3D CFD Analysis of the Influence of Some Geometrical Engine Parameters on Small PFI Engine Performances - The Effects on Tumble Motion and Mean Turbulent Intensity Distribution

In scooter/motorbike engines coherent and stable tumble motion generation is still considered an effective mean in order to both reduce engine emissions and promote higher levels of combustion efficiency. The scientific research also assessed that squish motion is an effective mean for speeding up the combustion in a combustion process already fast. In a previous technical paper the authors demonstrated that for an engine having a high C/D ratio the squish motion is not only not necessary but also detrimental for the stability of the tumble motion itself, because there is a strong interaction between these two motions with the consequent formation of secondary vortices, which in turn penalizes the tumble breakdown and the turbulent kinetic energy production. The present paper starts with the development of a theoretical model which will be applied to different engine classes demonstrating how the relative importance of the tumble and squish varies changing the typical design parameters. The final achievements will be to show that the squish flow importance in promoting the turbulence reduces moving to small bore square engines. CFD simulation of the intake flow will be then performed with AVL Fire v. 2010 for investigating in more detail the stroke influence on the in-cylinder turbulence generation.