EP0481975B1 - Process and device for lambda value control - Google Patents

Abstract

In a process for lambda value control, not only is the actual lambda control value measured by means of a lambda sensor responsive to abrupt charges, in order to determine control deviations, but an averaged lambda value is used as an actual lambda measurement value and compared with a target lambda value. The target lambda value is predetermined on the basis of various operating parameters so that under all operating conditions it is equal to to the lambda value which ensures optimal conversion of all pollutants. When the actual lambda value deviates from the predetermined value, at least one control parameter for the two-point control is modified so as to obtain the desired value. This process and a device for implementing it thereby enable the characteristics of a two-point control to be adjusted under all operating conditions so as to obtain the desired average lambda value for optimal conversion of pollutants.

Description

The invention relates to a method and a device for setting the lambda value for the air / fuel mixture to be supplied to an internal combustion engine.

State of the art

The lambda values of a fuel mixture are regulated in order to set optimal conversion conditions for a catalytic converter which is arranged in the exhaust gas duct of an internal combustion engine. The conversion takes place only in a narrow range of lambda values. Where the center of the range is best depends on the operating status. This is because the different pollutants, i.e. carbon monoxide, hydrocarbons and nitrogen oxides, occur in different concentrations in different operating states and because the usual catalysts convert these pollutants best into non-harmful gases at different lambda values. For example, nitrogen oxides are optimally converted at lambda values that are richer than the stoichiometric value, while carbon monoxide and hydrocarbons are better converted in the lean range. Since the main focus is on the removal of nitrogen oxides, catalysts are mainly operated in the slightly rich range.

The concentration of carbon monoxide is based essentially on inhomogeneous mixture distribution and on fluctuations in the mixture composition from cycle to cycle. The effects mentioned also influence the emission of hydrocarbons, which moreover depends strongly on the combustion temperature, and increases with decreasing combustion temperature. In contrast, the emission of nitrogen oxides decreases with decreasing combustion temperature. The mixture distribution and fluctuations thereof as well as the respective combustion temperature depend on the speed and the load. The different pollutant composition in different operating states therefore requires different lambda values to be set in the different operating states.

Different lambda values can be set by changing at least one control parameter of the means used for the two-point control. This measure is described in DE 25 45 759 A1 (US-4,210,106). In practical applications, e.g. B. an extended integration time in the bold direction stored in a characteristic or a map addressable via values of operating variables.

DE-A1-32 24 347 discloses a "device for reducing exhaust gas pollutant components of an internal combustion engine", each with an O₂ probe before and after the catalyst. The downstream probe serves as a guide probe in order to readjust an average lambda value in the event of deviations from the optimal composition of the air-fuel ratio. This can be achieved by modifying the control parameters.

It has been found that with the measure described it is not always possible in practice to set precisely the mean lambda values which lead to an optimized conversion rate for the various pollutants for a given operating state.

The invention is based on the object of specifying a method for regulating the lambda value with which the desired average lambda value can be set with great accuracy for all operating states. The invention is also based on the object of specifying a device for carrying out such a method.

Advantages of the invention

The method according to the invention is given by the features of claim 1 and the device according to the invention by the features of claim 5. Advantageous further developments and refinements are the subject of the dependent claims.

The method according to the invention is characterized in that it not only determines the current actual lambda value with the aid of whose two-point control takes place, but also uses the mean lambda value as the actual lambda measurement value, which is used with a predetermined lambda measurement setpoint to form a measurement deviation it is compared on the basis of which measurement deviation at least one control parameter is changed in such a way that an actual lambda measurement value should occur which reduces the measurement deviation mentioned. A control parameter is therefore no longer only determined as a function of values of operating variables in the respective operating state in order to achieve a certain average lambda value at which the catalytic converter converts optimally, but it is additionally monitored whether the desired value is actually achieved and if not , the specified control parameter is changed in such a way that the actual lambda measurement value actually desired for optimal conversion should occur.

The actual lambda measurement value, which is the mean lambda value, can either be determined by averaging the oscillating lambda value as supplied by the lambda probe used for control, or the lambda value behind the catalytic converter can be measured with a second probe will.

The actual lambda measurement value is preferably determined with such a probe if such a probe is present anyway is, e.g. B. to monitor the catalyst activity. If there is no second probe behind the catalytic converter, it is generally more advantageous to form the actual lambda measurement value by averaging the lambda value used for control.

Which of the various control parameters is changed due to the determined measurement deviation depends on the vibration behavior of the controlled overall system. Should z. For example, if the lambda value is shifted a little further in the bold direction, either the additional integration time can be extended or the proportional jump in the bold direction can be increased. The first measure leads to an increase in the oscillation time of the two-point control, while the second measure leads to a shortening. The latter results in faster correction of errors, but with the disadvantage of a higher tendency to vibrate. This makes it clear that the overall behavior of the controlled system must be taken into account when selecting the control parameter or the control parameters to be changed.

drawing

Fig. 1a, b

ein Diagramm des zeitlich schwingenden Lambdawertes bei Zweipunktregelung und ein zeitkorreliertes Diagramm des Verlaufs des Stellwertes;

Fig. 2a, b

Diagramme entsprechend denen von Fig. 1, jedoch unter Verwendung einer zusätzlichen Integrationszeit zum Gewinnen des Stellwertes;

Fig. 3a, b

Diagramme entsprechend denen von Fig. 1, jedoch unter Verwendung eines Proportionalsprunges zum Gewinnen des Stellwertes;

Fig. 4

ein Funktionsdiagramm in Form eines Blockfunktionsbildes zum Erläutern eines Verfahrens mit Regelparametern, die aufgrund einer Meßabweichung veränderbar sind, die mit Hilfe eines mittleren Lambdawertes gebildet wird, der von einer Sonde hinter einem Katalysator gemessen wird; und

Fig. 5

eine Variante des Funktionsablaufs von Fig. 2, gemäß der der mittlere Lambdawert durch Mitteln des Lambdawertes gewonnen wird, der zur Zweipunktregelung verwendet wird.

The invention is explained in more detail below with reference to exemplary embodiments illustrated by figures.
Show it:

1a, b

a diagram of the time-oscillating lambda value with two-point control and a time-correlated diagram of the course of the manipulated variable;

2a, b

Diagrams corresponding to those of FIG. 1, but using an additional integration time to obtain the manipulated variable;

3a, b

Diagrams corresponding to those of Figure 1, but using a proportional jump to gain the manipulated variable.

Fig. 4

a functional diagram in the form of a block functional diagram for explaining a method with control parameters that can be changed due to a measurement deviation that is formed using an average lambda value that is measured by a probe behind a catalytic converter; and

Fig. 5

2, according to which the average lambda value is obtained by averaging the lambda value that is used for the two-point control.

Description of exemplary embodiments

Before going into details of the invention, the problems on which the invention is based are first explained starting from the upper part of FIG. 4 and with the aid of FIGS. 1-3.

4, a horizontal, dash-dotted line is drawn in the lower part. Functions above this line are known from the prior art, while functions drawn below the line, along with lines of influence reaching into the upper part, are new.

The main functional flow in Fig. 4 is as follows. Depending on the values of the speed n and the load L, preliminary fuel injection times TIV are determined by a pilot control 10. These are converted into injection times TI by a link 11, which will be discussed in more detail below an injection device 14 arranged in the intake manifold 12 of an internal combustion engine 13. The fuel quantity injected into the intake air flow results in a specific lambda value, which is measured as the actual lambda control value by a lambda probe 16 arranged in the exhaust gas duct 15 of the internal combustion engine 13. This actual lambda control value is compared with a lambda control setpoint, which is supplied by a means 17 for outputting the setpoint. 4 that this value should be a reference voltage UREF of 450 mV. This indicates that in practice it is often not lambda values that are compared with one another, but rather voltages such as those emitted by a lambda probe at a certain lambda value. The comparison between the lambda control setpoint and the lambda control actual value takes place in a comparison step 18, in which the control deviation is formed as the difference between the values mentioned. On the basis of the control deviation, a lambda control 19 determines a manipulated value in the form of a control factor FR, by which the provisional injection time TIV is multiplied in the link 11. If the actual lambda control value remains below the lambda control setpoint, this means that the mixture burned in the internal combustion engine 13 is too lean. A control factor FR> 1 is then output, whereby a longer actual injection time TI is formed from the preliminary injection time TIV.

Possible courses of the actual lambda control value are shown in FIGS. 1a - 3a and associated courses of control factors FR in FIGS. 1b - 3b.

The explanation of FIG. 1 begins with a point in time at which the actual lambda control value, hereinafter referred to as the probe voltage, drops from rich to lean, ie from a value that indicates a mixture that is richer than a mixture corresponds, which leads to the reference voltage UREF, to a mixture that is leaner. The probe voltage passes through the reference voltage in leaps and bounds. The same applies to the return from lean to rich. At the moment when the probe voltage passes through the reference voltage during the jump from rich to lean, the lambda control 19 reverses the integration direction for gaining the control factor FR from the control deviation, so that the control factor is increased from values below 1. The time between the reversal of the direction of integration and reaching the control factor 1 is shown in FIG. 1b and designated TM. After reaching value 1, however, the integration continues because the probe voltage still indicates the value lean, although the control factor already leads to a rich mixture on the suction side. However, this rich mixture is only detected by the lambda probe 16 after a dead time TT. With this dead time TT, the probe voltage jumps from lean to rich. This jumping results in a new reversal of the direction of integration. The control factor FR is now reduced so that it reaches the value 1 again after a time TF has elapsed since the jump. If the control factor FR is further reduced, a lean mixture is set on the suction side, but this only leads to a jump in the measurement signal on the lambda probe 16 only after the dead time TT has elapsed, this time from rich to lean. In Fig. 1b, as is also assumed in Figs. 2b and 3b, it is assumed that the integration time TAUF for upward integration and the integration time TAB for downward integration are the same. Then the time intervals TM + TT and TF + TT are equal, so that the control factor oscillates symmetrically around the value 1 and the probe voltage symmetrically around the reference voltage UREF. It should be noted that with a reference voltage UREF between approximately 400 mV and 550 mV, as is used in practice, the control factor is not symmetrical by the value 1, but oscillates symmetrically around a value slightly less than 1. The mean lambda value is therefore slightly lean. In addition, the jump behavior of the probe leads to a shift into lean, in which the measured value jumps faster from lean to rich when the mixture changes abruptly than with a reverse change.

The effects just mentioned therefore lead to a lean shift. On the contrary, however, as mentioned above, in order to reduce the proportion of nitrogen oxides in the exhaust gas, it is desirable for the lambda value to be slightly rich on average. Lambda control 19 must be operated accordingly. Various methods are available for this, two of which are explained with reference to FIGS. 2 and 3.

According to FIG. 2b, according to the one measure, when the probe voltage jumps from lean to rich, the direction of integration is not immediately reversed from enriching to emaciating, but it is further enriched over a delay time TV before the jump in the probe voltage follows the control direction. Between the point in time of the jump in the probe voltage and the point in time in which the control factor passes through the value 1, not only the time TF passes, but also the time 2TV + TF. The control factor FR is therefore in the range of values> 1 during the time period TT + 2TV + TF. In contrast, the range for values <1 remains unchanged over the time period TT + TM. The measure leads to an averaged control factor> 1, which is shown in FIG. 2b by a dash-dotted line. By choosing the delay time TV differently, the average control factor and thus the average lambda value can be shifted differently in the direction of bold. The stronger the shift, the more the period of the control oscillation increases.

Likewise, a shift in the bold direction, but with a lowering of the period of the control oscillation, can be achieved by a measure as illustrated with reference to FIGS. 3a and b. When the probe voltage jumps from rich to lean, the control factor FR is suddenly increased by a proportional portion PAUF before the upward integration follows with the integration time TAUF. Due to the sudden upward change, only a short time TM 'passes between the change in the probe voltage from rich to lean and the point in time at which the control factor FR reaches the value 1 coming from smaller values. Values <1 for the control factor FR thus only exist during the time period TT + TM ', which is shorter than the time period TT + TM according to the method in FIG. 1b. The time period TT + TF remains unchanged. The larger the upward jump PAUF, the more the average control factor shifts to values> 1, which results in an increasingly richer average lambda value. The oscillation period of the two-point control, however, continues to decrease.

4, and still in part above the dash-dotted line, shows how the knowledge described is used to set an average lambda value for each operating state, which leads to optimal pollutant conversion. This is because there is a means 20 for setting the delay time TV depending on values of the speed n and the load L. This makes it possible to change the delay time TV as a function of the operating point, that is to say to carry out the method explained with reference to FIGS. 2a and b.

From the means 20 for setting the delay time TV in FIG. 4 are a means 21 for setting the size of the upward jump PAUF, a means 22 for setting the size of a downward jump PAB, a means 23 for setting the upward integration time IAUF and a means 24 for Setting the downward integration time IAB shown. Of the means 21 and 22 for setting the jump sizes, only dashed lines are drawn to the lambda control 19. This is because, in practice, these quantities are only changed with the delay time TV in exceptional cases. This is related to the vibration behavior of the entire controlled system. As explained with reference to FIGS. 2 and 3, the introduction of a delay time leads to an increased oscillation period, while the introduction of an upward jump and correspondingly a downward jump leads to a shortening of the oscillation period. As a rule, it only makes sense for a certain type of internal combustion engine to use only one of the two measures for shifting the average lambda value in the direction of rich. Different integration times IAUF and IAB could also be used for such a shift. However, these integration times are generally changed as a function of speed in order to keep the amplitude of the control oscillation essentially constant for all operating states.

In addition to the functions described so far, a pilot control adaptation 25 and a compensation 26 are also shown in FIG. 4. The latter serves to influence the influence of measured quantities on the injection time, e.g. B. to compensate for the influence of the battery voltage. The pilot control adaptation, on the other hand, serves to compensate for the influence of unmeasured disturbance variables, e.g. B. Air pressure or temperature fluctuations.

As explained above, in a two-point control the control value, in the example the control factor FR, and the lambda value oscillate around respective average values. At least one control parameter, in the example the delay time TV, is changed depending on the respective operating state in such a way that an average lambda value for optimal pollutant conversion should stop. In practice, however, this is not always achieved, which leads to poorer exhaust gas quality than is desired.

Very good exhaust gas quality in all operating states can be achieved with the aid of a lambda measuring probe 28 arranged behind the catalytic converter 27, a means 29 for outputting the measurement setpoint, a measured value comparison step 30 and a controller adaptation 31. In the measured value comparison step 30, the actual lambda measurement value, as supplied by the lambda measurement probe 28, is compared with the lambda measurement setpoint from the means 29 for outputting the measurement setpoint to form a measurement deviation. The measurement deviation is fed to the controller adaptation 31. If the measurement deviation is negative, that is to say the actual lambda measurement value is greater than the desired lambda measurement value, this is a sign that the average lambda value as it occurs behind the catalytic converter 27 is too rich. This means that the delay time TV is to be reduced, which is shown in FIG. 4 by a down arrow. This humiliation step can be a fixed step size or a step size determined according to a predefined calculation method, e.g. B. have a step size proportional to the measurement deviation. Which step size is best used depends on the vibration behavior of the entire controlled system.

In Fig. 4, not only is a solid line of influence drawn from the controller adaptation 31 to the means 20 for setting the delay time TV, but there are also dashed lines between the controller adaptation 31 and the means for setting the upward jump PAUF, the means 22 for setting the downward jump PAB , the means 23 for setting the upward integration time IAUF, the means 24 for setting the downward integration time IAB and the means 29 for measuring setpoint output. There are different reasons for the dashed lines. The line to the means 21 for setting the upward jump PAUF is dashed, since in the example it is assumed that the delay time TV is changed in order to set the desired average lambda value. As explained above, in practice, changes are typically only made to one of the various control parameters in the case of conventional methods in which the changes are made solely on the basis of values of operating variables. Accordingly, when regulating changes according to the invention, it is expedient to influence only one of the control parameters in each case.

The integration times IAUF and IAB are expediently not used to set the desired average lambda value, since, as explained above, these variables are typically changed to set a constant amplitude of the control oscillation at different speeds. The clarity of the regulation is made more difficult if these variables are changed depending on different values. In the event of special conditions, however, changing the integration times, depending on the measurement deviation, can be particularly useful.

The average lambda value can also be changed by shifting the reference voltage for the two-point control. Due to the jumping behavior of the lambda probe 16, however, there are only slight displacement options.

linearisiert.If a second measuring probe 28 is not to be used for reasons of cost, a method according to FIG. 5 is advantageous. According to this method, the average lambda value is not determined by measurement downstream of the catalytic converter 27, but rather by an averaging 32, the actual lambda measurement value is determined from the actual lambda controller value by the lambda probe 16 by averaging. The averaging takes place e.g. B. by averaging over an entire oscillation of the lambda controller actual value, that is, for. B. from a jump from lean to rich to the next jump from lean to rich. In this case, the measured values before the averaging advantageously correspond to the non-linear characteristic ${\text{U}}_{\text{λ}}\text{= f (λ)}$

linearized.

The means 29 for outputting the measurement setpoint value preferably has a memory in which lambda measurement setpoint values are stored in an addressable manner via values of operating variables. The setpoints are determined in such a way that they correspond to the mean lambda value which leads to optimal pollutant conversion in the respective operating state. Addressing operating variables are preferably the speed n and a variable dependent on the load L, for. B. the accelerator pedal position, the throttle valve angle or the intake air mass. However, the setpoints can also be determined on characteristic curves or by calculations based on a formula.

All of the means, method steps and memory mentioned are preferably formed by the hardware and software of a microcomputer, as is typically used in automotive electronics.

Claims (6)

1. Method for controlling the lambda value of the air/fuel mixture to be supplied to an internal combustion engine with the aid of

   - a means (18, 19) for two-point control with specified control parameters, to which means the signal of a lambda control probe has to be supplied for forming the control deviation, which probe has step behaviour, in which

   - a required measured lambda value (29) is determined for the respectively existing operating condition,

   - an average lambda value is used as the actual measured lambda value,

   - the measurement deviation is calculated between the required measured lambda value and the actual measured lambda value (30),

   - and at least one control parameter is varied as a function of the measurement deviation in such a way as to set an actual measured lambda value which reduces the measurement deviation quoted, and in which

   - a delay time is provided as the at least one control parameter, which delay time comes into effect in the two-point control after the control deviation flips over, and

   - the delay time is asymmetrically configured.
2. Method according to Claim 1, characterised in that the average lambda value is determined by measurement by means of a lambda measurement probe (28) behind a catalyser.
3. Method according to Claim 1 or 2, characterised in that the variable control parameter is varied in steps, the width of which is proportional to the measurement deviation.
4. Method according to Claim 1 or 2, characterised in that the variable control parameter is varied in steps of a specified fixed width.
5. Appliance for controlling the lambda value of the air/fuel mixture to be supplied to an internal combustion engine with the aid of

   - a means (18, 19) for two-point control with specified control parameters, to which means the signal of a lambda control probe has to be supplied for forming the control deviation, which probe has step behaviour,

   - with means, which determine a required measured lambda value (29) for the respectively existing operating condition, in which

   - an average lambda value is used as the actual measured lambda value,

   - with means, which calculate the measurement deviation between the required measured lambda value and the actual measured lambda value (30),

   - and with means, which vary at least one control parameter as a function of the measurement deviation in such a way as to set an actual measured lambda value which reduces the measurement deviation quoted, and

   - a delay time is provided as the at least one control parameter, which delay time comes into effect in the two-point control after the control deviation flips over, and

   - with means, which configure the delay time asymmetrically.
6. Appliance according to Claim 5, characterised in that a means (29) for determining the required measured lambda values has a memory which stores required measured lambda values addressable by means of values of operating parameters.

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