Detroit, Michigan – General Motors is developing a low-cost system to handle emissions from higher-efficiency engines, according to a report by the Green Car Congress.

Engineers at the automaker’s R&D centre are working on a passive ammonia selective catalytic reduction (SCR) system, which reduces nitrogen oxide (NOx) emissions produced by lean-burn spark ignition direct injection (SIDI) gasoline engines.

SIDI engines can produce better fuel economy and improved thermal efficiency, but can have increased pollutant formation. They require either combustion control, or an exhaust after-treatment similar to those used on diesel vehicles, such as urea injection.

The passive ammonia SCR concept uses a conventional three-way catalyst upstream of an SCR catalyst. Short periods of rich engine operation generate ammonia on the three-way catalyst, which is stored on the SCR and is used during subsequent lean operation to convert the NOx. The researchers found that limiting ammonia formation only to periods of richer operation, such as acceleration, resulted in no significant fuel penalty.

One limiting factor is the declining ability of SCR catalysts to store ammonia at higher temperature. The researchers are looking at placing multiple SCR catalysts in line, with each working as the catalyst ahead of it becomes too hot to work. The company said that there are plans to further develop the system for U.S. applications.

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