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Labyrinth seals as a type of non-contact seal have been used very often in gas turbine and in many kind of fluid machines. They have various type of shapes. Most of them had been designed from the view point of very practical use and it is difficult to explain their characteristics. However, one of the fundamental design policies of labyrinth seal is to design the configuration which can produce larger differential leakage in the direction of fluid flow. Especially, if there is a periodical variation of pressure difference between both sides of labyrinth seal or there is reciprocal pressure difference, it is very significant in the seal performance to have large difference of leakage in the direction of flow. In this research work, variation of leakage and fluid resistance depending on the flow direction were investigated for the narrow flow path.

In order to clarify theoretically the advantages of crankcase-supercharging, the proportion of the drive load of crankcase-supercharger to the output of engine and the increase ratio of mean effective pressure were studied through thermodynamic theory, and then suitable inlet systems were also examined. The validity of estimated values were checked with the experimental data. As a result, it became theoretically clear that a mean effective pressure rise of about 9% to 17% could be obtained by crankcase-supercharging. And it was estimated theoretically that comparing the crankcase pumping load with the output of engine, the former became only about 2.5% of the latter and the suitable inlet plenum volume is about 150 times the displacement volume. Additionally, estimated values are proved to be adequate for practical use by comparing with experimental results.

In order to boost the torque of a 4-stroke cycle diesel engine, a crankcase-supercharged 4-stroke cycle diesel engine was made and tested in the firing mode. As a result, a remarkable reduction of smoke density and brake specific fuel consumption compared with the naturally aspirated baseline diesel engine was obtained. Additionally, a Jet Air Port (JAP), which was opened and closed by piston movement near bottom dead center, was installed. The JAP utilized air furnished by the crankcase-supercharging system. A reduction of smoke density and brake specific fuel consumption compared with the naturally-aspirated baseline diesel engine was found.

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 investigate the transient characteristics of crankcase-supercharged 4-stroke cycle engines, such engines were installed in both a motorcycle and a passenger car for on-road test. It was found that, in the starting and the passing acceleration tests, the engine speeds of the crankcase-supercharged engines increased more rapidly than that of the naturally-aspirated engine. Additionally,in 0-200m acceleration tests, the crankcase-supercharged motorcycle's time was 4.7 sec (approximately 16%) faster than that of the naturally-aspirated motorcycle.

In order to improve the volumetric efficiency of a crankcase-supercharged 4-stroke cycle engine in the high speed range, rotary disc valves were used at both the inlet and the outlet sides of the crankcase. The effects of the size of the opening and the phasing of the disc valves on volumetric efficiency were investigated and compared with a system using reed valves. It was found that a volumetric efficiency higher than 120 % could be obtained over a speed of 3000-6000 rpm, which was not possible by using the reed valve system. SUPERCHARGING SYSTEMS utilizing Roots type blowers and/or exhaust turbochargers have been used effectively for increasing the brake mean effective pressure of 4-stroke cycle engines. The authors built a prototype cranckcase-supercharged 4-stroke cycle engine, which used the underside of the piston as a supercharging pump, and confirmed the effectiveness of the concept by motoring and firing tests.

4 crankcase-supercharged 4-stroke spark-ignition engine was made and tested in both the motoring and firing modes. It was found that a 20 to 30 % torque rise compared with the naturally-aspirated baseline engine was obtained in the low and middle engine speed ranges without any sacrifice in brake specific fuel consumption. Additionally, the throttle response of the modified engine was better than that of the naturally-aspirated baseline engine, and there was no deterioration in part-load brake specific fuel consumption.

Two hybrid Combustion engines, wherein a premised gasoline homogeneous charge was ignited by a small amount of injected diesel fuel under high compression ratio, were made and tested by modifying open chamber and prechamber 4-stroke cycle diesel engines. It was found that the premixed gasoline was effective not only for decreasing the fuel consumption but also for reducing the smoke density both in the heavy and over-load regions. The mixing ratio of the gasoline and diesel fuels, the injection timing of the diesel fuel and the compression ratio were found to be the main parameters affecting the performance of these hybrid combustion engines. The effect of introducing a small amount N2 gas for suppressing the diesel knock in the heavy load region also was examined.

As a mechanical supercharging system for 4-stroke cycle engines, crankcase-supercharging seems attractive because of its low production cost and simple construction. Therefore, a crankcase-supercharged 4-stroke cycle spark-ignition engine was developed and tested in both motoring and firing modes. It was found that the motoring volumetric efficiency could be increased over the entire engine speed range, but the increase was more noticeable at lower engine speeds. In the firing mode it was proved that a favorable throttle response and a 20 to 30% torque rise could be obtained by crankcase-supercharging without sacrificing the brake specific fuel consumption of the naturally aspirated baseline condition.

On 4-stroke cycle engines, the authors reported that under certain conditions the cylinder pressure, measured during the suction stroke with a low-pressure pickup, dropped below the critical pressure. Noting this considerable negative cylinder pressure during the suction stroke, the authors devised a Jet Air Suction Port (JASP), which was opened and closed by piston movement near bottom dead center. As a result, it was possible not only to burn lean mixtures steadily, but also to decrease the fuel consumption 10 to 30 % at idle by the jet air flow generated automatically from the JASP. Reductions in THC, CO and NO at idle were also noted.