Cavitation, primary break-up and flash boiling of gasoline, iso-octane and n-pentane with a real-size optical direct-injection nozzle

Abstract

Improvements to the direct-injection spark-ignition combustion system are necessary if the potential reductions in fuel consumption and emissions are to be fully realized in the near future. One critical link in the optimization process is the design and performance of the injectors used for fuel atomization. Multi-hole injectors have become the state-of-the-art choice for gasoline direct injection engines due to their flexibility in fuel targeting by selection of the number and angle of the nozzle holes, as well as due to their demonstrated stability of performance under a wide range of operating conditions. Recently there has been increased attention devoted to the study of the flow through the internal passages of injectors because of the presence of particular fluid phenomena, such as large scale vortical motion and cavitation patterns, which have been shown to influence the characteristics of primary break-up. Understanding how cavitation can be used to improve spray atomisation is essential for optimising mixture preparation quality under early injection and stratified engine operating conditions but currently no data exist for injector-body temperatures representative of real engine operation, particularly at low-load conditions that can also lead to phase change due to fuel flash boiling. This paper 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, primary break-up. This was achieved by the use of a real-size transparent nozzle of a gasoline injector from a modern direct-injection combustion system. Gasoline, iso-octane and n-pentane fuels were used at 20 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.3

Numerical investigation of VOC levels in the area of petrol stations

“…Additionally ethanol has shown similar behaviour to gasoline in terms of spray effects related to temperature and pressure conditions (e.g. see Aleiferis et al 2010a-b;Serras-Pereira et al, 2010;Aleiferis and van Romunde, 2013). However, iso-octane was also selected as a single component for the current vapour dispersion study since it is the compound with similar total percentage in gasoline to that of the sum of pentanes (C5); it is also a typical fuel used as substitute for gasoline for various research purposes.…”

An analysis of spray development with iso-octane, n-pentane, gasoline, ethanol and n-butanol from a multi-hole injector under hot fuel conditions

“…one at the intake side and the other at the exhaust side of the engine. More details about the injector geometry, nozzle-hole angles and spray formation in a quiescent environment and in a running DISI engine can be found in previous studies by the current authors [3,4,[8][9][10].…”

“…All fuel injection events were conducted in a quiescent pressure chamber [9,10]. Its octagonal shape and its six optical windows enable simultaneous multi-technique characterisation such as imaging with back lighting or side lighting, and the use of on-axis and off-axis techniques for droplet sizing and velocity measurements (Phase Doppler and Malvern).…”

“…The different plumes in which the measurements were taken may also have contributed to the difference in measured droplet sizes. More specifically, the fuel flow through the nozzle's interior geometry can lead to effects on the degree of atomisation of the fuel, as observed in optical nozzle work [9,10]. To this end, the difference in interior turning angles of the nozzle holes for plume 2 and Plume 3 is likely to have played a role in the difference in measured droplet sizes, albeit not quantified in detail yet.…”

“…Therefore, it seems plausible that this relationship could be used to predict the average SMD for typical DISI engine sprays if the values of constants were further refined. However, it would probably be also desirable to include the cavitation number in (1), as in-nozzle phase-change phenomena have been explicitly observed in multi-hole injectors of the same type [9,10].…”