Assessment and experimental validation of a 3D acoustic model of a motorcycle muffler

The intake and exhaust lines provide the main abatement of the acoustic emissions of an Internal Combustion Engine (ICE). Many different numerical approaches can be used to evaluate the acoustic attenuation, which is commonly expressed by the Transmission Loss. One-dimensional (1D) and three-dimensional (3D) simulations are conventionally carried out only considering the acoustic domain of the muffler or of the air-box. The walls of the acoustic filter are considered fully rigid and the interaction between the acoustic waves and the structure is consequently negligible. Moreover, the effect of the manufacturing characteristics and the attenuation of the acoustic waves due to the fluid viscous-thermal effects are also commonly disregarded in the numerical analysis of the filters. In addition, the presence of a catalytic converter or a filter cartridge may have an influence on the numerical results. All these aspects, however, can remarkably affect the matching between simulations and experiments both at high frequencies and at medium-low ones. In the present study, the effect of the aforementioned issues on the prediction capabilities of a 3D FEM model are analysed, with particular attention to the prediction of the acoustic attenuation of a commercial muffler. In detail, simplified models were developed in order to include the manufacturing features into acoustic simulations with almost negligible increase of the computational time. Numerical models were applied to a commercial motorcycle muffler featuring three chambers and a catalytic converter, and then validated by means of a specific experimental campaign that was carried out on an acoustic test rig. The tests were run at ambient temperature and no-flow conditions, obtaining good agreement between theory and experiments.