BURNER FOR AN EXHAUST-GAS AFTERTREATMENT SYSTEM, AND EXHAUST-GAS AFTERTREATMENT SYSTEM FOR AN INTERNAL COMBUSTION ENGINE COMPRISING SUCH A BURNER

Abstract

A burner for an exhaust-gas aftertreatment system. The burner includes a housing forming a combustion chamber, the housing having an outlet connected or connectable to an exhaust-gas line of the exhaust-gas aftertreatment system; a fuel feed device for feeding fuel into the combustion chamber; a fresh air feed device for feeding fresh air into the combustion chamber; and an ignition unit for igniting a fresh air-fuel mixture arranged in the combustion chamber. The ignition unit has a glow plug, which is arranged in the combustion chamber in such a way that a longitudinal center axis of the glow plug is oriented obliquely to a cross-sectional plane of the combustion chamber.

Description

FIELD

The present invention relates to a burner for an exhaust-gas aftertreatment system, comprising a housing forming a combustion chamber, the housing having an outlet connected or connectable to an exhaust-gas line of the exhaust-gas aftertreatment system, comprising a fuel feed device for feeding fuel into the combustion chamber, comprising a fresh air feed device for feeding fresh air into the combustion chamber, and comprising an ignition unit for igniting a fresh air-fuel mixture arranged in the combustion chamber.

The present invention also relates to an exhaust-gas aftertreatment system for an internal combustion engine.

BACKGROUND INFORMATION

For achieving current emission limits, the use of catalysts in exhaust-gas aftertreatment systems of internal combustion engines is conventional. The catalysts make a conversion of gaseous pollutants, such as NOx, HC and CO, into harmless products such as N2, H2O and CO2 possible. In order for these catalytic reactions to proceed sufficiently quickly, the temperature of the catalyst should exceed the so-called light-off temperature of typically 300° C. to 400° C. In order to quickly achieve this state, in particular in the case of a cold start of the internal combustion engine, so-called internal engine catalyst heating measures are often used. In this case, the degree of efficiency of the internal combustion engine is worsened by late ignition angles, whereby the exhaust-gas temperature and the enthalpy input into the catalyst are increased.

In addition to these internal engine catalyst heating measures, external catalyst heating measures are also described in the related art. For example, German Patent Application No. DE 195 04 208 A1 describes an exhaust-gas aftertreatment system in which a burner is assigned to the exhaust-gas line of the exhaust-gas aftertreatment system. The burner has a housing forming a combustion chamber. The burner also has a fuel feed device and a fresh air feed device. The fuel feed device is designed to feed fuel into the combustion chamber. The fresh air feed device is designed to feed fresh air into the combustion chamber. When the burner is in operation, the fresh air and the fuel flow through the combustion chamber as a fresh air-fuel mixture. Furthermore, an ignition unit is provided for igniting the fresh air-fuel mixture arranged in the combustion chamber. An outlet of the housing is fluidically connected to the exhaust-gas line of the exhaust-gas aftertreatment system. Fresh air-fuel mixture burned in the combustion chamber is accordingly fed through the outlet into the exhaust-gas line in order to heat the exhaust-gas line and in particular a catalyst of the exhaust-gas line.

SUMMARY

A burner according to the present invention having may have an advantage that the burner has a particularly advantageous ignition behavior. In particular, the ignition of the fresh air-fuel mixture is realized more quickly in comparison to conventional burners, so that the catalyst can also be heated more quickly. According to an example embodiment of the present invention, it is provided for this purpose that the ignition unit has a glow plug, which is arranged in the combustion chamber in such a way that a longitudinal center axis of the glow plug is oriented obliquely to a cross-sectional plane of the combustion chamber. It has been shown that an ignition unit having a glow plug is particularly advantageous with regard to quick and reliable ignition of the fresh air-fuel mixture. When the burner is in operation, the fresh air-fuel mixture typically flows through the combustion chamber in a spiral or swirl flow. The orientation according to the present invention of the glow plug achieves advantageous contact between the glow plug and the fresh air-fuel mixture so that the fresh air-fuel mixture is effectively heated by the glow plug. According to an example embodiment of the present invention, the longitudinal center axis of the glow plug is oriented obliquely to the cross-sectional plane of the combustion chamber. A cross-sectional plane is a plane that is perpendicular to the longitudinal center axis of the combustion chamber. Oblique orientation is assumed if the longitudinal center axis of the glow plug is neither parallel nor perpendicular to the cross-sectional plane. Another key advantage of a glow plug is its resistance to moisture. Due to its dead volume, a burner of an exhaust-gas aftertreatment system in passive operation tends to allow condensate to collect in the combustion chamber and also in the fuel feed device. If the burner is then started, this condensate can also be transported to the glow plug. However, the glow plug is at most slightly impaired by the condensate or moisture. Preferably, the glow plug protrudes through a housing wall of the housing into the combustion chamber. However, the glow plug may also be arranged entirely in the combustion chamber. The combustion chamber is preferably cylindrical. Particularly preferably, the combustion chamber has a circular cross-section. This ensures a particularly uniform swirl flow in the combustion chamber when the burner is in operation. However, the combustion chamber may also have a non-circular cross-section, for example an oval cross-section or a rectangular cross-section having rounded corners. The fuel feed device is preferably designed to meter the fuel directly into the combustion chamber. By effectively heating the fresh air-fuel mixture, the metered fuel is also effectively evaporated in this case. Preferably, the glow plug is arranged in such a way that the fresh air-fuel mixture only reaches the glow plug after the fresh air-fuel mixture has been deflected within the combustion chamber. The fuel is thus not metered directly onto the glow plug by the fuel feed device, but only reaches the glow plug as a swirl flow. Metering directly onto the glow plug could impair the ignition behavior of the burner.

According to a preferred embodiment of the present invention, it is provided that the glow plug is a ceramic glow plug. Ceramic glow plugs can reach particularly high temperatures, which is advantageous with regard to the ignition behavior of the burner. For example, a ceramic glow plug can achieve a constant high temperature of more than 1250° C. so that a particularly quick and reliable ignition of the fresh air-fuel mixture can be achieved. In addition, ceramic glow plugs are characterized in that they reach their operating temperature quickly. Typically, a ceramic glow plug reaches a temperature of 1000° C. within 1.4 s to 1.7 s. In addition, a ceramic glow plug shows hardly any aging effects over its service life. Furthermore, any aging effects that do occur can be easily compensated by calibration.

According to a preferred embodiment of the present invention, it is provided that an angle between the longitudinal center axis of the glow plug and the cross-sectional plane of the combustion chamber is between 20° and 80°. Such an orientation of the glow plug achieves a particularly advantageous contact between the glow plug and the fresh air-fuel mixture so that the heating of the fresh air-fuel mixture is optimized. The angle is particularly preferably between 50° and 80°. Preferably, the glow plug is arranged in such a way that a free end of the glow plug faces the outlet.

According to a preferred embodiment of the present invention, it is provided that the glow plug is oriented in such a way that the longitudinal center axis of the glow plug and a longitudinal center axis of the combustion chamber lie in a common imaginary plane. Such an integration of the glow plug into the combustion chamber is structurally simple to realize.

Preferably, in addition to the glow plug, the ignition unit has a spark plug fluidically connected downstream of the glow plug. The additional spark plug can further improve the ignition behavior of the burner. The spark plug is fluidically connected downstream of the glow plug. The spark plug is thus arranged between the glow plug and the outlet of the housing. The fresh air-fuel mixture thus first reaches the glow plug, is heated by the glow plug and then ignited by the spark plug connected downstream of the glow plug. Preferably, the spark plug is arranged in such a way that a longitudinal center axis of the spark plug is oriented in parallel with the cross-sectional plane of the combustion chamber. According to an alternative embodiment, the spark plug is preferably omitted. This also makes it possible to realize a burner with advantageous ignition behavior.

According to a preferred embodiment of the present invention, it is provided that the ignition unit has a tunnel-shaped first air guide element, and that the glow plug extends at least in portions through the first air guide element. Through the tunnel-shaped first air guide element, the fresh air-fuel mixture can be precisely fed to the glow plug. In addition, the tunnel-shaped first air guide element can adjust the flow direction of the fresh air-fuel mixture to the orientation of the glow plug. Furthermore, the tunnel-shaped first air guide element can also reduce the flow velocity of the fresh air-fuel mixture in the area of the glow plug. This has the result that the convective heat dissipation from the glow plug is reduced, which is advantageous for the ignition behavior of the burner.

According to a preferred embodiment of the present invention, it is provided that the tunnel-shaped first air guide element has a first side wall and a second side wall arranged at a distance from the first side wall in the circumferential direction of the combustion chamber, wherein the side walls each have a first end facing away from the outlet, and wherein the first end of the first side wall is at a greater distance from the outlet than the first end of the second side wall is. The first side wall thus extends further away from the outlet than the second side wall does. As mentioned above, the fresh air-fuel mixture typically flows through the combustion chamber in a spiral or swirl flow when the burner is in operation. Since one of the side walls extends further away from the outlet than the other of the side walls does, the entry of the fresh air-fuel mixture into the tunnel-shaped first air guide element is simplified. The first side wall forms an impact surface on which the fresh air-fuel mixture impinges. By impinging on the impact surface, the influence of the swirl on the fresh air-fuel mixture is reduced so that the fresh air-fuel mixture is precisely fed to the glow plug.

According to a preferred embodiment of the present invention, it is provided that the ignition unit has a second air guide element having a ramp surface oriented obliquely to the cross-sectional plane of the combustion chamber, and that the glow plug extends at least in portions along the ramp surface. The second air guide element, which is preferably shaped like a wedge or a jump ramp, adjusts the flow direction of the fresh air-fuel mixture in the area of the glow plug to the orientation of the glow plug. This allows the glow plug to heat the fresh air-fuel mixture particularly effectively. Preferably, the ramp surface is oriented in parallel with the longitudinal center axis of the glow plug. Particularly preferably, the second air guide element has a groove-shaped depression, which extends through a portion of the ramp surface opposite the glow plug and is oriented in parallel with the longitudinal center axis of the glow plug. Such a depression prevents turbulence of the fresh air-fuel mixture between the glow plug and the ramp surface of the second air guide element. If both the second air guide element and the aforementioned spark plug are present, the presence of the second air guide element also results in the advantage that the fresh air-fuel mixture can be precisely fed to the ignition electrodes of the spark plug through the second air guide element. This further optimizes the ignition behavior of the burner. Although the second air guide element is referred to as the second air guide element in the context of the disclosure, the presence of the second air guide element does not require the presence of the first air guide element. Preferably, however, the ignition unit has both the first air guide element and the second air guide element, wherein the second air guide element in this case also particularly preferably extends at least in portions through the tunnel-shaped first air guide element.

According to a preferred embodiment of the present invention, it is provided that the ignition unit has a carrier, which is arranged in an opening of a housing wall of the housing, and that the glow plug, the spark plug, the first air guide element and/or the second air guide element are arranged on the carrier. The carrier structurally simplifies the integration of the ignition unit into the burner. In addition, the elements forming the ignition unit can be easily handled together as an assembly before installation in the burner. Preferably, the carrier has a first opening assigned to the glow plug, wherein the glow plug protrudes through the first opening. Preferably, the carrier has a second opening assigned to the spark plug, wherein the spark plug protrudes through the second opening.

Preferably, the first air guide element and/or the second air guide element are formed in one piece with the carrier. This reduces the number of individual parts, which can lower production costs. In particular, only the first air guide element or only the second air guide element is formed in one piece with the carrier.

According to a preferred embodiment of the present invention, it is provided that the fuel feed device and the fresh air feed device together form a two-fluid nozzle. If fresh air and fuel are fed into the combustion chamber through the two-fluid nozzle, the fuel is broken up into fine droplets by the fresh air so that the fuel is finely distributed in the fresh air-fuel mixture. This allows the evaporation of the fuel by the glow plug to be accelerated, which ultimately also accelerates the ignition of the fresh air-fuel mixture.

Preferably, the fresh air feed device has a sleeve-shaped fresh air feed chamber, wherein the fresh air feed chamber radially encloses the housing, and wherein the ignition unit protrudes radially through the fresh air feed chamber. The fresh air thus flows past the housing in the area of the sleeve-shaped fresh air feed chamber, which has the result that the fresh air is heated by waste heat from the housing before the fresh air is then fed into the combustion chamber. This ensures particularly stable operation of the burner.

An exhaust-gas aftertreatment system according to the present invention for an internal combustion engine includes the burner according to the present invention. This also results in the advantages already mentioned. Further preferred features and combinations of features result from what was described above and from the rest of the disclosure herein. Preferably, the exhaust-gas aftertreatment system has an exhaust-gas line having a catalyst, wherein the exhaust-gas line is fluidically connected to the combustion chamber through the outlet of the housing of the burner.

The present invention is explained in more detail below with reference to the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exhaust-gas aftertreatment system in a schematic representation.

FIG. 2 is a sectional view of a burner of the exhaust-gas aftertreatment system, according to an example embodiment of the present invention.

FIG. 3 is a schematic sectional view of the burner, according to an example embodiment of the present invention.

FIG. 4 shows an ignition unit of the burner, according to an example embodiment of the present invention.

FIG. 5 is a further representation of the ignition unit, according to an example embodiment of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 shows an exhaust-gas aftertreatment system 1 in a schematic representation. The exhaust-gas aftertreatment system 1 has an exhaust-gas line 2 having a catalyst 3. The exhaust-gas aftertreatment system 1 is assigned to an internal combustion engine 4 of a motor vehicle not shown in detail. An outlet of the internal combustion engine 4 is fluidically connected to the exhaust-gas line 2 so that exhaust gas arising during operation of the internal combustion engine 4 can be or is fed into the exhaust-gas line 2. The exhaust gas of the internal combustion engine 4 flows through the catalyst 3, wherein the catalyst 4 converts gaseous pollutants of the exhaust gas, such as NOx, HC or CO, into harmless products. The efficiency of the catalyst 3 corresponds to the temperature of the catalyst 3. In particular, at catalyst temperatures below a so-called light-off temperature, the aforementioned pollutants may not be completely converted by the catalyst 3. This may, for example, occur during a cold start of the internal combustion engine 4. In order to accelerate the heating of the catalyst 3, in particular during a cold start of the internal combustion engine 4, the exhaust-gas aftertreatment system 1 has a burner 5. The structure of the burner 5 is explained in more detail below with reference to FIGS. 2 and 3. For this purpose, FIG. 2 is a sectional view of the burner 5. FIG. 3 shows a schematic longitudinal section through the burner 5.

The burner 5 has a housing 6, which forms or encloses a combustion chamber 7. An outlet 8 of the housing 6 is fluidically connected to the exhaust-gas line 2. A fresh air-fuel mixture burned in the combustion chamber 7 can thus be fed into the exhaust-gas line 2 in order to heat the catalyst 3 thereby. The housing 6 is cylindrical. In the present case, the housing 6 has a circular cross-section. Due to this design of the housing 6, the combustion chamber 7 is also cylindrical and has a circular cross-section. In a cross-section, the section plane is oriented perpendicularly to the longitudinal center axis 9 of the housing 6 or of the combustion chamber 7.

The burner 5 also has a fuel feed device 10, which is designed to feed fuel 11 into the combustion chamber 7. In the present case, the fuel feed device 10 is designed to meter the fuel 11 directly into the combustion chamber 7. For this purpose, the fuel feed device 10 has a fuel line 12, which opens directly into the combustion chamber 7 through a fuel inlet 13. The burner 5 also has a fresh air feed device 14, which is designed to feed fresh air 15 into the combustion chamber 7. For this purpose, the fresh air feed device 14 has a fresh air line 16, which is connected to the combustion chamber 7 through a fresh air inlet 17. In the present case, the fresh air inlet 17 is ring-shaped and encloses the fuel inlet 13. The fuel inlet 13 and the fresh air inlet 17 are located axially opposite the outlet 8 with respect to the longitudinal center axis 9 of the combustion chamber 7 or of the housing 6. The fresh air line 16 has a sleeve-shaped fresh air feed chamber 18, which radially encloses the housing 6. In the present case, the fuel feed device 10 and the fresh air feed device 14 together form a 3 two-fluid nozzle 19. If fuel 11 and fresh air 15 are fed into the combustion chamber 7 through the two-fluid nozzle 19, the fuel 11 is broken up into fine droplets by the fresh air 15 so that a fresh air-fuel mixture 20 is obtained, in which the fuel 11 is finely distributed. The fresh air-fuel mixture 20 is only indicated schematically in FIG. 3. Preferably, the fuel feed device 10 and the fresh air feed device 14 are designed to feed the fuel 11 and the fresh air 15 into the combustion chamber 7 in such a way that the fresh air-fuel mixture 20 flows through the combustion chamber 7 in a spiral or swirl flow 21.

As can be seen in FIG. 2, the housing 6 in the present case is of multi-part design. The housing 6 has a cylindrical first housing part 22. The combustion chamber 7 is substantially formed by the first housing part 22. The housing 6 also has a sleeve-shaped first adapter 23, which is arranged on an end face of the first housing part 22 facing the two-fluid nozzle 19. The housing 6 also has a sleeve-shaped second adapter 24, which is arranged on an end face of the first housing part 22 assigned to the exhaust-gas line 2. The outlet 8 is formed here by the second adapter 24. FIG. 3 shows the housing 6 as a single piece for the sake of simplicity.

The burner 5 also has an ignition unit 25 for igniting the fresh air-fuel mixture 20 arranged in the combustion chamber 7. FIG. 4 is a perspective view of the ignition unit 25. FIG. 5 is a plan view of the ignition unit 25. The ignition unit 25 has a glow plug 26, which protrudes into the combustion chamber 7 through an opening 27 of a housing wall 28 of the housing 6. In the present case, the opening 27 is formed in the first housing part 22. The glow plug 26 is arranged in such a way that a longitudinal center axis 29 of the elongate glow plug 26 is oriented obliquely to a cross-sectional plane A-A′ of the combustion chamber 7. A free end 30 of the glow plug 26 faces the outlet 8. Preferably, the longitudinal center axis 29 of the glow plug 25 is oriented obliquely in such a way that an angle α between the longitudinal center axis 29 and the cross-sectional plane A-A′ of the combustion chamber 7 is between 20° and 80°. In the present case, the angle α is approximately 70°. In the present case, the glow plug 26 is also arranged in such a way that the longitudinal center axis 29 of the glow plug 26 and the longitudinal center axis 9 of the combustion chamber 7 lie in a common imaginary plane. This plane corresponds to the longitudinal section plane of the longitudinal section shown in FIG. 3. In the present case, the glow plug 26 is a ceramic glow plug 26. Ceramic glow plugs are particularly suitable for use in the burner 5 due to their high operating temperatures.

The ignition unit 25 has a spark plug 31 in addition to the glow plug 26. The spark plug 31 is fluidically connected downstream of the glow plug 26. The spark plug 31 is arranged between the glow plug 26 and the outlet 8. The fresh air-fuel mixture 20 introduced into the combustion chamber 7 through the two-fluid nozzle 19 thus reaches first the glow plug 26 and only thereafter the spark plug 31. The spark plug 31 also protrudes through the opening 27 into the combustion chamber 7. In the present case, the spark plug 31 is arranged in such a way that a longitudinal center axis 32 of the spark plug 31 is oriented in parallel with the cross-sectional plane A-A′.

The ignition unit 25 also has a tunnel-shaped first air guide element 33. The tunnel-shaped first air guide element 33 has a first side wall 34, a second side wall 35 at a distance from the first side wall in the circumferential direction of the housing 6, and a cover 36. The first air guide element 33 is arranged in such a way that the glow plug 26 extends in portions through the first air guide element 33. The side walls 34 and 35 are thus arranged on both sides of the glow plug 26, and the cover 36 covers the glow plug 26. The first side wall 34 has a first end 37 facing away from the outlet 8. The second side wall 35 has a first end 38 facing away from the outlet 8. As can be seen in the figures, the first end 37 of the first side wall 34 is at a greater distance from the outlet 8 than the first end 38 of the second side wall 35 is. The first side wall 34 thus extends further in the direction of the two-fluid nozzle 19 than the second side wall 35 does. This has the result that the swirl flow 21 can enter the tunnel-shaped first air guide element 33 more easily. When the burner is in operation 5, the swirl flow 21 impinges on an inner surface 39 of the first side wall 34. As a result, the flow direction of the swirl flow 21 is changed and the fresh air-fuel mixture 20 is precisely fed to the glow plug 26. The inner surface 39 thus forms an impact surface 39 for the fresh air-fuel mixture 20 flowing as a swirl flow 21 through the combustion chamber 7.

The ignition unit 25 also has a wedge-shaped second air guide element 40. The second air guide element 40 has a ramp surface 41, which is oriented obliquely to the cross-sectional plane A-A′. In the present case, the ramp surface 41 is oriented in parallel with the longitudinal center axis 29 of the glow plug 26. The second air guide element 40 is arranged in such a way that the glow plug 26 extends in portions along the ramp surface 41. The ramp surface 41 changes the flow direction of the fresh air-fuel mixture 20 in the area of the glow plug 26 in order to adjust the flow direction to the orientation of the longitudinal center axis 29 of the glow plug 26. By changing the flow direction, it is also ensured that the fresh air-fuel mixture 20 is precisely fed to the ignition electrodes 42 of the spark plug 31. As can be seen in FIG. 4, the second air guide element 40 has a groove-shaped depression 43, which extends through a portion of the ramp surface 41 opposite the glow plug 26 and is oriented in parallel with the longitudinal center axis 29 of the glow plug 26.

The ignition unit 25 also has a carrier 44, which is arranged in the opening 27 of the housing wall 28. The carrier 44 carries the glow plug 26, the spark plug 31, the first air guide element 33 and the second air guide element 40. The carrier 44 has a first opening 45 assigned to the glow plug 26. The glow plug 26 protrudes through the first opening 45 into the combustion chamber 7. The carrier 44 also has a second opening 46 assigned to the spark plug 31. The spark plug 31 protrudes through the second opening 46 into the combustion chamber 7. The tunnel-shaped first air guide element 33 is here manufactured separately from the carrier 44. The wedge-shaped second air guide element 40 is here formed in one piece with the carrier 44.

According to the exemplary embodiment shown in the figures, the ignition unit 25 has the glow plug 26, the spark plug 31, the first air guide element 33 and the second air guide element 40. However, one or more of the elements 31, 33 and 40 of the ignition unit 25 may also be omitted.

According to a further exemplary embodiment, the ignition unit 25 has only the glow plug 26 of the elements 26, 31, 33 and 40.

According to a further exemplary embodiment, the ignition unit 25 has only the glow plug 26 and the wedge-shaped second air guide element 40 of the elements 26, 31, 33 and 40.

According to a further exemplary embodiment, the ignition unit 25 has only the glow plug 26, the tunnel-shaped first air guide element 33 and the wedge-shaped second air guide element 40 of the elements 26, 31, 33 and 40.

According to a further exemplary embodiment, the ignition unit 25 has only the glow plug 26 and the spark plug 31 of the elements 26, 31, 33 and 40.

According to a further exemplary embodiment, the ignition unit 25 has only the glow plug 26, the spark plug 31 and the wedge-shaped second air guide element 40 of the elements 26, 31, 33 and 40.

Claims

1-13. (canceled)

14. A burner for an exhaust-gas aftertreatment system, comprising: a housing forming a combustion chamber, the housing having an outlet connected or connectable to an exhaust-gas line of the exhaust-gas aftertreatment system;a fuel feed device configured to feed fuel into the combustion chamber;a fresh air feed device configured to feed fresh air into the combustion chamber; andan ignition unit configured to ignite a fresh air-fuel mixture arranged in the combustion chamber;wherein the ignition unit has a glow plug, which is arranged in the combustion chamber in such a way that a longitudinal center axis of the glow plug is oriented obliquely to a cross-sectional plane of the combustion chamber.

15. The burner according to claim 14, wherein the glow plug is a ceramic glow plug.

16. The burner according to claim 14, wherein an angle between the longitudinal center axis of the glow plug and the cross-sectional plane of the combustion chamber is between 20° and 80°.

17. The burner according to claim 14, wherein an angle between the longitudinal center axis of the glow plug and the cross-sectional plane of the combustion chamber is between 50° and 80°.

18. The burner according to claim 14, wherein the glow plug is oriented in such a way that the longitudinal center axis of the glow plug and a longitudinal center axis of the combustion chamber lie in a common imaginary plane.

19. The burner according to claim 14, wherein the ignition unit has a spark plug fluidically connected downstream of the glow plug.

20. The burner according to claim 14, wherein the ignition unit has a tunnel-shaped first air guide element, and the glow plug extends at least in portions through the first air guide element.

21. The burner according to claim 20, wherein the first air guide element has a first side wall and a second side wall arranged at a distance from the first side wall in a circumferential direction of the combustion chamber, wherein the first and second side walls each have a first end facing away from the outlet, and wherein the first end of the first side wall is at a greater distance from the outlet than the first end of the second side wall is from the outlet.

22. The burner according to claim 20, wherein the ignition unit has a second air guide element having a ramp surface oriented obliquely to the cross-sectional plane of the combustion chamber, and in that the glow plug extends at least in portions along the ramp surface.

23. The burner according to claim 22, wherein the ignition unit has a carrier which is arranged in an opening of a housing wall of the housing, and the glow plug and/or the spark plug and/or the first air guide element and/or the second air guide element are arranged on the carrier.

24. The burner according to claim 23, wherein the first air guide element and/or the second air guide element are formed in one piece with the carrier.

25. The burner according to claim 14, wherein the fuel feed device and the fresh air feed device together form a two-fluid nozzle.

26. The burner according to claim 14, wherein the fresh air feed device has a sleeve-shaped fresh air feed chamber, wherein the fresh air feed chamber radially encloses the housing, and wherein the ignition unit protrudes radially through the fresh air feed chamber.

27. An exhaust-gas aftertreatment system for an internal combustion engine, comprising: a burner for the exhaust-gas aftertreatment system, including: a housing forming a combustion chamber, the housing having an outlet connected or connectable to an exhaust-gas line of the exhaust-gas aftertreatment system;a fuel feed device configured to feed fuel into the combustion chamber;a fresh air feed device configured to feed fresh air into the combustion chamber; andan ignition unit configured to ignite a fresh air-fuel mixture arranged in the combustion chamber;wherein the ignition unit has a glow plug, which is arranged in the combustion chamber in such a way that a longitudinal center axis of the glow plug is oriented obliquely to a cross-sectional plane of the combustion chamber.