Research And Application Of Dominant Noise Contribution Mode Of Engine Thin-wall Parts

With the scientific and technological level of vehicles developed and the social consumption ability continuously improved, the competition in the automotive industry is becoming increasingly fierce. Therefore, the requirement of NVH performance of vehicles is much higher than before. As the main radiated noise source in engine, the radiated noise of thin-wall parts accounts for 40～50% of the engine noise. Furthermore the engine is one of the mainest sources of vehicle’s vibration excitation and noise radiation, so it has the important theoretical significance and practical value to conduct a comprehensive and thorough research on vibro-acoustic characteristics of thin-wall parts. Taking the timing cover as an example, the vibro-acoustic characteristics of thin-wall parts was studied from a new perspective in this paper, with the purpose of low noise structure design and low loudness structure design. This research included two aspects.That target modes are tracked by modal method is uncertain when low noise structure design is carried out, thus the method of dominant noise contribution mode has been put forward. Contribution of modes to the radiation noise at any frequency was analyzed and dominant noise-modes were identified by the new method, which could provide guidance and evidence for low noise structure design. Firstly, the first-order tetrahedral finite element model of timing cover was established, and the element stiffness matrix and the element mass matrix were calculated. Then the global stiffness matrix and global mass matrix were assembled, and the modal parameters were calculated by solving the eigenvalue equation. The excitations of bolt holes got from multi-body dynamics calculation were loaded in idealized spring elements that were used to simulate constrained conditions, and the dynamics response of timing cover was resolved by mode superposition method. The accuracy of the finite element model and the dynamics response analysis were verified by modal test and surface velocity test. Timing cover of complicated shape was discretized into a few piston sound sources, and surface velocities got from dynamics were mapped to acoustic element to predict the radiated noise. The radiated noise power was calculated with the method of surface vibration velocity, and the accuracy of acoustic prediction was verified. The dominant noise contribution mode analysis based on coupling method was carried out by programing, and 4 operational modes that made great contribution to noise were identified. Stiffeners were arranged to constrain the 4 modal shapes, then the results revealed that radiated noise power of timing cover decreased by 1.17dB.The structure design of thin-wall parts was done with the purpose of improving the sound quality. The specific loudness of radiated noise was analyzed, and 3 constraint modes of the model which made main contribution to loudness were identified, then stiffeners were placed based on the multi-objective topology optimization method. The results revealed that radiated noise loudness of timing cover decreased by 6.7%, therefore comfort of radiation noise was improved obviously.