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This paper discussed the sound quality which assumed important factor in the development of outboard engines in the 183 to 257 kW class in the future. Many kinds of industrial product development dealt with sound quality, and there were many examples using sound quality index adapted customer requirements or products usage. In case of outboard engine development, there were examples of noise reduction and compliance with noise regulations, but there was almost no example of sound quality development. This research proceeded a questionnaire survey of 90 boat owners who were listening to several cruising engine sounds in main market, US. From this result, authors discussed customer trend and extracted 3 sound quality indexes, luxury, deep and sporty, which were demanded in our target class. Next step was that authors made simulation sounds referring 3 sound quality indexes to verify customer’s trend.

One of the issues involved in compression ignition combustion is the increase in combustion noise from engine mechanical systems caused by rapid combustion. When the fuel used is natural gas, with its high ignition temperature, the compression is increased relative to gasoline, so that combustion becomes even more rapid. The present research pursues the issue of noise by clarifying the distinctive features of combustion noise through tests focused on the two topics of stroke-bore ratio (S/B ratio) and ignition timing for engine structures deformation mode. From these results, we verified combustion noise trend and occurrence factor.

For a small general purpose engine, the authors have studied on “combustion noise”, the mechanical noise originating from combustion. The purpose of this study is to clarify the mechanisms of combustion noise generation. The engine used in this study was a 4-stroke air-cooled single-cylinder engine with the typical characteristics of 3.5 kW-class small general purpose engines, which was specifically designed for experiments. We analyzed the operational behaviors of parts such as the crankshaft, the flywheel and the crankcase during the time of occurrence of combustion noise. Results of the analysis showed that the primary component of combustion noise in small general purpose engines is radiated from the flywheel connected to the crankshaft, and that the vibration mode that radiates the noise is excited by bending deformation of the crankshaft under explosion load. Based on these results, this paper will also discuss the effect of the stiffness of the main bearings on combustion noise.

Research has been conducted on an extended expansion engine, using a multiple linkage system to increase the thermal efficiency of general-purpose engines. In this research, first, the test engine was subjected to an engine acoustics measurement to clarify its noise characteristics. Then, based on the analysis results of the noise characteristics, we propose the direction to reduce the noise of the multiple linkage system engine. When the test engine was operated under a no-load firing condition, rattle noise was observed. Also it was found that the timing of the occurrence of the rattle noise was near after top dead center (ATDC) 90 degrees in crank angle (degCA). Focusing on the gear for linkage drive as a cause of the rattle noise, the authors formulated the torque acting on the gear.

Research has been conducted on an extended expansion engine using a multiple linkage system to increase the thermal efficiency of general-purpose engines. Previously reported engine increased mode fuel economy by 12.9%, while generated 1.93 times the summation of inertia compared to a conventional engine with the same output power. The newly-proposed multiple linkage system operates through a longer stroke term at a crank angle of 191.6°, so both of the piston speed and acceleration become lower. The summation of inertia in the newly-proposed multiple linkage system was decreased by 46.3% compared to that of the previous multiple linkage system.

The purpose of this paper is to clarify the vibration properties of the extended expansion engine with a multiple linkage system. The mass systems were used to obtain a substitution for the center of gravity of each component part in order to formulate the movement of the multiple linkages. Attention was focused on the maximum value for the summation of inertia forces generated by the component parts of the linkages as an indicator to use in evaluating the vibration properties of the multiple linkage system. In order to minimize this value, the optimum weight of the crankshaft counterweight and its position of gravity center were examined. The calculated results of these indicators show that the maximum value for the summation of inertia forces in the previously proposed engine is 35% larger than the value in an existing engine of the same power.

An air-cooled 4 stroke single-cylinder general purpose engine with 438 cm3 of displacement was researched to improve fuel efficiency. As a technique for improving fuel efficiency, 1) Intake valve were narrowed and spark plug was moved closer to the bore center. As a result, the new engine shortened the mass combustion rate by 13%, compared to existing engines, and improved fuel efficiency in the EPA general-purpose mode by 1.1%.2) Taking aim at reducing the piston system friction, piston ring tension was lowered 45% compared to that of existing engines in the same class by using a centrifugal casting sleeve with the air-cooled engine. This has resulted in a 5% reduction in friction mean effective pressure, and improved fuel efficiency in the EPA general-purpose mode by 0.9%. 3) Electronic control technology was used, and ignition timing was decided based on engine speed and throttle opening data.