METHOD AND DEVICE FOR MONITORING THE OPERABILITY OF AN EMISSION CONTROL SYSTEM

Abstract

In a method and a device for monitoring the operability of an emission control system of an internal combustion engine, at least two catalytic converters are situated in succession in an exhaust duct. For tracking control, breakthrough detection for diagnosing the first catalytic converter, and for a second balancing for the storage capacity of oxygen or rich gas of the second catalytic converter, a two-point lambda probe be used and, for the latter, tolerance and aging effects are compensated. This results in particular in cost advantages in emission control systems for fulfilling stricter emission and diagnostic requirements.

Claims

1. A method for monitoring an operability of an emission control system of an internal combustion engine that, in its central section includes first and second catalytic converters situated in an exhaust duct in succession, a first lambda probe being situated, with respect to a direction of flow of exhaust gas, downstream of an engine block and upstream of the first catalytic converter, a first exhaust-gas probe being situated downstream of the first catalytic converter and upstream of the second catalytic converter, and a second exhaust-gas probe being situated downstream of the second catalytic converter, wherein the lambda probe is used to perform a lambda control and a balancing for a storage capacity of oxygen or rich gas of the first catalytic converter, and the second exhaust-gas probe is implemented as a first two-point lambda probe and is used to perform a catalytic converter diagnosis of the second catalytic converter, the method comprising: using the first exhaust-gas probe, which is implemented as a second two-point lambda probe, for a tracking control and breakthrough detection for a diagnosis of the first catalytic converter and for a second balancing for the storage capacity of oxygen or rich gas of the second catalytic converter; and compensating for tolerances and aging effects of the second two-point lambda probe.

2. The method of claim 1, wherein the lambda probe upstream of the first catalytic converter is a wide-band lambda probe.

3. The method of claim 1, wherein the compensation includes adapting an offset of a probe characteristic of the second two-point lambda probe by an adjustment at high excess air while the internal combustion engine is running or is at a standstill.

4. The method of claim 3, wherein the compensating includes applying a correction of a temperature-related shift of the probe characteristic with the aid of an active measurement of a sensor element temperature while the internal combustion engine is running or is at a standstill.

5. The method of claim 3, wherein the compensating includes correcting a calculation of a lambda value in order to take into account a current exhaust-gas composition and a varying cross sensitivity vis-à-vis different exhaust-gas components.

6. The method of claim 1, wherein the compensating includes shifting a lambda-one point of a probe characteristic of the second two-point lambda probe via a tracking control of the first two-point lambda probe.

7. The method of claim 6, wherein the compensating includes applying a correction of a temperature-related shift of the probe characteristic with the aid of an active measurement of the sensor element temperature while the internal combustion engine is running or is at a standstill.

8. The method of claim 1, wherein the emission control system includes a further catalytic converter and a further two-point lambda probe directly upstream of the further catalytic converter for a balancing of a storage capacity of oxygen or rich gas of the additional catalytic converter and compensation measures are taken for the further two-point exhaust-gas probe.

9. The method of claim 1, wherein the internal combustion engine is part of a vehicle and at least one of the catalytic converters is implemented in combination with a particulate filter.

10. An emission control system of an internal combustion engine, the emission control system comprising: an engine control unit; a first catalytic converter situated in an exhaust duct; a second catalytic converter situated in the exhaust duct in sequence with the first catalytic converter; a first lambda probe that is situated, with respect to a direction of flow of exhaust gas, downstream of an engine block and upstream of the first catalytic converter, and that is configured to perform a lambda control and a balancing for a storage capacity of oxygen or rich gas of the first catalytic converter, and that is configured to supply signals to the engine control unit; a first exhaust-gas probe that is downstream of the first catalytic converter and upstream of the second catalytic converter, that is configured to supply signals to the engine control unit, and that is a first two-point lambda probe configured for a tracking control, a breakthrough detection for a diagnosis of the first catalytic converter, and a balancing for a storage capacity of oxygen or rich gas of the second catalytic converter; and a second exhaust-gas probe that is downstream of the second catalytic converter, that is a second two-point lambda probe, that is configured to perform a catalytic converter diagnosis of the second catalytic converter, and that is configured to supply signals to the engine control unit; wherein the engine control unit includes a storage device and a comparator unit and is configured to perform a catalytic converter diagnosis and execute measures for compensating tolerance and aging effects of the first two-point lambda probe.

11. The system of claim 10, wherein the emission control system includes a third catalytic converter and a third two-point lambda probe directly upstream of the third catalytic converter for a balancing of a storage capacity of oxygen or rich gas of the third catalytic converter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The FIGURE is a schematic representation of an internal combustion engine having an emission control system by which the method of the present invention can be implemented, according to an example embodiment of the present invention.

DETAILED DESCRIPTION

[0030] The FIGURE schematically shows an internal combustion engine 1 that includes an engine block 10, an exhaust duct 20, a lambda probe 30 that is, with respect to the direction of flow of exhaust gas, downstream of the engine block 10 and that is implemented as a wide-band lambda probe, and a first catalytic converter 40 situated downstream of the lambda probe 30. Lambda probe 30 makes possible lambda control 100 and is used for a first balancing 110 for diagnosing the storage capacity of the first catalytic converter.

[0031] The internal combustion engine 1 further includes, downstream from first catalytic converter 40, a first two-point lambda probe 50 used for tracking control 120 and for breakthrough detection for the catalytic converter diagnosis of first catalytic converter 40. The internal combustion engine 1 further includes a second catalytic converter 60 downstream of the first two-point lambda probe 50. The first two-point lambda probe 50 is additionally used for a second balancing 130 of the storage capacity of the second catalytic converter 60. Downstream of the second catalytic converter 60, the internal combustion engine 1 further includes a second two-point lambda probe 70 for breakthrough detection for a catalytic converter diagnosis 140 of second catalytic converter 60. Lambda probes 30, 50, and 70 are connected to an engine control unit 80, in which on the one hand the lambda control and on the other hand the diagnostic methods regarding the monitoring of the operability of the emission control system are implemented in hardware and/or software.

[0032] The present invention is analogously also applicable to emission control systems having more than two monitored catalytic converters 40, 60, a two-point lambda probe being used for balancing the storage capacity of oxygen or rich gas of the subsequent catalytic converter in the direction of flow of the exhaust gas.

[0033] For measuring the oxygen storage capacity of a catalytic converter 40, 60, the latter is first freed completely of oxygen. This is achieved by a preconditioning with a mixture of sufficiently “rich,” i.e., low, lambda (typically λ=0.95). The rich preconditioning occurs in a controlled manner on the basis of the lambda signal of the probe upstream from the catalytic converter. Subsequently, a switch is performed to a mixture with “lean” lambda, i.e., higher lambda (typically λ=1.05). This lambda value too is adjusted by the probe upstream from the catalytic converter.

[0034] This lean lambda value is maintained until the lambda probe downstream from the catalytic converter indicates a breakthrough of a lean exhaust gas mixture. The oxygen quantity introduced into the catalytic converter between the switchover to the lean lambda value and the breakthrough of the lean mixture is balanced and corresponds to the oxygen storage capacity of catalytic converter 40, 60. It is possible to apply this method analogously to the determination of the rich gas storage capacity of a catalytic converter 40, 60. Here, following a lean preconditioning where λ>1, the oxygen quantity discharged from catalytic converter 40, 60 during the rich phase, where λ<1, is balanced.

[0035] A lambda probe is suitable as a probe upstream from the catalytic converter 40, 60 for adjusting the lambda value in the rich preconditioning and for adjusting the lean lambda value as well as for balancing 110, 130 the entered oxygen quantity during the measurement of the oxygen storage capacity of catalytic converter 40, 60 if it has a sufficiently precise lambda signal in a wide lambda range (typically in a range between λ=0.95 and λ=1.05). A two-point lambda probe 50, 70 does not fulfill this requirement without additional measures.

[0036] The use of a first (as shown in the FIGURE) two-point lambda probe 50 for the second balancing 130 for the diagnosis of second catalytic converter 60 presupposes the compensation of tolerance and aging effects, which result in a shift of the actual probe characteristic vis-à-vis the reference probe characteristic stored in the control unit or engine control unit 80. Only then will the lambda signal of this first two-point lambda probe 50 fulfill the mentioned requirements regarding accuracy.

[0037] DE102012211687A1, DE102012211683A1, DE102013216595A1, DE102014210442A1 and DE102012221549A1 describe methods that allow for a compensation of tolerance and/or aging effects that result in such a characteristic curve shift. The methods described there, one or more of which, according to example embodiments of the present invention, are applied to the first two-point lambda probe 50 individually or in combination, include: adaptation of a constant offset of the probe characteristic by an adjustment at high excess air while the engine is running or is at a standstill; compensation of the shift of the lambda-one point of the probe characteristic via a tracking control using the second two-point lambda probe 50; compensation of a temperature-related shift of the probe characteristic with the aid of an active measurement of the sensor element temperature while the engine is running or is at a standstill; and taking into account the current exhaust-gas composition and different cross sensitivities of the probe vis-à-vis various exhaust-gas components in the conversion of the probe voltage into a lambda value.

[0038] In a particularly preferred variant of the method of the present invention, only a constant offset of the probe characteristic is adapted. This is possible to achieve in a comparatively simple manner by an adjustment at high excess air, such as occurs for example at an overrun fuel cutoff, and in many cases already results in a sufficient accuracy of the lambda signal so as to be able to use it for balancing the oxygen input and discharge for diagnosing the second catalytic converter 60. At the same time, this adaptation improves the accuracy of the breakthrough detection in the diagnosis of first catalytic converter 40 and the accuracy of the tracking control 120 with the aid of the first two-point lambda probe 40.

[0039] By combining the adaptation of a constant offset of the probe characteristic with one or more of the above-mentioned methods it is possible to improve the accuracy of the lambda signal of the first two-point lambda probe 50 downstream from first catalytic converter 40 further, in the event that even higher requirements are placed on the accuracy of this lambda signal.

[0040] Following the compensation, the lambda signal of the first two-point lambda probe 50 is used, as described above, for the measurement of the oxygen storage capacity of second catalytic converter 60. In the alternative method variant already mentioned above, the lambda signal is used for measuring the rich gas storage capacity. For both measurements, active lambda adjustment, triggered specifically for the catalytic converter diagnosis, and/or the use of lambda adjustment that are already present anyway are provided.