DE102011018770A1 - Exhaust system for a motor vehicle and method for operating an exhaust system - Google Patents

Abstract

The invention relates to an exhaust system (10) for a motor vehicle, with a metering device (14) for introducing a reducing agent for the exhaust gas aftertreatment in an exhaust pipe (12). At least one ventilation element (18, 26, 28, 34) is used to introduce a gas (24, 36) into a section of the exhaust pipe (12) that can be acted upon by the reducing agent. The at least one ventilation element (18, 26, 28, 34) can be used to set a higher content of the gas (24, 36) in a region of the section near a wall (22) of the exhaust pipe (12) than in one of the walls ( 22) further away area of the section of the exhaust pipe (12). The invention also relates to a method for operating such an exhaust system (10).

Description

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, for the exhaust aftertreatment by means of SCR technology (SCR = selective catalytic reduction, selective catalytic reduction) urea in the exhaust gas to convert to carbon dioxide and ammonia. 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.

The EP 1 316 688 A2 describes an exhaust system in which as a reducing agent urea solution is introduced into a mixing chamber, in which additional charge air is 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 the setting in the mixing section or hydrolysis low oxygen partial pressure, since the wall near portion of the portion 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. To prevent here that after a relatively short period of 10,000 km or less show undesirable corrosion damage to the exhaust system, the setting of the local, namely the wall of higher content of the gas shown as 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 being exposed to the mixture than would be the case if the ventilation element were arranged 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 are arranged spaced apart in the section both perpendicularly to the flow direction and in the flow direction, the effects described above can be achieved with a comparatively small number of ventilation elements.

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, at least two ventilation elements to arrange in the portion of the exhaust pipe which can be acted upon by the reducing agent such that the gas streams emerging from the respective ventilation elements combine with one another during the 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 is also possible to provide 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 gas line.

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 feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of the figures and / or in the figures alone can be used not only in the respectively specified combination but also in other combinations or in isolation, without the scope of To leave 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:

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

2 in a sectional view, a selective arrangement of air nozzles in the exhaust pipe;

3 in a further sectional view arranged on an upper side of the exhaust pipe air nozzle with a flat outlet opening;

4 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 schematically in a sectional view of the exhaust pipe according to 1 , wherein two air nozzles are provided at the level of a metering device for the urea solution;

6 in a sectional view, a distribution of air nozzles with circular outlet openings over the circumference of the exhaust pipe;

7 a further sectional view of the exhaust pipe with circumferentially distributed air nozzles with flat outlet openings; and

8th in a further sectional view, the exhaust pipe with an air nozzle whose outlet opening is circumferentially formed circumferentially.

From an exhaust system 10 for a car is in 1 an exhaust pipe 12 shown in which by means of a metering device 14 an aqueous urea solution is introduced. In one of the metering device 14 downstream hydrolysis pathway, the urea in the hot exhaust gas is converted to carbon dioxide and ammonia. In the area of this hydrolysis path there is a significant reduction in the oxygen partial pressure due to the formation of a reducing mixture 16 ,

At the exhaust system 10 according to 1 provides a downstream of the metering device 14 arranged air nozzle 18 for that this mixture 16 in the area of the hydrolysis line or at least to a much reduced extent in contact with an inner side 20 a wall 22 the exhaust pipe 12 comes. A from the air nozzle 18 exiting airflow 24 Rather, it ensures an oxygen enrichment in a region of the exhaust pipe near the wall 12 , This can be a repassivation of the preferably high-alloy steel, from which the wall 22 the exhaust pipe 12 is formed.

At the in 1 shown exhaust system 10 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 essentially prevents this air nozzle 18 on the corresponding side of the exhaust pipe 12 is arranged.

Two more air nozzles 26 . 28 are according to 1 downstream of the first air nozzle 18 arranged, and with respect to the flow direction of the exhaust gas through the exhaust pipe 12 at the same height. Will also about these air jets 26 . 28 which face each other in the exhaust pipe 12 protrude, air in the exhaust pipe 12 not only a one-sided, but a two-sided shielding of the wall is possible 22 from the mixture 16 to reach. In addition, they provide the downstream of the first air nozzle 18 arranged two air nozzles 26 . 28 for that with increasing distance from the air nozzle 18 weakening shielding of the wall 22 from the mixture 16 is restored.

The exhaust gas is in this case both upstream of the metered addition of the aqueous urea solution and downstream of the two air nozzles 26 . 28 as diverted by in 1 indicated flow arrows 30 is illustrated. To prevent in a downstream of the metering device 14 existing deflection 32 the mixture 16 in contact with the inside 20 the Wall 22 device, here is another air nozzle 34 provided, which one on the wall 22 along flowing airflow 36 provides.

Should, as in 2 shown the wall 22 the exhaust pipe 12 only to one side of the mixture 16 can be shielded, so on this side of the exhaust pipe 12 a single air nozzle 18 with one according to 2 be provided round outlet opening. But also other air nozzles 38 . 40 adjacent to the air nozzle 18 be provided, which also on this side of the exhaust pipe 12 are arranged. So can over these three air jets 18 . 38 . 40 each with a round outlet opening a comparatively large area of the wall 22 from the mixture 16 be shielded. The air nozzles 18 . 38 . 40 In this case, they may in particular be arranged so that the air streams emerging from them merge with one another.

At the in 3 shown embodiment of the exhaust system 10 are on one side of the exhaust pipe 12 not provided with several air nozzles, this is indicated by the single air nozzle 18 a wide, flat outlet opening, via which an equally flat air flow can be provided in order to the wall 22 to achieve a surface enrichment of the oxygen content.

At the in 4 shown embodiment of the exhaust system 10 is to shield the wall 22 from the mixture 16 on one side of the - in the example shown round - exhaust pipe 12 also only one air nozzle 18 intended. The shape of the outlet opening of this air nozzle 18 However, this corresponds to the contour of the inside of the wall 22 the exhaust pipe 12 ,

At the in 5 shown embodiment of the exhaust system 10 is the first air nozzle 18 at the height of the metering device 14 in the exhaust pipe 12 arranged, and the exiting from this airflow 24 shields the mixture 16 to a first side of the exhaust pipe 12 back from the wall 22 from. At the same height is an air nozzle 42 provided, from which a stream of air 44 exit. This shields the wall 22 also to an opposite second side of the exhaust pipe 12 from the mixture 16 from. By this two-sided enrichment of oxygen in the near wall region of the exhaust pipe 12 can be a particularly extensive shielding of the wall 22 from the mixture 16 and a particularly extensive repassivation of the steel of the exhaust pipe 12 to reach. In the deflection area 32 is the other air nozzle 34 intended.

In the embodiment according to 6 are in addition to on two opposite sides arranged two air nozzles 18 . 44 two more air nozzles 46 . 48 provided, which as well as the air nozzles 18 . 44 have round outlet openings. The two air nozzles 46 . 48 are also opposite each other and perpendicular to the flow direction of the exhaust gas through the exhaust pipe 12 from the two air nozzles 18 . 44 equidistant in the exhaust pipe 12 arranged. The four air nozzles 18 . 48 . 44 . 46 So are circumferentially circumferentially evenly distributed in the exhaust pipe near the wall 12 arranged. It can also be between these air jets 18 . 48 . 44 . 46 even more air jets 50 be also provided with round outlet opening to a circumferentially particularly far-reaching wrapping of the mixture 16 with air and thus a particularly extensive shielding and Repassivieren the wall 22 to reach.

One such the mixture 16 Circumferentially enclosing surrounding air jacket can also by a smaller number of air nozzles 18 . 48 . 44 . 46 be achieved, which each have wide and flat outlet openings, as shown in the 7 is shown. Here, the flat outlet openings take the air nozzles exemplified in a number of four 18 . 48 . 44 . 46 almost the entire inside surface of the wall 22 one.

At the in 8th shown embodiment of the exhaust system 10 has the only air nozzle 18 a circumferentially encircling outlet opening. So can by means of the air nozzle 18 exiting air flow a the mixture 16 enveloping air jacket in the - in the example round - 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 oxygen sensor (not shown) may be provided which is coupled to a controller (not shown). Here, a control circuit implemented in the control device can then ensure that the 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 from the mixture 16 shields. In particular, the air nozzles 18 . 26 . 28 . 34 . 38 . 40 . 44 . 46 . 48 . 50 in this case air in the exhaust pipe 12 bring in that a content of at least 0.1 vol .-% to 5 vol .-% oxygen in the exhaust pipe 12 is maintained.

The present described introducing air into the exhaust pipe 12 makes it possible, using SCR (Selective Catalytic Reduction) technology to reduce the nitrogen oxide content in the exhaust, to use fuels of comparatively high sulfur content, such as with a sulfur content of more than 500 ppm, without being contaminated oxygen depleted region downstream of the metering device 14 to a corrosion of the wall 22 exhaust pipe 12 comes.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• EP 1316688 A2 [0003]

Claims (10)

Exhaust system for a motor vehicle, with a metering device ( 14 ) for introducing a reducing agent for the exhaust aftertreatment in an exhaust pipe ( 12 ), and with 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 which can be acted upon by the reducing agent ( 12 ), characterized in that by means of the at least one ventilation element ( 18 . 26 . 28 . 34 . 38 . 40 . 44 . 46 . 48 . 50 ) in one of a wall ( 22 ) of the exhaust pipe ( 12 ) near the section of the gas higher content ( 24 . 36 ) is adjustable than in one of the wall ( 22 ) further away from the section 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 ) in the flow direction of the exhaust gas through the exhaust pipe ( 12 ) at the level and / or downstream of the metering device ( 14 ) is arranged. 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 stream, in particular air stream ( 24 . 36 ) and / or compressed air flow, in the section of the exhaust pipe ( 12 ) is formed whose direction of the flow direction of the exhaust gas through the exhaust pipe ( 12 ) is substantially the same. 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 ) - transversely, 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 with the reducing agent portion of the exhaust pipe ( 12 ) are arranged. 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 that 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 the contour of an inner side ( 20 ) the Wall ( 22 ) of the exhaust pipe ( 12 ) corresponds. 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 portion of the exhaust pipe that can be acted upon by the reducing agent such that the exhaust gases from the respective ventilation elements ( 18 . 38 . 40 . 44 . 46 . 48 . 50 ) escaping gas streams are to be combined. Exhaust system according to one of claims 1 to 6, characterized in that the at least one ventilation element ( 18 . 44 . 46 . 48 . 50 ) is arranged in such a way in the portion of the exhaust pipe which can be acted upon by the reducing agent that by means of one of the at least one ventilation element ( 18 . 44 . 46 . 48 . 50 ) leaving a gas mixture comprising the reducing agent and exhaust gas ( 16 ) is at least substantially umhüllbar. 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 portion of the exhaust gas line which can be acted upon by the reducing agent ( 12 ) is arranged. Exhaust system according to one of claims 1 to 8, characterized 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 - in dependence on at least one operating parameter of the exhaust system and / or an internal combustion engine and / or - in dependence on a measured content of the gas in the exhaust pipe is adjustable. 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 wherein in which can be acted upon by the reducing agent 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 one of a wall ( 22 ) of the exhaust pipe ( 12 ) near the section of the gas higher content ( 24 . 36 ) is set as in one of the wall ( 22 ) further away from the section of the exhaust pipe ( 12 ).

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