The accumulation of soot and lubrication-derived ash particles
in a diesel particulate filter (DPF) increases exhaust flow
restriction and negatively impacts engine efficiency. Previous
studies have described the macroscopic phenomenon and general
effects of soot and ash accumulation on filter pressure drop. In
order to enhance the fundamental understanding, this study utilized
a novel apparatus that of a dual beam scanning electron microscope
(SEM) and focused ion beam (FIB), to investigate microscopic
details of soot and ash accumulation in the DPF. Specifically, FIB
provides a minimally invasive technique to analyze the interactions
between the soot, ash, catalyst/washcoat, and DPF substrate with a
high degree of measurement resolution.
The FIB utilizes a gallium liquid metal ion source which
produces Ga+ ions of sufficient momentum to directionally mill
away material from the soot, ash, and substrate layers on a nm-μm
scale. As the FIB cuts into the sample, uncovering intra-layer
details, the coupled high resolution SEM imaging and energy
dispersive x-ray (EDX) analysis provide both morphological and
chemical data. This tool was applied to investigate soot and ash
accumulation in the DPF, with a specific focus on characterizing
interactions between the soot/ash/DPF interfaces, such as soot
penetration into the ash layer, as well as soot and ash
accumulation in the DPF surface pores. In particular, ash and soot
particle size, layer pore structure, and the extent of penetration
or intra-layer mixing, are all parameters directly impacting DPF
pressure drop, which may be quantified using this technique. The
work in this study leveraged existing databases of aged DPFs
containing various levels of soot and ash, originating from field
trials and controlled laboratory testing. Results obtained with
this technique provide a fresh and complementary perspective, as
well as additional details useful to understand the macroscopic
observations of the combined ash and soot effects on diesel
particulate filter pressure drop.