Experimental Investigation of Combined LNT + SCR Diesel Exhaust Aftertreatment

Abstract

In the present work, an exhaust aftertreatment system combining lean NOX trap (LNT) plus exhaust bypass, passive selective catalytic reduction catalyst (SCR) and engine-independent LNT reductant supply by onboard exhaust fuel reforming was developed. Further, it was experimentally investigated in steady-state operation on an engine test bench and on road in a demonstrator vehicle. The intrinsic NH3 formation during the LNT regeneration was intensively studied as one key function for passive SCR without active urea dosing. LNT regeneration duration and temperature are the most significant parameters for NH3 emission. In steady-state operation, the passive SCR could be forced to a contribution to the total NOX conversion of up to 35 %. At 170 °C, 50 % steady-state total NOX conversion was achieved, and at 250 °C and a fuel consumption penalty of <2 %, complete NOX conversion could be demonstrated. Compared to conventional LNT operation with frequent engine enrichment, the engine-independent LNT regeneration with reformate gas is energetically more efficient and advantageous regarding CO2 emissions. Additionally, by shifting NOX reduction from LNT to SCR, fuel consumption penalty due to LNT enrichment can be reduced. Finally, the combination system was demonstrated in transient test cycles and by real driving operation on road.