Title of Invention

CATALYST CONVERTER FOR A SMALL ENGINE

Abstract

A catalyst converter for cleaning an exhaust-gas stream of a small engine, comprising a body with channels through which the exhaust-gas stream can flow, wherein the body is formed by a base metal being not itself able to carry out catalytic conversion but being able to form a catalytically active oxide in sheet form, the oxide of which base metal has a catalytic activity with regard to a gas constituent In the exhaust-gas stream.

Full Text

-1A- This invention relates to a catalyst converter for a small engine. The present invention relates to a catalyst converter for cleaning an exhaust-gas stream of a small engine and to a process for producing a catalyst converter for a small engine. Owing to the stringent exhaust gas legislation which has developed over recent decades, catalyst engineering for internal combustion engines, in particular in motor vehicles, has been continuously developed to achieve ever higher efficiencies and an ever shorter ignition delay. With this in mind, development has been aimed at forming large-area catalysts which are able to efficiently catalyze as close as possible to 100% of the exhaust-gas constituents to be converted. On the one hand, this is achieved by the further development of even more resistant catalyst beds, which have a large surface area, as disclosed, for example, in WO 96/16188. On the other hand, with increasing use of catalyst technology, knowledge of catalytic materials which can be used, as well as the performance and processing options for these materials, increases. The aim of each of these developments in the field of catalytic conversion of exhaust gas from an internal combustion engine is to ensure a conversion rate which is as high as possible. Catalyst technology is also to some extent already employed in the field of small engines. DE 37 29 477 C2 has disclosed a catalyst which is arranged in a muffler housing and through which the exhaust gas emanating from the motor saw flows. This catalyst can also be retrofitted to a motor saw by means of the muffler. One object of the present invention is to provide a catalyst converter for a small engine which, on the one hand, provides an adequate catalytic conversion rate - 2 - and, on the other hand, can be employed in an exhaust-gas system of a small engine, and it should be possible to produce the catalyst converter in a small number of operating steps. Another object of the invention is to provide a process for producing a catalyst converter for a small engine, in which process a catalyst can be produced without a large number of expensive production stations. A further object of the invention is to define a suitable material for the catalyst converter. This object is achieved by means of a catalyst converter having the features of claim 1, by means of a process for producing a catalyst converter having the features of claim 21, and by means of a foil having the features of claim 23. Advantageous configurations, features and refinements are given in the respective dependent claims. A catalyst converter for cleaning an exhaust-gas stream of a small engine comprises a base metal, the ox_i.de of which has a catalytic activity with regard to a gas constituent in the exhaust-gas stream, the base metal being in sheet form and forming a body with* channels through which the exhaust-gas stream can flow. A catalyst of this nature, which provides the catalytic activity, as it were, in or on itself, represents a completely new direction. While catalyst beds coated with a washcoat and noble metals . were previously used to achieve the highest possible conversion rate, a catalyst according to the invention provides a less expensive solution. Compared to other catalysts, under certain circumstances this catalyst may have a lower exhaust-gas conversion rate. However, this rate is still sufficient for a satisfactory proportion of the exhaust gas in an exhaust-gas system of a small engine to be converted. Advantageously, the catalyst is arranged in a muffler for the small engine. This allows the catalyst to be - 3 - held at a distance from the external walls of the muffler. As a result, it is possible to avoid heat being transferred to an outer casing by direct contact. In catalysts used hitherto, the problem arose that they became very hot during the catalytic conversion, in particular as a result of a very rich exhaust gas in, for example, small two-stroke engines. The exhaust gas emitted could become too hot and hence represent a risk, for example, to an operator or the environment.. The catalyst proposed has a satisfactory catalytic activity. However, this is selected in such a way that the exhaust gas does not exceed a maximum temperature. For -motor saws, which are put down on the ground, for example in forests, a maximum value for this temperature is approximately 750 °C. One form of the catalyst therefore has an average conversion rate of at least 20% and at most approximately 50% of the total of convertible exhaust-gas constituents in the exhaust-gas stream in the catalyst. In a particularly advantageous configuration, the catalyst which, as it were, itself provides the catalytic activity is an alloy. It has proven advantageous for the alloy to contain at least one constituent selected from the following group: titanium, vanadium, zinc, . iron, chromium, copper, nickel, cobalt, and .these constituents may, of course, also each on their own represent the material for the catalyst. These constituents are arranged in the order given showing a decreasing catalytic activity of their oxides. Tests have shown that there are particular alloys which firstly have a desired long-term stability, in particular with regard to the high temperatures which prevail in "the exhaust-gas system, and which secondly have the required catalytic activity. Alloys comprising the following constituents have proven particularly suitable: nickel and .chromium or vanadium and iron or cobalt" and iron or chromium and iron or titanium 'arid iron, or combinations of these - 4 - alloying constituents. Titanium proved particularly-advantageous when it represented a very high proportion of the alloy. In addition to, alloys of this kind having the components just listed, certain two- or three-component alloys have also proven suitable. These alloys may also contain production-related impurities. The following alloy proportions in the individual alloys have proven advantageous in tests: NiCr 8,0 20. Since the base metal of the catalyst is not itself able to carry out the catalytic conversion, but rather a catalytically active oxide first has to be formed, in a refinement this has to be ensured before the metal is used as the catalyst. As a result, sufficient formation of oxide is ensured right from the outset. In another refinement, the metal is prepared in advance to a sufficient extent for it to be ensured, given predeterminable operation of the small engine, that an oxide layer which is required for the catalytic reaction will form as quickly as possible. To this end, either the small engine has to operate, as far as possible, with an excess of oxygen, or the, oxygen, which is most expediently air, is fed to the metal via a suitable feed line. This results in the growth of a layer of oxide, in particular oxides of the following composition: V2O5, Cr2O3, Fe6O3 and/or TiO2. These oxides have a particularly good catalytic activity and can also be formed in a controlled manner in an alloy. For the oxide formation itself, it has proven advantageous for the metal to be formed as a foil. On the one hand, this allows it to be formed particularly well into a body with channels through which the gas can flow, and on the other hand the foil can be prefabricated to such an extent that no significant further measures are required apart from a subsequent formation of oxide in order to ensure the catalytic - 5 -ability of the catalyst. The long-term stability of the catalyst is dependent on sufficient metal material being available to be able to compensate again and again for material lost during the operating period for the required number of operating hours of the small engine. This is, ensured by a suitable wall thickness. It is therefore advantageous for a wall of the metal, in particular of a foil, which is arranged in the exhaust-gas stream to have a thickness of at least 0.05 mm and preferably of from 0-06.5 mm to at least 0.11 mm. Foils of this thickness have proven to be extremely easy to shape during production of the catalyst without their stability being too low. However, depending on the application it is also possible to employ higher thicknesses. The thickness of the foil does not have to be equal throughout, but rather may be adapted to the region subsequently provided in the catalyst for part of the foil. A further parameter which can be used, to control the catalytic conversion rate of the catalyst is the specific surface area which the said catalyst provides. Measurements have shown that a specific surface area o'f at least 7 m2/g, advantageously of from 10 m2/g to 20 m2/g, ensures a satisfactory catalytic conversion rate. In this case, the specific surface area is defined by the three-dimensional structure of the catalyst. Therefore, a porous surface on the metal is advantageous for a high specific surface area. A porosity of this nature can be affected by the use of suitable metal alloys or alloying constituents and by means of a suitable production process. Important factors influencing the porosity have proven to be the temperature and also the temperature profile over time. By means of a suitable interaction between the temperature and the period for which the temperature is maintained, it is possible, for example, to affect the - 6 - crystal structure of the metal or of the alloy in such a way that a special layer of oxide having the required properties is formed. In the case of an exhaust-gas stream of from 5 to 25 m3 per hour with the following exhaust-gas constituents: 6% by volume of CO, 1% by Volume of HC, the catalyst has a catalytically active surface of preferably approximately 10 to 20 m2/g, and at least of 7 m2/g. An exhaust-gas stream of this nature is typical of small engines, in particular hand-held small engines. The catalytically active surface is sufficient to achieve an adequate conversion rate without causing an undesirably high rise in temperature. In order, on the one hand, for the oxide to be available in sufficient quantity and, on the other hand, for the catalyst still to satisfy the required strength values corresponding to its geometry, it is advantageous for the catalytic conversion if the metal of the catalyst or the desired alloying constituent of the alloy is oxidized to an extent of approximately at least 20% and preferably to an extent of between 25% and 50%. It has also proven expedient in tests for the metal of the catalyst to have a layer of oxide which has a thickness of at least 2 mm and preferably of from 3 mm to 5 mm. These values may range both above and below these levels, depending on the metal or alloying constituent used. For example, if titanium is used, oxidation to an extent of at least 3.5% and preferably to an extent of from 5% to 10% has proven suitable. Particularly in the case of small engines which have to be accommodated in casings where space is very limited so as to be easy to handle, it is necessary to ensure that heat is dissipated from the catalyst, once ignited, to the environment in order to provide cooling. If the catalyst has an excessively high conversion rate, the flow of heat produced is under - 7 - certain circumstances too high. In order to avoid this, in one design of the catalyst the conversion rate is limited, as described above, to at least 20% and at most 50%. An advantageous refinement has a cooling device on the "catalyst. This cooling device is able to dissipate or reduce the heat produced during the catalytic reaction. The cooling device allows the catalyst and/or, the exhaust gas emanatinq from the catalyst to be cooled to a sufficient, extent to -preclude any danger from the heat produced. A cooling device of this kind is described, for example, in WO 96/23133, while a further possibility for carrying out cooling is disclosed in DE^IL_&8_07 068, and further designs emerge from DE 40 17 267 Al and DE 37 29 477 Cl. These corresponding configurations can also advantageously be employed in this proposed catalyst, for which .reason reference is made to the said documents.. With regard to the feature of the cooling device, the contents of these documents also form part of this description. The cooling device is such that, in a further refinement of the catalyst, the latter may have a conversion rate of greater than 50% and the additional heat produced can be dissipated by means of the cooling device, thus reducing the temperature. The invention also provides a process for producing a catalyst converter with a honeycomb-like geometry for cleaning an exhaust-gas stream of a small engine. The catalyst is composed of a metal foil. The metal foil is heat treated until a layer of oxide ^ comprising the metal from the foil itself which subsequently serves as the catalyst is formed. Advantageously, a foil which contains chromium as alloying constituent or virtually pure titanium is used. On the one^ hand, an alloy of this nature can be readily oxidized, and the oxide in -turn can be used as catalyst, and, on the other hand, the alloy is also easy to shape. - 8 - To form the layer of oxide, the foil is preferably held at a temperature of from approximately 400°C to approximately 1100 C for a certain period of time. The time depends essentially on parameters such as the material, the oxidation atmosphere, the surface condition, the desired form of lattice of the material and the like,) and may therefore adopt extremely divergent values. However, this time is selected in such a way that it is ensured that a sufficient thickness of the oxide layer is formed. A temperature range which can be employed for a very large number of metals lies between 500°C and 850°C. In certain cases, attention should also be paid to the material surrounding the catalyst, so that it is entirely possible for other temperatures within the said ranges to be advantageously suitable. For example, in the case of ferritic outer casings, temperatures of up to 850°C may readily be used, in which case temperature holding times of approximately half an hour or an hour may be sufficient. During the oxidation, it is necessary to take into account the fact that at a high temperature the formation of the oxide is accompanied by a high density. Moreover, the result may be a specific surface area which is lower than the specific surface area obtained at a lower temperature for forming the layer of oxide. At a lower temperature, the oxide also has a lower density. This relationship, in combination with the time, can be exploited for the individual materials in such a way that it is in each case possible to form a suitable oxide layer of desired specific surface area and density on the metal. Allowing a layer of oxide to grow can also be controlled in a targeted manner by means of the nature and manner of heat treatment of the foil. The oxidation is promoted by the use of an atmosphere which has a - 9 - higher oxygen content than air. As a result, it is possible for the metal foil to form the desired oxide layer and thickness without undesirable components also building up in this layer. By way of example, it is also possible to employ a protective gas which contains at least 5% of oxygen as the atmosphere. This protective gas is selected in such a way that it does not allow any undesirable reactions on the surface, as is the case, for example, with an inert gas. Atmospheric constituents which interfere with the oxidation as a result of their reductive action, such as CO or hydrocarbons, are avoided as a result. Furthermore, allowing the layer of oxide to grow can also be controlled in a,targeted manner by applying to the material to be oxidized a substance which gives off oxygen when heated. This applied substance can be sprayed on or applied by some, other treatment. It is also possible to promote oxidation by influencing the surface to be oxidized, for example by means of roughening or other surface treatment methods. A further configuration of the process provides for the honeycomb-like geometry to be produced from the foil and for this body then to be oxidized. Conversely, it is also possible to oxidize the foil first and then to produce the honeycomb-like geometry. Further advantageous refinements and configurations are shown in the drawing, in which: Fig. 1 shows a production line for the oxidation of a metal foil. Fig. 2 shows a section through an oxidized metal foil, Fig. 3 shows a foil catalyst with a honeycomb-like geometry, and - 10 - Fig. 4 shows a small engine with an exhaust-gas system. Fig. 1 shows a belt conveyor line 1, on which a foil 2 made of base metal is conveyed continuously in the direction of the arrow. A cover 3 is arranged above the belt conveyor line 1. A heating appliance 4 is situated in the cover. This appliance irradiates the heat produced directly onto the foil 2. A special atmosphere 5 which allows or assists with the oxidation of a layer onto the foil 2 is situated in the cover 3. The oxidation depends on various parameters, for example the belt speed of the belt conveyor line 1, the composition of the atmosphere 5, the heat emitted by the heating appliance 4, the material used for the foil 2, and if appropriate corresponding further oxidation aids. However, in addition to this continuous process for producing a foil 2 with an oxidized surface, the process can also be carried out discontinuously. For this purpose, in one design the foil 2 is placed in a drum and held in a corresponding furnace under the action of heat. Fig. 2 shows a diagrammatic section through the foil 2. A layer of oxide 7 has formed on a surface 6. This layer is able to catalytically convert exhaust gases. Fig. 3 shows an embodiment of a honeycomb 8 comprising foil 2 with a layer of oxide 7. This honeycomb 8 can be employed in particular in a muffler for a small engine, its two lateral supports 9, comprising pinched-together foil 2, being used for mounting it in the muffler and for guiding a gas stream flowing around the honeycomb 8. Fig. 4 shows a small engine 10, to which an exhaust-gas system 11 is connected. A first exhaust-gas stream 12, which is indicated by an arrow, emerges from the small engine 10. The first exhaust-gas stream 12 flows - 11 - through a muffler 13 in which the catalyst is arranged. A second, catalytically converted exhaust-gas stream 14 emerges from the ' muffler 13. Under certain circumstances, there may be a difference in mass between the first exhaust-gas stream 12 and the second exhaust-gas stream 14. This is caused by the fact that the exhaust-gas system 11 has a cooling device 15. Surrounding air 18 flows in from outside the exhaust-gas system 11 via a first valve 16 and a second valve 17, thus reducing the exhaust-gas temperature. The second valve 17, with its flow paths, is able to cool the catalyst situated in the muffler 13 from the outside as well. The catalyst comprising a base metal, the oxide of which has a catalytic activity, allows a cost-effective production sequence, for which reason this catalyst is used in particular in a hand-held appliance with a small engine. - 12 - List of reference numerals 1 Belt conveyor line 2 Foil 3 Cover 4 Heating appliance 5 Atmosphere 6 Surface 7 Layer of oxide 8 Honeycomb 9 Support 10 Small engine 11 Exhaust-gas system 12 First exhaust-gas stream 13 Muffler 14 Second exhaust-gas stream 15 Cooling device 16 First valve 17 Second valve 18 Surrounding air 1 -13-WE CLAIM 1. A catalyst converter for cleaning an exhaust-gas stream (12) of a small engine (10), comprising a body with channels through which the exhaust- gas stream can flow, wherein the body is formed by a base metal being not itself able to carry out catalytic conversion but being able to form a catalytically active oxide in sheet form, the oxide of which base metal has a catalytic activity with regard to a gas constituent In the exhaust-gas stream (12). 2. The catalyst converter as claimed in claim 1, wherein the catalyst is arranged in a muffler (13) for the small engine (10). 3. The catalyst converter as claimed in claim 1 or 2, wherein the metal is an alloy. 4. The catalyst converter as claimed in claim 1, 2 or 3, wherein the alloy contains at least one constituent selected from the following group: Ti, Va, Zn, Fe.Cr, Cu.Ni, Co. 5. The catalyst converter as claimed in claim 4, wherein the, alloy has the following constituents: Ni and Cr or Va and Fe or Co and Fe or Cr and Fe or Ti. 6. The catalyst converter as claimed in one of claims 4 and 5, wherein the alloying constituent represents a proportion of the alloy of between 2% and 20%, preferably between 5% and 10%. 7. The catalyst converter as claimed in one of the preceding claims, wherein the metal, before being used as a catalyst, Is already oxidized and has formed a catalytically active oxide. 8. The catalyst converter as claimed in one of the preceding claims, wherein the metal is formed as a foil (2). 9. The catalyst converter as claimed in one of the preceding claims, wherein a wall of the metal in the exhaust-gas stream (12) has a thickness of at least 0.05 mm, advantageously of from 0.065 to at least 0.11 mm. 9. -14- 10.The catalyst converter as claimed in one of the preceding claims, wherein (he catalyst has a specific surface area of at least 7 m2/g, advantageously of from 10 m2/g to 20 m2/g. 11.The catalyst converter as claimed in one of the preceding claims, wherein a surface (6) on the metal Is porous. 12.The catalyst converter as claimed in one of the preceding claims, wherein the metal Is oxidized to an extent of approximately at least 20%, preferably to an extent of between 25% and 50%, or, In the case where titanium is used, to an extent of at least 3.5% and preferably to an extent of between 5% and 10%. 13.The catalyst converter as claimed in one of the preceding claims, wherein the metal has an oxide layer (7) which has a thickness of at least 2 mmt preferably of from 3 mm to 5 mm. 14. The catalyst converter as claimed in one of the preceding claims, wherein the catalyst has an average conversion rate of at least 20% and at most approximately 50% of the total of convertible exhaust-gas constituents in the exhaust-gas stream (12) upstream of the catalyst. 15. The catalyst converter as claimed in one of the preceding claims, wherein a cooling device (15) is connected to the catalyst. 16. A process for producing a catalyst converter as claimed In claim 1 with a honeycomb-like geometry for cleaning an exhaust-gas stream (12) of a small engine (10), the catalyst being composed of a metal foil (2), made of an alloy of Fe with Cr, Co and/or Ti for the catalyst comprising: - heat treatment of the metal foil (2) until a layer (7) of oxide comprising the metal from the foil (2) itself which subsequently serves as the catalyst is formed; - formation of the honeycomb-like geometry from respectively the metal foil (2) either before or after said heat treatment. 17. The process as claimed In claim 16, where the foil (2) Is held at a temperature of from approximately 400°C to approximately 500°C for at least 30 minutes. -15- 18. The process as claimed in claim 17, where an oxide layer (7) of V2O5, Cr2O3, FeeO3, C03O4 and/or TiO2 is allowed to grow. 19.The process as claimed in one of claims 16 to 18, where oxidation is carried out in an atmosphere which contains at least 5% of oxygen. 20.The process as claimed in one of claims 16 to 19, comprising production of the honeycomb-like geometry from the foil (2) and then oxidation of the body, or vice versa. 21.A catalyst converter as claimed In one of the preceding claims comprising a foil (2), the foil (2) having an alloy with Ni and Cr or V and Fe or Cr and Fe or Co and Fe or predominantly with Ti. 22. A process as claimed in one of the proceeding claims, wherein the foil (2) has an alloy with Ni and Cr or V and Fe or Cr and Fe or, Co and Fe or predominantly with Ti. 23.The catalyst converter or process as claimed in claim 21 or claim 22, wherein the foil (2) has a thickness of at least 0.05 mm, in particular of from 0.065 to at least 0.11 mm. Dated this 14th day of Jury 1998. A catalyst converter for cleaning an exhaust-gas stream of a small engine, comprising a body with channels through which the exhaust-gas stream can flow, wherein the body is formed by a base metal being not itself able to carry out catalytic conversion but being able to form a catalytically active oxide in sheet form, the oxide of which base metal has a catalytic activity with regard to a gas constituent In the exhaust-gas stream.

Documents:

01217-cal-1998 abstract.pdf

01217-cal-1998 claims.pdf

01217-cal-1998 correspondence.pdf

01217-cal-1998 description(complete).pdf

01217-cal-1998 drawings.pdf

01217-cal-1998 form-1.pdf

01217-cal-1998 form-2.pdf

01217-cal-1998 form-3.pdf

01217-cal-1998 form-5.pdf

01217-cal-1998 pa.pdf

01217-cal-1998 priority document other.pdf

01217-cal-1998 priority document.pdf

1217-cal-1998-granted-abstract_.pdf

1217-cal-1998-granted-acceptance publication_.pdf

1217-cal-1998-granted-claims_.pdf

1217-cal-1998-granted-correspondence_.pdf

1217-cal-1998-granted-description (complete)_.pdf

1217-cal-1998-granted-drawings_.pdf

1217-cal-1998-granted-form 1_.pdf

1217-cal-1998-granted-form 2_.pdf

1217-cal-1998-granted-form 3_.pdf

1217-cal-1998-granted-form 5_.pdf

1217-cal-1998-granted-letter patent_.pdf

1217-cal-1998-granted-pa_.pdf

1217-cal-1998-granted-priority document_.pdf

1217-cal-1998-granted-reply to examination report_.pdf

1217-cal-1998-granted-specification_.pdf

1217-cal-1998-granted-translated copy of priority document_.pdf