Evolution of Lean-NOx Traps on PFI and DISI Lean Burn Vehicles

This paper charts the development of three three-way catalyst (TWC) and four lean-NOx trap (LNT) formulations in four vehicle systems over a four-year period. All LNTs were installed in an underbody location behind a close-coupled TWC on vehicles equipped with either port fuel injection (PFI) or direct injection spark ignition (DISI) engines. In addition to the standard regulatory European drive cycles, a series of steady-state tests were conducted to determine changes in LNT NOx efficiency with increasing NOx storage, and changes in the levels of individual nitrogen-containing exhaust components. Each vehicle system was subjected to a durability cycle up to an equivalent of 80,000 km. The early LNT formulations on systems ‘1’ and ‘2’ suffered from inadequate thermal durability with system efficiencies for NOx deteriorating to ≤ 55% after vehicle aging under lean operating drive cycle conditions (from ≥ 80% when fresh). As a result, improvements were made to the LNT formulations on systems ‘3’ and ‘4’. Only minor deterioration in lean NOx performance was observed after vehicle aging on these systems (≥ 90% NOx efficiency after aging). These improvements have been such that ECE Stage III and IV NOx emission targets have been achieved after aging with PFI and DISI lean-burn gasoline technology, respectively.