Probably the strongest reason in-favor of ammonia fuel is its zero-carbon footprint. Ammonia is low temperature combustion delivering low thermal NOx. The present work provides a numerical study using two different combustion models i.e.; Finite Rate and Eddy Dissipation Concept (EDC). The first model, Finite Rate, considers very fast chemistry with no consideration of turbulence. The second model, EDC, connects turbulent mixing time scale with chemical reaction. The mixing time scale is the time needed for turbulent eddy to release its entire energy though, kinetic time scale is the time needed for reactants to reach equilibrium. Here, an ammonia/n-heptane fueled internal combustion engine is studied. The reaction mechanism includes 74 species and 495 reactions. The results are compared against state-of-the-art experiments. The study is conducted following RANS approach. The hybrid model kelvin-Helmholtz/Rayleigh-Taylor tackles heptane spray atomization. For the kinetic-controlled case, the reaction rate results in faster combustion. The effects of turbulent mixing impacts strongly the reaction rates progress. It matches the experiment better. The effects of turbulent mixing on kinetics need to be considered to correctly model the combustion.