1980-02-01

Three-Way Conversion Catalysts: Effect of Closed-Loop Feed-Back Control and Other Parameters on Catalyst Efficiency 800462

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 NOx CO and HC. In all evaluations, as the precious metal concentration increased from 5 g/ft3 to 40 g/ft3, the NOx 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 NOx 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 NOx 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 NOx 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 NOx CO and HC emissions from the engine.

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