Method for treating exhaust gas from an internal combustion engine of a vehicle and exhaust system for a vehicle

Abstract

A method for treating the exhaust gas of an internal combustion engine (12) of a vehicle, in which the exhaust gas of the internal combustion engine (12) is fed via an exhaust tract (14) to an exhaust aftertreatment device (16) designed to reduce the nitrogen oxide content of the exhaust gas, wherein oxygen and / or an oxygen-containing medium (26) is introduced into the exhaust tract (14) upstream of the exhaust aftertreatment device (16) when the content of at least one component of the exhaust gas becomes or threatens to become lower than a predetermined threshold value of the content of the at least one component, characterized in that the oxygen and / or the oxygen-containing medium (26) is introduced into the exhaust tract (14) when the oxygen content in the exhaust gas becomes or threatens to become lower than a predetermined threshold value of the content, wherein the oxygen and / or the oxygen-containing medium (26) is introduced into the exhaust tract (14).if the oxygen content in the exhaust gas becomes or threatens to become less than 5%.

Description

The invention relates to a method for treating the exhaust gas of an internal combustion engine of a vehicle. In this method, the exhaust gas of the internal combustion engine is fed via an exhaust system to an exhaust aftertreatment device, which is designed to reduce the nitrogen oxide content of the exhaust gas. The invention further relates to an exhaust system for a vehicle. It is known from the prior art to use an SCR catalyst (selective catalytic reduction) to reduce the nitrogen oxide content of the exhaust gas of an internal combustion engine in a vehicle. In this catalyst, ammonia reacts with nitrogen oxides from the exhaust gas in a selective catalytic reduction reaction to form nitrogen and water. A urea-water solution is typically injected into the exhaust system upstream of the SCR catalyst as the source of the ammonia, and the ammonia is released from the urea in the hot exhaust gas. Furthermore, US Patent 2009 / 0 260 349 A1 discloses a long-life catalyst system for reducing emissions from the exhaust stream of a gasoline engine, comprising a light-off catalyst closely coupled to the engine, a selective catalytic reduction catalyst located downstream of the light-off catalyst, a reducing agent injection system located between the light-off catalyst and the selective catalytic reduction catalyst, and an air injection system located between the light-off catalyst and the reducing agent injection point to inject air into the exhaust stream under certain engine conditions to improve the durability of the selective catalytic reduction catalyst. Furthermore, an emission treatment system for NOx reduction in a diesel exhaust stream from a diesel engine is known from WO 2010 / 108 083 A1, comprising a lean NOx trap (LNT) arranged downstream of the engine and an air injector for reducing hydrocarbons, which is arranged downstream of the NOx trap. In such exhaust gas treatment processes, it has been observed that the reaction of nitrogen oxides with ammonia in the exhaust gas aftertreatment system is not always satisfactory. The object of the present invention is therefore to provide a method of the type mentioned above by means of which an improvement in the reduction of nitrogen oxides can be achieved. This problem is solved by a method with the features of claim 1 and by an exhaust system with the features of claim 8. Advantageous embodiments with expedient further developments of the invention are specified in the dependent claims. In the method according to the invention, oxygen or an oxygen-containing medium is introduced into the exhaust gas tract upstream of the exhaust aftertreatment device. This occurs when the concentration of at least one component of the exhaust gas falls below, or threatens to fall below, a predetermined threshold value for the concentration of that component. This is based on the understanding that enriching the exhaust gas with oxygen upstream of the exhaust aftertreatment device improves the reduction of nitrogen oxides in the exhaust gas by the exhaust aftertreatment device. This ensures a particularly high level of activity from the exhaust aftertreatment device. This is especially true when the internal combustion engine is operated with a lower air-fuel ratio (λ) compared to normal operation with a very high excess of air. Furthermore, oxygen or an oxygen-containing medium is introduced into the exhaust system when the oxygen content in the exhaust gas falls below, or threatens to fall below, a predetermined oxygen threshold. This is because conventional SCR catalysts exhibit a significant dependence of nitrogen oxide conversion on the oxygen content in the exhaust gas. This manifests itself in a significant reduction in the conversion of nitrogen oxides to nitrogen and water with ammonia at very low oxygen levels. Particularly under richer operating conditions of the combustion engine compared to normal operation with a very high excess of air, a very low oxygen content in the exhaust gas can occur. If such a low oxygen content is reached or imminent, the oxygen content in the exhaust gas is increased upstream of the exhaust aftertreatment system.This can improve the conversion of nitrogen oxides. Additionally or alternatively, oxygen or an oxygen-containing medium is introduced into the exhaust system when the nitrogen dioxide content in the exhaust gas falls below, or threatens to fall below, a predetermined nitrogen dioxide threshold. This is because conventional SCR catalysts also exhibit a significant dependence of nitrogen oxide conversion on the ratio of nitrogen dioxide to nitrogen oxides, i.e., NO₂ to NOₓ. Here, too, a deficiency of one exhaust gas component, namely a deficiency of nitrogen dioxide, results in a significant decrease in nitrogen oxide conversion by the exhaust aftertreatment system.Furthermore, comparatively rich operating conditions of the internal combustion engine can also be responsible for the reduction of the ratio of nitrogen dioxide to nitrogen oxide in the exhaust gas, since under these conditions the formation of nitrogen dioxide at an oxidation catalyst upstream of the exhaust aftertreatment device is slowed down. Therefore, the actual or anticipated levels of oxygen and nitrogen dioxide in the exhaust gas are preferably taken into account when deciding whether the oxygen content should be increased upstream of the exhaust gas aftertreatment system. Furthermore, oxygen and / or an oxygen-containing medium is introduced into the exhaust gas tract as soon as the oxygen content in the exhaust gas falls below 5% or threatens to fall below 5%, taking into account the oxygen content immediately upstream of the exhaust aftertreatment system. The dependence of the conversion of nitrogen oxides in the exhaust aftertreatment system on the oxygen content becomes even more critical when it drops below 3%. Consequently, in this case, the oxygen content upstream of the exhaust aftertreatment system is increased, in particular, when the oxygen content in the exhaust gas falls below 3% or threatens to fall below 3%. The stated oxygen content values ​​correspond to exhaust gas air-fuel ratio (λ) values ​​of approximately ≤ 1.25 for 5% oxygen content and approximately ≤ 1.1 for 3% oxygen content. Avoiding such low oxygen levels in the exhaust gas at the inlet of the exhaust aftertreatment system, or at least reacting to such low oxygen levels, therefore leads to a particularly significant improvement in nitrogen oxide reduction within the exhaust aftertreatment system. Additionally or alternatively, the oxygen content upstream of the exhaust aftertreatment system is increased if the nitrogen dioxide content in the exhaust gas falls below or threatens to fall below 25%. In particular, if the nitrogen dioxide content in the exhaust gas drops below 10%, oxygen and / or an oxygen-containing medium is introduced into the exhaust tract to improve the conversion of nitrogen oxides in the exhaust aftertreatment system. Preferably, by introducing oxygen and / or an oxygen-containing medium into the exhaust gas tract, an oxygen content of at least 5% is maintained in the exhaust gas at an inlet to the exhaust gas aftertreatment system. This ensures, to a particularly high degree, that no significant reduction in the conversion of nitrogen oxides occurs due to an excessively low oxygen content in the exhaust gas. It is particularly advantageous if oxygen or air is introduced into the exhaust system before the oxygen or nitrogen dioxide content in the exhaust gas actually reaches low levels. This prevents a reduction in the activity of the exhaust aftertreatment system from occurring in the first place. Accordingly, in a further advantageous embodiment of the invention, oxygen or the oxygen-containing medium is introduced into the exhaust system when the load and / or speed of the internal combustion engine increases by a predetermined amount within a predetermined time period. An increase in engine load or torque, as well as an increase in engine speed, can lead to a decrease in the oxygen content at the inlet of the exhaust aftertreatment system. Preferably, the supply of oxygen or air is increased immediately upon request for increased engine load or engine speed. In particular, the air supply is increased when the rated load of the internal combustion engine is requested, for example, when the vehicle's accelerator pedal is depressed. The introduction of oxygen or an oxygen-containing medium into the exhaust system can also be triggered by a predetermined increase in the nitrogen oxide content of the exhaust gas within a predetermined time period. A sudden increase in the nitrogen oxide content of the exhaust gas can also lead to a decrease in the oxygen or nitrogen dioxide content upstream of the exhaust aftertreatment system without the introduction of oxygen or air upstream of the system. It can be provided that the oxygen content upstream of the exhaust aftertreatment system is increased if the load, engine speed, or nitrogen oxide content of the exhaust gas increases by at least 20% within a given time period. If the introduction of oxygen or air into the exhaust tract is made dependent on the increase in the magnitude of the load, the oxygen content upstream of the exhaust aftertreatment system can be increased, particularly when the load increases by at least 20% of the rated load of the internal combustion engine. Such a significant increase is particularly reliably detectable, and a corresponding decrease in the oxygen content or the nitrogen dioxide content in the exhaust gas upstream of the exhaust aftertreatment system must be expected if this is not counteracted by increasing the oxygen content. Oxygen, or rather an oxygen-containing medium, is introduced into the exhaust system particularly when the increase in the value occurs within less than 5 seconds, and especially within less than 3 seconds. This ensures a particularly effective response to a sudden increase in the aforementioned parameters. It is further preferred that oxygen or an oxygen-containing medium is introduced into the exhaust gas tract when the temperature of the exhaust gas aftertreatment system exceeds a certain temperature threshold. This ensures that the exhaust gas aftertreatment system is active, i.e., that it is actually reacting the nitrogen oxides in the exhaust gas with the ammonia. The supply of oxygen or air into the exhaust gas tract upstream of the exhaust gas aftertreatment system can be specifically linked to the condition that the temperature of the exhaust gas aftertreatment system exceeds approximately 250 °C. Since the vehicle is typically equipped with a metering device that introduces a reducing agent solution for exhaust aftertreatment upstream of the aftertreatment system into the exhaust tract, air can be introduced into the exhaust gas as an oxygen-containing medium using this metering device. This allows the same metering device to be used for both metering the reducing agent solution, such as a urea-water solution, and for adding air to the exhaust tract, thus eliminating the need for a separate supply unit. However, if the metering device is not designed to introduce air or oxygen into the exhaust system in addition to the reducing agent solution, a separate air supply unit can be used. This applies particularly if such a supply unit is already planned, for example, to increase the oxygen content in the exhaust gas upstream of an oxidation catalyst located before the exhaust aftertreatment system. A supply unit designed to introduce fuel into the exhaust gas can also be used to introduce air or oxygen into the exhaust system. The metering device or supply unit can be powered by a compressor, a compressed air reservoir, or a secondary air pump. This is particularly relevant if such a supply system is already present in the vehicle. The exhaust system according to the invention for a vehicle comprises an exhaust tract leading from an internal combustion engine of the vehicle to an exhaust aftertreatment device designed to reduce the nitrogen oxide content of the exhaust gas. Upstream of the exhaust aftertreatment device, at least one device for introducing oxygen and / or an oxygen-containing medium into the exhaust tract is provided. A control device serves to actuate the at least one device depending on the existing or impending concentration of at least one component in the exhaust gas, which is lower than a predetermined threshold value for the concentration of the at least one component.By issuing control commands via the control device, it can be ensured that the device designed to introduce oxygen or the oxygen-containing medium into the exhaust gas tract enriches the exhaust gas locally with oxygen, thus leading to an improvement in nitrogen oxide reduction by means of the exhaust gas aftertreatment device. The advantages and preferred embodiments described for the method according to the invention also apply to the exhaust system according to the invention and vice versa. The features and combinations of features mentioned above in the description, as well as the features and combinations of features mentioned below in the figure description and / or shown in the figures alone, can be used not only in the combination specified in each case, but also in other combinations or on their own, without leaving the scope of the invention. Further advantages, features, and details of the invention will become apparent from the claims, the following description of preferred embodiments, and the drawings, in which identical or functionally equivalent elements are provided with identical reference numerals. Figure 1 shows an exhaust system for a vehicle in which air can also be introduced into an exhaust pipe upstream of an SCR catalyst by means of a metering device for a urea-water solution; Figure 2 shows an exhaust system in which the oxygen content upstream of the SCR catalyst can be increased by introducing air by means of a supply unit arranged upstream of an oxidation catalyst; Figure 3 shows a graph illustrating a sudden increase in engine speed, engine load, or nitrogen oxide concentration in the exhaust gas as a trigger for introducing air upstream of the SCR catalyst; Figure 4 shows...Figure 4 shows the behavior of an iron-zeolite catalyst with respect to the conversion of nitrogen oxides as a function of the oxygen content in the exhaust gas at different nitrogen dioxide concentrations at 200 °C; Figure 5 shows the behavior of a copper-zeolite catalyst with respect to the conversion of nitrogen oxides as a function of the oxygen content in the exhaust gas at different nitrogen dioxide concentrations at 200 °C; Figure 6 shows the behavior of a vanadium catalyst with respect to the conversion of nitrogen oxides as a function of the oxygen content in the exhaust gas at different nitrogen dioxide concentrations at 200 °C; Figure 7 shows the behavior of the iron-zeolite catalyst according to Figure 4 at 300 °C; Figure 8 shows the behavior of the copper-zeolite catalyst according to Figure 5 at 300 °C; and Figure 9 shows the behavior of the vanadium catalyst according to Figure 6 at 300 °C. Exhaust gas from a vehicle's internal combustion engine 12 is routed through an exhaust system 10, shown schematically in Fig. 1, and treated therein. The exhaust system 10 comprises an exhaust pipe 14, which leads from the internal combustion engine 12 to an SCR catalyst 16. Upstream of the SCR catalyst 16, an aqueous urea solution 20 is introduced into the exhaust gas flowing through the exhaust pipe 14 by means of a metering unit 18. For this purpose, the metering unit 18 has a first connection 22 through which it is supplied with the urea-water solution 20. In the hot exhaust gas, the urea from the urea-water solution 20 is converted to ammonia. This reacts in the SCR catalyst 16 with nitrogen oxides contained in the exhaust gas in a selective catalytic reduction reaction to form nitrogen and water. It has been shown that this conversion of nitrogen oxides in the exhaust gas depends on the oxygen content and the nitrogen dioxide content in the exhaust gas upstream of the SCR catalyst 16. At low oxygen and nitrogen dioxide content upstream of the SCR catalyst 16, a significant reduction in the nitrogen oxide conversion of the SCR catalyst 16 is observed. This applies to iron-zeolite-based SCR catalysts (see Fig. 4 and Fig. 7), copper-zeolite-based SCR catalysts (see Fig. 5 and Fig. 8), and vanadium-based SCR catalysts (see Fig. 6 and Fig. 9). In this case, the exhaust gas is enriched with air, and thus with atmospheric oxygen, upstream of the SCR catalyst 16 in order to improve nitrogen oxide reduction at the SCR catalyst 16. In the exhaust system 10 shown in Fig. 1, this is achieved using the same metering unit 18, which also serves to add the urea-water solution 20 to the exhaust gas. This integrated metering unit 18 has an additional connection 24 through which air 26 can enter the metering unit 18 and be introduced into the exhaust gas upstream of the SCR catalyst 16 by means of the metering unit 18. A control unit 27 controls the metering unit 18 to introduce air 26 into the exhaust pipe 14. The control unit 27 processes input variables in the manner described below. In the exhaust system 10 shown in Fig. 2, an oxidation catalyst 28 is arranged upstream of the SCR catalyst 16, and a particulate filter 30 can optionally be provided between the oxidation catalyst 28 and the metering unit 18. Upstream of the oxidation catalyst 28 and downstream of the combustion engine 12 is a feed unit 32, through which air 26 can also be introduced into the exhaust gas. If such a feed unit 32 is provided, the metering unit 18 arranged downstream of the particulate filter 30 can also be configured solely for supplying the urea-water solution 20 into the exhaust gas line 14. In the present case, however, air 26 can also be introduced into the exhaust gas by means of the metering unit 18. Upstream of the oxidation catalyst 28, a fuel metering unit 34 is provided, by means of which fuel 36 can be introduced into the exhaust line 14. This can be done, for example, to raise the temperature of the exhaust aftertreatment devices, such as the oxidation catalyst 28, the particulate filter 30, or the SCR catalyst 16, to a value at which a significant conversion of exhaust gas components occurs after a cold start of the combustion engine 12. The fuel 36 can also be metered to regenerate the particulate filter 30, i.e., to burn off the soot retained in it. In particular, if the supply unit 32 is missing and the metering unit 18 does not have the connection 24 for air 26, air 26 can be introduced into the exhaust line 14 via the fuel metering unit 34.Alternatively, the air 26 can be introduced into the exhaust gas via the supply unit 32, the metering unit 18, or the fuel metering unit 34. For the sake of clarity, the control unit that activates the respective unit for introducing the air 26 into the exhaust gas is not shown in the exhaust system 10 according to Fig. 2. The supply of air and thus oxygen is generally provided when the content of available oxygen in the exhaust gas downstream of the oxidation catalyst 28 and upstream of the SCR catalyst 16 or downstream of the particulate filter 30 and upstream of the SCR catalyst 16 drops to a predetermined low value or threatens to drop. A low value can be defined as less than 5% oxygen in the exhaust gas, and in particular less than 3%. An oxygen content of less than 5% in the exhaust gas can occur with an air-fuel ratio (λ) of approximately 1.25, and an oxygen content of 3% with an air-fuel ratio (λ) of approximately 1.1. Additionally or alternatively, it is determined whether the nitrogen dioxide content in the exhaust gas falls to a predetermined low value of less than 25%, in particular less than 10%. If low oxygen and nitrogen dioxide levels are present simultaneously, this has a significant impact on the conversion of nitrogen oxides in the SCR catalyst 16. Such low conversion rates are prevented by introducing air 26 or oxygen into the exhaust gas line 14 upstream of the SCR catalyst 16. Preferably, sufficient air 26 is supplied to increase the oxygen content in the exhaust gas at the inlet of the SCR catalyst 16 to at least 5%. Preferably, air 26 is introduced into the exhaust gas before critical, i.e., low, oxygen levels are reached upstream of the SCR catalyst 16. This air supply occurs, for example, when an engine load, i.e., engine torque, or engine speed is suddenly increased. This is illustrated in Figure 3. In a graph shown in Fig. 3, time t is plotted on an abscissa 38 and a parameter, which can be the engine torque M, the engine speed n, or the nitrogen oxide content NOx in the exhaust gas, is plotted on an ordinate 40. A curve 42 illustrates the time course of this parameter. If, within a short time interval Δt, the parameter under consideration increases by a predetermined amount 44, then, without the supply of air upstream of the SCR catalyst 16, the oxygen content or the nitrogen dioxide content would fall below the threshold values ​​at which a significant deterioration in the conversion of the nitrogen oxides at the SCR catalyst 16 occurs. The increased air supply upstream of the SCR catalyst 16 is therefore preferably initiated directly upon request for an increase in engine torque M or engine speed n. In particular, the introduction of air 26 or oxygen upstream of the SCR catalyst 16 is triggered when the rated load of the internal combustion engine 12 is requested by the driver of the vehicle pressing the accelerator pedal. However, even in the event of a sudden increase in the nitrogen oxide content in the exhaust gas, the air 26 is supplied on the inlet side of the SCR catalyst 16. A value of at least 20% can be provided as amount 44, whereby, in the event of an increase in the requested engine torque, 20% of the rated load, i.e., the maximum torque of the combustion engine 12, is used as the amount 44 of the sudden increase, at the occurrence of which the exhaust gas is enriched with the air 26 or with oxygen. To determine whether there is a sudden increase in the parameter(s) mentioned, the analysis can be focused on a time interval Δt of less than 5 s, in particular less than 3 s. Preferably, it is also taken into account whether the SCR catalyst 16 has reached its activation temperature, i.e., a temperature at which at least 50% of the nitrogen oxides are converted. Such activity of the SCR catalyst 16 is present, for example, at a temperature of 250 °C at the SCR catalyst 16. Figure 4 illustrates the behavior of an iron-zeolite-based SCR catalyst 16 with regard to the conversion of nitrogen oxides, expressed as a percentage on the ordinate 46, as a function of the oxygen content, which is shown on the abscissa 48. A first curve 50 describes the conversion at 200 °C and a nitrogen dioxide content of 0% in the nitrogen oxides, and a second curve 52 describes the conversion at 200 °C and a nitrogen dioxide content of 50% in the nitrogen oxides. Fig. 5 shows analogous curves 54, 56 for a copper-zeolite-based SCR catalyst 16 and Fig. 6 further curves 58, 60 for a vanadium-based SCR catalyst 16. Here, curves 54, 58 show the conversion of nitrogen oxides at a nitrogen dioxide content of 0% and curves 56, 60 the conversion of nitrogen oxides at a nitrogen dioxide content of 50%. For all three catalyst technologies, at low temperatures (e.g., around 200 °C), a more or less pronounced dependence of the conversion on the oxygen content can be observed in the absence of nitrogen dioxide. Particularly with the copper-zeolite-based SCR catalyst 16, a very strong dependence of the conversion on the oxygen content in the exhaust gas is evident at an oxygen content of less than 5%. This can lead to a reduction in nitrogen oxide conversion of approximately 10% to 20% compared to conditions with an oxygen content of 5% or more. Once an oxygen concentration of 5% to 8% in the exhaust gas is established, no significant influence of the oxygen content on nitrogen oxide conversion is observed for the various types of SCR catalyst 16 described here. Figure 7 illustrates the behavior of the iron-zeolite-based SCR catalyst 16 at 300 °C and 0% nitrogen dioxide (curve 62), 50% nitrogen dioxide (curve 66), and 75% nitrogen dioxide (curve 64). In the absence of nitrogen dioxide (curve 62), a strong dependence of the conversion on the oxygen content is evident. Fig. 8 illustrates, using curves 70, 68, 72, the corresponding conversion behavior of the copper-zeolite based SCR catalyst 16 at the different nitrogen dioxide and oxygen concentrations. As illustrated by corresponding curves 78, 74, 76 in Fig. 9 for the vanadium-based SCR catalyst 16, the dependence of the nitrogen oxide conversion on the nitrogen dioxide content and the oxygen content is significantly less pronounced for these two latter SCR technologies at the considered temperature of 300 °C than for the iron-zeolite-based SCR catalyst 16. By introducing oxygen or air 26 upstream of the SCR catalyst 16, an improved conversion of nitrogen oxides at the SCR catalyst 16 is achieved, particularly at comparatively low exhaust gas temperatures. Reference symbol list 10 Exhaust system 12 Internal combustion engine 14 Exhaust pipe 16 SCR catalyst 18 Dosing unit 20 Urea-water solution 22 Connection 24 Connection 26 Air 27 Control unit 28 Oxidation catalyst 30 Particulate filter 32 Supply unit 34 Fuel meter 36 Fuel 38 Abscissa 40 Ordinate 42 Curve 44 Magnitude 46 Ordinate 48 Abscissa 50 Curve for iron-zeolite-based SCR catalyst at 0% nitrogen dioxide content and 200°C 52 Curve for iron-zeolite-based SCR catalyst at 50% nitrogen dioxide content and 200°C 54 Curve for copper-zeolite-based SCR catalyst at 0% nitrogen dioxide content and 200°C 56 Curve for copper-zeolite-based SCR catalyst at a nitrogen dioxide content of 50% and 200°C 58 Curve for vanadium-based SCR catalyst at 0% nitrogen dioxide content and 200°C 60 Curve for vanadium-based SCR catalyst at 50% nitrogen dioxide content and 200°C 62 Curve for iron-zeolite-based SCR catalyst at 0% nitrogen dioxide content and 300°C 64 Curve for iron-zeolite-based SCR catalyst atNitrogen dioxide content of 75% and 300°C 66 Curve for iron-zeolite-based SCR catalyst at nitrogen dioxide content of 50% and 300°C 68 Curve for copper-zeolite-based SCR catalyst at nitrogen dioxide content of 50% and 300°C 70 Curve for copper-zeolite-based SCR catalyst at nitrogen dioxide content of 0% and 300°C 72 Curve for copper-zeolite-based SCR catalyst at nitrogen dioxide content of 75% and 300°C 74 Curve for vanadium-based SCR catalyst at nitrogen dioxide content of 50% and 300°C 76 Curve for vanadium-based SCR catalyst at nitrogen dioxide content of 75% and 300°C 78 78 Curve for vanadium-based SCR catalyst at nitrogen dioxide content of 0% and 300°C

Claims

A method for treating the exhaust gas of an internal combustion engine (12) of a vehicle, in which the exhaust gas of the internal combustion engine (12) is fed via an exhaust tract (14) to an exhaust aftertreatment device (16) designed to reduce the nitrogen oxide content of the exhaust gas, wherein oxygen and / or an oxygen-containing medium (26) is introduced into the exhaust tract (14) upstream of the exhaust aftertreatment device (16) when the content of at least one component of the exhaust gas becomes or threatens to become lower than a predetermined threshold value of the content of the at least one component, characterized in that the oxygen and / or the oxygen-containing medium (26) is introduced into the exhaust tract (14) when the oxygen content in the exhaust gas becomes or threatens to become lower than a predetermined threshold value of the content, wherein the oxygen and / or the oxygen-containing medium (26) is introduced into the exhaust tract (14).if the oxygen content in the exhaust gas becomes or threatens to become less than 5%. Method according to claim 1, characterized in that by introducing the oxygen and / or the oxygen-containing medium (26) into the exhaust gas tract (14) an oxygen content in the exhaust gas at an inlet to the exhaust gas aftertreatment device (16) of at least 5% is set. Method according to one of claims 1 or 2, characterized in that the oxygen and / or the oxygen-containing medium (26) is introduced into the exhaust tract (14) when a load and / or a rotational speed of the internal combustion engine (12) increases by a predetermined amount (44) within a predetermined time period (Δt). Method according to one of claims 1 to 3, characterized in that the oxygen and / or the oxygen-containing medium (26) is introduced into the exhaust gas tract (14) when the nitrogen oxide content of the exhaust gas increases by a predetermined amount (44) within a predetermined time period (Δt). Method according to claim 3 or 4, characterized in that when the amount (44) increases by at least 20% within a time interval (Δt) of less than 5 s, in particular less than 3 s, the oxygen and / or the oxygen-containing medium (26) is introduced into the exhaust gas tract (14). Method according to one of claims 1 to 5, characterized in that the oxygen and / or the oxygen-containing medium (26) is introduced into the exhaust gas tract (14) when a temperature of the exhaust gas aftertreatment device (16) is greater than a temperature threshold, in particular greater than about 250 °C. Method according to one of claims 1 to 6, characterized in that air (26) is introduced into the exhaust tract (14) as an oxygen-containing medium by means of a metering device (18) designed for metering a reducing agent solution (20) for exhaust aftertreatment into the exhaust tract (14) and / or a supply unit (32, 34) designed for introducing air (26) and / or fuel (36) into the exhaust tract (14). Exhaust system (10) for a vehicle, comprising an exhaust tract (14) leading from an internal combustion engine (12) of the vehicle to an exhaust aftertreatment device (16) designed to reduce the nitrogen oxide content of the exhaust gas, and comprising at least one device (18, 32, 24) for introducing oxygen and / or an oxygen-containing medium (26) into the exhaust tract (14) upstream of the exhaust aftertreatment device (16), and comprising a control device (27) for controlling the at least one device (18, 32, 24) depending on a present or impending content of at least one component of the exhaust gas, which is lower than a predetermined threshold value of the content of the at least one component, characterized in that the oxygen and / or the oxygen-containing medium (26) is introduced into the exhaust tract (14).when the oxygen content in the exhaust gas becomes or threatens to become lower than a predetermined threshold value, wherein the oxygen and / or the oxygen-containing medium (26) is introduced into the exhaust gas tract (14) when the oxygen content in the exhaust gas becomes or threatens to become lower than 5%.

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