This work presents a method for simultaneously capturing visible and infrared images along with pressure data in an optical Diesel engine based on the International 4.5L VT275 engine. This paper seeks to illustrate the merits of each imaging technique for visualizing both in-cylinder fuel spray and combustion. The engine was operated under a part load, high simulated exhaust gas recirculation operating condition.Experiments examining fuel spray were conducted in nitrogen. Overlays of simultaneously acquired infrared and visible images are presented to illustrate the differences in imaging between the two techniques. It is seen that the infrared images spatially describe the fuel spray, especially fuel vapors, and the fuel mixing process better than the high-speed visible images.Combustion experiments were completed with the goal of demonstrating how infrared and high-speed visible simultaneous imaging techniques can assist in interpretation of the fuel-air mixing and combustion processes. The images produced from this experiment are overlaid in order to illustrate the differences in the techniques when applied to Diesel combustion. Combustion is seen to start at the edge of the combustion bowl at numerous individual points. In-cylinder temperatures are estimated from infrared data. The pressure data of the minimum and maximum indicated mean effective pressure cycles are examined along with the visible high-speed images for these cycles.The simultaneous data acquisition techniques presented here can achieve data which are otherwise nearly impossible to acquire. The infrared pictures spatially show the location of evaporated fuel and the start of combustion not observed in the visible, high-speed frames. However, the visible, high-speed frames provide details relevant to the evolution of the combustion events within an individual cycle. Pressure data is coupled with individual cycles for further analysis. Simultaneously recording the images allows for comparisons of the different imaging techniques while removing inaccuracies that would occur for images acquired under conditions related to different engine cycles.