A New Hybrid Approach for the Thermo-Acoustic Modelling of Engine Exhaust Systems

The time-domain method of characteristics and the frequency-domain linear acoustics method are the two different methods of thermo-acoustic modelling of engine exhaust systems in order to predict exhaust noise. To overcome the disadvantages of both the time-domain and the frequency-domain approaches, a hybrid approach has been developed which couples the acoustical description of the muffler piping system to the acoustic source more realistically than the usual time invariant linear model. A time domain model and the corresponding frequency domain model of a linear dynamical system are interrelated by the Fourier transform pair. In the present work, the cylinder/cavity is analysed in the time domain to calculate exhaust mass flux history at the exhaust valve by means of the method of characteristics, solving a number of equations simultaneously at the valve junction. This analysis has been done by making use of an interrelationship between progressive wave variables of linear acoustic theory and those of the method of characteristics. In this approach, nonlinear propagation in the exhaust pipe is neglected and radiation impedance at the end of the exhaust tail pipe is duly taken into account. Apart from this, actual reflection of the forward wave due to the presence of muffler is incorporated to make the analysis more realistic. Damping effects present in the fluid have also been taken into account. Computational results have been corroborated by experimental data for a single-cylinder, four-stroke cycle diesel engine