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United States Patent Application 20160319727
Kind Code A1
NIKOLAUS; Norbert ;   et al. November 3, 2016

METHOD FOR ON-BOARD DIAGNOSIS OF AN OXIDATION CATALYST IN AN EXHAUST-GAS SYSTEM OF AN INTERNAL COMBUSTION ENGINE OF A VEHICLE

Abstract

A method for on-board diagnosis of an oxidation catalyst in an exhaust-gas system of an internal combustion engine of a vehicle having an engine control device and a temperature sensor in the exhaust gas system upstream from a downstream side of said oxidation catalyst. The method includes operating the internal combustion engine is in an overrun mode, and injecting a quantity of fuel upstream from the oxidation catalyst. The change of temperature values over time is measured by the temperature sensor, and at least one physical quantity that characterizes the heat release in said oxidation catalyst is calculated on the basis of said amount of injected fuel. The method further includes ascertaining whether the calculated quantity falls within a set of values, and, if so, declaring that said oxidation catalyst is considered to be defective.


Inventors: NIKOLAUS; Norbert; (Vordorf, DE) ; SCHELLENBERG; Wadim; (Gifhorn, DE) ; ALICH; Frank; (Tiddische, DE) ; ZAISER; Maic; (Hannover, DE)
Applicant:
Name City State Country Type

Volkswagen Aktiengesellschaft

Wolfsburg

DE
Family ID: 1000001657005
Appl. No.: 14/701159
Filed: April 30, 2015


Current U.S. Class: 1/1
Current CPC Class: F01N 11/005 20130101
International Class: F01N 11/00 20060101 F01N011/00

Claims



1. A method for on-board diagnosis of an oxidation catalyst in an exhaust-gas system of an internal combustion engine of a vehicle, wherein the vehicle has an engine control device and a temperature sensor located in the exhaust gas system upstream from a downstream side of said oxidation catalyst, said method comprising the following steps: operating said internal combustion engine in an overrun mode; injecting an amount of fuel upstream from said oxidation catalyst; measuring, via said temperature sensor, a change of temperature values over time; calculating, via said engine control device, at least one physical quantity that characterizes a heat release in said oxidation catalyst on the basis of said amount of injected fuel; determining, via said engine control device, whether the calculated quantity falls within a set of values; and declaring, via said engine control device, that said oxidation catalyst is defective if the calculated quantity falls within the set of values.

2. The method for the on-board diagnosis of an oxidation catalyst according to claim 1, whereby declaring that said oxidation catalyst is defective comprises generating an error signal in said engine control device.

3. The method for the on-board diagnosis of an oxidation catalyst according to claim 1, wherein measuring the change of temperature values comprises measuring the change at a predetermined location in the oxidation catalyst.

4. The method for the on-board diagnosis of an oxidation catalyst according to claim 1, wherein said at least one physical quantity is a measure of a thermal energy released in said oxidation catalyst corresponding to said amount of injected fuel.

5. The method for the on-board diagnosis of an oxidation catalyst according to claim 4, further comprising, determining, during the calculation of said physical quantity, differences between the measured temperature values and reference temperature values.

6. The method for the on-board diagnosis of an oxidation catalyst according to claim 1, wherein said at least one physical quantity is a measure of a temperature gradient, or said at least one physical quantity is a quantity change that is a measure of a change of values of temperature gradients.

7. The method for the on-board diagnosis of an oxidation catalyst according to claim 1, wherein a temperature threshold is prescribed for a measured temperature, and further comprising determining a time span until the measured temperature exceeds the temperature threshold, wherein the determined time span is said at least one physical quantity.

8. The method for the on-board diagnosis of an oxidation catalyst according to claim 1, wherein calculating at least one physical quantity comprises calculating a plurality of physical quantities, and wherein each physical quantity constitutes a measure of a physical quantity of a chemical conversion, and the plurality of physical quantities are mutually different from one another.

9. The method for the on-board diagnosis of an oxidation catalyst according to claim 8, wherein the plurality of physical quantities comprises one, several or all of the physical quantities selected from the group consisting of: a measure of thermal energy released in said oxidation catalyst on the basis of said amount of injected fuel, a measure of a temperature gradient, a measure of a change of values of temperature gradients, and a measure of a time span until a measured temperature has exceeded a temperature threshold.

10. A vehicle having an internal combustion engine, an exhaust-gas system with an oxidation catalyst, and an engine control device comprising at least one computer and at least one memory element, wherein said engine control device is configured to carry out the method for the on-board diagnosis of the oxidation catalyst according to claim 1 during at least a partial execution of said computer.
Description



FIELD OF THE INVENTION

[0001] The invention relates to a method for the on-board diagnosis of an oxidation catalyst in an exhaust-gas system of an internal combustion engine of a vehicle. The invention also relates to a vehicle configured to implement the method of on-board diagnosis according to certain aspects of the present invention.

BACKGROUND OF THE INVENTION

[0002] To an increasing extent, it is necessary to monitor the components of the exhaust-gas system of an internal combustion engine of a vehicle while driving in order to detect possible malfunctions so that, if necessary, countermeasures can be taken. In an exemplary embodiment, primarily the proper functioning of the catalysts, especially oxidation catalysts, is of particular importance. At present, a widespread, known diagnostic strategy involves the passive evaluation of the amount of exothermic heat converted in the oxidation catalyst during the regeneration of a particle filter. This strategy, however, only detects a total failure of the oxidation catalyst. At the present time, the diagnostic strategy for detecting that an oxidation catalyst is only partially damaged involves evaluating the exothermic reaction of the unburned fuel in the oxidation catalyst during low-load operation. This approach, however, greatly worsens the emissions and causes problems with the running behavior of the internal combustion engine.

[0003] German patent application DE 42 11 092 A1 discloses a method and a device to assess the functionality of a catalyst in the exhaust-gas system of an internal combustion engine. In some embodiments of this method, the internal combustion engine is operated starting from a given state, for instance, a cold start, at which the catalyst temperature lies below the temperature at which a new catalyst starts the conversion process. The change of the temperature over time (e.g., the course or development of temperature over time) is estimated by employing a model into which are entered momentary values of operating quantities of the internal combustion engine and of the catalyst. The actual temperature of the catalyst is examined when the temperature estimated on the basis of the model has reached a comparison temperature that lies above the temperature at which conversion starts. If the actual temperature lies above the comparison temperature, the catalyst is considered to be functional. In actual practice, the described approach involves an unacceptably long measurement time amounting to several minutes.

SUMMARY OF THE INVENTION

[0004] It is an objective of the present invention to put forward a method for the on-board diagnosis of an oxidation catalyst entailing only a slight worsening of the emissions.

[0005] This objective is achieved by means of a method for the on-board diagnosis (OBD) according to aspects of the present invention as described below.

[0006] The inventive method for the on-board diagnosis (OBD) of an oxidation catalyst in the exhaust-gas system of an internal combustion engine of a vehicle having an engine control device--which especially comprises a computer and a memory element--and a temperature sensor in the exhaust gas system upstream (relative to the direction in which the exhaust gas flows through the exhaust-gas system) from the downstream side of said oxidation catalyst encompasses the following steps.

[0007] The internal combustion engine is operated in an overrun mode. During the overrun mode, in particular, no fuel is fed to the engine in order to generate a torque. Preferably, the overrun mode of operation according to the present invention takes place following an under load operation, in which a torque is generated due to fuel combustion in the combustion chambers of the internal combustion engine, in which the exhaust-gas system of the internal combustion engine has already reached an operating temperature, especially stationarily, in which chemical conversions take place in the oxidation catalyst, especially above the light-off temperature of the oxidation catalyst.

[0008] A quantity of fuel is injected upstream from the oxidation catalyst. Here, the fuel can be injected into a combustion chamber of the internal combustion engine or into the exhaust-gas system upstream from the oxidation catalyst, for example, into an exhaust-gas manifold or into an exhaust-gas line. The injection can take place in one or more events (e.g., at different times).

[0009] The change, especially the change (e.g., course or development) over time, of temperature values is measured by means of a temperature sensor, especially after the amount of fuel has been injected. In particular, the temperature sensor is in operational connection and/or signal connection with the engine control device, so that the measured change (e.g., course or development) of the temperature values over time can be acquired and evaluated (e.g., analyzed) in the engine control device.

[0010] According to certain aspects of the present invention, at least one physical quantity, in an exemplary embodiment, at least one value of the physical quantity, that characterizes or describes the heat release in the oxidation catalyst on the basis of the amount of injected fuel is calculated by, for example, the engine control device. In certain embodiments, a temperature sensor detects a change of temperature over time and transmits a signal corresponding to the change in temperate over time to the engine control device, which utilizes the signal to calculate at least one physical quantity (e.g., at least one value of a physical quantity) that characterizes or describes the heat release in the oxidation catalyst on the basis of the amount of injected fuel.

[0011] It is ascertained, for example in the engine control device, whether the calculated physical quantity falls within a set of values, particularly a prescribed set of values. For instance, the calculated value of the quantity is compared to a threshold value on a scale, or else it is checked whether the calculated value corresponds to each element of the set of values. That is, in certain embodiments, the engine control device analyzes the calculated value and determines whether or not the calculated value falls within a range of predetermined values such as, for example, comparing the calculated value to a threshold value.

[0012] If the calculated quantity falls within the set of values, then the oxidation catalyst is declared to be defective by setting the value of a variable, especially in the engine control device. For example, the calculated value falls within the set of values if a comparison of the threshold values shows that the limit prescribed by the threshold value has been exceeded.

[0013] Advantageously, the method according to the present invention makes it possible to detect oxidation catalysts that are already only partially damaged, without interfering with the operation of the internal combustion engine in a way that would worsen the emissions. The method according to the present invention can be carried out within a very short time period such as, for example, in less than one minute. Consequently, the emissions caused by the diagnosis, especially the emission of undesired combustion products, are kept to a minimum.

[0014] In certain embodiments, the internal combustion engine is a self-igniting internal combustion engine or a diesel engine. In these embodiments, the oxidation catalyst is also referred to as a diesel oxidation catalyst. In an exemplary embodiment, the vehicle can be a land-based vehicle that does not run on rails such as, for example, a passenger car or a truck. The changes are especially changes over time.

[0015] In a particularly preferred refinement of the method according to the present invention, the declaration that the oxidation catalyst is defective comprises the generation of an error signal by the engine control device. The error signal may be written, for example, into an error memory or else output to, for example, a display element.

[0016] Preferably, the change, especially a change over time, of temperature values (e.g., the course or development of temperature over time) is measured at a given place (e.g., location) in the oxidation catalyst.

[0017] A number of physical quantities that, in conjunction with the method according to the present invention, allow reliable on-board diagnosis, have proven to be particularly advantageous.

[0018] In a first embodiment, the at least one physical quantity is a measure of the thermal energy released in the oxidation catalyst on the basis of the amount of injected fuel. In an exemplary embodiment, the physical quantity can be the released thermal energy itself.

[0019] For purposes of calculating the measure of thermal energy released, it is advantageous to determine differences between the measured temperature values and the reference temperature values. The reference temperature values can be part of a change of predicted temperature values over time that are calculated on the basis of a model and that constitute a temperature change over time in the oxidation catalyst without injection of the amount of fuel.

[0020] In a second embodiment, the at least one physical quantity is a measure of the temperature gradient or the at least one physical quantity is a quantity change that is a measure of a change for values of temperature gradients.

[0021] In a third embodiment of the method according to the present invention for the on-board diagnosis of an oxidation catalyst, a temperature threshold, in particular a local temperature maximum is or has been prescribed for a measured temperature. A time span until the measured temperature exceeds the temperature threshold, in particular the local temperature maximum is then determined. In this embodiment, the time span determined by means of this approach is the at least one physical quantity.

[0022] In some embodiments of the method according to the present invention, a plurality of physical quantities is calculated. Here, each physical quantity constitutes a measure of a physical quantity of the chemical conversion, and the physical quantities vary alternatingly (e.g., are mutually different from one another). In an exemplary embodiment, the plurality of physical quantities can be one, several or all of the physical quantities from among the set of the following physical quantities: a measure of the thermal energy released in the oxidation catalyst on the basis of the amount of injected fuel, a measure of a temperature gradient, a measure of a change of values of one or more temperature gradients, and a measure of a certain time span until the measured temperature has exceeded a temperature threshold, in particular a local temperature maximum.

[0023] In conjunction with the present invention, there is also a vehicle having an internal combustion engine, an exhaust-gas system with an oxidation catalyst, and an engine control device. The engine control device may comprise at least one computer, controller or processor and at least one memory element or computer readable storage medium (e.g., a non-transitory computer readable storage medium). According certain aspects of the present invention, there is a program (e.g., a computer program or software) that is stored in the memory element and that, during an at least partial execution in the computer or processor, carries out or implements the steps of the method for the on-board diagnosis of an oxidation catalyst, the method having the features or feature combinations of the present invention as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The principles and operation of the systems and methods, as well as other advantages and advantageous embodiments and refinements, according to the present invention are may be better understood with reference to the drawings, and the following description, it being understood that these drawings are given for illustrative purposes only and are not meant to be limiting, wherein:

[0025] FIG. 1 shows a diagram of the topology of an internal combustion engine with an exhaust-gas system and an engine control device for carrying out the on-board diagnosis according to the invention;

[0026] FIG. 2 shows changes of the temperature as a function of the time during the injection of an amount of fuel;

[0027] FIG. 3 shows a flow chart of a preferred embodiment of the method according to the invention;

[0028] FIG. 4 shows additional changes of the temperature as a function of the time during the injection of an amount of fuel; and

[0029] FIG. 5 shows a flow chart of a preferred refinement of the method according to the invention.

[0030] It will be appreciated that, for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE INVENTION

[0031] In the following detailed description, numerous specific details re set forth in order to provide a thorough understanding of the invention. However, it will be understood by those of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention.

[0032] Unless specifically state otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as "processing", "computing", "storing", "determining", or the like, refer to the action and/or processes of a computer or computing system, or similar electronic computing device, that manipulate and/or transform data represented as physical, such as electronic, quantities within the computing system's registers and/or memories into other data similarly represented as physical quantities within the computing system's memories, registers or other such information storage, transmission or display devices.

[0033] Embodiments of the present invention may include apparatuses for performing the operations herein. Such apparatuses may be specially constructed for the desired purposes, or may comprise controllers, computers or processors selectively activated or reconfigured by a computer program stored in the computers. Such computer programs may be stored in a computer readable storage medium (e.g., a non-transitory computer readable storage medium), such as, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs) electrically programmable read-only memories (EPROMs), electrically erasable and programmable read only memories (EEPROMs), magnetic or optical cards, or any other type of media suitable for storing electronic instructions, and capable of being coupled to a computer system bus. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein.

[0034] Embodiments of the invention may include electronic circuitry, wired or wireless transmitters and receivers, input-output ("I/O") interfaces/devices, and one or more controllers. A receiver may be used, for example, to receive control information (e.g., to change a mode of operation, to change the value of a parameter, etc.) and various messages.

[0035] Embodiments of the invention may include an article such as a computer or processor readable medium, or a computer or processor storage medium, such as for example a memory, a disk drive, or a USB flash memory, encoding, including or storing instructions, e.g., computer-executable instructions that, when executed by a processor or controller, carry out methods disclosed herein. Processors may include any standard data processor, such as a microprocessor, multiprocessor, accelerator board, or any other serial or parallel high performance data processor.

[0036] FIG. 1 schematically and partially shows the topology of an internal combustion engine 12 with an exhaust-gas system 14 and an engine control device 16 for carrying out the method for on-board diagnosis in a vehicle 10, according to certain aspects of the present invention, wherein several individual details are not depicted. The internal combustion engine 12 is preferably a self-igniting internal combustion engine or a diesel internal combustion engine. It may be charged once or multiple times, especially by means of a turbocharger (not shown in FIG. 1), and may have one or more exhaust-gas return lines. There is an oxidation catalyst 18 in the exhaust-gas system 14. The oxidation catalyst 18 has a temperature sensor 20 that is in signal connection with an engine control device 16, so that the development (e.g., the change) over the time of the temperature can be measured and processed in the engine control device 16 for the on-board diagnosis.

[0037] The embodiment of the internal combustion engine 12 shown in FIG. 1 is an internal combustion reciprocating engine with four cylinders as combustion chambers and with direct fuel injection into the combustion chambers by means of injectors 22. Optionally, a fuel metering module 24 is arranged in the exhaust-gas system 14 upstream from the oxidation catalyst 18. Fresh air reaches the combustion chambers via an intake pipe 26. One or more additional exhaust-gas treatment components 28 may be arranged downstream from the oxidation catalyst in the exhaust-gas system 14. These additional exhaust-gas treatment components 28 can be, for example, filter elements, reduction catalysts or the like.

[0038] The engine control device 16 comprises at least one computer with a processor and at least one memory unit, for example, a hard disk and/or a RAM. The on-board diagnosis is carried out in the engine control device 16. The method is carried out by means of software (e.g., computer readable instructions), a process in which various hardware components are actuated by the engine control device 16.

[0039] Before a particularly preferred embodiment of the method according to the invention is described, it should be pointed out that shifting the light-off temperature towards higher temperatures is associated with a deterioration of the conversion of hydrocarbons and carbon monoxide by the oxidation catalyst in the emission-relevant range. For this reason, a damaged part or volume of the oxidation catalyst that is to be detected by the on-board diagnosis has a light-off temperature that differs significantly from the light-off temperature of a part or volume of the oxidation catalyst that the on-board diagnosis considers to be intact. If fuel (mainly hydrocarbons) reaches the oxidation catalyst within a temperature range between the light-off temperature of the part of the oxidation catalyst that the on-board diagnosis considers to be intact and the light-off temperature of the part of the oxidation catalyst that is to be considered to be defective, then monitoring the exothermic reaction that is taking place in the oxidation catalyst by means of a temperature sensor installed in the exhaust-gas system makes it possible to draw conclusions about the conversion capability of the oxidation catalyst.

[0040] In a preferred embodiment, the temperature sensor 20 may be installed either downstream from the oxidation catalyst 18 in the exhaust-gas system 14 (not the variant shown in FIG. 1) or else it is accommodated in the exhaust-gas system 14 in such a way that it protrudes into the oxidation catalyst 18 (variant shown in FIG. 1). The introduction of the hydrocarbons preferably takes place either through a late post-injection of fuel into a combustion chamber of the internal combustion engine 12 by means of an injector 22 or else through a separate metering module 24 that is installed in the exhaust-gas system 14 upstream from the oxidation catalyst 18 that is to be monitored (optional alternative, shown in FIG. 1).

[0041] In an especially advantageous manner, the fuel injection for the on-board diagnosis is carried out during the overrun mode of operation of the internal combustion engine. Essentially stationary flow conditions prevail in the oxidation catalyst during the overrun mode of operation. During this on-board diagnosis, any additional emissions are slightly or, preferably, completely avoidable if the hydrocarbons of the fuel can be completely converted in the oxidation catalyst. If the internal combustion engine has one or more exhaust-gas return lines, these are switched off in the overrun mode of operation for the on-board diagnosis according to the invention. Another advantage discovered was that there are no perceptible influences on the acoustics or on the running behavior during the overrun mode of operation.

[0042] FIG. 2 shows, by way of an example, qualitative changes of the temperature (T) as a function of the time (t) when an amount of fuel is injected at a pulse 30 during the overrun mode of operation. The solid-line curve 32 depicts the change over time of a calculated reference temperature 32. Such a temperature change occurs during the overrun mode of operation when no fuel is being injected, in other words, when no hydrocarbons are being converted. Then, if fuel is only injected for purposes of the on-board diagnosis according to the invention, this results in different changes of the measured temperatures over time as a function of the conversion capacity of the oxidation catalyst. The dotted-line curve 34 depicts the temperature change over time of a defective part or volume of the oxidation catalyst. This curve largely coincides with the solid-line curve 32 since no hydrocarbons are being converted and thus no exothermic reaction is taking place. The broken-line curve 36 is the qualitative change of the temperature over time of an intact part or volume of the oxidation catalyst. In comparison to the course of the dotted-line curve 32, a peak of the broken-line curve 36 that rises after the pulse 30 can be clearly seen. Only at a later point in time (t) do the broken-line curve 36 and the dotted-line curve 32 coincide, when the conversion of the fuel in the oxidation catalyst has taken place. The area 38 (e.g., the area under the curve) between the broken-line curve 36 and the dotted-line curve 32 is a measure of the amount of heat (Q) released in the exothermic reaction.

[0043] For purposes of the on-board diagnosis, the gradient of the measured temperature downstream from the part of the oxidation catalyst and/or the amount of heat (Q) generated by means of the fuel that was injected in the part of the oxidation catalyst for the on-board diagnosis can be taken into consideration. This amount of heat (Q) is determined on the basis of the difference between the temperature measured downstream from the part of the oxidation catalyst and a calculated reference temperature. In an exemplary embodiment, the amount of heat may be calculated as the integral over time of the time-dependent product of the mean specific thermal capacity of the exhaust-gas mass flow, of the exhaust-gas mass flow and of the difference between the measured temperature and the reference temperature.

[0044] FIG. 3 shows a flow chart of a preferred embodiment of the method according to certain aspects of the present invention as may be implemented in the form of software (e.g., computer readable instructions) in the engine control device 16. Merely for the sake of clarity, the method is described here in conjunction with the on-board diagnosis of the total volume of the oxidation catalyst, although on-board diagnosis of partial volume is also within the scope of the invention.

[0045] The sequence begins with the start 40 of the method for the on-board diagnosis. First of all, in the decision 42, it is determined whether the temperature of the oxidation catalyst--and if applicable other parameters--falls within a prescribed range. If the decision 42 is negative, the method ends and the sequence returns to the start 40. If the decision 42 is positive, the method continues with the decision 44 as to whether the internal combustion engine is in the overrun mode of operation. If the decision 44 is negative, the method ends and the sequence returns to the start 40. If the decision 44 is positive, the method continues with the active intervention 46 in which a quantity of fuel is injected upstream from the oxidation catalyst. This is followed by a decision 48 as to whether or not the active intervention has been successful. If the decision 48 is negative, the method ends and the sequence returns to the start 40. If the decision 48 is positive, the method continues with the start of the evaluation 50 of the measured change of the temperature over time.

[0046] Then, two processes take place in the embodiment shown: a calculation 52 of the amount of heat (Q) and a calculation 54 of the temperature gradient. The calculation 52 is followed by a decision 56 as to whether the calculated amount of heat (Q) is above a threshold. If this is the case, a declaration 58 is made to the effect that the oxidation catalyst is considered to be intact. The calculation 54 is followed by a decision 60 as to whether the calculated temperature gradient is above a threshold. If this is the case, a declaration 62 is made to the effect that the oxidation catalyst is considered to be intact. The negative outcomes of the decision 56 and of the decision 60 are processed with an AND-link 64, not with an exclusive AND-link: if at least one of the decisions 56, 60, preferably if both decisions 56, 60, turns out to be negative, then a declaration 66 is made to the effect that the oxidation catalyst is considered to be defective. If applicable, an error signal is then generated by the engine control device 16.

[0047] In an advantageous refinement for enhancing the diagnostic sensitivity or the diagnostic selectivity, it is also possible to monitor only a partial volume or several partial volumes of the oxidation catalyst. In a preferred embodiment regarding the on-board diagnosis of a partial volume of two same-sized partial volumes of the oxidation catalyst, the temperature sensor is arranged in the center of the diesel oxidation catalyst. As already explained on the basis of the flow chart shown in FIG. 3, the amount of heat released due to the exothermic reaction and the temperature gradient that occurs are evaluated according to certain aspects of the present invention. Furthermore, in order to further increase the sensitivity of the diagnosis, the monitored partial volume can be provided with a catalytic coating that differs from that of the complementary partial volume, whereby the other coating is the first to be damaged. In such an embodiment, this is especially the partial volume of the oxidation catalyst situated upstream since experiments have shown that this partial volume is often the first one to be damaged, whereas the partial volume of the oxidation catalyst situated downstream from the temperature sensor still makes a significant contribution to the reduction of the emissions.

[0048] In addition or as an alternative to the monitoring of the temperature gradient and/or of the amount of heat in case of an oxidation catalyst that has become damaged only in the upstream part, such an on-board diagnosis can be carried out in such a way that an evaluation is carried out of the time span until the generated amount of heat becomes measurable downstream from the diagnosed part. This can be done, for instance, by means of an evaluation of the temperature maximum or of the heat flow maximum in the appertaining change or by means of another temperature-related quantity as the measure. Preferably, a comparison is made here to the anticipated temperature change or heat flow change.

[0049] FIG. 4 schematically shows three qualitative changes of temperatures (T) as a function of the time (t) in the case of the injection of an amount of fuel at a pulse 30 in the overrun mode of operation in order to explain the approach of the on-board diagnosis through an evaluation of the point in time when a temperature increase is detected at the temperature sensor. The broken-line curve 68 depicts a change of a calculated reference temperature over time. For a partial volume of the oxidation catalyst that the on-board diagnosis considers to be intact, the measured change of the temperature over time depicted by the solid-line curve 70 is expected. In contrast to this, a partial volume of the oxidation catalyst that the on-board diagnosis considers to be defective shows a temperature change over time that is depicted by the dotted-line curve 72. The broken-line curve 68 exhibits a maximum 74 at a point in time t2, while the dotted-line curve 72 exhibits a maximum 76 at a point in time t1.

[0050] The time difference 78 between the measured temperature maximum and the expected temperature maximum, which is to be expected on the basis of the change of the reference temperature over time, is evaluated in the embodiments of the inventive on-board diagnosis of the examined partial volume of the oxidation catalyst. In one variant of this embodiment, the change of the heat flow is evaluated as an alternative to the change of the temperature over time.

[0051] At this juncture, it should be pointed out that, in the case of damage to the upstream part of the oxidation catalyst, the hydrocarbons of the injected fuel are only converted in the downstream part of the oxidation catalyst. For this reason, the temperature rise on the downstream side of the temperature sensor occurs sooner than in case of an oxidation catalyst that the on-board diagnosis considers to be intact. This shortening of the time span is thus utilized as a criterion in the appertaining embodiments of the on-board diagnosis. Advantageously, this approach is not dependent on the absence of heat generation in order to identify a defective part of the oxidation catalyst.

[0052] FIG. 5 is a flow chart of a preferred, expanded embodiment of the method according to the invention as can be implemented in the form of software (e.g., computer readable instructions) in the engine control device 16. According to certain aspects of the present invention, the expansion of the on-board diagnosis in the overrun mode of operation comprises an evaluation of the point in time when the temperature maximum occurred.

[0053] The sequence begins with the start 80 of the method for the on-board diagnosis. In the decision 82, it is determined whether the temperature of the oxidation catalyst--and if applicable other parameters--falls within a prescribed range. If the decision 82 is negative, the method ends and the sequence returns to the start 80. If the decision 82 is positive, the method continues with the decision 84 as to whether the internal combustion engine is in the overrun mode of operation. If the decision 84 is negative, the method ends and the sequence returns to the start 80. If the decision 84 is positive, the method continues with the active intervention 86 in which a quantity of fuel is injected upstream from the oxidation catalyst. This is followed by a decision 88 as to whether the active intervention has been successful. If the decision 88 is negative, the method ends and the sequence returns to the start 80.

[0054] If the decision 88 is positive, the method continues with the evaluation 90 as to whether a temperature maximum is present in the measured change of the temperature over time. If this is not the case (negative decision 90), an evaluation 92 takes place by means of a calculation of the amount of heat and/or a calculation of the temperature gradient; in this context, see again the corresponding part of the flow chart of FIG. 3 starting with evaluation 50. If the decision 90 is positive, an evaluation 92 takes place of the time span as of the injection of the quantity of fuel until the temperature maximum has occurred. Subsequently, in step 94, a decision is made as to whether the ascertained time span is above a threshold. If this is not the case (negative decision 94), a declaration 95 is made to the effect that the oxidation catalyst is considered to be defective. If the ascertained time span is above said threshold, in step 96, a declaration is made to the effect that the oxidation is considered to be intact.

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