Determination of the acoustical impedance of an internal combustion engine exhaust

In this investigation, the acoustical impedance of an internal combustion engine is measured and simulated. The measurements are based on the two microphone transfer function method, which is described in the standard 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 the deviation between the microphone responses. The method is applied to measure the acoustical impedance of an engine. To prevent the engine to disturb the reflection coefficient during the measurements, the engine is run with an electric motor and the intake is sealed. The simulations are carried out using electrical analog circuits. The circuits are built by analyzing the geometrical data of the engine parts and the exhaust manifold contributing to the acoustical impedance. The simulation results correlate well with the measured impedances. Some special cases are simulated, to determine the engine components primary contributing to the acoustical impedance.