Diesel Boost Pressure Control using Flatness-Based Internal Model Control 2006-01-0855

As a result of the increased complexity of today's power trains, the traditional ways of designing engine control systems essentially through ad hoc methods and experimental tuning will no longer provide the desired level of performance. Further, it is too time-consuming due to the calibration process. In this paper, a novel model-based controller is described which accommodates many of today's demands on controller development of the automotive industry.

The control problem treated here is a boost pressure control of a turbocharged diesel engine with a variable nozzle turbine (VNT). Depending on the injected fuel and the current speed of the diesel engine, the boost pressure has to follow a desired trajectory. Since the system is essentially nonlinear, a robust nonlinear controller is used.

The tracking problem is treated by a control method which combines the Internal Model Control (IMC) structure with the flatness-based approach to design feedforward controllers. The main idea of IMC is to include the model of the plant into the feedback controller. If the model perfectly represents the plant and no disturbances occur, the IMC structure degenerates to a pure feedforward control. Flat systems are characterized by the fact that their input can be expressed explicitly in terms of internal system dynamics, which results in a simple method for designing a feedforward controller. In order to extend this concept to nonlinear systems, a flatness-based feedforward controller is proposed as IMC controller. It is shown that this new concept provides an efficient controller design for certain nonlinear systems and ensures robustness and offset-free tracking. Simulations and testbed results on a turbocharged diesel engine demonstrate the feasibility of this control scheme which results in impressive control performance.