Methanol spray characterisation with different fuel additives under flash boiling conditions

This study explores the macroscopic spray behaviour of methanol and its oxygenated blends MTBE5 (5 vol% methyl tert-butyl ether), A5 (5 vol% acetone), and E5 (5 vol% 2-ethylhexyl nitrate) at a fixed low ambient pressure of 40 mbar while varying liquid injection pressure from 40 to 100 bar. By keeping the flash-boiling environment and injector configuration fixed, the study provides a controlled comparison of how additive chemistry alters methanol spray development. High-speed Schlieren imaging and image-based analysis were used to examine, four key spray features; namely cone angle, spray penetration, spray area, and spray width. The results show that higher injection pressures generally promote broader and more uniform macroscopic spray development, however, the additive-specific response remained distinct under identical operating conditions. Among the blends, MTBE5 showed an approximate 8% increase in cone angle during injection, forming wide and stable sprays due to its favourable balance of low surface tension and sustained lateral plume growth. The A5 blend, by contrast, produced sprays with the strongest forward momentum and most consistent penetration, particularly at moderate pressures (40–80 bar), however, it showed reduced stability at the higher pressures. Pure methanol displayed narrower but moderately stable sprays, while E5 generated the widest sprays but with noticeable instability in the breakup region and spatial distribution. These results show that chemically distinct oxygenated additives do not modify methanol spray behaviour in the same way; rather, each additive produces a different balance of lateral expansion, axial penetration, and repeatability under the same flash-boiling condition. This provides a controlled basis for additive selection in methanol-fuelled injection systems.