Honeycomb wall-flow filters are used to remove carbonaceous soot from exhaust of diesel engines. FIG. 1A shows a conventional honeycomb wall-flow filter 100 having an inlet end 102, an outlet end 104, and an array of interconnecting porous walls 106 extending longitudinally from the inlet end 102 to the outlet end 104. The interconnecting porous walls 106 define a grid of inlet channels 108 and outlet channels 110. At the inlet end 102, the outlet channels 110 are end-plugged with filler material 112 while inlet channels 108 are not end-plugged. Although not visible from the figure, at the outlet end 104, the inlet channels 108 are end-plugged with filler material while the outlet channels 110 are not end-plugged. Each inlet channel 108 is bordered on all sides by outlet channels 110 and vice versa. FIG. 1B shows a close-up view of the cell structure used in the honeycomb filter. The porous walls 106 defining the inlet and outlet channels (or cells) 108, 110 are straight, and the inlet and outlet cells 108, 110 have a square cross-section and equal hydraulic diameter.
Returning to FIG. 1A, diesel exhaust flows into the honeycomb filter 100 through the unplugged ends of the inlet channels 108 and exits the honeycomb filter through the unplugged ends of the outlet channels 110. Inside the honeycomb filter 100, the diesel exhaust is forced from the inlet channels 108 into the outlet channels 110 through the porous walls 106. As diesel exhaust flows through the honeycomb filter 100, soot and ash particles accumulate on the porous walls 106, decreasing the effective flow area of the inlet channels 108. The decreased effective flow area creates a pressure drop across the honeycomb filter, which leads to a gradual rise in back pressure against the diesel engine. When the pressure drop becomes unacceptable, thermal regeneration is used to remove the soot particles trapped in the honeycomb filter. The ash particles, which include metal oxide impurities, additives from lubrication oils, sulfates and the like, are not combustible and cannot be removed by thermal regeneration. During thermal regeneration, excessive temperature spikes can occur, which can thermally shock, crack, or even melt, the honeycomb filter.
It is desirable that the honeycomb filter has sufficient structural strength to withstand thermal regeneration. To avoid the need for frequent thermal regeneration, it is also desirable that the honeycomb filter has a high capacity for storing soot and ash particles. For a cell structure in which the inlet and outlet channels have equal hydraulic diameter, the effective flow area of the inlet channels can easily become much smaller than that of the outlet channels, creating a large pressure drop across the honeycomb filter. One solution that has been proposed to reducing this pressure drop involves making the hydraulic diameter (or effective cross-sectional flow area) of the inlet channels larger than that of the outlet channels. In this way, as soot and ash particles accumulate on the inlet portion of the porous walls, the effective flow area of the inlet channels will tend to equalize with that of the outlet channels.
For the conventional honeycomb cell structure shown in FIG. 1B, the hydraulic diameter of the inlet cells 108 can be made larger than the outlet cells 110 by reducing the hydraulic diameter of the outlet cells 110. FIG. 1C shows the honeycomb cell structure of FIG. 1B after reducing the hydraulic diameter of the outlet cell 110 such that the outlet cell 110 now has a smaller hydraulic diameter in comparison to the inlet cell 108. Another modification that can be made is to increase the hydraulic diameter of the inlet cells 108. This modification has the advantage of increasing the effective surface area available for collecting soot and ash particles in the inlet portion of the honeycomb filter, which ultimately increases the overall storage capacity of the honeycomb filter. FIG. 1D shows the honeycomb cell structure of FIG. 1C after increasing the hydraulic diameter of the inlet cell 108. Without changing the cell density of the honeycomb filter, any increase in the hydraulic diameter of the inlet cell 108 would produce a corresponding decrease in the thickness of the wall between the adjacent corners of inlet cells 108 (compare t2 in FIG. 1D with t1 in FIG. 1C). As the wall between the corners of the inlet cells become thinner, the structural strength of the honeycomb filter decreases, making the honeycomb filter more susceptible to thermal shock and cracking during thermal regeneration.
From the foregoing, there is desired a method of improving the storage capacity of the honeycomb filter while maintaining good flow rates through the honeycomb filter without significantly reducing the structural strength of the honeycomb filter.