Methanol as an octane booster for gasoline fuels

Octane rating is an essential property for gasoline-type fuels because it is the major factor that determines the maximum compression ratio (CR) allowed in spark-ignition engines. In practice, a base gasoline, which is a mixture of various refinery streams, is dosed with octane-booster additives for meeting the minimum octane requirements set by regulation standards. Coal-based methanol is widely used in China as a gasoline octane-booster. In addition to its high octane rating, methanol's cooling effect derived from its low latent heat and its high octane sensitivity also contribute to engine knocking suppression in spark-ignition direct-injection (DISI) engines. Thus, fully understand the role of methanol addition in gasoline is highly relevant to estimate the potential thermal efficiency gain for methanol blends. The novelty of this paper is that methanol's chemical effect, cooling effects and octane sensitivity effects on engine knocking suppression are quantitatively converted to an effective octane index (EOI), a parameter for comprehensively describing the anti-knocking properties of a gasoline-type fuel. Then Delta EOI of methanol blends (EOI_blend - EOI_base) was used for estimating engine thermal efficiency gain in compression ratio (CR) matched DISI engines. Results suggest that methanol's cooling effect is as significant as its high octane rating. For example, the cooling effect of methanol corresponds to 19 unit of octane in a blend with 70 vol% methanol (M70). The indicated thermal efficiency gain is up to 30% for M70 with RON81.1 base gasoline. The reduction in the lower heating value (LHV) of methanol blends lead to higher fuel consumption, which can be partially or even entirely offset by the improved indicated thermal efficiency. Overall, this paper highlighted the effectiveness of methanol as an octane and thermal efficiency booster.