Chemical kinetic basis of synergistic blending for research octane number

Abstract

Utilizing kinetic simulations with the Co-Optimization of Fuels & Engines (Co-Optima) mechanism, the research octane number (RON) of various synergistic blendstocks at several blend levels in a four-component surrogate were predicted and compared against measured values. The blendstocks investigated include dimethylfuran (DMF), 2-methylfuran (2MF), prenol, 2-methyl-2-butene (2M2B), and ethanol—selected because of their nonlinear blending or synergistic behavior. The RON predictions are in excellent agreement with measured values for DMF and ethanol (within 2 RON units), with less satisfying results for 2MF and 2M2B (as much as 6 and 12 RON units off respectively). The predictions for prenol do not even capture the synergistic blending behavior observed experimentally, reflecting the fact that the kinetic model for prenol does not include sufficiently accurate low-temperature chemistry. The kinetic model was interrogated to understand the most important reactions consuming the blendstock and surrogate components, to understand the most important reactions responsible for the synergistic blending behavior. Better synergistic blenders scavenge hydroxyl radicals (OH) by addition reactions rather than hydrogen-abstraction (H-abstraction) reactions. In addition, those blendstocks, such as 2M2B, can form resonance-stabilized radical products, leading to superior RON boosting compared to ethanol. DMF and 2MF were shown to have the highest RON-boosting ability due to the rapid reaction of the OH addition products to other species that pull the OH addition equilibrium toward products.