The Control Strategy Of VNT Based On Energy Balance Model

Nowadays,the stringent emission regulations make major OEM unable to help themselves when developing new projects.How to ensure that the engine can ensure its power and drivability without violating emission regulations has developed into an urgent problem that needs to be clarified at the beginning of the project.Among them,small-displacement high-performance turbocharged engines are gradually favored by OEMsdue to their natural low-emission advantages.However,due to the characteristics of the turbine itself and the limitation of the small-displacement engine and its functional ability,problems such aspoor response of the turbine at low speed and difficult to control at high speed,resulting in unstable boost pressure,will be brought about in the actual driving process.As a result,how to preciselycontrol the turbocharger’s turbocharging conditionshas become an important investment object in the process of engine research and development.The previous turbocharger control strategy is more like a black box in form,and the focus is only on its input and output.From the point of view of the ECU（Electronic Control Unit）,the input is the real-time boost pressure,demand value,the output is the duty cycle of the actuator,and does not care about the physical quantities in between.This kind of fuzzy control method is difficult to achieve high control precision.This topic is based on a new generation engine platform project of a large state-owned enterprise,with the help of its engine hardware conditions equipped with VNT（Variable Nozzle Turbocharger）in order to improve the control accuracy of turbocharger andrealize efficient turbocharging.In this thesis,the model-based control method is applied,and by analyzing the principle of internal combustion engine,a turbocharger mathematical model based on the hydrodynamics andthermodynamic changes of different working fluids in the turbocharging process is proposed.Taking thismodel as the core,a set of control strategy based on VNT energy balance model is developed.The above-mentioned mathematical model of the turbocharger includes a turbine module,a compressor module and an intermediate shaft module connecting the two of the turbochargers.At the same time,each module is divided into a physical modeling subsystem that derives the energy conversion and heat transfer of the working medium,and a demand modeling subsystem that calculates the target value according to the demand.In the working process,the physical modeling subsystem took the lead based on the principles of thermodynamics,fluid mechanics and so on,taking the initial state of the working medium inside the entrance of each component of the turbocharger as the input,deduced the state parameters of the working medium at the outlet corresponding to each component,and the energy loss of the working medium during the flow process.The system of equations describing the change can accurately capture the change of working medium after the turbocharging request is sent out,which is helpful for the demand modeling subsystem to quickly calculate the target value of turbocharger nozzle ring actuator opening which can meet the demand of turbocharging.This model also includes the detailed process of turbocharger transmission and heat transfer during the turbocharging process.Finally,to avoid the influence of uncertain factors on the control strategy,the adaptive process of the turbocharger is used to limit its output parameters beyond the actual controllable range.The method of closed-loop control of turbocharger is studied,and PID（Proportion Integral Differential,proportional-integral-differential）controller is selected for its feedback control,to improve the control accuracy of turbocharger boost pressure.Through the hardware-in-the-loop test and the real vehicle NEDC（New European Driving Cycle）cycle condition test,the results show that the control strategy studied in this topic can not only meet the turbocharging demand during vehicle driving,but also improve the turbocharging performance.It has a certain effect in terms of response speed and adjustment of turbocharging error.