Effect of Alcohol Fuel Addition on the Combustion Characteristics of Isooctane

• The effects of methanol and ethanol addition on the combustion and extinction characteristics of iso-octane were investigated under both lean and rich conditions using a constant-volume combustion chamber and a counterflow burner. • Under lean conditions, alcohol addition enhanced both laminar and turbulent combustion, whereas under rich conditions only laminar burning velocity increased. • Lewis number reduction is considered to enhance stretch-modulated local burning velocity, increasing turbulent burning only under lean conditions. • Flame extinction resistance increases with increasing methanol and ethanol blending ratios. • Critical Karlovitz number at extinction confirmed that lean-condition extinction enhancement is governed by Lewis number effects rather than reaction rate alone. The objective of this study is to systematically clarify the effects of alcohol addition on the turbulent combustion and extinction characteristics of gasoline-based fuels from the perspective of Lewis number effects. Iso-octane was used as the base fuel, and fuels blended with methanol and ethanol were experimentally evaluated in terms of laminar burning characteristics, turbulent combustion behavior, and extinction phenomena. The results showed that alcohol addition increases laminar burning velocity and also promotes combustion in turbulent flames. Analysis of these combustion-enhancing mechanisms from the perspective of the Lewis number suggests that the reduction in the Lewis number associated with alcohol addition mitigates the decrease in local burning velocity under flame stretch, thereby facilitating turbulent combustion. Furthermore, extinction experiments using counterflow flames revealed that alcohol addition increases the extinction strain rate, improving flame extinction resistance. Evaluation based on the Karlovitz number during extinction further indicated that this improvement in extinction resistance is strongly associated with molecular diffusion effects resulting from changes in the Lewis number. Taken together, these results experimentally demonstrate that Lewis number effects play a critical role in enhancing turbulent combustion and flame extinction resistance through alcohol addition.