Direct-injected (DI) Compressed Natural Gas (CNG) engines are emerging as a promising technology for highly efficient and low-emission future engines. Because of high knock resistance, it is possible to operate CNG engines at higher compression ratio and hence gain higher thermal efficiency similar to diesel engines. Direct gas injection provides an additional boost to volumetric efficiency reducing throttling losses as compared to port-injected gas engines. Additionally, in contrast to diesel fuel, CNG burns without soot emissions due to high hydrogen to carbon ratio, and is available at low cost. Despite such obvious advantages, DI CNG engines have not been extensively used so far, partly due to design problems. More specifically, the design of direct injection systems for a compressible gas is challenging due to high Mach number flows and the occurrence of shocks. An outwardly opening poppet valve design is widely used for CNG DI. The opening of the valve and the resulting gas flow through such valves is driven by the upstream injection pressure. The formation of a hollow cone gas jet resulting from this configuration, its subsequent collapse and its mixing is challenging to characterize using experimental methods. Therefore, numerical simulations can be helpful to understand the process, and later to develop models for full engine simulations. In this paper, the results of high-fidelity Large-Eddy Simulation (LES) of a stand-alone injector are discussed to better understand the evolution of the hollow cone gas jet. The hollow cone gas jet is characterized in terms of several parameters such as axial and radial jet penetration, jet area, and mixing in terms of cumulative area fraction against mass fraction of injected gas. Different grid resolutions have been used to study the effect on axial and radial jet penetration, as well as mixing. The temporal evolution of the axial and radial jet penetration is consistent with previously published experimental data for a similar hollow cone injector. Simulations overpredict the axial jet penetration because of initial non-linear behaviour of the jet evolution. We investigate this using less accurate but computationally cheap Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations with and without valve motion. It is found that the valve opening has a profound impact on initial stages of the gas jet formation and is responsible for the linear jet evolution observed in experiments. In addition, we investigate the applicability of different turbulence models on gas jet formation and mixing in this case.