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Abstract



Modeling and control of actuators and co-surge in turbocharged engines


The torque response of the engine is important for the driving experience of a vehicle. In spark ignited engines, torque is proportional to the air flow into the cylinders. Controlling torque therefore implies controlling air flow. In modern turbocharged engines, the driver commands are interpreted by an electronic control unit that controls the engine through electromechanical and pneumatic actuators. Air flow to the intake manifold is controlled by an electronic throttle, and a wastegate controls the energy to the turbine, affecting boost pressure and air flow. These actuators and their dynamics affect the torque response and a lot of time is put into calibration of controllers for these actuators. By modeling and understanding the actuator behavior this dynamics can be compensated for, leaving a reduced control problem, which can shorten the calibration time. Electronic throttle servo control is the first problem studied. By constructing a control oriented model for the throttle servo and inverting that model, the resulting controller becomes two static compensators for friction and limp-home nonlinearities, together with a PD-controller. A gain-scheduled I-part is added for robustness to handle model errors. The sensitivity to model errors is studied and a method for tuning the controller is presented. The performance has been evaluated in simulation, in test vehicle, and in a throttle control benchmark. A model for a pneumatic wastegate actuator and solenoid control valve, used for boost pressure control, is presented. The actuator dynamics is shown to be important for the transient boost pressure response. The model is incorporated in a mean value engine model and shown to give accurate description of the transient response. A tuning method for the feedback (PID) part of a boost controller is proposed, based on step responses in wastegate control signal. Together with static feedforward the controller is shown to achieve the desired boost pressure response. Submodels for an advanced boost control system consisting of several vacuum actuators, solenoid valves, a vacuum tank and a vacuum pump are developed. The submodels and integrated system are evaluated on a two stage series sequential turbo system, and control with system voltage disturbance rejection is demonstrated on an engine in a test cell. Turbocharged V-type engines often have two parallel turbochargers, each powered by one bank of cylinders. When the two air paths are connected before the throttle an unwanted oscillation can occur. When the compressors operate close to the surge line and a disturbance alters the mass flow balance, the compressors can begin to alternately go into surge, this is called co-surge. Measurements on co-surge in parallel turbocharged engines are presented and analyzed. A mean value engine model, augmented with a Moore-Greitzer compressor model to handle surge, is shown to capture the co-surge behavior. A sensitivity analysis shows which model parameters have the largest influence of the phenomena. The compressor operation in the map during co-surge is studied, and the alternating compressor speeds are shown to have a major impact on the continuing oscillation. Based on the analysis, detection methods and a controller are proposed, these detect co-surge and control the turbo speeds to match during co-surge. The controller is evaluated both in simulation and on a test vehicle in a vehicle dynamometer, showing that co-surge can be detected and the oscillations quelled.

Andreas Thomasson

2014

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