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Under diesel engine conditions the exit velocity of diesel fuel out of an injection nozzle depends on the temporal increase of the supply pressure. Therefore, heading ‘slow’ fuel elements are caught up by ‘fast’ elements leaving the nozzle at a later time. Similar processes occur in modulated flow. This kind of interaction (‘collision’) of liquid elements produces a free internal stagnation point, which moves along the spray axis. From the free stagnation point the liquid is ejected in radial direction, which results in mushroom shaped structures along the spray. A similar structure is observed at the tip of the spray. The spray structures have been observed in modulated laminar laboratory jets as well as in turbulent diesel injection sprays under atmospheric conditions. A model is presented which describes the propagation speed of the stagnation points and the radial and axial flow velocities out of the stagnation points. It also predicts the net mass flux out of the structures.

For high-speed imaging a newly developed eight-fold CCD camera, which permits framing rates of up to one million pictures per second, was used to obtain pictures of the injected sprays during the operation of a diesel engine. For the particular case studied here the framing rate was set at 50,000 pictures per second. This rate was sufficient to resolve the temporal development of the sprays in the transparent version of the four-cylinder, in-line, 1.9 litre DI production diesel engine of Volkswagen. The advantage of the camera is that it needs no light pulses for illumination, but can operate with a continuous light source. Each of the CCD chips is arranged around a central eight face reflecting pyramid, which splits the light coming from the camera lens to each CCD chip. The chips can be shuttered freely (asynchronously) at programmable inter-frame spacings thus permitting operation with continuous illumination. In this particular case a 30 Watt halogen lamp was used.