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The gas components CO, CO2, HC, NOx and the AFR in the exhaust from a SI engine, both upstream and down-stream of a Pd/Rh catalytic converter, have been monitored using fast response analyzers. Regular sequential step changes in the upstream air/fuel ratio (AFR), between two pre-set levels, have been implemented with both long and short periods between the steps. For transitions from rich to lean conditions, and vice-versa, several distinct zones for the output emissions characteristics, corresponding to different states of the catalyst surface, have been identified. These results suggest that, under reducing conditions, hydrogen is stored on the catalyst surface whereas under oxidizing conditions oxygen is stored by two different processes. These chemical insights facilitate the development of realistic models for tailpipe emissions from engines which are perturbed from steady state running.

The transient response of a catalytic converter to fluctuations in exhaust gas composition has a significant impact on tailpipe emissions. Advanced emission control strategies therefore need to incorporate a model for such behavior, which must also be sufficiently simple for practical implementation in-vehicle. To this end, a variety of semi-empirical models have been developed, including most recently a number of oxygen “storage-dominated” models. In this paper a new storage-dominated model is developed, which includes for the first time the effects of space velocity. The parameters of model may be estimated using the invariant embedding method.