In this paper, the unsteady behaviour of a compression-ignited (CI) engine combustion chamber is studied by analysing the results of a Computational Fluid Dynamics (CFD) model through the application of different flow decomposition techniques, aiming to resolve the underlying modal structure of the process. Experimental validation for the combustion simulation is provided, and a methodology for extracting coherent pressure information is proposed in order to provide a suitable input for different analysis methods. These range from straightforward Fourier transform techniques to more sophisticated modal decomposition approaches. In particular Proper Orthogonal Decomposition (POD) is shown to provide valuable insight into the time-spatial structure of the combustion flow field, allowing the establishment of correlations between pressure modes and physical parameters of the combustion, such as the injection timing or the chamber geometry. Dynamic Mode Decomposition (DMD) on the other hand is proven to successfully highlight the link between the frequency of the unsteady energy components and their spatial distribution within the chamber. Advantage is then taken of the modal characterization of the unsteady behaviour in the chamber to showcase how physical parameters such as the spray angle can be modified to optimize the acoustic signature of the combustion process, helping CI internal combustion engines reduce their acoustic environmental impact (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.