Air/fuel mixture ratio control system for internal combustion engine with feature of learning correction coefficient including altitude

What is claimed is:

1. An air/fuel ratio control system for controlling a mixture ratio of an air/fuel mixture to be introduced into a combustion chamber in an internal combustion engine, comprising:

an air/fuel mixture induction system for introducing an intake air and a fuel for forming an air/fuel mixture to be supplied into an engine combustion chamber, said air/fuel mixture delivery system incorporating a fuel metering means for delivering a controlled amount of fuel;

a first sensor means for monitoring a preselected basic first engine operation parameter to produce a first sensor signal indicative thereof;

a second sensor means for monitoring an air/fuel mixture ratio indicative parameter for producing a second sensor signal variable of the value indicative of a deviation from a threshold value representative of a stoichiometric value;

third means for deriving a basic fuel metering amount on the basis of said first sensor signal value;

fourth means for deriving a air/fuel ratio dependent correction factor variable of the value thereof depending upon said second sensor signal value;

fifth means for deriving an air density dependent first correction coefficient on the basis of said air/fuel ratio dependent correction factor for air/fuel ratio dependent correction of said basic fuel metering amount, which first correction coefficient is commonly applicable for correction of said basic fuel metering amount in over all engine driving ranges, said fifth means updating said first correction coefficient when a first learning condition is satisfied;

a sixth means for deriving a second correction coefficient which is variable depending upon the engine driving range on the basis of said air/fuel ratio dependent correction factor, said sixth means setting a plurality of said second correction coefficient in relation to respectively corresponding engine driving range and updating each of said second correction coefficient with an instantaneous value derived based on said air/fuel ratio dependent correction factor in the corresponding engine driving range;

a seventh means for detecting engine driving condition on the basis of said first sensor signal values and governing said fifth and sixth means for selectively operating one of said fifth and sixth means depending upon the detected engine driving condition; and

an eighth means for correcting said basic fuel metering amount with said correction coefficient to control said fuel metering means for delivering the fuel in the amount corresponding to the corrected fuel metering amount to said air/fuel mixture delivery system.

2. An air/fuel ratio control system as set forth in claim 1, wherein said seventh means detects said engine driving condition satisfying a predetermined feedback control condition for producing a feedback condition indicative signal to selectively enable said fifth and sixth means for updating one of said first and second correction coefficient and to disable said fifth and sixth means when said feedback condition is not satisfied.

3. An air/fuel ratio control system as set forth in claim 1, which further comprises a ninth means for detecting engine driving condition in high speed and high load, which is out of said feedback condition, to measure an elapsed period where said high speed and high load condition is maintained, said ninth means modifying said first correction coefficient when the measured elapsed time becomes longer than or equal to a predetermined period.

4. An air/fuel ratio control system as set forth in claim 3, wherein said ninth means cyclically modifies said first correction coefficient by a predetermined value while the engine is maintained at said high speed and high load condition.

5. An air/fuel ratio control system as set forth in claim 4, wherein said second sensor means varies polarity of said second sensor signal value when air/fuel ratio varies across a stoichiometric value, and which further comprises a tenth means for measuring an elapsed time in which the polarity of said second sensor signal value is held unchanged to detect abnormality of said second sensor means.

6. An air/fuel ratio control system as set forth in claim 5, wherein said tenth means disables said ninth means when abnormality of said second sensor means is detected.

7. An air/fuel ratio control system as set forth in claim 1, wherein said first sensor means includes means for monitoring an engine load indicative parameter and means for monitoring an engine speed indicative parameter, and said seventh means derives a first criterion on the basis of an engine speed derived on the basis of the monitored engine speed indicative parameter and compares an engine load derived based on said engine load indicative parameter with said first criterion for enabling said fifth means when said engine load is greater than or equal to said first criterion, and enabling said sixth means when said engine load is smaller than said first criterion.

8. An air/fuel ratio control system as set forth in claim 7, wherein said seventh means further derives a second criterion on the basis of said engine speed, which second criterion is set at a greater value than said first criterion and compares said engine load with said second criterion so as to disable said fifth means when said engine load is greater than said second criterion.

9. An air/fuel ratio control system as set forth in claim 1, wherein said second sensor means varies polarity of said second sensor signal value when air/fuel ratio varies across a stoichiometric value, and said fifth and sixth means, as being triggered by said seventh means, being responsive to change of polarity of said second sensor signal to update said first and second correction coefficients.

10. An air/fuel ratio control system as set forth in claim 1, which further comprises a detector means detective of engine driving condition satisfying a predetermined feedback control condition for producing a feedback condition indicative signal to operate said eighth means in feedback mode for correcting said basic fuel metering amount with said first and second correction coefficients and to operate said eighth means in open loop mode for disabling correction of said basic fuel metering amount utilizing said first and second correction coefficients, and said seventh means selectively enables said fifth and sixth means for updating said first and second correction coefficients while said eighth means operates in feedback mode.

11. An air/fuel ratio control system as set forth in claim 10, wherein said fourth means is active in presence of said feedback condition indicative signal to cyclically derive said correction factor, and said sixth means is active for deriving said second correction coefficient on the basis of said correction factor only when said feedback condition indicative signal is present.

12. An air/fuel ratio control system as set forth in claim 11, wherein said fourth means samples upper and lower peak values of said second sensor signal value for deriving said correction factor by averaging said upper and lower peak values.

13. An air/fuel ratio control system as set forth in claim 1, wherein said first sensor means monitors an engine speed indicative parameter and an engine load indicative parameter so that said third means derives said basic fuel metering amount on the basis of said engine speed indicative parameter and said engine load indicative parameter, and said fifth means detects said engine driving range on the basis of said engine speed and said basic fuel metering amount.

14. An air/fuel ratio control system as set forth in claim 13, wherein said first sensor means monitors a throttle valve angular position and derives said engine load indicative parameter on the basis of said throttle valve angular position and said engine speed.

15. An air/fuel ratio control system for controlling a mixture ratio of an air/fuel mixture to be introduced into a combustion chamber in an internal combustion engine, comprising:

an air/fuel mixture induction system for introducing an intake air and a fuel for forming an air/fuel mixture to be supplied into an engine combustion chamber, said air/fuel mixture delivery system incorporating a fuel metering means for delivering a controlled amount of fuel;

a first sensor means for monitoring a preselected basic first engine operation parameter to produce a first sensor signal indicative thereof, said first sensor signal including an engine load indicative component;

a second sensor means for monitoring an air/fuel mixture ratio indicative parameter for producing a second sensor signal variable of the value indicative of a deviation from a threshold value representative of a stoichiometric value;

third means for deriving a basic fuel metering amount on the basis of said first sensor signal value;

fourth means for deriving a air/fuel ratio dependent correction factor variable of the value thereof depending upon said second sensor signal value;

fifth means for deriving an air density dependent first correction coefficient on the basis of said air/fuel ratio dependent correction factor for air/fuel ratio dependent correction of said basic fuel metering amount, which first correction coefficient is commonly applicable for correction of said basic fuel metering amount in over all engine driving ranges, said fifth means updating said first correction coefficient when a first learning condition is satisfied;

a sixth means for deriving a second correction coefficient which is variable depending upon the engine driving range on the basis of said air/fuel ratio dependent correction factor, said sixth means setting a plurality of said second correction coefficient in relation to respectively corresponding engine driving range and updating each of said second correction coefficient with an instantaneous value derived based on said air/fuel ratio dependent correction factor in the corresponding engine driving range;

a seventh means, associated with said fifth means, for deriving an altitude dependent correction value for modifying said first correction value on the basis of said engine load component of said first sensor signal and a tendency of air/fuel ratio adjustment in a given cycles of said second sensor signal variations;

an eighth means for correcting said basic fuel metering amount with said correction coefficient to control said fuel metering means for delivering the fuel in the amount corresponding to the corrected fuel metering amount to said air/fuel mixture delivery system.

16. An air/fuel ratio control system as set forth in claim 15, wherein said seventh means increases altitude dependent correction value according to increase of said engine load.

17. An air/fuel ratio control system as set forth in claim 15, wherein said seventh means detects tendency of richer side or leaner side air/fuel ratio control depending upon distribution of richer side variation and leaner side variation of given number of said second correction coefficients updated by said sixth means in most recent given updating cycles.

18. An air/fuel ratio control system as set forth in claim 17, wherein said seventh means increases said altitude dependent correction value according to increase of tendency of either richer side or leaner side air/fuel control which is greater than the other.

19. An air/fuel ratio control system as set forth in claim 15, wherein said seventh means detects tendency of richer side or leaner side air/fuel ratio control depending upon distribution of said second correction values residing richer side and leaner side of a predetermined threshold value.

20. An air/fuel ratio control system as set forth in claim 19, wherein said seventh means increases said altitude dependent correction value according to increase of tendency of either richer side or leaner side distribution which is greater than the other.

21. An air/fuel ratio control system as set forth in claim 15, wherein said seventh means derives said altitude dependent correction value constituting a first component variable according to variation to variation of said engine load and a second component derived depending upon tendency of richer side or leaner side air/fuel ratio control depending upon distribution of richer side variation and leaner side variation of given number of said second correction coefficients updated by said sixth means in most recent given updating cycles.

22. An air/fuel ratio control system as set forth in claim 21, wherein said altitude dependent correction value is an average value of said first and second components.

23. An air/fuel ratio control system as set forth in claim 15, wherein said seventh means derives said altitude dependent correction value constituting a first component variable according to variation to variation of said engine load and a second component derived depending upon tendency of richer side or leaner side air/fuel ratio control depending upon distribution of said second correction values residing richer side and leaner side of a predetermined threshold value.

24. An air/fuel ratio control system as set forth in claim 23, wherein said altitude dependent correction value is an average value of said first and second components.

25. An air/fuel ratio control system as set forth in claim 15, wherein said seventh means derives said altitude dependent correction value constituting a first component variable according to variation to variation of said engine load, a second component derived depending upon tendency of richer side or leaner side air/fuel ratio control depending upon distribution of said second correction values residing richer side and leaner side of a predetermined threshold value, and a third component derived depending upon distribution of richer side variation and leaner side variation of given number of said second correction coefficients updated by said sixth means in most recent given updating cycles.

26. An air/fuel ratio control system as set forth in claim 25, wherein said altitude dependent correction value is an average value of said first, second and third components.

27. An air/fuel ratio control system as set forth in claim 15, wherein said first sensor means includes means for monitoring an engine load indicative parameter and means for monitoring an engine speed indicative parameter, and said seventh means derives a first criterion on the basis of an engine speed derived on the basis of the monitored engine speed indicative parameter and compares an engine load derived based on said engine load indicative parameter with said first criterion for enabling said fifth means when said engine load is greater than or equal to said first criterion, and enabling said sixth means when said engine load is smaller than said first criterion.

28. An air/fuel ratio control system as set forth in claim 15, wherein said second sensor means varies polarity of said second sensor signal value when air/fuel ratio varies across a stoichiometric value, and said fifth and sixth means, as being triggered by said seventh means, being responsive to change of polarity of said second sensor signal to update said first and second correction coefficients.

29. An air/fuel ratio control system as set forth in claim 15, which further comprises a detector means detective of engine driving condition satisfying a predetermined feedback control condition for producing a feedback condition indicative signal to operate said eighth means in feedback mode for correcting said basic fuel metering amount with said first and second correction coefficients and to operate said eighth means in open loop mode for disabling correction of said basic fuel metering amount utilizing said first and second correction coefficients, and said seventh means selectively enables said fifth and sixth means for updating said first and second correction coefficients while said eighth means operates in feedback mode.

30. An air/fuel ratio control system as set forth in claim 29, wherein said fourth means is active in presence of said feedback condition indicative signal to cyclically derive said correction factor, and said sixth means is active for deriving said second correction coefficient on the basis of said correction factor only when said feedback condition indicative signal is present.

31. An air/fuel ratio control system as set forth in claim 30, wherein said fourth means samples upper and lower peak values of said second sensor signal value for deriving said correction factor by averaging said upper and lower peak values.

32. An air/fuel ratio control system as set forth in claim 15, wherein said first sensor means monitors an engine speed indicative parameter and an engine load indicative parameter so that said third means derives said basic fuel metering amount on the basis of said engine speed indicative parameter and said engine load indicative parameter, and said fifth means detects said engine driving range on the basis of said engine speed and said basic fuel metering amount.

33. An air/fuel ratio control system as set forth in claim 32, wherein said first sensor means monitors a throttle valve angular position and derives said engine load indicative parameter on the basis of said throttle valve angular position and said engine speed.

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an air/fuel mixture ratio control system for an internal combustion engine. More specifically, the invention relates to a learning control system for controlling air/fuel ratio in a fuel injection internal combustion engine, which air/fuel ration control includes lambda (.lambda.) control for performing FEEDBACK or CLOSED LOOP control on the basis of oxygen concentration contained in an exhaust gas. Further particularly, the invention relates to an air/fuel ratio learning control system including altitude depending control, which can precisely adjust air/fuel ratio depending upon density of air to be introduced for forming the air/fuel mixture.

2. Description of the Background Art

In the recent years, there have been proposed various air/fuel control systems for internal combustion engines. Some of the recently developed air/fuel ratio control systems incorporate learning control feature to continuously update correction coefficient for correcting a basic fuel injection amount based on oxygen concentration in an exhaust gas in order to maintain air/fuel ratio at a stoichiometric value. In case that air density dependent air/fuel ratio is concerned, the correction coefficient may be uniformly updated based on an oxygen concentration indicative sensor signal value (hereafter O.sub.2 sensor signal) regardless of the engine driving range, in theory. However, in practice, because of tolerance in fuel injection valves, throttle body and other engine components, which causes deviation between arithmetically obtained basic fuel injection amount and practically required fuel injection amount, uniformly updating or learning of the correction coefficient regardless of engine driving range is practically not possible. By this, it is practically required to set learned correction coefficient for respective engine driving range.

In this view, learning control systems with FEEDBACK control feature for controlling air/fuel ratio have been recently proposed in the Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 60-90944 and the Japanese Patent First Publication (Tokkai) Showa No. 61-190142. In the disclosed system, a basic fuel injection amount is derived on the basis of preselected basic fuel injection control parameter or parameters, such as an intake air flow rate, an engine revolution speed and so forth. The basic fuel injection amount thus derived is modified employing a feedback correction coefficient which is derived on the basis of oxygen sensor in an exhaust system and composed of a proportional (P) component and an integral (I) component. By modifying the fuel injection amount on the basis of the feedback correction coefficient, air/fuel ratio can be FEEDBACK controlled toward a stoichiometric value. Furthermore, the disclosed system derives a learnt correction coefficient with respect to mutually distinct various engine operation range. In practice, the learned correction coefficient is determined by deriving a difference between the feedback correction coefficient and a predetermined reference value. This learned correction coefficient is used in OPEN LOOP mode air/fuel ratio control to derive the fuel injection amount. The learned correction coefficient may also be used in the FEEDBACK or CLOSED LOOP mode air/fuel ratio control together with the feedback correction coefficient.

Such a system assures to perform air/fuel ratio control in the FEEDBACK mode operation to maintain the air/fuel ratio precisely at the stoichiometric value. Furthermore, since the learned correction coefficient may serve to maintain desired air/fuel ratio even in OPEN LOOP mode operation.

However, in the aforementioned type of learning control system, drawback may be encountered in an engine driving condition where the engine driving or operation range frequently fluctuates. For example, in hill or mountain climbing, the air/fuel ratio control mode is held in transition mode condition between FEEDBACK mode and OPEN LOOP mode to too frequently change engine driving range to update learned correction coefficient during FEEDBACK mode operation. Therefore, the learned correction coefficient may not reflect the instantaneous air density. This causes delay in FEEDBACK mode control after the driving condition returns to stable state satisfying FEEDBACK condition. Furthermore, in the OPEN LOOP control, the air/fuel ratio tends to deviate far from the stoichiometric value to degrade drivability.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an air/fuel ratio control system which can precisely control fuel delivery amount at any altitude condition and can provide sufficiently high response characteristics to altitude change.

Another object of the invention is to introduce a control feature in the air/fuel ratio control for optimizing air/fuel control at any environmental condition.

In order to accomplish aforementioned and other objects, an air/fuel ratio control system, according to the present invention, controls fuel delivery amount on the basis of oxygen concentration in an exhaust gas. An air/fuel ratio dependent correction value is derived on the basis of the oxygen concentration. The air/fuel ratio control is performed in feedback mode and open loop mode. In feedback mode, fuel delivery amount is corrected utilizing a correction value which includes a learned component. Learning of the learnt component is performed during feedback mode operation. The learned component comprises an uniformly applicable air density dependent factor and a engine driving range dependent factor which is set with respect to each of the engine driving ranges. Learning of the air density factor and engine driving range dependent factor are selectively performed depending upon the engine driving condition.

This introduces altitude dependent air/fuel ratio control. According to the invention, altitude dependent control can be taken place even in engine high speed and high load condition for improving response characteristics in air/fuel ratio control at any altitude condition.

In the alternative, the control feature may be introduced in the air/fuel ratio control for optimizing air/fuel ratio control at any altitude.

According to one aspect of the invention, an air/fuel ratio control system for controlling a mixture ratio of an air/fuel mixture to be introduced into a combustion chamber in an internal combustion engine, comprises an air/fuel mixture induction system for introducing an intake air and a fuel for forming an air/fuel mixture to be supplied into an engine combustion chamber, the air/fuel mixture delivery system incorporating a fuel metering means for delivering a controlled amount of fuel, a first sensor means for monitoring a preselected basic first engine operation parameter to produce a first sensor signal indicative thereof, a second sensor means for monitoring an air/fuel mixture ratio indicative parameter for producing a second sensor signal variable of the value indicative of a deviation from a threshold value representative of a stoichiometric value, third means for deriving a basic fuel metering amount on the basis of the first sensor signal value, fourth means for deriving a air/fuel ratio dependent correction factor variable of the value thereof depending upon the second sensor signal value, fifth means for deriving an air density dependent first correction coefficient on the basis of the air/fuel ratio dependent correction factor for air/fuel ratio dependent correction of the basic fuel metering amount, which first correction coefficient is commonly applicable for correction of the basic fuel metering amount in over all engine driving ranges, the fifth means updating the first correction coefficient when a first learning condition is satisfied, a sixth means for deriving a second correction coefficient which is variable depending upon the engine driving range on the basis of the air/fuel ratio dependent correction factor, the sixth means setting a plurality of the second correction coefficient in relation to respectively corresponding engine driving range and updating each of the second correction coefficient with an instantaneous value derived based on the air/fuel ratio dependent correction factor in the corresponding engine driving range, a seventh means for detecting engine driving condition on the basis of the first sensor signal values and governing the fifth and sixth means for selectively operating one of the fifth and sixth means depending upon the detected engine driving condition, and an eighth means for correcting the basic fuel metering amount with the correction coefficient to control the fuel metering means for delivering the fuel in the amount corresponding to the corrected fuel metering amount to the air/fuel mixture delivery system.

The seventh means may detect the engine driving condition satisfying a predetermined feedback control condition for producing a feedback condition indicative signal to selectively enable the fifth and sixth means for updating one of the first and second correction coefficient and to disable the fifth and sixth means when the feedback condition is not satisfied.

The air/fuel ratio control system further comprises a ninth means for detecting engine driving condition in high speed and high load, which is out of the feedback condition, to measure an elapsed period where the high speed and high load condition is maintained, the ninth means modifying the first correction coefficient when the measured elapsed time becomes longer than or equal to a predetermined period. The ninth means cyclically modifies the first correction coefficient by a predetermined value while the engine is maintained at the high speed and high load condition. The second sensor means varies polarity of the second sensor signal value when air/fuel ratio varies across a stoichiometric value, and which further comprises a tenth means for measuring an elapsed time in which the polarity of the second sensor signal value is held unchanged to detect abnormality of the second sensor means. The tenth means disables the ninth means when abnormality of the second sensor means is detected.

The first sensor means preferably includes means for monitoring an engine load indicative parameter and means for monitoring an engine speed indicative parameter, and the seventh means derives a first criterion on the basis of an engine speed derived on the basis of the monitored engine speed indicative parameter and compares an engine load derived based on the engine load indicative parameter with the first criterion for enabling the fifth means when the engine load is greater than or equal to the first criterion, and enabling the sixth means when the engine load is smaller than the first criterion. The seventh means further derives a second criterion on the basis of the engine speed, which second criterion is set at a greater value than the first criterion and compares the engine load with the second criterion so as to disable the fifth means when the engine load is greater than the second criterion.

The second sensor means may vary polarity of the second sensor signal value when air/fuel ratio varies across a stoichiometric value, and the fifth and sixth means, as being triggered by the seventh means, being responsive to change of polarity of the second sensor signal to update the first and second correction coefficients.

The air/fuel ratio control system further comprises a detector means detective of engine driving condition satisfying a predetermined feedback control condition for producing a feedback condition indicative signal to operate the eighth means in feedback mode for correcting the basic fuel metering amount with the first and second correction coefficients and to operate the eighth means in open loop mode for disabling correction of the basic fuel metering amount utilizing the first and second correction coefficients, and the seventh means selectively enables the fifth and sixth means for updating the first and second correction coefficients while the eighth means operates in feedback mode. The fourth means is active in presence of the feedback condition indicative signal to cyclically derive the correction factor, and the sixth means is active for deriving the second correction coefficient on the basis of the correction factor only when the feedback condition indicative signal is present. The fourth means samples upper and lower peak values of the second sensor signal value for deriving the correction factor by averaging the upper and lower peak values. The first sensor means monitors an engine speed indicative parameter and an engine load indicative parameter so that the third means derives the basic fuel metering amount on the basis of the engine speed indicative parameter and the engine load indicative parameter, and the fifth means detects the engine driving range on the basis of the engine speed and the basic fuel metering amount. The first sensor means monitors a throttle valve angular position and derives the engine load indicative parameter on the basis of the throttle valve angular position and the engine speed.

According to another aspect of the invention, an air/fuel ratio control system for controlling a mixture ratio of an air/fuel mixture to be introduced into a combustion chamber in an internal combustion engine, comprises an air/fuel mixture induction system for introducing an intake air and a fuel for forming an air/fuel mixture to be supplied into an engine combustion chamber, the air/fuel mixture delivery system incorporating a fuel metering means for delivering a controlled amount of fuel, a first sensor means for monitoring a preselected basic first engine operation parameter to produce a first sensor signal indicative thereof, the first sensor signal including an engine load indicative component, a second sensor means for monitoring an air/fuel mixture ratio indicative parameter for producing a second sensor signal variable of the value indicative of a deviation from a threshold value representative of a stoichiometric value, third means for deriving a basic fuel metering amount on the basis of the first sensor signal value, fourth means for deriving a air/fuel ratio dependent correction factor variable of the value thereof depending upon the second sensor signal value, fifth means for deriving an air density dependent first correction coefficient on the basis of the air/fuel ratio dependent correction factor for air/fuel ratio dependent correction of the basic fuel metering amount, which first correction coefficient is commonly applicable for correction of the basic fuel metering amount in over all engine driving ranges, the fifth means updating the first correction coefficient when a first learning condition is satisfied, a sixth means for deriving a second correction coefficient which is variable depending upon the engine driving range on the basis of the air/fuel ratio dependent correction factor, the sixth means setting a plurality of the second correction coefficient in relation to respectively corresponding engine driving range and updating each of the second correction coefficient with an instantaneous value derived based on the air/fuel ratio dependent correction factor in the corresponding engine driving range, a seventh means, associated with the sixth means, for deriving an altitude dependent correction value for modifying the second correction value on the basis of the engine load component of the first sensor signal and a tendency of air/fuel ratio adjustment in a given cycles of the second sensor signal variations, an eighth means for correcting the basic fuel metering amount with the correction coefficient to control the fuel metering means for delivering the fuel in the amount corresponding to the corrected fuel metering amount to the air/fuel mixture delivery system.

The seventh means may derive the altitude dependent correction value constituting a first component variable according to variation to variation of the engine load and a second component derived depending upon tendency of richer side or leaner side air/fuel ratio control depending upon distribution of richer side variation and leaner side variation of given number of the second correction coefficients updated by the sixth means in most recent given updating cycles. In the alternative, the seventh means may derive the altitude dependent correction value constituting a first component variable according to variation to variation of the engine load and a second component derived depending upon tendency of richer side or leaner side air/fuel ratio control depending upon distribution of the second correction values residing richer side and leaner side of a predetermined threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for explanation and understanding only.

In the drawings:

FIG. 1 is a diagram of the preferred embodiment of a learning air/fuel ratio control system according to the invention;

FIG. 2 is a block diagram of a control unit employed in the preferred embodiment of the air/fuel ratio control system of the invention;

FIG. 3 is a flowchart of a routine for deriving and setting a fuel injection pulse width representative of a fuel injection amount;

FIG. 4 is a block diagram of an input/output unit in the control unit to be employed in the preferred embodiment of the air/fuel ratio control system of FIG. 2;

FIG. 5 is a flowchart of a routine for discriminating engine operating condition for governing control operation mode between FEEDBACK control mode and OPEN LOOP control mode;

FIG. 6 is a flowchart of a routine for deriving feedback correction coefficient composed of a proportional component and an integral component;

FIG. 7 is a flowchart of a learning governing routine for governing learning of K.sub.ALT and K.sub.MAP ;

FIG. 8 is a flowchart of a K.sub.ALT learning routine for updating a map storing an air density dependent uniform correction coefficients;

FIG. 9 is a flowchart showing a K.sub.MAP learning routine for updating an engine driving range based correction coefficients;

FIG. 10 is a timing chart showing operation of the preferred embodiment of the air/fuel ratio control system of the invention;

FIG. 11 is a flowchart of an automatically modifying routine for K.sub.ALT for modifying the K.sub.ALT automatically;

FIG. 12 is a chart showing FEEDBACK control range which is defined in terms of engine speed N and engine load Tp;

FIG. 13 is a chart showing range to perform learning of K.sub.ALT which is defined by throttle angular position .theta..sub.th, intake air flow rate Q and engine speed N;

FIGS. 14 and 15 are flowcharts showing a sequence of K.sub.MAP learning routine as modification of the routine of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, particularly to FIGS. 1 and 2, the preferred embodiment of an air/fuel ratio control system, according to the invention, is applied to a fuel injection internal combustion engine which is generally represented by the reference numeral "1". The engine 1 has an air induction system including an air cleaner 2, a throttle body 3 and an intake manifold 4. A throttle valve 5 is disposed within the throttle body 3 for adjusting induction rate of an air/fuel mixture.

In the shown embodiment, a fuel injection valve 6 is disposed within the throttle body 3 and upstream of the throttle valve 5. Therefore, the air/fuel mixture is formed at the position in the induction system upstream of the throttle valve. The air/fuel mixture flows through the throttle body 3 and introduced into an engine combustion chamber via the intake manifold 4 and an intake port which is open and closed by means of an intake valve.

The air/fuel mixture introduced into the engine combustion chamber is combustioned by spark ignition taken place by means of an ignition plug 7 which receives an ignition power from an ignition coil unit 8 via a distributor 9.

The engine 1 also has an exhaust system including an exhaust manifold 10, an exhaust duct 11, a catalytic converter unit 12 and a muffler 13.

In order to monitor the angular position of the throttle valve 5, a throttle angle sensor 15 is associated with the throttle valve 5 to produce a throttle angle indicative signal .theta..sub.th having a value indicative of the monitored throttle angle. In practice, the throttle angle sensor 15 comprises a potentiometer producing analog form throttle angle indicative signal having a voltage variable depending upon the throttle valve angular position. Also, an an engine idling condition detector switch 16 is associated with the throttle valve 5 for detecting fully closed or approximately fully closed position of the throttle valve. The engine idling condition detector switch 16 outputs an engine idling condition indicative signal IDL which is held LOW level while the throttle valve 5 is not in fully closed or approximately fully closed position and is held HIGH level while the throttle valve is maintained at fully closed or approximately fully closed position.

A crank angle sensor 17 is coupled with the distributor 9 for monitoring a crank shaft angular position. For this, the crank angle sensor 17 has a rotary disc which is so designed as to rotate synchroneously with rotation of a rotor of the distributor. The crank angle sensor 17 produces a crank reference signal .theta..sub.ref at each of predetermined angular position and a crank position signal .theta..sub.pos at every time of predetermined angle of angular displacement of the crank shaft. In practice, the crank reference signal is generated every time the crank shaft is rotated at an angular position corresponding on 70.degree. or 66.degree. before top-dead-center (BTDC) in compression stroke of one of engine cylinder. Therefore, in case of the 6-cylinder engine, the crank reference signal .theta..sub.ref is produced at every 120.degree. of the crank shaft angular displacement. On other hand, the crank position .theta..sub.pos, is generated every given angular displacement, i.e. 1.degree. or 2.degree., of the crank shaft.

An engine coolant temperature sensor 18 is disposed within an engine cooling chamber to monitor a temperature of an engine coolant filled in the cooling chamber. The engine coolant temperature sensor 18 is designed for monitoring the temperature of the engine coolant to produce an engine coolant temperature indicative signal Tw. In practice, the engine coolant temperature sensor 18 produces an analog form signal having a voltage variable depending upon the engine coolant temperature condition. A vehicle speed sensor 19 monitors a vehicle speed for producing a vehicle speed indicative signal Vs. Furthermore, the shown embodiment of the air/fuel ratio control system includes an oxygen sensor 20 disposed in the exhaust manifold 10. The oxygen sensor 20 monitors oxygen concentration contained in the exhaust gas to produce an oxygen concentration indicative signal V.sub.ox indicative of the monitored oxygen concentration. The oxygen concentration indica