The Shell autoignition model: applications to gasoline and diesel fuels

The applications of the Shell model to modelling autoignition in gasoline a nd diesel engines are reported. The complexities of modelling autoignition in diesel sprays have been highlighted. In contrast to autoignition in gaso line engines, autoignition of diesel fuel sprays takes place at a wide rang e of equivalence ratios and temperatures. This makes it necessary to impose flammability limits to restrict the range of equivalence ratios in which t he autoignition model is active. The autoignition chemical delay for n-dode cane is shown to be much less than the physical delay due to the droplet tr ansit 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 autoi gnition delay for n-dodecane are not available, this study of the applicabi lity of the Shelf model to diesel fuels is based on data for rr-heptane. Th e ignition time delays for premixed rr-heptane predicted by calculations us ing the kinetic rate parameters corresponding to the primary reference fuel , RON70, show good agreement with experimental results when A(f4) (preexpon ential factor in the rate of production of the intermediate agent) was chos en in the range between 3 x 10(6) and 6 x 10(6). 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 acco unted for. The I-elation between the two temperatures is approximated by a linear function. It is considered that this approach can be extended to II- dodecane. (C) 1999 Elsevier Science Ltd. All rights reserved.