Combustion and Combustion Chamber For a Low Heat Rejection Engine 960506
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.
Observation of combustion process in the LHR engine suggested that insufficient fuel-air mixing may be attributed to an increase in gas viscosity and deterioration of air entrainment of fuel spray as a result of high combustion temperature.
In order to improve combustion in a LHR diesel engine, a centrally located pre-combustion chamber in the cylinder head and throat holes drilled in it as radiating to cylinder inner surface has been developed.
The LHR engine with the new pre-combustion chamber realized about 10% improvement in fuel economy compared to a conventional DI engine and could meet the Japan emission standard.