An optically accessible single-cylinder high-speed direct-injection (HSDI) diesel engine equipped with a Bosch common rail injection system was used to study effects of injection pressures on the in-cylinder spray and combustion processes. An injector with an injection angle of 70 degrees and European low sulfur diesel fuel (cetane number 54) were used in the work. The operating load was 2.0 bar IMEP with no EGR added in the intake. The in-cylinder pressure was measured and the heat release rate was calculated. High-speed Mie-scattering technique was employed to visualize the liquid distribution and evolution. High-speed combustion video was also captured for all the studied cases using the same frame rate. NOx emissions were measured in the exhaust pipe. The experimental results indicated that for all of the conditions the heat release rate was dominated by a premixed combustion pattern. Two-stage low temperature reaction was seen for early injection timings. For early pre-TDC injection strategies, liquid fuel impingement is observed that results in fuel film combustion flame (pool fires) following an HCCI-like, low luminosity combustion flame. Increasing injection pressure reduces soot luminosity implying less soot generation in the combustion processes. Interesting results were seen for NOx emissions. For very early pre-TDC injection timings, NOx emissions are near zero. When the injection timing was retarded, NOx emissions first increase, and then decrease. For post-TDC injection timings, the NOx emissions are quite low. For all the injection timings, increasing injection pressure leads to higher NOx emissions due to fast combustion process resulted from better mixing process. In terms of the combined effects of fuel consumption, soot luminosity, and NOx emissions, retarded post-TDC injection timings results in better overall performance.