Thermal Durability of Wall-Flow Ceramic Diesel Particulate Filters

The maximum use temperature of a diesel particulate filter is often thought to be limited only by the melting point of the filter material itself. This paper suggests that the maximum practical use temperature for filters is limited not by the intrinsic filter melting temperature, but by the temperature at which the metal oxide ash collected from the engine exhaust sinters and adheres to the filter wall, or the temperature at which the filter undergoes eutectic melting by reaction with the ash. Ash sintering and adherence without reaction with the filter material may result in loss of filter permeability and a permanent pressure drop increase. Chemical reactions between the ash and the filter can result either in pinholes through the walls, which compromise filtration efficiency, or glazing on the surface of the walls, which increases back pressure. These metal oxide deposits that comprise the ash are derived from engine wear, oil additives, fuel additives, and corrosion of the manifold and other exhaust system components.

Excessive temperatures can be avoided by employing sophisticated regeneration controls and/or by employing filters that have a high volumetric heat capacity and/or a high thermal conductivity. Several new monolithic DPFs are under development that exhibit a unique combination of high thermal shock resistance and high volumetric heat capacity. Temperatures generated within these filters during uncontrolled regenerations are substantially lower than those observed for other ceramic oxide filters, resulting in little or no reaction between the filter and ash under severe operating conditions.