JPH0694831B2 - Failure detection method for air-fuel ratio controller - Google Patents

Description

The present invention relates to an internal combustion engine that controls a proportional control valve that adjusts the amount of secondary air to the engine based on the output signal of an oxygen concentration detector in the exhaust gas of an internal combustion engine. The present invention relates to a failure detection method for an air-fuel ratio control device for an engine.

(Prior art and its problems) Based on the output signal of the oxygen concentration detector provided in the exhaust system of the internal combustion engine, the valve opening provided in the air supply passage supplied to the engine is proportional to the magnitude of the supply current. An air-fuel ratio control device for an internal combustion engine, which controls a proportional control valve for adjusting the secondary air amount, is known, for example, from Japanese Patent Laid-Open No. 55-119941.

According to such a conventional air-fuel ratio control device, a series circuit is provided in which the solenoid of the proportional control valve and the drive transistor are connected in series between the power supply that supplies a predetermined voltage and the ground.
The valve opening of the proportional control valve, and thus the amount of secondary air supplied to the engine, is proportional to the amount of current supplied to the solenoid controlled by the duty control of the drive transistor.

However, when the inside of the drive transistor is short-circuited or the harness between the drive transistor and the solenoid is short-circuited to the ground side due to biting or the like, the solenoid is held in a biased state, that is, substantially 100 Since the proportional control valve is opened to the maximum opening because it is energized with a duty ratio of%, as a result, a large amount of secondary air is supplied to the engine and the air-fuel ratio of the air-fuel mixture is excessively lean (over lean). This causes deterioration of the combustion state of the engine.

(Object of the Invention) The present invention has been made to solve the above-described problems, and accurately detects a failure of an air-fuel ratio control device using a proportional control valve that controls the amount of secondary air to an internal combustion engine. An object of the present invention is to provide a failure detection method for an air-fuel ratio control device, which prevents the deterioration of the combustion state of the engine due to the over- leaning of the air-fuel ratio of the air-fuel mixture that occurs at the time of such a failure.

(Means for Solving Problems) In order to achieve the above object, in the present invention, an air supply passage supplied to an engine based on an output signal of an oxygen concentration detector provided in an exhaust system of an internal combustion engine. In a failure detection method for an air-fuel ratio control device for an internal combustion engine, which controls a proportional control valve, the valve opening of which is proportional to the magnitude of a supply current, a first transistor between a power supply supplying a predetermined voltage and a ground, A circuit is formed by connecting the proportional control valve and the second transistor in series in this order, and connecting the second transistor and the ground in the conductive and non-conductive states of the first and second transistors, respectively. A failure detection method for an air-fuel ratio control device for an internal combustion engine, comprising detecting a voltage value at a point and detecting a failure of the air-fuel ratio control device based on the detected voltage value. .

Embodiments of the Invention Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is an overall configuration diagram showing a gaburettor type internal combustion engine incorporating an air-fuel ratio control device for carrying out the method of the present invention.

In FIG. 1, reference numeral 5 is, for example, an internal combustion engine of four cylinders, and an intake pipe 4 of this engine 5 has an air intake port 1,
A well-known carburetor 3 having an air cleaner 2 and a venturi 7 is provided. A throttle valve 6 is provided on the downstream side of the venturi 7 of the intake pipe 4. Reference numeral 8 is a secondary air supply passage. One end of the secondary air supply passage 8 communicates with the air cleaner 2 on the upstream side of the venturi 7, and the other end communicates with the downstream side of the throttle valve 6 of the intake pipe 4, A linear solenoid type solenoid valve 9 as a proportional control valve is interposed in the middle thereof. The solenoid 9a of the solenoid valve 9 is connected to a control circuit (hereinafter referred to as "ECU") 20. Solenoid 9a is E
When the CU 20 controls the energization, the solenoid valve 9 controls the secondary air supply amount in proportion to the supplied current amount. On the other hand, an absolute pressure (P _B ) sensor 10 is provided downstream of the throttle valve 6 of the intake pipe 4, and an absolute pressure signal detected by this absolute pressure sensor 10 is sent to the ECU 20.

The engine body 5 is provided with an engine water temperature (T _W ) sensor 12, which is composed of a thermistor or the like, is mounted inside the engine cylinder peripheral wall filled with cooling water, and supplies the detected water temperature signal to the ECU 20.

An engine speed sensor (hereinafter referred to as "Ne sensor") 11 is mounted around a cam shaft (not shown) or around a crank shaft of the engine, and an engine speed signal, that is, a predetermined crank angle position for every 180 ° rotation of the engine crank shaft. It outputs a pulse signal generated by the pulse signal, which is sent to the ECU 20.

A three-way catalyst 33 is arranged in the exhaust pipe 15 of the engine 5 to purify HC, CO, and NO _X components in the exhaust gas. An O ₂ sensor as an oxygen concentration detector is provided on the upstream side of the three-way catalyst 33.
14 is attached to the exhaust pipe 15, and this sensor 14 detects the oxygen concentration in the exhaust and supplies the detection signal to the ECU 20.

Further, the ECU 20 is connected to a power supply 16 such as a battery which supplies an operating voltage of a predetermined voltage.

With ECU20 will determine the engine operating condition based on engine parameter signals such as from various sensors described above, controls the amount of fuel supplied from the vaporizer (not shown) to the venturi 7 according to the determined engine operating conditions, O ₂ By changing the duty ratio of the solenoid valve 9 according to the output signal of the sensor 14, the secondary air supply amount is controlled, and thus the air-fuel ratio is controlled to a required value.

FIG. 2 is a diagram showing a circuit configuration inside the ECU 20 of FIG.
The engine speed signal from the sensor 11 is waveform-shaped by the waveform shaping circuit 24 and then supplied to the Me counter 25. The Me counter 25 counts the time interval from the input of the previous predetermined position signal from the Ne sensor 11 to the input of the present predetermined position signal, and the count value Me is proportional to the reciprocal of the engine speed Ne. The Me counter 25 supplies this count value Me to a central processing unit (hereinafter referred to as "CPU") 29 via a data bus 32.

Output signals from various sensors such as the absolute pressure (P _B ) sensor 10, the engine water temperature sensor 12, and the O ₂ sensor 14 are corrected to a predetermined voltage level by the level correction circuit 21, and then sequentially input / output by the multiplexer 22. It is supplied to the converter 23. The A / D converter 23 sequentially converts the output signals from the aforementioned sensors into digital signals and supplies the digital signals to the CUP 29 via the data bus 32.

The CPU 29 is further connected via a data bus 32 to a read only memory (hereinafter referred to as “ROM”) 30, a random access memory (hereinafter referred to as “RAM”) 31, and drive circuits 28a and 28b. The calculation result and the like in the CPU 29 are temporarily stored, and the ROM 30 stores a failure detection program for the air-fuel ratio control device, which will be described later, executed by the CPU 29.

The CPU 29 determines the operating state of the engine according to the output signals from various engine parameter sensors as described above,
A control signal is supplied to the fuel amount control valve (not shown) of the carburetor according to the determined engine operating state, and the O ₂ sensor 14
The duty ratio of the solenoid valve 9 is calculated according to the output signal of
The duty ratio control signal Io _UT corresponding to the calculated value is supplied to the solenoid valve 9 control drive circuit 28b via the data bus 32. The drive circuit 28b supplies to the solenoid valve 9 a drive signal for energizing the solenoid valve 9 at a duty ratio according to the control signal Io _UT . Further, as will be described later in detail, the CPU 29 sends a failure detection signal for selectively energizing or deactivating a driving transistor therein, which will be described later, to the failure detection driving circuit 28a.

FIG. 3 shows a control circuit of the solenoid valve 9, that is, the drive circuits 28a and 29 described above.
It is a circuit diagram which shows the connection of b and the solenoid valve 9. As shown in the figure, the first transistor T of the failure detection drive circuit 28a is provided between the power supply 16 that supplies the predetermined voltage + Vcc and the ground.
r ₂ , solenoid 9a of solenoid valve 9, drive circuit for controlling solenoid valve 9
The second transistor Tr ₁ of 28b and the resistor R are connected in series. Specifically, the first transistor Tr ₂ of the failure detection drive circuit 28a has a harness 31 whose emitter is connected to the power supply 16 and whose collector is connected to one end of the solenoid 9a of the solenoid valve 9.
The bases are connected to failure detection signal output terminals 29a of the CPU 29, respectively. On the other hand, the second transistor Tr ₁ of the drive circuit 28b for controlling the solenoid valve 9 is connected to the harness 32 whose collector is connected to the other end of the solenoid 9a, whose emitter is grounded at one end and the other end of the resistor R, and whose base is the amplifier AMP. Are connected to the output side of each. The non-inverting input terminal of the amplifier AMP is CPU29
The duty ratio control signal Io _UT output terminal 29b is connected to the inverting input terminal at the connection point between the emitter of the second transistor Tr ₁ and the resistor R, respectively. Further, a failure detection point A is provided at a connection point between the emitter of the second transistor Tr ₁ and the other end of the resistor R, and the point A is the CPU 29.
Connected to the failure detection signal input terminal 29c.

In the above-mentioned configuration, during the above-described normal secondary air amount control, the CPU 29 supplies a low level signal to the first transistor Tr ₂ of the failure detection drive circuit 28a to keep it in the conductive state and at the same time the solenoid valve. 9. The duty ratio control signal Io _UT calculated according to the output signal of the O ₂ sensor 14 is supplied to the second transistor Tr ₁ of the control drive circuit 28b via the amplifier AMP, and the transistor Tr ₁ is supplied with the duty ratio of the signal. Energize ₁ . On the other hand, as described later, the duty ratio control signal
When Io _UT indicates the duty ratio O, the CPU 29 makes the transistors Tr ₁ and Tr ₂ conductive or non-conductive by a predetermined method and detects a failure by the voltage A / D at the failure detection point A.

4 and 5 are program flow charts showing an embodiment of the failure detection method of the present invention.

First, in step 1, it is determined whether the engine ignition switch is closed (ON), and the answer is affirmative (Yes).
If it is, it is determined in the next step 2 whether or not the engine is cranking. When this answer is affirmative (Yes), in step 3, the engine speed is the predetermined speed N _IG2 (for example, 20
00 rpm) and the answer is negative (No)
If so, the process proceeds to step 9 described later. This step 3
Is effective when it is erroneously determined whether cranking is in progress due to a failure of the starter switch used for the determination in step 2. If the answer to step 3 is affirmative (Yes), it is considered that the determination result of step 2 is an error, and the next step is the same as the case where the answer to step 2 is negative (no other than cranking). In step 4, a failure diagnosis subroutine of the air-fuel ratio control device is executed.

FIG. 5 shows a flow chart of a failure diagnosis subroutine of the air-fuel ratio control device executed in step 4 of FIG.

First, at step 11, it is determined whether the air-fuel ratio control device is in a resting state (resting mode), that is, the duty ratio control signal of the solenoid valve 9.
It is determined whether or not Io _UT shows a duty ratio of 0. If the answer is affirmative (Yes), in step 12, the timer value indicated by the Ts _A timer, which is a down counter started when the ignition switch is turned on, is determined. The Ts _A timer is set to a predetermined time Ts _A0 (2.0 seconds) at the time of initialization, that is, when the engine is started, and after the start, the initial value is decreased with the passage of time.
In step 12, whether Ts _A value is smaller than Ts _A0 −0.5 (1.5 seconds), that is, 0 after the ignition switch is turned on.
Determine if 5 seconds have passed. This answer is affirmative (Yes)
, The Ts _A value is Ts _A0 −1.0 (1.0 second) in the next step 13.
It is determined whether it is larger than that, that is, whether the elapsed time after the ignition switch is turned on is within 1.0 second. When this answer is negative (No), Ts _A value is 0 (0.0 seconds) in the next step 14.
Or not, that is, after turning on the ignition switch
Determine if the Ts _A timer has timed out after 2.0 seconds have elapsed.

If the answer to step 12 is negative (No), or step 14
When the answer to is affirmative (Yes) (when the elapsed time after turning on the ignition switch is 0.5 seconds or less or 2.0 seconds or more)
Or, if the answer to step 11 is negative (No), step 15
Continue below. In step 15, it is determined whether or not the output current Do _UT of the amplifier AMP of the drive circuit 28b is larger than 400 mA.

When the answer to step 15 is negative (No), it is determined at step 16 whether the voltage A / D at the failure point A in FIG. 3 is higher than a predetermined value AD ₁₅ ′ (for example, 0.95V). Step 1 above
Answer 5 is negative (No) and the output current Do _UT of the amplifier AMP is 400 m
If it is smaller than A, the voltage A / D at the failure detection point A should be a minute value if the control circuit of FIG. 3 is in a normal state. Therefore, when the answer to step 16 is negative (No), it is determined that the control circuit of FIG. 3 is in a normal state, and T _F s # ₁₅ for determining the elapse of a predetermined time is set by a timer and started (step 17). . Then, proceed to step 27 onward. On the other hand, when the answer to step 16 is affirmative (Yes), that is, when the voltage A / D is higher than the predetermined value AD ′ ₁₅ , an abnormality has occurred in the portion on the earth side of the solenoid valve 9 in the control circuit of FIG. _No. 2 transistor Tr ₁ is short-circuited) or the impedance of the solenoid 9a is abnormally small, so it is considered that the solenoid 9a is defective. Therefore, in the next step 22, T _F s # ₁₅
It is determined whether or not the timer value of the timer has become 0, and if the answer is affirmative (Yes), in step 23, both the first and second transistors are made non-conductive (compensation operation at the time of abnormality). OFF) to set the Ts _A timer value to hexadecimal equivalent value FF _H corresponding to infinity, deactivate solenoid 9a, and step 27
Continue below. When the answer to step 22 is negative (No), in order to avoid erroneous diagnosis due to noise or the like, the compensation operation at the time of abnormality (step 23) is not executed, and the process proceeds to the next step 27 and subsequent steps.
If the answer to step 15 is affirmative (Yes), it is assumed that the answer is normal and the process proceeds to step 17.

When the answer to step 13 is affirmative (Yes) (when the elapsed time after the ignition switch is turned on is 0.5 seconds or more and 1.0 seconds or less), the process proceeds to step 18 and thereafter. In step 18, the second transistor Tr ₁ of the drive circuit 28b shown in FIG. 3 is turned off (OF
F) to turn on the first transistor Tr ₂ of the drive circuit 28a of FIG. 3 in step 19 and then to step 2
When 0, the voltage A / D at the failure point A in FIG.
Determine if it is higher than ₁₅ (eg 1.3V). In steps 18 and 19 above, the second transistor Tr ₁ is turned off,
When the first transistor Tr ₂ is turned on, the voltage A / D at the failure detection point A should be 0 volt if the control circuit shown in FIG. 3 is in a normal state. Therefore, when the answer to step 20 is negative (No), the control circuit of FIG. 3 is regarded as in a normal state and the T _F s # ₁₅ timer is set and started (step 21), and then the process proceeds to step 27 and thereafter. . On the other hand, the answer in step 20 is affirmative (Yes), that is, the voltage A / D.
Is higher than a predetermined value AD ₁₅ , it is considered that an abnormality has occurred in the portion of the control circuit of FIG. 3 closer to the ground than the solenoid valve 9 (for example, the inside of the second transistor Tr ₁ is short-circuited). Continue below 22.

When the answer to step 14 is negative (No) (when the elapsed time after the ignition switch is turned on is 1.0 second or more and 2.0 seconds or less), the process proceeds to step 24 and thereafter. In step 24, the first transistor Tr ₂ of the drive circuit 28a in FIG. 3 is made non-conductive (OFF), and in step 25, the output current Do _UT of the amplifier AMP in FIG. 3 is kept closed by the control valve 9. A small predetermined value Do _UTF s
After setting (for example, 150 mA), it is determined in step 26 whether or not the voltage A / D at the failure point A in FIG. 3 is higher than a predetermined value AD ₁₄ (for example, 0.5 v). When the steps 24 and 25 are executed, the voltage A / D at the failure detection point A should be 0 volt if the control circuit of FIG. 3 is in a normal state. Therefore, if the answer in step 26 is negative (No),
It is assumed that the control circuit shown in FIG.
Go to 17 and below. On the other hand, the answer in step 26 is affirmative (Yes),
That is, when the voltage A / D is higher than the predetermined value AD ₁₄ , it is considered that an abnormality has occurred in the part closer to the power source than the solenoid valve 9 in the control circuit of FIG. 3 (for example, the inside of the first transistor Tr ₂ is short-circuited). Therefore, the process proceeds to step 22 onward.

From step 27 onward, another abnormality detection process is performed. First, in step 27, it is determined whether or not the air-fuel ratio control device is in the idle state as in step 11, and if the answer is affirmative (Yes), the Ts _A value is Ts _A0 −0.5 in step 28.
It is determined whether or not (1.5 seconds), that is, whether the elapsed time after the ignition switch is turned on is within 0.5 seconds.
If the answer is yes, go to step 30
Output current Do _UT is set to the predetermined value Do _UTF s, and in step 31, the first transistor Tr ₂ of the drive circuit 28a in FIG. 3 is turned on (ON), and in step 32, the failure point in FIG. It is determined whether the voltage A / D at A is lower than a minute predetermined value AD ₁₅ (for example, 0.5 V). When the steps 30 and 31 are executed, the voltage A / D at the failure detection point A should show a predetermined value which is not minute if the control circuit of FIG. 3 is in a normal state. Therefore, when the answer to step 32 is negative (No), it is determined that the control circuit of FIG. 3 is in a normal state, and a T _F s # ₁₄ timer is set and started (step 29). This subroutine ends. on the other hand,
The answer in step 32 is affirmative (Yes), that is, the voltage A / D is the predetermined value AD.
When it is lower than ₁₄ , an abnormality has occurred in the control circuit of FIG. 3 (for example, the first transistor Tr ₂ is short-circuited or disconnected to the ground side, the second transistor Tr ₁ is disconnected, or the solenoid 9a Is disconnected or short-circuited to the ground side).
# _{14 It} is determined whether or not the timer value has become 0. If the answer is affirmative (Yes), as in the case of step 23, both of the first and second transistors are set as the compensating operation at the time of abnormality in step 34. Set the timer value of the Ts _A timer to the hexadecimal value FF _H corresponding to infinity by turning off the solenoid 9
Deactivate a and end this subroutine. If the answer to step 33 is negative (No), step 34 is not executed and this subroutine is immediately terminated. Step 28
When the answer is negative (No), after executing the step 29, the present subroutine is ended.

Returning to FIG. 4 again, after executing step 4, the process proceeds to step 5. In step 5, it is determined whether or not the Ts _A value is smaller than Ts _A0 -0.5 (1.5 seconds), that is, whether 0.5 seconds has elapsed after the ignition switch was turned on. If this answer is negative (No), the Ts _A value is the hexadecimal value F in step 6.
Determine O _H greater than or not, the answer is in affirmative (Yes) is detected failure of the air-fuel ratio control system in fault diagnosis subroutine of FIG. 5 described above, Ts _A value is set to FF _H This means that the first transistor Tr ₂ is turned off (OFF) (step 7), and this program ends. That is, since the first transistor Tr ₂ is turned off at the time of starting the engine this time when a failure is detected during the last engine operation, it is possible to prevent a failure such as burning of Tr ₂ in advance. On the other hand, when the answer to step 6 is negative (No), the failure of the air-fuel ratio control device has not been detected, so the first transistor Tr ₂ of the device is turned on (on), (step 8), This program ends. Also, step 5
If the answer is affirmative (Yes), the normal operation or the abnormal compensation operation is performed in step 4, so that this program does not need to perform these processes and the program is immediately terminated.

On the other hand, if the answer to step 1 or 3 is negative (No), it is set to 1 when the engine is in the high temperature restart state in another program other than this program in step 9, and is set in any other state. It is determined whether the flag F _HOT set to 0 is 1 or not. When this answer is affirmative (Yes), the process proceeds to step 5 and subsequent steps. In this case, step 5 has not been executed yet during this engine operation, but Ts _A
The value at the time of the last engine operation is set, that is, the Ts _A value is set to FF _H if the drive transistor etc. is abnormal, and is set to a value smaller than FO _H if normal. . Therefore, when the abnormality of the second drive transistor Tr ₁ or the like is detected during the previous engine operation, the answer to step 6 is affirmative (Yes),
In step 7, the first transistor Tr ₂ is made non-conducting, which prevents the second transistor Tr ₁ from burning. On the other hand, when the failure of the second drive transistor Tr ₁ or the like is not detected during the previous engine operation, the answer to step 6 is negative (No), and at step 8, the first transistor Tr ₂ Is brought into a conducting state, whereby the engine can be smoothly started at the time of high temperature restart. On the other hand, this answer is negative (N
In the case of o), since the engine is in the cold start state, the first transistor Tr ₂ is brought into a non-conducting state (OFF), and this program ends.

As an effect peculiar to the above-described embodiment, it is possible to detect a failure at a plurality of points in the control circuit of the solenoid valve 9 only by detecting the voltage at a single point in the control circuit, so that the failure detection system can be configured. It can be simplified.

(Effect of the Invention) As described above, according to the present invention, the valve provided in the air supply passage supplied to the engine is based on the output signal of the oxygen concentration detector provided in the exhaust system of the internal combustion engine. In a failure detection method for an air-fuel ratio control device for an internal combustion engine, which controls a proportional control valve whose opening degree is proportional to the magnitude of a supply current, a first transistor and the proportional control valve are provided between a power supply that supplies a predetermined voltage and a ground. And a second transistor are connected in series in this order to form a circuit, and the voltage value at the connection point between the second transistor and the ground in the conductive and non-conductive states of the first and second transistors, respectively. Is detected and a failure of the air-fuel ratio control device is detected based on the detected voltage value. Therefore, an air-fuel ratio control device using a proportional control valve for controlling the amount of secondary air supplied to the internal combustion engine. The occurrence of a failure in a plurality of parts of the drive circuit section of the proportional control valve in
It is possible to detect accurately, and as a result, it is possible to prevent malfunction of the proportional control valve at the time of such a failure and prevent deterioration of the combustion state of the engine due to over leaning of the air-fuel ratio of the air-fuel mixture.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of an internal combustion engine equipped with an air-fuel ratio control device to which the method of the present invention is applied, FIG. 2 is a block diagram showing an internal configuration of the control circuit (ECU) shown in FIG. 1, and FIG.
FIG. 4 is a circuit diagram showing the connection between the control circuit and the solenoid valve 9, and FIGS. 4 and 5 are program flow charts showing an embodiment of the failure detection method according to the present invention. 4 ...... intake pipe 5 ...... internal combustion engine, 9 ...... solenoid valve (proportional control valve), 9a ...... solenoid, 14 ...... O ₂ sensor (oxygen concentration sensor), 15 ...... exhaust pipe, 16 ...... power, 20 ...... control circuit, 28a, 28b ...... driving circuit, Tr ₂ ...... first transistor, Tr ₁ ...... second transistor, R ...... resistance, a ......
Failure detection point.

Claims (6)

[Claims] 1. A proportional control in which an opening degree of a valve provided in an air supply passage supplied to an engine is proportional to a magnitude of a supply current, based on an output signal of an oxygen concentration detector provided in an exhaust system of an internal combustion engine. In a failure detection method for an air-fuel ratio control device for an internal combustion engine, which controls a valve, a first transistor, the proportional control valve, and a power supply for supplying a predetermined voltage to a ground.
And a second transistor are connected in series in this order to configure a voltage value at the connection point between the second transistor and the ground in the conductive and non-conductive states of the first and second transistors, respectively. A method for detecting a failure of an air-fuel ratio control device for an internal combustion engine, comprising detecting and detecting a failure of the air-fuel ratio control device based on the detected voltage value. 2. A voltage value at a connection point between the second transistor and the ground is set to a predetermined value when a predetermined current is supplied to the base of the second transistor when the proportional control valve is in an operating state. The method of detecting a failure of an air-fuel ratio control device for an internal combustion engine according to claim 1, wherein when it is out of the range, it is determined that the air-fuel ratio control device is out of order. 3. When the proportional control valve is in a resting state, the first transistor is made conductive and at the same time a predetermined current is supplied to the base of the second transistor, The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein it is determined that the air-fuel ratio control device is out of order when the voltage value at the connection point with the ground is out of a predetermined range. Failure detection method. 4. When the proportional control valve is in a resting state, the first transistor is made conductive and the second transistor is made nonconductive at the same time.
The invention is characterized in that when the voltage value at the connection point between the second transistor and the ground is out of a predetermined range, it is determined that the air-fuel ratio control device is out of order.
A method for detecting a failure of an air-fuel ratio control device for an internal combustion engine according to the above item. 5. When the proportional control valve is in a rest state, the first transistor is made non-conductive, and at the same time a predetermined current is supplied to the base of the second transistor, the second transistor is turned off. The air-fuel ratio of the internal combustion engine according to claim 1, wherein when the voltage value of the connection point between the transistor and the ground is out of a predetermined range, it is determined that the air-fuel ratio control device is out of order. Failure detection method for control device. 6. The internal combustion engine according to claim 3, wherein the predetermined current is set to a value that does not substantially open the proportional control valve. Failure detection method for air-fuel ratio control device.