A Study on Diesel Spray Characteristics for Small- Quantity Injection

Abstract

Multi-stage injection with pilot injection and post injection has been widely used for the noise and emissions reduction of diesel engines. Considering many parameters to be decided for optimal combustion, computer simulations such as three dimensional computational fluid dynamics (3D-CFD) and lower dimensional codes should play a role for optimal selection of intervals and quantity ratios. However, the data for the sprays are insufficient for reproducing the actual fuel-air mixture formation process related to pilot and post injection. Hence, there is a need for experimental data with a small-quantity injection. The small-quantity injection is characterized with an injection rate shape similar to a triangle rather than a rectangle. This study is mainly focused on the spray characteristics of diesel sprays in which the entire process is dominated by unsteady injection processes. The effects of injection parameters and nozzle hole diameter on spray penetration, spray angle, and fuel concentration are studied with the help of a rapid compression and expansion machine. A hybrid of shadowgraph and Mie scattering imaging set-up is used to visualize both spray liquid phase and vapor phase at the same time. A high-speed camera with a frame rate of 90, 000 fps is used to acquire spray images. Two injectors with a nozzle hole diameter of 0.12 mm and 0.14 mm are used. The studies are performed at the injection pressure of 40, 80, and 120 MPa while environmental temperature of 850 K. The experimental results show that the development of the spray tip is proportional to t and then to t1/2 at the later stage, and after the end of the injection, the spray tip penetration is found to follow t1/4. Also, the spray liquid penetration, spray dispersion, and air-fuel mixing processes are evaluated and compared with various injection parameters. In addition, the instantaneous behavior of the near-nozzle spray angle is studied carefully in order to provide reliable input data for spray models.