摘要
发动机的振动和噪声是影响汽车行驶舒适性的主要因数。随着人们对汽车舒适性要求的不断提高，发动机的减振降噪研究也日益受到重视。
众所周知，长期的噪声作用，使人产生紧张感，心情烦躁、易怒、易疲劳等，所以本文以一新开发的直列式四缸汽油机为例，系统的阐述了发动机结构振动噪声的计算机仿真分析过程。旨在降低该发动机的振动噪声，节省同类发动机的开发周期，为降低发动机的振动噪声提供指导或参考作用，并且为发动机的结构噪声分析提供思路和方法。
发动机结构是一个很复杂的机械系统，可以看成多自由度振动系统，其激振力和结构响应之间的关系可通过脉冲响应函数、频率响应函数或传递函数来确定，也可以通过有限元计算或实验模态分析的方法来确定发动机整机结构的主要阶次模态参数，其中固有频率和振型对控制发动机振动和噪声有特别重要的意义。这是因为结构以某一阶模态振动时，将在其辐射噪声频谱上出现一个峰值。如果某一峰值过高，则将对整个结构辐射噪声的强度产生较大影响，这时可考虑根据该阶振动的形状采取相应的措施，以改变该阶模态的固有频率、阻尼、刚度、质量分配等，使结构的固有频率移向不易振动的区域。所以发动机的模态分析是结构振动噪声分析中不可缺少的一步，它不仅证实了有限元模型的正确性，还为以后的结构修改提供可靠的依据。
有限元法将部件离散为许多单元，具有单元质心和单元刚度。但发动机整机有限元模型的离散单元过多，增加了计算量，而且还受到计算机硬件的限制。于是我们采用了结构动力学中的矩阵凝聚和矩阵恢复法。在矩阵的凝聚和恢复过程中，计算的精确与否，与主自由度的选取有直接的、密不可分的联系。所谓主自由度节点，就是将一定区域中的所有单元压缩到一个节点上，一个部件可以由合适多的、能反映其动力学特性（位移、变形和受力）的节点组成，这些节点称为主自由度节点，相应的其它节点称为从自由度节点。通过矩阵凝聚，缩减计算自由度，节省了大量的分析时间。
发动机的振动噪声主要是由于气体爆发压力所引起的振动并通过缸盖和活塞-连杆-曲轴-机体的途径向外辐射的噪声，和由于活塞对缸套的敲
击，配气机构、轴承等运动件之间机械碰撞所产生的振动激发的噪声。所以发动机振动噪声分析除考虑缸体、缸盖、油底壳外，还有为降低原有四缸发动机振动噪声而增加的梯形框架；另外，为了更接近发动机的真实工况，本文还考虑了曲轴部分、止推轴承、油膜阻尼等因素的影响。因而，发动机的结构噪声分析又是一个多体动力学的问题。本文将经典的有限元方法和多体动力学方法相结合，考虑结构之间的非线性因素，较为完善的分析了发动机整机的动态相应特性。
众所周知，声压级、声强级和声功率级是评价噪声的客观度量。本文由于时间的局限性，只是对发动机的表面振动速度级进行了求解，而且振动速度级与声压级存在一定的换算关系。虽然它不能直接作为发动机结构辐射噪声强弱的评判标准，但是可以间接的评价发动机噪声在某一部位的高低。
对发动机噪声的评价，除考虑其辐射噪声能量总水平外，还应考察以下噪声特性：噪声级及其随发动机工作状态的变化关系，发动机外表面各点噪声级数值的分布状态，空间各点的噪声频谱以及发动机工作过程各阶段的瞬时声压级。通过这些信息，不但可以比较和评价发动机辐射噪声的大小，还可以深入研究辐射声能在频域上的分布情况，判断发动机工作循环中辐射噪声最大的阶段，以便分析产生高噪声的原因，提出噪声控制措施，降低发动机表面噪声，为汽车的降噪带来良好的效果。
本文通过发动机的声谱分析，得知发动机在平而大的薄壁处振动较大，因此，增加这些地方的刚度或阻尼是降低振动噪声的有效途径。本文提供了在发动机缸盖和缸体上加筋后改进发动机的振动速度级与所分析的发动机（称为分析样机）的速度级的对比，证明了增加局部刚度可以降低整机噪声。而且本文在最后同时对四种发动机的振动速度级做了比较，证实梯形框架的使用大大降低了原有四缸发动机的振动噪声。
但由于时间及硬件各方面的不足，下面工作还有待完善：利用本文已求得的节点速度级，通过SYSNOISE软件中的边界有限元法,求出发动机噪声的声压级,这才是发动机在声场中的真实反映。
The vibration and the acoustic noise of the automobile are the main factors that affect the comfort for car-driving. Because people have an increasing request of the comfort, the noise-reducing analysis of engine becomes more and more important.
It is known that long-term noise can make people nervous, agitated, easy to be angry and tired and etc. This paper takes a four-cylinder in-line engine designed newly as an example, describing the computer simulation course of the engine structure vibration noise. It aims at reducing the vibration noise of this engine, saving the development period of the similar engine, offering guidance or conference for reducing the vibration noise of engine, and supplying the thought and method for the acoustic analysis of the engine structure.
The engine structure is a very complicated mechanical system which can be regarded as a vibration system with many degrees of freedom. The relations between shaking forces and structure response can be decided by the pulse function, frequency response function or transmission function, and can also be calculated through finite element method or modal experiment method that can confirm engine main modal parameter, among them natural frequencies and main vibration types are very significant to control the engine vibration and noise. Because the structure vibrates at some modal, a vibration peak value will appear at its noise frequency spectrum. If a certain peak value is too high, it will have greater influence on the intensity of the vibration noise of the whole structure, the corresponding measure can be taken according to the form of the structure vibration to change the modal natural frequency, damping, stiffness and quality of the step, making the natural frequency of the structure move to the hard vibration area.
So the modal analysis of the engine is an indispensable step, it not only verifies the correctness of the finite element model but also offers the reliable basis for modifying structure later.
Finite element method divides structure into a lot of entities, which have element mass and stiffness. But there are too many dispersed units of the finite element model in the complete engine, which increases the calculating amount, and calculation can still be limited by computer hardware. Then this paper has adopted the matrix condense and recovery in dynamics. In the course of matrix condense and recovery, the choice of the main nodes of degree of freedom decides the calculating accuracy. What is the main node of degree of freedom? It is that all elements in certain area are compressed to a node. One part can be made up of some main nodes which reflect its dynamics characteristic (displacement, deformation and strength), these nodes are called the main node of degree of freedom, other nodes are called the secondary node of degree of freedom. By condensing matrix and reducing the calculating degree of freedom, a large amount of analysis time is saved.
The vibration noise of engine is caused by gas eruption pressure which is transmitted through cylinder head and piston - conrod - crankshaft - body outwards to radiate noise, and the beat between jar set and piston and the vibration produced machinery for sport pieces or crankshaft can cause noise. So the vibration noise of the engine is analyzed considering block, cylinder head, oil pan, and also including the ladder frame in order to reduce original engine vibration noise. In addition, in order to close to true operating situation of engine, this paper considers the influence of crankshaft, bearing, oil film damping, etc. Therefore, the noise analysis of the engine is a multi-body dynamics problem. This paper combines multi-body dynamics methods with the classical finite element method, considering the non-linear
factor between the structures, and analyzes the engine dynamics response better.
As is known, sound pressure grade, sound intensity grade and grade of power of sound are the objective criterions of the noise. Because of the limitation of time, the paper has
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