This thesis presents a detailed three-dimensional Computational Fluid Dy- namics (CFD) analysis of a diesel engine optimized for hybrid powertrain applications. The main objective of the study is to investigate the influence of combustion chamber geometry on in-cylinder air motion, mixture formation, and combustion processes. Combustion efficiency and pollutant formation were evaluated to gain a deeper understanding of the effects of chamber design on engine performance. The case study focuses on a 2.2-liter, four-cylinder, direct-injection diesel engine intended for Light Duty Vehicle (LDV) applications, developed in collaboration with Stellantis. The simulations were carried out using the STAR-CCM+ software equipped with the In-Cylinder add-on.