Abstract
The increasing demand for more fuel-efficient and low-emission combustion engines in vehicles makes turbocharging a key technology. Low CO2 emission vehicles require downsized and down-speeded engines with higher boost demand which is achieved by using two turbochargers in series (R2S™). The demands on the control system also increase with the increasing complexity of the turbocharging system. The number of actuators rises and the manner how to control the boost pressure changes significantly.
The concept of flatness based control requires a self-contained analytic description of the system. The proposed analytic model already takes into account the special requirements of a flatness-based controller design.
Therefore, in the first step, physical modelling of the non-linear control system, consisting of a R2S™ turbocharger and engine, occurs. A theoretical physical approach is selected here; however, the response of various sub-models, i.e. turbine and compressor is emulated by static maps. Based on the complexity of the overall system and the fact that individual components occur more than once and only differ in their parameters, in the first step the behaviour of the individual components is described, which are subsequently combined to an overall system. Moreover, this results in a modular design, which easily facilitates subsequent extensions and enhancements.
The derived models for the turbine and compressor of the turbocharger itself are compared to the measured characteristic maps. The filling and emptying of the volumes in the intake and exhaust side are described by balance equations for the mass and enthalpy. The findings made for the tenth order space state model of the overall system are simplified to a fifth order model. The reduced and the complete model are compared to real engine measurements. The aim of the modelling is to show that the R2S™ system is flat. Flat systems are especially suited for tracking control.
Keywords: Modelling, Turbocharger, Flatness, Boost Pressure Control, Tracking Control