The injection process is one of the relevant phenomena occurring in modern diesel engine considering also that the fuel properties have been continuously changing. The interaction mechanisms between fuel quality, engine technologies, and emissions need to be understood to find the most effective approach towards innovative combustion strategies and low-emission diesel engines. In this work, the injection process occurring in a direct injection (DI) Common Rail (CR) diesel engine has been investigated from the start of injection up to the start of luminous combustion using different techniques. 2D visible and infrared imaging has been applied in single cylinder optical engine for several injection strategies typical of passenger car engine (homologation cycle NEDC). The spray penetration and cone angle obtained by 2D imaging are compared with the results of a 1D consolidated model for transient spray developed by the Sandia National Laboratory. Some modifications to the model boundary conditions have been made in order to assess the effect of fuel-wall interaction. This provided qualitative information of the impinged fuel inside the piston bowl. Finally, a good correlation between the fuel mass impinging on the chamber wall and the particulate matter (PM) emissions measured at the engine exhaust was found. The results of this study could be helpful for the determination of the best injection strategies in a diesel engine in order to reduce PM emissions. (C) 2015 Elsevier Ltd. All rights reserved.