Research On EGR Control Strategy Of Vehicle Diesel Engine Based On Double Closed Loop

Recently,with the serious shortage of energy and environmental pollution,coupled with increasingly stringent emission regulations,internal combustion engine development is facing unprecedented challenges,energy-saving emission reduction is imminent.Diesel engines with high compression ratio and excess air coefficient has better power,fuel economy and lower emission of CO and HC compared to gasoline engines.However,diesel engines emit more NOx and PM.In order to meet the stricter emission legislations,two main technical routes for diesel emission control were proposed.The first one is electronically controlled common rail system with SCR system,the second one is EGR+DOC/DPF/POC.EGR system is simple and reliable while SCR system is complex and expensive.In this paper,EGR control strategy was studied and a double closed loop EGR control model was built based on a 2.0L vehicle diesel engine.Firstly,the bench test of the diesel engine was carried out,and the influence of the EGR rate on the economy,power and emission performance of the diesel engine was analyzed.The optimal EGR rate was obtained considering the performance of the engine.EGR control strategy was implemented as follows according to the results of bench tests:Under start and idle condition,EGR valve needs to be closed in order to make the engine start smoothly and have a stable operation because combustion temperature is low and NOx emissions was less at this condition.At low speed and low load condition,EGR should not be used or EGR rate should be controlled within 10% otherwise the increasing EGR rate will lead to an increase in the fuel consumption and a loss in output torque.At low speed and high load condition with high NOx emission,EGR rate is supposed to put into a high value because high EGR rate has little impact on fuel consumption,output torque at this condition.At medium or high speed and low load condition,NOx emissions is large,high EGR rate should be used.At medium or high speed and high load condition,a small EGR rate or a closed EGR valve is needed.Under overrun,long idle,shift operation,too high or too low cooling water temperature and etc.,EGR valve should be closed to ensure the stable speed and normal operation of the engine.Secondly,based on the above EGR control strategy and compared with the traditional single position closed-loop control,a double closed loop EGR control model was built by using MATLAB/Simulink,including a closed-loop control module based on the intake air flow and a closed-loop control module based on the EGR valve position.The former one is composed of a target air intake module,an actual intake air module and an EGR closed loop control module based on the intake air quantity which compares the target intake air flow with the actual intake air flow through the PID.The latter one includes an EGR opening control module,an electronic EGR valve module and a closed loop control module based on the EGR position opening.Thirdly,the double closed-loop model and single position closed-loop model simulated,and the comparison between simulation results and experiments under transcient conditions.The results shows that simulation results can match well with the experimental data and the error at low load condition is relatively large but within 5%,which can meet the needs of practical applications.Finally,taking EGR valve as an example,the object code was generated through the Real-Time Workshop and the accuracy and feasibility of the code generation are verified.This lays the theoretical foundation for applying this EGR control model to real vehicles.