The Flow Patterns and Sprays of Variable Orifice Nozzle Geometries for Diesel Injection

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. They have been used in this study, and enable the spray details and the underlying causes to be established. Laser light sheet illumination and laser Doppler velocimetry have shown the cavitation, flow pattern and velocity profiles inside the orifice. The paper describes and explains the new and unusual spray characteristics which were identified in the work. In addition, the characteristics of a number of different variable area nozzle geometries are reviewed and compared.