The dual-fuel engine represents in principle a simple flexible approach to employing gaseous fuels in conventional diesel engines. Compared to the use of hydrogen in spark ignition engines, there is relatively limited information about its effect when present as a supplementary fuel in suitably modified conventional compression ignition engines. This is especially for engines of the IDI type and when employing only low concentrations of hydrogen in the intake air while retaining the injection of large diesel fuel quantities.In the present contribution, a 3D-CFD model based on KIVA 3, developed with a “reduced” detailed chemical kinetics of 294 elementary reaction steps with 79 chemical species for diesel fuel combustion which includes 20 steps for the oxidation of hydrogen, is outlined. The model which accounts for turbulence during combustion in engines with swirl pre-chambers was suitably verified experimentally and then employed to examine the combustion characteristics of a diesel engine with the addition of relatively small concentrations of hydrogen. The model was employed to investigate the effects of changes in a number of relatively not so easy to investigate and control experimentally parameters on the combustion processes and NOx emissions when adding low concentrations of hydrogen to the intake of an IDI diesel engine. The results showed that changes in the chamber surface temperatures, liquid fuel spray injection characteristics, and the presence of inert gas and chemically active intermediate products in the intake charge can have significant effects on engine combustion, performance and emissions.