Instantaneous Flow Rate Testing with Simultaneous Spray Visualization of an SCR Urea Injector at Elevated Fluid Temperatures

Selective Catalytic Reduction (SCR) diesel exhaust aftertreatment systems are virtually indispensable to meet NOx emissions limits worldwide. These systems generate the NH3 reductant by injecting aqueous urea solution (AUS-32/AdBlue®/DEF) into the exhaust for the SCR NOx reduction reactions. Understanding the AUS-32 injector spray performance is critical to proper optimization of the SCR system. Specifically, better knowledge is required of urea sprays under operating conditions including those where fluid temperatures exceed the atmospheric fluid boiling point. Results were previously presented from imaging of an AUS-32 injector spray which showed substantial structural differences in the spray between room temperature fluid conditions, and conditions where the fluid temperature approached and exceeded 104° C and “flash boiling” of the fluid was initiated. Subsequent testing was conducted using a novel instantaneous flow meter (dINJ) which gave further insight into injector flow behavior under flash boiling conditions. The current work presents results obtained with the dINJ flow meter which for the first time are combined with simultaneous high speed video imaging to give a unique ability to analyze the injector flow and spray behavior. A description of the dINJ flow meter is provided. Testing was conducted at liquid spray and flash boiling operating points using a prototype design actively heated injector. A detailed analysis is presented of the injector flow and spray characteristics during the opening and closing transients, and some conclusions are drawn confirming previous observations of fluid evacuation from the sac volume after injector closing.