| Installing an exhaust muffler is an effective way to control the exhaust noise of a diesel engine.However,the traditional resistant mufflers cannot meet the comprehensive requirements of good acoustic performance and low exhaust resistance at the same time.Therefore,multi-unit coupling of the split-stream rushing muffler unit and the traditional resistant muffler unit has been proposed in this project to reduce the airflow velocity in the new muffler so that the comprehensive performance can be improved.Based on this idea,a new type of diesel engine muffler was trial-produced,and the coupling performance analyses of the new type muffler were completed.In this dissertation,the exhaust noise characteristics of the diesel engines were obtained through the exhaust noise tests and spectrum analyses of different diesel engines.Based on the exhaust noise characteristics of the diesel engines and the acoustic performance of different noise reduction structures,structural coupling models for a new type of diesel engine muffler were proposed according to the multi-unit coupling theory.Through the combination of mathematical modeling,numerical simulation and experimental verification,the coupling study of the acoustic performance and exhaust resistance performance of the new muffler was completed,and the comprehensive performance of the new muffler of the diesel engine was evaluated.The main conclusions are as follows:(1)The exhaust noise tests at different speeds of the diesel engine were used to obtain the exhaust noise spectrums.It was determined that the target silencing frequency of the diesel engine was the low-frequency band within 200Hz.The control of low-frequency noise within 50Hz should be taken into special consideration.Based on this exhaust noise characteristic,new muffler models of coupled structure for diesel engine were developed by combining the theory of the multi-unit coupling theory and split-stream rushing mechanism.(2)The coupling study on acoustic performance for diesel engine muffler was carried on.The one-dimensional transfer matrix method was used to establish a mathematical model of the new muffler transfer matrix.The transmission loss curves of the new muffler models at three inlet velocities of 30m/s,40m/s,and 50m/s were obtained through Matlab programming.The results show that the transmission loss of the new muffler is a wide-band and M-type distribution.The new coupling structure widens the muffler frequency band and increases noise reduction of muffler at low frequency.The transmission loss in the 2000Hz bandwidth exceeds 20dB,which ensures good acoustic performance in the entire frequency band.With the increase of the inlet velocity,the transmission loss of the new type muffler is slightly enhanced,and its acoustic performance is stable.A multi-factor test design was carried out using orthogonal experiments to obtain the sensitivity of each structural factor to the transmission loss of the new diesel engine muffler.Taking the average value of the transmission loss TL within 0-200Hz of the new muffler as the evaluation index.The results show that the primary and secondary effects of the structural parameters on the transmission loss are the inner cavity diameter D2,the cone ring angle α,the diameter of intubation D4,and the length of intubation L11.The optimal parameter combination of transmission loss obtained is D2=70;α=50°;D4=58;L11=155.The optimal structural parameter combination obtained by mathematical modeling based on the transfer matrix method,MATLAB numerical simulation and orthogonal test realizes the rapid optimization of structural models for the new muffler which shortens the design cycle o f the new type of muffler for diesel engines.(3)In terms of the coupling research on the exhaust resistance performance of the new muffler,the internal flow field of the new muffler was simulated by Fluent software,and the velocity field,pressure field and turbulent kinetic energy field at the inlet airflow velocity of 30m/s,40m/s,and 50m/s were obtained,and the prediction of airflow regeneration noise at five measurement points inside and outside the new muffler at three velocities was completed.The results show that the coupling structure reduces the airflow velocity in the new muffler quickly,achieving the expected velocity reduction effect,and also effectively controlling the pressure loss.The turbulent kinetic energy inside the coupled structure mainly occurs in the split-stream rushing area,but the turbulence intensity is not high.The sound pressure level of the airflow regeneration noise of the new muffler increases with the increase of the inlet velocity,which verifies the correlation between the regeneration noise and the decrease of the airflow velocity.The airflow regeneration noise at outlet measurement point of new muffler is small,indicating that the coupling structure effectively suppresses the airflow regeneration noise.(4)The exhaust muffler test system of the internal combustion engine was used to complete the pressure loss and insertion loss tests of a new muffler at inlet velocity of 30m/s,40m/s,and 50m/s.The results show that the higher the airflow velocity at the inlet of the new muffler,the more obviously the airflow velocity in the muffler decreases,and the more obviously the corresponding pressure loss decreases.The principle of reducing the pressure loss by reducing the airflow velocity inside the muffler through the new coupling structure is verified.The pressure loss test results are consistent with the simulation analysis conclusions,and the relative error is within 10%.The new type muffler has an average insertion loss of 20.1dB within 2000Hz at three inlet velocities,which is ideal for noise attenuation performance.The average insertion loss within 200Hz is 19.16dB,which can ensure the noise reduction effect of low frequency band for diesel engine.The tests of the transmission loss of the new muffler and the original muffler were completed by the two-load method.The results show that the measured values of the transmission loss of the new muffler in the test band are in good agreement with the predicted values,and the relative error is within 5%,which verifies the two-load method can accurately test the transmission loss of the new muffler by the muffler test system.The test results of the transmission loss of the new coupled muffler is obviously better than that of the original muffler.Compared with the original muffler,transmission loss improvement rate of the new muffler within 0-2000Hz and 0-200Hz is 15.04%and 35.37%respectively.(5)The exhaust noise and fuel consumption tests of the tractor with the new muffler and the original muffler show that the acoustic performance,exhaust resistance performance and fuel economy of the new muffler are better than the original muffler.The muffler can achieve the comprehensive goal of good acoustic performance and low exhaust resistance by reducing the airflow velocity.In the exhaust noise tests within 2000 Hz at idle speed of 750 r/min,intermediate speed of 1500 r/min,and rated speed of 2300r/min,the average insertion loss of the new muffler is higher than that of the original muffler.The average insertion loss of the new muffler within 200Hz and 50Hz is 27.07dB and 24.65dB,which are 33.94%and 68.95%higher than the original muffler respectively,and it has taken advantage of the low-frequency noise reduction from the new coupling structure.In fuel consumption tests at idle speed,intermediate speed,and rated speed,the 60s fuel consumption L and hourly fuel consumption G of the new muffler are lower than the original muffler,and the average fuel consumption reduction rate is 19.16%,further verifying that the new muffler has a lower pressure loss than the original muffler.The research results provide a reference for the development of a new type of exhaust muffler for high-efficiency diesel engines. |
