JPH06137193A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To spread an applicable range relating to feedback control by improving detecting sensitivity in the vicinity of theoretical air-fuel ratio, so that air-fuel ratio can be surely controlled to a window of the theoretical air-fuel ratio and further the actual air-fuel ratio itself can be detected also in the vicinity of the theoretical air-fuel ratio. CONSTITUTION:Resistors R1, R2 are connected between output terminals 32a, 32b of a switching circuit 31, to make a resistance value switchable in accordance with opening/closing a switch S1, and since it is opened when the target air-fuel ratio is the theoretical air-fuel ratio, the resistance value acting between the output terminals 32a, 32b is increased to increase a gain of fluctuation of an output voltage value V relating to a fluctuation of air-fuel ratio, so that the air-fuel ratio can be detected by very high sensitivity. Since the output voltage value V in proportion to concentration of oxygen in exhaust gas is obtained, PID control, modern control, etc., can be executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control system for an internal combustion engine, and more particularly to controlling the air-fuel ratio to a lean side of the theoretical air-fuel ratio when the internal combustion engine is in a predetermined operating range. The present invention relates to an air-fuel ratio control device to be operated.

[0002]

2. Description of the Related Art In recent years, for the purpose of improving fuel efficiency and reducing emissions, an air-fuel ratio control device has been implemented which employs so-called lean control for controlling the air-fuel ratio of an internal combustion engine to be leaner than the stoichiometric air-fuel ratio. In such air-fuel ratio control, normal air-fuel ratio control and lean control are selectively executed according to the operating region of the internal combustion engine. As is well known, during normal air-fuel ratio control, the fuel injection amount is set according to the operating region of the internal combustion engine, and during lean control, a constant (<1.0 is mapped to each operating region of the internal combustion engine. ) Is multiplied by the fuel injection amount to correct the air-fuel ratio to the lean side. Then, in parallel with this processing, the output voltage value proportional to the oxygen concentration in the exhaust gas from the lean sensor provided in the exhaust passage of the internal combustion engine, and the target air-fuel ratio (theoretical air-fuel ratio during normal air-fuel ratio control,
During lean control, a voltage value corresponding to the leaner air-fuel ratio) is compared to perform feedback control of the fuel injection amount.

As a lean sensor used for such air-fuel ratio feedback, a sensor using an oxygen ion conductive solid electrolyte such as zirconia is known. This lean sensor is constructed by coating one side of a zirconia plate with a thin film forming a cathode and coating the other side of the zirconia plate with a thin film forming an anode.The lean sensor is mounted in an internal combustion engine with the cathode exposed in the exhaust passage. It When a voltage is applied between the cathode and the anode, the lean sensor outputs a current proportional to the oxygen concentration in the exhaust gas in contact with the cathode,
In a conventional general air-fuel ratio control device, this output current is converted into a voltage value by a single resistor so as to be compared with the voltage value of the target air-fuel ratio.

By the way, the detection range of the air-fuel ratio required for the lean sensor covers a wide range from around 14.7 of the theoretical air-fuel ratio to around 22 on the lean side. Therefore, when the resistance value is set so as to be applicable to all the regions, for example, like the detection characteristic shown by the broken line in FIG.
The gain of the fluctuation of the output voltage value with respect to the fluctuation of the air-fuel ratio becomes low over the entire region. However, in normal air-fuel ratio control, the air-fuel ratio is set in a narrow region (window) near the theoretical air-fuel ratio.
Therefore, it is necessary to control the air-fuel ratio to a high level, so that a particularly high sensitivity is required for the detection of the air-fuel ratio, and this requirement cannot be satisfied with the above-mentioned low-gain output voltage value.

Therefore, as an air-fuel ratio control device with improved detection sensitivity near the stoichiometric air-fuel ratio, for example, Japanese Patent Publication No. 4-11.
The one described in Japanese Patent No. 81 has been proposed.

FIG. 7 is a circuit diagram showing a schematic circuit configuration around a lean sensor in this conventional air-fuel ratio control device.

As shown in the figure, one output terminal 52a is connected to one end of the lean sensor 51, and the lean sensor 5
The movable contact a of the switch S2 is connected to the other end of the switch 1. The movable contact a can be selectively switched to a pair of fixed contacts b and c, one fixed contact b is connected to the other output terminal 52b via a power source 53, and the other fixed contact c is a power source. It bypasses 53 and is connected to the other output terminal 52b. A resistor R3 that converts the output current of the lean sensor 51 into a voltage value is connected between the output terminals 52a and 52b, and the output voltage value V is input to the air-fuel ratio control device.

During lean control, the movable contact a of the switch S2 is fixed to the fixed contact b by the air-fuel ratio controller.
Switched to the side. Therefore, the voltage of the power source 53 is applied to both ends of the lean sensor 51, a current proportional to the oxygen concentration in the exhaust gas is output, and the output terminals 52a and 52 are output.
A voltage value V corresponding to the output current is generated between b and the resistor R3, and the fuel injection amount can be feedback-controlled based on the output voltage value V. That is, the lean sensor 51 at the time of lean control has the same function as the lean sensor of the air-fuel ratio control device described above, and detects the air-fuel ratio by the so-called limiting current method principle.

During normal air-fuel ratio control, the movable contact a of the switch S2 is switched to the fixed contact c side by the air-fuel ratio control device. Therefore, the lean sensor 5
No. 1 does not apply the voltage of the power source 53, and an electromotive force is generated due to the concentration difference between the oxygen concentration in the exhaust gas contacting the cathode and the oxygen concentration in the atmosphere contacting the anode, and the output terminals 52a, 5
Between 2b, a high or low voltage value V is generated depending on whether the air-fuel ratio of the exhaust gas is richer or leaner than the stoichiometric air-fuel ratio. Therefore, the fuel injection amount can be feedback-controlled based on the output voltage value V. In other words, the lean sensor 51 at the time of this normal air-fuel ratio control has the same function as that of a general O ₂ sensor, and is rich in the air-fuel ratio based on the theoretical air-fuel ratio due to the so-called oxygen concentration battery principle. Or detect lean. In this way, the output voltage value V of the lean sensor 51 greatly changes at the stoichiometric air-fuel ratio as a boundary, so that highly sensitive air-fuel ratio detection required during normal air-fuel ratio control is realized and the air-fuel ratio is theoretically calculated. It becomes possible to control to the air-fuel ratio window.

[0010]

In the air-fuel ratio control device described in the prior art publication, as described above, the lean sensor 51 during the normal air-fuel ratio control is made to exhibit the same function as a general O ₂ sensor. , The voltage value V corresponding to the rich or lean of the air-fuel ratio of the exhaust gas based on the stoichiometric air-fuel ratio is output. That is, since the actual air-fuel ratio itself cannot be detected, it is possible to positively execute high-response feedback control using the deviation between the actual air-fuel ratio and the target air-fuel ratio, such as PID control and modern control. could not.

Therefore, the present invention improves the detection sensitivity near the stoichiometric air-fuel ratio so that the air-fuel ratio can be reliably controlled to the stoichiometric air-fuel ratio window, and the stoichiometric air-fuel ratio as well as the lean side air-fuel ratio can be controlled. An object of the present invention is to provide an air-fuel ratio control device for an internal combustion engine, which can detect the actual air-fuel ratio itself even in the vicinity and can expand the range of application for feedback control.

[0012]

An air-fuel ratio control system for an internal combustion engine according to the present invention, as shown in FIG.
Of the air-fuel ratio detecting means M2 provided in the exhaust passage for outputting a current substantially proportional to the oxygen concentration in the exhaust gas, and the air-fuel ratio detecting means M2 for converting the output current of the air-fuel ratio detecting means M2 into a voltage value. Connected to the output end of the variable resistance means M3 capable of changing the resistance value, and a target air-fuel ratio for setting the target air-fuel ratio from the stoichiometric air-fuel ratio to the lean-side air-fuel ratio in accordance with the operating region of the internal combustion engine M1. Fuel ratio setting means M4
And when the target air-fuel ratio set by the target air-fuel ratio setting means M4 is the lean side air-fuel ratio, the variable resistance means M
3 is switched to a low resistance value, and when the target air-fuel ratio is the stoichiometric air-fuel ratio, the output voltage gain switching means M5 for switching the variable resistance means M3 to a high resistance value, the output voltage value of the air-fuel ratio detection means M2 and the An injection amount feedback control means M6 for feedback controlling the fuel injection amount to the internal combustion engine M1 based on the target air-fuel ratio.

[0013]

In the present invention, the target air-fuel ratio is set by the target air-fuel ratio setting means M4 from the stoichiometric air-fuel ratio to the lean side air-fuel ratio in accordance with the operating region of the internal combustion engine M1, and the target air-fuel ratio is lean side air-fuel ratio. At the fuel ratio, the output voltage gain switching means M5 switches the resistance value of the variable resistance means M3 to a low value, and when the target air-fuel ratio is the stoichiometric air-fuel ratio, the resistance value of the variable resistance means M3 is switched to a high value. . The variable resistance means M3 converts the output current of the air-fuel ratio detection means M2, which is approximately proportional to the oxygen concentration in the exhaust gas, into a voltage value, and based on the output voltage value and the target air-fuel ratio, the injection amount. Feedback control means M6
Thus, the fuel injection amount to the internal combustion engine M1 is feedback-controlled.

Here, when the target air-fuel ratio is the lean side air-fuel ratio, the resistance value of the variable resistance means M3 is low,
The gain of the fluctuation of the output voltage value with respect to the fluctuation of the air-fuel ratio is lowered, and the lean side air-fuel ratio can be detected. on the other hand,
When the target air-fuel ratio is the theoretical air-fuel ratio, the variable resistance means M3
Since the resistance value of the air-fuel ratio is high, the gain of the fluctuation of the output voltage value with respect to the fluctuation of the air-fuel ratio is increased, the air-fuel ratio can be detected with extremely high sensitivity, and the output voltage that is approximately proportional to the oxygen concentration in the exhaust gas. Since the value is obtained, it is possible to execute high-response feedback control such as PID control or modern control.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment embodying the air-fuel ratio control system for an internal combustion engine of the present invention will be described below. FIG. 2 is a schematic configuration diagram of an air-fuel ratio control device for an internal combustion engine which is an embodiment of the present invention.

As shown in the figure, an internal combustion engine 1 to which the air-fuel ratio control system of this embodiment is applied is a 4-cycle 4-cylinder spark ignition type internal combustion engine mounted on a vehicle. An air cleaner 3 is provided on the upstream side of the intake passage 2 of the internal combustion engine 1, and intake air that has passed through the air cleaner 3 is supplied into the combustion chamber 5 of each cylinder via the intake passage 2 and the intake valve 4. An air flow meter 6 for detecting the intake air amount is provided on the downstream side of the air cleaner 3 in the intake passage 2, and a throttle valve 7 for adjusting the intake air amount according to the accelerator operation by the driver is provided on the downstream side thereof. There is. Throttle valve 7
An intake pressure sensor 17 for detecting intake pressure is provided on the downstream side, and a fuel injection valve 8 is provided on the most downstream side of the intake passage 2 for each cylinder. The fuel injection valve 8 injects fuel in synchronization with the rotation of a crank shaft (not shown), and the fuel is mixed with intake air passing through the intake passage 2 and introduced into the combustion chamber 5 as a mixture.

An ignition plug 9 is provided in the combustion chamber 5 of each cylinder, and an ignition current from an ignition coil 10 is distributed to the ignition plug 9 by a distributor 11 in synchronization with the rotation of a crankshaft. The air-fuel mixture introduced into the combustion chamber 5 is ignited by the ignition plug 11, and while burning, pushes down the piston 12 to apply torque to the crankshaft, and thereafter to the outside via the exhaust valve 13 and the exhaust passage 14. Is discharged. The lean sensor 1 is installed in the exhaust passage 14.
5, the lean sensor 15 outputs a current proportional to the oxygen concentration in the exhaust gas. Further, the distributor 11 is provided with a crank angle sensor 16 and outputs a pulse signal every 30 degrees in synchronization with the rotation of the crankshaft.

The electronic control unit 21 of the internal combustion engine 1 is a CPU
22, a ROM 23, and a RAM 24 constitute a logical operation circuit, which is connected to an input / output unit 26 via a common bus 25. The air flow meter 6, the fuel injection valve 8, the ignition coil 10, the crank angle sensor 16 and the intake pressure sensor 17 are connected to the input / output unit 26, respectively, and the lean sensor 15 is connected via a switching circuit 31. The CPU 22 performs input / output with the outside via the input / output unit 26. Further, the ROM 23 stores various programs for controlling the operating state of the internal combustion engine 1, for example, programs for controlling the injection amount of the fuel injection valve 8 and the ignition timing control of the spark plug 9, and the CPU 22 follows these programs. Execute the process. The RAM 24 is the CPU 2
2 temporarily stores the processing data to be executed.

Then, the CPU 22 selectively executes normal air-fuel ratio control and lean control according to the operating region of the internal combustion engine 1. As is well known, during normal air-fuel ratio control,
The basic fuel injection time τ0 is set according to the operating region of the internal combustion engine 1, and during lean control, the constant k (<1.0) mapped to each operating region of the internal combustion engine 1 is used as the basic fuel injection time τ0. To correct the air-fuel ratio to the lean side.
In parallel with this processing, the output voltage value V proportional to the oxygen concentration in the exhaust gas from the lean sensor 5 and the target air-fuel ratio (theoretical air-fuel ratio during normal air-fuel ratio control, (Air-fuel ratio on the leaner side during lean control)
The target fuel injection time τ0 is feedback-controlled by comparing it with the target output voltage value V0 corresponding to.

FIG. 3 is a circuit diagram showing a lean sensor and a switching circuit in an air-fuel ratio control system for an internal combustion engine which is an embodiment of the present invention.

Next, the lean sensor 15 and the switching circuit 31.
The configuration of the lean sensor 1 will be described as shown in the figure.
One output terminal 32a is connected to one end of 5, and the other output terminal 32b is connected to the other end of the lean sensor 15 via a power supply 33. Between both output terminals 32a, 32b
Is connected in parallel with a pair of resistors R1 and R2, and one resistor R1 is connected in series with a switch S1, and these power source 33, resistors R1 and R2, and switch S1 are connected.
The switching circuit 31 is constituted by the above. The output voltage value V between the output terminals 32a and 32b is the input / output unit 2
It is input to the CPU 22 via 6.

The switch S1 of this switching circuit 31 is a CPU
Switching control is performed by the CPU 22, and the CPU 22 opens the switch S1 as shown by the solid line during normal air-fuel ratio control, and closes the switch S1 as shown by the broken line during lean control. Hereinafter, a state in which the switch S1 is opened in this way is referred to as a normal detection state, and a state in which the switch S1 is closed is referred to as a lean detection state. In the present embodiment, as will be described below, the lean sensor 15 is made to detect the air-fuel ratio by the so-called limiting current method principle in both normal air-fuel ratio control and lean control.

FIG. 4 is an explanatory view showing a map for setting the output voltage value of the lean sensor from the air-fuel ratio in the air-fuel ratio control system for an internal combustion engine which is an embodiment of the present invention.

In the normal detection state in which the switch S1 is opened as described above, the output current of the lean sensor 15 is converted into a voltage value only by the resistor R1. The output voltage value V at this time can be represented by V = i × Ra. Here, i is the output current of the lean sensor, and Ra is the resistance value of the resistor R1.

The characteristic of the output voltage value V at this time is that the detectable range of the air-fuel ratio is limited to the vicinity of the theoretical air-fuel ratio as shown by the solid line in FIG. The gain of the fluctuation of V is high, and the air-fuel ratio can be detected with extremely high sensitivity. The map shown in this figure is stored in the ROM 23 in advance and used for the processing of the CPU 22 as described later.

Further, in the lean detection state in which the switch S1 is closed, the output current of the lean sensor 15 is converted into a voltage value by the combined resistance of the resistors R1 and R2.
The output voltage value V at this time can be represented by V = i × 1 / (1 / Ra + 1 / Rb). However, Rb is the resistance value of the resistor R2.

As a matter of course, the combined resistance of the resistors R1 and R2 at this time is lower than the resistance value of the resistor R1 used in the above-described normal detection state, so that the output voltage value V also decreases. Therefore, as shown by the broken line in FIG. 4, the characteristic of the output voltage value V is that the gain of the fluctuation of the output voltage value V with respect to the fluctuation of the air-fuel ratio becomes low, and the detection sensitivity of the air-fuel ratio is low, but the detectable air-fuel ratio is low. The range becomes wider, and it becomes possible to detect even the air-fuel ratio on the lean side, which is necessary for lean control.

Next, the air-fuel ratio control executed by the CPU 22 of the air-fuel ratio control device for an internal combustion engine configured as described above will be described.

FIG. 5 is a flow chart showing the air-fuel ratio control process executed by the CPU of the air-fuel ratio control apparatus for an internal combustion engine which is an embodiment of the present invention, and FIG. 6 is the air-fuel ratio of the internal combustion engine which is an embodiment of the present invention. It is explanatory drawing which shows the map for setting the target air-fuel ratio of a control apparatus.

The routine shown in FIG. 5 is executed every 360 degrees CA in synchronization with the rotation of the internal combustion engine 1. First, the CPU
In step S1, the engine speed Ne is calculated from the pulse signal from the crank angle sensor 16 in step S1, and the engine speed N is calculated.
The target air-fuel ratio is set from e and the intake pressure Pm detected by the intake pressure sensor 17. This target air-fuel ratio is set as shown in FIG.
The map is stored in advance in the ROM 23 as shown in FIG. The numerical value in the figure shows the target air-fuel ratio, and as is clear from the figure, the target air-fuel ratio is set to the theoretical air-fuel ratio in the region where the engine speed Ne is less than about 1000 rpm and more than about 4000 rpm, and about 1000- In the 4000 rpm range, the target air-fuel ratio is set to the lean side air-fuel ratio.

Next, in step S2, the CPU 22 sets the basic fuel injection time τ0 required for controlling the actual air-fuel ratio to the stoichiometric air-fuel ratio according to a map (not shown) from the engine speed Ne and the intake pressure Pm. Further, step S3
Then, it is determined whether or not the target air-fuel ratio set in step S1 is the lean side air-fuel ratio. If the target air-fuel ratio is the lean side air-fuel ratio, the lean control is executed in the processing from step S4. First, in step S4, the basic fuel injection time τ0 is multiplied by a constant k (<1.0) mapped for each operating region of the internal combustion engine 1 to correct the air-fuel ratio to the lean side. Then, in step S5, as shown by the broken line in FIG. 3, the switch S1 is closed to switch to the lean detection state, and step S6
Then, the target output voltage value V0 corresponding to the target air-fuel ratio is calculated according to the characteristic indicated by the broken line in the map of FIG.

Then, in step S7, the deviation (DIF) between the current output voltage V of the switching circuit 31 and the target output voltage value V0 is calculated.
L) is calculated, and the correction coefficient of the basic injection time τ0 is calculated according to the deviation. High-response feedback using PID (proportional / integral / derivative) is performed to correct the basic injection time. The feedback amount is calculated by calculating the P term (proportional term) in step S8, the I term (integral term) in step S9, the D term (differential term) in step S10, and the deviation between the output voltage V and the target voltage value V0. Based on DIFL, the feedback amount f is the sum of the P, I, and D terms in step S11, and then the basic injection time τ in step S12.
The fuel injection time τ is calculated by multiplying 0 by the feedback amount f, and this routine is once ended. Therefore, during the lean control, feedback control of the fuel injection time τ is executed according to the characteristic indicated by the broken line in the map of FIG. 4, and the air-fuel ratio is controlled to the lean side air-fuel ratio.

Further, when the target air-fuel ratio is the stoichiometric air-fuel ratio in step S3, the normal air-fuel ratio control is executed in the processing from step S13. First, in step S13, as shown by the solid line in FIG. 3, the switch S1 is opened to switch to the normal detection state, and in step S14 the current output voltage value V of the switching circuit 31 and the target voltage value V0 corresponding to the stoichiometric air-fuel ratio.
The deviation (DIFS) from 0 volt is calculated, and the correction coefficient of the basic injection time τ0 is calculated according to the deviation. For correction of the basic injection time, high-response feedback using PID (proportional / integral / derivative) is performed.

The feedback amount is calculated in step S15.
At step S16, the I term (integral term) at step S16, and the D term (differential term) at step S17 based on the deviation DIFS between the output voltage V and the target voltage value V0. In step S18, the sum of the P, I, and D terms is set as the feedback amount f, and then in step S12, the basic injection time τ0 is multiplied by the feedback amount f to calculate the fuel injection time τ, and this routine is once terminated. To do. Therefore, during this normal air-fuel ratio control, feedback control of the fuel injection time τ is executed according to the characteristics shown by the solid line in the map of FIG. 4, and the air-fuel ratio is controlled to the stoichiometric air-fuel ratio. Further, as described above, since the gain of the fluctuation of the output voltage V with respect to the fluctuation of the air-fuel ratio is high, the air-fuel ratio can be detected with extremely high accuracy.

Further, not only the PID control shown in this embodiment but also the feedback using the modern control theory can be implemented as the feedback method, and the fuel can be determined by determining rich or lean with respect to the target air-fuel ratio. It is also possible to execute so-called PI control in which the injection time τ is increased or decreased by a uniform value.

As described above, in this embodiment, the internal combustion engine M1
The internal combustion engine 1 functions as, the lean sensor 15 as the air-fuel ratio detection means M2, and the resistance R as the variable resistance means M3.
1, the resistor R2 and the switch S1 function. Further, the CPU 22 when executing the process of step S1 as the target air-fuel ratio setting means M4, and the CPU 22 when executing the processes of step S5 and step S11 as the output voltage gain switching means M5 are the injection amount feedback control means M.
6 as steps S7 to S10 and step S
CPU when executing the processing from 12 to step S14
22 function respectively.

As described above, the air-fuel ratio control system for the internal combustion engine of this embodiment is provided in the exhaust passage 14 of the internal combustion engine 1 and the lean sensor 15 for increasing the output current in proportion to the oxygen concentration in the exhaust gas. , The output terminals 32a, 3 of the switching circuit 31 for converting the output current of the lean sensor 15 into a voltage value.
2b, the resistances R1 and R2, whose resistance values are switched according to the opening and closing of the switch S1, and the target air-fuel ratio from the theoretical air-fuel ratio to the lean side air-fuel ratio, depending on the operating region of the internal combustion engine 1. And when the target air-fuel ratio is the lean side air-fuel ratio, the switch S1
Is closed and the combined resistance of the resistors R1 and R2 is applied between the output terminals 32a and 32b, and when the target air-fuel ratio is the theoretical air-fuel ratio, the switch S1 is opened to connect the resistor R1 between the output terminals 32a and 32b. And the output voltage value V between the output terminals 32a and 32b of the switching circuit 31.
And a CPU 22 for feedback-controlling the fuel injection amount to the internal combustion engine 1 based on the target output voltage value V0 corresponding to the target air-fuel ratio.

Therefore, during normal air-fuel ratio control in which the target air-fuel ratio is set to the stoichiometric air-fuel ratio, the output terminal 32
The resistance R1 acts between a and 32b to increase the gain of the fluctuation of the output voltage value V with respect to the fluctuation of the air-fuel ratio. Therefore, the air-fuel ratio can be detected with extremely high sensitivity, and the actual air-fuel ratio can be reliably controlled to the theoretical air-fuel ratio window.

Further, since the output voltage value V proportional to the oxygen concentration in the exhaust gas is obtained, high response feedback control using the deviation between the output voltage value V and the target output voltage value V0, such as PID control or It is possible to execute modern control and the like, and it is possible to greatly expand the range of application to feedback control.

[0040]

As described above, according to the air-fuel ratio control system for an internal combustion engine of the present invention, when the target air-fuel ratio is set to the stoichiometric air-fuel ratio, the resistance value of the variable resistance means is high, The gain of the fluctuation of the output voltage value with respect to the fluctuation of the fuel ratio is increased. Therefore, the air-fuel ratio can be detected with extremely high sensitivity, and the air-fuel ratio can be reliably controlled to the theoretical air-fuel ratio window. In addition, since an output voltage value that is approximately proportional to the oxygen concentration in the exhaust gas is obtained, it is possible to execute feedback control with high response, such as PID control or modern control, which greatly expands the range of application for feedback control. Can be expanded to.

[Brief description of drawings]

FIG. 1 is a claim correspondence diagram conceptually showing the content of one embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of an air-fuel ratio control device for an internal combustion engine that is an embodiment of the present invention.

FIG. 3 is a circuit diagram showing a lean sensor and a switching circuit in an air-fuel ratio control device for an internal combustion engine which is an embodiment of the present invention.

FIG. 4 is an explanatory diagram showing a map for setting the output voltage value of the lean sensor from the air-fuel ratio in the air-fuel ratio control device for an internal combustion engine which is an embodiment of the present invention.

FIG. 5 is a flowchart showing an air-fuel ratio control process executed by a CPU of an air-fuel ratio control device for an internal combustion engine which is an embodiment of the present invention.

FIG. 6 is an explanatory diagram showing a map for setting a target air-fuel ratio of the air-fuel ratio control device for an internal combustion engine which is an embodiment of the present invention.

FIG. 7 is a circuit diagram showing a schematic circuit configuration around a lean sensor in a conventional air-fuel ratio control device.

[Explanation of symbols]

 M1 internal combustion engine M2 air-fuel ratio detection means M3 variable resistance means M4 target air-fuel ratio setting means M5 output voltage gain switching means M6 injection amount feedback control means 1 internal combustion engine 15 lean sensor R1, R2 resistance S1 switch

Claims (1)

[Claims] 1. An air-fuel ratio detecting means, which is provided in an exhaust passage of an internal combustion engine and outputs a current substantially proportional to an oxygen concentration in exhaust gas, and an output current of the air-fuel ratio detecting means for converting into a voltage value. Variable resistance means connected to the output end of the air-fuel ratio detection means and capable of changing the resistance value, and a target air-fuel ratio is set from the theoretical air-fuel ratio to the lean side air-fuel ratio according to the operating region of the internal combustion engine. When the target air-fuel ratio setting means and the target air-fuel ratio set by the target air-fuel ratio setting means are lean side air-fuel ratios, the variable resistance means is switched to a low resistance value, and the target air-fuel ratio is the theoretical air-fuel ratio. At this time, based on the output voltage gain switching means for switching the variable resistance means to a high resistance value, and the output voltage value of the air-fuel ratio detecting means and the target air-fuel ratio, the fuel injection amount to the internal combustion engine is feedback-controlled. Air-fuel ratio control system for an internal combustion engine, characterized by comprising an injection amount feedback control means for.