Abstract
A new combustion model has been introduced for the prediction of turbulent propagating combustion in SI engines. The model consists of turbulent dissipation combustion and laminar flamelet combustion. The model assumes that the turbulent dissipation combustion takes place in Kolmogorov scale eddies and the laminar flamelet combustion works in the rest of the volume. The turbulent dissipation combustion is modeled by the fractal dimension that expresses a volume ratio of Kolmogorov scale eddies, while the laminar flamelet combustion is formulated by the fractal dimension that expresses a fractal nature of a wrinkled flame front surface. The model is called Hybrid Fractal Flame Model(HFFM). The volume ratio of Kolmogorov scale eddies depends on the turbulence Reynolds number. A laminar part of combustion is expressed by the fractal flame growth model, which was developed at Ritsumeikan University and is categorized as a laminar flamelet model. The laminar flamelet model also was tested to see the difference from the HFFM. Computations were performed using these two models and the results were compared with measured pressures and heat release rates for different engine speeds and excess air ratios. It is shown that the HFFM agrees well with the measurements for 1000, 1500 and 1800 rpm under the excess air ratio of 1.0, whereas the laminar flamelet model underestimates as engine speed increases. The analysis of the computed results indicates that the turbulent dissipation heat release rate occupies a larger part of the total heat release rate as the engine speed increases; from around 20% at 1000 rpm to 50% at 1800 rpm. It is also shown that the results computed by both the models for the excess air ratios of 1.0, 1.1 and 1.2 at 750 rpm agree generally well with the measurements.
Keywords: SI Engine, Combustion Model, Turbulent Dissipation, Laminar Flamelet, Fractal Dimension