Presented is the model based diagnostics of a three-way catalyst (TWC). The proposed TWC model relates measurable engine inputs (engine air mass (AM) and catalyst temperature) to a metric that quantifies TWC oxygen storage capacity. The TWC model structure is based on the dynamics of the TWC and identified using orthogonal least squares (OLS). The model coefficients are estimated using an instrumental variables four step (IV4) approach. TWC diagnostics is realized by means of an information synthesis (IS) technique where changes in the adapted TWC model coefficients are utilized to estimate TWC health. The approach is experimentally validated on a federal test procedure (FTP) drive cycle for healthy (full useful life, FUL) and failed (threshold) TWCs. The results will show that a 100% accurate classification in TWC health estimation (FUL or threshold) is produced for the catalysts tested.

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