Effect of Substituted Phenol Fuel Additives on Ignition Delay of a Toluene Reference Fuel

Aromatics have long been used in pump-grade gasoline to inhibit engine knock and enhance a fuel’s octane number, therefore this study focuses on how the addition of aromatics at 2% by mole affects the ignition characteristics of a Toluene Reference Fuel (TRF). The additives investigated in this study are the substituted phenols p-cresol and 2,6-xylenol. In addition to fuel composition, exhaust gas recirculation dilution can be used to lower the combustion temperature and consequently lengthen the ignition delay time of a given fuel-air mixture. This study replicated exhaust gas recirculation dilution by using N₂, as it was inert and did not interfere with reactions between the fuel and oxidizer. Determination of whether the similar structures of p-cresol and 2,6-xylenol result in different autoignition inhibiting characteristics was performed on a rapid compression machine. Each fuel mixture was tested over a range of engine-relevant compressed temperatures, equivalence ratios, and N₂ dilution ratios at 20 bar compressed pressure, to see how the ignition delay time varied with each condition. The measurements of the p-cresol and 2,6-xylenol mixtures were then compared to the toluene reference fuel, and it was found that the additives had the strongest lengthening effect on ignition delay time for stoichiometric and lean mixtures at the lowest temperatures investigated. Additionally, it was found that increasing the N₂ dilution in a stoichiometric fuel-air mixture was associated with increasingly long ignition delay times.