OPTEC - K.U.Leuven Center of Excellence: Optimization in Engineering

Diesel engines are nowadays equipped with an exhaust gas recirculation valve (EGR) and a variable geometry turbocharger (VGT) to influence the emissions and the torque output. The engine control problem can be stated as an optimization problem including fuel consumption, torque and emmissons. Due to complications in measuring the emissions in a production car an equivalent problem, known as the air path control problem, will be formulated. In the air path control subsitute quantities are used; the two most used quantities are the fresh air mass flow through the compressor (MAF) and the inlet manifold absolute pressure (MAP). These quantities are easily measured in a production car engine and directly affect the torque, fuel consumption and emissions.

A standard engine control unit (ECU) uses two single input single output (SISO) gain scheduled control loops to control MAP with VGT and MAF with EGR, which again requires a huge amount of tuning. This control strategy ignores that EGR also influences MAP and VGT also influences MAF via the so called cross coupling e.g. ignoring that the air path control system is a coupled MIMO system. Therefore it seems obvious to make use of a MIMO control strategy.

For air path modelling in the literature first principles are commonly used leading to mean value models (MVM) which are usually simplified for control design e.g. locally linearized except in the NMPC case. This motivates to directly use data based linear system identification techniques for the modelling. Due to the nonlinear behaviour of the air path no single linear model will give a satisfying quality which leads e.g multi linear identification techniques, LPV identification, a clustering technique for piecewise affine identification or recursive online estimation.

Experiment results, performed on a dynamical engine test bed, show an immense improvement in terms of MAF and MAP tracking of an explicit MPC controller compared to a standard ECU controller. As an alternative for explicit MPC, which is designed to be real time applicable, an online active set strategy is used for fast solving of the quadratic programs online. This leads to a reduction in computing power and therefore to higher possible complexity e.g. higher control horizons.

Results also claim that for affecting emissions (mainly nitrous oxides and particulate matter) in a positive way it is not enough to change only the control scheme also the setpoints for MAF and MAP need to be modified.

Slides