Potential Application of SCR Technology to GDI Engines

Authors

Dr. H. S. Gandhi, Dr. J. R. Theis, Ford Motor Company, Dearborn

Year

2008

Print Info

Fortschritt-Berichte VDI, Reihe 12, Nr. 672

Summary

Stratified-charge direct-injection gasoline (GDI) engines can operate at lean A/F ratios for improved fuel economy. Previously, lean NOx traps (LNT) have been used on GDI engines for lean NOx control. Selective catalytic reduction (SCR) with NH3 has several advantages over a LNT, including a broader temperature window and increased robustness to the feedgas NOx level. SCR catalysts also do not need the rich purges required by NOx traps. Finally, SCR catalysts use inexpensive base metals instead of the precious metals used in NOx traps. However, a concern was that hot rich exhaust conditions (e.g., during trailer tow) could degrade the zeolite-based SCR catalysts being developed for mobile applications. Therefore, laboratory samples of an iron/zeolite SCR catalyst and a copper/zeolite SCR catalyst were aged at high temperatures using different A/F ratio schedules. Both SCR formulations exhibited high NOx performance after aging, indicating that both catalysts were sufficiently durable for GDI engines. The effects of sulfur on the SCR catalysts were investigated. An optimized combination of Fe and Cu SCR catalysts provided robust NOx performance with different NO concentrations at different temperatures. Finally, the effects of the NH3/NO ratio on the NOx conversion and NH3 slip were evaluated.

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