Multi-dimensional modeling of the air/fuel mixture formation process in a PFI engine for motorcycle applications

The preparation of the air-fuel mixture represents one of the most critical tasks in the definition of a clean and efficient SI engine. Therefore it becomes necessary to consolidate the numerical methods which are able to describe such a complex physical process. Within this context, the authors developed a CFD methodology into an open-source code to investigate the air-fuel mixture formation process in PFI engines. Attention is focused on moving mesh algorithms, Lagrangian spray modeling and spray-wall interaction modeling. Since moving grids are involved and the mesh quality during motion strongly influences the computed in-cylinder flow-field, a FEM-based automatic mesh motion solver combined with topological changes was adopted to preserve the grid quality in presence of high boundary deformations like the interaction between the piston bowl and the valves during the valve-overlap period. The fuel spray was modeled by using the Lagrangian approach, and the spray sub-models (atomization and breakup) were tuned according to experimental validations carried out in previous works. Specific submodels were implemented to describe the impingement of fuel spray with the engine walls. The evolution of the resulting liquid film was also taken into account by solving the mass and momentum equations with the Finite-Area discretization method. The proposed methodology was applied to simulate a single-cylinder SI engine for motor-scooter applications at a low load operating condition. This operating point was chosen since these engines often run very close to idle conditions when they are used in the urban areas.