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Several aspects of the acoustic design optimization of induction systems are considered. The important role of the inlet manifold in the induction system is shown by constructing mathematical models of two levels of sophistication. A plane wave representation of the manifold is adequate when the geometry of the manifold supports only longitudinal acoustic modes. When the geometry supports transverse modes it is necessary to include the acoustic modal structure in the mathematical model comparing three mathematical models. The manifold with a conventional runner arrangement is shown to introduce harmonics of the firing frequency other than the generally assumed multiples of the number of cylinders. A manifold with runners coupled in a common plane is considered as a means of eliminating undesirable harmonics. It is shown by considering the ninth harmonic (order 4.5), that harmonics other than the integer multiples of the number of cylinders are drastically reduced in amplitude.

Two problems that arise in induction system acoustic analysis are addressed in the context of the Acoustic Wave Finite Element (AWFE) procedure. A source model that makes use of measured data to scale the source amplitude is shown to provide good correlation between the predicted and measured inlet manifold noise of a six cylinder engine. Three-dimensional effects due to complex geometry in intake manifolds and air cleaners are modeled using an extension of the AWFE method that introduces three-dimensional wave structure within elements that are comparable to the acoustic wave length. Comparisons are made of the acoustic output of induction system components modeled using simple plane wave elements and using elements with wave structure.