A Study on Generation Mechanisms of Combustion Induced Noise in Diesel Engine

在柴油机结构中，与燃烧冲击密切相关的激发源主要有三种，即：(1)气缸压力迅速上升引起的气体激励；(2)撞击轴承或齿轮处的元件引起的冲击；(3)活塞拍打引起的脉冲激励。研究了这三种激励引起的脉冲响应与发动机噪声之间的多相干分析，以阐明每种激励对发动机总噪声的贡献。在分析中，采用残差谱法去除其他激励的影响。通过这些分析表明，(1)与发动机噪声的低频和中频分量有很大的相干性，(2)和(3)与发动机噪声的高频分量有明显的相关性。将这些方法应用于某小型发动机，明确了产生发动机加速噪声的主要原因是活塞撞击。考虑润滑油温度、进给量、油膜压力、曲轴位移等因素，研究了轴承内润滑油膜对燃烧噪声的影响。从这些实验中可以清楚地看出，轴承润滑油对发动机燃烧噪声的贡献在总体上接近2dB。提出了振动强度测量作为识别发动机结构冲击源的一种方法。给出了强度矢量方向在柴油机各外壁上的分布。通过这些测量，明确了燃烧冲击的传递路径。

In diesel engine structure, there are three main excitation sources whichare strongly related with the combustion impact, that is, (1) gas excitation caused by rapid rise of cylinder pressure, (2) shocks caused by striking the elements at bearings or gears, (3) impulsive excitations caused by piston-slaps. The multi-coherence analysis between impulsive responses caused by these three excitations and the engine noise was examined to clarify each contribution to the total engine noise. In this analysis, the residual spectrum method was employed remove the influences of other excitations.It was clarified through these analyzes that (1) had a large coherence with the low and middle frequency components, beside (2) and (3) had a distinguished corelation with the high frequency component of the engine noise. These methods were applied to a small sized engine, and it was clarified that the predominant cause of the acceleration noise of engine was pistonslaps.The influence of lubricating oil film in bearings to combustion induced noise was investigated considering the temperature and feed of oil, the pressure of oil film, and the displacement of crank shaft. From these experiments, it became clear that the contribution of lubricating oil in bearings to the combustion induced noise of engine was atmost 2dB in over-all value.Vibration intensity measurement was proposed as a means for the identification of impact sources in an engine structure. The destributions of the dircetion of intensity vectors were shown on each outer wall of a diesel engine. Through these measurements, the transmission paths of combustion impact were clarified.