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The purification performances of some kinds of catalytic converters ( Pt, Rh, Pd, Pd/Rh, Pd/Pt, Pt/Rh/Pd, Pt_Pd and Pd_Cu) were investigated to select suitable catalytic converters for natural gas fueled automotive engines. Pd series catalysts showed better performance among the noble metal catalysts for oxidation of unburned methane in exhaust gas. The optimum loading of Pd catalyst is the range of 1.6 to 3.2 g/L. The dual-bed catalyst, Pt_Pd, consisting of a Pt catalyst in the front and a Pd catalyst in the rear, showed a performance better than Pd series catalysts. When aged to an accumulated running distance of 50000 miles, the catalytic activity of the Rh catalysts is much reduced, but those of Pd and Pt catalyst are affected little by aging. The aged Pd/Rh catalyst showed superior emissions' purification performance at the stoichiometric condition, but poor at lean mixture conditions.

The purification performances of catalytic converters (Pt, Rh, Pd, Pd/Rh, Pt/Pd, Pt/Rh/Pd, Pt̲Pd and Pd̲Cu) were investigated to select suitable one for newly developed liquified natural gas(LNG) fueled vehicle. Two types of the construction of the catalytic converter, single-bed type and dual-bed type using two different catalysts in series, were used. A natural gas engine that has been modified the 3 cylinder gasoline spark ignition engine of 0.55 liter displacement equipped with newly developed manifold injectors was used for the investigation. The conversion performance of the exhaust gas at different catalyst temperatures was discussed with main emphases on methane conversion. The durability of the catalyst and the optimum loading of Pd catalyst were also investigated. To understand the suitable operating condition for the developed LNG engine, the effect of air excess ratio and engine load were examined.