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In this paper, a numerical model is used to study the influence of several relevant parameters on the behaviour of a turbocharged Diesel engine as an exhaust noise source, with two main objectives: first, determine if it is possible to reduce exhaust noise at the source itself, thus simplifying the task of exhaust system design; and secondly, to asses up to which extent simple linear source models may be used to predict exhaust noise in these engines. The results obtained indicate that, on the one hand, exhaust noise is sensitive to the variation of certain engine design parameters and, on the other hand, that for certain running conditions simple source models may give an acceptable estimation of the actual engine behaviour as a noise source.

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.

The use of computer calculation tools in order to reduce the cost of the development of optimized exhaust systems has turned out to be a generalized industrial practice. Therefore, considerable efforts are devoted to the development of suitable calculation tools, which are representative of the real phenomena taking place in the exhaust systems. In the present paper, the results of the application of a hybrid linear/nonlinear calculation method to the prediction of the exhaust noise radiated by I.C. engines are presented. First, a brief description of the method is given. Then, comparison is shown between the results of the calculation and experimental measurements, both for in-duct pressure and for noise radiated. The agreement obtained indicates that this method may be used as a design tool in the frame of the new methodologies presently arising in exhaust system development.

The influence of manifold geometry on exhaust noise is studied. First, a linear description of the problem is presented, so that potential relevant factors may be identified. Then a full non-linear simulation is performed, for a simple geometry, in order to check, in more realistic conditions, the ideas obtained from the linear theory. The results indicate that, although some qualitative trends may be obtained from the linear analysis, the role of back-reaction of the manifold on the engine (a non-linear coupling effect) may be determinant.

One of the main problems in the design of exhaust silencers is the attenuation of low frequency noise. At these frequencies is where the influence of the engine has more importance; moreover, low frequency noise has the possibility of interaction with the mechanical resonances of the exhaust line, producing additional noise and vibration highly disturbing. A suitable solution to this problem is the use of the interferencial behaviour between two acoustic parallel paths, which produces high attenuation at a given frequency associated with the difference between the acoustic lengths of both paths. In the present paper, a general expression for the 4-pole transfer matrix of an interferencial system with two arbitrary branches is presented, which is applied to a simple but realistic exhaust silencer. Results are compared with the transmission loss measured with a modified impulse method, with good agreement between the model and the measurements.

Traditionally, combustion noise in Diesel engines has been quantified by means of a global noise level determined in many cases through the estimation of the attenuation curve of the block using the traditional discrete Fourier transform technique. In this work, the wavelet transform is used to establish a more reliable correlation between in-cylinder pressure (sources) and noise (effect) during the combustion of a new generation 2 liter DI Diesel engine. Then, in a qualitative sense, the contribution of each source intrinsic to the combustion process is determined for four engine operating conditions and two injection laws. The results have shown high variations in both the in-cylinder pressure and noise power harmonics along the time, which indicates the non-stationary character of this process.