New Developments in Diesel Oxidation Catalysts and Diesel Particulate Filters 2003-26-0017
Stringent emissions legislation for diesel-powered vehicles, soon to go into effect, has led to new advances in both Diesel Oxidation Catalysts (DOC) and Diesel Particulate Filters (DPF). This paper reviews some of the new developments in DOC support design which lead to improved light-off behavior and higher overall emissions performance through lower thermal mass, higher Geometric Surface Area (GSA) and larger Open Frontal Area (OFA) than those afforded by the standard cordierite 400/6.5 cell configuration. The four different DOC supports examined in this paper include 400/4, 200/8, 300/8 and 400/6.5 - the last one serving as baseline. Both the modeling studies, verified independently by tests on engine bench and chassis dynamometer, and performance and durability parameters computed from cell configuration and physical properties clearly demonstrate the beneficial effects of lower thermal mass on light-off, higher geometric surface area on overall emissions performance, and larger open frontal area on pressure drop. In view of low operating temperature the thermal durability of DOCs is excellent; also, their mechanical durability is adequate.
The major developments in diesel particulate filter relate to their thermal durability during regeneration. Key among these are new materials like RC 200/19 cordierite with higher heat capacity and SiC with higher thermal conductivity and higher heat capacity than the standard 100/17 cordierite filter. Analyses of thermal stresses during regeneration of 144 mm round × 152 mm long filters show that SiC filter would experience one to two orders of magnitude higher stresses than the standard 100/17 cordierite filter due, primarily, to high CTE and E-modulus of the former notwithstanding its order of magnitude higher conductivity. Consequently, stress-relief mechanisms like segmentation are required to limit regeneration stresses in SiC filters to safe levels to ensure long-term thermal durability. Such design modifications, however, may induce new defect sites while adding to the cost. The RC 200/19 cordierite filter, on the other hand, combines the advantages of high heat capacity, low CTE and low E-modulus which help limit the regeneration stresses without stress-relief mechanisms.