Emissions Implications of a Twin Close Coupled Catalyst System Designed for Improved Engine Performance on an In-line 4 Cylinder Engine

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.

Whilst steady state emissions seem to be strongly affected by the system configuration, results from engine-based Oxygen Storage Tests indicate that system robustness to AFR transients is less strongly affected, with the 4-2CCC-1 system and the 4-1semi-CCC system indistinguishable from one another. ECE Stage III drive-cycle emission data correlate well with the results of the AFR Bias Sweep and Oxygen Storage Tests.

There is minimal effect of catalyst precious metal loading on the AFR bias sweep performance of the 4-2CCC-1 system. It is suggested that increased catalyst volume would be required to offset the effect of exhaust flow pulsations. The results suggest that catalyst volume should be a function of cylinder capacity rather than engine capacity.