JPH0718837B2 - Air-fuel ratio detector - Google Patents

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 detecting device, and more particularly to an air-fuel ratio detecting device suitable for use in controlling the air-fuel ratio of an internal combustion engine over a wide range.

[0002]

2. Description of the Related Art Conventionally, as an air-fuel ratio detecting device for an internal combustion engine, a device for detecting a stoichiometric air-fuel ratio has been widely used for controlling an internal combustion engine.

However, in recent years, in order to improve the fuel consumption rate, development of an air-fuel ratio detecting device for lean burn control has been advanced. As an example, for example, US Pat.
As described in Japanese Patent Laid-Open No. 55-125448, there is known one which detects a lean air-fuel ratio by the limiting current value flowing when oxygen diffused from a diffusion resistor is pumped out by a solid electrolyte cell.

US Pat. No. 4,158,166 (Japanese Patent Laid-Open No. 53-66292)
It is known that the rich air-fuel ratio is detected by reacting carbon monoxide and the like diffused from the diffusion resistor with oxygen pumped into the solid electrolyte cell and limiting current flowing through the solid electrolyte cell at this time. ing.

[0005]

However, the above-mentioned prior art is not capable of detecting a wide air-fuel ratio range from lean to rich, and is not sufficient to detect the air-fuel ratio with high accuracy.

An object of the present invention is to provide an air-fuel ratio detecting device capable of detecting the air-fuel ratio with high accuracy.

[0007]

The above-mentioned object is to form a solid electrolyte, a first electrode formed on one surface of the solid electrolyte, and a first electrode formed on the other surface of the solid electrolyte, and contact the atmosphere. A second electrode, a diffusion resistor formed on the first electrode for controlling the diffusion of gas to the first electrode, and a heater for heating the solid electrolyte, and from a rich region to a lean region In the air-fuel ratio detecting device for detecting the air-fuel ratio of the above, the measurement of the electromotive force generated between the first electrode and the second electrode, and the measured electromotive force is set to a predetermined value. The detection means that controls the voltage applied between the first electrode and the second electrode and alternately measures the current flowing at that time, and the solid state from the electromotive force and the current measured by the detection means. A means for determining the internal resistance of the electrolyte, and The resistance value of the internal resistance obtained is achieved by a control means for controlling the energization of the heater to be constant.

[0008]

The internal resistance of the solid electrolyte is calculated based on the electromotive force value and the current value detected by time division, and the heater energization is controlled so that the resistance value of the internal resistance becomes constant. It is possible to detect the air-fuel ratio with high accuracy in a wide range from the range to rich.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

First, an example of the sensor portion used in the present invention will be described with reference to FIG. Blind tubular detector 10
Is arranged in a protective tube 12 having a hole 11 and is fixed in a plug body 14 having a screw 13. Then, it is attached to the exhaust pipe 15 through which the exhaust gas flows. Further, 16 is an electrode terminal, 17 is a heater terminal, and the detection unit is connected to an electronic circuit via these terminals. In addition, a rod-shaped heater (such as a W heater formed on an alumina rod) for heating the zirconia solid electrolyte 10 which is a bag-shaped detection unit is mounted inside. This heater raises the temperature of the zirconia solid electrolyte of the detection part to at least 600 ° C. or higher,
This is to reduce the impedance. Detector 1
Reference numeral 0 is a bag-shaped zirconia solid electrolyte having electrodes (made of platinum or the like) on the inside and the outside, respectively, and a porous diffusion resistor formed on the outside electrode. The atmosphere is introduced inside the solid electrolyte and the outside is exposed to the exhaust gas atmosphere.

Another example of the sensor portion used in the present invention is shown in FIG. In this figure, the zirconia solid electrolyte is a flat plate,
It shows the case where the shunt resistance element is composed of one hole. The atmosphere is introduced into the first electrode 22 portion via the passage 32 in the zirconia solid electrolyte 20. Residual oxygen and unburned gas in the exhaust gas pass through the diffusion resistance member 24 in the shape of a hole to the diffusion chamber 31.
It diffuses into the second electrode 23 inside. The zirconia solid electrolyte 20 is heated and controlled to a high temperature by the heater 212 in the alumina insulating layer 211 fixedly attached thereto.

In any case, the zirconia solid electrolyte has a pair of electrodes, one of which has an atmosphere introduced therein, and the other has a structure in which exhaust gas diffusely flows in through a diffusion resistor. The shape is basically good.

Next, the operating principle of the present invention will be described with reference to FIG.

In FIGS. 3 (a) and 3 (b), reference numeral 1 is an oxygen ion conductive solid electrolyte, and platinum electrodes 3 are provided on both surfaces thereof.
a and 3b are formed. The electrode 3a contacts the atmosphere,
The electrode 3b is in contact with exhaust gas via the porous diffusion resistor 2.

In this detection device, as shown in FIG. 1A, a voltage V is applied to the solid electrolyte 1 to flow a current I _P ,
The operation of moving oxygen into and out of the diffusion resistor 2 and the operation of measuring only the electromotive force E generated in the solid electrolyte 1 as shown in FIG. This is done by doing things. The absolute value and the direction of the current I _P shown in (a) of the figure change depending on the air-fuel ratio.
This is because the value of the voltage V is increased / decreased and oxygen in and out of the diffusion resistor 2 is taken in and out so that the electromotive force E measured during the period shown in FIG. . Looking at the terminal voltage of the electrode 3a as shown in FIG.
Voltage period t _i to flow a current I _P is (as V _1, V ₂₎ which changes by the air-fuel ratio. On the other hand, the voltage during the period t _e for measuring the electromotive force E is kept constant E even if the air-fuel ratio changes. That is, the applied voltage V is changed so that the electromotive force E becomes constant.

Next, the relationship between the change in the oxygen concentration distribution in the diffusion resistor 2 and the movement of oxygen and I _P when the air-fuel ratio changes will be described with reference to FIG. FIG. 4A shows the operation in the lean region, in which the movements of I _P and O ₂ and the distribution of the O ₂ concentration when the air-fuel ratio is λ are shown by solid lines and the case of λ ′ is shown by dotted lines. Indicated by. Here, it is assumed that λ '>λ> 1. When the air-fuel ratio is λ, the oxygen concentration in the exhaust gas is P _O2
Therefore , the oxygen concentration on the exhaust side of the diffusion resistor 2 is P _O2 . Here, since the current I _{P is made} to flow so that the electromotive force E measured in another period in advance becomes constant, oxygen in the diffusion resistor 2 is extracted to the atmosphere side, and the oxygen concentration near the electrode 3b is always constant. It becomes a constant value P _O. Here, for example, when the electromotive force E is 0.5 V, P _O is 10 ^-12 %
It will be about. Next, when the air-fuel ratio changes to λ ′ (λ ′> λ), the oxygen concentration on the exhaust side of the diffusion resistor 2 increases to P _O2 ′, but again, the electromotive force E is made constant so that
Since a current I _P ′ larger than I _P is passed, the oxygen concentration near the electrode 3b is maintained at a constant value P _O. Thus, when the oxygen concentration changes from P _O2 to P _O2 ′ (P _O2 ′> P _O2 ), it is necessary to move a large amount of oxygen in order to keep P _O constant. That is, by changing the voltage V to V '(V'> V), increase the I _P to _{I P '(I P'>} I P). Therefore, the current value at this time, that is, the voltage value becomes proportional to the air-fuel ratio.

FIG. 4B shows the operation in the rich region. Since a combustible gas such as CO is generated in the rich region where the air-fuel ratio is λ ″ (λ ″ <1), in order to make the oxygen concentration near the electrode 3b P _O , O ₂ is changed from the atmosphere side to the exhaust side. Need to be sent to. Therefore, the polarity of the voltage applied to the solid electrolyte 1 is reversed, and the current I _P ″ flows in the direction opposite to the lean region. By this operation, the oxygen concentration near the electrode 3b is P _O.
Kept in. The solid line shown in the diffusion resistor 2, shows the concentration distribution of carbon monoxide CO, the exhaust side with P _CO, electrode 3b side becomes P _O of approximately zero. When the air-fuel ratio becomes smaller than λ ″, a large amount of CO exists and the diffusion resistor 2
Oxygen concentration in the vicinity of the electrode 3b is P _O
In order to keep at, it is necessary to send in more oxygen. Therefore, V ″ is increased and I _P ″ is increased. That is, if the air-fuel ratio changes, V ″ changes.

According to the above principle, the air-fuel ratio can be measured in the rich to lean region, but it is necessary to reverse the current direction here. In the present invention, this operation is
It was automatically performed without detecting the point of λ = 1.0. This will be described below.

FIG. 5A shows the operation in the lean region. The exhaust side electrode 3b is connected to a potential ground 4 having a certain constant potential V _PG . I _{P in the} lean area
To flow in the direction of the arrow, the voltage V _{P of the} electrode 3a is made higher than V _PG . That is, when V _P > V _PG , oxygen in the diffusion resistor 2 is extracted to the atmosphere side. Next, FIG. 5 (b)
At the rich time shown in FIG. 5, when V _P is made small so that V _P <V _PG , I _P flows in the opposite direction to the lean time, and oxygen is sent into the diffusion resistor 2.

As described above, the direction of I _P can be automatically reversed by connecting the electrode 3b to the potential ground 4 and raising or lowering the voltage on the electrode 3a side.
The configuration of this potential ground will be described later.

FIG. 6 shows the entire structure of the drive circuit of the detection apparatus according to the embodiment of the present invention. 5 is ON for A terminal and B terminal
Although the microcomputer outputs the -OFF signal alternately, it may be a normal oscillation circuit using a capacitor and a resistor. When the ON signal is output to the A terminal, the switch S
W ₁ is turned on (conduction), an OFF signal is output to the B terminal during this period, and the switch SW ₂ is turned off (non-conduction).
When the switch SW ₁ is turned on, a current I flows through the solid electrolyte 1.
_P flows. The next period, A, ON-OFF state of the B terminal is reversed, the switch SW ₂ is ON, the switch SW ₁ is turned OFF. The period in which the switch SW ₂ is ON is a period in which the electromotive force E is measured. The electromotive force E detected during this period is held by the hold circuit 6, and the value of the electromotive force E is maintained even when the switch SW ₂ is turned off. Then, the differential integrator circuit 7 is compared electromotive force E and the reference value E _ref, in the case of E <E _ref continues to increase the output V _PH of the differential integrating circuit 7. Here, when the switch SW ₁ is turned on, the output V
_P is applied. When E> E _ref , the output V _P of the differential integrator 7 continues to decrease. As described above, the output V _PH is raised and lowered so that the electromotive force E becomes E _ref and the current I _P
To control. As described above, in the rich region, the output V _P is reduced so that the output V _P becomes smaller than V _PG .

As the output of the circuit, a fixed resistor R ₁ is provided between the potential ground 4 and the electrode 3b, the terminal voltage of which is held by the hold circuit 8, and the output V
_out . The output V _out has a value proportional to the current value I _P because the resistance R ₁ is a fixed resistance.

FIG. 7 shows the signal levels of the respective parts when the air-fuel ratio changes from rich to lean and the switches SW ₁ ,
The operation of SW ₂ is shown. (A) is an actual change in the air-fuel ratio, which changes from λ <1 to λ> 1. (B) is the voltage V _P ′ applied to the electrode 3a via the switch SW ₁ , and (c) is the electromotive force E input to the differential integration circuit 7.
showed that. When the air-fuel ratio suddenly changes to the lean side, the amount of oxygen in the diffusion resistor 2 increases, so the electromotive force E becomes E _ref.
Will be smaller than. Therefore, the output V of the differential integration circuit 17
_P continues to increase. When the air-fuel ratio eventually falls to a constant value, the output V _P also converges to a constant value. At this time, the electromotive force E converges on E _ref . (D) and (e) are the periods during which the switches SW ₁ and SW ₂ are ON, and are ON-OFF
The cycle may be set sufficiently shorter than the normal air-fuel ratio change time. Further, (f) shows V _out, and V _out <V _{PG in} the rich region, but V _out > V _PG when changing to the lean region.

As described above, the circuit _raises and lowers the output V _P so that the electromotive force E becomes E _ref . When the air-fuel ratio changes from lean to rich, the output V _P decreases and the output V _out also decreases.

FIG. 8 shows the relationship of V _out with respect to the air-fuel ratio (excess air ratio) λ. At λ = 1.0, theoretically, I
_{Since P} = 0, _Vout = _VPG . When λ <1, V _out <V _PG , and when λ> 1, V _out > V _PG . That is, the direction of the current I _P is automatically inverted at the boundary of λ = 1.0, and the air-fuel ratio from rich to lean can be continuously measured.

Next, the physical meaning of the operation of this detecting device will be described. FIG. 9 shows the characteristics of the current value I when the voltage V applied to the solid electrolyte is increased. As the voltage V increases from zero, the current I increases in proportion to the voltage V due to oxygen ion conduction in the range of (a). If the voltage V is further increased, the flow of oxygen is regulated by the action of the diffusion resistor 2, and even if the voltage V is increased, the current I
Does not change (range (b)). The current I at this time is a value called a limiting current value.

Now, the operating principle of the circuit of FIG. 6 when the air-fuel ratio has a certain value will be shown by the actual characteristics of FIG. Assuming that the output V _P of the differential integrator circuit 7 is now in the range of voltage (a), the electromotive force E generated between the terminals at this time is almost zero. Therefore, the differential integration circuit 7 increases the output V _P. Eventually, when the output V enters the range of (b), an electromotive force starts to be generated between the terminals, and when the electromotive force reaches E _ref , the increase of the output V _P stops and converges to V _C. At this time, a current I _P flows through the solid electrolyte 1. The current I _P is equal to the limit current value described above and is a value proportional to the air-fuel ratio.

The characteristics shown by the dotted line in FIG. 9 are those when the temperature of the solid electrolyte 1 is low. here,
When the output V _P remains V _C , the current value becomes I _L and does not show the limit current value. However, in the circuit of the present invention, when V _P = V _C , it is determined that E≈0. _P is further increased and the electromotive force E converges to a value V _C ′ at which E _ref becomes E _ref .
Therefore, the current value flowing through the solid electrolyte 1 at this time is also the limiting current value. As described above, the detection device according to the present invention always causes the solid electrolyte 1 to pass a current value corresponding to the limiting current value even if the temperature of the solid electrolyte 1 changes.

FIG. 10 shows that the temperature of the solid electrolyte 1 is constant,
The characteristics when the air-fuel ratio λ changes are shown below.
We have shown about ₁ , λ ₂ , λ ₃ and λ ₄ (λ ₁ > λ ₂ > λ ₃ >
λ ₄ ). When the air-fuel ratio is λ ₁ , the electromotive force between the terminals is E
To be _ref, it is necessary to apply a voltage V _P1 to the solid electrolyte 1, the output V _PH is increased to V _P1, converges to V _P1. The current value at this time is I _P1 , which matches the limiting current value at the air-fuel ratio λ ₁ . When the air-fuel ratio becomes λ ₂ , the output V _PH decreases and converges to V _P2 , and the current value becomes I _P2 . Hereinafter, similarly when the air-fuel ratio is λ ₃ and λ ₄ ,
The respective limiting current values I _P3 and I _P4 are passed through the solid electrolyte 1. Since the circuit shown in FIG. 6 outputs an output proportional to this current value, the limiting current value can be constantly monitored.

FIG. 11 shows changes in output when the temperature T of the solid electrolyte 1 is changed. The air-fuel ratio is constant here. 1B shows the characteristics when a constant voltage V _C is constantly applied to the solid electrolyte 1 as shown in FIG. 9, and FIG. 1A shows the output characteristics of the detection device according to the present invention. As described above, even if the temperature T changes, the limit current value can be constantly monitored, so that the temperature dependence of the output is small.

FIG. 12 shows output characteristics when the air-fuel ratio λ is changed with the temperature T kept constant. (B) is a characteristic when the voltage applied to the solid electrolyte is constant. As can be seen from FIG. 10, for example, when the voltage is constant at V _P3 , the limit current value cannot be measured as the air-fuel ratio λ increases, and as shown in FIG. , The gain for the air-fuel ratio λ becomes low. The characteristic (a) is obtained by the detection device of the present invention,
Even if the air-fuel ratio λ changes, the voltage V _P is changed and the limiting current value is constantly measured. Therefore, even if λ increases, the gain of the output value with respect to λ does not change.

FIG. 13 shows an example of a concrete configuration of the circuit of the detection device according to the embodiment of the present invention. 5 is a microcomputer or an oscillator. Hold circuit 6
Is composed of a capacitor C ₁ and a buffer amplifier A ₁ . The electromotive force E held by the hold circuit 6 is input to the differential integration circuit 7. In this differential integration circuit 7, the electromotive force E and the reference voltage E _ref are compared, and when E <E _ref ,
The voltage V _P continues to increase, and when E> E _ref , the voltage V _P continues to decrease. This integration operation is continued until the electromotive force E converges to E _{ref, and} when converged, the voltage V _P also converges to a certain constant value. This voltage V _P is applied to the electrode 3a via the buffer amplifier A ₂ and the switch SW ₁ . On the other hand, the potential ground 4 is composed of a buffer amplifier A ₃ and outputs a constant value of the voltage V _PG . Here, when the switch SW ₁ is turned on and the voltage V _P is applied to the solid electrolyte 1, when V _P > V _PG , the current passes through the buffer amplifier A ₂ and the solid electrolyte 1, and the buffer amplifier A
Dropped to ground within ₃ . However, the potential ground remains constant at V _PG . When V _P <V _PG , the current passes through the buffer amplifier A ₃ and the solid electrolyte 1 and is dropped to the ground in the buffer amplifier A ₂ . However, the output of the buffer amplifier A ₂ is kept constant at V _P. Further, the output circuit 8 holds the terminal voltage of the resistor R ₁ when the switch SW ₁ is turned on by the capacitor C ₁ and the buffer amplifier A ₄ , and outputs (V _out ). The above circuit makes it possible to detect the air-fuel ratio according to the present invention. FIG. 14 shows the result of measurement of actual engine exhaust according to the above-described embodiment of the present invention. The sensor portion of the detection device was attached to the exhaust manifold of the engine, and the engine was operated at a rotation speed of 1250 rpm and a constant torque of 3 kg · m. The horizontal axis of FIG. 14 is the air-fuel ratio λ, and the vertical axis is the output. As can be seen from FIG. 14, the air-fuel ratio in a wide range from λ <1 to λ> 1 can be measured, but the gain of the output value changes at the boundary of λ = 1. This is the limiting current I when λ> 1.
_P depends on the oxygen concentration in the exhaust gas, but at λ <1, the current I _P is the combustible gas CO, HC, H _{2 in the} exhaust gas.
It depends on. Particularly, in a flammable gas, the diffusion rate of H ₂ is very fast (about 4 times that of CO and O ₂ ), so that a large current value is required and the output is greatly inclined.

Next, a temperature control method for keeping the internal resistance r constant in order to keep the temperature of the solid electrolyte 1 constant will be described. FIG. 15A shows the operation when λ> 1. In this case, the current I _P flows in the direction of the arrow in the figure, the voltage on the electrode 3a side is V _P , the voltage on the 3b side is V _H , and the resistance R ₁
Let V _PG be the potential ground 4 side of V _P > V _H
> V _PG . In this case, there is the following relationship between V _P , V _H , and V _PG .

[0034]

[Formula 1] V _P −V _H = E + rI _P (Formula 1)

[0035]

V _H −V _PG = r _C · I _P (Equation 2) where E: electromotive force, r _C : resistance value of resistor R ₁ (Equation 1)
From the formula, the internal resistance r is

[0036]

[Equation 3]

Here, V _H is known because it is always measured as an output, and E is known because it is controlled so as to always have a constant value. here,

[0038]

[Formula 4] V = V _H + E (Formula 4)

[0039]

[Equation 5]

From the equation (2), I _P is

[0041]

[Equation 6]

Therefore, since V _H , V _PG and r _C are known, I _P is also known. Therefore, the internal resistance r of the solid electrolyte 1 is calculated from the equations (6) and (5).

Next, the operation when λ <1 is shown in FIG.
Shown in. Here, the current I _P flows in the direction opposite to that of (a), so that the voltage of each part is V _P <V _H <V _PG . Also here, when the calculation as in the formulas 1 to 6 is performed, the result is as follows. The relationship between V _P , V _H and V _PG is

[0044]

[Equation 7] V _P −V _H = E−rI _P (Equation 7)

[0045]

[Equation 8] V _PG −V _H = r _C · I _P (Equation 8) Here, when the equation (7) is rewritten using the relationship of the equation (4),

[0046]

[Equation 9]

It becomes Also, I _P is calculated from the equation (8).

[0048]

[Equation 10]

It becomes Here, since V _PG , V _H , and r _C · V * are known, the internal resistance r is obtained from the equations (9) and (10). This internal resistance r is calculated, the heater is controlled, and the internal resistance r is always kept constant. FIG. 16 shows a flow of heater control. This calculation is based on λ> 1 and λ <1
Since it is different, I will make this judgment first. The magnitude relationship between V _H and V _PG changes depending on the direction of the current I _P , and this is used for the determination (step 101). For λ> 1, V _H > V
_Since it becomes _PG , the current I _P is calculated by the equation (6) (step 102). Next, the internal resistance r is calculated from this current _IP and the equation (5) (step 103). When the internal resistance r is controlled, it is compared with the resistance value r _ref (step 104). Here, if r> r _ref ,
Since the temperature T of the solid electrolyte 1 is lower than the set value, a Hi signal for turning on the heater is output to heat the solid electrolyte 1 (step 105). When r <r _ref , T is higher than the set temperature, so the Lo signal for turning off the heater is output (step 10).
6). Then, the process returns to step 101.

On the other hand, when V _H <V _PG , since λ <1.0, the current I _P is calculated by the equation (10) (step 107). The internal resistance is calculated from this _IP and the equation (9) (step 108). As described above, after the internal resistance r is obtained, the process proceeds to step 4, and the heater is controlled by the same flow as above. The above flow is
Although it can be processed by an analog circuit, it is more advantageous to perform A / D conversion and input to the microcomputer 5 for digital processing.

FIG. 17 is a wiring diagram of a circuit for controlling the heater. The above-described outputs V _PH and V _H are input to the AD converter 42 from the drive circuit 41 of the detection device. V _PH , V _H
The digital value corresponding to is input to the microcomputer 5. The flow shown in FIG. 16 is executed in the microcomputer 5, and the Hi and Lo signals are output to the comparator 43. When Hi signal is output, the comparator 43 is Lo signal is output to the base of the transistor Tr _1, the transistor Tr ₁ is conductive, the heater 4
An electric current flows to 0, and the solid electrolyte 1 is heated.

On the other hand, when the Lo signal is output from the microcomputer 5, the comparator 43 outputs the Hi signal, the transistor Tr ₁ is turned off, and the heater 4 is turned off.
No current flows to 0. The heater is controlled by the above hardware, and the temperature of the solid electrolyte 1 is always controlled to be constant.

FIG. 18 shows another method for controlling the heater. In this method, as shown in FIG. 18A, in addition to the period t _i of flowing the current I _P and the period t _e of measuring the electromotive force E, the internal resistance r of the solid electrolyte 1 is set to a constant value of current. This is a method of newly providing a period t _h for measuring by flowing a value. As shown in FIG. 18B, the internal resistance r is measured by flowing the constant current I _H and detecting the terminal voltage V _HC at this time during the period t _h . At this time, as shown in FIG. 18C, the current I _H flows in a direction in which oxygen is sent into the diffusion resistor 2. Therefore, the terminal voltage V _HC is expressed as follows.

[0054]

V _HC = rI _H −E (Equation 11)

[0055]

[Formula 12] V _HC ∝r ... (Formula 12) As can be seen from the formula (12), the voltage V _HC is I _H , E.
Is a function of the internal resistance r. That is,
If this voltage V _HC is measured and the heater is controlled so as to have a constant value, highly accurate temperature control becomes possible. Note that FIG.
The solid line characteristics shown in (a) and (b) are for λ> 1, and the dashed line characteristics are for λ <1. FIG.
As shown in (b), in the case of λ <1, the direction of the current I _P is opposite to λ> 1, and thus it is shown as a negative value.

In the period t _h , the current I _H is shown in FIG.
Flowing in the direction of 8 (c) also serves to protect the solid electrolyte 1 in the rich region since oxygen is sent to the exhaust side.

FIG. 19 shows a circuit configuration for realizing the operation of FIG. From the A terminal of the microcomputer 5, a signal that is turned on for t _i is output, from the B terminal, a signal that is turned on for t _e is output, and from the C terminal, a signal that is turned on for t _h is output. It For this reason, t _h
During this period, only the switch SW ₃ is in the conductive state. When the switch SW ₃ is turned on, the constant current I _H flows from the constant current source 44 to the solid electrolyte 1. The terminal voltage V _HC at this time is sampled and held by the capacitor C ₃ and the buffer amplifier A ₅ . Therefore, the output of the buffer amplifier A ₅ is maintained at V _HC even when the switch SW ₃ is turned off. This output V _HC is compared with the reference voltage V _ref, and V
_{If HC} > V _ref , the comparator 45 outputs an OFF signal to the base of the transistor Tr ₂ and
With r ₂ in a conducting state, a current is passed through the heater 40 to heat the solid electrolyte 1. On the other hand, when V _HC <V _ref , the comparator 45 outputs an ON signal, so that the transistor Tr ₂ becomes non-conductive and no current flows to the heater 40. It should be noted that this V _ref is determined in advance by the equation (11), the value corresponding to the internal resistance r to be set. With the above method, highly accurate temperature control can be performed while monitoring the internal resistance r of the solid electrolyte 1.

The overall structure of another embodiment of the present invention will be described with reference to FIG. With the potential ground circuit 84 including the voltage source 64 of the potential V _PG and the buffer amplifier 65,
The output voltage of the amplifier 65 is held at a constant value higher than the circuit ground by the potential V _PG . As a result, since the electric potential of the electrode 53 contacting the exhaust gas atmosphere via the porous diffusion resistor 54 is always higher than the circuit ground, the positive / negative pump current I moving in the zirconia solid electrolyte 50 is obtained. It becomes possible to measure _P. The resistor 66 is the pump current I
It is a detection resistance of _P , and I _P is converted into output voltage e ₀ ,
The signal is transmitted from the output circuit 88 including the buffer amplifier 67 and the capacitor 62 to the engine control microcomputer.

It is assumed that the pulse train A of the time division signal generator 85 is off and B is on. At this time, the switches 68 and 69 made of CMOS or the like are turned off.
When the switch 69 is turned off, the pump voltage I _P flowing through the zirconia solid electrolyte 50 is forced to zero because the excitation voltage supplied from the integrating circuit 87 to the electrode 52 disappears. At this time, since the voltage difference between the electrodes is only the component of the electromotive force eλ, this is detected by the differential amplifier circuit 89 including the resistors 71 to 74 and the amplifier 75. As a result, the electromotive force eλ can be detected with high accuracy without being affected by the ohmic loss overvoltage. Since the switch 76 is in the ON state, the eλ value detected by the differential amplifier circuit 89 is quickly transferred to the hold circuit 86 including the capacitor 77 and the amplifier 78. This eλ value is input to the integrating circuit 87 and compared with the reference voltage E _ref . The time constant τ of the integrator circuit 87 is determined by the values of the resistor 79 and the capacitor 80, and is set to a value of several to several tens ms. Further, the reference voltage E _ref is 0.3 to 0.6.
Set to the value of the bolt. When eλ <E _ref , electrode 5
It acts so that the excitation voltage applied to 2 becomes large.
On the other hand, when eλ> E _ref , it acts so that the excitation voltage becomes small.

Next, the pulse train A of the time division signal generator 85
When B and B are reversed, the switch 69 is turned on,
The excitation voltage is applied to the electrode 52 from the integrating circuit 87. e
lambda <When E _ref, because the excitation voltage increases, pulling a large amount of oxygen electrode 53 parts when the lean state, acts to reduce the oxygen supply to the electrode 53 parts when rich conditions, Iramuda is E _ref The excitation voltage is feedback-controlled so that

On the contrary, when eλ> E _ref , the excitation voltage becomes small, so that a small amount of oxygen is extracted from the electrode 53 in the lean state, and a large amount of oxygen is supplied to the electrode 53 in the rich state. Similarly, the excitation voltage is feedback-controlled so that eλ becomes E _ref . They are,
It is feedback-controlled to an equilibrium state by an electrochemical O ₂ pump action. Note that the switch 76 is in the OFF state so that the excitation voltage is not sampled. On the contrary, the switch 68 is in the ON state, the voltage corresponding to the pump current I _P detected by the detection resistor 66 is sampled, and the output voltage V is output from the output circuit 88 including the capacitor 62 and the buffer amplifier 67. It is output as _out.

As described above, the air-fuel ratio in the rich range, the stoichiometric air-fuel ratio, and the lean range can be continuously detected by repeating the cycle alternately.

The time for detecting the electromotive force depends on the diffusion resistor 54.
Since the electromotive force eλ changes little by little due to the diffusion of gas in the part, it must be set to several ms or less.

Further, when the diffusion resistor 54 is manufactured so that the limiting current value I _P has a large value, when the switch 69 is composed of CMOS or the like, the voltage drop caused by its internal resistance becomes large. A switch such as a transistor is desirable. At this time, it is necessary to pass currents in both directions (positive and negative), and two transistors must be used in pairs to enable bidirectional current movement.

Further, the integrating circuit 87 may be composed of a differential amplifier whose gain is appropriately selected.

In any case, with the configuration of FIG. 20, the excess air ratio λ could be detected with high accuracy even if the excitation voltage acting between both electrodes was several times larger than the electromotive force eλ.

As a result, even when the temperature of the zirconia solid electrolyte 50 was low (in the experiment, 600 ° C. or higher), the electric power of the heater was reduced and the durability thereof was improved.

When the sensor is continuously placed in a rich environment for several tens of minutes to several hours, the electrode 53
Probably because the zirconia solid electrolyte 50 in the vicinity of the part locally shifts to electron conduction, hysteresis occurs in the output characteristics. An example of such an evaluation result is shown in FIG. For this, a synthesis gas was used, and the excess air ratio λ was decreased from λ = 1.5 at a uniform speed, and was decreased to λ = 0.73 in about 10 minutes. Then, the hysteresis characteristic was obtained by continuously standing in a rich atmosphere of λ = 0.73 for about 70 minutes, then increasing λ at a uniform speed, and increasing to λ = 1.5 in about 10 minutes. It is shown. As shown in the figure, a hysteresis phenomenon occurred in the output voltage characteristic. Regarding the magnitude of the hysteresis, the longer the time of leaving in the rich atmosphere and the smaller the λ value when leaving, the greater the tendency of the hysteresis to be.

Even the actual engine may be exposed to the rich atmosphere for a long time depending on the operating condition. Therefore, it is needless to say that the occurrence of the hysteresis phenomenon is not preferable in terms of control.

From this experimental tendency, basically,
It is expected to reduce the time of the excitation voltage applied to the zirconia solid electrolyte 50 in the rich atmosphere. It was decided to apply this countermeasure to the circuit configuration shown in FIG.

FIG. 22 shows this countermeasure method. The time division signal generator 85 is controlled according to the magnitude of the output voltage V _out ,
This is a method in which the pulse trains A and B are sometimes paused as shown in the section Z. As a result, the switch 69 in FIG. 20 is turned off for a relatively long time (several tens to several hundreds ms) according to the section Z, and the application of the excitation voltage to the zirconia solid electrolyte 50 is stopped during this period. The length of the section Z is longer as V _out is smaller, that is, the λ value is smaller, and it is effective to increase the number of times. The measurement of the instantaneous excess air ratio becomes momentary to some extent, but this can be improved by improving the control algorithm so that there is no problem in actual use.

Next, the advantages of application when the present detection device uses the air-fuel ratio control of the engine will be described.

In this detection device, when the diffusion resistor 2 changes with time due to a cause such as clogging, the output also changes with time. However, at λ = 1.0, since I _P = 0, the output V _out becomes V _PG . That is, at this time, since I _P is not flowing, the output is always V _PG regardless of the state of the diffusion resistor 2. That is, at λ = 1, even if I _P is flowed and oxygen is not moved, the diffusion resistor 2
The oxygen concentration distribution inside is always P _O (P _O ≈0). This is because the oxygen concentration in the exhaust gas is already P _O. As described above, at λ = 1.0, since oxygen does not move or diffuse, even if the diffusion resistor 2 changes with time, V
_out is always V _PG constant.

Here, even if the output changes with time, λ
The output at = 1.0 remains V _PG . That is, in this detection device, the air-fuel ratio of λ = 1.0 can be measured with the same accuracy as that of the conventional oxygen sensor that detects the stoichiometric air-fuel ratio. Therefore, when the control air-fuel ratio of the engine is changed by the load, the correction amount to be added to the injection amount of the fuel injection valve is determined when controlling λ = 1.0.

Here, the injection width T _P is

[0076]

Equation 13] _{T P = T B (1 +} K 1 + K 2 ......) ... ( Equation 13) where, K _1: air-fuel ratio detector according to the correction factor K ₂ and later: correction by other air-fuel ratio detection coefficient (e.g. water temperature correction Etc.) When λ = 1.0, K ₁ is determined and this correction coefficient K ₁ is also used when controlling to another air-fuel ratio.

As described above, highly accurate air-fuel ratio correction can be performed.

According to the embodiments of the present invention described above,
It has the following effects.

(1) It is possible to detect over a wide air-fuel ratio range from lean to rich. (2) The electromotive force Eλ is detected, and the excitation voltage V _P is controlled so that this Eλ becomes constant. Therefore, in FIG.
As described in FIG. 11, the measurement is possible even when the temperature of the solid electrolyte is low (about 600 ° C.) and the internal impedance is relatively large. Therefore, the required power of the heater can be reduced, and the durability of the sensor portion can be improved.

(3) Further, as explained in FIG. 12, since the excitation voltage V _P is varied, the output gain does not change even if the air-fuel ratio becomes large.

(4) When the solid electrolyte is used for a long time in the rich region, the solid electrolyte becomes electronically conductive and causes a hysteresis in the output characteristics. In such a case as well, the time division type By setting the current flowing through the solid electrolyte to zero, electron conduction can be prevented.

(5) Using the internal resistance value of the solid electrolyte,
By controlling the energization of the heater, the temperature of the solid electrolyte can be kept constant and the detection accuracy can be improved.

[0083]

According to the present invention, it is possible to detect the air-fuel ratio in a wide range from lean to rich.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view of an example of a sensor unit used in the present invention.

FIG. 2 is a partial cross-sectional view of another example of the sensor unit used in the present invention.

FIG. 3 is an explanatory view of the principle of the present invention.

FIG. 4 is an explanatory view of the principle of the present invention.

FIG. 5 is an explanatory view of the principle of the present invention.

FIG. 6 is a conceptual diagram of an embodiment of the present invention.

FIG. 7 is an explanatory diagram of operation and characteristics of one embodiment of the present invention.

FIG. 8 is an explanatory diagram of operation and characteristics of the embodiment of the present invention.

FIG. 9 is an explanatory diagram of operation and characteristics of one embodiment of the present invention.

FIG. 10 is an explanatory diagram of operation and characteristics of the embodiment of the present invention.

FIG. 11 is an explanatory diagram of operation and characteristics of one embodiment of the present invention.

FIG. 12 is an explanatory diagram of the operation and characteristics of the embodiment of the present invention.

FIG. 13 is a configuration diagram of an embodiment of the present invention.

FIG. 14 is an experimental result diagram.

FIG. 15 is an explanatory diagram of another embodiment of the present invention.

FIG. 16 is an explanatory diagram of another embodiment of the present invention.

FIG. 17 is an explanatory diagram of another embodiment of the present invention.

FIG. 18 is an explanatory diagram of another embodiment of the present invention.

FIG. 19 is an explanatory diagram of another embodiment of the present invention.

FIG. 20 is an explanatory diagram of another embodiment of the present invention.

FIG. 21 is an explanatory diagram of another embodiment of the present invention.

FIG. 22 is an explanatory diagram of another embodiment of the present invention.

[Explanation of symbols]

2, 24, 54 ... Diffusion resistor, 4 ... Potential ground circuit, 5 ... Microcomputer, 6 ... Sample hold circuit, 7 ... Differential integration circuit, 8 ... Output circuit, 22, 2
3, 52, 53 ... Electrodes, 40 ... Heaters.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical location 9218-2J 327 Q 9218-2J 327 N (72) Inventor Minoru Osuka 4026 Kuji Town, Hitachi City, Ibaraki Prefecture Hitachi, Ltd., Hitachi Research Laboratory (72) Inventor, Yoshishige Oyama 4026, Kuji Town, Hitachi City, Ibaraki Prefecture Hitachi, Ltd., Hitachi Research Laboratory (56) References JP-A-57-187646 (JP, A) JP-A-57 -192852 (JP, A)

Claims (1)

[Claims] 1. A solid electrolyte, a first electrode formed on one surface of the solid electrolyte, a second electrode formed on the other surface of the solid electrolyte and in contact with an atmosphere, and An air-fuel ratio for detecting an air-fuel ratio from a rich region to a lean region, the diffusion resistor being formed on one electrode and controlling diffusion of gas to the first electrode, and a heater for heating the solid electrolyte. In the detection device, measurement of an electromotive force generated between the first electrode and the second electrode, and the first electrode and the second electrode so that the measured electromotive force has a predetermined value. A means for controlling the voltage applied to the electrodes of the solid electrolyte and the means for alternately measuring the current flowing at that time, and means for obtaining the internal resistance of the solid electrolyte from the electromotive force and the current measured by the detecting means. , The resistance value of the calculated internal resistance Air-fuel ratio detecting apparatus characterized by a control means for controlling the energization of the heater to be constant.