Abstract: The three dimensional model of a threecylinder diesel engine block is established by using UG 5.0 software, and the modal analysis of the engine block is carried out under four different constraint conditions using Lanczos method. Through the analysis, the inherent frequencies and the modal shapes of the first 10 order modes are obtained. The results are compared one another. Based on the comparisons, it is discovered that the inherent frequencies and the corresponding vibration modes are quite different for different constraint conditions. In addition, the weak portions of the engine block vibration are found by modal analysis, and the corresponding modifications are put forward, which may provide a reference for further structural design and dynamic response analysis of the diesel engine block.

Key words: vibration and wave, diesel engine block, modal analysis, constraint, finite element method

摘要： 利用UG5.0软件建立了某三缸柴油机机体的三维实体模型，应用Ansys有限元分析中常用的Lanczos法，采用4种不同约束条件对其进行了模态分析，分别计算出机体结构的前10阶约束模态的固有频率和相应振型，并对分析结果进行了比较。通过振型比较分析，发现施加不同约束固有频率提高的幅度不同，振型也有较大变化。另外针对机体振动的薄弱部位，提出了相应的改进措施，从而为该柴油机机体结构的改进设计及机体的动态响应分析提供可靠的依据。

关键词: 振动与波, 柴油机机体, 模态分析, 约束, 有限元法