Predictions of tumble and turbulence in four-valve pentroof spark ignition engines

Abstract

A predictive model for tumble charge motion and turbulence in four-valve pentroof engines has been developed. The model formulation is based on a mean flow analysis of tumble motion in conjunction with k-∊ turbulence equations. All major phenomena, including fluid shear, affecting mean vortex behaviour and turbulence generation are included. The predictions are made for both intake and compression periods of the engine cycle. The model predictions have been compared with earlier analytical investigations in two engines and are found to be in good qualitative and quantitative agreement. The distinct stages characterizing mean vortex and its turbulence have been identified in this work. Further, the mechanism responsible for turbulence enhancement through tumble has been synthesized and related to these stages. A preliminary parametric study with intake valve lifts and pentroof angles is carried out to demonstrate the capability of the model in design studies. It is revealed that a large-angled pentroof retains significant vortex structures even at top dead centre. In an optimized geometry, these structures may play a role in improving overall mass burn rates. The chamber geometry is found to have a significant influence on bulk motion and turbulence levels at ignition. The intake system, however, influences the formation of tumble vortices during the intake stroke.