The present disclosure relates to catalytic converters for treatment of exhaust, e.g., from internal combustion engines. More particularly, the disclosure relates to a method of manufacturing catalytic converters having fragile substrates.
Newer “thin-wall” substrates in catalytic converters offer significant advantages over traditional catalytic converter substrates, including a greater geometric surface area per unit volume and faster light-off due to the lower thermal mass of the substrate. As is generally understood in the art, faster light-off translates to higher conversion efficiency since catalytic converters are not effective during a cold engine start until they reach operation, or light-off temperature. However, thin wall substrates are significantly more fragile and are subject to fracture during stressful manufacturing operations that including stuffing, sizing, and burnoff operations.
One method currently employed to manufacture a catalytic converter includes wrapping the substrate in a matting material, or mat, and stuff the substrate and mat into a metal housing through the use of a stuffing cone, the cone serving to compress the matting so that it can slide into the housing (see FIG. 1). The mat serves to support the substrate, insulate the housing from the high temperatures reached within substrate, and protect the substrate from shocks and vibrations. Although converter housings are most often cylindrical, they can be other shapes as well, such as having elliptical or oval cross sections.
Depending on the type of mat, whether it is intumescent or non-intumescent, the necessary stuffing pressure varies. Intumescent mats are called such because they swell under high temperature. This swelling is a property of a component of the mat, typically vermiculite. Non-intumescent mats do not contain vermiculite. These matting materials are well known in the art and are available from 3M, Minneapolis Minn. as well as from Unifrax Co., Niagra Falls, N.Y. The swelling property of intumescent mats is useful because it helps to maintain a positive pressure between the substrate and the housing during the thermal cycle imposed on the converter in normal use. In use, the diameter of the metal housing increases due to thermal expansion to a greater degree than that of the ceramic substrate. Thus, to maintain a positive pressure, it is advantageous to employ a mat that swells to fill the growing gap as the temperature rises. Non-intumescent mats must be stuffed under much greater force to a high level of compression in order to ensure a continued positive pressure between the substrate and housing during use. This high-force stuffing is more time consuming and takes considerable energy, which significantly increases the overall production cost of the converter.
Once the substrate and matting material is stuffed into the housing, the housing may be sized and appropriate connections are formed for assembly into an exhaust system. Sizing operations, when necessary, compensate for variations in substrate diameters, and may comprise compressing the housing to produce an overlapped seam, and then welding, or a housing may be reduced by drawing or compressing the housing using a pipe-sizer.
Exhaust pipe connections may be formed in or welded onto either end of the housing. The connections include portions having varying cross-sections to conform the stream entering the converter to the shape of the substrate, thereby allowing exhaust to flow smoothly from the engine into and out of the converter, and through the remaining exhaust system to the tail pipe.
After the exhaust pipe connections are formed on the housing, the converter is ready to be assembled into an engine. During the converter's first use, the converter is heated to normal operating temperature, which may be anywhere from 300° C. to more than 500° C. This first use or heating drives off organic binders within the mat and causes the intumescent material within the mat to greatly expand, thus increasing the pressure within the confines between the housing and substrate. Some substrates, particularly the newer, more fragile substrates, can fail under this pressure, rendering the entire converter unusable.
A graph showing estimated matting pressure verses time during the manufacturing method described above is provided in FIG. 5. Starting at the left side of the graph, the intumescent matting is stuffed under very low pressure, i.e., less than 10 pounds per square inch (psi). The pressure is greatly increased to about 150 psi during the sizing operation. After sizing, the matting responds by relaxing somewhat, reducing the stress therein and the pressure to about 100 psi. The exhaust pipe connections are then formed in or welded to the housing, which does not affect the pressure of the matting. Finally, the converter is heated, e.g., during its first use, which causes swelling of the matting, which increases the pressure by about 80 psi to 180 psi. The pressure may be even higher locally within the matting material due to variations in the matting or the substrate itself. The fragile ceramic substrate is sometimes unable to stand up to these high pressures and fail.
It would be desirable to reduce the likelihood of breaking the ceramic substrate during the production or first use of a catalytic converter.
The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by sizing a housing of a catalytic converter over a substrate and intumescent mat subsequently to heating the converter, the heating causing the intumescent mat to at least reach a temperature at which the intumescent mat swells.
BRIEF DESCRIPTION OF THE DRAWINGS
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.
The present invention will now be described by way of example with references to the accompanying drawings, in which:
FIG. 1 shows a flow chart illustrating a method to produce a catalytic converter;
FIG. 2 shows a first step in the construction of a catalytic converter, according to one embodiment of the disclosed method;
FIG. 3 shows a subsequent step in the construction of a catalytic converter, according to one embodiment of the disclosed method;
FIG. 4 shows a subsequent step in the construction of a catalytic converter, according to one embodiment of the disclosed method;
FIG. 5 shows a pressure-time diagram illustrating the advantages of the method shown in FIG. 1; and
FIG. 6 shows a pressure-time diagram illustrating the disadvantage of another method.
A flow chart diagramming a method for manufacturing a catalytic converter is shown in FIG. 1. The method will be described to some extent by reference to FIGS. 2, 3, and 4. It has been found that by heating the material prior to sizing reduces the internal matting pressure against the fragile substrate and therefore reduces the likelihood of breakage thereof.
Thus, in the preferred process, in a first step 12, intumescent matting 27 is wrapped around a substrate 25, and the substrate 25 with matting 27 is stuffed into a housing 29. This operation is ordinarily conducted through the use of a stuffing cone 30, as shown in FIG. 1. The stuffing cone compresses matting 27 to a diameter the same as or slightly smaller than the smallest potential diameter of housing 29, according to manufacturing tolerances, thus allowing matting 27 and substrate 25 to slide into place within housing 29.
The stuffing operation is done under low pressure, and low mount Gap Bulk Density (GBD). The GBD defines the level of mat compression in grams per cubic centimeter. The preferred mount density for the stuffing operation is less than about 0.7 g/cm3. It is also preferred that the mount density be greater than about 0.6 g/cm3.
After stuffing, substrate 25 and matting 27 are positioned within housing 29 as shown in FIG. 2, and the assembly is heated in an oven to undergo burn-off and expand step 14 (FIG. 1). It is preferred that the assembly be heated to a temperature greater than about 500 degrees Celsius, During this step, organic binders are burned off and the vermiculite or other intumescent component of matting 27 swells.
Following burn-off/expand step 14, housing 29 is sized by step 16 to bring the GBD to approximately 1.0 g/cm3. The target size for each housing may vary depending upon the size of the substrate. Alternatively, the target size may be a dimension that is common for all converters being manufactured that is optimally determined to satisfy GBD requirements within reasonable tolerances.
There are several known methods for sizing housing 29. A preferred sizing technique is described in commonly-assigned U.S. patent application Ser. No. 09/141,299, filed Aug. 27, 1998 by Michael R. Foster, et al., which is incorporated herein by reference in its entirety. In this method, each substrate 25 is individually measured to determine its dimensions prior to stuffing into a housing that does not have any slits. The housing is then sized by compressing it from all directions in a radial press, thereby plastically deforming the housing until it reaches the target size. Such sizing devices are generally known for expanding and diametrically compressing pipes.
Another method that can be used, sometimes referred to as a tourniquet or shoebox method, is best for cylindrical housings having one or two slits and includes compressing or tightening the housing, e.g., with a strap or press, until the proper size is reached or the compressing force reaches a selected stop force, then welding the seem or seams. Housing 29 is reduced in size until the target size is reached or a selected stop-force sensed by the sizing machine. In this manner, sizing step may compensate for variations in the size of the substrate.
After the sizing step 16, mat 27 undergoes a relaxation step 18 in which the mat material partially relaxes, reducing the internal pressure. Finally, the connection ends 32 are added to housing 29 by form/weld step 20. Form/weld step 20 may comprise any known method of forming connection ends onto housing 29, either by welding them to the housing 29 or by deforming housing 29 to shape the connection ends. In one preferred embodiment, housing 29 extends some distance on either side of substrate 25 as shown in FIG. 2, and undergoes a spin-form process in which rollers progressively shape either end until connection ends 32 are formed as seen in FIG. 3.
FIG. 4 shows a diagram showing estimated pressure changes in an example according to the method described above. Initial mounting pressure is shown at the left side of the diagram to be less than 10 psi. Burn-off/expansion increases the pressure to about 80 psi. Note that a similar pressure increase occurs in the current process shown in FIG. 5 during the burn-off/expand step 14. Subsequent to the burn-off expand step 14, pressure increases again during sizing step 16, during which internal mat pressure increases to about 150 psi. This same pressure is attained in the current method shown in FIG. 5 because, as noted above, the internal pressure subsequent to the sizing operation is dependent upon the GBD of the mat. Relaxation step 18 reduces internal pressure by about 50 psi to about 100 psi. Since matting 27 has already been expanded in expansion step 14, subsequent heating in use of the device will not increase the pressure within the mat to such a degree that substrate 25 is likely to fail.
While preferred embodiments have been shown and described, various modifications and substitutions maybe made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.