Autoignition Control Using an Additive with Adaptable Chemical Structure. Part 2. Development of a PRF Kinetic Model Including 1,3-Cyclohexadiene Mechanism and Simulations of Ignition Control

Abstract

Autoignition control of fuel and air mixtures was simulated using an additive able to change its molecular structure upon light irradiation. This control was assumed to be feasible through the photochemical isomerization of 1,3-cyclohexadiene (1,3-CHD) to cis-1,3,5-hexatriene (1,3,5-HT). 1,3-CHD was present in a molar concentration of 1% in a PRF fuel and was transformed into 1,3,5-HT in varying amounts prior to ignition, in an attempt to control autoignition timing. The autoignition delays were calculated using a newly developed chemical kinetic mechanism for the low temperature combustion of PRF/1,3-CHD/1,3,5-HT mixtures in air. Validations for PRF/air mixtures were performed by simulations based on the new mechanism developed in the current work against ignition delay times (IDT) of the literature measured in rapid compression machines. The agreement between simulations and experiments for the pure compounds reinforced the accuracy of the mechanism, which led to an investigation of its impact on the IDTs for the addition of 1,3-CHD to PRF90. The computations showed that 1,3-CHD was an ignition enhancer, with a similar boosting effect to that of 2-ethylhexyl nitrate. Simulations predicted that the extent of ignition enhancing of 1,3-CHD can be controlled by the pressure because of the specific combustion chemistry that rules the ignition enhancing capacity of this compound. Additions of 1,3-CHD were found to promote the reactivity of a PRF90 to a greater extent than the addition of 1,3,5-HT. Simple single-zone modeling showed that ignition in a homogeneous charge compression ignition (HCCI) engine may be controlled if the fuel is photochemically isomerized using light-irradiation prior to the combustion process.