Acoustic Analysis of a Compact Muffler for Automotive Application

A production muffler of a 2.2 liter compression ignition engine is analyzed using plane wave (Transfer Matrix) method. The objective is to show the usefulness of plane wave models to analyze the acoustic performance (Transmission Loss, TL) of a compact hybrid muffler (made up of reactive and dissipative elements). The muffler consists of three chambers, two of which are acoustically short in the axial direction. The chambers are separated by an impervious baffle on the upstream side and a perforated plate on the downstream side. The first chamber is a Concentric Tube Resonator (CTR). The second chamber consists of an extended inlet and a flow reversal 180-degree curved outlet duct. The acoustic cavity in the third chamber is coupled with the second chamber through the acoustic impedances of the end plate and the perforated plate. The effect of manufacturing defects in the form of leakages and weep holes (to reduce internal corrosion) which are commonly found in automotive mufflers is investigated by modelling it as perforations. The back pressure of the muffler is predicted by using the friction factor for perforated pipes in the flow resistance network of the muffler. The TL of the muffler is measured without mean flow inside a semi-anechoic room by using the two load method. The back pressure is measured on board the vehicle at different mass flow rates and temperatures. The predicted and experimental results are found to be in reasonable agreement. The effect of leaks, weep holes and plastic cover of glass wool are found to be significant.