Lean-burn SIDI engine technology offers improved fuel economy;
however, the reduction of NOx during lean-operation
continues to be a major technical hurdle in the implementation of
energy efficient technology. There are several aftertreatment
technologies, including the lean NOx trap and active
urea SCR, which have been widely considered, but they all suffer
from high material cost and require customer intervention to fill
the urea solution. Recently reported passive NH₃-SCR system - a
simple, low-cost, and urea-free system - has the potential to
enable the implementation of lean-burn gasoline engines.
Key components in the passive NH₃-SCR aftertreatment system
include a close-coupled TWC and underfloor SCR technology. NH₃ is
formed on the TWC with short pulses of rich engine operation and
the NH₃ is then stored on the underfloor SCR catalysts. On
subsequent lean operations during the NEDC transient cycles, the
NOx that breaks through the TWC converter is reduced by
stored NH₃ on the SCR catalysts. The design of the TWC is crucial
for NH₃ generation and critical for HC/CO emission reduction due to
the low exhaust temperatures. In summary, the passive NH₃ SCR has
been demonstrated as an efficient and low cost lean NOx
aftertreatment technology for lean-burn SIDI. These and other
results including steady-state results will be presented.