The more important results are presented from a 2-year research program involving, it is believed, the first high-speed photographic records of events occurring within a very large engine cylinder. This research was performed on a co-operative basis by The Cooper-Bessemer Corporation and Battelle Memorial Institute. Involved in this study was a Cooper-Bessemer GMV engine, a two-cycle, spark-ignited engine of 14-in. bore and 14-in. stroke, which operates on natural gas. This engine is widely used for pumping natural gas through cross-country pipe lines. The photographs were made with the Battelle Isotran camera. Schlieren photographs were taken of combustion, exhaust blowdown, scavenging, gas turbulence, and fuel injection, through large glass windows mounted at various positions in the cylinder head and cylinder wall. All of these phenomena were rendered clearly visible in slow-motion pictures, and exact data were secured for all gas and flame velocities, as well as the crank angles involved in these events. The most important indication from the photographs is the basic nature of flow of scavenging air through the cylinder volume in turbulent jets, which apparently cannot be circumvented.