Diesel particle filters (DPFs) efficiently eliminate soot, fuel-, and oil-ash emissions of diesel engines, but little data are available with respect to long term aging or deterioration effects of DPFs under real world operating conditions. Aging of wash coat- and catalyst-materials, catalyst poisoning, ash sintering, adsorption and long lasting storage of semi- or non-volatile substances can take place, which over time may influence filtration and conversion properties of DPFs.
Herein we report to what extent DPF aging may affect particle filtration characteristics. We compared particle number concentrations (PN), and particle mass (PM) emissions after a 2000 operating hours endurance test (VFT2). Such a controlled field test is required by VERT verification procedures, which lately were published as a national standard (SNR 277205). Five different DPF systems, all operated for >2000 hrs, either in city bus, refuse truck or construction engine applications, were compared with respect to PN- and PM-filtration efficiencies. In general, only small effects were found for gaseous emissions with no deterioration for particle number-based filtration efficiencies (PNFEs). In all cases excellent PNFEs of 99.43 ± 0.62 were reached, which is valid for all particle size classes from 10 to 300 nm.
However, particle-mass based filtration efficiencies (PMFEs) were considerably lower and varied to a large extent. In cases of precious metal coated DPFs or DPFs operated after diesel oxidation catalysts (DOCs), even negative PMFEs were noticed, indicating that some additionally formed particles or aerosols must be released from such DPFs, most pronounced under full load conditions.
Store- and release-effects of sulphates, formed via SO2→SO3 oxidation on precious metal catalysts, was identified as the main cause for these phenomena. Hence, it can be concluded, that particle mass measurements are more sensitive to such sulphate formation artefacts than solid particle number measurements. We therefore recommend, as foreseen in the VERT procedures, that solid particle number measurements should be used for filter characterization.