Abstract
KEYWORDS – SCR, flash-boiling, injection rate analysis, shot-to-shot dispersion, PFI
ABSTRACT
In the present paper an experimental campaign aimed at analyzing the hydraulic behavior of a SCR injector operated with high temperature Urea-Water Solution (UWS) is presented. In previous analyses, the effects exerted by UWS high temperature on the spray evolution were investigated in terms of spray shape and sizing. It was observed that increasing the injected liquid temperature so to approach flash-boiling conditions results in an increase of the spray cone angle and in reduced penetration. Moreover, the spray sizing is significantly reduced improving the UWS to ammonia conversion rate.
The main objective of the present research is the investigation of the effects exerted by flash-boiling on the injector hydraulics in terms of mean and shot-to-shot injected mass and injection rate.
In order to investigate the injector actual hydraulic behavior when injecting high temperature fluid, an innovative instrument named dINJ Injection Analyzer was developed at the University of Perugia. This instrument detects the shot-to-shot injection rate profile and injected volume while preserving the normal operation conditions for the injector, i.e. with the injected spray developing in a gaseous environment. The proposed instrument is based on the detection of the pressure time-history in a closed vessel acting as isolated fuel rail during the injection process. According to this approach the typical constraints of conventional Injection Analyzers requiring a pressurized volume (a closed vessel or a pipe) are overcome. A low pressure injector, as long as a GDI injector, can be operated with the prescribed pressure differential, maintained throughout the injection process. Moreover, the injection in a gaseous environment allows both the onset of peculiar flow conditions (flash-boiling) and the application of additional diagnostics as imaging. In the paper, after a preliminary validation of the proposed instrument with a Port-Fuel-Injection (PFI) injector, the hydraulic analysis of a SCR doser supplied with UWS at temperature ranging from ambient to 130 °C is presented and discussed, analysing the mean injected mass trend with temperature, the shot-to-shot dispersion and the injection rate profile demonstrating the effect of the flash-boiling phenomenon on the discharge process. A partial limitation of the current analysis is given by the ambient pressure and temperature conditions maintained in the vessel downstream the injector nozzle. The investigation will be completed in the future by an upgrade of the experimental setup in order to control not only the pressure and temperature of the UWS in the nozzle but also the pressure and temperature level of the gaseous environment in which the spray evolves.