NOx Performance Degradation of Aftertreatment Architectures Containing DOC with SCR on Filter or Uncatalyzed DPF Downstream of DEF Injection 2019-01-0740
SCR on filter, also known as SCRoF, SCRF, SDPF, is widely utilized to meet the stringent light duty Euro 6 emission regulations. DOC- SCR on filter|SCR architectures, offers a balance of NOx performance at cold start and highway driving conditions. However, DOC|DPF-SCR architectures are most commonly applied to meet the high way heavy duty emission regulations.
Off-road engines operate under higher loads at extended times with higher exhaust temperatures and engine out NOx emissions. Three aftertreatment architectures with diesel particulate filters(DPF) were evaluated as candidates to meet Stage V emission regulations. The studied systems were DOC|uncatalyzed DPF-SCR, DOC-SCR on filter|SCR and DOC-uncatalyzed DPF|SCR, respectively. Emissions performance testing utilized 4.5L non-EGR engine, and thermal aging and ash loading utilized a 6.8L engine mounted on a dynamometer as the test bed for emission evaluations. During operation under a steady state condition at constant DEF dosing rate, an unexpected but continuous NOx performance degradation was observed on systems containing SCR on filter and uncatalyzed DPF when exhaust temperature exceeds 450°C. Two systems exhibiting NOx performance degradation share a common design feature: DEF injection is upstream of SCR on filter or uncatalyzed DPF. Similar NOx performance degradation was observed with different samples of DOC tested. Degraded NOx performance was mostly recovered through DEF injection quantity increases from the initial value. In the case of DOC-SCR on filter system, when steady state testing was repeated without a DOC, NOx performance remains stable. In the case of DOC-uncatalyzed DPF|SCR system with degraded NOx performance, when the uncatalyzed DPF was relocated to a position before the DEF injection NOx performance was restored. Thermal aging with ash loading performance loss results for SCR on filter will also be reported. Chemical analysis of SCR on filter and uncatalyzed DPF revealed the presence of measurable amounts of platinum (Pt) and palladium (Pd), providing a strong hypothesis linking the NOx performance degradation to increased parasitic NH3 oxidation by O2 when DEF is introduced upstream of the filter.
Eric Hruby, Shyan Huang, Ramesh Duddukuri, Danan Dou