A Fractal-Based SI Engine Model: Comparisons of Predictions with Experimental Data

A quasidimensional engine simulation which uses the concepts of fractal geometry to model the effects of turbulence on flame propagation in a homogeneous charge SI engine has been developed. Heat transfer and blowby/crevice flow submodels are included in this code and the submodels chosen are found to be reasonable. The model predictions of cylinder pressure histories are then compared with experimental data over a range of loads, equivalence ratios, and engine speeds. The model is not adjusted in any manner to yield better agreement with the data, other than by tuning the simple turbulence model used so as to yield agreement with data for the nonreacting flow. However, current information about the flame wrinkling scales in an engine is inadequate. Therefore, predictions are made for three different assumptions about the flame wrinkling scales which span the range of physically possible scales. It is found that the fractal burning model yields predictions of the rate of early pressure rise that agree extremely well with the data for all operating conditions investigated independent of the assumption made about the flame wrinkling scales, and that over the complete range of loads and equivalence ratios examined the assumption that the ratio of the flame wrinkling scales is approximately equal to the ratio of the integral length scale to the Kolmogorov scale yields very accurate predictions of the peak cylinder pressure, the position of peak pressure, and the cylinder pressure history. It is not claimed that the excellent predictions resulting from this assumption have a physical basis, but this result is interesting and perhaps useful. The model predictions are consistent for all loads and equivalence ratios, but the character of the trends is different for the single data set available at a different speed. More experimental data are needed for comparison with the model predictions. Most importantly, it is shown that use of fractal geometry to model turbulent combustion in SI engines is a promising new engineering tool. However, many unresolved issues regarding the fractal characteristics of flames in engines remain unanswered and merit experimental investigation.