AIR/FUEL RATIO CONTROL APPARATUS FOR INTERNAL COMBUSTION ENGINES

Abstract: In feedback control for the air/fuel ratio based on a sensor (4) producing an output signal proportional to a change in the air/fuel ratio, a control constant modifier (10) is operative to vary the proportional component and the integration component in a feedback system (7, 8, 9) in accordance with an air/fuel ratio set for controlling, thereby assuring stability of ultimate fuel control.

Description: BACKGROUND OF THE INVENTION This invention relates to an air/fuel ratio control apparatus for internal combustion engines. Conventionally, the air/fuel ratio control apparatus uses a conventional oxygen sensor the output signal of which changes stepwise at an air/fuel ratio λ equal to one (stoichiometric air/fuel ratio), as described in JP-A-51-106828. Accordingly, if the conventional air/­ fuel ratio control apparatus is incorporated with another type of air/fuel ratio sensor the output of which changes in proportion to the air/fuel ratio, the control system will operate erroneously. More particularly, in the sensor producing an output signal proportional to an air/­ fuel ratio, the gain relative to the air/fuel ratio is smaller in the lean ragion and larger in the rich region. Therefore, if the proportional constant of the control system, for example, remains unchanged throughout the rich and lean regions, an erroneous operation such as hunting will result within either one of the rich and lean regions. SUMMARY OF THE INVENTION An object of this invention is to provide an air/fuel ratio control apparatus which can use the sensor having its output signal proportional to the air/fuel ratio without causing hunting and the like. This invention features that with the sensor producing a proportional detection signal used, the pro­ potional or integration constant of a proportional or integration circuit of the air/fuel ratio control system is changed in compliance with a value of air/fuel ratio set for controlling. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic block diagram showing the overall construction of an air/fuel ratio control appara­ tus according to an embodiment of the invention; Fig. 2 is a graph showing a sensor output characteristic; Fig. 3 is a sectional view showing the con­ struction of an air/fuel ratio sensor; Figs. 4 to 7 are illustrative of the principle of one type of air/fuel ratio sensor; Fig. 8 is a graph showing a characteristic of the sensor of Figs. 4 to 7; Fig. 9 is illustrative of the principle of another type of sensor; Fig. 10 graphically illustrates a character­ istic of the Fig. 9 sensor; Fig. 11 is a graph showing various character­ istics of the sensor; Figs. 12 to 14 are graphs showing closed loop control of the air/fuel ratio and closed loop control characteristics; Fig. 15 is a block diagram of an air/fuel ratio closed loop control system; Fig. 16 is a diagram showing signal waveforms appearing in the system of Fig. 15; Fig. 17 is a circuit diagram for implementation of the Fig. 15 system according to an embodiment of the invention; Fig. 18 is a circuit diagram showing a modifica­ tion of part of the Fig. 17 circuit; Fig. 19 is a flow chart for implementation of the Fig. 15 system according to another embodiment of the invention; Fig. 20 illustrates map data used in the Fig. 19 embodiment; Fig. 21 is a graph showing actual output charac­ teristics of the air/fuel ratio sensor; Fig. 22 is a circuit diagram for implementation of the Fig. 15 system according to further embodiment of the invention; and Fig. 23 is a graph showing the ideal relation between the gain of air/fuel ratio sensor and the propor­ tional gain. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Fig. 1, there is illustrated, in block form, an air/fuel ratio control apparatus embody­ ing the invention. The apparatus comprises a fuel feeder 1 for feeding fuel to an engine 2, and an air/fuel ratio sensor 4 mounted in an exhaust pipe 3. A drive circuit 5 for the sensor 4 transmits a signal proportional to an air/fuel ratio. Based on this signal, a difference generator 6 determines a difference between a set value and a detected value, and the difference is applied to a proportional circuit 7 and an integration circuit 8. A control signal generator 9 is responsive to output signals from the proportional circuit 7 and integration circuit 8 to generate and output a control signal to the fuel feeder 1 such as an electronic fuel injection valve. Thus, the control apparatus of the above construction is well adapted to implement proportional and integration closed loop control of the air/fuel ratio. This apparatus may otherwise be added with a differentiation control system to perform PID control. Since, as shown in Fig. 2, the air/fuel ratio sensor 4 has different output gains relative to air/fuel ratio λ for a lean region where λ > 1.0 and for a rich region where λ < 1.0, the proportional constant of pro­ portional circuit 7 and the integration constant of integration circuit 8 for closed loop control at a point A within the lean region must be changed from those for closed control at a point B within the rich region. Especially, the proportional control system is predominant in the closed loop control and so stability of the closed loop control system is greatly affected by changes in the proportional constant. Therefore, the apparatus of Fig. 1 further comprises a control constant modifier 10 adapted to issue commands which change the control constant of the proportional circuit 7 mainly and also the control constant of the integration circuit 8 in compliance with a value of air/fuel ratio set for controlling. Components of the apparatus shown in Fig. 1 will now be described in greater detail. Th air/fuel ratio sensor 4 is constructed as shown in Fig. 3, having a solid electrolyte 11, a diffu­ sion resistor 12 of a porous material, a heater 13 for heating the solid electrolyte 11, and a protective tube 14. When the solid electrolyte 11 having ability to conduct oxygen ions is heated to about 600 to 1000°C by means of the heater 13 and current or voltage is supplied to electrodes provided, as will be described later, on the opposite side surfaces of solid electrolyte 11, an amount of oxygen, which is proportional to amounts of electricity supplied to the opposite side surfaces, pro­ pagates through the solid electrolyte 11. The amount of oxygen prevailing in the diffusion resistor 12 is then controlled by utilizing this oxygen pumping effect such that the partial pressure of oxygen inside the diffusion resistor 12 is always constant. Then, the amounts of electricity consumed to establish the constant oxygen partial pressure are in proportion to an air/fuel ratio. In the solid electrolyte 11, atmospheric air is admitted to the interior side surface and an exhaust gas is admitted to the exterior side surface. Fig. 4 is illustrative of the principle of air/­ fuel ratio detection. In particular, an encircled portion in Fig. 3 is enlarged for illustration at (a) in Fig. 4. An electrode 15a is provided on a side surface exposed to atmosphere and an electrode 15b is provided on the opposite side surface exposed to the exhaust gas. As is seen at (a) in Fig. 4, an electromotive force E developing in the solid electrolyte 11 is measured. By measuring this E, the oxygen partial pressure inside the diffusion resistor 12 can be measured. More specifically, when an amount of oxygen is charged into or discharged from the interior of the diffusion resistor 12 so as to keep constant the E representative of the oxygen partial pressure, this amount of propagating oxygen is determined by amounts of electricity supplied to the opposite side surfaces, which amounts are in turn proportional to an air/fuel ratio under measurement and set for controlling. The amount of oxygen inside the diffusion resistor 12 is controlled as shown at section (b) in Fig. 4. The elec­ trode 15b is applied with a fixed voltage V P through a buffer amplifier 16 and the electrode 15a is applied with a voltage V D through a buffer amplifier 17. When V D > V P is established by changing the voltage V D , a current I is passed in a direction of solid arrow and the oxygen O₂ in the diffusion resistor 12 is consequently discharged in a direction of solid arrow. As a result, the amount of oxygen inside the diffusion resistor 12 is decreased. When the voltage V D is decreased to establish V D < V P , a current I is passed in a direction of dotted arrow and the oxygen O₂ is charged in a direction of dotted arrow, thus increasing the amount of oxygen inside the diffusion resistor 12. In this way, the electromotive force E representative of the partial pressure of oxygen within the diffusion resistor 12 can be controlled so as to be always constant. The measurement of the E and the application of the voltage V D are alternately effected on the time division basis. The alternate operations are accomplished for the lean region as illustrated in a tim­ ing chart of Fig. 5 wherein V D > V P is established during a period for the application of V D to obtain the constant electromotive force E. For the rich region, V D < V P is established to obtain the constant electromotive force E as illustrated in a time chart of Fig. 6. In Figs. 5 and 6, the level of V D settled for the constant E is in pro­ portion to an air/fuel ratio set for controlling. The aforementioned alternate operations are carried out with a specified circuit arrangement as shown in Fig. 7. Initially, an electromotive force E is measured by closing switches 19a and 19b and opening switches 18a and 18b under the direction of a controller 80 and held by a circuit comprised of an amplifier 20. Subsequently, a differential integration circuit comprised of an ampli­ fier 21 compares the measured E with a reference voltage E ref (constant) and then integrates a difference between the E and E ref to increase or decrease the voltage V D in accordance with a time constant. Specifically, for E > E ref , the voltage V D is decreased for E < E ref , increased.Thereafter, the thus varied voltage V D is applied to the solid electrolyte 11 by closing the switches 18a and 18b and opening the switches 19a and 19b. With the circuit arrangement constructed as above, even when the air/furl ratio changes, the voltage V D is controlled for increase or decrease to always make the E equal to the E ref and hence it is placed in proportion to an air/fuel ratio under measurement. By opening a switch 19c simul­ taneously with the switch 19a and 19b, the voltage V D is held by a hold circuit comprised of an amplifier 22 and delivered as an output signal V out . The output signal V out is related to the air/fuel ratio λ as graphically illustrated in Fig. 8. At λ = 1.0 (stoichiometric air/­ fuel ratio), the V out equals V P which is time-invariable. The V out is subject to gains which are different for the rich and lean regions. In particular, the sensor is more sensitive in the rich region and less sensitive in the lean region. In addition to the above method, various methods have been proposed for measurement of the air/fuel ratio over a wide range covering the rich and lean regions, including one that is executed with an arrangement as shown in Fig. 9. The arrangement of Fig. 9 includes solid elec­ trolytes 23 and 24, a diffusion hole 25 and a chamber 26. When a fixed current I B is supplied to the solid electro­ lyte 24 in a direction of arrow, oxygen O₂ in atmosphere is charged into the chamber 26. With the other solid electrolyte 23 applied with a fixed voltage V S of 0.2 to 1.0 V, the diffusion hole 25 functions to generate a so-called marginal current I S which is proportional to an amount of oxygen inside the chamber 26. In the lean region, the I S takes a value which is proportional to the sum of an amount of oxygen charged by the I B and an amount of oxygen contained in an exhaust gas diffusing into the chamber through the diffusion hole 25. In the rich region, the oxygen charged by the I B is consumed by a combustible gas of CO, HC and H₂ diffusing into the chamber 26 and the I S takes a value which is proportional to an amount of the remaining oxygen. As the air/fuel ratio λ falls below 1.0 with the content of the combustible gas increased, the I S decreases. Thus, the output signal V out corresponding to the I S is related to the air/fuel ratio λ as graphically illustrated in Fig. 10. For I B=0 , the I S can be measured only within the lean region, as indicatid by a curve (a). Where the I B is a positive fixed value, a characteristic curve (b) is obtained, indicating that the measurement of the air/fuel ratio can be permitted over a wide range. In this method, the V out is subject to gains relative to the λ which are different for the rich and lean regions. Characteristics of the air/fuel ratio sensor will be more detailed with reference to Fig. 11. In the lean region, the oxygen partial pressure, P O2 , increases as the air/fuel ratio λ increases, causing the V out to increase. In the rich region, the partial pressure of CO, HC and H₂ combustible gas, P CO + P HC + P H2 , increases as the λ decreases, causing the V out to decrease. Especially, in the rich region where λ < 1.0, the gain is different from that for the lean region to deviate from a dotted-line extension (a) because a consti­ tuent H₂ gas has 3 to 4 times the diffusion speed of the remaining gas O₂, CO or HC and consumes a great amount of oxygen charged into the diffusion resistor through the solid electrolyte. Therefore, the gain K R for the rich region becomes larger than the gain K L for the lean region. This invention contemplates improvements in air/­ fuel ratio control based on the sensor having sensitivity which, as has been explained hereinbefore, is different for the rich and lean regions. When using the sensor, the air/fuel ratio can be controlled through proportional and integration closed loop control as will be described with reference to Figs. 12, 13 and 14. Fig. 12 illustrates at section (a) a control signal for controlling the amount of fuel. Since combustion conditions in the engine slightly vary even under normal operation, the control signal also varies slightly as indicated at (a) in Fig. 12 to correct a variation in combustion. Fig. 12 also indicates at (a) that values of the proportional and integration constants remain unchanged throughout the lean and rich regions. In such a case, owing to the different gains devoted to the V out , an erroneous operation takes place in either one of the lean and rich regions.For example, if the control constants are set to meet the lean region, then controlling with the same control constants will lead to too high a proportional gain in the rich region and hunting will result as shown at (b) in Fig. 12. Even without the occurrence of hunting, the ultimate air/fuel ratio will deviate from a commanded air/fuel ratio by a stationary difference e in the rich region as indicated at (c) in Fig. 12. To solve the above problems, it is necessary to change the control constants such that they meet both the rich and lean regions. To this end, according to this invention, the proportional gain is varied complementarily to the differ­ ent gains K R and K L shown in Fig. 13, that is, made smaller for the rich region than for the lean region as shown at (a) in Fig. 14. By using the varying proportional gain, the ultimate air/fuel ratio freed from hunting or station­ ary difference e can be obtained even in the rich region as shown at (b) in Fig. 14. Especially where the varying proportional gain requires the integration time constant to vary, the integration constant needs to be varied for the rich and lean regions. The proportional and integration control system is generally and schematically illustrated for clarity of explanation in a block diagram of Fig. 15. Referring to Fig. 15, an engine 30 illustrated as a block includes a fuel feed system and is controlled in terms of air/fuel ratio. A circuit 27 for deriving and producing a differ­ ence a has a proportional gain of K₁ This difference is multiplied by a constant gain K₂ at a block 28 to prepare a proportional component b of the control signal. The difference a is also integrated at a block 29 to prepare an integration component c of the control signal. The integration component is added to the input signal to remove an offset. When the input signal is applied stepwise as indicated at (a) in Fig. 16, this signal is processed into a difference which in turn is multiplied by the gain K₁ to provide the difference signal a as indicated at (b) in Fig. 16. The difference signal a is further multiplied by the gain K₂ to provide the signal b as indicated at (c) in Fig. 16. The difference signal a is also integrated with a time constant T I to provide the signal c as indicated at (d) in Fig. 16. The proportional signal b and the integration signal c are added together to provide a signal d as indicated at (e) in Fig. 16. If the pro­ portional gain K₂ of block 28 is decreased, the signal b is decreased as indicated by a dotted line at (c) in Fig. 16 and on the other hand, if the integration time constant T I of block 29 is increased, the signal c is decreased as indicated by a dotted line at (d) in Fig. 16 with the result that the sum signal d is also decreased as indicated by a dotted line at (e) in Fig. 16. In this manner, various results can be obtained by changing the proportional and integration constants, more specifically, by changing the proportional gain K₂ and integration time constant T I . Thus, in accordance with the present invention, the two values for the rich region are made different from those for the lean region. Fig. 17 illustrates, in block form, an embodi­ ment of a circuit for implementation of the Fig. 15 control system. Referring to Fig. 17, a difference circuit C₁ produces a difference between the output signal V out and a voltage V ref which is the sum of a commanded value and a fixed value. The output signal of the difference circuit C₁ is multiplied by the proportional gain K₂ at an ampli­ fier circuit C₂. This amplifier circuit C₂ produces an amplified signal which contains an amplified AC component and an amplified DC component and so, the amplified DC component (V ref ") is subtracted at a subtraction circuit C₃ to provide a difference signal representative of deviation from the commanded value and which is multiplied by the proportional gain K₂. The output signal of the difference circuit C₁ is also supplied to a differential integration circuit C₄ so as to be compared with a voltage V ref ′ corresponding to the commanded value, so that an integrated signal in accordance with a difference representative of deviation from the commanded value is delivered out of the differen­ tial integration circuit C₄. The integrated signal is increasing when the output signal of the difference circuit C₁ is larger than the V ref ′ and is decreasing when the output signal is smaller than V ref ′, indicating that correct integration operations are being carried out. The thus obtained proportional component and integration component are added together at an adder circuit C₅ to provide a control signal. In the circuit of Fig. 17, the proportional constant and the integration constant can be varied as will be described below. Since the proportional constant is defined by a resistance ratio between resistances of resistors R₁ and R₂ included in the amplifier circuit C₂, the proportional constant can be varied by turning on or off a switch S₁ to connect or disconnect a resistor R₃ in parallel relationship with the resistor R₂. The switch S₁ is operated by a command from the control constant modifier 10. Similarly, the integration constant, defined by resistances of resistors R₄ and R₅ and capacitances of capacitors C₁₀ and C₁₁ of the integration circuit C₄, can be varied by turning on or off a switch S₂ to connect or disconnect a resistor R₆ in parallel relationship with the resistor R₄ and by turning on or off a switch S₃ to con­ nect or disconnect a resistor R₇ in parallel relationship with the resistor R₅. These switches S₂ and S₃ are also operated by commands from the control constant modifier 10. In this manner, the control constants can be varied to meet an air/fuel ratio to be controlled. The integration constant of the integration circuit C₄ may also be varied using a modified circuit as shown in Fig. 18. In this modification, the integration constant can be varied by turning on or off a switch S₄ to connect or disconnect a capacitor C₁₂ in parallel relationship with the capacitor C₁₀ and by turning on or off a switch S₅ to connect or disconnect a capacitor C₁₃ in parallel relationship with the capacitor C₁₁. These switches S₄ and S₅ are again operated by commands issued from the control constant modifier 10. Without resort to the analog circuit as shown in Fig. 17, the proportional and integration control can be performed and the control constants therefor can be varied using a microcomputer in accordance with a flow chart as illustrated in Fig. 19. More particularly, a commanded air/fuel ratio λ to be controlled is first read out of a map graphically illustrated in Fig. 20, as indicated at a step 100. An output value V out* of the air/fuel ratio sensor correspond­ ing to the commanded λ is then set as indicated at a step 200. Subsequently, the output value V out* is compared with a value V out1 in a step 300 and when the V out* exceeds the V out1 , the proportional constant is set to K P in a step 400 and the integration constant is set to K I in a step 500. If it is decided in a step 600 that the V out* falls within a range V out1 > V out* ≧ V out2 , preset proportional constant K P is incremented by ΔK P in a step 700 and the preset integration constant K I is incremented by ΔK I in a step 800. If it is decided in a step 900 that the V out* falls within a range V out2 > V out* ≧ V out3 , the preset proportional constant K P is further incremented by ΔK P ′ in a step 1000 and the preset integration constant K I is further incremented by ΔK I ′ in a step 1100. The above operations are repeated to obtain optimum control constants which meet the air/fuel ratio to be controlled. There is illustrated in Fig. 21 an actual V out versus air/fuel ratio characteristic plotted by measuring an engine exhaust gas. Since the partial pressures P O2 and P CO + P HC + P H2 change curvedly with respect to the air/fuel ratio, the V out also changes curvedly. In other words, the V out is subject to gains relative to the air/­ fuel ratio which not only change at the boundary between the rich and lean regions but also slightly vary in the lean region itself and in the rich region itself. Under the situation, it is ideal that the control constants should be varied continuously or in analog fashion with respect to changes in the air/fuel ratio to be controlled. Fig. 22 shows a circuit arrangement to this end. For the purpose of varying the proportional constant of an amplifier circuit C₂, a transistor T r1 is connected to substitute for the resistor R₂ included in the amplifier circuit C₂ of Fig. 17. The resistance of the transistor T r1 is varied in analog fashion with the value of a voltage V₃ applied to its base and the proportional constant consequently varies in analog fashion. In an integration circuit C₄, transistors T r2 and T r3 are connected to substitute for the resistors R₄ and R₅ included in the integration circuit C₄ of Fig. 17. Similarly, the resistances of the transistors T r2 and T r3 are varied in analog fashion with values of voltages V₁ and V₂ applied to their bases and hence the integration constant of the integration circuit C₄ varies in analog fashion. A voltage generator 30 responds to commands from the control constant modifier 10 to generate the voltages V₁, V₂ and V₃ and to change their levels in accordance with an air/fuel ratio set for controlling. If the voltages V₁, V₂ and V₃ are controlled to propor­ tionate the V out , exact analog operations can be per­ formed. In this manner, some control constants can be varied, especially, in exact analog fashion. Typically, as graphically shown in Fig. 23, the V out is subject to gains relative to the air/fuel ratio which vary curvedly in analog fashion so as to be larger in the rich region than in the lean region and therefore, it is ideal that the proportional gain should be varied complementarily to the V out to trace an analog curve which is of smaller values in the rich region than in the lean region. The proportional gain can be varied to comply with the analog curve of Fig. 23 by making use of changes in resistance of the transistor used in the cir­ cuit of Fig. 22. As has been described, according to the inven­ tion, even when the air/fuel ratio closed loop control is performed using an air/fuel ratio sensor having a non-­ linear output characteristic relative to the air/fuel ratio, optimum control constants can always be obtained to permit stable controlling at all the values of air/­ fuel ratio.

Claims: 1. An air/fuel ratio control apparatus for an internal combustion emgine comprising: air/fuel ratio sensor means (4, 5, 6) for detecting an amount of oxygen or combustible component remaining in an exhaust gas to produce an output signal proportional to an air/fuel ratio in said engine (2); means (1, 6, 7, 8, 9) responsive to the output signal from said air/fuel ratio sensor means to control the air/fuel ratio of a mixture gas fed to said engine on the closed loop control basis; and means (7, 8, 10) for varying air/fuel ratio control constants used for the closed loop control in accordance with an air/fuel ratio set for controlling. 2. An air/fuel ratio control apparatus according to Claim 1 wherein said air/fuel ratio control constants includes an integration constant and a proportional constant, and either one or both of the two constants are varied in accordance with the air/fuel ratio set for controlling. 3. An air/fuel ratio control apparatus according to Claim 2 wherein the proportional and integration constants for a first region where the air/fuel ratio is larger than the stoichiometric air/fuel ratio is made different from those for a second region where the air/­ fuel ratio is smaller than the stoichiometric air/fuel ratio. 4. An air/fuel ratio control apparatus according to Claim 3 wherein the proportional and integration con­ stants are varied continuously in accordance with the air/fuel ratio set for controlling. 5. An air/fuel ratio control apparatus according to Claim 3 wherein the proportional and integration constants for said first region are made larger than those for said second region. 6. An air/fuel ratio control apparatus using an air/fuel ratio sensor (4) for detecting an amount of oxygen or combustible component remaining in an exhaust gas to produce an output signal proportional to an air/fuel ratio in an engine (2) and performing closed loop control of the air/fuel ratio of a mixture gas fed to said engine on the basis of the output signal from said air/fuel ratio sensor, wherein air/fuel ratio control constants used for the closed loop control are varied in accordance with an air/fuel ratio set for controlling.

Inventors: OSUGA, MINORU, OYAMA, YOSHISHIGE
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: F02D41
IPC codes: F02D41