A combustion simulation code for the prediction of heat release, flame propagation speed and pollutant formation in SI engines was developed and assessed. It is based on a multizone combustion model that takes the non-uniform spatial distribution of the in-cylinder burned-gas thermochemical properties into account. The multizone approach for burning rate calculation is coupled with a CAD procedure for the evaluation of burned-gas front area and radius. Specifically developed sub-models for determining CO and NO formation are included in the code.
An original model based on the fractal geometry concept was used to describe the entrainment of fresh mixture through the flame front. The proposed approach takes account of the effects of both radical species and heat transfer across the flame front by fine-scale turbulence eddies, in addition to the increase of the actual flame front area caused by the wrinkling effect of turbulence, which is the only effect considered in the conventional approach. The model presents a new definition for the outer cutoff length scale, based on the flame front area. The conventional fractal approach, as well as the widely-used non-fractal one, are still embedded in the code for comparison.
The predictive model was applied to a multivalve, fast-burn chamber SI engine fuelled by either gasoline or CNG under a significant sample of operating conditions. The calculation results were compared to experimental data. The developed code was shown to be an effective and reliable tool for SI engine combustion simulation. In fact, the authors' flame-propagation model led to an accurate prediction of the flame-turbulence interaction during the overall combustion period, from early flame development to flame extinction. In addition, it does not require flame kernel growth and burnout sub-models as conventional correlations do for combustion stages in which fine-scale turbulence is reduced and the outer length-scale cutoff is linked to the flame size.