The topic of this paper is the analytical and computational modeling of the gas injection process in a large bore natural gas fueled engine. At high injection pressures, the overall gas injection and mixing process includes compressible flow features such as rarefaction waves and shock formation. The injection geometries examined in the paper include both a two dimensional slot and an axisymmetric nozzle. The computations examine the effect of the injection pressure/back pressure ratio, with ratios ranging from 3 to 80. The computational modeling was validated by comparison with results obtained from a 2D analytical method of characteristics solution. The validation process evaluated factors such as pressure and Mach number profiles, jet boundary shape and shock location.