Method for removing nitrogen oxides from an exhaust gas of a hydrogen engine, and hydrogen engine

Abstract

The invention relates to a method for removing nitrogen oxides from an exhaust gas of an internal combustion engine (1), operated using hydrogen as fuel, by means of an exhaust gas purification system (2) which has an SCR catalyst (4) and is connected to the internal combustion engine (1), wherein a metered amount of an ammonia-containing reducing agent to be supplied, at the inlet of the SCR catalyst (4), to the exhaust gas is determined. According to the invention, an air-fuel ratio λ of an air-fuel mixture combusted in the internal combustion engine (1) is determined and, if there are predefinable or predefined operating conditions of the internal combustion engine (1), the metered amount is determined on the basis of the determined air-fuel ratio λ. The invention further relates to a hydrogen engine (1) having a connected exhaust gas purification system (2) which has an SCR catalyst (4) and a metering unit (5) for supplying, on the inlet side of the SCR catalyst (4), an ammonia-containing reducing agent into an exhaust gas emitted by the hydrogen engine (1), and having a control unit (10) for determining a metered amount of the reducing agent for reducing nitrogen oxides present in the exhaust gas of the hydrogen engine (1).

Description

[0001] Method for removing nitrogen oxides from an exhaust gas of a hydrogen engine and hydrogen engine

[0002] The invention relates to a method for removing nitrogen oxides from an exhaust gas of an internal combustion engine operated with hydrogen as fuel, having the features of the preamble of claim 1, and to a hydrogen engine having the features of the preamble of claim 10.

[0003] WO 2022 / 000012 A1 discloses a method for removing nitrogen oxides from the exhaust gas of an internal combustion engine fueled by hydrogen, in which method an ammonia-containing reducing agent is supplied to an SCR catalyst to reduce the nitrogen oxides. For corresponding SCR methods for reducing nitrogen oxides, it has been proposed on various occasions to determine a metered amount of the reducing agent based on the signal from a nitrogen oxide sensor arranged upstream of the SCR catalyst. However, the inventors of the present invention have recognized that in internal combustion engines fueled by hydrogen, this may result in insufficient nitrogen oxide reduction under certain operating conditions, such as a sudden demand for increased engine load.

[0004] The object of the present invention is therefore to provide a method and a hydrogen engine in which these problems are at least largely avoided. This object is achieved by a method having the features of claim 1 and by a hydrogen engine having the features of claim 10.

[0005] In the method according to the invention for removing nitrogen oxides from the exhaust gas of an internal combustion engine powered by hydrogen as fuel, with an exhaust gas purification system having an SCR catalyst and connected to the internal combustion engine, a metered amount of an ammonia-containing reducing agent to be supplied to the exhaust gas at the inlet of the SCR catalyst is determined. According to the invention, an air-fuel ratio X of an air-fuel mixture combusted in the internal combustion engine is determined, and if predeterminable or predetermined operating conditions of the internal combustion engine exist, the metered amount is determined based on the determined air-fuel ratio X. A metering unit is continuously controlled so that respectively determined metered amounts of the reducing agent are supplied to the exhaust gas. The reducing agent is supplied at the inlet of the SCR catalyst into an exhaust line of the exhaust gas purification system.In this context, the inlet-side supply refers to a supply located upstream of the SCR catalyst located in the exhaust line and at a short distance from the catalyst. The distance to the exhaust gas inlet side of the SCR catalyst can range from a few centimeters to a few decimetres.

[0006] The air-fuel ratio or combustion air ratio X is the ratio of the quantities of air and fuel supplied to the combustion engine relative to the stoichiometric ratio which theoretically just allows complete combustion of the fuel. A lean air-fuel ratio therefore has an X value of > 1.0, a rich air-fuel ratio an X value of < 1.0. An X value of 1.0 indicates the stoichiometric air-fuel ratio. Here and in the following, a quantity is understood to mean a quantity related to a unit of time. The ammonia-containing reducing agent is a reagent which contains ammonia in free or chemically bound form. Possible reducing agents are pure ammonia or urea, preferably in the form of an aqueous solution.In the case of a reducing agent containing ammonia in chemically bound form, ammonia is released from this in the hot exhaust gas by hydrolysis and / or thermolysis, which acts as the actual reducing agent in a chemical reduction of the nitrogen oxides in a catalytically assisted selective reduction reaction on the SC R catalyst.

[0007] An internal combustion engine powered by hydrogen as fuel can also be referred to as a hydrogen engine. This engine preferably operates according to the Otto cycle principle. It is preferable to operate the internal combustion engine exclusively with hydrogen as fuel, meaning no other fuels are used.

[0008] The inventors have recognized that under certain critical operating conditions of the hydrogen engine, determining the dosage amount of the reducing agent based on a signal from a sensor detecting the nitrogen oxide concentration in the exhaust gas yields unsatisfactory nitrogen oxide reduction results at the SCR catalyst. These critical operating conditions include, in particular, sudden or rapid changes in engine load. As has been determined, sensor-based dosage amount determination can then be inaccurate due to system or sensor inertia or dead times, which can result in unacceptably increased nitrogen oxide tailpipe emissions. In this context, a nitrogen oxide tailpipe emission is defined as the nitrogen oxide concentration or amount of nitrogen oxide in the exhaust gas released into the environment. However, it has been shown that the air-fuel ratio X can be determined almost instantaneously.If, under the aforementioned critical operating conditions, the metered quantity is determined based on the determined air-fuel ratio X, an increase in raw nitrogen oxide emissions can be responded to very quickly by increasing the metered quantity accordingly, thus preventing nitrogen oxide breakthrough through the SCR catalyst. Raw nitrogen oxide emissions are defined as the nitrogen oxide concentration or quantity of nitrogen oxide in the exhaust gas that has not been treated with regard to nitrogen oxide reduction.

[0009] In an embodiment of the method, the predeterminable or predetermined operating conditions under which the reducing agent dosage amount is determined based on the air-fuel ratio X include a decrease in the air-fuel ratio X. A decrease in the air-fuel ratio X is associated with an increase in the raw nitrogen oxide emissions. Because the air-fuel ratio X can be determined almost instantaneously, an X-based dosage amount determination can be used to react very quickly to the increase in the raw nitrogen oxide emissions by correspondingly increasing the dosage amount, and a nitrogen oxide breakthrough through the SCR catalyst can be avoided. The dosage amount determination based on the determined air-fuel ratio X can be carried out in particular when the air-fuel ratio X decreases if the decrease exceeds a predeterminable temporal gradient.

[0010] It is advantageous if, in a further embodiment of the method, the predeterminable or predetermined operating conditions, under which the reducing agent dosage amount is determined based on the air-fuel ratio X, include a drop in the air-fuel ratio X below a predeterminable X threshold value (Xs). This takes into account the fact that when the air-fuel ratio X drops below a certain value, the raw nitrogen oxide emissions rise sharply. The procedure according to the invention therefore makes it possible to react to sharply rising raw nitrogen oxide emissions almost instantaneously with an appropriate increase in the reducing agent dosage amount.

[0011] In a further embodiment of the method, the X threshold is approximately 2.5 or less. The X threshold is preferably specified individually for an internal combustion engine or depending on the engine type. In particular, the type of mixture formation can be taken into account. The X threshold is preferably selected to be lower for an internal combustion engine with external mixture formation or multipoint hydrogen injection than for an internal combustion engine with internal mixture formation or low-pressure injection. For an internal combustion engine with external mixture formation or multipoint injection, for example, an X threshold of approximately 2.2 or less can be selected.

[0012] Advantageously, in an embodiment of the method, if the air-fuel ratio is below the X-threshold, a metered amount is determined based on a characteristic curve representing the raw nitrogen oxide emissions of the internal combustion engine as a function of the air-fuel ratio. This metered amount is sufficient to enable at least a substantial reduction of the nitrogen oxide quantity in the exhaust gas at the SCR catalyst, which quantity results from the determined air-fuel ratio based on the characteristic curve. The characteristic curve can be determined in advance individually or type-specifically for the corresponding internal combustion engine. The metered amount thus determined can, if necessary, be corrected depending on influencing factors such as catalyst temperature.

[0013] In a further embodiment, the predeterminable or predetermined operating conditions under which the reducing agent dosage amount is determined based on the air-fuel ratio X include the presence of raw nitrogen oxide emissions from the combustion engine that exceed a predeterminable limit value. This takes into account the fact that the measuring range of conventional nitrogen oxide sensors has an upper limit. The measuring range limit may be sensor-specific and can be, for example, 3000 ppm. Above this limit, the nitrogen oxide concentration in the exhaust gas can no longer be correctly determined by such a nitrogen oxide sensor. A reduction quantity determination based on the signal from the nitrogen oxide sensor is therefore inevitably incorrect in this case.However, if, as provided according to the invention, a dosing quantity determination is carried out based on the determined air-fuel ratio X when the limit value, which is preferably predetermined by the measuring range limit of the nitrogen oxide sensor, is exceeded, an accurate dosing quantity determination can be carried out even at high nitrogen oxide concentrations in the exhaust gas.

[0014] In a further embodiment of the method, the internal combustion engine is operated at least predominantly with a lean air-fuel mixture. Preferably, the internal combustion engine is operated exclusively or almost exclusively with a lean mixture. It may be provided that the internal combustion engine is operated at least predominantly or exclusively with an air-fuel ratio with an X value of more than 1.3.

[0015] In a further embodiment of the process, stored hydrogen in liquid form is used as the fuel. This offers advantages in terms of energy density compared to pressurized gaseous hydrogen, which can of course also be used. The liquid hydrogen is preferably stored on board a motor vehicle powered by the combustion engine, in a dedicated cryogenic tank.

[0016] In a further embodiment of the invention, the ammonia-containing reducing agent is used exclusively as a reducing agent for the nitrogen oxides present in the exhaust gas of the combustion engine. This avoids the increased expense of using an additional reducing agent.

[0017] The hydrogen engine according to the invention has a connected exhaust gas purification system with an SCR catalyst and a metering unit for supplying an ammonia-containing reducing agent to an exhaust gas emitted by the hydrogen engine on the inlet side of the SCR catalyst, as well as a control unit for determining a metered amount of the reducing agent for reducing nitrogen oxides present in the exhaust gas of the hydrogen engine. According to the invention, means are provided for determining an air-fuel ratio X of an air-fuel mixture combusted in the hydrogen engine, wherein the control unit is designed to determine the metered amount based on the determined air-fuel ratio X when predeterminable or predetermined operating conditions of the internal combustion engine exist. The hydrogen engine can be either an internal combustion engine with external or one with internal mixture formation.Typically, the means for determining the air-fuel ratio can determine it almost instantaneously. Conventional measuring devices for the amount of air and fuel supplied to the engine can be used as means for determining the air-fuel ratio. The control unit continuously records operational variables of the hydrogen engine and the exhaust gas purification system, such as engine load, speed, combustion air volume, fuel consumption, exhaust gas temperatures, etc., and uses these to determine, among other things, control variables for controlling the operation of the engine and exhaust gas purification system. In particular, a control variable is determined that activates the dosing unit accordingly to deliver the determined dosing quantity.

[0018] In an embodiment of the invention, the control unit is designed to determine the metered amount based on the determined air-fuel ratio if the determined air-fuel ratio X drops below a predeterminable X threshold value (Xs) and / or if the hydrogen engine's nitrogen oxide raw emissions exceed a predeterminable limit value. If the air-fuel ratio drops very rapidly below the X threshold value (Xs), for example due to a sudden increase in engine load, the almost instantaneous determination of the air-fuel ratio makes it possible to react very quickly to an increased nitrogen oxide raw emission associated with the drop in the air-fuel ratio with an adapted increase in the reducing agent metered amount, thus reliably preventing nitrogen oxide breakthrough through the SCR catalyst.Likewise, incorrect dosing of the reducing agent can be avoided if the raw nitrogen oxide emission exceeds the limit value.

[0019] In an embodiment of the invention, a nitrogen oxide sensor is provided in the exhaust gas purification system upstream of the SCR catalyst and the control unit is designed to determine the dosage amount in a predeterminable X range above the X threshold value based on a signal from the nitrogen oxide sensor.

[0020] In a further embodiment of the invention, the hydrogen engine is designed as a spark-ignition internal combustion engine. It is advantageous if, in a further embodiment of the invention, liquid hydrogen is provided as the fuel for the hydrogen engine.

[0021] In a further embodiment of the invention, the reducing agent used to reduce nitrogen oxides present in the exhaust gas of the hydrogen engine is exclusively a reducing agent containing ammonia. Preferably, an aqueous urea solution stored in a suitable storage tank is used as the reducing agent. The reducing agent or urea solution is metered into an exhaust line of the exhaust gas purification system via a metering unit controlled by the control unit, a short distance from the inlet side of the SCR catalyst.

[0022] The above-mentioned and further features and advantages of the invention will become apparent from the following description of preferred, non-limiting embodiments of the invention with reference to the accompanying drawings. The features and combinations of features mentioned above in the description, as well as the features and combinations of features mentioned below in the descriptions of the figures and / or shown alone in the figures, can be used not only in the respective combinations specified, but also in other combinations or on their own, without departing from the scope of the invention. They show:

[0023] Fig. 1 is a schematic representation of an advantageous embodiment of a hydrogen engine with connected exhaust gas purification system, and

[0024] Fig. 2 is a representation of a characteristic curve which schematically shows the dependence of the raw nitrogen oxide emission of the hydrogen engine on the air-fuel ratio X. Fig. 1 shows, merely schematically and in a highly simplified manner, an example of an advantageous embodiment of an exhaust gas purification system 2 connected to an internal combustion engine 1 of a motor vehicle (not shown in detail). The internal combustion engine 1 in the present case is an internal combustion engine operated exclusively with hydrogen as fuel, hereinafter referred to simply as a hydrogen engine. The hydrogen engine 1 can be an engine with compression ignition. In the present case, however, an engine with spark ignition is assumed. For the hydrogen engine 1, an external or, alternatively, an internal mixture formation can be provided.In the case of external mixture formation, it is preferably provided that the hydrogen stored in liquid form is taken from a corresponding storage tank and injected by means of injectors into an intake manifold upstream of an inlet valve of a respective engine cylinder (multi-point injection). In the case of internal mixture formation, direct injection into a respective combustion chamber of the hydrogen engine 1 is provided. Details of these technical embodiments have been omitted. Irrespective of the type of mixture formation, at least predominantly lean operation, i.e. operation with excess air, is provided for the hydrogen engine 1. It can be provided to operate the hydrogen engine 1 exclusively with a lean air-fuel ratio X of, for example, greater than X = 1.3.

[0025] Exhaust gas emitted by the hydrogen engine 1 enters the exhaust gas purification system 2 via an exhaust line 3. In accordance with the exhaust gas flow direction indicated by an arrow, the exhaust gas flows successively through an oxidation catalyst 8, an SCR catalyst 4, and an ammonia slip catalyst 9, preferably designed as an oxidation catalyst. The catalysts 8, 4, 9 are preferably designed as ceramic honeycomb bodies with a plurality of parallel channels and a catalytically active coating. A coating comprising one or more precious metals from the platinum group is provided for the oxidation catalyst 8. A similar coating can be provided for the ammonia slip catalyst 9. The SCR catalyst 4 can have a preferably zeolitic coating containing iron, copper, or vanadium.By means of the oxidation catalyst 8, nitrogen monoxide (NO) contained in the exhaust gas can be oxidized to nitrogen dioxide (NO2), thus increasing the NO2 - NO ratio of the nitrogen oxides. This enables improved nitrogen oxide reduction in the SCR catalyst 4. For this nitrogen oxide reduction, a reducing agent containing ammonia in free or bound form, preferably an aqueous urea solution, is supplied to the exhaust gas purification line 3. This is carried out by means of a dosing unit 5 arranged a short distance from the inlet side of the SCR catalyst 4 in the exhaust gas purification system 2. The reducing agent is stored in a corresponding storage tank and is supplied to the dosing unit 5 via suitable conveying means, which is not shown in detail. By means of the ammonia slip catalyst 9, any ammonia slipping through the SCR catalyst 4 can be oxidized.

[0026] To control the reducing agent dosage quantity or to monitor the nitrogen oxide conversion in the SCR catalyst 4, nitrogen oxide sensors 6, 7 are provided downstream of the oxidation catalyst 8 and upstream of the dosing unit 5 or downstream of the SCR catalyst 4 to detect the nitrogen oxide concentration in the exhaust gas.

[0027] It is understood that Fig. 1 merely shows an advantageous embodiment of a suitable exhaust gas purification system 2 by way of example. However, other embodiments are also possible. For example, the oxidation catalyst 8 can optionally be omitted. For nitrogen oxide reduction, a two-stage SCR system with two SCR catalysts arranged one behind the other, each with an associated dosing unit, can also be provided. Likewise, the ammonia slip catalyst 9 can be directly connected to the SCR catalyst 4 or be designed as an end region of the SCR catalyst 4. In this case, the nitrogen oxide sensor 7 is arranged downstream of the ammonia slip catalyst 9.

[0028] As far as the monitoring and control of the hydrogen engine 1 and the exhaust gas purification system 2 is concerned, a control unit 10 is provided for this purpose. For this purpose, the control unit 10 receives input signals 11, processes them, and generates output signals 12 from them. The input signals 11 include, in particular, signals from the nitrogen oxide sensors 6, 7 installed in the exhaust gas purification system 2, as well as other sensors that may be present, for example for temperature or ammonia content in the exhaust gas. However, the control unit 10 can also process input signals representing operating variables of the hydrogen engine 1, such as measured variables for air and fuel quantity, engine load, speed, etc. The output signals 12 include, in particular, control signals for controlling the dosing unit 5. The control unit 10 can, for example, be designed as a microcontroller that responds to characteristic maps orCharacteristic curves for dependencies of operating variables of the hydrogen engine 1 or the exhaust gas purification system 2 can be accessed. The control and regulation processes effected by the control unit 10 are preferably carried out by a stored program.

[0029] A procedure according to the invention for operating the hydrogen engine 1 and the exhaust gas purification system 2 is discussed below with reference to Fig. 2. In this case, Fig. 2 shows a diagram of a characteristic curve 13 typical for a hydrogen engine for the dependence of the raw nitrogen oxide emissions on the air-fuel ratio X at which the hydrogen engine is operated. A dashed line 14 indicates a break point or an X value Xs near a break point of the characteristic curve 13, from which the raw nitrogen oxide emissions begin to rise steeply from low values ​​with decreasing X values. This X value, referred to here as the X threshold value Xs, may be decisive for the basis on which the metered amount of the reducing agent is determined, as explained in more detail below. Depending on the design of the hydrogen engine 1, the X threshold value Xs can vary.Typically, the X threshold Xs is around 2.5 or lower.

[0030] During operation of the hydrogen engine 1, operating variables of the hydrogen engine 1 and the exhaust gas purification system 2, such as nitrogen oxide concentration and temperature of the exhaust gas, exhaust gas mass flow, engine speed, engine load, air-fuel ratio, etc., are continuously recorded or determined and processed by the control unit 10, and a demand-based metered amount for the reducing agent is determined and set. Especially in the range of low to medium engine load or with comparatively small increases in engine load, the hydrogen engine 1 is operated with comparatively high air-fuel ratios in an X range above the X threshold value Xs. In this case, the reducing agent metered amount is preferably determined based on the signal of the nitrogen oxide sensor 6 arranged upstream of the SCR catalyst 4.For this purpose, the control unit 10 preferably determines a nitrogen oxide flow rate and, based on the signal from the nitrogen oxide sensor 6, which correlates with the nitrogen oxide concentration in the exhaust gas, and the exhaust gas mass flow, the resulting reducing agent dosage amount to be set, and controls the dosing unit 5 to deliver it. Preferably, a reducing agent dosage amount is set that achieves an approximately stoichiometric ratio of ammonia to nitrogen oxide. In this case, particularly at comparatively high X values ​​of, for example, more than approximately 3, which, according to characteristic curve 13, result in very low nitrogen oxide raw emissions, dosing may be omitted if necessary, i.e., a dosage amount of zero may be set at least temporarily.

[0031] As a result of correspondingly strong, particularly rapid, increases in engine load, the air-fuel ratio can fall below the threshold value Xs. In order to react quickly enough to a resulting increase in raw nitrogen oxide emissions with a corresponding increase in the reducing agent dosage amount, the dosage amount to be set is determined in these cases based on the current air-fuel ratio. For this purpose, the raw nitrogen oxide emissions resulting from the air-fuel ratio are determined using characteristic curve 13. From this, a reducing agent dosage amount is determined that is sufficient to reduce the nitrogen oxides present in the exhaust gas as much as possible at the SCR catalytic converter 4. This procedure has the advantage of avoiding dead times and delays that can occur when determining the reducing agent dosage amount based on the signal from the nitrogen oxide sensor 6.In addition, incorrect dosing due to possible incorrect measurements of the nitrogen oxide sensor, which can occur at elevated nitrogen oxide concentrations, can be avoided.

[0032] If the air-fuel ratio increases again to a value in a range above the threshold value Xs, for example due to a further reduction in engine load demand, the system preferably switches back to determining the reducing agent dosage amount based on the signal from the nitrogen oxide sensor 6. This range can be directly adjacent to the threshold value Xs or at a predeterminable distance.

[0033] It should also be mentioned that a determined reducing agent dosage amount can generally be corrected, if necessary, taking into account other influencing variables, such as the temperature of the SCR catalyst 4. It can also be provided to forgo switching from a sensor-based determination of the reducing agent dosage amount to one based on the air-fuel ratio if the air-fuel ratio decreases comparatively slowly. Determination of the reducing agent dosage amount based on the air-fuel ratio can then only be provided if the X value decreases rapidly with a limit value exceeding a predeterminable temporal gradient.Regardless of a decrease in the air-fuel ratio, it is preferably provided to generally determine the reducing agent dosage amount based on the air-fuel ratio when the raw nitrogen oxide emissions of the hydrogen engine 1 exceed a predeterminable limit. This can be determined using characteristic curve 13. The limit can be based on an upper measuring range limit for the nitrogen oxide sensor 6 and can be, for example, 3000 ppm. Furthermore, it can be provided that, instead of a characteristic curve 13 representing the correlation between raw nitrogen oxide emissions and the air-fuel ratio, a calculation model taking this correlation into account is used.

Claims

Patent claims 1. A method for removing nitrogen oxides from an exhaust gas of an internal combustion engine (1) operated with hydrogen as fuel, having an exhaust gas purification system (2) having an SCR catalyst (4) and connected to the internal combustion engine (1), wherein a metered amount of an ammonia-containing reducing agent to be supplied to the exhaust gas at the inlet of the SCR catalyst (4) is determined, characterized in that an air-fuel ratio X of an air-fuel mixture combusted in the internal combustion engine (1) is determined and, if predeterminable or predetermined operating conditions of the internal combustion engine (1) exist, the metered amount is determined based on the determined air-fuel ratio X.

2. Method according to claim 1, characterized in that the predeterminable or predetermined operating conditions include a decrease in the air-fuel ratio X.

3. Method according to claim 2, characterized in that the predeterminable or predetermined operating conditions comprise a drop in the air-fuel ratio X below a predeterminable X threshold value (Xs).

4. Method according to claim 3, characterized in that the X threshold value (Xs) is about 2.5 or less.

5. Method according to claim 3 or 4, characterized in that if an air-fuel ratio below the X-threshold value (Xs) is present, based on a characteristic curve (13) which represents a raw nitrogen oxide emission of the internal combustion engine (1) as a function of the air-fuel ratio, a metered amount is determined which is sufficient to enable at least a substantial reduction of a nitrogen oxide quantity in the exhaust gas at the SCR catalyst (4) resulting from the determined air-fuel ratio on the basis of the characteristic curve (13).

6. Method according to one of claims 1 to 5, characterized in that the predeterminable or predetermined operating conditions comprise the presence of a nitrogen oxide raw emission of the internal combustion engine (1) which exceeds a predeterminable limit value.

7. Method according to one of claims 1 to 6, characterized in that the internal combustion engine (1) is operated at least predominantly with a lean air-fuel mixture.

8. Method according to one of claims 1 to 7, characterized in that hydrogen stored in liquid form is used as fuel.

9. Method according to one of claims 1 to 8, characterized in that exclusively the ammonia-containing reducing agent is used as the reducing agent for the nitrogen oxides present in the exhaust gas of the internal combustion engine (1).

10. Hydrogen engine (1) with connected exhaust gas purification system (2) with an SCR catalyst (4) and a dosing unit (5) for supplying an ammonia-containing reducing agent into an exhaust gas emitted by the hydrogen engine (1) on the inlet side of the SCR catalyst (4) and with a control unit (10) for determining a dosing quantity of the reducing agent for reducing nitrogen oxides present in the exhaust gas of the hydrogen engine (1) characterized in that Means are provided for determining an air-fuel ratio X of an air-fuel mixture burned in the hydrogen engine (1), wherein the control unit (10) is designed to determine the metered quantity based on the determined air-fuel ratio X when predeterminable or predetermined operating conditions of the internal combustion engine (1) exist.

11. Hydrogen engine (1) according to claim 10, characterized in that the control unit (10) is designed to determine the metering quantity based on the determined air-fuel ratio when the determined air-fuel ratio X falls below a predeterminable X threshold value (Xs) and / or when the nitrogen oxide raw emission of the hydrogen engine (1) exceeds a predeterminable limit value.

12. Hydrogen engine (1) according to claim 10 or 11, characterized in that a nitrogen oxide sensor (6) is provided in the exhaust gas purification system (2) upstream of the SC R catalyst (4) and the control unit (10) is designed to determine the metered amount based on a signal from the nitrogen oxide sensor (6) in a predeterminable X range above the X threshold value (Xs).

13. Hydrogen engine (1) according to one of claims 10 to 12, characterized in that the hydrogen engine (1) is designed as an internal combustion engine with spark ignition.

14. Hydrogen engine (1) according to one of claims 10 to 13, characterized in that liquid hydrogen is provided as fuel for the hydrogen engine (1).

15. Hydrogen engine (1) according to one of claims 10 to 14, characterized in that as a reducing agent for reducing nitrogen oxides present in the exhaust gas of the hydrogen engine (1), only the reducing agent containing ammonia is provided.

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