Effect of fuel temperature on in-nozzle cavitation and spray formation of liquid hydrocarbons and alcohols from a real-size optical injector for direct-injection spark-ignition engines

High-pressure multi-hole injectors for direct-injection spark-ignition engines offer some great benefits in terms of fuel atomisation, as well as flexibility in fuel targeting by selection of the number and angle of the nozzle holes. The flow through the internal passages of injectors is known to influence the characteristics of spray formation. In particular, understanding how in-nozzle cavitation phenomena can be used to improve atomisation is essential for improving mixture preparation quality under homogeneous or stratified engine operating conditions. However, no data exist for injector body temperatures representative of real engine operation, especially at low-load conditions with early injection strategies that can also lead to phase change due to fuel flash-boiling upon injection. This challenge is further complicated by the predicted fuel stocks which will include a significant bio-derived component presenting the requirement to manage fuel flexibility. The physical/chemical properties of bio-components, like various types of alcohols, can differ markedly from gasoline and it is important to study their effects. This work outlines results from an experimental imaging investigation into the effects of fuel properties, temperature and pressure conditions on the extent of cavitation, flash-boiling and, subsequently, spray formation. This was achieved by the use of real-size transparent nozzles, replica of an injector from a modern direct-injection spark-ignition combustion system. Gasoline, iso-octane, n-pentane, ethanol and butanol were used at 20, 50 and 90 °C injector body temperatures for ambient pressures of 0.5 bar and 1.0 bar in order to simulate early homogeneous injection strategies for part-load and wide open throttle engine operation. The fuel matrix also included a blend of 10% ethanol with 90% gasoline (E10) because the vapour pressure of E10 is higher than the vapour pressure of either ethanol or gasoline and the distillation curve of E10 reflects strongly this effect. Therefore, the distillation curves of the fuels, the vapour pressures, as well as density, viscosity and surface tension were obtained and the Reynolds, Weber, Ohnesorge and Cavitation numbers were considered in the analysis. The in-nozzle flow regime and spray formation was found to be sensitive to the fuel temperature and gas pressure as a result of the vapour pressure and temperature relationships.