Injection pressure and hydraulic nozzle flow rate are significant parameters that influence mixture formation and combustion in the direct injection (DI) diesel combustion process. An injection pressure increase enhances mixture formation as well as combustion. Decreasing the hydraulic flow rate, by means of reducing spray hole orifice diameter, affects mixture formation primarily through improved gas entrainment processes. Both injection pressure increase and hydraulic flow rate decrease therefore benefit mixture formation and combustion, which cause less soot emissions. Thus, the application of higher exhaust gas recirculation rates to decrease nitrogen oxide emissions is possible. The objective of this paper is to determine the effects of hydraulic flow rate variation in combination with very high injection pressures.Engine tests are carried out using a single-cylinder research engine, which is based on a light-duty DI diesel engine. With a displacement of 755 cm 3 per cylinder, the engines range between typical passenger car (' 500 cm 3 ) and medium-sized heavy-duty (' 1000 cm 3 ) engines. The engine is equipped with a prototype common rail fuel injection system that is capable of delivering system pressures larger than 250 MPa. Four different hydraulic nozzle layouts are tested. The effects are presented for one part load and one full load operating condition. For each engine operating point, exhaust gas recirculation variations with different injection pressure levels and boost conditions are conducted. Evaluation of engine test results is performed at a constant soot/nitrogen oxide emission ratio of 1:10, which is relevant in engine calibration. Hydraulic measurements and the application of an empirical spray model support the interpretation of the engine test results.It is observed that a reduced hydraulic flow rate is beneficial for part load emission performance. It was found that a lower hydraulic flow rate causes a higher mean relative air/fuel ratio within the fuel spray and thus induces less soot formation. Moreover, the premixed fraction of combustion is enhanced through both injection pressure and hydraulic flow rate increases. In full load operation, the beneficial effect of reduced flow rates by means of smaller nozzle orifices cannot be detected. Parameters such as injection duration, combustion duration and spray momentum during spray/wall interaction are identified as more important. Boost pressure variation has only a minor effect on part load, whereas full load results are greatly influenced. In part load and full load operation, a limitation of the potential to reduce emissions is not observed within the investigated injection pressure range.