Visualization of in-cylinder reaction processes and performance analysis of a direct-injection Diesel engine equipped with a high injection pressure (HIP) unit were conducted. The study was directed towards evaluation of high-power-density (HPD) engine design strategies, which utilize more intake air operating at rich overall fuel-air ratios.Two separate engine apparatus were used in this study: a Cummins 903 engine and a single-cylinder optical engine equipped with the same family engine components including the cylinder head. The engines were mated with an intensifier-type HIP fuel system fabricated at Rutgers which can deliver fuel injection pressure of over 200 MPa (30,000psi).The one-of-a-kind high-speed four-band infrared (IR) imaging system was used to obtain over fifteen hundred sets of spectral digital movies under varied engine design and operating conditions for the present analysis. In order to analyze and present this large amount of results, a new data analysis and presentation method was developed (called Rutgers Animation Program, RAP for short). RAP is a computer program permitting simultaneous display of as many as twenty-eight (28) sets of digital movies over a single PC screen for mutual comparison in a controlled manner.Among the findings from the study is that the in-cylinder imaging performed within the first several cycles may not represent what is happening in a typical warm engine. In addition, the spray development and subsequent reactions of the HIP unit may not be comparable to those of low-pressure conventional injector units. In-cylinder processes and engine performance of an HPD engine affected by various factors are also described in the paper which include: the injection pressure, intake air temperature, and overall air-fuel ratio.