Exhaust system for receiving an exhaust stream generated by an internal combustion engine, motor vehicle and method for treating an exhaust stream generated by an internal combustion engine

Abstract

Exhaust system (1) for receiving an exhaust gas flow (3) generated by an internal combustion engine (2), comprising: - an LNT catalyst (4), - an SCR catalyst (5) arranged downstream of the LNT catalyst (4), - a bypass (6) to bypass the SCR catalyst (5), - a bypass valve (7) for splitting the exhaust gas flow (3) between the bypass (6) and the SCR catalyst (5), - a lambda sensor (8, 9) arranged upstream of the LNT catalyst (4) or downstream of the LNT catalyst (4) and upstream of the bypass valve (7), - a temperature sensor for determining the temperature of the exhaust gas flow (3) and - a control unit (10) which is configured and designed to output a control signal to the bypass valve (7) depending on sensor signals from the lambda sensors (8, 9) and the temperature sensor, wherein a control signal to activate the bypass (6) is output if a combustion air ratio λ is detected by means of the lambda sensor (8) arranged downstream of the LNT catalyst (4). ab < 1 is detected and the temperature of the exhaust gas flow (3) determined by means of the temperature sensor exceeds a limit temperature or a control signal to activate the bypass (6) is output if a combustion air ratio λ determined by means of a auf , which is determined by means of the lambda sensor (9) arranged upstream of the LNT catalyst (4), estimated time t1 at which the air-fuel ratio downstream of the LNT catalyst (4) λ ab< 1 will be reached and the temperature of the exhaust gas flow (3) determined by means of the temperature sensor exceeds a limit temperature.

Description

[0001] The present invention relates to an exhaust system for receiving an exhaust gas flow generated by an internal combustion engine, a motor vehicle with such an exhaust system and methods for treating an exhaust gas flow generated by an internal combustion engine.

[0002] The reduction of nitrogen oxides (NO x ) in the exhaust stream of lean-burn engines, i.e. internal combustion engines operated with a lean air-fuel mixture with a combustion air ratio λ > 1, in an overall oxidising environment represents a major challenge under various operating conditions.

[0003] Nitrogen oxide storage catalysts, also known as NSR catalysts (NO x storage and reduction catalyst) or LNT catalyst (lean NO xtrap) represent one of the two main technologies for the catalytic conversion of nitrogen oxides. The alternative main technology is based on the selective catalytic reduction (SCR) of nitrogen oxides using ammonia, which can be obtained, for example, from a supplied urea solution.

[0004] LNT catalysts can store nitrogen oxides even at low temperatures, and storage efficiency increases up to temperatures of approximately 200 to 250 °C. The adsorbed, i.e., stored, nitrogen oxides can then be reduced to nitrogen by flushing the LNT catalyst with a rich mixture at higher temperatures. The hydrocarbons contained in the rich mixture, along with carbon monoxide and hydrogen, act as reducing agents.

[0005] SCR technology, on the other hand, typically cannot achieve high reaction rates for nitrogen oxide reduction at low temperatures unless an optimal NO:NO2 ratio is present and ammonia is available to promote rapid reaction. However, once temperatures exceed 200 °C, SCR catalysts demonstrate excellent performance in nitrogen oxide conversion.

[0006] In order to meet increasingly stringent emissions regulations, a combination of the two main technologies has become attractive, as described, for example, in US 2016 / 0 376 962 A1. This enables nitrogen oxides to be converted effectively over a much wider temperature range. However, the combination of an LNT catalyst with an SCR catalyst also brings with it problems. While the SCR catalyst is designed for use with a lean exhaust gas mixture, i.e., an excess of oxygen, the LNT catalyst requires a rich exhaust gas mixture, at least temporarily, for its regeneration, i.e., the reduction of the stored nitrogen oxides and the release of the reduced compounds into the exhaust stream to restore the nitrogen oxide storage capacity.

[0007] Intermittent operation with a rich exhaust gas mixture causes reductant slip. In other words, the rich exhaust gas mixture also reaches the SCR catalyst located downstream of the LNT catalyst. This rich exhaust gas mixture negatively impacts the performance of the SCR catalyst, particularly at high temperatures, such as those encountered during desulfurization (DeSOx). This impairment of the SCR catalyst's performance is permanent and is due to the deactivation of the catalyst material, e.g., due to sintering processes, and / or the reduction of copper in the SCR catalyst.

[0008] Such a reduction in the performance of the SCR catalyst is also difficult to predict and can therefore only be inadequately taken into account when predicting the optimal amount of urea solution to be added. If the performance of the SCR catalyst is reduced due to contact with a rich exhaust mixture, but the same amount of urea solution continues to be added, an undesirable release of ammonia occurs.

[0009] DE 10 2013 106 323 A1 describes that hydrocarbons escaping from an oxidation catalyst can cause a deterioration in the operation of an SCR catalyst, thereby reducing its efficiency and service life. To improve the operation of an SCR catalyst, a system and method have been developed in which the hydrocarbons can bypass the SCR catalyst via a bypass. The bypass is activated if the temperature of the exhaust gas purification device is below a threshold temperature and the hydrocarbon concentration is above a threshold.

[0010] In contrast to the described problem of the impact of a rich exhaust gas mixture on the SCR catalyst, which can cause irreversible damage to the SCR catalyst, the escaping hydrocarbons described in DE 10 2013 106 323 A1 only lead to a temporary deterioration in the activity of the SCR catalyst, as the hydrocarbons only temporarily occupy the active sites of the catalyst. Such temporary occupation is presumably based on physical adsorption at low temperatures.

[0011] The object of the present invention is therefore to reduce or avoid the adverse effects of a rich exhaust gas mixture, such as that used for the regeneration of an LNT catalyst, on the SCR catalyst.

[0012] This object is achieved by the subject matter of the independent claims. Advantageous developments of the invention are specified in the subclaims.

[0013] The basic idea of ​​the invention is to protect the SCR catalyst from a rich exhaust gas mixture at high temperatures by bypassing the exhaust gas flow around the SCR catalyst during rich conditions and high temperatures. This prevents the rich exhaust gas mixture from entering the SCR catalyst at high temperatures, thus preventing irreversible damage to the SCR catalyst.

[0014] LNT and SCR catalysts can thus be used together for the catalytic treatment of the exhaust stream. This enables effective catalytic treatment of the exhaust stream within a wide temperature range, allowing the nitrogen oxides contained in the exhaust stream to be largely removed while simultaneously protecting the SCR catalyst from irreversible damage and the associated deterioration of its catalytic activity.

[0015] An exhaust system according to the invention for receiving an exhaust gas flow generated by an internal combustion engine comprises an LNT catalyst, an SCR catalyst arranged downstream of the LNT catalyst, a bypass for bypassing the SCR catalyst, a bypass valve for dividing the exhaust gas flow between the bypass and the SCR catalyst, and a lambda sensor. The lambda sensor is arranged either upstream of the LNT catalyst or downstream of the LNT catalyst and upstream of the bypass valve.

[0016] An internal combustion engine, sometimes also referred to as an internal combustion engine, is a combustion engine that converts the chemical energy contained in fuel into mechanical work. The internal combustion engine can be a compression-ignition or spark-ignition engine, for example. The fuel used can be gasoline or diesel, for example. The flow directions indicated refer to the direction of the exhaust gas flow from the internal combustion engine toward the exhaust.

[0017] The exhaust system consists of an exhaust line through which the exhaust gas flows and in which the catalysts and sensors are arranged. This allows the catalysts to also be flowed through by the exhaust gas flow, and the properties of the exhaust gas flow, such as its composition, temperature, etc., can be determined using the sensors. For the definition and function of an LNT or SCR catalyst, please refer to the introductory explanations.

[0018] The exhaust system has a bypass, i.e., a bypass line, that can be used to bypass the SCR catalyst. In other words, the exhaust gas flow can optionally flow through either the bypass or the SCR catalyst. The bypass valve can be used to determine the distribution of the exhaust gas flow between the bypass or the SCR catalyst. For example, the bypass valve can be controlled depending on sensor signals, e.g., from the lambda sensor described below, so that the exhaust gas flow is distributed between the bypass or the SCR catalyst depending on the sensor signals.

[0019] According to the invention, a lambda sensor is arranged upstream of the LNT catalyst or downstream of the LNT catalyst and upstream of the bypass valve, and thus also upstream of the SCR catalyst. A lambda sensor is a sensor that supplies measurement signals from which the combustion air ratio λ, i.e. the ratio of the air mass actually available for combustion to the minimum stoichiometric air mass required for complete combustion, is determined. The lambda sensor can, for example, be designed as a lambda probe that compares the residual oxygen content in the exhaust gas with the oxygen content of the current atmospheric air, from which the combustion air ratio can be determined. The lambda sensor can also be designed as a nitrogen oxide sensor, since such nitrogen oxide sensors can output the combustion air ratio in addition to determining the nitrogen oxide content.

[0020] The lambda sensor can be used to determine the combustion air ratio upstream or downstream of the LNT catalyst.

[0021] If the lambda sensor is located downstream of the LNT catalyst, it can be determined whether the exhaust gas after flowing through the LNT catalyst is a rich exhaust gas mixture (λ ab < 1), a stoichiometric exhaust gas mixture (λ ab = 1) or a lean exhaust gas mixture (λ ab > 1). If a rich exhaust gas mixture is fed to the LNT catalyst for regeneration, the lambda sensor can be used to determine the point in time at which regeneration of the LNT catalyst can be assumed, since a rich exhaust gas mixture can also be detected downstream at this point in time. This point in time is also referred to as lambda breakthrough.

[0022] If the lambda sensor detects a combustion air ratio λ ab< 1, the bypass valve can be activated if the temperature of the exhaust gas flow exceeds a certain limit temperature. The exhaust gas flow then flows through the bypass instead of the SCR catalyst. The bypass can, for example, remain activated as long as the lambda sensor detects a combustion air ratio λ ab < 1 detected.

[0023] However, if a stoichiometric combustion air ratio or a combustion air ratio λ ab > 1 is detected by the lambda sensor located downstream of the LNT catalyst, there is no danger to the SCR catalyst and the exhaust gas flow can be passed through the SCR catalyst for aftertreatment to remove nitrogen oxides.

[0024] Alternatively, the lambda sensor can be located upstream of the LNT catalyst so that the combustion air ratio λ auf can be determined. If a combustion air ratio λ auf< 1, a rich exhaust gas mixture is fed into the exhaust system, e.g., for the regeneration of the LNT catalyst. It is to be expected that after a certain period of time, a combustion air ratio λ ab < 1. This time period and thus also the time t1 at which a combustion air ratio λ ab< 1 is also to be expected downstream of the LNT catalyst, can be determined, for example, using model calculations that can be performed online. Such model calculations can take into account, for example, the amount of nitrogen oxides and / or oxygen stored in the LNT catalyst, the rate of decrease in the stored nitrogen oxides and / or oxygen as a function of temperature and the combustion air ratio λ, the amount of reducing compounds fed to the LNT catalyst, which can be determined from the combustion air ratio λ, and the temperature.

[0025] When this time t1 is reached, the bypass can be activated via the bypass valve if the temperature of the exhaust gas flow exceeds a threshold temperature. Optionally, a time t0 can be set before time t1, whereby the bypass is activated as soon as time t0 is reached, so that the exhaust gas flow flows through the bypass instead of the SCR catalyst from time t0 onwards.

[0026] If the bypass is activated as soon as the time t0 is reached, an improved protection of the SCR catalyst against a rich exhaust gas mixture can be achieved, since the bypass is not activated only when a combustion air ratio λ ab < 1 or reaching the time t1, but already at time t0. A possible penetration of a rich exhaust gas mixture into the SCR catalyst in the reaction time of the bypass valve, ie between the detection of a combustion air ratio λ ab< 1 or reaching the time t1 and activating, ie opening, the bypass, can be prevented.

[0027] In other words, the activation of the bypass can be either upon detection of a combustion air ratio λ ab < 1 by means of the lambda sensor arranged downstream of the LNT catalyst, at time t1 or at time t0, wherein the times t1 and t0 are determined on the basis of the combustion air ratio λ determined by means of the lambda sensor arranged upstream of the LNT catalyst auf be determined.

[0028] If the lambda sensor upstream of the LNT catalyst detects a stoichiometric combustion air ratio or a combustion air ratio λ auf > 1, there is no danger to the SCR catalyst and the exhaust gas flow can be passed through the SCR catalyst for aftertreatment to remove nitrogen oxides.

[0029] With both lambda sensor arrangement options, contact of the SCR catalyst with a rich exhaust mixture at high temperatures can be avoided, as the rich exhaust mixture is routed through the bypass. This prevents permanent deterioration of the catalyst activity of the SCR catalyst due to contact with a rich exhaust mixture. The efficiency of the SCR catalyst in removing nitrogen oxides from the exhaust stream can be maintained over a longer period of time. Furthermore, the SCR catalyst can be monitored more precisely, e.g., to determine the correct amount of urea solution to be added and to prevent the undesirable release of excess ammonia, as a limitation of the catalyst activity due to contact with a rich exhaust mixture, which would otherwise have to be considered, is avoided.In other words, the prediction of the required amount of urea solution better matches the actual amount required.

[0030] According to the invention, the exhaust system further comprises a temperature sensor for determining the temperature of the exhaust gas stream. Such a temperature sensor can be arranged, for example, upstream or downstream of the LNT catalyst.

[0031] The temperature sensor allows the activation or deactivation of the bypass to be controlled depending on the temperature of the exhaust gas flow. The bypass is designed to be activated or remain activated only when a threshold temperature is exceeded. This threshold temperature can be in the range of 500 °C, for example, 500 °C.

[0032] This advantageously prevents irreversible damage to the SCR catalyst under rich conditions and high temperatures. However, if the limit temperature is not exceeded, activation of the bypass can be omitted even under rich conditions, as the risk of irreversible damage is lower.

[0033] In other words, a compromise can be found between possible irreversible damage to the SCR catalyst and inadequate aftertreatment of the exhaust gas flow by not treating it in the SCR catalyst but passing it through the bypass.

[0034] According to various embodiments, the exhaust system can have a supply device for supplying an ammonia-forming agent to the exhaust gas flow, which can be arranged, for example, downstream of the bypass valve and upstream of the SCR catalyst. The supply device can also be deactivated, for example, when the exhaust gas flow flows through the bypass, since the SCR catalyst then no longer requires ammonia. This prevents unwanted release of ammonia into the environment.

[0035] According to the invention, the exhaust system further comprises a control unit which is configured and designed to output a control signal to the bypass valve, e.g., for activating or deactivating the bypass by opening or closing the bypass valve, depending on sensor signals from the lambda sensor(s) or depending on sensor signals from the lambda sensor(s) and the temperature sensor.

[0036] The control unit receives signals from the lambda sensors as input data, processes this input data and sends control signals to the bypass valve as an actuator in response to the processed input data based on instructions or a programmed code according to one or more routines.

[0037] The control unit outputs control signals as explained above depending on the combustion air ratios determined by the lambda sensors, i.e. a control signal to activate the bypass if a combustion air ratio λ ab < 1 is detected or a determined by means of the combustion air ratio λ auf Estimated time t1 at which the combustion air ratio downstream of the LNT catalyst (4) λ ab will be < 1.

[0038] In addition, the bypass is only activated or left in an activated state if the temperature of the exhaust gas flow determined by the temperature sensor exceeds the limit temperature.

[0039] Optionally, the following can be output: a control signal to prepare the bypass valve for activation of the bypass if a combustion air ratio λ auf < 1 is detected; a control signal to activate the bypass if the time t0 is reached and / or a control signal to deactivate the bypass if a combustion air ratio λ ab ≥ 1 is detected or time t2 is reached.

[0040] The bypass valve can advantageously be controlled automatically using the control unit, so that a quick change between activated and deactivated bypass can be realized.

[0041] Optionally, the control unit can also be configured and designed to output a control signal to the supply device depending on sensor signals from the lambda sensors or depending on sensor signals from the lambda sensors and the temperature sensor. This allows the supply device to be controlled so that it is only active, i.e., supplies an ammonia-forming agent to the exhaust gas stream, when the exhaust gas stream also flows through the SCR catalyst. Therefore, the same parameters as for controlling the bypass valve can be used to activate or deactivate the supply device.

[0042] A motor vehicle according to the invention has an exhaust system as described above. A motor vehicle is understood to be a vehicle powered by an internal combustion engine, e.g., a land vehicle, aircraft, or water vehicle. Therefore, the above explanations for the exhaust system according to the invention also serve to describe the motor vehicle according to the invention. The advantages of the motor vehicle according to the invention correspond to those of the exhaust system according to the invention and its variants.

[0043] A method according to the invention for treating an exhaust gas stream generated by an internal combustion engine comprises the following steps: passing the exhaust gas stream through an LNT catalyst, determining the combustion air ratio λ ab in the exhaust gas flow downstream of the LNT catalyst and upstream of an SCR catalyst, passing the exhaust gas flow through the SCR catalyst if the combustion air ratio λ ab≥ 1, and passing the exhaust gas flow through a bypass bypassing the SCR catalyst if the combustion air ratio λ ab < 1.

[0044] The method also includes determining the temperature of the exhaust gas stream. The determined temperature is compared with a previously defined limit temperature, e.g., a limit temperature in the range of 500°C. The exhaust gas stream is only routed through the bypass that bypasses the SCR catalyst if the temperature of the exhaust gas stream exceeds a limit temperature.

[0045] If the limit temperature is not exceeded or is no longer exceeded, the bypass can be omitted or deactivated. This ensures that the exhaust gas flow only flows through the bypass when the limit temperature is exceeded and is therefore not aftertreated in the SCR catalyst. This is particularly advantageous in preventing irreversible damage to the SCR catalyst caused by contact with a rich exhaust gas mixture at high temperatures. The limit temperature can be selected to strike a compromise between irreversible damage to the SCR catalyst and aftertreatment of the exhaust gas flow to prevent the emission of air pollutants. Consequently, the exhaust gas flow is routed through the bypass instead of the SCR catalyst if a rich exhaust gas mixture is detected downstream of the LNT catalyst and the temperature of the exhaust gas flow exceeds the limit temperature.This can prevent a rich exhaust gas mixture from entering the SCR catalyst at high temperatures.

[0046] The method according to the invention can be carried out, for example, using the exhaust system according to the invention explained above. Therefore, the above explanations for the exhaust system according to the invention also serve to describe the method according to the invention. The advantages of the method according to the invention correspond to those of the exhaust system according to the invention and its corresponding embodiments.

[0047] According to various embodiments, the method may further comprise determining the combustion air ratio λ auf in the exhaust gas flow upstream of the LNT catalyst and preparing to pass the exhaust gas flow through the bypass if the combustion air ratio λ auf < 1.

[0048] The routing of the exhaust gas flow through the bypass can be prepared, for example, by preloading a bypass valve. This can shorten the bypass valve's response time.

[0049] A further method according to the invention for treating an exhaust gas flow generated by an internal combustion engine comprises the following steps: Determining the combustion air ratio λ auf in the exhaust gas stream upstream of an LNT catalyst; passing the exhaust gas stream through the LNT catalyst; if the combustion air ratio λ auf ≥ 1, passing the exhaust gas flow through an SCR catalyst downstream of the LNT catalyst; if the combustion air ratio λ auf < 1, estimating the time t1 at which the combustion air ratio downstream of the LNT catalyst λ abwill be < 1, if time t1 is not reached, passing the exhaust gas flow through an SCR catalyst downstream of the LNT catalyst and, from time t1 is reached, passing the exhaust gas flow through a bypass bypassing the SCR catalyst.

[0050] The method also includes determining the temperature of the exhaust gas stream. The determined temperature is compared with a previously defined limit temperature, e.g., a limit temperature in the range of 500°C. The exhaust gas stream is only routed through the bypass that bypasses the SCR catalyst if the temperature of the exhaust gas stream exceeds a limit temperature.

[0051] If the limit temperature is not exceeded or is no longer exceeded, the bypass can be omitted or deactivated. This ensures that the exhaust gas flow only flows through the bypass when the limit temperature is exceeded and is therefore not subjected to post-treatment in the SCR catalyst. This advantageously prevents irreversible damage to the SCR catalyst caused by contact with a rich exhaust gas mixture at high temperatures. The limit temperature can be selected to strike a compromise between irreversible damage to the SCR catalyst and post-treatment of the exhaust gas flow to prevent the emission of air pollutants.

[0052] In other words, there are several ways to determine the combustion air ratio λ ab, namely detection by means of a sensor or estimation by means of a model. For example, the combustion air ratio determined from the measurement signal of a lambda sensor, e.g., a lambda sensor located upstream of the LNT catalyst or downstream of the LNT catalyst, can be used together with another combustion air ratio obtained by means of a model to control the bypass valve, i.e., to decide whether the exhaust gas flow flows through the SCR catalyst or the bypass.

[0053] Optionally, a time t0 can be set before time t1. Reaching time t0 can then be used instead of reaching time t1 to decide whether the exhaust flow is routed through the SCR catalyst or the bypass. If time t0 is not reached, the exhaust flow is routed through the SCR catalyst. From time t0 onward, the exhaust flow is routed through the bypass, bypassing the SCR catalyst.

[0054] Consequently, if a rich exhaust gas mixture is detected upstream of the LNT catalyst and the subsequently specified time t1 or t0 is reached, and the exhaust gas temperature exceeds the threshold temperature, the exhaust gas flow is routed through the bypass instead of the SCR catalyst. This prevents a rich exhaust gas mixture from entering the SCR catalyst at high temperatures.

[0055] The further method according to the invention can also be carried out, for example, using the exhaust system according to the invention explained above. In this respect, the above explanations for explaining the exhaust system according to the invention also serve to describe the method according to the invention. The advantages of the method according to the invention correspond to those of the exhaust system according to the invention and its corresponding embodiments.

[0056] According to various embodiments, the methods according to the invention can further comprise determining the combustion air ratio λ auf in the exhaust stream upstream of the LNT catalyst. The combustion air ratio λ auf in the exhaust gas stream upstream of the LNT catalyst can be carried out for the first time, repeatedly or continuously, depending on the sequence of the previous process steps.

[0057] If the combustion air ratio λ auf≥ 1, a time t2 is estimated at which the combustion air ratio downstream of the LNT catalyst λ ab ≥ 1. Once time t2 is reached, it can be assumed that there is no longer a rich exhaust gas mixture downstream of the LNT catalyst. The exhaust gas flow is stopped passing through the bypass that bypasses the SCR catalyst when time t2 is reached.

[0058] This prevents contact of the SCR catalyst with a rich exhaust gas mixture. On the other hand, efficient exhaust gas aftertreatment of a stoichiometric or lean exhaust gas mixture can be achieved in the SCR catalyst. Determining the combustion air ratio λ ab in the exhaust gas stream downstream of the LNT catalyst and any sensors required for this can advantageously be omitted.

[0059] According to various embodiments, the methods according to the invention can further comprise determining a combustion air ratio λ ab in the exhaust gas stream downstream of the LNT catalyst and upstream of the SCR catalyst. The combustion air ratio λ ab in the exhaust gas stream downstream of the LNT catalyst can be carried out for the first time, repeatedly or continuously, depending on the sequence of the previous process steps.

[0060] As long as the combustion air ratio λ ab < 1, the exhaust gas flow continues to be directed through the bypass that bypasses the SCR catalyst. If the combustion air ratio λ ab≥ 1, the exhaust gas flow is stopped passing through the bypass. In other words, if a rich exhaust gas mixture is no longer detected downstream of the LNT catalyst after the exhaust gas flow has been passed through the bypass, the bypass valve is closed again and the exhaust gas flow flows through the SCR catalyst again. According to further embodiments, the methods can further comprise supplying an ammonia-forming agent to the exhaust gas flow if the exhaust gas flow is passed through the SCR catalyst and terminating the supply of an ammonia-forming agent to the exhaust gas flow if the exhaust gas flow is passed through the bypass. This can ensure that ammonia is only formed when it is needed in the SCR catalyst for exhaust gas aftertreatment. An undesirable release of ammonia can therefore be avoided.

[0061] In other words, the combustion air ratio λ abIt can be determined whether the exhaust gas flow should continue to be passed through the bypass or whether an operating phase with a rich exhaust gas mixture, e.g. for the regeneration of the LNT catalyst, has ended and the exhaust gas flow can therefore be passed through the SCR catalyst again in order to be able to remove nitrogen oxides from the exhaust gas flow by means of the SCR catalyst.

[0062] Further advantages of the present invention will become apparent from the following description and the figures. The invention is explained in more detail with reference to the figures and the following description. They show: Fig. 1 a schematic representation of an exemplary exhaust system with an internal combustion engine; Fig. 2 a flow diagram of a process for treating an exhaust gas stream; Fig. 3 a flow diagram of another method for treating an exhaust gas stream; Fig. 4 a possible further course of the proceedings according to Fig. 3; and Fig. 5 Dependence of the proportion of converted nitrogen oxides on temperature for different SCR catalysts.

[0063] Fig. Figure 1 schematically shows an exhaust system 1 connected to an internal combustion engine 2. The internal combustion engine 2 can be designed as a compression-ignition engine and run on diesel fuel, for example. The internal combustion engine generates an exhaust gas flow 3, which is received by the exhaust system 1. Described in the flow direction of the exhaust gas flow 3, the exhaust system 1 has an LNT catalyst 4, a lambda sensor 8, a bypass valve 7, and an SCR catalyst 5. The SCR catalyst 5 can be bypassed by means of the bypass 6. For this purpose, the exhaust gas flow 3 can be controlled by means of the bypass valve 7 so that it flows through either the SCR catalyst 5 or the bypass 6.

[0064] In addition, the exhaust system 1 has another lambda sensor 9, which is located upstream of the LNT catalyst 4. The lambda sensors 8, 9 are designed as lambda probes. Alternatively, only one lambda sensor 8, 9 may be present.

[0065] Both lambda sensors 8, 9 and the bypass valve 7 are signal-connected to the control unit 10, which can output a control signal to the bypass valve 7 depending on the signals from the lambda sensors 8, 9.

[0066] Optionally, the SCR catalyst 5 can also have a particulate filter functionality and be designed as an SDPF catalyst, i.e., a particulate filter with SCR functionality. A feed device 11 for an ammonia-forming agent to the exhaust gas stream 3, e.g., urea solution, can be arranged between the bypass valve 7 and the SCR catalyst 5, so that the SCR catalyst 5 can receive the amount of ammonia required to convert the nitrogen oxides. The feed device 11 can optionally also be controlled by means of the control unit 10. For example, the feed device 11 can only be active, i.e., feed an ammonia-forming agent to the exhaust gas stream 3, when the exhaust gas stream 3 flows through the SCR catalyst 5. If, however, the bypass 6 is activated, the feed can be stopped, since the SCR catalyst 5 does not require any ammonia during this period.

[0067] The combustion air ratio λ of the exhaust gas stream 3 leaving the internal combustion engine 2 can be determined by a corresponding engine control system. For example, the combustion air ratio λ can be greater than 1 in a normal operating mode, i.e., the internal combustion engine 2 is operated with a lean air-fuel mixture. The exhaust gas stream 3 is then aftertreated in the exhaust system 1 by means of the LNT catalyst 4 and the SCR catalyst 5. Optionally, further exhaust gas aftertreatment devices, e.g., particulate filters, can be present. It is also possible to recirculate a portion of the exhaust gas, i.e., to feed it mixed with fresh air to the internal combustion engine 2 as intake air.

[0068] To regenerate the LNT catalyst, the combustion engine 2 can be temporarily operated with a rich air-fuel mixture. Accordingly, the exhaust gas stream 3 leaving the combustion engine 2 then also has a combustion air ratio λ less than 1. The completion of the regeneration can be determined based on a combustion air ratio λ ab < 1 can be detected downstream of the LNT catalyst 4. The operation of the combustion engine 2 with the rich air-fuel mixture can take place until the end of the regeneration, but also for a longer period of time, ie, beyond the lambda breakthrough.

[0069] By means of the exhaust system 1 it can be prevented that an exhaust gas mixture with a combustion air ratio λ ab < 1, which is produced during operation of the combustion engine 2 with a rich air-fuel mixture, enters the SCR catalyst 5. For this purpose, the exhaust system 1, as described below with reference to the Fig. 2, Fig. 3 and Fig. 4 described.

[0070] Optionally, the exhaust system 1 can include a temperature sensor for determining the temperature of the exhaust gas stream 3, for example, arranged upstream or downstream of the LNT catalyst 4 (not shown). The temperature sensor can also be signal-connected to the control unit 10, so that the control unit 10 can output a control signal to the bypass valve 7 depending on the signals from one or both of the lambda sensors 8, 9 and the temperature sensor.

[0071] Fig. Figure 2 shows a flow diagram of a method for treating an exhaust gas flow 3 generated by an internal combustion engine 2. After starting the internal combustion engine 2, it is initially operated in a lean operating mode, ie, with a lean air-fuel mixture. The combustion air ratio λ is measured by means of the additional lambda sensor 9. aufin the exhaust gas stream 3 upstream of the LNT catalyst 4. If the combustion air ratio λ auf not less than 1, the lean operating mode is continued without further measures. However, if a combustion air ratio λ auf < 1, the bypass valve 7 is prepared for an impending activation of the bypass 7.

[0072] Furthermore, the combustion air ratio λ ab downstream of the LNT catalyst 4 and upstream of the SCR catalyst 5 by means of the lambda sensor 8. As long as the combustion air ratio λ ab is not less than 1, the bypass valve 7 remains in a prepared but closed state. The exhaust gas flow 3 flows through the SCR catalyst 5. If a combustion air ratio λ ab< 1, the bypass valve 7 is opened so that the exhaust gas flow 3 now flows through the bypass 6 instead of the SCR catalyst 5. The exhaust gas flow 3 flows through the bypass 6 until the combustion air ratio λ ab is no longer less than 1. If a combustion air ratio λ ab ≥ 1, the bypass valve 7 is closed and the exhaust gas flow 3 flows through the SCR catalyst 5.

[0073] Furthermore, a determination of the temperature of the exhaust gas flow 3 is provided, wherein the exhaust gas flow 3 is only passed through the bypass 6 bypassing the SCR catalyst 5 if the temperature of the exhaust gas flow 3 exceeds a limit temperature of, for example, 500 °C. Before the step "Open bypass valve so that the exhaust gas flow flows through the bypass" in Fig. 2, a query of the temperature of the exhaust gas flow 3 can be provided. If the temperature exceeds the limit temperature, the bypass valve is opened; otherwise, the temperature of the exhaust gas flow 3 is determined and compared with the limit temperature as long as a combustion air ratio λ ab less than 1 is determined.

[0074] The sequence of an alternative method for treating an exhaust gas flow 3 generated by an internal combustion engine 2 is shown in Fig. 3 schematically shown. In contrast to the procedure according to Fig. 2 is at a combustion air ratio λ auf < 1, the time t1 is estimated using a model at which the combustion air ratio λ ab downstream of the LNT catalyst 4 will be less than 1. In addition, the time t0 is determined, which is before the time t1.

[0075] If time t0 has not yet been reached, bypass valve 7 is closed or is closed, so that exhaust gas flow 3 flows through SCR catalyst 5. Reaching time t0 is checked again. If time t0 is reached, bypass valve 7 is opened, so that exhaust gas flow 3 flows through bypass 6 instead of SCR catalyst 5.

[0076] The further possible course of the procedure according to Fig. 3 shows Fig. 4. It is assumed that the exhaust gas flow 3 flows through the bypass 6. The bypass valve 7 is therefore open. In a next step, the combustion air ratio λ auf determined. If λ auf < 1, i.e. if a rich exhaust gas mixture is present upstream of the LNT catalyst 4, the bypass valve 7 remains open and the exhaust gas flow 3 continues to flow through the bypass 6.

[0077] However, if λ auf< 1, ie if there is a stoichiometric or lean exhaust gas mixture upstream of the LNT catalyst 4, the time t2 is estimated at which the combustion air ratio downstream of the LNT catalyst λ ab ≥ 1.

[0078] If time t2 has not yet been reached, bypass valve 7 remains open and exhaust gas flow 3 continues to flow through bypass 6. However, if time t2 has been reached, bypass valve 7 is closed, meaning the flow of exhaust gas flow 3 through bypass 6 is terminated. Exhaust gas flow 3 now flows through SCR catalyst 5.

[0079] Alternatively, following the procedure according to Fig. 3 the combustion air ratio λ ab downstream of the LNT catalyst 4 and upstream of the SCR catalyst 5 (in Fig. 4 not shown), e.g. by means of the lambda sensor 8. If a combustion air ratio λ ab≥ 1, the bypass valve 7 is closed again and the exhaust gas flow 3 flows through the SCR catalyst 5.

[0080] The alternative procedures according to Fig. 3 and Fig. 4 may optionally comprise determining the temperature of the exhaust gas flow 3, wherein the determined temperature is as Fig. 2 can be used to decide whether to activate or deactivate the bypass.

[0081] Fig. Figure 5 shows the proportion of converted nitrogen oxides as a function of temperature for SCR catalysts 5 under different conditions. It can be seen that the proportions of converted nitrogen oxides are approximately the same for an unused SCR catalyst 5, i.e., one preconditioned at 600 °C, and for an SCR catalyst 5 exposed to lean conditions at 650 °C.

[0082] In contrast, Fig. 5 that the proportion of converted nitrogen oxides is significantly reduced in an SCR catalyst 5 which is alternately exposed to lean and rich conditions at 650 °C, ie an SCR catalyst 5 used according to the state of the art.

[0083] The invention ensures that the SCR catalyst 5 is not exposed to damaging rich conditions. The proportion of converted nitrogen oxides thus corresponds to the curve “SCR lean conditions at 650 °C” in Fig. 5. Compared to the curve “SCR lean-rich conditions at 650 °C”, a significant improvement is evident, ie a higher proportion of nitrogen oxides is converted and pollutant emissions are reduced.

[0084] Furthermore, the invention simplifies the monitoring of the SCR catalyst 5. Typically, the ammonia storage capacity of the SCR catalyst 5 is monitored to supply an optimal amount of urea solution to the SCR catalyst 5 and prevent unwanted ammonia release. The ammonia storage capacity is predicted using a model that takes into account the supplied amounts of nitrogen oxides and ammonia.

[0085] In addition, the normal aging of the SCR catalyst 5 under lean conditions due to high temperatures and the presence of water is taken into account. However, deterioration of the SCR catalyst 5 due to the supply of a rich exhaust mixture is not considered.

[0086] This leads to a deviation of the actual ammonia storage capacity from the predicted ammonia storage capacity in the event of deterioration due to a rich exhaust gas mixture. By largely preventing such deterioration with the invention, the invention leads to improved agreement between the predicted and actual ammonia storage capacity. As a result, the supplied amount of urea solution also corresponds more closely to the required amount of urea solution, and an undesirable release of ammonia can be avoided. List of reference symbols 1 exhaust system 2 combustion engine 3 Exhaust gas flow 4 LNT catalyst 5 SCR catalyst 6 Bypass 7 Bypass valve 8 Lambda sensor 9 additional lambda sensors 10 Control unit 11 Feeding device λ abCombustion air ratio downstream of the LNT catalyst λ auf Combustion air ratio upstream of the LNT catalyst t0 time before time t1 t1 Time at which the combustion air ratio downstream of the LNT catalyst λ ab will be < 1 t2 Time at which the combustion air ratio downstream of the LNT catalyst λ ab ≥ 1 will be

Claims

[1] Exhaust system (1) for receiving an exhaust gas flow (3) generated by an internal combustion engine (2), comprising: - an LNT catalyst (4), - an SCR catalyst (5) arranged downstream of the LNT catalyst (4), - a bypass (6) to bypass the SCR catalyst (5), - a bypass valve (7) for dividing the exhaust gas flow (3) between the bypass (6) and the SCR catalyst (5), - a lambda sensor (8, 9) arranged upstream of the LNT catalyst (4) or downstream of the LNT catalyst (4) and upstream of the bypass valve (7), - a temperature sensor for determining the temperature of the exhaust gas flow (3) and - a control unit (10) which is designed and configured to output a control signal to the bypass valve (7) as a function of sensor signals from the lambda sensors (8, 9) and the temperature sensor, wherein a control signal for activating the bypass (6) is output if a combustion air ratio λ ab < 1 is detected and the temperature of the exhaust gas flow (3) determined by means of the temperature sensor exceeds a limit temperature or wherein a control signal for activating the bypass (6) is output if a determined combustion air ratio λ auf , which is determined by means of the lambda sensor (9) arranged upstream of the LNT catalyst (4), estimated time t1 at which the combustion air ratio downstream of the LNT catalyst (4) λ ab< 1, is reached and the temperature of the exhaust gas flow (3) determined by means of the temperature sensor exceeds a limit temperature. [2] Exhaust system (1) according to claim 1, comprising: - a further lambda sensor (8, 9) arranged upstream of the LNT catalyst (4) or downstream of the LNT catalyst (4) and upstream of the bypass valve (7). [3] Exhaust system (1) according to one of the preceding claims, further comprising: - a supply device (11) for supplying an ammonia-forming agent to the exhaust gas stream (3). [4] Exhaust system (1) according to one of the preceding claims, wherein the control unit (10) is further configured and designed to output a control signal to the supply device (11) as a function of sensor signals from the lambda sensors (8, 9) or as a function of sensor signals from the lambda sensors (8, 9) and the temperature sensor. [5] Motor vehicle with an exhaust system (1) according to one of the preceding claims. [6] Method for treating an exhaust gas stream (3) generated by an internal combustion engine (2), comprising the following steps: - passing the exhaust gas flow (3) through an LNT catalyst (4), - Determine the combustion air ratio λ ab in the exhaust gas stream (3) downstream of the LNT catalyst (4) and upstream of an SCR catalyst (5), - Determining the temperature of the exhaust gas flow (3) and - Passing the exhaust gas flow (3) through the SCR catalyst (5) if the combustion air ratio λ ab ≥ 1, and - guiding the exhaust gas flow (3) through a bypass (6) bypassing the SCR catalyst (5) if the combustion air ratio λ ab < 1 and the temperature of the exhaust gas stream (3) exceeds a limit temperature. [7] The method of claim 6, further comprising: - Determine the combustion air ratio λ auf in the exhaust gas stream (3) upstream of the LNT catalyst (4) and - Preparing the exhaust gas flow (3) to pass through the bypass (6) if the combustion air ratio λ auf < 1. [8] Method for treating an exhaust gas stream (3) generated by an internal combustion engine (2), comprising the following steps: - Determine the combustion air ratio λ auf in the exhaust gas stream (3) upstream of an LNT catalyst (4), - Passing the exhaust gas flow (3) through the LNT catalyst (4), - Determining the temperature of the exhaust gas stream (3), - if the combustion air ratio λ auf ≥ 1, passing the exhaust gas flow (3) through an SCR catalyst downstream of the LNT catalyst (4), - if the combustion air ratio λ auf< 1, estimating a time t1 at which the combustion air ratio downstream of the LNT catalyst (4) λ ab will be < 1, - if the time t1 is not reached, passing the exhaust gas flow (3) through an SCR catalyst (5) downstream of the LNT catalyst (4), and - from reaching the time t1, passing the exhaust gas flow (3) through a bypass (6) bypassing the SCR catalyst (5) if the temperature of the exhaust gas flow (3) exceeds a limit temperature. [9] Method according to claim 8, comprising: - Setting a time t0 before time t1, - if the time t0 is not reached, passing the exhaust gas flow (3) through the SCR catalyst (5), and - from the time t0 is reached, the exhaust gas flow (3) is passed through the bypass (6) bypassing the SCR catalyst (5). [10] Method according to one of claims 6 to 9, further comprising: - Determining a combustion air ratio λ auf in the exhaust gas stream (3) upstream of the LNT catalyst (4), - if the combustion air ratio λ auf ≥ 1, estimating a time t2 at which the combustion air ratio downstream of the LNT catalyst (4) λ ab ≥ 1, - Terminating the conduction of the exhaust gas flow (3) through the bypass (6) bypassing the SCR catalyst (5) if the time t2 is reached. [11] Method according to one of claims 6 to 10, further comprising: - Determining a combustion air ratio λ ab in the exhaust gas stream (3) downstream of the LNT catalyst (4) and upstream of the SCR catalyst (5) and - Terminating the flow of the exhaust gas (3) through the bypass (6) bypassing the SCR catalyst (5) if the combustion air ratio λ ab ≥ 1. [12] Method according to one of claims 6 to 11, further comprising: - supplying an ammonia-forming agent to the exhaust gas stream (3) if the exhaust gas stream (3) is passed through the SCR catalyst (5), and - stopping the supply of an ammonia-forming agent to the exhaust gas stream (3) if the exhaust gas stream (3) is passed through the bypass (6).

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