Analysis of Generation Mechanisms of Combustion Induced Diesel Engine Noise

运转中的发动机结构会受到各种各样的冲击。其中，燃烧冲击由于燃烧室内压力的快速上升传递到结构的各个部分，引起振动并从其表面辐射噪声。燃烧噪声的频率特性受气缸压力过程、结构振动响应特性和外表面噪声辐射系数的强烈影响。本研究试图阐明发动机燃烧噪声产生的特点。研究了发动机的传输路径和噪声辐射面的传输辐射系数，该系数被定义为燃烧冲击对燃烧噪声的转化率。在lpg -氧-氮混合气单次爆炸激发的静止发动机和运行发动机上研究了这些系数。在同一台发动机上得到的这些系数的特性是一致的，因此可以说，运行中的发动机的燃烧噪声的传递和辐射与静止的发动机相似。利用改进的试验发动机结构，分别获得了发动机表面和三个传动路径（即气体激励路径、活塞-气缸路径和活塞-曲轴路径）下这些系数的特性。并对燃烧噪声的产生机理进行了详细的研究，分别计算了振动传递系数和噪声辐射系数。

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.