The simulation of the combustion process in high performance SI (Spark Ignition) engines requires an accurate prediction of the turbulent flow field in the combustion chamber at the intake valve closing.
An innovative multi-cycle simulation methodology has been applied to a high performance motorbike engine. The main goal is to fully reproduce the complex gas dynamics occurring in the intake system, which is modeled upstream up to the trumpet and includes a large plenum emulating the bike air-box. The advantage in considering the entire pipe system consists mainly of the possibility to avoid using 1D boundary
conditions at some intermediate section. While that approach would be obviously more convenient in order to contain the CAD model size and hence the mesh element number, it could lead to an ill-conditioning of the flow field inside the intake port and to possible inaccuracies in the prediction of turbulent quantities and charge motion indices inside the combustion chamber. These errors would ultimately influence the mixture ignition and the flame front propagation during the combustion analysis.
In order to deal with such a complex sequence of topologies and key-frame events (e.g. activation/deactivation of the intake/exhaust ports), a flexible simulation workflow was developed for ANSYS CFX. Key-grids are generating according to a semi-automated, versatile and robust meshing process, based on ANSYS ICEM-CFD, which allows combining different mesh types like tetrahedral elements with prism inflation layers at walls, prism extrusion layers in the cylinder region and hexahedral cells in the valve-gap region and for the plenum.
The aim of this paper is to give an overview of the whole methodology, the meshing-related issues and some results like pressure and mass flow-rates at some relevant sections of the intake system, in-cylinder charge motion indices and
internal EGR (Exhaust Gas Recirculation) mass fraction.
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