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 the formation of near-nozzle deposits that have been observed on existing underfloor SCR systems.
The current work presents in-situ time lapse imaging of an underfloor mounted AUS-32 exhaust-mounted urea dosing unit. The operating conditions under examination are representative of low-load low speed urban driving interspersed with high temperature exposures typical of periodic DPF regeneration.
Analysis is provided of various phenomena leading to the creation of urea-based thermal decomposition products and their deposition on the near-nozzle injector surfaces. Further imaging during the high temperature exposure reveals the subsequent polymerization of liquid films generated during the lower temperature operation. Burnoff of solid urea buildup is also observed during the high temperature events.
The present test campaign provides new insights on the various mechanisms leading to the near-nozzle deposits, including indications of urea vapor transport back to the dosing unit injection point.