EP0939207B1 - New catalytic unit for the treatment of the exhaust gas of an internal combustion engine - Google Patents

Description

The present invention relates to the field of gas treatment exhaust from an internal combustion engine.

More specifically, the invention relates to the catalytic depollution of these exhaust gas.

A major problem related to depollution can arise, in certain applications of the strong exothermicity of reactions at the level of catalysts usually placed in the exhaust line. We will call in the rest of the text "catalyst" or "monolith" any element comprising a mechanical support on which the catalyst itself is disposed. Exothermicity causes significant elevations in temperature of the monoliths; these temperatures are dangerous for the mechanical strength of the support (metal, ceramic or other). In addition, these high temperatures are detrimental to the catalytic activity of the catalyst deposited on the support.

In order to avoid damage to the monoliths, several concepts have already been offered.

A first way consists in equipping the exhaust line with two catalysts. US Patent 5,377,486 is an example. In such lines a main monolith, in which the gas will constantly pass exhaust is preceded by a monolith called "light-of" or "priming" in which the gases will pass when the engine starts, so when they do not are not yet very hot. Although small, the monolith "of light off "is however effective from the first moments of system operation. When the gases are going to heat up, a winnowing pilot will allow to bypass this first catalyst to send gas directly in the main monolith.

Document JP-08 189344 thus allows winnowing according to the temperature of the gases leaving the engine.

Another known concept, as described in US Pat. No. 3,796,546, consists in passing the gases by different paths, according to their temperature. Thus, according to this document, the gases pass either successively through two catalysts either do not cross either.

The problem at the origin of the present invention is as follows:

It may happen that the composition of the gases has an adverse effect on the catalytic depollution. We can indeed be in the presence of a gas exhaust simultaneously containing a large quantity of reducers (CO, HC) and a large amount of oxidants (oxygen).

This will lead to a strong oxidation activity of the reducers which will cause a significant increase in gas temperature (exothermic reaction).

In addition, the gaseous mixture, although containing a lot of oxygen, may however not contain enough of it to oxidize both the carbon monoxide and the hydrocarbons (to CO ₂ and H ₂ O). Part of the hydrocarbons is then only partially oxidized to form carbon monoxide in the catalyst. The catalytic efficiency with respect to CO will therefore be a compromise between the CO oxidized to CO ₂ and the CO reformed by this partial oxidation of the hydrocarbons. This process is favored at high temperature.

We can represent this phenomenon by the CO / HC ratio at the outlet of the catalyst (compare to that existing at the inlet).

It is therefore a question of achieving a compromise between the temperature on the one hand and the CO / HC ratio at the outlet of the exhaust line on the other hand. The ideal situation would be one where we would have neither carbon monoxide nor hydrocarbons at the outlet. This being impossible, a compromise must be found in particular to achieve the best CO / HC ratio.

Future standards, in most industrialized countries go, in situations like this, be difficult to respect. A catalyst is going to have to be mounted in the exhaust lines. The problem of the cost of a catalyst therefore becomes a determining factor.

• soit un monolithe de section faible, et plutôt allongé ; cette solution crée des variations locales de température plutôt défavorables à l'efficacité du catalyseur.
• Un monolithe de grande section et de faible longueur peut aussi être choisi mais cette solution présente le même inconvénient que la précédente.
• Il est encore connu, par le brevet français FR 2 687 431 par exemple, de revêtir la paroi interne de la ligne d'échappement d'une couche catalytique. Ce concept permet de limiter les élévations de température ; cependant tout le gaz n'atteint pas forcément la paroi ainsi revêtue de sorte que l'activité catalytique peut être considérée comme faible.

Finding a minimum cost means having only one piece. Given the space constraints, we can choose:

• either a monolith of weak section, and rather elongated; this solution creates local temperature variations which are rather unfavorable to the efficiency of the catalyst.
• A monolith with a large section and a short length can also be chosen, but this solution has the same drawback as the previous one.
• It is also known, from French patent FR 2 687 431 for example, to coat the internal wall of the exhaust line with a catalytic layer. This concept makes it possible to limit temperature rises; however, not all of the gas necessarily reaches the wall thus coated so that the catalytic activity can be considered to be weak.

This problem is more acute in the exhaust lines two-stroke engines because on the one hand the pollutant emissions are very important. On the other hand, congestion is a crucial problem. Other says, the current catalyst solutions in 2-stroke engines are only not satisfactory.

Future standards for pollutant emissions will make necessary and compulsory a catalytic converter in two-stroke engines.

We already know monoliths placed in the line silencers exhaust of two-stroke engines. They are commonly found in shape of a ferrule which envelops a ceramic or metallic support itself covered with a catalyst. The supports are often only partially enclosed in the shell due to temperature problems. This is to reduce the risk of temperature rise inherent in the catalytic reaction.

It is also known from document JP-A-04140413 to have, in a ferrule, two catalytic elements one after the other and at distance from each other so as to create an empty volume between these two elements, volume in which the exhaust gases from the first element are cooled before passing through the second catalytic element.

This known arrangement has the disadvantage not only of not not being able to control the exothermicity of the catalytic elements but also to require a long time before the second element is operational.

The present invention notably represents a solution to problem of monoliths placed at the exhaust of two-stroke engines; the temperature rise is favorably reduced according to the invention; of the more the space constraints are respected; moreover the present invention can be added to an existing exhaust, quick and easy.

Advantageously, the present invention makes it possible to use certain characteristics of metal monoliths known to eliminate problems of excessive exothermicity. It also helps maintain sustainably catalytic performance.

Thus, the subject of the invention is a catalytic element intended to eliminate pollutants from exhaust gases from internal combustion engines, comprising at least one monolith wrapped in a ferrule, said monolith comprising first and second catalytic elements separated by an empty volume delimited by said ferrule and a face of each of said elements, characterized in that the shell comprises openings placed at the level of the empty volume and through which is introduced a coolant of the exhaust gases having passed through the first catalytic element and in that said empty volume is also coated internally with a catalytic layer.

According to one embodiment of the invention, at least one of the elements protrudes over part of its length, outside the shell.

According to another embodiment of the invention, none of the said elements does not protrude from the shell.

The volumes of each of said elements may be different.

Likewise, the cell density of each of the monoliths can be different.

According to the invention, the formulation of each of the monoliths can be different. Without departing from the scope of the invention, it can also be the same.

The openings may have inclined edges so as to guiding the gas flow inside said empty volume.

In addition, the shell includes, at the empty volume and downstream of said openings, at least one circumferential groove hollowed out towards inside the shell.

According to a preferred embodiment of the invention, the element catalytic is placed inside a silencer that is part of the line exhaust.

The invention further relates to a silencer comprising an element catalytic and further comprising at least one inlet and at least an outlet for the exhaust gases, a double envelope delimiting a intermediate volume and the interior volume of the silencer, a wall intended to separate said interior volume into two parts, upstream and downstream and on which the catalytic element is mounted in such a way that the first catalytic element is located in the upstream part and that the second catalytic element is located in the downstream part, said openings of the catalytic element being located on the side downstream ; the internal envelope has openings located on the downstream side of so that a fluid flowing in the intermediate volume can penetrate in said downstream volume through said openings and then through the second catalytic element.

The fluid in the intermediate volume can be air and / or gases exhaust to be treated.

• La figure 1 est un schéma longitudinal d'une ligne d'échappement comprenant un élément catalytique de l'état de la technique ;
• La figure 2 est une perspective simplifiée d'un élément catalytique selon la figure 1 ;
• La figure 3 est un schéma longitudinal d'une ligne d'échappement comprenant un élément catalytique selon un mode de réalisation de l'invention ;
• La figure 4 est une perspective simplifiée d'un élément catalytique selon le mode de réalisation de l'invention ;
• les figures 5A, 5B, 5C représentent des variantes de l'élément catalytique selon le mode de réalisation de l'invention ;
• La figure 6 est une coupe longitudinale partielle d'un silencieux selon l'invention.

Other characteristics, details and advantages of the invention will appear better on reading the description which follows, given by way of illustration and in no way limiting with reference to the appended figures:

• Figure 1 is a longitudinal diagram of an exhaust line comprising a catalytic element of the prior art;
• Figure 2 is a simplified perspective of a catalytic element according to Figure 1;
• FIG. 3 is a longitudinal diagram of an exhaust line comprising a catalytic element according to an embodiment of the invention;
• Figure 4 is a simplified perspective of a catalytic element according to the embodiment of the invention;
• FIGS. 5A, 5B, 5C represent variants of the catalytic element according to the embodiment of the invention;
• Figure 6 is a partial longitudinal section of a silencer according to the invention.

Figure 1 is a diagram of an exhaust line which includes a pipe 1 having a first end 2 connected to the outlet of the engine (not shown); at its second end 3 the pipe opens, according to what embodiment of the invention, in a casing 4 which is usually used silent. More precisely, the pipe 1 exceeds for a certain length inside the housing 4.

In addition, the casing 4 contains a partition wall 6 which defines two volumes:

The first 7 contains the end 3 of the pipe.

The second volume 8 of the casing 4 includes an opening 9 for the outlet of the exhaust gases decontaminated towards the atmosphere.

On the partition wall 6 is provided a passage which contains a catalytic element 10.

The catalytic element 10 is shown in more detail in Figure 2 where we can see that it includes a ferrule 11 which surrounds the monolith itself. This includes two catalytic elements 12, 13 separated by a empty 14. The empty volume 14 is thus delimited by a part of the shell 11 and by one face of each of the catalytic elements 12, 13.

Thus the exhaust gases emerging in the closed volume 7 pass successively through the first catalytic element 12, the empty volume 14 then the second catalytic element 13 before reaching the volume 8 from which they exit to the atmosphere via the opening (or openings) 9.

Furthermore, the internal wall of the empty volume 14 is coated with a catalytic layer. This so-called "combi" concept allows gases to pass successively through a catalytic channel and then through a multi-channel honeycomb monolith, also with activity Catalytic. This concept is very interesting for the treatment of gases during operating at low speed since it promotes the initiation of reactions catalyst.

Figures 3 and following relate to an embodiment of the invention which differs from the previous one in that the shell 11 comprises lights 15 at the empty volume 14. Thus, part of the gases exhaust does not pass through the first catalytic element 12 since it arrives directly in the empty volume. This proportion of gases exhaust system cools the exhaust gases that have passed through the first catalytic element.

All of the exhaust gases therefore pass through the second catalytic element 13 with a lower temperature than in the mode of previous achievement. In addition, this introduction of exhaust gases modifies the emission ratio between carbon oxides and hydrocarbons.

Figures 5A, 5B and 5C show variants of the invention.

FIG. 5A illustrates an embodiment with lights 15 which occupy a large area; thus the gas penetration rate directly into volume 14 is relatively large. Of course, this permeability will be adapted to the envisaged flow.

FIG. 5B shows curvatures around the openings 15, which allow to better direct the gas flow inside the shell 11, in the volume 14.

According to FIG. 5C, the shell 11 has a specific profile with at least minus a circumferential groove 16 hollowed inwards. Groove 16 is located downstream of the openings 15 relative to the direction of circulation of the gas. This venturi type profile therefore makes it possible to accelerate the gases as soon as they introduction into volume 14 and / or promoting the mixing of the two flows gaseous.

In Figure 5C, it appears that the first catalytic element 12 has a smaller volume than the second catalytic element 13.

It is indeed not necessary that the respective volumes of the elements 12 and 13 are the same. The volume of each element catalytic 12, 13 will be calculated to optimize the gas temperatures obtained at the output of each of them, in particular to avoid peaks of temperatures detrimental to their integrity.

Similarly, the distance between the monoliths can be defined so that obtain sufficient cooling of the gases leaving the first element catalytic 12 until the entry of the second 13, and to prevent again excessively high temperatures in the second catalytic element.

The respective volumes of each catalytic element which exceed of the shell 11 are to be treated with the same objective.

The same adjustments can be made, as required, from either to reduce excessively high carbon oxide emissions or on the contrary to favor even partial oxidation of hydrocarbons y understood if it leads to an increase in the emission of oxides of carbon.

In order to promote the oxidation of hydrocarbons, it is also possible act on the section / length ratio of each catalytic element and on the cell densities used, as well as of course on the formulation Catalytic.

Temperature, overall catalytic efficiency and ratio CO / HC emissions can be optimized by making holes on the ferrule 11, which allow part of the gas to bypass the first element catalytic 12. The number, size and shape of the orifices is adapt according to the proportion of gas that one wishes to deviate from first catalytic element 12.

In this embodiment, all the gas not passing through the first catalytic element, the catalytic reactions occurring in these will lead to lower temperatures at the outlet. The gas inlet "fresh" through the orifices 15 further cools the gas entering the second catalytic element 13, which in turn should operate at a temperature lower, even if these reintroduced "fresh" gases are to be cleaned up and can "re-increase" the operating temperature of this second catalyst. The "two-stage" depollution will also modify the balance sheet between CO emissions and HC emissions, the direction of variation being depending on the proportion of gas diverted.

Thus, an optimum can be obtained, according to the invention to obtain a overall compromise satisfying both temperatures and pollutant emissions.

According to one embodiment of the invention, the catalyst element is placed in the exhaust line silencer.

Figure 6 partially shows a silencer that looks like the one shown schematically in Figure 3. The differences lie in the presence of a double envelope 4, 4 ′ delimiting an annular space 24 in which transits a fluid: air and / or exhaust gases. This fluid flows according to arrows B and enters volume 8 through holes 17 of the inner envelope 4.

Furthermore, the catalytic element 10 here being provided with openings 15 located at the orifices 17, the fluid coming from the annular space 24 can flow into the empty volume 14 by venturi effect and then into the second monolith 13. This is particularly interesting in engines where the dynamic aspect is very important.

Without departing from the scope of the invention, the catalytic element can be integrated into a tubular zone of the exhaust line or even to the outlet of the diffuser.

The preferred applications of the invention relate to engines 2 times of transport vehicles, but also lawn mowers, chainsaws, or other motorized vehicles.

Claims (12)

1. Catalytic converter element designed to eliminate the contaminants from the exhaust gases of internal combustion engines, comprising at least one monolith (10) encased in a tube (11), said monolith comprising a first catalytic element (12) and a second catalytic element (13) separated by an empty space (14) delimited by said tube and one face of each of said elements, characterised in that the tube (11) comprises openings (15) placed level with the empty space (14) and through which is introduced a fluid for cooling the exhaust gases having passed through the first catalytic element (12), and in that said empty space is also covered internally with a catalytic layer.
2. Catalytic converter element according to claim 1, characterised in that at least one of the elements (12, 13) extends outside the tube (11) over a portion of its length.
3. Catalytic converter element according to claim 1, characterised in that none of said elements (12, 13) extends outside the tube (11).
4. Catalytic converter element according to any one of the preceding claims, characterised in that the volumes of each of said elements (12, 13) are different.
5. Catalytic converter element according to any one of the preceding claims, characterised in that the cell density of each of the elements (12, 13) is different.
6. Catalytic converter element according to any one of the preceding claims, characterised in that the formulation of each of the elements (12, 13) is different.
7. Catalytic converter element according to any one of the preceding claims, characterised in that the openings (15) of said tube (11) exhibit edges which are inclined so as to guide the gaseous flow inside said empty space (14).
8. Catalytic converter element according to any one of the preceding claims, characterised in that level with the empty space (14) and downstream of said openings (15), the tube (11) comprises at least one circumferential groove (16) formed towards the inside of the tube (11) and designed to produce a venturi effect.
9. Use of a catalytic converter element according to any one of the preceding claims inside a silencer forming part of the exhaust system.
10. Silencer comprising a catalytic converter element according to any one of claims 1 to 8 and additionally comprising at least one inlet (3) and at least one outlet (9) for the exhaust gases, a double casing (4, 4') delimiting an intermediate space (24) and the internal space of the silencer, a wall (6) designed to separate said internal space into two portions, an upstream portion (7) and a downstream portion (8), and on which the catalytic converter element (10) is mounted so that the first catalytic element (12) is situated in the upstream portion (7) and the second catalytic element (13) is situated in the downstream portion (8), in that said openings (15) of the catalytic element are situated on the downstream side (8), in that the internal casing (4) also exhibits openings (17) situated on the downstream side (8) so that a fluid flowing in the intermediate space (24) can pass into said downstream space (8) through said openings (17) then through the second catalytic element (13).
11. Silencer according to claim 10, characterised in that the fluid consists of air.
12. Silencer according to any one of claims 10 or 11, characterised in that the fluid consists of exhaust gases.

Images