Study On Improving Muffler Performance Based On Pipe Orifice Acoustic Reflection Characteristics

The intake and exhaust noise is the main noise source of engine.The installation of muffler is a common method to control the intake and exhaust noise,and muffler has always been a research hotspot.From the research situation of muffler,people have done a lot of research work on the improvement of internal structure of muffler,but the influence of outlet end on the performance of muffler has not been paid enough attention.There is a certain reflection characteristic when the sound wave propagates to the pipe orifice in the pipe,so that the sound wave will not radiate out completely at the pipe orifice,and some sound waves will be reflected back into the pipe.This phenomenon is conducive to improving the muffler performance.This paper studies the influence of the pipe orifice sound reflection characteristic on the muffler to improve the muffler performance.When using the acoustic software virtual.lab to calculate the performance of muffler,the characteristic impedance of air is usually substituted into the outlet boundary to define the outlet non reflection.It is assumed that the sound waves radiate out at the nozzle completely,and the sound reflection characteristics of the nozzle are not considered.By calculating the actual acoustic impedance of the nozzle instead of the air characteristic impedance as the outlet boundary condition,the calculation results are more reasonable and can reflect the sound reflection at the nozzle.The common resonant anechoic components are selected as the research object to study the influence of the sound reflection from the nozzle on its anechoic performance.According to the theory of sound reflection characteristics of pipe orifice,reducing the pipe diameter can increase the reflection coefficient.Therefore,the method of changing single outlet pipe into multi outlet pipe to keep the same total flow area is proposed to reduce the pipe diameter and improve the noise attenuation performance.Firstly,the single outlet pipe is added to the double outlet pipe,and the calculation results and test results verify that the overall noise elimination performance of the double outlet pipe is improved compared with the single outlet pipe.Further demonstration of the improvement,the double outlet pipe is added to the three outlet pipe,and the results show that the overall noise elimination performance of the three outlet pipe is improved compared with the double outlet pipe.The influence of the length of the outlet pipe on the noise elimination performance is studied.The results show that the performance is improved in the low frequency range after the length of the outlet pipe is lengthened.The noise elimination performance fluctuates obviously in the medium and high frequency range.The noise elimination performance is improved after the length of the outlet pipe is changed in some local frequency bands.The relevant experimental verification is carried out.The impedance matching of double outlet tubes with different lengths is changed due to the influence of sound reflection from the tube orifice.The performance change of resonance silencing elements with different tube lengths is studied.The results show that the double outlet tubes with different tube lengths can also improve the silencing performance in the local frequency range.In contrast,if the air characteristic impedance is used as the outlet boundary condition for calculation,the calculation results will not change whether the total flow area is kept the same,the number of outlet pipes is increased,or the length of outlet pipes is changed,which can not guide the use of the nozzle acoustic reflection characteristics to improve the performance of muffler.On the basis of the above research,the performance of the muffler on an engine is improved by using the sound reflection characteristics of the nozzle.The muffler is composed of an expansion cavity and four resonators in parallel,and the noise is mainly eliminated by four resonance peaks.However,the noise elimination valley between the resonance peaks makes the performance of some local frequency domains poor,especially at 1250 Hz.In order to improve the muffler,firstly,the total flow area is kept unchanged,and the single outlet pipe muffler is changed into double outlet pipe muffler considering the sound reflection of pipe orifice,which improves the overall noise reduction performance.Then the sound reflection characteristics are used to improve the noise elimination effect at1250 Hz.By adjusting the length of the double outlet pipe outside the cavity,the noise elimination performance near the frequency point is improved.This measure does not need to directly change the internal structure of the muffler,and effectively uses the advantages of the sound reflection characteristics of the pipe orifice.In order to further improve the performance of the muffler,a double insertion tube with a length of 30 mm is added into the muffler cavity,which improves the performance in the range of medium and high frequency,but does not eliminate the noise elimination trough at 1250 Hz.Therefore,we continue to use the sound reflection characteristics of the nozzle to change the length of the double outlet tube outside the cavity,so as to improve the overall noise elimination effect of the muffler.The performance of the muffler is obviously improved by using the sound reflection characteristics of the nozzle.