DE10243343B4 - Method for evaluating a three-way catalyst - Google Patents

Abstract

Method for evaluating a three-way catalytic converter (7) located in the exhaust gas tract (3) of an internal combustion engine (1), in which
a) repeatedly measured downstream of the catalyst (7) an N 2 O concentration and from this an N 2 O emission value (N 2 O) is formed,
b) checking that the emission value (N2O) exceeds a threshold value (SW1),
c) if the threshold value is exceeded, a time duration (t) which has elapsed since the internal combustion engine (1) was put into operation, or a temperature value (T) of the catalytic converter (1) is detected as a further measured variable,
d) on the basis of this further measured variable (t, T) the evaluation of the catalyst (1) is carried out.

Description

The The invention relates to a method for evaluating an im Exhaust tract of an internal combustion engine lying three-way catalyst.

Three-way catalysts are an essential component of exhaust aftertreatment at lambda = 1 regulated internal combustion engines, since they simultaneously pollutants Reduce NOX, HC and CO. In modern internal combustion engines is To ensure low exhaust emissions as constant as possible monitoring of such a catalyst required.

In EP 1 167 710 A2 For example, a method and a device for increasing a catalyst temperature of a catalytic converter arranged in an exhaust system of an internal combustion engine are described, wherein an energy input into the exhaust system is increased by at least one motor and / or non-motor measure. The condition of the catalyst is monitored by means of a NO _x sensor.

In EP 1 136 670 A2 and DE 100 14 239 A1 a method and a device for monitoring the operation of a 3-way catalytic converter in the exhaust passage of an internal combustion engine are described. In this case, at least one lambda probe and an NO _x -sensitive measuring device downstream of the 3-way catalyst are arranged in the exhaust passage. A lambda range is preset and, given the presence of a lambda value within the lambda range, a magnitude of the NO _x signal and / or the value of a gradient of the NO _x signal. detected in a predetermined measuring interval (reference values). Furthermore, the reference values are compared with threshold values and, when the threshold values are exceeded, damage to the 3-way catalytic converter is indicated.

In EP 1 164 268 A2 is an arrangement for monitoring a regenerable NO _x storage in an exhaust line, in particular a diesel or lean-burn engine, through which flows a NO _x -containing gas, described, in which a temperature measurement is carried out in the exhaust line, from which on the operability, the Operating state and / or on the storage capacity of the NO _x storage is closed.

In DE 199 18 756 A1 are described an arrangement for cleaning an exhaust gas of an internal combustion engine, in particular of motor vehicles, with a arranged in an exhaust passage catalyst system for reducing a nitrogen oxide (NO _x ) content of the exhaust gas and a method for operating such an arrangement. It is envisaged that the catalyst system comprises a first NO _x storage catalytic converter in the flow path of the exhaust gas and spaced therefrom a second NO _x storage catalytic converter.

In DE 198 51 564 A1 A method for operating and verifying the activity of an arranged in an exhaust passage of an internal combustion engine NO _x storage catalytic converter is described. The aging state of the NO _x storage catalyst is expressed by an aging factor that describes the current storage capacity of the NO _x storage catalyst reduced by aging influences. The aging factor is compared successively in time with a plurality of thresholds of different height, wherein the height of the thresholds decreases with increasing running distance of the NO _x storage catalytic converter. If the respective threshold values are undershot, different operating strategies for the NO _x storage catalytic converter and / or the internal combustion engine are initiated.

In DE 198 44 178 A1 is a method of assessing the operability of a catalyst, which is traversed by the exhaust gases of a combustion process is described. It comprises the steps of: Performing a regeneration attempt to eliminate reversible loss of functionality, testing the functionality and assessing the functionality on the basis of the above test after the regeneration attempt.

In DE 199 31 321 A1 It is proposed to use a NOx sensor for determining the NOx concentration downstream of the catalyst to be diagnosed for checking a three-way catalytic converter. From the measured NOx concentration, it can be concluded that the HC conversion properties of the three-way catalyst are due to the close relationship between NOx and HC conversion.

Regarding the Harmful component HC, it is state of the art, for monitoring and evaluating a catalyst to measure its oxygen storage capacity. As from the publication Braun, "Design of catalyst sensors and software for future OBD Requirements in USA and Europe ", MTZ58, 1997, is known Measurement of the oxygen storage capacity of a catalyst an evaluation be used, which upstream of a catalyst and a downstream of the catalyst arranged lambda probe accesses.

The use of the oxygen storage capacity to evaluate the catalyst is based on the assumption that the decrease of an efficiency of the catalyst and thus an emission deterioration of an internal combustion engine is accompanied by a sinking oxygen storage capacity ty. However, a direct inference to the conversion process of the catalyst is not possible with this diagnostic or evaluation method. This is especially unfortunate with low emission concepts because, to achieve very low exhaust emissions, the combination of oxygen storage capacity and catalytic converter efficiency has heretofore not been detectable with the required accuracy. Also, the publication Rabl, "The formation of nitrous oxide (N ₂ O) on three-way catalysts," Progress Reports VDI, Series 12, No. 380, VDI-Verlag 1999, page 112, noted this, this publication on page 16 it is explained that three-way catalysts in the so-called light-off state, in which stand-by readiness is reached, release a maximum of N 2 O.

Of the Invention is based on the object, a method for evaluation specify a three-way catalyst, with the recognition of a defective catalyst possible is.

These Task is solved by a method for evaluating one in the exhaust tract of an internal combustion engine lying three-way catalyst, in which repeated downstream of the Catalyst measured an N2O concentration and formed an N2O emission value is being checked, whether the emission value exceeds a threshold, with threshold exceeded as another measure one Time that has elapsed since the internal combustion engine was put into operation, or a temperature value of the catalyst is detected and based on this further measure the rating the catalyst is made.

The Invention thus focuses on the knowledge that the NOX conversion in three-way catalysts over simultaneous oxidation of the reducing constituents HC, CO and H2 and reduction of the oxidizing constituents of NOX and O2. there extends the reduction of NOX not always directly to the inert components N2 and H2O, but in the so-called light-off region in which the catalyst its operability begins to reach, is formed in an intermediate step N2O. outgoing from the realization that the occurrence of N2O is thus characteristic for the Light-off area is, according to the invention, the measurement the N2O concentration now used to evaluate the catalyst in terms of his aging condition.

The Measurement according to the invention the N2O concentration allows it, a measure of the existence of the light-off state of the catalyst to win, depending on Condition of the catalyst to different operating conditions of the Internal combustion engine is achieved. For older catalysts takes it after commissioning of the engine longer, until the catalyst is ready. In a first variant of the invention, therefore, the time duration since commissioning of the engine to evaluate the catalyst used.

A alternative measurand, on which supported the rating can be, the operating temperature of the catalyst in the light-off state, because older catalysts a higher one Operating temperature to reach the light-off state need. The regulations the temperature value of the catalyst can in many ways respectively. For example, arithmetic investigations with the help of a Temperature model of operating parameters of the internal combustion engine known; also can temperature sensors be used. Such can both the temperature directly at the catalyst and downstream of it of sensing.

Consequently are both the period of time since the internal combustion engine was put into operation as well as the operating temperature of the catalyst are suitable as further measured variables, evaluated with the detection of N2O concentration for the evaluation of the catalyst to become.

The Evaluation of the catalyst on the basis of the further measured variable is the Professional then just possible, For example, in the evaluation can be checked whether the period or the temperature exceeds a limit. In case of a limit violation then the catalyst is classified as defective, since the light-off state too late was achieved.

The N2O concentration increases after commissioning of an internal combustion engine until it reaches the light-off state to a maximum value and then falls off quickly. For the threshold value check according to the invention, therefore, any N 2 O emission value is suitable which enables a detection of the light-off state. For this purpose, for example, the N2O measured value itself can be used. The light-off state is then safely reached when this N2O measured value falls back below the threshold value once the maximum has been reached, ie when the threshold value is exceeded by larger N2O measured values in the direction of smaller N2O measured values. At this time, the maximum of N 2 O emission indicative of the light-off condition has already occurred during operation of the engine. In this procedure, the detection reliability of the light-off state can be further improved by subjecting the N 2 O measured value to low-pass filtering or smoothing. Alternatively, it is also possible to determine from the course of the N 2 O measured value by differentiation the maximum of the N 2 O emission. In this way can also the Light off state can be detected.

In Another alternative is the N2O emission reading Integration of the N2O measured value via certain time intervals are obtained. If this N2O emission value exceeds one Maximum value, this is a sign for the light-off state.

It Further methods are conceivable, with which the N2O emission value from the N2O measurement, which then the light-off state reliable indicating, can be won. It is always essential that the N2O emission value a statement about it allows that the light-off state is present or (already) reached.

ever according to the type of internal combustion engine, the maximum of the N2O emission is only then characteristic of the light-off state when the internal combustion engine is inside a certain operating parameter range is located. This is for example in internal combustion engines of the case, in addition to stoichiometric mixture too can be operated with lean or rich mixture. It is therefore especially for such internal combustion engines preferred that the detection of the N2O concentration only then done is when the exhaust of the internal combustion engine in a certain Oxygen content range is. Then with the N2O emission the Light-off state reliable be detected. The waiver of the measurement or the subsequent determination of the N2O emission value in operating ranges where no reliable light-off state detection possible is, saves processing overhead in the control of the internal combustion engine.

One Area where a reliable Detection of the light-off state possible is an operation with a lambda value close to 1. It is therefore preferable that the oxygen content range is a lambda value between 0.9 and 1.2, in particular between 0.99 and 1.01, particularly preferred between 0.995 and 1.005.

Of the Threshold value used in the evaluation of the N2O emission value is of course suitable to the N2O emission value to choose. For example, it may be an N2O concentration, a temporal N2O concentration change or act a N2O mass.

The The invention will be described below with reference to the drawings even closer explained. In the drawings shows:

1 a block diagram of an internal combustion engine with a catalyst in the exhaust system,

2 a block diagram of a method for evaluating the catalyst of the internal combustion engine of 1 and

3 another method for evaluating the catalyst of the internal combustion engine 1 ,

In 1 is schematically an internal combustion engine 1 shown, the combustion air via an intake 2 receives. Exhaust gases in the internal combustion engine 1 expiring combustion are in an exhaust tract 3 pushed out. The internal combustion engine 1 runs under the control of a control unit 4 , which among other things an injection system 5 controls which (via unspecified lines) of the internal combustion engine 1 Fuel is allocated.

The control unit 4 controls the injection system 5 taking into account the values of a lambda probe 6 on, the oxygen content in the exhaust system 3 the internal combustion engine 1 measures. The control unit 4 regulates the internal combustion engine 1 doing so so that the internal combustion engine 1 performs a stoichiometric combustion. Under operating conditions around Lambda = 1 around has one in the exhaust tract 3 lying three-way catalyst 7 a maximum efficiency and can thus convert pollutants located in the exhaust NOX, HC and CO, into harmless compounds.

The control unit evaluates the compliance with limit values in this regard 4 the catalyst 7 during operation of the internal combustion engine 1 in a so-called on-board diagnosis. This is the control unit 4 with an N2O probe 8th connected in the exhaust tract 3 downstream of the catalyst 7 measures the N2O concentration. Further, the controller detects 4 by means of one in a catalyst 7 attached temperature sensor 9 the temperature of the catalyst 7 ,

The control unit 4 leads to the evaluation of the catalyst 7 the following in 2 illustrated method from:
At the start of the evaluation process, a time counter is set to zero in a step S0, so that a starting point is given for a time t. This initialization takes place at the start of the internal combustion engine 1 instead of. Subsequently, in a step S1, a query is made as to whether an N 2 O emission value in the form of a concentration has fallen below a threshold value SW. The step S1 involves a prior measurement of the N2O concentration by means of the N2O probe 8th , If it falls below, it is then checked whether the N2O concentration falls below the threshold value SW1 from a larger value. If this is not the case (n-branch), the time counter is incremented in a step S2, so that the time t is increased since commissioning, and jumped back before step S1.

If, on the other hand, a corresponding threshold underrun has been detected (J branch) in step S1, it is next checked in step S3 whether the time t is above a threshold value SW2. If this is not the case (N branching), a functional catalyst is formed in step S4 7 diagnosed. Otherwise (J branch), in a step S5, the catalyst 7 detected as defective and a corresponding storage in a fault memory of the control unit 4 performed. Alternatively, a corresponding indication can also be presented to a driver or an operating state restriction in the internal combustion engine 1 be noted that only a limited operation allows, despite a defective catalyst 7 if possible, to be able to comply with exhaust emission limits.

In addition to the in 2 shown queries in steps S1 and S3 can still be checked in step S1 before detecting the N2O concentration, whether the internal combustion engine 7 is within a certain lambda range around lambda = 1. Only if this is the case, the measured value for the concentration N2O is updated.

An alternative embodiment for the method for evaluating the catalyst 7 is in 3 shown schematically.

This method differs from that already mentioned in that not the time t, but the temperature T of the catalyst 7 is evaluated in a second query.

In this method, the catalyst temperature is measured in a step S6. This is done by means of the temperature sensor 9 , Subsequently, in a step S7 identical to step S1 of FIG 2 is, checked whether the N2O emission value, ie the N2O concentration, fell below the threshold value SW1. If this is not the case (N-branching), the system jumps back to step S6, otherwise (J-branching) proceeds to S8, in which it is checked whether the temperature T is above a threshold value SW3.

Is the operating temperature of the catalyst 7 below this threshold (N-branch), a functioning catalyst is detected in step S9. On the other hand, if the temperature is above the threshold value, that is, the catalyst has 7 reaches its light-off state only at an excessive operating temperature, (J branch), a defective catalyst is detected in step S10. The measures introduced in this case correspond to those of step S5 of FIG 2 ,

Claims (6)

Method for the evaluation of an exhaust tract ( 3 ) an internal combustion engine ( 1 ) three-way catalyst ( 7 ), in which a) repeatedly downstream of the catalyst ( 7 b) it is checked whether the emission value (N2O) exceeds a threshold value (SW1), c) if the threshold value is exceeded, as a further measured variable a time duration (t), since commissioning of the internal combustion engine ( 1 ), or a temperature value (T) of the catalyst ( 1 ), d) on the basis of this further measured variable (t, T) the evaluation of the catalyst ( 1 ) is made. The method of claim 1, wherein in the evaluation tested in step d) whether the time duration (t) or the temperature value (T) has a threshold value (SW2, SW3), and in case of passing detected on a catalyst defect. Method according to one of the above claims, wherein as the N2O emission value a current N2O measured value, an integrated N2O value or a fluctuation range of the N2O measured value is used. Method according to one of the preceding claims, in which step a) is carried out only when the exhaust gas of the internal combustion engine ( 1 ) is in a certain oxygen content range. The method of claim 4, wherein the region is a Lambda value between 0.9 and 1.2, in particular between 0.99 and 1.01. Method according to one of the above claims, wherein the threshold as a N2O concentration, a temporal N2O concentration change or an N2O mass is used.