Method and control unit for calculating the raw NOx emissions of an internal combustion engine

Abstract

Procedure for calculating NO x -Raw emissions (NOx) x,AKT ) of an internal combustion engine (10) from operating parameters of the internal combustion engine (10), wherein an oxygen concentration (O₂) is a key operating parameter 2,AKT ) is determined in a suction pipe (20) and the calculation of the NO x -Raw emissions (NOx) x,AKT ) depending on the determined oxygen concentration (O 2,AKT ) is characterized in that a correction function takes into account the dependence of the raw NOx emission (NO x, AKT ) from a temperature prevailing in the intake manifold (20) (T AKT ) taken into account.

Description

State of the art

[0001] The present invention relates to a method for calculating a raw NOx emission of an internal combustion engine according to the preamble of claim 1. Furthermore, the invention relates to a computer program according to the preamble of claim 9 and a control unit according to the preamble of claim 10.

[0002] In the exhaust aftertreatment of diesel engines and gasoline direct injection engines, selective catalytic reduction (SCR) is often used to reduce nitrogen oxide emissions (NO x ) is used. A reducing agent, usually ammonia (NH3), must be added to the exhaust stream. The amount of reducing agent added depends largely on the amount of NO emitted. x If the amount of reducing agent is too small, too little NO xHowever, if too much reducing agent is added, it will also be emitted and thus represents an environmental pollution that must be avoided. Usually, the current NO x -Emissions are recorded using a sensor.

[0003] As long as the sensor is not ready for operation after a cold start of the internal combustion engine, the reducing agent dosage NO x -raw emission values, which are calculated using models. The NO is also used for on-board diagnostic monitoring. x The sensor is compared with calculated values. In recent applications, the sensor is even completely replaced by a calculation model for cost reasons.

[0004] The described use cases require a high accuracy in the modelling of NO x -raw emission values. From DE 10 2009 055 058 A1 a NO x-Raw emission model is assumed to be known, which shows a relationship between the NO x The raw emissions and the oxygen concentration in the intake manifold of the internal combustion engine are stored in characteristic maps as a function of the engine's operating point and the current injection pattern or operating mode. The operating point is essentially characterized by the engine's speed and load. This model takes environmental influences such as pressure and temperature into account through correction factors that are applied to the base values ​​stored in the characteristic maps.

[0005] However, even under constant ambient conditions, the temperature after the intercooler, i.e. in the intake manifold, can vary, which has a significant impact on the NO x-raw emissions. Factors that influence this include, for example, the accumulated heat when stationary, changes in the efficiency of the intercooler when the temperature of the cooling medium changes, etc. The well-known NO x The raw emission model only takes these influences into account indirectly, since a change in temperature is also accompanied by a change in the oxygen concentration in the intake manifold.

[0006] The known model, however, does not take into account that a temperature change after the intercooler causes a temperature change in the combustion chamber charge before compression begins, and thus a change in the combustion temperature. Because an increased combustion temperature promotes the oxidation of atmospheric nitrogen (Zeldovich reaction), the combustion temperature has a significant influence on the NO x -Emission. Depending on the operating point, this effect can clearly outweigh the aforementioned effect caused by the change in oxygen concentration.

[0007] From the published patent application DE 10 2009 036 060 A1, a determination of raw NOx emissions from an internal combustion engine is known by measuring the exhaust gases of the internal combustion engine during operation on a test bench.

[0008] German Patent Application DE 100 43 690 A1 discloses a method for determining the NOx mass flow at the inlet of a NOx storage catalyst in the exhaust gas of an internal combustion engine based on the engine's operating parameters. This method takes into account the intake air temperature and the engine oil temperature.

[0009] German Patent Application DE 198 51 319 A1 discloses a method for determining raw NOx emissions from an internal combustion engine. This method incorporates operating parameters such as fuel mass and engine speed. Disclosure of the invention

[0010] The invention represents a significant improvement of the already known method for calculating NO x-Raw emissions in the exhaust tract of an internal combustion engine. A key idea is to improve the accuracy of the known method in real vehicle operation by taking into account the influence of the temperature after the charge air cooler, or in the intake manifold of the internal combustion engine, on the NO x -raw emissions are taken into account.

[0011] It has been shown that a temperature increase after the intercooler affects the NO x -raw emissions. This is caused by two mechanisms: Firstly, the increased temperature after the intercooler reduces the filling of the combustion chambers. This also reduces the exhaust gas recirculation (EGR) rate via the air mass control. Because there is less recirculated exhaust gas in the combustion chamber, the proportion of fresh air and thus the oxygen content is higher. A higher oxygen content causes higher NO x-emission. This mechanism is taken into account by the known model, as it calculates the NO x -Emission calculated as a function of the oxygen concentration in the intake manifold.

[0012] The second mechanism, even with a constant oxygen concentration in the intake manifold, leads to an increased combustion temperature via an increased temperature of the combustion chamber filling and thus to an increase in NO x -Raw emissions. The so-called Zeldovich mechanism leads to the formation of nitrogen monoxide at high combustion temperatures via the formation of oxygen radicals and subsequent oxidation of atmospheric nitrogen. This produces nitrogen radicals, which in turn react with oxygen molecules to form nitrogen monoxide and oxygen radicals. This mechanism is based on the already known NO x Calculation model not shown. By applying the known NO xcalculation model, this second mechanism can be clearly separated from the first.

[0013] According to the invention, the temperature influence of the Zeldovich reaction during operation without exhaust gas recirculation (i.e. with constant oxygen concentration in the intake manifold) on the NO x -emission. This temperature influence can easily be transferred to the operation of the internal combustion engine with exhaust gas recirculation. Subsequently, by applying the well-known NO x The influence of oxygen concentration is taken into account in the calculation model. The correction function according to the invention thus enables more precise modeling and, in combination with the already known calculation model, contributes to an improvement in modeling in real vehicle operation.

[0014] A further significant advantage is that the output value of the correction function according to the invention is used as an input value for other, already known correction functions. By appropriately arranging the correction functions in the model kernel, or in the main program, interactions between the correction functions are avoided. Thus, the invention makes it possible to apply the correction function according to the invention independently of other correction functions. Overall, the method according to the invention improves the accuracy of the modeling result in real operation.

[0015] A further advantage is that the correction function is very easy to calculate. This is because the influence of oxygen concentration on NO can be easily separated. x-Evaluation of the influence of combustion temperature without exhaust gas recirculation. This significantly reduces the application effort compared to other known models.

[0016] Further advantages and advantageous embodiments of the invention can be found in the following drawings, their description, and the patent claims. All features disclosed in the drawings, their description, and the patent claims may be essential to the invention both individually and in any combination.

[0017] They show: Fig. 1 the technical environment of the invention; Fig. 2a empirically determined relationship of NO x -Raw emission with the oxygen volume fraction and the temperature in the intake manifold; Fig. 2b modeled relationship of NO x -Raw emission with the temperature in the intake manifold; Fig. 3 shows a first embodiment of the invention in a functional block diagram; Fig. 4 shows a further embodiment in a functional block diagram; Fig. 5 Integration of the method according to the invention into a known NO x -Raw emission model in functional block representation.

[0018] Fig. 1 shows an internal combustion engine 10, which serves in particular to drive a motor vehicle. The internal combustion engine 10 shown operates with direct injection of fuel via injectors 12 into combustion chambers 14 of the internal combustion engine 10 according to the Otto combustion process, the diesel combustion process, or another combustion process, for example, a CAI combustion process (CAI = Controlled Auto Ignition). In the Otto combustion process, the combustion chamber charge of air and injected fuel is ignited by a spark plug 16. Each combustion chamber 14 is movably sealed by a piston 18 and filled with air via an intake manifold 20. Upstream of the intake manifold 20, a charge air cooler 22 is arranged, which cools the air before it enters the intake manifold 20. A turbocharger arranged upstream of the charge air cooler 22 is not shown.A temperature sensor 24 is arranged between the intercooler 22 and the combustion chamber 14. This sensor measures the temperature of the air after it exits the intercooler 22 and thus before it enters the combustion chamber 14. The fuel-air mixture combusted in the combustion chamber 14 is expelled into an exhaust system 26. The change in the combustion chamber charge (gas exchange) is controlled by gas exchange valves 28, 30, which are actuated synchronously with the movement of the piston 18.

[0019] When the exhaust gas recirculation valve 34 is open, exhaust gas is returned to the combustion chamber 14 via an exhaust gas recirculation 32 in order to reduce the NO x -Reduce raw emissions from the internal combustion engine.

[0020] The temperature of the air after leaving the charge air cooler 22 is called T akt It can be further differentiated into a temperature T n,LLK before the exhaust gas recirculation 32 and a temperature T n,misch after exhaust gas recirculation 32. If the temperature Takt As mentioned, both temperatures T n,LLK and T n,misch The temperature T n,misch after the exhaust gas recirculation 32 can be determined either by a separate temperature sensor (not shown) or by a corresponding conversion of the temperature T detected by the temperature sensor 24 n,LLK before the exhaust gas recirculation 32.

[0021] The exhaust system also includes exhaust aftertreatment components not shown, such as particulate filters, catalysts, dosing systems for reducing agents, exhaust gas sensors 36, etc.

[0022] The internal combustion engine 10 is controlled by a control unit 38, which processes, among other things, signals from the exhaust gas sensor 36, a speed sensor 40, the temperature sensor 24, and other sensors for additional operating parameters. Such additional operating parameters include, for example, the accelerator pedal position or the mass of the fresh air drawn in. From these signals, the control unit 38 generates control signals for injectors 12, for any spark plugs 16 present, for the exhaust gas recirculation system 34, and possibly other control signals for actuators not shown in the figure, which are present in modern internal combustion engines 10 and are therefore familiar to those skilled in the art.

[0023] Furthermore, the control unit 38 is configured, in particular programmed, to carry out the method according to the invention or one of its embodiments. The control unit 38 is configured, in particular, to x-To model raw emissions as a function of the oxygen concentration in the intake manifold 20. The value of the oxygen concentration in the intake manifold 20 is calculated anyway in known control units 38 of internal combustion engines 10 or is present in the control units 38. Key input variables for this are the intake fresh air mass, the exhaust gas recirculation rate, as an intermediate variable, the injected fuel quantity, and an oxygen concentration in the exhaust gas that is modeled or measured with the exhaust gas sensor 36.

[0024] An essential prerequisite for a concrete modeling of NO x -raw emissions as a function of the oxygen concentration or the temperature in the intake manifold 20 is the knowledge of the quantitative relationships.

[0025] Fig. Figure 2a shows such empirically determined relationships. The NO x-Concentration in ppm in the raw emissions of the internal combustion engine 10 is plotted against the volume fraction of oxygen in % in the intake manifold 20. Both the oxygen concentration and the NO x -Concentration are plotted logarithmically.

[0026] An evaluation of the empirically obtained data shows that an increase in the temperature in the intake manifold 20 leads, on the one hand, to an increased oxygen concentration in the intake manifold 20 and thus to an increase in the NO x-raw emissions. Assuming air mass control of the internal combustion engine 10, the increased oxygen concentration can be explained as follows: At elevated temperatures downstream of the charge air cooler 22, the charge volume of the combustion chambers 14 is reduced. This, in turn, results in a reduction in the exhaust gas recirculation rate. Since the fresh air has a higher oxygen concentration than the recirculated exhaust gas, a higher oxygen concentration is achieved in the intake manifold 20. This effect is represented by an arrow 42.

[0027] On the other hand, the increased temperature in the intake manifold 20 at constant oxygen concentration also leads to an increase in NO x-raw emissions. The relationships are as follows: An increase in temperature in the intake manifold leads to an increase in the temperature of the combustion chamber charge at the start of compression, thereby increasing the temperature during combustion. High combustion temperatures promote the so-called thermal nitrogen oxide formation, in which radicals are formed from atmospheric nitrogen and oxygen, which react with each other to form nitrogen monoxide (NO). These reactions, also known as the Zeldovich mechanism, are the dominant process in the combustion chamber for the formation of nitrogen oxide during combustion. The combustion temperature primarily influences the rate at which nitrogen monoxide is formed. Thus, an increased temperature at a constant oxygen concentration in the intake manifold 20 leads to increased nitrogen oxide emissions. An arrow 44 symbolizes this effect in Fig. 2a.

[0028] The two effects described above can be clearly separated from each other. This is demonstrated by the following Fig. 2b clearly: Fig. Figure 2b shows the relationship between the nitrogen oxide concentration (NOx) in ppm, plotted logarithmically on the ordinate, and the oxygen concentration in mass percent, also plotted logarithmically on the abscissa. Measurements yielded parallel straight lines 45a, 45b, and 45c for different temperatures downstream of the intercooler and in the intake manifold 20, respectively, whose gradient is not affected by temperature changes.

[0029] An evaluation of the empirically obtained data has shown that this relationship can be explained by the formula NOx,Cor=NOx,Bas⋅eC⋅(TAkt−TRef) can be represented.

[0030] In it is NO x, Cor , the corrected NO x -Concentration, which depends on the current temperature T Aktafter the intercooler and the NO concentration measured under reference conditions x, Bas can be calculated. NO x, Bas is the NO x -Raw emission which is at a reference temperature T Ref after the intercooler 22. The condition without exhaust gas recirculation is suitable as a reference condition, since the oxygen concentration in the intake manifold 20 is constant.

[0031] C is a parameter that has a constant value on a test bench. Generally, different internal combustion engines 10 will result in a different value for C, which may also depend on the operating point of the internal combustion engine 10.

[0032] This means that both the corrected NO x -Concentration NO x, Cor as well as the recorded concentration of NO x, Bas This refers to an oxygen concentration of 23.15 mass%. This means that the internal combustion engine 10 operates without exhaust gas recirculation.

[0033] Fig. Figure 3 shows a functional block diagram of a first exemplary embodiment of the method according to the invention with the parameter C as a constant. Block 46 symbolizes the formation of a variable representing the load L of the internal combustion engine 10. The load L can be a fuel metering signal or a value of the internal torque of the internal combustion engine 10 that is already calculated by the control unit 38. The usual methods for determining both variables are known to those skilled in the art.

[0034] In block 48, the rotational speed n of the internal combustion engine 10 is calculated from signals from the rotational speed sensor 40.

[0035] In block 50 the current temperature T is calculated AKT after the charge air cooler 22 or in the intake manifold 20 of the internal combustion engine 10 from the signals of the temperature sensor 24 arranged there.

[0036] In a map 52, reference values ​​NO determined in reference states are x,Bas , the NO x-Raw emissions stored depending on load and speed.

[0037] A map 54 contains load and speed dependent stored values ​​of the respective associated temperature T Ref . The value of parameter C is stored in function block 56. Blocks 46 to 56 thus supply all the variables required for the application of equation 1 to block 58, which calculates the corrected nitrogen oxide concentration NO x, Cor according to equation 1. This corrected NO x -Concentration is passed to the emissions calculation model.

[0038] Fig. 4 shows a further embodiment in the case of a load and / or speed-dependent parameter C. The process according to the invention is carried out in accordance with the method described in Fig. 3 of the previously described embodiment. With the difference that instead of a constant value in function block 56, load- and / or speed-dependent values ​​for C are stored in a characteristic map.

[0039] The correction function according to the invention Fig. 3 or Fig. 4 is, as follows in Fig. 5, the known overall function for the NO x -model implemented. The known overall function calculates the NO x -Raw emissions according to the following equation 2: NOx,AKT=NOx,REF⋅(O2,AKTO2,REF)α

[0040] In it is NO x, AKT the current NO x -Concentration in ppm, which depends on the current oxygen concentration O2, AKT in mass percent and the concentrations NO recorded under reference conditions x, REF and O 2, REFThe exponent α depends on the speed and load of the internal combustion engine 10 and, to a first approximation, is independent of the current O2 concentration. The reference concentrations NO x, REF and O 2, REF depend on the speed and the load. Examples of the known overall function also include load and speed dependent correction functions for α and O 2, REF before.

[0041] According to the invention, in equation 2 explained above, instead of NO x, REF the result from equation 1, i.e. NO x, Cor used.

[0042] Fig. Figure 5 shows a functional block diagram of the known overall function with the correction function according to the invention implemented. Block 46, as in the previous figures, represents the generation of a variable representing the load L of the internal combustion engine 10. Block 48 calculates the rotational speed n of the internal combustion engine 10 from the signals of sensor 40.

[0043] In block 60 the oxygen concentration O is calculated 2, akt in the intake manifold 20 of the internal combustion engine 10. The oxygen concentration is essentially the quotient of the oxygen flow into the intake manifold 20 in the numerator and the filling mass flow (fresh air and recirculated exhaust gas) into the combustion chambers 14 of the internal combustion engine 10 in the denominator.

[0044] The filling mass flow is determined by the general gas equation depending on the known intake manifold volume, the measured intake manifold pressure, the measured temperature and the general gas constant.

[0045] The oxygen flow into intake manifold 20 is determined as the sum of the measured fresh air mass flow weighted by the known oxygen concentration of the fresh air (23.15 mass percent) and the mass flow of the recirculated exhaust gas weighted by the oxygen concentration in the exhaust gas. The oxygen concentration in the exhaust gas is determined from the signal of exhaust gas sensor 36. The mass flow of the recirculated exhaust gas is determined by subtracting the measured fresh air mass flow from the charge mass flow calculated from the general gas equation.

[0046] The oxygen concentration in the exhaust gas is lower than the oxygen concentration in the fresh air, so the resulting oxygen concentration in the intake manifold is the average of the oxygen concentrations in the exhaust gas and fresh air, weighted by the fresh air and exhaust gas masses involved. Such calculations are performed by modern control units anyway and are familiar to experts.

[0047] A map 62 contains load and speed dependent stored values ​​of the respective oxygen concentration O 2, REF . Values ​​of the exponent α are also stored in map 64, depending on load and speed. In block 66, as in the previous Fig. 3 and Fig. 4 explains the corrected nitrogen oxide concentration NO x, Cor The blocks 60 to 66 thus supply all the quantities required for the application of equation 1 to block 68, which calculates the current NO x-Concentration NO x, AKT represented by equation 1.

[0048] In the known overall function, the relationship between NO x -Raw emissions and oxygen concentration in the intake manifold 20 are stored in characteristic maps depending on the respective operating point, the operating mode, and the injection profile of the internal combustion engine 10. Depending on the currently prevailing ambient conditions (ambient pressure and temperature), corrections are then made to the current values ​​taken from the characteristic maps (not explicitly shown in the figure).

[0049] Fig.Figure 5 shows an embodiment of the known overall function represented by equation 2. As already explained above, the gradient of the straight line 45 is not changed by a change in the fresh air temperature in the intake manifold 20, so that the correction function according to the invention (equation 1) is advantageously applied to the raw NOx emission without exhaust gas recirculation. And then the nitrogen oxide value NO corrected with the correction function according to the invention can be x, Cor serve as the starting value for the known overall function (equation 2), i.e. for operation with exhaust gas recirculation. This means that in equation 2, NO x, REF by NO x, Cor By applying equation 2 to the initial value of equation 1, both equations and the associated corrections can be used to model the NO x -Raw emissions are applied independently of each other.

Claims

[1] Method for calculating NO x -Raw emissions (NO x,AKT ) of an internal combustion engine (10) from operating parameters of the internal combustion engine (10), wherein an oxygen concentration (O 2,AKT ) in a suction pipe (20) and the calculation of the NO x -Raw emissions (NO x,AKT ) depending on the determined oxygen concentration (O 2,AKT ) is carried out, characterized by that a correction function is a dependence of the raw NOx emission (NO x, AKT ) from a temperature (T AKT ) taken into account. [2] Method according to claim 1, characterized by that the correction function satisfies the following equation: NOx,Cor=NOx,Bas⋅eC⋅(TAkt−TRef) where NO x,Cor , the corrected NO x -Concentration, which depends on the current temperature T Aktafter the charge air cooler (22) and the NO concentration measured under reference conditions x,Bas the NO x -Raw emission at temperature T Ref after the intercooler (22) and C is a constant. [3] Method according to claim 1 or 2, characterized by that the corrected NO x -Concentration (NO x, Cor ) the NO x -Raw emissions (NO x, AKT ) is calculated. [4] Method according to one of the preceding claims, characterized by that the parameter C is an internal combustion engine-specific constant. [5] Method according to one of the preceding claims, characterized by that the parameter C is a load and / or speed dependent variable [6] Method according to one of the preceding claims, characterized by that the parameter C is determined by a map access or calculated during operation of the internal combustion engine (10). [7] Method according to one of the preceding claims, characterized by that the temperature (T akt ) of the air is equal to the temperature (T n,LLK ) before the exhaust gas recirculation (32). [8] Method according to one of the preceding claims, characterized by that the temperature (T akt ) of the air is equal to the temperature (T n,misch ) after the exhaust gas recirculation 32. [9] Computer program, characterized by that it is programmed for use in a method according to one of the preceding claims. [10] Control and / or regulating device for an internal combustion engine (10), characterized by that it is programmed for use in a method according to one of claims 1 to 8.

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