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A newly developed OHC (Over-Head Camshaft) prototype of a six-cylinder in-line diesel engine (with bore size: 114mm, stroke size: 130mm) was studied, comparing with the previous version of OHV (Over-Head Valve) type engine (with bore size: 110mm, stroke size: 130mm). It was found that the new type of cylinder block (with 130.8 kg of mass) has significantly lower natural frequencies than those for the previous type of cylinder block (with 133.2 kg of mass). Furthermore, slightly more predominant engine noise and vibration were induced in the new engine. The vibration behavior and the excitation force transmission characteristics were investigated by EMA (Experimental Modal Analysis). We performed a series of impact tests for (1) free-free cylinder block, (2) free-free crankshaft substructure with torsional damper and flywheel attached, and (3) the case where (1) and (2) are assembled together.

This paper describes the investigation of the mechanisms of engine front noise generation and the corresponding countermeasures employed in the development of Hino's medium duty diesel engine. The engine front noise, which had a noise peak in the 630 Hz 1/3 octave band, was investigated by experiment and it was concluded that there were two mechanisms as follows: 1) Combustion pressure excites the crankshaft. Noise is generated by the crankshaft pulley which vibrates with the crankshaft system mode shapes. 2) The cavity between the torsional damper and the timing gear case resonates as a result of the vibration of the torsional damper. Noise caused by the acoustic resonance is emitted to the front of the engine. Using both experimental and analytical methods, crankshaft vibration and acoustic resonance were reduced, thus yielding a substantial noise reduction.