High efficiency of three-way automotive catalysts is achieved by regulating cylinder air–fuel ratio in a narrow band around the stoichiometry. Due to a delay present in the exhaust gas oxygen sensor signal, performance of the air–fuel ratio regulation depends on accuracy of the cylinder air-charge estimate. Variable cam timing (VCT) introduces a challenge to the air-charge estimator due to its effect on engine pumping and, in some cases, due to unmeasured back-flow of the exhaust gas into the intake manifold. The objective of this paper is to illuminate some of these issues and suggest methods to improve accuracy of air-charge estimation in VCT engines.
