Development of an Improved Fractal Model for the Simulation of Turbulent Flame Propagation in SI Engines 2005-24-082

The necessity for further reductions of in-cylinder pollutant formation and the opportunity to minimize engine development and testing times highlight the need of engine thermodynamic cycle simulation tools that are able to accurately predict the effects of fuel, design and operating variables on engine performance.

In order to set up reliable codes for indicated cycle simulation in SI engines, an accurate prediction of heat release is required, which, in turn, involves the evaluation of in-cylinder turbulence generation and flame-turbulence interaction. This is generally pursued by the application of a combustion fractal model coupled with semi-empirical correlations of available geometrical and thermodynamical mass-averaged quantities. However, the currently available correlations generally show an unsatisfactory capability to predict the effects of flame-turbulence interaction on burning speed under the overall flame propagation interval.

Therefore, in the present paper, a new correlation that improves the turbulent burning speed calculation is developed. It features an original definition of the outer turbulence cutoff length scale, based on the flame front area, and takes account of the increased transfer across the flame front of both radical species and heat for high in-cylinder densities. The correlation has been applied to calculate the burning speeds in the cylinder of a naturally aspirated bi-fuel engine for a wide range of engine speeds (N 2000-4600 rpm), loads (bmep 200-790 kPa), relative air-fuel ratios (RAFR 0.80-1.30) and spark-advances (SA ranging from 8 deg retard to 2 deg advance with respect to MBT), under both gasoline and CNG operations.

The computed burning speeds were then compared to those stemming from the traditional correlation reported in the literature and to the experimental flame propagation data. These latter were extracted from the measured in-cylinder pressure traces by means of a diagnostics technique previously developed by the authors. The results indicate that the burning speeds calculated through the authors' model are in better agreement with the experimental outcomes than those derived from the traditional correlation widely applied in the literature.