WO2022000012A1

WO2022000012A1 - Motor vehicle having an internal combustion engine which is operated with carbon-free fuel and which has an exhaust gas system connected thereto

Info

Publication number: WO2022000012A1
Application number: PCT/AT2021/060233
Authority: WO
Inventors: Bernhard Raser; Josef MACHERHAMMER; Klaus HADL
Original assignee: Avl List Gmbh
Priority date: 2020-07-03
Filing date: 2021-07-02
Publication date: 2022-01-06
Also published as: DE112021001728A5; AT524011B1; AT524011A1; CN115667683A

Abstract

The invention relates to a motor vehicle having an internal combustion engine which is operated with carbon-free fuel, such as hydrogen or ammonia, and an exhaust gas system (1) connected thereto, said exhaust gas system having at least one first nitrogen oxide reduction catalytic converter (2), which preferably contains vanadium. According to the invention, the exhaust gas system (1) further has a heat exchanger (3), which can be used to recover energy, wherein an adjustable portion of the exhaust gas emitted from the internal combustion engine can be conveyed through the heat exchanger (3) before being supplied to the nitrogen oxide reduction catalytic converter (2). Two nitrogen oxide reduction catalytic converters can also be arranged one behind the other, wherein the reduction medium can be the fuel. The operating method according to the invention teaches that the portion of the exhaust gas conveyed through the heat exchanger (3) in connection with an operation of the heat exchanger (3) and/or an energy recovery device (6) is chosen such that a temperature of the first nitrogen oxide reduction catalytic converter (2) is kept at least predominantly below a definable upper temperature limit.

Description

Motor vehicle with an internal combustion engine operated with carbon-free fuel with an exhaust system connected to it

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

A motor vehicle of the generic type is known from DE 102007021 827 A1.

The corresponding internal combustion engine is operated with hydrogen as a carbon-free fuel. On the outlet side of the internal combustion engine, hydrogen extracted from a storage tank is fed to the exhaust system. A catalytic converter located downstream in the exhaust system removes nitrogen oxides contained in the exhaust gas by catalyzed reduction with the supplied hydrogen.

The object of the invention is to provide a motor vehicle with an internal combustion engine operated with carbon-free fuel which, in contrast, enables improved energy utilization and exhaust gas purification.

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

The motor vehicle according to the invention has an internal combustion engine which is operated with a carbon-free fuel. An exhaust system connected to the internal combustion engine has a particularly first nitrogen oxide reduction catalyst and also a heat exchanger, with exhaust gas emitted by the internal combustion engine being able to be passed through the heat exchanger with an adjustable proportion before being fed to the nitrogen oxide reduction catalyst. The internal combustion engine is preferably designed as a spark-ignited reciprocating piston engine.

Due to the heat exchanger provided according to the invention, exhaust gas from the internal combustion engine can be tempered, in particular cooled, before it reaches the nitrogen oxide reduction catalytic converter. While a first specifiable portion of the total, The exhaust gas given off by the internal combustion engine is passed through the heat exchanger, the remaining second portion reaches the nitrogen oxide reduction catalytic converter, bypassing the heat exchanger. The specifiable and adjustable first portion of the total exhaust gas emitted by the internal combustion engine can assume values between 0% and 100%. The part of the exhaust gas flow passed through the heat exchanger is brought together again with the exhaust gas flow bypassing the heat exchanger and a common, temperature-controlled total exhaust gas flow is fed to the nitrogen oxide reduction catalytic converter. In this way, the nitrogen oxide reduction catalyst can be kept at least predominantly in its operating temperature range. Preferably, heat is withdrawn from the exhaust gas passed through the heat exchanger and the total exhaust gas flow fed to the nitrogen oxide reduction catalytic converter is therefore cooled. Thus, an effectiveness of the nitrogen oxide reduction catalyst can be achieved even with high output of the internal combustion engine with emission of exhaust gas with correspondingly high temperatures lying above the operating temperature range of the nitrogen oxide reduction catalyst. This enables an effective nitrogen oxide reduction in an extended engine operating range. Furthermore, the use of catalyst materials with a relatively low operating temperature range is also made possible. The upstream bypassable heat exchanger can also prevent the nitrogen oxide reduction catalyst from reaching or exceeding a degradation temperature limit. This also improves the durability of the nitrogen oxide reduction catalytic converter.

In an embodiment of the invention, the exhaust system has an energy recovery device that can generate useful energy from the exhaust gas passed through the heat exchanger, extracted thermal energy. The energy recovery device can be designed, for example, as a thermoelectric generator that generates useful electrical energy. A device can also be provided which generates useful mechanical energy through a thermodynamic cycle, for example a Rankine process. For this purpose, heat is extracted from the exhaust gas in the heat exchanger and fed to an operating medium. The operating medium is directed to the energy recovery device, where the absorbed heat is partially converted into useful mechanical energy. Electrical or mechanical energy generated in the energy recovery device can be used in auxiliary equipment in the motor vehicle, for example. This improves the overall energy utilization of the motor vehicle.

In a further embodiment of the invention, a second nitrogen oxide reduction catalytic converter is provided, which is connected upstream of the first nitrogen oxide reduction catalytic converter in terms of flow. This makes it possible for at least one of the two nitrogen oxide reduction catalysts to be at operating temperature when there is a temperature gradient typically present along the exhaust gas path. In particular, as a result of the heat exchanger, it is also possible to influence the temperature gradient in this sense. The operating range of the internal combustion engine in which nitrogen oxide can be effectively removed from the exhaust gas is thus greatly expanded. It can be advantageous to provide catalytic converter materials with different operating temperature ranges for the first and second nitrogen oxide reduction catalytic converters. For example, the second nitrogen oxide reduction catalyst can have a lower operating temperature range than the first nitrogen oxide reduction catalyst. As a result, the second nitrogen oxide reduction catalyst is quickly available for nitrogen oxide reduction after a cold start of the internal combustion engine. If the second nitrogen oxide reduction catalytic converter warms up to a temperature above its operating temperature range during a warm-up, then the first nitrogen oxide reduction catalytic converter has meanwhile reached operating temperature 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. For example, the second nitrogen oxide reduction catalytic converter can have a smaller volume than the first nitrogen oxide reduction catalytic converter. This enables the second nitrogen oxide reduction catalytic converter to be heated up to operating temperature particularly quickly. The second nitrogen oxide reduction catalyst can, for example, have less than 80%, 60% or 40% of the volume of the first nitrogen oxide reduction catalyst or an even smaller volume. Furthermore, it can be provided that the second nitrogen oxide reduction catalytic converter is arranged close to the engine, for example in an engine compartment of the motor vehicle, while the first nitrogen oxide reduction catalytic converter is arranged remote from the engine in an underbody area of the motor vehicle. In a further embodiment of the invention, the heat exchanger is fluidically connected upstream of the first nitrogen oxide reduction catalyst and downstream of the second nitrogen oxide reduction catalyst. In terms of flow technology, the heat exchanger is thus arranged between the second nitrogen oxide reduction catalytic converter arranged further upstream and the first nitrogen oxide reduction catalytic converter arranged further downstream. This has the advantage that when the second nitrogen oxide reduction catalytic converter is already above its operating temperature, if the first nitrogen oxide reduction catalytic converter is threatened with excessive further heating, its effectiveness can still be maintained by operating the heat exchanger accordingly.

In a further advantageous embodiment of the invention, the reducing agent for reducing nitrogen oxides contained in the exhaust gas on the first and / or on the second nitrogen oxide reduction catalytic converter is the fuel used to operate the internal combustion engine. This means that a storage container for a separate reducing agent can be omitted or at least made smaller. 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-working fuel injection valve is preferably provided for at least one cylinder of the internal combustion engine, via which the fuel can be introduced directly into the corresponding combustion chamber. This saves a separate supply device for supplying the reducing agent to the exhaust gas. In addition, a particularly homogeneous distribution of the reducing agent in the exhaust gas is made possible in this way. The fuel used as a reducing agent is preferably introduced into the combustion chamber or combustion chambers by a post-injection or injection of fuel carried out so late in the working cycle that it is expelled chemically at least approximately unchanged with the exhaust cycle. This preferably takes place in a crank angle range of 120 ° to 180 ° after top dead center.

In a further embodiment of the invention, the fuel for operating the internal combustion engine is hydrogen. This avoids climate-damaging carbon dioxide emissions. Emissions of soot particles, hydrocarbons and carbon monoxide are also avoided. In one, preferably at least predominantly envisaged lean operation of the internal combustion engine, only nitrogen oxides are produced as pollutants during fuel combustion, but these can be removed from the exhaust gas by a nitrogen oxide reduction catalytic converter.

In a further embodiment of the invention it is provided that the second nitrogen oxide reduction catalyst is a lean Denox catalyst which can catalyze a reduction of nitrogen oxides with hydrogen as the reducing agent. This embodiment is particularly advantageous in connection with an internal combustion engine designed as a hydrogen engine, since the fuel used to operate the engine can then also be used as a reducing agent for reducing nitrogen oxides on the second nitrogen oxide reduction catalytic converter. For this purpose, the hydrogen can be added to the exhaust gas either via a combustion chamber of the internal combustion engine or via an engine-external metering device. The lean Denox catalyst preferably has a catalytic coating which contains noble metals, in particular of the platinum group, such as platinum and / or rhodium. In general, a catalytic coating is provided which can catalyze a nitrogen oxide reduction with hydrogen as a reducing agent even when there is an excess of oxygen in the exhaust gas. In particular in the case of a lean Denox catalyst containing platinum and / or rhodium and / or palladium, the operating temperature range is relatively low. A lower temperature limit of the operating range can be around 80 ° C. An upper temperature limit of the operating range can be around 250 ° C. The lean Denox catalytic converter is ready for operation very quickly after a cold start of the internal combustion engine.

In a further embodiment of the invention, a metering device for introducing an ammonia-containing reducing agent into the exhaust gas is provided on the inlet side of the first nitrogen oxide reduction catalyst. Ammonia can be contained in the reducing agent in free or bound form. A metering device for introducing an aqueous urea solution into the exhaust gas is preferably provided. 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 seen in the flow direction after the merging point of exhaust gas passed through the heat exchanger and bypassing the heat exchanger. When using an ammonia-containing reducing agent, the first nitrogen oxide reduction catalyst can be designed as a classic SCR catalyst, for example as a copper or iron-containing zeolitic catalyst. In particular, it is provided in a further embodiment of the invention that the first nitrogen oxide reduction catalytic converter contains vanadium. Vanadium is preferably present as an oxide, in particular as vanadium pentoxide. Further constituents effective with regard to promoting the selective nitrogen oxide reduction ability, such as oxides of tungsten, molybdenum and / or titanium, can of course also be provided. From an application point of view, the operating temperature range of the intended nitrogen oxide reduction catalytic converter is typically in a medium range from approximately 180 ° C.-200 ° C. to approximately 420 ° C.-480 ° C. Considerable vanadium-containing nitrogen oxide reduction catalysts can, however, have a comparatively low temperature limit with regard to stability or degradation. For this reason, it is preferably provided that the temperature of the exhaust gas entering the nitrogen oxide reduction catalytic converter is kept at least predominantly below 350 ° C., in particular below 300 ° C., by cooling by means of the heat exchanger.

In a further embodiment of the invention, the fuel for operating the internal combustion engine is ammonia. This also avoids harmful carbon emissions. Furthermore, the fuel for operating the internal combustion engine can advantageously also be used as a reducing agent for reducing nitrogen oxides in the exhaust system. This eliminates the need for a separate reducing agent storage tank. Ammonia as a nitrogen oxide reducing agent can be supplied to a nitrogen oxide reduction catalyst of the exhaust system by a motor, specifically by being introduced into a combustion chamber of the internal combustion engine late in the work cycle. This preferably takes place in a crank angle range of 120 ° to 180 ° after top dead center. A supply to the exhaust gas from outside the engine can, however, additionally or alternatively take place by means of a separate metering device.

The operating method according to the invention for a motor vehicle carried out as described above provides that the proportion of the exhaust gas passed through the heat exchanger in connection with operation of the heat exchanger and / or the energy recovery device is selected such that a temperature of the first nitrogen oxide reduction catalytic converter is at least predominantly below one specifiable upper temperature limit is maintained. The predeterminable temperature limit can be selected as a function of an upper operating temperature limit or a temperature stability limit of the nitrogen oxide reduction catalytic converter. An upper temperature limit of 350 ° C. to 300 ° C., in particular 300 ° C., is preferred. If the exhaust gas, which reaches a branching point for dividing the exhaust gas flow into a portion flowing through the heat exchanger and a portion bypassing the heat exchanger, has a temperature that exceeds the upper temperature limit, a certain portion of the exhaust gas is passed through the heat exchanger by means of an adjusting element. A division of the exhaust gas flow into a portion passed through the heat exchanger and a portion bypassing the heat exchanger preferably takes place as a function of the exhaust gas temperature and the intended upper temperature limit. In addition, the cooling effect of the heat exchanger can be adjusted accordingly by influencing the amount of operating medium flowing through the heat exchanger and absorbing heat. In particular, the cooling effect can be increased if necessary by increasing the flow of operating medium. As a result, it can be reliably avoided that exhaust gas entering the second nitrogen oxide reduction catalytic converter exceeds the upper temperature limit. A high effectiveness of the reducing agent catalyst can thus be achieved and its degradation can be avoided.

In an embodiment of the operating method according to the invention, the first and / or the second nitrogen oxide reduction catalytic converter is supplied with reducing agent for reducing nitrogen oxides contained in the exhaust gas from an external storage container. A reducing agent supply from an external storage container can be provided in particular for the first nitrogen oxide reduction catalytic converter. If a second nitrogen oxide reduction catalytic converter is present, a reducing agent supply from an external storage container can additionally or alternatively be provided for this. A particularly suitable reducing agent for reducing nitrogen oxides on the first and / or on the second nitrogen oxide reduction catalytic converter is aqueous urea solution. However, ammonia, in particular gaseous ammonia, or hydrogen can also be provided as the 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 such a late point in time of a working cycle that the fuel is expelled from the internal combustion engine at least approximately chemically unchanged and supplied to the second nitrogen oxide reduction catalyst as a reducing agent to reduce nitrogen oxides contained in the exhaust gas. In this case, the fuel used to operate the internal combustion engine also acts as a reducing agent for reducing nitrogen oxides, at least on the second nitrogen oxide reduction catalytic converter. A late introduction of gaseous ammonia or hydrogen as fuel into a combustion chamber or into several combustion chambers is preferably carried out in a crank angle range of 120 ° to 180 ° after top dead center. This ensures that fuel introduced into the combustion chamber no longer or only insignificantly takes part in the combustion and is pushed out of the combustion chamber into the exhaust system with practically no chemical changes.

The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of the figures and / or shown alone in the figures can be used not only in the specified combination, but also in other combinations or on their own, without the frame to leave the invention. Above and other features and advantages of the invention emerge from the following description of preferred, non-limiting embodiment examples of the invention with reference to the accompanying drawings. Show:

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

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

3 shows a schematic representation of a third advantageous embodiment of an exhaust system of the motor vehicle according to the invention, and 4 shows a schematic representation of a first advantageous embodiment of an exhaust system of the motor vehicle according to the invention,

Fig. 1 shows only schematically and greatly simplified a first example of an advantageous embodiment of an exhaust system 1 connected to an internal combustion engine of a motor vehicle. The motor vehicle is not shown here. The internal combustion engine, also not shown, is designed as an internal combustion engine operated with carbon-free fuel. A design as a hydrogen engine is preferred. However, it can also be designed as an ammonia motor. In any case, at least predominantly lean operation, i.e. operation with excess air, is provided for the internal combustion engine.

Exhaust gas emitted by the internal combustion engine enters the exhaust system 1 via an exhaust line 13. Corresponding to the exhaust gas flow direction marked with an open arrow, the exhaust gas initially flows through a catalytic converter, referred to here as a second nitrogen oxide reduction catalytic converter 8. Downstream of the second nitrogen oxide reduction catalytic converter 8, a bypass line 4 is provided which bypasses an exhaust line section 13 ′. A first partial exhaust gas flow 5 can be passed through the bypass line 4 via actuating means 9, 9 ', while the remaining portion of the total exhaust gas flow is routed as a second partial exhaust gas flow 10 through the exhaust gas pipe portion 13', which is fluidically parallel to the bypass line 4. At the adjusting means 9 ', the bypass line 4 opens again into the exhaust gas line 13 and the partial exhaust gas flows 5, 10 are brought together again. The total exhaust gas flow formed again by the combined exhaust gas partial flows 5, 10 is then passed through a catalytic converter, referred to here as the first nitrogen oxide reduction catalytic converter 2.

In the bypass line 4, a heat exchanger 3 is arranged, which is used to transfer heat from the first exhaust gas substream 5 to a working fluid. The working fluid is conducted in a working fluid circuit 7 on the one hand through the heat exchanger 3 and on the other hand through an energy recovery device 6. The energy recovery device 6 can convert heat supplied with the working fluid into useful energy. The energy recovery system preferably works device 6 as a thermodynamic cycle or as part of a thermodynamic cycle and can generate useful mechanical energy from the heat extracted from the first exhaust gas partial flow 5 by means of the working fluid. As a result, the working fluid is cooled and is fed back to the heat exchanger 3, where it can again absorb heat from the first partial exhaust gas flow 5. Funding means, not shown in more detail, are preferably provided here, which can keep the working fluid circuit 7 running and adjust its strength. As a result, a control of the heat flow taken from the exhaust gas partial flow 5 can be controlled. Furthermore, a control of the heat extracted from the exhaust gas is provided by actuating the adjusting means 9, 9 ', via which the proportion of the first exhaust gas partial flow 5 in the total exhaust gas flow can be adjusted as required. One of the two adjusting means 9, 9 'can also be omitted.

Upstream of the first nitrogen oxide reduction catalytic converter 2 and downstream of the junction of the first exhaust gas partial flow 5 and second exhaust gas partial flow 10, a first reducing agent addition device 11 is provided, which can supply a reducing agent for reducing nitrogen oxides to the exhaust gas as required. Similarly, upstream of the second nitrogen oxide reduction catalytic converter 8, a second reducing agent adding device 12 is provided, with which a reducing agent for reducing nitrogen oxides can also be added to the exhaust gas. In the present case, the respective reducing agent is taken from a storage container (not shown in greater detail). Hydrogen, ammonia or aqueous urea solution can be used as reducing agents. The same reducing agent can be provided for the first nitrogen oxide reduction catalytic converter 2 and the second nitrogen oxide reduction catalytic converter 8. However, different reducing agents can also be used. For example, urea solution can be provided as a reducing agent for the first nitrogen oxide reduction catalytic converter 2 and hydrogen or ammonia for the second nitrogen oxide reduction catalytic converter 8 or vice versa. The use of hydrogen as a reducing agent is provided in particular when the internal combustion engine is designed as a hydrogen engine. The use of ammonia as a reducing agent is provided in particular when the internal combustion engine is designed as an ammonia engine. In both cases, the fuel used for the internal combustion engine can be used as a reducing agent in an advantageous manner. The catalyst material used for the first nitrogen oxide reduction catalyst 2 and for the second nitrogen oxide reduction catalyst 8 is advantageously selected depending on the type of reducing agent used in each case. A low-temperature version for the second nitrogen oxide reduction catalytic converter 8 is particularly preferred, since this is preferably installed close to the engine. In this way, it is ready for use particularly quickly after a cold start. A preferred operating temperature range is between about 100.degree. C. and about 250.degree. This can be achieved when using hydrogen as the reducing agent by means of a lean Denox catalyst which, for example, has a catalyst material containing platinum, palladium and / or rhodium. In an advantageous embodiment with the use of ammonia or aqueous urea solution as the reducing agent for the first nitrogen oxide reduction catalytic converter 2, this is preferably designed as a vanadium-containing catalytic converter.

During operation of the internal combustion engine, the second nitrogen oxide reduction catalyst 8 reducing agent is supplied in a regulated manner via the second reducing agent addition device 12, provided that the second nitrogen oxide reduction catalyst 8 is ready for operation. If no nitrogen oxide reduction or only an incomplete nitrogen oxide reduction can take place through the second nitrogen oxide reduction catalytic converter 8, reducing agent is fed to the first nitrogen oxide reduction catalytic converter 2 via the first reducing agent addition device 11, provided that the first nitrogen oxide reduction catalytic converter 2 is ready for operation. The provided heat exchanger 3 in conjunction with the energy recovery device 6 enables the first nitrogen oxide reduction catalyst 2 to be kept in its operating temperature range even when the exhaust gas flowing out of the second nitrogen oxide reduction catalyst 8 is at a higher temperature. In such a case, the actuating means 9, 9 'are actuated in such a way that the exhaust gas flowing into the first nitrogen oxide reduction catalyst 2 is a temperature within the operating temperature range, but preferably below, by dividing the exhaust gas flow into the first exhaust gas partial flow 5 and the second exhaust gas partial flow 10 of 300 ° C.

The amount of heat withdrawn from the first exhaust gas partial flow 5 can additionally be adjusted by operating the energy recovery device 6 or by controlling the working fluid circuit. In FIGS. 2 to 4, exhaust systems similar to that of FIG. 1 are shown, the corresponding components, insofar as they correspond to the parts of FIG. 1, being identified by the same reference numerals. Due to the similarities with the exhaust system shown in Fig. 1, only differences in this regard will be discussed below.

In the exhaust system 1 shown in FIG. 2, in contrast to the exhaust system 1 of FIG. In this way, the exhaust gas fed to the second nitrogen oxide reduction catalytic converter 8 can already be cooled if necessary. Accordingly, it is advantageous to use a low-temperature version, for example a lean Denox catalyst, as the second nitrogen oxide reduction catalyst 8. The exhaust system 1 shown in FIG. 2 is operated in a manner analogous to the procedure described above, it being possible in the present case to set or limit the temperature of the second nitrogen oxide reduction catalytic converter 8, which is arranged further upstream in terms of flow. Due to a temperature gradient typically occurring along the exhaust gas flow path, exceeding an undesirably high temperature of the first nitrogen oxide reduction catalytic converter 2 can therefore be avoided with even greater certainty.

The exhaust system 1 shown in Fig. 3 differs from the one shown in Fig. 1 Darge presented only by eliminating the external reducing agent supply to the second nitrogen oxide reduction catalyst 8, ie by eliminating the second reducing agent supply device 12. In this case, the second nitrogen oxide reduction catalyst receives reducing agent for catalytic reduction of Nitrogen oxides from the combustion engine. For this purpose, when the internal combustion engine is operated with hydrogen or ammonia as fuel, in addition to a regular, torque-generating fuel injection or injection, there is a late injection or injection of fuel towards the end of the work cycle. This is preferably not torque effective and preferably takes place in a crank angle range between 120 ° and 180 ° after top dead center. The late fuel introduced into the combustion chamber is expelled as part of the total exhaust gas with the expulsion cycle and then passed on to exhaust system 1. On the second nitrogen oxide reduction catalytic converter 8, the fuel component contained in the exhaust gas and functioning as a reducing agent causes a catalytic reduction of the nitrogen oxides also contained in the fuel. The further operation of the exhaust system 1, in particular with regard to an exhaust gas temperature setting by means of the heat exchanger 3 and the energy recovery device 6, takes place analogously to the mode of operation explained in connection with FIG. 1.

Similarly, the exhaust system 1 shown in FIG. 4 differs from the one shown in FIG. 2, likewise only in that the external reducing agent supply to the second nitrogen oxide reduction catalytic converter 8 is omitted. In the embodiment according to FIG. 4, the second nitrogen oxide reduction catalytic converter 8 receives reducing agent for reducing nitrogen oxides the internal combustion engine as above in connection with Fig.

3 described. Otherwise, the corresponding exhaust system 1 is operated analogously to that of FIG. 2.

It goes without saying that the exhaust system 1 according to FIGS. 1 to 4 includes sensors for temperature and exhaust gas components, not shown separately, which are necessary to detect the operating state of the exhaust system 1 or its components and corresponding control signals for controlling the operation of the exhaust gas systems 1 to generate.

LIST OF REFERENCE NUMERALS Exhaust system First nitrogen oxide reduction catalytic converter Heat exchanger bypass line First exhaust gas partial flow Energy recovery device Working fluid circuit Second nitrogen oxide reduction catalytic converter Actuating means 0 Second exhaust gas partial flow 1 First reducing agent adding device2 Second reducing agent adding device3 Exhaust gas line 3 'Exhaust gas line section

Claims

1. Motor vehicle with an internal combustion engine operated with carbon-free fuel with an exhaust system (1) connected to it, which has an in particular first nitrogen oxide reduction catalyst (2), characterized in that the exhaust system (1) also has a heat exchanger (3), with exhaust gas emitted by the internal combustion engine can be passed through the heat exchanger (3) with an adjustable proportion before being fed to the nitrogen oxide reduction catalytic converter (2).

2. Motor vehicle according to claim 1, characterized in that the exhaust system (1) has an energy recovery device (6) which can generate useful energy from the heat energy extracted from the exhaust gas passed through the heat exchanger (3).

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

4. Motor vehicle according to claim 3, characterized in that the heat exchanger (3) fluidically connected upstream of the first nitrogen oxide reduction catalyst (2) and downstream of the second nitrogen oxide reduction catalyst (8).

5. Motor vehicle according to one of claims 1 to 4, characterized in that the reducing agent for reducing nitrogen oxides contained in the exhaust gas on the first and / or on the second nitrogen oxide reduction catalyst (2, 8) is the fuel used to operate the internal combustion engine.

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

7. Motor vehicle according to one of claims 1 to 6, characterized in that the fuel for operating the internal combustion engine is hydrogen.

8. Motor vehicle according to one of claims 3 to 7, characterized in that the second nitrogen oxide reduction catalyst (8) is a lean Denox catalyst which can catalyze a reduction of nitrogen oxides with hydrogen as a reducing agent.

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

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

11. Motor vehicle according to one of claims 1 to 9, characterized in that the fuel for operating the internal combustion engine is ammonia.

12. Operating method for a motor vehicle according to one of claims 1 to 11, characterized in that the proportion of the exhaust gas passed through the heat exchanger (3) in connection with an operation of the heat exchanger (3) and / or the energy recovery device (6) is selected is that a temperature of the first nitrogen oxide reduction catalytic converter (2) is kept at least predominantly below a predeterminable upper temperature limit.

13. Operating method according to claim 12, characterized in that the first and / or the second nitrogen oxide reduction catalyst (2, 8) reducing agent for reducing nitrogen oxides contained in the exhaust gas is fed from an external storage container.

14. Operating method according to claim 12 or 13, characterized in that fuel is supplied to at least one combustion chamber of the internal combustion engine at such a late point in time of a working cycle that the fuel is expelled from the internal combustion engine at least approximately chemically unchanged and is supplied to the second nitrogen oxide reduction catalyst (8) as a reducing agent Reduction of nitrogen oxides contained in the exhaust gas is supplied.