RELATION BETWEEN CRANKSHAFT TORSIONAL VIBRATION AND ENGINE NOISE

This paper presents the results of the latest investigations carried out at Isuzu Motors Ltd. into the mechanism of radiated noise from an acoustically modified diesel engine. Although the overall noise level of this engine is quite low compared with those of conventional design because noise in the acoustically important range from 800 to 1250 Hz has already been sufficiently reduced by structure modification, predominant peaks appear at 315 Hz and at 1600 Hz third octave bands which now govern the overall noise. To achieve further reduction of engine noise, these two peaks have to be adequately reduced. Therefore the noise generating mechanisms at 315 Hz were studied in detail on the running engine as well as experimentally using modal analysis program and by theoretical analysis. This study revealed the relationship between the vibration behaviour of the crankcase and a torsional vibration of the crankshaft. Also presented are the results of a study into the means of reducing noise peak.