Gaseous fuels, such as natural gas or blends of methane and hydrogen, have emerged as a viable alternative to conventional gasoline and diesel fuels in internal combustion engines, as they have the potential to reduce CO2, NOx, unburnt hydrocarbon and soot emissions. Diffusion controlled combustion of gas can be readily adapted from standard diesel technologies and direct injection systems avoid mixture preparation difficulties associated with low volumetric efficiency. However, high injection pressures must be employed to overcome typical end-of-compression cylinder pressure levels and high pressure ratios must be applied to counteract the lower density of the gaseous fuel. As a result of the high pressures, real-gas effects become important, whereas as a result of the high pressure ratios, the nozzles are usually operated at under-expanded conditions. The present study investigates numerically the effect of geometric variations and back-pressure levels on the injection of CH4, H2 and N2 into air at rest at moderately supercritical pressure ratios. Nozzle flow, mass flow rate and mixture formation in the main chamber are examined by means of detailed URANS computations that account for real gas effects via appropriate treatment for the equation of state, the caloric and transport properties as well as the mixing rules. Furthermore, as the detailed simulations are computationally expensive, an approach is advanced to develop inlet boundary conditions and model real-gas effects that accurately reproduce the mixing field and can be employed at typical spatial and temporal resolution of engine simulations without considerable cost in turn-around time and complexity.