DE102012001044A1 - Method for determining operability of resistive soot sensor for monitoring particulate filter in motor car, involves outputting notification if change in signal falls below threshold value, where notification indicates malfunction of sensor - Google Patents

Abstract

The method involves monitoring a signal of a soot sensor correlating with a resistance of a sensitive region (1) of the soot sensor during forced regeneration of a particulate filter in case of self-diagnosis operation of the soot sensor, where the sensitive region comprises an interdigital electrode structure that is arranged between two electrodes (2, 3). A notification is output if a change in the signal falls below predeterminable threshold value during the self-diagnosis operation of the soot sensor, where the notification indicates a malfunction of the soot sensor.

Description

The invention relates to a method for determining a functional capability of a resistive soot sensor provided for monitoring a particulate filter, which is arranged in the exhaust gas line of an internal combustion engine on the output side of the particulate filter.

When using a particulate filter for filtering out soot particles from the exhaust gas of motor vehicle internal combustion engines, it is advantageous to monitor the filter function of the particulate filter in order to be able to detect any filter damage that may occur. For this purpose, it is known to arrange a soot sensor on the output side of the particulate filter in the exhaust system. If an increased soot emission is detected by the sensor, then the particulate filter is diagnosed as defective. In the case of resistive soot sensors, an excessively high soot content in the exhaust gas attributable to a filter damage is recognized by the fact that soot contained in the exhaust gas is deposited on the sensitive sensor region and due to its electrical conductivity causes a change in resistance which can be detected. Not least because of legal requirements, there is a need to also monitor the soot sensor itself in terms of its operability in order to be sure that the monitoring of the particulate filter is reliable.

This is in the DE 10 2009 058 260 A1 proposed to divide the sensitive area of a resistive soot sensor into two areas. By means of a cover layer, for example, it is ensured that no soot can deposit on one of the two sensitive areas. By comparing the signals obtained from the regions, it can be verified whether a signal of the uncovered region is actually due to precipitation of carbon black.

As a difficulty, it is to be regarded in the known method that the output side of a fully functional particulate filter practically no more soot is contained in the exhaust gas. Thus, it takes a very long time until a measurable change in resistance of sensitive area which can be acted upon by soot occurs. At least during this time, therefore, it can not be determined easily whether the soot sensor is functioning properly. Furthermore, it may happen that particles contained in the exhaust gas, electrically non-conductive particles, such as ash particles, deposit on the sensitive area and form an electrically non-conductive coating. This has the consequence that a soot deposit can no longer cause a change in resistance of the sensitive area and thus the functionality of the soot sensor is no longer present.

The object of the invention is to provide a monitoring method for a soot sensor, which avoids the above-mentioned disadvantage and allows a more reliable functional test.

This object is achieved by a method having the features of claim 1.

For the inventive method for determining a functional capability of a resistive soot sensor provided for monitoring a particulate filter, it is characteristic that in a self-diagnostic operation of the soot sensor, a signal of the soot sensor correlating with a resistance of a sensitive region of the soot sensor is monitored during a forced regeneration of the particulate filter. In the event that a change in the signal occurring during the self-diagnostic operation falls below a predefinable limit value, a message indicating a malfunction of the soot sensor is output as the diagnosis result.

The inventors have found that in a forced particulate filter regeneration, the exhaust gas on the output side of the particulate filter has surprisingly a soot content that can be detected with a sufficiently sensitive and properly functioning resistive soot sensor. This phenomenon is exploited according to the invention for functional testing of the soot sensor. If the soot sensor in the self-diagnostic mode active during a forced particulate filter regeneration does not show any signal change due to the presence of soot in the exhaust gas, for example as a result of an electrically non-conductive passivation layer present on the sensitive area, then a non-functioning soot sensor can be deduced. However, if a corresponding signal change occurs, it can be assumed that the soot sensor is functioning properly. Thus, the soot sensor can be reliably diagnosed at the recurrent intervals of forced particulate filter regeneration. Since usually forced particulate filter regeneration occur at more or less regular intervals of about thousand to several thousand kilometers of driving distance, a plurality of soot sensor checks is made possible over the term of the corresponding motor vehicle.

For the purposes of the invention, a forced particulate filter regeneration is to be understood as an operating state deliberately brought about by special measures, in which soot deposited on the particulate filter can burn off by reaction with oxygen present in the exhaust gas. Typically, this operating condition is achieved by, for example, changing the Operating conditions of the internal combustion engine, a sufficiently hot exhaust gas having a temperature of about 700 ° C or more is generated and the particulate filter is thus brought to a temperature above the Rußabbrenntemperatur. This operating state is generally requested when it is determined that the amount of soot deposited on the particulate filter has exceeded a predefinable limit. At the latest when the conditions for soot burn-off are met, the soot sensor is switched to its own diagnostic mode of operation and the output signal is monitored. Occurs in the course of the particulate filter regeneration, which typically lasts between 5 min and 15 min, with active self-diagnostic operation a specifiable change of the output signal compared to the signal at the beginning of the self-diagnostic mode, the function of the soot sensor is recognized as being correct. Otherwise, a message indicating a malfunction of the soot sensor is output.

In an embodiment of the method, a soot sensor is used whose sensitive region has an interdigital electrode structure in which an electrical voltage of predeterminable magnitude is applied between two electrodes of the interdigital structure and the current flowing between the two electrodes is monitored. An interdigital electrode structure is understood to mean a structure of two comb-shaped electrodes, in which the tines of the comb structures of the two electrodes intermesh without touching each other. The spacing of the tines is low and is typically in a range below one millimeter, the comb structure forms the sensitive area of the soot sensor. From a certain amount, soot depositing on this area forms a conductive bridge between adjacent tines of the two electrodes, which changes the resistance between the electrodes in a detectable manner. Such sensors have proven to be particularly well suited for the detection of soot contained in engine exhaust gas. As for the resistance of the sensitive area characteristic sensor signal of the current flowing from one electrode to another electrical current is evaluated at an applied between the electrodes electrical voltage of predeterminable size.

In a further embodiment of the method, an electrical operating voltage with a predetermined size is provided for the soot sensor and a malfunction of the soot sensor is displayed when a limit value is fallen below, which corresponds to an electrical resistance of the sensitive region in the range of two to six megohms. Preferably, a malfunction is indicated when there is a resistance of less than four megohms due to the operating voltage applied between the electrodes of the soot sensor and a sensed electrical current.

In a further refinement of the method, the sensitive region of the soot sensor is heated up immediately before or after an activation of the self-diagnostic operation in such a way that soot deposited on the sensitive region is burnt off. In this way, the sensor for a self-diagnosis is brought into a defined state, in particular in an at least approximately soot-free state. As a result of Rußabbrands immediately before or after the activation of the self-diagnostic operation previously present on the sensitive area soot can not affect the diagnosis result. For Rußabbrand a preferably provided for the soot sensor heater is activated so that the sensitive region of the soot sensor is brought to a temperature of about 800 ° C for about one to two minutes. The heater is deactivated for the actual self-diagnosis phase.

In a further refinement of the method, when the limit value is undershot in the self-diagnostic mode, an evaluation is carried out with respect to a validity of the diagnostic result, and the message concerning a malfunction of the soot sensor is suppressed if the diagnosis result is found to be invalid. The evaluation is preferably carried out directly after the self-diagnostic operation, but preferably at a time at which the process of the forced particulate filter regeneration is completed. By evaluating the diagnostic result, a misdiagnosis can be avoided, but at least a likelihood of misdiagnosis can be minimized. For the evaluation, it is preferably provided that the course of the forced particulate filter regeneration is monitored with respect to an operating variable correlated with a soot emission and the invalidity of the diagnostic result is determined if the monitored operating variable is expected to cause the soot emission to fall below a predefinable limit value. In this way, it can be ruled out that, in the self-diagnostic mode, because of an excessively low soot particle emission during the particle filter regeneration, it would not be possible or unlikely to require soot amount separation to respond to the sensor.

In a preferred embodiment, the evaluation includes monitoring the course of the forced particulate filter regeneration and determining the invalidity of the diagnostic result if predefinable limit values with respect to a duration and / or a temperature of the particulate filter during regeneration are undershot. In particular, it can be determined whether a particulate filter regeneration by a premature termination or has remained incomplete due to too low a temperature. In these cases it is assumed that the soot particle exposure of the soot sensor during the particulate filter regeneration or during the self-diagnostic operation was too low to be able to reliably determine a malfunction of the sensor. It is understood that in addition to the regeneration duration or temperature, further operating variables, such as, for example, an exhaust gas mass flow, a concentration of one or more exhaust gas components such as oxygen, nitrogen oxide, ammonia, carbon monoxide can be monitored and used to evaluate the diagnostic result.

If the evaluation of the diagnostic result that the predefinable limit value has not fallen below until the end of the self-diagnostic operation that this result is valid, a signal indicating a defective soot sensor is output. Preferably, an optical and / or acoustic fault message display is activated and made an entry in an electronic fault memory.

Further advantages, features and details of the invention will become apparent from the following description of preferred embodiments and from the drawings. The above. Features and combinations of features mentioned in the description as well as the features and combinations of features mentioned below in the description of the figures and / or shown alone in the figures can be used not only in the respectively indicated combination but also in other combinations or alone, without the scope of the invention leave.

The figures show in:

1 a schematic representation of an advantageous embodiment of a resistive soot sensor to be monitored,

2 a schematic representation of a characteristic of the soot sensor according to 1 ,

According to the merely rough schematic representation in 1 is a sensitive area 1 of the resistive soot sensor to be monitored by an interdigital structure of two electrodes 2 . 3 educated. As shown, in such an embodiment, fingers or prongs of the two electrodes engage 2 . 3 combed into each other without touching. The electrodes 2 . 3 are on a heat resistant substrate 4 made of electrically non-conductive material such as ceramic applied. The substrate 4 is arranged on the output side and at a short distance to a particulate filter, not shown in the exhaust stream of an internal combustion engine such that the sensitive region of flowing out of the particulate filter exhaust gas can be flowed or flows around. The electrodes 2 . 3 are connected to a control and evaluation unit, not shown. From this is an electrical operating voltage of predeterminable size to the electrodes 2 . 3 created and monitored the flow of current. The operating voltage is preferably a direct electrical voltage. A preferred value for the operating voltage is in the range of 10 to 80 volts, more preferably a value of about 45 volts. The substrate 4 is preferably formed heatable, including an electrical heater on the underside of the substrate 4 or in the substrate 4 can be arranged integrated.

When soot deposits on the substrate, a conductive amount will be formed between adjacent fingers or tines of the electrodes at an appropriate amount 2 . 3 out. Consequently, due to the applied operating voltage, an electric current between the electrodes 2 . 3 flow, which is monitored and measured as a measurement signal. If the current exceeds a predefinable value, soot loading of the sensitive area is concluded. In the normal measuring mode in which the particulate filter is monitored with regard to its filter function, in such a case an error message indicating a filter defect can be output.

To clarify the facts, an in 2 schematically illustrated characteristic of the soot sensor reference. In the characteristic curve gives a trace 5 a typical course of a measuring current I as a function of a soot loading B of the sensitive area 1 again. As can be seen, the measuring current I at low loading B is very small and increases only at a certain loading B, but then increases rapidly with increasing loading B. At very high loads B, the measuring current I can go into a saturation course, but this is not shown here. It is preferably provided to specify an administrable limit value I _G , which is assigned to a critical soot loading B _k . In this case, the limit values I _G and B _k are selected so that in the case when the limit value I _{G is} reached in a measurement cycle of a given duration, a critical soot charge B _{k has been} deposited, which causes an inadmissibly high soot content in the exhaust gas or an inadmissibly high Soot emission and thus an impermissibly low particle retention function of the particulate filter corresponds. In this case, a malfunction of the particulate filter is preferably signaled.

For a determination of the functionality of the soot sensor according to the invention, the procedure is analogous in principle, which is why continue to the 1 and 2 Reference can be made. in the In contrast to an operation of the sensor provided in normal operation of the particulate filter, however, a self-diagnostic operation is awaited until a forced particulate filter regeneration is requested. It is assumed below that a requested forced particulate filter regeneration is started and performed as intended. For this purpose, the particulate filter is heated in a conventional manner to a temperature required for Rußabbrand of typically 700 ° C or more and maintained at the regeneration temperature for a predetermined period of time. Following a request for forced particulate filter regeneration, the normal monitoring operation of the soot sensor with respect to particulate filter damage is disabled and the self-diagnostic operation mode is activated. Preferably, after activation of the self-diagnostic operation, the soot sensor itself is also regenerated with respect to possibly deposited soot. The soot sensor regeneration may also occur in time between the deactivation of the normal particulate filter monitoring operation and before activation of the self-diagnostic operation. It can also be provided that the soot sensor regeneration in connection with a self-diagnosis is completely dispensed with.

For soot sensor regeneration, the heater of the soot sensor is actuated such that the sensitive area 1 the soot sensor assumes a temperature of about 800 ° C for about 1 to 2 minutes. Thereafter, the heater is turned off or operated so that the sensitive area 1 assumes a predeterminable temperature or its temperature is in a predeterminable range of about 200 ° C to 400 ° C. Then, with the particle filter regeneration running, and with the electrodes 2 . 3 applied electrical measuring voltage of, for example, 45 V across the electrodes 2 . 3 flowing current continuously measured. If it is determined that a predeterminable limit value I _G for the measurement current I is reached or exceeded, the soot sensor is determined to be correct, subject to the result of a self-diagnosis preferably subsequent result evaluation. If the limit value I _G for the measuring current I is not reached, the soot sensor is determined to be defective, subject to the result of a preferably subsequent evaluation of the result of the self-diagnosis.

The limit value I _G for the measurement current may correspond to the limit value I _G predetermined for a particulate filter monitoring. Preferably, however, it is above it. It is preferable that the limit value I _{G is} in the initial range of the steep rise of the characteristic according to lane 5 is placed. The limit value I _G for the measuring current can for example be correlated with a critical soot loading B _k , which is obtained with a predefinable soot emission of about 5 mg per km of driving distance to 30 mg per km of driving distance. At a measuring voltage of 45 V, the limit value I _G for the measuring current I, depending on the type of sensor, is preferably in the range between 5 μA and 30 μA, particularly preferably between 10 μA and 20 μA.

The self-diagnosis of the soot sensor is temporally preferably completely within the regeneration phase of the particulate filter. The self-diagnostic operation may be terminated when the particulate filter regeneration is terminated. Alternatively, a predefinable period of time may also be provided for the self-diagnostic operation. This time span may be based on a soot emission expected during particle filter regeneration. For example, the duration of the self-diagnosis of the soot sensor may be linked to an expected soot emission value, which exceeds the soot emission value by a multiple within a measurement cycle for determining the functionality of the particulate filter in the case of a proper particulate filter.

In the following, the preferably provided evaluation of the diagnostic result following the self-diagnostic operation will be discussed. The evaluation is carried out in particular if the limit value I _G in the self-diagnostic mode has not been reached and thus a malfunction of the soot sensor can be assumed. For the result evaluation, values of a plurality of operating parameters during the particle filter regeneration are preferably evaluated. In this case, it is checked whether the values are each within a predetermined value range. In particular, it may be provided to estimate the soot emission during the self-diagnostic operation. If the estimated value for the soot emission is below a predefinable threshold value, the diagnostic result is invalidated or suppressed. The procedure is analogous if the duration of the particle filter regeneration or the self-diagnostic operation has fallen below or falls below a predefinable threshold and / or if a temperature of the particulate filter temporarily falls below or falls below a predefinable threshold during the particulate filter regeneration or self-diagnostic operation. In addition, an invalidation of the diagnostic result may be provided if the exhaust gas volume flow or a concentration of one or more of the exhaust gas components HC, CO, O2, NOx, NH3 was temporarily outside a predetermined value range. In this way, a further improvement in the reliability of the diagnosis result is made possible.

It should also be pointed out that in addition or as an alternative to a soot sensor self-diagnosis during a forced particulate filter regeneration, such may also be provided during an engine operating phase, in which one or more Exhaust after-treatment components of a rapid heating with a heating gradient of predeterminable size to be subjected. For this purpose, operating phases come into consideration, in which a transition from a low-load operation to a high-load operation or in which a regeneration of an optionally provided nitrogen oxide storage catalytic converter or an oxidation catalytic converter is undertaken.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102009058260 A1 [0003]

Claims (7)

Method for determining a functional capability of a resistive soot sensor provided for monitoring a particulate filter, which is arranged in the exhaust tract of an internal combustion engine on the output side of the particulate filter, characterized in that in a self-diagnostic operation of the soot sensor with a resistance of a sensitive area ( 1 ) of the soot sensor correlating signal (I) of the soot sensor is monitored during a forced regeneration of the particulate filter and in the event that a occurring during the self-diagnostic operation change of the signal (I) below a predetermined limit (I _G ), as a result of diagnosis a message is output, which indicates a malfunction of the soot sensor. A method according to claim 1, characterized in that a soot sensor is used whose sensitive area ( 1 ) has an interdigital electrode structure in which between two electrodes ( 2 . 3 ) of the interdigital structure an electrical voltage of predeterminable size is applied and that between the two electrodes ( 2 . 3 ) flowing electricity is monitored. A method according to claim 1 or 2, characterized in that for the soot sensor, an electrical operating voltage is provided with a predetermined size and a malfunction of the soot sensor is displayed when a limit (I _G ) is exceeded, which is an electrical resistance of the sensitive area in the range from two to six megohms equals. Method according to one of claims 1 to 3, characterized in that immediately before or after activation of the self-diagnostic operation of the sensitive area ( 1 ) of the soot sensor is heated in such a way that on the sensitive area ( 1 ) is burned deposited soot. Method according to one of Claims 1 to 4, characterized in that, when the limit value _{( IG ) is} undershot in the self-diagnostic mode, an evaluation is carried out with respect to a validity of the diagnostic result and the message relating to a malfunction of the soot sensor is determined when the diagnosis result is invalid is suppressed. A method according to claim 5, characterized in that the course of the forced particulate filter regeneration is monitored with respect to a correlated with a soot emission operating size and the invalidity of the diagnostic result is determined if the monitored operating variable is expected that the soot emission below a predetermined limit. A method according to claim 5 or 6, characterized in that the course of the forced particulate filter regeneration is monitored and the invalidity of the diagnostic result is determined if predetermined limits are exceeded with respect to a duration and / or a temperature of the particulate filter in the regeneration.

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