Effect of water content in ethanol on spray formation at subcooled and flash-boiling conditions

Abstract

Direct-injection spark-ignition engines can be more sensitive to fuel type as the time available for spray formation and mixing is rather limited. The challenge is further complicated by fuel stock which includes a significant bio-derived component in some markets to manage fuel sustainability. The thermophysical properties of bio-components like ethanol differ markedly from typical hydrocarbons found in gasoline like iso-octane. Moreover, the production of anhydrous ethanol fuel involves separating water and ethanol by distillation and dehydration in an energy intensive process. This study presents results from an optical investigation into the spray formation process of hydrous ethanol fuels with 4–15% water content per volume in direct comparison to anhydrous ethanol and iso-octane. Injection tests were carried out with 20–110°C fuel temperature at 1.0 bar and 0.5 bar gas pressure to mimic thermodynamic conditions of cold-start and fully-warm engine at low-load and wide-open-throttle operation. High-speed spray imaging was conducted along with droplet sizing by phase Doppler anemometry. The thermophysical properties of all fuels, including vapour pressure, density, viscosity and surface tension, as well as distillation curves, were obtained over a range of temperatures and the respective Reynolds, Weber and Ohnesorge numbers were considered in the analysis. At 20°C iso-octane and anhydrous ethanol exhibited similar spray plume penetration, whilst increasing water content in ethanol reduced spray penetration. At higher temperatures up to 90°C, anhydrous ethanol showed lower penetration than iso-octane whilst the hydrous fuels still exhibited lower penetration than anhydrous ethanol in sequence of increasing water content. At 110°C, the ethanol fuels showed signs of plume merging and spray collapse, particularly at 0.5 bar gas pressure, with clear differences from iso-octane. The degree of plume merging and collapse was stronger with increased water content. The cone angle for the hydrous fuels was higher than that of pure ethanol, with the 4% water content ethanol that was close to the azeotropic point showing wider cone angle at temperatures higher than 20°C, being distinctly higher at 90°C. Iso-octane had consistently smaller droplet Sauter mean diameter than the ethanol fuels. As the percentage of water content in ethanol increased there was an increase in droplet sizes at all conditions.