Influence of Methanol-to-Gasoline Fuel Formulation on Knock Propensity and Flame Speed

<div class="section abstract"><div class="htmlview paragraph">Drop-in gasoline fuels that originate from renewable, low-net-carbon sources, such as methanol-to-gasoline (MTG), are an important bridge in the transition between traditional fossil fuels and electrification of the transportation sector. The composition of these fuels can be tuned by adjusting the settings of the chemical processes used to create them, which can be leveraged to formulate optimized fuels for higher knock resistance or higher flame speed.</div><div class="htmlview paragraph">This study investigated how the distribution of hydrocarbon classes and molecular structure of a renewable MTG gasoline surrogate affected knock and flame speed using chemical kinetic modeling. The original MTG surrogate was modified by increasing the relative amount of a certain hydrocarbon class while the concentration of other hydrocarbon classes is reduced equally. Increasing normal- and iso-alkanes increased reactivity and penalized octane sensitivity, olefins increased octane sensitivity while keeping the research octane number constant, and increasing cyclo-alkanes or aromatics decreased reactivity with the fuel being more sensitive to cyclo-alkanes. To optimize octane rating, short normal-alkanes and long, highly-branched iso-alkanes are preferred, with octane rating being very sensitive to the structure of iso-alkanes. Increased branching also improves octane rating for cyclo-alkanes, olefins, and aromatics. Regarding flame speed, normal-alkanes tend to increase flame speed at engine-relevant conditions because they form radicals that accelerate the flame. However, aromatics and cyclo-alkanes, which are promising octane boosters, showed lower flame speeds. Based on this understanding, an optimized MTG was formulated and compared against the effect of ethanol blending on the original MTG, with the optimized fuel showing similar performance as MTG with 52.5%_vol ethanol. Chemical kinetic analyses showed that the chemistry that controls octane rating is different from that that controls flame speed, opening the door to fuels that simultaneously improve both knock and deflagration characteristics.</div></div>