NOx Reduction using A Dual Stage Catalyst System with Intercooling in Vehicle Gasoline Engines Under Real Driving Conditions

Paper #:
  • 2018-01-0335

  • 2018-04-03
Selective Catalytic Reduction (SCR) of NOx is used in diesel fueled mobile applications where urea is an added reducing agent. We show that the Ultera® dual stage catalyst aftertreatment system (reference 2017-01-0920) intriniscally performs the function of the SCR method in nominally stoichiometric gasoline vehicle engines without the need for an added reductant. NOx is reduced during the low temperature operation of the dual stage system, benefiting from the typically periodic transient operation (acceleration & decelerations) inherent in mobile applications, as commonly expected and observed in real driving. The primary objective of the dual stage aftertreatment system is to remove NMOG and CO slip from the vehicle's TWC by oxidizing these constituents in the second stage catalyst. The system includes an interstage cooler and air injection to reduce the exhaust temperature to 350 °F - 450 °F to avoid the reformation of NOx. However, measurements show that a secondary benefit of this system is realized in mobile applications. During accelerations, the air/fuel ratio (AFR) control of many vehicles operates slightly rich producing ammonia in the TWC. Research has demonstrated that ammonia storage is optimum at lower temperatures, and at approximately 400 F, the second stage catalyst ammonia storage capacity is approximately 15 times more than typical SCR catalysts operating at higher temperatures. Consequently, during deceleration when the exhaust is oxygen-rich and NOx is high, the ammonia stored in the second stage acts as the reductant for NOx. Test results over standard FTP and US06 drive cycles have demonstrated that this mechanism is responsible for an additional NOx removal of up to 30%. Optimization of this system to improve the simultaneous hydrocarbon and NOx removal includes alternate temperature and airflow setpoints during accelerations vs. decelerations, and alternative catalyst formulations to improve storage and the SCR operation.
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