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The noise-generating mechanism of a reciprocating air compressor for heavy duty vehicles during idling was investigated. It was elucidated that the gear rattling noise of the air compressor drive gear train caused by the negative value of the air compressor drive torque was a major noise source. To completely suppress the gear rattling phenomenon, a new loading device with an air cylinder that cancels the negative value of the air compressor drive torque was fabricated. When the loading device was worked, the impulsive sound level was reduced to 10 dB(A). It was found that the impulsive sound level during gear rattling is closely related to the difference in gear teeth velocity between the crankshaft gear and the air compressor drive gear, as one of the characteristics that are needed to obtain a guide for carrying out estimations in the calculation simulation.

The vibration mode analysis of an automotive diesel engine for the purpose of obtaining a lower level of noise emission will be described in the paper. Since the noise emission corresponds to the vibration of certain components of the engine, the vibration of engine structure was analyzed in several running conditions. Vibration distribution shows a natural vibration mode which was proven by coincidence with the result obtained by modal analysis. When measurement of the vibration mode is very difficult due to the high mechanical impedance and especially the big damping factor of the component, a new method of vibration mode analysis for the engine component excited by cylinder pressure was proven to be useful. Some attempts for controlling the noise and an analysis of damping effects will also be described in the paper.

Noise reduction of diesel engines installed in heavy duty vehicles is one of the highest priorities from the viewpoints of meeting the regulations for urban traffic noise abatement and noise reduction in the cabin for lightening fatigue with comfortable long driving. It is necessary that noise reduction measures then be applied to those causes. Noise reduction measures for the diesel engine can be classified into five categories on the noise radiation block-diagram. These are, reduction of combustion and mechanical forces, deformation and vibration control of cylinder block, vibration control of fastened components, prevention of standing wave and close fitting shields. All noise reduction measures for the diesel engine researched for the purpose of practical use are fully described in this paper.