Method and system for controlling the mixture air-to-fuel ratio

A closed loop air-to-fuel ratio control system having an oxygen responsive sensor and a feedback circuit is disclosed. The sensor is disposed in an exhaust passage to produce a voltage indicative of the oxygen concentration in the exhaust gas. This voltage is integrated in the feedback circuit to cause the mixture air-to-fuel ratio to be corrected in response thereto. During specific engine conditions such as engine idling and low temperature of the exhaust gas, the integration is stopped to thereby switch off the closed loop to an open loop. The engine, on this occasion, can be supplied with the air-fuel mixture of an arbitrary ratio irrespective of the oxygen concentration in the exhaust gas, resulting in the optimum air-to-fuel control well-matched to the engine conditions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an air-to-fuel ratio control system for an 
internal combustion engine, wherein a closed feedback loop for controlling 
the mixture ratio is switched off to an open loop during specific engine 
conditions. 
2. Description of the Prior Art 
A closed loop feedback control system for internal combustion engines has 
been highly appreciated from the point that the air-to-fuel ratio of the 
mixture to be supplied to the engine can be controlled to the 
stoichiometric ratio at which exhaust emissions therefrom becomes 
tolerable. It is a well-known matter, on the other hand, that the engine 
requires the air-fuel mixture other than the stoichiometric mixture upon 
specific engine conditions such as idling and that the oxygen responsive 
sensor is inoperative under the low ambient temperature. The closed loop 
feedback control, for this reason, must be switched off under these 
conditions. 
SUMMARY OF THE INVENTION 
It is, therefore, a primary object of the invention to switch off a closed 
loop feedback control system to an open loop control system under specific 
engine conditions. 
It is another object of the invention to stop the integration operation of 
the feedback control system to thereby switch off the closed loop system. 
It is a further object of the invention to stop the integration operation 
while the oxygen sensor is inoperative. 
It is a still further object of the invention to stop the integration 
operation while a throttle valve is fully opened and closed. 
It is a still further object of the invention to stop the integration 
operation while the engine is in at least one preselected conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the embodiment shown in FIG. 1, an oxygen responsive sensor 
(O.sub.2 -sensor) 1 is provided in an exhaust passage 6 of an internal 
combustion engine 5. The engine 5 is provided with a mixture supply 
controller 4, which detects operating conditions of the engine 5 and in 
turn supplies air-fuel mixture thereto. The mixture supply controller 4 is 
constructed as a well-known fuel injection controller, in this embodiment, 
which determines the fuel injection duration in accordance with the engine 
conditions such as air amount sucked into the engine 5 and rotational 
speed thereof. 
The O.sub.2 -sensor 1 is connected, via a comparison circuit 2, to an 
integration circuit 3 which is connected to the mixture supply controller 
4 for correcting the mixture air-to-fuel ratio to the stoichiometric ratio 
in response to the sensor output voltage. A closed feedback loop 
comprising the oxygen sensor 1, the comparison circuit 2 and the 
integration circuit 3 is switched off from the controller 4 under 
preselected specific engine conditions. A condition detector 8 coupled to 
the engine 5 detects the preselected engine conditions and a halt circuit 
7 halts the integration operation of the integration circuit 3 in response 
to a detection signal from the condition detector 8. 
The output voltage characteristics of the O.sub.2 -sensor 1 is shown in 
FIG. 2, wherein the abscissa and the ordinate respectively represent the 
mixture air-to-fuel ratio and the output voltage. As can be seen from FIG. 
2, the output voltage level of the O.sub.2 -sensor 1 is high and low for 
the lesser ratio (air number .lambda.&lt;1 or rich mixture) and the greater 
ratio (.lambda.&gt;1 or lean mixture), respectively, and it abruptly changes 
from one level to the other at the stoichiometric ratio (.lambda.=1) above 
which oxygen is present in the exhaust gas. 
In FIG. 3 showing a detail circuit construction of the feedback loop, 
V.sub.B and GND designate the voltage potential of a storage battery (not 
shown) and the ground potential, respectively. The comparison circuit 2 is 
constructed with resistors 2a to 2p, a zener diode 2q, transistors 2r to 
2u and an operational amplifier 2v. The amplifier 2v receives the set 
voltage V.sub.sl divided by the resistors 2a and 2b at the positive 
terminal (+) via the resistor 2i and the voltage developing at the 
junction of the resistors 2f and 2g at the negative terminal (-) via the 
resistor 2h. The set voltage V.sub.sl is selected to be proportional to 
the sensor output voltage V.sub.s (FIG. 2) indicative of the 
stoichiometric ratio of the air-fuel mixture. The transistor 2s connected 
to the resistor 2g receives at the base thereof the output voltage of the 
O.sub.2 -sensor 1, whereas the transistor 2r connected to the resistor 2 f 
receives at the base thereof a constant voltage regulated by the zener 
diode 2q and the resistors 2c, 2d and 2e. 
Receiving the low level output voltage indicative of the lean mixture from 
the O.sub.2 -sensor 1, the transistor 2s is rendered conductive to provide 
the negative terminal of the amplifier 2v with a voltage lower than the 
set voltage V.sub.sl. Receiving the high level output voltage indicative 
of the rich mixture from the O.sub.2 -sensor 1, the transistor 2s is 
rendered nonconductive to provide with a voltage higher than the set 
voltage V.sub.sl. The amplifier 2v, comparing the two input voltages, 
produces a high level and a low level comparison-resultant voltages when 
the negative terminal voltage is lower and higher than the positive 
terminal voltage V.sub.sl, respectively. 
The comparison-resultant voltage is applied to the bases of the transistors 
2t and 2u, the collectors thereof being connected to each other, through 
the respective resistors 20 and 2p to control the on-off condition 
thereof. The resistors 2k, 2l, 2m and 2n are connected in series across 
the V.sub.B line and the GND line and the emitters of the transistors 2t 
and 2u are connected to the junction of the resistors 2k and 2l and to the 
junction of the resistors 2m and 2n, respectively. 
The transistor 2t becomes conductive in response to the low level 
comparison-resultant voltage and an electric current i1 flows therethrough 
to the integration circuit 3, whereas the transistor 2u becomes conductive 
in response to the high level comparison-resultant voltage and an electric 
current i2 flows therethrough from the integration circuit 3. 
The integration circuit 3 is constructed with an operational amplifier 3a, 
a capacitor 3b and resistors 3c, 3d and 3e. The negative terminal (-) and 
the positive terminal (+) of the amplifier 3a are connected to the 
collectors of the transistors 2t and 2u via the resistor 3c and to the 
junction of the resistors 2l and 2m to be provided with the set voltage 
.sup.V B/.sub.2 through the resistor 3d, respectively. Connected between 
the input terminal (-) and the output terminal of the amplifier 3d is the 
capacitor 3b for integrating the currents i1 and i2. The integration 
circuit 3 produces an integration-resultant voltage which increases while 
the current i2 is integrated and decreases while the current i1 is 
integrated. 
The integration-resultant voltage repetitively becomes higher and lower 
than the set voltage .sup.V B/.sub.2 provided that the engine 5 is 
supplied with the air-fuel mixture of the stoichiometric ratio. Correcting 
the mixture air-to-fuel ratio in accordance with the integration-resultant 
voltage, more particularly increasing and decreasing the fuel amount while 
the voltage is higher and lower than the set voltage .sup.V B/.sub.2 
respectively, the air-to-fuel ratio of the mixure supplied from the 
mixture supply controller can be controlled to the stoichiometric ratio. 
The integration operation of the integration circuit 3 is controlled by the 
halt circuit 7 and the condition detector 8. The halt circuit 7 is 
constructed with transistors 7a and 7b and resistors 7c, 7d and 7e. The 
condition detector 8 is constructed with resistors 8a to 8n, a 
thermally-sensitive resistor 80, an operational amplifier 8p, transistors 
8q and 8r, on-off switches 8s and 8t and diodes 8u, 8v and 8w. 
The thermally-sensitive resistor 80 having a negative temperature 
coefficient is positioned in the exhaust passage to detect the ambient 
exhaust temperature of the O.sub.2 -sensor 1 which is inoperative, as well 
known, under the temperature 450.degree..about.600.degree. C. A voltage 
indicative of the ambient temperature and developing at the junction of 
the resistors 8a and 8o and a set voltage determined by the resistors 8b 
and 8c to be corresponding to the ambient temperature over which the 
O.sub.2 -sensor 1 becomes operative are applied to the amplifier 8p. 
Inasmuch as the former voltage decreases as the ambient temperature rises, 
the amplifier 8p produces a high level voltage only while the O.sub.2 
-sensor is in the inoperative condition. 
The on-off switches 8s and 8t are connected to the V.sub.B line via the 
respective resistors 8g and 8h. The switch 8s closes only while a throttle 
valve (not shown) is fully closed due to engine idling and engine 
deceleration, whereas the switch 8t closes only while the throttle valve 
is fully opened due to engine acceleration. 
The three diodes 8u, 8v and 8w constitute an OR logic gate which passes 
only the high level voltage to cause the transistor 8q conductive. The 
transistor 8r, as a result, is rendered nonconductive only while the 
exhaust temperature is low, the throttle valve is fully closed or the 
throttle valve is fully opened. 
The transistors 7a and 7b of the halt circuit 7, connected to the 
transistor 8r of the condition detector 8 to be responsive thereto, is 
rendered conductive upon receipt of the high level voltage applied through 
the resistors 7d and 7e. The transistors 7a and 7b, in the conduction 
state, causes the capacitor 3b of the integration circuit 3 to be 
discharged therethrough and halts the above-described integration 
operation. The integration-resultant voltage to be applied to the mixture 
supply controller 4 is eventually maintained to the set voltage .sup.V 
B/.sub.2 with which correction of the air-to-fuel ratio is not made any 
longer. 
Thus switching off the closed feedback loop to the open loop, erroneous 
feedback control resulting from inoperativeness of the O.sub.2 -sensor 1 
can be prevented until the O.sub.2 -sensor 1 becomes operative and the 
engine 5 can be supplied with the air-fuel mixture of the ratio other than 
the stoichiometric ratio during the engine acceleration and idling. It 
should be understood herein that halting the integration operation in the 
feedback loop can be made responsive to other engine conditions such as 
engine coolant temperature and engine rotational speed without departing 
from the scope of this invention.