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The introduction of the LDCR common rail injection system has opened up new possibilities in controlling the details of the injection rate and the spray characteristics. In particular, there is potential to optimize engine performance across the speed and load range, if a nozzle can be developed which has the facility to vary the final orifice area over the operating range of the engine. There are a number of different geometries which may achieve the required effects. Two possible methods are to throttle either the entrance or the exit of the nozzle holes to a greater or lesser extent, according to the engine running condition. The paper describes an investigation of the spray characteristics of entry and exit throttled orifices, and how they are affected by pressure levels and degrees of opening. In previous studies, large scale transparent models have accurately reproduced the different spray characteristics observed with actual nozzles.

The paper describes how laser light sheet illumination was used to study the onset and development of cavitation in a scaled up plain orifice nozzle. In addition, measurements were taken using laser Doppler velocimetry and the refractive index matching technique, and these establish the velocity profiles within the orifice under non-cavitating conditions. The light sheet makes visible new detail in the cavitating flow field and additional stages in the cavitation process are identified. The mechanism which causes hydraulic flip is demonstrated and confirms the authors' hypothesis from previous studies. An investigation into the form which the cavitation takes is included: flow conditions are demonstrated in which the cavitation produces an opaque mass of small bubbles, and alternative conditions in which large transparent vapour pockets are produced.

An investigation into the different flow structures which exist in the holes of direct injection nozzles and their corresponding large scale acrylic models is described. Some effects of different experimental methods are discussed. Results obtained under steady flow conditions are compared with those from transient tests. The correct use of large scale models is found to enable the interpretation and clarification of the results obtained with actual nozzles. The effect of flow cavitation in the nozzle holes is clarified. New information concerning the state of the injected fuel is included. The relationships between cavitation and the spray abnormality, hydraulic flip, and between cavitation and atomization are established. The spray characteristics of single hole nozzles are contrasted with those of standard multihole sac type nozzles.