ABSTRACT 

The combustion process is one of main sources of noise in Diesel engines, due to the sudden increase of pressure caused by auto-ignition in the combus­tion chamber. This fast increase starts an oscillation process of burnt gases in the combustion chamber, known as resonance, which is transmitted through the block and affects negatively the engine acoustic quality.. 
Several experimental procedures exist, directed at studying the noise caused by the Diesel engines combustion process. However, their application in combustion chamber design is not ideal, because of the cost related to prototypes realization, assembly and experimental testing.. 
In comparison to experimental methods, theoretical methods are faster and provide large amounts of spatial and temporal data about resonance. The modal theory and the CFD (Computational Fluid Dynamics) methods are mainly used. The first method provides an estimation of the propagation modes, but its use is limited to cylindrical combustion chamber geometry. The CFD method can be used for any combustion chamber geometry; however, the prediction of local pressure effects, with the combustion models available nowadays is not coherent with auto-ignition oscillation levels. 
This thesis contributes to the development of a methodology to improve the predictions obtained by theoretical-numerical methods, exceeding their geometrical and predictive limitations. This methodology consists in simulating thermodynamic auto-ignition conditions, using pressure and temperature sources at a temporal instant, without considering the reactive process of fuel­air mixture. 
In the first part of this thesis, in order to simplify the simulations, it is assumed that the combustion process occurs at TDC, and the piston motion is not considered. Based on this assumption, the resonance sensitivity to changes in bowl geometry, air velocity and temperature, and auto-ignition conditions is studied. Although these calculations provide information about which engine operation parameters have significant influence on resonance, this methodology is limited, because it does not consider the temporal variation of pressure caused by the piston motion. Moreover, the auto-ignition simulation process at an instant is not coherent with the physics of this process and results in reduced levels of oscillations in a very limited temporal frame. 
For this reason, in the second part of this thesis, relevant modifications are integrated to the methodology, in order to solve these limitations. 
On the one hand, the piston motion is simulated by using a cells activation-deactivation model implemented in the CFD program. This model allows calculating the compression and expansion processes, and thus the effects of temporal pressure variations, caused by the piston motion. 
On the other hand, the thermodynamic auto-ignition simulation is improved, by the automatic activation and deactivation of temporal sources that are introduced in the energy and species equations during the calculation. A program was developed specifically for this purpose, and integrated in the CFD code. These source terms are obtained from the in-cylinder pressure analysis by a thermodynamic combustion model and a special processing program developed within this thesis. 
This improved method allows obtaining pressure propagation levels and temperature levels inside the cylinder consistent with the physics of the problem, validating the method with experimental measurements and studying qualitatively and quantitatively the influence of injection, load, regime and geometrical parameters of the combustion chamber on the modes of resonance and engine noise. 
Finally, the methodology developed within this thesis has allowed studying the resonance phenomenon and the effects of various engine operation parameters on its acoustic quality. Moreover, it provides a basis for future research on auto-ignition simulation, with the objective to improve current combustion models, particularly the prediction of local pressure effects , which are the cause of resonance and contribute to deteriorating the acoustic quality of Diesel engines.