We experimentally investigated the influence of shifting natural frequencies of the internal transmission system depending on the connecting-rod specifications on the characteristics of noise radiated from a single-cylinder diesel engine. We used FFT analysis to investigate the influence of shifting natural frequencies of the internal transmission system on the radiated noise characteristics. By changing the thinned portion of the connecting-rod, we confirmed that the natural frequency of the piston-connecting-rod-coupled vibration differed from another natural frequency of the engine structure, and thus the engine noise was reduced. This research studied the time-frequency characteristics of combustion impact and engine noise by wavelet analysis of in-cylinder pressure and sound pressure. We examined the vibration-transmission characteristics through the relationship between the maximum engine noise power and the maximum combustion energy in the same cycle for the main frequencies of combustion noise. Two types of connecting-rods were used: the original connecting-rod and a connecting rod that has a short-thinned portion (STP). By shortening the thinned portion of the connecting-rod, the natural frequency of the piston-connecting-rod-coupled vibration became around 2800 Hz from 2500 Hz, and the vibration transmission efficiency, η, around 2500 Hz became small. By shortening the thinned portion of the connecting-rod, the decay rate, c, around 2500 Hz became large while the decay rate around 2800 Hz did not change.
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