Patent Application
20080241007
The technical field of this invention is catalytic converters for engine exhaust gases.
One type of catalytic converter for engine exhaust gases provides a catalyst element comprising a monolithic, catalyst coated, ceramic substrate. The catalyst element is contained in a metal case having openings at opposing ends to allow exhaust gas flow through the substrate. Because the substrate is extremely hot during use, insulation is used between the substrate and the case to reduce heat transfer to the case. The insulation typically takes the form of an insulating mat wrapped around the substrate. In addition, inner metallic end cones or end rings are used to direct the flow of hot exhaust gases away from the insulation and through the substrate, so as to reduce heating of the case and prevent erosion of the insulating mat by this hot exhaust gas flow.
This construction has several problems. Hot exhaust gases can escape the substrate and flow through the volume containing the insulating mat. These gases can loosen components or pieces of the mat insulation material, which can get around the inner end cone or end ring and plug the face of high cell density catalysts. In addition, end cones require complex, expensive dies. Furthermore, the insulating mat must hold the substrate firmly in place within the case; and this can require high forces capable of destroying fragile ceramic substrates.
The catalytic converter of this invention cylindrically encloses a ceramic catalyst element in a metallic sheath that grips the catalyst element and has opposing open ends with at least one of these ends projecting axially beyond the axial end of the catalyst element. An insulating mat cylindrically surrounds the sheath; and a case cylindrically surrounds the insulating mat. The case has opposing axial end portions defining openings for gas flow; and the end of the metallic sheath that projects axially past the end of the catalyst element extends substantially to the inlet end of the case such that the sheath directs substantially all gas flow through the catalyst element and shields the insulating mat from the gas flow.
In a preferred embodiment of the invention, the opposing open axial ends of the metallic sheath are beaded or otherwise structured to be axially resilient relative to a remainder of the metallic sheath, and these ends engage an end portion of the outer case structure to more completely prevent hot, high energy gases from impinging on the insulating mat and to provide additional axial support for the catalyst element, so that the constrictive forces exerted by the mat on the catalyst element may be lowered.
The catalytic converter may be manufactured by the following method. A catalyst element is provided having a cylindrical shape with opposing axial ends. A rectangular metallic sheet, which may be provided with a resilient structure at one or both of opposing rectangular ends, is rolled into a cylindrical metallic sheath with one or both of the first and second axial ends extending beyond the axial length of the catalyst element. The sheath is wrapped and tightened around the catalyst element to grip and retain the catalyst element therein; and parts of the sheath are preferably permanently joined to provide a sheathed catalyst structure. A heat insulating mat is wrapped around the cylindrical metallic sheath; and the sheathed catalyst structure, wrapped by the heat insulating mat, is inserted in a cylindrical case such that the first beaded axial end of the sheath, if so designed, resiliently engages a first axial end portion of the cylindrical case. Finally, a second, opposing axial end portion is formed on the case such that the second beaded axial end of the sheath, if so designed, resiliently engages the second axial end portion of the cylindrical case, with the sheath defining a flow chamber containing the catalyst element between a pair of openings in the case for substantially all gas flow through the catalytic converter and optionally supporting the catalyst element in the case.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Referring to
Outer case 12 contains a generally cylindrical, monolithic catalyst element 30 with a cross-section that is preferably, but not necessarily, circular. Catalyst element 30 may be any such element, preferably comprising a ceramic substrate with a flow-through structure and coated with a catalytic substance to promote chemical activity in gasses flowing therethrough. Typically, although not exclusively, such substrates have a multi-path structure to maximize the catalyst coated area to which the gasses are exposed. Catalyst element 30 may have essentially flat axial ends and preferably extends most of the axial distance from end plate 16 to end plate 18. Such catalyst elements are well known in the art and will not be further described in detail except where necessary to further describe this invention.
Catalyst element 30 is retained in an inner sheath 40, which is made of a thin sheet metal alloy capable of withstanding the high temperatures and corrosive atmosphere inside catalytic converter 10. Examples of such an alloy are 439, fecrolloy, inconel or 321SS; and other examples may come to the mind of one skilled in the art. The axially central portion 42 of sheath 40 has a generally cylindrical shape matching that of the outer perimeter of catalyst element 30 and is wrapped tightly around catalyst element 30 to retain it. The inner surface of sheath 40 may be knurled or otherwise roughened to improve this retention, and the outer surface of sheath 40 may also be roughened, for reasons that will be apparent below.
At each axial end 44, 46 of inner sheath 40, it is provided with an axially resilient structure to provide a resilient engagement with the adjacent end plate 16, 18 respectively. The axially resilient structure may preferably, but not exclusively, take the form of a bead forming a roll in the metal that is convex as seen radially toward the axis and concave when viewed in the opposite direction, as shown in
An insulating mat 50 is wrapped about sheath 40 and has a radial thickness extending between sheath 40 and cylindrical case member 22 of outer case 20. Mat 50 is made generally of a heat insulating material normally used to provide heat insulation between a hot catalyst element 30 and outer case 20 but may have a lower mount density and be less tightly wrapped, since sheath 40 axially retains catalyst element 30. Mat 50 may or may not contain vermiculite particles, depending on the quality of the seal between sheath 40 and end plates 16, 18. Mat 50 is located in an annular space outside sheath 40 that is generally isolated from the hot exhaust gas flow through catalytic converter 10. Thus mat 50 is protected from damage by hot exhaust flow, and catalyst element 30 is protected from blockage by pieces of mat 50 that might otherwise be separated and blown into the direct exhaust path through the catalyst element. Mat 50 also, through the tension of its wrapping, assists sheath 40 in gripping catalyst element 30 and further provides resistance to movement of the sheath 40 and catalyst element 30 perpendicular to the axis. A roughened outer surface of sheath 40 may assist in this resistance to movement by providing a better grip between metallic sheath 40 and insulating mat 50.
The beading of the ends 44, 46 of metallic sheath 40 may not be required in all embodiments of the invention if (1) the constrictive force of insulating mat 50 applied to catalyst element 30 through metallic sheath 40 is sufficient to provide axial support for the catalyst element within case 50, and if (2) the ends of metallic sheath 40 are not required to actually engage case 50, especially at the inlet end, in order to adequately prevent entry of hot, high energy gases into the insulating mat. In some embodiments, such engagement will not be necessary if the end of metallic sheath 40 comes close to case 50 without engaging it, since very little gas flow will find its way through the gap, and such gases that do enter the insulating mat through that gap will have reduced kinetic energy due to the gap constriction and will thus not unduly erode or otherwise damage the insulating mat. Thus, this invention in its broadest sense requires that the inlet side end of the metallic sheath extends substantially to the case structure, where the word “substantially” means engaging or at least close enough to satisfy the design requirements of this paragraph.
In addition, if additional axial support of the catalyst element is not required from sheath engagement with the case in a particular embodiment, it may not be necessary to extent the sheath beyond the end of the catalyst element at the outlet end of the catalytic converter, since there is little problem with gases entering the insulating mat at this end. But, since the gases at the outlet end of the converter tend to be quite hot, it will be preferable in many embodiments to provide such extension of the sheath at the outlet end to improved thermal isolation of the case from the hot gases at the outlet end of the catalytic converter.
The method of manufacturing a catalytic converter as disclosed above will be described with reference to the flow chart of
The method continues at step 304 with rolling of a sheet into a cylindrical sheath closely matching the cylindrical shape of the catalyst element, with a roughened surface of the sheet preferably forming the inner surface of the sheath. Such a cylindrical sheath is shown in
In step 310, an insulating mat is wrapped around the sheath. This insulating mat is made of a heat insulating material or materials to insulate the outer case from the sheathed catalyst element, which becomes very hot in use. It further provides assistance to the sheath in radially fixing the catalyst element in the case. It may, however, be made less dense and need not be wrapped as tightly as it would be in a conventional converter of the prior art if the sheath includes resilient structure at its axial ends and is designed to engage the case and thus bear most of the retention loads in the axial direction. Alternatively, the insulating mat, if wrapped sufficiently tightly, may be sufficient to firmly hold the sheath on the catalyst element and may thus dispense with the need for permanent joining of parts of the metallic sheath by welding or other means.
The catalytic converter is finished in two final steps. In step 312, the sheathed catalyst element, wrapped in the insulating mat, is inserted axially into a case with a closing member or portion at one end to a specified position or, depending on the converter design, until one of the beaded axial ends of the sheath engages the closed axial end member or portion of the case. In step 314, the case is closed at the end through which the sheathed catalyst element was inserted. This is accomplished by attaching another end member or forming another end portion on the case in engagement with the other one of the beaded axial ends of the sheath. This step may be controlled to provide a resilient axial load on the sheath within a predetermined range to ensure positive axial and radial retention of the catalyst element within the case. It may be noted that, in the method described herein, the overlapping ends of the sheath, when joined by welding, may produce a slight discontinuity in the beaded ends that engage the end portions of the case, especially in embodiments such as those of
