JPH0545183B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to an air-fuel ratio measuring device,
In particular, the present invention relates to an air-fuel ratio measuring device that can sufficiently protect a sensor (air-fuel ratio detector) constituting the measuring device from the risk of damage.

(Prior Art) It has been known that zirconia, which is a ceramic, is an electrical insulator at room temperature, but becomes a conductor at high temperatures (approximately 500° C. or higher). It is also known that zirconia at high temperatures has the property of selectively permeating only oxygen (hereinafter referred to as "selective oxygen permeation property").

From the selective oxygen permeability characteristic, an electromotive voltage can be obtained that corresponds to the difference in oxygen concentration on both main surfaces of the zirconia plate. In other words, if the electromotive force and the oxygen concentration on one main surface of the zirconia plate are known, the oxygen concentration on the opposite main surface can be calculated from a certain formula (Nernst's formula), so zirconia can be used as an oxygen concentration detection element. can be used.

Furthermore, when a current is passed through the zirconia plate between its two main surfaces, the zirconia plate has the function of moving oxygen in a direction opposite to the direction of the current. That is, it has a pumping action of pumping oxygen from one main surface side of the zirconia plate to the other main surface side.

Conventionally, there has been known an air-fuel ratio detector that combines the oxygen concentration detection function and pumping function of zirconia.

FIG. 2 is a circuit diagram showing an example of a conventional air-fuel ratio measuring device using the air-fuel ratio detector.

In the figure, 1 is an air-fuel ratio detector (hereinafter simply referred to as a detector). This detector 1 includes a first zirconia 2 that performs an oxygen concentration detection operation, a second zirconia 3 that performs a pumping operation, a hollow chamber 4 provided between the first and second zirconia, and the second zirconia 3 that performs a pumping operation. 1 and 2nd zirconia 2,3
Thin film electrodes 5 made of, for example, platinum are formed on both main surfaces of each of the electrodes.

A small hole 6 is provided in the second zirconia 3. Note that the portion of the small pores 6 may be formed of a porous coating film having characteristics equivalent to those of the small pores 6.

Further, among the electrodes 5, the electrodes 5b and 5c inside the first and second zirconia 2 and 3 are grounded, and the electrode 5a outside the first zirconia 2 is grounded.
is the (-) input terminal of the drive amplifier 7, and the outer electrode 5d of the second zirconia 3 is connected to the drive amplifier 7.
are connected to the respective output terminals.

The (+) input terminal of the drive amplifier 7 has a reference voltage.
Er is applied. Note that the detector 1 is
It can be attached to and detached from the circuit shown in the figure.

The air-fuel ratio measuring device having the above configuration can detect the air-fuel ratio from, for example, exhaust gas of an internal combustion engine. During air-fuel ratio detection, the outer main surface of the first zirconia 2 (electrode 5a side) is in contact with the atmosphere, and the outer main surface of the second zirconia 3 (electrode 5a side) is in contact with the atmosphere.
d side) is brought into contact with the exhaust gas flowing through by concentration diffusion. Therefore, there is a small hole 6 in the hollow chamber 4.
Exhaust gas flows in through.

Below, the second case is divided into the case of lean combustion with a large air-fuel ratio and the case of rich combustion with a small air-fuel ratio.
The operation of the diagram will be explained.

(1) Operation during lean burn During lean burn, a relatively large amount of residual oxygen exists in the exhaust gas. Such exhaust gas flows into the hollow chamber 4.
When the oxygen flows into the hollow chamber 4, the difference in concentration between the oxygen concentration Cv in the hollow chamber 4 and the oxygen concentration Cr in the atmosphere becomes relatively small. Therefore, the electromotive force generated in the first zirconia 2, that is, the voltage E between the electrodes 5a and 5b (between the oxygen concentration detection electrodes) becomes smaller than the reference voltage Er.

Therefore, during lean combustion, the output of the drive amplifier 7 becomes positive, and a pumping current in the direction of arrow A (positive direction) corresponding to the difference between the reference voltage Er and the electromotive force E flows through the second zirconia electrode 5d. 3.

As a result, the oxygen in the hollow chamber 4 is pumped out to the outer main surface side of the second zirconia 3 by the pumping action of the zirconia. and,
When the amount of oxygen pumped out by the pumping current in the positive direction and the amount of oxygen flowing into the hollow chamber 4 become balanced, the pumping current stabilizes at a certain constant value.

(2) Operation during rich combustion During rich combustion, relatively large amounts of hydrogen and carbon monoxide are present in the exhaust gas. When such exhaust gas flows into the hollow chamber 4 through the small hole 6, it combines with oxygen in the hollow chamber 4 to become water and carbon dioxide. As a result, the oxygen concentration Cv in the hollow chamber 4 decreases significantly, and the difference in concentration from the oxygen concentration Cr in the atmosphere increases.

For this reason, the voltage (electromotive voltage generated in the first zirconia 2) E applied to the (-) input terminal of the drive amplifier 7 becomes larger than the reference voltage Er. Therefore, during rich combustion, the output of the drive amplifier 7 becomes negative, and a pumping current in the direction of arrow B (negative direction) corresponding to the difference between the reference voltage Er and the electromotive voltage E is supplied to the second zirconia 3.

As a result, oxygen (oxygen in the exhaust gas) on the outer main surface side of the second zirconia 3 is taken into the hollow chamber 4 by the pumping action of the zirconia.
When the amount of oxygen taken in by the negative pumping current and the amount of oxygen combined with hydrogen or carbon monoxide are balanced, the pumping current settles to a certain value.

In the air-fuel ratio measuring device configured and operated as described above, the air-fuel ratio is calculated from a well-known formula based on the pumping current during the operation of (1) and (2) above. , the carbon monoxide concentration can be calculated, and the calculated value can be calculated by calculation.

By the way, when using the detector 1, there is a restriction that excessive power must not be supplied to the second zirconia 3. The reason is that if excessive power is supplied, the second zirconia 3 will be heated and the electrode 5d, and therefore the detector 1, will be thermally damaged.

The power consumption in the second zirconia 3 is determined by the product of the pumping current and the voltage between the electrodes 5d and 5c (between the pumping current supply electrodes).

As is clear from the operation description in FIG. 2, the pumping current has a positive or negative value determined in proportion to the oxygen concentration, hydrogen, or carbon monoxide concentration of the exhaust gas (measured gas). In addition, the electrode 5
The voltage between electrodes d and 5c is mainly determined by the product of the pumping current and the internal resistance between electrodes 5d and 5c.

By the way, the internal resistance between the electrodes 5d and 5c mainly depends on the temperature of the detector 1, and when the temperature of the detector 1 is low (approximately 600 degrees Celsius or less), the internal resistance is extremely large, especially at room temperature. It becomes 10MΩ.

Therefore, in order to eliminate excessive power supply to the second zirconia 3 and avoid heat damage to the detector 1, the detector 1 is turned on at the same time as the air-fuel ratio measuring device is turned on.
The operating temperature (usually around 700°C) is controlled by a heater built into the detector itself or externally attached.
It is heated to.

However, in the following case, the internal resistance between the electrodes 5d and 5c is large and excessive power is supplied to the second zirconia 3, so the supply of pumping current is blocked and the detector 1 It is necessary to prevent heat damage.

(a) When the detector 1 has not reached the above operating temperature after the power is turned on to the air-fuel ratio measuring device.

(b) When the air-fuel ratio measuring device is replaced with a new detector 1 while it is in use, and the detector 1 has not yet reached the operating temperature described above.

(c) When the heater that heats detector 1 is disconnected.

(d) When detector 1 is excessively cooled by the gas to be measured.

Note that in the above case, the detector 1 does not operate correctly, and therefore it is not possible to calculate an accurate air-fuel ratio.

Furthermore, in the following cases, the pumping current becomes excessive, so it is necessary to limit this to prevent thermal damage to the detector 1.

(e) When the concentration of oxygen, hydrogen, or carbon monoxide in the gas to be measured is excessive.

FIG. 3 is a circuit diagram showing an example of a conventional air-fuel ratio measuring device devised to prevent thermal damage to the detector 1 as much as possible. In the figure, the same reference numerals as in FIG. 2 represent the same or equivalent parts.

The feature of FIG. 3 is that a pumping current limiting circuit 8 and a switch circuit 10 which is opened and closed by the output signal of the timer 9 are provided between the output terminal of the drive amplifier 7 and the electrode 5d.

The timer 9 starts counting by a known appropriate means when the air-fuel ratio measuring device is powered on, and outputs a count-up signal to the switch circuit 10 when the scheduled time has elapsed. As a result, the switch circuit 10
is closed after a predetermined period of time (usually 10 to 30 seconds) from when the power is turned on until the detector 1 reaches its operating temperature. As a result, a pumping current is supplied to the second zirconia 3.

Further, the pumping current is limited to the predetermined value by the pumping current limiting circuit 8 when the positive and negative currents become larger than the predetermined value.

Therefore, according to the circuit shown in FIG. 3, in the case a, the pumping current can be prevented from being supplied, and also in the case e, where the pumping current becomes excessive, the pumping current is reduced to the planned value. can be limited to. Therefore, thermal damage to the detector 1 caused by these is prevented.

(Problems to be Solved by the Invention) However, the circuit shown in FIG. In this case, there was a problem that the detector 1 would be damaged by heat.

The present invention has been made to solve the above-mentioned problems.

(Means and operations for solving the problems) In order to solve the above problems, the present invention provides a method for solving the problems described above, in which the pumping current supply electrode is disposed between the output side of the drive amplifier and one of the pumping current supply electrodes. a switching means for selectively connecting one side to the output side of the drive amplifier or the constant current circuit; and the switching means is connected to the constant current circuit in the counting standby state and the time counting state, and to the output side of the detector after counting up. A voltage comparator is provided between the output side of the switching means and the timer and puts the timer in a counting standby state or a time counting state. When the voltage between the supply electrodes is outside the range of the expected upper limit voltage and lower limit voltage that may damage the pumping current supply electrode, the timer is placed in a counting standby state and the voltage is within the range of the upper limit voltage and lower limit voltage. The present invention is characterized in that the timer is placed in a time counting state, and the timer is placed in a counting standby state at the same time as the air-fuel ratio measuring device is powered on.

(Example) The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention. In the figure, the same reference numerals as in FIG. 3 represent the same or equivalent parts.

In FIG. 1, a changeover switch 11 is provided between an electrode 5d and a drive amplifier 7, and a timer 1
In response to the output signal No. 2, the movable contact 11a is switched to one of the fixed contacts 11b and 11c.

That is, when the timer 12 is in the counting standby state or the time counting state, the movable contact 11a is in contact with the fixed contact 11c, and the resistor R1 and the power source 13
A constant current circuit 15 and an electrode 5d are connected.

Furthermore, when the timer 12 completes counting (counts up) the scheduled time and outputs a count-up signal, the movable contact 11a contacts the fixed contact 11b, so the output terminal of the drive amplifier 7 and the electrode 5d is connected.

The current supplied from the power supply 13 to the second zirconia 3 is controlled by a resistor so that it has a sufficiently smaller value (within a few μA) than the current output from the pumping current limiting circuit 8 (positive and negative current of about several mA). Regulated by R1.

The voltage Eo of the power supply 13 is determined by the voltage comparator 1 described later.
4 is set sufficiently higher than the upper limit voltage (positive voltage) Vp. Note that the timer 12 enters a counting standby state by a known appropriate means at the same time as the power of the air-fuel ratio measuring device is turned on.

When the voltage applied to the second zirconia 3 exceeds a predetermined upper limit voltage Vp or is smaller than a lower limit (negative voltage) Vm, the voltage comparator 14 puts the timer 12 into a counting standby state, and When the voltage is within the range of the lower limit voltages Vp and Vm, the timer 12 operates to be in a clock counting state.

The absolute values of the upper and lower limit voltages Vp and Vm are larger than the absolute value (about 2V) of the voltage that can occur in the second zirconia 3 under normal operating conditions of the detector 1, and are larger than the maximum voltage of the pumping current limiting circuit 8. The power determined by the product of the output current and the voltage generated across the second zirconia 3 at this time is set to be equal to or lower than the voltage at which the detector 1 is equal to the maximum power that will not damage it. According to the inventor's experiments, it is usually about 3 to 6V.

Further, the (-) input terminal side of the drive amplifier 7 is grounded through a resistor R2. The value of this resistance R2 is determined between the oxygen concentration detection electrodes of the detector 1 (electrodes 5a, 5
(between b) at room temperature (several tens of M
Ω) and sufficiently larger than the value (several KΩ) during heating (600°C or higher). Normally, it can be set to about 1MΩ.

Next, the operation of this embodiment will be explained.

(1) When the air-fuel ratio measuring device is powered on, the timer 12 enters a counting standby state and the changeover switch 11 enters the state shown, so that the electrode 5d is connected to the constant current circuit 15. Immediately after the power is turned on, the detector 1 is at approximately room temperature, so the internal resistance between the electrodes 5d and 5Cc is extremely high.

As a result, the voltage generated between the electrodes 5d and 5c due to the minute current supplied from the power supply 13 through the resistor R1 is the upper limit voltage Vp of the voltage comparator 14.
The value is higher than . As a result, the timer 1
2 maintains a counting standby state and does not start counting operation.

(2) When current is applied to the air-fuel ratio measuring device, a heater (not shown) built in or externally attached to the detector 1 starts heating the detector 1. This causes the temperature of detector 1 to rise,
Accordingly, the internal resistance between the electrodes 5d and 5c rapidly decreases.

When the internal resistance between the electrodes 5d and 5c reaches a certain value (for example, 10KΩ) and the voltage between the electrodes 5d and 5c becomes below the upper limit voltage Vp, the voltage comparator 14 changes the timer 12 from the counting standby state to the time counting state. do. This causes the timer 12 to start counting.

(3) When the scheduled time for which the detector 1 will reach its operating temperature has elapsed, the timer 12 outputs a count-up signal. As a result, changeover switch 1
The first movable contact 11a switches to the fixed contact 11b side.

Thereby, the electrode 5d is connected to the output terminal of the drive amplifier 7 through the pumping current limiting circuit 8, and the normal operation of the air-fuel ratio measuring device is started. Note that the internal resistance between the electrodes 5d and 5c at the operating temperature is usually about 10Ω.

Next, a description will be given of the operation when replacing the detector 1 while the air-fuel ratio measuring device is in use, that is, while the measuring device is powered on.

When the detector 1 is disconnected from the circuit shown in Figure 1 for replacement, etc., the (-) input terminal side of the drive amplifier 7 becomes ground potential due to the resistor R2, so the reference voltage on the (+) input terminal side It will be lower than Er.

As a result, the voltage applied to the voltage comparator 14 becomes higher than the upper limit voltage Vp, the timer 12 enters a counting standby state, and the changeover switch 11 enters the illustrated state. Since the voltage Eo of the power supply 13 is set sufficiently higher than the upper limit voltage Vp as described above,
This state is maintained.

When the replaced new detector 1 is connected, the air-fuel ratio measuring device starts normal operation through the operations (2) and (3) described above.

Note that since the resistance R2 is set to be sufficiently larger than the internal resistance between the electrodes 5a and 5b during normal operation of the air-fuel ratio measuring device, it has no effect on the operation of the measuring device during normal operation. I won't give it.

Next, the operation when the detector 1 is excessively cooled by the gas to be measured will be described.

As described above, when the detector 1 is excessively cooled by the gas to be measured, the internal resistance between the electrodes 5d and 5c increases as the temperature of the detector 1 decreases.

At this time, assuming that the pumping current output from the drive amplifier 7 does not change, the voltage between the electrodes 5d and 5c increases as this internal resistance increases,
When the voltage exceeds the upper and lower limit voltages Vp and Vm of the voltage comparator 14, the timer 12 enters a counting standby state, and the changeover switch 11 enters the state shown in the figure.
That is, the normal operating state of the air-fuel ratio measuring device is interrupted.

From this state, the temperature of the second zirconia 3 rises and the voltage between the electrodes 5d and 5c increases to the voltage comparator 14.
When the voltage falls within the range of the upper and lower limits of voltage Vp and Vm, the timer 12 immediately shifts to a counting operation state. Thereafter, when the scheduled time has elapsed and a count-up signal is output from the timer 12 and the movable contact 11a switches to the fixed contact 11b, the air-fuel ratio measuring device enters the normal operating state.

At this time, if the temperature of the detector 1 has not returned to a level at which the air-fuel ratio measuring device can maintain the normal operating state, the timer 12 is in the counting standby state → the movable contact 1
Switching 1a to fixed contact 11c side → timer 1
2 calculation operation start → count up signal output from timer 12 → fixed contact 11b of movable contact 11a
The cyclic operation of switching to the side and waiting for counting by the timer 12 is repeated.

Note that when the detector 1 is in a supercooled state due to the gas to be measured and the oxygen concentration, hydrogen, or carbon monoxide concentration of the gas to be measured is excessively high, the temperature will be lower than in other cases of supercooling. Therefore, even if the increase in the internal resistance between the electrodes 5d and 5c due to the temperature drop of the detector 1 is small, the voltage between the electrodes 5d and 5c will exceed the upper limit voltage Vp or the lower limit voltage Vm, so the timer 12 It is easy to understand that the counting wait state is reached even earlier.

Next, the operation when the heater that heats the detector 1 is disconnected will be described.

When the heater is disconnected, the temperature of the detector 1 decreases, and the internal resistance between the electrodes 5d and 5c increases accordingly. With this increase in internal resistance, the electrode 5
When the voltage between d and 5c increases and exceeds the upper limit voltage Vp or lower limit voltage Vm of the voltage comparator 14, the timer 12 is placed in a counting standby state and the changeover switch 11 is placed in the state shown.

If the heater is disconnected, the internal resistance does not decrease, so this state continues thereafter. Note that if the heater is disconnected when the air-fuel ratio measuring device is powered on, the timer 12 is put into a counting standby state at the same time as the power is powered on.
This state will continue.

In the above description, the (-) input terminal side of the drive amplifier 7 is grounded through the resistor R2, but the resistor R2 is not necessarily necessary.

That is, even if there is no resistor R2, when the detector 1 is disconnected, the voltage comparator 14 is connected from the drive amplifier 7.
This is because the voltage applied to the sensor normally exceeds the upper limit voltage Vp or is smaller than the lower limit voltage Vm, so that the detector operates normally when replaced.

However, when the resistor R2 is provided, the (-) input terminal side of the drive amplifier 7 is not in a floating state, so that the operation of the drive amplifier 7 becomes stable when the detector is replaced.

Furthermore, when the timer 12 is forced into a counting standby state by the voltage comparator 14, an indicator indicating this can be provided to indicate whether the detector 1 is disconnected or the heater is disconnected. In addition, if a lamp serving as an indicator repeatedly turns on and off within a relatively short period of time, it can be easily detected that the temperature of the gas to be measured is too low.

(Effects of the Invention) As is clear from the above description, according to the present invention, the following effects are achieved.

(1) Even if a new detector is replaced and connected while the air-fuel ratio measuring device is in use, the heater that heats the detector is disconnected, or the detector is excessively cooled by the gas being measured, the pumping current Because excessive power is not supplied to the supply electrode,
The detector can be completely protected.

(2) Also, when the timer is forcibly reset by the voltage comparator, if an indicator is provided to indicate this, either the detector is disconnected or the heater is disconnected. It can be easily determined that the temperature of the gas to be measured is too low.

(3) Furthermore, since the present invention does not measure the air-fuel ratio when the detector is not in a normal operating state, control operations based on erroneous counts will not occur even when the present invention is installed in an actual vehicle. The reliability of the vehicle can be improved without causing any damage.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of the present invention. FIG. 2 is a circuit diagram showing an example of a conventional air-fuel ratio measuring device. FIG. 3 is a circuit diagram showing another example of a conventional air-fuel ratio measuring device. 1...Detector, 2...First zirconia, 3...Second
Zirconia, 4... Hollow chamber, 5,5a-5d... Electrode, 6... Small hole, 7... Drive amplifier, 8... Pumping current limiting circuit, 11... Changeover switch, 12...
Timer, 13...Power supply, 14...Voltage comparator, 15...
Constant current circuit, R1, R2...resistance, Er...reference voltage.

Claims (1)

[Scope of Claims] 1. A zirconia plate for oxygen concentration detection, a zirconia plate for pumping, an oxygen concentration detection electrode formed on both main surfaces of the zirconia plate for oxygen concentration detection, and an electrode formed on both main surfaces of the zirconia plate for pumping. a pumping current supply electrode, a hollow chamber formed between the opposing surfaces of both the zirconia plates, and a pumping current supply electrode for introducing the gas to be measured on the outer main surface side of the pumping zirconia plate into the hollow chamber. The air-fuel ratio detector consists of a gas to be measured introducing means formed on a zirconia plate, one input terminal is connected to one of the oxygen concentration detection electrodes, the other input terminal is connected to a reference voltage, and the voltage between the oxygen concentration detection electrodes is and a drive means for outputting positive and negative pumping currents to the pumping zirconia plate according to the difference between the voltage of a switching means provided between the pumping current supply electrodes for selectively connecting one of the pumping current supply electrodes to the output side of the drive means and one of the constant current circuit; , a timer that controls switching to the output side of the driving means after counting up;
A voltage comparator is provided between the output side of the switching means and the timer and puts the timer into a counting standby state or a time counting state, and the voltage comparator is configured to detect the voltage between the pumping current supply electrodes. When is outside the range of the scheduled upper limit voltage and lower limit voltage, the timer is placed in a counting standby state; when it is within the range of the upper limit voltage and lower limit voltage, the timer is placed in a time counting state; An air-fuel ratio measuring device characterized in that it enters a counting standby state at the same time as the measuring device is powered on. 2. Claim 1, characterized in that, when the absolute value of the positive and negative pumping currents of the drive means becomes larger than a predetermined value, pumping current limiting means is provided for limiting the absolute value to a predetermined value. The air-fuel ratio measuring device described in . 3. One input terminal side of the driving means is grounded via a resistor, and the value of the resistor is smaller than the value of the internal resistance between the oxygen concentration detection electrodes at room temperature,
The air-fuel ratio measuring device according to claim 1 or 2, wherein the internal resistance is set to be larger than the value of the internal resistance during heating. 4. Claim 1, characterized in that the timer is provided with a display means for displaying that the timer is forced into a counting standby state by a voltage comparator.
The air-fuel ratio measuring device according to item 2 or 3.