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

Abstract

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 promotion of a stable and coherent tumble structure is largely believed in literature to enhance in-cylinder turbulence accelerating combustion process. In small PFI engine layout and weight constraints limit the adoption of more advanced concepts. In previous technical papers the authors demonstrated the influence of head shape and squish area on tumble vortex formation, development, breakdown and on final value of turbulence close to spark plug for small PFI engines. The main result of the this research was that the combustion chamber having the less squish area resulted to have the highest level of turbulence close to spark plug at ignition time. The geometry under analysis in the current paper is a 3-valves pent-roof motorcycle engine. 3D CFD simulations were ran at 6500rpm with AVL FIRE code. The chosen engine geometry was the geometry found to be the best set-up in terms of turbulence and combustion performances in the previous paper. In the present paper the head shape and the squish area were kept constant and the following engine parameters were varied: the intake duct angle (the angle of the intake duct entering the head was reduced of 6%, i.e. it was more directed toward the exhaust side of the chamber), the piston shape, and finally the compression ratio (it was reduced of 9%). The main goal of the current analysis is to understand which of these parameters is predominant in accelerating combustion for directing engine design toward the best set-up.