Effects of Fuel Composition on Burning Rate and Flame Structure in Normal-Gravity Combustion of Gasoline–Isobutanol Droplets

Isobutanol is one of the four isomers of butanol (C4H9OH) and has strong potential as an alternative fuel for internal combustion engines because of its favorable properties. Compared with ethanol, it has a higher heating value, lower hygroscopicity, and lower vapor pressure, while maintaining a high octane number. Moreover, advances in biotechnology have improved its production from biomass, supporting its use as a renewable fuel and gasoline additive. The present study examined the droplet combustion behavior of pure gasoline (octane number 87), pure isobutanol, and their blends. The volumetric ratios of gasoline to isobutanol studied in this work were 100:0, 90:10, 50:50, 20:80, and 0:100. Initial droplet diameters (D0) ranged from ∼0.3 mm to ∼0.7 mm. Droplets were suspended on 14 μm diameter silicon carbide (SiC) fibers to maintain stable positioning. Ignition was achieved using symmetric spark electrodes placed on opposite sides of the droplet. All experiments were conducted under normal gravity conditions. High-speed imaging was used to record droplet regression and flame evolution. The results showed that the isobutanol blend ratio had little effect on the burning rate (K). The value of K remained approximately 0.57 mm2/s for droplets with a D0 of about 0.5 mm, with only slight variation among the tested blends. This behavior is consistent with classical droplet combustion theory, as isobutanol and gasoline have similar liquid densities and gas-phase properties. On the other hand, isobutanol droplets had very different sooting dynamics from gasoline since soot formation depends strongly on fuel composition. Increasing the isobutanol content in the blends reduced flame luminosity, indicating a reduction of sooting propensity. The flame structure also changed with the blend ratio. Under normal-gravity conditions, gasoline-rich blends exhibited more elongated flame structures, whereas increasing the isobutanol content produced progressively more symmetric flames, consistent with suppressed soot formation and the associated luminous wake. No clear dependence of K on D0 was observed within the range of droplet sizes reported in this study.