Abstract:
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.

Description:
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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram illustrating an embodiment of this invention: 
     FIG. 2 is a graph showing an output voltage characteristics of an oxygen responsive sensor: and 
     FIG. 3 is an electric wiring diagram of the feedback loop shown in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the embodiment shown in FIG. 1, an oxygen responsive sensor (O 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 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 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 2  -sensor 1 is high and low for the lesser ratio (air number λ&lt;1 or rich mixture) and the greater ratio (λ&gt;1 or lean mixture), respectively, and it abruptly changes from one level to the other at the stoichiometric ratio (λ=1) above which oxygen is present in the exhaust gas. 
     In FIG. 3 showing a detail circuit construction of the feedback loop, V 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 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 sl  is selected to be proportional to the sensor output voltage V 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 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 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 sl . Receiving the high level output voltage indicative of the rich mixture from the O 2  -sensor 1, the transistor 2s is rendered nonconductive to provide with a voltage higher than the set voltage V 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 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 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  V  B/ 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  V  B/ 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  V  B/ 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 2  -sensor 1 which is inoperative, as well known, under the temperature 450°˜600° 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 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 2  -sensor is in the inoperative condition. 
     The on-off switches 8s and 8t are connected to the V 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  V  B/ 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 2  -sensor 1 can be prevented until the O 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.