Engineers at GM’s R&D Center are developing a new low-cost concept for NOx control from lean-burn spark ignition direct injection (SIDI) engines: passive ammonia SCR.
Lean-burn (i.e., a high air-to-fuel ratio) SIDI (e.g., gasoline) engines, with a higher charge mass and higher temperature spread, can deliver improved thermal efficiency and better fuel economy. However, SIDI engines also have the potential for increased pollutant formation, and require either careful control of combustion for reduced engine-out emissions, or an exhaust after-treatment systems similar to those under development for diesel vehicles.
There are basically two technologies currently used for NOx reduction: Lean NOx Traps, and Urea Selective Catalytic Reduction. Each has drawbacks, noted GM’s Wei Li in a presentation at the recent Department of Energy DEER conference in Dearborn, Michigan.
Lean NOx Trap (LNT) catalysts suffer from high platinum group meals (PGM) cost, poor thermal durability, sulfur poisoning and a requirement for active desulfation.
While urea SCR systems offer pretty high efficiency with a wide temperature window, they require a second fluid tank with injection system, a warming solution to keep the urea from freezing, and periodic customer intervention.
GM’s passive ammonia SCR concept places a conventional three-way catalyst upstream of an SCR catalyst. Short periods of rich engine operation generate ammonia (NH3) on the three-way catalyst (TWC), which is stored on the SCR. During subsequent lean operations, the NOx that breaks through the TWC converters is converted by the NH3 stored on the SCR catalysts.
GM’s work with the concept to date has been focused on proof-of concept: the ability to make ammonia, to store it on the SCR catalyst and to convert NOx, as well as on catalyst durability under the higher temperatures of gasoline operation.
GM found that if ammonia formation is limited only to periods of richer operation such as acceleration, GM found no significant fuel penalty.
One limiting factor is the declining capability of SCR catalysts to store ammonia at higher temperature. GM found that at about 400 ° C, there is very little capacity left. To get around that, Li said, GM looked at placing multiple SCR catalysts in line. SCR 1, closer to the engine, operates at an optimal temperature for most of the ECE driving cycle. Once the exhaust temperature gets too hot for SCR 1 to work, SCR 2, further away from the engine, picks up the slack. If you use a combination of catalysts, Li said, you have a potential to expand the temperature window for the technology.
Current testing shows the potential to meet Euro 6 targets with very little fuel economy penalty, and with no ammonia slip.
GM has concluded that passive ammonia SCR is a very-efficient low-cost alternative for lean-burn SIDI engines. There are plans in-house, Li said, to further develop the system for US applications, looking at a SULEV emissions target (Bin 2). (Earlier post.)
Wei Li, Kevin Perry, Kushal Narayanaswamy, Chang Kim and Paul Najt. Passive NH3 SCR for Lean-burn SIDI Engines (DEER 2009)
Wayne Leighty, Christopher Yang and Joan Ogden (2007) Advanced ICE Vehicles: An Assessment of the Technologies for Next Generation Vehicles. (Advanced Energy Pathways (AEP) Project Task 4.1 Technology Assessments of Vehicle Fuels and Technologies, Public Interest Energy Research (PIER) Program, California Energy Commission)