Optimization Of Nonlinear Vibration And Acoustics Performances For A Passenger Car Exhaust System

With the advent of the new era,customers demand more in vehicles? performance and quality.Vehicle riding comfort,as an important indicator of vehicle performance evaluation,gets increasing attention in the market competition.Nowadays,there are more and more cars driving on the road,and then in order to control the pollution and damage caused by vehicles to the ecological environment,the government introduces increasingly stringent environmental regulations.The exhaust system,as one of the important parts during automobile NVH design,undertakes multiple missions,such as purifying vehicle exhaust,isolating vibration impact and reducing exhaust noise.In this paper,two exhaust systems designed at the initial stage of the exhaust system of a vehicle are presented,which show significant noise and poor acoustic attenuation performance in bench test.The acoustic coupling models of both two exhaust systems and the engine are established and corrected in GT-Power.Through a comparative analysis of the two exhaust systems? test results in tailpipe noise and transmission loss,it is found that in the first scheme the noise values in whole and of 4&6 orders exceed the limits,and in second scheme the noise values do not exceed the limit but are high,which is mainly caused by the lack of tramsmission loss of the muffler in the middle and low frequency.Combined with Fluent,the hydrodynamic performance of exhaust system under 5500r/min is simulated,and through comparison it is found that the backpressure meets the demand,but the turbulent kinetic energy and the regeneration noise distribution inside the muffler is high,and the inner acoustic cavity need to be optimized to improve the hydrodynamic performance.According to the analysis results in acoustics and flow field,with a guidance of pipe acoustic theory and experience,the acoustic cavity structure of the muffler is changed and reanalyzed of transmission loss and tailpipe noise after optimization.The simulation results show that the transmission loss of the optimized structure is effectively improved in the middle frequency band of 200-600 Hz and low frequency band of 0-60 Hz,the tailpipe noises of each order are obviously reduced,and the total noise value is reduced effectively in the middle and low frequency and in satisfied development target.In addition,the tailpipe noise is tested by the bench and the real vehicle,and the test results show that the tailpipe noise of the optimized exhaust system is eliminated effectively and the power loss is also within the target range.Then,the vibration performance of the optimization scheme is analyzed.Through the modal test to validate the established finite element model,the calculation results show that the exhaust system can avoid the idle excitation frequency from the engine,but the dynamic forces of each isolator are not uniform.And then the influence of the nonlinearity of the rubber isolator upon the static performance of the exhaust system is studied,and the results show that the nonlinearity of the rubber isolator has a slight influence on the calculation result under the static load of dead weight and significant influence when calculating the envelope surface of the exhaust system,which indicates the nonlinear stiffness characteristic of the rubber isolator should be taken into account when calculating the exhaust system envelope surface.Finally,the position of the hanger is improved with the guidance of ADDOFD method,the dynamic and static performances of the exhaust system are improved obviously,which is helpful to improve the vehicle riding comfort and performance of exhaust system.This paper through GT-Power and Fluent?s acoustic simulation achieves the optimization of the tailpipe noise of the exhaust system.It is a strong indication that the combination of the two software can provide an accutate basis for the optimization of the acoustic performance of the exhaust system.It has certain reference value to consider the nonlinear effect of the isolator in the severe deformation while optimizing the exhaust system or other related parts.The ADDOFD method can provide guidance to improve the position of the hanger.