Development of a real-size optical injector nozzle for studies of cavitation, spray formation and flash-boiling at conditions relevant to direct-injection spark-ignition engines

Abstract

High-pressure multi-hole injectors for direct-injection spark-ignition engines have shown enhanced fuel atomisation and flexibility in fuel targeting by selection of the number and angle of the nozzle holes. The nozzle internal flow is known to influence the characteristics of spray formation; hence, understanding its mechanisms is essential for improving mixture preparation. However, currently, no data exist for fuel temperatures representative of real engine operation, especially at low-load high-temperature conditions with early injection strategies that can lead to phase change due to fuel flash-boiling upon injection. This challenge is further complicated by the predicted fuel stocks, which may include new (e.g. bio-derived) components. The physical/chemical properties of such components can differ markedly from gasoline, and it is important to have the capability to study their effects on in-nozzle flow and spray formation, taking under consideration their different chemical compatibilities with optical materials as well. The current article presents the design and development of a real-size quartz optical nozzle, 200 µm in diameter, suitable for high-temperature applications and also compatible with new fuels such as alcohols. First, the internal geometry of a typical real multi-hole injector was analysed by electron microscopy. Mass flow was measured, and relevant fluid mechanics dimensionless parameters were derived. Laser and mechanical drilling of the quartz nozzle holes were compared. Abrasive flow machining of the optical nozzles was also performed and analysed by microscopy in comparison to the real injector. Initial validation results with a high-speed camera showed successful imaging of microscopic in-nozzle flow and cavitation phenomena, coupled to downstream spray formation, under a variety of conditions including high fuel temperature flash-boiling effects. The current work used gasoline and iso-octane to provide proof-of-concept images of the optical nozzle, and future work will include testing of a range of fuels, some of which will also be bio-derived.