Research On Characteristics Of Dual-Mode Muffler

The muffler is the key device for controlling the exhaust noise of the car.In order to ensure better noise reduction effect,the structure of the muffler is often complicated,which may lead to poor exhaust gas,increase back pressure and power loss of the engine,and reduce the fuel economy of the vehicle.At high speed,the airflow of a fast speed and the complicated structure of the muffler is likely to cause large airflow regeneration noise.The dual-mode muffler has good muffling performance when the valve closes at low speed,and the valve opens at high speed,which helps to reduce pressure loss and airflow regeneration noise,balance the contradiction between acoustic performance and aerodynamic performance,and improve the vehicle NVH(Noise,Vibration,Harshness)performance.Firstly,in order to study the muffling performance of dual-mode mufflers at different valve opening degrees,the transmission loss of mufflers at different valve opening degrees was measured with double-load method on the muffler test bench.The test results show that the transmission loss trend is basically the same at different valve opening degrees,but the noise reduction decreases gradually and the transmission loss tends to move to high frequency with the increase of valve opening degree.Secondly,in order to study the airflow regeneration noise of the dual-mode muffler,The flow regeneration noise and tailpipe noise were measured under the valve closed and fully opened conditions at different inlet flow rates with the methods of dual-microphone transfer function and the muffler static transmission loss.The results show that the incident sound power level of air regeneration noise at the outlet of muffler and tailpipe noise are reduced when the valve is fully opened compared with that when the valve is closed,which directly proves that the dual-mode muffler is helpful to reduce the air regeneration noise.The aerodynamic noise of the muffler is simulated based on the broadband noise source model,and the sound power level distribution of the muffler was obtained.The simulation results show that the sound power level of the muffler gradually decreases as the valve opening degree increases.Then,in order to study the aerodynamic performance of the dual-mode muffler at different valve opening degrees,the back pressure and pressure loss at different inlet flow rates were measured by cold flow test.The test results show that the back pressure and pressure loss of the muffler decrease as the valve opening degree increases.The flow field simulation of the muffler was carried by Fluent,and the distribution of pressure,velocity and turbulent kinetic energy in the muffler were obtained.The results show that pressure,flow velocity and turbulent kinetic energy of muffler decrease with the increase of valve opening degrees.The opening of the valve is helpful to improve the flow field performance of muffler.The improvement of the flow field of the muffler is obvious at the initial stage of opening,and the extent of improvement continues to increase with the increase of the opening degree of the valve.Then,in order to study the performance of the dual-mode muffler under the actual working conditions,the relationship between the inlet flow velocity of the muffler and the dynamic opening degree of the valve was obtained through cold flow test,and the back pressure,pressure loss and tailpipe noise of the muffler under the actual working conditions were measured.The results show that the valve opening degree of the dual-mode muffler will not be very large under the actual working conditions,but the back pressure,pressure loss and tail pipe noise of the muffler reduced under the actual working conditions compared with that when the valve is closed.Based on the experimental results,the flow field and aerodynamic noise are simulated.The pressure,velocity,turbulent kinetic energy and sound power level distribution of the muffler were obtained.The results show that the pressure,airflow velocity,turbulent kinetic energy and sound power level are reduced compared with that when the valve is closed.Finally,the flow field and acoustic performance of a dual-mode muffler were analyzed,and the structure optimization was carried out to overcome its shortcomings.The flow field and acoustic performance of the muffler before and after the optimization were compared when the valve is closed and fully opened.The results show that the optimization is effective.