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. The causes of unequal sprays in two stage VCO injectors are studied, and a new flow singularity, which causes a spray abnormality and flow rate discontinuity is identified.