In the study, the influence of nozzle hole geometry, rail pressure and injection timing on combustion was analyzed in an optical access single-cylinder diesel engine equipped with a high pressure common-rail injection system. The K factor represents the taper ratio of the nozzle hole of the injector. The injection rate showed that tapered nozzle hole with bigger K factor result in a higher injection speed. A tapered nozzle produced some favorable results; a greater heat release and decreased THC (Total Hydrocarbon) and PM (Particulate Matter), despite an increase in NOx (Nitrogen Oxides) level.