Computational assessment of combustion noise of automotive compression-ignited engines

Resumen

The ever-increasing demands of industry are changing the way we understand society and the environment in which we live. In the face of the need for rapid and globalised trade, a number of sustainability issues are emerging which, on the one hand, encourage sectors such as transport to radically increase their activities, but, on the other hand, cause a negative impact on terrestrial ecosystems. In this context, the negative effects of environmental and noise pollution are reaching really worrying limits, these being especially visible in the main urban areas where the authorities are even restricting the circulation of vehicles powered with thermal engines. In particular, the noise produced by the fuel burning in vehicles powered by reciprocating internal combustion engines, being one of the main acoustic sources ahead of others such as aviation or railways, is being the focus of recent studies to reduce its harmful effects on the population. The main objective of this thesis focuses on the study and characterization of combustion as a source of noise emissions. Specifically, this research focuses on addressing the physical phenomena associated with noise generation in compression-ignited engines, as well as proposing some guidelines in order to better understand and improve -from the point of view of noise emissions and fuel consumption- the design of current engines. In a first approach, experimental techniques are used to characterise the source of the acoustic disturbances by recording the instantaneous pressure inside the combustion chamber. Although the information provided by these methods is relevant, there are some limitations to recreate the spatiality of the acoustic field and, therefore, make it difficult to understand the non-stationary phenomena associated with it. For this reason, in subsequent studies the Computational Fluid Dynamics or CFD approach is utilized, thereby overcoming the limitations of experimental techniques and allowing a complete visualization of the problem. As a preliminary and indispensable step, we proceed to implement and validate the CFD model to ensure a good accuracy in the results and a reasonable calculation time. The application of frequency analysis and modal decomposition methods has made it possible to study the pressure field inside the chamber and thus better understand its behaviour. In this way, it has been possible to find relationships between the combustion and the spectral response of the internal acoustic field. The pressure oscillation patterns show that the most energetic structures, and thus contributing the most to the acoustic emission, are centred on macroscopic structures of similar size to the chamber geometry. In addition, the ignition position of the fuel has been shown to have a direct effect on the amplitude of the resonant modes and their spatial distribution. Finally, regarding the evaluation of different strategies for mitigating noise, different studies are proposed in which the trends in noise emission are analysed by modifying the sound source through the injection configuration and the geometry of the combustion system.