TWC+LNT/SCR Systems for Satisfying Tier 2, Bin 2 Emission Standards on Lean-Burn Gasoline Engines

Abstract

A laboratory study was performed to assess the potential capability of TWC+LNT/SCR systems to satisfy the Tier 2, Bin 2 emission standards for lean-burn gasoline applications. It was assumed that the exhaust system would need a close-coupled (CC) TWC, an underbody (U/B) TWC, and a third U/B LNT/SCR converter to satisfy the emission standards on the FTP and US06 tests while allowing lean operation for improved fuel economy during select driving conditions. Target levels for HC, CO, and NO x during lean/ rich cycling were established. Sizing studies were performed to determine the minimum LNT/SCR volume needed to satisfy the NO x target. The ability of the TWC to oxidize the HC during rich operation through steam reforming was crucial for satisfying the HC target. Temperature studies indicated that the CC TWC needed to operate at a minimum of 500°C to provide good steam reforming activity, while the LNT/SCR needed to operate between 300 and 350°C to satisfy the NO x slip target while minimizing the slip of NH 3 , N 2 O, and HC during the purges. Sulfur poisoning increased the HC slip by degrading the steam reforming reaction, and the sulfur increased the NO x slip by decreasing the NO x storage capacity of the LNT. Both the TWC and LNT/SCR could be desulfated with rich exhaust at 700°C. However, it was projected that the ability to obtain 700°C at the underbody LNT/SCR location would be difficult without additional exhaust hardware, such as fuel injectors or air pumps. Consequently, development of the LNT/SCR system was terminated in favor of a passive TWC+SCR approach because of its superior sulfur tolerance. Investigations into the passive TWC+SCR approach are discussed in a companion SAE paper.

Passive TWC+SCR Systems for Satisfying Tier 2, Bin 2 Emission Standards on Lean-Burn Gasoline Engines

“…Thus, it was concluded that the SCR catalyst was not significantly affected by sulfur poisoning, but the NH 3 production and steam reforming capability of the TWC were degraded significantly by the sulfur during continuous operation at temperatures near 500°C. It was shown in the companion paper on the TWC+LNT/SCR system [16] that the poisoned TWC could be desulfated with rich exhaust at 700°C. Such conditions are expected to be encountered routinely by close-coupled TWCs during cold-starts (from rapid heating strategies), accelerations, and high load operation.…”

“…In the companion paper on the TWC+LNT/SCR system [16], it was shown that the HC slip could satisfy the target level of 1.0 mg/mile as long as the temperature of the TWC was sufficiently high (e.g., 500°C) to provide good steam reforming during rich operation. These requirements also apply to the TWC+SCR system.…”

“…In the companion paper on the TWC+LNT/SCR system [16], exhaust modeling was used to determine that the LNT/SCR converter needed to be placed approximately 44″ downstream of the U/B TWC to maintain the LNT/SCR converter in its optimum temperature range of 300-350°C during the FTP. The SCR converter needs to be placed in a similar location to maintain its temperature near 300 to 320°C during the FTP.…”

“…The SCR catalyst then uses the stored NH 3 to reduce NO x during lean operation. A third approach referred to as a TWC+LNT/SCR system combines a LNT with a SCR catalyst for lean NO x control [4,5,6,7,8,9,10,11,12,13,14,15,16]. This LNT/SCR system was shown to demonstrate equivalent NO x conversion, lower NH 3 emissions, and lower PGM costs compared to a LNT of the same total volume [15].…”

“…This paper summarizes the work on the passive TWC+SCR system. A companion paper describes the work on the TWC+LNT/SCR approach [16]. For both systems, high emphasis was placed on achieving the HC targets as well as the NO x targets due to the low emission levels allowed for the Tier 2, Bin 2 standards.…”