Abstract
Keywords - diesel engine, noise, cavitation, main bearing, oil film
Abstract - This paper presents an analysis of noise occurrence at the 7th main bearing of an in-line six-cylinder diesel engine, and proposes a design to prevent the noise. The frequency of the noise was over 5 kHz, and the noise occurred unperiodically. Engine experiments suggested that cavitation in the bearing oil film was a major cause.
The mechanism of noise occurrence was assumed to be as follows. The noise occurred when the following conditions were combined: (1) cavitation appeared in the oil film at the main bearing, (2) main journal vibration in the radial direction induced further appearance and collapse of the cavitation.
The mechanism was verified as follows. Firstly, the existence of cavitation at the time of noise occurrence was predicted by elastohydrodynamic lubrication analysis with mass conservation in the oil flow. Secondly, instability of the main journal-oil film system was predicted by stability analysis. When the system became unstable and combustion pressure fluctuated simultaneously, the main journal vibrated violently. These results showed that the combination of two conditions contributed to the appearance and collapse of the cavitation and noise occurrence, then it verified the validity of the assumed mechanism.
Finally, the observation of excited oil film and the measurement of noise were conducted simultaneously by making a test rig based on the mechanism in order to investigate the relation between the cavitation and noise. Some cavitation of lightning shape was observed in the oil film, and the noise occurred when the cavitation reached the end of a circular piece, that is, the atmosphere. The reaching of cavitation was prevented by cutting a ring groove at the end of circular piece, and the noise was reduced. The noise reduction was confirmed on a real engine by using a main bearing with grooves at both ends.
This technique will be useful for producing high-quality cars with no high-frequency noise.