Towards a Predictive CFD Approach for Assessing Noise in Diesel Compression Ignition Engines.

Abstract

Combustion noise has become an aspect of great importance for engine manufacturers due to its impact on the customers' comfort and health. Furthermore, the increasingly important contribution of turbocharger noise in downsized engines aggravates this problem due to the increment of the compression ratios required. Limitations on the experimental techniques have led to develop numerical methodologies for analyzing and understanding both noise sources and their effects on the acoustic field. However, the complex phenomena associated to the noise generation compromise the accuracy of these approaches, and represent a challenge in the field of the Computational Fluid Dynamics (CFD) modelling. This paper presents a CFD methodology for assessing Diesel combustion as a noise source in compression ignition engines. The model is validated by simulating a steady operation condition at medium speed and medium load, and positive results are obtained in both temporal and frequency domains. Moreover, it allows predicting the main combustion related parameters and the metrics associated to the external acoustic field as well. The simulation results shed some light on the most dominant processes in the noise generation and its propagation inside the combustion chamber. The paper also includes a sensitivity study of the main operation settings (split between pilot injections, exhaust gas recirculation, intake and injection pressure) obtained by using a Design of Experiments (DoE) approach. In addition to determining the most convenient operation strategy, this technique is extremely useful to establish cause/effect relationships between the inputs (combustion settings) and outputs (noise source). Results show that by increasing the split between the two pilots and decreasing the gap between the second pilot and the main injection, it is possible to reduce the noise emissions, while improving the indicated efficiency. Additionally, the increase of the intake pressure improves the traditional trade-off trend between combustion noise and efficiency.