Engine block dynamic response via combustion noise radiation

Besides pollutant emissions, fuel consumption and performance, vehicle NVH constitutes a further object during engine development and optimization. In recent years, research activity for diesel engine noise reduction has been devoted to investigate aerodynamic noise due to intake and exhaust systems and surface radiated noise. Most of the attention has been concerned with the identification and analysis of noise sources in order to evaluate the individual contribution (injection, combustion, piston slap, turbocharger, oil pump, valves) to the overall noise with the aim of selecting appropriate control strategies. Several studies have been devoted to analyze combustion process that has a direct influence on engine noise emission; the impact of injection strategies on the combustion noise has been evaluated and approaches able to separate engine combustion and mechanical noise components have been presented. This work fits in a research program that aims at analyzing the engine noise emission for both the combustion diagnosis via non-intrusive sensors and the definition of potential strategies for combustion related noise emission improvement. Previous works have been devoted to analyze the entire noise generation process of a diesel engine in order to identify the contribution of the different sources to the overall emission and to investigate the correlation between noise radiation and in-cylinder pressure measurement aimed at assessing the in-cylinder pressure development during the combustion process via its related emission. Therefore, during such activity, the attention was focused on the excitation forces and on their effect, the radiated noise. This work has been devoted to deeper insight into the structural response of the engine, that represents the transfer function between in-cylinder pressure and radiated noise. A small displacement common-rail diesel engine has been fully instrumented and its noise radiation has been investigated by analyzing the signals of microphones placed in selected locations around the engine. A method in the frequency domain is presented for evaluating the engine structural response function; the engine is considered as a black box in which the input is the in-cylinder pressure and the output is the noise radiation. Once the dynamic response of the engine is estimated, combustion development can be characterized via the engine noise radiation, thus allowing to get indication about possible strategies for noise emission improvement, that could be obtained through structural modification and/or combustion process control by injection strategy variation.