| The fractal engine model, which uses fractal geometry to model the effects of turbulence on flame propagation in a 4-stroke, homogeneous charge, SI engine, has been developed. This model is the first to recognize recent experimental evidence that SI engine combustion should be classified within the category of wrinkled laminar flamelets among the various regimes of turbulent premixed combustion. Thus, the present model assumes that the dominant effects of turbulence on the combustion process in an engine are flame wrinkling and flame stretch. Initial predictions were made after making various simplifications in the formulation of the model. The sensitivity of the predictions to the fractal dimension, to the effects of flame stretch, and to the ratio of the maximum-to-minimum flame wrinkling scales was investigated. After this sensitivity analysis was performed, submodels for heat transfer and blowby/crevice flow were included in the model and tested against available data. The resulting predictions are compared with experimental cylinder pressure histories over a range of loads, equivalence ratios, and engine speeds. 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. It is shown that fractal geometry may be a powerful new tool for modeling premixed turbulent flames. It is also noteworthy that this is the first quasidimensional engine code to account for stretch, the first engine code of any type to take advantage of recent experimental data to improve the heat loss model, and the first engine code of any type to use fractals to simulate the wrinkling effect. During the course of this investigation, it was found that current information about the flame wrinkling scales is inadequate (compared to other modeling factors, such as stretch and fractal dimension). Therefore, a combustion model which explicitly refers to the local structure of the wrinkled flame and that characterizes it by statistical fractals was developed as a means to resolve this problem. |
