Acoustic Modeling and Optimization of Induction System Components

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. The difficult problem of generating a good source model for the inherently nonlinear aeroacoustic source at the inlet valve is approached by the use of a simple linear model with source strength and source impedance adjusted to match with data over limited frequency ranges. The car body transfer function is shown to be an important element in the design optimization process which is based on the reduction of driver's ear noise, rather than on snorkel noise. The design process is systematized by using a SIMPLEX optimization scheme in conjunction with a plane wave modeling code. The design procedure is shown to depend heavily on access to acoustic data from a prototype vehicle. This data is required to calibrate the noise source, to establish background noise levels which limit the benefits achievable by induction system optimization, and to provide a transfer function for the car body.