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As alternative fuel, natural gas holds a dominant position with widely distributed resources in the world and a low CO2 emission rate compared with the other fuels for automobile. Natural gas should be used in liquid phase (LNG) especially in automobile use due to considerations of energy density. Research and development was conducted for a practical LNG vehicle equipped with a LNG engine and LNG supply and engine control systems. The LNG engine system was given a high compression ratio, manifold gas injection, and spark ignition for the effective use of natural gas. For the components of LNG supply system, LNG tank, vaporizer, LNG control valve and gas injector were developed. The LNG tank and the LNG control valves were given super insulation constructions to prevent gas boil off. LNG was vaporized using heat from the radiator. The gas injector is controlled by a solenoid coil for optimum fuel supply. A computer using fuzzy logic was developed in part for the engine control system.

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.

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.