Effect of pressure cycling on gas exchange in a transparent fuel injector 2019-01-2280
Gas ingested into the sac of a fuel injector after the injector needle valve closes is known to have crucial impacts on initial spray formation and plume growth in a following injection cycle. Yet little research has been attempted to understand the fate sac gases during pressure expansion and compression typical of an engine. This study investigated cavitation and bubble processes in the sac including the effect of chamber pressure decrease and increase consistent with an engine cycle. A single axial-hole transparent nozzle based on the Engine Combustion Network (ECN) Spray D nozzle geometry was mounted in a vessel filled with nitrogen, and the nitrogen gas pressure was cycled after the end of injection. Interior nozzle phenomena were visualized by high-speed longdistance microscopy with a nanosecond pulsed LED back-illumination. Experimental results showed that the volume of gas in the sac after the needle closes depends upon the vessel gas pressure. Higher back pressure results in less cavitation and a smaller volume of non-condensable gas in the sac. But a pressure decrease mimicking the expansion stroke causes the gas within the sac to expand significantly, proportional to the pressure decrease, while also evacuating liquid in front of the bubble. The volume of the gas in the sac increases during the expansion cycle due both to isothermal expansion as well as desorption of inherent dissolved gas in the fuel. During the compression cycle, the volume of bubbles decreases and additional non-condensable ambient gas is ingested into the sac. As the liquid fuel is nearly incompressible, the volume of both liquid and gas essentially remains constant during compression.