The Shell autoignition model: applications to gasoline and diesel fuels

The applications of the Shell model to modelling autoignition in gasoline and diesel engines are reported. The complexities of modelling autoignition in diesel sprays have been highlighted. In contrast to autoignition in gasoline engines, autoignition of diesel fuel sprays takes place at a wide range of equivalence ratios and temperatures. This makes it necessary to impose flammability limits to restrict the range of equivalence ratios in which the autoignition model is active. The autoignition chemical delay for n-dodecane is shown to be much less than the physical delay due to the droplet transit time, atomization, heating, evaporation and mixing. This enables the use of the less accurate but more computer efficient Shell model for diesel fuel chemical autoignition. Since experimental data for the chemical autoignition delay for n-dodecane are not available, this study of the applicability of the Shell model to diesel fuels is based on data for n-heptane. The ignition time delays for premixed n-heptane predicted by calculations using the kinetic rate parameters corresponding to the primary reference fuel, RON70, show good agreement with experimental results when A_f4 (preexponential factor in the rate of production of the intermediate agent) was chosen in the range between 3×10⁶ and 6×10⁶. It is pointed out that the difference between the end-of-compression temperature, as predicted by the adiabatic law, and the actual end-of-compression temperature, taking into account the exothermic reactions at the end of compression, needs to be accounted for. The relation between the two temperatures is approximated by a linear function. It is considered that this approach can be extended to n-dodecane.