CFD simulering av en fyrtakts Otto motorcykel motor

Sammanfattning: This thesis deals with the numerical 3D simulation of a four stroke, spark-ignited, internal combustion engine, which is mounted on a high performance motorcycle. The work, carried out in collaboration with Politecnico di Milano and MV Agusta, focuses on the simulation of the intake, compression and expansion stroke, including combustion simulation, using an open source CFD code. Simulation of internal combustion engines offers a great challenge in the field of CFD research. Moving boundaries of the solution domain, caused by the motion of the piston and valves, leads to deformation of the computational grid, with decreasing quality of the solution as result. Based on this, a previously developed mesh motion strategy was employed where a number of meshes are used to cover the whole simulation cycle. Within each mesh interval the internal cell points of the mesh are moved to account for the boundary motion of the piston and valves. This strategy was previously applied to simplified geometries as well as real engine geometries. The purpose of this work is to show its validity for a real and complex engine geometry, covering a longer simulation interval then previously. After an initial description of the mesh creation process, studies of the in-cylinder charge motion, created during the intake stroke, were made to investigate the presence of a tumble motion. The result shows that a strong tumble motion is not generated. This could also be because of numerical diffusion caused by the mesh structure. Furthermore, investigations were made to see why the present engine has a tendency to knock. The simulations showed that the in-cylinder flow field creates a temperature distribution in the cylinder that is not uniform, possible increasing the knock risk close to the exhaust valves. Lastly, simulations of the combustion process were made using the Weller combustion model. Problems causing a calculated turbulence intensity that was too low for proper flame propagation were apparent, leading to inconsistency between the calculated cylinder pressure of the CFD calculation and the calibrated 1D calculation.