All vehicle manufacturers implement an air-to-fuel ratio (AFR) control system for emissions reduction in gasoline engines. When using a model based control structure, it is vital to capture the underlying dynamics of the plant as accurately as possible, thus facilitating a robust control design that meets the emissions regulation requirements. One of the leading sources of uncertainty in the engine model is the variable plant delay. Although the delay could be modeled using a look-up table of steady-state delay values, during transients when AFR control is most important the steady-state delay poorly approximates the true delay. An exhaust geometry based delay model was developed previously within the framework of a model based control design for AFR control of stoichiometric engines. In this paper, it is shown that using this model the delay can be predicted with a significantly higher accuracy especially during transients, thus improving emissions performance. Because the plant delay plays a destabilizing role in feedback control, the utility of such a model is also to minimize phase errors between the predicted and measured equivalence ratio (EQR) in a reference tracking control setting.

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