The present invention generally relates to a catalytic converter for the purification of exhaust gases and, more particularly, to a catalytic converter utilizing solid honeycomb type catalyst carriers.
The type of catalytic converter to which the present invention pertains is largely used in an automobile for the substantial purification, or minimization, of pollutants present in exhaust gases emitted from an automobile internal combustion engine. In general, in a catalytic converter utilizing a solid honeycomb type catalyst carrier, the carrier has numerous design limitations and, because of these design limitations, a carrier having a relatively small length for a given pattern of flow of exhaust gases therethrough is considered to be most desirable. Speaking conversely, if the carrier has a relatively large length, not only does the manufacture of the carrier by the use of the existing extrusion technique involve difficulties, but similar difficulties are also involved in depositing a catalyst on surfaces of macropores, which communicate with generally parallel closely adjacent flow passages in the carrier, at a portion substantially intermediate of the length of the carrier. In addition, the greater the length of the carrier, the more likely the occurrence of uneven distribution of heat energies evolved in the carrier, which may in turn result in undesirable thermal deformation of the carrier thereby reducing the durability of the catalytic converter as a whole.
In order to improve the handling capacity of the catalytic converter with respect to a relatively large amount of exhaust gases, the employment of two or more catalyst carriers in one catalytic converter has fairly recently been practised. Shown in FIGS. 1 to 3 of the accompanying drawings is one example of the prior art catalytic converter utilizing two catalyst carriers, reference to which will now be made.
Referring to FIGS. 1 to 3, particularly to FIG. 1, the prior art catalytic converter comprises a cylindrical metallic casing 10 having generally frusto-conical metallic end closure members 11 and 12 secured at respective ends of the casing 10 and protruding outwardly therefrom in alignment with the longitudinal axis of the casing 10. Each of the end closure members 11 and 12 is in the form of a duct, and the end closure members 11 and 12 provide respective inlet and outlet openings of the casing 10. The casing 10 has a pair of catalyst carriers 13 and 14 installed therein in spaced relation to each other and in axially aligned relation to the casing 10, each of the catalyst carriers 13 and 14 being of a honeycomb type having a plurality of generally parallel closely adjacent flow passages 13a or 14a extending longitudinally of the carrier 13 or 14. Each of the carriers 13 and 14 also has a catalyst deposited on surfaces of macropores communicating with the flow passages 13a or 14a and surfaces defining the flow passages 13a and 14a.
The prior art catalytic converter shown is so designed that the exhaust gases entering the casing 10 through the end closure member 11 can flow into the space S between the carriers 13 and 14 through the flow passages 13a in the carrier 13 in contact with the catalyst and, after having been stirred in the space S to enable the exhaust gases to be introduced uniformly into the flow passages 14a in the carrier 14, flow towards the end closure member 12 and then towards the atmosphere through the flow passages 14a in the carrier 14 in contact with the catalyst.
In order to avoid any possible vibrations of the catalyst carriers 13 and 14 relative to the casing 10 which would result in damage to the catalyst carriers 13 and 14, the prior art catalytic converter further comprises an annular spacer ring 15 for retaining the catalyst carriers 13 and 14 in substantially permanently spaced relation to each other and a cushioning layer 16 or 17 for each catalyst carrier 13 or 14 positioned within an annular clearance between the casing 10 and the corresponding catalyst carrier 13 or 14. The annular spacer ring 15 is of a type having an outer diameter substantially equal to the inner diameter of the casing 10 and having a substantially intermediate portion 15a radially inwardly recessed to provide a generally U-sectioned spacer. Within the casing 10, this annular spacer ring 15 is installed in such a manner that the opposed end portions of the spacer ring 15 overlie respectively the opposing inner ends of the associated catalyst carriers 13 and 14 through the cushioning layers 16 and 17 on the catalyst carriers 13 and 14 and the spacer 15a protrudes a predetermined distance radially inwardly into the space S to keep the catalyst carriers 13 and 14 positioned on respective sides of such spacer 15a.
Each of the cushioning layers 16 and 17 is made of a web of metallic mesh fabric or a knitted web of ceramic fibers and is mounted on the corresponding catalyst carrier 13 or 14 by wrapping the cushioning web therearound prior to the catalyst being inserted into the casing 10.
Because of the employment of the separate cushioning layers 16 and 17, the illustrated prior art catalytic converter requires time-consuming and cumbersome assemblage which tends to adversely affect the cost of the catalytic converter. Furthermore, since the cushioning layers 16 and 17 on the respective catalyst carriers 13 and 14 are mounted in compressed sandwich fashion within the casing 10, a portion of one or both of the cushioning layers 16 and 17 tends to be squeezed into the space S, in a manner such as shown in FIG. 3, through a clearance between the spacer ring 15 and the corresponding catalyst carrier 13 and 14. This often happens when the catalyst converter is assembled by inserting the catalyst carrier 13 or 14 with the corresponding cushioning layer 16 or 17 thereon into the casing 10, then inserting the annular spacer ring 15 and finally inserting the other catalyst carrier 14 or 13 with the corresponding cushioning layer 17 or 16 thereon into the casing 10.
Other examples of prior art catalytic converters utilizing two or more catalyst carriers are disclosed in the U.S. Pat. Nos. 3,937,617 and 3,969,083, both patented in 1976. The catalytic converter disclosed in the first mentioned U.S. patent is substantially similar to that discussed above with reference to FIGS. 1 to 3, except that the annular spacer ring used therein has a rectangular cross section. The catalytic converter constructed according to the first mentioned U.S. patent has the cushioning layers held within the casing in compressed sandwich fashion after having been inserted into the casing together with the catalyst carrier and they are bonded to the casing by the utilization of a plug welding technique which applies weld deposits from apertures in the casing into the cushioning layers thereby simultaneously connecting the cushioning layers to the wall of the casing and closing the apertures in the casing.
As compared with the prior art catalytic converter of the construction shown in FIGS. 1 to 3, the prior art catalytic converter disclosed in the first mentioned U.S. patent appears to be satisfactory and effective in terms of rigid retention of the catalyst carrier within the casing, but the employment of the plug welding method causes the assemblage of the catalytic converter to be not only time-consuming and cumbersome, but also expensive.
The second mentioned U.S. patent discloses a catalytic converter utilizing at least two catalyst carriers housed within the casing in spaced relation to each other and spaced from each other by means of an annular spacer ring of substantially U-shaped cross section while the outer peripheries of the respective catalyst carriers are held in direct contact with the casing. In order to attain a rigid connection between the catalyst carriers in spaced relation to each other, the peripheral edges of the catalyst carriers, which face each other in the assembled condition of the catalytic converter, are chamfered on one hand and a pair of cushioning rings, each made of a knitted sleeve of metallic wire compressed into a generally triangular cross-sectional configuration, are positioned within respective triangular-section spaces defined by the chamfered peripheral edge of the corresponding catalyst carrier, the casing and the annular spacer ring.
In the prior art catalytic converter disclosed in the second mentioned U.S. patent, since the catalyst carriers are in direct contact with the casing, there is the possibility that the thermal efficiency of the catalyst carriers will be adversely be affected to such an extent as to reduce the purifying efficiency of the catalytic converter.