State of the art of rational design of efficient mufflers

An efficient exhaust muffler needs to satisfy the rather conflicting requirements of adequate insertion loss (IL) with limited back pressure, weight, size and cost. The unmuffled exhaust noise of an internal combustion engine peaks at the engine firing frequency and the first few harmonics thereof. Designing the muffler (a wide-band low pass acoustical filter) for such low frequencies is quite a challenge. Therefore, design of efficient mufflers has remained an art as well as science. During the last two decades or so, however, (a) multi-hole cross-flow perforates, (b) multiply connected perforated pipes and baffles, and (c) the double-tuned, grazing-flow concentric tube resonators have emerged as promising constituent elements. In this paper, elements (a) and (c) have been combined ingeniously to synthesize efficient muffler configuration for diesel generator (DG) sets as well as automotive engines. Relatively simpler lumped parameter approximation for perforates opening into cavities, electro-acoustic analogies, and the simple expansion chamber equivalence of the double-tuned concentric tube resonator have been used to develop a rational design methodology. A novel feature of this methodology is incorporation of the given back-pressure limit into the acoustic design process. The resulting muffler configurations are robust and can be scaled up (and down) easily for IL as well as back pressure.