An exhaust gas emitted from diesel engines contains fine particles (particulate matter) based on carbon such as soot and high-boiling-point hydrocarbons. When such exhaust gas is released in the atmosphere, it may adversely affect human beings and the environment. For this reason, a ceramic honeycomb filter, which may be called “honeycomb filter” in short later, has been disposed in an exhaust pipe connected to a diesel engine to purify an exhaust gas by removing particulate matter. As shown in FIGS. 8(a) and 8(b), a conventional honeycomb filter 20 comprises a ceramic honeycomb structure comprising porous cell walls 2 forming a lot of flow paths 3, 4, and an outer peripheral wall 1, and plugs 6a, 6b alternately sealing both ends 8, 9 of the flow paths 3, 4 in a checkerboard pattern. The outer peripheral wall 1 of the honeycomb filter is fixed to a holding member (not shown) formed by a metal mesh or a ceramic mat, and placed in a metal container (not shown).
In the honeycomb filter 20, an exhaust gas flows into the flow paths 3 that are open at an exhaust-gas-entering-side end 8 and sealed at an exhaust-gas-exiting-side end, as shown by the dotted arrow. Particulate matter in the exhaust gas is captured by pores in the cell walls 2 while the exhaust gas passes through the pores. The purified exhaust gas exits from the flow paths 4 that are sealed at an exhaust-gas-entering-side end and open at an exhaust-gas-exiting-side end 9. As the particulate matter is continuously captured in the cell walls 2, the pores of the cell walls are clogged, resulting in increased pressure loss. The honeycomb filter is regenerated by burning the accumulated particulate matter by a burner or a heater. However, since energy is consumed to burn the particulate matter, a regeneration interval is preferably as long as possible. Required to that end are that the honeycomb filter has small initial pressure loss, and that the pressure loss of the honeycomb filter does not increase drastically after the honeycomb filter has captured particulate matter.
The honeycomb filter is required to have small pressure loss while maintaining high capturing efficiency of particulate matter. As schematically shown in FIG. 2, it is considered that the pressure loss of the honeycomb filter is a sum of an inlet loss P1 generated when an exhaust gas flows into the inlet-side end 8, an outlet loss P2 generated when the exhaust gas exits from the outlet-side end 9, a cell wall loss P3 generated when the exhaust gas passes through the cell walls 2, and a flow path loss P4 generated by friction with the cell walls while the exhaust gas flows through the flow paths 3, 4. The cell wall loss P3 is a major fraction of the pressure loss of the filter, in particular largely contributing to increase in the pressure loss after the particulate matter is captured. Accordingly, technologies have been proposed to reduce the cell wall loss.
JP 2003-40687 A discloses a honeycomb filter having a porosity of 55 to 65%, and an average pore size of 15 to 30 μm, the total area of pores exposed to a cell wall surface being 35% or more of the area of the cell wall. This reference describes that the adjustment of the porosity, etc. of cell walls makes it possible to have high particulate-matter-capturing efficiency and low pressure loss, and that the permeability of the cell walls influencing the cell wall loss P3 is preferably 1.5 to 6 μm2. Although JP 2003-40687 A describes how to reduce the cell wall loss P3, it fails to teach the adjustment of the length and cross section area of flow paths to reduce the flow path loss P4.
JP 2002-239322 A discloses a porous ceramic honeycomb structure comprising cell walls having a thickness of 0.1 to 0.3 mm and a pitch of 1.4 to 3 mm, and flow paths each having a cross section area of 1.3 mm2 or more and a side as long as 1.15 mm or more, the surface area of the filter per a unit volume being 7 cm2/cm3 or more. This reference describes that these adjusted parameters enable highly efficient capturing of particulate matter with low pressure loss, and that with too small a pitch of cell walls, the exhaust gas entering an inlet-side end 8 undergoes a large inlet loss P1. This description suggests that the outlet loss P2 also increases. However, JP 2002-239322 A fails to teach how the total pressure loss of the honeycomb filter changes when the pitch of cell walls is made as small as 3 mm or less, which leads to a smaller cell wall loss P3 and larger inlet and outlet losses P1, P2. With the flow path loss P4 not considered, it is impossible to know from the description of JP 2002-239322 A how the total pressure loss of the honeycomb filter changes, for instance, when the flow paths are elongated with the pitch of cell walls unchanged, although it increases the total area of cell walls 2, presumably leading to a smaller cell wall loss P3 and a larger flow path loss P4.
WO 2003/074848 discloses a honeycomb filter, in which the length 1 (mm) of a longer side of a cross section of each flow path and the length L (mm) of the flow path satisfy the relation of 60≦L/1≦500, and the surface roughness Ra of flow path walls is 100 μm or less. WO2003/074848 describes that there is a large flow path loss P4, when the flow paths are too long, or when the area of a cross section perpendicular to the longitudinal direction of each flow path, which may be called simply “cross section area of flow path,” is too small (a small cell wall pitch with the cell wall thickness unchanged). However, no attention is paid to an increased flow path loss P4 and a decreased cell wall loss P3 due to an increased total area of cell walls, in the case of long flow paths or a small cell wall pitch, and it is not known from the description of WO 2003/074848 how the total pressure loss of the honeycomb filter changes by increase in the flow path loss P4 and decrease in the cell wall loss P3.
JP 2003-515023 A, a prior art reference indicating the relation between the length of flow paths and the pressure loss of a honeycomb filter, discloses a ceramic filter having a bulk density of at least about 0.50 g/cm3, and a length/diameter ratio not more than about 0.9. Showing the relation between the length of the honeycomb filter and its pressure loss when a cell wall thickness, a cell wall pitch and a honeycomb filter volume are constant, JP 2003-515023 A teaches that as the length of the honeycomb filter becomes smaller (in this case, the cross section area perpendicular to the flow paths increases because of a constant volume), the pressure loss of the honeycomb filter decreases. Namely, although the cell wall loss P3 does not change because of a constant total area of cell walls, the flow path loss P4 decreases as the flow paths become shorter, resulting in a decreased total pressure loss. When the length of the honeycomb filter (flow path length) changes with the cross section area of the honeycomb filter unchanged, the flow path loss P4 decreases, and the cell wall loss P3 increases. However, it is not known from the description of JP 2003-515023 A how the total pressure loss changes.
As described above, although it can be presumed from the cell wall pitch and the flow path length whether each of four types of loss P1 to P4 increases or decreases, it is not easy to determine how the pressure loss of the honeycomb filter obtained by totaling four types of loss changes.
JP 9-299811 A describes a honeycomb structure having a ratio L/d of a length L to a diameter d in a range of 0.4 to 1.3, a cell wall thickness of 0.1 mm or less, and the number of flow paths of 100/cm2 or more. However, this honeycomb structure is not constructed to have reduced pressure loss despite high spalling resistance together with high exhaust-gas-cleaning performance. Accordingly, no hint can be obtained from JP 9-299811 A about how to design a cell wall pitch and a flow path length to reduce the pressure loss of the honeycomb filter.
As described above, although it can be presumed whether each of four types of loss P1 to P4 increases or decreases depending on the cell wall pitch and the flow path length, it is not easy to know how the pressure loss of the honeycomb filter, a total of the four types of loss, changes. Accordingly, the development of honeycomb filters has been conducted by a trial-and-error method of repeatedly producing various honeycomb filters until preferred properties are achieved.
The cell wall pitch and the flow path length affect temperature elevation while regenerating a honeycomb filter (burning particulate matter); the longer the flow paths, the higher the temperature near an exhaust-gas-exiting-side end 9, causing melting damage.