EP1227234B1 - Method and apparatus for operating a NOx-sensor located in the exhaust line of an internal combustion engine - Google Patents

Description

The invention relates to a method and a device for Operating an exhaust gas sensor and in particular a Method and device for operating NOx sensors, arranged in the exhaust path of an internal combustion engine are.

In exhaust aftertreatment systems of internal combustion engines NOx catalysts are used to filter the high nitrogen oxide content of internal combustion engines with a lean air / fuel mixture. The so-called air ratio λ provides the specification of the air / fuel ratio. The air ratio λ is the air / fuel ratio normalized to stoichiometric conditions, which indicates how many kilograms of air are required for the complete combustion of one kilogram of fuel. Stoichiometric conditions of this air-fuel ratio are thus present for λ = 1.
In the prior art NOx storage catalysts are known, which are operated alternately in two modes of operation. In a first operating mode, the so-called sorption phase, NOx contained in the exhaust gas of the internal combustion engine is stored by the NOx storage catalyst under oxidizing exhaust gas conditions, ie in the lean operation of the engine. However, since the storage capacity of a storage catalyst is limited, a regeneration phase or desorption phase is initiated at certain times, in which the air ratio of the air / fuel mixture supplied to the engine and thus the air ratio of the exhaust gas leaving the engine for a short time to less than 1 lowered becomes. This is also referred to as enrichment of the air / fuel mixture or the exhaust gas. During this short operating phase, reducing conditions exist in the exhaust gas before entering the storage catalytic converter. Under the reducing conditions during the retting phase, the stored nitrogen oxides are then released.

To control the operation of such NOx catalysts is a NOx-sensitive sensor, or sensor in the exhaust path arranged. The output of this sensor is then to Loading rate determination of the catalyst and for determining the Start and end of the regeneration phase used.

The known to date NOx sensors show, however, when new a satisfactory measurement accuracy, but they are not resistant to aging. The NOx sensor in the exhaust path changes its Measuring characteristic due to contamination by various pollutants or deposits of Combustion residues. In particular, the shifts Zero point of the measurement curve during aging considerably. A realistic shift of 10% corresponds approximately a content of more than 50 ppm NOx. Moreover, the changes Slope of the trace. The accuracy in the field of Zero point, however, is for a series use of NOx sensors very important because in an arrangement after the catalyst predominantly small NOx concentrations have to be measured (<50 ppm). A slight inaccuracy of the measured values leads consequently a large measurement uncertainty.

DE-198 59 580 A1 proposes in this regard, the Working temperature of the sensor element of the NOx sensor slowly over the life of the sensor element away increase. This temperature correction becomes the resistance of the sensor element is lowered, since the ion conductivity with the temperature increases. By increasing the, Operating temperature of the sensor element is achieved that at a measuring electrode applied a maximum voltage Value does not exceed, to a decomposition at NOx at this Electrode to prevent which decomposed NOx molecules then no longer available at the actual measuring electrode stood. However, such prevention of measurement errors is very expensive, because an electric heater for heating the sensor element is required.

It is also known that the method is in an engine operating condition is performed, in which the NOx concentration in the exhaust gas their minimum reached, i. Zero is as in DE-198 10 483, EP-1 041 254 and DE-199 26 139 described.

It is thus an object of the present invention to provide a To provide a method and a device by which a functional long term use of a in the exhaust path of a Internal combustion engine arranged NOx sensor is possible, so that despite insufficient aging stability of the NOx sensor of Series use is possible.

• Einstellen eines Betriebszustands des Verbrennungsmotors mit bekanntem NOx-Gehalt im Abgas;
• Erfassen mindestens eines NOx-Sensor-Signalwertes bei dem bestimmten Motorbetriebszustand des Verbrennungsmotors;
• Vergleichen des mindestens einen mit dem NOx-Sensor gemessenen Signalwertes mit einem den bekannten NOx-Gehalt entsprechenden Signalwertes bei dem jeweiligen bekannten NOx-Gehalt;
• Ermitteln der Differenz zwischen dem gemessenen NOx-Gehalt und dem bekannten NOx-Gehalt;
• Bestimmen eines Korrekturwertes aus der mindestens einen ermittelten Differenz; und
• Korrigieren der von dem NOx-Sensor gemessenen Signalwerte um den Korrekturwert.

For this purpose, the method according to the invention has the following steps:

• Setting an operating state of the internal combustion engine with known NOx content in the exhaust gas;
• Detecting at least one NOx sensor signal value at the determined engine operating condition of the internal combustion engine;
• Comparing the at least one signal value measured with the NOx sensor with a signal value corresponding to the known NOx content at the respective known NOx content;
• Determining the difference between the measured NOx content and the known NOx content;
• Determining a correction value from the at least one determined difference; and
• Correcting the signal values measured by the NOx sensor by the correction value.

According to the invention, a device for determining is also provided the NOx content in the exhaust path of an internal combustion engine provided with a NOx sensor of which the NOx content in Exhaust path is detected and an associated NOx sensor signal value is generated, and with a control device that with the NOx sensor is connected, wherein the control device so is formed that of her a predetermined operating condition the internal combustion engine with known NOx content in the exhaust gas is adjustable, with at least one NOx sensor signal value at the predetermined engine operating condition of Internal combustion engine is supplied from the NOx sensor that of her the at least one supplied NOx sensor signal value with a signal value corresponding to the known NOx content comparable and the difference between the measured NOx content and the known NOx content is determined that of you from the at least one determined difference Correction value can be determined, and that of her from the NOx sensor received NOx sensor signal values under determination of respective NOx content signal values corresponding to the respective NOx contents in the exhaust gas, by means of the correction value are correctable.

Advantageous developments of the invention are in the respective subclaims described.

At very high NOx levels, the slope is logarithmic NOx sensor values approach zero and forms a maximum Limit. The method can therefore preferably also in maximum limit of the NOx measurement curve are completed, i. in certain engine operating conditions at certain locations of the Exhaust system where an extremely high NOx concentration occurs.

1. in der Schubphase des Verbrennungsmotors;

2. am Ende einer Regenerationsphase, bei der der Motor mit einem fetten, unterstöchiometrischen Luft-Kraftstoff-Verhältnis läuft; ,

3. wenn der Verbrennungsmotor ausgeschaltet ist, wobei eine Sekundärluftpumpe die Abgasleitung spülen kann;

4. unmittelbar nach dem Kaltstart, wobei der NOx-Sensor dann vor dem Motorstart vorgeheizt werden könnte;

5. im Leerlauf des Verbrennungsmotors mit der Luftzahl λ=1 oder λ<1, wobei eine geringe NOx-Emission im NOx-Speicher-Katalysator vollständig umgesetzt werden kann;

6. nach der Regeneration des NOx-Speicher-Katalysators im Niedrig- oder Teillastbetrieb mit einer Luftzahl von λ>1. Maximale NOx-Konzentrationen treten im Abgas bei folgenden Betriebszuständen des Verbrennungsmotors auf:

7. beim Betrieb mit einer Luftzahl λ=1,1, wobei durch geeignete Zündwinkelverstellung und Rücknahme der Abgasrückführrate die NOx-Emission zusätzlich erhöht werden könnte;

8. bei klopfender Verbrennung;

9. beim Abstellen des Verbrennungsmotors, wenn die letzten Verbrennungen "fett" ablaufen, d.h. λ<1, und im NOx-Speicher-Katalysator adsorbiertes NOx desorbiert; aufgrund des kleinen Massenstroms bildet sich dann hinter dem NOx-Katalysator eine extrem hohe NOx-Konzentration.

Minimal or zero NOx concentrations occur in the exhaust gas under the following operating conditions:

1. in the overrun phase of the internal combustion engine;

2. at the end of a regeneration phase, when the engine is running at a rich, stoichiometric air-fuel ratio; .

3. when the internal combustion engine is switched off, wherein a secondary air pump can flush the exhaust pipe;

4. immediately after the cold start, whereby the NOx sensor could then be preheated prior to engine start;

5. at idle of the internal combustion engine with the air ratio λ = 1 or λ <1, with a low NOx emission in the NOx storage catalyst can be fully implemented;

6. after the regeneration of the NOx storage catalyst in low or partial load operation with an air ratio of λ> 1. Maximum NOx concentrations occur in the exhaust gas in the following operating states of the internal combustion engine:

7. when operating with an air ratio λ = 1.1, which could be additionally increased by appropriate Zündwinkelverstellung and withdrawal of the exhaust gas recirculation rate, the NOx emission;

8. with knocking combustion;

9. when the internal combustion engine stops, when the last burns are "rich", ie, λ <1, and desorbed NOx adsorbed in the NOx storage catalyst; Due to the small mass flow, an extremely high NOx concentration then forms behind the NOx catalyst.

In a preferred embodiment of the invention is the Correction value used to correct the NOx sensor measured signal values is used, from a plurality of Differences between the measured NOx content and a calculated corresponding known NOx content, the Differences in each case at different operating points of the Internal combustion engine can be determined.

a) nach dem Verbrennungsmotor und vor dem Vorkatalysator,

b) nach dem Vorkatalysator und vor dem NOx-Speicher-Katalysator, oder

c) nach dem NOx-Speicher-Katalysator.

The NOx sensor may be positioned at three locations in the exhaust path of the internal combustion engine. Either

a) after the combustion engine and before the pre-catalyst,

b) after the pre-catalyst and before the NOx storage catalyst, or

c) after the NOx storage catalyst.

The arrangement of the NOx sensor before the pre-catalyst is suitable, for the above engine operating conditions 1, 3, 4 and 7 to 9.

The arrangement of the NOx sensor in front of the NOx storage catalyst, in particular between pre-catalyst and NOx storage catalyst, is suitable for the above engine operating conditions 1 to 5 and 7 to 9.

The arrangement of the NOx sensor after the NOx storage catalyst is suitable for all the above engine operating conditions 1 to 9.

The NOx curve can be measured at certain intervals and upon occurrence of the appropriate particular ones described above Engine operating conditions via the engine control or Calibrated computer device and adjusted with it.

• Fig. 1 eine schematische Darstellung mit den erforderlichen Komponenten und den Verbindungen für ein Verfahren und eine Vorrichtung zum Eichen eines NOx-Sensors.

In the following, a preferred embodiment of the present invention will be explained in more detail with reference to a drawing. In the drawing shows

• Fig. 1 is a schematic representation of the required components and the connections for a method and apparatus for calibrating a NOx sensor.

The internal combustion engine 1 can be either a gasoline or a Diesel engine, and it gives up its engine operating condition at least one sensor to the engine control or Computer unit 5 on. In the exhaust path after the Internal combustion engine 1 is a pre-catalyst 2 and behind a NOx storage catalyst 3. A NOx sensor 4A, 4B, 4C may be at the location between the engine 1 and the Pre-catalyst 2 or at the location between the pre-catalyst 2 and the NOx storage catalyst 3 or at the place after NOx storage catalyst (3) be positioned. But it can Also, multiple NOx sensors 4A-4C are positioned at multiple locations be. The NOx sensor 4 outputs its measured value to the Motor control or computer unit 5 on. The air mass sensor 6 is used for motor control.

The internal combustion engine 1 is controlled by the engine control 5 in FIG Dependence of the driving commands of the vehicle operator and in Dependence of the required engine operating conditions of the or NOx catalyst 3 controlled and regulated. The NOx storage catalyst 3 stores the NOx emissions through adsorption in a storage phase and reduces this in the Regeneration phase, so that the NOx emissions are minimized become. The NOx sensor 4C after the NOx storage catalyst 3 shows when exceeding a NOx sensor limit of the Engine control 5 that the storage phase of the NOx catalyst 3 finished and this is so saturated. The Engine controller 5 has stored a NOx trace. she controls the calibration of the NOx sensor 4 described above Method.

The engine controller 5 represents a control device which to determine a NOx trace of known NOx levels Has stored operating conditions, which NOx curve of the fully functional NOx sensors at these particular Operating conditions would have to capture. To worsen the To compensate for the function of the NOx sensor, controls the Motor control 5 in the manner described above the engine a predetermined operating state, for which they the NOx graph and compares one from the NOx sensor actual NOx measured value in this operating state or more detected NOx readings with the pre-stored NOx values. From the difference between actually recorded NOx readings and the pre-stored Values, the engine controller 5 determines a correction value for Correction of the NOx levels detected by the NOx sensor. This Correction value is used to equalize the measured values of the NOx sensor used until a new correction value after described procedure is determined. A determination a respective correction value is preferably at intervals from xx sec (insert preferred period).

advantages

The previously known NOx sensors can thus be used in series production. There is no additional cost to increase the quality of the aging stability of the NOx sensors.
The use of NOx sensors provides the benefits of fuel reduction and meeting stringent emission limits. The method and the device require no additional space and thus no significant additional weight. In addition, the invention requires little additional cost, since all the essential components for the control of the NOx catalyst and the diagnosis already exist.

Claims (10)

1. A method for coordinating measurement values of an NOx sensor (4A, 4B, 4C) arranged in the exhaust line of an internal-combustion engine (1) with the signal values output at an earlier time by the NOx sensor with respect to the same NOx content, comprising the following method steps:

   adjusting a predetermined operating state of the internal-combustion engine with a known NOx content in the exhaust gas;

   detecting at least one NOx sensor signal value in the predetermined engine operating state of the internal-combustion engine;

   comparing at least one signal value measured by the NOx sensor with a signal value corresponding to the known NOx content and determining the difference between the measured NOx content and the known NOx content;

   determining a correction value from the at least one determined difference; and

   correcting the signal values measured by the NOx sensor by the correction value,

   wherein, in the predetermined engine operating state, the NOx content in the exhaust gas is very high and the increase in the logarithmic NOx content value approaches the limit value zero, in other words the NOx sensor is operated at saturation.
2. A method according to claim 1, wherein the predetermined engine operating state is operation at an air ratio λ = 1.1.
3. A method according to claim 1, wherein the predetermined engine operating state is a knocking combustion.
4. A method according to claim 1, wherein the predetermined engine operating state is the switch-off phase of the internal-combustion engine with last combustions operated in rich mode.
5. A method according to any one of claims 1 to 4, wherein the NOx sensor is arranged upstream from a primary catalytic converter and/or upstream from an NOx storage catalytic converter.
6. A device for determining the NOx content in the exhaust gas line of an internal-combustion engine (1), comprising an NOx sensor (4A, 4B, 4C), by which the NOx content in the exhaust gas line is detected, and an associated NOx sensor signal value is generated, and comprising a control device (5) connected to the NOx sensor, wherein the control device is designed in such a way that a predetermined operating state of the internal-combustion engine with a known NOx content in the exhaust gas can be adjusted by it, wherein at least one NOx sensor signal value is supplied to it by the NOx sensor at the predetermined operating state of the internal-combustion engine, in that the at least one supplied NOx sensor signal value can be compared with a signal value corresponding to the known NOx content and the difference between the measured NOx content and the known NOx content can be determined by the control device, in that a correction value can be determined by it from the at least one determined difference, and in that the NOx sensor signal values obtained by the NOx sensor can be corrected by the control device by means of the correction value with the determination of respective NOx content signal values which correspond to the respective NOx contents in the exhaust gas, wherein the control device is designed in such a way that an engine operating state with a very high NOx content in the exhaust gas is set by it as the predetermined engine operating state, at which the increase in the logarithmic NOx content value approaches the limit value zero, in other words the NOx sensor is operated at saturation.
7. A device according to claim 6, wherein the control device is designed in such a way that the engine operation is set by it at an air ratio λ = 1.1 as the predetermined engine operating state.
8. A device according to claim 6, wherein the control device is designed in such a way that an engine operation with knocking combustion is set by it as the predetermined engine operating state.
9. A device according to claim 6, wherein the control device is designed in such a way that the turn-off phase of the internal-combustion engine with last combustions operated in the rich mode is set by it as the predetermined engine operating state.
10. A device according to any one of claims 6 to 9, wherein the NOx sensor is arranged upstream from a primary catalytic converter and/or upstream from an NOx storage catalytic converter.

Images