Order Dispersion Method by Crankpin Arrangement into Enhancements of Structural Reliability and Sound Quality for V8 Outboard Motor

This paper provides insight into the utility of the low vibration crankshaft for outboard motors, especially focuses on the enhancements of the structural reliability of the crankshaft and the sound quality of the outboard motor. In previous research, we developed the crankshaft which can balance by itself for V8 outboard motor with V bank angle of 60 degrees. We specifically showed the theoretical basis of the balancing and the measurement results of actual vibration levels on boat. Meanwhile, note that the crankshaft has a distinctive structure of crankpin offset angle of 60 degrees, so that combustion interval becomes unequal. As to the combustion, however, we just mentioned the effects on the engine output performance, not the practicality. In this paper, we clarify the following two benefits of unequal interval combustion in terms of the structural reliability and the sound quality. One is that the order dispersion resulting from unequal interval combustion can reduce the excitation force in stress. Taking the resonance magnification into account in the strength evaluation, regardless of the resonant frequency in the normal rotation range, we could secure the structure without using components such as dynamic dampers. The other is that the dispersion enhances the sound quality of outboard motors. It allows excitation energy to be distributed to low frequency side, leading to thicker sound. Through the survey and the sound quality evaluation, it is verified that most boat users tend to prefer the sound mentioned above. Thus, this result showed one direction toward attractive sound quality.