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Control-Oriented Modeling of a LNT-SCR Diesel After-Treatment Architecture
Abstract:
Lean NOx trap (LNT) and Selective Catalytic Reduction catalysts
(SCR) are two leading candidates for diesel NOx after-treatment.
Each technology exhibits good properties to reduce efficiently
diesel NOx emissions in order to match the forthcoming EURO 6
standards.
NOx reduction in LNT is made through a two-step process. In
normal (lean) mode, diesel engine exhausts NOx is stored into the
NOx trap; then when necessary the engine runs rich during limited
time to treat the stored NOx. This operating mode has the benefit
of using onboard fuel as NOx reducer. But NOx trap solution is
restrained by limited active temperature windows. On the other
hand, NH₃-SCR catalysts operate in a wider range of temperature and
do not contain precious metals. However, NH₃-SCR systems
traditionally use urea-water solution as reducing agent, requiring
thus additional infrastructure to supply the vehicles with enough
reducer. These pros and cons are quite restrictive in classical LNT
or NH₃-SCR architecture.
The present paper presents an after-treatment architecture
combining a NOx trap and a passive NH₃-SCR. Synergy of the two
systems is possible if the SCR takes advantage of the LNT ability
to produce Ammonia (NH₃). Indeed, during the rich phases (purges),
small amounts of Ammonia are formed as by-product, which can be
used in the downstream catalyst as the NOx reducing agent.
A major difficulty to operate the proposed architecture is the
real-time management of the NOx purge: for future control and
diagnosis applications, it is crucial to have accurate but
low-complexity models. A complete reduced model of the
physicochemical phenomena involved is proposed in the present
paper. Based on simplified chemical assumptions and time scale
separation, the latter is suitable for on-board diagnostics and
model-based control. Validation has been achieved through extensive
experiments.