NOx Performance Degradation of Aftertreatment Architectures Containing DOC with SCR on Filter or Uncatalyzed DPF Downstream of DEF Injection

Eric Hruby; Shyan Huang; Ramesh Duddukuri; Danan Dou

doi:10.4271/2019-01-0740

SCR on filter, also known as SCRoF, SCRF, SDPF, has been utilized to meet the stringent light duty Euro 6 emission regulations. Close-coupled DOC-DEF-SCR on filter with underfloor SCR architectures, offer a balance of NOx performance at cold start and highway driving conditions. In contrast, the DOC-DPF-DEF-SCR architecture has been most commonly selected to meet the on-road and non-road heavy duty emission regulations worldwide.

Diesel engines applied to off road vehicles can operate under higher loads for extended times, producing higher exhaust temperatures and engine out NOx emissions. New European Stage V emission regulations will mandate diesel particulate filter (DPF) adoption because of particulate number and more stringent particulate mass requirements. Three aftertreatment architecture choices with diesel particulate filters (DPF) were evaluated as candidates to fulfill the Stage V emission regulations. The studied architectures were: DOC-uncatalyzed DPF-DEF-SCR, DOC-DEF-SCR on filter-SCR and DOC-DEF-uncatalyzed DPF-SCR, respectively.

During steady state operations, an unexpected continuous and repeatable NOx performance degradation occurred at inlet temperatures exceeding 450 °C on systems containing either SCR on filter or uncatalyzed DPF located downstream of the DEF injection. NOx performance degradations were observed over multiple tests with different samples of DOC. Additional reference testing on two architectures, DOC-uncatalyzed DPF-DEF-SCR and DOC-DEF-SCR systems, did not exhibit NOx performance degradation when exposed to the same conditions.

Additional experiments were conducted to investigate the cause of NOx performance degradations. In the case of DOC-DEF-SCR on filter architecture, when tested without a DOC upstream, performance remained stable throughout testing, and did not show degradation. In the case of a degraded DOC-DEF-uncatalyzed DPF|SCR architecture when reconfigured by relocating the uncatalyzed DPF to a position before the DEF injection, demonstrated NOx performance was fully restored, and did not exhibit further degradation. These architectural studies helped isolate the cause of degradation to the filter downstream of the DEF injection system.

NOx performance changes of SCR on filter due to thermal aging and ash loading were also evaluated. Significant NOx performance loss was observed at exhaust temperatures above 450 °C after thermal aging, while expected levels of NOx performance losses were observed after ash loading.

Chemical analyses of several SCR on filter and uncatalyzed DPF’s samples from this study revealed measurable amounts of platinum (Pt), providing a strong hypothesis linking the NOx performance degradation to increased parasitic NH₃ oxidation by O₂, for both DOC-DEF-SCR on filter-SCR and DOC-DEF-uncatalyzed DPF-SCR architectures.

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NOx Performance Degradation of Aftertreatment Architectures Containing DOC with SCR on Filter or Uncatalyzed DPF Downstream of DEF Injection 2019-01-0740

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