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The combustion process of a homogeneous hydrogen charge in a small-bore compression ignition engine with diesel-pilot ignition was simulated using the CONVERGE computational fluid dynamics code. Analysis of the simulation results aimed to understand the processes leading to NOx production and heat loss in this combustion strategy, and their dependence on the hydrogen fuel energy fraction. Previous experimental results demonstrated promising performance, but this comes with a penalty in increased NOx emissions and potentially higher heat losses. The present study aims to enhance understanding of the mechanisms governing these phenomena. The simulated engine was initialised with a lean homogeneous hydrogen-air mixture at BDC and n-dodecane was injected as a diesel surrogate fuel near TDC. The simulations were validated based on experimental results for up to 50% hydrogen energy fraction, followed by an exploratory study with variation of the energy fraction from 0% to 90%.