Measurement of the acoustic impedance of an internal combustion engine

Abstract

The specific acoustic impedance ζ of an internal combustion engine has been studied experimentally to promote the computer-aided muffler design to a higher level of sophistication. Measurements were performed with a specially prepared impedance tube facility capable of accommodating the hot and moving exhaust gases. At low engine speeds (RPM) the ζ trace on the complex ζ plane follows an irregular path that cannot be readily modeled in terms of simpler acoustic components. At higher RPM, however, the trace becomes a spiral, centered at and slowly converging towards ζ = 1, indicating a resemblance to an open-ended pipe segment. The measured ζ has been used in designing a muffler for this internal combustion engine with favorable results.