EP0706608B1 - Exhaust pipe for a catalytic exhaust device - Google Patents

Description

The present invention relates to an exhaust manifold for an engine catalyzed exhaust system thermal.

Due to the introduction of new emission standards, vehicle exhaust systems to heat engines must be fitted with catalysts, whose goal is to actively participate in the reduction of releases to the atmosphere of more or less combustion gases toxic, in order to best preserve and respect the environment.

For this, the catalyst or catalytic converter of the devices exhaust is connected to the outlet of the tubing exhaust, the inlet of which is fixed to the manifold combustion gases from the engine. A silencer ends, moreover, the devices being connected to the catalyst by an exhaust pipe. Structurally, a catalyst consists of a rigid envelope inside which a block or monolith of ceramic with covered cells is arranged alumina and precious metals (platinum, rhodium, etc ...) which act, by catalysis, in particular on the carbon monoxide, nitrogen oxides and hydrocarbons unburnt, to annihilate their harmful constituents and transform them into non-harmful waste.

Furthermore, it is known that the catalyst is only effective when it reaches a certain temperature (several hundred degrees), i.e. when the engine is running at least for several minutes, so that the monolith is sufficiently heated by the gases to initiate catalysis reactions. Therefore, as long as the monolith does not reach a given temperature, the gases leaving the engine, although passing through the catalyst, does not are not processed. In addition, as the catalyst is often distant from the collector due to vehicle design and safety criteria, its rise in temperature, which is carried out by the gases passing through the tubing, is all the more longer.

• d'une part, les échanges thermiques s'effectuent tout d'abord au travers de la tubulure d'échappement ayant une capacité thermique élevée en raison de l'épaisseur de sa paroi de l'ordre de 2 à 3 millimètres, de sorte que le temps de montée en température du catalyseur est donc trop important, au moment du démarrage à froid du véhicule ;
• d'autre part, la capacité thermique de la tubulure est élevée et le tube composite entoure ladite tubulure, de sorte que l'énergie calorifique des gaz, lorsque le moteur est chaud, n'est pas assez évacuée, en risquant de surchauffer le catalyseur si la température des gaz atteint environ 1000°C.

Also, to overcome these drawbacks, it has already been proposed to surround the exhaust manifold externally with a composite thermal insulation tube, consisting of an internal tube, an external tube and a thermally insulating material provided between the inner and outer tubes. In reality, it turns out that the catalyst is not effective more quickly, however, because these composite tubes exhibit the following behavior:

• on the one hand, the heat exchanges are carried out firstly through the exhaust pipe having a high thermal capacity due to the thickness of its wall of the order of 2 to 3 millimeters, so that the temperature rise time of the catalyst is therefore too great, at the time of the cold start of the vehicle;
• on the other hand, the heat capacity of the tubing is high and the composite tube surrounds said tubing, so that the heat energy of the gases, when the engine is warm, is not sufficiently evacuated, risking overheating the catalyst. if the gas temperature reaches around 1000 ° C.

The object of the present invention is to remedy these drawbacks and relates to an exhaust manifold for a catalytic gas exhaust device, equipped with a composite tube whose design greatly favors the temperature rise of the catalyst at start-up engine, while not interfering with heat exchange when the engine is warm.

For this purpose, the exhaust manifold for device catalysis gas exhaust comprising a manifold gas and a catalyst, said tube being located between said manifold and said catalyst and provided with a tube composite which consists of an inner tube and a tube external defining between them a substantially annular space, is remarkable, according to the invention, in that said composite tube is housed inside said tubing exhaust, the outer tube coming substantially at contact of said tubing, and in that said tubes internal and external have thin walls whose thickness is less than 0.3 millimeter.

So, as the composite tube is located inside the tubing, this arrangement allows a temperature rise fast catalyst at cold start of the vehicle and after each stop, since the heat energy gas is almost directly transferred to the catalyst, without having to overcome thermal capacity important of the tubing. The catalyst monolith is then operational in a reduced priming time.

On the other hand, when the vehicle engine is hot, and since the composite tube is thin and configurable in heat transfer, giving the latter a low thermal resistance, the composite tube is not an obstacle to heat exchanges between the gases and the exhaust manifold which can thus freely dissipate the heat towards the exterior by conventional heat exchanges. So, overheating of the monolith which can lead to its destruction is avoided. Consequently, by the use of tubes internal and external with thin walls, giving the tube composite low mass and therefore low resistance thermal at high temperature, and by the arrangement of said composite tube inside said exhaust manifold, a rapid activation of the catalyst is allowed when the engine is cold, while avoiding the risk of overheating when the engine is warm.

Note that document WO 91/02143 describes a tubing exhaust made up of internal and external tubes, thin-walled grooves, which define a space between them substantially annular filled with fiber-based insulation refractory.

Advantageously, in the tubing of the invention, the thickness of the walls of said inner and outer tubes is lower to 0.3 millimeter, preferably of the order of 0.15 to 0.20 millimeter. So the thermal capacity is still scaled down. Preferably, said internal and external tubes are made of stainless steel.

Furthermore, in said annular space delimited by said outer and inner tubes, is arranged a material to low thermal capacity and low density, occurring in particulate or fibrous form. So this matter main purpose is to transfer to the tubing exhaust, via the external tube, the forces generated by the passage of gases under pressure and acting on the thin internal tube, to avoid deformation of the latter. The intermediate material therefore plays the role of spacer to maintain an acceptable mechanical strength during the audit composite tube and it must be non-conductive of heat so as not to increase the heat capacity of the tube composite.

For example, the thermal capacity of said material can be of the order of 0.25 kcal / kg and its density at most equal at 0.3.

When said material occurs more particularly in the form of particles, rings are provided on the ends of said inner and outer tubes, sealing said annular space to enclose said material. Preferably, in each end of said tubes, are housed a flexible insulating ring, mounted in said space and coming to contact of said material with low thermal capacity and low density, and a thermally resistant ring rigid, mounted in said space and coming into contact with said flexible insulating ring. Flexible insulating rings allow in particular the free expansion of the external tubes and internal, while the rigid rings provide the centering said tubes.

When said material occurs more particularly in the form of fibers, fibrous windings can be attached to the outside of said inner tube and spaced apart others by free intervals, said outer tube coming substantially into contact with the fibrous windings. By example, these are defined by a plurality of rings fibrous surrounding fixedly and at regular intervals said internal tube, and having a trapezoidal section or the like, rings being attached around said fibrous rings to engage in said outer tube and be fixed, by at least one of them, to said tube external.

Due to the different shapes and lengths of the tubing the composite tube is advantageously made up by a plurality of individual elements, susceptible to be assembled with each other. So we can adapt at best the composite tube to the tubing concerned. Good obviously each individual element includes tubes external and internal with thin walls, and a material with low heat capacity and low density disposed between said tubes. Furthermore, when two individual elements are assembled, the corresponding ends of the tubes interns are nested one inside the other, while the corresponding ends of the outer tubes are in abutment one against the other.

The figures in the accompanying drawing will make it clear how the invention can be realized. In these figures, references identical denote similar elements.

Figure 1 shows schematically an exhaust device catalysis gas including the exhaust manifold, according to the invention, is provided with a composite tube.

FIG. 2 represents an exemplary embodiment of one of the individual elements constituting said composite tube.

FIG. 3 shows individual elements of said tube, mounted in said exhaust manifold.

Figure 4 shows another embodiment individual elements of said tube, mounted in said exhaust manifold.

The exhaust device 1 illustrated in Figure 1 usually includes a manifold 2 of gases from the engine thermal 3, an exhaust pipe 4 connected to the collector, a catalyst or catalytic converter 5 connected to turn to the exhaust manifold and an exhaust pipe 6 connected to said catalyst and comprising a silencer 7. Such a catalyzed exhaust device thus makes it possible, as previously mentioned, to reduce emissions harmful gases leaving the engine, to the outside.

To ensure a rapid rise in temperature of the catalyst 5, the exhaust manifold 4 is provided with a composite tube 8 which consists of an inner tube 10, a tube external 11 and of a low density material 12, not heat conductive, arranged in the annular space 9 delimited by the two tubes 10 and 11, preferably concentric and of circular cross section, like said tubing.

According to the invention, the composite tube 8 is housed inside of the exhaust manifold 4 and the walls 10A and 11A which make up the inner and outer tubes are thin to have a thickness of less than 0.3 millimeter and, preferably between 0.15 and 0.20 millimeters. As we can see in Figures 1 and 3, the composite tube 8 consists of a plurality of elements or sections individual 14 assembled one after the other in the straight parts of the exhaust manifold 4 which usually has a bend, to ensure the connection between the outlet of the collector 2 and the inlet of the catalyst 5. In FIG. 1 schematically showing the device 1, the tube 4 is straight, but it goes without saying that, in reality, it is bent.

Dimensionally, the composite tube 8, formed of the elements individual 14, has an outside diameter, defined by the tube external 11 of each element, at most equal to the diameter inside the wall 4A of the pipe 4 to allow their mounting in the latter. Also, to keep the same gas flow section from the engine, the diameter inside the tubing 4 is increased, by about example of 10 millimeters, so that the inside diameter of the inner tube of each element is then identical to that current tubing.

Structurally, the inner 10 and outer 11 tubes of each element are made of stainless steel thus resistant to the high temperatures of the exhaust gases. The material with low thermal capacity and low density can be, for its part, of the particulate type, that is to say consisting in particular of microspheres of SiO2 or the like compacted or not, or of the fibrous type, that is to say - Say comprising in particular long fibers of SiO2 or Al2O3, for example. This material must be refractory, resistant to temperatures of 1000 ° C or more, light and relatively flexible, and capable of transmitting the mechanical forces from the internal tube to the external tube and, therefore, to the tubing, without penalizing the thermal capacity of the composite tube, in particular that of the internal tube 10. For this, the density of the material is less than 300 kg / m ³ , while its thermal capacity can be of the order of 0.25 kcal / kg or less.

According to the embodiment of the element 14 shown in FIG. 2, the material 12 is of the particulate type. In this case, each element 14 of the composite tube 8 includes rings at its annular ends delimited by the internal tubes 10 and external 11. More particularly, two rings 15 are mounted near the ends 10B, 10C, 11B, 11C said tubes, in the annular space 9, to contain thus the material 12 in the element 14. These rings 15 are also made of a flexible insulating material, or semi-flexible, allowing free expansion of the tube internal 10 relative to external tube 11, both axial than diametral, due to temperature differences appearing between the two tubes. In addition, two others rings 16 are also mounted at the ends 10B, 10C, 11B, 11C of each element coming into contact with the rings insulators 15. The rings 16 are made of a material rigid, such as a dense ceramic based on alumina, thermally resistant and not very sensitive to thermal shock, and they hold the insulating rings 15, ensure the centering of tubes 10 and 11, one with respect to the other and allow the relative longitudinal elongations and transverse since they are mounted with play in the annular space 9 of the tubes.

Also, to immobilize said rings axially by relative to the element tubes, it is provided on one side of element 14, an external radial projection 10D, formed at near the end 10B of the inner tube, and on the other side of the element 14, an internal radial projection 11D formed in the vicinity of the end 11B of the outer tube.

In addition, we note that the end 10B is in the extension of the wall 10A of the inner tube 10, while the opposite end 10C is slightly enlarged. Likewise, the end 11B of the outer tube 11 ends in a flap 11E at right angles facing inwards, while the opposite end 11C extends the wall 11A. Preferably, the flap 11E of the outer tube is in the same diametral plane as the external projection 10D of the internal tube, just as the internal projection 11D is located approximately to the right of the change of section of the internal tube between its wall 10A and its end 10C. Therefore, the rings 15 and 16, as well as the material 12, are maintained axially in place in the annular space 9 of each item 14.

The assembly of the individual assembled elements 14 is shown in Figure 3. The end 10B of an element 14 engages then with gentle friction in the enlarged end 10C of a other contiguous element, which ensures their nesting until the folded end 11E of the outer tube 11 of the element abuts against the end 11C of the other element.

The composite tube 8 with individual elements thus constitutes a modular system which makes it easy to "paper" inside the exhaust manifold 4.

The advantages of arranging such a tube composite 8 with low thermal capacity relate in particular to rapid priming of the catalyst monolith allowing the almost instantaneous elimination of toxic emissions from gas. Indeed, during the transitional period starting at from engine start up to a few minutes, the heat exchange and, therefore, exhaust gas temperature are minimized during their journey through the tubing, by the inner composite tube. However, during the established or cruising period, the thinness of the tube composite does not hinder the evacuation of heat energy to the exhaust manifold whose heat exchanges are managed by conductivity, radiation and convection outwards, which prevents overheating of the catalyst monolith.

Tests have also shown that the rise in temperature gases at the inlet of the catalyst, with a pipe exhaust fitted with the composite tube of the invention, was five times faster than with exhaust manifold usual.

According to the embodiment of the element 14 shown in FIG. 4, the material 12 is of the fibrous type. In this case, the space annular 9 contains windings in the form of rings 12A, long fibers (continuous wicks) thus providing a acceptable radial stiffness to avoid deformation of the inner tube 10. In particular, these fiber rings 12A are regularly spaced from each other along the outer wall 10A of the inner tube 10, providing identical intervals between them. They also present a substantially trapezoidal section so that the large base of each of them is properly attached to the wall 10A of the internal tube by means of an adhesive, such as high temperature ceramic glue. On the small bases of said rings, corresponding to the winding of the last row of turns of said fibers, are added rings 17 which are preferably split for facilitate their installation, said split rings 17 being there also fixed by a high temperature glue on the corresponding small bases of said rings.

Once the assembly "internal tube 10 - rings 12A - 17 "rings produced, the assembly thus assembled is introduced in the outer tube 11 which has, unlike that illustrated in FIGS. 2 and 3, side slits semi-opening 11F at its ends 11B, 11C allowing to facilitate assembly.

When the aforementioned assembly is properly installed relative to the outer tube 11, the central ring 12A and its attached ring 17 is located substantially in the plane median of the outer tube, from which are respectively from both sides the semi-through slots 11F. Welding points 18 then immobilize the outer tube 11 of the assembly to constitute the individual element 14 of the composite tube 8. Of course, this realization does not require not to use rings 15 and 16 and radial projections 10D and 11D to hold the material 12. On the other hand, the ends of the outer tube 11 can be both angled inward to form flaps 11E coming substantially against corresponding flaps, when the elements 14 are fitted together others.

During operation, the differential elongations tubes 10 and 11, along the longitudinal axis, are allowed by the sliding of the rings 17, entrained by the rings 12A, along the outer tube 11. These differential elongations are further divided respectively by share and on the other side of the median plane of the outer tube, due to the rigid attachment of the central ring 17 thereto, which is mechanically more satisfactory. As for the extensions radial, less ample, they are absorbed by the rings of fibers, not integral with each other.

The advantages of this variant of the composite tube, illustrated in Figure 4, are analogous to those produced by the previous realization illustrated on Figures 2 and 3. However, this variant allows easily mechanically and thermally optimize the tube composite by playing in particular on the shape (section) of fibrous rings, their number in other words their pitch, the arrangement wicks (tangent or crossed) and nature fibers.

Claims (12)

1. Exhaust piping for a catalytic gas exhaust system including a gas manifold (2) and a catalyser (5), the said piping (4) being situated between the said manifold and the said catalyser and provided with a composite tube (8) which consists of an internal tube and of an external tube defining between them a substantially annular space, characterized in that the said composite tube (8) is housed inside the said exhaust piping (4), the external tube coming substantially into contact with the said piping, and in that the said internal tube (10) and external tube (11) exhibit thin walls (10A, 11A), the thickness of which is less than 0.3 millimetre.
2. Piping according to Claim 1, characterized in that the thickness of the walls of the said internal tube (10) and external tube (11) is of the order of 0.15 to 0.20 millimetre.
3. Piping according to one of Claims 1 or 2, characterized in that the said internal tube (10) and external tube (11) are made of stainless steel.
4. Piping according to one of Claims 1 to 3, characterized in that, in the said annular space (9) delimited by the said external tube (11) and internal tube (10), there is arranged a substance (12) with low heat capacity and low density, in the form of particles or fibres.
5. Piping according to Claim 4, characterized in that the heat capacity of the said substance is of the order of 0.25 kcal/kg and its density is at most equal to 0.3.
6. Piping according to one of Claims 4 or 5, characterized in that rings (15, 16) are provided at the ends of the said internal tube (10) and external tube (11), closing off the said annular space (9) in order to contain the said substance (12).
7. Piping according to Claim 6, characterized in that, in each end of the said tubes (10, 11) there are housed a flexible insulating ring (15), which is housed in the said space and comes into contact with the said substance (12) of low heat capacity and low density, and a rigid, thermally resistant ring (16) which is mounted in the said space and comes into contact with the said flexible insulating ring (15).
8. Piping according to one of Claims 4 or 5, characterized in that fibrous windings of substance (12) are fixed to the outside of the said internal tube (10), and spaced apart by empty spaces, the said external tube coming substantially into contact with the said fibrous windings of substance.
9. Piping according to Claim 8, characterized in that the said windings are defined by a plurality of fibrous bands (12A) surrounding the said internal tube fixedly and at regular spacings, and exhibiting a trapezoidal or similar cross section, rings (17) being attached around the said fibrous bands in order to fit inside the external tube (11) and be fixed, by at least one of these, to the said external tube.
10. Piping according to any one of Claims 1 to 9, characterized in that it consists of a plurality of individual elements (14) which can be joined together.
11. Piping according to Claim 10, characterized in that each individual element (14) comprises an external tube (11) and an internal tube (10), both with thin walls, and a substance (12) with low heat capacity and low density arranged between the said tubes.
12. Piping according to Claim 11, characterized in that, when two individual elements are joined together, the corresponding ends (10B, 10C) of the internal tubes fit one inside the other, whereas the corresponding ends (11B, 11C) of the external tubes butt up against each other.

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