Analysis of Generation Mechanisms of Combustion Induced Diesel Engine Noise

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A running engine structure is subjected to various kinds of impacts. Among them, the combustion impact, due to a rapid pressure rise in the combustion chamber is transmitted to each portion of the structure, causes vibration and radiates noise from its surfaces. The frequency characteristic of a combustion induced noise is strongly influenced by the cylinder pressure process, the characteristic of vibration response of the structure and the noise radiation coefficients of the outer surfaces.In this research, an attempt was made to clarify the characteristics of combustion noise generation of an engine. The transmission-radiation coefficients which are defined as a conversion ratio of the combustion impact to the combustion noise are examined for the transmission paths and the noise radiating surfaces of the engine. These coefficients were studied with both a stationary engine excited by a single explosion of LPG-oxygen-nitrogen mixture and an operating engine. The characteristics of these coefficients obtained in the same engine coincided with each other, so it can be said that the transmission and radiation of combustion noise of an operating engine are analogous with those of a stationary engine.Using modified structures of a test engine, the characteristics of these coefficients are separately obtained for engine surfaces and for three transmission paths each, i.e. the gas excitation path, the piston-cylinder path and the piston-crankshaft path.The generation mechanism of combustion induced noise were also investigated in detail calculating the vibration transmission coefficient and the noise radiation coefficient separately.