An Investigation of the Ignition Delay Character of Different Fuel Components and an Assessment of Various Autoignition Modelling Approaches

An understanding of the ignition delay behaviour of spark ignition fuels, over a wide range of temperatures and pressures, was an essential prerequisite for an ongoing pursuit to develop a fundamentally-based predictive octane model for gasoline blends. The ignition delay characteristics of certain model fuel compounds such as linear and iso-paraffins, olefins, aromatics and alcohols were investigated by means of chemical kinetic modelling, employing CHEMKIN 3.7 using detailed molecular oxidation mechanisms obtained from the literature. The complexity of these mechanisms necessitated the parallel investigation of reduced kinetic models in some of the applications. Reduced kinetic models were also used to describe the blending behaviour of selected binary combinations of the model fuels.

The complex ignition delay response in the temperature/pressure domain that was predicted by the detailed kinetic analyses was reduced to a simple system of three, coupled Arrhenius equations. This simplified expression was used to emulate experimental data that were obtained for the model fuels in a combustion bomb apparatus, the IQT™, as well as data from a single cylinder CFR engine under knocking conditions. A combination of the various approaches has led to new insights regarding the blending behaviour of various classes of fuel molecules in regard to their collective resistance towards autoignition. This is a critical requirement for understanding and modelling the chemical ignition delay as reflected by octane numbers.