Research On Noise Control And Prediction Of Sound Radiation For I.C.Engines

Energy shortage and environment pollution are the two serious problems which faced by both the automobile and the engine industries. Together with the reduction of the pollution exhaustion, noise reduction is one of the highest challenging targets for ICE development. Aiming at the key problems of engine noise, this thesis presents a deep study of noise control and sound radiation prediction which integrate multiple methods, such as theory, experiment and virtual design. This paper is divided into two parts according to the main content.Aimed at the noise control technique, the former part of this paper mainly studied noise identification and low-noise engine design. In the aspect of the noise identification, sound radiation efficiency, the key issue of using the surface vibration technique, was investigated by the Discrete Calculation Method (DCM). Some conclusions can be drawn that the DCM which is in high precision can be used in any parts of ICE. These results are also the basis of noise radiation prediction which will be shown in the latter part. Some methods such as partial cylinder misfire, surface vibration technique and near field scan technique were adopted to identify the functional noise and the surface radiated noise of the engine. After that, Analytical Hierarchy Process (AHP) was applied to identify the complex diesel engine noise source. Based on coherent power spectrum analysis, a relationship between the functional noise and the surface radiated noise was established. The functional noise can be controlled by corresponding strategy which is related to surface radiated noise.In the aspect of low-noise engine design, the relationship between three- dimensional vibration of the crankshaft and surface vibration of the crankcase was studied. The result shows that surface vibration of the crankcase is induced by 3—D vibration of the crankshaft and the surface vibration of the crankcase can be depressed by controlling the vibration of the crankshaft and changing the transfer function. But for a long time, people have paid a little attention to the coupled vibration of crankshaft. This paper presents a model of coupled torsional-axial vibration based on Rayleigh differential method. Applied with a vector matrix, a single order or a whole system vibration can be analyzed. Compared with the experiment results, the calculated results are to be in agreement in main order. This is to say, the model can be used to design torsional vibration absorber. Through sensitivity analysis andparameter optimization design, an absorber was designed and an experiment verified its validity.The noise reduction research on the thin-wall part is carried out in two aspects: the first aspect is aiming at the reducing vibration mechanism of composite vibration-damping steel plates. The impact of frequency, temperature and lubricating oil on the damp factor are also introduced. The second aspect is the research on the mechanism of structure stiffness. Through sensitivity analysis, the dominating factor of mode stiffness was founded and the result was verified by test. According to the analysis and the summarization of the noise reduction tasks, the author puts forward a new design method based on principle of stimulation source and transfer path.The latter part of this paper is mainly on the prediction of sound radiation, the content includes load calculation of main bearing based on multiply dynamics, dynamics response of combined structure and prediction of sound radiation based on boundary element method (BEM).Used the multi-body dynamic analysis and considered the flexibility of crankshaft, the piston-crankshaft system can be simulated as a dynamic system consisting of both the flexible bodies and the rigid bodies. A numerical method is contributed to simulate the influence of the crankshaft on the main bear dynamic behavior. The result shows that the working condition of crankshaft becomes more complex if considering the flexibility of crankshaft which is the result of the deformation of flexible crankshaft.Based on joint parameter method, mode analysis is carried out to combined structure of a diesel engine. Applied with the exciting forces and boundary condition on the model, the time-history and frequency domain results can be obtained by performing transient dynamic FEM analysis using MSC/NASTRAN. The vibration experiment and mode experiment results verified the validity of the model.A prediction model of noise radiation is set up based on BEM and the sound radiation is calculated by direct BEM. The results of sound pressure level, sound intensity vectors and the contributing factors of main radiating surfaces could be obtained and this method provided a basis on noise evaluation during design period.