This paper describes a preliminary characterization of in-cylinder spray and combustion behavior from a high-pressure common rail injection system. The engine used in the tests was a single-cylinder optical research diesel engine, adequately developed in a full-fired version, equipped with a common rail injection system. An elongated piston allows for the optical access to the combustion chamber for diagnostic applications.Characteristic of the optical engine is the availability to investigate different combustion system designs due to an interchangeable head-cylinder group. The system configuration tested in the present work corresponds to a four-cylinder engine of 1930 cc of displacement that is representative in the class of light duty d.i. diesel engine.Spray and combustion evolutions were visualized through a high-speed CCD camera synchronized with a copper vapor laser acting as light source. Tests were performed at fixed engine speed and injected fuel mass (1500 rpm and 12.4 mg/stroke) but varying injector configuration (holes number and size).As expected, pilot activation strongly affects spray and combustion evolution of subsequent main injection. Pilot seems to be characterized by small penetration and quick evaporation, burning almost completely before the main injection occurs.Spray distribution of main injection appears to be influenced by pilot activation and injector configuration. In particular, the change of the nozzle configuration results in different spray patterns.Due to a marked influence of pilot injection on whole combustion process, pilot parameter setting values should be chosen adequately in view of engine emission optimization.