FR2966512A1 - DEVICE FOR INTRODUCING A LIQUID ADDITIVE IN A THERMAL ENGINE EXHAUST LINE - Google Patents

Abstract

The invention relates to the introduction and vaporization of a liquid additive, in particular based on urea, in an exhaust line (2) of a heat engine, upstream of a catalyst (3). A receiver element (5), of tubular form, is placed inside the exhaust line (2), coaxially with it, to collect the additive drops sprayed by at least one injector (4) and forming a liquid additive film (9) on the inner wall of the tubular element (5). An annular space (8) traversed by the exhaust gas is formed between the tubular element (5) and the inner wall of the exhaust line (2). The vaporization of the liquid additive is thus favored. Application: automotive engines, especially diesel engines.

Description

The present invention relates generally to a device for introducing a liquid additive into an exhaust line of a heat engine, in particular a diesel engine. This invention relates more particularly to the automotive industry and the depollution.

It is recalled that combustion, in internal combustion engines, produces nitrogen oxides which are highly polluting for the atmosphere. We are therefore looking for ways to remove nitrogen oxides from the exhaust gas. A known solution for the removal of these nitrogen oxides is to react with ammonia in a catalyst. For this purpose, a liquid additive consisting of a mixture of water and urea is injected into the exhaust line, upstream of the catalyst. Under the effect of heat, the mixture vaporizes and the urea is transformed into gaseous ammonia, which then enters the catalyst and reacts with the nitrogen oxides.

Since any residual liquid entering the catalyst will limit the effectiveness of the catalyst, it should be ensured that the mixture of water and urea has completely vaporized before reaching the catalyst. According to the most common technique, the urea liquid additive is introduced into the exhaust line by means of an injector, so as to form drops which are supposed to vaporize, the additive that can be heated prior to injection - see, for example, WO 2007/104353 and WO 2009/141051. As these documents show, the injector usually delivers the drops of additive in the central axis of the exhaust line. Such a solution, certainly simple, retains disadvantages. In particular, some drops sprayed by the injector in the axis of the exhaust line will fall on the inner wall of the exhaust pipe, where they accumulate in the form of a liquid film. However, the exhaust pipe being strongly cooled from the outside, its temperature is too low and unsuitable for a total vaporization of the additive. In addition, even if they do not fall on the wall of the exhaust pipe, the largest drops entrained towards the catalyst do not have time to vaporize and strike and clog the catalyst, formed by small holes.

In view of this, an interesting idea is to inject small drops into the exhaust line to increase the heat exchange surface with the exhaust gas. However, the small drops are easily driven by the flow of exhaust gases and are found at a certain distance from the injector, driven at the same speed as the exhaust gas. The heat exchange between these gases and the drops then becomes ineffective, so that the drops vaporize with difficulty. Thus, it seems preferable that the drops sprayed by the injector are fixed, which creates a relative speed with respect to the flow of the exhaust gas. However, it is not interesting that the "fixed" position of these drops is located on the inner wall of the exhaust pipe, for the reasons already set out above. Patent document WO 2008/135176 already proposes to project the drops of additive on a receiving element placed inside the exhaust pipe, so as to prevent these drops from falling on the inner wall of the exhaust pipe , wall whose temperature is not always ideal for rapid vaporization. The receiving element of the drops here has the shape of a cup, or the shape of a plate arranged inclined inside the exhaust pipe. The cup or plate is placed just below the injector. Such a proposition remains imperfect and insufficient as to the result obtained. In particular, given its position, the receiving element such as cup or plate mainly recovers large drops that fall by gravity. The small and medium drops, driven by the flow of the exhaust gases, do not fall on the receiving element and instead are likely to fall on the inner wall of the exhaust pipe. Only the very small drops are driven towards the catalyst and quickly evaporated. In the case where the receiving element is a cup, it collects a certain volume of liquid additive, spread over a small area which is not conducive to rapid evaporation. In the case where the receiver element is a plate, it can be assumed that the spread of the drops of additive is better, but the vaporization takes place to the detriment of the free circulation of the exhaust gases, the significant inclination of the plate forming an obstacle in the gas flow. In no case does the document WO 2008/135176 seek to optimize the distribution of the drops collected by the receiver element, in particular by the formation of a liquid "film". The object of the present invention is to remedy the drawbacks previously set forth, and its purpose is therefore to provide a device for introducing liquid additive, of the type in question here, which provides a complete vaporization of the additive before it reaches the catalyst, in order to optimize the operation of the catalyst. For this purpose, the subject of the invention is a device for introducing and vaporizing a liquid additive, in particular based on urea, into an exhaust line of a heat engine, the device comprising means such as minus an injector for supplying and diffusing the liquid additive into the exhaust line upstream of a catalyst, and a receiving element placed inside the exhaust line for recovering and vaporizing the diffused liquid additive, this device being essentially characterized by the fact that the receiving element is shaped as a substantially tubular element placed inside the exhaust line, coaxially with this exhaust line and in connection with the means for supply and diffusion of the liquid additive, so as to obtain the formation of a liquid additive film on the inner wall of said tubular element, an annular space traversed by the exhaust gas was provided between this tubular element and the inner wall of the exhaust line. Thus, the drops of liquid additive are diffused not on the inner wall of the exhaust pipe, but on the inner wall of a tubular element placed inside the exhaust pipe, without contact with the wall of this pipe. pipe, the tubular element being "licked" externally by a fraction of the flow of the exhaust gas. The walls of the tubular element therefore remain very hot, which favors the evaporation of the liquid additive film formed on the inner wall of this tubular element. In addition, the tubular element surrounding the injector, this element recovers all large or medium additive drops, preventing them from falling on the wall of the exhaust pipe, and distributes these drops in a film which extends over a maximum area, thus promoting heat exchange and evaporation. Only the smallest drops with an axial direction of ejection can go through the tubular element and exit at the downstream end of it, but these very small drops are not a problem because their mass is low and they are quickly evaporated. .

The arrangement of the tubular element, coaxial with the exhaust pipe, is also advantageous in that it does not hinder the flow of the exhaust gas flow; in particular, the tubular element can be thin, it creates very little loss of load.

The small thickness of this tubular element furthermore promotes the communication of the heat of the gases passing outside said element towards the liquid additive film formed on the inner wall of this element. Other advantageous arrangements can also contribute to the effectiveness of the device, object of the invention.

According to a first advantageous arrangement, the additive injector is arranged, relative to the tubular element, so as to project drops of additive to the upper part of this tubular element. Thus, the liquid additive film is formed in the upper part of the tubular element, whose axis is substantially horizontal, and the liquid descends by gravity on both sides and towards the lower part of the tubular element. The liquid additive then spreads over the entire inner wall of the tubular element, thus on a maximum surface, which is particularly important when a large amount of liquid additive must be introduced and vaporized. According to another advantageous arrangement, the tubular element is followed, beyond its downstream end, by at least one other tubular element of slightly greater diameter, which is also placed inside the exhaust line, a space annulus traveled by the exhaust gas is still formed between the other tubular element and the inner wall of the exhaust line.

The additional tubular element makes it possible to recover the drops released at the downstream end of the first tubular element, avoiding their contact with the colder inner wall of the exhaust pipe. Indeed, at the downstream end of the first tubular element, the liquid film can tear and turn into drops again. The smaller of these drops are easily evaporated, but larger drops may reform a liquid film on the inner wall of the exhaust pipe; the additional tubular element avoids this contact with the wall of the exhaust pipe, by "recovering" the larger drops, and the presence of this additional tubular element increases the extent of the liquid additive film. Several additional tubular elements succeeding one another in the direction of the axis of the exhaust line towards the catalyst can accentuate this result, practically without creating additional pressure drop if these tubular elements are all thin. Another type of solution, to increase the wetted surfaces by the liquid film without increasing the pressure drops, is based on the principle of capillarity, and more particularly the phenomenon called "capillary bridge". It is recalled that when two liquid films are placed sufficiently close to each other, the two films will touch each other in places, this effect being known as a capillary bridge. A first solution exploiting this phenomenon is to provide, on the tubular element and in particular in the downstream part of this element, longitudinal slots creating between them tongues or points of material capable of generating capillary bridges. Other solutions, the general idea of which is still to form "voids" in any part of the tubular element to promote the formation of capillary bridges, are conceivable. Thus, the tubular element can be made, in its entirety or at least partially, as a porous or braided structure. A porous or braided structure may also be provided only in the downstream portion of the tubular member. Each pore here can be filled with liquid and create a capillary bridge. A final solution consists in providing, extending the tubular element in its downstream part, substantially parallel or intersecting wires which catch the liquid by capillarity while increasing the liquid-gas contact surface. Advantageously, the son are in the form of spiral or helix which increases the length of these son for a given size. The capillary phenomenon can also be used for the supply and diffusion of the liquid additive, in order to "wet" the inner wall of the tubular element over the entire circumference of this element. It is thus possible to suppress the generation of drops and to directly bring the liquid additive onto the tubular element. In particular, the tubular element may include, towards its upstream end, an annular channel of small width hollowed in its inner wall, a liquid additive inlet being provided at at least one point of the annular channel. The liquid additive is thus injected into a "capillary" channel which retains the liquid and distributes it circumferentially. So that the liquid additive wets the wall of the tubular element, preferably downstream of the annular channel, it is advisable to promote the passage of the liquid downstream of said channel, either by ad hoc arrangements or by providing a difference in diameter between the region of the tubular element located upstream of the annular channel and the zone of the tubular element located downstream of the annular channel, the downstream zone being of larger internal diameter. If the tubular element is made as a porous or braided structure, in its entirety or at least up to the zone of introduction of the liquid additive, the aforementioned annular channel can be removed, the liquid additive being directly supplied and diffused in the porous structure, which ensures its spreading by capillarity. In a basic version, the tubular element is a simple cylindrical tube of constant diameter along its entire length. However, in an improved embodiment of the invention, the tubular element comprises a convergent conical upstream portion, followed by a divergent conical downstream portion, in the manner of a Venturi, the supply of liquid additive being made in the narrowest zone of the tubular element. This embodiment has a triple advantage: The liquid additive being fed into the smaller diameter area, the liquid film can be formed more easily than over a large diameter area. - The Venturi effect accelerates the exhaust gas that passes through the tubular element, the suction of the liquid additive is improved, in proportion to the increase in gas velocity. As the liquid film advances over the divergent downstream portion, the more the liquid-gas contact surface increases, the more the film thickness decreases, which promotes evaporation. As a complementary advantageous effect, it will be noted that the suction of liquid additive in proportion to the speed, therefore to the flow rate of the exhaust gases, may constitute a simple but sufficient solution, in the case of certain engines, to dose the mixture. liquid additive in line with the instantaneous needs. Another variant, which also deviates from the strictly cylindrical shape, consists in producing the tubular element with longitudinal conformations such as ribs, grooves or corrugations, which provides an increase in the exchange surface, without increasing the loss of charge.

In any case, the invention will be better understood with the aid of the description which follows, with reference to the appended schematic drawing showing, by way of examples, some embodiments of this device for introducing a liquid additive. in a heat engine exhaust line: FIG. 1 partially shows, and in partial section, an exhaust line receiving the device that is the subject of the present invention, in a first embodiment; Figure 2 is a similar to Figure 1, showing a first variant of the device of the invention; Figure 3 is a similar to the previous, showing an embodiment with two tubular elements; Figure 4 shows, in perspective, the tubular element alone in another embodiment, exploiting the phenomenon of "capillary bridge"; Figure 5 is a perspective view of a variant of the tubular element of Figure 4; Figures 6 and 7 show, in perspective, two other embodiments of this tubular element, with son; Figure 8 shows a tubular element with an integral porous structure; Figure 9 shows a tubular element with a braided structure; Figure 10 shows a partially porous tubular member; Figure 11 shows, in longitudinal section, a tubular element with capillary feed channel; Figure 12 shows, on an enlarged scale, detail A of Figure 11; Figure 13 is a view similar to Figure 11 showing a variant of the tubular capillary feed tube member; Figure 14 is an overall view, similar to Figure 1, of another embodiment of the device of the invention; Figure 15 shows, on a larger scale and in longitudinal section, the tubular element of the embodiment of Figure 14: Figure 16 is an overall view, similar to Figure 1, of an embodiment with tubular element shaped in Venturi; Figure 17 shows, in longitudinal section and on a larger scale, this tubular element alone; Figure 18 is a perspective view of a tubular element with ribs or longitudinal grooves; Figure 19 shows, in perspective, a "tubular" semi-cylindrical element; Figure 20 shows, in perspective, a tubular element fed by a plurality of additive injectors; Figure 21 shows a tubular member provided with external helical ribs; Figure 22 shows a tubular member provided with internal helical ribs.

Figure 1 shows a section of exhaust pipe 2, belonging to the exhaust line of a motor vehicle engine, not shown. In the exhaust pipe 2 is placed a catalyst 3. In operation, the exhaust pipe 2 is traversed by the flow F of the exhaust gas. Before the catalyst 3, an injector 4 is installed so as to introduce an additive into the exhaust pipe 2. The injector 4 is connected to an unillustrated liquid additive circuit, allowing the dispensing of metered quantities of additive. In particular, the injector 4 is designed here to spray drops of liquid additive. Inside the exhaust pipe 2, and coaxially with this pipe, is placed a tubular element 5 with a thin wall. The tubular element 5, open at its upstream end 6 and at its downstream end 7, has a diameter smaller than the diameter of the exhaust pipe 2, so that an annular space 8 is provided between this tubular element 5 and the internal wall of the exhaust pipe 2. The injector 4 through the wall of the tubular element 5, in the upper upstream portion of the tubular element 5, so as to project the drops 30 of liquid additive inside of the tubular element 5. The liquid additive, thus introduced, forms a liquid film 9 which spreads on the inner wall of the tubular element 5. At least the large drops of additive are thus recovered on the inner wall of the tubular element 5, while the small drops can be driven forward by the fraction of the flow F of the exhaust gas flowing through the tubular element 5.

Another fraction of the flow F of the exhaust gas flows through the annular space 8 formed between the tubular element 5 and the inner wall of the exhaust pipe 2. The liquid additive, spread out, into a film 9 on the inner wall of the tubular element 5 heated by the exhaust gas flowing through the annular space 8, is largely evaporated on this wall. The small drops of additive, which are carried downstream without touching the wall of the tubular element 5, also evaporate rapidly. At the downstream end 7 of the tubular element 5, the non-evaporated fraction of the liquid film 9 can be sprayed again into droplets which themselves vaporize easily. Thus the additive will ideally reach the catalyst 3 in the fully vaporized state. FIG. 2, in which the elements corresponding to those described above are designated by the same references, shows a variant in which the injector 4 passes through the wall of the tubular element 5 in the lower upstream portion thereof, and projects thus the liquid additive upwards. The liquid film 9 then tends to form on the upper part of the tubular element 5, then to fall by gravity to spread over the lateral and lower parts of the tubular element 5. The surface of the liquid film 9 is thus maximized. In the embodiment of FIG. 3, the tubular element 5 is coaxially followed, beyond its downstream end 7, by another similar tubular element 10, of slightly larger diameter but still providing an annular space 11 between this annular element 10 and the inner wall of the exhaust pipe 2. In operation, the liquid film 9 can tear from the wall of the first tubular element 5 at its downstream end 7, and turn into drops, these drops are "recovered By the next tubular element 10, on the inner wall of which they form again a film which evaporates. According to this principle, two or more successive tubular elements, possibly shorter, may be arranged coaxially inside the exhaust pipe 2 upstream of the catalyst 3. FIG. 4, in which the tubular element 5 is represented alone, shows that the downstream portion of this element may have longitudinal slots 12 regularly spaced around the circumference, which create between them parallel tabs 13, which promotes the formation of "capillary bridges" increasing the extent of the liquid film formed by the additive. In a variant illustrated in FIG. 5, the slots 12 have a "V" shape and thus create points 14 between them, again generating capillary bridges of liquid additive. As shown in FIG. 6, the downstream part of the tubular element 5 may also, in place of the tongues or points, comprise wires 15, forming here a bundle of close and substantially parallel wires, which make it possible to increase the liquid surface by capillary effect. In a variant illustrated in Figure 7, the downstream portion of the tubular element 5 comprises interwoven son 16, in particular formed spirally or helically, this to increase the length of the son in the same size. Apart from the slits or wires, it is heretofore assumed that the tubular element 5 has a solid wall on which the liquid additive film 9 is formed. In other embodiments, the element tubular 5 has a perforated structure, promoting the capillary phenomenon. Thus, Figure 8 shows a tubular member 5 integrally formed by a porous structure, and Figure 9 shows a tubular member 5 integrally formed by a braided structure. The perforated structure may also concern only part of the tubular element 5. Thus, FIG. 10 shows a tubular element 5 which has a full upstream portion 17 and a porous downstream portion 18. Referring to FIGS. 11 and 12, the injection of drops is replaceable by feeding the liquid additive into an annular supply channel 19, hollowed in the inner wall of the annular element 5, at a short distance from the upstream end 6 of this element. As shown more particularly in Figure 12, the annular channel 19 has a small width e so as to form a capillary channel. In addition, the annular channel has an asymmetrical section, with an internal diameter difference d between the zone upstream of the annular channel 19 and the zone situated downstream from this channel, the downstream zone being of larger internal diameter. . Thus, the liquid additive being brought to a point of the annular channel 19, it is distributed over the entire length of this channel and, thanks to the difference in diameters, is spread over the entire circumference of the inner wall of the channel. tubular element 5, downstream of the channel 19.

In a variant illustrated in Figure 13, the annular supply channel 19 is formed in an inclined plane, so that the liquid additive introduced has a movement component directed downstream. Other forms and arrangements are possible for the annular channel 19, for example a variable height. FIGS. 14 and 15 illustrate another solution for the direct supply of the liquid additive to the tubular element 5. This is, as in the case of FIG. 10, a tubular element 5 which has a upstream part 17 full, and a downstream portion 18 porous. The liquid additive is distributed in the junction zone between the upstream 17 and the porous downstream portion 18, and this additive migrates, by capillarity, to the porous downstream portion 18, where it vaporizes. In a variant not illustrated, the liquid additive can also be injected directly, in the same manner, into a tubular element made integrally as a porous structure. The quantity of liquid additive, absorbed by the porous structure or the porous part, depends directly on the amount evaporated. The arrival of the liquid additive can thus be regulated without additional metering means, or with simplified means, which cut off the arrival of the additive in the stopping phases of the engine. While the preceding examples comprise a tubular element 5 of cylindrical shape, FIGS. 16 and 17 show an embodiment in which the tubular element 5 is shaped in Venturi, with from upstream to downstream: - A convergent conical part 21 ; - A tighter intermediate portion 22; - A diverging conical portion 23. The introduction of the liquid additive is carried out in the constricted intermediate portion 22, by one of the previously indicated solutions, such as an injector 4 projecting drops of liquid additive or by a capillary annular channel 19. The liquid additive film 9 is thus formed in the constricted part 22, and therefore the part of smaller internal diameter, and this film is then driven downstream on the diverging part 23, which progressively reduce its thickness. The quantity of liquid additive sucked is here directly related to the flow rate of the exhaust gas flowing through the tubular element 5 shaped Venturi, which also ensures a dosage and a regulation of the amount of additive absorbed by the device. FIG. 18 shows that the generally cylindrical tubular element 5 may however comprise ribs or longitudinal grooves 24, conferring on it a corrugated profile and thus increasing the surface of the liquid additive film and also the exchange surface thermal, to promote the vaporization of the additive. As shown in FIG. 19, the "tubular" element 5 can be of semi-cylindrical shape, in particular by occupying the lower part of the exhaust pipe (not shown), while the injector 4 is situated in the upper part , which may be sufficient to collect the liquid additive and form the film, the operation is not changed. The injector 4 or other means of supplying the liquid additive having heretofore been considered as unique and situated at a precise point, FIG. 20 illustrates the possibility of providing several injectors or means for supplying the liquid additive. . Thus, three injectors 4 are provided towards the upstream part of the tubular element 5, these injectors being arranged at angular intervals of 120 ° relative to each other, and thus being oriented towards different zones of the inner wall of the tubular element 5. 5. The additive drops are sprayed simultaneously on these areas and the liquid additive film is then directly created on a major part of the surface of the tubular element 5. Finally, the wall of the element tubular 5 may comprise, externally or internally, helical ribs which generate a swirling flow of the exhaust gas, in the exhaust pipe 2. FIG. 21 shows a tubular element 5 thus provided with external helical ribs 25, while the FIG. 22 shows a tubular element 5 provided with internal helical ribs 26. The tubular element 5 is also used to rotate the mixing the exhaust gas and the vaporized additive, which increases the flow length and promotes the mixing of the two gases, beyond the tubular element 5, before they reach the catalyst. It goes without saying, and as it follows from the foregoing, the invention is not limited to the embodiments of this device for introducing a liquid additive which have been described above, as a examples; it embraces the contrary, all variants of implementation and application respecting the same principle. It is thus, in particular, that one would not depart from the scope of the invention by realizing the tubular element in all materials and with all textures, and also by conferring on this element all forms, for example a section elliptical and not circular, which can promote the formation of the liquid additive film under the effect of gravity. In the same vein, the mode of introduction of the additive into the tubular element may also vary, and finally the various features here described and illustrated separately may give rise to any combination to the extent that they are compatible. 10

Claims (15)

CLAIMS1 Device for introducing and vaporizing a liquid additive, in particular based on urea, into an exhaust line (2) of a heat engine, the device comprising means such as at least one injector (4) for supplying and diffusing the liquid additive into the exhaust line upstream of a catalyst (3), and a receiving element placed inside the exhaust line (2) and provided for recovering and vaporizing the diffused additive, characterized in that the receiving element is shaped as a substantially tubular element (5) placed inside the exhaust line (2), coaxially with this exhaust line and in connection with the means (4) for supplying and diffusing the liquid additive, so as to obtain the formation of a liquid additive film (9) on the inner wall of said tubular element (5), an annular space ( 8) traversed by the exhaust gases being formed between this tubular element (5) and the inner wall of the exhaust line (2). 2. Device according to claim 1, characterized in that the additive injector (4) is arranged to project drops of additive to the upper part of this tubular element (5). 3. Device according to claim 1 or 2, characterized in that the tubular element (5) is followed, beyond its downstream end (7), by at least one other tubular element (10) of slightly greater diameter, which is also located inside the exhaust line (2), an annular space (11) traversed by the exhaust gas is still formed between this other tubular element (10) and the inner wall of the line d exhaust (2). 4. Device according to any one of claims 1 to 3, characterized in that there are provided, on the tubular element (5) and in particular in the downstream part of this element, longitudinal slots (12) creating between them tabs (13) or spikes (14) of material, capable of generating capillary bridges. 5. Device according to any one of claims 1 to 3, characterized in that the tubular element (5) is formed, in its entirety or at least partially, as a porous or braided structure. 6. Device according to claim 5, characterized in that a porous or braided structure is provided only in the downstream part (18) of the tubular element (5). 10 7. Device according to any one of claims 1 to 3, characterized in that are provided, extending the tubular element (5) in its downstream portion, son (15,16) substantially parallel or intersecting, which hang the liquid by capillarity. 15 8. Device according to claim 7, characterized in that the son (16) are in the form of spiral or helix. 9. Device according to any one of claims 1 to 8, characterized in that the tubular element (5) comprises, towards its upstream end (6), an annular channel (19) of small width (e) dug in its inner wall, an inlet of liquid additive being provided at at least one point of the annular channel (19). 10. Device according to claim 9, characterized in that an internal diameter gap (d) is provided between the zone of the tubular element (5) situated upstream of the annular channel (19) and the zone of the tubular element (5) located downstream of the annular channel (19), the downstream zone being of larger internal diameter. 30 11. Device according to claim 5, characterized in that, the tubular element (5) being formed as a porous structure, in its entirety or at least up to the zone of introduction of the liquid additive, the additive liquid is directly supplied and diffused into the porous structure, which ensures its spreading by capillarity. 35 12. Device according to any one of claims 1 to 11, characterized in that the tubular element (5) comprises a convergent conical portion 510amont (21), followed by a divergent conical downstream portion (23), in the manner a Venturi, the supply of liquid additive being made in the narrowest zone (22) of the tubular element (5). 13. Device according to any one of claims 1 to 12, characterized in that the tubular element (5) is formed with longitudinal conformations (24) such as ribs, grooves or corrugations to increase the exchange surface. 14. Device according to any one of claims 1 to 13, characterized in that the wall of the tubular element (5) comprises, externally and / or internally, helical ribs (25, 26) generating a swirling flow. 15