EP0157004B1 - Lambda-controlled mixture-measuring system for an internal-combustion engine - Google Patents

Abstract

The invention is directed to a mixture metering arrangement for an internal combustion engine and includes an exhaust-gas sensor which is exposed to the exhaust gas of the internal combustion engine. The exhaust-gas sensor indicates the air ratio lambda and preferably has a two-level characteristic. The sensor output signals are acted upon by a follow-on controller which is preferably a PI-controller. The controller output quantity acts upon the mixture composition in a corrective fashion. In this arrangement, the control oscillation of the controller output quantity is adjusted to a predetermined amplitude by means of a superposed control. In particular, the integral component of the control oscillation is influenced in a manner causing it to have the same amplitude as the proportional component while in the steady operating condition. Thus, it is possible to maintain the maximum control frequency in any operating range of the internal combustion engine so that the controller always operates at its optimum. In addition, the effects of deviations occurring from one engine to another or from one exhaust-gas sensor to another as well as of long-term variations are suppressed.

Description

State of the art

The invention relates to a mixture metering system for an internal combustion engine according to the preamble of the main claim. Such a mixture metering system is known, for example, from DE-OS 31 24 676 or the corresponding US Pat. No. 4,442,817. There, the mixture composition is precontrolled as a function of various operating parameters of the internal combustion engine, a superimposed lambda control having a corrective effect on these precontrol values. Since the internal combustion engine as a controlled system exhibits a dead time behavior, which is mainly due to the gas runtime through the internal combustion engine and the response time of the lambda probe, and the output signal of the lambda probe in particular has an almost binary signal, the lambda control occurs Continuous vibration, the frequency of which is given by the dead time and the amplitude of which is given by the control parameters. The general rule is that with increasing value of the control amplitude a faster correction of disturbances is guaranteed. However, with increasing control amplitude, the internal combustion engine runs unevenly, which is due to the change in torque caused by the control. Furthermore, undesirable exhaust gas peaks can occur, in particular during dynamic transition situations when the internal combustion engine is operating, due to excessive control vibrations. This is due to the fact that the lambda control can then run briefly against its stop. In the document cited in the prior art, it is now proposed to reduce the slope of the I component of the control oscillation of the P-I controller during stationary or quasi-stationary operating states of the internal combustion engine to a minimum value in successive correction cycles. If a stationary operating state of the internal combustion engine occurs, the slope of the I component is reset to a predetermined maximum value. This is a pure control of the integrator slope, with which not all long-term or short-term drift phenomena occurring over the life of the internal combustion engine can be compensated. In addition, this controlled adjustment of the integrator slope is only effective during stationary operating states.

A lambda control system is known from GB-A-2 007 407, in which the amplitude of the control oscillation can be limited to specific values. For this purpose, operational amplifiers 108, 120 are used as threshold switches, which influence the integrator of the lambda control so that its output signal is kept constant (page 5, line 21) when a certain amplitude of the control oscillation is exceeded or undershot. In the subject of this GB-A 407, the control vibration is therefore only influenced to a limited extent by interrupting the further integration process from a certain height or amplitude. As a result, phases with a constantly high or constantly low lambda value can occur within a control oscillation in this known lambda control device, which may not be desirable for reasons relating to exhaust gas technology.

Furthermore, US-A-4 145 999 discloses a lambda control system in which the integrator slope is influenced depending on the frequency of the control oscillation. Naturally, this solution completely disregards the amplitude of the control oscillation, so that the actual conditions are not taken into account in the lambda control. However, this is essential with regard to a mixture control based on the actual conditions.

Advantages of the invention

The mixture metering system according to the invention for an internal combustion engine with the features of the main claim, on the other hand, allows a considerably reduced application effort, since the system automatically adapts to sample variations from engine to engine, from lambda probe to lambda probe and to long-term changes in engine and probe. Furthermore, the mixture metering system according to the invention finds an optimal compromise between the running behavior of the internal combustion engine and the exhaust gas emission.

Furthermore, it proves to be advantageous that the control oscillation has equal amplitude values of the P and I components in the steady state. This ensures that the control frequency of the lambda control assumes an optimal value.

Another advantage arises in the event that the mean value of the control oscillation is deliberately shifted by asymmetrical P or 1 components. This asymmetrical control oscillation for generating a lambda shift is necessary because, due to the almost binary signal of the lambda probe, the shift cannot be set via other lambda target values. However, the value of the lambda shift is dependent on the amplitude of the control oscillation, so that this disturbing dependency does not have an effect on an oscillation amplitude regulated to constant values.

Further advantages of the invention result in connection with the subclaims from the following description of the exemplary embodiment.

drawing

An embodiment of the invention is shown in the drawing and explained in more detail in the following description. 1 shows a basic illustration of an electronically controlled mixture metering system for an internal combustion engine, FIG. 2 shows a rough overview of a lambda control with a microcomputer, FIGS. 3a to c show the output signals of a lambda controller according to the prior art, FIG. 4 shows a controller as a block diagram for the mixture metering system according to the invention and FIG. 5 the output signals of the controllers of FIG.

Description of the embodiments

The following exemplary embodiments are described in connection with a fuel injection system. However, the lambda control as such is independent of the type of mixture metering, so that the invention can also be used in connection with carburetor systems, for example.

In FIG. 1, 10 denotes a timing element that receives input signals from a load sensor 11 and from a speed sensor 12 and outputs pilot values of duration tp for the injection pulses on the output side. This is followed by a correction stage 13, in which the pilot control values are influenced as a function of, for example, the internal combustion engine temperature or acceleration processes and in particular as a function of a lambda control. The one with t _; Corrected pulses designated are finally supplied to at least one injection valve 14 in the area of the intake manifold of the internal combustion engine, not shown. An exhaust gas probe denoted by 15 emits its output signal to a controller 16, preferably having a PI behavior. In this case, a lambda correction signal F _{r is} formed in this, also as a function of further internal combustion engine parameters supplied via a control input 17, and is fed as an input signal to the correction stage 13. This basic arrangement shown in FIG. 1 is already known as such and is essentially intended to explain the actual invention.

As a result of the growing demands on mixture metering systems for internal combustion engines, computer-controlled solutions are predominantly used today, so that a rough overview will also be given of such embodiments with reference to FIG. 2 with the most important components. 20 denotes an arithmetic logic unit which is coupled via a data, control and address bus 21 to a memory 22 and to an input / output unit 23. In addition to a signal from the exhaust gas probe 15, this block 23 is supplied with various input variables I _k, and in addition it outputs various output variables O _k . B. an injection period. The functioning of this arrangement according to FIG. 2 depends crucially on the computer programming. Nowadays, the programming itself does not pose any problems for the person skilled in the field of electronic control systems for internal combustion engines, so that the invention is not to be illustrated in the following in the form of a program but on the basis of block diagrams which are conventional per se. It is irrelevant for the basic idea of the invention whether the later embodiment is implemented in hardware or by means of corresponding programming of a (tC).

The diagrams in FIG. 3 serve to explain the functioning of mixture metering systems in accordance with the prior art. The lambda correction signal F _r , which influences the pilot control values for the injection quantity, is plotted as a function of the time t, represented in arbitrary units. In the special case at hand, the signal form consists of an I component and a P component. Since the output variable of the exhaust gas probe used in the present exemplary embodiment, which is designed in particular as an oxygen probe, assumes essentially only two values, namely a high output level for a rich air-fuel mixture and a low output level for a lean air-fuel mixture, the resulting signal form results of the correction factor F _r as follows: If the output variable of the oxygen probe jumps from rich to lean or lean to rich, a P component at the output of the controller 16 becomes effective. The integral behavior of the controller takes effect during the time the special signal remains in one of the two output states. The time period in which an integral behavior of the controller is effective depends on the dead time behavior of the controlled system, which is essentially due to the gas throughput times through the internal combustion engine. The permanent vibrations shown in FIGS. 3a to 3c therefore arise. It can be seen from FIG. 3a that the mean value of the control oscillation is at the value F _r = 1 for times t <t _a . This means that the pilot control value for the injection quantity has been correctly selected for the operating conditions currently prevailing and the lambda control therefore does not have to intervene in a corrective manner. Between the times t _a and t _{b there} is an incorrect pilot control value for the fuel metering under changed operating conditions, so that the lambda control must intervene in a corrective manner. Because the output signal of the probe remains at one of the two possible levels, the output variable F _{r of} the controller is changed via the integral component until the fuel injection quantity has been corrected to the desired value via the correction stage 13. In the present case, the mean value of the correction factor F _r fluctuates by values F _r > 1, which suggests that the pilot control value corresponds to an insufficient fuel quantity. It can also be seen from this example that an incorrect setting of the pilot control values does not lead to an increase in the control oscillation of the controller in the steady state. The Amplitude components of the control oscillation of the correction factor F _r , which can be attributed to the P or I component, always take on the same values with a suitable choice and constant dead time in the steady state, whereby an optimal control frequency is achieved.

This situation changes completely if, as shown in FIG. 3b, the dead time of the controlled system is changed. The dead time of the controlled system is very dependent on the speed and load, so that such changes in the dead time of the controlled system occur very frequently. As a result of an increase in the dead time between the times t _a and t _b , the amplitude of the control oscillation is increased, which results solely from the integral behavior of the controller 17. In the extreme case of very large amplitudes, this can be manifested by changes in the torque of the internal combustion engine and an associated uneven running with increased exhaust gas emission. Furthermore, the optimum relationship between the amplitudes of the control oscillation based on the P and I components can no longer be maintained. This results in a reduction in the frequency of the control oscillation, which leads to an even more sluggish behavior of the overall arrangement.

Another disadvantage of known arrangements is to be illustrated by the diagram in FIG. 3c. The fact that in this example the P component in the fat-lean jump of the output signal of the oxygen probe is set to identical zero results in a lambda shift which is deliberately brought about in some cases and is desired. The lambda shift is generally achieved by different P components between the rich-lean jump and the lean-rich jump. The P component does not have to be zero. The measure for this shift results from the difference of the surface portions above and below the line at F _r = 1. Also in this example, an increase in the dead time of the control loop during the period between t _a and t; is assumed. The increase in the amplitude of the control oscillation also results in a changed displacement of the control factor F _r , so that this deliberately induced lambda displacement becomes dependent on the amplitude of the control oscillation.

From all of the above-mentioned disadvantages of the prior art, there is a demand for an adjustment of the slope of the I component of the controller 17 in such a way that the slope is corrected as a function of the previous control amplitude until the predetermined desired amplitude of the control oscillation is reached . The exact value of the amplitude of the control oscillation must be determined in individual cases. For large control amplitudes, the disturbance that occurs in each case is eliminated very quickly, but the internal combustion engine also runs smoothly with increasing control amplitude. Amplitudes of the control oscillation that are just below the limit are to be set that impair the driving comfort of a vehicle equipped with such an internal combustion engine. Secondly, the flue gas fluctuations that occur should be taken into account, although here the buffer effect of a downstream catalytic converter largely averages out these fluctuations. The upper limit for the amplitude of the control vibration is therefore determined either by the driving behavior or an upper threshold value for the exhaust gas emission. For the relevant specialist, the dimensioning of this value of the amplitude of the control oscillation is a mere routine work and presents no problems for him.

FIG. 4 shows an exemplary embodiment of the controller of the mixture metering system according to the invention. The output signals of the exhaust gas probe 15 reach a comparison device 41, in which they are compared with a predetermined target value 42. The result of this comparison operation serves as the input variable of the controller 16, the output signals F _{r of} which serve to correct, for example, the injection duration. The controller 16 consists of a P-channel 43 and an I-channel 44 connected in parallel with it, which is preceded by a correction stage 45.

The output signals of the exhaust gas probe 15 are also fed to two monoflop stages 46 and 47, which actuate two switches 48 and 49 on the output side. The monoflop stage 46 is sensitive to the positive edges and the monoflop stage 47 to the negative edges of the output signal of the exhaust gas probe 15. Via the switches 48 and 49, the output signal F _{r of} the controller 16 is applied to the input of two sample and hold units 50 and 51, respectively. The output signals of these sample and hold units 50 and 51, together with the signals of the P-channel 43 of the controller 16, reach a comparison stage 52. In a division stage 53, the quotient is formed from the output signal of the comparison stage 52 and a predetermined target value 54. This quotient is compared in a comparison stage 55 with a target value 56 and this result is fed to a multiplier stage 57 in addition to other variables. The output variable of the multiplier 57 reaches a counter 59 via a V / F converter 60 and a switch 58. The counting direction of the counter 59 depends on the respective position of the switch 58, this switch 58 each time the output variable of the exhaust gas probe changes on a flank 15 is operated. The counter reading of the counter 59 influences the correction stage 45 and the multiplier stage 57. Furthermore, the multiplier stage 57 can be supplied with a further variable G _f . It proves to be useful in many applications, the correction stage 45 with signals from a load detection stage 61, the corresponding machine parameters such. B. O _L , a, n or p are supplied to act.

The arrangement works as follows: by means of the sample and hold amplifiers 50, 51, the amplitudes of the control oscillation are stored at the switching points of the output variable of the exhaust gas probe 15. These values are formed in the comparison stage 52, so that the amplitude of the control oscillation is available at the output. The amplitude of the integral part alone To determine, the P component of the control oscillation is also subtracted in comparison stage 52. In various cases, it can prove to be advantageous to set the P component to be subtracted to be purely arithmetical, since this may reduce the computational effort. After a division of the output variable of the comparison stage 52 by a predetermined target variable 54 and a comparison of this quotient in the comparison stage 55 with a target value 56, which in particular assumes the value 1, multiplication by the output variable of the counter 59 takes place in the multiplier stage 57. The multiplier 57 influences the counting speed of the counter 59 via the voltage / frequency converter 60. The correction stage 45 influences the slope of the I component of the control oscillation as a function of the counter reading of the counter 59.

A pre-control of the I component of the control oscillation depending on the load of the internal combustion engine can be carried out via a load detection stage 61, to which machine parameters such as the speed n, the throttle valve position a or the air flow rate Q are supplied. Such an arrangement is shown for example in DE-OS 22 29 928.

berechnet werden. Mit der Beziehung

ergibt sich für die Änderung der Integratorsteigung ΔS der Zusammenhang

The described arrangement essentially has three functions, namely the measurement of the integrator slope, the comparison with the nominal slope and the correction of the integrator slope. Since the control oscillation has its greatest value Ar within a cycle at the point in time at which the output variable of the exhaust gas probe has a lean to rich and its smallest value A _u in the event of a rich-lean jump, these extreme values can be stored and a difference subsequently formed the actual amplitude A _i = A _o -A _{u are} formed. If the proportional component P is also subtracted, the amplitude component of the control oscillation is obtained which is solely due to the I component I _i = A _i - P. The P component can assume asymmetrical values, ie the P value in the lean-rich jump can differ from that in the rich-lean jump in the output signal of the exhaust gas probe. The slope of the I component for a new cycle can be determined from the slope of the I component of a previous cycle

be calculated. With the relationship

the relationship results for the change in the integrator slope ΔS

Since there is a shift in the mean vibration value when there are changes in the pilot control values or in the event of sudden disturbances, and therefore the deviation from the old mean vibration value must be covered by an additionally extended integrator runtime, the increase in amplitude may result in inaccuracies in the calculation of the new integrator slope. In order to largely suppress this effect, a weight factor G _{f is} introduced, which essentially assumes values G _f <1. This measure reduces the maximum possible rate of change in the integrator slope, so that this slope takes on the desired value only after several oscillations. This significantly suppresses the influence of a single slope that is increased by a mean shift in the vibration.

Another possibility of suppressing this mean value shift is to compare the total amplitudes A = I + P instead of comparing the amplitudes I caused solely by the I component of the control oscillation, for which purpose the P component to be subtracted in the comparison stage 52 is set to identical zero. This would also result in a reduction in the rate of change of the integrator slope. In addition, such a version could reduce the computing effort. At this point it should be emphasized again that a person skilled in the art, with knowledge of the described relationships, with his specialist knowledge can implement both an analog and a digital microcomputer-controlled embodiment of the present invention.

The following is a flow chart for a computer-controlled embodiment of the invention that speaks for itself and requires no further explanation:

• A_u - unterer Amplitudenwert
• A_o - oberer Amplitudenwert
• F_r - Ausgang des λ-Reglers
• P_n - negativer Proportionalanteil
• P_p - positiver Proportionalanteil
• I_i - Ist-Amplitude
• S_neu - neur Integratorsteigungskorrektur
• S_alt - alte Integratorsteigungskorrektur
• G_f - Gewichtungsfaktor
• I_so - Sollamplitude
• S_v - Vorsteurung der Integratorsteigung
• U_soll - Sollspannung der λ-Sonde
• Ust - Istspannung der x-Sonde

Abbreviations:

• A _u - lower amplitude value
• A _o - upper amplitude value
• F _r - output of the λ controller
• P _n - negative proportional component
• P _p - positive proportional component
• I _i - actual amplitude
• S _new - new integrator slope correction
• S _old - old integrator slope correction
• G _f - weighting factor
• I _so - target amplitude
• S _v - pre-control of the integrator slope
• U _soll - target voltage of the λ probe
• Ust - actual voltage of the x-probe

5 shows the output signal F _{r of} a controller in the mixture metering system of an internal combustion engine as a function of time. At time t _A there is a sudden change in the pilot control value and the dead time of the control loop (combination of the effects of FIGS. 3a and 3b). Despite the simultaneous change of these two quantities, the system according to the invention achieves an adjustment of the integrator slope after approximately three oscillation cycles in such a way that the target amplitude of the control oscillation is reached.

Seen as a whole, the maximum control frequency can be achieved with this system according to the invention, since the amplitudes of the P and I components of the control oscillation are adjusted to values of equal size by adapting, in particular, the integrator slope, and the controller therefore always works at its optimum. Variations in specimens from one engine to another or one exhaust gas probe to another and long-term changes in the engine and exhaust gas probe are no longer of disadvantageous importance due to the adapting slope of the integrator. Although exemplary embodiments of the invention have been explained in connection with an injection system, it does not matter to the essence of the invention in what special way the mixture preparation is carried out.

Claims (9)

1. Mixture metering system of an internal-combustion engine with an exhaust-gas probe which is exposed to the exhaust of the internal-combustion engine, indicates the air/fuel ratio lambda, preferably has a two-point characteristic and the signals of which are subjected to a downstream control function having integral action, preferably proportional-integral action, and in which the output variable of this control function acts on the mixture composition in a correcting manner, with means of recording the at least integral component of the amplitude of the output variable of the control function as well as of a cascade control of the integral action of this control function, characterized in that the increase in the I component of the control function can be set in many different ways by means of the cascade control as a function of the at least integral component of the measured amplitude.

2. Mixture metering system according to Claim 1, characterized in that the proportional component included in the control oscillation is fixed to a predetermined value or is set in relation to the measured amplitude of the control oscillation.

3. Mixture metering system according to at least one of Claims 1 or 2, characterized in that the integral component included in the control oscillation has in the steady state the same amplitude as the proportional component.

4. Mixture metering system according to at least one of Claims 1 to 3, characterized in that the actual value of the increase in the integral component of the control oscillation is determined in time sequence, compared with a set value and over this set/actual value comparison the increase in the integral component is corrected.

5. Mixture metering system according to at least one of Claims 1 to 3, characterized in that the actual values of the amplitude of the control oscillation are determined in time sequence, compared with a set value and over this set/actual value comparison the increase in the I component is corrected.

6. Mixture metering system according to at least one of Claims 1 to 5, characterized in that a precontrol of the actual value of the increase in the integral component of the control oscillation is carried out as a function of internal-combustion engine operating chaiacteristics dependent at least on the load condition of the internal-combustion engine.

7. Mixture metering system according to at least one of Claims 1 to 6, characterized in that the influence of the cascade control on the integral component can be influenced by a weighting factor (G_f).

8. Mixture metering system according to Claim 7, characterized in that the weighting factor (G_f) is set to values G_f < 1.

9. Mixture metering system according to at least one of the preceding claims, characterized in that the output variable of the control function acts in a correcting manner on the precontrol values for the mixture composition, which are dependent on operating parameters of the internal-combustion engine, such as speed, rate of air flow or the load.

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