Air/fuel ratio control system for internal combustion engines, having secondary air supply control

An air/fuel ratio control system for use with an internal combustion engine, which includes a three-way catalyst arranged in the exhaust system of the engine, an electrical circuit operatively connecting an O.sub.2 sensor with fuel quantity adjusting means in a manner effecting feedback control of the air/fuel ratio of a mixture produced by the fuel quantity adjusting means and being supplied to the engine, in response to an output signal produced by the O.sub.2 sensor, and secondary air supply means for supplying atmospheric air into the exhaust system at a zone upstream of the O.sub.2 sensor. The secondary air supply means is adapted to operate only during open loop control of the air/fuel ratio.

BACKGROUND OF THE INVENTION 
This invention relates to an air/fuel ratio control system for performing 
feedback control of the air/fuel ratio of an air/fuel mixture being 
supplied to an internal combustion engine, and more particularly to a 
device provided in such system for controlling a secondary air valve so as 
to supply secondary air into the exhaust system of the engine under 
particular open loop control conditions for enhanced purification of 
exhaust gas ingredients by means of a three-way catalyst. 
An air/fuel ratio control system for use with an internal combustion engine 
having an intake pipe has already been proposed by the assignee of the 
present application, which comprises a three-way catalyst provided in the 
exhaust system of the engine, an O.sub.2 sensor for detecting the 
concentration of oxygen in exhaust gases emitted from the engine, fuel 
quantity adjusting means for producing an air/fuel mixture being supplied 
to the engine, and an electrical circuit operatively connecting the 
O.sub.2 sensor with the fuel quantity adjusting means in a manner 
effecting feedback control operation to control the air/fuel ratio of the 
mixture to a predetermined value in response to an output signal produced 
by the O.sub.2 sensor. 
Internal combustion engines for automotive vehicles are generally provided 
with three-way catalysts in their exhaust systems for purifying 
ingredients of HC and CO in the exhaust gases. It is also known to use a 
secondary air valve for supplying atmospheric air into the engine exhaust 
system at a zone upstream of the O.sub.2 sensor to cause combustion of the 
HC and CO ingredients in the exhaust gases under an oxidizing atmosphere 
for better purification of these ingredients by a three-way catalyst 
arranged in the exhaust system. 
However, according to the aforementioned air/fuel ratio control system, the 
air/fuel ratio of the mixture is controlled by means of feedback to a 
predetermined value in response to the output signal of the O.sub.2 
sensor, as noted above. Therefore, if the supply of secondary air into the 
engine exhaust system is carried out during the feedback control 
operation, the output signal of the O.sub.2 sensor which is located 
downstream of the secondary air supplying zone in the engine exhaust 
system does not represent a proper air/fuel ratio on the basis of which 
the air/fuel ratio is to be controlled, thus impeding accurate air/fuel 
ratio feedback control. 
OBJECT AND SUMMARY OF THE INVENTION 
It is therefore the object of the invention to provide an air/fuel ratio 
control system for internal combustion engines, which is arranged to 
effect the supply of secondary air into the engine exhaust system in 
synchronism with air/fuel ratio control in open loop mode, with no 
influence upon the air/fuel ratio feedback control. 
According to the invention, there is provided an air/fuel ratio control 
system for performing feedback control of the air/fuel ratio of an 
air/fuel mixture being supplied to an internal combustion engine having an 
intake system and an exhaust system, which includes a three-way catalyst 
provided in the exhaust system of the engine, an O.sub.2 sensor also 
provided in the exhaust system at a location upstream of the three-way 
catalyst for detecting the concentration of an exhaust gas ingredient 
emitted from the engine, fuel quantity adjusting means for producing the 
mixture being supplied to engine, and an electrical circuit operatively 
connecting the O.sub.2 sensor with the fuel quantity adjusting means in a 
manner effecting feedback control operation to control the air/fuel ratio 
of the mixture to a predetermined value in response to an output signal 
produced by the O.sub.2 sensor. The air/fuel ratio control system is 
characterized by comprising in combination: secondary air supply means for 
supplying atmospheric air into the exhaust system of the engine at a 
location between the engine and the O.sub.2 sensor, an engine speed sensor 
for detecting the rotational speed of the engine, a pressure sensor for 
detecting absolute pressure in the intake system of the engine, means for 
determining activation of the O.sub.2 sensor, means connected to the 
engine speed sensor, the pressure sensor and the O.sub.2 sensor activation 
determining means, for producing a feedback control interrupting signal 
when there occurs at least one of conditions of engine idle, engine 
deceleration and deactivation of the O.sub.2 sensor, and means responsive 
to the feedback control interrupting signal for actuating the secondary 
air supply means.

DETAILED DESCRIPTION 
The air/fuel ratio control system according to the invention will now be 
described in detail with reference to the accompanying drawings wherein an 
embodiment of the invention is illustrated. 
Referring now to FIG. 1, there is illustrated the whole system of the 
invention. Reference numeral 1 designates an internal combustion engine. 
Connected to the engine 1 is an intake manifold 2 which is provided with a 
carburetor generally designated by the numeral 3. The carburetor 3 has 
fuel passages 5, 6 which communicate a float chamber 4 with the primary 
bore 3.sub.1 of the carburetor 3. These fuel passages 5, 6 are connected 
to an air/fuel ratio control valve generally designated by the numeral 9, 
via air bleed passages 8.sub.1, 8.sub.2. The carburetor 3 also has fuel 
passages 7.sub.1, 7.sub.2 communicating the float chamber 4 with the 
secondary bore 3.sub.2 of the carburetor 3. The fuel passage 7.sub.1, on 
one hand, is connected to the above air/fuel ratio control valve 9 via an 
air passage 8.sub.3 and, on the other hand, opens in the secondary bore at 
a location slightly upstream of a throttle valve 30.sub.2 in the secondary 
bore 3.sub.2. The fuel passage 7.sub.2 communicates with the interior of 
an air cleaner 47 via an air passage 8.sub.4 having a fixed orifice. The 
control valve 9 is comprised of three flow rate control valves, each of 
which is formed of a cylinder 10, a valve body 11 displaceably inserted 
into the cylinder 10, and a coil spring 12 interposed between the cylinder 
10 and the valve body 11 for urging the valve body 11 in a predetermined 
direction. Each valve body 11 is tapered along its end portion 11a remote 
from the coil spring 12 so that the effective opening area of the opening 
10a of each cylinder 10, in which the tapered portion 11a of the valve 
body is inserted, varies as the valve body 11 is moved. Each valve body 11 
is disposed in urging contact with a connection plate 15 coupled to a worm 
element 14 which is axially movable but not rotatable about its own axis. 
The worm element 14 is in threaded engagement with the rotor 17 of a pulse 
motor 13 which is arranged about the element 14 and rotatably supported by 
radial bearings 16. Arranged about the rotor 17 is a solenoid 18 which is 
electrically connected to an electronic control unit (hereinafter called 
"ECU") 20. The solenoid 18 is energized by driving pulses supplied from 
ECU 20 to cause rotation of the rotor 17 which in turn causes movement of 
the worm element 14 threadedly engaging the rotor 17 in the leftward and 
rightward directions as viewed in FIG. 1. Accordingly, the connection 
plate 15 coupled to the worm element 14 is moved leftward and rightward in 
unison with the movement of the worm element 14. 
The pulse motor 13 has its stationary housing 21 provided with a permanent 
magnet 22 and a reed switch 23 arranged opposite to each other. The plate 
15 is provided at its peripheral edge with a magnetic shielding plate 24 
formed of a magnetic material which is interposed between the permanent 
magnet 22 and the reed switch 23 for movement into and out of the gap 
between the two members 22, 23. The magnetic shielding plate 24 is 
displaced in the leftward and rightward directions in unison with 
displacement of the plate 15 in the corresponding directions. The reed 
switch 23 turns on or off in response to the displacement of the plate 24. 
That is, when the valve body 11 of the air/fuel ratio control valve 9 
passes a reference position which is determined by the positions of the 
permanent magnet 22, reed switch 23 and magnetic shielding plate 24, the 
reed switch 23 turns on or off depending upon the moving direction of the 
valve body 11, to supply a corresponding binary output signal to ECU 20. 
Incidentally, the pulse motor housing 21 is formed with an air intake 25 
communicating with the atmosphere. Air is introduced through a filter 26 
mounted in the air intake 25, into each flow rate control valve in the 
housing 21. 
On the other hand, an O.sub.2 sensor 28, which is made of stabilized 
zirconium oxide or the like, is mounted in the peripheral wall of the 
exhaust manifold 27 of the engine 1 in a manner partly projecting in the 
manifold 27. The sensor 28 is electrically connected to ECU 20 to supply 
its output signal thereto. An atmospheric pressure sensor 29 is arranged 
to detect the ambient atmospheric pressure surrounding the vehicle, not 
shown, in which the engine 1 is installed, and also electrically connected 
to ECU 20 to supply its output signal thereto. Further provided is a 
pressure sensor 43 which is arranged to detect the intake pressure 
(absolute pressure) in the intake manifold 2 through a conduit 44 which 
opens in the inner wall of the manifold 2 at a zone downstream of the 
throttle valves 35.sub.1, 35.sub.2. The pressure sensor 43 is also 
electrically connected to ECU 20 to supply its output signal thereto. A 
thermistor 45 is inserted in the peripheral wall of the engine cylinder, 
the interior of which is filled with engine cooling water, to detect the 
temperature of the engine cooling water as the engine temperature and 
supply its output signal to ECU 20 to which it is connected. 
Reference numeral 30 designates a secondary air valve. This valve 30 has a 
housing 30d which is connected to one end of a conduit 31 which in turn 
has its other end opening in the inner wall of the exhaust manifold 27. 
The housing 30d is provided therein with a partition wall 30f having a 
through hole 30f'. The partition wall 30f has a reed 30a mounted on its 
side surface facing the conduit 31 to close and open the through hole 
30f'. Thus, atmospheric air is allowed to flow only toward the exhaust 
manifold 27 through the through hole 30f'. The wall of the housing 30d 
remote from the exhaust manifold 27 is formed with an opening 30d' which 
is closable by a diaphragm 30b mounted on the housing 30d. Further, a 
cover 30e is mounted on the housing 30d in a fashion enclosing the 
diaphragm 30b, with its one end connected to the housing 30d and its other 
end to a corresponding end of a conduit 32, respectively. Interposed 
between the cover 30e and the diaphragm 30b is a spring 30c which urges 
the diaphragm 30b in the direction of closing the opening 30d'. The 
diaphragm 30b and the cover 30e form a valve for closing the opening 30d', 
in cooperation with the spring 30c, and also define a negative pressure 
chamber 30g therebetween to provide negative pressure-actuated means for 
actuating the above valve for closing the opening 30d'. 
The secondary air valve 30 is, on one hand, connected to the exhaust 
manifold 27 of the engine 1, through the conduit 31, and, on the other 
hand, to the air cleaner 47, that is, communicating with the atmosphere, 
through a conduit 37. The above-mentioned conduit 32 communicates the 
valve 30 with a control valve 33 which is formed of a solenoid controlled 
valve. The valve 33 in turn communicates with the intake manifold 2 at a 
zone downstream of the throttle valves 35.sub.1, 35.sub.2 via a conduit 
34. The conduit 31 opens in the exhaust manifold 27 at a location upstream 
of the O.sub.2 sensor 28. A three-way catalyst 36 is arranged across the 
exhaust output downstream of the O.sub.2 sensor 28. The solenoid 
controlled valve 33 is a three way valve which is comprised of a valve 
body 33a disposed for interrupting the communication between the conduits 
32, 34, a spring 33b disposed to permanently urge the valve body 33a in 
its closing direction, a solenoid 33c disposed to be energized by a 
control signal from ECU 20, and an air intake 33d arranged for 
communication with the conduit 32 when the valve is closed. When the 
solenoid 33c is energized, the valve 33 allows negative pressure produced 
in the intake manifold 2 at a zone downstream of the throttle valves 
35.sub.1, 35.sub.2 during operation of the engine to be introduced into 
the secondary air valve 30. The introduced negative pressure retracts the 
diaphragm 30b of the valve 30 to allow air (secondary air) to be 
introduced into the valve 30 through the conduit 37. Then, when negative 
pressure is produced in the exhaust manifold 27, this air urges the reed 
30a into its open position to be introduced into the exhaust manifold 27. 
This introduction of secondary air into the exhaust manifold 27 causes 
dilution of the exhaust gases with the secondary air to place the 
three-way catalyst 36 under an oxidizing atmosphere wherein HC and CO in 
the exhaust gases are well burned to obtain good purification of the 
exhaust gases. 
However, if the above secondary air introduction is carried out by the reed 
valve 30 during air/fuel ratio feedback control based upon the detected 
value signal outputted from the O.sub.2 sensor 28 which will be 
hereinlater referred to, the detected value signal of the O.sub.2 sensor 
which is located downstream of the opening of the conduit 31 does not 
represent a proper air/fuel ratio, on the basis of which feedback control 
of the air/fuel ratio is to be carried out. Therefore, during feedback 
control of the air/fuel ratio the reed valve 30 is held inoperative by 
means of the solenoid controlled valve 33 which is actuated by the control 
signal outputted from ECU 20, that is, the valve 30 is made to operate 
only when particular open loop control conditions are met where the 
air/fuel ratio feedback control is not carried out. 
In FIG. 1, reference numeral 38 designates an ignition plug, 39 a 
distributor, 40 an ignition coil, 41 an ignition switch, and 42 a battery, 
respectively. The distributor 39 has a drive shaft, not shown, which is 
arranged for rotation at a speed proportional to the engine speed, and 
accordingly pulses are produced in the ignition coil 40, which correspond 
in frequency to switching of contact points or the output of a contactless 
pickup, the contact points or the contactless pickup being arranged to 
operate in synchronism with the rotation of the above drive shaft. The 
above pulses produced in the coil 40 are supplied to ECU 20. It will be 
noted that in the illustrated embodiment the distributor 39 and the 
ignition coil 40 form an engine rpm sensor. 
Details of the air/fuel ratio control which can be performed by the 
air/fuel ratio control system according to the invention will now be 
described by reference to FIG. 1 which has been referred to hereinabove. 
Initialization 
Referring first to the initialization, when the ignition switch 41 in FIG. 
1 is set on, ECU 20 is initialized to detect the reference position of the 
actuator or pulse motor 13 by means of the reed switch 23 and hence drive 
the pulse motor 13 to set it to its best position (a preset position) for 
starting the engine, that is, set the initial air/fuel ratio to a 
predetermined proper value. The above preset position of the pulse motor 
13 is hereinafter called "PS.sub.CR ". This setting of the initial 
air/fuel ratio is made on condition that the engine rpm Ne is lower than a 
predetermined value N.sub.CR (e.g., 400 rpm) and the engine is in a 
condition before firing. The predetermined value N.sub.CR is set at a 
value higher than the cranking rpm and lower than the idling rpm. 
The above reference position of the pulse motor 13 is detected as the 
position at which the reed switch 23 turns on or off, as previously 
mentioned with reference to FIG. 1. 
Then, ECU 20 monitors the condition of activation of the O.sub.2 sensor 28 
and the coolant temperature Tw detected by the thermistor 45 to determine 
whether or not the engine is in a condition for initiation of the air/fuel 
ratio control. For accurate air/fuel ratio feedback control, it is a 
requisite that the O.sub.2 sensor 28 is fully activated and the engine is 
in a warmed-up condition. The O.sub.2 sensor, which is made of stabilized 
zirconium dioxide or the like, has a characteristic that its internal 
resistance decreases as its temperature increases. If the O.sub.2 sensor 
is supplied with electric current through a resistance having a suitable 
resistance value from a constant-voltage regulated power supply provided 
within ECU 20, the electrical potential or output voltage of the sensor 
initially shown a value close to the power supply voltage (e.g., 5 volts) 
when the sensor is not activated, and then, its electrical potential 
lowers with the increase of its temperature. Therefore, according to the 
invention, the air/fuel ratio feedback control is not initiated until 
after the conditions are fulfilled that the sensor produces an activation 
signal when its output voltage lowers down to a predetermined voltage Vx 
(e.g., 0.5 volt) a timer finishes counting for a predetermined period of 
time t.sub.x (e.g., 1 minute) starting from the occurrence of the above 
activation signal, and the coolant temperature Tw increases up to a 
predetermined value Twx at which the automatic choke is opened to an 
opening for enabling the air/fuel ratio feedback control. 
During warming-up operation of the engine 1 where the O.sub.2 sensor 28 is 
not yet activated and the temperature of the engine cooling water is still 
low, unburned ingredients are emitted in large quantities from the engine 
1. The secondary air valve 30 is opened during such warming-up operation 
to allow the three-way catalyst 36 to operate under an oxidizing 
atmosphere to largely reduce the amount of the unburned ingredients. 
Further, this secondary air supply enables detection of the activation of 
the O.sub.2 sensor in the lean or large air/fuel ratio region of the 
exhaust gases. If detection of the activation of the O.sub.2 sensor is 
effected in the rich or small air/fuel ratio region to the contrary, the 
reference voltage Vx for comparison with the output voltage V of the 
O.sub.2 sensor has to be set at a higher value and also the counting 
period of time t.sub.x of the timer has to be larger than in the case of 
detecting the O.sub.2 sensor activation in the lean region, which 
necessitates retarded commencement of the air/fuel ratio feedback control 
so as to cope with variations in performance between engines to be used 
with the air/fuel ratio control system. This retarded air/fuel ratio 
control commencement leads to an increase in the amount of detrimental 
ingredients in the exhaust gases, depending upon the performance of an 
engine concerned. However, this disadvantage can be avoided by the 
detection of the O.sub.2 sensor activation in the lean region. 
During the above stage of the detection of activation of the O.sub.2 sensor 
and the coolant temperature Tw, the pulse motor 13 is held at its 
predetermined position PS.sub.CR. The pulse motor 13 is driven to 
appropriate positions in response to the operating condition of the engine 
after initiation of the air/fuel ratio control, as hereinlater described. 
Basic Air/Fuel Ratio Control 
Following the initialization, the program proceeds to the basic air/fuel 
ratio control. 
ECU 20 is responsive to various detected value signals representing the 
output voltage of the O.sub.2 sensor 28, the absolute pressure in the 
intake manifold 2 detected by the pressure sensor 43, the engine rpm Ne 
detected by the rpm sensor 39, 40, and the atmospheric pressure P.sub.A 
detected by the atmospheric pressure sensor 29, to drive the pulse motor 
13 as a function of these signals to control the air/fuel ratio. More 
specifically, the basic air/fuel ratio control comprises open loop control 
which is carried out at wide-open-throttle, at engine idle, and at engine 
deceleration, and closed loop control which is carried out at engine 
partial load. All the control is initiated after completion of the 
warming-up of the engine. 
First, the condition of open loop control at wide-open-throttle is met when 
the differential pressure P.sub.A -P.sub.B (gauge pressure) between the 
absolute pressure P.sub.B detected by the pressure sensor 43 and the 
atmospheric pressure P.sub.A (absolute pressure) detected by the 
atmospheric pressure sensor 29 is lower than a predetermined value 
.DELTA.P.sub.WOT. ECU 20 compares the difference in value between the 
output signals of the sensors 29, 43 with the predetermined value 
.DELTA.P.sub.WOT stored therein, and when the relationship of P.sub.A 
-P.sub.B &lt;.DELTA.P.sub.WOT stands, drives the pulse motor 13 to a 
predetermined position (preset position) PS.sub.WOT and holds it there, 
which is a position best appropriate for the engine emissions to be 
obtained at the time of termination of the wide-open-throttle open loop 
control. At wide-open-throttle, a known economizer, not shown, or the like 
is actuated to supply a rich or small air/fuel ratio mixture to the 
engine. 
The condition of open loop control at engine idle is met when the engine 
rpm Ne is lower than a predetermined idle rpm N.sub.IDL (e.g., 1,000 rpm). 
ECU 20 compares the output signal value Ne of the rpm sensor 39, 40 with 
the predetermined rpm N.sub.IDL stored therein, and when the relationship 
of Ne&lt;N.sub.IDL stands, drives the pulse motor 13 to a predetermined idle 
position (preset position) PS.sub.IDL which is best suitable for the 
engine emissions and holds it there. 
The condition of open loop control at engine deceleration is fulfilled when 
the absolute pressure P.sub.B in the intake manifold is lower than a 
predetermined value PB.sub.DEC. ECU 20 compares the output signal value 
P.sub.B of the pressure sensor 43 with the predetermined value PB.sub.DEC 
stored therein, and when the relationship of P.sub.B &lt;PB.sub.DEC stands, 
drives the pulse motor 13 to a predetermined deceleration position (preset 
position) PS.sub.DEC best suitable for the engine emissions and holds it 
there. 
The ground for this condition of open loop control at engine deceleration 
lies in that when the absolute pressure P.sub.B in the intake manifold 
drops below the predetermined value, unburned HC is produced at an 
increased rate in the exhaust gases, to make it impossible to carry out 
the air/fuel ratio feedback control based upon the detected value signal 
of the O.sub.2 sensor with accuracy, thus failing to control the air/fuel 
ratio to a theoretical value. Therefore, according to the invention, the 
open loop control is employed, as noted above, when the absolute pressure 
P.sub.B in the intake manifold detected by the pressure sensor 43 is 
smaller than the predetermined value PB.sub.DEC, where the pulse motor is 
set to the predetermined position PS.sub.DEC best suitable for the engine 
emissions obtained at the time of termination of the deceleration open 
loop control. At the beginning of engine deceleration, a shot air valve, 
not shown, is actuated to supply air into the intake manifold to prevent 
the occurrence of unburned ingredients in the exhaust gases. 
During operations of the above-mentioned open loop control at 
wide-open-throttle, at engine idle, at engine deceleration, the respective 
predetermined positions PS.sub.WOT, PS.sub.IDL, PS.sub.DEC for the pulse 
motor 13 are compensated for atmospheric pressure P.sub.A, as hereinlater 
described. 
On the other hand, the condition of closed loop control at engine partial 
load is met when the engine is in an operating condition other than the 
above-mentioned open loop control conditions. During the closed loop 
control, ECU 20 performs selectively feedback control based upon 
proportional term correction (hereinafter called "P term control") and 
feedback control based upon integral term correction (hereinafter called 
"I term control"), in response to the engine rpm Ne detected by the engine 
rpm sensor 39, 40 and the output signal of the O.sub.2 sensor 28. To be 
concrete, the integral term correction is used when the output voltage of 
the O.sub.2 sensor 28 varies only at the higher level side or only at the 
lower level side with respect to a reference voltage Vref, wherein the 
position of the pulse motor 13 is corrected by an integral value obtained 
by integrating the value of a binary signal which changes in dependence on 
whether the output voltage of the O.sub.2 sensor is at the higher level or 
at the lower level with respect to the predetermined reference voltage 
Vref, to thereby achieve stable and accurate position control of the pulse 
motor 13. On the other hand, when the output signal of the O.sub.2 sensor 
changes from the higher level to the lower level or vice versa, the 
proportional term correction is carried out wherein the position of the 
pulse motor 13 is corrected by a value directly proportional to a change 
in the output voltage of the O.sub.2 sensor to thereby achieve air/fuel 
ratio control in a manner prompter and more efficient than the integral 
term correction. 
As noted above, according to the above I term control, the pulse motor 
position is varied by an integral value by integrating the value of a 
binary signal corresponding to the change of the output voltage of the 
O.sub.2 sensor. According to this I term control, the number of steps by 
which the pulse motor is to be displaced per second differs depending upon 
the speed at which the engine is then operating. That is, in a low engine 
rpm range, the number of steps by which the pulse motor is to be displaced 
is small. With an increase in the engine rpm, the above number of steps 
increases so that it is large in a high engine rpm range. 
Whilst, according to the P term control which, as noted above, is used when 
there is a change in the output voltage of the O.sub.2 sensor from the 
higher level to the lower one or vice versa with respect to the reference 
voltage Vref, the number of steps by which the pulse motor is to be 
displaced per second is set at a single predetermined value (e.g., 6 
steps), irrespective of the engine rpm. 
The air/fuel ratio control at engine acceleration (i.e., off-idle 
acceleration) is carried out when the engine rpm Ne exceeds the 
aforementioned predetermined idle rpm N.sub.IDL during the course of the 
engine speed increasing from a low rpm range to a high rpm range, that is, 
when the engine speed changes from a relationship Ne&lt;N.sub.IDL to one 
Ne.gtoreq.N.sub.IDL. On this occasion, ECU 20 rapidly moves the pulse 
motor 13 to a predetermined acceleration position (preset position) 
PS.sub.ACC, and thereafter initiates the aforementioned air/fuel ratio 
feedback control. This predetermined position PS.sub.ACC is compensated 
for atmospheric pressure P.sub.A, too, as hereinlater described. 
The above-mentioned predetermined position PS.sub.ACC is set at a position 
where the amount of detrimental ingredients in the exhaust gases is small. 
Therefore, particularly at the so-called "standing start", i.e., 
acceleration from a vehicle-stopping position, setting the pulse motor 
position to the predetermined position PS.sub.ACC is advantageous to 
anti-exhaust measures, as well as to achievement of accurate air/fuel 
ratio feedback control to be done following the acceleration. This 
acceleration control is carried out under a warmed-up engine condition, 
too. 
In transition from the above-mentioned various open loop control to the 
closed loop control at engine partial load or vice versa, changeover 
between open loop mode and closed loop mode is effected in the following 
manner: First, in changing from closed loop mode to open loop mode, ECU 20 
moves the pulse motor 13 to an atmospheric pressure-compensated 
predetermined position PSi(P.sub.A) in a manner referred to later, 
irrespective of the position at which the pulse motor was located 
immediately before entering the open loop control. This predetermined 
position PSi(P.sub.A) includes preset positions PS.sub.CR, PS.sub.WOT, 
PS.sub.IDL, PS.sub.DEC and PS.sub.ACC, each of which is corrected in 
response to actual atmospheric pressure as hereinlater referred to. 
Various open loop control operations can be promptly done, simply by 
setting the pulse motor to the above-mentioned respective predetermined 
positions. 
On the other hand, in changing from open loop mode to closed loop mode, ECU 
20 commands the pulse motor 13 to initiate air/fuel ratio feedback control 
with I term correction. That is, there can be a difference in timing 
between the change of the output signal level of the O.sub.2 sensor from 
the high level to the low level or vice versa and the change from the open 
loop mode to the closed loop mode. In such an event, the deviation of the 
pulse motor position from the proper position upon entering the closed 
loop mode, which is due to such timing difference, is much smaller in the 
case of initiating air/fuel ratio control with I term correction than that 
in the case of initiating it with P term correction, to make it possible 
to resume early accurate air/fuel ratio control and accordingly ensure 
highly stable engine emissions. 
To obtain optimum exhaust emission characteristics irrespective of changes 
in the actual atmospheric pressure during open loop air/fuel ratio control 
or at the time of shifting from open loop mode to closed loop mode, the 
position of the pulse motor 13 needs to be compensated for atmospheric 
pressure. According to the invention, the above-mentioned predetermined or 
preset positions PS.sub.CR, PS.sub.WOT, PS.sub.IDL, PS.sub.DEC, PS.sub.ACC 
at which the pulse motor 13 is to be held during the respective open loop 
control operations are corrected in a linear manner as a function of 
changes in the atmospheric pressure P.sub.A, using the following equation: 
EQU PSi(P.sub.A)=PSi+(760-P.sub.A).times.Ci 
where i represents any one of CR, WOT, IDL, DEC and ACC, accordingly PSi 
represents any one of PS.sub.CR, PS.sub.WOT, PS.sub.IDL, PS.sub.DEC and 
PS.sub.ACC at 1 atmospheric pressure (=760 mmHg), and Ci a correction 
coefficient, representing any one of C.sub.CR, C.sub.WOT, C.sub.IDL, 
C.sub.DEC and C.sub.ACC. The values of PSi and Ci are previously stored in 
ECU 20. 
ECU 20 applies to the above equation the coefficients PSi, Ci which are 
determined at proper different values according to the kinds of open loop 
control to be carried out, to calculate by the above equation the position 
PSi(P.sub.A) for the pulse motor 13 to be set at a required kind of open 
loop control and moves the pulse motor 13 to the calculated position 
PSi(P.sub.A). 
By correcting the air/fuel ratio during open loop control in response to 
the actual atmospheric pressure in the above-mentioned manner, it is 
possible to obtain not only conventionally known effects such as best 
driveability and prevention of burning of the ignition plug in an engine 
cylinder, but also optimum emission characteristics by setting the value 
of Ci at a suitable value, since the pulse motor position held during open 
loop control forms an initial position upon entering subsequent closed 
loop control. 
The position of the pulse motor 13 which is used as the actuator for the 
air/fuel ratio control valve 9 is monitored by a position counter provided 
within ECU 20. However, there can occur a disagreement between the counted 
value of the position counter and the actual position of the pulse motor 
due to skipping or racing of the pulse motor. In such an event, ECU 20 
operates on the counted value of the position counter as if it were the 
actual position of the pulse motor 13. However, this can impede proper 
setting of the air/fuel ratio during open loop control where the actual 
position of the pulse motor 13 must be accurately recognized by ECU 20. 
In view of the above disadvantage, according to the air/fuel ratio control 
system of the invention, in addition to detection of the initial position 
of the pulse motor 13 by regarding as the reference position (e.g., 50th 
step) the position of the pulse motor at which the reed switch 23 turns on 
or off when the pulse motor is driven, which was previously noted with 
reference to the initialization, the position counter has its counted 
value replaced by the number of steps corresponding to the reference 
position (e.g., 50 steps) stored in ECU 20 upon the pulse motor 13 passing 
the switching point of the reed switch 23, to thus ensure high reliability 
of subsequent air/fuel ratio control. 
Control of Secondary Air Valve 
The secondary air valve 30, which serves to create an oxidizing atmosphere 
in the interior of the three-way catalyst 36 for efficient purification of 
HC, CO ingredients in the exhaust gases as previously noted, needs to be 
held inoperative during air/fuel ratio feedback control for the reason 
previously mentioned. To this end, according to the invention, the 
secondary air valve 30 is operated in synchronism with the open loop 
control operations so as to avoid concurrence of the operation of the 
valve with the closed loop control operation. More specifically, the 
secondary air valve 30 is operated when there is fulfillment of any one of 
the conditions of open loop control operations at engine idle, at engine 
deceleration, at non-activation of the O.sub.2 sensor and at engine low 
temperature (before warming-up of the engine). To this end, ECU 20 
energizes the solenoid controlled valve 33 to actuate the secondary air 
valve 30 when any one of the following conditions a-c is fulfilled: 
a. The aforementioned timer has not finished counting as yet, which is 
triggered by an activation signal from the O.sub.2 sensor 28 to start 
counting for one minute for instance, or the engine coolant temperature Tw 
is lower than a predetermined value (e.g., 35.degree. C.). 
b. The engine rpm Ne is lower than a predetermined value (e.g., 1,000 rpm). 
c. The absolute pressure P.sub.B in the intake manifold is lower than a 
predetermined value (e.g., 200 mmHg), that is, the negative pressure in 
the intake manifold is larger than the predetermined value. 
Operation of the secondary air valve 30 at fulfillment of any one of the 
above requirements a, b, c will bring about the following results: 
(i) The condition a corresponds to prewarming-up condition of the engine. 
Under such condition, CO, HC ingredients are present in large quantities 
in the exhaust gases. Purification of these ingredients can be effectively 
carried out by the three-way catalyst 36 due to operation of the valve 30. 
(ii) The condition b corresponds to idling condition of the engine where 
NOx is present in small quantities in the exhaust gases. Purification of 
CO and unburned HC ingredients which are produced at engine idle is made 
by the three-way catalyst 36 due to operation of the valve 30. 
(iii) The condition c corresponds to decelerated condition of the engine 
where NOx is present in small quantities in the exhaust gases. 
Purification of CO and unburned HC ingredients which are produced at 
engine deceleration is effected by the three-way catalyst due to operation 
of the valve 30. 
The above values Tw, Ne, P.sub.B are detected, respectively, by the engine 
cooling water temperature sensor 45, rpm sensor 39, 40 and pressure sensor 
43, all shown in FIG. 1. 
FIG. 2 is a block diagram illustrating the interior construction of ECU 20 
used in the air/fuel ratio control system having the above-mentioned 
functions according to the invention. In ECU 20, reference numeral 201 
designates a circuit for detecting the activation of the O.sub.2 sensor 28 
in FIG. 1, which is supplied at its input with an output signal V from the 
O.sub.2 sensor. Upon passage of the predetermined period of time Tx after 
the voltage of the above output signal V has dropped below the 
predetermined value Vx, the above circuit 201 supplies an activation 
signal S.sub.1 to an activation determining circuit 202. This activation 
determining circuit 202 is also supplied at its input with an engine 
coolant temperature signal Tw from the thermistor 45 in FIG. 1. When 
supplied with both the above activation signal S.sub.1 and the coolant 
temperature signal Tw indicative of a value exceeding the predetermined 
value Twx, the activation determining circuit 202 supplies an air/fuel 
ratio control initiation signal S.sub.2 to a PI control circuit 203 to 
render same ready to operate. Reference numeral 204 represents an air/fuel 
ratio determining circuit which determines the value of air/fuel ratio of 
engine exhaust gases, depending upon whether or not the output voltage of 
the O.sub.2 sensor is larger than the predetermined value Vref, to supply 
a binary signal S.sub.3 indicative of the value of air/fuel ratio thus 
obtained, to the PI control circuit 203. On the other hand, an engine 
condition detecting circuit 205 is provided in ECU 20, which is supplied 
with an engine rpm signal Ne from the engine rpm sensor 39, 40, an 
absolute pressure signal P.sub.B from the pressure sensor 43, an 
atmospheric pressure signal P.sub.A from the atmospheric pressure sensor 
29, all the sensors being shown in FIG. 1, and the above control 
initiation signal S.sub.2 from the activation determining circuit 202 in 
FIG. 2, respectively. The circuit 205 supplies a control signal S.sub.4 
indicative of a value corresponding to the values of the above input 
signals to the PI control circuit 203. The PI control circuit 203 
accordingly supplies to a change-over circuit 209 to be referred to later 
a pulse motor control signal S.sub.5 having a value corresponding to the 
air/fuel ratio signal S.sub.3 from the air/fuel ratio determining circuit 
204 and a signal component corresponding to the engine rpm Ne in the 
control signal S.sub.4 supplied from the engine condition detecting 
circuit 205. The engine condition detecting circuit 205 also supplies to 
the PI control circuit 203 the above control signal S.sub.4 containing a 
signal component corresponding to the engine rpm Ne, the absolute pressure 
P.sub.B in the intake manifold, atmospheric pressure P.sub.A and the value 
of air/fuel ratio control initiation signal S.sub.2. When supplied with 
the above signal component from the engine condition detecting circuit 
205, the PI control circuit 203 interrupts its own operation. Upon 
interruption of the supply of the above signal component to the control 
circuit 203, a pulse signal S.sub.5 is outputted from the circuit 203 to 
the change-over circuit 209, which signal starts air/fuel ratio control 
with integral term correction. A preset value register 206 is provided in 
ECU 20, in which are stored the basic values of preset values PS.sub.CR, 
PS.sub.WOT, PS.sub.IDL, PS.sub.DEC and PS.sub.ACC for the pulse motor 
position, applicable to various engine conditions, and atmospheric 
pressure correcting coefficients C.sub.CR, C.sub.WOT, C.sub.IDL, C.sub.DEC 
and C.sub.ACC for these basic values. The engine condition detecting 
circuit 205 detects the operating condition of the engine based upon the 
activation of the O.sub.2 sensor and the values of engine rpm Ne, intake 
manifold absolute pressure P.sub.B and atmospheric pressure P.sub.A to 
read from the register 206 the basic value of a preset value corresponding 
to the detected operating condition of the engine and its corresponding 
correcting coefficient and apply same to an arithmetic circuit 207. The 
arithmetic circuit 207 performs arithmetic operation responsive to the 
value of the atmospheric pressure signal P.sub.A, using the equation 
PSi(P.sub.A)=PSi+(760-P.sub.A).times.Ci. The resulting preset value is 
applied to a comparator 210. 
On the other hand, a reference position signal processing circuit 208 is 
provided in ECU 20, which is responsive to the output signal of the 
reference position detecting device (reed switch) 23, indicative of the 
switching of same, to produce a binary signal S.sub.6 having a certain 
level from the start of the engine until it is detected that the pulse 
motor reaches the reference position. This binary signal S.sub.6 is 
supplied to the change-over circuit 209 which in turn keeps the control 
signal S.sub.5 from being transmitted from the PI control circuit 203 to a 
pulse motor driving signal generator 211 as long as it is supplied with 
this binary signal S.sub.6, thus avoiding the interference of the 
operation of setting the pulse motor to the initial position with the 
operation of P-term/I-term control. The reference position signal 
processing circuit 208 also produces a pulse signal S.sub.7 in response to 
the output signal of the reference position detecting device 23, which 
signal causes the pulse motor 13 to be driven in the step-increasing 
direction or in the step-decreasing direction so as to detect the 
reference position of the pulse motor 13. This signal S.sub.7 is supplied 
directly to the pulse motor driving signal generator 211 to cause same to 
drive the pulse motor 13 until the reference position is detected. The 
reference position signal processing circuit 208 produces another pulse 
signal S.sub.8 each time the reference position is detected. This pulse 
signal S.sub.8 is supplied to a reference position register 212 in which 
the value of the reference position (e.g., 50 steps) is stored. This 
register 212 is responsive to the above signal S.sub.8 to apply its stored 
value to one input terminal of the comparator 210 and to the input of a 
reversible counter 213. The reversible counter 213 is also supplied with 
an output pulse signal S.sub.9 produced by the pulse motor driving signal 
generator 211 to count the pulses of the signal S.sub.9 corresponding to 
the actual position of the pulse motor 13. When supplied with the stored 
value from the reference position register 212, the counter 213 has its 
counted value replaced by the value of the reference position of the pulse 
motor. 
The counted value thus renewed is applied to the other input terminal of 
the comparator 210. Since the comparator 210 has its other input terminal 
supplied with the same pulse motor reference position value, as noted 
above, no output signal is supplied from the comparator 210 to the pulse 
motor driving signal generator 211 to thereby hold the pulse motor at the 
reference position with certainty. Subsequently, when the O.sub.2 sensor 
28 remains deactivated, an atmospheric pressure-compensated preset value 
PS.sub.CR (P.sub.A) is outputted from the arithmetic circuit 207 to the 
one input terminal of the comparator 210 which in turn supplies an output 
signal S.sub.10 corresponding to the difference between the preset value 
PS.sub.CR (P.sub.A) and a counted value supplied from the reversible 
counter 213, to the pulse motor driving signal generator 211, to thereby 
achieve accurate control of the position of the pulse motor 13. Also, when 
the other open loop control conditions are detected by the engine 
condition detecting circuit 205, similar operations to that just mentioned 
above are carried out. 
Referring to FIG. 3, there is shown a block diagram of a device which is 
provided in ECU 20 in FIG. 1 and operable to actuate the secondary air 
valve 30 in synchronism with fulfillment of open loop control conditions 
to avoid that the valve 30 is operated during air/fuel ratio feedback 
control. 
The rpm sensor 39, 40 in FIG. 1 is connected to the input of an F-V 
(frequency-to-voltage) converter 214 for converting the engine rpm into a 
direct current voltage, which in turn has its output connected to the 
input of a comparator 215. This comparator 215 is arranged to compare an 
output voltage produced by the F-V converter 214 with a reference voltage 
corresponding to a predetermined engine rpm (1,000 rpm) to determine 
fulfillment of the aforementioned open loop control condition at engine 
idle. The comparator 215 is adapted to produce a binary output of 1 when 
the output voltage of the F-V converter 214 is lower than the reference 
voltage. The pressure sensor 43 in FIG. 1 is connected to the input of 
another comparator 216 which is arranged to compare the output voltage of 
the pressure sensor 43 with a reference voltage corresponding to a 
predetermined absolute pressure (200 mmHg) to determine fulfillment of the 
aforementioned open loop control condition at engine deceleration. This 
comparator 216 is adapted to produce a binary output of 1 when the output 
voltage of the pressure sensor 43 indicative of the absolute pressure in 
the intake manifold is lower than the reference voltage. The O.sub.2 
sensor 28 and the thermistor 45 in FIG. 1 are connected to the activation 
determining circuit 202, respectively, by way of the O.sub.2 sensor 
activation detecting circuit 201 and directly, as previously noted with 
reference to FIG. 2. As previously mentioned, the activation detecting 
circuit 201 is adapted to supply the activation signal S.sub.1 to the 
activation determining circuit 202 upon passage of the predetermined 
period of time t.sub.x (1 minute) after the output voltage of the O.sub.2 
sensor 28 has dropped below the predetermined value Vx which is set so as 
to determine fulfillment of the air/fuel ratio control initiating 
condition. When supplied with the temperature signal Tw from the 
thermistor 45, indicative of a value exceeding the predetermined value 
(35.degree. C.) which is set so as to determine fulfillment of the 
air/fuel ratio control initiating condition in addition to the above 
activation signal S.sub.1, the activation determining circuit 202 supplies 
the air/fuel ratio control initiating signal S.sub.2 to the PI control 
circuit 203 in FIG. 2, as previously noted. The circuit 202 is, however, 
adapted to produce a binary signal of 1 so long as only one of the above 
signals S.sub.1 and Tw is supplied thereto. 
The comparators 215, 216 and the activation determining circuit 202 have 
their respective outputs connected to corresponding input terminals of an 
OR circuit 217. This OR circuit 217 has its output connected to the input 
of a driving circuit 218 which in turn has its output connected to the 
solenoid 33c of the solenoid controlled valve 33 for control of the 
secondary air valve 30, to energize or deenergize the same solenoid. The 
above circuits 214 through 217 form part of the engine operating condition 
detecting circuit 205 in FIG. 2. 
The operation of the above arrangement will now be described. At the start 
of the engine for instance, the activation determining circuit 202 is not 
yet supplied with the signal S.sub.1 from the activation detecting circuit 
201. At this stage, the circuit 202 produces a binary signal of 1 to cause 
the OR circuit 217 to produce a binary output of 1 when the temperature 
signal Tw outputted from the activation detecting circuit 201 shows a 
value lower than the predetermined value (35.degree. C.). Accordingly, the 
driving circuit 218 energizes the solenoid 33c of the solenoid controlled 
valve 33 to cause the secondary air valve 30 to be opened to introduce 
atmospheric air (secondary air) into the exhaust manifold 27. The above 
binary signal of 1 outputted from the OR circuit 217 is also fed as the 
feedback control interrupting signal S.sub.4 to the PI control circuit 203 
in FIG. 2 to interrupt the air/fuel ratio feedback control. 
When the engine rpm signal Ne outputted from the engine rpm sensor 39, 40 
has a value lower than the predetermined value (1,000 rpm) (i.e., at 
engine idle), or when the signal P.sub.B representing the absolute 
pressure in the intake manifold 2, which is outputted from the pressure 
sensor 43, has a value lower than the predetermined value (200 mmHg) 
(i.e., at engine deceleration), the comparators 215, 216 both produce 
binary outputs of 1 to cause energization of the solenoid 33c of the 
solenoid controlled valve 33 through the OR circuit 217 and the driving 
circuit 218, to thereby actuate the secondary air valve 30 to introduce 
atmospheric air (secondary air) into the exhaust manifold 27. Also on this 
occasion, the output of 1 of the OR circuit 217 is fed as the feedback 
control interrupting signal S.sub.4 to the PI control circuit 203 to 
interrupt the air/fuel ratio feedback control. 
When none of the aforementioned three conditions for actuation of the 
solenoid controlled valve 33 is fulfilled, the comparators 215, 216 and 
the activation determining circuit 202 all produce binary outputs of 0 to 
keep the solenoid 33c of the solenoid controlled valve 33 deenergized to 
suspend the secondary air introduction, while simultaneously allowing the 
air/fuel ratio feedback control to be continued.