EP2786003B1 - Method and apparatus for controlling an air fuel ratio of an internal combustion engine - Google Patents

Description

The invention relates to a method for controlling an air-fuel ratio of an internal combustion engine as a function of a composition of its exhaust gas and a correspondingly configured control device.

It is known to control internal combustion engines depending on a composition of their exhaust gases or regulate, wherein the corresponding exhaust gas component is measured by means of a suitable exhaust gas probe. In particular, the air-fuel ratio with which the engine is operated, regulated by the oxygen content of the exhaust gas is measured by means of a lambda probe in the exhaust system. This procedure is generally referred to as lambda control. In this case, the lambda probe provides an actual probe signal which is dependent on the oxygen content of the exhaust gas and which is usually a probe voltage. This probe signal is converted by means of a stored characteristic or a corresponding calculation rule into the lambda value and this is used for the control.

However, a conversion of the probe signal into a lambda value is made more difficult in practice by the fact that the probe signal not only depends on the exhaust gas composition but is also influenced by additional disturbing influences which cause the characteristic curve not to be constant under all conditions. In the case of jump probes, for example, it is known that the probe temperature, that is to say the temperature of the measuring element of the probe, has an influence on the accuracy of the conversion rule or the characteristic curve. This has an effect especially in the rich lambda range, ie at lambda values <1. Furthermore, changes in the characteristic curve resulting from increasing aging of the measuring element of the probe over the operating time. In addition, various exhaust gas components such as lead, manganese, phosphorus or zinc can cause progressive poisoning of the measuring element and thus to a change in the characteristic curve.

Out DE 100 36 129 A it is known to compensate for influences of temperature on the probe signal. For this purpose, the probe temperature is determined as a function of the internal resistance of the probe from a stored characteristic curve. Using a three-dimensional Characteristic map, which maps a correction voltage in dependence on a current probe actual voltage and the previously determined probe temperature, is then used to determine the actual correction voltage, which is added to the actual probe actual voltage, in order to obtain a corrected probe voltage.

DE 199 19 427 A describes a method for correcting a characteristic curve of a broadband lambda probe which is installed upstream of an exhaust gas catalytic converter, wherein in a fuel cut-off phase of the internal combustion engine the sensor signal of the lambda probe is evaluated and the signal level thus determined is used for the correction of the slope of the characteristic curve.

Out DE 10 2007 015 362 A For example, a method for calibrating a jump lambda probe arranged upstream of a catalytic converter is known. For this purpose, a correction signal is determined from a measurement signal provided by a downstream reference lambda probe and used for the characteristic adaptation of the jump lambda probe.

A disadvantage of all known methods is that even the corrections have only a limited accuracy and therefore deviations of the corrected characteristic of the exact characteristic can remain. This circumstance is taken into account in the prior art in that lambda desired values or lambda threshold values to be regulated, the achievement of which trigger a change in the air-fuel mixture, are defined with a safety margin taking into account the uncertainty. This safety distance is usually dimensioned so that the largest assumed inaccuracy of the characteristic is taken into account.

A typical example of this procedure is the enrichment of a motor, which is made to protect components from overheating. In this case, the combustion and thus the exhaust gas temperature is lowered by additional addition of fuel and thus prevents overheating example of turbochargers or catalysts. The Gemischanfettung for component protection usually takes place when reaching a permissible limit temperature, for example, of 900 ° C, with a target lambda value of, for example, 0.9 is set by additional fuel addition, which ensures an effective cooling effect. If, for example, a maximum tolerance band of 2% is calculated for the lambda probe used, the lambda threshold of 0.88 is conventionally set for the engine in order to remain safely below the necessary limit of lambda 0.9 under all conditions. However, this results in the majority of the engines that have a Lambda probe with lower tolerance deviation, a higher enrichment and thus a higher fuel consumption than would actually be needed.

The object of the present invention is therefore to provide a method and a device for regulating an air-fuel ratio of an internal combustion engine and in which the safety distance to be maintained by threshold values for the exhaust gas composition, in particular lambda threshold values, is determined according to actual requirements and thus the fuel consumption is reduced.

This object is achieved by a method for controlling an air-fuel ratio of an internal combustion engine and by a corresponding control device having the features of the independent claims.

• Bestimmung der Abgaszusammensetzung, indem mittels einer Abgassonde ein von der Abgaszusammensetzung abhängiges Ist-Sondensignal erfasst wird und mittels einer Kennlinie oder einer Rechenvorschrift die Abgaszusammensetzung in Abhängigkeit von dem Ist-Sondensignal bestimmt wird,
• Vergleich der ermittelten Abgaszusammensetzung mit einem Soll- oder Schwellenwert, wobei zur Berücksichtigung zumindest einer Störgröße auf das Ist-Sondensignal ein Sicherheitsabstand festgelegt wird, der auf die Kennlinie oder Rechenvorschrift, auf das Ist-Sondensignal oder auf den Soll- oder Schwellenwert angewendet wird, und
• Auslösen einer Beeinflussung des dem Verbrennungsmotor zugeführten Luft-Kraftstoff-Verhältnisses, wenn die ermittelte Abgaszusammensetzung den Soll- oder Schwellenwert erreicht.

The method according to the invention comprises the steps:

• Determination of the exhaust gas composition by detecting an exhaust gas composition-dependent actual probe signal by means of an exhaust gas probe and determining the exhaust gas composition as a function of the actual probe signal by means of a characteristic curve or a calculation rule;
• Comparison of the determined exhaust gas composition with a setpoint or threshold value, wherein a safety distance is set to take into account at least one disturbance on the actual probe signal, which is applied to the characteristic or calculation rule, to the actual probe signal or to the setpoint or threshold, and
• Triggering an influence on the air-fuel ratio supplied to the internal combustion engine if the determined exhaust gas composition reaches the setpoint or threshold value.

According to the invention, an assessment of a current accuracy of the at least one disturbance variable and / or an actual influence of the at least one disturbance variable on the probe signal is undertaken and the safety distance caused by the at least one disturbance variable is established as a function of the result of the evaluation.

While in the prior art, the safety margin is thus always set constant and in the amount of its highest possible value in the sense of a worst-case scenario, according to the invention, its variable definition. This makes it possible to set the safety margin as minimal as the present situation allows, to the target value actually to be met (for example, the target or threshold value). The method thus not only allows a higher accuracy of the regulation of a desired value, but also a fuel economy. Preferably, the disturbance value evaluated in the context of the invention comprises a temperature of the exhaust gas probe and / or an aging of the exhaust gas probe and / or a chemical poisoning of the exhaust gas probe. The influence of these disturbances on the probe signal, in particular of lambda probes, is known in the prior art. As already described above, according to the invention, however, it is not assumed that their greatest possible uncertainty or their greatest possible influence on the probe signal for these disturbances, but this / this is currently evaluated.

According to a preferred embodiment of the invention, a spread is determined for the evaluation of the current accuracy of the at least one disturbance, within which values of this disturbance lie, which were detected in a past period. The safety distance is then determined as a function of the spread, it being understood that the safety level is chosen the greater, the greater the spread. For example, if the disturbance is the temperature of the probe, then for a predetermined past period of time it is determined what variance the sensed temperature values had from the true value. If only a small variance of the determined temperature has been found in the past, the safety distance can be set correspondingly small.

According to a further advantageous embodiment of the invention, a duration is determined for the assessment of the current accuracy of the at least one disturbance, which has elapsed since a past calibration of a detection system of this disturbance. The safety distance is then determined as a function of the duration determined in this way, the safety distance being chosen to be greater with increasing duration, since an increasingly imprecise disturbance variable detection can be assumed. Using the example probe temperature as a disturbance variable, this means that it is checked how long the last calibration of the temperature measurement took place. If the temperature detection takes place, for example, via the internal resistance of the measuring element of the probe according to FIG DE 100 36 129 A , it is determined when the last calibration of the internal temperature resistance characteristic curve has taken place. The longer the calibration, the greater the safety margin.

In this context, it can also be checked whether in the past a need for a calibration was found, but this has not been done yet. In such a case, a high uncertainty of the currently determined disturbance is assumed and set the safety distance correspondingly large.

According to a further advantageous embodiment of the method, an absolute height of the currently detected disturbance variable is determined for the evaluation of the current influence of the at least one disturbance variable on the probe signal, and the safety distance is established as a function of the absolute altitude. For example, if the absolute value of the internal resistance of the measuring element of the probe in a range in which a temperature determination can be very inaccurate, for example at resistance values close to zero, is assumed by a relatively high error of the temperature determination and set a correspondingly high safety margin.

According to a further embodiment of the method, the safety distance is also determined depending on an operating point of the internal combustion engine, in particular as a function of an engine speed and / or an engine load. For this purpose, a map can be used which represents the safety distance as a function of the speed and / or the load. In this way influences can be taken into account which can not be quantified in the evaluation.

The method can be used particularly advantageously in connection with the performance of a mixture enrichment for component protection of the internal combustion engine and / or the exhaust system against overheating. In this case, the lambda input provided for the mixture enrichment is preferably determined according to the method. In this context, the method makes it possible to set the lambda input for component protection as lean as possible, that is to say with the smallest possible safety distance to the target value, thereby minimizing the additional fuel consumption required for component protection.

Furthermore, the method according to the invention can advantageously also be used within the scope of the lambda control of the internal combustion engine, wherein the lambda desired value to be adjusted is determined in the manner according to the invention. Here, the invention enables a particularly precise lambda control.

The invention further relates to a control device for controlling an air-fuel ratio of an internal combustion engine, which is set up to carry out the method according to the invention as described above.

Further advantageous embodiments of the invention are the subject of the remaining dependent claims.

Figur 1

eine Verbrennungskraftmaschine mit einer Regelvorrichtung gemäß der vorliegenden Erfindung,

Figur 2

ein Fließdiagramm eines Verfahrensablaufs zur Durchführung einer Gemischanfettung zum Bauteileschutz vor Überhitzung und

Figur 3

Kennlinien einer Sprung-Lambdasonde für verschiedene Temperaturen.

The invention will be explained in more detail in embodiments with reference to the accompanying drawings. Show it:

FIG. 1

an internal combustion engine with a control device according to the present invention,

FIG. 2

a flow diagram of a procedure for performing a mixture enrichment for component protection against overheating and

FIG. 3

Characteristics of a jump lambda probe for different temperatures.

FIG. 1 shows an internal combustion engine 10, the fuel supply via a fuel injection system 12 takes place. In the injection system 12 may be a port injection or a cylinder direct injection. The internal combustion engine 10 is also supplied via an intake manifold 14 with combustion air. Optionally, the amount of air supplied via a arranged in the intake manifold 14 controllable actuator 16, such as a throttle valve, are regulated.

An exhaust gas generated by the internal combustion engine 10 is released into the environment via an exhaust passage 18, whereby environmentally relevant exhaust gas constituents are converted by a catalyst 20.

Within the exhaust duct 18, an exhaust gas probe 22 is arranged at a position close to the engine, which is in particular a lambda probe, typically a jump lambda probe. If appropriate, a further exhaust gas probe 24 may be arranged downstream of the catalytic converter 20, which may likewise be a lambda probe, in particular a broadband lambda probe, or an NO _x sensor. The signals of the exhaust probes 22 and 24 are transmitted to a motor controller 26. Other signals not shown sensors also go into the engine control 26. The engine controller 26 controls in dependence on the incoming signals in a known manner to various components of the internal combustion engine 10. In particular, as a function of the probe signal U _actual (probe voltage) of the lambda probe 22 close to the engine, a control of the air-fuel mixture to be supplied to the internal combustion engine is effected, for which the engine control 26 regulates a fuel quantity to be supplied via the fuel injection system 12 and / or an air quantity to be supplied via the intake system 14 , The engine controller 26 includes a control device 28, which for carrying out the method according to the invention for controlling the Air-fuel ratio of the internal combustion engine 10 is set up. For this purpose, the control device 28 includes a corresponding algorithm in computer-readable form and suitable characteristics and maps.

The present method is described below using the example of the motor control to perform the component protection against overheating based on FIG. 2 explained.

This in FIG. 2 The illustrated method starts from a state in which the temperature T _M (see FIG. 1 ) of a component, for example of intake or exhaust valves of the engine 10 or an exhaust gas turbocharger or the catalyst 20, exceeds a permissible temperature, and thus the implementation of a Gemischanfettung for the purpose of component protection is required.

The method starts in step 100, where, for the purpose of detecting the temperature of the lambda probe 22, the internal resistance R _{i of} the measuring element of the probe 22 is read in. In the subsequent step 102, the sensor temperature T _{S of} the probe 22 is determined as a function of the internal resistance R _i . For this purpose, it is possible, for example, to resort to a characteristic curve which maps the probe temperature T _S as a function of the internal resistance R _i . Such a method for determining the probe temperature is for example off DE 100 36 129 A1 known. Of course, however, other methods for determining the probe temperature can be used in the context of the present invention.

In a parallel (or subsequent) process train, the exhaust gas composition-dependent probe signal U _{actual of} the lambda probe 22 is read. Subsequently, in step 112, the determination of the exhaust gas composition, in particular of the actual lambda value λ _actual , in dependence on the probe signal U _actual as well as the probe temperature T _S determined in step 102. For this purpose, it is possible to resort to a stored characteristic map which maps the lambda value λ _actual as a function of the probe signal U _actual and the probe temperature T _S. FIG. 3 shows an example of such a map in which the characteristics of the jump lambda probe for three different probe temperatures T _S are shown. It can be seen that, in particular for fat lambda values λ _actual <1, the probe voltage U _actual depends strongly on the temperature.

In a step 104 following step 102, according to the invention, an assessment is made of a current accuracy of the disturbing probe temperature ΔT _S or an actual influence of this disturbance variable on the probe signal U _actual . For example, on this location, the spread of the measured resistance value δR _i or the derived probe temperature δT _{S are determined} in a predetermined past period. Further embodiments of the evaluation carried out in step 104 have already been explained above. In dependence on the scattering range ΔT _{S of} the probe temperature determined in step 104, the safety distance ΔS is determined in a subsequent step 106, the safety distance ΔS being selected to be greater, the greater the spread ΔT _{S of} the probe temperature. Here, for example, a linear relationship can be used.

The method then proceeds to step 108, where a setpoint for the lambda preset λ _setpoint for the mixture enrichment for the purpose of component protection is determined. In particular, in step 108, the previously determined safety distance .DELTA.S is deducted from the lambda _target specification .lambda.- _{target to be} maintained for the component protection. If the target λ _target for component protection is, for example, 0.9 and if a safety distance ΔS of 0.02 has been determined in step 106, the lambda _setpoint λ _{setpoint is} 0.88. Notwithstanding the embodiment described above, it is understood that the lambda deviation .DELTA.S can also be a factor which is multiplied by the lambda target.

In the subsequent steps 114 to 120, a control of the air-fuel mixture to be supplied to the internal combustion engine 10 takes place in accordance with the lambda _desired preset λ _set determined in step 108, as is generally known in the prior art. For this purpose, a query takes place in step 114, in which the actual lambda value λ _actual determined in step 112 _is compared with the desired lambda value λ _Soll determined in step 108. In particular, it can be checked in step 114 whether the difference λ _actual -λ _setpoint > 0. If this query is affirmative, ie the actual lambda value is greater (leaner) than desired, the method proceeds to step 116, where an amount of fuel m _KS supplied to the internal combustion engine 10 is increased by a predetermined increment of the fuel quantity ΔKS in order to enrich the air To achieve fuel mixture. Otherwise, if the query is denied in step 114, that is, the actual lambda value λ _actual is smaller (richer) than the desired lambda value λ _setpoint , the method proceeds to step 118, where the fuel quantity m _{KS is} decreased by a corresponding increment ΔKS in order to achieve a leaning of the engine. In step 120, the supply of the fuel to the internal combustion engine 10 takes place in accordance with the fuel quantity m _KS ascertained in step 116 or 118.

The method then returns to step 110 in order to detect the probe signal U _actual again, in step 112 to determine the actual lambda value λ _actual as a function of the probe signal U _actual and in step 114, the actual lambda value λ _actual again to be compared with the target specification λ _target . This cycle is repeated during the entire component protection measure until the component temperature T _{M has reached} a permissible value. The query cycle for checking the component temperature T _M is in FIG. 2 not shown.

In this case, it is possible, but not necessary, for steps 104 to 108 to be carried out during each passage, since a change in the safety distance ΔS and thus of the desired lambda value λ _nominal does not usually change in the short term. By contrast, the implementation of steps 100 and 102 for determining the probe temperature T _S in each interrogation cycle, especially in the case of the mixture enrichment for component protection makes sense, since a sinking temperature is also to be expected from the sensor.

In the in FIG. 2 the process sequence shown, the safety distance .DELTA.S is applied to the target lambda value λ _target , so as to set the desired lambda value λ _target for the lambda control. However, it is understood that, in deviation from this example, a corresponding safety distance ΔS can also be applied to the characteristic curve used in step 112 in order to adapt it to take account of a lambda scattering which reflects the uncertainty of the temperature determination. Alternatively, it is also conceivable to determine the actual lambda value λ _actual determined in step 112 as described above and to apply the safety distance ΔS to the actual lambda value λ _actual thus determined. All of these variants are to be regarded as equivalent.

In a preferred embodiment of the invention, the safety distance .DELTA.S is additionally made dependent on which absolute value assumes the current desired lambda value of the engine. This can take into account that some disturbances gain influence in certain areas. For example, the probe temperature T _{S influences} the characteristic curve significantly more with rich lambda values than with lean lambda values (see FIG FIG. 3 ). Thus, in step 106 in FIG FIG. 2 The function used to determine the safety distance ΔS takes into account the current lambda value in such a way that as the lambda values decrease, the safety distance ΔS is increased.

While based on the FIG. 2 the consideration of the probe temperature TS was shown as a disturbance in the lambda detection, this can alternatively or additionally also for the disturbance variable of the aging of the lambda probe 22. For this purpose, the aging of the lambda probe 22, for example by means of the downstream lambda probe 24 (see FIG. 1 ), which acts as a reference probe here. In particular, a deviation from the average mixture value can be determined via the signal of the Breibandlambdasonde 24 and the characteristic curve of the lambda probe 22 are corrected accordingly. Corresponding methods for taking account of such aging effects and for correcting the characteristic are known in the prior art. Other methods for determining an aging correction value may also be used within the scope of the present invention.

According to the present invention, the inaccuracies of the aging of the exhaust gas probe, in spite of the characteristic curve correction in the conversion of the probe signal U _actual into the actual lambda value λ _actual , can now be evaluated on the basis of the thus determined aging correction value. If, for example, the probe 22 has not yet aged and the conversion rule or characteristic curve used in step 112 is stored correctly, there will be virtually no deviation of the actual value determined in step 112 from an actual lambda value. Thus, there is no need to correct the target lambda value λ _{target to be set} for the component protection. Thus, the safety distance .DELTA.S can be set equal to zero in step 106 in extreme cases.

In contrast, an aging correction value will result on aging of the lambda probe 22 and concomitant correction of the probe characteristic. From the extent of this correction value is now inventively evaluated, for example, which tolerance in the determined actual lambda value despite characteristic correction can still remain. Depending on this, the safety distance .DELTA.S, that is the additionally necessary enrichment for the component protection is determined.

In a further embodiment, influences which can not be quantified explicitly with evaluation variables but nevertheless can disturb the lambda determination 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 distance operating point-dependent from a speed-load characteristic map.

The particular advantage of the method according to the invention can therefore be seen in the fact that fuel savings are achieved for the statistical majority of the probes which have little or no aging and for the majority of the operating conditions in which the influence of signal-distorting disturbance variables is small. This is achieved by not considering the full theoretically possible tolerance band of the measuring error, but always the one that is needed.

LIST OF REFERENCE NUMBERS

10

internal combustion engine

12

Fuel injection system

14

Intake

16

actuator

18

exhaust duct

20

catalyst

22

Exhaust gas sensor / oxygen sensor

24

gas probe

26

motor control

28

control device

Claims (7)

1. A method for regulating an air-fuel ratio of an internal combustion engine (10), comprising the steps:

   Detection (110) of an actual probe signal (U_Ist) of an exhaust gas probe (22) dependent on the exhaust gas composition of the exhaust gas of an internal combustion engine (10),

   determination (112) of the exhaust gas composition (λ_Ist) as a function the actual probe signal (U_Ist) by means of a characteristic curve or a calculation rule,

   comparison (114) of the determined exhaust gas composition (λ_Ist) with a target value (A_Soll) or threshold value,

   manipulation (116, 118, 120) of the air-fuel ratio supplied to the internal combustion engine (10), if the determined exhaust gas composition (λ_Ist) attains or exceeds the target value (λ_Soll) or threshold value,

   wherein a safety margin (ΔS), which takes into account an impact of at least one disturbance variable on the actual probe signal (U_Ist), is defined (106), and

   the safety margin (ΔS) is applied (108) to the characteristic curve, the calculation rule, the actual probe signal (U_Ist), the target value (λ_Soll) or threshold value,

   characterized in that

   an evaluation (104) of the current accuracy of the at least one disturbance variable and/or a current impact of the at least one disturbance variable on the probe signal (U_Ist) is performed and

   the safety margin (ΔS) owing to the at least one disturbance variable as a function of the evaluation is variably defined (106), wherein

   for the evaluation of the current accuracy of the at least one disturbance variable a spreading width of detected values of said disturbance variable in a past period of time is determined and the safety margin (ΔS) is defined as a function of the spreading width, or

   for the evaluation of the current accuracy of the at least one disturbance variable a duration is determined, which has elapsed since a past calibration of a detection system of said disturbance variable, and the safety margin (ΔS) is defined as a function of the duration, or

   for the evaluation of the current impact of the at least one disturbance variable on the probe signal an absolute height of the currently detected disturbance variable is determined and the safety margin (ΔS) is defined as a function of the absolute height.
2. The method according to Claim 1, characterized in that the disturbance variable comprises at least one of the following: Temperature of the exhaust gas probe (22), aging of the exhaust gas probe (22) and a chemical poisoning of the exhaust gas probe (22).
3. The method according to any one of the preceding claims, characterized in that the target value or threshold value is a lambda specification for a mixture enrichment for component protection against overheating.
4. The method according to any one of the preceding claims, characterized in that the target value comprises a lambda specification to be regulated as part of a lambda regulation.
5. The method according to any one of the preceding claims, characterized in that the safety margin (ΔS) is further defined as a function of an operating point of the internal combustion engine (10), in particular as a function of an engine speed and/or an engine load.
6. The method according to any one of the preceding claims, characterized in that the exhaust gas probe (22) is a lambda probe, in particular a jump lambda probe.
7. A regulating device (28) for regulating an air-fuel ratio of an internal combustion engine (10), which is configured for implementing the method according to any one of Claims 1 to 6.

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