Air-fuel ratio control system for internal combustion engines capable of controlling air-fuel ratio in accordance with degree of warming-up of the engines

While an exhaust gas sensor having an output characteristic linear with respect to the concentration of a specific component in exhaust gases from an internal combustion engine, the opening of an automatic choke valve is controlled in response to the degree of warming-up of the engine. From the time of activation of the exhaust gas sensor to the time of completion of warming-up of the engine, the automatic choke valve and an air-fuel ratio control valve arranged in an air passage bypassing a throttle valve in an intake passage of the engine, are driven so as to achieve a desired air-fuel ratio, respectively, when the difference between the desired air-fuel ratio and actual one is larger than a predetermined value, and when the difference is smaller than the predetermined value. After completion of warming-up of the engine, the air-fuel ratio control valve is driven in response to operating conditions of the engine so as to achieve the desired air-fuel ratio.

BACKGROUND OF THE INVENTION 
This invention relates to an air-fuel ratio control system for internal 
combustion engines, which is capable of appropriately controlling the 
air-fuel ratio in accordance with the degree of warming-up of the engines. 
It has been generally carried out to set the air-fuel ratio of a mixture 
supplied to an internal combustion engine to such a small value as can 
secure stable operation of the engine during cold starting of the engine 
and/or during warming-up of the engine immediately following the cold 
starting of the engine, and on the other hand, to set the air-fuel ratio 
to such large values as correspond to operating conditions of the engine 
after the warming-up of the engine has been completed, since the operation 
of the engine becomes stable after completion of the warming-up. In order 
to satisfy required operating characteristics of an internal combustion 
engine such as driveability, fuel consumption and exhaust emission 
characteristics at the same time, it is prerequisite that the air-fuel 
ratio should be accurately controlled in accordance with warming-up 
conditions of the engine. On the other hand, exhaust gas sensors for 
sensing the concentration of a specific component in exhaust gases from 
internal combustion engines, as represented by an O.sub.2 sensor, are 
generally used in feedback control of the air-fuel ratio. Such exhaust gas 
sensors need to be activated before they normally operate, that is, the 
temperature of the sensors per se has to be elevated up to a prescribed 
activating temperature. Therefore, conventionally, when the engine is 
started while it is in a cold state, the air-fuel ratio feedback control 
cannot be effected due to the exhaust gas sensor being inactive. 
There have been proposed air-fuel ratio control systems which are intended 
to appropriately control the air-fuel ratio by taking into account the 
aforementioned difference in required air-fuel ratio between during cold 
starting of an internal conbustion engine and after completion of the 
warming-up of the engine, e.g. by Japanese Patent Publication (Kokoku) No. 
57-7297 (hereinafter called "Conventional System 1") and Japanese 
Provisional Patent Publication (Kokai) No. 58-20950 (hereinafter called 
"Conventional System 2"). 
According to Conventional System 1, the air-fuel ratio is controlled by an 
automatic choke valve when the ambient temperature of an internal 
combustion engine is below a first predetermined value, by an air-fuel 
ratio control valve which regulates an amount of air introduced into an 
air bleed of a carburetor, in response to the ambient temperature when the 
ambient temperature is above the first predetermined value and below a 
second predetermined value, and by the air-fuel ratio control valve in 
response to the output from an exhaust gas sensor when the ambient 
temperature is above the second predetermined value, respectively. That 
is, according to Conventional System 1, when the engine ambient 
temperature is in an intermediate range between the first predetermined 
value and the second predetermined value, the air-fuel ratio is not 
controlled in feedback mode responsive to the output from the exhaust gas 
sensor but it is controlled in response to the engine ambient temperature 
by the air-fuel ratio control valve, which, however, results in the 
air-fuel ratio not being accurately controlled to a desired value. 
Further, when the engine ambient temperature is in the intermediate range, 
the air-fuel ratio has to be controlled to a relatively small or rich 
value required by the engine so as to prevent stalling of the engine which 
is being warmed up, which is disadvantageous in respect of fuel economy. 
On the other hand, according to Conventional System 2, an O.sub.2 sensor 
which has an output characteristic linear with respect to the 
concentration of oxygen in exhaust gases is used as the exhaust gas sensor 
to sensor the actual air-fuel ratio, and the air-fuel ratio is controlled 
to a desired value in feedback manner such that an air-fuel ratio control 
valve which regulates the fuel supply amount is controlled in response to 
the result of comparison between the actual air fuel ratio and the desired 
one. However, if the air-fuel ratio control valve is controlled by means 
of digital computation, the resolution power of control of the valve is 
set as a function of a quotient resulting from equal division of the 
controlling range of the air-fuel ratio by a given number. Therefore, as 
the controlling range of the air-fuel ratio becomes larger, the resolution 
power becomes degraded. If Conventional System 2 is applied both during 
and after warming-up of the engine, the controlling range of the air-fuel 
ratio is larger as compared with the case where it is applied only after 
warming-up of the engine, and accordingly the resolution power, i.e. the 
control accuracy becomes degraded, thus making it difficult to control the 
air-fuel ratio to a desired value. After warming-up of the engine in 
particular, although the engine operation becomes stable so that the 
variation width of the air-fuel ratio becomes small, the air-fuel ratio 
has to be controlled in a fine manner so as to secure required 
driveability and exhaust emission characteristics of the engine, which, 
however, cannot be achieved owing to the above-mentioned degraded 
resolution power. 
SUMMARY OF THE INVENTION 
It is the object of the invention to provide an air-fuel ratio control 
system for an internal combustion engine, which is capable of controlling 
the air-fuel ratio to a desired one in an accurate manner to thereby 
achieve required driveability, fuel consumption and exhaust emission 
characteristics of the engine, both during and after warming-up of the 
engine. 
In order to achieve the object, the present invention provides an air-fuel 
ratio control system for an internal combustion engine, the system having 
an automatic choke valve arranged in an intake passage of the engine, an 
exhaust gas sensor arranged in an exhaust passage of the engine and having 
an output characteristic linear with respect to the concentration of a 
specific component in exhaust gases from the engine, an air passage 
bypassing a throttle valve in the intake passage, an air-fuel ratio 
control valve arranged in the air passage and disposed to be driven in 
response to an output from the exhaust gas sensor for controlling the 
air-fuel ratio of a mixture supplied to the engine, and temperature 
sensing means for sensing the degree of warming-up of the engine. 
The air-fuel ratio control system according to the invention is 
characterized by the combination comprising: determining means for 
determining whether the exhaust gas has been activated; means for 
controlling the opening of the automatic choke valve in response to the 
degree of warming-up of the engine sensed by the temperature sensing means 
while the determining means determines that the exhaust gas sensor is 
inactive; means for determining the difference between a desired value of 
the air-fuel ratio and an actual value thereof sensed by the exhaust gas 
sensor, and for driving the automatic choke valve when the determined 
difference is larger than a predetermined value, and the air-fuel ratio 
control valve when the determined difference is smaller than the 
predetermined value, respectively, from the time the determining means 
determines for the first time that the exhaust gas sensor has become 
activated to the time the temperature sensing means detects completion of 
warming-up of the engine; and means for driving the air-fuel ratio control 
valve in response to operating conditions of the engine so as to achieve a 
desired value of the air-fuel ratio, after the temperature sensing means 
detects completion of warming-up of the engine. 
Preferably, the air-fuel ratio control system according to the invention 
includes setting means for setting a desired value of the air-fuel ratio 
in dependence on the degree of warming-up of the engine sensed by the 
temperature sensing means, from the time the determining means determines 
for the first time that the exhaust gas sensor has become activated to the 
time the temperature sensing means detects completion of warming-up of the 
engine. 
More preferably, the above setting means sets the desired value of the 
air-fuel ratio in dependence on engine coolant temperature and intake air 
temperature. 
Preferably, the above-mentioned means for controlling the opening of the 
automatic choke valve controls the opening of the automatic choke valve in 
dependence on intake air temperature and atmospheric pressure, while the 
determining means determines that the exhaust gas sensor is inactive. 
Also preferably, the opening of the air-fuel ratio control valve is held at 
one of maximum and minimum values thereof and the opening of the automatic 
choke valve is varied so that the actual value of the air-fuel ratio 
becomes equal to the desired value thereof when the difference between the 
desired value of the air-fuel ratio and the actual value thereof is larger 
than the predetermined value, from the time the determining means 
determines for the first time that the exhaust gas sensor has become 
activated to the time the temperature sensing means detects completion of 
warming-up of the engine. 
Preferably, the opening of the air-fuel ratio control valve is set to 
values corresponding to the difference between the desired value of the 
air-fuel ratio and the actual value thereof and the opening of the 
automatic choke valve is held at an immediately preceding value thereof 
when the difference between the desired value of the air-fuel ratio and 
the actual value thereof is smaller than the predetermined value, from the 
time the determining means determines for the first time that the exhaust 
gas sensor has become activated to the time the temperature sensing means 
detects completion of warming-up of the engine. 
The opening of the air-fuel ratio control valve is set to values 
corresponding to the difference between the desired value of the air-fuel 
ratio and the actual value thereof and the opening of the automatic choke 
valve is held at a maximum value thereof after the temperature sensing 
means detects completion of warming-up of the engine. 
The above and other objects, features and advantages of the invention will 
be more apparent from the ensuing detailed description taken in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION 
The invention will now be described in detail with reference to the 
drawings showing an embodiment thereof. 
Referring first to FIG. 1, there is shown the whole arrangement of an 
air-fuel ratio control system for an internal combustion engine according 
to the invention. Connected to the engine 1 which may be a four-cylinder 
type is an intake pipe 2 which is provided therein with a venturi 3 
forming part of a carburetor. One end of a fuel passage 4 opens, as a 
nozzle 5, into the interior of the intake pipe 2 at the venturi 3, and the 
other end of the fuel passage 4 leads to a float chamber, not shown, of 
the carburetor. 
An automatic choke valve 6 is arranged in the intake pipe 2 upstream of the 
venturi 3, and a throttle valve 7 downstream of the venturi 3, 
respectively. The automatic choke valve 6 comprises a bimetal 6a, and a 
heater 8, and is disposed to have its opening decreased as the temperature 
of the bimetal 6a becomes lower. The heater 8 is electrically connected to 
an electronic control unit (hereinafter called "the ECU") 20 to have its 
energizing duty ratio D controlled by the latter so as to have its heat 
generation amount controlled accordingly, whereby the temperature of the 
bimetal 6 is controlled to thereby control the opening degree of the choke 
valve 6 as well as the opening and closing speeds thereof. The heater 8 is 
preferably formed by a well-known PTC heater, which, when its temperature 
rises above a predetermined value, suddenly increases in electric 
resistance so that the amount of current flowing therethrougn suddenly 
decreases and accordingly the heat generation amount is restricted below a 
predetermined value. 
In FIG. 1, reference numeral 9 designates a secondary air supply passage, 
which communicates at one end thereof with the interior of the intake pipe 
2 upstream of the choke valve 6 and at the other end with the interior of 
the intake pipe 2 downstream of the throttle valve 7, thus bypassiong the 
throttle valve 7. Arranged across the secondary air supply passage 9 is an 
electromagnetic valve 10 which is a normally closed and two-position type 
on-off valve and has a solenoid 10a disposed to be energized with a duty 
ratio set by the ECU 20 so that the valve opening is controlled to thereby 
control the amount of secondary air supplied to the engine 1 through the 
secondary air supply passage 9. 
A throttle valve opening (.theta.th) sensor 11 is connected to the throttle 
valve 7, for supplying an electric signal indicative of the sensed opening 
of the valve 7 to the ECU 20. An absolure pressure (PBA) sensor 12 and an 
intake air temperature (TA) sensor as a temperature sensing means are 
provided for sensing absolute pressure and intake air temperature in the 
intake pipe 2 downstream of the throttle valve 7, respectively, and 
supplying electric signals indicative of the sensed PBA and TA values to 
the ECU 20. 
An engine coolant temperature (TW) sensor 14 as another temperature sensing 
means is mounted on the cylinder block of the engine 1. To be specific, 
the sensor 14 is embedded in the peripheral wall of a cylinder, not shown, 
of the engine 1 filled with engine coolant, for supplying an electric 
signal indicative of the sensed coolant temperature value to the ECU 20. 
An engine rotational speed (Ne) sensor 15 is arranged in facing relation 
to a camshaft or a crankshaft of the engine 1, neither of which is shown, 
and adapted to generate a pulse corresponding to one of predetermined 
crank angle positions each time the crankshaft rotates through 180 
degrees, the pulse being supplied to the ECU 20 as a TC signal pulse. 
An atmospheric pressure (PA) sensor 16 is electrically connected to the ECU 
20 for supplying an electric signal indicative of the sensed atmospheric 
pressure value thereto. 
A three-way catalyst 18 is arranged within an exhaust pipe 7 of the engine 
1 for purifying HC, CO and NOx components in exhaust gases emitted from 
the engine 1. An O.sub.2 sensor 19 as an exhaust gas sensor is inserted 
into the exhaust pipe 17 for sensing the concentration of oxygen in the 
exhaust gases. The O.sub.2 sensor 19 employed by the present invention has 
such an output characteristic that the output voltage from the sensor 19 
varies linearly with respect to the oxygen concentration in the exhaust 
gases, in other words, an output characteristic which is linear with 
respect to the air-fuel ratio of a mixture supplied to the engine 1. The 
O.sub.2 sensor 19 generates an electric signal indicative of the sensed 
air-fuel ratio and supplies same to the ECU 20. The O.sub.2 sensor 19 has 
a built-in heater, not shown, which generates heat when supplied with 
current from the ECU 20 to heat the sensor body so that the sensor becomes 
activated within a short period of time, e.g. after the lapse of about 10 
seconds since an ignition switch, not shown, of the engine 1 is turned on 
even at cold starting of the engine 1. 
The ECU 20 is mainly composed of input circuit means 20a having functions, 
e.g. of shaping the waveforms of input signals from part of the 
above-mentioned various sensors, shifting the levels of output voltages 
from part of the sensors into a predetermined level, converting analog 
output signals from part of the sensors into respective digital signals, a 
central processing unit (hereinafter called "the CPU") 20b, memory means 
20c storing control programs executed within the CPU 20b, and for storing 
results of various calculations executed within the CPU 20b, and output 
circuit means 20d having functions, e.g. of supplying driving signals to 
the heater 8 and the electromagnetic valve 10, etc. Further, a timer, not 
shown, is provided within the ECU 20 as determining means for determining 
the state of activation of the O.sub.2 sensor 19. 
FIG. 2 is a flowchart of a control program for carrying out the air-fuel 
ratio control by means of the air-fuel ratio control system of the 
invention shown in FIG. 1, The present program is executed upon generation 
of each TDC signal pulse. 
First at a step 201, it is determined whether or not the O.sub.2 sensor 19 
has been activated. The determination is made by determining whether or 
not a predetermined period of time (e.g. 10 seconds) has elapsed from the 
time the ignition switch has been turned on. If the answer to the question 
of the step 201 is negative, that is, if the O.sub.2 sensor 19 is 
inactive, a rich air-fuel ratio is then required, and the program proceeds 
to a step 202 wherein the valve opening or energizing pulse duty ratio E 
of the electromagnetic valve 10 is held at 0 to make the secondary air 
supply amount zero. Then, the program proceeds to a step 203 to determine 
a basic value DBASE of the energizing duty rato of the heater 8 from the 
intake air temperature TA sensed by the intake air temperature sensor 13. 
FIG. 3 shows, by way of example, a table of the relationship between 
intake air temperature TA and the duty ratio basic value D.sub.BASE, 
according to which the basic value D.sub.BASE is set to smaller values as 
the intake air temperature becomes lower, that is, as the degree of 
warming-up of the engine 1 is lower. Then, a step 204 is executed to 
determine an atmospheric pressure-dependent correction value D.sub.PA from 
the atmospheric pressure PA sensed by the atmospheric pressure sensor 16. 
FIG. 4 shows, by way of example, a table of the relationship between 
atmospheric pressure PA and the atmospheric pressure-dependent correction 
value D.sub.PA, according to which the correction value D.sub.PA is set to 
zero and 50%, respectively, when the atmospheric pressure PA is 760 mmHg 
or higher and when it is 450 mmHg or lower, and is determined by means of 
an interpolation method when the atmospheric pressure PA is between 450 
mmHg and 760 mmHg so as to decrease with increase in the atmospheric 
pressure PA. This atmospheric pressure-dependent correction of the duty 
ratio D of the heater 8 is effective for preventing the mixture from 
becoming overrich due to decrease in the air density due to a drop in the 
atmospheric pressure PA. Then, the program proceeds to a step 205 wherein 
the duty ratio D of the heater 8 is calculated by the use of the following 
equation (1) using the basic value D.sub.BASE and the atmospheric 
pressure-dependent correction value D.sub.PA read at the steps 204 and 
204, respectively: 
EQU D=D.sub.BASE +D.sub.PA (1) 
Then the program proceeds to a step 223 to energize the heater 8 with the 
duty ratio D thus calculated, followed by termination of the program. As 
is learned from the above, when the O.sub.2 sensor 19 is determined to be 
inactive, the electromagnetic valve 10 is kept closed to make the seconary 
air supply amount zero, while the duty ratio D of the heater 8 is set to a 
value corresponding to intake air temperature TA and atmospheric pressure 
PA, whereby the engine 1 is supplied with a mixture having an appropriate 
small or rich air-fuel ratio set in accordance with the degree of 
warming-up of the engine as well as atmospheric pressure PA. 
If the answer to the question of the step 201 is affirmative or Yes, that 
is, if the O.sub.2 sensor 19 has become activated, the program proceeds to 
a step 206 wherein it is determined whether or not the engine coolant 
temperature TW is higher than a predetermined value TW1 (e.g. 80.degree. 
C.). This determination is intended to determine whether or not the engine 
1 has become fully warmed up. If the answer is negative or No, that is, if 
TW=TW1 stands and accordingly it is determined that the engine 1 has not 
been warmed up as yet, the program proceeds to a step 207 wherein a basic 
value A/F.sub.BASE of the desired air-fuel ratio is determined from the 
engine coolant temperature TW sensed by the engine coolant temperature 
sensor 14. FIG. 5 shows, by way of example, a table of the relationship 
between engine coolant temperature TW and the basic value A/F.sub.BASE, 
according to which the basic value A/F.sub.BASE is set to smaller values 
as the engine coolant temperature becomes lower, and it is set to values 
along a curve converging to a final desired air-fuel ratio A/F.sub.LMT 
(e.g. 14.7) after completion of warming-up of the engine when the engine 
coolant temperature TW exceeds the aforementioned predetermined value TW1. 
The program then proceeds to a step 208 wherein a correction value 
.DELTA.A/F for the desired air-fuel ratio is determined from the intake 
air temperature TA. FIG. 6 shows, by way of example, a table of the 
relationship between the correction value .DELTA.A/F and intake air 
temperature TA. The correction value .DELTA.A/F is set to 0, -3.0, and 
+1.0, respectively, when the intake air temperature TA is equal to 
+35.degree. C., below -25.degree. C., and above +45.degree. C., and it is 
determined by means of an interpolation method so as to increase with rise 
in the intake air temperature when the intake air temperature TA lies 
between -25.degree. C. and +35.degree. C. and between +35.degree. C. and 
+45.degree. C. 
The program then proceeds to a step 209 to calculate the desired air-fuel 
ratio A/F.sub.REF by the use of the following equation (2) using the basic 
value A/F.sub.BASE and the correction value .DELTA.A/F read at the steps 
207 and 208, respectively: 
EQU A/F.sub.REF =A/F.sub.BASE +.DELTA.A/F (2) 
Thus, the desired air-fuel ratio A/F.sub.REF is set in response to the 
engine coolant temperature TW and the intake air temperature TA such that 
it becomes smaller as the values of these temperatures are lower, that is, 
as the degree of warming-up of the engine is lower. Since the desired 
air-fuel ratio A/F.sub.REF is set in dependence on two different 
temperature parameters of engine coolant temperature TW and intake air 
temperature TA, which are sensed at different locations of the engine 1 
and both represent the warming-up condition of the engine 1, the desired 
air-fuel ratio can be set to a more appropriate value as compared with 
setting of the desired air-fuel ratio in dependence on a single 
temperature parameter, e.g. engine coolant temperature TW. 
The basic value A/F.sub.BASE of the desired air-fuel ratio and the 
correction value .DELTA.A/F may be set in dependence on the intake air 
temperature TA and the engine coolant temperature TW, respectively, i.e. 
in a manner reverse to the above described embodiment. 
Then, the program proceeds to a step 210 wherein it is determined whether 
or not the actual air-fuel ratio A/F sensed by the O.sub.2 sensor 19 is 
larger than the desired air-fuel ratio A/F.sub.REF set at the step 209. If 
the answer is affirmative or Yes, it is determined at a step 211 whether 
or not the difference between the actual and desired air-fuel ratios is 
smaller than a predetermined value .alpha.1 (e.g. 1.0-2.0). If the answer 
is negative or No, that is, if the actual air-fuel ratio A/F is larger 
than the desired air-fuel ratio A/F.sub.REF, and at the same time the 
difference between the two air-fuel ratios is relatively large, it is then 
necessary to largely reduce or enrich the actual air-fuel ratio. 
Therefore, a step 212 is called for wherein the duty ratio E of the 
electromagnetic valve 10 is held at the minimum value applied during 
warming-up of the engine 1, e.g. 20%, and the duty ratio D of the heater 8 
is set to a value smaller than an immediately preceding value set at the 
time of generation of an immediately preceding TDC signal pulse, by a 
predetermined value D1 (e.g. 0.01-1.0%), followed by execution of the 
aforementioned step 223 and termination of the program. 
If the answer to the question of the step 211 is affirmative or Yes, that 
is, if the actual air-fuel ratio A/F is larger than the desired one 
A/F.sub.REF and at the same time the difference between the two air-fuel 
ratios is relatively small, the program proceeds to a step 214 wherein the 
duty ratio E of the electromagnetic valve 10 is set to a value dependent 
upon the air-fuel ratio difference, within a range from 20 to 40% in such 
a manner that it is set to smaller values as the air-fuel ratio difference 
is larger. Then at a step 215 the duty ratio D of the heater 8 is set to 
the same value set at the time of generation of an immediately preceding 
TDC signal pulse, followed by execution of the step 223 and termination of 
the program. Thus, the actual air-fuel ratio is controlled to the desired 
one with accuracy. 
If the answer to the question of the step 210 is negative or No, that is, 
if the actual air-fuel ratio is smaller than the desired air-fuel ratio 
A/F.sub.REF, the program proceeds to a step 216 wherein it is determined 
whether or not the air-fuel ratio difference is smaller than a 
predetermined value .alpha.2 (e.g. 1.0-2.0). If the answer is affirmative 
or Yes, that is, if the actual air-fuel ratio A/F is smaller than the 
desired one A/F.sub.REF and at the same time the air-fuel ratio difference 
is relatively small, steps 217 and 218 are executed to control the duty 
ratios E and D, respectively, just in the same manner as the 
aforedescribed steps 214 and 215. 
If the answer to the question of the step 216 is negative or No, that is, 
if the actual air-fuel ratio A/F is smaller than the desired one 
A/F.sub.REF and at the same time the air-fuel ratio difference is 
relatively large, it is then necessary to largely increase or lean the 
actual air-fuel ratio A/F. Therefore, a step 219 is called for wherein the 
duty ratio E of the electromagnetic valve 10 is held at the maximum value 
applied during warming-up of the engine, e.g. 40% to thereby supply the 
maximum quantity of secondary air to the engine 1. Then at a step 220 the 
duty ratio D of the heater 8 is set to a value larger than an immediately 
preceding value set at the time of generation of an immediately preceding 
TDC signal pulse, by a predetermined value D2 (e.g. 0.01-1.0%), followed 
by execution of the aforementioned step 223 and termination of the 
program. 
As described above, according to the invention, from the time of activation 
of the O.sub.2 sensor 19 to the time of completion of warming-up of the 
engine 1 the desired air-fuel ratio A/F.sub.REF is set to appropriate 
values in dependence on two parameters representative of the degree of 
warming-up of the engine 1, e.g. engine coolant temperature TW and intake 
air temperature TA. Further, depending upon the magnitude of the 
difference between the actual air-fuel ratio A/F and the desired one 
A/F.sub.REF, the air-fuel ratio is controlled in different manners. That 
is, when the air-fuel ratio difference is relatively large, the valve 
opening of the choke valve 6, which is adapted to change the actual 
air-fuel ratio A/F at a large rate, is feedback-controlled, whereas when 
the air-fuel ratio difference is relatively small, the duty ratio E of the 
electromagnetic valve 10, which is adapted to change the actual air-fuel 
ratio A/F in a fine manner, is feedback-controlled. By virtue of this 
controlling manner, the air-fuel ratio range that is to be controlled by 
the electromagnetic valve 10 can be made moderately narrow, making it 
possible to control the actual air-fuel ratio A/F to the desired one 
A/F.sub.REF with high accuracy as well as with high responsiveness, with 
the aid of the choke valve 6. 
If the answer to the question of the step 206 is affirmative or Yes, that 
is, TW TW1 stands so that it is judged that the engine 1 has been warmed 
up, the program proceeds to a step 221 wherein the duty ratio E of the 
electromagnetic valve 10 is controlled to values within a range from 0 to 
100% depending upon the difference between the actual air-fuel ratio A/F 
and the desired one A/F.sub.REF. Also on this occasion the desired 
air-fuel ratio A/F.sub.REF is set in dependence on the engine coolant 
temperature TW and the intake air temperature TA in accordance with the 
tables of FIGS. 5 and 6, as in the control during warming-up of the engine 
1. The program then proceeds to a step 222 wherein the duty ratio D of the 
heater 8 is held at 100%, followed by execution of the step 223 and 
termination of the program. In this way, after completion of warming-up of 
the engine 1, the choke valve 6 is kept fully open, i.e. kept inoperative, 
and at the same time air-fuel feedback control is carried out based upon 
the output from the O.sub.2 sensor 19, whereby the actual air-fuel ratio 
A/F is controlled to the desired air-fuel ratio A/F.sub.REF with high 
accuracy. 
Since the heater 8 is formed of a PTC heater, even if the duty ratio D is 
held at 100%, the heat generation amount is so small that the choke valve 
6 is kept fully open without hindrance. 
The air-fuel ratio control system according to the invention, which 
performs the air-fuel ratio control in the above described manner, 
provides excellent results as follows: 
(i) Since the valve opening of the automatic choke valve 6 is controlled to 
a value corresponding to the degree of warming-up of the engine, that is, 
to the output from the temperature sensing means 13 while the O.sub.2 
sensor 19 is inactive, an appropriate air-fuel ratio can be achieved; 
(ii) Since from the time of activation of the O.sub.2 sensor 19 to the time 
of completion of warming-up of the engine the desired air-fuel ratio is 
set in dependence on the degree of warming-up of the engine as well as on 
a plurality of temperature parameters (e.g. TW and TA), the desired 
air-fuel ratio can be set to an appropriate value. 
(iii) By virtue of the use of the exhaust gas sensor which has an output 
characteristic linear with respect to the actual air-fuel ratio, the 
actual air-fuel ratio can be sensed with accuracy so that the air-fuel 
ratio can be accurately controlled to the desired one; 
(iv) Since from the time of activation of the O.sub.2 sensor 19 to the time 
of completion of warming-up of the engine the automatic choke valve 6 is 
driven when the difference between the actual air-fuel ratio and the 
desired air-fuel ratio is large, whereby the actual air-fuel ratio is 
largely changed to promptly bring the actual air-fuel ratio to the desired 
one, whereas the air-fuel ratio control valve 10 is driven when the 
air-fuel ratio difference is small, whereby the actual air-fuel ratio is 
finely adjused to the desired one, the air-fuel ratio range that is to be 
controlled by the air-fuel ratio control valve can be made moderately 
narrow, thereby securing required control accuracy; 
(v) Since after completion of warming-up of the engine the air-fuel ratio 
control valve is driven so that the actual air-fuel ratio is brought to 
the desired one, the actual air-fuel ratio can be controlled with accuracy 
both during warming-up of the engine and after completion of the 
warming-up in cooperation with the results (i)-(iv), thereby enabling to 
satisfy all requirements in respect of driveability, fuel consumption and 
exhaust emission characteristics of the engine.