Accumulation of lubricant and fuel derived ash in the diesel particulate filter (DPF) during vehicle operation results in a significant increase of pressure drop across the after-treatment system leading to loss of fuel economy and reduced soot storage capacity over time. Under certain operating conditions, the accumulated ash and/or soot cake layer can collapse resulting in ash deposits upstream from the typical ash plug section, henceforth termed mid-channel ash deposits. In addition, ash particles can bond (either physically or chemically) with neighboring particles resulting in formation of bridges across the channels that effectively block access to the remainder of the channel for the incoming exhaust gas stream. This phenomenon creates serious long-term durability issues for the DPF, which often must be replaced. Mid-channel deposits and ash bridges are extremely difficult to remove from the channels as they often sinter to the substrate.
The current study is comprised of analyzing field-return/field-aged DPF units exhibiting variations in ash bridging, characterized by high resolution X-ray CT, XRD, XRF and SEM-EDS. X-ray CT with a transmission X-ray source (voxel size ~700nm) was utilized for direct and accurate 3D visualization of the individual ash particles (which have an average size of 1-2μm), catalyst substrate structure and the ash which penetrates into the substrate surface pores. Data from X-ray imaging and ash chemical makeup, combined with data from field operation provide a deeper insight into understanding mechanisms that are responsible for mid-channel deposits. Details about the sample preparations necessary for X-ray CT, the combination of CT data and other characterization techniques, the comparison between on-road/off-road operating conditions and long-term durability implications of the DPF due to mid-channel ash deposits and ash bridging will be discussed. In addition, an open source CFD tool is discussed in the context of simulating 3D flow within DPF inlet channels which contain mid-channel deposits.