Sappok, A., Kamp, C., and Wong, V., "Sensitivity Analysis of Ash Packing and Distribution in Diesel Particulate Filters to Transient Changes in Exhaust Conditions," SAE Int. J. Fuels Lubr. 5(2):733-750, 2012, doi:10.4271/2012-01-1093.
Current CJ-4 lubricant specifications place chemical limits on diesel engine oil formulations to minimize the accumulation of lubricant-derived ash in diesel particulate filters (DPF). While lubricant additive chemistry plays a strong role in determining the amount and type of ash accumulated in the DPF, a number of additional factors play important roles as well. Relative to soot particles, whose residence time in the DPF is short-lived, ash particles remain in the filter for a significant fraction of the filter's useful life. While it is well-known that the properties (packing density, porosity, permeability) of soot deposits are primarily controlled by the local exhaust conditions at the time of particle deposition in the DPF, the cumulative operating history of the filter plays a much stronger role in controlling the properties and distribution of the accumulated ash. Results of this work indicate that short-duration, transient, high temperature events can have a profound impact on ash packing and DPF pressure drop, while exposure to transient high flow rate conditions produce only marginal changes.The tests conducted in this work utilized core samples, removed from DPFs containing known ash levels, to study the effect of transient changes in exhaust conditions on ash properties using a flow bench. Use of the core samples enabled more precise control of DPF exposure to transient variations in exhaust flow and temperature conditions. In addition to the filter performance characterization, application of advanced diagnostics including scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), and x-ray diffraction (XRD) provide insights into the changes in ash properties induced by the variations in exhaust conditions, useful to explain the observed results. Enhanced understanding of the fundamental mechanisms controlling ash properties and their impact on DPF performance is useful to not only extend the filter's service life, but more importantly to improve both DPF control and on-board diagnostic capabilities.