Injection Orifice Shape: Effects on Spray Characteristics and Heat-Release Rate in a Large-Size Single-Cylinder Diesel Engine

A series of experimental studies of diesel spray and combustion characteristics was carried out using circular, elliptic and step orifices. The experiment was performed on a 3-litre single-cylinder engine with optical access. In the engine tests, an elliptic-orifice nozzle with an aspect ratio of approximately 2:1, and a step-orifice nozzle were compared with circular-orifice nozzles. All orifices had sharp-edged inlets. The nozzles were tested at injection pressures extending from 300 to 1300 bar. The nozzles were evaluated in respect of initial spray tip velocity, penetration, spray cone angle, spray width, intermittency and heat-release.

Substantial differences were observed in the spray characteristics: At an injection pressure of 300 bar, the spray width increased twice as fast in the minor axis plane of the elliptic orifice and step orifice than the circular orifices. The elliptic and step nozzles showed substantially larger close-to-nozzle spray cone angles in both the minor and major axis plane than that of the circular orifice nozzles. The spray cone angle was 93% larger in the minor axis plane than that of one of the circular orifice nozzles. The spray cone angles and the spread were larger in the minor axis plane than in the major axis plane, and the elliptic spray became circular at around 2.5 mm downstream from the nozzle tip. However, the differences seem to be mitigated at injection pressures above 500 bar. In spite of the significant differences in the spray characteristics, the orifice shapes have only shown significant effects on the heat-release at the low injection pressure (300 bar).