Research On Coordinated Control Strategy Of Diesel Engine Air System

With the exhaustion of conventional energy sources and increasingly strict emission regulations,the internal combustion engine is forced to develop in the direction of low pollution,low fuel consumption and high specific power.In order to achieve the goals of reducing pollutant emissions and improving fuel economy at the same time:on the one hand,it is necessary to increase the exhaust gas recirculation(Exhaust Gas Recirculation,EGR)rate to control nitrogen oxides(Nitrogen Oxides,NO_x)emissions and ensure air-fuel ratio control particulates(Particulate matter(PM)emissions;on the other hand,accurately matching the diesel engine air system,optimizing exhaust back pressure and reducing the pumping loss of the diesel engine is one of the important ways to improve economy.The traditional air system composed of a fixed-section supercharger and EGR has great difficulty in optimizing the exhaust back pressure of the diesel engine in the full operating range due to the fixed turbine section.The air system composed of Variable Nozzle Turbocharger(VNT)and EGR can not only improve the air-fuel ratio and EGR rate of the diesel engine by adjusting the VNT opening,but also optimize the exhaust back by adjusting the VNT opening Pressure to improve the economy of diesel engines.Both EGR and VNT are driven by exhaust gas.When the EGR rate increases,the amount of exhaust gas that drives the turbine decreases,the turbine power decreases,resulting in a decrease in boost pressure and a decrease in intake air.As the EGR rate increases,it is necessary to reduce the VNT opening degree and increase the boost pressure and intake air amount.On the contrary,VNT opening degree needs to be increased.Therefore,the EGR rate control needs to simultaneously control the VNT opening degree and the EGR valve opening degree,but there is a nonlinear relationship between the EGR valve opening degree and the VNT opening degree.Therefore,a coordinated control strategy of EGR and VNT based on intake manifold pressure and intake mass flow is proposed.This paper has conducted the following studies:1)Theoretical analysis of EGR control strategy and establishment of EGR control model based on MATLAB/Simulink.The EGR control strategy uses double closed-loop control based on air volume and EGR valve position to ensure the accuracy and effectiveness of the EGR control system.Closed-loop control based on air volume includes EGR open-loop calculation,target air volume calculation,PID adaptive control,monitoring and switching modules.Closed-loop control based on EGR valve position includes EGR actual opening calculation,PID adaptation,and EGR duty cycle calculation.2)Theoretical analysis of the calculation process of VNT opening degree and the establishment of VNT control model based on MATLAB/Simulink.Due to the nonlinearity and hysteresis of the VNT diesel engine air system,the control method is divided into open-loop control or open-loop and closed-loop control.When the diesel engine is in transient operating conditions,the VNT opening degree is calculated through open-loop control to solve the need for quick response.When the diesel engine is in steady-state operating conditions,PID closed-loop correction is performed according to the deviation of the target supercharging pressure and the actual supercharging pressure,and the opening degree of the VNT supercharger under the current operating conditions is calculated.The VNT supercharger is controlled by converting the VNT opening degree into a duty ratio through closed-loop control based on the opening degree.By integrating the designed EGR control strategy model and VNT control strategy model with the diesel engine control system independently developed by the laboratory,and carrying out bench tests to verify the effectiveness of the EGR and VNT control strategies.The test results show that EGR control strategy and supercharging control strategy can accurately control EGR rate and supercharging opening under different working conditions of diesel engine to meet the requirements of diesel engine emission control.Through the control strategy of EGR valve sticking protection,the stuck state of EGR valve can be effectively monitored and the power-off measures can be taken to protect the EGR valve.