Reduced emissions and increased efficiencies of natural gas pipeline compressor engines are becoming increasingly important. Response to these needs requires detailed understanding of in-cylinder phenomena. The large bore optically accessible engine at Colorado State University was developed for this purpose. That is, to facilitate the study of in-cylinder phenomena in large bore natural gas engines in support of the development of retrofit technologies. This work is comprised of two separate investigations: (1) the study of mixing in a 4-stroke KVS-DT configuration optical engine with high pressure fuel injection and (2) visualization of combustion in a 2-stroke cycle optical engine with GMV cylinder geometry. Necessary optical engine modifications to accommodate 4-stroke operation and combustion are described. Planar laser induced fluorescence is utilized to image mixing for the first investigation; optimum timing for high pressure fuel injection in 4-stroke large bore engines is evaluated. To study combustion, direct imaging of ignition and flame propagation is carried out with a color Kodak Motion-Quarter Ultra high speed video camera.