Apparatus for controlling the fuel injection quantity

Disclosed is an apparatus for controlling the fuel injection quantity for an internal combustion engine. The apparatus includes an operating condition detecting unit for detecting the operating conditions of an internal combustion engine on the basis of the engine speed and the intake air amount or the internal pressure of an intake pipe, a purge control variable calculating unit for calculating the amount of the purge control on the basis of the detected operating conditions, an A/F ratio learning and calculating unit for learning the deviation from the desired A/F ratio control by inputting the effect of the disturbance due to the canister purge and performing the A/F ratio feedback control, a memory unit for storing the deviation learned by the calculating unit, and a fuel injection quantity calculation means unit for determining the fuel injection quantity by the amount of correction based on the result of the learned deviation and the A/F ratio deviation obtained by the A/F ratio control.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for controlling the fuel 
injection quantity, in which an air/fuel (A/F) ratio feedback control and 
an A/F ratio open control can be switched. 
2. Description of the Prior Art 
Generally, a feedback control is performed in an internal combustion 
engine, in which a basic fuel injection quantity is computed on the basis 
of the intake air amount or the internal pressure of an intake pipe and 
the engine speed, and is corrected on the basis of the output of an 
O.sub.2 sensor installed in an exhaust pipe of the internal combustion 
engine. As a result, the fuel to be supplied into the engine can be 
adjusted to satisfy the desired A/F ratio such as the theoretical A/F 
ratio. For example, when the disturbance of the A/F ratio control such as 
the canister purge occurs, the coefficient (hereinafter, referred to as an 
.alpha. signal) which corrects the fuel injection quantity so as to obtain 
the theoretical A/F ratio, is computed in a control unit. The desired A/F 
ratio can be obtained by the A/F ratio feedback control in which the 
calculated coefficient value is used. Accordingly, when the canister purge 
is larger than the theoretical A/F ratio, the .alpha. signal is outputted 
in order to decrease the fuel injection quantity more than the case where 
the canister purge is not performed. 
There is shown the prior countermeasure to the disturbance of the A/F ratio 
control in the laid-open patent application 2-70953 (1990), in which 
disclosed is such a technique that calculates the deviation from the 
standard value of the .alpha. signal, corrects it every operation regions 
so that the .alpha. signal may be stabilized at the standard value, and 
learns them. Therefore, even if the disturbance of the A/F ratio occurs, 
it is possible to control accurately the A/F ratio and supply stably the 
fuel injection, independent of the disturbance of A/F ratio control and 
the operation region, by detecting, correcting and learning the 
disturbance of A/F ratio in the control unit. 
However, accurate control can be achieved only within the region of A/F 
ratio feedback control in which the oxygen sensor is activated. Therefore, 
during A/F ratio open control such as an A/F ratio lean burn control, the 
accuracy of A/F ratio control may be remarkably decreased due to the 
effect of A/F ratio control disturbance such as the canister purge. 
Generally, the A/F ratio of the internal combustion engine is adjusted to 
the theoretical A/F ratio by the A/F ratio feedback control. Then, the 
condition for shifting to the A/F ratio open control is satisfied and the 
open control is performed. After shifting to the open control, the control 
must be performed independent of the value learned so far, because of the 
disturbance of canister purge. That is, the A/F ratio may be gradually 
varied, because the concentration of the gas vapor in the purge air from 
the canister, which is supplied to the internal combustion engine along 
with the intake air during the open control, may be changed over time. 
When the gas concentration of canister purge is decreased, the A/F ratio 
becomes lean, thus the driving performance may be deteriorated due to 
misfire, torque-down, and so on. On the other hand, when the gas 
concentration of canister purge is increased, the A/F ratio becomes rich, 
thus the components of exhaust gas may be deteriorated. 
As described above, the learning correction amount in the A/F ratio 
feedback control can not be renewed in the A/F ratio open control. 
Therefore, there is such a problem that when the disturbance of the A/F 
ratio control is changed over time during the A/F ratio open control, the 
accurate correction can not made, the driving performance may be 
deteriorated, and the components of exhaust gas may also get worse. 
Now, the gas concentration during the purge may be effected by various 
factors such as the temperature, the amount of gas and the gas 
characteristics. Therefore, it is very difficult to predict the gas 
concentration. As a result, when shifting to the A/F ratio open control, 
it is essential to recognize quickly and accurately and correct suitably 
the effect of the disturbance of A/F ratio. Furthermore, it is also 
essential to adjust the fuel injection quantity in accordance with the 
time varying amount of the disturbance of A/F ratio during the open 
control. 
SUMMARY OF THE INVENTION 
The present invention is made on the basis of the point of view that the 
change in the A/F ratio due to the effect of purge can be known only 
during the A/F ratio feedback control. According to the present invention, 
the effect of purge can be detected during the A/F ratio feedback control, 
and the fuel injection quantity to be supplied during the open control can 
be controlled on the basis of the magnitude of the detected effect. 
It is an object of the present invention to provide an apparatus for 
controlling the fuel injection quantity which can correct the disturbance 
of A/F ratio control during the A/F ratio open control. 
It is a further object of the present invention to provide an apparatus for 
controlling the fuel injection quantity which can increase the accuracy of 
A/F ratio control. 
It is another object of the present invention to provide an apparatus for 
controlling the fuel injection quantity which can maintain the good 
driving performance, and which can prevent the exhaust gas from 
deteriorating, that is, which is capable of more effectively lowering 
emission levels. 
The foregoing objectives are achieved in an apparatus for controlling the 
fuel injection quantity which comprises operating condition detecting 
means for detecting the operating condition of an internal combustion 
engine on the basis of the engine speed and the intake air amount or the 
internal pressure of an intake pipe, purge control variable calculating 
means for calculating the amount of purge control on the basis of the 
detected operating conditions, A/F ratio learning and calculating means 
for learning the deviation from the desired A/F ratio control by inputting 
the effect of disturbance due to the canister purge and performing the A/F 
ratio feedback control, memory means for storing the deviation learned by 
the calculating means, and fuel injection quantity calculation means for 
determining the fuel injection quantity by the amount of correction based 
on the result of the learned deviation and the A/F ratio deviation 
obtained by the A/F ratio control. 
More concretely, an apparatus for controlling the fuel injection quantity 
is comprised of means for detecting the A/F ratio by using an oxygen 
sensor, means for detecting the intaked air amount or the internal 
pressure of an intake pipe in an internal combustion engine, means for 
detecting the deviation from the desired A/F ratio by using the output of 
the oxygen sensor, means for detecting the operating conditions of the 
engine on the basis of the engine speed and the intake air amount or the 
internal pressure of the intake pipe, means for determining the fuel 
injection quantity on the basis of the operating conditions detected by 
the detecting means, means for determining the amount of canister purge 
from the operating conditions, means for learning the deviation from the 
desired value for the A/F ratio, means for dividing the learned deviation 
amount at the predetermined ratio in accordance with the operating 
conditions and for learning each of them as different factors, means for 
determining the A/F ratio control due to the canister purge in each of the 
operating region, and means for correcting the fuel injection quantity on 
the basis of the learned deviation amount and the A/F ratio deviation 
amount obtained from the output of the oxygen sensor. 
In the apparatus for controlling the fuel injection quantity constructed as 
the above, the effect of the disturbance of A/F ratio control can be 
obtained by detecting the control amount of the disturbance and the 
deviation amount from the desired A/F ratio during the A/F ratio feedback 
control. The fuel injection quantity can be corrected according to the 
operating conditions by using the A/F ratio open control, on the basis of 
the learned deviation amount and the A/F ratio deviation amount obtained 
from the output of the oxygen sensor. When the disturbance may change over 
time during the open control, it is possible to maintain the good and long 
operation performance by learning again the magnitude of its effect after 
the predetermined time elapsed. 
These and other objects, features and advantages of the present invention 
will become more apparent from the detailed description of the preferred 
embodiments taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of the present invention will be explained 
hereinafter with reference to the accompanying drawings. 
First, the whole construction of an apparatus for controlling the fuel 
injection quantity according to the present invention will be explained. 
FIG. 1 shows the whole construction of the apparatus for controlling the 
fuel injection quantity according to the present invention, which is used 
to an internal combustion engine. FIG. 2 shows an electric circuit used in 
the apparatus for controlling the fuel injection quantity according to the 
present invention, in which input/output signals are illustrated in 
detail. 
Referring to FIG. 1, there is provided an airflow sensor (thermal airflow 
meter) 2 for detecting the airflow mass or flow rate intaked into an 
internal combustion engine 18 after being filtered by an air cleaner 1. 
Air intake passageways (air intake pipes) from the air cleaner 1 are 
connected, via a duct 31 and a collector 20, to intake pipes, each 
connected to each of cylinders of the internal combustion engine 18. A 
throttle body 4 housing a throttle valve 30 for controlling the flow rate 
to be intaked, is formed in the neighborhood of the connection part 
between the duct 31 and the collector 20. The duct 31 is provided with an 
idle speed control (ISC) 3 for controlling the engine idling speed by 
adjusting the airflow rate by-passing the throttle valve 30. The air to be 
intaked into the internal combustion engine 18 is intaked from an inlet of 
the air cleaner 1, through the duct 31 and the airflow sensor 2 and 
further through the throttle body 4 or the ISC valve 3, into the collector 
20. Where, the intake air is distributed into the intake pipes such as a 
pipe 32 and is induced into the cylinders. 
A swirl control valve (SCV) 17 is provided in the intake pipe 32 and at an 
inlet part of the engine 18, which is opened and closed by a control 
signal from a control unit 7 mentioned later. Thus, the intake air is 
swirled and intaked into the cylinder. 
A fuel injector (fuel injection valve) 5 provided at an intake port 21 of 
the intake pipe 32, for injecting the fuel is connected via a piping 33 to 
a fuel tank 11. In the piping 33, there are provided a fuel pump 10, a 
fuel damper 22 and a fuel filter 23 for supplying the sucked and 
pressurized fuel from the fuel tank 11 to the injector 5. In a fuel piping 
system, there is provided a fuel pressure regulator 12 for adjusting the 
fuel pressure so that the fuel pressure supplied to the injector 5 may be 
always maintained constantly. 
The control unit 7 receives a signal from the various sensors, executes the 
predetermined arithmetic processing and controls an amount of fuel to be 
supplied, an ISC air, an ignition timing and so on. Further, the control 
unit 7 receives an output signal indicative of an amount of air from the 
airflow sensor 2 and an output signal indicative of the temperature of 
cooling water from a water temperature sensor 13. And further, the control 
unit 7 receives an output signal from a throttle sensor (throttle valve 
position sensor) 24 provided inside the throttle body 4, for detecting the 
position (opening) of throttle valve 30. 
While, as an ignition assembly, there are provided an ignition coil 19, an 
ignition plug 14 and a distributor for distributing a high-voltage current 
to the ignition plug 14. The distributor 6 includes a crank angle sensor 
(not shown), which outputs a reference signal REF indicative of a 
rotational position of a crank axle and a position signal POS for 
detecting a rotational speed (rotational frequency). These signals are 
inputted into the control unit 7. The ignition plug 14 induces an electric 
energy with high voltages generated by the ignition coil 19 when an 
ignition signal from the control unit 7 was provided to it, whereby a fuel 
mixture intaked through an intake valve 34a into the cylinder is ignited. 
The exhaust gas is exhausted through a exhaust valve 34b. 
A charcoal canister 9 is provided in a piping connecting the fuel tank 11 
to the throttle body 4. The charcoal canister adsorbs temporarily the fuel 
vapour generated in the fuel tank 11, and purges the desired amount of 
fuel vapor by using a purge valve 8 in accordance with the operating 
conditions. 
An exhaust pipe 35 is provided with a catalyst 16, and an oxygen 
concentration sensor 15 in the upstream of the catalyst. The oxygen sensor 
15 detects whether the actual A/F ratio is richer than the theoretical A/F 
ratio or leaner, and outputs the resultant signal to the control unit 7. 
As shown in FIG. 2, a main part of the control unit 7 comprises a driver 
circuit 201 for converting low-power signals from the various sensors into 
high-power signals, respectively, an interface circuit 202 for converting 
an analog input/output signals into digital signals which are able to 
execute a digital arithmetic processing, an arithmetic circuit 203 
including a microcomputer for executing the digital arithmetic processing 
or another circuit corresponding to the microcomputer, a non-volatile ROM 
204 and a volatile RAM 205 for storing constants, variables and programs 
used in the arithmetic processing in the arithmetic circuit 203, and a 
backup circuit 206 for keeping the content of the volatile RAM 205. The 
control unit 7 shown in FIG. 2 receives output signals derived from the 
various sensors such as the thermal airflow meter 2, the throttle valve 
position sensor 24, the oxygen concentration sensor 15 and a crank angle 
sensor, and executes the predetermined arithmetic processing. The 
resultant control signals are supplied to the fuel injection valve 5, the 
ignition coil 19 and the ISC valve 3, whereby the fuel injection, the 
ignition and the ISC are controlled. 
FIG. 3 is a block diagram illustrating the setting of the fuel injection 
quantity. In block 301, the operating conditions of the internal 
combustion engine such as a load, the rotational speed, the cooling water 
temperature and a battery voltage are detected, on the basis of the 
outputs of sensors for detecting the engine speed, the intake amount or 
the intake negative pressure, the cooling water temperature and the 
battery voltage. In block 304, the canister purge amount is calculated on 
the basis of the operating conditions. In block 302, the effect of 
disturbance due to the canister purge is inputted and the deviation from 
the desired A/F control is learned by the A/F feedback control. The 
deviation is stored in block 303. Finally, in block 305, the fuel 
injection quantity is calculated by using the correction amount based on 
the results of the blocks 301 to 304. 
Next, the control of the fuel injection quantity will be explained in 
detail, in which the A/F ratio open control is performed. 
FIG. 4 is a general flow chart illustrating the operation of the fuel 
injection quantity control which is performed every predetermined time. 
First, in step 401, it is determined whether the A/F ratio control is the 
open control. If it is the A/F ratio open control, the processing shifts 
to step 402, in which the time TIME is counted, at which the A/F ratio 
control is during the open control and the disturbance of the A/F ratio 
may occur, in order to count the time until when it is possible to ignore 
the effect which the disturbance due to the purge exerts on the A/F ratio. 
It is possible to fetch sufficiently the effect of the disturbance of A/F 
ratio by setting the time at which the disturbance of A/F ratio can be 
sufficiently stabilized as above. Therefore, it is prevented to mistake 
the effect of disturbance. In step 403, it is determined whether the TIME 
is larger than the predetermined value TALPHA#, thus whether the effect 
that the disturbance exerts on the A/F ratio can be ignored. It is 
appreciated that the value of TALPHA# can be preset to such the value that 
the disturbance which changes over time during the open control does not 
exert the bad influence upon the accuracy of A/F ratio open control. If 
TIME.gtoreq.TALPHA#, it is determined that it is necessary to check the 
extent of purge effect to the A/F ratio. The processing shifts to step 404 
in which the open control is stopped and the feedback control is started. 
Next, it is determined in step 405 whether the learning of A/F ratio 
converged. This is used when the extent of the effect of disturbance is 
calculated in step 406, in order to confirm that the center of control is 
obtained. If the value of learning at each region does not converge, the 
control of learning is performed in order to obtain the center of control. 
In step 406, MALPHAT (the average of the coefficient .alpha.) 
representative of the effect of A/F ratio disturbance is calculated. More 
concretely, the MALPHAT is used in order to detect the effect of 
disturbance by the difference from the center of control of the 
coefficient of feedback. It is determined whether the calculation of 
MALPHAT was completed in step 407, because it takes a long time to 
calculate the MALPHAT. In step 408, the correction value of fuel injection 
quantity corresponding to the amount of disturbance is determined, and the 
A/F ratio open control is started again on the basis of the obtained 
correction value. 
While in the preferred embodiment of the present invention, the time 
between the execution of open controls has been used in order to overcome 
the time variation of the A/F ratio disturbance, it should be understood 
that numerous modifications may be made thereto. For example, instead of 
the time count, it is possible to use the total sum of engine speed during 
the open control, the amount of intake air, or the total amount of 
disturbance control to the A/F ratio control during the open control, and 
in their case the same performance is obtained. 
FIG. 5 is a timing chart illustrating the correction of the fuel injection 
quantity. As described above, the learning is performed till ALPHA=1.0, at 
which the ALPHA representative of the deviation to the desired A/F ratio 
converges. In a period for learning, the disturbance of A/F ratio control 
i.e. the canister purge is cut. Whereby the scattering of components and 
the deviation of the center of control changed according to the 
environment conditions are compensated and the center of control is 
keeped. After it is determined that the learning control has converged, 
the learning control is prohibited, the purge is permitted, and MALPHAT 
representative of the average of the deviation of A/F ratio is calculated. 
In the present embodiment, the MALPHAT is obtained from the average at the 
predetermined portion of the value .alpha..sub.max just before the 
proportional control part (P part) of lean correction is added to ALPHA 
and the value .alpha..sub.min just before the proportional control part (P 
part) of rich correction is added to ALPHA. 
Next, FIG. 6 is a general flow chart illustrating the determination of the 
correction value of the fuel injection quantity. It must be necessary to 
determine the MALPHAT in such a state that the A/F ratio feedback control 
is suitable and stable, because it is necessary to obtain accurately the 
effect of disturbance. Namely, when the oxygen sensor works normally and 
the operating conditions are in a transitional state, the determination of 
MALPHAT must be excluded from the flow. As an example of method of 
determining whether or not the oxygen sensor works normally, there is a 
method of detecting the inversion cycle (inversional period) of the oxygen 
sensor during the feedback control. A flow for calculating the MALPHAT by 
using the method will be explained in detail hereinafter. 
In step 601, the output of oxygen sensor is read. it is determined whether 
the inversion cycle TO.sub.2 is equal to or less than the predetermined 
value KTO.sub.2 # in step 602. It is desired that the KTO.sub.2 # value is 
substantially equal to the inversion cycle of oxygen sensor in such a 
condition that the A/F ratio feedback control is performed normally. If 
the inversion cycle TO.sub.2 of oxygen sensor is more than the 
predetermined value KTO.sub.2 #, then it is determined that the A/F ratio 
feedback control is not suitable, and the calculation is cancelled. In 
step 603, the average value MALP of the maximum value .alpha..sub.max of 
ALPHA and the minimum value .alpha..sub.min is calculated every time when 
the A/F ratio deviation ALPHA is inverted. When the A/F ratio feedback is 
not stable, for example, due to the lag of the corollary of disturbance, 
the value of MALP is liable to vary. Therefore, the relative difference 
.DELTA.MALP between the current MALP value and the former value is 
obtained in step 604, in order to detect the effect of disturbance under 
the stable conditions. In step 605, If .DELTA.MALP.gtoreq.KMALP#, it is 
determined that the A/F ratio feedback is not stable, and the flow is 
closed without shifting to the next step. On the other hand, if it is 
determined that .DELTA.MALP.ltoreq.KMALP# in step 604, the processing 
shifts to step 606 in which the output value TVO of the throttle sensor is 
read. In step 607, it is determined whether the variation value .DELTA.TVO 
of TVO per hour is equal to or less than the predetermined value KTVO#. If 
.DELTA.TVO is more than KTVO# and the operating condition is in the 
transient state, it is similarly determined that the A/F ratio feedback is 
not stable, and the flow is closed without shifting to the next step. It 
should be understood that numerous modifications may be made thereto. For 
example, instead of the TVO in step 607, it is possible to use the 
variation of engine speed per hour, the load variation of of the internal 
combustion engine, and also in their case or in a case in which a 
plurality of variation are combined, the same performance is obtained. 
Next, in step 608, the MALPHA is obtained by using the following equation 
(1) on the basis of the obtained MALP value. 
EQU MALPHA.sub.(new) =xXMALPHA.sub.(old) +(1-x)XMALP (1) 
in the equation (1), a weighting coefficient x is multiplied by the former 
value of MALPHA, and next (1-x) is multiplied by the resultant MALPHA, 
because the deviation .alpha. may vary complicately and the value of 
MALPHA also varies according to the value of .alpha.. By the repetition of 
the calculation, it becomes possible to obtain the average value of 
MALPHA, indicative of the extension of the effect of disturbance. 
In step 609, the values of MALP and MALPHA are stored the predetermined 
times in a memory, in order to calculate for equation (1). In step 610, 
the average value of MALPHA during the predetermined time TALPHA# is 
obtained in order to increase the reliability of the value of MALPHA 
representative of the extension of effect of the disturbance. The value of 
MALPHA is used as the extension of effect of the disturbance of A/F ratio. 
By using the following equation (2) as a COEF (multiplied by the fuel 
injection quantity) when the MALPHAT is converted to the fuel injection 
quantity, the desired A/F ratio is obtained, where COEF.sub.(old) is a 
coefficient of correction which is not including the correction to the 
disturbance of A/F ratio. 
EQU COEF.sub.(new) =COEF.sub.(old) XMALPHAT (2) 
In FIG. 6, the smoothing of variables is performed in steps 608 and 610 in 
order to obtain the effect of disturbance. However, it is possible to 
exclude either step. If the A/F ratio feedback of the internal combustion 
engine is always stable, it is possible to exclude both steps 608 and 610. 
Hereinbefore, the example has been explained in which the extension of the 
disturbance of A/F ratio is equal at each operation region. Another 
example will be explained hereinafter, which the extension of the 
disturbance of A/F ratio is different at each operation region. 
If the extension of the disturbance of A/F ratio is different at each 
operation region, it is impossible to use the same MALPH value at each 
operation region. FIG. 7 is a graph showing the extension of effect at 
each operation region in which a canister purge is picked up as an example 
of the disturbance of A/F ratio control. The purge factor J is a rate 
of the purge amount Qp to the intake air amount Qa and is defined by the 
following equation (3). 
EQU J=Qp/Qa (3) 
In FIG. 7, a region A is a purge factor control region where it is possible 
to control the purge amount Qp in proportion to the intake air amount Qa 
according to the equation (3), and the control of purge amount is 
performed by a control means for the variable amount of canister purge. A 
region B is that where it is impossible to control the purge factor in 
proportion to the intake air amount Qa, because the Qa reaches the maximum 
value due to the full open of the purge valve. A region C is that where 
the purge is not performed. 
In the region A, the extension of the effect on the A/F ratio control is 
the same, it is therefore possible to use the same MALPHA value. In the 
region C, the purge is not performed, therefore, the value of MALPHA is 
set to 1.0. It is appreciated that the control under the conditions of 
canister purge cut is the same as that in the region C. 
While, in the region C, it is impossible to increase the intake air amount 
Qa more than the amount when the purge factor Qp has reached the maximum 
value. Thus, it is impossible physically to control the purge factor J. 
When the purge valve is fully open, the purge amount Qp depends on the 
negative pressure of intake in the engine. Therefore, as the negative 
pressure of intake becomes close to the atomospheric pressure, the purge 
factor J decreases. 
As described above, the MALPHAT determined at the region A can not be 
applied to the region B as it is, since the purge factor J in the 
region B is different from that in the region A. Accordingly, in the case 
that the extension of effect of the disturbance of A/F ratio is different 
at each operation region, it is necessary to correct the MALPHAT at each 
operation region. The correction can be made in the following way. That 
is, the purge factors J are obtained under the different operating 
conditions, Then, the variation amount of each of ALPHAs is calculated, 
which correspond to the obtained purge factors. 
FIG. 8 is a general flow chart illustrating an example of such the 
correction method. The flow of FIG. 8 is a modification of the flow shown 
after the step 603 in FIG. 6. In step 801, it is determined whether the 
canister purge is being performed. If the purge is cut, then the flow is 
closed without shifting to the next step. In step 802, the open area PAS 
of the purge valve is determined, which controls variably the purge amount 
necessary to obtain the value of the purge factor J at a certain 
operating condition. The PAS can be obtained by retrieving a table TPAS of 
the open area of the purge valve corresponding to a drive signal. If the 
valve open characteristics is changed by the battery voltage or the 
environmental temperature, the PAS value can be corrected by detecting the 
battery voltage or the temperature of the cooling water. In step 803, the 
purge factor J can be calculated by using the following equation (4), 
on the basis of the value of the obtained J. 
##EQU1## 
where, # is a constant, and TP is indicative of the load of the 
internal combustion engine. 
In the above embodiment, the purge factor has been determined by using the 
equation (4). However, it should be understood that numerous modifications 
may be made thereto. For example, the purge factor can be determined by 
setting a map with an engine speed axis and a load axis, setting constants 
corresponding to the predetermined purge factors, and retrieving the map, 
because the purge factor J depends on the engine speed and the load of 
the engine. 
As described above, the effect of purge is different at each operation 
region. Therefore, in step 804, the average value of the maximum value 
.alpha..sub.max and the minimum value .alpha..sub.min (see FIG. 6) is 
divided by the J, whereby the varied amount of the deviation ALPHA to a 
unit purge factor is obtained and it becomes possible to determine the 
correction value of fuel injection quantity corresponding to the purge 
factor at each operating condition. The step 804 continues to the flow 
shown after the step 604 in FIG. 6. 
Referring to FIG. 9, An example of the method of correcting the fuel 
injection quantity by using the MALPHAT will be explained hereinafter. 
In step 901 of the correction routine for the fuel injection quantity, it 
is determined whether or not the control is the A/F ratio open control. If 
not the open control, the flow is closed without shifting to the next 
step. In step 902, the MALPHAT and the J are read. Then, in step 903, 
the COEF.sub.(new) is calculated by multiplying COEF.sub.(old) by the 
MALPHAT and the PRAJ. The COEF.sub.(new) will be used for the fuel 
injection hereinafter. 
If the switching from the A/F ratio feedback control to the A/F ratio open 
control or from the open control to the feedback control exerts the bad 
effect on the driving characteristic, because of the desired A/F ratios of 
the feedback control and the open control being different from each other, 
it is essential to avoid such the control. FIG. 10 is a timing chart 
illustrating a point of time when the A/F ratio control mode is switched, 
and shows an example of the method for avoiding such the control. The 
timing chart of FIG. 10 shows the state after the open control was 
performed for the TALPHA#. Assumed that the open control is suddenly 
shifted to the feedback control. A torque difference corresponding to the 
difference between the A/P ratios occurs, thus exerting the bad effect on 
the driving characteristic. It is, therefore, necessary gradually to vary 
the A/F ratio to the desired A/F ratio of feedback control by the 
predetermined value MALPD1# every the predetermined time TIALP1#. The 
MALPHA is calculated after the shift to the feedback control, and in order 
to maintain the driving characteristic after the permission condition for 
the open control was established again, the correction value of fuel 
injection quantity is gradually varied by the predetermined value MALPD1# 
every the predetermined time TIALP1# until the A/F ratio reaches the 
desired A/F ratio of feedback control. 
FIG. 11 is a general flow chart illustrating the switching operation of the 
A/F ratio control, and shows an example of the method of maintaining the 
driving performance after switching. The flow of FIG. 11 is inserted 
between the step 403 of FIG. 4 and the step 404. In step 1101, the effect 
of disturbance of the control of A/F ratio, MALPHAT, is calculated by 
using the following equation (5). 
EQU MALPHAT.sub.(new) =MALPHAT.sub.(old) XMALPD1# (5) 
where the value of MALPD1# may be determined by the estimation of driving 
performance. 
In step 1102, it is determined whether the value of MALPHAT=1.0 was 
cleared. When the value of MALPHAT was cleared, the coefficient of 
correction for the deviation of A/F ratio, ALPHA(.alpha.) is returned to 
the value equal to the ALPRET#. It should be noted that the extension of 
the effect of the disturbance of A/F ratio may be different from that just 
before the feedback control is shifted to the open control. The value of 
ALPRET# can be obtained by experiments. In step 1104, The A/F ratio 
control is returned to the feedback control. The processing shifts to step 
1105 in which the MALPHAT is calculated again. In step 1106, the 
completion of the calculation of MALPHAT is determined. In step 1107, when 
the condition of shift to the open control is established, the effect of 
the disturbance of the control of A/F ratio, MALPU, is calculated by using 
the following equation (6). 
EQU MALPU.sub.(new) =MALPU.sub.(old) XMALPU1# (6) 
where the value of MALPU1# may be determined from the estimation of driving 
performance by experiments. In step 1108, the equation (6) is calculated 
until the value of MALPU reaches the value of MALPHAT obtained in the step 
1106. Whereby the active A/F ratio during the open control can be 
controlled to the desired value. In step 1109, the A/F ratio open control 
is permitted on the basis of the correction value of fuel injection 
quantity obtained above. 
While the correction value of fuel injection quantity has been gradually 
varied when the feedback control is switched to the open control, it 
should be understood that numerous modifications may be made thereto. For 
example, it is possible to vary the ignition timing. The ignition timing 
is temporarily allowed to be delayed, and then is allowed gradually to be 
returned to the optimum ignition timing (MBT). Further, the same 
performance is achieved by an air amount control by a supplemental air 
control valve. If the output torque increases by the switching of A/F 
ratio control, the supplemental air amount is decreased according to the 
amount of the variation of torque. If the output torque decreases, the 
supplemental air amount is increased according to the amount of the 
variation of torque. 
FIG. 12 is a timing chart illustrating an example of effects of the 
correction of the fuel injection quantity, and shows the result when the 
factor of disturbance of A/F ratio is caused to become lean after the A/F 
ratio open control is started. In the prior system, the A/F ratio becomes 
more than the allowable upper limit (ex. the output A/F=25.0) after a 
certain time lapsed, and become overlean. On the other hand, while the A/F 
ratio becomes lean after a certain time lapsed in the present invention, 
the feedback control is started before the A/F ratio becomes more than the 
allowable upper limit, and the open control is started again after the 
recognition of the effect of disturbance. It should be noted that the 
value of A/F ratio is within the predetermined value. Accordingly, the 
present invention can provide the apparatus for controlling the fuel 
injection quantity for the internal combustion engine which is capable of 
more effectively lowering emission levels in order to meet current and 
future low emission vehicle and its regulations.