JP4289736B2 - Method for determining functionality of NOx storage catalyst - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a diagnosis of a NOx storage catalyst by a NOx sensor arranged behind the catalyst in the flow direction.
[0002]
[Prior art]
NOx storage catalysts are used for the conversion of harmful substances in the combustion process within the combustion range of lean fuel / air mixtures (λ> 1). In this range, the three-way catalyst no longer meets the requirements for exhaust gas quality. In this case, NOx storage catalysts are used which store nitrogen oxides released in lean engine operation as well as in gasoline engines as well as in diesel engines. By operating the engine in a rich range (λ <1), the stored nitrate is released and reduced to nitrogen.
[0003]
Ideally, the engine is operated lean in the first process until the NOx storage catalyst is full, i.e. no more nitrogen oxides can be stored. Ideally, this is followed by a second process having a rich operation for the time required for regeneration of the NOx storage catalyst.
[0004]
The active storage location is damaged by the aging of the NOx storage catalyst. Therefore, the storage capacity of the NOx catalyst continuously decreases as the deterioration with time progresses.
German Patent Publication No. 19635977 proposes to monitor the NOx storage catalyst by monitoring the filling degree at that time. Data on the degree of filling at that time, that is, the degree of filling of the NOx storage catalyst with nitrogen oxides, is used for control purposes. When the current storage charge measurement indicates that the storage capacity is full, a rich pulse is generated, i.e. engine operation with the rich mixture for regeneration of the storage catalyst is started.
[0005]
From SAE Paper 960334, a NOx sensor with a substantially linear signal characteristic is known.
Regulatory requirements stipulate onboard diagnostics for automotive components such as catalysts associated with hazardous emissions.
[0006]
When degraded, the active storage location of the NOx storage catalyst is damaged, thereby reducing the storage and release characteristics of the NOx catalyst. In addition to deterioration due to heat, poisoning due to, for example, sulfur uptake occurs. At this time, the catalyst will store less nitrate than in the fresh state. Thus, catalyst back-emission increases and regeneration must occur frequently to maintain the same average conversion capacity.
[0007]
[Problems to be solved by the invention]
It is an object of the present invention to provide a method for determining the functionality of a NOx storage catalyst.
[0008]
[Means for Solving the Problems]
The problem is that the exhaust gas is adjusted to contain more NOx in the first process than in the second process, and the exhaust gas is adjusted to contain a reducing agent in the second process. NOx catalyst in a method for determining the functionality of a NOx storage catalyst to which exhaust gas is supplied from a combustion process, using a NOx sensor disposed at the rear of the NOx storage catalyst in the flow direction, wherein repetitive switching from the first process to the second process is performed Is determined based on the NOx sensor signal, and is solved by the NOx storage catalyst functionality determination method of the present invention.
[0009]
The present invention is based on the finding that the decrease in functionality of the NOx storage catalyst is represented in a NOx concentration time diagram that can be measured behind the catalyst.
When the functionality is reduced due to deterioration at a predetermined NOx supply mass intake amount mno1, the catalyst rearward nitrogen oxide emission mno2 rises. This relationship can be used for diagnosis.
[0010]
For example, during storage, the measurable NOx concentration behind the catalyst increases more and more rapidly with the progress of catalyst degradation. During the regeneration process, the measurable NOx concentration behind the catalyst decreases more and more rapidly with the progress of catalyst degradation. In other words, the slope of the NOx concentration measured behind the catalyst becomes steeper as the catalyst progresses.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 shows in detail the catalyst 2, exhaust gas sensor 3, NOx sensor 4, control device 5, fuel supply means 6 and other internal combustion engines such as load L and rotational speed n and possibly temperature, throttle valve position, etc. 1 shows an internal combustion engine 1 with various sensors 7, 8, 9 for operating parameters and an error lamp 10, for example as an error indication means and / or storage means.
[0012]
From the said input signal and possibly other input signals, the control device 5 forms a fuel supply signal, in particular for operating the fuel supply means 6. The fuel supply means 6 may be formed not only as so-called intake pipe injection but also as direct injection of gasoline or diesel into the combustion chambers of the individual cylinders. The mixture composition may be changed by changing the injection pulse width for operating the fuel supply means 6.
[0013]
The essence of the method according to the invention firstly relates in this environment to the cooperation of the control device 5 and the NOx sensor 4 arranged behind the catalyst.
FIG. 2 is a signal characteristic diagram (FIG. 2A) of the NOx sensor 4 disposed behind the catalyst and an attached air-fuel ratio λ (FIG. 2) measured by the exhaust gas sensor 3 disposed in front of the catalyst. 2 (B)) shows the process switching.
[0014]
Assume that at time t = 0, the NOx storage catalyst is empty. In the subsequent first phase Ph1, the internal combustion engine is operated with a lean mixture (λ> 1). This corresponds to step 3.1 in FIG. The nitrogen oxides released in this case are stored in the storage catalyst. Ideally, the first process (lean process), also called a storage process, is terminated when the storage catalyst 2a is full.
[0015]
In this case, the storage catalyst is considered full when the NOx sensor signal reaches, for example, the upper threshold UL. See step 3.2 in FIG.
This first process is followed by a second process Ph2, in which the storage catalyst is regenerated, which is represented by step 3.3 in FIG. The second process is also called a regeneration process. In this embodiment, the regeneration is performed by a rich engine operation in which the λ value in the process Ph2 is smaller than 1. In this case, an internal combustion engine operating with a fuel rich mixture releases unburned HC and CO as a reducing agent. Under the action of the catalyst, the reducing agent reacts with the nitrogen oxides being stored to produce water, CO ₂ and N _2, water, CO ₂ and N ₂ is discharged together with the exhaust gases. This allows the storage catalyst to newly accept nitrogen oxides, i.e., be regenerated. During regeneration, the NOx content of the exhaust gas behind the storage catalyst gradually decreases. As soon as the NOx sensor signal reaches the lower threshold LL, the shift to lean operation and new storage of NOx in the storage catalyst are performed. See step 3.4 in FIG. Switching between the processes Ph1 and Ph2 is performed continuously by the control device 5.
[0016]
Deterioration shortens storage time and regeneration time. This is shown schematically in FIG. 2 by shortening the period. In practice, this shortening is very slow. On the other hand, the positions of the upper threshold and the lower threshold remain constant.
[0017]
Steady increases and decreases in NOx concentration behind the storage catalyst are characteristic of the known NOx storage catalyst. Since the storage rate of NOx continuously decreases as the degree of filling increases, the concentration of NOx in the exhaust gas that can be measured behind the storage catalyst increases as the degree of filling increases.
[0018]
The concept of diagnosis is based on the measurement of NOx emission behind the catalyst by a NOx sensor.
In the first embodiment, the measurement of the curve diagram shown in FIG. 2 in the fresh state, the storage of this curve diagram, the measurement of the curve diagram at a later time, and the curve diagram obtained later are stored. Comparison with the existing curve diagram is performed. If the deviation exceeds a predetermined scale, the catalyst is considered degraded.
[0019]
The curve diagram shown in FIG. 2 can be reconstructed from a pair of predetermined characteristic values of the NOx concentration at a predetermined time, for example. The pair of characteristic values are represented by, for example, inversion points O1, O2,..., U1, U2,.
[0020]
Instead of comparing a number of individual points on a curve, for example, the slope of the curve, ie the quotient of the difference between two NOx values and the time interval at which these values were measured, may be evaluated. For example, the gradient G in the release or regeneration process can be calculated by G = (LL−UL) / (t2−t1). See steps 4.1 and 4.2 in FIG.
[0021]
In step 4.3, the slope is compared with a predetermined limit value G_Schwell. If this limit value is exceeded, an error indication is given by the warning lamp MIL (reference numeral 10 in FIG. 1) after optionally performing a statistical test in step 4.4.
[0022]
The limit value can be determined as follows, for example. An initial gradient G0 is determined for the new catalyst. The limit value is determined as an offset or a factor of eg 1.5 times the initial slope.
[0023]
Instead of storing an initial curve diagram, a curve diagram may be modeled. When based on a functional catalyst, an expected value for the NOx concentration behind the catalyst can be formed from engine operating parameters such as a diagram of load, rotational speed, λ, and λ value in front of the catalyst. If the actual measured NOx concentration is unacceptably different from the modeled diagram, this is evaluated as an indication for a degraded catalyst.
[0024]
The slope may be determined and evaluated separately for storage and regeneration processes, or may be determined as the average value of the slope in both processes over one or more cycles of storage and regeneration.
[0025]
Similarly, the length of one or more storage or regeneration processes, the period time of a storage / regeneration cycle, or the frequency of periodic NOx concentration oscillations may be used as a measure for the slope.
[0026]
For example, the length of the regeneration process is determined both by the release potential of the stored catalyst. In this case, the starting point is that the NOx concentration decreases according to the characteristic time diagram during the regeneration when λ is 1 or less. Thereby, the maximum allowable reproduction time can be defined. When the regeneration time exceeds a predetermined allowable regeneration time without the NOx concentration falling below the threshold, the catalyst is considered degraded.
[0027]
Other embodiments are based on the formation of instantaneous NOx mass flow or integrated NOx mass flow behind the catalyst. The NOx mass flow mno2 behind the catalyst can be evaluated based on the NOx concentration measured behind the catalyst, optionally using the intake mass flow (sensor 7) or load signal and / or rotational speed signal simultaneously.
[0028]
The supplied mass take-in amount mno1 into the catalyst can be evaluated by a model. Therefore, the bench test determines the nitrogen oxide supply emission of an engine without using exhaust gas aftertreatment means for one model series engine, stores it in the characteristic curve group, and the other engine of this model series. It can be used for modeling in later driving.
[0029]
The quotient mno2 / mno1 or the integral quotient of these values is a measure for the storage capacity of the catalyst as a function of degradation. When the storage catalyst is healthy, this quotient is ideally equal to zero. As the deterioration progresses, the quotient approaches a value of 1, at which the inlet and outlet emissions are equal, indicating a complete reduction in conversion capacity. A healthy catalyst and a deteriorated catalyst can be distinguished from each other by a predetermined limit value determined in order to satisfy legal regulations.
[0030]
Values are calculated only in stratified operation, but the others are independent of the operating point. The stratified operation is an operation having stratified filling in the cylinder. This is understood as a spatially non-uniform fuel / air mixture composition within the cylinder. For example, the mixture in the spark plug region is rich to ensure reliable ignition and in other regions it is lean to reduce fuel consumption. On average, in stratified operation, the mixture is lean (1 <λ <about 3). For example, operation with a homogeneous mixture distribution that produces high power is distinguished from this.
[0031]
Integral formation has the advantage of being very insensitive with respect to disturbances such as sensor signal changes or NOx feed mass changes and thus represents an advantageous method that is not affected by disturbances. Furthermore, by limiting the amount of feed mass taken into the catalyst for NOx, model formation is reduced, which in turn makes the method unaffected by disturbances.
[0032]
Furthermore, on the assumption that the catalyst has functionality, the filling amount, that is, the filling degree, can be calculated from the supplied mass intake amount. As noted above, storage capacity decreases with increasing fill. Therefore, the NOx emission behind the catalyst rises as the filling amount increases. A validity comparison between the calculated charge and the measured NOx concentration behind the catalyst can be used for diagnosis as well.
[0033]
When the NOx concentration exceeds a reasonable measure, for example with respect to the calculated charge, the catalyst is degraded.
In all examples, it is common that a NOx sensor is used behind the catalyst for diagnosis. The characteristic value of the NOx concentration behind the catalyst is derived from the NOx sensor signal.
[Brief description of the drawings]
FIG. 1 is a technical peripheral view showing the operation of the present invention.
FIG. 2A is a signal time diagram of a NOx sensor disposed behind a catalyst in various aging states of the catalyst, and FIG. 2B is an accessory measured by an exhaust gas sensor disposed in front of the catalyst. FIG. 6 is a time diagram of the air-fuel ratio λ.
FIG. 3 is a flow diagram illustrating an example of a mixture control scheme adapted to the function of the NOx storage catalyst.
FIG. 4 is a flowchart of an embodiment showing the process of the method according to the present invention.
[Explanation of symbols]
1 Internal combustion engine 1a Combustion chamber 2 Catalyst 2a First part of catalyst (NOx storage catalyst)
2b Second part of catalyst 3 Exhaust gas sensor 4 NOx sensor 5 Control device 6 Fuel supply means 7, 8, 9 Sensor 10 Error lamp (alarm lamp)

Claims (1)

The exhaust gas is adjusted to contain a larger amount of NOx in the first process than in the second process, and the exhaust gas is adjusted to contain a reducing agent in the second process. In a method for determining the functionality of a NOx storage catalyst to which exhaust gas is supplied from a combustion process, by a NOx sensor arranged at the rear of the NOx storage catalyst in the flow direction, wherein the NOx catalyst functionality is determined. The determination based on the NOx sensor signal, the filling state of the catalyst with nitrogen oxides from the engine and catalyst operating parameters are modeled, and the expectation for the NOx concentration behind the catalyst from the modeled filling state A value is formed and compared with the NOx concentration measured behind the catalyst and at least one deviation between the expected value and the measured value exceeds the threshold Come, the functionality of the determination method of the NOx storage catalyst, wherein the the catalyst is evaluated to have deteriorated, the.

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