Metal working – Method of mechanical manufacture – Catalytic device making
Reexamination Certificate
2000-03-24
2002-05-21
Cuda-Rosenbaum, Irene (Department: 3726)
Metal working
Method of mechanical manufacture
Catalytic device making
C029S428000
Reexamination Certificate
active
06389694
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing a metal carrier of a catalytic converter to be used for an exhaust gas purifying device of internal combustion engine.
2. Related Art Statement
Conventionally, it has been practiced to manufacture a metal carrier of a catalytic converter to be used for an exhaust gas purifying device of internal combustion engine, by alternatively overlapping a pair of flat metal foil
2
and wavy metal foil
3
such as made of heat-resistant stainless steel, spirally winding them in this state, and joining respective top portions of waves of the wavy metal foil
3
to a corresponding surface of the flat metal foil
2
such as by brazing, as shown by a reference numeral
1
in FIG.
1
.
The thus manufactured catalyst carrier
1
made of metal has a number of exhaust gas passages defined by the flat metal foil
2
and wavy metal foil
3
. As shown in
FIG. 2
, wash coat liquid
4
is applied onto the surfaces of the exhaust gas passages with dipping, and then dried. Thereafter, catalyst is carried on the surfaces of the wash coat liquid
4
to thereby manufacture an exhaust gas purifying catalyst.
Once the exhaust gas gets into the exhaust gas passages of the exhaust gas purifying catalyst, reaction target substance within exhaust gas moves to a surface of the catalyst due to diffusion so that a predetermined chemical reaction progresses. As a result, generated substance moves from the catalyst into the exhaust gas, and is then discharged into the atmosphere.
Thus, exhaust gas purifying rate or speed is limited by: a transfer rate of the reaction target substance onto a catalyst surface; a chemical reaction rate at the catalyst surface; and a transfer rate of a generated substance from the catalyst surface. If the exhaust gas purifying rate is fast, the exhaust gas purifying catalyst may have a short length, and if the exhaust gas purifying late is slow, it is necessary to provide an exhaust gas purifying catalyst having a length sufficient for purifying a harmful substance within exhaust gas.
(1) It is a matter of course that an exhaust gas purifying catalyst preferably to have a higher reaction efficiency and having a shorter axial length is preferable.
On the other hand, in addition to the aforementioned demand, exhaust gas purifying catalyst is preferable to satisfy the following conditions.
(2) It is said that, in an automobile in which an exhaust gas purifying catalyst is used, a ratio of harmful exhaust substance to be discharged just after starting of engine, in relation to an entire quantity of harmful exhaust substance, is 50% or more. Therefore, it is extremely important that a temperature rising rate of exhaust gas purifying catalyst just after starting of engine is high so as to contribute to removal of harmful exhaust substance.
By the way, commonly used platinum based catalyst normally functions at a temperature of 350° C. or higher. It is accordingly preferable that an exhaust gas purifying catalyst reaches this activating temperature as soon as possible after starting of engine.
Meanwhile, as previously described in relation to
FIG. 1
, in a conventional exhaust gas purifying catalyst, catalyst carrier
1
has a structure having overlapped flat metal foil
2
and wavy metal foil
3
. Therefore, as shown by &agr; in
FIG. 2
, it is impossible to avoid occurrence of surfaces intersecting with each other at an acute angle interiorly of the exhaust gas passage.
It is also impossible to avoid that an unnecessarily large amount of wash coat liquid
4
is coated between the surfaces intersecting with each other at an acute angle (&agr;) within the exhaust gas passage, since the wash coat liquid
4
is coated onto an inner surface of the exhaust gas passage by a dipping method as described above so that the wash coat liquid
4
concentrates to the aforementioned area due to a surface tension.
As such, in the conventional catalyst carrier
1
for an exhaust gas purifying catalyst, in addition to a problem of cost increase due to adhesion of excessive amount of wash coat liquid, there have been caused such problems that:
Deterioration of catalyst reaction efficiency due to reduction of catalyst carrying surface area inevitably leads to a long and large exhaust gas purifying catalyst, so that the aforementioned demand (1) is not satisfied; and
Increase of heat capacity due to adhesion of excessive amount of wash coat liquid prolongs a time required for temperature rise up to the activating temperature of exhaust gas purifying catalyst after starting of engine, so that the aforementioned demand (2) is not fully satisfied.
Further, it has been also confirmed that the following problems are caused in the conventional catalyst carrier
1
for exhaust gas purifying catalyst.
Namely, flow rate of exhaust gas entering the catalyst carrier
1
is not uniform. Generally, high speed exhaust gas flows such as from an exhaust pipe having a diameter of about 60 mm or less into the catalyst carrier
1
having a diameter of approximately 100 mm, so that the flow rate is high at a center portion and low at a peripheral portion of the catalyst carrier
1
.
At the center portion of catalyst carrier
1
at which the flow rate is high, temperature of wall surface rises within a short period of time just after starting of engine. However, at the peripheral portion of catalyst carrier
1
at which the flow rate is low, wall surface does not reach an activating temperature unless a considerable period of time has passed after starting of engine, resulting in that the temperature elevation of wall surface just after starting of engine is delayed during which unpurified harmful substance continues to flow out.
To solve the aforementioned problems, as described in Japanese Patent Application Opened No. 309277/93, there has been proposed a countermeasure to penetratingly form a number of holes in a flat metal foil and a wavy metal foil so as to diffuse the exhaust gas in a radial direction within the carrier.
However, in case of penetratingly forming a number of holes newly in the flat metal foil and wavy metal foil, it is required to provide another process for forming the holes, leading to increase of cost. Among other things, it has been confirmed that the degree of performance improvement of the peripheral portion of carrier is low relative to the increased cost, thus this is not practical.
It was pointed out in the above that the heat capacity of the conventional catalyst carrier
1
is one of the reasons which make the delay of the temperature rise of catalyst after starting of engine. In addition, a low heat transfer rate from exhaust gas to wall surface of carrier is the other reason which makes the delay of the temperature rise of catalyst.
Considering here a heat transfer rate of exhaust gas to a wall surface of carrier, it is apparent that: the shorter the distance between the exhaust gas and the catalyst surface, the shorter the period of time required that all of reactants reach the catalyst surface and are substituted by reaction products by transference of reactants within the exhaust gas passage.
To shorten the distance between exhaust gas and catalyst surface, it is sufficient to reduce a cross sectional area of the exhaust gas passage insofar as the cross sectional shapes are identical. Further, concerning a cross sectional shape of exhaust gas passage, the object of interest is achieved by flattening the cross sectional shape to thereby shorten a distance between opposing wall surfaces of exhaust gas passage.
Concerning the latter cross sectional shape of exhaust gas passage, in “Analytical Investigation of the Performance of Catalytic Monoliths of Varying Channel Geometries Based on Mass Transfer Controlling Conditions”, Society of Automotive Engineers, Automotive Engineers Congress, Feb. 25, 1974, there has been presented a calculation result obtained by: successively changing a cross sectional shape of exhaust gas passage such as into triangular, circular, square, rectangular sha
Cuda-Rosenbaum Irene
Knobbe Martens Olson & Bear LLP
Nagoya University
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