Evaluation of Adding an Olefin to Mixtures of Primary Reference Fuels and Toluene To Model the Oxidation of a Fully Blended Gasoline

The impact of adding an olefin to ternary mixtures of toluene and primary reference fuels to mimic the oxidation of a fully blended gasoline was examined with kinetic modeling. Reactions for the oxidation of 2,4,4-trimethyl-1-pentene (DIB-1), which is the major constituent in diisobutylene (DIB), were added to a previously developed semidetailed mechanism for ternary mixtures. The merged kinetic mechanism was revised and successfully checked for validity against data for neat fuel components as well as fuel mixtures at conditions relevant to engine combustion. The validated kinetic model was then used to model a fully blended research gasoline. By using a nonlinear-by-volume blending model for octane numbers, a four-component surrogate fuel was formulated which consisted of 51% isooctane, 18% n-heptane, 26.4% toluene, and 4.6% DIB-1 by liquid volume. The surrogate fuel reflected molecular-structure class composition, research octane number, motor octane number, density, and H/C ratio of the target gasoline. Ignition delay times for gasoline measured in a shock tube, rapid compression machine, and an HCCI engine were then compared to simulated results using the quaternary mixture and ternary mixtures with similar octane numbers and H/C ratio as the target gasoline. Adding DIB-1 to a ternary mixture had a small but significant effect on the autoignition of gasoline surrogate fuels. The quaternary mixture showed better agreement when compared to measurements, especially at higher temperatures. The simulated ignition delays at shock tube and rapid compression machine conditions were also well-correlated with the combustion phasing in an HCCI engine defined as the temperature required at bottom dead center to achieve 50% heat release (CA50) at top dead center. Similar results were achieved when comparing with other published mechanisms. Simulations with neat and binary mixtures combined with a rate-of-production and sensitivity analysis with multicomponent mixtures show that the reason for the increased reactivity and shorter ignition delay when adding DIB-1 to the ternary mixture is that DIB-1 promotes toluene ignition more than isooctane at these conditions.