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The horse power and emissions of a heat insulated engine with high heat insulation rate* over 70% was investigated, as a result, a deterioration of power and emissions has become clear to be caused by change of combustion in the tests. Therefore, the observation of the combustion process in high temperature air has been carried out in the engine by using a high speed camera. We have investigated the combustion process included progress of injected fuel spray, ignition, extension of flame, and formation of soot in a vessel charged with air of high temperature and pressure with the gas condition in an engine cylinder. The test result showed that diesel fuel did not penetrate straight forward in the high temperature air, and the tip of the jet stream spread in such a shape like a lump of cloud and the spray is ignited in a short ignition delay. The flame extended widely in the free jet stream, however we observed soot formations here and there in the flame.

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.