Internal combustion engine

Abstract

Internal combustion engine comprising an engine controller (3), a knock sensor (4), and at least one actuator (7,8,9) for influencing an engine variable, preferably a boost pressure generated by a charge air system (2), wherein the engine controller (3) is configured to receive knock measurement signals from the knock sensor (4), wherein the engine controller (3) is configured to - during operation of the internal combustion engine (1) actuate the at least one actuator (7,8,9) such that the engine variable is changed, preferably reduced, while keeping a desired power output and / or an actual power output within a predefined range, - correlate at which value of the engine variable knock starts to manifest in the knock measurement signals during the changing of the engine variable thereby identifying a knock border, and - set a first engine variable reference, preferably first boost pressure reference, at a predetermined distance from the identified knock border.

Description

[0001]Internal combustion engine The present invention concerns internal combustion engines according to the classifying portion of claim 1 or 10, and computer program products and methods for controlling an internal combustion engine. In view of ever stricter requirements regarding emissions and increasing expectations on the performance of internal combustionengines the accurate and stable control of a desired operatingpoint of the internal combustion engine is very important. At the same time there are many factors that can lead to a drift of the operating point even if one or more of the main parameters, such as engine speed and / or charge pressure are kept constant, for example because of variations in the fuel composition or aging of the internal combustion engines. Examples of adjustments that can be made on modern internal combustion engines are fuelling, exhaust gas recirculation rate,throttle valve, compressor bypass and wastegate valve positions ofa turbocharger, and ignition timing.Regarding the example of the exhaust gas recirculation rate, it isknown from US 7913675 B2 to determine the density of the cylindercharge and to determine the exhaust gas recirculation rate on this basis. However, in that disclosure the fuelling state of the internal combustion engine is not taken into account such that a high quality of the control and calibration of the operating point cannot be expected. The object of the invention is therefore to provide an internalcombustion engine, a computer program product for operating an internal combustion engine, and a method for operating an internal combustion engine, where the drift of the operating point isreduced, or a more stable operating point is possible.In a first aspect of the invention this object is achieved by the features of claim 1, namely by an internal combustion engine comprising an engine controller, a knock sensor, and at least oneactuator for influencing an engine variable, preferably a boostpressure generated by a charge air system, wherein the enginecontroller is configured to -receive knock measurement signals from the knock sensor,- during operation of the internal combustion engine actuatethe at least one actuator such that the engine variable ischanged, preferably reduced, while keeping a desired poweroutput and / or an actual power output within a predefined range, -correlate at which value of the engine variable knockstarts to manifest in the knock measurement signals during the changing of the engine variable thereby identifying aknock border, and -set a first engine variable reference at a predetermineddistance from the identified knock border. In the first aspect of the invention the object is achieved by the features of claim 17, namely by a computer program product for controlling an internal combustion engine comprising instructions which cause an executing computer to perform the following: -during operation of the internal combustion engineoutputting actuation signals to at least one actuator capable of influencing an engine variable such that theengine variable is changed, preferably reduced, whilekeeping a desired power output and / or an actual power output within a predefined range,- receiving knock measurement signals from a knock sensor asinput, -correlating at which value of the engine variable knockstarts to manifest in the knock measurement signals duringthe changing of the engine variable thereby identifying aknock border, and -setting a first engine variable reference at apredetermined distance from the identified knock border. In the first aspect of the invention the object is achieved by thefeatures of claim 20, namely by a method for controlling aninternal combustion engine, wherein the following steps are performed: -during operation of the internal combustion engine usingat least one actuator capable of influencing an enginevariable such that the engine variable is changed, whilekeeping a desired power output and / or an actual power output within a predefined range, -receiving knock measurement signals from a knock sensor asinput, -correlating at which value of the engine variable engineknock starts to manifest in the knock measurement signalsduring the changing of the engine variable therebyidentifying a knock border, and -setting a first engine variable reference at apredetermined distance from the identified knock border. In a second aspect of the invention the object is achieved by the features of claim 10, namely by an internal combustion engine comprising an engine controller, a NOx sensor for detecting a NOx concentration in an exhaust gas of the internal combustion engine,and at least one actuator for influencing an engine variable, inparticular a boost pressure generated by a charge air system,wherein the engine controller is configured to- receive NOx measurement signals from the NOx sensor,- carry out closed loop control of the NOx concentrationduring operation of the internal combustion engine with the NOx measurement signals as NOx feedback signal for the closed loop control, -use the engine variable or another variable that influencesthe engine variable as actuating variable for the closedloop control of the NOx concentration, and -determine and / or store an actual and / or set engine variableas a second engine variable reference when the NOx feedbacksignal is stabilised at a NOx reference value. In the second aspect of the invention the object is achieved bythe features of claim 18, namely by a computer program product,comprising instructions which cause an executing computer to perform the following: -receiving NOx measurement signals from a NOx sensor asinput, -carrying out closed loop control of a NOx concentrationduring operation of the internal combustion engine with the NOx measurement signals as NOx feedback signal for theclosed loop control, -using an engine variable or another variable thatinfluences the engine variable as actuating variable forthe closed loop control of the NOx concentration byoutputting actuation signals to at least one actuator capable of influencing the engine variable, and- determining and / or storing an actual and / or set enginevariable as a second engine variable reference when theNOx feedback signal is stabilised at a NOx reference value. In the second aspect of the invention the object is achieved bythe features of claim 21, namely by a method for controlling an internal combustion engine, wherein the following steps are performed: -carrying out closed loop control of a NOx concentrationduring operation of the internal combustion engine with a NOx feedback signal for the closed loop control, -using an engine variable or another variable thatinfluences the engine variable as actuating variable forthe closed loop control of the NOx concentration by outputting actuation signals to at least one actuator capable of influencing the engine variable, and -determining and / or storing an actual and / or set boostpressure as a second engine variable reference when theNOx feedback signal is stabilised at a NOx reference value. In particularly preferable embodiments of the invention the internal combustion engine comprises a charge air system, preferably a turbo charger, and / or the engine variable is a boost pressure generated by the charge air system. In such embodiments the first engine variable reference and / or the second engine variable reference become the first boost pressure reference and / or the second boost pressure reference, respectively. The changing of the engine variable according to the first aspect of the invention is particularly preferably carried out so that the engine variable approaches the knock border, preferably in a steady and / or linear manner. In case the engine variable is a boost pressure that means that the boost pressure is reduced. The engine variable is preferably an engine variable which defines the operating point of the internal combustion engine. In particularly preferred embodiments the engine variable is a boost pressure. The boost pressure can in many instances be viewed as a measure of the mass being charged into the area where the combustion takes place, e.g., a cylinder of the internal combustion engine.Alternatively, or additionally, the engine variable can be anexhaust gas recirculation rate, an NOx emission or differentemission variable at the exhaust outlet, and / or a lambda (air fuelequivalence ratio, i.e., scaled to 1 at the stoichiometric air fuel ratio) setpoint, the latter preferably for lean burn engines. It is also conceivable to use combinations and / or derived quantities of the mentioned variables as engine variable. A basic idea of the invention is to perform a calibration of the boost pressure reference, or generally the engine variablereference, which can then for example serve as a definition of theoperating point of the internal combustion engine. According to the invention a more stable operation point withrespect to the distance of the operating point of the internalcombustion engine from the knock border and resulting emissions of the internal combustion engine is possible. In the first aspect of the invention the setting of the first engine variable reference is made in relation to the knock border.The setting of the first engine variable reference according tothe first aspect of the invention can be carried out whenever thereduction of the engine variable while keeping the power output ina predefined range will result in knock appearing in the measurement signals of the knock sensor. In preferred embodiments the setting of the first engine variable reference according to the first aspect of the invention may be carried out while the internal combustion engine operates at or above 60% and / or 70% and / or 80% power output and / or load capacity.The setting of the second engine variable reference according tothe second aspect of the invention may be performed independently of the possibility of knock or the presence of a knock sensor.However, in its second aspect the invention requires NOxmeasurement signals stemming from a NOx sensor. In preferred embodiments the setting of the second engine variable reference according to the second aspect of the invention may be performed when the internal combustion engine operates at 30% to 70%, particularly preferably 40% to 60% and particularly preferably 45% to 55%, power output and / or load capacity. The charge air system which provides the boost pressure canparticularly preferably comprise a turbo charger, i.e., an exhaustgas turbine coupled by a turbo shaft to a compressor for compressing the air or air / fuel mixture in an intake manifold of the internal combustion engine thereby producing the boost pressure. Additionally, or alternatively, the charge air system can comprise an electric motor for driving the compressor. Examples for actuators which are suitable for influencing theengine variable, such as the boost pressure, would be a wastegatevalve, a compressor bypass valve, or a throttle valve. In preferred embodiments of the invention the boost pressure is reduced while the desired power output and / or the actual power output (or equivalently load capacity) is kept within a predefined range. Independently of the choice of the engine variable, the predefined range could for example be defined by a deviation of less than 10% and / or less than 5% and / or less than 3% and / or less than 1%, from the desired and / or actual power output of the internal combustion engine just before the reducing of the boost pressure started. In particularly preferred embodiments the predefined range ischosen such that the desired or actual power output is essentiallymaintained, i.e., constant. The first aspect of the invention furthermore defines that when the knock starts to manifest in the knock measurement signals andwhat the boost pressure at this point in time was are determinedwhich can be understood as the correlation of the manifestation of the knock signal with the reduction of the boost pressure. In preferable embodiments this correlating step is performed as part of a quasi-stationary changing of the engine variable so that the changed engine variable and the knock signal can be correlated directly. Of course, much more complex determination schemes can be envisaged where the process of the changing of the engine variable and one or more levels of knock can be considered and taken into account. In additional or alternative embodiments, the correlating mentioned may include noting or determining the time at which the knock signals start to manifest and to then determine what desiredand / or actual engine variable was present at this point in time.The manifestation of engine knock in the knock measurement signalscan for example be defined by a threshold value on the knockmeasurement signals. Also, here much more involved schemes can beenvisaged, for example where the threshold value needs be exceeded for a certain amount of time, or the like. It should be noted that this correlation can be carried out once or, if the changing process of the engine variable is repeated to increase the accuracy of the procedure, several times, or with respect to several measurements. The predetermined distance can for example be determined on a testbed engine where the optimal and / or desired operating point as well as the knock border can be measured accurately. According to the invention the operating point of the internal combustion engine can be calibrated so that the distance from the knock border can be kept constant. Alternatively, or additionally, the distance of the operating point to the knock border could also be determined online and used as the predetermined distance according to the invention, e.g., bycalibrating an NOx concentration in the exhaust gas according togiven boost pressures, and measuring the distance to the knockborder. The distance of the operating point from the knock bordercan also depend on the fuel used. The knock border can for example be expressed as an absolute value for the boost pressure or as a boost pressure offset from a current operating point (analogous for engine variables other than the boost pressure). The predetermined distance of the first boost pressure reference can preferably be between 50mbar and 100mbar, preferably between 60mbar and 90mbar and particularly preferably substantially 80mbar. Together with the setting of the first engine variable reference at a predetermined distance from the identified knock border it is optionally possible to change or set an ignition timing reference. Different ignition timings can also be seen as defining a certain distance of the operating point of the internal combustion engine from the knock border.In the second aspect of the invention closed loop control of theNOx concentration in the exhaust gas of the internal combustion engine is provided where the boost pressure is used as an actuatingvariable, and the reference is preferably identified in the firstaspect of the invention. Once the NOx concentration stabilises the resulting value of theengine variable can be used as second engine variable reference.Different criteria are conceivable for the stabilisation of the NOx concentration. E.g., there could be predefined time span afterwhich the NOx concentration is simply assumed to be stable orverified to be below a predefined threshold. In other embodiments the NOx measurement signals could be evaluated during the time span and the criterion for the stability of the NOx measurement signals could be that the deviation from a mean NOx concentration is less than 10% and / or less than 5% and / or less than 3% from a mean NOx concentration during the time span. In other embodiments an upper limit for a standard deviation of the NOx concentration could be imposed. Evidently, many different embodiments are conceivable in this respect. The length of the time span can be between 1 second and 10 minutes. The first aspect of the invention can be carried out before and / or after the second aspect of the invention. The internal combustion engine can be a gas engine, configured to combust molecular hydrogen, natural gas, methane, or other gases with a significant hydrocarbon component, or mixtures of the mentioned gases. The internal combustion engine can be mechanically coupled to a generator so as to drive the generator for producing electric energy from the mechanical energy delivered by the internal combustion engine. Gas engines to which generators are coupled for generating electric energy are commonly called gensets. The internal combustion engine can be of piston-cylinder type with a plurality of piston-cylinder units, preferably more than eight or ten. It should be noted that some engines, in particular gas engines, are configured to operate with a wide variety of fuels, e.g., from biogas, e.g., with methane numbers between 135 and 145, to different types of natural gas, e.g., with methane numbers between 60 and 100, and to associated gas (associated to petroleum industry) or liquid propane, e.g., with methane numbers between 34 to 60. All these types of fuel result in different knock borders.Therefore, it is abundantly clear that a calibration of the engineaccording to the invention is of great benefit. The internal combustion engine according to the invention can comprise at least one piston-cylinder unit (as mentioned), a charging system (e.g., a turbo charger, an electrically driven compressor, or mixed forms) with for example one, two, three, or four stages, and / or an exhaust gas aftertreatment system.The engine controller according to the invention can be embodiedas a main engine controller integrally embodied with the internal combustion engine. Alternatively, some or all functions of the engine controller can be realised by a computer server located remotely from the internal combustion engine. The engine controller can be the computer executing the computer program products according to the invention. In particularly preferred embodiments the internal combustion engine comprises an exhaust gas recirculation system. Preferably, the exhaust gas recirculation system has a exhaust gas recirculation valve for admitting exhaust gas from an exhaust gas conduit into an intake manifold or an intake conduit. The exhaust gas recirculation system can be low-pressure or a high-pressure exhaust gas recirculation system.In preferred embodiments the engine controller can be configured to increase the exhaust gas recirculation rate if the boostpressure is raised, and vice versa, as long as the engine poweroutput and / or load capacity is kept essentially constant. For example, when the internal combustion engine is operated at stoichiometric conditions with EGR and a three-way catalyst, thefuel mass may directly be defined by the engine power setpoint andthe air mass may then be defined by the operation at Lambda = 1(or close to this setpoint). Then the change of the boost pressure setpoint only influences the amount of dilution via EGR and therefore defines the EGR rate. For example, when the engine is operated at lean burn conditions without EGR and without a three-way catalyst (but possibly othercatalyst systems), the fuel mass may be directly defined by theengine power setpoint. Then the change of the boost pressure setpoint only influences the amount of dilution via additional airleading to Lambda > 1.The knock sensor and / or the NOx sensor can be present for each cylinder individually or, e.g., common sensors for groups of cylinders or all cylinders can be used. In preferred embodiments knock sensors are present for each cylinder individually.For the purpose of carrying out the invention as described abovethe measurement signal of the knock sensor and / or the NOx sensorcan be averaged if there are more than one knock sensor and / or NOxsensor. In preferred embodiments the NOx sensor can be one common sensor for the whole internal combustion engine, preferably downstream ofthe internal combustion engine, before an exhaust gasaftertreatment system. Additional NOx sensors for monitoring canbe applied after the aftertreatment system, but may not necessarily be used in the current invention. In particularly preferred embodiments the NOx sensor is a sensor configured for direct measurement of the NOx concentration. In other embodiments the NOx sensor is configured for indirect measurement of the NOx concentration. For example, the sensor could measure the amount of NOx and the concentration could be calculated with a known amount of exhaust gas. In other embodiments the NOx sensor could be a virtual NOx sensor. For example, for certain engines the temperature of the combustion is known to correlate quite strictly with the NOx concentration in the exhaust gas. Thus, for example, a measurement of the temperature of the combustion could be used to derive the amount or concentration of NOx in the exhaust gas. In yet other embodiments a close correlation between an exhaust gas recirculation rate and / or a lambda value for high lambda engines and the NOx concentration, possibly also taking into account a combustion temperature, can be employed to derive a virtual NOx amount or concentration in the exhaust gas. Protected is furthermore -an arrangement comprising an internal combustion engineaccording to the invention and a generator for creating electrical energy mechanically driven by the internal combustion engine, and -transitory or non-transitory data storage having stored acomputer program product according to the invention. Everything that is disclosed with respect to the prior art can also be used with the invention. The charge air system can be a system for charging only air (air charged engine) or can be a system for charging an air / fuel mixture (mixture charged system). In particularly preferred embodiments the internal combustion engine comprises a closed or open loop control of the engine variable which is carried out essentially all the time during the operation of the internal combustion engine. Possible exceptions include the calibration according to the invention and / or startup procedures and / or shutdown procedures. Preferable embodiments of the invention are defined in the dependent claims. The knock border may be dependent on a load and / or power output of the internal combustion engine and the engine controller can preferably be configured to perform the step of changing the enginevariable – and preferably identifying the knock border – while theinternal combustion engine operates at or above 80 % power output and / or load capacity of the internal combustion engine. The predefined range for the desired power output and / or the actual power output is defined by a deviation from a reference power value, preferably by a deviation of less than 10% and / or less than 5% and / or less than 3% and / or less than 1% from the reference power value.The engine controller can comprise a high-level control, whereinpreferably an output of the high-level control is an input for acontrol of an exhaust gas recirculation (EGR) rate of the internalcombustion engine.The engine controller can comprise a high-level control, whereinpreferably an output of the high-level control is an input for acontrol of a fuel dosage system and / or for a control of an air / fuelequivalence ratio. The air / fuel equivalence ratio is the ratio of the amount of air to the amount of fuel normalised such that at stoichiometric ratiothe air / fuel equivalence ratio is one. The air / fuel equivalenceratio is also called lambda. In preferred embodiments a NOx sensor for detecting a NOx concentration in an exhaust gas of the internal combustion engine is provided, wherein the engine controller is configured to -receive NOx measurement signals from the NOx sensor, and- set a NOx value measured after setting, and preferablytracking, of the boost pressure reference – preferablyafter a stabilisation time period for the NOx concentration –as NOx reference value.The time period mentioned here can be the same or different compared to the time span mentioned in the introductory part. The engine controller can comprise a boost pressure controllerthat receives a reference from a high-level controller (that isinfluenced by the first engine variable reference and / or the secondengine variable reference according to the invention) and actuatesan actuator that influences the boost pressure. The at least one actuator for influencing the engine variable, preferably the boost pressure can comprise one or more of the following: a compressor bypass valve, a wastegate valve, a throttle valve, an actuator for actuating a variable turbine geometry, a blowoff valve, an electric motor for driving the compressor.The knock sensor can comprise or be an acceleration sensor and / oran in-cylinder pressure sensor and / or a strain gauge configured tomeasure a pressure in a cylinder (in which the combustion takesplace) and / or an ion current sensor. All the mentioned types ofsensors are considered to be types of knock sensors in the context of the invention. The engine controller can be configured to carry out the closed loop control of the NOx concentration when the internal combustion engine operates at 30% to 70%, particularly preferably 40% to 60% and particularly preferably 45% to 55%, power output and / or load capacity. The NOx sensor can preferably be arranged upstream of an exhaust gas catalytic converter system for reducing emissions of the internal combustion engine. The engine controller can be configured to extrapolate the secondengine variable reference to 50% power output and / or load capacity,preferably making use of the first engine variable reference validfor 80%, particularly preferably 90% and particularly preferably 95%, or more power output and / or load capacity of the internal combustion engine. The first aspect and the second aspect of the invention can becombined to create engine variable references for a wide range ofpower outputs and / or load capacities. Interpolation may be used tofind an engine variable reference for an engine power output and / orload capacity different from the ones for which the first andsecond engine variable references where determined. Also, morethan two points could be used to define the map of boost pressure references. The engine controller can be configured to further modify the firstengine variable reference and / or the second engine variablereference in dependence on a power output and / or a load and / or a desired NOx concentration of the exhaust gas and / or an ignition timing point and / or an engine speed and / or a charge temperature and / or an exhaust gas temperature and / or the setting of a variable valve timing system of the internal combustion engine and / or anambient air temperature and / or an ambient air humidity and / or afuel composition and / or a fuel quality.The setting of the first engine variable reference according tothe invention can be understood such that it is used subsequently to carrying out the first aspect of the invention as the effectivereference for the engine variable of the internal combustionengine. Alternatively, or additionally, the first engine variablereference or a modified first engine variable reference can, e.g.,be determined and / or stored in the engine controller and / or another computer executing a computer program product according to the invention.The second engine variable reference or a modified second enginevariable reference can be determined and / or stored according tothe invention. This can happen, e.g., in the engine controller and / or another computer executing a computer program product according to the invention. In principle, it is of course also conceivable to use the secondengine variable reference subsequently to carrying out the secondaspect of the invention as the effective reference for the enginevariable of the internal combustion engine.The engine controller can be configured to interpolate between the(modified or unmodified) first engine variable reference and the(modified or unmodified) second engine variable reference based ona desired power and / or load of the internal combustion engine, andpreferably to use an interpolated engine variable referenceresulting from the interpolation between the first engine variable reference and the second engine variable reference for a closed oropen loop control of the engine variable, preferably the boostpressure. The engine controller can be configured to extrapolate theidentified knock border and / or the first engine variale referenceto 100% power output and / or load capacity, preferably making useof a second engine variable reference valid for 30% to 70%,particularly preferably 40% to 60% and particularly preferably 45% to 55%, power output and / or load capacity of the internal combustion engine.The calibration of the engine variable setpoint with this inventionis preferably performed periodically, e.g., once a day or once aweek, while the resulting engine variable setpoint lookup tablewith interpolation is preferably used continuously throughout theengine operation for the boost pressure control loop.Further details and advantages of the invention are apparent from the figures and the corresponding description of the figures. The figures show:Fig. 1 a diagram visualising an embodiment according to theinvention,Fig. 2 a diagram visualising an embodiment according to theinvention,Fig. 3 an embodiment of an internal combustion engine accordingto the invention,Fig. 4 an embodiment of an internal combustion engine accordingto the invention,Fig. 5 an embodiment of a genset according to the invention,Fig. 6 an example of a control scheme for controlling aninternal combustion engine,Fig. 7a diagrams visualising an embodiment according to theinvention, andFig. 7b diagrams visualising another embodiment according to theinvention. The embodiments described here generally use the boost pressure as engine variable. Equivalent embodiments could be defined with the EGR rate, NOx or other emissions, and / or lambda as additional or alternative engine variables.Figures 1 and 2 show diagrams of the boost pressure p2’ versus thepower output P (or load capacity) of an internal combustion engine 1. The solid line in each diagram depicts a relationship between the power output and the boost pressure which is used according to thedisclosure of EP 2977596 A1 to set an operating point of theinternal combustion engine 1 by setting the appropriate boostpressure reference (as substitution variable for NOx emissions oran NOx concentration) for the desired or actual power output of the internal combustion engine 1. The solid line is given by three reference points at three different power outputs and linear interpolation between these points. Here, the first point is at zero power output and the other two points are at power output P1 and P2. In this particular example P1 is at half load (50% power output). In this particular example P2 is at full load (100% power output). In this embodiment the ultimate goal is to correct the reference points (e.g., at 0, P1 and P2) for the effects of variations in the fuel composition or aging of the internal combustion engine 1. With respect to the larger loads this is done according to the first aspect of the invention for which the embodiment of Fig. 1 is an example. With respect to the smaller loads this is done according to the second aspect of the invention for which the embodiment of Fig. 2 is an example. In the embodiment of Fig. 1 the internal combustion engine runs initially at a power output pact-knockwith boost pressure as given by the solid line. According to the first aspect of the invention at least oneactuator (see 7,8,9 in Fig. 3 or 4 and 6) is actuated such thatthe boost pressure is reduced, while keeping a desired power output and / or an actual power output within a predefined range, e.g., in the range of 80% to 100% power output. In this example this is more specifically performed such that the power output stays essentially constant. Keeping the power output essentially constant at reduced boostpressure can for example – depending on the type and operation ofthe internal combustion engine 1 – be achieved with a reducedexhaust gas recirculation rate or a lower lambda. The reduction of the boost pressure is performed until the knock measurement signals show that knock starts to occur. The boost pressure at the time at which knock starts to manifest in the knock measurement signals (correlating at which boost pressure knock starts to manifest in the knock measurement signals during the reduction of the boost) is denoted in Fig. 1 as ^p2’act-knock. This value identifies the knock border for the present internal combustion engine 1 at the present conditions (power output, age, fuel composition, environmental conditions). For example, it can be defined that the internal combustion engine 1 should be operated at a distance of 80mbar of boost pressure from the knock border. Then the first boost pressure reference, denoted by R1 in Fig. 1, can be set at ^p2’act-knock + 80mbar for the specific power output atwhich the procedure was carried out. After the NOx measurementsignal stabilises, the actual value (measured by NOx sensor 6) isstored as a reference for further calibration steps. It is afterwards possible to modify the first boost pressure reference by extrapolation (dashed line) based on the reference at P1 and the first boost pressure reference to a modified first boost pressure reference valid for, e.g., the power output value at P2. In Fig. 1 this extrapolated point is denoted by R1’. In the embodiment of Fig. 2 the internal combustion engine 1 runs initially at a power output pact-NOxwith boost pressure as given bythe solid line. In this embodiment the following procedure can betriggered when pact-NOxis between 45% and 55% load capacity of the internal combustion engine 1. At this point the usual boost pressure control is suspended, andinstead a closed loop control of the NOx concentration is carriedout with the NOx measurement signals as NOx feedback signal for the closed loop control. Here, the boost pressure is used as actuating variable for the closed loop control of the NOx concentration, concretely by using the actuators for the boostpressure control (see 7,8,9 in Fig. 3 or 4 and 6). The referencefor the NOx concentration has been derived by the previous step. After the NOx measurement signal stabilises (see example criteriain the general part of the description) the actual (measured byboost pressure sensor 30) and / or set boost pressure (in the closedloop control) is stored or determined as a second boost pressurereference at the power output pact-NOx. This is denoted as R2 in Fig.2. Also, in this case it is possible to extrapolate (see dashed line Fig. 2) the second boost pressure reference using the first boost pressure reference (or the initial boost pressure reference) and the second boost pressure reference to receive an modified second boost pressure reference valid for power output P1. This is denoted as R2’ in Fig. 2. The new relationship for choosing the boost pressure reference actually used in the operation of the internal combustion engine 1 based on the power output is then for example given by the reference at zero, and the modified second boost pressure reference, and the modified first boost pressure reference. For power output values between zero and P1, or between P1 and P2, linear interpolation can be used to derive the appropriate value. In other embodiments it is possible to simply use the first boost pressure reference R1 if the power output is at or near Pact-knock.Fig. 3 shows a schematic depiction of an embodiment of an internalcombustion engine 1 according to the invention.A gas dosage valve 12 is provided for mixing a fuel and air toproduce a combustible air / fuel mixture. The fuel can for example be natural gas including methane and / or molecular hydrogen. Alternatively, or additionally the fuel may comprise other hydrocarbons.The air / fuel mixture is compressed in compressor 13 of the chargeair system, here a turbocharger, so that the air / fuel mixture ischarged into a piston-cylinder unit 19 or a plurality of piston-cylinder units 19 while under a boost pressure p2’ (potentiallytogether with recirculated exhaust gas, see below).In a bypass conduct bypassing the compressor 13 of the turbochargerthere is a compressor bypass valve 16 which can be used to directan amount of the air / fuel mixture around the compressor 13 so asto lower the boost pressure p2’.In the conduct connecting the compressor 13 of the turbocharger tothe piston-cylinder unit(s) 19 there is a throttle valve 18.In this embodiment the engine is mixture charged as the gas dosagevalve 12 is upstream of the compressor 13. In other embodimentsthe gas dosage valve 12 can be arranged downstream of thecompressor 13 (air charged engine).The turbocharger in this example is a single stage turbocharger. In other embodiments according to the invention there can two,three, four or more turbocharger stages, or generally chargestages. The internal combustion engine 1 could also include a blowoff valve for rapidly discharging charged air and / or charged air / fuel mixture into the environment or other separate volumes. However, in this embodiment such a blowoff valve is not included. The cylinder charge, which is the air / fuel mixture charged intothe piston-cylinder unit(s) 19 under the boost pressure p2’ isignited using an ignition device 20, in this case a system comprising a spark plug for each piston-cylinder unit 19. In principle, the invention can also be used with compression ignition engines and / or engines operated with liquid fuel and / or dual fuel engines.The piston-cylinder unit(s) 19 can comprise a pre-combustionchamber.After combustion in the piston-cylinder unit(s) 19 the exhaustgases remaining in the cylinder are expelled therefrom. There is an exhaust gas recirculation passage in which an exhaustgas recirculation valve 21 is arranged.The exhaust gas recirculation valve 21 can be used to recirculatepart of the exhaust gas into the mass flow of the air / fuel mixture directed into the piston-cylinder unit 19, such that the cylinder charge comprises recirculated exhaust gas next to the air / fuel mixture. The exhaust gas which is not recirculated is decompressed in theexhaust turbine 14 of the turbocharger. The exhaust turbine 14drives the turbocharger shaft 15 which in turn drives thecompressor 13 of the turbocharger.Alternatively, or additionally to the exhaust turbine 14 the chargeair system 2 can comprise an electric drive for the compressor 13of the turbocharger.There is a bypass conduct bypassing the exhaust turbine 14 and awaste gate valve 17 is arranged in this bypass of the exhaustturbine 14 so that part of the exhaust gas can be routed past theexhaust turbine 14.The exhaust gas passing through the exhaust turbine 14 and / or thewastegate valve 17 is then subjected to aftertreatment in theexhaust gas aftertreatment system 22 which can comprise differentkinds of catalytic converters. For example, if the internal combustion engine 1 is operated with essentially stoichiometric lambda the exhaust gas aftertreatmentsystem 22 can comprise a three-way catalytic converter.In other embodiments where the internal combustion engine 1 is operated with a lean burn concept the exhaust gas aftertreatmentsystem 22 can comprise a selective catalytic reaction catalyticconverter and / or an oxidation catalytic converter and / or a thermal oxidiser. In embodiments where the internal combustion engine 1 is operated with a lean burn concept the exhaust gas recirculation valve 21 and the corresponding conduct may not be present. In embodiments where the internal combustion engine 1 can be operated both stoichiometrically and with a lean burn concept asdesired the exhaust gas recirculation valve 21 can preferably bekept closed as long as the internal combustion engine 1 is operated at lambda greater than one (lean operation).The engine controller 3 is in signal communication with theactuators of the internal combustion engines, which in this embodiment comprise the gas dosage valve 12, the exhaust gas recirculation valve 21, the at least one ignition device 20, the compressor bypass valve 16, the throttle valve 18, and the wastegate valve 17. The engine controller 3 is furthermore in signal communicationwith the knock sensor 4, and the NOx sensor 6 for receiving therespective measurement signals, and the boost pressure sensor 30. However, the signal communication of the engine control 2 with theactuators and the sensors is not depicted in Fig. 3 for the sakeof clarity the figure.The engine control 3 of this embodiment is also in signalcommunication with a multitude of further measurement devices which can be used to implement closed loop control, both for thehigh-level control 5 and the low-level control 26 (see Fig. 6).Measurement device which can for example be used in this capacity are -pressure sensors, upstream of the compressor 13 and / orupstream and / or downstream of the exhaust turbine 14, and / or -temperature sensors, upstream and / or downstream of thecompressor 13 and / or upstream and / or downstream of theexhaust turbine 14, and / or -in cylinder pressure sensors and / or- lambda sensors in the exhaust gas conduct and / or- oxygen concentration sensors and / or- engine speed sensorsThe knock sensor 4 and / or the NOx sensor 6 and / or and / or the boostpressure sensor 30 the further measurement devices mentioned above can be embodied as in principle known in the prior art. Also here, the signal communication of the engine controller 3 with the sensors is not depicted in Fig. 4 for the sake of clarity of the figure.The engine controller 3 is in this example adapted to perform theembodiments according to the invention described in connection with Fig. 1 and Fig. 2.In the embodiment of Fig. 3 the exhaust gas recirculation valve 8recirculates exhaust gas from upstream of the turbine 14 to avolume, e.g., into an intake manifold, downstream of the compressor13 when at least partially opened. This is called high-pressureexhaust gas recirculation.Fig. 4 shows an embodiment where the exhaust gas is recirculatedfrom downstream of the turbine 14 to upstream of the compressor13, i.e., a low-pressure exhaust gas recirculation. The other features of the embodiment according to Fig. 4 are analogous to the embodiment of Fig. 3.Fig. 5 schematically shows a genset with an internal combustionengine 1 according to the invention which is coupled mechanicallyto a generator 11 for creating electrical energy.The generator 11 may be connected to a power supply grid 23. Thepower supply grid 23 can for example be a public power supply grid, an island grind, or a microgrid.In these embodiments the engine controller 3 may also receivemeasurement values from the generator 11 and / or power supply grid23, such as a frequency ^ or voltage or current.Fig. 6 shows schematically a cascaded control scheme for operatingan internal combustion engine 1 according to the embodiment ofFig. 3 or 4.It comprises a high-level control 5 and number of low-levelcontrols 26. The low-level controls 26 receive setpoints from thehigh-level control 25 and output command values to the actuatorsof the internal combustion engine 1.The high-level control 5 receives and / or generates boost pressurereferences according to the invention, and during normal orotherwise operation, and may receive and / or generate otherreference values, e.g., for engine speed and / or generator frequency and / or engine power output. The high-level control can be embodied to perform and / or command the procedures described in connection with Fig. 1 (in particular reducing the boost pressure, setting of the first boost pressure reference) and / or Fig. 2 (in particular closed loop control of the NOx emissions, setting of the second boost pressure reference). Through setting of the, preferably piecewise linear, control relationship between the power and the boost pressure reference, a stationary boost pressure reference at a given load is provided and as a result a desired working point of the internal combustion engine 1 regarding emissions on the one hand and combustion stability on the other hand is defined (see in this context also EP 2977596 A1). It serves as basis for the calculation of the setpoints given to the low-level controls 26. It should be mentioned that the stationary boost pressure reference resulting from the, preferably piecewise linear, control relationship between the power and the boost pressure, and the boost pressure setpoint sp2’mentioned below are different elements of the cascaded control of this embodiment. The stationary boost pressure reference serves as a reference to define the desired operation point proportional to the load and usually does not change during stationary operation of the internal combustion engine. The boost pressure setpoint sp2’can change dynamically to support the speed and power control of the engine during transients and to stabilize the engine at the desired operation point defined by speed and boost pressure reference.The high-level control 5 in this embodiment includes a model-basedcontroller 24. Regarding the setup of the model-based controllerit is referred to the as yet unpublished International Patent Applications PCT / AT2023 / 060134 and PCT / AT2023 / 060230.The at least one high-level control 5 is configured to control apower output and a speed and a boost pressure of the internal combustion engine 1 by directly generating setpoints for the low- level controllers 17.This the model-based controller 24 comprises an appropriate costfunction which. Based on the measurements and the minimisation ofthe cost function the high-level control 24 computes setpoints forgas dosage, EGR mass flow, boost pressure and ignition timing. The gas dosage setpoint sgasis given to the gas dosage control 26. Based on the gas dosage setpoint sgasthe gas dosage control outputsa gas dosage command value ugas to the gas dosage valve 12 of theinternal combustion engine 1. The recirculated exhaust gas setpoint sEGRis given to the EGR control 27. Based on the recirculated exhaust gas setpoint sEGRtheEGR control 27 outputs a recirculated exhaust gas command valueuEGR to the exhaust gas recirculation valve 21 of the internalcombustion engine 1. The gas dosage setpoint sgasin this embodiment is given as a lambda setpoint but could in principle also be given as a mass flow parameter. The recirculated exhaust gas setpoint sEGRin this embodiment is given as mass flow parameter but can for example are be given relative to the mass flow of air and / or air fuel mixture in the intake manifold (fraction). The boost pressure setpoint sp2’is given to the boost pressure control 28. Based on the boost pressure setpoint sp2’the boost pressure control 28 outputs a compressor bypass valve command valueucbv to the compressor bypass valve 16 of the internal combustionengine 1, a throttle valve command value utvto the throttle valve18 of the internal combustion engine 1, and wastegate valve commandvalue uwgv to the wastegate valve 17 of the internal combustionengine 1. The ignition timing setpoint sITis given to the ignition timing control 29. Based on the ignition timing setpoint sITthe ignitiontiming control 29 outputs an ignition timing command value uIT tothe ignition device 20, in this embodiment a system comprising a spark plug for each piston-cylinder unit 19, of the internal combustion engine 1. The gas dosage control 26, the EGR control 27, the boost pressurecontrol 28 and / or the ignition control 29 can be open loopcontrollers. When the second aspect of the invention is carried out the closed loop control of the boost pressure is preferably carried out by the boost pressure control 28.The boost pressure controller 25 can preferably be a closed loopcontroller with a measured boost pressure as a feedback parameter. The gas dosage control can be a closed loop control with measured lambda or oxygen fraction in the exhaust as a feedback parameter.The modules (at least reference numerals 5 and 26 to 29) can beimplemented as hardware modules in reality. However, in preferred embodiments of the invention these modules are implemented as software modules being executed on the engine controller 3. Mixed implementations are of course also conceivable. Fig. 7a shows four diagrams of engine parameters during carryingout of an embodiment according to the invention in its firstaspect. The diagrams show the engine power, the boost pressure, the NOxconcentration in the exhaust gas (e.g., the NOx measurementsignals), and the EGR rate or, alternatively, the amount of air in the cylinder charge. In the embodiment of Fig. 7a the boost pressure is reduced while the power is kept essentially constant. In order to maintain the power output while the boost pressure is reduced For example, when the internal combustion engine is operated atstoichiometric conditions with EGR, the fuel mass may directly bedefined by the constant engine power setpoint and the airmass maybe defined by the operation at Lambda = 1 (or close to thissetpoint). Then the change of the boost pressure setpoint only influences the amount of dilution via EGR and therefore definesthe EGR rate (see fourth diagram in Fig. 7a).For example, when the engine is operated at lean burn conditionswithout EGR, the gas mass may be directly defined by the constantengine power setpoint. Then the change of the boost pressure setpoint only influences the amount of dilution via additional air(see fourth diagram in Fig. 7a) leading to Lambda > 1.At some point during the reduction of the boost pressure knock starts to manifest in separate knock measurement signals which are not shown in Fig. 7a. This is marked with an arrow and “Knock detected” in the second diagram in Fig. 7a. The boost pressure at this point (or the delta from the original boost pressure or boost pressure reference) can be viewed as the knock border. The first boost pressure reference is then set at a predetermined distance from the knock border. After this calibration the first boost pressure reference or a modified first boost pressure reference is then set as the reference for the boost pressure control of the internal combustion engine 1 such that the boost pressure is raised after the point at which knock was detected as can be seen in the second diagram of Fig. 7a. A NOx sensor can also be utilised in the first aspect of the invention. As can be seen in the third diagram of Fig. 7a, before the calibration is started by reducing the boost pressure theactual NOx concentration in the exhaust gas tracks an NOx referenceNOxref. After the calibration through reducing the boost pressure and setting the appropriate first boost pressure reference a new stable NOx concentration is established. After a certain time period, in which the NOx concentration stabilises, the new stable NOx concentration can be set or stored as new NOx reference NOxref. This new reference can be used in the second aspect of the invention.Fig. 7b shows another embodiment of the invention in its secondaspect, wherein the diagrams show the same parameters as the diagrams in Fig. 7a. Here, closed loop control of the NOx concentration during operation of the internal combustion engine 1 with the NOx measurement signals as NOx feedback signal for the closed loop control. As the third diagram shows the closed loop control manages to stabilise the NOx measurement signal at a given NOx reference NOxref(e.g., the NOx reference mentioned in connection with Fig. 7a).As actuating variable the closed loop control uses the boostpressure, here for example concretely the boost pressure setpoint sp2’. As can be seen from the second diagram in Fig. 7b the boost pressure consequently changes and also stabilises to a new value. After a pre-determined time span the boost pressure can be assumed to be stabilised (as also the engine power is kept constant in this embodiment). The actual value of the boost pressure or the boost pressure setpoint sp2’can then be used second boost pressure reference. As mentioned before in other embodiments of the invention the operating point of the internal combustion engine 1 could be defined alternatively or additionally by other engine variables than the boost pressure. Instead of the first and / or second boost pressure references other first and / or second engine variable references can then be achieved according to the invention. List of reference signs:1 internal combustion engine2 charge air system3 engine controller4 knock sensor5 high-level control6 NOx sensor7 compressor bypass valve8 wastegate valve9 throttle valve10 arrangement11 generator12 gas dosage valve13 compressor14 compressor15 turbocharger shaft16 compressor bypass valve17 wastegate valve18 throttle valve19 piston-cylinder unit20 ignition device21 exhaust gas recirculation valve22 exhaust gas aftertreatment system23 power supply grid24 model-based controller25 low-level control26 gas dosage control27 EGR control28 boost pressure control29 ignition timing control30 boost pressure sensorPact-knock power output during the first aspect of theinventionPact-NOx power output during the second aspect of theinvention ^p2’act-knockactual boost pressure defining the knock borderR1 first boost pressure referenceR1’ modified first boost pressure referenceR2 second boost pressure referenceR2’ modified second boost pressure referencesgasgas dosage setpoint sEGRrecirculated exhaust gas setpoint sP2’boost pressure setpoint sITignition timing setpoint ugasignition timing command value uEGRrecirculated exhaust gas command value ucbvcompressor bypass valve command value utvthrottle valve command value uwgvwastegate valve command value uITignition timing command value

Claims

Claims1. Internal combustion engine comprising an engine controller(3), a knock sensor (4), and at least one actuator (7,8,9) forinfluencing an engine variable, preferably a boost pressure generated by a charge air system (2), wherein the enginecontroller (3) is configured to receive knock measurement signals from the knock sensor (4), characterised in that theengine controller (3) is configured to -during operation of the internal combustion engine (1)actuate the at least one actuator (7,8,9) such that the engine variable is changed, preferably reduced, whilekeeping a desired power output and / or an actual poweroutput within a predefined range,- correlate at which value of the engine variable knockstarts to manifest in the knock measurement signals duringthe changing of the engine variable thereby identifying aknock border, and- set a first engine variable reference, preferably firstboost pressure reference, at a predetermined distance fromthe identified knock border.

2. Internal combustion engine according to claim 1, wherein theinternal combustion engine comprises a charge air system (2), preferably a turbo charger, and / or the engine variable is a boost pressure generated by the charge air system.

3. Internal combustion engine according to one of the precedingclaims, wherein the knock border is dependent on a load and / orpower output of the internal combustion engine (1) andpreferably the engine controller (3) is configured to performthe step of changing the engine variable – and preferablyidentifying the knock border – while the internal combustionengine (1) operates at or above 80 % power output and / or loadcapacity of the internal combustion engine (1).

4. Internal combustion engine according to one of the precedingclaims, wherein the predefined range for the desired poweroutput and / or the actual power output is defined by a deviation from a reference power value, preferably by a deviation of less than 10% and / or less than 5% and / or less than 3% and / or less than 1% from the reference power value.

5. Internal combustion engine according to one of the precedingclaims, wherein the engine controller (3) comprises a high- level control (5), wherein preferably an output of the high-level control (5) is an input for a control of an exhaust gasrecirculation rate of the internal combustion engine (1).

6. Internal combustion engine according to one of the precedingclaims, wherein the engine controller (3) comprises a high- level control (5), wherein preferably an output of the high-level control (5) is an input for a control of a fuel dosagesystem and / or for a control of an air / fuel equivalence ratio.

7. Internal combustion engine according to one of the precedingclaims, wherein a NOx sensor (6) for detecting a NOxconcentration in an exhaust gas of the internal combustion engine (1) is provided, wherein the engine controller (3) isconfigured to -receive NOx measurement signals from the NOx sensor (6),and -set a NOx value measured after setting, and preferablytracking, of the boost pressure reference – preferablyafter a stabilisation time period for the NOx concentration– as NOx reference value.

8. Internal combustion engine according to one of the precedingclaims, wherein the at least one actuator (7,8,9) forinfluencing the engine variable comprises one or more of thefollowing: a compressor bypass valve (7), a wastegate valve(8), a throttle valve (9), an actuator for actuating a variableturbine geometry, a blowoff valve, an electric motor for driving the compressor.

9. Internal combustion engine according to one of the precedingclaims, wherein the knock sensor (4) comprises or is anacceleration sensor and / or an in-cylinder pressure sensor and / or a strain gauge configured to measure a pressure in a cylinder and / or an ion current sensor.

10. Internal combustion engine, in particular according to one ofthe preceding claims, comprising an engine controller (3), aNOx sensor (6) for detecting a NOx concentration in an exhaust gas of the internal combustion engine (1), and at least oneactuator (7,8,9) for influencing an engine variable, inparticular a boost pressure generated by a charge air system (2), wherein the engine controller (3) is configured to receiveNOx measurement signals from the NOx sensor (6), characterisedin that the engine controller (3) is configured to- carry out closed loop control of the NOx concentrationduring operation of the internal combustion engine (1) withthe NOx measurement signals as NOx feedback signal for theclosed loop control, -use the engine variable or another variable that influencesthe engine variable as actuating variable for the closed loop control of the NOx concentration, and -determine and / or store an actual and / or set value of theengine variable as a second engine variable reference whenthe NOx feedback signal is stabilised at a NOx reference value.

11. Internal combustion engine according to claim 10, wherein theengine controller (3) is configured to carry out the closed loop control of the NOx concentration when the internalcombustion engine (1) operates at 30% to 70%, particularlypreferably 40% to 60% and particularly preferably 45% to 55%, power output and / or load capacity.

12. Internal combustion engine according to claim 10 or 11, whereinthe engine controller (3) is configured to extrapolate the second engine variable reference to 50% power output and / orload capacity, preferably making use of the first enginevariable reference valid for 80%, particularly preferably 90%and particularly preferably 95%, or more power output and / or load capacity of the internal combustion engine (1).

13. Internal combustion engine according to one of the precedingclaims, wherein the engine controller (3) is configured tofurther modify the first engine variable reference and / or thesecond engine variable reference in dependence on a poweroutput and / or a load and / or a desired NOx concentration of the exhaust gas and / or an ignition timing point and / or an engine speed and / or a charge temperature and / or an exhaust gas temperature and / or the setting of a variable valve timing system of the internal combustion engine and / or an ambient airtemperature and / or an ambient air humidity and / or a fuelcomposition and / or a fuel quality.

14. Internal combustion engine according to one of the precedingclaims, wherein the engine controller (3) is configured to extrapolate the identified knock border and / or the first engine variable reference to 100% power output and / or load capacity,preferably making use of a second engine variable referencevalid for 30% to 70%, particularly preferably 40% to 60% andparticularly preferably 45% to 55%, power output and / or load capacity of the internal combustion engine (1).

15. Internal combustion engine according to at least claim 1 andclaim 9, optionally according to one of the claims 2 to 8 or10 to 12, wherein the engine controller (3) is configured tointerpolate between the first engine variable reference andthe second engine variable reference based on a desired powerand / or load of the internal combustion engine (1), andpreferably to use an interpolated engine variable referenceresulting from the interpolation between the first engine variable reference and the second engine variable referencefor a closed or open loop control of the engine variable.

16. Arrangement comprising an internal combustion engine (1)according to one of the preceding claims and a generator (11) for creating electrical energy mechanically driven by the internal combustion engine (1).

17. Computer program product for controlling an internalcombustion engine (1), in particular according to one of theclaims 1 to 15, and / or an arrangement (10) comprising an internal combustion engine (1), in particular according toclaim 16, comprising instructions which cause an executing computer to perform the following: -during operation of the internal combustion engine (1)outputting actuation signals to at least one actuator(7,8,9) capable of influencing an engine variable such thatthe engine variable is changed, while keeping a desiredpower output and / or an actual power output within a predefined range, -receiving knock measurement signals from a knock sensor(6) as input,- correlating at which value of the engine variable knockstarts to manifest in the knock measurement signals duringthe changing of the engine variable thereby identifying aknock border, and -setting a first engine variable reference at apredetermined distance from the identified knock border.

18. Computer program product, in particular according to claim 17,for controlling an internal combustion engine (1), inparticular according to one of the claims 1 to 15, and / or an arrangement (10) comprising an internal combustion engine (1),in particular according to claim 16, comprising instructions which cause an executing computer to perform the following: -receiving NOx measurement signals from a NOx sensor (6) asinput, -carrying out closed loop control of a NOx concentrationduring operation of the internal combustion engine (1) withthe NOx measurement signals as NOx feedback signal for theclosed loop control, -using an engine variable or another variable thatinfluences the engine variable as actuating variable forthe closed loop control of the NOx concentration byoutputting actuation signals to at least one actuator (7,8,9) capable of influencing the engine variable, and- determining and / or storing an actual and / or set value ofthe engine variable as a second engine variable referencewhen the NOx feedback signal is stabilised at a NOx reference value.

19. Transitory or non-transitory data storage having stored acomputer program product according to claim 17 or 18.

20. Method for controlling an internal combustion engine, inparticular according to one of the claims 1 to 15, and / or anarrangement (10) comprising an internal combustion engine (1),in particular according to claim 16, wherein the following steps are performed: -during operation of the internal combustion engine (1)using at least one actuator (7,8,9) capable of influencingan engine variable such that the engine variable ischanged, while keeping a desired power output and / or anactual power output within a predefined range, -receiving knock measurement signals from a knock sensor(6) as input, -correlating at which value of the engine variable engineknock starts to manifest in the knock measurement signalsduring the changing of the engine variable therebyidentifying a knock border, and -setting a first engine variable reference at apredetermined distance from the identified knock border.

21. Method, in particular according to claim 20, for controllingan internal combustion engine (1), in particular according toone of the claims 1 to 15, and / or an arrangement comprising (10) an internal combustion engine (1), in particular accordingto claim 16, wherein the following steps are performed: -carrying out closed loop control of a NOx concentrationduring operation of the internal combustion engine (1) with aNOx feedback signal for the closed loop control,- using an engine variable or another variable thatinfluences the engine variable as actuating variable forthe closed loop control of the NOx concentration by outputting actuation signals to at least one actuator (7,8,9) capable of influencing the engine variable, and- determining and / or storing an actual and / or set boostpressure as a second engine variable reference when theNOx feedback signal is stabilised at a NOx reference value.

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