyst.By introducing the concept of"efficiency temperature",it is more accurate to evaluate the catalyst temperature.When the"efficiency temperature"is used to evaluate the temperature state of the catalyst,the measurement value from the upstream and downstream temperature sensor can be used as the reference of the closed-loop feedback.The real-time conversion efficiency can be corrected by the calculation when using"efficiency temperature"as the evaluation value of the efficiency;?4?Establish a NH₃ coverage prediction model.In this paper,the extended Kalman filter algorithm is used to estimate the coverage of NH₃,and the state of NH₃ stored is controlled by the research results of the dynamic parameters variation,reducing the dependence of prediction time and test results,which make the results of the model more reliable and dynamic response fast;?5?Study and establish a model for controlling the amount of urea dosing.Combining the model of catalytic system and the calibration results of the diesel engine-aftertreatment system,the sub model of SCR upstream NO_X concentration and of chemical reaction mass ratio are established.A feed forward control model of urea dosing amount is established based on the control objective of conversion efficiency and NH₃ coverage.In order to compensate for the phenomenon of the NO_X original exhaust shift caused by the aging of the diesel engine,the NO_X concentration self-correcting model is established.Based on the ammonia cross-sensitivity of NO_X sensor,the sampling value of NO_X sensor is linearized by means of mathematical method through the determination of the main exhaust components to meet the requirement of higher injection control precision,realizing the closed-loop control of the supply of urea;?6?Research on the periodic fluctuation injection control strategy.Through the test,it is found that in a certain operating condition,maximizing the performance of the catalyst as well as the energy efficiency of the reducing agent are difficult to achieve simultaneously by constant injection amount,only by making the urea dosing amount vary with the optimal curve of dynamic parameters under the current operating conditions,the relative parameters of the catalyst can be maintained at a high level of activity in the controllable range,thus achieving higher conversion efficiency and energy efficiency ratio.The periodic fluctuation injection control strategy is designed for that purpose,and the NH₃ coverage and energy efficiency of the reducing agent are both as the control target included in the overall control strategy of the Urea-SCR system.The experimental results show that the control strategy can bring about the dual increase of energy efficiency of the reducing agent?average 32%,maximum 106.5%?and DeNOx effect?average 23%at low temperature,maximum 45.5%?,while reducing the leakage level of NH_3,improving the service quality of the system.