The Passive Ammonia SCR System (PASS) concept is an integrated aftertreatment system that relies on the intrinsic performance characteristics of a close-coupled threeway catalyst (TWC) and an underfloor selective catalytic reduction (SCR) catalyst to holistically address both stoichiometric and oxygen-rich SI engine aftertreatment. The PASS concept combines the performance capabilities of a conventional TWC system and a urea-based SCR system without the cost and complexity of a dosing system. In its simplest form, PASS is an oxygen tolerant aftertreatment system that has been shown to work effectively for a range of homogeneous, stoichiometric SI engine applications. In addition, the PASS concept can be easily adapted for use on lean-burn applications and has been shown to work effectively for a range of stratified-charge SI engine applications. Critical to the performance of PASS are the efficient generation of ammonia on the close-coupled TWC catalyst and the storage of ammonia on the underfloor SCR catalyst. As such, achieving maximum performance from PASS is dependent on fully optimizing the performance and robustness of the key components - the close-coupled TWC and the underfloor SCR - for the specific application.
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