Kamp, C., Sappok, A., Wang, Y., Bryk, W. et al., "Direct Measurements of Soot/Ash Affinity in the Diesel Particulate Filter by Atomic Force Microscopy and Implications for Ash Accumulation and DPF Degradation," SAE Int. J. Fuels Lubr. 7(1):307-316, 2014, doi:10.4271/2014-01-1486.
Inorganic engine lubricant additives, which have various specific, necessary functions such as anti-wear, leave the combustion chamber bound to soot particles (approximately ≤1% by mass) as ash , and accumulate in aftertreatment components. The diesel particulate filter (DPF) is especially susceptible to ash-related issues due to its wall-flow architecture which physically traps most of the soot and ash emissions. Accumulated lubricant-derived ash results in numerous problems including increased filter pressure drop and decreased catalytic functionality. While much progress has been made to understand the macroscopic details and effects of ash accumulation on DPF performance, this study explores the nano- and micron-scale forces which impact particle adhesion and mobility within the particulate filter.Several recent studies have revealed several important mechanisms influencing the nature of the soot and ash deposits, which if manipulated, could yield means of actively minimizing the ash-related impact on aftertreatment system performance [1, 2, 3, 4, 5,13]. The aim of this study is to measure and manipulate the attractive forces relevant to the soot/ash/DPF system and to identify their role in ash accumulation and catalyst aging. Interaction force measurements can be made directly in nano- and micron-scale agglomerated particle systems by use of atomic force microscopy (AFM). In this case single particles and small micron-sized particle clusters of both soot and ash are attached to AFM tipless cantilevers, where the particles act as the AFM probe tips. AFM force measurement is then used to measure several specific attractive interaction forces including those at the following interfaces: soot-soot, soot-ash, soot-DPF, ash-ash and ash-DPF. In this case, ‘attractive interaction forces’ includes the summation of most likely Van der Waals and electrostatic interaction. The aim of introducing this new experimental approach is to first, contribute to the fundamental understanding of the ash agglomeration process within emissions aftertreatment systems, specifically the DPF, and second, to suggest potential design characteristics and strategies for both lubricant formulations and aftertreatment system components to reduce the effects of lubricant-derived ash accumulation.