Passive mufflers are widely employed to reduce industrial and domestic ventilation noise as well as vehicle exhaust noise. Their basic geometry is formed by a simple expansion chamber and the performance is controlled by using complex geometries or by adding porous materials inside the chamber. However, when a clean absorbent system is desirable or when the muffler must support high air flux, it is not possible to add those fibrous materials and the use of micro perforated panels (MPP) as another alternative to improve the acoustic performance become important. The purpose of this work is not only to optimize the acoustic performance of low cost simple geometry mufflers using MPP but also to find the best shape design under a limited space constraint aiming at improving the acoustic performance of automotive engines. In this paper, on the basis of plane wave theory, the four-port system matrix for two wave guides coupled via a MPP tube is derived and used to compute the two-port transfer matrix for an expansion chamber muffler with a MPP tube. Two different procedures to optimize the muffler acoustic performance; the acoustical based and the numerical based methods are presented under the same boundary conditions at a targeted frequency of 1500 Hz. Different methods to improve the MPP wall impedance are presented and compared. New optimized muffler is proposed and used to study the acoustic performance of four-cylinder diesel engine and compared with its performance using the existing straight through resonator muffler. It has been shown that the new optimized muffler reduces the engine noise around 6 dB(A), and also reduces the brake specific fuel consumption of the same engine about 8 percent at same operating conditions.

MPP, numerical assessment, dissipative muffler, shape optimization, engine noise, pressure drop, fuel consumption