Three-Way Conversion Catalysts: Effect of Closed-Loop Feed-Back Control and Other Parameters on Catalyst Efficiency

Monolithic three-way conversion (TWC) catalysts, at different precious metal concentrations, were aged on an engine dynamometer with fuel doped with lead (0.012 g Pb/gal). These catalysts were subsequently evaluated on an engine dynamometer to examine the effects of air/fuel ratio set point, temperature, and air/fuel ratio amplitude and frequency on the conversion efficiencies for NO_x CO and HC. In all evaluations, as the precious metal concentration increased from 5 g/ft³ to 40 g/ft³, the NO_x CO and HC conversions increased. Also, the smallest effect of precious metal loading on catalyst efficiency was found at the smallest air/fuel amplitude (±0.3 A/F). The highest overall conversions of NO_x CO and HC were obtained at the stoichiometric control point for perturbations of ±0.3 A/F amplitude. Therefore, it appears that a considerable savings in precious metals can be realized if the A/F amplitude of a closed-loop feed-back control system is small (±0.3 A/F). On the other hand, the higher the oscillation frequency of the feed-back control system, the higher the NO_x CO and HC conversions. Thus, an additional savings in precious metals may be achieved if the control frequency is increased. Since the A/F perturbation amplitude and frequency, catalyst temperature range, A/F control point, and catalyst precious metal concentration all combine to affect the NO_x CO and HC conversion efficiency, the selection of the precious metal loading of the TWC catalyst, the catalyst strategy (single or dual-bed system), and the A/F set point largely depend on the characteristics of the feed-back control system and the concentration of NO_x CO and HC emissions from the engine.