EP0375758B1 - A method and device for lambda control with several probes - Google Patents

Abstract

A method for lambda control operates with two control circuits for different groups of cylinders. Two-point control is operated in each control circuit, causing control oscillation each time. The phase shift between the two oscillations is found and set to a predetermined value. If the predetermined value of the phase shift corresponds to half an oscillation period, the exhaust gas from one group of cylinders changes from rich to lean when the exhaust gas from the other changes from lean to rich, and vice versa. If the two quantities of exhaust gas are mixed upstream of a catalyser, the latter receives a gas mixture with a lambda value of substantially 1. The method thus makes it possible to attain an oscillation amplitude in the exhaust gas lambda value which is smaller than those of the air-fuel mixture fed to both groups of cylinders. The result is the improved conversion of the pollutants. A device for implementing said method has two means of two-point control, one to determine the actual and one to set the target phase shift.

Description

The invention relates to a method and a device for lambda control for an internal combustion engine with at least two lambda sensors.

State of the art

Several lambda sensors on an internal combustion engine are used in two fundamentally different arrangements. In one arrangement, the plurality of probes are located in successive locations in the exhaust gas duct of the internal combustion engine. In the other arrangement, the lambda probes are attached to the same position in different subchannels of the exhaust gas duct system. The invention relates to an arrangement of the latter type.

Such an arrangement is provided, for example, for the methods and devices according to DE-A1-22 55 874 and DE-A1-27 13 988. In each case one lambda probe is arranged in the exhaust subchannel in each case one half of a V-engine in front of that point before the two subchannels are brought together to form a manifold in which a catalyst is arranged. In the method according to DE-A1-22 55 874, the signals from the two probes serve to supply actual values for two separate control loops, one of which is assigned to one engine half. In the method according to DE-A1-27 13 988, the signals from both probes serve as actual values for a single control loop. Depending on the type of signals from the two probes, a decision is made as to whether the controller increases, decreases or leaves the manipulated value unchanged. In addition, depending on a summed control deviation, the manipulated variable is wobbled so that the mixture, overlaid with the usual oscillation of the two-point controller, constantly and quickly changes between rich and lean.

US Pat. No. 4,703,735 discloses a method in which one exhaust gas probe is arranged in each of the initially separated exhaust pipes of the two cylinder banks of a V-type engine, each of which serves as a control sensor for the lambda = 1 mixture control for the associated cylinder bank. The two control loops are controlled so that there is a phase difference of 180 ° between their control vibrations. However, the method described allows improvements, in particular in the manner in which the synchronization of the control vibrations is carried out.

The known methods and devices serve to supply air / fuel mixtures which lead to exhaust gas of such a type that pollutants still present in the exhaust gas can be optimally converted by a catalytic converter.

The invention has for its object to provide a method for lambda control for an internal combustion engine with at least two lambda sensors in the same position, which allows even better pollutant conversion than previous methods. 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 or 2 and the device according to the invention by the features of claim 6 or 7. Advantageous embodiments of the method are the subject of the dependent claims.

The invention uses the following consideration. With a two-point lambda control, the lambda value continuously oscillates between rich and lean. The greater the amplitude of these vibrations, the lower the relative pollutant conversion of the catalytic converter. If two control loops are now used instead of one control loop, it must be possible to adapt the vibrations in the two circles to one another such that the mixture in one control loop just swings in the rich direction when the mixture in the other control loop swings in the lean direction. The exhaust gases of the rich mixture come together in the collecting pipe in front of the catalytic converter and lead there to exhaust gas with approximately lambda value 1. The lambda value of the exhaust gas will hardly oscillate any more if the phase shift is approximately half a period of oscillation. If it is more or less, there is still an oscillation of the lambda value, but with a considerably lower amplitude than is present without a phase shift of the two control oscillations. The amplitude can be determined by the extent of the phase shift. A low residual vibration is desirable for some catalysts, since they only work optimally if they can work oxidizing during one half cycle of the control vibration and reducing during the other half cycle.

The known methods mentioned at the outset are intended for engines in which, due to their structure, very long exhaust gas ducts exist, especially for V-engines. The method according to the invention, on the other hand, has advantages for all types of motors, that is to say also for. B. in a four-cylinder in-line engine. A probe can be arranged in each outlet connection, to which a control loop is assigned. The phase shifts between the four control loops are set so that mixed exhaust gas with essentially lambda value 1 is produced in the collecting duct, or that for the reason mentioned above there is still a small residual lambda vibration in the mixed exhaust gas.

The phase correction can be carried out particularly easily if reference is made continuously to the phase of a specific control loop. The regulation, on the other hand, becomes faster if the earliest vibration is variably referred to.

drawing

• Fig. 1 ein als Blockschaltbild dargestelltes Verfahren zur Lambdaregelung mit zwei Sonden und zwei Regelkreisen, zwischen denen eine vorgegebene Phasenverschiebung eingestellt wird;
• Fig. 2a - c zeitkorrelierte Diagramme des Signales von einer Lambdasonde, des zugehörigen Stellwertes und des tatsächlichen Lambdawertes am Ort der Sonde;
• Fig. 3 ein Diagramm des zeitlichen Verlaufs des Lambdawertes von zwei Einzelabgasen und des Lambdawertes des gemischten Abgases bei einer Phasenverschiebung von einer halben Schwingungsperiode;
• Fig. 4 ein Diagramm entsprechend dem von Fig. 3, jedoch mit einer Phasenverschiebung von weniger als einer halben Schwingungsperiode;
• Fig. 5a und b Diagramme der zeitlichen Verläufe zweier Stellwerte mit zwei unterschiedlichen Arten der Phasenverschiebung für ein verspätetes Signal;
• Fig. 6 ein Diagramm entsprechend dem von Fig. 5a, jedoch betreffend die Phasenverschiebung eines voreilenden Signales;
• Fig. 7 ein Diagramm entsprechend dem von Fig. 5a, jedoch mit einer größeren Phasenverschiebung und mit einer Korrektur der Phase in zwei Schritten;
• Fig. 8a und b Diagramm entsprechend denen von Fig. 5a bzw. b, wobei jedoch die voreilende Phase Bezugsphase ist;
• Fig. 9 ein als Blockschaltbild dargestelltes Teilverfahren zur Phasenberechnung und Phasenkorrektur, bei dem von Stellwerten statt von Regelabweichungen, wie beim Verfahren von Fig. 1, ausgegangen wird;
• Fig. 10 ein Diagramm betreffend den zeitlichen Verlauf der der Stellwerte von zwei phasenverschobenen Regelkreisen, wobei ein Signal nacheilt; und
• Fig. 11 ein Diagramm entsprechend dem von Fig. 10, wobei jedoch das dort nacheilende Signal nun voreilt.

The invention is explained in more detail below on the basis of exemplary embodiments illustrated by figures. Show it:

• 1 shows a method for lambda control with two probes and two control loops, between which a predetermined phase shift is set, shown as a block diagram;
• 2a-c show time-correlated diagrams of the signal from a lambda probe, the associated control value and the actual lambda value at the location of the probe;
• 3 shows a diagram of the time profile of the lambda value of two individual exhaust gases and the lambda value of the mixed exhaust gas with a phase shift of half an oscillation period;
• FIG. 4 shows a diagram corresponding to that of FIG. 3, but with a phase shift of less than half an oscillation period;
• 5a and b show diagrams of the time profiles of two manipulated values with two different types of phase shift for a delayed signal;
• FIG. 6 shows a diagram corresponding to that of FIG. 5a, but relating to the phase shift of a leading signal;
• FIG. 7 shows a diagram corresponding to that of FIG. 5a, but with a larger phase shift and with a correction of the phase in two steps;
• Figures 8a and b are diagrams corresponding to those of Figures 5a and b, however, the leading phase is the reference phase;
• FIG. 9 shows a partial method for phase calculation and phase correction, shown as a block diagram, in which control values are used instead of control deviations, as in the method of FIG. 1;
• 10 shows a diagram relating to the time course of the control values of two phase-shifted control loops, with one signal lagging; and
• 11 is a diagram corresponding to that of FIG. 10, but the signal lagging there now leads.

Description of exemplary embodiments

The method according to the block diagram of FIG. 1 works on an internal combustion engine 12 with a first cylinder bank 13.I and a second cylinder bank 13.II. A first injection valve arrangement 15.I is present in the intake duct 14.I for the first cylinder bank 13.I. A corresponding second injection valve arrangement 15.II is located in the second intake duct 14.II. A first lambda probe 17.1 is arranged in the exhaust gas duct 16.I and a second lambda probe 17.2 is accordingly arranged in the second exhaust gas duct 16.II. The two exhaust gas sub-channels open into a collecting channel 18 in which a catalytic converter 19 is arranged.

The method for regulating the mixture composition for the first cylinder bank 13.I is explained in broad outline below with additional reference to FIG. 2. The first injection valve arrangement 15.I is supplied with a first injection time signal TI.I, which is formed by multiplying a signal TIV (n, L) for a predetermined injection time by a control factor FR.I in a multiplication step 20.I. The control factor FR.I is the manipulated variable output by a control step 21.I in response to a control deviation signal RAW.I. The control deviation value RAW.I is formed by subtracting the signal Lambda-Ist.I from the first lambda probe 17.I from a lambda setpoint in a subtraction step 22.I.

Corresponding method steps are carried out in the control circuit which adjusts the mixture supplied to the second cylinder bank 13.II. Actual control loops are structured in a considerably refined manner. In particular, different disturbance variable correction steps are available, and adaptation methods are used which have the purpose of continuously adapting different correction values to changing conditions.

The signal λ _{s of} a lambda probe, such as is used for two-point control, has jump behavior from the transition from rich to lean, as is shown in FIG. 2a. The actual course of lambda, which leads to this jump signal, is shown in FIG. 2c. 2b, which shows the timing of a control factor FR, be it the control factor FR.I for the first control loop or the factor FR.II for the second control loop to understand how the actual signal curve comes about. If the probe signal λ _s jumps from rich to lean or vice versa, the control deviation signal RAW passes through the value 0 in one direction or the other. When passing through 0, the integration direction of the control step 21 changes, as a result of which enrichment takes place as soon as the probe signal has jumped to lean, and is thinned out as soon as it has jumped to rich. As soon as the control factor FR reaches the value 1, the mixture supplied to a cylinder bank has the lambda value 1, provided the pilot control value TIV (n, L) (n = speed; L - load-indicating signal) is correctly determined, which is assumed here. If the control factor FR is further integrated, a rich lambda value is set. However, this is measured by the lambad probe only after a dead time TT, which can be seen from the lagging phase shift TT of the probe signal λ _s compared to the control factor signal FR from FIGS. 2a and b. The signal curve according to FIG. 2c has the same phase shift compared to that of FIG. 2b. Otherwise, the waveforms of FIGS. 2c and b are shown identically. This is due to the fact that, with a correctly determined pilot control value and without further corrective measures, the lambda value on the injection side corresponds to the value of the control factor FR. Under these conditions, FIG. 2b therefore shows not only the time profile of the control factor, but also the time profile of the control factor, but also the time profile of the lambda value on the injection side. The time profile of the lambda value on the exhaust gas side according to FIG. 2c is shifted by the dead time TT. In actual operation, the turning points are also somewhat flattened, but this is not important for the explanation of the following.

The representation of FIGS. 3 and 4 corresponds to that of FIG. 2c, however, with the addition that instead of the course of the lambda signal for a single control loop, the courses for two control loops are shown. In Fig. 3 it is assumed that the lambda signal λ Ist.II for the second control loop compared to the lambda signal λ Ist.I for the first control loop is shifted by a phase shift PS which corresponds to half the oscillation period SP. 4, however, the phase shift PS is only a quarter of an oscillation period. When shifted by half a period, the mixture in the first control loop reaches the greatest value in the rich direction when the mixture in the second control loop reaches the greatest value in the lean direction and vice versa. It also applies to the rest of the time course that the lambda values are opposite to each other in relation to the lambda value 1. The consequence of this for the lambda value λ.18 in the collecting channel 18 is that it remains essentially at the value 1. If, on the other hand, the phase shift is more or less - as shown in FIG. 4 - than a half oscillation period, the lambda value λ.18 of the mixed exhaust gas in the collection channel 18 also oscillates by the value lambda = 1 Corresponds to individual vibrations. The oscillation amplitude of the lambda value of the mixed exhaust gas can be determined by the extent of the phase shift. The value to be used in practice depends on the properties of the type of catalyst used in each case. If the latter requires a certain oscillation amplitude of the lambda value for alternating oxidation and reduction, a corresponding lambda shift is set. If the catalytic converter does not require a lambda value oscillation, one is Phase shift of half an oscillation period is preferred.

In order to be able to carry out a phase shift in the manner described, the method according to FIG. 1 has a phase calculation step 23 which calculates the phase shift between the two control loop vibrations from the control deviation signals RAW.I and RAW.II. The actual phase shift value is compared in a phase correction step with the target phase shift value, and in the event of a deviation, the phase of one oscillation is shifted relative to the other in such a way that the desired phase shift value is set.

Some possibilities for calculating and correcting the phase shift will now be explained with reference to FIGS. 5-11. 5-8 relate to the method according to FIG. 1, while FIGS. 10 and 11 relate to a modified method, which is explained below with reference to FIG. 9.

5-8 as well as 10 and 11 differ from those of FIGS. 3 and 4 in that phase-shifted control factors (corresponding to FIG. 2b) are no longer shown, but phase-shifted. In all the figures mentioned, the course of the control factor FR.I is drawn through and the course of the control factor FR.II is shown in dash-dot lines. The course of the respective reference phase is shown in dashed lines. Reference points, from which the phase shift is measured, are given by dots shown in bold. In all cases it is assumed that the target phase shift should correspond to half an oscillation period.

5a and 6 are discussed first. In both cases, the phase of the signal FR.I is the reference phase and the jump from lambda to lean night is the reference point. This corresponds to the reversal point in the control factor from increasing to decreasing. The course of the signal FR.II is determined at each of these times in the signal FR.I. For this signal, the reversal point is not triggered directly by the jump in the associated probe signal, but with the help of the reference point in signal FR.I. According to FIG. 5a, this occurs with the lagging signal FR.II in that it is determined at the reference time that the associated probe signal has not yet jumped for the signal FR.II. The time Δ PS is then measured, which passes until the probe signal associated with the signal FR.II jumps. If this jump time were not delayed by the time period Δ PS compared to the reference point, but rather without delay, the signal FR.II would have already increased by the value Δ PS x IV in the time period Δ PS, where IV is the rate of integration. The signal FR.II is therefore increased by the specified value Δ PS x IV with the lapse of the time interval Δ PS, as a result of which the lagging is eliminated. If the signal FR.II is leading, the probe jump from rich to lean occurs before the probe jump from lean to rich for signal FR.I. In this case, the reversal of the signal FR.II is not yet permitted, but its value is reduced further until the signal FR.I is reversed in its direction of change, that is to say reaches the reference point. The elapsed time period is also referred to as Δ PS in FIG. 6. When the reference point occurs, the signal FR.II is raised by the value Δ PS x IV even in the case of leading, which eliminates the undesired phase shift Δ PS.

5b, like FIG. 5a, relates to the case of the lagging signal FR.II. However, the correction takes place differently than according to FIG. 5a. When the reference point appears in signal FR.I namely the value determined on which the signal FR.II is currently. This is compared with the value that the signal FR.II should have at its lower reversal point. If the measured value does not match the expected value, the signal FR.II is set to the expected value. The expected value may e.g. B. the inverse value in the previous oscillation, or it can be the values of the reference point for the signal FR.I mirrored at the value Lambda = 1.

Fig. 7 largely corresponds to Fig; 5a, but with the difference that the undesired phase shift Δ PS is approximately twice as large as in the case of FIG. 5a. The result of this is that the correction value Δ PS x IV is quite high. If this correction were carried out in a single step, this could lead to restless driving behavior. It is therefore provided in accordance with FIG. 7 that instead of a single large correction step, two smaller correction steps are used, each of which corresponds to the value Δ PS x IV / 2. The individual correction steps are carried out in predetermined successive periods, z. B. with each computer cycle to calculate the control factors, in the case of implementation by a microcomputer.

For the representations according to FIGS. 8a and b, it is assumed that reference is no longer made to the phase of the oscillation FR.I, but that reference is always made to the earliest signal. In the case of FIGS. 8 a and b, this is the signal FR.II, since, as shown in FIG. 6, it leads the signal FR.I. The signal for which a jump occurs first in the associated probe signal sets a time measurement to O, which measures the time period Δ PS that passes until the probe signal for the other control factor signal also jumps. 5a and 8a differ only in that in FIG. 8a the reference point lies on the leading signal FR.II. As soon as the Time period Δ PS jumps the probe signal for the signal FR.I, its value is reduced by the correction value Δ PS x IV. If the signal FR.II were to lag behind the signal FR.I at a later time, an image according to FIG. 5a would result. As FIGS. 8a and 5a correspond, FIGS. 8b and 5b correspond. According to FIG. 8b, the signal FR.I is raised to the expected amplitude as soon as the probe signal belonging to the control factor signal FR.II jumps from rich to lean.

5 and 6 it is therefore assumed that the signal FR.I continuously forms the reference signal for determining the phase shift. In contrast, in the ratio of FIGS. 5 and 8 it has been assumed that the earliest probe jump point is the reference point. In all cases it was assumed that only one specific direction of jump was used for the probe signal to form the reference point, however, each probe jump can be used.

As explained above with reference to FIGS. 2a and b, there is a fixed phase shift in the value of the dead time TT between the control deviation signal RAW and the control factor signal FR. This phase shift TT applies equally to both control factor signals FR.I and FR.II, so that it has no influence whatsoever on a mutual shift of these two signals to one another. The phase shift can thus not only be calculated with the aid of the step signals from the lambda sensors 17.I and 17.II, but also the control factors FR.I and FR.II can be compared directly with one another. This is shown in FIG. 9. The only difference from the corresponding part of the illustration in FIG. 1 is that the values of control factors FR.I and FR.II are supplied to the phase calculation step 23 instead of the values of the control deviations RAW.I and RAW.II. In FIG. 9, too, a solid line leads from the phase correction step 24 to the control step 21.II, on the other hand a dashed line to control step 21.I. This is to indicate that in all cases it is either possible to hold on to one of the control factor signals, in the example the control factor signal FR.I, and only to correct the other (FIGS. 5 and 6), or that it is possible to do so at the earliest To refer to the signal and correct the other. In this case, the phase correction step 24 must deliver a correction value to the first control step 21.I and another time to the control step 21.II.

In order to determine the phase shift between the control factor signals FR.I and FR.II, it is expedient to take the run through the fixed value 1. Correspondingly, in the representations of FIGS. 6 and 11, the reference point lies on the auxiliary line for the value mentioned. The reference point is the point in time at which one of the two signals FR.I and FR.II first reaches the value 1. In Fig. 10, this is the signal FR.I because the signal FR.II lags. In Fig. 11 it is the reverse. The time interval Δ PS is measured in each case, which takes place between the passage of the previous signal by the value 1 and the passage of the later signal by this value. Correspondingly, the lagging signal FR.II is lowered by the value Δ PS x IV in FIG. 10 so that it reaches the lower value that it would have had if it had already passed through the value 1 earlier in the decreasing state by the time period Δ PS would be, in time with the passage of FR.I from bottom to top. Conversely, in the sequence according to FIG. 11, the signal FR.I is increased by the value Δ PS x IV at the end of the time period Δ PS in order to eliminate its lagging compared to the signal FR.II. Even in cases according to those of FIGS. 10 and 11, the correction step can be broken down into a number of individual steps if a single correction step would be undesirably large.

Among the methods described, those that use one of the signal curves of the control factors as a permanent reference curve have the advantage of simplicity. On the other hand, the procedures that refer to the earliest phase are faster. The correction does not necessarily have to be made in steps, but can also be done by changing the integration times with which the control deviation values are integrated to form the control factors.

A device for executing the described methods and others is preferably provided by a microcomputer, to which the signals of the two lambda sensors are fed and which has two means for two-point control, one means for determining the actual phase shift and one means for setting the target phase shift between the two control loops. If there are more than two control loops with associated lambda probes, the device has a means for determining the actual phase shift as they exist between the control vibrations, which are generated by two means for two-point control, and the means for setting the target phase shifts designed so that it maintains a target phase shift between two associated control loops.

Claims (7)

1. Method for lambda control in an internal-combustion engine with at least two lambda probes in an identical position in different part channels of an exhaust gas system which together supply a catalyst, in which at least two different air/fuel mixtures for different cylinders or groups of cylinders are put under two-position control in different control circuits and where desired phase shifts are set between the control oscillations, and in which either the phase of one of the control circuits is used continuously as reference phase or the earliest phase in each case is used as reference phase, and in which the phase shifts of the other control circuits are set to the desired values,
characterised in that
the phase shift of the other control circuits is carried out by means of addition or subtraction of correction values which are determined from the product of phase-shift differences and a variable dependent on the integration speed of the other control circuits, the phase-shift difference being the time difference between measured and predetermined phase difference.

2. Method for lambda control in an internal combustion engine with at least two lambda probes in an identical position in different part channels of an exhaust gas system which together supply a catalyst, in which at least two different air/fuel mixtures for different cylinders or groups of cylinders are put under two-position control in different control circuits and where desired phase shifts are set between the control oscillations, and in which either the phase of one of the control circuits is used continuously as reference phase or the earliest phase in each case is used as reference phase, and in which the phase shifts of the other control circuits are set to the desired values,
characterised in that
the phase shift of the other control circuits is carried out by means of addition or subtraction of correction values, the correction values corresponding to the difference between actual values and expected values of the respective control factors (FR2,...) and the expected values being given by the respective reversal values of the preceding oscillation of the respective control factor or by the values of the reference signal at a reference time which are reflected at the value lambda = 1.

3. Method according to Claim 1 or 2, characterised in that, when a maximum value is exceeded, the correction value is broken down into a plurality of individual values, each of which corresponds at most to the maximum value, and in that these individual values are added or subtracted with a time offset relative to one another.

4. Method according to Claim 3, characterised in that the time offset corresponds to a computing cycle of a microcomputer.

5. Method according to one of Claims 1 to 4, characterised in that a phase shift of approximately half an oscillation period is set for two control circuits.

6. Device for lambda control in an internal-combustion engine with at least two lambda probes in an indentical position in different part channels of an exhaust gas system which together supply a catalyst, in which at least two different air/fuel mixtures for different cylinders or groups of cylinders are put under two-position control in different control circuits and which has means with which desired phase shifts are set between the control oscillations, and in which either the phase of one of the control circuits is used continuously as reference phase or the earliest phase in each case is used as reference phase, and in which the phase shifts of the other control circuits are set to the desired values,
characterised in that
means are present which effect the phase shift of the other control circuits by means of addition or subtraction of correction values which are determined form the product of phase-shift differences and a variable dependent on the integration speed of the other control circuits, the phase-shift difference being the time difference between measured and predetermined phase difference.

7. Device for lambda control in an internal-combustion engine with at least two lambda probes in an identical position in different part channels of an exhaust gas system which together supply a catalyst, in which at least two different air/fuel mixtures for different cylinders or groups of cylinders are put under two-position control in different control circuits and which has means with which desired phase shifts are set between the control oscillations, and in which either the phase of one of the control circuits is used continuously as reference phase or the earliest phase in each case is used as reference phase, and in which the phase shifts of the other control circuits are set to the desired values,
characterised in that
means are present which effect the phase shift of the other control circuits by means of addition of subtraction of corrections values, the correction values corresponding to the differences between actual values and expected values of the respective control factors (FR2,...) and the expected values being given by the respective reversal values of the preceding oscillation of the respective control factor or by the values of the reference signal at a reference time which are reflected at the value lambda = 1.

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