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The authors have been developing a ceramic heat insulated natural gas engine, in which cooling losses are greatly reduced by using heat insulation structure on the combustion chamber walls. High thermal efficiency is achieved by being able to operate this engine at high compression ratios. This engine is intended to use in co-generation systems and hybrid vehicles. In the engine, a new combustion system has been developed to enable the use of compressed natural gas (CNG) as fuel supplied at low pressure. The system consists of a pre-chamber and a throat valve on the passage between the pre- and main- chambers in order to make the engine ignite smoothly. This system uses low-pressure fuel, injected into the pre-chamber when the throat valve is closed. A homogeneous charge and large amounts of EGR are used in the main chamber.

The Japan Gas Association has been engaged in the development of a 200-kW-class ceramic natural gas engine system to be used as a co-generation power source, subsidized by Ministry of International Trade and Industry. The engine has several new concepts to achieve high efficiency and low emissions simultaneously and to enable to use natural gas as fuel supplied with low pressure in spite of diesel engine. The co-generation system needs de-NOx system. So, we developed a urea de-NOx catalyst system for high-temperature exhaust gas. This paper will describe the result in the fiscal year 1998.

For the purpose of eliminating a cooling device from conventional diesel engines, a heat insulation structure referred to as thermos structure was adapted in a low heat rejection (LHR) diesel engine. The thermosstructure is constructed by a combustion chamber wall made of Si3N4 monolithic ceramics having higher strength and fracture toughness at much higher temperature and the heat insulation layers combined with air gap and gaskets with low thermal conductivity that are located behind the combustion chamber wall. Although the insulated engine achieved reduced heat rejection from the combustion chamber with the thermos structure, improvement in fuel economy and exhaust emissions could not be realized in the case of a diesel engine with Direct Injection (DI) system.