Hydrogen – fuelled internal combustion engines are becoming an increasingly promising pathway toward low – carbon mobility, particularly for applications where compatibility with existing engine platforms is necessary. Hydrogen offers fast flame speeds, wide flammability limits, and carbon-free combustion, making it an attractive alternative to conventional fuels. However, its distinct physical and chemical properties significantly influence mixture formation, ignition behaviour, and flame propagation, requiring a detailed understanding of the in-cylinder combustion process. This thesis investigates the in-cylinder combustion behaviour of a hydrogen port fuel injection (PFI) engine through three – dimensional Computational Fluid Dynamics (CFD) simulations. The primary objective is to deepen the understanding of the combustion process and to assess the capability of numerical models to reproduce experimentally observed in – cylinder quantities under representative engine operating conditions.