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A hybrid combustion engine in which a premixed gasoline homogeneous charge was ignited by a small amount of injected diesel fuel under a high compression ratio was made by modifying a 4-stroke cycle diesel engine and was tested on a bus. It was found that the premixed of gasoline was effective not only for decreasing the fuel consumption but also for reducing the smoke density during an acceleration road test. The effect of introducing a small amount of N2 gas for supressing the diesel knock under heavy load operation was examined. These days diesel engines are widely used because of their excellent fuel economy. Particulates emitted from a diesel engines, consist of dry soot observed as smoke and the Soluble Organic Fraction (SOF) containing Polynuclear Aromatic Compounds (PAC).

In order to reduce diesel idle knock, the effects of EGR on the idling characteristics were investigated on three 4-stroke cycle engines having three different combustion chambers (precombustion, swirl and direct injection). It was found that EGR was effective not only for reducing idle knock, but also for decreasing fuel consumption, smoke density and exhaust emissions at idling operation regardless of the type of combustion camber. The same test results were obtained on a crankcase-scavenged 2-stroke cycle diesel engine (direct injection type) by means of inlet throttling.

In order to improve the part-load and idling characteristics of 2-stroke cycle diesel engines, the effect of throttling on engine performance was investigated on a separately scavenged engine with a Roots blower and on a crankcase-scavenged engine. For the first engine, it was found that decreasing the delivery ratio through bypassing a part of scavenging air into the inlet side, that is, by recirculation of scavenging air, made it possible not only to decrease the fuel consumption but also to raise the scavenging temperature which improved combustion. Also, a vehicle test using a bus powered by the same type of engine showed improved mileage through throttling. For the crankcase-scavenged engine, it was found that a slight increase in delivery ratio was very effective for improving the performance at low speed with heavy load, and throttling was also effective in improving not only the part-load but also the idling operation.

In the research of DME compression ignition engine, there are a lot of reports on the high fuel pressure systems which are used in the common-rail fuel injector and others for the DME mixture formation promotion. However, the initial development-cost of these fuel supply systems will be increased for small compression ignition engines. On the other hand, it has been understood that excellent thermal efficiency of DME compression ignition engine was obtained at the appropriate fuel injection timing by using the electronic controlled injector with low pressure injection. In this paper, the stabilization of combustion on DME compression ignition engine with low pressure injection was investigated for the influence of the fuel pressure and the combustion assistance with homogeneous charge.

Experiments of combustion observations and radical measurements in DME compression ignition engine were significant for the elucidation of DME combustion. There have been several reports on combustion observation studies of DME compression ignition engines. All of these reports were experimented with the same method observed due to the bottom view, and none of these studies was targeted on low pressure injection and/or small compression ignition engines. In this research, the combustion analysis of the small compression ignition engine with DME low pressure injection was attempted by the top view observation using the wide angle view engine scope. As a result, DME combustion using the ordinary diesel engine's piston chamber shape and material was observed. The production of the exhaust elements (OH, CHO, NO) was obtained to the time series on combustion process of the small compression ignition engine with DME low pressure injection.