Burner for an exhaust tract through which exhaust gas from an internal combustion engine of a motor vehicle can flow

Abstract

The invention relates to a burner (10) for an exhaust tract (25) through which exhaust gas (36) from an internal combustion engine of a motor vehicle flows, comprising a combustion chamber (12) in which a mixture comprising air and a fuel is to be ignited and thereby burned, comprising at least one housing element (18) which at least partially delimits the combustion chamber (12), and comprising at least one outlet element (20) which has at least one through-opening (22) via which burner exhaust gas (34) produced by the combustion of the mixture can be discharged from the combustion chamber (12) for heating the exhaust tract (25) of the internal combustion engine.The invention is characterized in that the through-opening (22) has at least one first longitudinal region (26) through which the burner exhaust gas (34) can flow, which has a first cross-section with a convergent course, and at least one second longitudinal region (28) through which the burner exhaust gas (34) can flow, which second cross-section with a divergent course.

Description

The invention relates to a burner for an exhaust tract through which exhaust gas from an internal combustion engine of a motor vehicle can flow, according to the preamble of patent claim 1, to an internal combustion engine having a burner for a motor vehicle and to a method for operating such a burner.Motor vehicles with internal combustion engines and exhaust systems are known from series vehicle construction, which are also referred to as exhaust tracts. The respective exhaust tract is through which exhaust gas of the respective internal combustion engine, also referred to as an internal combustion engine, can flow. The exhaust gas can also be referred to as engine exhaust gas and can arise during combustion processes in which a fuel-air mixture is to be burned or is burned. The internal combustion engine preferably has at least one combustion chamber in which the combustion processes take place. The combustion chamber is designed, for example, as a cylinder. The combustion chamber is formed separately from the burner. In particular, the combustion chamber and the burner are spaced apart from one another. In some operating states or operating situations of the respective internal combustion engine, a high temperature of the exhaust gas may be desirable in order to be able to quickly heat up and / or keep warm an exhaust gas aftertreatment device arranged in the exhaust tract, for example, wherein, however, the temperature of the exhaust gas of the internal combustion engine is only insufficiently high in these operating states or operating situations. For heating the exhaust tract, the exhaust tract has a burner. The burner can have at least one combustion chamber in which an air and a mixture, in particular a liquid mixture, comprising fuel is to be ignited and thereby burned. In addition, the burner has at least one housing element, which can be referred to in particular as a combustion chamber housing or burner housing or combustion chamber housing. The housing element delimits the combustion chamber at least partially, in particular predominantly or completely, for example in the radial direction of the combustion chamber and / or in the axial direction of the combustion chamber. Furthermore, the burner has at least one outlet element, in particular an outlet element which delimits the combustion chamber at least partially, for example in the axial direction of the combustion chamber. The outlet element has at least one through-opening, via which burner exhaust gas generated by the burning of the mixture can be discharged from the combustion chamber for heating the exhaust tract of the internal combustion engine, in particular for heating the exhaust gas aftertreatment device arranged in the exhaust tract of the internal combustion engine, and in particular can be introduced into the exhaust tract or at least one longitudinal region of the exhaust tract. In particular, the outlet element is arranged on an outlet side of the combustion chamber, wherein the outlet side is, for example, an axial end side of the combustion chamber or of the housing element. The outlet side can be referred to in particular as the gas outlet side.Such a passage opening known per se can have a cross section or cross section course widening or diverging in the flow direction, i.e. from the combustion chamber through the outlet element to the gas outlet side. A through-opening with only one divergent cross-sectional profile can also be referred to as a diffuser. This can result in the disadvantage that a wall region of the outlet element around an inlet opening into the through opening heats up particularly strongly when hot burner exhaust gas flows through it. As a result of this local, particularly high heating, thermal stresses and / or expansions can arise, which can lead to cracks in the outlet element. The inlet opening can be the cross section through which the flow or the burner exhaust gas, coming from the combustion chamber, enters the through opening in order to flow through it and flow out of the through opening on the gas outlet side.It is an object of the present invention to provide a burner for an exhaust tract through which exhaust gas from an internal combustion engine of a motor vehicle can flow, an internal combustion engine for a motor vehicle and a method for operating such a burner, such that a thermal load capacity of the burner can be particularly increased.This object is achieved according to the invention by a burner for an exhaust tract through which exhaust gas from an internal combustion engine of a motor vehicle can flow, having the features of patent claim 1, by an internal combustion engine having the features of patent claim 9 and by a method having the features of patent claim 10. Advantageous embodiments with expedient developments of the invention are specified in the other claims.A first aspect of the invention relates to a burner for an exhaust tract through which exhaust gas of an internal combustion engine of a motor vehicle can flow, having a combustion chamber in which a mixture comprising air and a fuel can be ignited and thereby burned, having at least one housing element which at least partially delimits the combustion chamber, and having at least one outlet element which has at least one through opening via which burner exhaust gas generated by the burning of the mixture can be discharged from the combustion chamber for heating the exhaust tract of the internal combustion engine. The motor vehicle is designed, for example, as a motor vehicle, in particular as a passenger car or as a commercial motor vehicle.In order to be able to particularly increase a thermal load capacity of the burner, in particular of the outlet element, it is provided according to the invention that the through-opening has at least one first longitudinal region through which the burner exhaust gas can flow, which first longitudinal region has a first cross section with a convergent course, and at least one second longitudinal region through which the burner exhaust gas can flow, which second longitudinal region has a second cross section with a divergent course.This results in the advantage that a heat input from the hot burner exhaust gas into the outlet element can be reduced compared to a through-opening with, for example, a constant or only divergent cross-sectional profile. In addition, there is an advantage that a temperature in the outlet element can be uniformly distributed. As a result, stress cracks as a result of thermal stresses and / or expansions can be prevented, in particular during heating and / or cooling of the outlet element. Stresses here mean, in particular, tensile stresses and / or compressive stresses.During operation of the burner, the combustion chamber can heat up particularly strongly, for example to up to 1150° C., for example on an outlet side which the outlet element can have, for example. Due to a possible temperature difference which can be particularly high and is, for example, almost 1,200 K, very high, in particular thermal, stresses can occur at a perforated outlet surface in the form of the outlet element having the at least one passage opening, which stresses can lead to cracks. The housing element can delimit the combustion chamber at least partially, in particular predominantly or completely, for example in the radial direction of the combustion chamber and / or in the axial direction of the combustion chamber. Furthermore, the burner can have at least one outlet element, in particular an outlet element which delimits the combustion chamber at least partially, for example in the axial direction of the combustion chamber. The outlet element can have at least one through-opening, via which burner exhaust gas generated by the burning of the mixture can be discharged from the combustion chamber for heating the exhaust tract of the internal combustion engine, in particular for heating the exhaust gas aftertreatment device arranged in the exhaust tract of the internal combustion engine, and in particular can be introduced into the exhaust tract or at least one longitudinal region of the exhaust tract. In particular, the outlet element can be arranged on an outlet side of the combustion chamber, wherein the outlet side can be, for example, an axial end side of the combustion chamber or of the housing element. The outlet side can be referred to in particular as the gas outlet side.The convergent course is understood here in particular to mean a cross-sectional course which narrows in the flow direction of the burner exhaust gas discharged or to be discharged from the combustion chamber via the passage opening. This means that the cross-sectional area through which flow can pass becomes narrower along a flow or in the flow direction or downstream of an inlet opening, which can also be referred to as the inflow cross-section, that is to say is narrower downstream than upstream or has a smaller cross-sectional area. Divergent means in particular that the cross-sectional area through which flow can pass widens in the flow direction of the burner exhaust gas which is discharged or is to be discharged from the combustion chamber via the passage opening, that is to say that the cross-section through which flow can pass is greater downstream than upstream. The cross-sectional area here denotes the flow-through area which is perpendicular to a middle flow direction. Thus, the respective cross-sectional profiles, i.e. the convergent cross-sectional profile and the divergent cross-sectional profile, mean a cross-sectional profile which extends in each case in the flow direction. Flow-through means here that the burner exhaust gas and / or a flame can flow through the passage opening or is flowed through during operation of the burner. The flame may be used here to mean exhaust gas which has not yet been completely burnt or burnt. The outlet element can have a wall region which has the through opening.The invention is based in particular on the following considerations and findings: In order to make the burner, in particular the outlet element, particularly thermally loadable, it is intended to attempt to keep a heat input into a wall region around the passage opening, or into the wall region which has the passage opening, as low as possible. The heat input, which can also be referred to as heat flux density, here denotes an amount of heat energy which is introduced or transferred from the flow into or onto the wall region depending on the time and per area of the wall region. The heat input from a flow into a wall region at least partially delimiting or guiding the flow depends, in addition to the temperature difference between the temperature of the flow and the temperature of the wall region, in particular the static temperature in each case, over a (thermal) boundary layer thickness, on a running length of the flow, the speed of the flow and on a pressure profile of the static pressure along the flow. Also or alternatively from the Reynolds number, which describes a ratio of inertial forces to toughness forces of the flow or along the flow. According to the Bernoulli equation, which originates from a constant total pressure, which describes the sum of static pressure and dynamic pressure in the case of loss-free flow and in the case of neglecting differences in height, in conjunction with a non-compressible flowing fluid and the maintenance of mass, a convergent cross-sectional profile leads to an acceleration of the flow, associated with a drop in the static pressure in the flow. As a result of the acceleration of the flow, the boundary layer thickness decreases, as a result of which the heat transfer from the flow into the wall region increases. Along a divergent cross-sectional profile, i.e. a cross-sectional profile along which the cross section widens, the static pressure increases along the flow, as a result of which the boundary layer thickness increases and the heat transfer from the flow into the wall region decreases. If the cross section continues to expand, as a result of which the static pressure or counterpressure continues to increase, the boundary layer will be detached from the wall region from a so-called detachment point. Further downstream of the separation point, a turbulent return flow region is formed between the flow and the wall region. Turbulent flows are distinguished in that, in addition to a movement of fluid particles in the flow direction or main flow direction, a movement of the fluid particles also takes place perpendicular or transversely to the main flow direction or flow direction. Turbulent flows are thus particularly well suited for mixing fluids and / or different heat ranges of fluids. As a further finding, the invention is based on the fact that for heating a wall region, the heat conduction away from the wall region into which heat is introduced into the material surrounding this wall region must be taken into account. This means that particularly thin wall regions overheat more quickly than thicker wall regions, since the heat introduced here can only be dissipated into the surrounding material via material that is poorly available.The invention also includes developments which result in additional advantages.According to a development of the burner, the first longitudinal region is arranged upstream of the second longitudinal region, in particular in the flow direction of the exhaust gas flowing through the passage opening.The first longitudinal region, along which the hot burner exhaust gas flows into the passage opening, has a convergent cross-sectional profile, although the heat input can increase as a result of a flow speed which is increased as a result. As a result of the convergent cross-sectional profile, an obtuse angle, that is to say greater than 90 degrees of arc, is provided between the wall region adjoining the flow and comprising the first longitudinal section and a wall region perpendicular to the inflow direction into the first longitudinal region, precisely in an inflow region or inflow region or around the inlet opening, as viewed in a section, for example perpendicular to the inlet opening through the passage opening. In contrast to a divergent cross-sectional profile, the introduced heat, which is introduced, for example, at the apex of the angle, has more material available via which it can be distributed away from the inflow region into surrounding wall material than in the case of a divergent cross-sectional profile directly or directly away from the inflow region in the direction of flow. In other words, in the case of a convergent inflow region, the heat introduced here can be dissipated better via heat conduction into the surrounding material. This results in the advantage that more heat can be introduced until a material-specific failure limit is reached, compared to an only divergent cross-sectional profile in the flow direction. In order to reduce the heat input from the flow into the wall region surrounding the through-opening overall, the second length region, which adjoins the first length region downstream, has a divergent cross-sectional profile.This results in the advantage that the heat input or a quantity of heat introduced into a wall region can be dissipated particularly well by heat conduction in the inflow region of the first longitudinal region of the through-opening, although here the heat input can be increased by the convergent cross-sectional profile. In the second length region, the heat input from the flow into the wall region surrounding or comprising the second length region can be reduced by a delay of the flow and an increase of the boundary layer thickness as a result of the static pressure increase along the flow. A flow direction through the at least one through opening from the combustion chamber into the exhaust tract is predefined for the burner exhaust gas, wherein the first longitudinal region is arranged in front of the second longitudinal region in the flow direction and can be flown through and / or is arranged upstream of the divergent cross-sectional profile. During operation of the burner, the burner exhaust gas thus flows first through the first longitudinal section and immediately subsequently through the second longitudinal section. The first longitudinal section upstream of the second longitudinal section in the flow direction can result in the advantage that flow losses when flowing through the entire through-opening are lower than if the through-opening is configured only as a diffuser. A reduction in flow losses can mean that more burner exhaust gas or a greater mass flow of burner exhaust gas can be discharged from the combustion chamber into the exhaust tract or is discharged from the combustion chamber during operation of the burner than in the case of higher flow losses.According to a development of the burner, the second longitudinal region has an opening angle greater than 8 degrees of arc, in particular greater than 30 degrees of arc and / or less than 100 degrees of arc. In other words, the opening angle of the second length section, which has the divergent cross-sectional profile, should be so large that the flow shuts off when a flow passes through the divergent cross-sectional profile. A separation of the flow when flowing through the second longitudinal section results in the advantage that the heat input from the flow in the region of the separated flow, i.e. between a return flow or a dead water region and the wall region along the second longitudinal section, is lower than if the flow were present there. In other words, a main flow is surrounded or enveloped by the return flow region, such that the return flow is located or formed between the main flow in a middle or a middle region of the second length section and the wall region around this length section. The return flow can thus thermally isolate the wall region from the main flow. This embodiment is based on the finding that such detachment can be expected from an opening angle of 8 degrees of arc. However, it has been shown that the detachment behaves particularly favorably starting from 30 degrees of opening angle, in particular when the opening angle is less than 100 degrees of opening. On the other hand, the separation results in the advantage that the turbulence in the flow after the exit from the second longitudinal section or from the through-opening and in particular into the exhaust tract is increased as compared to a flow which is present and exits over the entire second longitudinal section. Due to the increased turbulence of the return flow exiting on the gas outlet side together with the main flow, a better mixing of the burner exhaust gas with, for example, cooler engine exhaust gas can be achieved. Due to the better mixing, local temperature differences in the flow flowing through the exhaust tract downstream of the burner can be reduced or a heat profile or a heat distribution in the flow in the exhaust tract downstream of the burner can be homogenized. This makes it possible to avoid local or locally occurring overheating of the exhaust gas aftertreatment device when the flow impinges, which may represent a mixture between burner exhaust gas and engine exhaust gas. The exhaust gas aftertreatment device can be, in particular, a catalytic converter.According to a development of the burner, the through opening, in particular in the flow direction of the burner exhaust gas flowing through the through opening, has, downstream of the first longitudinal region and, in particular in the flow direction of the burner exhaust gas flowing through the through opening, has, upstream of the second longitudinal region, a third longitudinal region which has a third cross section with a constant course. In other words, when viewed in the flow direction, a third length region is to be arranged between the first length region and the second length region when flowing through the through opening, said third length region having a cross section with a constant profile. In particular, the third longitudinal section can be arranged upstream of the second longitudinal region and downstream of the first longitudinal region in the flow direction of the burner exhaust gas flowing through the through opening. The third length region can thus directly adjoin the first length region along the through-opening and the second length region can directly adjoin the third length region. The third length region with the constant cross-sectional profile, through which flow can pass, can have in particular a cross section which corresponds to the narrowest or smallest cross section of the first length region with the convergent cross-sectional profile and the narrowest or smallest cross section of the second length region with the divergent cross-sectional profile. Thus, according to this refinement, the through-opening has at least one convergent and at least one divergent region and, in between, at least one region with a constant cross section through which flow can pass.This results in the advantage that after a possible separation after an exit from the first length region before entering the second length region, the flow can again be applied to a wall of this third length region when flowing through the third length region, which wall can surround or enclose or encompass it. As a result, local temperature or heat input peaks in the second length range can be avoided or reduced. When selecting or configuring the third cross section, which can be in particular the narrowest cross section of the passage opening or when flowing through the passage opening, it should be taken into account that sound velocity or a Mach number of 1 in the flow can be achieved in dependence on a combustion chamber pressure in the combustion chamber of the burner and a pressure on the gas outlet side in this narrowest cross section. The through-opening can namely have the shape of a convergent-divergent nozzle, also called a Laval nozzle, as a result of which the nozzle blocks when the speed of sound is reached. A blocking of the nozzle means that the mass flow cannot be increased further by lowering the pressure on the gas outlet side. However, according to the invention, it can be provided that sonic velocity is achieved precisely in this narrowest cross section, since the maximum flow velocity can be achieved in this way at a given combustion chamber pressure and a pressure on the gas outlet side and the mass flow of the flow can thus become maximum.According to a further development of the burner, the outlet element has a plurality of passage openings via which the burner exhaust gas can be discharged from the combustion chamber for heating the exhaust tract of the internal combustion engine. In particular, it is provided that the outlet element has a plurality of structurally identical passage openings. The plurality of through openings results in the advantage that a mass flow of burner exhaust gas, which can be supplied to the engine exhaust gas or can be discharged or is to be discharged into the exhaust tract, can be increased.According to a development of the burner, the burner exhaust gas can be mixed with an engine exhaust gas flowing through the exhaust tract when being discharged from the outlet element. In other words, the burner exhaust gas exiting or discharged from the outlet element is intended to mix with the engine exhaust gas flowing through the exhaust tract, which exhaust gas can be discharged or discharged from at least one combustion chamber of the internal combustion engine which is separate and / or spaced apart from the burner. In particular, the increased turbulence of the burner exhaust gas flowing out of the burner on the gas outlet side is intended to promote or promote mixing of the same with the engine exhaust gas. Mixing the burner exhaust gas with the engine exhaust gas results in the advantage that the temperature and / or the gas mixture flowing through the exhaust tract downstream of the outlet element can be changed or increased. In addition, the mixing results in the advantage that the temperature of this gas mixture can be uniformly distributed, in particular over a cross section of the exhaust tract, that is to say over the gas mixture which flows through this cross section. A uniform distribution can mean here that no temperature differences greater than 50 Kelvin (K) form in a flow cross section.According to a development of the burner, the outlet element is arranged upstream of an exhaust gas aftertreatment device arranged in the exhaust gas tract, with respect to a flow direction of the exhaust gas tract. In other words, a mixture or a gas mixture of the burner exhaust gas and the engine exhaust gas can be supplied to the exhaust gas aftertreatment device. This results in the advantage that the temperature of the exhaust gas mixture which is supplied to the exhaust gas aftertreatment device can be increased or decreased by supplying the burner exhaust gas, compared to an inflow of pure, premixed engine exhaust gas.According to a further development of the burner, the outlet element has at least one curvature. In other words, the outlet element which has the one or more passage openings is curved, bulbous and / or curved, that is to say the outlet element is in particular not formed as a planar or flat plate. In particular, the outlet element can be curved away from the combustion chamber or the housing element outwards or into the exhaust tract.This results in the advantage that a larger surface area is available as a result of the curvature, over which thermal stresses can be distributed, compared to a planar plate. As a result, stress peaks as a result of thermal loads on the outlet element, for example as a result of hot burner exhaust gas flowing through the passage openings, can be reduced.The thermal load capacity of the burner according to the invention, in particular of the outlet element and / or of the housing element, can be particularly increased. This can be achieved in particular by the outlet element being curved and / or curved, as a result of which an expansion direction can be predefined for the outlet element under thermal load, in particular in order to contract and / or expand. By means of a predefined curvature or curvature of the outlet element, thermal expansions between the outlet element and the burner or the combustion chamber housing or the combustion chamber can take place uniformly in the region of the curvature, as a result of which thermal stresses and cracks can be avoided.A second aspect of the invention relates to an internal combustion engine for a motor vehicle having a burner, in particular according to the first aspect of the invention. Advantages and advantageous refinements of the first aspect of the invention are to be regarded as advantages and advantageous refinements of the second aspect of the invention and vice versa.The internal combustion engine is preferably designed for driving the motor vehicle. The internal combustion engine can comprise an exhaust tract and the burner according to the invention according to the first aspect of the invention. In addition, the internal combustion engine can also comprise the exhaust gas aftertreatment device.For example, the internal combustion engine, in particular in its completely produced state, has the burner, wherein the burner is preferably arrangeable or arranged in the exhaust tract of the internal combustion engine. The internal combustion engine preferably has at least one combustion chamber in which combustion processes take place in which a fuel-air mixture is to be burned or is burned. The combustion chamber is designed, for example, as a cylinder. The combustion chamber is formed separately from the burner. In particular, the combustion chamber and the burner are spaced apart from one another. The internal combustion engine preferably has at least one intake tract through which air can flow, via which intake tract air can be supplied to the combustion chamber. The exhaust gas of the internal combustion engine generated from the combustion of the fuel-air mixture in the combustion chamber can be discharged from the combustion chamber via the exhaust tract.A third aspect of the invention relates to a method for operating a burner, in particular according to the first aspect of the invention. Advantages and advantageous refinements of the first aspect of the invention are to be regarded as advantages and advantageous refinements of the third aspect of the invention and vice versa.During operation of the burner, a mixture comprising air and a fuel is ignited and burned in its combustion chamber. A burner exhaust gas and / or a flame which arises in this case flows through one or more passage openings in the outlet element into the exhaust tract and mixes there with engine exhaust gas discharged from the combustion chamber of the internal combustion engine. Downstream of the burner, a gas mixture of burner exhaust gas and engine exhaust gas can flow onto an exhaust gas aftertreatment device, in particular a catalytic converter, which is arranged in the exhaust tract.A further aspect relates to a motor vehicle having an internal combustion engine, in particular according to the second aspect of the invention. Advantages and advantageous refinements of the first, the second and the third aspect of the invention are to be regarded as advantages and advantageous refinements of the further aspect and vice versa.Further advantages, features and details of the invention are evident from the following description of preferred exemplary embodiments and on the basis of the drawings. The features and combinations of features mentioned above in the description and the features and combinations of features mentioned below in the description of the figures and / or shown alone in the figures can be used not only in the respectively specified combination but also in other combinations or alone without departing from the scope of the invention.The following are shown: FIG. 1 is a schematic sectional view of a burner according to the invention; and FIG. 2 shows a schematic detailed sectional view of an outlet element of the burner with passage openings; and FIG. 3 shows a schematic sectional view of a diffuser through which flow takes place; and FIG. 4 shows a schematic sectional view of a through-opening having three length regions, each having a cross-sectional profile.In the figures, identical or functionally identical elements are provided with identical reference symbols.FIG. 1 shows a schematic sectional view of a burner 10 for an exhaust tract 25 through which exhaust gas 36 of an internal combustion engine of a motor vehicle can flow. The motor vehicle can preferably be driven by means of the internal combustion engine.The exhaust tract 25 can be understood in particular as an exhaust system of the internal combustion engine. In some operating states or operating situations of the internal combustion engine, a high temperature of the exhaust gas 36 may be desirable in order to be able to quickly heat up and / or keep warm an exhaust gas aftertreatment device 40 arranged in the exhaust tract 25, for example, wherein, however, the temperature of the exhaust gas of the internal combustion engine is only insufficiently high in these operating states or operating situations. By means of the burner 10, the exhaust tract 25, and in particular the exhaust gas aftertreatment device 40, can be heated and / or kept warm. This means that the exhaust tract 25, in particular the exhaust gas aftertreatment device 40, can be heated and / or kept warm by means of the burner 10, for example in addition to heating and / or keeping warm by means of the exhaust gas 36. This exhaust gas 36 can be understood to mean, in particular, an exhaust gas 36 generated from a fuel-air mixture in a combustion chamber of the internal combustion engine. The combustion chamber, which is designed as a cylinder, for example, is designed separately from the burner 10.The internal combustion engine is designed, for example, as a diesel engine. The exhaust gas aftertreatment device 40 comprises, for example, at least one catalytic converter or is designed as a catalytic converter. The exhaust gas aftertreatment device 40 or the catalytic converter is designed, for example, as an oxidation catalytic converter, in particular a diesel oxidation catalytic converter, a nitrogen oxide storage catalytic converter, an SCR catalytic converter as a particle filter, in particular a diesel particle filter. The exhaust gas 36 of the internal combustion engine can be purified by means of the exhaust gas aftertreatment device 40.The burner 10 has at least one combustion chamber 12 in which an air and a mixture, in particular a liquid mixture, comprising fuel is to be ignited and thereby burned. This means that the burner 10 in the combustion chamber 12 can ignite and combust the mixture, in particular with the formation of a flame and in particular with the provision of a burner exhaust gas 34.For example, the fuel is the fuel which is also burned in the combustion chamber, which is designed as a cylinder, for example, of the internal combustion engine. Thus, the fuel may be used as the fuel. However, the burner exhaust gas 34 is not the exhaust gas 36 or engine exhaust gas generated in the combustion chamber of the internal combustion engine.In the exemplary embodiment, the burner 10 has a burner housing 13, in which an introduction element 14 is provided, by means of which or via which the fuel, in particular for forming the mixture, can be introduced into the combustion chamber 12, in particular directly. The introduction element 14 is designed, for example, as an injection valve.Furthermore, the burner 10 has an air feed 15 in the combustion housing 13, via which the air can be introduced into the combustion chamber 12 in order to form a mixture. The introduction element 14 preferably injects the fuel into the air in the air supply 15, after which the air is already mixed with the injected fuel in the air supply 15 and reaches directly into the combustion chamber 12.The air supply 15 can be fluidically separated from the combustion chamber 12 by a flap 16. For this purpose, the flap 16 is provided pivotably on the burner housing 13 in the combustion chamber 12. By means of pivoting the flap 16, the air feed 15 separated from the combustion chamber 12 can be connected again to the combustion chamber 12, so that air and fuel can be introduced from the air feed 15 into the combustion chamber 12.Furthermore, the burner 10 has at least one ignition device, which is provided in particular in the burner housing 13 and projects into the combustion chamber 12 and by means of which the mixture comprising the air and the fuel is to be ignited and thereby burned. The ignition device is designed, for example, as a spark plug, glow plug or glow plug. The ignition device is arranged, for example, in an installation position 17 provided with the reference numeral 17.The burner 10 has at least one housing element 18, which can be referred to in particular as a combustion chamber housing. The housing member 18 is attached to the burner housing 13. The housing element 18 and the burner housing 13 delimit the combustion chamber 12. Furthermore, in the exemplary embodiment, the housing element 18 can delimit the combustion chamber 12 at least partially, in particular predominantly or completely, in its radial direction 19. Furthermore, the housing element 18 has, for example, at least one outlet element 20 which at least partially delimits the combustion chamber 12 in its axial direction 21, in particular at one end or in the axial direction 21 adjoining the combustion chamber 12 or the housing element 18. The outlet element 19 and the housing element 19 can preferably be formed integrally together.FIG. 2 shows a schematic detailed sectional view of the outlet element 19 of the burner 10 with a plurality of preferably identically constructed through-openings 22. In the exemplary embodiment, the outlet element 20 comprises a plurality of through-openings 22. The central axis can extend in particular in the axial direction of the combustion chamber 12. The outlet element 20 can have an inflow side pointing or pointing toward the combustion chamber 12 and an outer gas outlet side 24 pointing toward the exhaust tract 25. Due to the structure of the burner shown in FIG. 1, a flow direction 32 can be predefined for the passage openings 22. In this case, the burner exhaust gas 34 or the flame should enter the passage opening or passage openings 22 coming from the combustion chamber 12 or from the inflow side, flow through the passage openings 22 and flow out or flow out on the gas outlet side 24 or flow into the exhaust tract 25.In the exemplary embodiment, the outlet element 20 can be curved. Preferably, the outlet element 20 is arched pointing outward away from the combustion chamber 12 toward the exhaust tract 25. The through openings 22 can be uniformly distributed on a circular path about the central axis or rotational symmetry axis of the housing element 18 and / or of the combustion chamber 12 and / or of the outlet element 20, that is to say arranged on a circular path at the same distance from one another in the circumferential direction. In this case, the respective flow directions 32 per passage opening 22 can be different, but each flow direction 32 is perpendicular to an inflow side or inlet side or inlet plane of a respective passage opening 22. The through-openings 22 can have a circular or rotationally symmetrical cross section. The outlet element 20 which is curved in the exemplary embodiment can be connected integrally to the housing element 18 and thus directly adjoin the combustion chamber 12 or the housing element 18. As shown in FIG. 2 by means of a cut through-opening 22, each through-opening 22 can be chamfered at least on the inflow side 23 and / or on the gas outlet side 24. Thus, at least one or more, in particular two, variable cross-sectional profiles along the flow direction 32 have. Coming from the inflow side 23, the flow can, for example, initially flow through a first longitudinal section 26 with a convergently running cross-sectional profile, which can be referred to as nozzle-shaped. The flow can then flow through a second longitudinal section 28, which widens towards the gas outlet side 24 and can be referred to as a diffuser.FIG. 3 shows a schematic sectional view of a diffuser D through which flow takes place. Based on FIG. 3, basic flow phenomena of a diffuser through-flow are to be illustrated. The diffuser D in FIG. 3 has three different cross-sectional profiles through which flow can pass. The diffuser D in FIG. 3 can be designed rotationally symmetrically about a central axis or axis of symmetry, not shown. In the flow direction 32, the flow can initially flow through a first region B 1 with a constant cross section, immediately followed in the flow direction 32 by a second region B 2 with a divergent cross section profile, which can represent the actual diffuser D. A third region B 3 with a constant cross-sectional profile can be attached to the second region B 2 directly in the flow direction 32, which third region can have a larger cross-sectional area through which flow can pass than the first region B 1. Along one or the flow directions 32, the flow can enter the diffuser D or the second region B 2 with a velocity profile 48 that can form in or along the first region B 1 as a function of the Reynolds number and the running length of the flow. As can be seen in FIG. 3, the velocity profile 48 of the flow cannot be homogeneous or uniformly distributed over the cross section, in that the velocity in the flow or in a flow cross section can increase away from a wall or a wall region toward the flow center.The region in the flow in which the velocity increases away from the wall can also be referred to as a boundary layer. Directly on the wall, the flow has no velocity or zero velocity due to the adhesion condition. The diffuser D, i.e. the second length region B 2, can have an opening angle 42. The diffuser D in FIG. 3 is characterized in that it has a divergent cross-sectional profile along the flow or in the flow direction 32. The diffuser D illustrated in FIG. 3 can be configured rotationally symmetrically with respect to a central axis, not illustrated. If the flow flows through the widening cross-sectional area or second area B 2, the static pressure in the flow can increase along the flow, i.e. in the flow direction 32, according to the Bernoulli equation. Since the velocity of the flow in a region near the wall or within the boundary layer can be lower than in a flow center, in this case, when the flow passes through the widening region or along the divergently running cross-sectional profile, kinetic energy of the flow in the region near the wall cannot be sufficient to flow against the increasing static pressure. As a result, the boundary layer of the flow may separate from the wall, which may form a return flow region 44. When forming a return flow region 44, the flow in the second region B 2 can be divided into two regions, namely into the return flow region 44 or the return flow region and into a main flow region which, in the case of a rotationally symmetrical diffuser D, can be surrounded by the return flow region 44. In FIG. 3, adjacent to the second region B 2 with the widening cross-sectional profile, a third region B 3 with a constant cross-sectional profile is shown. If the flow releases from the wall in the diffuser D, it can again bear against the wall along the flow after entering the third region B 3 with constant cross-sectional profile, wherein the application can specify a contact point or contact region which delimits or terminates the return flow region 44 as viewed in the flow direction 32. The point or, in the example, annular region at which the flow separates from the wall can be referred to as a separation point, or in this case as a separation region. The return flow region 44 may thermally isolate a diffuser wall or the wall region surrounding the main flow or the flow from, for example, hot flow. This means that a heat input or a heat flow from the flow into the wall region can be reduced or inhibited. The separation point or separation region depends substantially on the opening angle 42 of the diffuser D, not substantially on an inlet length which determines the height of the boundary layer and / or on a speed of the flow or the speed profile 48 of the flow and thus on the Reynolds number. The second region B 2, which has a divergent cross-sectional profile, can be adjoined by a longitudinal region, as illustrated in FIG. 3, having a constant cross section in the flow direction. Thus, the second length region 28 (FIG. 4 ) can have a flow profile as is illustrated in the divergent cross-sectional region illustrated in FIG. 3 as far as the parting plane 46. This means that, in addition to the main flow, the return flow region 44 on the gas outlet side 24 can also emerge from a respective passage opening 22. The return flow region 44 has the property of having a particularly high degree of turbulence compared to the main flow. Turbulence means that fluid particles move not only in a direction in the main flow direction but also in directions running transversely thereto. Turbulent flows therefore have the ability to mix particularly well with other flows or fluids.FIG. 4 shows a schematic sectional view of a through-opening 22 having three length regions, each having a cross-sectional profile. These three length ranges are to be distinguished from the ranges B 1, B 2 and B 3 introduced previously in connection with the description of FIG. 3 and refer only to the exemplary embodiment described with reference to FIG. 4 in the following. In the flow direction 32, i.e. coming from the combustion chamber 12, a first longitudinal region 26 can flow through first or first in the flow direction 32 in the exemplary embodiment. This length region can have a convergent cross-sectional profile. As shown in FIG. 4, this can mean that the cross section narrows or narrows in the flow direction or is formed in the shape of a nozzle. According to the Bernoulli equation, a nozzle flow has the property that the flow is accelerated, wherein the static pressure in the flow decreases along the flow as it flows through the convergent cross-sectional profile. Acceleration ensures, on the one hand, or ensures that the flow is applied to the wall or a wall region, wherein the boundary layer thickness decreases or increases more slowly with the running length of the flow along the flow direction, compared to a diffuser flow. As a result, a heat transfer from the flow into the wall region surrounding the flow can be increased. The first longitudinal region 26 and the entire through-opening 22 can have a rotationally symmetrical cross-sectional profile about a central axis not shown in FIG. 4. The cross-sectional profile of the first length section 26 can be linear, as illustrated in FIG. 4. However, the cross-sectional profile can also be square or parabolic. As shown in FIG. 4, an obtuse angle can form between an inflow side 23, from which the fluid flows into the through opening 22 and the first longitudinal region 26, that is to say an angle greater than 90 degrees of arc. This results in better heat conduction or heat conduction from the wall region surrounding the first longitudinal region 26 into the surrounding material which can be surrounded by the outlet element 20, compared to an acute angle which would form, for example, if a divergent cross-sectional profile would result or would be given directly starting from the inflow side. The first longitudinal region 26 can be adjoined by a third longitudinal region 30 directly adjoining it in the flow direction. This third longitudinal region 30 can have a constant cross-sectional profile, as illustrated in FIG. 4. This can result in the advantage that a flow separating at the transition between the first longitudinal region 26 and the third longitudinal region 30 can apply again before the second longitudinal region 28 directly adjoining one another in the flow direction is flowed through. The narrowest cross section of the third length region 30 can be equal to the narrowest cross section or cross section region or, in the case of a rotationally symmetrical through-opening 22, the narrowest or smallest diameter of the first length region 26 and of the second length region 28, respectively. The second longitudinal region 28, which can have a cross-sectional profile divergent in the flow direction, can have an opening angle 42 of at least 8° degrees of arc, a maximum of 100° degrees of arc, in particular an opening angle 42 of 40 degrees of arc. Along a flow through the second length region 28, the flow can become detached, as a result of which a turbulent return flow region 44 can form. As a result, since not as shown in FIG. 3, a region with a constant cross-sectional profile adjoins the divergent cross-sectional profile, this turbulent return flow region 44 can emerge from the passage opening 22 on the gas outlet side 24 with a main flow. The gas outlet side 24 may be surrounded by exhaust gas 36 or engine exhaust gas which is discharged from a combustion chamber of the internal combustion engine. The burner exhaust gas 34 can mix particularly well with this exhaust gas 36 after leaving the through opening 22 as a result of the high turbulence of the flow as a result of the return flow region 44. As a result, a temperature distribution in a mixed exhaust gas 38, i.e. a mixture of exhaust gas 36 and burner exhaust gas 34, can be particularly homogeneous, viewed or measured in a flow cross section. This can be particularly advantageous when an exhaust gas aftertreatment device 40 flows against it, which can be located downstream of the through opening 22 in the exhaust tract 25. At the same time, the mixed exhaust gas 38 can have a higher temperature than the exhaust gas 36.A particularly preferred exemplary embodiment is described below.A sharp edge at the inlet or inlet opening of the diffuser, which the second length region 28 may have, may tend to overheat. Therefore, a chamfer is also applied to an inner side of the combustion chamber 12, which chamfer the first longitudinal region 26 can have. This change in geometry also reduces flow losses of the exhaust gas 36 that can flow into the diffuser.The diffuser reduces a flow velocity of the exhaust gas 36 and increases a gas pressure or a static pressure in the flow. By reducing the gas velocity or the flow velocity, the heat transfer or the heat input into the combustion chamber wall, which may be exhibited by the housing element, is reduced (Reynolds number decreases) and thus fewer thermal stresses may arise, which may lead to component failure (cracks in this case). Another advantage of turbulence is uniform distribution of the hot exhaust gas 36 into the catalyst, which may constitute the exhaust aftertreatment element 40. If the exhaust gas 36 flows directly onto the catalyst surface, stress cracks could occur as a result of local overheating. The catalyst is heated more uniformly by the turbulent and thus well mixed exhaust gas, which can also be referred to as mixed exhaust gas 38, from the burner 10 (very hot, about 1200° C.) and internal combustion engine exhaust gas, which can also be referred to as exhaust gas 36 (cooler, about 500° C.). Since an angle of the diffuser, which may be exhibited by the second length region 28, is 40 degrees of arc (30 degrees of arc to 100 degrees of arc) and thus significantly above the critical 8 degrees of arc limit for separation, turbulences, dead space areas and friction losses may occur.The following technical problem can be solved. Crack formation as a result of local component overheating at the combustion chamber outlet, which the outlet element 16 can have, and at the catalytic converter can be avoided. In this case, diffuser flows in aircraft engines or power plant combustion chambers can be known. Due to the diffusor-shaped configuration of the passage opening 22, which can be exhibited by the outlet element 20, the flow velocity and thus also the heat transfer or heat input of the burner gas or burner exhaust gas 34 to a wall of the combustion chamber 12, which can be exhibited by the housing element 18, can be reduced. At the same time, owing to turbulence flows which arise, optimized mixing and equal distribution of burner exhaust gas 34 with engine exhaust gas or exhaust gas 36 can be enforced. Advantageously, crack formation as a result of thermal component stresses at the combustion chamber outlet opening and the catalytic converter can be avoided.List of reference characters10 Burner 12 Combustion chamber 13 Burner housing 14 Introduction element 15 Air feed 16 Flap 17 Installation position of an ignition device 18 Housing element 19 Radial direction of the combustion chamber 20 Outlet element 21 Axial direction of the combustion chamber 22 Passage opening 23 Inflow side 24 Gas outlet side 25 Exhaust tract 26 First longitudinal region 28 Second longitudinal region 30 Third longitudinal region 32 Flow direction 34 Burner exhaust gas 36 Exhaust gas 38 Mixed exhaust gas 40 Exhaust gas aftertreatment device 42 Opening angle 44 Return flow region 46 Separating plane 48 Speed profile B 1 First region B 2 Second region B 3 Third region D Diffuser

Claims

Burner (10) for an exhaust tract (25) through which exhaust gas (36) of an internal combustion engine of a motor vehicle can flow, having a combustion chamber (12) in which a mixture comprising air and a fuel can be ignited and thereby burned, having at least one housing element (18) which at least partially delimits the combustion chamber (12), and having at least one outlet element (20) which has at least one passage opening (22) via which burner exhaust gas (34), which is generated by the burning of the mixture, can be discharged from the combustion chamber (12) for heating the exhaust tract (25) of the internal combustion engine, characterized in that the passage opening (22) has at least one first longitudinal region (26) through which the burner exhaust gas (34) can flow and which has a first cross section with a convergent course, and at least one second longitudinal region (28) through which the burner exhaust gas (34) can flow, which has a second cross section with a divergent profile.Burner (10) according to claim 1, characterised in that the first length region (26) is arranged upstream of the second length region (28).Burner (10) according to one of the preceding claims, characterized in that the second longitudinal region (28) has an opening angle greater than 8 degrees of arc, in particular greater than 30 degrees of arc, and / or less than 100 degrees of arc.Burner (10) according to one of the preceding claims, characterized in that the through-opening (22) has, downstream of the first longitudinal region (26) and upstream of the second longitudinal region (28), a third longitudinal region (30) which has a third cross section with a constant profile.Burner (10) according to one of the preceding claims, characterized in that the outlet element (20) has a plurality of passage openings (22), via which the burner exhaust gas (34) can be discharged from the combustion chamber (12) in order to heat the exhaust tract (25) of the internal combustion engine.Burner (10) according to one of the preceding claims, characterized in that the burner exhaust gas (34), when being discharged from the outlet element (20), can be mixed with an exhaust gas (36) flowing through the exhaust tract (25).Burner (10) according to one of the preceding claims, characterized in that the outlet element (20) is arranged upstream of an exhaust gas aftertreatment device (40) arranged in the exhaust gas duct (25) with respect to a flow direction of the exhaust gas duct (25).Burner (10) according to one of the preceding claims, characterized in that the outlet element (30) has at least one curvature.Internal combustion engine for a motor vehicle having a burner (10) according to one of the preceding claims.Method for operating a burner (10) according to one of claims 1 to 8.

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