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The development of internal combustion engines is challenged by increasingly stringent emission limits. This challenge can only be solved by a deeper knowledge of the combustion process itself. However, the combustion is strongly influenced by the processes inside the injector nozzle. This begins with the internal flow of the injector, which significantly influences the spray formation. One of the key parameters here is the appearance of cavitation caused by the nozzle geometry, needle lift and injection pressure up to 3000 bar. However, there is lack of knowledge about the real flow inside injectors at these high pressure conditions. The cavitation effects in the injection port area destabilize the emergent fuel jet and improve the jet break-up. Therefore internal flow, the jet break-up, the atomization and the mixture formation are closely connected in the combustion chamber and therefore have a direct impact on emissions, fuel consumption and performance of an engine.
In this work, the flow in a nozzle of a real diesel fuel injector is analyzed with high-resolution neutron imaging. Due to the different mechanisms of interactions between neutrons and the material, the steel structure of the nozzle is relatively transparent to neutrons, while the fuel possesses high neutron attenuation [1]. This distinguishes the neutron imaging from x-ray measurements, in which the fuel is of a very low contrast. Thanks to these advantages of the neutron imaging in comparison with other methods (e.g visible light microscopy with glass structures [2]), the nozzle can be investigated under realistic pressure and flow conditions.
Recently, the high resolution neutron imaging instrument (`Neutron Microscope') has been developed at Paul Scherrer Institut (PSI) [3]. This allows the acquisition of neutron radiographies with the spatial resolution down to 5 micrometers [4]. An injection nozzle has been enclosed in a specially tailored chamber that allowed placing the nozzle in close proximity of the detector and tested in-situ Neutron Microscope.
The injector was operated under steady state condition at lowered injection pressures up to 170 bar. In this pilot high resolution investigation water, with gadolinium salt as a tracer, was used to optimize the contrast of the images. The nozzle was modified to a single hole nozzle due to an easier observation. First examinations show various cavitation phenomena. Different characteristics of cavitation in the spray hole were detected, and its influence on the spray formation was clearly visualized simultaneously.
