GB2247413A

GB2247413A - Electrically heated catalytic converter

Info

Publication number: GB2247413A
Application number: GB9117846A
Authority: GB
Inventors: Paul Anthony Battiston; David Russell Lancaster; David Rudolph Sigler; Daniel William Wendland
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1990-08-31
Filing date: 1991-08-19
Publication date: 1992-03-04
Anticipated expiration: 2011-08-19
Also published as: DE4128924A1; GB9117846D0; GB2247413B; CA2049634A1

Abstract

An electrically heated catalytic converter (10) for use in purifying the exhaust gases of an internal combustion engine has a metal foil catalyst substrate (20) constructed of a corrugated foil strip (22) which is repeatedly folded back on itself to form a monolith brick (20). The fail strip (22) is coated with an electrically insulating coating (26, 28) to prevent shorting between adjacent layers of the monolith (20), thereby allowing the resistance of the monolith (20) to be tailored to allow the use of a voltage source exceeding the 12 volts typically used in automotive vehicle electrical systems. Ceramic mat (32) provides insulation between the monolith (20) and the casing (14). <IMAGE>

Description

-I- ELECTRICALLY HEATED CATALYTIC CONVERTER The present invention relates

to an electrically heated catalytic converter for use in purifying the exhaust gas of an internal combustion engine.

Some automotive vehicle exhaust systems incorporate catalytic converters and other exhaust emission control systems to assist in purifying the exhaust gases passing therethrough. A catalytic converter is a device which has a catalyst dispersed on the surface of a support structure through which the exhaust gases flow. As the exhaust gases pass through the catalytic converter, unburned hydrocarbons, carbon monoxide and oxides of nitrogen from the exhaust gases react with the catalyst to form carbon dioxide, water vapor and nitrogen respectively. For these reactions to occur, the exhaust must be held at the proper chemical composition, and the catalyst must be at a sufficiently high temperature. During warm-up after a cold start, pollutants flow through the converter substantially unreacted until the catalyst warms up to a light-off temperature at which reactions become significant. The efficiency of the catalyst then continues to increase until the converter reaches its fully warmed up operating temperature.

To reduce the amount of pollutants emitted from the tailpipe during the warm-up period, various schemes have been proposed to raise the converter temperature more quickly. These have included moving the converter closer to the engine, insulating the exhaust system between the engine and the converter, reducing the thermal mass of the exhaust system between the engine and the converter, and electrically heating the converter.

f US Patent 3,770,389 discloses an electrically heated converter constructed from plate-fin corrugated metal foil which is coated with alumina and wound into a spiral. The coating provides insulation between the adjacent foil layers and carries the noble metals used as the catalyst. The foil is heated by a voltage applied across its longitudinal extent.

A problem with the converter of US-3,770,389 is that electrical shorting between the foil layers can occur. Furthermore, tests have found that wound converters of the type of US-3,770,389 can fail by unwinding and extrusion of the - foil layers, causing durability concerns. Methods Used to prevent unwinding such as brazing or mechanical pinning of the foil can not be used since they result in metal-to-metal contact between the foil layers. Moreover, wound converters are limited to having either circular or oval cross-sections, and fourth, the plate-fin construction where the adjacent foil layers are in line contact increases the chances for shorting and also increases the thermal mass of the device which results in a longer time to reach the light-off temperature.

Another problem with the above mentioned prior art devices is that they require a very large current, about 500 to 600 amperes, from the vehicle's 12 volt battery. Such current ratings are undesirable from the standpoint of effect on battery life. Additionally, such low voltage/high current systems are undesirable from an efficiency standpoint since a significant portion of the system resistance and power dissipation occurs in the conductors.

The present invention seeks to provide an improved catalyst support for use in a catalytic 1 converter.

1 Accordingly, an aspect of the present invention provides a catalyst support for use in a catalytic converter comprising a metal foil monolith including a corrugated foil strip repeatedly folded back on itself across an inlet area of the monolith; an oxide layer covering substantially the entire surface of the foil strip; a heat resistant, electrically insulating layer covering the oxide layer to insulate electrically adjacent layers of the foil strip so as to establish a high resistance electrical path along the length of the foil strip; and electrical leads each connected to a respective end of the foil strip, wherein a voltage can be applied across the electrical leads to heat the catalyst support.

The metal foil strip is preferably a corrugated Fe-Cr-Al alloy strip. The preferred corrugation is an alternating chevron, or herringbone pattern which allows the foil strip to be repeatedly folded back on itself to form a monolith brick without the problem of the adjacent layers nesting within one another. Additionally, an advantage of a herringbone corrugation is that it reduces the possibility of electrical shorting between foil layers since contact between adjacent layers is limited to point-to-point contact rather than the line contact which occurs when the conventional plate-fin construction is used. Also, for a given inlet area, the herringbone corrugation usually requires less foil than a similarly performing plate-fin corrugation and, thus, can reduce thermal mass.

The resistance of the foil monolith is preferably tailored to match the voltage and power 1 available from the power supply, thereby achieving maximum heating efficiency. Such resistance tailoring may be a function of the resistivity and dimensions of the foil which, when a herringbone corrugation is adopted, must take into account the geometry of the herringbone corrugation. Where the resistance of the monolith is tailored for a given power source, a high voltage, low current system can be used. The use of such a power supply can eliminate several problems inherent in using the 12 volt power supply of the automobile battery.

The foil strip is coated with an insulating layer to insulate adjacent foil layers when folded. The metal foil strip has an oxide layer, preferably an aluminium oxide layer, covering substantially the entire surface of the strip. Although several oxide layer morphologies may perform satisfactorily (eg nodules, rosettes or blades), a layer having high aspect ratio whiskers is preferred. A heat resistant, electrically insulating layer can then be applied over this aluminium oxide layer.

The insulating layer may comprise an alumina gel coat followed by an alumina washcoat.

In practice, whiskers of the aluminium" oxide layer establish a capillary action which helps to ensure that the alumina gel coat solution, which is preferably applied before the washcoat, uniformly coats and tenaciously bonds to the foil surface. The gel coat can provide an excellent base for the application of the washcoat since the dried gel goes partially back into solution upon contact with the wet washcoat, thereby forming an excellent bond between the coatings.

Electrical leads are attached to each end of the foil strip and the foil monolith is preferably 1 wrapped in a nonconducting material (eg ceramic mat) to insulate the monolith from its outer shell. To heat the monolith, a voltage is applied across its length.

The invention can provide a heated metal monolith having a highly durable configuration which is not limited to the oval or round shapes of prior, spiral-wound foil converters. Additionally, the use of a high resistance design, which is resistance tailored using specific dimensional design criteria, allows the use of a high voltage, low amperage electrical source.

According to another aspect of the present invention, there is provided a catalytic converter comprising a catalyst support as herein defined and a voltage source adapted to produce a voltage greater than 12 volts.

An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawings, in which:Figure 1 is a perspective view, partially in section, of an embodiment of catalytic converter; Figure 2 is a schematic view of an electrically heated metal foil monolith of the catalytic converter of Figure 1; Figure 3 is a partial sectional view of several layers of the electrically heated metal foil monolith of Figure 2; Figure 4 is a plan view, partially in section, of several layers of the electrically heated metal foil monolith of Figure 2; and Figure 5 is a partial end view of the electrically heated metal foil monolith of Figure 2.

Figure 1 shows a catalytic converter assembly 10 used for purifying the exhaust gases 1 emitted from an internal combustion engine. The assembly 10 has an electrically heatable catalyst support unit 12 contained in canister 14, which comprises upper and lower mating shells. The canister 14 is configured to allow the catalytic converter assembly 10 to be coupled in- line to an engine exhaust system (not shown), and is-generally constructed of stainless steel or any other suitable material which is durable in an under-body exhaust environment.

When installed in this manner, exhaust gases exiting the engine enter the canister 14 at inlet 16, pass through the catalyst support unit 12, where harmful exhaust constituents are converted to less harmful substances, and subsequently exit the canister at outlet 18.

Although the catalytic converter assembly 10 is shown with a single, electrically heated catalyst support unit 12, it is contemplated that similar units with multiple catalyst supports, both heated and unheated, may be used. In such instances, the electrically heated catalyst support unit 12 is placed upstream of the unheated catalyst supports, or in other words in an inlet area of the catalyst support, so that heating of unit 12 will assist light-off (ie commencement of significant chemical reactions) of the unheated supports.

Additionally, several methods are well known for mounting such a catalyst support unit within a canister, such as the canister 14, and as such, will not be described further.

The electrically heated catalyst support unit 12 has a metal foil monolith 20 constructed of a metal foil strip 22 which is repeatedly folded back on itself across the frontal, or inlet area of the 1 monolith 20, as is best seen in Figure 2. The metal foil strip 22 preferably comprises an Iron-Chromium-Aluminium (Fe-Cr-Al) alloy. The strip 22 may also contain other elements, as required, to tailor its metallurgical and electrical properties to predetermined applications.

By varying the lengths of the respective foil layers, the monolith 20 can be formed to have virtually any cross-section.

The foil strip 22 has a corrugation formed therein which is configured in an alternating chevron pattern, as shown in Figures 3 and 4. The use of such a pattern prevents adjacent layers from nesting within one another as the layers are folded and, additionally, the layers contact one another in a point-to-point relationship, thereby minimizing the contact area between adjacent plates and the potential for electrical shorting therebetween. Other corrugation patterns may also be used with similar results.

An electrical lead 23, 24 is attached at each end of the foil monolith 20. During operation of the converter 10, a voltage source (not shown) may be applied across the leads 23, 24 of the monolith 20 to heat the catalyst support, thereby reducing the time to reach the light-off temperature.

In order to use the supplied power efficiently, the dimensions of the foil strip 22 of the monolith 20 are chosen so as to tailor its electrical resistance to a predetermined value on the basis of the following equation:

1 R = pL/8w where: p = foil resistivity L = foil length (total) w = foil width & = foil thickness and where: L [(4/b 2 + 1/a 2)]A where: a = amplitude b = wavelength of the corrugation pattern, and A = frontal area Use of the above equations in determining the dimensions of the monolith allows the choice of a power configuration best suited to the particular application. In the present case, it is contemplated that a monolith resistance of approximately 2.5 ohms will require the use of a relatively low current, in the range of 25 to 32 amperes, and a relatively high voltage, in the range of 62 to 80 volts, which will result in a power rating for the converter of 1500 to 2500 watts. This is desirable from the standpoint of safety and durability.

As will be apparent, the above configuration differs greatly from the 500+ amperes which would be required in an electrically heated converter operating at the 12 volt level-of the automobile with a monolith having an electrical resistance in the order of milliohms.

In order for the foil strip 22 to present a high resistance path across its length while folded- J t 1 in the configuration shown, electrical insulation must be provided between its adjacent layers. The insulation illustrated in Figure 5 must be effective without interfering with the flow of exhaust gases through the monolith 20 and must be highly resistant to degradation of the type which would subject adjacent foil layers to contact one another so as to cause an electrical short.

The insulating coating applied to the foil strip 22 is formed of several layers. The first, or base layer (not shown), is an aluminium oxide which covers substantially the entire foil strip 22. The aluminium oxide layer is oxidized on the surface of the foil strip 22 using a process similar to that described in US Patent 4,588,499 and in US Patent 4,318,828 the disclosure of each of which is incorporated herein by reference. The oxide may take several forms, however, a high-aspect-ratio whisker is preferred. The whiskers act in a capillary manner to absorb and hold subsequent coating layers and, as a result, contribute to an overall electrically insulating layer which is highly resistant to spalling and loss.

The second layer applied to the foil strip 22 is an alumina gel coat 26 (Figure 5). The gel is similar to that described in the US Patent 4,318, 828 and is typically made by combining 5 %wt alumina monohydrate with water and 5 %wt nitric acid. Several layers of the gel coat 26 may be applied to the folded, whiskered strip in its uncompressed state (described below) to assure adequate coverage of all the foil layers.

The third layer applied to the foil strip 22 is an alumina washcoat 28. The washcoat is of the type described in US Patent 4,318,828 and may, or may 1 not, contain noble metals. The washcoat 28 may also take the form of other commercially available washcoats which have characteristics which allow suitable banding with the alumina gel coat 26. There are various possibilities for the application of the above-described insulating layers to the strip 22. It is preferable, however, to apply the alumina gel coat 26 to the folded, uncompressed foil monolith 20. In other words, the gel coating 28 is applied after the foil strip 22 has been folded to the shape of the monolith 20, but before further steps, described in detail below, are performed. Application of the gel coat 26 to the foil strip 22 when in uncompressed form assures that the gel coat 26 will coat the foil strip 22 uniformly, thereby providing good insulation between adjacent layers of the strip 22. Several coats of the alumina gel 26 may be applied as desired. Following the application of the alumina 20 gel layer 26, the foil monolith 20 is compressed to the size required to fit into the catalyst support unit shell 30. The shell 30, shown in Figure 1, generally comprises a two-piece clamshell assembly secured around the monolith 20 and acts to hold the monolith 20 rigidly within the canister 14. other canister configurations, such as single piece, tubular shells with individual end cones, may also be used. In order to prevent electrical shorting between the metal foil monolith 20 and the shell 30, an insulating material, such as ceramic mat 32 is wrapped around the outer periphery of the compressed foil monolith 20 prior to its placement within the shell 30. The assembled monolith 20 may receive an Z 1 additional layer of the alumina gel coat 26, which will penetrate the insulating material, to give it additional strength and rigidity, as well as to reduce its ability to absorb the subsequently applied noble metals.

The alumina gel coat 26 provides a good base for the subsequent application of the washcoat 28 since the dried gel goes partially back into solution upon contact with the wet washcoat. As a result, a very tenacious bond between the two coatings is formed.

Although the above description is directed towards a single-strip foil monolith, in order to achieve greater flexibility in choosing the most appropriate resistance for the monolith 20, it is contemplated that more than one foil strip 22 may be used in a single monolith 20, as shown in Figure 2. The monoliths are electrically connected in parallel in order to reduce the overall resistance of the catalytic converter 10. In an alternative, the monoliths may be electrically connected in series (not shown), in order to increase the resistance of the catalytic converter.

1 claims:

1. A catalyst support for use in a catalytic converter comprising a metal foil monolith including a corrugated foil strip repeatedly folded back on itself across an inlet area of the monolith; an oxide layer covering substantially the entire surface of the foil strip; a heat resistant, electrically insulating layer covering the oxide layer to insulate electrically adjacent layers of the foil strip so as to establish a high resistance electrical path along the length of the foil strip; and electrical leads each connected to a respective end of the foil strip, wherein a voltage can be applied across the electrical leads to heat the catalyst support.

2. A catalyst support according to claim 1, wherein the oxide layer comprises a high aspect ratio aluminium oxide whisker.

3. A catalyst support according to claim 1 or 2, wherein the electrically insulating layer comprises an alumina gel coat covering the oxide layer; and an alumina washcoat covering the alumina gel coat.

4. A catalyst support according to claim 1, 2 or 3, comprising a ceramic mat around the exterior of the metal foil monolith; and a retaining shell provided around the metal foil monolith and ceramic mat; the ceramic mat being adapted to provide electrical insulation between the metal foil monolith and the retaining shell.

5. A catalyst support according to any preceding claim, wherein the metal foil monolith is adapted to have an electrical resistance based on the equation:

ZI is 1 R = pL/6w, the terms of which are as herein defined.

6. A catalyst support according to any preceding claim, wherein the electrical resistance of the metal foil monolith is such as to be adapted for use with a voltage source providing a voltage greater than 12 volts.

7. A catalyst support according to any preceding claim, wherein the foil strip is corrugated so as to have an alternating chevron pattern.

8. A catalyst support according to any preceding claim, comprising two or more metal foil strips connected together so as to be electrically in parallel.

9. A catalyst support according to any one of claims 1 to 7, comprising two or more metal foil strips connected togethe so as to be electrically in series.

10. A catalytic converter comprising a catalyst support according to any preceding claim and a voltage source adapted to produce a voltage greater than 12 volts.

Published 1992 at The Patent Office, Concept House. Cardiff Road. Newport, Gwent NP9 I RH. Further copies may be obtained from Sales Branch, Unit 6. Nine Mile Point. Cwmfelinfach, Cross Keys. Newport. NP1 7HZ. Printed by Multiplex techniques ltd. St Mary Cray, Kent.