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Journal Article

Passive Ammonia SCR System for Lean-burn SIDI Engines

Lean-burn Spark Ignition Direct Injection (SIDI) engines offer potential fuel economy savings, however, lack of cost-effective lean NOx aftertreatment systems has hindered its broad application. Lean NO Trap (LNT) and Urea Selective Catalytic Reduction (SCR) technologies have been widely investigated as possible solutions, but they both have considerable drawbacks. LNT catalysts suffer from high Platinum Group Metals (PGM) cost, poor thermal durability, sulfur poisoning and active SO regeneration requirements. Urea SCR systems require a secondary fluid tank with an injection system, resulting in added system cost and complexity. Other concerns for urea SCR include potential freezing of the urea solution and the need for customers to periodically fill the urea reservoir. In this paper we report a low-cost, high efficiency concept that has the potential to be a key enabler for lean-burn gasoline engines.
Technical Paper

Experimental Study of NOx Reduction by Passive Ammonia-SCR for Stoichiometric SIDI Engines

As vehicle fuel economy requirements continue to increase it is becoming more challenging and expensive to simultaneously improve fuel consumption and meet emissions regulations. The Passive Ammonia SCR System (PASS) is a novel aftertreatment concept which has the potential to address NOx emissions with application to both lean SI and stoichiometric SI engines. PASS relies on an underfloor (U/F) SCR for storage of ammonia which is generated by the close-coupled (CC) TWCs. For lean SI engines, it is required to operate with occasional rich pulses in order to generate the ammonia, while for stoichiometric application ammonia is passively generated through the toggling of air/fuel ratio. PASS serves as an efficient and cost-effective enhancement to standard aftertreatment systems. For this study, the PASS concept was demonstrated first using lab reactor results which highlight the oxygen tolerance and temperature requirements of the SCR.
Technical Paper

The Effects of CO, H2, and C3H6 on the SCR Reactions of an Fe Zeolite SCR Catalyst

Selective Catalytic Reduction (SCR) catalysts used in Lean NOx Trap (LNT) - SCR exhaust aftertreatment systems typically encounter alternating oxidizing and reducing environments. Reducing conditions occur when diesel fuel is injected upstream of a reformer catalyst, generating high concentrations of hydrogen (H₂), carbon monoxide (CO), and hydrocarbons to deNOx the LNT. In this study, the functionality of an iron (Fe) zeolite SCR catalyst is explored with a bench top reactor during steady-state and cyclic transient SCR operation. Experiments to characterize the effect of an LNT deNOx event on SCR operation show that adding H₂ or CO only slightly changes SCR behavior with the primary contribution being an enhancement of nitrogen dioxide (NO₂) decomposition into nitric oxide (NO). Exposure of the catalyst to C₃H₆ (a surrogate for an actual exhaust HC mixture) leads to a significant decrease in NOx reduction capabilities of the catalyst.