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Understanding Chemical Composition and Phase Transitions of Ash from Field Returned DPF Units and Their Correlation with Filter Operating Conditions
Abstract:
Current and future diesel engine oil categories have specifications that impose limits on SAPS (Sulfated Ash, Phosphorous and Sulfur) levels that help to minimize accumulation of ash in the DPF originating from oil consumption in the engine. Metallic species in the oil formulation, mostly from detergents and anti-wear additives, have a significant impact on ash behavior when subjected to high temperatures during active regeneration of the filter. Certain compounds in the oil, especially derivatives of ZDDP (Zinc Dialkyl Dithiophosphate), interact with filter substrate and sinter at temperatures that the DPF is exposed to during active regeneration. This phenomenon of sintering or ‘ash wetting’ leads to ash-bridges in the DPF that result in reduction of soot storage capacity, higher back-pressure and possibly reduced active surface area for catalytic conversion of engine-out emissions. Changes in ash chemistry when subjected to high temperature transient events is irreversible and necessitates wet cleaning of the DPF to recover most of the filter performance.
Ash samples were collected from field returned DPF’s that were categorized into line-haul and a mix of ‘stop and go’ duty-cycles; these trucks had two different heavy-duty diesel engine types. Several techniques for materials and chemical characterization such as high-temp XRD (X-ray Diffraction), Raman Spectroscopy, ICP-OES, SEM-EDS amongst others were employed in the current study for an in-depth understanding of the composition, phase transition and crystal lattice structure of the samples. Additionally, DPF ash from two different oil formulations were evaluated for their wetting behavior on the filter substrate and changes in ash composition were documented. Ash that was rich in phosphorous compounds exhibited phase transitions which were different from the ash that was rich in calcium compounds. Crystallographic planes from phosphate compounds were identified and correlated with oil formulations and vehicle operating conditions.
Data generated in this study provides insights into ash morphology, chemistry and compositional changes when subjected to high temperatures in the DPF operating range. Analysis of ash from field returned DPF’s can be helpful in understanding filter operating history (regeneration conditions, duty cycles, engine oil type etc.), which then could be used as inputs for recommending filter service intervals, specific cleaning techniques and even DPF calibration based on mileage and duty-cycles.