Method and device for regulating an air-fuel ratio of an internal combustion engine

Abstract

The invention relates to a method and to a regulating device for regulating an air-fuel ratio of an internal combustion engine (10), wherein an exhaust-gas composition of an exhaust gas of the internal combustion engine (10) is determined by virtue of an actual probe signal, which is dependent on the exhaust-gas composition, being detected by means of an exhaust-gas probe (22) and the exhaust-gas composition being determined as a function of the actual probe signal by means of a characteristic curve or a calculation rule, and wherein the determined exhaust-gas composition is compared with a setpoint value or a threshold value, the attainment or exceedance of which triggers a manipulation of the air-fuel ratio supplied to the internal combustion engine (10), wherein, in order to take into consideration at least one disturbance variable which affects the actual probe signal, a safety margin (S) is defined which is applied to the characteristic curve or calculation rule, to the actual probe signal or to the setpoint value or threshold value. It is provided that an evaluation of a present accuracy of the at least one disturbance variable and/or of a present influence of the at least one disturbance variable on the probe signal is performed, and the safety margin (S) owing to the at least one disturbance variable is defined as a function of the evaluation.

Claims

1. A method for regulating an air/fuel ratio of an internal combustion engine, comprising: determining a composition of an exhaust gas of the internal combustion engine by detecting an actual probe signal of the exhaust gas by means of an exhaust gas probe, and by applying a characteristic curve or a calculation rule expressing the exhaust gas composition as a function of the actual probe signal using a regulating device, wherein the determined exhaust gas composition has a lambda value, comparing the lambda value of the determined exhaust gas composition with a setpoint value or a threshold value, influencing the air/fuel ratio fed to the internal combustion engine, when the lambda value of the determined exhaust gas composition reaches or exceeds the setpoint value or the threshold value, defining a safety interval (S) to take into account at least one interference variable which effects a deviation of the lambda value of the determined exhaust gas composition from a lambda value of an exact exhaust gas composition by acting on the actual probe signal, wherein the safety interval (S) ensures that the lambda value of the exact exhaust gas composition is lower than a target lambda value, and applying the safety interval (S) to the characteristic curve, to the calculation rule or to the actual probe signal or defining the setpoint value or threshold value for the exhaust gas composition as a function of the target lambda value and the safety interval, evaluating a current accuracy level of at least one of the at least one interference variable and a current influence of the at least one interference variable on the probe signal, and defining the safety interval (S), which is conditioned by the at least one interference variable, as a function of the evaluation.

2. The method as claimed in claim 1, wherein the interference variable comprises at least one of the following: temperature of the exhaust gas probe, aging of the exhaust gas probe and chemical contamination of the exhaust gas probe.

3. The method as claimed in claim 1, wherein evaluating the current accuracy level of the at least one interference variable, comprises determining a variance of detected values of the interference variable in a preceding time period, and defining the safety interval (S) as a function of the variance.

4. The method as claimed in claim 1, wherein evaluating the current accuracy level of the at least one interference variable, comprises determining a period that has passed since a preceding calibration of a detection system for the interference variable, and defining the safety interval (S) as a function of the period.

5. The method as claimed in claim 1, wherein evaluating the current influence of the at least one interference variable on the probe signal, comprises determining an absolute magnitude of the currently detected interference variable, and defining the safety interval (S) as a function of the absolute magnitude.

6. The method as claimed in claim 1, wherein the setpoint value or threshold value is a predefined lambda value for mixture enrichment in order to protect components against overheating.

7. The method as claimed in claim 1, wherein the setpoint value comprises a predefined lambda value which is to be adjusted within the scope of a lambda control process.

8. The method as claimed in claim 1 further comprising defining the safety interval (S) as a function of an operating point of the internal combustion engine.

9. The method as claimed in claim 8 wherein the operating point of the internal combustion engine is an engine speed and/or an engine load.

10. The method as claimed in claim 1, wherein the exhaust gas probe is a lambda probe.

11. The method as claimed in claim 10, wherein the lambda probe is a step-change lambda probe.

12. A regulating device for regulating an air/fuel ratio of an internal combustion engine, the device configured to carry out the method as claimed in claim 1.

13. A method for regulating an air/fuel ratio of an internal combustion engine, comprising: determining a composition of an exhaust gas of the internal combustion engine by detecting an actual probe signal of the exhaust gas by means of an exhaust gas probe, and by applying a characteristic curve or a calculation rule expressing the exhaust gas composition as a function of the actual probe signal using a regulating device, defining a safety interval (AS) to take into account at least one interference variable acting on the actual probe signal and applying the safety interval (AS) to the characteristic curve or calculation rule, to the actual probe signal or to a setpoint value or threshold value for the exhaust gas composition, evaluating a current accuracy level of at least one of the at least one interference variable and a current influence of the at least one interference variable on the probe signal, wherein evaluating the current accuracy level of the at least one interference variable comprises determining a period that has passed since a preceding calibration of a detection system for the interference variable, defining the safety interval (AS), which is conditioned by the at least one interference variable, as a function of the evaluation and of the period, comparing the determined exhaust gas composition with the setpoint value or a threshold value, and influencing the air/fuel ratio fed to the internal combustion engine, when the determined exhaust gas composition reaches or exceeds the setpoint value or the threshold value.

14. The method as claimed in claim 13, wherein the interference variable comprises at least one of the following: temperature of the exhaust gas probe, aging of the exhaust gas probe and chemical contamination of the exhaust gas probe.

15. The method as claimed in claim 13, wherein evaluating the current accuracy level of the at least one interference variable, comprises determining a variance of detected values of the interference variable in a preceding time period, and defining the safety interval (S) as a function of the variance.

16. The method as claimed in claim 13, wherein evaluating the current influence of the at least one interference variable on the probe signal, comprises determining an absolute magnitude of the currently detected interference variable, and defining the safety interval (S) as a function of the absolute magnitude.

17. The method as claimed in claim 13, wherein the setpoint value or threshold value is a predefined lambda value for mixture enrichment in order to protect components against overheating.

18. The method as claimed in claim 13, wherein the setpoint value comprises a predefined lambda value which is to be adjusted within the scope of a lambda control process.

19. The method as claimed in claim 13 further comprising defining the safety interval (S) as a function of an operating point of the internal combustion engine.

20. The method as claimed in claim 13, wherein the exhaust gas probe is a lambda probe.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in more detail below in exemplary embodiments and with reference to the associated drawings, of which:

(2) FIG. 1 shows an internal combustion engine having a regulating device according to the present invention,

(3) FIG. 2 shows a flowchart of a method sequence for carrying out mixture enrichment in order to protect components against overheating, and

(4) FIG. 3 shows characteristic curves of a step-change lambda probe for various temperatures.

DETAILED DESCRIPTION OF THE INVENTION

(5) FIG. 1 shows an internal combustion engine 10 whose fuel supply is provided via a fuel injection system 12. The injection system 12 can be an intake manifold injection system or a cylinder direct injection system. The internal combustion engine 10 is also supplied with combustion air via an intake manifold 14. If appropriate the quantity of air which is fed in can be regulated by means of a controllable actuating element 16, for example of a throttle valve, which is arranged in the intake manifold 14.

(6) Exhaust gas which is generated by the internal combustion engine 10 is discharged into the surroundings via an exhaust gas duct 18, wherein exhaust gas components which are relevant environmentally are converted by a catalytic converter 20.

(7) Arranged within the exhaust gas duct 18 at a position near to the engine is an exhaust gas probe 22, which is, in particular, a lambda probe, typically a step-change lambda probe. If appropriate, a further exhaust gas probe 24 can be arranged downstream of the catalytic converter 20, which further exhaust gas probe 24 can also be a lambda probe, in particular a broadband lambda probe, or an NO.sub.x sensor. The signals of the exhaust gas probes 22 and 24 are transmitted to an engine controller 26. Further signals of sensors which are not illustrated are also input into the engine controller 26. The engine controller 26 actuates various components of the internal combustion engine 10 in a known fashion as a function of the signals which are input. In particular, the air/fuel mixture which is to be fed into the internal combustion engine is regulated as a function of the probe signal U.sub.act (probe voltage) of the lambda probe 22 which is near to the engine, for which purpose the engine controller 26 regulates a quantity of fuel which is to be fed in via the fuel injection system 12 and/or a quantity of air which is to be fed in via the intake system 14. The engine controller 26 comprises a regulating device 28 which is configured to carry out the method according to the invention in order to regulate the air/fuel ratio of the internal combustion engine 10. For this purpose, the regulating device 28 contains a corresponding algorithm in a computer-readable form as well as suitable characteristic curves and characteristic diagrams.

(8) The present method will be explained below using the example of the regulation of the engine in order to protect components against overheating, with reference to FIG. 2.

(9) The method illustrated in FIG. 2 assumes a state in which the temperature T.sub.M (see FIG. 1) of a component, for example of inlet valves or outlet valves of the engine 10 or of an exhaust gas turbocharger or of the catalytic converter 20, exceeds a permissible temperature and therefore the execution of mixture enrichment for the purpose of protecting components is required.

(10) The method starts in step 100 where, for the purpose of detecting the temperature of the lambda probe 22, the internal resistance R.sub.i of the measuring element of the probe 22 is input. In the subsequent step 102, the sensor temperature T.sub.S of the probe 22 is determined as a function of the internal resistance R.sub.i. For this purpose, it is possible to have recourse for instance to a characteristic curve which maps the probe temperature T.sub.S as a function of the internal resistance R.sub.i. Such a method for determining the probe temperature is known, for example, from DE 100 36 129 A1. However, within the scope of the present invention, it is, of course, also possible to use other methods for determining the probe temperature.

(11) In a parallel (or subsequent) method strand, the probe signal U.sub.act, dependent on the exhaust gas composition, of the lambda probe 22 is input. Subsequently, in step 112 the exhaust gas composition, in particular the actual lambda value .sub.act, is determined as a function of the probe signal U.sub.act and of the probe temperature T.sub.S determined in step 102. For this purpose, it is possible to have recourse to a stored characteristic diagram which maps the lambda value .sub.act as a function of the probe signal U.sub.act and of the probe temperature T.sub.S. FIG. 3 shows by way of example such a characteristic diagram in which the characteristic curves of the step-change lambda probe are represented for three different probe temperatures T.sub.S. It is apparent that, in particular, for rich lambda values .sub.act<1, the probe voltage U.sub.act depends strongly on the temperature.

(12) In a step 104, which is subsequent to step 102, according to the invention, a current accuracy level of the interference variable comprising the probe temperature T.sub.S or of a current influence of this interference variable on the probe signal U.sub.act is evaluated. For example, at this point the variance of the measured resistance value R.sub.i or of the probe temperature T.sub.S which is derived therefrom can be determined in a predetermined preceding time period. Further refinements of the evaluation taking place in step 104 have already been explained above. In a subsequent step 106, the safety interval S is determined as a function of the variance T.sub.S, determined in step 104, of the probe temperature, wherein a larger value is selected for the safety interval S the larger the variance T.sub.S of the probe temperature. Here, for example a linear relationship can be used.

(13) The method then goes to step 108, where a setpoint value for the predefined lambda value .sub.setp for the mixture enrichment for the purpose of protecting components is defined. In particular, in step 108 the previously determined safety interval S is subtracted from the predefined lambda target value .sub.target which is to be complied with for protection of the components. If the predefined target value .sub.target for protecting components is, for example, 0.9 and if a safety interval S of 0.02 was determined in step 106, a predefined lambda setpoint value .sub.setp of 0.88 results. In contrast to the embodiment described above, the lambda deviation S can, of course, also be a factor which is multiplied by the predefined lambda target value.

(14) In the now subsequent steps 114 to 120, the air/fuel mixture which is to be fed into the internal combustion engine 10 is regulated in accordance with the predefined lambda setpoint value .sub.setp which is determined in step 108, as is generally known in the prior art. For this purpose, in step 114 an interrogation occurs in which the actual lambda value .sub.act which was determined in step 112 is compared with the setpoint lambda value .sub.setp which was determined in step 108. In particular, in step 114 it is possible to check whether the difference .sub.act.sub.setp>0. If this interrogation receives a positive response, i.e. the current lambda value is larger (more lean) than desired, the method goes to step 116 where a quantity of fuel m.sub.KS which is fed into the internal combustion engine 10 is increased by a predetermined increment of the quantity of fuel KS in order to bring about enrichment of the air/fuel mixture. On the other hand, if the interrogation in step 114 receives a negative response, that is to say the actual lambda value .sub.act is smaller (richer) than the setpoint lambda value .sub.setp, the method goes to step 118, where the quantity of fuel m.sub.KS is reduced by a corresponding increment KS in order to bring about adjustment of the engine in the lean direction. In step 120, the fuel is fed into the internal combustion engine 10 in accordance with the quantity of fuel m.sub.KS which was determined in step 116 or 118.

(15) The method then goes back to step 110 in order to detect the probe signal U.sub.act again, to determine the actual lambda value .sub.act as a function of the probe signal U.sub.act in step 112, and to compare the actual lambda value .sub.act again with the predefined setpoint value .sub.setp in step 114. This cycle is repeated during the entire measure for protecting components until the component temperature T.sub.M has reached a permissible value. The interrogation cycle for checking the component temperature T.sub.M is not illustrated in FIG. 2.

(16) In this context it is possible, but not necessary, that the steps 104 to 108 are carried out at every pass, since a change in the safety interval S and therefore in the setpoint lambda value .sub.setp does not usually change in the short term. In contrast, it is appropriate to carry out the steps 100 and 102 for determining the probe temperature T.sub.S in every interrogation cycle, particularly in the case of mixture enrichment in order to protect components, since here it is to be expected that the temperature of the sensor will also drop.

(17) In the method sequence illustrated in FIG. 2, the safety interval S is applied to the target lambda value .sub.target in order to define in this way the setpoint lambda value .sub.setp of the lambda control process. However, it is, of course, possible, in contrast with this example, to apply a corresponding safety interval S also to the characteristic curve applied in step 112 in order to adapt it in such a way that lambda variation which reflects the uncertainty of the determination of the temperature is taken into account. Alternatively, it is also possible to determine the actual lambda value .sub.act, determined in step 112, in the way described above and to apply the safety interval S to the actual lambda value .sub.act determined in this way. All of these variants are to be considered as equivalent.

(18) In one preferred refinement of the invention, the safety interval S is additionally made dependent on the absolute value assumed by the current setpoint lambda value of the engine. In this way it is possible to take into account the fact that many interference variables acquire influence in certain ranges. For example, the probe temperature T.sub.S influences the characteristic curve characteristics significantly more in the case of rich lambda values than in the case of lean ones (see FIG. 3). As a result, the function, applied in step 106 in FIG. 2, for determining the safety interval S can take into account the current lambda value in such a way that the safety interval S is made larger as the lambda values become smaller.

(19) Whereas it was illustrated with respect to FIG. 2 how the probe temperature T.sub.S is taken into account as an interference variable in the detection of lambda, this can alternatively or additionally also take place for the interference variable of the aging of the lambda probe 22. For this purpose, the aging of the lambda probe 22 is detected, for example, by means of the lambda probe 24 which is connected downstream (see FIG. 1) and which functions here as a reference probe. In particular, a deviation from the mean mixture value can be determined by means of the signal of the broadband lambda probe 24, and the characteristic curve of the lambda probe 22 can be correspondingly corrected. Corresponding methods for taking into account such aging effects and for correcting the characteristic curve are known in the prior art. Other methods for determining an aging correction value can also be applied within the scope of the present invention.

(20) According to the invention, an evaluation is now performed, on the basis of the aging correction value determined in this way, as to which inaccuracies can result during the conversion of the probe signal U.sub.act into the actual lambda value .sub.act aging of the exhaust gas probe despite the characteristic curve correction. If the probe 22 is, for example, not yet aged at all and if the conversion rule or characteristic curve which is used in step 112 is correctly stored, there will be virtually no deviation of the actual value, determined in step 112, from an actual lambda value. As a result, there is no need for correction of the target lambda value .sub.target which is to be adjusted for protection of the components. As a result, the safety interval S in step 106 can, in an extreme case, be set equal to zero.

(21) In contrast, in the case of aging of the lambda probe 22 and associated correction of the probe characteristic curve an aging correction value will occur. On the basis of the magnitude of this correction value, the invention now evaluates, for example, what tolerance can still remain in the determined actual lambda value despite characteristic curve correction. As a function of this, the safety interval S, that is to say the enrichment which is additionally necessary for protection of the components, is defined.

(22) In a further refinement, influences which cannot be explicitly quantified with evaluation variables but which can nevertheless have a disruptive influence on the determination of lambda are taken into account. In particular, the influence of the operating point of the internal combustion engine 10 can be evaluated here, for example by determining an additional safety interval as a function of the operating point from a rotational speed/load characteristic diagram.

(23) The particular advantage of the method according to the invention can therefore be considered that a saving in fuel is achieved for the statistical majority of probes which have no aging, or only a small degree of aging, and for the majority of operating conditions under which the influence of signal-falsifying interference variables is low. This is achieved in that the full theoretically possible tolerance range of the measurement error is not taken into account in a global fashion, but instead the tolerance range which is actually necessary is always taken into account.

LIST OF REFERENCE NUMERALS

(24) 10 Internal combustion engine 12 Fuel injection system 14 Intake system 16 Actuating element 18 Exhaust gas duct 20 Catalytic converter 22 Exhaust gas probe/lambda probe 24 Exhaust gas probe 26 Engine controller 28 Regulating device