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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.