Ash Transport in Diesel Particle Filters 2012-01-1732
Lubricant oil derived ash deposits still represent a major issue in diesel particulate filter operation in vehicles. In literature various ash deposition patterns are described. The two boundary deposition patterns are (a) wall layer and (b) filling at the back end of the inlet channels. The patterns are often associated with different regeneration methods. Continuous regeneration is supposed to result in a homogeneous ash layer, whereas periodic (active) regeneration is reported to result in back end filling. The current contribution describes the basic mechanisms associated with ash transport phenomena in particle filters. On the basis of (a) frequency of ash exposure to flow (b) ash particle structure re-entrainment and finally (c) axial ash transport the different deposition pattern can be explained. Exposure to flow accomplished by periodical soot removal, either by passive or active regeneration is the first step. Subsequent re-entrainment of deposited particle structures exposed to flow depends on the level of axial flow velocity and particle properties, amongst which the agglomerate/particle structure size is considered a key parameter allowing re-entrainment in the flow. It is postulated, that the particle agglomerate structure can be influenced by frequency of regeneration and regeneration conditions (especially temperature, NOx/PM). Since most diesel particle filters are operated in axial inlet velocity ranges above 10 m/s the particle structures can be re-entrained in the flow and finally be transported by pneumatic conveying in the inlet channel to the filter end. On the basis of an analysis of multiple filters from real field operation under different operational conditions the mechanisms laid out are explained. Operational areas are identified, that outline the parameters needed to be control in order to achieve optimum ash deposition patterns in particulate filters, finally allowing to extend filter cleaning intervals or to optimize the filter sizing.