The paper presents a large eddy simulation investigation on the effect of fuel injection pressure on mixing, in an optical heavy-duty diesel engine. Recent investigation on impinging wall jets at constant-volume and quiescent conditions exhibited augmented air entrainment in wall jets with increasing injection pressure, when compared with a free jet. The increased mixing rates were explained as owing to enhanced turbulence and vortex formation in the jet-tip in the recirculation zone. A recent investigation carried out in an optical heavy-duty diesel engine indicated however a negligible effect of injection pressure on the mixing in the engine environment. The effect of enhanced turbulence and vortex formation of the jet-tip in the recirculation zone is believed weaker than the effect of engine confinement, due to the presence of fuel from adjacent jets limiting the mixing the fuel with the ambient gas. The aims of this paper are to investigate this issue and to look into more details about the nature of the mixing process in diesel engines. Two fuel injection pressures, 2000 bar and 2500 bar for a constant swirl level of 0.5 and inter-jet angles 45° and 135° were employed. The simulations were performed using OpenFoam, accounting for the mesh-motion and fuel spray treatment. The results exhibit the same trend found in the optical experiments. It was shown that increasing the mass flow rate by increasing injection velocity resulted in augmented turbulence levels in the recirculation zone; however, nearly injection pressure independent equivalence ratio was exhibited during quasi-stationary flow conditions.