WO2012146267A1

WO2012146267A1 - Exhaust gas system for a motor vehicle, and method for operating an exhaust gas system

Info

Publication number: WO2012146267A1
Application number: PCT/EP2011/006122
Authority: WO
Inventors: Arnd Bressel; Heinz-Dieter Hesse
Original assignee: Daimler Ag
Priority date: 2011-04-27
Filing date: 2011-12-07
Publication date: 2012-11-01
Also published as: CN103502594B; DE102011018770A1; CN103502594A; RU2545267C1

Abstract

The invention relates to an exhaust gas system (10) for a motor vehicle, comprising a metering device (14) for introducing a reducing agent for the exhaust gas aftertreatment in an exhaust gas line (12). At least one ventilating element (18, 26, 28, 34) is used to introduce a gas (24, 36) into an exhaust gas line (12) portion that can be supplied with the reducing agent. A higher concentration of the gas (24, 36) can be set in a portion region near a wall (22) of the exhaust gas line (12) than in an exhaust gas line (12) portion region that is further away from the wall (22) by means of the at least one ventilating element (18, 26, 28, 34). The invention further relates to a method for operating such an exhaust gas system (10).

Description

Exhaust system for a motor vehicle and method for operating an exhaust system

The invention relates to an exhaust system for a motor vehicle, which comprises a metering device, by means of which a reductant provided for the exhaust gas aftertreatment can be introduced into an exhaust gas line. At least one ventilation element of the exhaust system serves to introduce a gas into a section of the exhaust gas line that can be acted upon by the reducing agent. Furthermore, the invention relates to a method for operating such an exhaust system.

It is known from the prior art to convert urea in the exhaust gas line into carbon dioxide and ammonia by means of SCR technology (SCR = selective catalytic reduction) for exhaust gas aftertreatment. The urea, which can be metered into the exhaust gas in the form of an aqueous urea solution, is decomposed in the region of a mixing section or hydrolysis section of the exhaust system with the release of ammonia. In one of the hydrolysis line downstream SCR catalyst, the ammonia is then reacted with nitrogen oxides from the exhaust gas, so that a reduction in the nitrogen oxide content of the exhaust gas is achieved.

EP 1 316 688 A2 describes an exhaust system in which a urea solution is introduced as a reducing agent into a mixing chamber into which charge air is additionally introduced. The charge air is provided by the compressor of an exhaust gas turbocharger. The injected via a metering valve in the mixing chamber reducing agent is thus mixed with a charge air stream and introduced as an aerosol from the mixing chamber into the exhaust pipe.

A disadvantage here is the fact that unwanted damage to the exhaust system can occur. Object of the present invention is therefore to provide an exhaust system of the type mentioned above and a method for operating such an exhaust system, which avoids occurrence of damage.

This object is achieved by an exhaust system having the features of patent claim 1 and by a method having the features of patent claim 10. Advantageous embodiments with expedient developments of the invention are specified in the dependent claims.

The exhaust system according to the invention for a motor vehicle comprises a metering device for introducing a reducing agent for the exhaust gas aftertreatment into an exhaust gas line. Furthermore, the exhaust gas system comprises at least one ventilation element, which is designed to introduce a gas into a section of the exhaust gas line that can be acted upon by the reducing agent. In this case, a higher content of the gas can be set by means of the at least one ventilation element in a region of the section which is near a wall of the exhaust gas line than in a region of the section of the exhaust gas line which is further away from the wall. In other words, the gas shields a mixture of reducing agent and exhaust gas from the wall of the exhaust pipe.

This is based on the finding that it can come in the field of treatment of the reducing agent, ie when using urea as a reducing agent in the hydrolysis, due to the formation of reducing gas mixtures to a significant reduction in the oxygen partial pressure. Due to the use of a fuel having a comparatively high sulfur content, aggressive acid condensates may additionally occur in this oxygen-depleted region. This, in conjunction with the low oxygen content, can lead to extremely severe corrosion of the wall of the exhaust pipe. Such corrosion is prevented by the introduction of the gas in the near wall region of the portion of the exhaust pipe.

Also, an exhaust gas recirculation, which already leads to a reduced oxygen content in the intake air of the internal combustion engine, is unproblematic in view of adjusting in the mixing section or hydrolysis low oxygen partial pressure, since the wall near portion of the section of the

Mixture of reducing agent and exhaust gas is shielded. It does not need to be made to increase the quality of the material used for the exhaust pipe in order to protect it from corrosive attack by the mixture of the reducing agent and the exhaust gas.

By preventing the occurrence of corrosion damage, a particularly long mileage of the exhaust system can be achieved. This is true even if the

Internal combustion engine whose exhaust gas flows through the exhaust system is operated with fuel, which has a comparatively high sulfur content, for example, a sulfur content of more than 500 ppm. Such fuels are readily available in a number of non-European countries. In order to prevent unwanted corrosion damage to the exhaust system after only a comparatively short running time of 10,000 km or less, the setting of the locally higher content of the gas on the wall has proven to be promising.

The exhaust system can be provided both for a passenger car and for a commercial vehicle.

By means of the ventilation element, in particular air and / or compressed air can be introduced into the section of the exhaust pipe. The air or compressed air or compressed air in this case serves to specifically increase the oxygen partial pressure in the contact region to the metallic wall of the exhaust gas line in the section which is particularly heavily loaded by the mixture of the reducing agent and the exhaust gas. Then, an oxygen-rich layer shields the wall in the section from the mixture. This can be achieved by using a high alloy steel for the exhaust pipe repassivation of the steel. Such passivation by an oxide layer of at least one alloying constituent of the steel particularly effectively prevents the corrosion of the exhaust pipe in the section acted upon by the reducing agent.

In an advantageous embodiment of the invention, the at least one ventilation element is arranged in the flow direction of the exhaust gas through the exhaust pipe at the height of the metering device. As a result, the region of the section immediately downstream of the metering device can already be protected from direct application of the mixture of exhaust gas and reducing agent. In addition, by means of a single ventilation element, a larger area of the exhaust pipe can be shielded from exposure to the mixture than this would be the case with the arrangement of the ventilation element upstream of the metering device. However, even if the at least one ventilation element is arranged a little way downstream of the metering device, such a shield can be achieved with great effectiveness, since propagation of the mixture containing the reducing agent towards the wall of the exhaust pipe is only fully carried out some distance downstream of the metering device comes.

In order to shield the wall of the exhaust pipe in the acted upon with the reducing agent portion of the mixture, the at least one venting element can guide the gas flow in the circumferential direction of the exhaust pipe along the wall. However, this is not particularly easy to accomplish in terms of flow guidance.

According to a further advantageous embodiment of the invention, therefore, the at least one ventilation element for introducing a gas flow is formed in the portion of the exhaust pipe, the direction of the flow direction of the exhaust gas through the exhaust pipe is substantially equal. Thus, an enrichment of the gas in the near-wall region can be achieved particularly easily over a comparatively long distance.

If at least two ventilation elements are arranged transversely to the flow direction of the exhaust gas spaced apart in the acted upon with the reducing agent portion of the exhaust pipe, can be achieved in a particularly large area close to the wall enrichment of the gas or reduce contact of the wall with the reducing acting mixture or avoid , This is especially true when the two ventilation elements are arranged perpendicular to the flow direction of the exhaust gas spaced from each other.

It is also possible for at least two ventilation elements in the flow direction of the exhaust gas to be arranged spaced apart from one another in the section through the exhaust gas line.

This can take into account the fact that the effect of a further upstream ventilation element decreases with increasing distance from the impingement point.

If two ventilation elements, which are arranged transversely to the flow direction of the exhaust gas spaced apart in the section to come to lie on two opposite sides of the exhaust pipe, so it can be a shielding of the mixture to achieve two sides. If the ventilation elements both Spaced perpendicular to the flow direction as well as in the flow direction spaced from each other are arranged in the section, so can be achieved with a comparatively small number of ventilation elements, the effects described above.

In a further advantageous embodiment of the invention, an outlet opening of the at least one ventilation element has a width which is greater than a height of the outlet opening. By means of such a ventilation element, a planar gas flow can be generated particularly well, which is particularly effective in shielding the wall from the mixture.

Additionally or alternatively, the shape of an outlet opening of the at least one ventilation element at least partially correspond to the contour of an inner side of the wall of the exhaust pipe. In this way, even with only one ventilation element seen in the circumferential direction of the exhaust pipe particularly extensive shielding of the wall can be achieved by the mixture.

Furthermore, it is additionally or alternatively possible to arrange at least two ventilation elements in the portion of the exhaust gas line which can be acted upon by the reducing agent in such a way that the gas streams emerging from the respective ventilation elements combine with one another during operation of the ventilation elements. Thus, even with the use of particularly simple design ventilation elements a very good surface loading of the near-wall area can be achieved with the gas.

As a further advantage, it has been shown that when the at least one ventilation element is arranged in the acted upon with the reducing agent portion of the exhaust pipe, that by means of emerging from the at least one vent gas flow, a mixture comprising the reducing agent and exhaust gas can be at least substantially enveloped. As a result, a particularly comprehensive shielding of the wall of the mixture can be achieved. Also, in the circumferential direction of the exhaust pipe whose wall surface can be subjected to the gas.

As a further advantage, it has been shown that the at least one ventilation element is arranged in a deflection region of the acted upon by the reducing agent portion of the exhaust pipe. In fact, in the region of such a bend there is also a deflection of the exhaust gas flow. As a result, the mixture of the reducing agent and the exhaust gas flows comparatively directly toward the wall. Here, the ventilation element ensures that the mixture is shielded from the wall. It can also at least one first ventilation element at the level or downstream of the metering device and at least one second ventilation element in the deflection region of the section of the exhaust pipe.

Finally, it has proven to be advantageous if the exhaust system comprises a control device, by means of which the pressurization of the at least one ventilation element with a gas flow of constant size is adjustable. In this case, the gas flow can always be adjusted when the metering device meters reducing agent into the section of the exhaust gas line. However, the actuation of the section with the gas flow via the at least one ventilation element can also be map-controlled, that is, as a function of at least one operating parameter of the exhaust system and / or the internal combustion engine. For example, the gas flow through the ventilation element can be varied as a function of the exhaust gas mass flow or as a function of the air ratio during the combustion of the fuel or as a function of a rotational speed of the internal combustion engine or the like. Such control of the gas flow based on maps is technically particularly easy to implement.

However, it can also be provided to regulate the admission of the at least one ventilation element to the gas flow by adjusting the gas flow as a function of a measured content of the gas in the exhaust gas line. In this case, for example, an oxygen sensor provides information about the content of oxygen in the portion of the exhaust gas line which is acted upon by the reducing agent. If this is particularly low and is, for example, less than 0.1% by volume, it can be ensured by pressurizing the ventilation element with a gas stream that this oxygen-poor mixture of reducing agent and exhaust gas does not come into contact with the wall.

However, it is also possible to set a desired oxygen content in the section which can be acted upon by the reducing agent, for example a minimum content of 0.1% by volume to 5% by volume of oxygen, by introducing air or compressed air into the exhaust pipe via the ventilation element and so on the oxygen content in the exhaust pipe is increased overall. Again, the desired oxygen content depending on operating parameters of the exhaust system and / or the internal combustion engine can be specified as a variable setpoint. In the method according to the invention for operating an exhaust system for a motor vehicle, a reducing agent for the exhaust gas aftertreatment is introduced into an exhaust gas line. A gas is introduced into a section of the exhaust gas line which can be acted upon by the reducing agent. In this case, a higher content of the gas is adjusted by means of at least one ventilation element in a region of the section which is near a wall of the exhaust gas line than in a region of the section of the exhaust gas line which is further away from the wall. By shielding the wall of the exhaust pipe from the mixture of reducing agent and exhaust gas can be corrosion damage the exhaust system avoided.

The advantages and preferred embodiments described for the exhaust system according to the invention also apply to the method according to the invention.

The features and combinations of features mentioned above in the description as well as the features and feature combinations mentioned below in the description of the figures and / or shown alone in the figures are not only in the respectively indicated combination, but also in other combinations or in

Used alone, without departing from the scope of the invention.

Further advantages, features and details of the invention will become apparent from the claims, the following description of preferred embodiments and with reference to the drawings, in which the same or functionally identical elements are provided with identical reference numerals. Showing:

Figure 1 is a schematic sectional view of an exhaust pipe into which an aqueous urea solution is metered, being introduced by means of air nozzles air into the exhaust pipe and walls of the exhaust pipe are protected against direct contact with a mixture of hydrolysis products and exhaust gas.

Fig. 2 is a sectional view of a selective arrangement of air nozzles in the

Exhaust pipe;

3 shows a further sectional view of an air nozzle arranged on an upper side of the exhaust pipe and having a flat outlet opening; 4 shows in a further sectional view the outlet opening of an air nozzle whose shape is adapted to the contour of an inner side of the wall of the exhaust pipe;

5 is a schematic sectional view of the exhaust gas line according to FIG. 1, two air nozzles being provided at the level of a metering device for the urea solution;

Fig. 6 is a sectional view of a distribution of air nozzles with circular

Outlets over the circumference of the exhaust pipe;

Fig. 7 is a further sectional view of the exhaust pipe with circumferentially distributed

Air nozzles with flat outlet openings; and

Fig. 8 is a further sectional view of the exhaust pipe with an air nozzle, whose

Outlet opening is circumferentially formed circumferentially.

From an exhaust system 10 for a motor vehicle, an exhaust pipe 12 is shown in Fig. 1, in which by means of a metering device 14, an aqueous urea solution is introduced. In one of the metering 14 downstream hydrolysis of the urea is reacted in the hot exhaust gas to carbon dioxide and ammonia. In the area of this hydrolysis it comes to a significant reduction in the

Oxygen partial pressure due to the formation of a reducing mixture 16.

In the case of the exhaust system 10 according to FIG. 1, an air nozzle 18 arranged downstream of the metering device 14 ensures that this mixture 16 does not come into contact with an inner side 20 of a wall 22 of the exhaust gas line 12 or at least to a much reduced extent in the region of the hydrolysis section. Rather, an air stream 24 emerging from the air nozzle 18 ensures an enrichment of oxygen in a region of the exhaust pipe 12 close to the wall. Thus, a repassivation of the preferably high-alloy steel can take place, from which the wall 22 of the exhaust pipe 12 is formed.

In the exhaust system 10 shown in Fig. 1 is a contact of the mixture 16 with the wall 22 to one side of the exhaust pipe 12 through the provision of the air nozzle 18 in Substantially prevented, since this air nozzle 18 on the corresponding side of the

Exhaust pipe 12 is arranged.

Two further air nozzles 26, 28 are arranged as shown in FIG. 1 downstream of the first air nozzle 18, and with respect to the flow direction of the exhaust gas through the

Exhaust pipe 12 at the same height. If air is also introduced into the exhaust pipe 12 via these air nozzles 26, 28, which protrude into the exhaust pipe 12 opposite one another, not only a one-sided, but a two-sided shielding of the wall 22 from the mixture 16 can be achieved. In addition, these two air nozzles 26, 28 arranged downstream of the first air nozzle 18 ensure that the shielding of the wall 22 weakening with increasing distance from the air nozzle 18 is restored by the mixture 16.

The exhaust gas is in the present case deflected both upstream of the metered addition of the aqueous urea solution and downstream of the two air nozzles 26, 28, as illustrated by flow arrows 30 indicated in FIG. In order to prevent the mixture 16 from coming into contact with the inside 20 of the wall 22 in a deflection region 32 present downstream of the metering device 14, a further air nozzle 34 is provided here, which provides an air flow 36 flowing along the wall 22.

If, as shown in Fig. 2, the wall 22 of the exhaust pipe 12 are shielded only to one side of the mixture 6, so may be provided on this side of the exhaust pipe 2, a single air nozzle 18 with a round as shown in FIG. 2 outlet opening. However, other air nozzles 38, 40 may be provided adjacent to the air nozzle 18, which are also arranged on this side of the exhaust pipe 12. Thus, a comparatively large area of the wall 22 can be shielded from the mixture 16 via these three air nozzles 18, 38, 40, each with a round outlet opening. The air nozzles 18, 38, 40 can in this case in particular be arranged such that the air streams emerging from them merge with one another.

In the embodiment of the exhaust system 10 shown in Fig. 3 are not provided on one side of the exhaust pipe 12 a plurality of air nozzles, but the single air nozzle 18 has a wide, flat outlet opening, via which a flat air flow can be provided to in the area the wall 22 to achieve a surface enrichment of the oxygen content. In the embodiment of the exhaust system 10 shown in FIG. 4, only one air nozzle 18 is provided for shielding the wall 22 from the mixture 16 on one side of the exhaust pipe 12, which is round in the example shown. However, the shape of the outlet opening of this air nozzle 18 here corresponds to the contour of the inside of the wall 22 of the Abgasieitung 12th

In the embodiment of the exhaust system 10 shown in Fig. 5, the first air nozzle 18 is disposed at the level of the metering device 14 in the Abgasieitung 2, and the exiting air flow 24 shields the mixture 16 to a first side of the Abgasieitung 12 out of the wall 22 off. At the same height an air nozzle 42 is provided, from which an air flow 44 exits. This shields the wall 22 also to an opposite second side of the Abgasieitung 12 from the mixture 16 from. By this two-sided enrichment of oxygen in the region near the wall of the exhaust line 12, a particularly extensive shielding of the wall 22 from the mixture 16 and a particularly extensive repassivation of the steel of the exhaust line 12 can be achieved. In the deflection region 32, the further air nozzle 34 is provided.

In the embodiment according to FIG. 6, in addition to two air nozzles 18, 44 arranged on two opposite sides, two further air nozzles 46, 48 are provided which, like the air nozzles 18, 44, have round outlet openings. The two air nozzles 46, 48 are also arranged opposite each other and perpendicular to the flow direction of the exhaust gas through the Abgasieitung 12 of the two air nozzles 18, 44 equidistant in the Abgasieitung 12. The four air nozzles 18, 48, 44, 46 are thus circumferentially circumferentially distributed evenly distributed in the Abgasieitung 12 near the wall. It can also between these air nozzles 18, 48, 44, 46 further air nozzles 50 also be provided with round outlet opening to a circumferentially particularly extensive enveloping the mixture 16 with air and thus to achieve a particularly extensive shielding and Repassivieren the wall 22.

Such an air jacket circumferentially surrounding the mixture 16 can also be achieved by a smaller number of air nozzles 18, 48, 44, 46, which each have wide and flat outlet openings, as shown in FIG. Here, the flat outlet openings of the air nozzles 18, 48, 44, 46 shown by way of example in a number of four occupy almost the entire inside surface of the wall 22.

In the embodiment of the exhaust system 10 shown in FIG. 8, the single air nozzle 18 has a peripheral outlet opening. So can by means of the this air nozzle 18 exiting air flow, the mixture 16 einhüllender air jacket in the - round in the example - exhaust pipe 12 can be generated.

When introducing air or compressed air into the exhaust pipe 12, a constant or a map-controlled variable air flow can be specified. Alternatively, in the

Exhaust pipe 12 an (not shown) oxygen sensor may be provided which is coupled to a (not shown) control means. Here then implemented in the control device control circuit ensure that a wall 22 shielding air flow through the air nozzles 18, 26, 28, 34, 38, 40, 44, 46, 48, 50 is set, which the wall 22 of the Mixture 16 shields. In particular, the air nozzles 18, 26, 28, 34, 38, 40, 44, 46, 48, 50 can in this case introduce air into the exhaust pipe 12 in such a way that a content of at least 0.1% by volume to 5% by volume. Oxygen in the exhaust pipe 12 is maintained.

The presently described introduction of air into the exhaust conduit 12 makes it possible, using SCR (Selective Catalytic Reduction) technology to reduce the nitrogen oxide content in the exhaust gas, to have fuels of comparatively high sulfur content, such as more than 500 ppm, without causing corrosion of the wall 22 exhaust pipe 12 in the oxygen-depleted region downstream of the metering device 14.

Claims

claims

An exhaust system for a motor vehicle, comprising a metering device (14) for introducing a reducing agent for the exhaust aftertreatment into an exhaust pipe (12), and having at least one ventilation element (18, 26, 28, 34, 38, 40, 44, 46, 48, 50 ) for introducing a gas (24, 36) into a section of the exhaust gas line (12) that can be acted upon by the reducing agent,

characterized in that

by means of the at least one ventilation element (18, 26, 28, 34, 38, 40, 44, 46, 48, 50) in a region of the exhaust pipe (12) near a wall (22)

Section is a higher content of the gas (24, 36) is adjustable than in a of the wall (22) further removed portion of the portion of the exhaust pipe (12).

Exhaust system according to claim 1,

characterized in that

the at least one ventilation element (18, 26, 28, 34, 38, 40, 44, 46, 48, 50) is arranged at the level and / or downstream of the metering device (14) as viewed in the flow direction of the exhaust gas through the exhaust line (12) ,

Exhaust system according to claim 1 or 2,

characterized in that

the at least one ventilation element (18, 26, 28, 34, 38, 40, 44, 46, 48, 50) for introducing a gas flow, in particular air flow (24, 36) and / or

Compressed air flow, in the portion of the exhaust pipe (12) is formed, whose direction of the flow direction of the exhaust gas through the exhaust pipe (12) is substantially equal.

4. Exhaust system according to one of claims 1 to 3,

characterized in that

at least two ventilation elements (18, 26, 28, 34, 38, 40, 44, 46, 48, 50)

transverse, in particular perpendicular, to the flow direction of the exhaust gas and / or in the flow direction of the exhaust gas through the exhaust pipe (12)

spaced apart in the acted upon by the reducing agent

Section of the exhaust pipe (12) are arranged.

5. Exhaust system according to one of claims 1 to 4,

characterized in that

an outlet opening of the at least one ventilation element (18, 44, 46, 48) has a width which is greater than a height of the outlet opening and / or in that the shape of an outlet opening of the at least one

Ventilation element (18) at least partially corresponds to the contour of an inner side (20) of the wall (22) of the exhaust pipe (12).

6. Exhaust system according to one of claims 1 to 5,

characterized in that

at least two ventilation elements (18, 38, 40, 44, 46, 48, 50) are arranged in the section of the exhaust pipe that can be acted upon by the reducing agent such that the ventilation elements (18, 38, 40, 44, 46, 48, 50) emerging gas streams are to be combined with each other.

7. Exhaust system according to one of claims 1 to 6,

characterized in that

the at least one venting element (18, 44, 46, 48, 50) is arranged in the section of the exhaust pipe that can be acted upon by the reducing agent, by means of a gas flow emerging from the at least one venting element (18, 44, 46, 48, 50) the reducing agent and exhaust gas comprehensive mixture (16) is at least substantially umhüllbar.

8. Exhaust system according to one of claims 1 to 7,

characterized in that

the at least one ventilation element (34) in a deflection region (32) of the the reducing agent acted upon portion of the exhaust pipe (12) is arranged.

9. Exhaust system according to one of claims 1 to 8,

marked by

a control device, by means of which the loading of the at least one ventilation element (18, 26, 28, 34, 38, 40, 44, 46, 48, 50) with a

gas flow

constant size and / or

depending on at least one operating parameter of the exhaust system and / or an internal combustion engine and / or

is adjustable in dependence on a measured content of the gas in the exhaust pipe.

10. A method for operating an exhaust system for a motor vehicle, in which a reducing agent for the exhaust aftertreatment in an exhaust pipe (12) is introduced, and in which one with the reducing agent

acted upon portion of the exhaust pipe (12) a gas (24, 36) is introduced,

characterized in that

by means of at least one ventilation element (18, 26, 28, 34, 38, 40, 44, 46, 48, 50) in a region of the section close to a wall (22) of the exhaust pipe (12) a higher content of the gas (24, 36 ) is set as in a region of the portion of the exhaust pipe (12) farther from the wall (22).