Two-dimensional laser-sheet imaging and high-speed cinematography have been used to examine the combustion process in a newly constructed, optically accessible, direct-injection Diesel engine of the “heavy-duty” size class. The design of this engine preserves the intake port geometry and basic dimensions of a Cummins N-series production engine. It also includes several unique features to provide considerable optical access. An extended piston with piston-crown window and a window in the cylinder head allow the processes in the combustion bowl and squish region to be observed simultaneously. Windows at the top of the cylinder wall provide orthogonal-optical access with the capability of allowing the laser sheet to enter the cylinder along the axis of the spray. Finally, this new engine incorporates a unique separating cylinder liner that permits rapid cleaning of the windows.Studies were performed at a medium speed (1200 rpm) using a Cummins closed-nozzle fuel injector. High-speed cinematography was used to visualize the combustion process for two combustion bowl geometries and two fueling rates. These photographs show the onset of luminous (sooting) combustion and the temporal evolution of the combusting fuel jet as it interacts with the wall of the combustion bowl and penetrates into the squish region. Luminous combustion was seen in the squish region for all conditions photographed, but the amount varied significantly with fuel load and combustion-bowl geometry. Two-dimensional images of the soot distribution were obtained using a combination of laser-induced incandescence (LII) and elastic (Mie) scattering. These images show that soot is distributed throughout the cross section of the fuel jet with the closed-nozzle injector used in this study. They also provide information about the relative soot concentrations and particle size distributions.