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 phenomenon has been considered that the spray of fuel is obstructed to penetrate into the air with high viscosity increased by higher temperature about 250°C than those of cooled engine. In order to improve the combustion at higher temperature, we tested pre-combustion chamber engine, heat insulated combustion chamber walls, which in turn, may allow us to improve the combustion in the engine.On the other hand, we have investigated combustion process in other fuels such as methanol which has lower cetane number than diesel fuel in the air vessel. The combustion process of methanol fuel injected into the heat insulated vessel had become clear to be improved to the combustion level of diesel fuel injected to the ambience air of cooled engine.