The present paper describes an experimental investigation over the impact of diesel injector nozzle flow number on spray formation and combustion evolution for a modern EURO5 light-duty diesel engine. The analysis has been carried out by coupling the investigations in non evaporative spray bomb to tests in optical single cylinder engine in firing conditions.The research activity, which is the result of a collaborative project between Istituto Motori Napoli - CNR and GM Powertrain Europe, is devoted to understanding the basic operating behaviour of low flow number nozzles which are showing promising improvements in diesel engine behaviour at partial load. In fact, because of the compelling need to push further emission, efficiency, combustion noise and power density capabilities of the last-generation diesel engines, the combination of high injection pressure fuel pumps and low flow number nozzles is general trend among major OEMs. Therefore, aim of this paper is to provide a deeper understanding about the link between the nozzle flow number, the spray and mixture formation and the consequent combustion behaviour for different nozzle geometries and engine operating conditions typical of diesel engine operating. Main deliverables of the above mentioned activity will be guidelines for a balanced nozzle flow number selection and references for upgrading 3D-CFD simulations spray models.Spatial and temporal spray characterizations have been evaluated for three different nozzle flow numbers for a 2.0L diesel engine in various operating conditions. The results confirm that by reducing the flow number, for low injected quantities typical of low load and speed engine conditions, better fuel/air mixing is achieved. On the contrary, increasing the injected quantity (as for medium/high load and speed conditions) flow number reduction requires engine re-calibration in order to keep the advantages of mixture formation.