CLEERS Teleconference: Eric Hruby, John Deere

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

Eric Hruby
John Deere

Abstract:

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-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-SCR architecture has been most commonly selected to meet the highway and non-road heavy duty emission regulations worldwide.

Engines in off road vehicles typically 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-SCR, DOC-SCR on filter|SCR and DOC-uncatalyzed DPF|SCR, respectively.

Emission tests were performed on a 4.5L engine, while separate ash loading and thermal aging tests were carried out on a 6.8L engine connected to engine dynamometers. During steady state operations at fixed DEF dosing rates, an unexpected continuous and repeatable NOx performance degradations were observed on several systems containing either SCR on filter or uncatalyzed DPF when exhaust temperatures exceed 450°C. These systems shared a common design feature where DEF injection was upstream of either the SCR on filter or the uncatalyzed DPF components. Similar NOx performance degradations were observed when different samples of DOC were tested. Two reference architectures, DOC|uncatalyzed DPF – SCR and DOC-SCR systems, did not exhibit NOx performance degradations when tested under same conditions.

Several experiments to investigate the cause of NOx performance degradations focused on these two architectures. In the case of DOC-SCR on filter architecture, when steady state testing was repeated without a DOC, NOx performance remained stable. In the case of a DOC-uncatalyzed DPF|SCR architecture which displayed degraded NOx performance, reconfiguring the system by relocating the uncatalyzed DPF to a position before the DEF injection enabled the NOx performance to be fully restored.

NOx performance changes of SCR on filter due to thermal aging and ash loading were also evaluated. Thermal aging was accelerated through repeated active regenerations initiated by in-cylinder post injection upstream of DOC to maintain 600 °C inlet temperature to the SCR on filter. Large NOx performance loss was observed at exhaust temperatures above 450 °C after thermal aging.

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