Probably the strongest reason in favor of hydrogen, compared to fossil fuels energy sources, is its zero-carbon footprint. This paper explores the impact of hydrogen-substitution on ammonia/heptane combustion. Here, heptane represents diesel. The onset of hydrogen and ammonia come through intake valve. Heptane is injected, afterwards, when the piston reaches the top dead center during the compression stroke. Two simulations, Ammonia/heptane and ammonia-hydrogen/heptane, are performed. Ammonia/heptane combustion is used as a reference to evaluate the impact of hydrogen. The first simulation is 40% ammonia and 60% heptane. In the second simulation, 15% of total energy substitutes the 60% of heptane, where ammonia energy kept similar. Hydrogen is joined to tackle ammonia difficulties e.g.; poor ignition quality, low flame speed, high latent heat of vaporization. The combustion has evolved partially premixed. The study is performed through RANS using the RNG K-epsilon model. The finding results report that both combustion phasing and combustion duration are improved. Hydrogen, with 15% of total energy, does not impact ignition.