An experimental study was carried out to visualize the spray and combustion inside an AVL single-cylinder research diesel engine converted for optical access. The injection system was a hydraulically-amplified electronically-controlled unit injector capable of high injection pressure up to 180 MPa and injection rate shaping. The injection characteristics were carefully characterized with injection rate meter and with spray visualization in high-pressure chamber. The intake air was supplied by a compressor and heated with a 40kW electrical heater to simulate turbocharged intake condition. In addition to injection and cylinder pressure measurements, the experiment used 16-mm high-speed movie photography to directly visualize the global structures of the sprays and ignition process.The results showed that optically accessible engines provide very useful information for studying the diesel combustion conditions, which also provided a very critical test for diesel combustion models. The high IMEP conditions were successfully achieved in the optical engine experiments, which show sensitivity to injection pressure, charge condition, and combustion chamber geometry. Sprays and combustion visualization for both single and multi-stage injections was conducted in this study. Nozzle hole flow area, injection pressure, and length of rate-shaping pipe are the dominating parameters that control the injection rate, and the rate shape is affected mainly by common rail pressure, especially the pressure rising rate, and length of rate-shaping pipe.