Researchers from Sandia National Laboratories and Toyota Motor have used a surrogate for a methanol-to-gasoline (MTG) fuel to study numerically the effects of gasoline composition on knock propensity and on the sensitivity of knock to thermal and fuel stratification, to oxygen dilution and to nitric oxide from exhaust gas recirculation of residual gases. Their paper is published in the journal Fuel.

Low-carbon gasoline-like fuels such as methanol-to-gasoline (MTG) could be a promising approach to achieve rapid greenhouse gas emission reduction of the transportation sector. The MTG process converts methanol to di-methyl ether (DME) using a methanol dehydration catalyst. Olefins are then firstly formed over a zeolite catalyst, followed by formation of gasoline-range paraffinic and aromatic hydrocarbons.

The relative yield of various hydrocarbons and, therefore, the composition of the final gasoline product, can be modified by adjusting the residence time, temperature, and pressure in the catalyst as well as the shape selectivity imposed by the catalyst structure. Diesel fuel and jet fuel can be also synthesized through this process by subsequent oligomerization of the light olefins produced from DME. Unlike Fischer-Tropsch-based processes that only produce alkanes and require additional refining to obtain diesel-like or gasoline-like fuel, MTG-based technologies produce market-ready gasoline, which results in lower production costs, according to the team.

Although MTG that meets the ASTM D4814 standard for automotive spark-ignition engine fuel can be produced readily, it is unclear how the composition of MTG may affect engine performance and emissions.

The team compared the MTG surrogate to a petroleum-based regular E10 gasoline (PACE-20). A premium-grade MTG fuel was also formulated by adding ethanol to the MTG surrogate, and results were compared against those of four premium-grade, gasoline-like fuels representative of future alternative gasoline formulations.

Despite the fact that the MTG surrogate has a RON 1.1 units higher than that of PACE-20, it may show higher knock propensity at medium temperature conditions due to a less intense NTC behavior. MTG autoignition was more temperature- and equivalence ratio-sensitive than that of PACE20, suggesting that MTG can benefit more from naturally-occurring thermal stratification or from induced fuel stratification of the end gas to mitigate knock intensity.

The sensitivity of autoignition reactivity to oxygen dilution and to NO concentration was higher for MTG than for regular gasoline at medium loads, but the opposite trend was observed at high loads due to the effect of pressure on the low-temperature chemistry of regular gasoline.

Approximately 14 %vol ethanol content was required to upgrade the octane rating of MTG from regular grade to premium grade. Adding 13.6 %vol ethanol made the fuel autoignition less sensitive to both oxygen dilution and NO content (ignition time varies approx. 17 % and 50 % less with oxygen dilution and NO addition, respectively, when adding ethanol at high engine loads).

—MacDonald et al.

Resources

• James MacDonald, Dario Lopez-Pintor, Naoyoshi Matsubara, Koji Kitano, Ryota Yamada, A chemical kinetic analysis of knock propensity of methanol-to-gasoline fuel, Fuel, Volume 382, Part B, 2025, doi: 10.1016/j.fuel.2024.133787