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Three exhaust emissions, hydrocarbons, carbon monoxide and oxides of nitrogen, from the automotive spark-ignition engines are presently subject to regulatory control. Of these harmful pollutants, NOx emissions are the hardest to control under current status of emission control technology. Accordingly, exhaust gas recirculation (EGR) has been receiving continued efforts as one of promising NOx control. This paper reports the effects of EGR on the mechanism of NOx reduction and engine fuel economy, on the basis of research made in the following areas: (1). NOx formation in a combustion vessel. (2). Studies on EGR effects in a single-cylinder engine. (3). Effects of EGR on NOx and HC emissions and fuel economy.

Three Japanese automobile manufacturers-Mitsubishi Motors Corp., Nissan Motor Co., Ltd., and Toyo Kogyo Co., Ltd.-have been making efforts over the past three years to design and develop effective thermal reactor-exhaust gas recirculation and catalytic converter systems suitable for small engines. The work is being done by members participating in the IIEC (Inter-Industry Emission Control) Program, and the exhaust emission levels of the concept vehicles developed by these companies have met the goal established by the IIEC Program at low mileage. Each system, however, has a characteristic relationship between exhaust emission level and loss of fuel economy. Much investigation is required, particularly with respect to durability, before any system that will fully satisfy all service requirements can be completed. This paper reports the progress of research and development of the individual concept vehicles.

During the past four years, Mitsubishi Motors Corporation has been active in the design and development of catalytic converter systems in the IIEC Program. Experience indicates that an effective dual catalytic converter, consisting of a NOx catalyst and a HC/CO catalyst, is indispensable to achieve the stringent IIEC targets, established on the basis of 1976 Federal Standards. However, there still remains a major difficulty of meeting the NOx target since NO is regenerated in the HC/CO catalyst in the dual converter system. This paper describes our major efforts in seeking a breakthrough to control the NOx emission, and covers some noteworthy findings on transient NO concentrations at the inlet and outlet of HC/CO converter in an experimental dual converter.