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The efficiency of engine operations, i.e. cold start, transient response and operating at idle, depends on the capability of the injection fuel system to promote a homogeneous mixture formation through an efficient interaction with engine fluid dynamics and geometry. The paper presents the development and the application of a methodology for running a CFD PFI engine simulation. A preliminary assessment of the wall-film and droplet-wall interaction sub models has been carried out in order to validate the methodology. Then a three-step numerical procedure has been adopted. The first two steps are aimed to properly initialize the secondary breakup model depending on the type of injector installed on board in order to achieve accurate predictions of spray characteristics.

In the next incoming future the necessity of reducing the raw emissions leads to the challenge of an increment of the thermal engine efficiency. In particular it is necessary to increase the engine efficiency not only at full load but also at partial load conditions. In the open literature very few technical papers are available on the partial load conditions analysis. In the present paper the analysis of the effect of the throttle valve rotational direction on the mixture formation is analyzed. The engine was a PFI 4-valves motorcycle engine. The throttle valve opening angle was 17.2°, which lays between the very partial load and the partial load condition. The CFD code adopted for the analysis was the FIRE AVL code v. 2013.2. The exhaust, intake and compression phases till TDC were simulated: inlet/outlet boundary conditions from 1D simulations were imposed.