Research On VNT Match And Control Strategy Of High Pressure Common Rail Diesel Engine

In the high pressure common rail diesel engine, the traditional turbochargers have someproblems that it is impossible to achieve enough torque at high and low speed synchronously,and turbine lag, matching problems of turbocharger and EGR, turbocharger and injectionparameters. VNT is a kind of mechanism whose diameter ratio（A/R）of turbine is variable byadjusting the blade opening, and it could effectively improve the torque contradiction at highand low speed, at the same time also can improve transient response and turbine lag. Inaddition, through the control strategy design of VNT and comprehensive consideration ofenvironmental or cooling water temperature, ambient pressure, inlet pressure, EGR ratio andinjection parameters corrections, it can achieve accurate VNT opening so that can accuratelymeet the combustion requirement of air quantity.VNT electronic control requires not only ECU hardware interface, perfect driving circuitand an executing mechanism, but also need perfect control strategy to ensure the stability andaccuracy of VNT working in normal operating mode or under special conditions. It also canguarantee the working reliability, such as in the conditions of cold start, transient condition,system errors, big system deviation etc.This paper focuses on a research on turbocharger matching and control from two aspects.First of all, when matching different A/R turbochargers, there is a torque contradiction athigh and low speed. At low speed, performance of the small A/R is better, but at high speedthe big is better. Therefore, VNT is better than traditional turbocharger by adjusting the VNTopening to realize the switch between small and big A/R turbine. The output torque of thespeed adaptability is improved when the match is VNT, the fuel consumption was decreased.,and the interval of economy was wider.Secondly, before the research of VNT control strategy, there are a series of bench testsabout the coupling relationship between turbocharger and EGR, turbocharger and injectionparameters to select the optimal EGR ratio and injection parameters.1）The bench test is to analyze the effect of EGR rate on exhaust gas parameters. Exhaustparameters affect the entrance state of the turbine, thus affecting the inlet flow rate and inletpressure, and the inlet flow rate changes will directly affect the implementation of EGR rate.2）In addition, through the experimental study, the main injection timing and pressure have important effects on combustion process, and combustion will directly affect the exhaustparameters. Then the entrance state of the turbine affects the characteristics of turbocharger.The research shows that small load should be by the larger EGR rate, high load should beused for low EGR ratio by the analysis of influence on the intake and exhaust parameters. Theuse of a smaller main injection timing and the rail pressure is essential for improving theintake and exhaust parameters and the ability of the turbocharger.Thirdly, the study of the inlet and exhaust parameters, torque and economic performancevarying with the VNT opening is based on the experimental research of turbochargercoordination. Therefore, through the above work, it could determine the open-loop VNTopening and the target intake pressure, providing experimental basis for the design of VNTcontrol strategy.Finally, the strategy that an open loop and a closed loop is combined respectively settingthe blade opening and inlet pressure as the control target value at the steady-state and transientconditions is designed based on the above three tests. In the working process, the controlsystem is switching between open loop and closed loop. Then, the next work is the modulepartition of control model, delimiting the function and inputs and outputs of each sub-module.In the end, with the help of Simulink software, the building and validation of simulationmodel of control strategy is accomplished. The error is within5%so that the model could beused as the basis of the later research.