Feasibility Investigation of a High-Efficiency NOx Aftertreatment System for Diesel Engines

A high-efficiency NOx aftertreatment system has been proposed for use in Diesel engines. This system includes a Lean NOx Trap (LNT) in series with a Selective Catalyst Reduction (SCR) catalyst [6], [7], [8], and is hereinafter referred to as the LNT-SCR system. The combined LNT-SCR system can potentially overcome many of the drawbacks of LNT-only and SCR-only operation and achieve very high NOx conversion efficiency without external addition of ammonia (or urea). A laboratory test procedure was developed to validate the LNT-SCR system concept, and a series of tests was conducted to test the NOx conversion of this system under various conditions. A Synthetic Gas Reactor (SGR) system was modified to accommodate LNT and SCR catalyst cores and synthetic gas mixtures were used to simulate rich-lean regeneration cycles from a diesel engine. A Fourier Transform Infrared (FTIR) system was used to measure gas compositions within the LNT-SCR system.

Results from these preliminary tests validated the LNT-SCR concept operation and demonstrated that the LNT-SCR system can achieve higher NOx conversion efficiency than the LNT-only system at low temperatures (350°C and lower). However, at high exhaust temperatures (400°C and higher) the NOx conversion efficiency of the LNT-SCR system was lower than that of the LNT-only system. A considerable amount of work is still required to formulate a high-efficiency NOx aftertreatment system suitable for production vehicle applications, but these results indicate that the LNT-SCR concept works and merits further consideration.