This paper deals with a numerical investigation of a single cylinder diesel engine equipped with mechanical fuel direct injection system and focuses on the fuel injection system modelling with the aim of predicting the performance of the entire injection system, the spray characteristics, the interaction among spray-cones, combustion chamber flows and geometry. In the simulations, two different codes have been used. With the former one, AMESim code, the complete injection system has been analysed and the single components have been selected and modelled. The results obtained from the injection system simulation, in terms of injection needle lift, injection flow rate, pressure time evolution, have been used to initialize the latter computation tool, FIRE code, in which 3D flow numerical investigation of the internal injector flow has been performed. Since such a flow is directly linked to the spray modelling, the primary break-up effects have been taken into account. The details of the adopted modelling strategy have been shown and the results of each simulation step have been presented. In order to highlight the relationship among the nozzle flow condition and the spray formation-vaporization characteristics, a comparison between two different calculation setups has been shown. Moreover, a qualitative comparison among predictions and experimental data has been discussed.

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