1. Field of the Invention
The invention relates to a catalytic converter for the treatment of exhaust gas, and a process for the production of a catalytic converter. The catalytic converter is intended in particular for installation in the exhaust gas pipe of an internal combustion engine--for example of the gasoline engine of a road vehicle.
2. Description of the Prior Art A catalytic converter disclosed in British Patent publication 2 048 105 has an oblong metallic casing and two cores, which are arranged therein and each of which has a ceramic element having an approximately oval cross-section and passages for the exhaust gas, and a catalytically active coating. The casing has two shells with curved main sections, which together form a generally cylindrical lateral part of approximately, oval cross-section, and end walls which are connected to its two ends and are inclined toward the longitudinal axis of the catalytic converter and are provided with holes in the center. The shells have flanges which project outwardly away from their curved main sections and are welded to one another. These flanges are essentially parallel to a plane extending between the two shells, and rest against one another with flat surfaces in cross-sections at right angles to the weld seams. An intermediate layer is arranged between the inner surface of the casing and each core. The intermediate layer has a collar consisting of wire fabric and a collar of an elastic layer which inflates on heating.
The commercially available ceramic elements may have shapes differing greatly from the intended ideal shapes and may be, for example, more or less curved in the form of a banana instead of a cylinder. Furthermore, the actual dimensions may differ relatively greatly from the intended ideal dimensions. For the ideal cross-sectional dimensions of, typically, about 10 cm to 30 cm and the ideal lengths of about 30 cm to 60 cm, the deviations from the ideal dimensions are, for example, often more than 1 mm. In addition, the catalysts are usually heated to temperatures of about 750.degree. C. to 950.degree. C. during operation. This heating causes expansions, the metallic casing expanding to a substantially greater extent than the ceramic element.
Since the ceramic elements are brittle, they should be firmly held in the casing, both in the cold and in the warm state and also during the vibrations which occur during operation, but should not be subjected to excessive compressive forces. The above-mentioned intermediate layers of the catalytic converter are intended to compensate the deviations of the shape and of the dimensions from the ideal shape and the ideal dimensions, and also the different expansions of the metallic casing and of the ceramic element caused by heating. However, if the deviations are too great, they are frequently insufficiently compensated by the intermediate layer. If the ceramic elements are then subjected to an excessive compressive force by the casing, at least in localized areas, this can cause damage--such as cracks or fractures. If, on the other hand, the ceramic elements are held only loosely in the casing of a catalytic converter, for example, installed in a motor vehicle, they may be destroyed in a short time by the vibrations and impacts arising during use of the motor vehicle. Similar problems may also occur in the case of catalytic converters whose cores, instead of ceramic elements, have elements which consist of another material and restrict the passages for the exhaust gas to be treated. In the case of catalytic converters having casings whose shells have flanges parallel to a plane, there is therefore the danger that there will be a great deal of waste during production and/or that the catalytic converters will be damaged after being used for only a short time.
U.S. Pat. No. 4,925,634 discloses catalytic converters having casings whose shells, where edge sections are welded to one another in pairs, rest against one another at the surfaces which are at least approximately flat, and at right angles to a plane extending through the different edges. The casings contain a core and an intermediate layer arranged between it and the inner surface of the casing. In the production of a catalytic converters of this type, the core and the intermediate layer are placed between the two shells. The two shells are then inserted one into the other, pressed against one another with a predetermined compressive force and welded at their edge sections. Catalytic converters and production processes which are more or less similar are also disclosed in British Patent disclosure 2 047 557 and European Patent publication 0 278 455.
The shells for the casings of such catalytic converters are usually produced from originally flat pieces of sheet metal by deep drawing. However, it is practically impossible to produce, directly by deep drawing, shells having edge sections which are directly adjacent to their edges and have surfaces parallel to the displacement direction of the deep drawing die.
For the production of such shells, it is therefore necessary, for example, first to form shell-like workpieces during deep drawing, which workpieces have flanges projecting outwardly at right angles to the stated displacement direction. These flanges must subsequently be removed. Removal of the flanges requires at least one additional, relatively complicated cutting process and also results in loss of material. The production of shells having edge sections of the described type which can be inserted one into the other, is therefore relatively expensive.
In the case of catalytic converter whose shells have edge sections which are inserted one into the other and welded to one another in the manner described, they can be elastically deformed very slightly at the most, if at all. This also constitutes a certain disadvantage since the edge sections cannot contribute anything toward compensating the different changes, caused by temperature changes, in the cross-sectional dimensions of the cores and shells.