A common rail injection system for diesel engines has a potential to realize low emission and low fuel consumption diesel combustion. The core technology of this system is accumulated on spray characteristics such as mean droplet diameter, spray cone angle and tip penetration. And these characteristics are greatly influenced by internal flow in a diesel nozzle. In order to clarify the internal flow phenomenon such as cavitation, a real-size transparent nozzle had been developed, which can realize simultaneous cavitation visualization during a single-shot injection under high-density and room temperature condition using a high-speed camera. In this article, relationship between internal flow in the nozzle and spray combustion had been investigated. A high pressure and temperature environment in which the spray ignites were achieved in a rapid compression machine, and internal flow in the nozzle and the spray combustion were visualized. Regarding the evaluation of the spray combustion, the formation of the luminous flame triggered by the spray ignition was observed. Two cases of inlet radius of the nozzle hole (R = 0.016 and 0.033 mm) were prepared for this analysis. Under the case of larger inlet radius hole (R = 0.033 mm), probability of the string-type cavitation was greater, spray cone angle was larger and ignition delay was shorter.
In diesel engines for passenger cars, the spray and combustion characteristics are one of the important factors for determining the exhaust and engine performances. These characteristics are affected by the internal flow, the film-type and string-type cavitations in the diesel nozzle. Specifically, the string-type cavitation correlates well with the spray cone angle, and the string-type cavitation behavior destabilizes the spray formation and combustion. The string-type cavitation is influenced by the vortex flow in the nozzle sac. Therefore, it is important to clarify the effect of nozzle and needle shapes on the vortex flow for further development of the diesel injector. This study investigates the effect of needle tip shape on a vortex flow, string-type cavitation and the spray characteristics. This was realized by comparison of the three needle shapes which were chosen in order to control the vortex flow in nozzle sac. A real-size transparent nozzle technique was used to investigate the relationship between the vortex flow and cavitation behavior in the diesel nozzle. This technique allows a spray characteristic to be observed concurrently with the inner flow as well. In addition, the visualized vortex flow was analyzed in detail by the micro-particle image velocimetry method and computational fluid dynamics.
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