| 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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