A Simple Approach for the Estimation of the Exhaust Noise Source at the Valves
To be published on September 9, 2019 by SAE International in United States
Exhaust noise emission is the result of the propagation of pressure perturbations along the exhaust line. Such perturbations are primarily originated by the discharge of hot, high-speed gases through the exhaust valves. These gases do not simply displace the gases present in the exhaust port but compress them, giving rise to the perturbation mentioned above. Therefore, any attempt at the prediction of exhaust noise is based on the knowledge of the instantaneous mass flow rate across the exhaust valves. However, this magnitude is not readily accessible to measurements, and it is thus imperative to use predictive models. It is apparent that, while information on the instantaneous mass flow through the exhaust valves may be obtained from well-validated commercial gas-dynamic codes, the data required is not always available or fully defined at the time of starting the design of an exhaust line. It is therefore desirable to be able to estimate the instantaneous mass flow passing through the valve starting from 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. In this paper, a model is described that provides such an estimation. The model 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.