In order to satisfy China IV (equivalent to EU IV) emission regulations, an unconventional design concept was proposed with injector closely coupled with SCR can body. The benefit of this design is that the urea decomposition pipe was removed or drastically shortened, resulting in much smaller packaging space and lower cost of the whole system. However, the resulting short urea mixing distance generates concerns on low urea mixing efficiency and risks of urea deposits.
In particular, airless urea injectors tend to generate incomplete evaporation of urea water solution, resulting in high risks of urea deposits. New aftertreatment mixing structures need to be developed to resolve these technical challenges. To this end, stepwise and systematic enhancements of the design have been employed, resulting in multiple designs to eventually meet a set of performance targets, including emission reduction efficiency, reagent mixing, urea evaporation, ammonia and velocity distribution, back pressure, and urea deposits.
Two types of injectors are studied: (1) Bosch 2.2 airless injector; (2) Albonair air-assisted injector. With respect to droplet sizes, the former has Saunter Mean Diameter (SMD) of 70 μm vs. the latter's 30 μm. Evidently, compared with air-assisted injectors, airless injector has higher risks of urea deposits and less mixing efficiency with exhaust stream. To overcome these challenges from airless injector, significant development work has been performed. The optimized mixing structure is able to improve NOx reduction uniformity, eliminate urea deposits, and improve NOx conversion efficiency while satisfy existing installation packing space. This development demonstrates that good system performance can be achieved despite the challenges of meeting strict and often-conflicting performance targets. System perspectives combined with assessment and understanding of major components / mechanisms are critical to achieving aggressive performance targets while reducing development time for urea SCR systems.