EP1255917B1 - METHOD AND DEVICE FOR DETECTING A NOx CONCENTRATION OF AN EXHAUST GAS STREAM PERTAINING TO A COMBUSTION ENGINE - Google Patents

Abstract

The invention relates to a method and a device for detecting a NOx concentration of an exhaust gas stream of a combustion engine by means of a NOx-sensitive measuring device, especially a NOx sensor, that is arranged downstream in relation to a NOx storage catalyst in an exhaust gas section of the combustion engine. According to the invention, a search for a minimum (min) of a signal curve (SK) pertaining to the measuring device (18) is carried out in an operational phase of the combustion engine (10), whereby the NOx concentration passes through a value close to zero on the position of the NOx-sensitive measuring device (18) during said phase. The detected minimum (min) is taken over as the zero point for calibrating the measuring device (18).

Description

The invention relates to a method and a device for determining a NO _x concentration of an exhaust gas stream of an internal combustion engine by means of a downstream of a NO _x storage catalytic converter in an exhaust line of the internal combustion engine arranged NO _x -sensitive measuring device, in particular a NO _x probe.

For carrying out a post-treatment of exhaust gases of at least temporarily lean-running internal combustion engines, it is known to arrange NO _x storage catalysts in the exhaust duct of the internal combustion engine, which are operated alternately in NO _x storage phases and regeneration phases. For maximum reduction of NO _x emission, it is advantageous to control the NO _x storage catalytic converter or the internal combustion engine as a function of a NO _x concentration present behind the NO _x storage catalytic converter. This requires an accurate detection of a current NO _x concentration ahead, for which _NOx sensors are used.

Each NO _x sensor has an individual error measure, which leads to deviations of a displayed measured value from an actually present actual value of the NO _x concentration. These individual inaccuracies in the characteristic of the NO _x sensor lead to errors in the control of the internal combustion engine, so that, for example, a NO _x level of NO _x storage is detected inaccurately, whereby regeneration can be turned on too late. This leads to increased NO _x emissions.

EP 0 916 941 A discloses a method for determining a NO _x concentration in the exhaust gas of an internal combustion engine by means of a NO _x sensor arranged downstream of a NO _x storage catalytic converter in the exhaust gas line. In this case, a minimum search of a signal curve of the sensor is carried out in an operating phase of the internal combustion engine immediately after a stoichiometric operation in which the NO _x storage was emptied and the NO _x concentration of the NO _x sensor is close to zero, and the value found as a reference value adopted for the control of the NO _x storage catalytic converter. In the case of too high a value, it is proposed to limit a duration of the lean phase.

The invention has for its object to improve the accuracy of the determination of the NO _x content of an exhaust gas.

This object is solved by the features of the independent claims. The inventive method provides, in an operating phase of the internal combustion engine, in which it is known that the NO _x concentration at the position of the NO _x -sensitive measuring device passes through a value close to zero, a search for a low value, in particular a minimum Performing the signal curve of the measuring device to subject the found minimum on the basis of predetermined criteria of a plausibility check and, if it meets the criteria of the plausibility check, as a zero point for the calibration of the NO _x -sensitive measuring device.

According to a very preferred embodiment of the invention, the search for the minimum is performed directly or shortly after a NO _x regeneration or desulfurization of the NO _x storage catalytic converter. Following a NO _x or sulfur regeneration, an approximately 100% NO _x absorption can be assumed with sufficient accuracy even with a severely damaged catalyst. In this phase is to be expected at least for a limited duration with a NO _x concentration of 0 to 5 ppm downstream of the catalyst, so that obtained in this phase a minimum signal of NO _x -Messßeinrichtung with a negligible error as the zero point for a. Offset correction of the NO _x sensor can be adopted.

It is further preferred that for the search for the minimum signal values of the Measuring device at predetermined intervals or after specified exhaust or Reductant throughputs are detected. Each measured signal value is included compared to a last stored minimum. If the signal value is less than that last found minimum, it is saved as a new minimum. The search after the minimum becomes after expiration of a given duration or one predetermined exhaust or reducing agent flow rate ended. The described Method ensures that a minimum found in such a way to an absolute minimum in the Signal waveform of the measuring device and not about a local minimum, the For example, caused by an irregularity in the sensor characteristic can be.

Since a possible miscalibration of the NO _x -sensitive measuring device can have dramatic consequences on the control of the storage catalytic converter and the internal combustion engine, strict requirements must be placed on the safety of a found minimum. Therefore, a method according to the invention provides to subject the found minimum to a plausibility check. For this purpose, one or more criteria must be fulfilled before a minimum is assumed as the zero point for the calibration. According to a preferred embodiment, for example, it is required that the minimum found must fall below an upper threshold. Furthermore, falling below the upper threshold value may be required for a certain minimum duration. As a further criterion it can be specified that the found minimum is located at a sufficiently large distance to a start and an end point of the minimum search. In this way, it is checked whether the "bottom" in which the minimum is actually in the window of the minimum search.

It may also be advantageous to smooth the signal course before its evaluation, for example, by averaging adjacent measuring points. Further Mathematical methods for smoothing are known and intended will not be explained here.

An extremely advantageous embodiment of the method furthermore provides that the minimum search is aborted and / or a found minimum is not taken over as the zero point for the calibration of the measuring device if predetermined boundary conditions for operating parameters are not fulfilled during the minimum search. An important constraint relates to, for example, a temperature of the NO _x storage analyzer that should be in a range that ensures satisfactory NO _x absorption. Another boundary condition requires a lean operation of the internal combustion engine and / or the presence of a lean exhaust gas in the region of the NO _x storage catalytic converter and / or in the region of the measuring device. In addition, this may be required for a predetermined minimum duration. Further, it may be assumed that the internal combustion engine is not in a fuel cut-off phase and / or not in an idle state and / or not in a highly transient operating mode during the minimum search. In addition, a satisfactory NO _x storage behavior of a NO _x - storage catalytic converter can be required, which is determined for example by a diagnostic method. Also, limit may be set for a light emitted from the internal combustion engine, NO _x -Rohmassenstrom an upper and / or lower.

The device according to the invention provides a procedure stored in a control unit in digital form for carrying out the above-described method steps for determining the NO _x concentration. Preferably, the control unit is integrated in an engine control unit.

Further preferred embodiments of the invention will become apparent from the others, in the subclaims mentioned features.

Figur 1

eine schematische Darstellung eines Abgaskanals einer Verbrennungskraftmaschine;

Figur 2

eine Darstellung einer Sensorkennlinie eines NO_x-Sensors und

Figur 3

ein Sensorsignal in Abhängigkeit der Zeit während des Umschaltens zwischen Regeneration und Magerbetrieb.

The invention will be explained in more detail in embodiments with reference to the accompanying drawings. Show it:

FIG. 1

a schematic representation of an exhaust passage of an internal combustion engine;

FIG. 2

a representation of a sensor characteristic of a NO _x sensor and

FIG. 3

a sensor signal as a function of time during the switching between regeneration and lean operation.

The arrangement of an internal combustion engine 10 shown schematically in FIG. 1 has an exhaust tract 12 with an optional pre-catalyst 14 arranged therein and an NO _x storage catalytic converter 16. Downstream of the NO _x storage catalyst 16, an NO _x sensor 18 is also disposed in the exhaust passage 12. The NO _x sensor 18 outputs a measurement signal to an engine control unit 20, in which a control unit 22 is integrated. In dependence on a NO _x concentration determined by the NO _x sensor 18, the engine control unit 20 controls the internal combustion engine 10, for example by influencing an air-fuel mixture to be fed in.

Figure 2 shows a model a characteristic K of a NO _x sensor 18. It is here the output signal OUT of the sensor 18, which may for example be a voltage depending on a _NOx concentration shown in waste gas in the application of the NO _x sensor 18 linear characteristic K is characterized by the slope S, so their sensitivity, and by the so-called offset OS, which corresponds to an output signal in the absence of NO _x in the exhaust gas, for the calibration of the characteristic K of the NO _x sensor 18 is how for each straight line, the knowledge of two points on the characteristic K suffice. If the zero point of the characteristic curve is known as a special case, then this can at least be used for an offset correction. In order to detect the zero-point signal of the NO _x sensor 18, this must in a situation in which the NO _x concentration at the position of the sensor 18 is known or can be assumed to be approximately zero, the output signal OUT of NO _x - Sensors 18 are measured. The thus measured signal can then be set as the zero point of the sensor 18.

An operating situation in which a NO _x concentration behind the NO _x storage catalytic converter 16 can be assumed to be close to zero with a sufficiently high probability is shortly after a regeneration, in particular a NO _x regeneration, of the NO _x storage catalytic converter 16 given.

FIG. 3 shows an idealized chronological progression of a signal curve SK of an NO _x sensor 18 arranged downstream of an NO _x storage catalytic converter 16 during the changeover from a rich operating mode (regeneration phase R) into a lean operating mode M of the internal combustion engine 10. During the NO _x regeneration R remains the sensor signal OUT at a relatively high level. This is due on the one hand to an incomplete reduction of desorbed nitrogen oxides NO _x and on the other hand to a generation of ammonia NH ₃ during regeneration R, to which the NO _x sensor 18 also responds For a short while after completion of the regeneration R and switching the Internal combustion engine 10 in a lean mode M at a time t _A , the signal OUT remains depending on a dead volume of the exhaust passage and of a space velocity of the exhaust gas initially still at this relatively high level. In addition, an inherent inertia of the NO _x sensor 18 leads to a delay in the response of the signal curve SK, even if already present lean exhaust gas downstream of the NO _x storage catalytic converter sixteenth After a subsequent steep drop, the signal curve SK passes shortly after switching to the lean mode M a minimum min, then again very slowly increase due to increasing loading of the NO _x storage catalytic converter 16. A reliable prediction of the time of the minimum min is practically impossible, which is why the specification of a fixed period after switching from the regeneration mode R to the lean mode M for measuring the zero point signal of the NO _x sensor 18 appears to make little sense. For this reason, it is provided according to the present invention to observe the course of the signal curve SK over a longer period of operation after completion of the regeneration R and to determine the global minimum min in this phase. This is preferably done by detecting measurement signals in the predetermined time window and comparing them successively with each other. If a current measuring signal falls below a provisional minimum, the current signal is noted as a new minimum. In this way, so-called outliers or sharp voltage peaks that can lead to local minima are ignored. Thus, signal curves SK, which correspond to a less ideal course than shown, are also accessible to the evaluation. The described method thus also makes it possible, with a low signal-to-noise ratio, that is to say with a high background noise and with irregularities in the characteristic curve K, as is observed in particular for aged NO _x sensors 18, to reliably detect the minimum min ,

The minimum search is terminated after a predetermined period of time after switching to the lean mode M at the time t _E. Instead of a time period, it is equally possible to specify an enforced exhaust gas or reducing agent mass or a number of measuring points.

As already explained above, it is further provided that the minimum for calibration is only released if further boundary conditions are met. This applies, for example, to a temperature of the NO _x storage catalytic converter 16 which should be within a range that guarantees a reliable injection of NO _x. As an additional measure for validating a found minimum min, it is advantageous to check the found minimum min using predefined criteria. For example, in FIG. 3, a threshold value SW is shown whose falling below can be a prerequisite for the release of the found minimum min. It may also be required that the threshold SW must be undershot for a predetermined minimum duration.

If the given boundary conditions are fulfilled during the entire minimum search and if the found global minimum min meets the established criteria, the minimum min is released immediately as the zero point of the NO _x sensor 18 and an offset correction of the sensor 18 is carried out. The found minimum min remains valid until a new minimum is determined in a subsequent minimum search. A minimum search can - after observing the described boundary conditions - be carried out after each NO _x regeneration.

Claims (18)

1. Method for determining an NO_x concentration in an exhaust gas stream from an internal combustion engine (10) by means of an NO_x-sensitive measurement device (18), in particular an NO_x sensor, arranged downstream of an NO_x storage catalytic converter (16) in an exhaust section of the internal combustion engine (10), in which method, during an operating phase of the internal combustion engine (10) in which the NO_x concentration at the position of the NO_x-sensitive measurement device (18) reaches a value close to zero, a search is carried out for a low value, in particular a minimum (min), in a signal curve (SK) of the measurement device (18), and the value (min) which is found is used as reference point, in particular zero point, for a calibration of the measurement device (18), characterized in that the minimum (min) which is found is subjected to a plausibility check on the basis of predetermined criteria, and if it meets the criteria of the plausibility check is used as reference point for the calibration of the NO_x-sensitive measurement device (18).
2. Method according to Claim 1, characterized in that the search for the minimum (min) is carried out directly or shortly after an NO_x regeneration or a desulphurization of the NO_x storage catalytic converter (16).
3. Method according to either of Claims 1 and 2, characterized in that the search for the minimum (min) comprises the steps of

   (a) recording signal values from the measurement device (18) at predetermined time intervals or after predetermined throughputs of exhaust gas or reducing agent,

   (b) comparing a signal value with the last minimum (min) stored,

   (c) storing the signal value as new minimum (min) if this value is lower than the minimum (min) found last, and

   (d) ending the search after a predetermined time has elapsed or after a predetermined throughput of exhaust gas or reducing agent.
4. Method according to one of the preceding claims, characterized in that a criterion for the minimum (min) found is that it be below an upper threshold value (SW).
5. Method according to one of the preceding claims, characterized in that a criterion for the minimum (min) found is that it be below an upper threshold value (SW) for a minimum period of time.
6. Method according to one of the preceding claims, characterized in that a criterion for the minimum (min) found is that the minimum (min) be at a sufficient margin from a starting point and an end point of the search for a minimum.
7. Method according to one of the preceding claims, characterized in that the signal curve is smoothed before it is evaluated.
8. Method according to one of the preceding claims, characterized in that the search for a minimum is terminated and/or a minimum which has been found is not used as zero point for the calibration of the measurement device (18) if predetermined boundary conditions for operating parameters are not satisfied during the search for a minimum.
9. Method according to Claim 8, characterized in that one boundary condition is a predetermined temperature range for the NO_x storage catalytic converter (16) which ensures sufficient NO_x absorption.
10. Method according to Claim 8, characterized in that one boundary condition is that the internal combustion engine (10) be operating in a lean-burn mode and/or that the exhaust gas in the region of the NO_x storage catalytic converter (16) and/or in the region of the measurement device (18) be lean.
11. Method according to Claim 8, characterized in that one boundary condition is that an overrun cutoff phase and/or an idling mode and/or a very non-steady operating mode of the internal combustion engine (10) is not present during the search for a zero point.
12. Method according to Claim 8, characterized in that one boundary condition is a satisfactory NO_x storage performance by the NO_x storage catalytic converter (16), for example according to a diagnosis method.
13. Method according to Claim 8, characterized in that one boundary condition is a predetermined range for an untreated NO_x mass flow.
14. Method according to one of the preceding claims, characterized in that the search for a zero point is carried out after each NO_x regeneration or desulphurization.
15. Method according to one of the preceding claims, characterized in that the minimum (min) which has been found is released as a zero point for the calibration of the measurement device (18) immediately after it has been validated.
16. Method according to one of the preceding claims, characterized in that the minimum (min) which has been found retains its validity until a new minimum (min) has been determined.
17. Apparatus for determining an NO_x concentration in an exhaust gas stream from an internal combustion engine (10), comprising an NO_x-sensitive measurement device (18), in particular an NO_x sensor, arranged downstream of an NO_x storage catalytic converter (16) in an exhaust section of the internal combustion engine (10), and a procedure in digital form stored in a control unit (22) for carrying out the method steps of:

    (a) recording a signal curve (SK) of an output signal from the measurement device (18) which is dependent on the NO_x concentration for a predetermined time or during a predetermined throughput of exhaust gas or reducing agent in an operating phase of the internal combustion engine (10) in which the NO_x concentration at the position of the NO_x-sensitive measurement device (18) passes through a value close to zero;

    (b) determining a low value, in particular a minimum (min), of the signal curve (SK);

    (c) carrying out a plausibility check on the minimum (min) found on the basis of predetermined criteria; and

    (d) adopting the value (min) which has been found and validated by the plausibility check as a reference point, in particular as a zero point, for a calibration of the measurement device (18).
18. Apparatus according to Claim 17, characterized in that the control unit (22) is integrated in an engine control unit (20).

Images