US20120031075A1

US20120031075A1 - Method for determining oxygen storage capacity

Info

Publication number: US20120031075A1
Application number: US13/193,008
Authority: US
Inventors: Bodo Odendall
Original assignee: Audi AG
Current assignee: Audi AG
Priority date: 2010-08-04
Filing date: 2011-07-28
Publication date: 2012-02-09
Also published as: DE102010033335A1; DE102010033335B4

Abstract

The oxygen storage capacity of an oxygen store associated with a catalytic converter of a combustion engine is computed by forming an integral which begins at the time of a changeover in the exposure, e.g., from rich to lean, and ends when the output signal of a post-catalytic converter lambda probe is less than a threshold value. A correction is performed to take into a consideration a time offset in the signals of the post-catalytic converter lambda probe. In particular, the time offset is measured to determine a time at which the integration should have been terminated, wherein this time is inferred retroactively.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the priority of German Patent Application, Serial No. 10 2010 033 335.2, filed Aug. 4, 2010, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein.

BACKGROUND OF THE INVENTION

The invention relates to a method for determining the oxygen storage capacity of an oxygen store is associated with a catalytic converter.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
The starting point is a situation where a catalytic converter is arranged downstream of an internal combustion engine in the outflow direction of the exhaust gas, wherein an oxygen store is associated with the catalytic converter; in particular, the oxygen store may be integrated in the catalytic converter, but may also be provided as a separate component. A so-called pre-catalytic converter lambda probe is arranged upstream of the catalytic converter, whereas a post-catalytic converter lambda probe is arranged downstream of the catalytic converter. Lambda probes measure the air-fuel ratio (in the exhaust gas).
A method for determining the oxygen uptake storage capacity will now be described. To determine the oxygen removal capacity, it is only necessary to interchange “rich” with “lean” and vice versa, and in conjunction with the discussion of oxygen uptake, a method for determining the oxygen removal capacity will likewise assume a removal of oxygen.
To determine the oxygen (uptake) storage capacity, the oxygen store is initially exposed to rich exhaust gas, in order to remove the oxygen from the oxygen store. The fact that the exhaust gas is rich is determined from signals of the pre-catalytic converter lambda probe, meaning under control of the lambda probe.
When the oxygen store is almost completely emptied (according to a predetermined criterion), a changeover immediately occurs to an exposure of the oxygen store to lean exhaust gas, so as to fill the oxygen store again slowly with oxygen. In this case, the air-fuel ratio “lean” is also defined under control of the pre-catalytic converter lambda probe.
The quantity of oxygen taken up per time interval during refill is then integrated. If possible, the entire quantity of oxygen should be measured. Accordingly, the time interval for the integration starts at the time offset changeover (which is determined by a controller and therefore known). The time interval ends at a time when a full state of the oxygen store causes an (the) output signal of the post-catalytic converter program to fall below a threshold value. As long as the oxygen store is not completely filled, oxygen in the lean exhaust gas is removed from the lean exhaust gas by the oxygen store, and the exhaust gas is no longer lean when reaching the post-catalytic converter probe. When the oxygen store is then full at some point in time, additional oxygen is no longer stored, so that lean exhaust gas actually reaches the post-catalytic converter probe. The voltage in the voltage signal then typically falls below, for example, 0.4 V.
The method of the invention operates very well as long as the employed measuring devices are fully operational.
If the post-catalytic converter lambda probe has aged, it may particularly react with a delay. If the start of the time interval is the time when the changeover occurs, then the measurement begins at the correct time, but ends with a delay due to the aging of the post-catalytic converter lambda probe, because the time when the interval ends is determined by the signals of just this post-catalytic converter lambda probe.
It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide an improved catalytic converter which takes into consideration aging of the post-catalytic converter lambda probe when determining the oxygen storage capacity.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method for determining oxygen storage capacity of an oxygen store associated with a catalytic converter of an internal combustion engine, includes the steps of measuring, in an outflow direction of exhaust gas, an air-fuel ratio with a pre-catalytic converter lambda probe arranged upstream of the catalytic converter and with a post-catalytic converter lambda probe arranged downstream of a section of the catalytic converter, initially exposing the oxygen store, under control of the pre-catalytic converter lambda probe, to rich exhaust gas so as to extract as much oxygen as possible from the oxygen store or to lean exhaust gas so as to fill the oxygen store with as much oxygen as possible, thereafter changing over from the rich exhaust gas to lean exhaust gas so as to fill the oxygen store with oxygen or changing over from the lean exhaust gas to rich exhaust gas so as to remove oxygen from the oxygen store, integrating the filled or removed quantity of oxygen over a time interval, starting at a first time of the changeover and ending at a second time when an output signal of the post-catalytic converter lambda probe is less than or greater than a threshold value, indicating a full state or an empty state of the oxygen store, measuring a time offset between the first time and a third time when the mathematical sign of a slope of the output signal of the post-catalytic converter lambda probe changes, and correcting the obtained integral with the time offset.
The method is based on the observation that the changeover in the exposure—clearly recognizable at the end of the time interval—can already be adequately established almost directly from the output signal of the post-catalytic converter lambda probe. Ideally, a changeover in the exposure is directly accompanied by a change in the slope (first derivative with respect to time) of the output signal. However, a time delay in the change would indicate aging of the probe.
The time delay (“probe delay”) remains constant during the entire time. It is then possible to determine, based on the time offset at the time of the change, when the measurement should have ended. The measurement ends with a delay of exactly the time offset.
If plenty of storage capacity is available (e.g., in a ring memory), then intermediate values of the integral can be stored during the integration over time, and a final value can be obtained by subtracting the time offset from the time when the time interval ended. The intermediate value associated with the final value can hereby be used as a correction value. In this variant, the correct value for the integral is in a way stored.
However, sufficient storage capacity is not always available. If weight for a ring memory or another high-capacity data storage device needs to be reduced, then not all intermediate values of the integral can be stored and the value calculated for the “correct point in time” must be estimated. According to an advantageous feature of the present invention, the fraction of computed oxygen storage capacity, which has been included in the integral during the length of the time offset before the time interval ended, may be computationally estimated. Precisely this fraction is then subtracted from the integral for obtaining a correction value.
According to an advantageous feature of the present invention, the oxygen load of the oxygen store may increase continuously, so that the correction value can be obtained by a simple proportional calculation based on the elapsed time.
According to another advantageous feature of the present invention, the method of the invention may also be performed during arbitrary driving. In this case, the vehicle operator may change the exhaust gas mass flow during the measurement or the air-fuel ratio may change due to a change in the load-rotation speed point. In this case, it is no longer correct to assume that the slope in the oxygen load is constant.
According to yet another advantageous feature of the present invention, a change in the exhaust gas mass or in the air-fuel ratio during the time interval may be taken into consideration when estimating the fraction.
The effect of a time delay affects low pass filtering. According to an advantageous feature of the present invention, the time offset may determine, or more particularly represent, a filter constant for such (digital) low pass filter. If the quantity “oxygen uptake per time” is filtered with this digital low pass filter, then a result of the filtering is obtained at the end of this time interval which can be used as a slope of a straight line. The fraction of computed excess oxygen storage capacity can be obtained by multiplying the slope with a time offset.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:

FIG. 1 shows an arrangement adapted for use of the method according to the invention,

FIG. 2A shows the air-fuel ratio lambda as determined with the method according to the invention,

FIG. 2B shows response signals of a fully functional post-catalytic converter lambda probe with a time delay following the exposure according to FIG. 2A,

FIG. 2C shows the oxygen load that would be calculated based on the two response signals,

FIGS. 3A and 3B show diagrams corresponding to FIGS. 2A and 2B,

FIG. 3D shows an exemplary exhaust gas mass flow, as adjusted by the vehicle operator, and

FIG. 3C shows a diagram corresponding to FIG. 2C for the situation of FIG. 3D,

FIGS. 4A and 4B show diagrams corresponding to FIGS. 2A and 2B,

FIG. 4D shows another exemplary exhaust gas mass flow, as adjusted by the vehicle operator, and

FIG. 4C shows a diagram corresponding to FIG. 2C for the situation of FIG. 4D,

FIGS. 5A and 5B show diagrams corresponding to FIGS. 2A and 2B,

FIG. 5D shows a diagram corresponding to FIG. 4D,

FIG. 5E shows introduction of oxygen with digital low pass filtering, and

FIG. 5C shows a diagram corresponding to the FIGS. 2C, 3C and 4C for the situation of FIG. 5D and FIG. 5E.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout all the figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.
Turning now to the drawing, and in particular to FIG. 1, there is shown a schematic diagram of an internal combustion engine 1 with an exhaust gas system 2. The exhaust gas system 2 includes an exhaust gas catalytic converter 3, which is constructed, for example, as a three-way catalytic converter, as a NOx storage catalytic converter, or as an active particle filter, as well as an integrated oxygen store 4. The exhaust gas system 2 further includes a pre-catalytic converter lambda probe which is arranged upstream of the exhaust gas catalytic converter 3 and operates as a master probe, and a post-catalytic converter lambda probe 6 which is associated with the exhaust gas catalytic converter 3 and operates as a control probe.
In the present exemplary embodiment, the post-catalytic converter lambda probe 6 is arranged downstream of the exhaust gas catalytic converter 3. However, this post-catalytic converter lambda probe could also be arranged directly inside the exhaust gas catalytic converter 3, i.e., following a partial volume of the oxygen store 4.
In the following, it will be assumed that the exhaust gas of the internal combustion engine can be adjusted at least with a predetermined accuracy to a predetermined air-fuel ratio lambda.
The intent is here to determine the oxygen storage capacity of the oxygen store 4.
The oxygen storage capacity can be determined during uptake of oxygen and during removal of stored oxygen. In the following, determination of the oxygen storage capacity during uptake will be described.
Before the oxygen storage capacity during uptake of oxygen can be measured, the oxygen store 4 must first be completely emptied. To this end, the internal combustion engine 1 is operated so that the exhaust gas reaching the catalytic converter 2 is rich; a corresponding curve 10 is shown in FIG. 2A: initially, there is excess fuel in the exhaust gas. The excess fuel is combusted by removing oxygen from the oxygen store, which is then progressively emptied.
At some point, the oxygen store will be almost empty, whereafter a changeover can be made to exposure with lean exhaust gas, meaning with exhaust gas having an excess of air and hence also an excess of oxygen in relation to the fuel. The changeover from rich to lean is made at a time t₀. Such changeover causes a change in the mathematical sign of the time derivative of the signal of the post-catalytic converter lambda probe 6 according to curve 12 exactly at the time t₀. If the post-catalytic converter lambda probe 6 is not fully operational, but reacts with a time offset (“probe delay”), then for example curve 14 is applicable, wherein the time derivative changes its mathematical sign only at a time t₁after the time t₀.
The oxygen storage capacity during the oxygen uptake is to be determined, meaning from the time t₀. A prerequisite is that the exhaust gas mass flow is kept constant. Because the air-fuel ratio lambda is kept constant, a constant quantity of oxygen is taken up per time interval. The oxygen loading OSC therefore increases steadily with time, see curve 16.
The oxygen storage capacity OSC is generally computed with the following formula:
$O S C = \int_{t_{0}}^{t_{End}} 0, 23 (1 - λ (t)) \partial t * \dot{m} (t) \partial t,$
wherein the oxygen uptake is integrated over the entire time from t₀to t_End. If λ=cst. and {dot over (m)}=cst., then OSC increases linearly with the end time of the integral t_End, as also seen from curve 16.
The question now arises when the integration should be terminated.
Conventionally, the integration is terminated when the post-catalytic converter probe measures a voltage that is smaller than a predetermined threshold value, for example 0.4 V. In this case, the lean exhaust gas reaches, without releasing additional oxygen, directed to the post-catalytic converter probe, indicating that the oxygen store is completely full.
If the probe is fully functional, the time t₂is exactly the correct time. If the probe is not fully functional, then the voltage drops below 0.4 V at a time which is too late, namely not before the time t₃.
If one integrates starting at the time the time derivative changes, then for a constant lambda and exhaust gas mass flow, the integral is independent if the post-catalytic converter lambda probe 6 is fully functional or not. The integral from t₀to t₂is identical to the integral from t₁to t₃.
The situation is different when the lambda value and the exhaust gas mass flow change during the time interval: As can be seen from FIGS. 3A to 3D, when the exhaust gas mass flow {dot over (m)} changes according to curve 18, the slope is no longer constant when integrating according to the present formula: In the present example, the slope between the times t₀and t₁is greater than subsequently between the times t₁and t₂and/or t₃, respectively. The “correct” value for the oxygen storage capacity would be the value measured at point 20. If the post-catalytic converter lambda probe 6 has aged and is not fully functional, then an oxygen storage capacity according to point 22 would be measured.
Accordingly, the measured oxygen storage capacity would be too low if the exhaust gas mass flow decreases in the meantime.
As shown in FIGS. 4A to 4D, when the catalytic converter is exposed to an exhaust gas mass flow according to curve 24, the “correct” oxygen storage capacity measured at point 26 would be lower than the actual oxygen storage capacity measured at point 28.
To solve this problem, it is presently proposed to start measuring the oxygen storage capacity essentially at the time t₀of the changeover; in this case, one could not use the signal from the post-catalytic converter lambda probe 6, but would have to use the signal from the pre-catalytic converter lambda probe 5.
In this case, the oxygen storage capacity measured on the aged post-catalytic converter lambda probe would be too high, because the integral is always measured up to the time t₃. Accordingly, the quantity oxygen taken up between the times t₂and t₃should be taken into account in some way.
In a simplified embodiment, each intermediate value for the integral OSC is actually stored. Because the mathematical sign of the signal of the post-catalytic converter lambda probe changes according to FIG. 14, the time t₁can be determined, and hence also the spacing t₁−t₀. The time t₃is also known, so that the time t₂=t₃−t₁+t₀can be determined from the relationship t₃−t₂=t₁−t₀.
If the time t₂is known, then the actual value of the oxygen storage capacity can be inferred. In the simplest case, all intermediate values are stored when the quantity OSC is integrated, starting at the time t₀for several times t₁in discrete intervals which a relatively small in relation to the total time. By storing these intermediate values, the value of the integral at the time t₂can still be determined at the time t₃, thus allowing determination of the correct value for the oxygen storage capacity.
However, such a large quantity of data can not always be kept available. Therefore, the integral is preferably calculated until the time t₃, whereafter the magnitude of the integral at the time t₂is calculated backwards. In the case of curve 24, digital low pass filtering can be performed, with the filter constant determined by t₁−t₀. When filtering the quantity of taken-up oxygen with the low pass filter, the curve 30 is obtained. A point 32 is reached when computing the oxygen storage capacity OSC, and a slope of a segment 36 located between the point 32 and a point 38 still to be computed can be determined based on the value 34 determined from the curve 30 at the time t₃.
A value ΔOSC between the point 32 and the point 38 is obtained by multiplying the slope by t₃−t₂, or t₁−t₀. One arrives at the point 38 and knows the actual oxygen storage capacity once the oxygen storage capacity OSC has been computed from t₀to t₃and after subtracting the quantity ΔOSC.
While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.

Claims

1. A method for determining oxygen storage capacity of an oxygen store associated with a catalytic converter of an internal combustion engine, comprising the steps of:

measuring, in an outflow direction of exhaust gas, an air-fuel ratio with a pre-catalytic converter lambda probe arranged upstream of the catalytic converter and with a post-catalytic converter lambda probe arranged downstream of a section of the catalytic converter,

initially exposing the oxygen store, under control of the pre-catalytic converter lambda probe, to rich exhaust gas so as to extract as much oxygen as possible from the oxygen store or to lean exhaust gas so as to fill the oxygen store with as much oxygen as possible,

thereafter changing over from the rich exhaust gas to lean exhaust gas so as to fill the oxygen store with oxygen or changing over from the lean exhaust gas to rich exhaust gas so as to remove oxygen from the oxygen store,

integrating the filled or removed quantity of oxygen over a time interval, starting at a first time of the changeover and ending at a second time when an output signal of the post-catalytic converter lambda probe is less than or greater than a threshold value, indicating a full state or an empty state of the oxygen store,

measuring a time offset between the first time and a third time when the mathematical sign of a slope of the output signal of the post-catalytic converter lambda probe changes, and

correcting the obtained integral with the time offset.

2. The method of claim 1, further comprising the steps of storing intermediate values of the integral, subtracting the time offset from the second time for obtaining a final time, and using the intermediate value of the integral associated with the final time as a correction value for the obtained integral.

3. The method of claim 1, further comprising the steps of computationally estimating a fraction of computed oxygen storage capacity contributing to the integral for the duration of the time offset before the second time, and subtracting this fraction from the obtained integral for obtaining a correction value for the obtained integral.

4. The method of claim 3, wherein a change in the exhaust gas quantity occurring during the time interval is taken into consideration when estimating the fraction.

5. The method of claim 3, wherein a change in the air-fuel ratio occurring during the time interval is taken into consideration when estimating the fraction.

6. The method of claim 4, further comprising the steps of:

deriving a filter constant for a digital low pass filter from the time offset,

filtering the oxygen uptake as a function of time with the low pass filter, and

calculating the correction value for the obtained integral based on a result of the filtering and the time offset.

7. The method of claim 6, wherein the correction value for the obtained integral is calculated based on a filtering result at the end of the time interval and the time offset.

8. The method of claim 5, further comprising the steps of:

deriving a filter constant for a digital low pass filter from the time offset,

filtering the oxygen uptake as a function of time with the low pass filter, and

9. The method of claim 8, wherein the correction value for the obtained integral is calculated based on a filtering result at the end of the time interval and the time offset.