JPH0762661B2 - Exhaust concentration detector - Google Patents

Description

TECHNICAL FIELD The present invention relates to a structure of an exhaust gas concentration detector used in a fuel supply control device for controlling an air-fuel ratio of an air-fuel mixture of an internal combustion engine, In particular, the present invention relates to an exhaust gas concentration detector that can always accurately and surely detect a failure of the exhaust gas concentration detector from a change in the output voltage of the detector.

(Problems to be Solved by the Related Art and Invention) Generally, in order to control the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine to fall within a certain range around a desired value, it is included in exhaust gas. A specific component concentration, such as an oxygen gas concentration, is detected, an air-fuel ratio correction coefficient value is set according to the detected oxygen gas concentration, and the air-fuel ratio is corrected using this correction coefficient value. An oxygen gas concentration sensor (O ₂ sensor), which is an exhaust gas concentration detector for detecting the oxygen gas concentration from the exhaust gas of an internal combustion engine, is of a type including, for example, a zirconia solid electrolyte (ZrO ₂ ), and its electromotive force is Has a characteristic that it rapidly changes before and after the stoichiometric air-fuel ratio of the internal combustion engine, and the output signal of the O ₂ sensor has a high level on the exhaust gas rich side and has a low level on the lean side. The disconnection, short circuit and deterioration of the O ₂ sensor for detecting the oxygen gas concentration have a great influence on the air-fuel ratio control. Therefore, it is necessary to constantly monitor an exhaust gas concentration detection system including an exhaust gas concentration sensor such as an O ₂ sensor and make the air-fuel ratio control system function normally with a normal sensor signal.

As a means for detecting an abnormality in the exhaust gas concentration sensor for that purpose, conventionally, a technique for detecting a failure by detecting the reversal time of the output of the O ₂ sensor, that is, the reversal time interval from the rich side to the lean side or vice versa (for example, , Japanese Examined Sho 56-2910
No. 0), or a technique for detecting abnormality by comparing a voltage greater than the output voltage of the O ₂ sensor during normal operation (about 0.1 V to 1 V) with the O ₂ sensor output voltage (JP-A-53-95431). No. publication) is known.

The former one considers the inversion time of the output of the comparator to which the output of the O ₂ sensor is supplied by the amount of charge accumulated in the storage capacitor.If the inversion time is long, that is, if it does not invert easily, it is considered to be a failure. However, in such a method, when the air-fuel ratio (A / F) of the base is deviated, the rich or lean state caused by it is erroneously detected as a failure state of the O ₂ sensor. There is a risk of doing it.

That is, when the basic fuel amount is mapped and stored in advance, when the base map value is slightly deviated, specifically, it does not match the engine,
Alternatively, even if the maps in the engine and the electronic control unit are correct, if there are variations in the opening area of the injector or the set pressure of the pressure regulator, etc. As described above, the output of the O ₂ sensor changes very sharply in the stoichiometric air-fuel ratio (in other words, before and after that, it shows a predetermined level indicating rich or lean). Even if the shift (change) of F) occurs, for example, the rich state may continue.

Therefore, when the method of detecting the reversal time of the O ₂ sensor is used, it is erroneous that the O ₂ sensor is abnormal even if such a rich state continues, that is, even if the system system is abnormal. Make a judgment (actually, the O ₂ sensor is misdiagnosed as abnormal even though it is normal)
It will be. Therefore, it is easily affected by the air-fuel ratio of the base and the above-mentioned variations, and there is a problem in terms of accuracy and certainty of failure detection.

On the other hand, in the latter case, that is, when the output voltage from the exhaust gas concentration sensor becomes a high voltage of, for example, 6 V or more, the exhaust gas concentration sensor, that is, the O ₂ sensor Although it is possible to detect the disconnection of
When the O ₂ sensor is short-circuited, the output voltage will drop to 0.
It is not possible to detect abnormalities due to a short circuit in the O ₂ sensor.

(Object of the Invention) The present invention has been made in view of the above circumstances, and makes it possible to detect not only disconnection but also short-circuit failure.
An object of the present invention is to provide an exhaust gas concentration detector capable of improving the failure detection accuracy of the exhaust gas concentration detector including a short circuit.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a pair of electrodes on a solid electrolyte, one electrode is brought into contact with a reference gas, and the other electrode is brought into contact with exhaust gas, In an exhaust gas concentration detector for detecting the oxygen concentration in the exhaust gas by the voltage generated between the electrodes when a predetermined current is supplied from the current supply means, a predetermined electric resistor is connected in series to one of the current supply means and the electrode. It is designed to be installed in.

(Example) Hereinafter, one example of the present invention will be described in detail with reference to the drawings.

Figure 1 is a circuit configuration of the O ₂ and an equivalent circuit of the sensor, O ₂ sensor input circuit portion inside the electronic control unit including a checker circuit for fault diagnosis of the O ₂ sensor as the exhaust concentration detector according to the invention The figure is shown.

In the figure, reference numeral 100 represents an electronic control unit (hereinafter referred to as “ECU”) that executes a fuel injection time, an arithmetic process for calculating an air-fuel ratio, etc., and an O ₂ sensor abnormality detection process described below and other predetermined processes. An O ₂ sensor 10 is connected to the ECU 100, and an engine speed sensor, a throttle valve opening sensor, an intake pipe absolute pressure sensor, an engine cooling water temperature sensor, and other required sensors (all not shown) are connected to the ECU 100. To be done.

The ECU 100 shapes the input signal waveform from the O ₂ sensor 10 and various other sensors, corrects the voltage level to a predetermined level,
When the input signal is an analog signal, an input circuit having a function of converting the analog signal value into a digital signal value,
A central processing circuit (CPU), storage means for storing various calculation programs and calculation results including an O ₂ sensor failure detection program executed by the CPU, and drive signals to the fuel injection valve of the internal combustion engine, and , An output circuit for sending a switching control signal for turning on and off to a checker circuit switch described later used for O ₂ sensor failure diagnosis (all are not shown except a part of the input circuit of the O ₂ sensor 10). ).

The O ₂ sensor 10 connected to the ECU 100 as described above has one end grounded via the exhaust pipe wall of the engine,
The other end is connected to the ECU 100 through the signal line 1, but O ₂
When detecting disconnection and short circuit between the sensor 10 and the ECU 100, in order to improve the accuracy of failure detection including these disconnection and short circuit, the internal resistance for detection having a predetermined resistance value inside the O ₂ sensor 10 ( Electric resistance as a fixed value)
12 has been added and incorporated.

That, O ₂ sensor is basically represented by the electromotive force 11a and the element internal resistance 11b of the oxygen concentration detecting element unit, the O ₂
In the sensor 10, in addition to this, the electric resistor 12 is inserted and connected in series.

FIG. 2 shows an O ₂ sensor 10 provided with such an electric resistor 12.
An example of the structure of is shown.

As shown in the figure, the O ₂ sensor 10 is provided with a housing 10a having a mounting step portion on the tip end side (lower side in the figure) into which a part is inserted into the exhaust pipe. Housing 10a
A protective cover 10c having an appropriate number of vent holes 10b for circulating exhaust gas is provided at the tip of the housing 1a, and the housing 1a is provided on the other end side (upper end side in the figure) of the housing 1a.
A cylindrical cap 10d is fixed to the outer peripheral surface of 0a.

Inside the housing 10a and the protective cover 10c, for example, a zirconia element (solid electrolyte element) 10e as a cylinder-shaped oxygen concentration detection element, which has zirconium oxide as a main component and is closed at one end, is mounted.

The inner and outer surfaces of the zirconia element 10e are coated with platinum as an electrode, so that the exhaust gas is guided to the space 10f on the outer surface side and the atmosphere is guided to the space 10g on the inner surface side as the reference gas. It has become.

Further, an inner cylinder 10h is provided inside the cap 10d, a hollow long-axis insulator 10i is incorporated in the inner cylinder 10h, and a washer is provided above the long-axis insulator 10i. A hollow short-shaft insulator 10k is provided via 10j. Between the upper end surface of the short-axis insulator 10k and the inner surface of the top of the cap 10d, a disc spring 10l for holding an insulator is attached, and further, the output of the O ₂ sensor 10 is provided on the top of the cap 10d. A chromet 10n made of, for example, fluororubber is fixed for protecting the lead wire 10m for taking out.

The lead wire drawn from the O ₂ sensor 10 is only the illustrated lead wire 10 ml, the lead wire 10 m side is electrically connected to the electrode on the inner surface side of the zirconia element 10 e, and the electrode on the outer surface side of the zirconia element 10 e. Is grounded through the housing 10a and also through the exhaust pipe wall. For electrical connection between the lead wire 10m and the inner surface electrode of the zirconia element 10e,
The hollow portions of the insulators 10i, 10k are provided with the inner center electrode 10o and the inner center electrode 10p with a wave spring, and the electric resistor 12 as the above-mentioned internal resistance for detection is provided in the middle of these conductive paths. There is.

The installation position of the electric resistor 12 is preferably selected in a place that is not easily affected by temperature. Therefore, in the illustrated example, the zirconia element 10e which is the heat receiving part of the O ₂ sensor 10,
At the position opposite to the protective cover 10c side, the electric resistor 12
Are arranged.

The oxygen concentration in the exhaust gas detected by the O ₂ sensor 10 is
One of the two electrodes of the zirconia element 10e provided with a pair of electrodes on the inner and outer surfaces, that is, the inner electrode in this example, is brought into contact with the atmosphere as the reference gas, and the other electrode, that is, the outer electrode is used as the exhaust gas. This is performed by bringing them into contact with each other and detecting the oxygen concentration in the exhaust gas by the voltage generated between the inner and outer electrodes when a predetermined current is supplied from the current supply means as described later. That is, when the outer surface of the zirconia element 10e is exposed to the exhaust gas in the exhaust pipe and the inner surface is exposed to the atmosphere, the zirconia element 10e is a kind of electromotive force corresponding to the element concentration difference between the inner and outer surfaces thereof. It functions as an oxygen concentration battery, a voltage is generated between the electrodes according to the oxygen concentration in the exhaust gas, and when the temperature is higher than a certain temperature, the voltage suddenly changes at the theoretical air-fuel ratio. Exhibit characteristics. Utilizing such a characteristic, the oxygen concentration is detected by the change of the output voltage, and the detection output is supplied to the ECU 100 through the lead wire 10m to be used for the feedback control of the air-fuel ratio. Mold O ₂
In the sensor, an electric resistor 12 is added to the inside of the O ₂ sensor 10 in order to detect the disconnection or short-circuit failure more accurately when the failure occurs.

As described above, in the O ₂ sensor 10, the electric resistor 12 is incorporated in series with the zirconia element 10e which is the solid electrolyte element and mounted on the exhaust pipe, and the lead wire 10m is connected to the ECU 100 as the signal path l. Has been done.

Returning to FIG. ₁ , two capacitors C ₁ and C are provided inside the ECU 100.
_A low-pass filter composed of ₂ and a resistor R ₁ and an operational amplifier 110 are provided. The output voltage VO ₂ of the O ₂ sensor 10 is applied to the non-inverting input terminal of the operational amplifier 110 via the low-pass filter, and the operational amplifier 11
The signal is amplified by 0 and supplied to the multiplexer and A / D converter that form a part of the input circuit of the ECU 100.

The operation amplifier 110 is for supplying a current to the O ₂ sensor 10 and constitutes a current supply means.

A checker circuit switch (SW) is provided between the connection point between the capacitor C ₁ and the resistor R ₁ and the specified power supply voltage terminal.
A series circuit of a switch controllable switch 120 and a resistor R ₂ is connected, and the switch 120 is turned on / off by a control signal from a fail-safe drive circuit forming a part of the output circuit of the ECU 100. To be done. Switch 120
It may be an appropriate switch element.

In the chucker circuit with the above switch 120, O ₂
In the case where the signal of the sensor 10 does not change the predetermined time, it increases the current flow into the O ₂ sensor 10 by turning on the switch 120, failure detection of the O ₂ sensor 10 by checking the output voltage of the O ₂ sensor 10, That is, self-diagnosis is performed.

The failure detection is based on the following operation, which will be described with reference to FIGS. 3 and 4.

The chucker circuit switch 120 is normally turned off in the state where the self-diagnosis is not performed. switch
If 120 is off, the output voltage VO ₂ of the O ₂ sensor 10, O ₂
If the value of the electromotive force of the sensor 10 is Vs [V], the current flowing from the operational amplifier 110 is i [A], and the total resistance value of the element internal resistance 11b and the electric resistor 12 is r [KΩ]. , Is expressed by the following equation.

VO ₂ = Vs + i × r (2) When the O ₂ sensor 10 is activated, the current i is several 10 nA.
And i × r is several mV (if the resistance value of the electric resistor 12 to be added is set to an appropriate value (for example, several KΩ to several tens KΩ), i
Since xr can be set to such a value), VO ₂ ≈Vs, and therefore the output voltage value VO _2R when the air-fuel ratio (A / F) is rich and the output voltage VO _2L when it is lean are VO _2R ≒ 1
[V] and VO _2L ≈0 [V].

That is, during normal operation, as shown in FIGS. 3 and 4, a so-called pouring type (detection circuit configuration for supplying a constant current)
The output voltage VO ₂ is repeatedly inverted by the same operation as in the above case.

Next, considering the case where the switch 120 is turned on, when the switch 120 is turned on and a predetermined voltage is applied to the O ₂ sensor 10, a current i ′ flows through the resistor R ₂ , so that O ₂ The current flowing into the sensor 10 is i + which is the sum of i and i '.
i ′, and the O ₂ sensor output voltage VO ₂ in this case is expressed by the following equation.

VO ₂ = Vs + (i + i ′) × r (3) Assuming that the current i ′ flowing through the switch 120 is several tens μA, when the O ₂ sensor 10 is active, i × r is several mV as described above.
Therefore, the output voltage VO ₂ becomes VO ₂ ≈Vs + i ′ × r (4), and when the signal line 1 is normal, that is, in the state where there is no disconnection failure or short-circuit failure of the O ₂ sensor 10, FIG. As shown in FIG. 4 (a), when the switch 120 is turned on, the output voltage VO ₂ changes by ΔVO ₂ = i ′ × r. The amount of change ΔVO ₂ is about 0.1 to 1.0V.

That is, if the O ₂ sensor 10 is normal, it can be confirmed to be normal by observing the change in VO ₂ caused by turning on the switch 120.

On the other hand, when the O ₂ sensor 10 has a failure, even if the switch 120 is turned on, the above-mentioned change does not occur. Therefore, by checking this, the failure occurrence can be detected.

That is, when the signal line 1 is disconnected, the resistance on the sensor side when looking at the O ₂ sensor side from the ECU 100 becomes infinite due to the disconnection, so regardless of the current flowing to the sensor side, that is,
The ECU 10 regardless of whether the switch 120 is off or on.
The voltage at the input terminal of 0 is given by the following equation.

VO _{2 (ECU)} = ∞ × i≈∞ × (i + i ′) = ∞ (5) Actually, in the circuit configuration of FIG. 1, the plus side power supply voltage value (output Max value) of the operation amplifier 110 and Become (the third
Figure (b)).

Therefore, when a disconnection occurs in the O ₂ sensor 10 and the voltage rises to the positive side power supply voltage value and does not change as shown in FIG. No change occurs, and the disconnection fault can be clearly detected by distinguishing it from the normal state in FIG.

Moreover, even in the case of a short-circuit failure, it can be detected by the switch 120 in the same manner.

That is, even when the signal line is short-circuited to the ground (body earth), as shown in FIG.
Since the input terminal of U100 is held at the ground potential, VO ₂ ≈ 0 regardless of the current flowing to the sensor side, that is, regardless of whether the switch 120 is off or on, and there is no voltage change due to the switch 120 being on. . Therefore, it is possible to detect the short-circuit fault clearly distinguishing it from the normal case of FIG.

As described above, in the checker circuit using the switch 120 shown in FIG. 1, if the O ₂ sensor 10 is normal, the switch
Increasing the current pour 120 by turning on the O ₂ sensor 10, FIG. 3 (a), the change in Delta] VO ₂ occurs in the output voltage VO ₂ as shown in 4 (a), the other, O ₂ Disconnection on sensor 10,
If there is an abnormality in the short circuit, even if the switch 120 is turned on, the above-mentioned Δ is generated as shown in FIGS. 3 (b) and 4 (b).
No change in VO ₂ occurs. Therefore, under certain conditions, namely O
The failure of the O ₂ sensor 10 can be detected by turning on the switch 120 and observing the change of VO ₂ under the condition that the ₂ sensor voltage does not change for a predetermined time.

In the above-mentioned failure detection, a predetermined voltage is applied to the O ₂ sensor 10 by the switch 120, and the change amount ΔVO ₂ of VO ₂ at that time is detected, so that the same applies to both the disconnection failure and the short-circuit failure. Failure detection can be performed by the detection procedure of
Even if the air-fuel ratio (A / F) of the base is deviated, its influence can be eliminated and the failure can be reliably detected.

Especially when the fuel adheres to the intake pipe and the fuel becomes thick, as in the state after hot restart after high-speed running, or when running at high altitudes, the output of the O ₂ sensor is usually reversed. Even in the case where it is difficult to perform the above, it is possible to clearly distinguish and detect such a case from the true failure of the O ₂ sensor 10.

Further, as shown in the structure of FIGS. 1 and ₂ , the O ₂ sensor 10 has an electric resistor 12 connected in series between the current supply means and one of the electrodes of the zirconia element 10e. Since the configuration is provided, the voltage change can be accurately detected when the current is supplied by turning on the switch 120 under normal conditions, and as a result, more reliable failure detection can be performed.

That is, in general, the element internal resistance of the O ₂ sensor 10 tends to become extremely small as the resistance value of the O ₂ sensor element portion decreases when the temperature of the O ₂ sensor element becomes high. Even if the current i ′ is applied, the normal change ΔVO ₂ accompanying the turning on of the switch 120 may not occur due to the small internal resistance value of the element. In such a case, there is a possibility that the wire may be erroneously determined as a disconnection although it is normal as it is. Therefore, even when the temperature of the O ₂ sensor becomes high and the internal resistance of the O ₂ sensor itself becomes small, a current larger than the above-mentioned flow current i ′ is surely passed in order to surely obtain an output change of a certain value or more. In this case, it is possible to generate a change ΔVO ₂ sufficient to distinguish whether it is normal or not, but O ₂
If such an overcurrent is applied to the sensor, this may deteriorate the O ₂ sensor and cause the O ₂ sensor body to lose its function.

On the other hand, like the O ₂ sensor 10 shown in FIGS. 1 and 2, if the required electrical resistor 12 is added in series to the element internal resistance 11b, the O ₂ sensor 10 will be at a high temperature. Therefore, even when the resistance value of the element internal resistance 11b decreases, the decrease amount is reduced by the electric resistance 1 in series with the element internal resistance 11b.
2 can be compensated for by a constant current i ′ flowing through the O ₂ sensor 10 no matter what the engine load condition is, and the presence of the electric resistor 12 ensures that the electric current exceeds a certain level. The output change ΔVO ₂ of can be obtained.

Also, as shown in FIG. 2, if the electric resistor 12 is provided at a location that is not easily affected by temperature, the required resistance value can always be ensured regardless of temperature changes, and it is possible to ensure that there is no error. Since detection can be avoided and it is not necessary to pass a large current, it is possible to realize this while preventing deterioration of the O ₂ sensor 10.

(Effect of the Invention) According to the present invention, a pair of electrodes is provided on the solid electrolyte, one electrode is brought into contact with a reference gas, the other electrode is brought into contact with exhaust gas, and at the time of supplying a predetermined current from the current supply means. In the exhaust gas concentration detector that detects the oxygen concentration in the exhaust gas by the voltage generated between the electrodes, a predetermined electric resistor is provided in series with one of the current supply means and the electrode. Since the electric resistor is provided even at a high temperature, it is possible to accurately detect the voltage change at the time of supplying current, so that it is possible to improve the failure detection accuracy of the exhaust gas concentration detector including disconnection and short circuit. .

[Brief description of drawings]

1 is a circuit configuration diagram of a detector input circuit portion including an equivalent circuit and a checker circuit of an exhaust gas concentration detector according to the present invention, and FIG. 2 shows an example of a concrete structure of the O ₂ sensor of FIG. structure diagram, FIG. 3 is characteristic diagram showing the state of the O ₂ sensor output during the O ₂ sensor normal time with disconnection failure, Figure 4 is also a characteristic illustration of the case at the time of the O ₂ sensor normal time and short-circuit failure is there. 10 …… O ₂ sensor (exhaust gas concentration detector), 10e …… Zirconia element (solid electrolyte element), 10f, 10g …… Space, 10m…
… Lead wire, 10o, 10p …… Inner center electrode, 12 …… Electric resistor, 110 …… Operation amplifier, 120 …… Switch.

Claims (1)

[Claims] 1. A solid electrolyte is provided with a pair of electrodes, one electrode is brought into contact with a reference gas, the other electrode is brought into contact with exhaust gas, and a predetermined current is supplied from a current supply means between the electrodes. An exhaust gas concentration detector for detecting the oxygen concentration in the exhaust gas by voltage, wherein a predetermined electric resistor is provided in series with one of the current supply means and the electrode.