A Simple Approach for the Estimation of the Exhaust Noise Source at the Valves

Exhaust noise emission is the result of the propagation of pressure perturbations along the exhaust line, whose primary source is the instantaneous mass flow rate across the exhaust valves. In this paper, a model for the estimation of this magnitude is presented, which has two main objectives: the first one is to provide a representation of the engine as an exhaust noise source as independent as possible on the exhaust system; the second one to allow for the estimation of the exhaust mass flow in such cases where the full set of data required by a conventional gas-dynamic simulation is not available. The model presented uses a reduced set of geometrical and operation data, which can be either representative for a given engine family, or even target values for an engine still not fully defined. It is based on the estimation of in-cylinder variables at exhaust opening by means of a First-Law approach to the closed cycled, starting from rather general data on the energy balance of the engine. Then, conventional gas-dynamic equations are solved, but with the assumption of an anechoic termination downstream of the exhaust valve, so that the results are rendered independent of the particular exhaust geometry. The results of the model are validated against those of a conventional gas-dynamic code, showing that any differences lie within acceptable limits if the purpose is the assessment of the exhaust noise source.