Analysis and Design Requirements for Tandem TWC Systems

Two-brick (tandem) three-way underfloor catalyst systems provide greater emission reduction performance compared to comparable single brick TWC systems, which contain the same amount of platinum group metal (PGM) for the same catalyst volume. This superior emission reduction performance is speculated to be due to front catalyst activity promoted by heat transfer from reverse exhaust gas flow in the gap between the front and the rear catalyst of tandem TWC system (hereinafter, tandem gap). Furthermore, the following findings were obtained by conducting experiments with model catalysts. 1) During catalyst light-off phase, conversion efficiency strongly depends on activity of the front portion of catalyst where temperature rises rapidly. On the contrary, temperature rise for the rear catalyst is slow, thus PGM loaded therein has practically no effect to conversion efficiency at cold starts. 2) Gas diffusion properties inside washcoat greatly affect conversion efficiency, especially for HC, such that collision frequency between exhaust gas and active PGM site need to be increased at high space velocity (SV).