Investigations of the fuel injection process for large bore natural gas engines are performed on an off-engine test fixture. Two types of fuel injectors are studied, a low pressure (0.39 MPa) and a high pressure (3.5 MPa) gas injection valve. Planar laser induced fluorescence is implemented to image fuel jet concentration. The fuel jet from each injector is imaged from the start of injection though their valve-open duration. Effects of injection pressure, piston interaction, and injection event repeatability are investigated. The results are related to previous engine studies of high pressure fuel injection and Schlieren photography of in-cylinder events on a Cooper-Bessemer GMV large bore natural gas engines. The images indicate that the low pressure valve achieves more effective mixing during jet penetration. The high pressure injection event shows relatively little mixing during jet penetration. However, the high pressure jet has much more energy when it impinges on the piston. It is evident that the important time for mixing using high pressure injection occurs after piston impingement.