A series of experiments have been performed to examine the phenomenon of the after injection of diesel fuel at the closing stage of the needle motion in a single-spring diesel injector.
Spray behavior at the final stage of a diesel injection from a single-hole injector was examined. As the result, the after-injection and the dripping phenomena were observed at the final stage of injection, which may cause generation of the unburned hydrocarbons or the particulate materials. Needle bouncing within the injector was detected and turned out to be the major reason for the after-injection. Both the amplitude and the frequency of the needle bouncing increased with the increase of valve-opening pressure. When the valve-opening pressure was increased, spray became more dispersed compared to the case with the lower valve-opening pressure. The amount of the fuel discharged during the after-injection period also increased with the increase of valve-opening pressure. On the other hand, the dripping of the liquid fuel was suppressed when the ambient pressure was increased. Spray drop sizes at the final stage of an injection were measured by using the Malvern particle sizer. When the valve opening pressure was raised, the spray drops became finer. On the other hand, spray drops became coarser with the increase of the ambient pressure from 1 MPa to 2 MPa. In overall, the atomization performance was improved, but only marginally with the increase of the valve-opening pressure due to occurrence of the after-injection phenomenon.
A simplified analytical model has been proposed to describe the needle bouncing which was confirmed to be the primary cause of the after injection. The needle motion was well simulated by the proposed model. A series of parametric studies have been performed to see the effects of injector design parameters (such as damping coefficients, spring stiffnesses, equivalent stiffnesses, maximum needle lift and needle diameters). The needle bouncing tends to be supressed with the larger damping coefficient by the fluid shear at the needle surface. The needle bouncing can be reduced also by decreasing the material stiffness, initial displacement of the needle spring, equivalent stiffnesses of the needle seat and the injector body, and the maximum needle lift. Finally, the possibility of optimizing the injector design has been discussed for reduction of the after injection phenomenon.
