An Experimental Study of High Pressure Nozzles in Consideration of Hole-to-Hole Spray Abnormalities

The present study focuses on the causes of dissimilarity in the flow structures of sprays produced by different holes of the same direct injection high-pressure diesel nozzle. To assess the effect of nozzle geometry on the transient spray structure, photographs of the spray plumes produced by VCO, mini-sac and reduced sac nozzles at different delays from the start of injection were acquired. Injected fuel volume, feeding pressure, injection duration, spray penetration and cone angle were measured for all the investigated nozzles. A statistical analysis of the acquired images and data showed that sprays from the same hole were highly repeatable even with clear hole-to-hole variation of the spray structure. In particular, for the three investigated nozzle geometries the effect of nozzle flow rate, hole inlet and outlet diameter, needle geometry and working time under engine conditions were investigated. Microscope pictures of the nozzle holes were also acquired. Measurements showed that no correlation exists between spray structure and micro-defects in the holes geometry caused by drilling operations in the range of the investigated conditions (injection pressure ranging between 40 and 140 MPa). Whereas a strong dependence on needle dynamics and on the eccentricity between the nozzle and the needle due to asymmetric feeding conditions was observed.

A deterioration of the nozzle performances after a fatigue test of about 100 hour running in engine was observed for all the investigated nozzles due to the deposition of carbon particles on the interior wall of the nozzle holes.