Method for monitoring an electrically heated catalyst

Abstract

Method for monitoring an electrically heatable catalyst (28) in an exhaust system (20) of an internal combustion engine (10), wherein a temperature sensor (40) is arranged in the exhaust system (20) upstream of the electrically heatable catalyst (28), comprising the following steps: - electrically heating the electrically heatable catalyst (28), wherein - the temperature sensor (40) is heated by the heat radiation of the electrically heatable catalyst (28), whereby - the temperature measured by the temperature sensor (40) is corrected by the cooling effect of the exhaust gas mass flow, whereby - by heating the temperature sensor (40) a heat input of the electrically heatable catalyst (28) into the exhaust system (20) is determined.

Description

The invention relates to a method for monitoring an electrically heatable catalytic converter in an exhaust system of an internal combustion engine and to an exhaust system having an electrically heatable catalytic converter according to the preamble of the independent claims.The continuous strengthening of exhaust gas legislation places high demands on vehicle manufacturers, which are solved by corresponding measures for reducing the raw engine emissions and by corresponding exhaust gas aftertreatment. In order to be able to effectively implement the incompletely avoidable raw emissions by the after-engine, noble metal-coated catalytic converters are installed in the exhaust system of the internal combustion engine. In order for these catalysts to be able to react the pollutants, a minimum temperature level of the exhaust gas and of the catalyst is necessary. In order to bring the catalytic converter to an operating temperature as quickly as possible after a cold start of the internal combustion engine, engine heating measures such as an adjustment of the ignition angle or the fuel injection in the "late" direction or a substoichiometric operation of the internal combustion engine with simultaneous introduction of secondary air are used. In order to introduce even more thermal energy into the exhaust system in a targeted manner, it is possible to electrically heat the catalytic converter or to install an activatable exhaust gas burner on the exhaust system in order to introduce hot exhaust gas into the exhaust system and to heat the exhaust gas aftertreatment components arranged in the exhaust system independently of the internal combustion engine. As a result, the emissions can already be significantly reduced in the heating phase of the catalytic converter.In diesel engines, a late post-injection and / or a shift of the center of combustion in the "late" direction are known for increasing the exhaust gas temperature, wherein the thermal efficiency of the internal combustion engine decreases and more energy is introduced into the exhaust system. At the same time, by shifting the main injection in the "late" direction, the raw emissions of nitrogen oxides can be reduced.However, a disadvantage of the solutions known from the prior art is that internal heating measures become effective only after the internal combustion engine has been started. In addition, these internal engine heating measures can lead to a fuel injection into the engine oil, as a result of which the wear is increased or the oil change intervals have to be shortened.Furthermore, external heating means in the exhaust system, such as an exhaust gas burner or an electrical heating element, are known from the prior art. External heating means can heat the exhaust gas aftertreatment components substantially independently of the exhaust gas flow of the internal combustion engine and thus reach their operating temperature more quickly, as a result of which the tail pipe emissions can be reduced, in particular in a cold start phase of the internal combustion engine.DE 10 2016 122 304 A1 discloses a method for heating an electrically heatable catalytic converter in an exhaust gas duct of a motor vehicle having an internal combustion engine. In order to heat up the catalytic converter before the internal combustion engine is started, it is provided that the catalytic converter is already electrically heated before the engine start and already allows efficient exhaust gas after-treatment with the engine start. After an electrical preheating phase after the engine start, the catalytic converter is further heated by a combined electrical and chemical heating as a result of the exothermic reaction of unburned fuel components on a catalytically active surface of the electrically heatable catalytic converter.DE 10 2017 113 320 A1 discloses a method and a system for non-uniformly heating an electrically heatable catalytic converter in the exhaust system of an internal combustion engine. The method includes providing a catalyst from a plurality of electrically conductive sections and at least one catalyst heating element capable of heating at least one of the various electrically conductive sections of the catalyst to form at least one predetermined non-uniform heating pattern. A heater control device is provided to control operation of the at least one heating element of the catalyst. When the internal combustion engine is in a cold start condition, the heater control module causes at least one of the catalyst heaters to heat at least one of the electrically conductive portions of the catalyst to obtain a first predetermined non-uniform heating pattern.DE 10 2017 218 374 A1 discloses a device for determining a heating temperature of a heating element for an electrically heatable catalytic converter. The apparatus includes the heating element and the catalyst disposed within a catalyst housing, and a first temperature sensor and a second temperature sensor disposed within the catalyst housing. In this case, the first temperature sensor is arranged closer to the heating element than the second temperature sensor, with respect to an exhaust gas flow direction within the catalytic converter housing.DE 42 35 113 A1 describes a method for diagnosing the components of a heating circuit for an electrically heatable catalytic converter. In order to enable the most accurate possible diagnosis of the various components, it is provided that the temperature of the catalytic converter is determined and the time profile of this temperature is stored as an actual characteristic curve, wherein the actual characteristic curve is compared with a predetermined setpoint characteristic curve and the correspondence of the two characteristic curves is evaluated.The object of the invention is to make possible the simplest and cost-effective monitoring of the function of the electric heating element in an electrically heatable catalytic converter.According to the invention, this object is achieved by a method for monitoring an electrically heatable catalytic converter in an exhaust system of an internal combustion engine. In this case, a temperature sensor is arranged in the exhaust system upstream of the electrically heatable catalytic converter, with which a temperature in the exhaust system can be measured. The method comprises the following steps:electrically heating the electrically heatable catalytic converter, whereinthe temperature sensor is heated by the thermal radiation of the electrically heatable catalytic converter, whereinthe temperature measured by the temperature sensor is corrected by the cooling effect by the exhaust gas mass flow, whereina heat input of the electrically heatable catalytic converter into the exhaust system is determined by heating the temperature sensor.By means of a method according to the invention, it is possible to monitor the electrical heating power of an electrically heatable catalytic converter. In this case, the sensors already present in the exhaust system can be used, so that the method can be implemented easily and cost-effectively. No additional installation space is required for further sensors in the exhaust system. Furthermore, an on-board diagnosis of the electrically heatable catalytic converter can be carried out, so that a failure can be detected, which would lead directly to an increase in the tail pipe emissions.The features listed in the dependent claims allow advantageous improvements and further developments of the method specified in the independent claim for monitoring an electrically heatable catalytic converter.In a preferred embodiment of the invention, it is provided that the determined heating of the temperature sensor is compared with a heating at a given exhaust gas mass flow, which heating is to be expected at a defined heating power, and the operability of the electrically heatable catalytic converter is monitored on the basis of this comparison. In general, the greater the exhaust gas mass flow rate, the smaller the heating to be expected. As a result, quantitative analysis of the introduced heating power can be carried out. It is thus possible to detect in good time if the heating power of the electrically heatable catalytic converter decreases. In this case, a threshold value can be defined, from which replacement of the electric heating element of the electrically heatable catalytic converter is necessary.In a preferred embodiment of the invention, it is provided that the ascertained gradient of the heating of the temperature sensor is compared with a gradient to be expected at a defined heating power at a given exhaust gas mass flow, and on the basis of this comparison, the functionality of the electrically heatable catalytic converter is monitored. This results in a dependence on the exhaust gas mass flow of the internal combustion engine, in which it generally applies that with a larger exhaust gas mass flow, the gradient to be expected for the heating decreases. As a result, a quantitative analysis of the introduced heating power can be estimated already immediately after the beginning of the heating phase. It is thus possible to detect in good time if the heating power of the electrically heatable catalytic converter decreases.It is particularly preferred in this case if the defined heating power is the maximum electrical heating power of the electrically heatable catalytic converter. Since the heat flow radiated by a body increases exponentially with temperature, a greater heating power of the electrically heatable catalytic converter leads to a marked increase in the thermal radiation. In order to determine the temperature increase at the temperature sensor by the thermal radiation, the highest possible heating power, in particular the maximum heating power of the electrically heatable catalytic converter, is therefore useful for monitoring the functionality.In a further preferred embodiment of the invention, it is provided that the electrically heatable catalytic converter is electrically heated for at least 15 seconds, preferably for at least 30 seconds, particularly preferably for at least 45 seconds, wherein the temperature change of the temperature sensor to the heat input is determined. In order to heat the electrically heatable catalytic converter or a catalytic converter directly downstream of the electrically heatable catalytic converter to its light-off temperature, it is expedient to electrically heat the electrically heatable catalytic converter for at least 45 seconds in order to ensure an adequate energy input into the exhaust system. For analyzing the functionality, it is likewise helpful to electrically heat the electrically heatable catalytic converter at least for 20 seconds in order to ensure heating of the electrically heatable catalytic converter to its maximum possible temperature and to ensure a correspondingly high thermal radiation.According to the invention, an exhaust system for an internal combustion engine having at least one combustion chamber is proposed, wherein the internal combustion engine can be connected to the exhaust system by its outlet. At least one catalytic converter is arranged in the exhaust system and an electrically heatable catalytic converter is arranged upstream of the catalytic converter. A temperature sensor is arranged downstream of the outlet of the internal combustion engine and upstream of the electrically heatable catalytic converter. The temperature sensor and the electrically heatable catalytic converter are connected to a control unit which is configured to carry out a method according to the invention when a machine-readable program code is executed by the control unit. A temperature sensor downstream of the catalytic converter would only allow a greatly delayed determination of a temperature rise in the exhaust system, since the catalytic converter initially absorbs heat in order to heat itself. Furthermore, the exothermic, catalytic reactions on the catalyst also lead to an increase in the temperature, so that the amount of heat introduced by the electrically heatable catalyst would be difficult and time-delayed to determine. Although the convective portion of the heat transfer is dispensed with by arranging the temperature sensor upstream of the electrically heatable catalytic converter, an increase in the sensor temperature due to the thermal radiation is directly measurable.With such an exhaust system, monitoring of an electrically heatable catalytic converter in the exhaust system can be carried out in a comparatively simple and cost-effective manner. It can be detected here if the electrically heatable catalytic converter has a malfunction, in particular an inadequate heating power, and there is the risk that the malfunction results in an inadequate conversion of the pollutants in the exhaust gas of the internal combustion engine.In a preferred embodiment of the exhaust system, it is provided that the electrically heatable catalytic converter comprises an electric heating element, in particular an electric heating disk. By means of an electric heating element which is connected upstream of a catalyst, rapid heating of the catalyst can be ensured convectively by means of the exhaust gas stream. In this case, the heating element radiates heat, which leads to heating of the temperature sensor upstream of the electrically heatable catalytic converter.According to a preferred embodiment of the exhaust system, it is provided that the catalytic converter is designed as an oxidation catalytic converter or as a NOx storage catalytic converter. Since electrically heatable catalytic converters are often used in exhaust systems of diesel engines, it is expedient to use such an electrically heatable catalytic converter as the first catalytic converter in the flow direction of an exhaust gas of the internal combustion engine.Alternatively, it is advantageously provided that the catalyst is designed as a three-way catalyst or as a four-way catalyst. In spark ignition engines, the arrangement of an electrically heatable catalytic converter as a first catalytic converter directly in front of a catalytic converter close to the engine, in particular a three-way catalytic converter or four-way catalytic converter, is helpful in order to reduce cold start emissions. In this context, a catalytic converter close to the engine is understood to mean a catalytic converter whose inlet-side end side is arranged with an exhaust gas running length of less than 80 cm, preferably less than 50 cm, from an outlet of the internal combustion engine.In a further advantageous variant of the exhaust system, it is provided that the catalyst is designed as an SCR catalyst or as a particle filter with a catalytically active coating, in particular with a coating for selective, catalytic reduction of nitrogen oxides (SCR coating). In particular in the case of diesel engines, it may be helpful to arrange an electrically heatable catalytic converter directly upstream of an SCR catalytic converter or a particle filter with an SCR coating in order to heat the SCR catalytic converter or the particle filter to a temperature necessary for selective catalytic reduction. In order to be able to monitor such an electrically heatable catalytic converter, the proposed method is likewise suitable.In a preferred embodiment of the invention, it is provided that the temperature sensor is arranged at most 5 cm, preferably at most 3 cm, upstream of the electrically heatable catalytic converter. Since the thermal radiation decreases strongly with distance, it is advantageous to arrange the temperature sensor as close as possible to the electrically heatable catalytic converter in order to be able to measure heating due to the thermal radiation. In this case, the distance from a central axis of the temperature sensor to an inlet-side end face of the electrically heatable catalytic converter is measured. By removing a maximum of 5 cm, it can be ensured that a strong heating of the electrically heatable catalytic converter leads to a functionally reliable and reproducibly measurable temperature increase at the temperature sensor.In an advantageous embodiment of the exhaust system, it is provided that a turbine of an exhaust gas turbocharger is arranged in the exhaust system, wherein the temperature sensor is arranged downstream of the turbine and upstream of the electrically heatable catalytic converter. In order to determine a measurable effect for the heating of the temperature sensor by the thermal radiation of the electrically heatable catalytic converter, the temperature sensor is to be acted upon by an exhaust gas that is as homogeneous as possible. The turbine ensures that the exhaust gas stream is mixed, so that hot zones of the exhaust gas stream are mixed with comparatively cold zones and a thermally substantially homogeneous exhaust gas flows against the temperature sensor.The various embodiments of the invention mentioned in this application can be combined with one another with advantage unless stated otherwise in the individual case.The invention is explained below in exemplary embodiments with reference to the associated drawings. Identical components or components with the same function are identified with the same reference numerals in the different figures. The following are shown: FIG. 1 shows a first exemplary embodiment of an internal combustion engine having an exhaust system for carrying out a method according to the invention for monitoring an electrically heatable catalytic converter; FIG. 2 shows a further exemplary embodiment of an internal combustion engine having an exhaust system for carrying out a method according to the invention for monitoring an electrically heatable catalytic converter; and FIG. 3 shows a temperature profile at a temperature sensor in the exhaust system when carrying out a method according to the invention for monitoring an electrically heatable catalytic converter.FIG. 1 shows a schematic illustration of an internal combustion engine 10, the outlet 18 of which is connected to an exhaust system 20 of an exhaust gas aftertreatment system according to the invention for the exhaust gas aftertreatment of an exhaust gas stream of the internal combustion engine 10. The internal combustion engine 10 is designed as a diesel engine. The internal combustion engine 10 has a plurality of combustion chambers 12 in which a fuel-air mixture is burned. For this purpose, a fuel injector 14 is provided on each of the combustion chambers 12 in order to inject a fuel into the combustion chambers 12. The internal combustion engine 10 is preferably designed as an internal combustion engine 10 which is supercharged by means of an exhaust gas turbocharger 24, wherein a turbine 26 of the exhaust gas turbocharger 24 is arranged downstream of the outlet 18 and upstream of the exhaust gas aftertreatment components 28, 30, 32, 34, 36, 38, 46. The exhaust system 20 comprises an exhaust gas duct 22, in which an electrically heatable catalytic converter 28 is arranged downstream of the turbine 26 of the exhaust gas turbocharger 24 in the flow direction of an exhaust gas through the exhaust gas duct 22, and a catalytic converter 32, in particular an oxidation catalytic converter 34 or a NOx storage catalytic converter 36, which is preferably close to the engine is arranged downstream of the electrically heatable catalytic converter 28. Downstream of the oxidation catalytic converter 34 or the NOx storage catalytic converter 36, an SCR catalytic converter 48 or a particle filter 42 with a coating for selective, catalytic reduction of nitrogen oxides is arranged. The electrically heatable catalytic converter 28 comprises an electric heating element 30, in particular an electric heating disc 44, by means of which the exhaust gas aftertreatment components 32, 34, 36, 38, 46 arranged downstream of the electrically heatable catalytic converter 28 can be heated substantially independently of the exhaust gas flow of the internal combustion engine 10. The electrically heatable catalytic converter 28 is connected to a control unit 50 of the internal combustion engine 10, via which the heating power and / or the duration of the activation of the electrically heatable catalytic converter 28 can be controlled.Downstream of turbine 26 of exhaust gas turbocharger 24 and upstream of electrically heatable catalytic converter 28, a temperature sensor 40 is situated, with which a temperature in exhaust gas duct 22 upstream of electrically heatable catalytic converter 28 may be ascertained. The temperature sensor 40 is likewise connected to the control unit 50 and transmits a temperature measured in the exhaust gas duct 22 to the control unit 50. Alternatively or additionally, an electrically heatable catalytic converter 28 can also be arranged upstream of the SCR catalytic converter 48 or the particle filter 42.FIG. 2 shows an alternative exemplary embodiment of an exhaust system 20 for an internal combustion engine 10. The internal combustion engine 10 is designed in this exemplary embodiment as a spark-ignited spark ignition engine. The internal combustion engine 10 has at least one combustion chamber 12, on which a spark plug 16 for igniting a fuel-air mixture is arranged in the combustion chamber 12. The internal combustion engine 10 is preferably designed as a direct-injection internal combustion engine 10. For this purpose, a fuel injector 14 is arranged on the combustion chamber 12, with which fuel can be injected into the respective combustion chamber 12. Alternatively or additionally, a fuel injection valve for introducing the combustible fuel may also be provided in the intake tract of the internal combustion engine 10.The internal combustion engine 10 is connected with its outlet 18 to an exhaust system 20, in which a turbine 26 of an exhaust gas turbocharger 24 is arranged in the flow direction of an exhaust gas of the internal combustion engine 10 and an electrically heatable catalytic converter 28 is arranged downstream of the turbine 26, preferably as a first component for exhaust gas aftertreatment. The electrically heatable catalytic converter 28 comprises an electric heating element 30, in particular an electric heating disk 44, with which the components for exhaust gas aftertreatment 32, 34, 36, 38, 42, 46, 48, in particular a three-way catalytic converter 38 or a four-way catalytic converter 46, arranged downstream of the electrically heatable catalytic converter 28 can be heated substantially independently of the exhaust gas flow of the internal combustion engine 10. The electrically heatable catalytic converter 28 and / or the electrical heating element 30 are connected to a control unit 50 of the internal combustion engine 10, via which the heating power and heating duration of the electrically heatable catalytic converter 28 and / or of the electrical heating element 30 can be controlled.Downstream of the first catalytic converter 32, 38, 46, preferably in an underbody position of a motor vehicle, a further catalytic converter 38, 46, in particular a further three-way catalytic converter 38 or a further four-way catalytic converter 46, is provided.Downstream of turbine 26 of exhaust gas turbocharger 24 and upstream of electrically heatable catalytic converter 28, a temperature sensor 40 is situated, with which a temperature in exhaust gas duct 22 upstream of electrically heatable catalytic converter 28 may be ascertained. The temperature sensor 40 is likewise connected to the control unit 50 and transmits a temperature measured in the exhaust gas duct 22 to the control unit 50. Alternatively or additionally, an electrically heatable catalytic converter 28 can also be arranged upstream of the further catalytic converter 38, 46 in the underbody position of the motor vehicle.The emissions in the cold start phase of the internal combustion engine 10 and / or after a stop of the internal combustion engine 10 can be reduced by the electrically heatable catalytic converter 28. This applies both to internal combustion engines in motor vehicles and to internal combustion engines used in a stationary manner. In particular, an electrically heatable catalytic converter 28 can also be used in a motor vehicle with a hybrid drive, since in such a motor vehicle the exhaust gas aftertreatment components can cool down below their operating temperature during purely electric driving operation. Such an electrically heatable catalytic converter 28 must be monitored by an on-board diagnosis in order to detect a functional restriction or a failure which otherwise directly leads to an increase in the tail pipe emissions(s). Due to the limited installation space and the increased costs, additional sensors for monitoring the electrically heatable catalytic converter 28 should be dispensed with and already present sensors 40 should be used. The thermal radiation of the electrically heatable catalytic converter 28 and / or of the electrical heating element 30 leads to a measurable temperature rise at the temperature sensor 40 upstream of the electrically heatable catalytic converter 28. When electrically heatable catalytic converter 28 is activated, such a temperature rise can be measured immediately, since no further components between electrically heatable catalytic converter 28 and temperature sensor 40 have to be heated through.FIG. 3 shows the temperature profile at a temperature sensor 40 arranged directly upstream of the electrically heatable catalytic converter 28 as a function of the actuation of the electrically heatable catalytic converter 28. In this case, the electrical heating power (P H), the temperature (T S) at the temperature sensor 40 and the temperature (T nKat) downstream of the catalytic converter 32 are shown over time. As can be seen from FIG. 3, a high correlation is shown between the electrical heating power (P H) and the temperature (T S) at the temperature sensor 40 upstream of the electrically heatable catalytic converter 28, which can be used for monitoring and on-board diagnosis of the electrically heatable catalytic converter 28.List of reference characters10 Internal combustion engine 12 combustion chamber 14 fuel injector 16 spark plug 18 outlet 20 exhaust system 22 exhaust gas duct 24 exhaust gas turbocharger 26 turbine 28 electrically heatable catalytic converter 30 electric heating element 32 catalytic converter 34 oxidation catalytic converter 36 NOx storage catalytic converter 38 three-way catalytic converter 40 temperature sensor 42 particle filter 44 electric heating disk 46 four-way catalytic converter 48 SCR catalytic converter 50 control device P H electric heating power P def defined heating power P max maximum heating power T S temperature at the temperature sensor T nKat temperature downstream of the catalytic converter t time

Claims

Method for monitoring an electrically heatable catalytic converter (28) in an exhaust system (20) of an internal combustion engine (10), wherein a temperature sensor (40) is arranged in the exhaust system (20) upstream of the electrically heatable catalytic converter (28), comprising the following steps: - electrically heating the electrically heatable catalytic converter (28), wherein - the temperature sensor (40) is heated by the thermal radiation of the electrically heatable catalytic converter (28), wherein - the temperature measured by the temperature sensor (40) is corrected by the cooling effect by the exhaust gas mass flow, wherein - a heat input of the electrically heatable catalytic converter (28) into the exhaust system (20) is determined by heating the temperature sensor (40).Method for monitoring an electrically heatable catalytic converter (28) according to Claim 1, characterized in that the heating of the temperature sensor (40) determined is compared with heating to be expected at a defined heating power (P def) and the functionality of the electrically heatable catalytic converter (28) is monitored on the basis of this comparison.Method for monitoring an electrically heatable catalytic converter (28) according to Claim 1, characterized in that the ascertained gradient of the heating of the temperature sensor (40) is compared with a heating gradient to be expected at a defined heating power (P def) and the operability of the electrically heatable catalytic converter (28) is monitored on the basis of this comparison.Method for monitoring an electrically heatable catalytic converter (28) according to claim 2 or 3, characterised in that the defined heating power (P def) is the maximum electrical heating power (P max) of the electrically heatable catalytic converter (28).Method for monitoring an electrically heatable catalytic converter (28) according to one of Claims 1 to 4, characterized in that the electrically heatable catalytic converter (28) is electrically heated for at least 15 seconds, wherein the temperature change of the temperature sensor (40) to the heat input is determined.Method for monitoring an electrically heatable catalytic converter (28) according to one of Claims 1 to 5, characterized in that the electrically heatable catalytic converter (28) is electrically heated for at least five seconds, wherein the gradient of the temperature change of the temperature sensor (40) to the heat input is determined.Exhaust system (20) for an internal combustion engine (10) having at least one combustion chamber (12), wherein the internal combustion engine (10) can be connected by its outlet (18) to the exhaust system (20), wherein at least one catalytic converter (32, 34, 36, 38, 42, 46, 48) is arranged in the exhaust system (20) and an electrically heatable catalytic converter (28) is arranged upstream of the catalytic converter (32, 34, 36, 38, 42, 46, 48), wherein a temperature sensor (40) is arranged downstream of the outlet (18) and upstream of the electrically heatable catalytic converter (28), and having a control unit (50) which is configured to carry out a method according to one of Claims 1 to 4 when a machine-readable program code is executed by the control unit (50).Exhaust system (20) according to Claim 7, characterized in that the electrically heatable catalytic converter (28) comprises an electrical heating element (30), in particular an electrical heating disc (44).Exhaust system (20) according to Claim 7 or 8, characterized in that the catalytic converter (32, 34, 36, 38, 42, 46, 48) is designed as an oxidation catalytic converter (34), as a NOx storage catalytic converter (36), as a three-way catalytic converter (38), as a four-way catalytic converter (46), as a SCR catalytic converter (48) or as a particle filter (42) having a catalytically active coating, in particular having a coating for the selective, catalytic reduction of nitrogen oxides.Exhaust system (20) according to one of Claims 7 to 9, characterized in that a turbine (26) of an exhaust-gas turbocharger (24) is arranged in the exhaust system (20), wherein the temperature sensor (40) is arranged downstream of the turbine (26) and upstream of the electrically heatable catalytic converter (28).

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