CN115667683A

CN115667683A - Motor vehicle with an internal combustion engine driven by a carbon-free fuel and an exhaust system connected thereto

Info

Publication number: CN115667683A
Application number: CN202180036025.5A
Authority: CN
Inventors: B·拉塞尔; J·麦克哈默; K·哈德
Original assignee: AVL List GmbH
Current assignee: AVL List GmbH
Priority date: 2020-07-03
Filing date: 2021-07-02
Publication date: 2023-01-31
Also published as: DE112021001728A5; WO2022000012A1; AT524011B1; AT524011A1

Abstract

The invention relates to a motor vehicle having an internal combustion engine driven with a carbon-free fuel, such as hydrogen or ammonia, and having an exhaust system (1) connected thereto, which has at least a first nitrogen oxide reduction catalytic converter (2), which preferably contains vanadium. According to the invention, the exhaust system (1) also has a heat exchanger (3) which can be used for energy recovery, wherein the exhaust gas emitted by the internal combustion engine can be guided through the heat exchanger (3) in an adjustable ratio before being fed to the nitrogen oxide reduction catalyst (2). It is also possible to arrange two nitrogen oxide reduction catalysts in succession, wherein fuel can be used as reducing agent. The operating method according to the invention provides that, in the operating state of the heat exchanger (3) and/or of the energy recovery device (6), the proportion of the exhaust gas conducted through the heat exchanger (3) is selected in such a way that the temperature of the first nitrogen oxide reduction catalyst (2) is kept at least predominantly below a predefinable upper temperature limit.

Description

Motor vehicle with an internal combustion engine driven by a carbon-free fuel and an exhaust system connected thereto

Technical Field

The invention relates to a motor vehicle having an internal combustion engine driven with a carbon-free fuel, having the features of the preamble of claim 1, and to a method for operating such a motor vehicle having the features of the preamble of claim 12.

Background

A motor vehicle of the generic type is known from DE 10 2007 021 827 A1. The corresponding internal combustion engine operates on hydrogen as a carbon-free fuel. On the output side of the internal combustion engine, the hydrogen taken out of the storage tank is conveyed to the exhaust system. A catalyst provided downstream in the exhaust system removes nitrogen oxides contained in the exhaust gas by catalytic reduction using the supplied hydrogen.

Disclosure of Invention

The object of the present invention is to provide a motor vehicle having an internal combustion engine driven with a carbon-free fuel, which allows improved energy utilization and exhaust gas purification in comparison.

This object is achieved by a motor vehicle having the features of claim 1 and by a method having the features of claim 12.

The motor vehicle according to the invention has an internal combustion engine driven with a carbon-free fuel. An exhaust system connected to the internal combustion engine has, in particular, a first nitrogen oxide reduction catalyst and also has a heat exchanger, wherein the exhaust gas emitted by the internal combustion engine can be conducted through the heat exchanger in an adjustable ratio before being fed to the nitrogen oxide reduction catalyst. The internal combustion engine is preferably designed as a spark-ignition piston engine.

Due to the heat exchanger provided according to the invention, the exhaust gas of the internal combustion engine can be temperature-regulated, in particular cooled, before it reaches the nitrogen oxide reduction catalyst. A first predeterminable fraction of the total exhaust gas emitted by the internal combustion engine is conducted through the heat exchanger, while the remaining second fraction bypasses the heat exchanger and reaches the nitrogen oxide reduction catalyst. The predeterminable and adjustable first portion of the total exhaust gas emitted by the internal combustion engine can here assume values between 0% and 100%. The partial exhaust gas flows guided through the heat exchanger are in turn combined with the exhaust gas flows bypassing the heat exchanger, and the common, temperature-regulated total exhaust gas flow is fed to the nitrogen oxide reduction catalyst. In this way, the nitrogen oxide reduction catalyst can be kept at least predominantly in its operating temperature range. Preferably, heat is extracted from the exhaust gases which are led through the heat exchanger, so that the total exhaust gas flow which is fed to the nitrogen oxide reduction catalyst is cooled. Thus, even when the internal combustion engine outputs high power and the temperature of the exhaust gas discharged is correspondingly high and higher than the operating temperature range of the nitrogen oxide reduction catalyst, the effectiveness of the nitrogen oxide reduction catalyst can be obtained. This allows for efficient reduction of nitrogen oxides over an extended engine operating range. Furthermore, it also allows the use of catalyst materials with a lower operating temperature range. Also, the nitrogen oxide reduction catalyst can be prevented from reaching or exceeding the degradation temperature limit by the bypass heat exchanger disposed upstream. Thus, the long-term durability of the nitrogen oxide reduction catalyst is also improved.

In the design of the invention, the exhaust system has an energy recovery device which can utilize the thermal energy drawn from the exhaust gas conducted through the heat exchanger to produce useful energy. The energy recovery device can be designed, for example, as a thermoelectric generator which produces useful electrical energy. It is also possible to provide means for generating useful mechanical energy by means of a thermodynamic cyclic process, such as a rankine cycle. For this purpose, heat is extracted from the exhaust gas in a heat exchanger and supplied to the working medium. The working medium is directed to an energy recovery system where the absorbed heat is partially converted into useful mechanical energy. The electrical or mechanical energy generated in the energy recovery device can be used in a motor vehicle, for example in an auxiliary device. This improves the energy utilization of the motor vehicle overall.

In a further embodiment of the invention, a second nitrogen oxide reduction catalyst is provided, which is arranged upstream of the first nitrogen oxide reduction catalyst in terms of flow technology. This makes it possible to maintain at least one of the two nitrogen oxide reduction catalysts at operating temperature in the case of the temperature gradients typically present along the exhaust gas path. In particular, the temperature gradient can also be influenced in this sense because of the heat exchanger. Therefore, the operating range of the internal combustion engine in which nitrogen oxides are effectively removed from exhaust gas can be significantly extended. It may be advantageous to specify catalyst materials with different operating temperature ranges for the first and second nitrogen oxide reduction catalysts. For example, the second nitrogen oxide reduction catalyst may have a lower operating temperature range than the first nitrogen oxide reduction catalyst. The second nitrogen oxide reduction catalyst is thus available for nitrogen oxide reduction quickly after a cold start of the internal combustion engine. If the second nitrogen oxide reduction catalyst heats up during warm-up to a temperature above its operating temperature range, the first nitrogen oxide reduction catalyst is at an operating temperature during this period and can remove nitrogen oxides from the exhaust gas. It can also be provided that the volumes of the two nitrogen oxide reduction catalysts are selected to be different from one another. For example, the second nitrogen oxide reduction catalyst may have a smaller volume than the first nitrogen oxide reduction catalyst. Thereby allowing the second nitrogen oxide reduction catalyst to be heated to the operating temperature very quickly. The volume of the second nitrogen oxide reduction catalyst may be, for example, less than 80%, 60%, or 40% or even less than the volume of the first nitrogen oxide reduction catalyst. Furthermore, provision may be made for the second nitrogen oxide reduction catalyst to be arranged close to the engine, for example in the engine compartment of the motor vehicle, while the first nitrogen oxide reduction catalyst is arranged remote from the engine in the region of the vehicle floor.

In a further embodiment of the invention, the heat exchanger is arranged in terms of flow technology upstream of the first nitrogen oxide reduction catalyst and downstream of the second nitrogen oxide reduction catalyst. The heat exchanger is therefore arranged between the second nitrogen oxide reduction catalyst arranged further upstream and the first nitrogen oxide reduction catalyst arranged further downstream, as viewed in terms of flow technology. This has the advantage that, if the first nitrogen oxide reduction catalyst is subjected to too intensive a further heating when the second nitrogen oxide reduction catalyst is already above its operating temperature, its effectiveness can also be maintained by a corresponding operation of the heat exchanger.

In a further advantageous embodiment of the invention, the reducing agent for reducing the nitrogen oxides contained in the exhaust gas at the first and/or second nitrogen oxide reduction catalyst is a fuel which is used to drive the internal combustion engine. Thus, a storage container for the separate reducing agent can be dispensed with or at least be designed to be smaller. In this case, it is particularly advantageous if, in a further embodiment of the invention, the internal combustion engine is designed to supply the reducing agent to the exhaust system via a combustion chamber of the internal combustion engine. For this purpose, a fast-acting fuel supply valve is preferably provided for at least one cylinder of the internal combustion engine, by means of which fuel can be supplied directly into the respective combustion chamber. A separate supply device for supplying the reducing agent into the exhaust gas is thereby dispensed with. In addition, in this way, a particularly homogeneous distribution of the reducing agent in the exhaust gas is permitted. Preferably, the fuel used as reducing agent is introduced into the combustion chamber or chambers by means of fuel injection or fuel injection which is carried out later in the operating cycle, so that the fuel is discharged chemically at least approximately constantly with the exhaust stroke. This is preferably carried out in the range of crankshaft angles of 120 ° to 180 ° after top dead center.

In a further embodiment of the invention, the fuel used for operating the internal combustion engine is hydrogen. Thereby avoiding climate-hazardous carbon dioxide emissions. Also avoids the emission of carbon black particles, hydrocarbons and carbon monoxide. In the lean-burn operating mode of the internal combustion engine, which is preferably at least predominantly specified, the pollutants produced during the combustion of the fuel are exclusively nitrogen oxides, but nitrogen oxides can be removed from the exhaust gas by means of a nitrogen oxide reduction catalyst.

In a further embodiment of the invention, it is provided that the second nitrogen oxide reduction catalyst is a lean nox catalyst which is able to catalyze the reduction of nitrogen oxides with hydrogen as reducing agent. This embodiment is particularly advantageous in connection with an internal combustion engine designed as a hydrogen engine, since then the fuel used for engine operation can also be used as reducing agent for reducing nitrogen oxides at the second nitrogen oxide reduction catalyst. For this purpose, the exhaust gases can be supplied with hydrogen via the combustion chamber of the internal combustion engine or via a metering device outside the engine. The lean-oil denitrification catalyst preferably has a catalytic coating which contains precious metals and in particular platinum group metals, such as platinum and/or rhodium. Catalytic coatings are often provided which catalyze the reduction of nitrogen oxides with hydrogen as a reducing agent even when excess oxygen is present in the exhaust gas. In particular in the case of lean-nox catalysts which contain platinum and/or rhodium and/or palladium, the operating temperature range is then relatively low. The lower temperature limit of the operating range can be about 80 ℃. The upper temperature limit of the operating range may be about 250 ℃. Therefore, the lean nox catalyst is ready to operate soon after the cold start of the internal combustion engine.

In a further embodiment of the invention, a metering device for adding an ammonia-containing reducing agent to the exhaust gas is arranged on the inlet side of the first nitrogen oxide reduction catalyst. Ammonia can be contained in the reducing agent in free or bound form. Preferably, a metering device is provided for adding the aqueous urea solution to the exhaust gases. The urea solution is preferably conveyed from a separate storage container to the metering device by means of a conveying device. The metering device is preferably arranged after the point of convergence, viewed in the flow direction, of the exhaust gas conducted through the recuperator with the exhaust gas bypassing the recuperator.

When using a reducing agent containing ammonia, the first nitrogen oxide reduction catalyst can be designed as a classical SCR catalyst, for example as a copper-containing or iron-containing zeolite catalyst. In particular, in a further embodiment of the invention, the first nitrogen oxide reduction catalyst contains vanadium. The vanadium is preferably present here in the form of an oxide, in particular vanadium pentoxide. Other components effective in promoting selective nitrogen oxide reduction may of course also be specified, such as oxides of tungsten, molybdenum and/or titanium. From an application technology point of view, the predetermined operating temperature range of the nitrogen oxide reduction catalyst is typically in the average range from about 180 ℃ to 200 ℃ to about 420 ℃ to 480 ℃. However, contemplated vanadium-containing nitrogen oxide reduction catalysts may have lower temperature limits in terms of stability or quality degradation. It is therefore preferably provided that the temperature of the exhaust gas entering the nitrogen oxide reduction catalyst is at least predominantly maintained below 350 ℃, in particular below 300 ℃, by cooling by means of a heat exchanger.

In a further embodiment of the invention, the fuel used for operating the internal combustion engine is ammonia. Thereby also avoiding harmful carbonaceous emissions. Furthermore, the fuel used for operating the internal combustion engine can also be advantageously used as a reducing agent for reducing nitrogen oxides in the exhaust system. A separate reducing agent reservoir can thereby be dispensed with. Ammonia as nitrogen oxide reducing agent can be fed by the electric motor to a nitrogen oxide reduction catalyst of the exhaust system, in particular by being added later in the working cycle to the combustion chamber of the internal combustion engine. This preferably takes place in the range of crankshaft angles of 120 ° to 180 ° after top dead center. However, "supply to the exhaust gas outside the engine" may additionally or alternatively be performed by a separate metering device. The operating method according to the invention for a motor vehicle designed as described above provides that, in combination with the operation of the recuperator and/or the energy recovery device, the proportion of the exhaust gas conducted through the recuperator is selected such that the temperature of the first nitrogen oxide reduction catalyst is kept at least predominantly below a predefinable upper temperature limit. The predeterminable temperature limit may be selected as a function of an upper operating temperature limit or a temperature stability limit of the nitrogen oxide reduction catalyst. An upper temperature limit of 350 ℃ to 300 ℃, in particular 300 ℃, is preferred. If the exhaust gas reaching the dividing point for dividing the exhaust gas flow into the portion flowing through the heat exchanger and the portion bypassing the heat exchanger has a temperature exceeding the upper temperature limit, a certain portion of the exhaust gas is guided through the heat exchanger by means of the regulating element. The division of the exhaust gas flow into a portion flowing through the heat exchanger and a portion bypassing the heat exchanger is preferably carried out as a function of the exhaust gas temperature and the predetermined upper temperature limit. In this case, the heat exchanger cooling effect can also be adjusted accordingly by influencing the quantity of heat-absorbing working medium flowing through the heat exchanger. In particular, the cooling effect can be increased, if desired, by increasing the working medium flow. This reliably prevents the exhaust gas entering the second nitrogen oxide reduction catalyst from exceeding the upper temperature limit. Therefore, the efficiency of the reducing agent catalyst can be obtained and the quality thereof can be prevented from being degraded.

In a configuration of the operating method according to the invention, the first and/or second nitrogen oxide reduction catalyst is/are supplied with a reducing agent for reducing the nitrogen oxides contained in the exhaust gas from an external storage tank. In particular, for the first nitrogen oxide reduction catalyst, a supply of reducing agent from an external reservoir can be provided. When a second nitrogen oxide reduction catalyst is present, provision may additionally or alternatively be made for a supply of reducing agent from an external reservoir for this purpose. In particular, an aqueous urea solution is considered as a reducing agent for the nitrogen oxide reduction at the first and/or second nitrogen oxide reduction catalyst. However, ammonia and in particular gaseous ammonia or hydrogen can also be specified as reducing agent.

In a further embodiment of the operating method according to the invention, fuel is supplied to at least one combustion chamber of the internal combustion engine at a later point in the operating cycle when the fuel is discharged at least approximately chemically unchanged from the internal combustion engine and is fed to the second nitrogen oxide reduction catalyst as a reducing agent for reducing the nitrogen oxides contained in the exhaust gas. In this case, the fuel used for operating the internal combustion engine is also used at least at the second nitrogen oxide reduction catalyst as a reducing agent for reducing nitrogen oxides. The "feeding gaseous ammonia or hydrogen as fuel later into the combustion chamber or combustion chambers" is preferably carried out within a crank angle range of 120 ° to 180 ° after top dead center. It is thus ensured that the fuel fed into the combustion chamber no longer participates in the combustion, or only to a negligible extent, and is discharged from the combustion chamber into the exhaust system virtually unchanged chemically.

Drawings

The features and feature combinations mentioned above in the description and the features and feature combinations mentioned below in the description of the figures and/or shown in the figures individually can be used not only in the respectively indicated combination but also in other combinations or alone without going beyond the scope of the present invention. The above and other features and advantages of the invention result from the following description of a non-limiting preferred embodiment of the invention with reference to the accompanying drawings, in which:

figure 1 shows a schematic view of a first advantageous embodiment of the exhaust system of a motor vehicle according to the invention,

figure 2 shows a schematic view of a second advantageous embodiment of the exhaust system of a motor vehicle according to the invention,

figure 3 shows a schematic view of a third advantageous embodiment of the exhaust system of a motor vehicle according to the invention,

fig. 4 shows a schematic view of a first advantageous embodiment of the exhaust system of a motor vehicle according to the invention.

Detailed Description

Fig. 1 shows a first example of an advantageous embodiment of an exhaust system 1 for connection to an internal combustion engine of a motor vehicle only schematically and in a highly simplified manner. The illustration of a motor vehicle is omitted here. The internal combustion engine, which is also not shown, is designed as an internal combustion engine driven with a carbon-free fuel. Preferably designed as a hydrogen engine. But designs as ammonia engines are also possible. In any case, at least predominantly lean operation, i.e. air-rich operation, is specified for the internal combustion engine.

Exhaust gases from the internal combustion engine enter the exhaust system 1 via an exhaust line 13. In the exhaust gas flow direction indicated by the hollow arrow, the exhaust gas first flows through a catalytic converter, which is referred to here as the second nox reduction catalyst 8. Downstream of the second nox-reducing catalytic converter 8, a bypass line 4 is provided which bypasses the exhaust-gas line section 13'. The first exhaust gas partial flow 5 can be guided via the bypass line 4 by means of the adjusting means 9, 9', while the remaining part of the entire exhaust gas flow is guided as the second exhaust gas partial flow 10 via an exhaust gas line section 13' which is flow-technically parallel to the bypass line 4. At the regulating device 9', the bypass line 4 opens again into the exhaust gas line 13 and the exhaust gas

partial flows

5, 10 merge again. The total exhaust gas flow formed again from the merged exhaust gas

partial flows

5, 10 is then conveyed through a catalytic converter, which is referred to here as the first nitrogen oxide reduction catalyst 2.

The heat exchanger 3 is arranged in a bypass line 4 and serves to transfer heat from the first exhaust gas partial flow 5 to the working fluid. The working fluid is guided in the working fluid circuit 7 on the one hand through the heat exchanger 3 and on the other hand through the energy recovery device 6. The energy recovery device 6 may convert heat supplied with the working fluid into useful energy. The energy recovery device 6 preferably operates as a thermodynamic cycle process or as part of a thermodynamic cycle process and can generate useful mechanical energy from the heat drawn by the first exhaust gas partial flow 5 by means of the working fluid. The working fluid is thereby cooled and fed back to the heat exchanger 3, where it can again absorb heat from the first exhaust gas partial flow 5. Here, a delivery mechanism, not shown in detail, is preferably provided, which maintains the operation of the working fluid circuit 7 and can adjustably control its intensity. The heat flow drawn from the exhaust gas partial flow 5 can thereby be controlled. Furthermore, provision is made for the heat extracted from the exhaust gas to be controlled by operation of the regulating device 9, 9', by means of which the proportion of the first exhaust gas partial flow 5 to the total exhaust gas flow can be regulated as desired. One of the two adjusting mechanisms 9, 9' can also be dispensed with.

A first reducing agent addition device 11, which can supply reducing agent for reducing nitrogen oxides to the exhaust gas as required, is arranged upstream of the first nitrogen oxide reduction catalyst 2 and downstream of the point of confluence of the first exhaust gas partial flow 5 and the second exhaust gas partial flow 10. Similarly, a second reducing agent addition device 12 is arranged upstream of the second nitrogen oxide reduction catalyst 8, by means of which the exhaust gas can also be supplied with the reducing agent for reducing nitrogen oxides. The respective reducing agent is taken from a reservoir, which is not shown in detail. Hydrogen, ammonia or aqueous urea solutions may be considered as reducing agents. The same reducing agent can be provided for the first nitrogen oxide reduction catalyst 2 and the second nitrogen oxide reduction catalyst 8. But different reducing agents may also be used. For example, a urea solution can be provided as reducing agent for the first nitrogen oxide reduction catalyst 2, and hydrogen or ammonia can be provided as reducing agent for the second nitrogen oxide reduction catalyst 8, or vice versa. In particular, when the internal combustion engine is designed as a hydrogen engine, hydrogen is provided as the reducing agent. Especially when the internal combustion engine is designed as an ammonia engine, provision is made for ammonia to be the reducing agent. In both cases, the fuel used for the internal combustion engine can advantageously be used as reducing agent.

The catalyst materials used for the first nitrogen oxide reduction catalyst 2 and the second nitrogen oxide reduction catalyst 8 are preferably selected according to the type of reducing agent used for each. The low-temperature design for the second nitrogen oxide reduction catalyst 8 is particularly preferred, since it is preferably installed in the vicinity of the engine. In this way, it can be very quickly ready for use after a cold start. The preferred operating temperature range is then between about 100 ℃ to about 250 ℃. In the case of hydrogen as reducing agent, this can be achieved, for example, by an oxygen-rich denitrogenation catalyst having a catalyst material comprising platinum, palladium and/or rhodium. In an advantageous embodiment in which ammonia or an aqueous urea solution is used as the reducing agent for the first nitrogen oxide reduction catalyst 2, it is preferably designed as a vanadium-containing catalyst.

When the internal combustion engine is running, the second nox reduction catalyst 8 is supplied with reducing agent in a controlled quantity via the second reducing agent adding device 12 if the second nox reduction catalyst 8 is ready for operation. If no nitrogen oxide reduction or only an incomplete nitrogen oxide reduction is possible by means of the second nitrogen oxide reduction catalyst 8, the first nitrogen oxide reduction catalyst 2 is supplied with reducing agent via the first reducing agent adding device 11 as long as the first nitrogen oxide reduction catalyst 2 is ready for operation. In this case, the use of a predetermined heat exchanger 3 in combination with the energy recovery device 6 makes it possible to keep the first nitrogen oxide reduction catalyst 2 in its operating temperature range only when the exhaust gas flowing out of the second nitrogen oxide reduction catalyst 8 has a relatively high temperature. In this case, the regulating means 9, 9' are actuated in such a way that, by dividing the exhaust gas flow into the first exhaust gas partial flow 5 and the second exhaust gas partial flow 10, a temperature in the operating temperature range, but preferably below 300 ℃, is obtained for the exhaust gas flowing into the first nitrogen oxide reduction catalyst 2. The heat extracted from the first exhaust gas partial flow 5 can also be additionally regulated by the operation of the energy recovery device 6 or by the control of the working fluid circuit.

In fig. 2-4, the exhaust system is shown similarly to fig. 1, wherein corresponding components are respectively designated by the same reference numerals if they correspond to parts of fig. 1. Due to the similarity with the exhaust system shown in fig. 1, only the differences associated therewith will be described below.

In the exhaust system 1 shown in fig. 2, in contrast to the exhaust system 1 of fig. 1, the second nox-reducing catalyst 8 is arranged in terms of flow technology downstream of the merging of the first exhaust gas partial flow 5 and the second exhaust gas partial flow 10 and upstream of the first nox-reducing catalyst 2. The exhaust gases fed to the second nitrogen oxide reduction catalyst 8 can therefore already be cooled as required. Correspondingly, it is advantageous to use a low-temperature embodiment, for example an oxygen-rich denitrification catalyst, as the second nitrogen oxide reduction catalyst 8. The operation of the exhaust system 1 shown in fig. 2 is carried out analogously to the above-described procedure, wherein the temperature of the second nox-reducing catalyst 8, which is arranged further upstream in terms of flow technology, can already be regulated or limited. Thus, due to the temperature gradient that usually occurs along the exhaust gas flow path, an undesirably high temperature exceeding the first nitrogen oxide reduction catalyst 2 can be avoided more reliably.

The exhaust system 1 shown in fig. 3 differs from the exhaust system shown in fig. 1 only in that the external supply of reducing agent to the second nitrogen oxide reduction catalyst 8 is omitted, i.e. the second reducing agent adding device 12 is omitted. In this case, the second nitrogen oxide reduction catalyst receives a reducing agent for catalytically reducing nitrogen oxides flowing through the internal combustion engine. For this reason, when the internal combustion engine is operated on hydrogen or ammonia fuel, in addition to the conventional torque-generating fuel injection or injection, fuel is injected or injected later towards the end of the working cycle. This preferably does not generate torque and is preferably carried out in the range of crankshaft angles of 120 ° to 180 ° after top dead center. The fuel which is later fed into the combustion chamber is discharged as a part of the total exhaust gas using the exhaust stroke and is then further led to the exhaust system 1. At the second nitrogen oxide reduction catalyst 8, the proportion of fuel contained in the exhaust gas, which acts as a reducing agent, causes a catalytic reduction of the nitrogen oxides also contained in the fuel. A further mode of operation of the exhaust system 1 is in particular in relation to the setting of the exhaust gas temperature by means of the heat exchanger 3 and the energy recovery device 6 in a manner similar to the mode of operation explained in connection with fig. 1.

Similarly, the exhaust system 1 shown in fig. 4 also differs from the exhaust system shown in fig. 2 only in that the supply of external reducing agent to the second nitrogen oxide reduction catalyst 8 is omitted. As described above in connection with fig. 3, in the embodiment according to fig. 4, the second nitrogen oxide reduction catalyst 8 receives a reducing agent for reducing the nitrogen oxides flowing through the internal combustion engine. Otherwise, the operation of the respective exhaust system 1 is carried out in a manner similar to the operation of fig. 2.

It is clear that the exhaust system 1 according to fig. 1-4 comprises sensors, not shown separately, for temperature and exhaust gas composition, which are required for measuring the operating state of the exhaust system 1 or its component parts and for generating corresponding control signals for controlling the operation of the exhaust system 1.

List of reference numerals

1. Exhaust system

2. First nitrogen oxide reduction catalyst

3. Heat exchanger

4. Bypass line

5. First exhaust gas split

6. Energy recovery device

7. Working fluid circulation

8. Second nitrogen oxide reduction catalyst

9. Adjusting mechanism

9' adjusting mechanism

10. Second exhaust gas partial flow

11. First reducing agent adding device

12. Second reducing agent adding device

13. Waste gas line

13' exhaust line section

Claims

1. A motor vehicle having an internal combustion engine driven by a carbon-free fuel and having an exhaust system (1) connected thereto, which has in particular a first nitrogen oxide reduction catalyst (2), characterized in that the exhaust system (1) also has a heat exchanger (3), wherein the exhaust gases discharged by the internal combustion engine can be conducted through the heat exchanger (3) in adjustable proportions before being fed to the nitrogen oxide reduction catalyst (2).

2. A motor vehicle according to claim 1, characterized in that the exhaust system (1) has an energy recovery device (6) which can utilize the thermal energy extracted from the exhaust gases conducted through the heat exchanger (3) for the production of useful energy.

3. Motor vehicle according to claim 1 or 2, characterized in that a second nitrogen oxide reduction catalyst (8) is provided, which is arranged upstream of the first nitrogen oxide reduction catalyst (2) in terms of flow technology.

4. A motor vehicle as claimed in claim 3, characterized in that the heat exchanger (3) is arranged in terms of flow upstream of the first nitrogen oxide reduction catalyst (2) and downstream of the second nitrogen oxide reduction catalyst (8).

5. A motor vehicle according to any of claims 1 to 4, characterized in that the reducing agent for reducing nitrogen oxides contained in the exhaust gas at the first and/or second nitrogen oxide reduction catalyst (2;8) is the fuel for operating the internal combustion engine.

6. A motor vehicle according to claim 5, characterized in that the internal combustion engine is designed to supply reducing agent to the exhaust system (1) via the combustion chamber of the internal combustion engine.

7. A motor vehicle as claimed in any one of claims 1 to 6, characterized in that the fuel used for operating the internal combustion engine is hydrogen.

8. A motor vehicle according to any one of claims 3 to 7, characterized in that the second nitrogen oxide reduction catalyst (8) is a lean nox catalyst which is capable of catalytically reducing nitrogen oxides with hydrogen as a reducing agent.

9. Motor vehicle according to one of claims 1 to 8, characterized in that a metering device (11) for introducing reducing agent containing ammonia into the exhaust gas is provided on the input side of the first nitrogen oxide reduction catalyst (2).

10. Motor vehicle according to one of claims 1 to 9, characterized in that the first nitrogen oxide reduction catalyst (2) contains vanadium.

11. A motor vehicle as claimed in any one of claims 1 to 9 wherein the fuel used to operate the internal combustion engine is ammonia.

12. A method for operating a motor vehicle as claimed in one of claims 1 to 11, characterized in that the proportion of the exhaust gas conducted through the heat exchanger (3) is selected in the operating state of the heat exchanger (3) and/or of the energy recovery device (6) in such a way that the temperature of the first nitrogen oxide reduction catalyst (2) is at least predominantly kept below a predeterminable upper temperature limit.

13. The operating method according to claim 12, characterized in that the first and/or the second nitrogen oxide reduction catalyst (2;8) is supplied with a reducing agent from an external storage tank for the reduction of nitrogen oxides contained in the exhaust gas.

14. An operating method according to claim 12 or 13, characterised in that fuel is supplied to at least one combustion chamber of the internal combustion engine at a later point in the operating cycle when fuel is discharged from the internal combustion engine in an at least virtually chemically unchanged manner and is fed as a reducing agent to the second nox-reducing catalyst (8) for reducing the nox contained in the exhaust gas.