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The emission capability of an exhaust system tuned for improved engine performance from an in-line four-cylinder engine has been investigated. The exhaust system comprises two close-coupled catalysts; each located in separate exhaust streams and has been termed the 4-2 close-coupled catalysts (CCC) -1 system. It has been shown that, given equivalent total catalyst volume, this system configuration results in compromised high exhaust flow rate emissions performance compared with a single catalyst (4-1semi-CCC) system. This emissions performance deficit has been attributed to the effect of engine frequency flow pulsations, which result in relatively high peak space velocities in the 4-2CCC-1 system despite the mean space velocity being consistent. Engine-based AFR Bias Sweep tests suggest that hydrocarbon emissions are most strongly affected by this phenomenon. At lower exhaust flow rates, the difference in performance between the two systems is negligible.

A method for improving three-way catalyst (TWC) performance by superimposing a low frequency lean air-to-fuel ratio (AFR)bias perturbation onto the standard AFR oscillations is described. This observation of Catalyst Regeneration (CatRegen) has been attributed to a reactivation of poisoned precious metal sites on the catalyst surface. Preliminary tests under steady-state conditions show that there is a gradual reduction in TWC activity for NOx after a lean-rich transition, suggesting a temporary poisoning of the active precious metal sites on the TWC under rich conditions. This deactivation can be prevented by periodically exposing the catalyst to lean exhaust gas; which has led to the development of the CatRegen system.