Study On Closed-loop Control Strategy Of Universal SCR System

With the increasing vehicle ownership and stringent emission regulations year by year,engine exhaust pollution control has become more and more important in our industry.Selective catalytic reduction(SCR)technology is widely used in diesel engine NOx emissions control.But most of the traditional SCR control system was developed with vehicle electric control system at the same time,calibration work needs much time and the system is not universal.This paper enhances the Universal SCR control strategy which makes the initial map need much less calibration work and urea,and this paper also puts forward the closed-loop control strategy based on the initial map,which enhances the reliability of the initial map and makes NOx more controllable.The formation of the initial map is universal,the control strategy suits any type of engine.In the process of test we just adjust the working condition of the engine to complete calibration of preset condition points,and then automatically selects the most economic quantity of urea which meets emission regulations.Closed-loop control strategy based on the initial map can not only avoid inherent large dead time of pure closed-loop control,but also solve the problem such as much calibration work in open-loop control.Firstly,designing hardware and software of universal SCR system.Designing the whole control circuit,and according to the needed hardware devices with the correct connection method,integrating the electronic control system after finishing the physical connection.Injecting urea on-demand is a precondition for this test,and we respectively research the injection characteristics at 10 and 20 Hz frequency under the pressure of 4 bar and 6 bar.Through comparation,then discovering the good linear relationship between urea mass flow and PWM.With the pressure and injection frequency increasing,urea mass flow will also increase and ultimately selects the injection condition at 20 Hz frequency under the pressure 6 bar.In the designing of software,programming with the foundation of the Freescale MC9SXS128 microcontroller combining with CODE WARRIOR,including the sensor signal procedures and core control strategy.Using Lab VIEW to communicate with PC which ensures the data monitoring,acquisiting and storaging all the time.Secondly,building test bench,and then generating initial controllable map which is universal.After owning the hardware and software foundation,and successfully building test bench,we will complete the test according to control strategy.The strategy includes three parts: Calibration mode,operating automatically mode and the generation of map.Calibration mode only needs to adjust the working condition,the rest of work can be done by itself.According maximum conversion efficiency corresponding NOx value after catalyst of ESC13 to verify whether meet the requirement,and select the most economic amount on the basis of satisfactory result,the above process is done by the control unit itself.After test to calculate weighted emissions according the maximum conversion efficiency corresponding NOx value after catalyst of ESC13 is 1.561g/(kw.h),which demonstrates the catalyst meet the requirement.Selecting out the ESC13 most economical amount of weighted urea is 360.2mg/s,the corresponding weighted emissions is 3.364g/kw h.It satisfies the fourth emission regulation while the urea is minimal.At the same time,to select urea reasonably for the rest working conditions of other 6*6 and use MATLAB linear interpolation generate initial controllable map.Finally,PID closed-loop control combined initial map is put forward and parameter setting experiment is performed.Engine working condition changes frequently,only injecting urea according to a specific map can not meet the requirement.Through experiment exploration and method improvement,demonstrating that the validity of PID control which has good control effect.Under the effect optimal Kp,Ki parameter in A25 condition,the NOx value after catalyst will catch up with the target quickly.The average has 1.06 ppm error with target in sampling time,the standard deviation is 7.86,NOx value remains a small oscillation amplitude.