GB2173926A - Air/fuel ratio feedback control system effective even during engine warm-up - Google Patents

Abstract

The system for feedback control of the air/fuel ratio in an internal combustion engine uses an oxygen sensor (50) to detect the actual air/fuel ratio in the exhaust gas. The system also has an electronic control unit (100), which outputs a control signal (Si) to regulate, e.g., the amount of fuel injection, by producing a signal indicative of the deviation of the detected air/fuel ratio from a target value and performing proportional and/or integral treatment of the deviation signal. The control system further comprises means (40) e.g. an engine coolant temperature sensor to detect the degree of warm-up of the engine and means to variably determine the values of the proportional constant (Cp) and/or the integration constant (Ci) accordingly. The feedback control operation can be started soon after starting the engine even in the case of cold-starting. The oxygen sensor (50) is described in detail and comprises a sensor cell and a pump cell each comprising an ion conductive solid electrolyte. <IMAGE>

Description

SPECIFICATION Air/fuel ratio feedback control system effective even during engine warm-up This invention relates to a system for feedback control of the air/fuel ratio in an internal combustion engine, particularly an automotive engine, by using an oxygen sensor as an exhaust gas sensor to detect actual values of the air/fuel ratio.

In the current automotive internal combustion engines, it is popular to perform feedback control of the air/fuel ratio to meet the exhaust emission regulations and also to meet the growing demands for better fuel economy and improved driveability. Usually an oxygen sensor comprising a solid electrolyte cell is used to estimate actual values of the air/fuel ratio in the engine from the concentrations of oxygen in the exhaust gas, and a signal represenative of the amount of deviation of the actual air/fuel ratio from the target value is produced in an electronic control unit. In most cases a feedback signal to control the functions of a fuel feed device such as carburetor or fuel injector is produced by proportional and/or integral treatment of the air/fuel ratio deviation signal, as described in, e.g., SAE paper No. 740014, J. F.Cassidy, Jr., "Electronic Closed Loop Control for Automobilies", published February, 1974.

In the cases of automotive engines using fuel injectors, a standard amount of fuel injection Tp is varied according to the engine operating conditions and may be given by the equation (1): T" = K.(QJN) (1) wherein K is a constant, Q is the flow rate of air being taken into the engine, and N is the revolving speed of the engine.

When feedback control of the air/fuel ratio is performed, a corrected amount of fuel injection T, is computed by using a feedback signal to cancel deviations of the actual air/fuel ratio from the target value. For example, T, is given by the equation (2): T, = Tp X C X a + T, (2) where C is a weighting factor which is variable depending on some parameters of the engine operating conditions such as the temperature of the cooling water, degree of opening of the throttle valve, etc., a is a feedback correction factor computed by the aforementioned proportional and/or integral treatment of an air/fuel ratio deviation signal, and Ts is a correction factor for compensation of a delay in the response of the fuel injector to a control or command signal.In the conventional air/fuel ratio feedback control systems, the proportional constant and the integration constant in computing the feedback correction factor a are respectively fixed at predetermined values because, under normal operating conditions of the engine after sufficient warmup, a definite correlation exists between the actual air/fuel ratio in the engine and a signal produced by the operation of an oxygen sensor exposed to the exhaust gas and also because there is little difference between the actual air/fuel ratio and a calculational air/fuel ratio established by the controlled function of the fuel injectors. The aforementioned proportional constant and/or the integration constant will collectively be referred to as the constant, or constants, of feedback control. The speed of the feedback control depends on the values of these constants.

At starting an automotive engine provided with a conventional air/fuel ratio control system it is customary to defer the start of the feedback control operation until completion of the engine warm-up or a transiently accelerating condition. In this regard, a recent trend is to commence feedback control of the air/fuel ratio soon after starting the engine with a view to satisfying the demands for better fuel economy and driveability. However, it is not easy to perform proper feedback control of the air/fuel ratio before sufficient warm-up of the engine. A principal reason is that vaporization of the injected fuel remains incomplete at cold-starting of the engine and particularly in the winter season. That is, more than a negligible quantity of fuel in a liquid state adheres to the wall surfaces in the intake manifold and intake ports.Therefore, there arises discrepancy between an apparent or calculated air/fuel ratio established by the controlled operation of the fuel injectors and an effective air/fuel ratio at which combustion takes place and which is detected by the function of the oxygen sensor in the exhaust gas. Though the discrepancy lessens as the engine warms up, it does not vanish before the end of the warm-up stage of the engine operation and augments if the engine revolution is accelerated or decelerated. It is natural that until the effective and detectable air/fuel ratio comes into agreement with the calculated air/fuel ratio the accuracy and efficiency of the air/fuel ratio feedback control are inferior.If the actual air/fuel ratio deviates considerably from the target value during the warm-up period it takes a long time to converge the deviating air/fuel ratio to the target value since the constants of feedback are fixed so as to be optimum under normal operating conditions of the engine after warming up. If the constants of feedback are fixed otherwise in order to cope with deviations of air/fuel ratio before warmup of the engine, the feedback control operation after the warm-up will possibly cause significant hunting of the air/fuel ratio.

It is an object of the present invention to provide a system for feedback control of the air/fuel ratio in an internal combustion engine, which control system can quickly, smoothly and accurately correct deviations of the air/fuel ratio from the target value not only during normal operations of the engine after warming up but also before sufficient warm-up of the engine.

To accomplish the above object the present invention proposes to vary the values of the constants of feedback control according to the degree of warm-up of the engine.

More definitely, the invention provides a control system for feedback control of the air/fuel ratio of an air-fuel mixture supplied to an internal combustion engine, the control system comprising an oxygen sensor which is disposed in an exhaust passage of the engine and provides an air/fuel ratio signal representative of the actual air/fuel ratio in the engine, warm-up detection means for detecting the degree of warm-up of the engine, control means for producing and outputting an air/fuel ratio control signal by first producing a deviation signal indicative of the amount of deviation of the air/fuel ratio represented by the air/fuel ratio signal from a predetermined value and performing arithmetic treatment of the deviation signal by using at least one constant of feedback control, and adjusting means for variably determining the value of said at least one constant according to the degree of warm-up of the engine detected by the detection means.

The air/fuel ratio control system according to the invention is very suitable for application to automotive engines. It is a matter of course that the air/fuel ratio control signal produced in this system is used to regulate the rate of air intake into the engine and/or the rate of fuel feed to the engine. In this control system the values of the constants of feedback control such as a proportional constant and/or an integration constant are gradually varied during the warm-up stage of the engine operation so as to become optimum for feedback control of the air/fuel ratio in every region of the relatively low temperatures of the engine in warm-up operation. Accordingly feedback control of the air/fuel ratio can be commenced soon after starting the engine and can be performed accurately and smoothly even though the engine is not sufficiently warmed up.This is very favourable for fuel economy and exhaust emission control and also for driveability of the engine. After warm-up of the engine the constants of feedback can be kept fixed so as to be optimum under normal operating conditions of the engine. The control means and the constant adjusting means in the feedback control system can be integrated and embodied in a microcomputer. In the accompanying drawings: Fig. 1 is a block diagram showing the fundamental construction of an air/fuel ratio control system according to the invention; Fig. 2 is a diagrammatic illustration of an embodiment of the invention, which is an air/fuel ratio feedback control system for an automotive engine; Fig. 3 is a schematic and sectional view of an oxygen sensor used in the system of Fig.

2; Fig. 4 is an exploded view of the oxygen sensor of Fig. 3; Fig. 5 is a simplified circuit diagram of an air/fuel ratio detection circuit used in the system of Fig. 2; Fig. 6 is a graph showing the relationship between air/fuel ratio in the engine in Fig. 2 and a voltage signal produced in the circuit of Fig. 5; Fig. 7 is a chart illustrating deviations of actual air/fuel ratio at which combustion takes place in the engine during an initial stage of the feedback control operation from an apparent air/fuel ratio established by the feedback control; Fig. 8 is a chart illustrating the manners of convergence of actual air/fuel ratio in the engine to a target value at feedback control of the air/fuel ratio by conventional control systems; Fig. 9 is a flow chart showing a computer program stored in the control unit of the system of Fig. 2 for computing a feedback correction factor;; Fig. 10 is a chart showing the manner of determining the value of a constant, which becomes an important factor in both producing an air/fuel ratio control signal and computing the feedback correction factor, in dependence on the temperature of the engine cooling water in the operation of a control system according to the invention; and Fig. 11 is a chart showing the manner of determining the value of another important constant in dependence on the temperature of the engine cooling water in the operation of a control system according to the invention.

Fig. 1 shows the functional connections between the principal elements of an air/fuel ratio control system according to the invention.

The control system includes an oxygen sensor 10, which is disposed in the exhaust system of the internal combustion engine in which the air/fuel ratio is to be controlled as a means to detect the actual air/fuel ratio in the engine.

An electronic control unit 12 performs proportional and/or integral control by utilizing a voltage or current signal produced by the operation of the oxygen sensor 10 to produce either a fuel feed control signal or an air intake control signal, which is supplied to an electromechanical means 14 for minutely regulating the feed rate of fuel or air into the engine. Furthermore, the air/fuel ratio control system includes a temperature sensing means 16 to detect the degree of warm-up of the engine, and the control unit 12 is provided with a constant adjusting means 18 which has the function of optimumly determining the constant or constants to be employed in the proportional and/or integral control based on the information supplied from the warm-up detection means 16.

As an embodiment of the invention, Fig. 2 shows an automotive internal combustion engine 20 provided with an air/fuel ratio control system which accomplishes its purpose by controlling the amount of fuel injection into the engine. In the usual manner an intake passage 22 extends from an air cleaner 24 to the combustion chambers of the engine 20, and electromagnetically operated fuel injectors 26 open into the intake passage 22. In an exhaust passage 28, a catalytic converter 30 occupies an intermediate section for purifying the exhaust gas by means of a suitable catalyst such as a three-way catalyst.

In the intake passage 22 there is an airflow meter 32 which produces a signal representative of the flow rate Q, of air being taken into the engine, and a sensor 36 is coupled with throttle valve 34 to produce a signal representative of the degree of opening C5 of the throttle valve 34. A crank-angle sensor 38 is provided to produce a signal representative of the engine revolving speed N. A temperature sensor 40 is disposed in the cooling water jacket to produce a signal representative of the cooling water temperature Tw. In this embodiment the cooling water temperture sensor 40 is employed as means to detect the degree of warm-up of the engine 20.

An oxygen sensor 50 is disposed in the exhaust passage 28 at a section upstream of the catalytic converter 30 to estimate an actual air/fuel ratio in the combustion chamber from the concentration of oxygen in the exhaust gas. In the present invention the type of the oxygen sensor 50 is not specified, so that a wide selection can be made from conventional and recently developed oxygen sensors.

For example, the oxygen sensor 50 is comprised of an oxygen concentration cell using an oxygen ion conductive solid electrolyte and an oxygen ion pump cell which too uses a similar solid electrolyte. An air/fuel ratio detection circuit 80, which is a part of the control unit 12 in Fig. 1, measures the output voltage V of the concentration cell in the oxygen sensor 50 and supplies a controlled pumping current Ip to the pump cell in the sensor 50 so as to keep the output voltage V at a predetermined level. Furthermore. this circuit 80 produces a voltage signal V, which is representative of the magnitude of the controlled pumping current 1, and, therefore, is indicative of the actual air/fuel ratio.

The air/fuel ratio control system of Fig. 2 has a control unit 100 in which the control unit 12 and the constant adjusting means 18 shown in Fig. 1 are integrated. This control unit 100 is a microcomputer comprised of CPU 102, ROM 104, RAM 106 and I/O port 108. The ROM 104 stores programs of operations of CPU 102. The RAM 106 stores various data to be used in operations of CPU 102, some of which are in the form of map or table. The signals produced by the above described sensors 32, 36, 38 and 40 are input to the I/O port 108 along with the air/fuel ratio signal V, produced in the detection circuit 80. Based on the engine operating condition information gained from these input signals the control unit 100 provides a fuel injection control signal Sj to the injectors 26 so as to realize an intended air/fuel ratio.

The construction of the oxygen sensor 50 is, for example, as shown in Figs. 3 and 4.

This oxygen sensor 50 is a laminate-like assembly of thin layers including a substrate 52 of a ceramic material such as alumina. As shown in Fig. 4 a heater element 54 is attached to or embedded in the substrate 52.

On the substrate 52 there is another ceramic board 56, which is formed with a shallow channel 58 in its top surface so as to leave undepressed marginal regions on three sides.

A first layer or plate 60 of an oxygen ion conductive solid electrolyte, such as zirconia stablized with calcia or yttria, is bonded to the ceramic board 56 so that the channel 58 in the board 56 becomes a chamber which is open to the atmosphere only at one side of the rectangular assembly. The bottom face of the solid electrolyte plate 60 is locally laid with an anode layer 62 which is to be exposed to the air admitted into the chamber 58. A cathode layer 64 is formed on the top face of the solid electrolyte plate 60. A spacer sheet 68 is bonded to the solid electrolyte plate 60 so as to cover a roughly half area not containing the cathode layer 64. Usually the thickness L of the spacer 68 is about 0.1 mm. A second layer or plate 70 of an oxygen ion conductive solid electrolyte is bonded to the spacer 68 so as to lie opposite and parallel to the first solid electrolyte plate 60.As the result, a gap 72 of the given width L exists between the first and second solid electrolyte plates 60 and 70. The bottom face of the solid electrolyte plate 70 is locally laid with a cathode layer 76, which faces to, and is exposed in the gap 72. An anode layer 74 is formed on the top face of the solid electrolyte plate 70.

In using this oxygen sensor 50 in the air/fuel ratio control system of Fig. 2, the sensor 50 is disposed in the exhaust passage 28 such that the exhaust gas indicated by arrows G in Fig. 3 enters the aforementioned gap 72 while only the air (or an alternative oxygencontaining reference gas) is admitted into the chamber 58. The combination of the first solid electrolyte plate 60 and the anode and cathode layers 62 and 64 serves as an oxygen concentration cell which generates a variable electromotive force or voltage V5 according to a difference in oxygen partial pressure be tween the air existing on the anode side and the gas G existing on the cathode side. In the following description this combination will be called the sensor cell 66.

The combination of the second solid electrolyte plate 70 and the anode and cathode layers 74 and 76 will be called the pump cell 78. When an externally supplied DC current Ip flows across the solid electrolyte plate 70 from the anode 74 toward the cathode 76, there occurs migration of oxygen ions through the solid electrolyte plate 70 from the cathode side toward the anode side. Therefore, the flow of the current Ip in such a direction results in extraction of some oxygen from the gas G existing in the gap 72. When the current Ip flows in the reverse direction some oxygen ions are suplied through the solid electrolyte plate 70 to the gas G in the gap 72.

Thus, the pump cell 78 functions as an oxygen ion pump. Because of the narrowness of the gap width L, considerable resistance is offered to diffusion of the exhaust gas G into the gap 72. Therefore, the transfer of oxygen from or into the gap 72 by the action of the pump cell 78 is effective for varying the partial pressure of oxygen within the gap 72. For this reason the magnitude of the output voltage V5 of the sensor cell 66 can be varied by controlling the pumping current Ip.

The heater 54 is incorporated in the sensor 50 to heat both the first and second solid electrolyte plates 60 and 70 when the exhaust gas temperature is not sufficiently high since the solid electrolyte material used in the sensor 50 is usefully active only at fairly elevated temperatures.

Fig. 5 shows the construction of the air/fuel ratio detection circuit 80 in the system of Fig.

2. The circuit 80 includes a DC power source 82 which provides a target voltage V,. A differential amplifier 84 is used to compare the output voltage Vs of the sensor cell 66 of the oxygen sensor 50 with the target voltage V, and to produce a voltage signal AV which represents the difference V5-V,. There is a current supplying circuit 86 for supplying the pumping current Ip to the pump cell 78 in the oxygen sensor 50. This circuit 86 receives the output AV of the differential amplifier 84 and varies the polarity and magnitude of the current Ip so as to nullify the differential voltage AV by the function of the pump cell 78.More particularly, the current supplying circuit 86 functions so as to increase the pumping current Ip when the differential voltage AV is positive and to decrease the current Ip when AV is negative. In Fig. 5 the pumping current p is positve when flowing in the direction of the arrow in broken line and negative when flowing in the direction of the arrow in solid line. The path of the current Ip includes a resistance 88 which is used to detect the magnitude of the pumping current Ip. That is, a current detection circuit 90 produces a voltage signal Vj which is proportional to a voltage drop across the resistance 88. Naturally, V is proportional to Ip.

In the air/fuel ratio detection circuit 80 the target voltage V, is set at such a value that the output voltage V5 of the oxygen sensor 50 becomes equal to V, when the concentration of oxygen in the gas within the gap 72 in the oxygen sensor 50 is as expected under the desired air/fuel ratio condition or, in other words, when the oxygen partial pressure ratio between the anode 62 and the cathode 64 of the sensor cell 66 is as expected.Since the pumping current Ip is controlled so as to nullify the difference AV between Vs and V5 while V5 is deviating from V5 by changes in the oxygen concentration in the exhaust gas G diffused into the gap 72, the current Ip or indication voltage V, produced by the current detection circuit 90 varies with the actual air/fuel ratio of the mixture supplied to the engine. There is a definite relationship between the air/fuel ratio and the indication voltage Vl as shown in Fig. 6, wherein the air/fuel ratio on the abscissa is represented by excess air factor (;1).

Therefore, by utilizing the indication voltage V it is possible to accurately and continuously detect the actual air/fuel ratio over a wide range including both fuel-rich conditions and lean conditions.

The control unit 100 utilizes the output V, of the air/fuel ratio detection circuit 80 as an air/fuel ratio signal and makes proportional and integral treatments of a difference between this signal V, and a reference signal to thereby produce the fuel injection control signal S,, which corresponds to the corrected amount of fuel injection T, given by the equation (2). Accordingly deviations of the actual air/fuel ratio from the target value can be soon corrected.

As mentioned hereinbefore, at cold-starting of the engine particularly in the winter season there is some difference between an air/fuel ratio established, on calculation, by the controlled operation of the fuel injectors 26 and an air/fuel ratio at which combustion takes place in the engine and to which the above described air/fuel ratio signal V, corresponds.

This is because of adhesion of some fuel droplets to the inner surfaces of the intake manifold and intake ports. In the chart of Fig. 7, which is merely for convenience of explanation, the curve C represents the former air/fuel ratio (calculated) and the curve EC the latter air/fuel ratio (actually effective). If the engine is accelerated or decelerated before warm-up the discrepancy between the calculational air/fuel ratio and the actually effective air/fuel ratio further increases. Even when the engine is started in a hot state the effective air/fuel ratio, represented by the curve EW in Fig. 7, still differs from the calculational air/fuel ratio during an initial stage of the engine operation, though the discrepancy is smaller than in the case of cold-starting.As the engine sufficiently warms up the effective air/fuel ratio comes into agreement with the calculated air/fuel ratio.

If feedback control of the air/fuel ratio is commenced while discrepancy exists between the calculated air/fuel ratio and the actual air/fuel ratio detectable by the oxygen sensor without taking any countermeasure, it is difficult to smoothly correct deviations of the actual air/fuel ratio from the target value. More particularly, if the constants of feedback control such as the proportional constant in the proportional control operation and the integration constant in the integral control operation are kept fixed as in the conventional feedback control systems, feedback control of the air/fuel ratio cannot so successfully be achieved as is desired. Referring to Fig. 8 which is merely for convenience of explanation, the aim of the feedback control is to keep the actual air/fuel ratio in agreement with the target value represented by the curve A.If the constants of feedback control are fixed at relatively small values, a considerably long time passes before bringing the actual air/fuel ratio represented by the curve FS to the target value A during the warm-up stage of the engine operation. If the constants of feedback control are fixed at relatively large values, it is likely that hunting of the actual air/fuel ratio occurs as represented by the curve FL during operation of the warmed engine.

In view of the above explained matter, an air/fuel ratio control system according to the invention comprises means to variably determine the constants of feedback according to the degree of warm-up of the engine. The operations of the control unit 100 in Fig. 2, which includes such means, will be described with reference to Fig. 9. The flow chart is of one of the computer programs stored in the ROM 104. This program is repeatedly executed at a predetermined time interval.

At the initial step P1 the cooling water temperature T is read in. At the next step P2 it is ascertained whether warm-up of the engine has already been accomplished or not by comparing T with a predetermined temperature Th. For example, Th is in the range of from 50 to 90"C. If T is not higher than Th, the operation proceeds to the step P3 based on a judgement that the warm-up is still incomplete. At the step P3, an optimum value of the proportional constant Cp in the proportional control algorism is found by table lookup. The realtionship between Tw and Cp such as the one shown in Fig. 10 is stored as a table in the RAM 106. At the next step P4, an optimum value of the integration constant C, in the integral control algorism is found by table look-up.The stored relationship between T and Cp is, for example, as shown in Fig.

11. If Tw is higher than Th at the step P2, the operation proceeds to the step P5 assuming that the warm-up has already been completed.

At the step P5 the proportional constant Cp is set at a predetermined high-temperature value Cph, and at the next step P6 the integration constant Cj is set at a predetermined hightemperature value Cjh.

From either step P4 or step P6 the operation proceeds or jumps to the step P7 where the amount of deviation of the air/fuel ratio A(A/F) is computed according to the equation (3): A(A/F) = VcVj (3) wherein Vc is a voltage corresponding to the target value of the air/fuel ratio.

At the step P8, the proportional component Kp of the feedback correction factor a is computed according to the equation (4), and at the step P9 the integral component K1 of the same factor a is computed according to the equation (5).

Kp = Cp X A(A/F) (4) K, = Cj X fA(A/F).dt (5) At the next step P10 the feedback correction factor a is computed according to the equation (6).

a = Kp + K, + 1 (6) The thus computed correction factor a is used in a separate routine to compute the corrected amount of fuel injection Ti according to the equation (2) to thereby produce the fuel injection control signal S,. The weighting factor C in the equation (2) is determined according to the engine operating conditions estimated from the signals sent from, for example, the sensors 36, 38 and 40 in Fig. 2.

As will be understood from the foregoing description, the constants of feedback control Cp and C, are frequently varied until completion of warm-up of the engine so as to become optimum at the varying temperatures of the cooling water. After completion of the warmup these constants Cp and C, are respectively fixed at predetermined values Cph and Clh which are optimum under normal engine operating conditions. By virture of this measure, it is possible to smoothly and rapidly converge the actual air/fuel ratio to the target value even when the feedback control is started before warm-up of the engine and even when the target value of the air/fuel ratio is changed while the engine temperature is still low.

The above described manner of varying the values of the constants Cp and C, is not limitative. It is also possible to stepwise vary the value of each constant Cp, Cj by making an optimum selection from at least two fixed values each of which corresponds to a specific range of the engine temperature. As a modification in another aspect, the integral component K1 may be omitted from the equation (6) to compute the feedback correction factor a to the effect that only the changes in the value of the proportional constant Cp are reflected in the correction factor a. Needless to mention, the degree of the engine warm-up can be detected also by measuring the temperature in the intake or exhaust manifold, in the intake ports or of the intake air instead of the cooling water temperature. Also it will be understood that the type and construction of the oxygen sensor 50 illustrated in Figs. 3 and 4 are merely exemplary and are not the least limitative.

Claims (8)

1. A control system for feedback control of the air/fuel ratio of an air-fuel mixture supplied to an internal combustion engine, the control system comprising: an oxygen sensor which is disposed in an exhaust passage of the engine and provides an air/fuel ratio signal representative of the actual air/fuel ratio in the engine; detection means for detecting the degree of warm-up of the engine; control means for producing and outputting an air/fuel ratio control signal by first producing a deviation signal indicative of the amount of deviation of the air/fuel ratio represented by said air/fuel ratio signal from a predetermined value and performing arithmetic treatment of said deviation signal by using at least one constant of feedback control; and adjusting means for variably determining the value of said at least one constant according to the degree of warm-up of the engine detected by said detection means.

2. A control system according to Claim 1, wherein said detection means comprises means for measuring the temperature of a fluid flowing in the engine.

3. A control system according to Claim 2, wherein said fluid is cooling water.

4. A control system according to any of claims 1 to 3, wherein said arithmetic treatment comprises a proportional treatment, said at least one constant comprising a proportional constant in said proportional treatment.

5. A control system according to any of claims 1 to 3, wherein said arithmetic treatment comprises an integral treatment, said at least one constant comprising an integration constant in said integral treatment.

6. A control system according to any of the preceding claims, wherein said oxygen sensor comprises an oxygen concentration cell comprising an oxygen ion conductive solid electrolyte.

7. A control system according to any of the preceding claims, wherein said control means and said adjusting means are integrated in a microcomputer.

8. A control system for feedback control of the air/fuel ratio of an air-fuel mixture supplied to an integral combustion engine, substantially as described with reference to, and as illustrated in, the accompanying drawings.