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Two effects of rich-spike duration on NOx-storing have been analyzed. The first one, that NOx-storing speed decreases as rich-spike duration increases, is explained as the influence of NOx diffusion in wash-coat layer, which is quantified by a simple mathematical expression for NOx-storing rate. The second one, a peculiar behavior of NOx-storing in appearance of the outlet NOx concentration, is clarified: Heat produced directly or indirectly (via oxygen storage in ceria) by rich-spike warms up the downstream part, which releases excess NOx at the raised temperature. Contributions of the oxygen storage and the carbonate of NOx-storage material are also discussed.

Catalyst specifications and converter layouts were studied to identify the high conversion performance under various in-use driving conditions, high mileage intervals and extended life cycle. The effects of volumes, configuration, selection and loading distribution of precious metals, additive components and substrate type for catalyst were studied on engine dynamometers and vehicle tests to optimize a catalyst converter system. Moreover, model gas experiments were conducted to analyze deterioration mechanisms and conversion characteristics of catalysts. As a result, the concept of a divided catalyst converter system, which provides separate functions for a close-coupled and an under-floor catalyst, was found to be effective for the future exhaust system. For reducing HC emissions, the close-coupled catalyst should warm up quickly and resist a high temperature. The under-floor catalyst, located at a rather low temperature position, is durable and maintains high NOx conversion.

A new variable venturi carburetor has been developed, in order to achieve high metering accuracy, fuel economy, higher power and good driveability of the vehicle. This carburetor is a down-draft-type, and has better characteristics compared with the fixed venturi carburetor, such as better fuel atomization, lower cycle-to-cycle variation of combustion in lean mixture, faster response, and higher power. For this carburetor, a new type venturi and a new cold enrichment system has been developed. This venturi type has an exponential-profile with a nearly constant increasing rate of venturi opening area to the suction piston stroke. The new cold enrichment system controls the airfuel ratio in all conditions by changing air bleed quantities using the thermo-wax. With this system, cold drive-ability is improved greatly.