Publication11

A well-timed turbulence due to tumble in SI engines is found to be of substantial benefit to the engine combustion process. A mean flow analysis of tumble motion in conjunction with k-ε turbulence model has been developed to provide a detailed mechanism for turbulence enhancement due to tumble. Considering that the tumble phenomenon is highly geometry dependant, an attempt is made to relate tumble-generated turbulence to the parameters relating to intake conditions and combustion chamber geometry. Finally, a new parameter 'vortex life span' has been proposed to characterize tumble and its turbulence, which globally encompasses intake and combustion chamber related features. The sensitivity of this parameter is demonstrated at various operating conditions. It is found that the 'vortex life span' has an inverse relationship with commonly measured BDC tumble ratio and is more sensitive to the chamber geometry effects.

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. © 2001, Institution of Mechanical Engineers. All rights reserved.