Scale Model Experiment of Flows in D.I. Diesel Injection Nozzles
Keiya NISHIDA1, Takayuki MIZUGUCHI2, Takuo YOSHIZAKI3 and Hiroyuki HIROYASU4
Flows in the fuel injection nozzles for Direct Injection (D.I.) Diesel engines were studied by using transparent and scale up model nozzles to clarify the effects of the flows in the sac chamber and the discharge hole on the spray behaviors. Geometries of the model nozzles were scaled up ten times the actual nozzles and the injection pressure for the model nozzles was adjusted so as to achieve a Reynolds number at the discharge hole which was the same as an actual nozzle. Aluminum oxide (Al2O3) tracers were used to visualize the flow patterns in the sac chamber. Sequential images of the flow in the sac chamber and the issued spray during the opening and closing processes of the needle valve were taken by a high-speed video camera. For the Mini Sac Model, spiral air cavities develop from the exits of the discharge holes to the inside of the sac chamber when the needle lift is small. Such flow characteristics in the nozzle make the spray a hollow cone structure with fine droplets and a very large spray angle.
Key Words: Diesel Engine, Fuel Injection, Atomization, Spray, Hole Nozzle, Direct Injection
It has been known recently that the disintegrating processes of a fuel jet from the injection nozzle for a direct injection (D.I.) Diesel engine are controlled by not only the interfacial force between the jet and the surrounding gas, but also by the internal flow of the nozzle. Figure 1 illustrates the fuel flows in the hole type nozzle for a D.I. Diesel engine. The spray characteristics, such as break-up length, spray angle and mean diameter, are affected by the internal flows of the nozzle, which are defined by the nozzle internal geometry and the incoming fuel flow condition. In the needle seat, there are wall bounded shear turbulent flows and choke flows. These flows induce the turbulent and vortex flows in the sac chamber. The intense turbulence in the sac chamber and the discharge hole inlet geometry lead to the separating and cavitating flows in the discharge hole. These flows determine the spray characteristics.
One of the authors et al. [1, 2] studied the disintegrating processes of the liquid jet from a simple hole type nozzle. They clarified that the breakup length of the liquid jet was mainly controlled by the turbulence of the internal flow of the nozzle as compared with the interfacial force between the jet and the surrounding gas. Soteriou et al.  studied the effects of the cavitation and the hydraulic flip on the atomization of the spray from the nozzle for a D.I. Diesel engine. They showed that the geometry induced cavitation occurred in the nozzle hole and this cavitation has the predominant effect on the atomization of the spray. Chaves et al.  studied cavitation in the holes of a D.I. Diesel nozzle using a transparent model nozzle. They showed that the length of the cavitation film was not steady and the cavitation air film oscillated in length in spite of the steady upstream conditions. Xu and one of the authors et al.  revealed the effects of turbulence in the sac chamber of D.I. Diesel nozzle on the spray behaviors under the steady flow condition.
In our previous study of the steady flow in the D.I. Diesel nozzle , it was found that the intense turbulence in the sac chamber made the spray angle large. The cavitation which occurred in the sac chamber was unstable and caused the fluctuation of the spray. The full-hole cavitation in the discharge hole made both the spray angle and the discharge coefficient small.
Fig.1 Flows in D.I. Diesel Nozzle