The objective of this research is the determination of the acoustical impedance of an internal combustion engine exhaust. The impedance will be modeled in an electrical equivalent circuit. This circuit can then be used in conjunction with the electrical equivalent circuit for an exhaust system to predict the exhaust noise at the tailpipe opening.

The used measurement method is based on the two microphone method described in the ISO/FDIS 10543-2. The method uses the transfer function between two microphones positioned on a waveguide connected to the impedance to be measured. Some improvements are proposed to this procedure, concerning the calibration of the measurement setup. New corrections are proposed to eliminate the speed of sound, the microphone locations and the deviation between the microphone responses.

The electrical equivalent circuit is built by analyzing the engine parts contributing to the acoustical impedance. The electrical analog components are determined using the geometrical data of the engine and the exhaust manifold. The simulation results correlate well with the measured impedance. Some special cases are simulated, to determine the engine components, which contribute primarily to the acoustical impedance.

The experiments were limited to measure the output impedance of the engine. More generally, in the future, a measurement setup to determine the four impedance parameters, i.e. input impedance, output impedance, the transfer between input and output and vice versa, will be developed. These measurements will be used to generate T- or PI-equivalent electrical circuits.

• prof. dr. ir. Paul Sas
• dr. ir. Rene Boonen