GB2342597A

GB2342597A - Assessing deterioration of a NOx catalytic converter

Info

Publication number: GB2342597A
Application number: GB9921660A
Authority: GB
Inventors: Joachim Berger; Rene Schnek; Andreas Blumenstock; Klaus Winkler
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1998-09-25
Filing date: 1999-09-15
Publication date: 2000-04-19
Anticipated expiration: 2019-09-15
Also published as: JP4289736B2; GB9921660D0; DE19843871A1; JP2000104536A; GB2342597B; DE19843871B4

Abstract

The performance of a NO<SB>x</SB> catalytic converter for an i.c.e. degrades during its lifetime. The NO<SB>x</SB> trap is subject to repeated cycles of lean-burn fumes (NO<SB>x</SB> absorbed) and rich-burn fumes (NO<SB>x</SB> desorbed and reduced). As the NO<SB>x</SB> catalyst ages it is poisoned by SO<SB>x</SB>, and its cells become contaminated, so that it absorbs less NO<SB>x</SB> in each lean-burn phase. By placing a NO<SB>x</SB> sensor downstream of the NO<SB>x</SB> trap the ever-shortening time period of absorption and desorption steps can be measured, and from this the limited capacity of the NO<SB>x</SB> trap deduced. Real-time measurements for purging or charging periods, or for NO<SB>x</SB> mass flow, can be compared with threshold values determined when the NO<SB>x</SB> trap was new, and thereby indicate if the device is defective.

Description

Monitoring of an NOx-storage catalytic converter having an NOx-sensor connecte downstream The invention relates to the monitoring of an NOx-storage catalytic converter having an NOx-sensor which is disposed in the direction of flow downstream of the catalytic converter.

NOx-storage catalytic converters are used for the purpose of converting pollutants in the combustion processes in the range of lean fuel/air mixtures (lambda > 1). In this range the three-way catalytic converter no longer meets the requirements of exhaust gas quality.

In this case, NOx-storage catalytic converters are used both in the case of petrol and diesel engines and store the nitrogen oxides emitted during the lean engine operation.

When operating the engine in the rich range (lambda < 1), stored nitrates are released and reduced to form nitrogen.

Ideally, the engine is operated in a lean manner during a first phase, until the NOx-storage catalytic converter is full, i. e. until it is no longer able to store any further nitrogen oxides.

This is then ideally followed by a second phase having a rich operation for the particular period of time which is required for the purpose of regenerating the NOx-storage catalytic converter.

The ageing of the NOx-storage catalytic converter causes the active storage areas to become damaged. Therefore, the storage capability of the NOx-catalytic converter decreases continuously with age.

DE OS 1 96 35 977 proposes an NOx-sensor for the purpose of monitoring an NOx storage catalytic converter in the sense of monitoring its current charging level.

Knowledge of the current charging level, i. e. the level at which the NOx-storage device is filled with nitrogen oxides is utilized for control purposes. If the measurement of the current charging level of the storage device indicates that the storage capacity is exhausted, a rich pulse, i. e. an operation of the engine using a rich mixture is produced for the purpose of regenerating the storage catalytic converter.

The SAE Paper 960334 discloses an NOx-sensor with approximately linear signal characteristics Legislative requirements provide for an'On Board'monitoring of motor vehicle components which are relevant to the emission of pollutants, such as catalytic converters.

Upon ageing, the active storage areas of the NOx-storage catalytic converter are damaged, which causes the storage and discharge behaviour of the NOx-catalytic converter to deteriorate. In addition to thermal ageing, symptoms of contamination are also produced, for example, by virtue of sulphur charges. The catalytic converter will then store fewer nitrates than in the fresh state. Therefore, the emissions downstream of the catalytic converter increase and in order to maintain the same average conversion efficiency, a regeneration process must be implemented more frequently.

It is an object of the invention to provide a method and a device for the purpose of assessing the capability of an NOx-storage catalytic converter to function.

In accordance with the present invention there is provided a method of assessing the functionability of an NOx-storage catalytic converter which is supplied with exhaust gas from a combustion process, having an NOx-sensor which is disposed in the direction of flow downstream of the NOx-storage device, wherein the exhaust gas is influenced in the first phases in such a manner that it contains more NOx than in second phases and wherein the exhaust gas in the second phases is influenced in such a manner that it contains reducing agents and wherein a change from the first phase to the second phase is performed repeatedly, the functionability of the NOx-catalytic converter being judged on the basis of the signal of the NOx-sensor.

The invention is based upon the recognition that losses in the functionability of the NOxstorage catalytic converter are illustrated in the progression with respect to time of the NOx-concentration which can be measured downstream of the catalytic converter.

In the case of a predetermined NOx-raw mass charge mnol, the nitrogen oxide emissions mno2 downstream of the catalytic converter increase when the functionability decreases as a result of the ageing process. This behaviour can be diagnosed ; For example, the NOx-concentrations which can be measured in the storage phase downstream of the catalytic converter rise in an increasingly more rapid manner as the catalytic converter gets older. In the regeneration phase, the said concentrations fall in an increasingly more rapid manner as the catalytic converter gets older. In other words: the gradients of the NOx-concentrations measured downstream of the catalytic converter become steeper as the age of the catalytic converter increases.

Exemplified embodiments of the invention are explained hereinunder with reference to the drawings, in which Figure 1 shows the technical environment of the invention, Figure 2 shows the progression with respect to time of the signal of an NOx-sensor, which is disposed downstream of the catalytic converter, with the catalytic converter at different states of ageing, Figure 3 shows an example of a mixture control strategy which is tailored to suit the function of the NOx-storage catalytic converter, and Figure 4 shows one exemplified embodiment of the sequence of the method in accordance with the invention.

Figure 1 shows in detail an internal combustion engine 1 having a catalytic converter 2, an exhaust gas probe 3, an NOx-sensor 4, a control device 5, a fuel metering means 6, different sensors 7,8,9 for load L and rotational speed n and, where appropriate, further operating parameters of the internal combustion engine such as temperatures, throttle valve position etc. and a malfunction lamp 10 as an example of a means for indicating and/or storing a malfunction.

From the said input signals and, where appropriate, further input signals, the control device forms inter alia fuel metering signals by means of which the fuel metering means 6 is controlled. The fuel metering means 6 can be formed both for a so-called intake manifold injection procedure and for a direct petrol injection or diesel injection procedure into the combustion chambers of the individual cylinders. The mixture composition can be varied by way of a change in the injection pulse widths, by means of which the fuel metering means is controlled.

The core of the method in accordance with the invention relates primarily in this environment to the cooperation of the control device 5 with the NOx-sensor 4 which is disposed downstream of the catalytic converter.

Figure 2 illustrates the phase change with an illustration of the signal behaviour of the NOx-sensor 4 which is disposed downstream of the catalytic converter (Figure 2a) and with the associated fuel/air ratio lambda, as detected by the exhaust gas probe 3 which is disposed upstream of the catalytic converter (Figure 2b).

At time t=0, the NOx-storage catalytic converter is empty. In the subsequent first phase Phl the internal combustion engine is operated with a lean mixture (lambda > 1). This corresponds to the step 3.1 in Figure 3. The nitrogen oxides emitted are stored in the storage catalytic converter. The first phase (lean phase) which is also defined as the storage phase is terminated ideally when the storage catalytic converter 2a is full.

The storage catalytic converter is considered, for example, to be full, if the signal of the NOx-sensor achieves an upper threshold value UL. See step 3.2 in Figure 3.

The first phase is followed by a second phase Ph2, in which the storage catalytic converter is regenerated, which is represented by the step 3.3 in Figure 3. The second phase is also defined as a regeneration phase. In this exemplified embodiment, the regeneration process is performed in phase Ph2 when the engine is operating with a lambda value less than 1. The internal combustion engine which operates with a richer fuel mixture emits non-combusted HC and CO as a reducing agent. Under the influence of the catalytic converter the reducing agent reacts with the stored nitrogen oxides to produce water, C02 and N2, which are transported further with the exhaust gas. As a result, the storage device is then ready to receive nitrogen oxides once again, i. e it is regenerated. During the regeneration, the NOx-content of the exhaust gas downstream of the storage catalytic converter decreases in a continuous manner. As soon as the signal of the NOx-sensor achieves a lower threshold value LL, there is a change-over to the lean operation and NOx is stored once again in the storage catalytic converter. See step 3.4 in Figure 3. Between the phases Phl and Ph2 the control device 5 performs a continuous change-over procedure.

As a result of ageing, the storage and regeneration times are shortened. This is illustrated in a symbolic manner in Figure 2 by a reduction in the duration of the time period. In reality, the reduction occurs substantially more slowly. However, the position of the upper and lower threshold remains constant.

The constant increase and decrease in the NOx-concentration downstream of the storage catalytic converter is typical of known NOx-storage catalytic converters. The rate of NOx-storage falls in a continuous manner as the filling rate increases, so that the NOxconcentrations which can be measured downstream of the storage catalytic converter increase in the exhaust gas as the filling rate increases.

The ideas for the monitoring process are based upon a measurement of the NOxemissions downstream of the catalytic converter using the NOx-sensor.

In a first exemplified embodiment, the curve progression as shown in Figure 2 is measured in the fresh state, the curve progression is stored, the curve progression is measured at subsequent points in time and the curve progression which is recorded subsequently is compared to the stored curve progression. If the deviations exceed a predetermined measurement, the catalytic converter is considered to be defective.

The curve progression as shown in Figure 2 can be reconstructed at specific times, for example, from predetermined characteristic value pairs of the NOx-concentration.

Characteristic value pairs are represented, for example, by the reversal points 01, 02,..., U1, U2,... of the signal progression in Figure 2.

Instead of comparing a plurality of individual points of the curves it is possible, for example, to evaluate the ascending gradient thereof, i. e. the quotient of the difference between two NOx-values and the time difference, by means of which these values have been determined. For example, the ascending gradient G in the discharge or regeneration phase can be calculated to G = (LL-UL)/ (t2-tl). See Figure 4, steps 4.1 and 4.2.

The ascending gradient can be compared in step 4.3 to a predetermined limit value G Schwell. An error is indicated by a warning lamp MIL (no. 10 in Figure 1) in step 4.4 if this limit value is exceeded, where appropriate after statistical protection.

The limit value can be fixed, for example, in the following manner: The initial ascending gradient GO is determined in the case of a new catalytic converter. The limit value is fixed as an offset or factor, for example, as 1.5 times of the initial ascending gradient.

As an alternative to storing an initial curve progression, the curve progression can also be modelled. If a functional catalytic converter is taken as a basis, it is possible to form an expected value for the NOx-concentration downstream of the catalytic converter from the operating parameters of the engine, such as load, rotational speed, lambda, progression of the lambda value upstream of the catalytic converter. If the actually measured NOxconcentration deviates to an unacceptable extent from the modelled progression, it is judged to be a sign of a defective catalytic converter.

The ascending gradient can be determined and evaluated separately for the storage and regeneration phase or can also be determined as an average value of the ascending gradients in both phases over one or several storage and regeneration periods.

Likewise, the length of one or several storage or regeneration phases, the duration of the period of the storage/regeneration cycle or the frequency of the periodic NOxconcentration oscillations can be used as a measurement for the ascending gradient.

For example, the length of the regeneration phase is simultaneously determined by the discharge capability of the storage catalytic converter. In so doing, it is assumed that during the regeneration phase at lambda s 1 the NOx-concentration falls according to a characteristic progression with respect to time. Therefore, it is possible to define the maximum permissible regeneration periods. If the regeneration period has exceeded a predetermined permissible regeneration period but the NOx-concentration has not achieved a threshold value, the catalytic converter is considered to be defective.

Further exemplified embodiments are based upon the formation of the instantaneous or integrated NOx-mass flow downstream of the catalytic converter. The NOx-mass flow mn02 downstream of the catalytic converter can be estimated on the basis of the NOxconcentration, which is detected downstream of the catalytic converter, where appropriate by also using the intake air mass flow (sensor 7) or a load and/or rotational speed signal.

The raw mass charge mnol into the catalytic converter can be estimated by virtue of a model. Therefore, by virtue of the test bench trials the raw nitrogen oxide emission of the engine can be ascertained without any subsequent exhaust gas treatment measures for an engine of a constructional series, can be stored in characteristics maps and used for modelling during the subsequent operation of different engines of this constructional series.

The quotient mno2/nmo I or the quotient of the integrals of these variables is a measurement for the storage capability of the catalytic converter as dependent upon ageing. Ideally, the quotient in an effective storage catalytic converter equals zero. As the storage catalytic converter ages, this quotient moves towards the value 1, at which the equality of the input and output emissions signifies a complete failure in the ability of the catalytic converter to convert pollutants. By virtue of a predetermined limit value, which is intended to comply with legislative standards, it is possible to differentiate between effective and inadequate catalytic converters.

The variables are only calculated during the layered operation but this calculation process is otherwise dependent upon the operating point. The layered operation is the operation using a layered charge in the cylinder. This refers to a spatially non-homogenous fuel/air mixture composition in the cylinder. For example, the mixture in the region of the spark plug is rich, in order to guarantee a reliable ignition procedure and is lean in other regions, in order to reduce consumption. On average, the mixture in the layered operation is lean (l < lambda < ca. 3). Differentiated from this is the operation using a homogenous mixture distribution which, for example, provides a high level of output.

The process of forming the integral is associated with the advantage of extremely low sensitivity in relation to disruptions, for example, changes in the sensor signal or changes in the NOx-raw mass and constitutes therefore an advantageously robust method.

Moreover, the formation of the model is minimized by the limitation to the raw mass charge of NOx into the catalytic converter, which also enhances the robust nature of the method.

Furthermore, the raw mass charge can be used to calculate the charging, i. e. the filling level under the premise of a functional catalytic converter. As already described above, the storage capability falls as the charging increases. Consequently, the NOx-emission increases downstream of the catalytic converter as the charging increases. A plausibility comparison between the calculated charging and the measured NOx-concentration downstream of the catalytic converter can also be used for diagnostic purposes.

If the NOx-concentration exceeds a measurement which is plausible with respect to the calculated charging, the catalytic converter is defective.

The common aspect in all of the examples is the use of the NOx-sensor downstream of the catalytic converter for diagnostic purposes. The signal thereof is used to derive a characteristic variable of the NOx-concentration downstream of the catalytic converter.

Claims

Claims 1. A method of assessing the functionability of an NOx-storage catalytic converter which is supplied with exhaust gas from a combustion process, having an NOx sensor which is disposed in the direction of flow downstream of the NOx-storage device, wherein the exhaust gas is influenced in the first phases in such a manner that it contains more NOx than in second phases and wherein the exhaust gas in the second phases is influenced in such a manner that it contains reducing agents and wherein a change from the first phase to the second phase is performed repeatedly, the functionability of the NOx-catalytic converter being judged on the basis of the signal of the NOx-sensor.
2. A method according to claim 1, wherein at least one variable which represents the progression with respect to time of the signal of the NOx-sensor in a new catalytic converter is detected and stored, the process of detecting the at least one variable is repeated at later points in time, deviations between the variables, which variables are detected at different points in time, are ascertained and the catalytic converter is judged as being defective, if at least one deviation exceeds a predetermined threshold value.
3. A method according to claim 2, wherein the ascending gradient of the signal is ascertained as the variable which represents the progression with respect to time of the said signal of the NOx-sensor.
4. A method according to claim 2, wherein the limit value is ascertained by the addition of an offset value to the at least one variable, which represents the progression with respect to time of the signal of the NOx-sensor in a new catalytic converter, or that the limit value is ascertained as a product of a factor with the said at least one variable.
5. A method according to claim 1, wherein at least one variable, which represents the progression with respect to time of the signal of the NOx-sensor in a reference catalyst, is modelled from operating parameters of the engine, optionally supplemented by operating parameters of the catalytic converter, the said at least one variable is detected during the operation of the engine, deviations between the modelled and the detected variable are formed and the catalytic converter is judged to be defective, if at least one deviation exceeds a predetermined threshold value.
6. A method according to claim 3, wherein the ascending gradient is determined separately for the storage and regeneration phases or is also determined and evaluated as an average value of the ascending gradients in both phases over one or several storage and regeneration periods.
7. A method according to claim 3, wherein the length of one or several storage or regeneration phases, the duration of the period of the storage/regeneration cycle or the frequency of the periodic NOx-concentration oscillation are used as the measurement for the ascending gradient.
8. Method according to claim 1, wherein the length of the regeneration phase is detected, compared with a threshold value and that the catalytic converter is judged to be defective when the threshold value is exceeded.
9. A method according to claim 1, wherein the instantaneous filling state of the catalytic converter with nitrogen oxides is modelled from operating parameters of the engine and is modelled facultatively from operating parameters of the catalytic converter, an expected value for the NOx-concentration downstream of the catalytic converter is formed from the modelled filling state and compared with the NOx-concentration detected downstream of the catalytic converter and the catalytic converter is then judged to be defective, if at least one deviation between the expected value and the detected value exceeds a threshold value.
10. A method according to claim 1, wherein an NOx-mass flow (dmN02/dt) out of the storage device is calculated, an NOx-mass flow dmNOl/dt into the catalytic converter is determined, and the catalytic converter is judged to be defective, if the two mass flows do not differ to a sufficient extent from each other.
11. A method according to claim 11, wherein the quotient of the two mass flows is formed and the catalytic converter is considered to be defective, if the quotient deviates from the value 1 by less than a predetermined measurement.
12. A method according to claim 10 or 11, wherein integrals of the mass flows are formed and the catalytic converter is considered to be defective, if the integrals of the two mass flows do not differ to a sufficient extent from each other.
13. A method of assessing the functionability of an NOx-storage catalytic converter, substantially as hereinbefore described, with reference to the accompanying drawings.