Disturbance attenuation in time-delay systems — A case study on engine air-fuel ratio control

In this paper we analyze relative performance of several approaches to disturbance attenuation for systems with time delay including the conventional Proportional-Integral controller, the Smith Predictor, and the Model Reduction controller. The paper proposes a measure of disturbance attenuation capability and computes it analytically for each of the controllers considered. The results are applied to the air-fuel ratio regulation in automotive engines. To meet strict emission regulations, gasoline engines must operate at stoichiometric air fuel ratio over most of its operating range. A major component towards accomplishing this goal is the closed loop fuel controller. The feedback uses an exhaust-gas oxygen sensor which introduces a long transport delay. This paper discusses the air-fuel ratio regulation problem, explores options in control design for disturbance attenuation in system with time-delay, and shows simulation and experimental, in-vehicle validations.