Method for Operating an Exhaust System, in Particular of a Motor Vehicle

Abstract

A method for operating an exhaust system, where the exhaust system has a first selective catalytic reduction (SCR) catalytic converter close to the engine and a second SCR catalytic converter arranged downstream of the first. A respective quantity of reducing agent to be introduced into the exhaust gas by a respective dosing element is set depending on a first temperature of the first SCR catalytic converter, where during a period of time during which the first temperature exceeds a predeterminable first threshold value and a second temperature of the second SCR catalytic converter exceeds a predeterminable second threshold value, which is lower than the first threshold value, the introduction of the reducing agent into the exhaust gas with regard to the dosing elements takes place exclusively via the second dosing element such that during the period of time, no introduction of the reducing agent into the exhaust gas takes place.

Claims

1.-10. (canceled)

11. A method for operating an exhaust system (10) through which exhaust gas from an internal combustion engine is flowable, wherein the exhaust system has a first selective catalytic reduction (SCR) catalytic converter (14) close to the internal combustion engine, a second SCR catalytic converter (16) arranged downstream of the first SCR catalytic converter (14), a first dosing element via which a reducing agent is introducible into the exhaust gas at a first introduction point (E1) disposed upstream of the first SCR catalytic converter (14), a second dosing element (22) via which the reducing agent is introducible into the exhaust gas a second introduction point (E2) disposed downstream of the first SCR catalytic converter (14) and upstream of the second SCR catalytic converter (16), the method comprising: a respective quantity of the reducing agent to be introduced into the exhaust gas by a respective dosing element (20, 22) is set depending on at least a first temperature of the first SCR catalytic converter (14), wherein during a period of time during which the first temperature exceeds a predeterminable first threshold value and at least a second temperature of the second SCR catalytic converter (16) exceeds a predeterminable second threshold value, which is lower than the first threshold value, the introduction of the reducing agent into the exhaust gas with regard to the dosing elements (20, 22) takes place exclusively via the second dosing element (22), such that during the period of time, no introduction, which is caused by the first dosing element (20), of the reducing agent into the exhaust gas takes place; wherein from an average first temperature of more than 400 degrees Celsius at the first SCR catalytic converter, the system switches over to the second SCR catalytic converter and to the second dosing element assigned to it; wherein during the period of time, the second temperature is lower than the first threshold value and the first threshold value is 450 degrees Celsius and the second threshold value is 250 degrees Celsius.

12. The method according to claim 11, wherein during the period of time, a mass flow of the exhaust gas falls below a predeterminable third threshold value.

13. The method according to claim 12, wherein during a second period of time, during which the first temperature exceeds the first threshold value, the second temperature exceeds the second threshold value and the mass flow of the exhaust gas exceeds the third threshold value, the introduction of reducing agent into the exhaust gas takes place via both dosing elements (20, 22).

14. The method according to claim 13, wherein during a third period of time, during which the first temperature exceeds a predeterminable fourth threshold value, which is lower than the first threshold value, and falls below the first threshold and the second temperature falls below the second threshold value, the introduction of the reducing agent into the exhaust gas takes place via both dosing elements (20, 22).

15. The method according to claim 14, wherein during the third period of time, the quantity introduced into the exhaust gas by the first dosing element (20) is greater than the quantity introduced into the exhaust gas by the second dosing element (22).

16. The method according to claim 15, wherein a ratio of the amount introduced into the exhaust gas by the first dosing element (20) to the amount introduced into the exhaust gas by the second dosing element (22) is in a range of from 7/3 to 8/2 inclusive.

17. The method according to claim 14, wherein during a fourth period of time, during which the first temperature falls below the first threshold value and the second temperature exceeds the second threshold value, the introduction of the reducing agent into the exhaust gas takes place via both dosing elements (20, 22), wherein, during the fourth period of time, the quantity introduced into the exhaust gas by the first dosing element (20) is less than the quantity introduced into the exhaust gas by the second dosing element (22).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In the single FIGURE, the drawing shows a schematic representation of an exhaust system designed to carry out a method in accordance with the invention, in particular of a motor

DETAILED DESCRIPTION OF THE DRAWINGS

[0039] The single FIGURE shows, in a schematic depiction, an exhaust system 10 for a motor vehicle, in particular for an automobile. In its completely manufactured state, the motor vehicle comprises the exhaust system 10 and an internal combustion engine, also referred to as combustion motor, by means of which the motor vehicle can be driven. During a fired operation of the internal combustion engine, the latter provides exhaust gas whichas illustrated by an arrow 12 in the FIGUREflows into the exhaust system 10 and can flow through the exhaust system 10. The exhaust system 10 comprises a first SCR catalyst 14 for effecting or supporting the selective catalyst reduction (SCR). The exhaust system also comprises a second SCR catalytic converter 16, which is arranged downstream of the first SCR catalytic converter 14 in the direction of flow of the exhaust gas flowing through the exhaust system 10. The SCR can also be catalytically supported or effected by means of the second SCR catalytic converter 16. For catalytically effecting or supporting the SCR, the respective SCR catalytic converter 14 or 16 has at least one coating which is catalytically active for the SCR, such that the SCR can occur or take place in the respective SCR catalytic converter 14 or 16. In the scope of the SCR, any nitrogen oxides (NOx) contained in the exhaust gas react with ammonia (NH3), which originates from a reducing agent that is or was introduced into the exhaust gas, to form nitrogen and water, such that the nitrogen oxides contained in the exhaust gas are at least partially removed from the exhaust gas. This is also referred to as denitrification of the exhaust gas.

[0040] The exhaust system 10 comprises a dosing system, designated in its entirety as 18, by means of which, for example, within the scope of a respective introduction process, an advantageous or, in particular, at least necessary quantity of the reducing agent for denitrification of the exhaust gas can be or is introduced into the exhaust gas. For this purpose, the dosing system 18 comprises a first dosing element 20, by means of which the reducing agent can be introduced into the exhaust gas, in particular in the course of the respective introduction process, at at least one first introduction point E1. The introduction of the reducing agent into the exhaust gas is also referred to as dosing or metering. The dosing system 18 further comprises a second dosing element 22, which is arranged, for example, downstream of the first dosing element 20. By means of the second dosing element 22, the reducing agent can be introduced, in particular injected, into the exhaust gas at a second introduction point E2. It can be seen from the FIGURE that the first introduction point E1 is arranged upstream of the SCR catalytic converter 14, wherein the second introduction point E2 is arranged downstream of the SCR catalytic converter 14 and upstream of the SCR catalytic converter 16.

[0041] In the exemplary embodiment illustrated in the FIGURE, the exhaust system 10 also comprises a particulate filter 24, which is designed as a diesel particulate filter (DPF), in particular if the internal combustion engine is designed as a diesel engine. The particulate filter 24 is arranged upstream of the SCR catalytic converter 14. In particular, for example, the particulate filter 14 has a coating which is catalytically effective for the SCR, such that, for example, the particulate filter 24 is designed as an SCR particulate filter, in particular as an SCR diesel particulate filter (SDPF). Thus, for example, the particulate filter 24 also represents an SCR catalytic converter or a catalytic element which is catalytically effective for the SCR. The introduction point E1 is also arranged upstream of the particulate filter 24.

[0042] The exhaust system 10 also comprises an exhaust gas aftertreatment element 26, which is arranged upstream of the introduction point E1 and thus upstream of the SCR catalytic converter 14 and upstream of the particulate filter 24. The exhaust gas aftertreatment element 26 is designed, for example, as an oxidation catalytic converter or comprises such an oxidation catalytic converter, wherein the oxidation catalytic converter is designed as a diesel oxidation catalytic converter (DOC), in particular when the internal combustion engine is designed as a diesel engine. Alternatively or additionally, the exhaust gas aftertreatment element 26 is designed as a nitrogen oxide storage catalyst (NSK) or a passive nitrogen oxide absorber (PNA) or comprises such a nitrogen oxide storage catalyst or passive nitrogen oxide absorber. In particular, nitrogen oxides from the exhaust gas can be captured and stored by means of the nitrogen oxide storage catalyst or the passive nitrogen oxide absorber.

[0043] The exhaust system 10 also has an exhaust gas recirculation device 28, by means of which low-pressure exhaust gas recirculation (LP EGR) can be carried out. By means of the exhaust gas recirculation device 28, at least a part of the exhaust gas flowing through the exhaust system 10 or an exhaust pipe 30 of the exhaust system 10 can be branched off from the exhaust system 10 or from the exhaust pipe 30 and recirculated to the internal combustion engine. By means of the exhaust gas recirculation device 28, at least the part of the exhaust gas can be branched off from the exhaust pipe 30 at a branching point A, wherein the branching point A is arranged downstream of the SCR catalytic converter 14 and upstream of the SCR catalytic converter 16 and is thereby arranged closer to the SCR catalytic converter 14 than to the SCR catalytic converter 16. The exhaust gas recirculation device 28 comprises a valve element 32, for example designed as a flap, by means of which a quantity of the exhaust gas to be recirculated and thus a so-called exhaust gas recirculation rate (EGR rate) can be set.

[0044] In addition, the exhaust system 10 includes an ammonia slip catalyst 34, which is also referred to as ASC or ammonia trap catalyst. The ASC is arranged downstream of the second SCR catalyst 16. In the FIGURE, an arrow 36 illustrates a so-called ammonia or reducing agent slip, which is also referred to as slip quantity. The reducing agent or ammonia slip is formed by unused ammonia resulting from the reducing agent which is or was introduced into the exhaust gas and which has flowed through at least one of the SCR catalytic converters 14 and 16 but did not participate in the SCR and therefore was not used. In other words, the reducing agent or ammonia slip, which is also simply referred to as slip, comprises unused reducing agent or unused ammonia which has passed through at least one of the SCR catalysts 14 and 16 but has not participated in the SCR.

[0045] To avoid excessive emissions of unused reducing agent or ammonia, the reducing agent or ammonia slip, i.e, the reducing agent or ammonia forming the slip, is converted into nitrogen and water in the ASC or by means of the ASC. In the following, a method is described using the only FIGURE, by means of which the exhaust system 10 is operated and a particularly low-emission operation can be implemented.

[0046] In the method, for example, the total quantity of the reducing agent to be introduced into the exhaust gas flowing through the exhaust system 10, in particular in the course of a respective introduction process, by means of the dosing system 18 and necessary or advantageous for the denitrification of the exhaust gas, is divided between the dosing elements 20 and 22 and thus between the introduction points E1 and E2, such that respective quantities of the reducing agent to be introduced into the exhaust gas by means of the dosing elements 20 and 22, in particular by means of an electronic computing device not depicted in the FIGURE, are set or varied, wherein the quantities add up to the total quantity and, for example, are to be or are introduced into the exhaust gas by means of the dosing elements 20 and 22 within the framework of the respective introduction process.

[0047] In the method, it is provided that the respective quantity of reducing agent to be introduced into the exhaust gas by means of the respective doing element 20 or 22, in particular in the course of the respective introduction process, is adjusted depending on at least a first temperature of the first SCR catalytic converter 14. The first temperature is measured, for example, by means of at least one first temperature sensor not depicted in the FIGURE and/or calculated by means of a calculation model.

[0048] Furthermore, it is provided in the method that, during a period of time in which the first temperature exceeds a predeterminable first threshold value and at least one second temperature of the second SCR catalytic converter 16 exceeds a predeterminable second threshold value which is lower than the first threshold value and, for example, falls below the first threshold value, the introduction of the reducing agent or of the total quantity of the reducing agent into the exhaust gas takes place exclusively via the second dosing element 22 with respect to the dosing elements 20 and 22, such that during the period of time, no introduction of the reducing agent or of the total quantity of or part of the total quantity into the exhaust gas which is effected by means of the first dosing element 20 takes place. The second temperature is detected, for example, by means of at least one second temperature sensor and/or calculated by means of a calculation model.

[0049] In other words, when the first temperature exceeds the predeterminable or predetermined first threshold value and the second temperature is greater than the predeterminable or predetermined second threshold value and, for example, is lower than the first threshold value, the reducing agent is introduced into the exhaust gas exclusively via the second dosing element 22. In this way, for example, a nitrogen oxide conversion which does not take place or is only insufficient, which can be effected by means of the first SCR catalytic converter 14 and results, for example, from the first temperature exceeding the first threshold value, can be compensated for by means of the second SCR catalytic converter 16, such that the exhaust gas can be denitrified particularly advantageously. At the same time, the consumption of reducing agents can be kept within a particularly low range.

[0050] The first SCR catalytic converter 14, for example, is an SCR catalytic converter close to the engine, which is arranged substantially closer to the internal combustion engine than the second SCR catalytic converter 16. This is advantageous because the first SCR catalytic converter 14 can then be brought to a temperature that is advantageous for the conversion of nitrogen oxides particularly quickly. However, the SCR catalytic converter 14 can reach excessively high temperatures, such that it can only convert nitrogen oxides inadequately at such high temperatures. However, since the total quantity is then divided between dosing elements 20 and 22 in such a way that the total quantity is introduced into the exhaust gas exclusively by means of the dosing element 22 and not by means of the dosing element 20 during the respective introduction process, a sufficient nitrogen oxide conversion can be ensured by means of the SCR catalytic converter 16, such that excessive nitrogen oxide emissions can be avoided.

[0051] In order to avoid excessive reducing agent or ammonia slip, it is preferably provided that a distance B between the SCR catalytic converter 16, in particular between its outlet, is greater than 30 centimeters, in particular greater than 50 centimeters. Thus, for example, at the nominal power point of the internal combustion engine, a difference between the second temperature and a third temperature of the ASC is at least 50 degrees, in particular at least 100 degrees, whereby an excessive ammonia slip can be particularly advantageously avoided.

REFERENCE NUMERAL LIST

[0052] 10 exhaust system [0053] 12 arrow [0054] 14 first SCR catalytic converter [0055] 16 second SCR catalytic converter [0056] 18 dosing system [0057] 20 first dosing element [0058] 22 second dosing element [0059] 24 particulate filter [0060] 26 exhaust gas aftertreatment element [0061] 28 exhaust gas recirculation device [0062] 30 exhaust pipe [0063] 32 valve element [0064] 34 ammonia slip catalyst [0065] 36 arrow [0066] A branching point [0067] B distance [0068] E1 first introduction point [0069] E2 second introduction point