Macroscopic and microscopic characteristics of biofuel spray (biodiesel and alcohols) in CI engines: A review

Spray characteristics are among the most important factors affecting compression ignition (CI) engine performance and emission levels. Fuel flow visualisation and optical diagnostics have been widely employed as methods for investigating combustion processes in CI engines. Due to the increase in the number of renewable alternative fuels and more stringent legislation governing engine pollutant emissions, it is important to characterise the spray behaviour of these alternative fuels in CI engines under different operating conditions. This study reviews and summarises the practice of spray visualisation to determine macroscopic characteristics, such as spray tip penetration (S) and spray cone (?); and microscopic characteristics such as spray droplet diameter and Sauter Mean Diameter (SMD). It also provides literature reviews: experimental efforts regarding optical techniques; spray operating conditions (injection pressure, ambient conditions, and injector geometry), fuel types (biodiesel and alcohol blends) and numerical studies regarding software usage, spray models and simulation cases results. Increasing fuel injection pressure (FIP) initiates combustion due to improved spray penetration length and atomisation, however SMD decreases with increasing FIP. Fuel properties and ambient temperatures have a significant impact on both spray penetration length of the liquid phase and, two-phase interaction between liquid and vapour. Velocity of spray droplets is impacted by ambient gas and aerodynamic and viscous properties because of their impact on spray breakup processes. The higher viscosity and higher surface tension of the biodiesel blend resulted in a reduction of the Weber number and injection velocity with a corresponding increase in droplet size and SMD. All alcohol blends improved the atomisation performance of biodiesel and diesel fuel because alcohol blends have low kinematic viscosity, boiling point and surface tension leading to improved atomisation and vaporisation rate. Piezo-driven injection system atomisation performance is better than that of the solenoid-driven injection because of its faster response time. Finally, simulation studies of spray visualisation revealed good agreement with the experimental investigation so simulation studies can save time and effort.