DE102021209369A1 - Method of monitoring an oxidation catalyst - Google Patents

Abstract

The invention relates to a method for monitoring an oxidation catalytic converter, which is arranged in an exhaust line of an internal combustion engine, a model-based differential heat quantity (Q ^WPA-BPU ) being determined based on a mass flow of exhaust gas after passing through the oxidation catalytic converter and a model-based temperature difference, the specified, model-based Temperature difference corresponds to a difference between temperatures that occur in a first oxidation catalytic converter and second oxidation catalytic converter, which has a lower efficiency than the first oxidation catalytic converter, for the exhaust gas after passing through the oxidation catalytic converter, based on the mass flow of the exhaust gas after passing through the oxidation catalytic converter and a measured temperature difference a measured differential heat quantity (Q ^meas-BPU ) is determined, the measured temperature difference being a difference between a current, measured temperature and the temperature ent speaking, which occurs in the second oxidation catalytic converter for the exhaust gas after passing through the oxidation catalytic converter, and based on a comparison of a quotient (r _DOC ) from the measured differential heat quantity (Q ^mess-BPU ) and the modeled differential heat quantity (Q ^WPA-BPU ) with a predetermined quotient threshold value (rs) is closed on the functionality of the oxidation catalyst.

Description

The present invention relates to a method for monitoring an oxidation catalytic converter and a computing unit and a computer program for carrying it out.

Background of the Invention

Modern motor vehicles are often equipped with catalytic converters for after-treatment of an exhaust gas from an internal combustion engine. In many cases these catalysts are monitored and/or controlled.

In diesel engines in particular, incomplete combustion of the air-fuel mixture leads to combustion products such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides ( _NOx ). According to the current state of the art, applicable exhaust emission limit values for motor vehicles for such combustion products can only be met with catalytic exhaust gas aftertreatment. By using an oxidation catalytic converter, for example, the pollutant components hydrocarbons (HC) and carbon monoxide (CO) can be converted. A subsequent SCR catalytic converter can remove nitrogen oxides (NO _x ).

Disclosure of Invention

According to the invention, a method for monitoring an oxidation catalyst and a computing unit and a computer program for its implementation are proposed with the features of the independent patent claims.

Advantageous configurations are the subject of the dependent claims and the following description.

The invention deals with the monitoring of an oxidation catalytic converter, in particular a diesel oxidation catalytic converter, which is arranged in an exhaust system of an internal combustion engine and through which the exhaust gas flows. An SCR catalytic converter is preferably also arranged downstream of the oxidation catalytic converter in the exhaust system.

As mentioned, such an oxidation catalytic converter or diesel oxidation catalytic converter (DOC stands for “Diesel Oxidation Catalyst”) must be monitored with regard to its emission-reducing effect on hydrocarbons (HC) and carbon monoxide (CO). This means that a monitoring function that can distinguish a healthy oxidation catalyst from a non-healthy oxidation catalyst is desired. The CO-reducing effect of the oxidation catalytic converter is provided by a catalytic precious metal coating, which is able to oxidize the CO to CO ₂ with high efficiency in lean-burn operation, which is typical for diesel engines, even at very low exhaust gas temperatures. The HC-reducing effect of the oxidation catalytic converter is based on the one hand on the storage of the HC components in the exhaust gas at low exhaust gas temperatures (engine cold start) and on the other hand on the oxidation of the HCs to CO ₂ and H ₂ O at exhaust gas temperatures above the ignition temperature typical of a catalytic converter coating (so-called "Light Off").

Damage to the oxidation catalytic converter causes a reduction in HC storage capacity and an increase in the light-off for HCs. Conventional monitoring approaches for the oxidation catalytic converter are based on the fact that at low exhaust gas temperatures in the ignition temperature range, a high HC concentration is used to generate exothermic oxidation on the oxidation catalytic converter, which can be measured using an exhaust gas temperature sensor downstream of the oxidation catalytic converter. A sufficiently damaged oxidation catalytic converter will show a reduced exothermic reaction due to its increased ignition temperature and will thus be assessed as defective by the monitoring function.

Appropriate monitoring conditions (high HC concentration in the exhaust gas, low exhaust gas temperature) can occur for some engine/exhaust systems under regular engine operating conditions, e.g. during warm-up measures during an engine cold start. Alternatively, the monitoring conditions can also be created actively by temporarily switching over the engine operation. To calculate the exothermicity generated in the oxidation catalytic converter, a modeled temperature without HC conversion (i.e. a model of an oxidation catalytic converter that has no HC conversion is calculated for comparison) can be subtracted from a measured temperature downstream of the oxidation catalytic converter. Based on this temperature difference, the heat generated in the oxidation catalytic converter can be calculated, taking into account the exhaust gas mass flow.

This "measured" exotherm can be related to an expected exotherm. This can either be calculated directly from the calorific value of the HC converted on the oxidation catalytic converter or it is calculated on the basis of temperature models in a manner similar to that described above. From the temperature at the outlet of the oxidation catalytic converter of a model with an intact oxidation catalytic converter, the temperature described above of a model without Sales subtracted and thus the exotherm expected for an intact system can be calculated.

However, such methods have disadvantages with regard to the accuracy of the monitoring. Only catalysts with a very strong light-off shift can be detected (e.g. by more than 100°C). Only with these catalysts is the difference in the exotherm under all circumstances so great that the boundary catalyst can be distinguished from the intact oxidation catalyst. The closer the light-off of the borderline catalytic converter is to that of the intact oxidation catalytic converter (e.g. 40°C), the more likely it is that not only the catalytic converter but also the operating conditions that result from the driving style play a role in the exotherm. While driving in a city - in a simplified calculation - e.g. often leads to catalyst temperatures in the relevant range and thus to a measurable difference in exothermicity, driving on the motorway causes a rapid increase in temperature, with the shifted light-off having a minor influence on the exothermicity.

Against this background, the use of models for an intact and a non-intact oxidation catalytic converter is proposed within the scope of the invention. In order to cover as many cases as possible of intact and non-intact or defective oxidation catalytic converters, a so-called WPA template is used for the intact oxidation catalytic converter (WPA stands for "worst part acceptable"), i.e. an oxidation catalytic converter that or its functionality is just as is considered acceptable. Accordingly, a so-called BPU sample (BPU stands for "best part unacceptable") is used for the defective oxidation catalytic converter, i.e. an oxidation catalytic converter which or whose functionality is no longer considered acceptable. In principle, however, the method already works with models of two different oxidation catalysts, one of which has a lower efficiency than the other oxidation catalyst (with regard to the conversion of exhaust gases or HCs).

The method is based on the prediction (by means of models) of the temperature behind the oxidation catalyst for two cases. On the one hand, a system with an intact (first) oxidation catalytic converter is modeled, with the oxidation catalytic converter preferably aging at most as much as can be expected in the vehicle's lifetime (WPA). The modeled temperature is also referred to below as T ^ModWPA .

On the other hand, a system with an aged (second) oxidation catalytic converter is modeled, whereby the oxidation catalytic converter has aged so much that the emissions (just) no longer correspond to the OBD limit value (BPU). This modeled temperature is referred to as T ^ModBPU in the following. The specific models of how the efficiency of the oxidation catalyst affects the temperature in the exhaust gas are known in the art and are not the subject of the invention. Nevertheless, the models should be as accurate as possible. For example, a kinetic model can be used that depicts the reaction and storage processes using Arrhenius equations.

wobei c_p die spezifische Wärmekapazität des Abgases und dmldt den Massenstrom des Abgases angeben. Hierfür ist nur der aktuelle Massenstrom des Abgases zu erfassen bzw. zu bestimmen, z.B. unter Verwendung einer entsprechenden Messeinrichtung oder eines Sensors.A modeled differential temperature ΔT ^ModWPA-BPU can be determined from the difference between the two models or the temperatures obtained for both models . On the basis of this variable, it can be deduced how well the two catalysts (WPA and BPU) can be distinguished. From this modeled temperature difference, the difference in exhaust gas enthalpy in both cases, i.e. a differential heat quantity Q ^WPA-BPU , can be calculated using the following formula: $$Q^{WPA-BPu}={\displaystyle \int c_p\cdot \frac{\mathrm{i.e}m}{\mathrm{i.e}t}}\cdot \Delta T^{MO\mathrm{i.e}WPA-BPu}\cdot \mathrm{i.e}t$$

where c _p is the specific heat capacity of the exhaust gas and dmldt is the mass flow of the exhaust gas. For this purpose, only the current mass flow of the exhaust gas is to be recorded or determined, for example using an appropriate measuring device or a sensor.

In addition, a measured differential heat quantity Q ^mess-BPU is determined from a measured temperature difference, specifically from a measured (current) temperature in the exhaust gas after the oxidation catalytic converter and that for the second oxidation catalytic converter or the BPU model: $$Q^{mess-BPu}={\displaystyle \int c_p\cdot \frac{\mathrm{i.e}m}{\mathrm{i.e}t}}\cdot \left(T^{mess}-T^{MO\mathrm{i.e}BPu}\right)\cdot \mathrm{i.e}t$$

The diagnosis is then based on the quotient r _DOC from the two differential amounts of heat, i.e. a conversion rate: $${\mathrm{right}}_{DOC}=\frac{Q^{mess-BPu}}{Q^{WPA-BPu}}$$

The two variables Q ^WPA-BPU and ΔT ^ModWPA-BPU play a major role in evaluating the ability to distinguish or monitor and are also used for this within the scope of the invention. If there are large temperature differences between the WPA and BPU models, the absolute tolerances of the measured temperature play a smaller role, which results in a smaller scatter in the conversion rates. A sufficiently large expected value quantity difference Q ^WPA-BPU indicates that a differentiation based on the quantity of heat is possible for the existing driving profile. The quotient or the conversion rate r _DOC runs towards zero when the differential heat quantity measured is very low - this means that the oxidation catalytic converter is close to a BPU.

If the measured differential amount of heat corresponds at least to the difference between WPA and BPU, i.e. the oxidation catalytic converter can still be regarded as at least WPA or better, the quotient is one or greater. A threshold value for the quotient or the conversion rate (quotient or conversion threshold value) can then also be specified, so that when the quotient exceeds this threshold value, the oxidation catalytic converter is considered to be intact or functional. For example, the threshold can be 0.5. Otherwise it would be concluded that the oxidation catalytic converter is not functional or defective. In both cases, an entry can be made in a monitoring memory or the like; in the latter case, it is also expedient to request that the oxidation catalytic converter be replaced.

Conventional approaches try to derive the information about the differentiability, e.g. However, the prognosis of distinctness obtained from this is much less accurate.

The quotient (conversion rate) can be determined or calculated over the entire monitoring-relevant heating phase, ie while the expected heat quantity difference Q ^WPA-BPU is greater than 0, but can only be used under certain conditions for monitoring or assessing functionality. An assessment of the model-based differential heat quantity Q ^WPA-BPU and/or the modeled temperature difference ΔT ^ModWPA-BPU can ultimately be used to assess the confidence or to determine a confidence level of the monitoring. For example, the monitoring can only be used if the model-based differential heat quantity Q ^WPA-BPU exceeds a certain threshold value, i.e. a sufficiently precise statement about the functionality can be expected.

A mean (eg arithmetic mean) ΔT ^TModWPA-BPU* over the monitoring period can be formed to evaluate the temperature difference. Exemplary thresholds for a car are 50 KJ (as a heat quantity threshold) for the differential heat quantity and, for example, 30° C. (or 30 K) for the average temperature difference (as a temperature threshold). Instead of averaging, a ratio r _t of the time t _i , in which the difference is greater than a threshold (e.g. 50 K), to the total time t _G can be calculated and compared to a threshold (e.g. 0.5): $${\mathrm{right}}_t=\frac{t_i}{t_G}$$

um einen Wärmemengenanteil r_Q zu bestimmen, bei welchem die Temperaturdifferenz über der Schwelle liegt.Instead of the time, the heat quantity difference Q ^WPA-BPU can also be used as a reference: $${\mathrm{right}}_Q=\frac{Q_i}{Q^{WPA-BPu}}$$

to determine a heat quantity fraction r _Q at which the temperature difference is above the threshold.

The calculation and evaluation of the conversion rate can also be controlled using the model-based differential heat quantity Q ^WPA-BPU and the model-based temperature difference ΔT ^ModWPA-BPU . This means that when the threshold for the model-based differential heat quantity is exceeded, the evaluation is triggered directly. In this case, the temperature difference can directly enable or stop the integration (determination) of the quantities of heat. If it is enabled again, the integration will continue without a reset. A check of the size of the individual integration phases based on the change in the integrator for Q ^WPA-BPU prevents, for example, that many small (imprecise) phases lead to a large phase that is then evaluated.

As mentioned above, using a model for the BPU in particular is a big step towards more accurate monitoring. In addition to the use described for enabling and controlling the monitoring, the use of the temperature of the BPU modeled in this way enables the conversion rate of the oxidation catalytic converter to be normalized based on the difference between the WPA and the BPU. Thus, regardless of the operating conditions, a conversion rate value close to zero always indicates a BPU, while a conversion rate value close to one indicates an intact oxidation catalyst (regardless of the significance and robustness assessment described above).

In contrast, if normalized to an uncoated oxidation catalyst (ie, the temperature of the BPU would be the temperature that would be expected for an uncoated oxidation catalyst), the conversion rates would vary between zero and one depending on the operating conditions. By restrictions one would have to try to reduce the scatter. It is precisely this that leads to the fact that such a diagnosis cannot detect a BPU whose light-off is only slightly shifted.

The trustworthiness of the measured value at the current point in time can preferably also be taken into account directly when calculating the conversion rate. In one of the versions described above, the calculation is stopped below a threshold. An extension of this method would be a continuous weighting k of the integration based on the temperature difference ΔT ^ModWPA-BPU , resulting in the following calculation rule: $${\mathrm{right}}_{DOC}=\frac{{\displaystyle \int k\cdot \Delta T^{MO\mathrm{i.e}WPA-BPu}\cdot c_p\cdot \frac{\mathrm{i.e}m}{\mathrm{i.e}t}\cdot \left(T^{mess}-T^{MO\mathrm{i.e}BPu}\right)\cdot \mathrm{i.e}t}}{{\displaystyle \int k\cdot \Delta T^{MO\mathrm{i.e}WPA-BPu}\cdot c_p\cdot \frac{\mathrm{i.e}m}{\mathrm{i.e}t}\cdot \Delta T^{T^{MO\mathrm{i.e}WPA-BPu}}\cdot \mathrm{i.e}t}}$$

In a further preferred embodiment, a confidence factor—for example the weighting k mentioned—can also be used to carry out a weighting based on the reliability of the model values T ^ModWPA and T ^ModBPU . The contributing differential heat flows are therefore weighted. This confidence factor can depend on the model-based temperature difference. In addition, the temperature models use, for example, an HC model value as an input variable, the quality of which can vary, for example, depending on the operating point of the internal combustion engine (engine operating point). A confidence factor for the HC model k _HC can be specified in an application, for example, with a map depending on engine speed and engine torque.

However, the application of this confidence factor to the model temperature values in the above integral formula expediently requires a delay logic, since changes in the HC value only affect the temperatures with a certain delay due to the thermal inertia of the oxidation catalytic converter. A PT1 filter can be used for this, which filters the confidence factor of the HC model with the following thermal time constant: $$tPT{1}_{DOC}=\frac{m_{DOC}\cdot c{p}_{DOC}}{\frac{\mathrm{i.e}m}{\mathrm{i.e}t}\cdot c{p}_{Gas}}$$

Here m _DOC is the mass and cp _DOC is the specific heat capacity of the oxidation catalyst. The value cp _gas corresponds to the specific heat capacity of the exhaust gas. The lagged confidence factor k can thus be calculated with $$k=PT1\left(k_{HC},tPT{1}_{DOC}\right)$$

The method can run either during the initial warm-up after a cold start or during a post-heat event when the system cools down again. Other operating conditions in which HCs are burned on the oxidation catalytic converter are also conceivable, e.g. active HC injection for diagnostic purposes.

A computing unit according to the invention, e.g. a control unit of a motor vehicle, is set up, in particular in terms of programming, to carry out a method according to the invention.

The implementation of a method according to the invention in the form of a computer program or computer program product with program code for carrying out all method steps is advantageous because this causes particularly low costs, especially if an executing control unit is also used for other tasks and is therefore available anyway. Suitable data carriers for providing the computer program are, in particular, magnetic, optical and electrical memories, such as hard drives, flash memories, EEPROMs, DVDs, etc. It is also possible to download a program via computer networks (Internet, intranet, etc.).

Further advantages and refinements of the invention result from the description and the attached drawing.

The invention is shown schematically in the drawing using exemplary embodiments and is described below with reference to the drawing.

character list

• 1 shows schematically an internal combustion engine with an exhaust system in which an oxidation catalytic converter is arranged, in which a method according to the invention can be carried out.
• 2a and 2 B show temperature curves to explain the invention.
• 3a and 3b show temperature and heat quantity curves to explain the invention.
• 4 shows a sequence of a method according to the invention in a preferred embodiment.
• 5 shows a sequence of a method according to the invention in a further preferred embodiment.

embodiment(s) of the invention

In 1 an internal combustion engine 100 (in particular a diesel engine) with an exhaust system 110 is shown schematically, in which an oxidation catalytic converter 120 (DOC) is arranged, in which a method according to the invention can be carried out. Exhaust gas 112, which leaves the internal combustion engine 100, first passes through the exhaust system 110 to the oxidation catalytic converter 120 and then to an SCR catalytic converter 130 (SCR stands for “Selective Catalytic Reduction”), urea-water being Solution is sprayed into the exhaust gas.

A mass flow of the exhaust gas 112 can be calculated based on the fresh air flowing into the internal combustion engine and the injection quantity. The value of the exhaust gas mass flow can then be supplied by an engine model. In principle, however, it would also be conceivable to measure the mass flow of the exhaust gas 112 using a measuring device 126. Using the measuring devices or temperature sensors 122, 124, the temperature can be measured or determined before or after the oxidation catalytic converter 120, with the present invention in particular the temperature after the oxidation catalyst is relevant.

In addition, a computing unit 102 designed as an engine control unit is provided, by means of which a method according to the invention can also be carried out, in particular data or measured values of measuring devices 122, 124 and 126 can also be received.

In the 2a and 2 B temperature curves are shown to explain the invention; a temperature T and a temperature difference ΔT are plotted against time t.

In 2a the upper diagram shows typical temperature profiles after the oxidation catalytic converter when the exhaust system heats up.

The temperatures or temperature models T ^ModWPA and T ^ModBPU required for the calculations and the measured temperature T ^mess behind the oxidation catalytic converter are shown.

The lower diagram shows the temperature difference ΔT ^ModWPA-BPU of the model temperatures of T ^ModWPA and T ^ModBPU . In this example, the diagnosis (shaded area) is started when the difference rises above the threshold ΔT _S and is carried out until it falls below it again. Hysteresis can also be used instead of a single threshold. For the model-based temperatures T ^ModWPA and T ^ModBPU , it should be mentioned that these can vary depending on the current operating conditions, so that varying temperature differences can also result.

In 2 B the integration is carried out over the entire heating phase (shaded area). During the integration phase, the mean value ΔT ^{ModWPA- BPU*} of the temperature difference ΔT ^ModWPA-BPU is calculated and compared at the end with a threshold ΔT _S . The conversion rate result is only evaluated if the mean temperature difference is above this threshold at the end.

In the 3a and 3b temperature and heat quantity curves are shown to explain the invention; a temperature T and a heat quantity difference Q are plotted against time t.

In both 3a and 3b the temperature curves in the upper diagrams are comparable to the 2a , 2 B shown. In 3a The lower diagram shows the calculated, model-based differential heat quantities Q ^WPA-BPU between WPA and BPU. The amount of heat is evaluated at the end of the integration phase, ie it is checked whether the differential amount of heat exceeds a threshold value Q _S . In 3b on the other hand, the amount of heat is used directly to trigger the diagnosis.

In 4 a sequence of a method according to the invention is shown in a preferred embodiment. After the start 400, it is first checked in a step 402 whether general release conditions such as a current operating mode of the internal combustion engine are present. If so, in step 404 it is checked whether the model-based temperature difference ΔT ^ModWPA-BPU under the current conditions exceeds a specified temperature threshold value ΔT _S (cf 2a , 2 B and the associated explanations).

Only if this is the case, according to step 406, is an integration (for the first time or also continued) of the mass flow of the exhaust gas. Thus, according to the above formula $$Q^{WPA-BPu}={\displaystyle \int c_p\cdot \frac{\mathrm{i.e}m}{\mathrm{i.e}t}}\cdot \Delta T^{MO\mathrm{i.e}WPA-BPu}\cdot \mathrm{i.e}t$$

bestimmt werden. In Schritt 408 wird geprüft, ob die modellbasierte Differenzwärmemenge Q^WPA-BPU einen Schwellwert Q_S überschreitet (vgl. 3a, 3b). In einem Schritt 410 wird dann aus den beiden Differenzwärmemengen der Quotient bzw. die Umsatzrate r_DOC gemäß obiger Formel $$r_{DOC}=\frac{Q^{mess-BPU}}{Q^{WPA-BPU}}$$

bestimmt.The differential heat quantity Q ^WPA-BPU can be determined. Likewise, with the current, measured temperature, the measured differential heat quantity Q ^mess-BPU according to the above formula $$Q^{mess-BPu}={\displaystyle \int c_p\cdot \frac{\mathrm{i.e}m}{\mathrm{i.e}t}}\cdot \left(T^{mess}-T^{MO\mathrm{i.e}BPu}\right)\cdot \mathrm{i.e}t$$

to be determined. In step 408 it is checked whether the model-based differential heat quantity Q ^WPA-BPU exceeds a threshold value Q _S (cf. 3a , 3b) . In a step 410, the quotient or the conversion rate r _DOC is then calculated from the two differential amounts of heat according to the above formula $${\mathrm{right}}_{DOC}=\frac{Q^{mess-BPu}}{Q^{WPA-BPu}}$$

certainly.

If, according to step 412, this conversion rate is greater than a threshold value r _S , then according to step 414 it can be concluded that the oxidation catalytic converter is functional. Otherwise, according to step 416, it is concluded that the oxidation catalytic converter is not functional.

The process of diagnosis according to 4 thus applies to the case that the integration only runs when the temperature difference is above a threshold. The end of the integration or diagnosis is triggered as soon as the amount of heat exceeds a threshold.

In 5 a sequence of a method according to the invention is shown in a further preferred embodiment. After the start 500, it is first checked in a step 502 whether general release conditions such as a current operating mode of the internal combustion engine are present. If this is the case, in step 504 it is checked whether a heating phase is taking place, ie whether sufficient hydrocarbons (HC) are currently being converted at the oxidation catalytic converter. For this purpose, it can be checked, for example, whether a temperature increase can be measured downstream of the oxidation catalytic converter.

If this is the case, according to step 506 an integration (for the first time or also continued) of the mass flow of the exhaust gas takes place. As in step 406 (cf. 4 ) the model-based differential heat quantity Q ^WPA-BPU and the measured differential heat quantity Q ^mess-BPU are determined. In addition, a mean value ΔT ^{TModWPA-BPU * is determined for the model-based temperature difference ΔT ModWPA-BPU} (cf. also 2 B and the associated explanations).

In step 508 it is then checked whether a general termination criterion (eg system error, too long idling or overrun phases) is present. If not, it is checked in step 510 whether there is still a heating phase. If not, then it can first be checked in step 512 whether the model-based differential heat quantity Q ^WPA-BPU exceeds a threshold value Q _S (cf. 3a , 3b) . If this is the case, it is also checked in step 514 whether the mean value ΔT ^TModWPA-BPU* exceeds a threshold value ΔT _S (cf. 3a , 3b) . If not, the process ends in step 516.

Otherwise, according to step 518, the quotient or the conversion rate r _DOC is determined from the two differential amounts of heat. This step corresponds to step 410 from FIG 2 . The further course of the diagnosis is then also as in 2 , i.e. steps 520, 522, 524 correspond to steps 412, 414, 426.

The process of diagnosis according to 5 This also applies if the diagnosis runs during the entire heating phase and is only evaluated at the end to determine whether the mean temperature difference and the amount of heat are large enough. In this case, too, it is conceivable for several heating phases to be combined into one result. In this case, the curve is run through several times for each heating phase. If the heating phase is good enough, it (ie the integrators of the turnover rate calculation) is added to the overall result, otherwise discarded. Only when enough heating phases have been evaluated (either the number or the total amount of heat is greater than the threshold) is the total conversion rate evaluated.

Overall, it is thus possible in this way to check in a particularly simple and precise manner whether the oxidation catalytic converter used is still functional or is already no longer functioning.

Claims (14)

Method for monitoring an oxidation catalytic converter (120), which is arranged in an exhaust line (110) of an internal combustion engine (100), based on a mass flow of exhaust gas (112) after passing through the oxidation catalytic converter (120) and a model-based temperature difference (ΔT ^ModWPA-BPU ) a model-based differential heat quantity (Q ^WPA-BPU ) is determined, the model-based temperature difference (ΔT ^ModWPA-BPU ) corresponding to a difference between temperatures (T ^ModWPA , T ^ModBPU ) that occur in a first oxidation catalyst and a second oxidation catalyst that has a lower efficiency as the first oxidation catalytic converter, for the exhaust gas after passing through the oxidation catalytic converter, a measured differential heat quantity (Q ^mess-BPU ) being determined based on the mass flow of the exhaust gas (112) after passing through the oxidation catalytic converter (120) and a measured temperature difference, the measured temperature difference of a difference between hen a current, measured temperature (T ^mess ) and the temperature (r ^ModBPU ) corresponds to the second oxidation catalyst for the exhaust gas after passing of the oxidation catalytic converter (120), and based on a comparison of a quotient (r _DOC ) from the measured differential heat quantity (Q ^mess-BPU ) and the modeled differential heat quantity (Q ^WPA-BPU ) with a predetermined quotient threshold value (r _S ) on the Functionality of the oxidation catalyst (120) is closed. procedure after claim 1 , it being concluded (414) that the oxidation catalytic converter (120) is functional if the quotient exceeds the predetermined quotient threshold value. procedure after claim 1 or 2 , The quotient (r _DOC ) is only determined when the model-based differential heat quantity (Q ^WPA-BPU ) exceeds a predetermined heat quantity threshold value (Q _S ). The method according to any one of the preceding claims, wherein the model-based differential heat quantity (Q ^WPA-BPU ) and the measured differential heat quantity (Q ^mess-BPU ) are determined using the mass flow of the exhaust gas (112) when the model-based temperature difference (ΔT ^ModWPA-BPU ) is a predetermined one Temperature threshold (ΔT _S ) exceeded. Method according to one of the preceding claims, wherein when determining the model-based differential heat quantity (Q ^WPA-BPU ) and/or the measured differential heat quantity (Q ^mess-BPU ), a confidence factor is used in each case for weighting contributing differential heat flows. procedure after claim 5 , where the confidence factor is chosen depending on the model-based temperature difference (ΔT ^ModWPA-BPU ). procedure after claim 5 or 6 , wherein the confidence factor indicates an accuracy of an HC model and is preferably selected as a function of an operating point of the internal combustion engine, the confidence factor being filtered in particular with a PT1 filter, with a filter time constant reflecting the thermal inertia of the oxidation catalyst. A method according to any one of the preceding claims, wherein a value for the mass flow of the exhaust gas (112) is received from a model of the internal combustion engine. Method according to one of the preceding claims, in which a value for the measured temperature (T ^meas ) of the exhaust gas is received from a measuring device (124) for measuring the temperature in the exhaust gas flow. Method according to one of the preceding claims, in which a WPA model is used as the first oxidation catalyst and/or in which a BPU model is used as the second oxidation catalyst. Method according to one of the preceding claims, in which an SCR catalytic converter (130) is arranged in the exhaust line (110) downstream of the oxidation catalytic converter (120). Arithmetic unit (102) which is set up to carry out all method steps of a method according to one of the preceding claims. Computer program that causes a computing unit (102) to carry out all method steps of a method according to one of Claims 1 until 11 to be performed when it is executed on the computing unit (102). Machine-readable storage medium with a computer program stored on it Claim 13 .

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