Patent Application
20120222474
The invention relates to a method for dynamically diagnosing an exhaust gas probe disposed in an exhaust duct of an internal combustion engine, wherein the dynamic diagnosis is performed after a change in a lambda value of the exhaust gas, and on the basis of a comparison of a measured signal rise relative to an expected rise of the signal.
The invention further relates to a device for dynamically diagnosing an exhaust gas probe disposed in an exhaust duct of an internal combustion engine, the output signal thereof being fed to an engine controller connected to additional input signals providing at least additional information about intake air mass and fuel metering.
The storage capacity of an emission control system for oxygen is utilized for the purpose of receiving oxygen during lean phases and in turn dispensing oxygen during rich phases. As a result, oxidizable constituents of harmful gas contained in the exhaust gas can be converted. An exhaust gas probe disposed downstream of the emission control system thereby serves to monitor the oxygen storage capacity of said emission control system. The oxygen storage capacity has to be monitored within the scope of the on-board diagnostics system because said capacity represents a measurement for the conversion capacity of said emission control system. In order to determine said oxygen storage capacity, either the emission control system is initially saturated with oxygen in a lean phase and is subsequently emptied in a rich phase having an exhaust gas of a known lambda ratio with regard to the amount of exhaust gas passing through said emission control system or said system is initially emptied of oxygen in a rich phase and in a lean phase is subsequently filled with an exhaust gas of a known lambda ratio with regard to the amount of exhaust gas passing through said system. The lean phase is ended if the exhaust gas probe downstream of said emission control system detects the oxygen, which can no longer be stored by said emission control system. A rich phase is likewise ended if said exhaust gas probe detects the passage of rich exhaust gas. An output signal of said exhaust gas probe furthermore serves as additional information for a lambda control, which however primarily is based on an output signal of a lambda probe disposed upstream of said emission control system.
If the exhaust gas probe has aged, the output signal of the exhaust gas probe slows down in its reaction to changes in the exhaust gas composition, and deviations can result in the diagnosis of the emission control system, which can lead to an emission control system, which is no longer operating correctly, being mistakenly determined to be in working order. Tremendous importance is thus attached to a dynamic monitoring of the exhaust gas probe.
The evaluation of the reaction speed of the exhaust gas probe to a changing O2-concentration is understood by the term dynamic monitoring. Due to aging and contamination of the ceramic probe body, respectively the probe case, the possibility exists for the measurement of the O2 concentration of the exhaust gas to be considerably delayed. For this reason, the functions relevant to emissions, which require the lambda signal as an input signal, would be operated in a delayed fashion. As a result, emission threshold values can be exceeded, which have been specified by the lawmakers (European authorities or CARB, EPA).
A method of prior art for diagnosing an emission control system also evaluates, for example, the ratio of the amplitudes of the output signals of the lambda probe disposed upstream of the emission control system and the lambda probe disposed downstream of said system. An emission control system in working order dampens the amplitude of an oscillation of the oxygen content of the exhaust gas at the outlet of the internal combustion engine so that the ratio of the amplitudes upstream and downstream of said emission control system results in a high value. A delayed reaction of the exhaust gas probe disposed downstream of said emission control system leads, however, likewise to a reduction in the amplitude of the output signal thereof, whereby the oxygen storage capacity of said emission control system is evaluated as too high. An emission control system, which no longer meets the legal requirements, can thus under certain circumstances be mistakenly classified as being in good working order.
A dynamic diagnosis is made difficult because of the fact that the output signal of the exhaust gas probe is dependent upon the initial and the final lambda value when a rich to lean or lean to rich step change occurs. In addition, the influence of the emission control system described above must be taken into consideration, wherein the influences of temperature and the age of said emission control system must additionally be considered.
A method for dynamically diagnosing an exhaust gas probe is stated in the German patent publication DE 19722334. The exhaust gas probe is disposed in the exhaust gas downstream of an emission control system. The rate of change of an output signal of the exhaust gas probe is used as the evaluation criterion, said rate of change occurring, for example, after the beginning of a phase in overrun conditions. A disadvantage thereby is that this method only works when a high mass air flow (>>50 kg/h) occurs. This is because the influence of the catalytic converter can only then be disregarded. In such operating states, undesired conditions can, however, arise when returning to load conditions after being in the overrun mode.
In the German patent publication DE 10 2006 041 477 A1, a method for dynamically diagnosing an exhaust gas probe disposed in the exhaust duct of an internal combustion engine downstream of the emission control system is described, wherein the dynamic diagnosis is performed at the same time that a step change in the lambda value of the exhaust gas from rich to lean or from lean to rich occurs.
The present-day operation of dynamic monitoring calculates two O2 threshold values on the basis of the measured O2 concentration during a valid step load transition. The measured rise time of the O2 concentration from the first to the second threshold value is used as the evaluation criterion for the dynamic characteristics of the exhaust gas probe. If the measured rise time remains under a fixed threshold value, an intact message results, otherwise a fault is reported. The lambda signal is thereby qualified with respect to a fixed value in an operating range to be applied.
A disadvantage in this is that a setting of the operating range is dependent upon the component tolerances of the components upstream of the emission control system, including sensors and actuators. Possible drifting of the component characteristics is not taken into account with fixed threshold values. In addition, with these fixed threshold values, only a limited operating range for changes in load can be used for a dynamic diagnosis. Dynamic characteristics of the exhaust gas probe can also not always be correctly diagnosed with fixed threshold values. As a result, a dynamically defective exhaust gas probe can be evaluated as being in working order, which should be considered as critical in light of the massively increased or increasing legal requirements.
It is therefore the aim of the invention to provide a method for dynamically diagnosing an exhaust gas probe, which facilitates a greater and more reliable selectivity of the dynamic characteristics of the exhaust gas probe over an operating range and reduces the influence of the operating range on the release of a dynamic plausibility check by the probe output signal.
The aim relating to the method is thereby met by a target/actual comparison between a calculated O2 signal and an O2 signal measured by the exhaust gas probe, or between signals derived from said signals, being performed for a step load transition. Dynamic processes can be considered more reliably using the method than for the prior art, so that improved selectivity is made possible, independent of the operating point. The increased legal requirements with respect to the on-board diagnosis can thereby be fulfilled.
A preferred modification to the method thereby provides for the calculation of the O2 signal to be performed with the air mass and the injected fuel quantity.
If the calculated O2 signal and the measured O2 signal are, for example, filtered by means of a low-pass filter and hence a calculated and filtered O2 signal and a measured and filtered O2 signal are formed for the target/actual comparison, the diagnostic result can be less affected by disturbances which temporarily occur during signal transmission or during signal processing.
If, as the invention provides in a preferred modification to the method, the gradients of the calculated O2 signal and the measured O2 or the gradients of the filtered O2 signals are used for the target/actual comparison, particularly the dynamic characteristics of the exhaust gas probe can be directly analyzed. In comparison to a mere evaluation of the rise time between the aforementioned O2 thresholds, said characteristics of the exhaust gas probe can also be reliably determined as a function of the respective operating condition. The assessment of this relative change is vis-B-vis an evaluation of an absolute change in the signal fundamentally less susceptible to disturbances with respect to possible offset influences within the evaluation system and the involved sensors or actuators.
It is particularly advantageous if a target value, which is associated with the respective operating point of the internal combustion engine, is formed for the target/actual comparison and is subsequently compared with the actual value. Within the scope of the application, a dynamic diagnosis is thus made possible not only in a limited operating range, as is the case up until now, but in a range which is now considerably expanded; thus enabling the dynamic characteristics of the exhaust gas probe to be determined in a broad operating range of the internal combustion engine. Secondly, dynamic diagnosis results from different operating ranges can also be used for the assessment in order, for example, to check the individual results for plausibility or also to identify the operating conditions, in which a dynamic diagnosis should not take place. Should, for example, there turn out to be a defect in the dynamics of the exhaust gas probe, a dynamic protraction of the signal will not only appear for a step load transition in the present operating range but is detectable for step load changes in other operating ranges.
In a preferred modification to the method, the invention provides for a first and a second O2 threshold value of the calculated O2 signal to be determined for the step load transition on the basis of the signal curve of the calculated and filtered O2 signal. Provision is thereby made for the threshold value determination of the O2 threshold values to be performed again for each step load transition used for the dynamic diagnosis. A modification to the method furthermore provides that in the case of a valid step load transition, an O2 threshold value of the measured O2 signal is determined based on the measured O2 signal, the calculation thereof being performed identically to the calculation of the first O2 threshold value of the calculated O2 signal. In so doing, a percentagely identical threshold value is taken as a basis with regard to the respective signal deviation.
This respective recalculation of the O2 threshold values means on the one hand that the O2 threshold values can be adapted in each case to the operating range, in which the dynamic diagnosis takes place. On the other hand, an improved diagnosis can be achieved with these variable O2 threshold values in comparison to the rigidly predefined threshold values according to prior art in the event of a drift of the component characteristics. Furthermore, the influence of the vehicle driver, which stems from releasing the gas pedal differently in each case, can, for example, be avoided. This relates particularly to the compensation for the quantity gradient during a transition to overrun conditions.
For the dynamic diagnosis of an exhaust gas probe, a preferred modification to the method provides that for the calculated O2 signal or for the calculated and filtered O2 signal during the time of reaching the first O2 threshold value of said calculated O2 signal up until reaching the second O2 threshold value of said calculated O2 signal, an O2 gradient signal for the calculated value is integrated and the target value is derived from the result thereof. In addition, an integration period can be determined for said calculated O2 signal. Parallel to this, an O2 gradient signal for the measured value is integrated for the measured O2 signal or for the calculated and filtered O2 signal and the actual value is derived from the result of this integration. The integration period for the calculated O2 signal is thereby used as the integration period for the measured O2 signal. A trigger time is used as the starting point in time of the integration, said trigger time being determined if the measured O2 signal or the measured and filtered O2 signal exceeds the O2 threshold value of the measured O2 signal. The integrals calculated in this manner for the target value and the actual value particularly take into account the dynamic effects and are additionally robust against offsets and temporary signal disturbances.
The actual value and the target value can then be set in relationship to one another for the dynamic diagnosis, and a dynamic assessment of the exhaust gas probe can be derived from the result of this relationship, wherein the integral for the actual value becomes smaller in relation to the integral for the target value with worsening dynamics.
In an equally advantageous modification to the method, provision can be made for the dynamic assessment to be performed by direct comparison between the absolute O2 gradient signal for the calculated value and the absolute O2 gradient signal for the measured value. Provision can, for example, likewise be made for the dynamic assessment to be performed by direct comparison of the temporal profiles of the calculated O2 signal and the measured O2 signal or of the temporal profiles of the filtered O2 signals. Both modifications also meet the requirements for a reproducible selectivity of the dynamic monitoring, are, however, less complex and therefore can be used in simplified OBD units.
The aim relating to the device is thereby met in that the engine controller comprises devices for determining a calculated O2 signal from the information about the input air mass, for example, ascertained by evaluation of the signals of an air mass flow meter or by means of a model, and the fuel metering and also comprises devices for filtering and/or gradient forming and/or integrating the calculated O2 signal and an O2 signal measured by the exhaust probe, wherein for the purpose of a dynamic diagnosis a target/actual comparison between said calculated O2 signal and said O2 signal measured by the exhaust probe, or between signals derived from said signals, can be performed for a step load transition. The devices required for performing the method as, for example, low-pass filter units, differentiation units, integration units and units for calculating threshold values can thereby be implemented as a hardware or software solution within the higher-ranking engine controller and consequently form an important functional group within an on-board diagnostic device. Moreover, separate diagnostic devices are conceivable, which can communicate with the higher-ranking engine controller.
The invention is explained in detail below with the aid of an exemplary embodiment depicted in the figures. The following are shown:
The method according to the invention is disclosed using the time flow-charts 20 depicted in
In
If a step load transition occurs, a first O2 threshold value of the calculated O2 signal 32 as well as a second O2 threshold value of the calculated O2 signal 33 is determined on the basis of the calculated and filtered O2 signal 28. Parallel to this process, an O2 signal 27 measured by the exhaust gas probe 17 is converted into a measured and filtered O2 signal 29, the profile of which is likewise depicted here. From the measured and filtered O2 signal 29 and the calculated and filtered O2 signal 28, an O2 gradient signal 30, 31 is determined in each case for the calculated value and the measured value. In the case of a valid step load transition, an O2 threshold value of the measured O2 signal 34 is generated on the basis of the measured O2 signal 27. The calculation thereof is thereby identical to the calculation of the first O2 threshold value of the calculated O2 signal 32. A point in time of the threshold value calculation 25 can thereby be determined by the signal rise of the calculated O2 signal 26.
As can be seen, the profiles of the various signal values 21 in
The fundamental procedural approach for calculating the O2 threshold values 32, 33, 34 is illustrated in
Based on the signal deviation of the calculated O2 signal 38, a percentage threshold value 39 for the first O2 threshold value of the calculated O2 signal 32 is predefined. The second O2 threshold value of the calculated O2 signal 33 is correspondingly also predefined, the percentage threshold value 39 being thereby different from the first. The determination of the O2 threshold value of the measured O2 signal 34 is performed correspondingly. In so doing, the same percentage threshold value 39 is taken as a basis as was used in determining the first O2 threshold value of the calculated O2 signal 32.
The point in time of the threshold calculation 25 is predefined in the example shown from the beginning of the drop in the injected fuel quantity 35.
When performing the dynamic diagnosis, the invention provides in both figures that for the calculated O2 signal 26 during the time from reaching the first O2 threshold value of the calculated O2 signal 32 up until reaching the second O2 threshold value of the calculated O2 signal 33, the O2 gradient signal 30 for the calculated value is integrated and that a target value 42 is derived from the result of the integral formation. In addition, an integration period can be determined for the calculated O2 signal 40. Parallel to this, with regard to the measured O2 signal 27, the O2 gradient signal 31 for the measured value is integrated and an actual value 43 is derived from the result thereof. The integration period for the calculated O2 signal 40 is thereby used as the integration period for the measured O2 signal 41. A trigger time 44 is used as the starting point in time of the integration of the O2 gradient signal 31 for the measured value, said trigger time being determined if the measured O2 signal 27 exceeds the O2 threshold value of the measured O2 signal 34. The integrals calculated in this manner for the target value 42 and the actual value 43 can now be used for the quantitative dynamic diagnosis. The ratios of the target and actual values 42, 43 derived from the integrals can assume different values depending upon the sluggishness of the exhaust gas probe 17 and can be used directly as a measurement for the dynamics of said exhaust gas probe 17. The area ratio of the two areas for the target and the actual value 42, 43 in
In a modification to the method, which is not depicted, the respective filtered O2 signals 28, 29 can be evaluated as described above.
In comparison to prior art, the method according to the invention allows a dynamic diagnosis having greater selectivity to be performed, independent of the operating point. The increased legal requirements with respect to on-board diagnosis can thereby be fulfilled.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2009 028 367.6 | Aug 2009 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2010/060634 | 7/22/2010 | WO | 00 | 4/27/2012 |
