Persistent Memory Effects and the Mid- and Post-Brick Dynamic Behaviour of Three-Way Automotive Catalysts

This paper presents the results of an experimental study into the dynamic behaviour of a three-way automotive catalyst and its associated exhaust gas oxygen sensors. Motivated by issues of feedback sensor location, the study seeks to overlay the results of repeat experiments, with sensors and fast-response gas analyzers positioned at different locations, in order to obtain a detailed picture of system dynamics at different points within the catalyst. Initial results demonstrated that the dynamic response of the catalyst can be significantly affected by a persistent memory effect in addition to reversible deactivation dynamics and the familiar oxygen storage/release dynamics of the system. In particular, the effects of prior rich or stoichiometric operation are shown to persist even after extended periods of lean operation. This memory effect is important, not only because of its potential impact on conversion efficiency, but also because of its impact on the repeatability of experiments carried out under what would appear to be near-identical operating conditions. By pre-conditioning under rich conditions highly repeatable experiments were achieved. The results were combined to give a detailed picture of catalyst dynamics at pre-, mid- and post-catalyst locations, and provide insight into catalyst and (non-ideal) exhaust gas oxygen sensor behavior.