JPH0815216A - Oxygen content determining device - Google Patents

Abstract

PURPOSE:To increase the period of time when the oxygen content can be determined by reducing the time required for detecting the oxygen content after positive bias is applied in an oxygen content determining device. CONSTITUTION:At the time of determining the temperature of the sensor main body 20, at one time before the end of increasing an electric current flowing through the sensor main body 20 after the sensor main body 20 is negative biased by a bias control circuit 40, according to a detected current of a current detecting circuit 50 at this time, a microcomputer 70 estimates its saturation current to determine the temperature of the sensor main body 20. Immediately after the lapse of the above one time, the sensor main body 20 is positive-biased by the bias control circuit 40, and at one time before the end of decreasing a current flowing through the sensor main body 20, according to a detected current at this time, the microcomputer 70 estimates a limiting current after convergence to determine the air-fuel ratio according to the estimated limiting current.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxygen concentration determination device for determining the air-fuel ratio in the exhaust gas of an internal combustion engine, that is, the oxygen concentration, and in particular, it determines the oxygen concentration by utilizing a limiting current type oxygen sensor. The present invention relates to an oxygen concentration determination device suitable for.

[0002]

2. Description of the Related Art Conventionally, in this type of oxygen concentration determination device, as shown in, for example, Japanese Patent Laid-Open No. 163556/1984, the internal resistance of a limiting current type oxygen sensor changes with temperature. And that the current-voltage characteristic that specifies the temperature of the oxygen sensor, that is, the internal resistance passes through the origin, the oxygen sensor is positively biased for a first period by a positive voltage, while it is second biased by a negative voltage. Negatively biased during the first and second periods, the converged current flowing through the oxygen sensor is detected, and the oxygen concentration is detected based on the detected current in the first period. The internal resistance of the oxygen sensor is detected based on the detected current. Then, by heating the oxygen sensor so that the detected internal resistance becomes almost constant, the temperature of the oxygen sensor is controlled with high accuracy so as to maintain the oxygen sensor in the active region, and the air-fuel ratio is determined based on the detected oxygen concentration. Is determined stably.

Further, as shown in Japanese Patent Publication Nos. 1-28905 and 1-25419, attention is paid to the fact that the internal resistance and the temperature of the oxygen sensor have a one-to-one correspondence. By detecting the internal resistance, calculating the voltage applied to the oxygen sensor according to this detected value, and applying the voltage to the oxygen sensor based on this calculation, the operating temperature range and oxygen concentration measurable range are restricted. remove.

[0004]

However, in such a configuration, the above-described first period and second period, which are set to a time sufficient for the detection current to converge, are uniformly repeated. Therefore, even if the air-fuel ratio can be stably determined, the air-fuel ratio must always be set after the elapse of the first period and the second period set to a time sufficient for the detected current to converge. Therefore, there is a problem in that the determination time of the air-fuel ratio is delayed. Further, since the above-mentioned second period is repeatedly set regardless of the temperature decrease of the oxygen sensor, the application timing of the negative voltage to the oxygen sensor does not always coincide with the temperature decrease start timing of the oxygen sensor.
Therefore, if the temperature of the oxygen sensor drops too much by the time when the next negative voltage is applied after the air-fuel ratio can be stably determined, the response of the oxygen sensor to the temperature is slow, Even if the above temperature control is performed by applying a negative voltage, it takes a long time before the air-fuel ratio can be stably determined. In such a case, the first period and the second period described above are set to the time required for the internal resistance of the oxygen sensor to stabilize (the detected current converges), so that the time when the air-fuel ratio can be determined is further delayed. Is causing

Therefore, in order to cope with such a situation, the present invention predicts the current value after convergence with the current value during convergence after applying the positive bias and determines the oxygen concentration in a short time. Therefore, it is an object of the present invention to significantly reduce the time during which the oxygen concentration determination device cannot determine the oxygen concentration.

[0006]

To solve the above-mentioned problems, in the present invention, as illustrated in FIG. 1, a limiting current type oxygen sensor, a positive voltage is applied to the oxygen sensor, and a negative voltage is applied for a predetermined period. A voltage applying unit that switches and applies the voltage, a current detecting unit that detects a current flowing in the oxygen sensor by applying the voltage, and the negative current based on the detected current when the negative voltage is applied to the oxygen sensor for a predetermined period. Impedance detecting means for detecting the direct current impedance of the oxygen sensor, limiting current predicting means for predicting the limiting current after convergence by the detected current in the middle of convergence after applying the positive voltage to the oxygen sensor, and the predicted And an oxygen concentration determination means for determining the oxygen concentration based on the limiting current, the oxygen concentration determination device is provided.

[0007]

According to the present invention, the voltage applying means applies a positive voltage to the oxygen sensor and switches it to a negative voltage for a predetermined period of time to apply the voltage, and detects the current flowing through the oxygen sensor by applying the voltage. The impedance detector detects the DC impedance of the oxygen sensor based on the detected current when the negative voltage is applied to the oxygen sensor for a predetermined period. And
The limiting current predicting means predicts the limiting current after convergence by the detection current in the middle of convergence after applying the positive voltage to the oxygen sensor, and the oxygen concentration determining means determines the oxygen concentration based on the predicted limiting current.

[0008]

【Example】

[First Embodiment] A first embodiment of the present invention will be described below with reference to the drawings. FIG. 2 shows an example of an oxygen concentration determination device according to the present invention applied to an internal combustion engine 10. The oxygen concentration determination device is provided with a limiting current type oxygen sensor S, and this oxygen sensor S is attached inside an exhaust pipe 11 extending from an engine body 10a of the internal combustion engine 10. The oxygen sensor S is composed of a sensor body 20 and a cover 30 having a U-shaped cross section, and the sensor body 20 has a mounting hole formed in a part of the peripheral wall of the exhaust pipe 11 at the base end thereof. It is fitted in the portion 11 a and extends toward the inside of the exhaust pipe 11.

The sensor body 20 has a diffusion resistance layer 21 having a cup-shaped cross section, and the diffusion resistance layer 21 is fitted into the mounting hole portion 11a of the exhaust pipe 11 at the opening end portion 21a thereof. ing. The diffusion resistance layer 21 is formed by a plasma spraying method using ZrO ₂ or the like. Further, the sensor body 20 has a solid electrolyte layer 22, and the solid electrolyte layer 22 is formed of an oxygen ion conductive oxide sintered body in a cup-shaped cross section, and an exhaust gas side electrode having a cup-shaped cross section. It is evenly fitted to the inner peripheral wall of the resistance diffusion layer 21 via the layer 23, and the atmosphere-side electrode layer 24 is uniformly fixed to the inner surface of the solid electrolyte layer 22 in a cup-shaped cross section. In this case, the exhaust side electrode layer 23 and the atmosphere side electrode layer 24 are
In both cases, a precious metal having a high catalytic activity such as platinum is formed sufficiently porous by chemical plating or the like. The area and the thickness of the exhaust-side electrode layer 23 is a 10 to 100 mm ² and 0.5~2.0μ about, whereas, the area and the thickness of the atmosphere-side electrode layer 24 is 10 mm ² or more And 0.5
It is about 2.0 μ.

The sensor body 20 thus constructed generates a concentration electromotive force at the stoichiometric air-fuel ratio point and a limiting current corresponding to the oxygen concentration in the lean region from the stoichiometric air-fuel ratio point. In such a case, the limiting current corresponding to the oxygen concentration is determined by the area of the exhaust gas side electrode layer 23, the thickness of the diffusion resistance layer 21, the porosity and the average pore diameter. Further, although the sensor body 20 can detect the oxygen concentration with a linear characteristic, a high temperature of about 650 ° C. or higher is required to activate the sensor body 20, and the sensor body 20 has a high temperature. Since the active temperature range of the internal combustion engine is narrow, the active region cannot be controlled by heating only the exhaust gas of the internal combustion engine. Therefore, the heating control of the heater 26 described later is utilized. In the region richer than the stoichiometric air-fuel ratio, carbon monoxide (CO) which is unburned gas
Changes almost linearly with respect to the air-fuel ratio, and a limiting current corresponding to this is generated.

The voltage-current characteristic of the sensor body 20 with the temperature of the sensor body 20 as a parameter will now be described. The current-voltage characteristic is proportional to the oxygen concentration (air-fuel ratio) detected by the oxygen sensor S. It is shown that the relationship between the current flowing into the solid electrolyte layer 22 of the main body 20 and the voltage applied to the solid electrolyte layer 22 is linear. Then, when the sensor body 20 is in the active state at the temperature T = T1, a stable state is shown by the characteristic graph L1 as shown by the solid line in FIG. 3 (B). In such a case, the characteristic graph L1
The straight line portion parallel to the voltage axis V specifies the limit current of the sensor body 20. Then, the increase / decrease in the limit current corresponds to the increase / decrease in the air-fuel ratio (that is, lean / rich). When the temperature T of the sensor body 20 is at T2 lower than T1,
The current-voltage characteristic is specified by the characteristic graph L2 as shown by the broken line in FIG. In such a case, the straight line portion parallel to the voltage axis V of the characteristic graph L2 specifies the limiting current of the sensor main body 20 at T = T2, and this limiting current substantially matches the limiting current according to the characteristic graph L1.

Then, in the characteristic graph L1, if a positive applied voltage Vpos is applied to the solid electrolyte layer 22 of the sensor body 20, the current flowing through the sensor body 20 becomes the limiting current Ipo.
s (see point P1 in FIG. 3B). Further, if a negative applied voltage Vneg is applied to the solid electrolyte layer 22 of the sensor body 20, the negative temperature current specified by the point P2 in which the current flowing in the sensor body 20 is proportional to only the temperature without depending on the oxygen concentration. It becomes Ineg. Therefore, the temperature current Ineg at this time
It is possible to maintain the internal state of the sensor body 20 in an active state by making the internal resistance (DC impedance) of the sensor body 20 almost constant by controlling the heating of the heater 26 by utilizing the above.

Further, the sensor body 20 has a heater 26, which is housed in the atmosphere-side electrode layer 24, and the heat energy generated by the heater 26 causes the atmosphere-side electrode layer 2 to be heated.
4. The solid electrolyte layer 22, the exhaust gas side electrode layer 23 and the diffusion resistance layer 21 are heated. In such a case, the heater 26 has a heat generation capacity sufficient to activate the sensor body 20.
The cover 30 covers the sensor main body 20 and is fitted into a part of the peripheral wall of the exhaust pipe 11 at the opening thereof, and a small hole 31 is provided in a part of the peripheral wall of the cover 30. Thirty
Is provided so that the outside of the container communicates with the inside of the cover 30. As a result, the cover 30 is attached to the sensor body 20.
The sensor body 2 while preventing direct contact with exhaust gas of
Secure 0 insulation.

As shown in FIG. 2, the oxygen concentration determination device also includes a bias control circuit 40. The bias control circuit 40 includes a positive bias DC power supply 41, a negative bias DC power supply 42, and a changeover switch. It is configured by the circuit 43. The DC power supply 41 has its negative electrode connected to one end of the exhaust gas side electrode layer 23 via a lead wire 41a, while the DC power supply 42 has its positive electrode connected to the exhaust gas via a lead wire 41a. It is connected to one end of the side electrode layer 23. In the first switching state, the changeover switch circuit 43 connects only the positive electrode of the DC power supply 41 to the input terminal 51 of the current detection circuit 50, while in the second switching state, the negative side of the DC power supply 42 is connected. Only the side electrode is connected to the input terminal 51 of the current detection circuit 50, and is connected to the atmosphere side electrode layer 24 from the input terminal 51, the current detection circuit 50, and the lead wire 42a. Therefore, when the changeover switch circuit 43 is in the first changeover state, the DC power supply 41 positively biases the solid electrolyte layer 22 to cause a current to flow in the solid electrolyte layer 22 in the positive direction.
On the other hand, when the change-over switch circuit 43 is in the second change-over state, the DC power supply 42 negatively biases the solid electrolyte layer 22 to cause a current to flow in the solid electrolyte layer 22 in the negative direction. In this case, the terminal voltages of the DC power supplies 41 and 42 correspond to the above-mentioned applied voltages Vpos and Vneg, respectively.

The current detection circuit 50 detects a current flowing from the atmosphere-side electrode layer 24 of the sensor body 20 to the changeover switch circuit 43 or a current flowing in the opposite direction, that is, a current flowing in the solid electrolyte layer 22, and A-D Output to the converter 60. The AD converter 60 digitally converts the detected current from the current detection circuit 50 and outputs it to the microcomputer 70. The microcomputer 70 executes the computer program in cooperation with the AD converter 60 according to the flowchart shown in FIG. 4, and during this execution, the heating control circuit 80 and the fuel injection control device (hereinafter referred to as EFI). The arithmetic processing required to drive and control 90 is performed. However, the computer program described above is stored in advance in the ROM of the microcomputer 70.
Further, the heating control circuit 80 is the microcomputer 70.
The heating of the heater 26 is controlled according to the element temperature of the oxygen sensor S under the control of. The EFI 90 is controlled by the microcomputer 70 under the control of the internal combustion engine 1
Fuel injection control is performed according to internal combustion engine information such as zero exhaust gas amount, rotational speed, intake air flow rate, intake pipe negative pressure and cooling water temperature.

In the first embodiment constructed as described above, after the microcomputer 70 starts executing the computer program in step 100 according to the operation of the internal combustion engine 10 in accordance with the flowchart of FIG. It is assumed that the execution is repeated. At the present stage, it is assumed that the oxygen sensor S is in an active state and stable. And step 10
At 1, the temperature detection cycle is variably set based on the warm-up state and the operating state of the internal combustion engine, and then at step 102, it is determined whether or not the temperature detection cycle is variably set at step 101. If not, the determination of “NO” is repeated in step 102.

In such a state, the microcomputer 70 executes the computer program shown in FIG.
Of step 121 and step 123, and in step 121, the positive applied voltage V to the sensor body 20 is applied.
The positive bias command required to apply pos is output to the changeover switch circuit 43 of the bias control circuit 40. Here, the applied voltage Vpos may be a constant value, but as shown in FIG. 3B, the applied voltage Vpos necessary for detecting the limiting current changes according to the element temperature of the oxygen sensor S and the oxygen concentration. The applied voltage Vpos is changed according to the element temperature and the oxygen concentration (when the element temperature is low, the applied voltage Vpos is higher than when it is high).
Is set to a high value and the oxygen concentration is high (air-fuel ratio is lean)
The applied voltage Vpos is sometimes smaller than when the air-fuel ratio is thin (rich air-fuel ratio).
Is set to a high value). Then, the change-over switch circuit 43 enters the first change-over state in response to the positive bias command from the microcomputer 70, and connects the positive side electrode of the DC power supply 41 to the input terminal 51 of the current detection circuit 50. Therefore, the current Ipos from the DC power supply 41 flows as a limiting current through the current detection circuit 50, the conductor 42a, the atmosphere side electrode 24, the solid electrolyte layer 22, the exhaust gas side electrode 23 and the conductor 41a.

Next, the current detection circuit 50 detects the inflow current Ipos, and the AD converter 60 digitally converts the detected inflow current Ipos and outputs it to the microcomputer 70. Then, the microcomputer 70 inputs the same conversion current Ipos at step 123, estimates and sets the limit current Iposa after convergence, and sets it at step 124.
At, the oxygen concentration, that is, the air-fuel ratio is determined according to the limiting current Iposa estimated based on the oxygen concentration-limiting current data shown in FIG. However, the oxygen concentration-limit current data of FIG. 5 is stored in the ROM of the microcomputer 70 as data for specifying the linear relationship between the oxygen concentration in the exhaust gas at the temperature T = T1, that is, the air-fuel ratio and the limit current of the sensor body 20. It is stored in advance. When the air-fuel ratio is determined in this way, the microcomputer 70 performs the arithmetic processing required for the fuel injection control of the EFI 90 in step 125 in consideration of the determined air-fuel ratio. Therefore, the EFI 90 controls the fuel injection to the internal combustion engine 10 based on the same calculation process.

In such a state, step 101
Based on the determination of the temperature detection cycle set in step 2, the microcomputer 70 determines “YES” in step 102, and applies negative voltage Vneg to the sensor body 20 in step 112. The required negative bias command is output to the changeover switch circuit 43 of the bias control circuit 40 (see FIG. 6). Then, the changeover switch circuit 43 responds to the negative bias command from the microcomputer 70, and enters the second changeover state, and the DC power supply 4
The negative electrode of No. 2 is connected to the input terminal 51 of the current detection circuit 50. Therefore, the current Ineg from the DC power source 42 (see FIG.
Indicates a conducting wire 41a, an exhaust gas side electrode 23 of the sensor body 20, a solid electrolyte layer 22, an atmosphere side electrode 2
4, start to flow through the conductor 42a and the current detection circuit 50.

After the arithmetic processing in step 112 as described above, in step 115, the microcomputer 70 converts the conversion current Ineg from the AD converter 60 into the current In.
By setting ega, the saturated temperature current Inegs after the convergence is estimated based on the transient phenomenon equation representing the relationship between the current Ineg and the applied voltage Vneg according to the same current Inega. In such a case, the transient equation is constructed with the negative bias timing of the sensor body 20 as the initial condition, and the microcomputer 7
0 ROM is stored in advance. Thereafter, in step 116, the microcomputer 70 determines the temperature of the sensor body 20 based on the estimated saturation current-temperature characteristic data according to the estimated saturation current Inegs. However, the above-mentioned estimated saturation current-temperature characteristic data is stored in advance in the ROM of the microcomputer 70 as data representing a direct proportional relationship between the estimated saturation current | Inegs | and the temperature of the sensor body 20.

When the temperature of the sensor main body 20 is determined in this way, the microcomputer 70 makes a determination in step 1
In step 17, an arithmetic process is performed to control the heating of the heater 26 so as to maintain the determination temperature in step 116 at the temperature T1 (see the characteristic graph L1). Therefore, the heating control circuit 80 controls the heating of the heater 26 based on the heating control calculation process of the microcomputer 70. This allows
Even if the temperature of the sensor body 20 is temporarily lowered, it quickly returns to the temperature T1.

Therefore, the microcomputer 70 is
In step 120, "YES" is determined based on the determination that the air-fuel ratio is stable and the computer program proceeds to step 121 and subsequent steps. Then, in step 121, the microcomputer 70 outputs to the bias control circuit 40 a positive bias command required to apply the positive applied voltage Vpos to the sensor body 20. Then, the bias control circuit 40 applies the applied voltage Vpos from the DC power supply 41 to the sensor body 20 as described above. This means that the applied voltage Vpos is applied to the sensor body 20 immediately after the elapse of the predetermined period t1 for detecting the element temperature. For this reason, the current Ipos from the DC power supply 41 causes the conductor 41
a, the exhaust gas side electrode 23 of the sensor body 20, the solid electrolyte layer 22, the atmosphere side electrode 24, the lead wire 42a, and the current detection circuit 50, and starts flowing as a limiting current. In other words, as shown in FIG. 6, the current I flowing through the sensor body 20
Immediately after the elapse of the predetermined period t1, neg negates and rises as shown by the solid line in the figure to become the current Ipos, and thereafter starts to decrease exponentially.

[Details of Saturation Current Estimating Step 115] Next, details of the saturation current estimating step 115 will be described with reference to FIG. 7. In this embodiment, the value Inega of one period during the increasing process of the current Ineg is measured three times to estimate the saturation current Inegs of the current Ineg. By doing so, the saturation current is accurately measured. Inegs will be able to guess.

FIG. 6 shows the voltage V applied to the sensor body 20.
FIG. 3 is a diagram showing a current I flowing in the sensor 20 at that time. In FIG. 6, the current Ineg flowing when the applied voltage V is switched from Vpos to Vneg has a peak current value I ₀ , a saturation current value (converging current value) Inegs, and a time constant T.
It changes exponentially as shown in the following formula 1.

[0025]

[Equation 1] Ineg = Inegs + (I ₀ −Inegs) e ^{−t / T} where peak current value I ₀ and saturation current value (convergence current value)
If Inegs and time constant T are unknowns,
In order to obtain Inegs, it is necessary to detect the detected current values at three points on the Ineg curve, Inega1, Inega2 and Inega3. Then, the detected current values of the three detected points, Ine
From ga1, Inega2, and Inega3, the solution thereof, that is, Inegs, is obtained based on the following simultaneous equations of equation 2.

[0026]

[Number 2] _{Inega1 = Inegs + (I 0 -Inegs} ) e -t1a / T Inega2 = Inegs + (I 0 -Inegs) e -t1b / T Inega3 = Inegs + (I 0 -Inegs) e -t1c / T where, Inega1 , Inega2 and Inega3 are t1a and t1b after switching the applied voltage V from Vpos to Vneg, respectively.
And the current value Ineg after t1c time. For example,
To simplify the calculation, t1a = 0, t1b = t1c-t1b
Then, by substituting these values into Equation 2 to obtain Inegs, Inegs is obtained as in the following Equation 3.

[0027]

[Equation 3] Inegs = (Inega2 ² −Inega3 · Inega1) /
(2Inega2-Inega3-Inega1) Next, the operation of this embodiment will be described with reference to the flowchart of FIG.

After the arithmetic processing in step 112, the microcomputer 70 waits for a predetermined time t1a in step 113a. When the time waiting in step 113a is completed, the microcomputer 70 detects the current value in step 114a, and the microcomputer 70 detects the current value.
The converted current Ineg from 0 is set as the current Inega1. After that, the microcomputer 70 waits for a predetermined time t1b in step 113b. Step 113
When the time waiting in b is completed, the microcomputer 70 detects the current value in step 114b,
The converted current Ineg from the -D converter 60 is set as the current Inega2. Then, the microcomputer 70 waits for a predetermined time t1c in step 113c.

When the time waiting in step 113c is completed, the microcomputer 70 causes the step 114 to proceed.
At c, the current value is detected, and the converted current Ineg from the AD converter 60 is set as the current Inega3. Then, in step 115a, the saturation current I is calculated based on the simultaneous equations of Equation 2.
Calculate negs. In such a case, the simultaneous equations of the above equation 2 are configured with the negative bias timing of the sensor main body 20 as an initial condition, and are stored in advance in the ROM of the microcomputer 70. After that, the microcomputer 70
However, in step 116, the temperature of the sensor body 20 is determined based on the saturation current-temperature characteristic data according to the saturation current Inegs obtained in step 115a.

As described above, in the embodiment of FIG. 7, the value Inega of one period during the increasing process of the current Ineg is 3
This is because the saturation current Inegs of the current Ineg is estimated by performing the measurement once, and by doing so, the saturation current Inegs can be accurately estimated. further,
When the temperature of the sensor body 20 is determined, this time is set at a certain time (when the predetermined time t1 has elapsed) before the current Ineg flowing through the sensor body 20 is negatively biased after being negatively biased by the applied voltage Vneg. Current Ineg
Since the saturation current Inegs is estimated with a, and the temperature of the sensor body 20 is determined by this, the subsequent air-fuel ratio determinable timing can be realized abruptly.

[Details 1 of Limit Current Estimation Step 123]
Next, detail 1 of the limiting current estimation step 123 will be described with reference to FIG. In this embodiment, the value Ipos at one time during the decrease process of the current Ipos is measured three times to estimate the limiting current Iposa of the current Ipos. By doing so, the limiting current can be accurately obtained. Iposa can be guessed.

In FIG. 6, the applied voltage V is changed from Vneg to Vp.
The current Ipos flowing when switched to os changes exponentially as shown in the following Equation 4, which is the peak current value I ₀ ′, the saturation current value (convergence limit current value) Iposa, and the time constant T.

[0033]

[Equation 4] Ipos = Iposa + (I ₀ '−Iposa) e ^{−t / T} Here, when the peak current value I ₀ ′, the saturation current value (convergence limit current value) Iposa, and the time constant T are unknowns, respectively. , Iposa, it is necessary to detect the detected current values at three points on the Ipos curve, Ipos1, Ipos2, and Ipos3. Then, the detected current values of the three detected points, Ipo
The solution, that is, Iposa, is obtained from s1, Ipos2, and Ipos3 on the basis of the following simultaneous equations of equation 5.

[0034]

[Number 5] _{Ipos1 = Iposa + (I 0 '} -Iposa) e -t2a / T Ipos2 = Iposa + (I 0' -Iposa) e -t2b / T Ipos3 = Iposa + (I 0 '-Iposa) e -t2c / T here Then, Ipos1, Ipos2, and Ipos3 are values of the current value Ipos after t2a, t2b, and t2c time after switching the applied voltage V from Vneg to Vpos, respectively.

For example, to simplify the calculation, t2a =
Assuming that 0 and t2b = t2c-t2b, these values are substituted into Equation 5 to obtain Iposa, and Iposa is obtained as in Equation 6 below.

[0036]

[Equation 6] Iposa = (Ipos2 ² −Ipos3 · Ipos1) / (2
Ipos2-Ipos3-Ipos1) Next, the operation of this embodiment will be described with reference to the flowchart of FIG. After the calculation processing in step 121, the microcomputer 70 applies a negative bias voltage to the sensor body 20 in step 122a and then detects a predetermined time t11 (negative bias time t1 + positive bias voltage and then detects). The current is set to a predetermined time sufficient for the current to converge) or a predetermined time t11-t1 has elapsed after the positive bias voltage was applied, and "YES" is determined.
In case of, step 1
At 22b, the conversion current Ipos from the AD converter 60 is set as it is as the limiting current Iposa. Step 122
If “a” is “NO”, the process proceeds to step 123a and thereafter, and the limiting current after the convergence is estimated from the detected current during the convergence.

That is, in step 123a, the predetermined time t
Wait 2 hours. When the time waiting in step 123a is completed, the microcomputer 70 detects the current value in step 123b and the A / D converter 60 is detected.
The converted current Ipos from is set as the current Ipos1. After that, the microcomputer 70 waits for a predetermined time t2b in step 123c. Step 123
When the time waiting in c is completed, the microcomputer 70 detects the current value in step 123d, and A
The converted current Ipos from the −D converter 60 is set as the current Ipos2. Then, the microcomputer 70 waits for a predetermined time t2c in step 123e.

When the time waiting in step 123e ends, the microcomputer 70 causes the step
At f, the current value is detected, and the converted current Ipos from the AD converter 60 is set as the current Ipos3. Then, in step 123g, the saturation limit current Iposa is calculated based on the simultaneous equations of Equation 5. In such a case, the simultaneous equations of the equation 5 are configured with the positive bias timing of the sensor main body 20 as an initial condition, and are stored in advance in the ROM of the microcomputer 70. Then, in step 122c, the saturation limit current Iposa estimated in step 123g is set as the limit current Iposa. Then, in step 124, the microcomputer 70 responds to the limiting current Iposa estimated based on the oxygen concentration-limiting current data shown in FIG. 5 according to the limiting current Iposa set in step 122b or step 122c. Oxygen concentration, that is, the air-fuel ratio is determined.

As described above, in the embodiment of FIG. 8, the value of the current Ipos at one time during the decreasing process is measured three times to estimate the limiting current Iposa of the current Ipos. By doing so, the limiting current Iposa can be accurately estimated. Further, after the positive bias voltage is applied to the sensor body 20 and after a time sufficient for the detected current to converge, the converged detected current is directly used as the limit current and repeated while the positive bias voltage is applied. Since the set value is set and the oxygen concentration, that is, the air-fuel ratio is continuously determined, after the detection current converges, the oxygen concentration can be accurately determined repeatedly according to the change in the oxygen concentration.

Further, immediately after the elapse of the predetermined time t1, the sensor body 20 is positively biased by the applied voltage Vpos, and a predetermined time (t11-t1) which is a convergence time at which the current Ipos flowing through the sensor body 20 is completely reduced by this positive bias. Since the air-fuel ratio is determined based on the currents Ipos1 to Ipos3 up to this time at the time t33 in the middle of convergence before the elapse of time, the air-fuel ratio can be determined earlier than in the conventional case. In such a case, as described above, the rising peak level of the current Ipos in the present embodiment is maintained relatively lower than the rising peak level of the current Ipos in the conventional case, and the current Ipos in the case of the present embodiment. Is rapidly decreased as compared with the current Ipos in the conventional case, so that the air-fuel ratio can be judged much earlier.

In the first embodiment, the sensor body 20 is applied with the applied voltage Vpos immediately after the elapse of the predetermined time t1.
However, the present invention is not limited to this, and the applied voltage Vpos is not limited to this but after the predetermined time t1 has elapsed and before the predetermined time t11 has elapsed.
Therefore, the sensor body 20 may be positively biased. [Detail 2 of Limit Current Estimation Step 123] Next, detail 2 of the limit current estimation step 123 will be described with reference to FIG. This embodiment differs from the embodiment of FIG. 8 in step 12
2a is omitted, and instead step 122d is added after step 123g, and when it is determined in this step 122d that the previous value and the current value of the detected current Ipos are substantially equal, it is assumed that the detected current has converged to step 122b.
Then, the current detection current after convergence is set as it is as the limit current, and the detection current Ipos is set in step 122d.
When it is determined that the previous value and the current value of are not equal to each other, it is determined that the detected current is in the process of converging, the process proceeds to step 122c, and the value estimated in step 123g is set as the limiting current. Also in this embodiment, the same effect as in FIG. 8 can be obtained.

[Temperature detection cycle variable setting step 101
Details of Next] The details of the temperature detection cycle variable setting step 101 will be described with reference to FIG. First, in step 151, after the internal combustion engine is started, it is determined whether a predetermined time ta in which the engine temperature stabilizes (or a predetermined value or more in which the element temperature stabilizes) has passed. If the predetermined time ta has not elapsed,
After proceeding to step 152 and detecting the variation amount ΔT of the element temperature of the oxygen sensor S determined in step 116 of FIG. 4,
As shown in the map 1 of FIG. 11, the routine proceeds to step 153, where the element temperature change amount ΔT which is one of the engine warm-up states.
The detection period corresponding to the element temperature change amount ΔT is determined from the map 1 which is stored in advance in the ROM of the microcomputer 70. This map 1 is set so that the detection cycle becomes shorter as the element temperature change amount ΔT becomes larger. If the predetermined time ta has elapsed after the start of the internal combustion engine, the routine proceeds to step 154, where the change amount ΔQ of the intake air amount Q of the internal combustion engine is detected, and then step 1
As shown in the map 2 of FIG. 11, proceeding to 55, the change amount ΔQ of the intake air amount Q which is one of the engine operating state changes.
The detection period corresponding to the change amount ΔQ of the intake air amount Q is determined from the map 2 which is stored in advance in the ROM of the microcomputer 70. The map 2 is set to have a longer detection cycle than the map 1, and the map 2 is set to have a longer detection cycle as the change amount ΔQ of the intake air amount Q increases.

As a result, as shown in FIG. 12, the element temperature detection period corresponding to the element temperature change amount ΔT in the map 1 is prioritized during the predetermined time ta after the start when the element temperature change amount ΔT is relatively large. As the element temperature change amount ΔT increases, the element temperature detection frequency increases, and the element temperature can be detected quickly by following the element temperature change. Also, a predetermined time ta after the start
Since the element temperature change amount ΔT is relatively small after the elapse, the element temperature detection period corresponding to the change amount of the intake air amount Q in the map 2 is prioritized, and the element temperature detection frequency decreases when the air-fuel ratio changes a lot. Therefore, it becomes possible to detect the air-fuel ratio by quickly following changes in the air-fuel ratio.

Here, in the map 1 of FIG. 11, as the warm-up state, the change amount of the cooling water temperature of the internal combustion engine may be used instead of the element temperature change amount ΔT. In this case, in step 152 of FIG. 11, the element temperature change amount Δ
It goes without saying that instead of T, the amount of change in the cooling water temperature of the internal combustion engine is detected. Further, in the map 2 of FIG. 11, as the operation state change amount, instead of the change amount of the intake air amount Q, the change amount ΔA / F of the air-fuel ratio A / F, the change amount ΔPm of the intake pipe pressure Pm, the engine rotation speed Change amount ΔN of several Ne
Any one of e, the change amount ΔTAU of the fuel injection amount TAU, the change amount of the throttle opening, and the change amount of the vehicle speed may be used. In this case, step 154 of FIG.
In place of the change in the intake air amount Q, the air-fuel ratio A
/ F change amount ΔA / F, intake pipe pressure Pm change amount ΔP
m, change amount ΔNe of engine speed Ne, fuel injection amount TAU
It is needless to say that any one of the change amount ΔTAU, the change amount of the throttle opening, and the change amount of the vehicle speed is detected.

Other Embodiments In step 115a of FIG. 7, the saturation temperature current Inegs is obtained from the solution of equation 2, and in step 123g of FIGS. 8 and 9, the limiting current Iposa is obtained from the solution of equation 5. However, the saturation temperature current Inegs and the limit current Iposa may be obtained from the ROM map from the absolute value change amount of the detected current. Here, an example of obtaining the limit current Iposa by a map will be described. First, (Ipos1−Ipos2) = ΔI1 and (I
pos2-Ipos3) = ΔI2, and from this result | ΔI1
-ΔI2 | or (| ΔI1 -ΔI2 |) / | ΔI1 |
The absolute value change amount (change rate) of is calculated as shown in FIG. 13, and | ΔI 1 −Δ
I2 | or (| ΔI1 −ΔI2 |) / | ΔI1 | and I
The deviation i is determined based on the deviation i from pos3 to Iposa (see FIG. 6) characteristic (the deviation i is set to increase as the amount of change in absolute value increases), and the limit is calculated by calculating Iposa = Ipos3−i. The current Iposa is obtained. As a result, even if the slope of convergence of the detected current is not determined to be one, the limiting current Iposa can be accurately estimated without performing a complicated mathematical operation. Here, when the slope of convergence of the detected current is determined to be one, (Ipos1−Ipos2) = ΔI1
Using only the absolute value change amount of
It is also possible to obtain the deviation i based on the characteristic of the deviation i from Ipos2 to Iposa with respect to I1 and to obtain the limiting current Iposa by calculating Iposa = Ipos2-i. Further, in the above-described embodiment, the limit current Iposa is repeatedly detected until the next temperature detection timing even after the detected current converges. However, a negative bias and a positive bias are applied at a constant cycle of t33 in FIG. Alternating voltage is applied to the sensor body 20 so that the limit current Ipo
It is also possible to estimate sa and not detect the limiting current Iposa after convergence. In this case,
122a and 122b of FIG. 9 and step 122 of FIG.
Although d and 122b can be omitted, if this is done, the limiting current Iposa is not detected at all due to the detected current after convergence,
Since the limit current Iposa is only estimated by the detected current in the middle of convergence, even if there is a deviation between the estimated limit current and the actual limit current after convergence, it cannot be considered.

In carrying out the present invention, the determination of the oxygen concentration in the exhaust gas of the internal combustion engine 10 is not limited to
The present invention may be applied to the determination of the oxygen concentration in each gas.

[0047]

As described above, since the limiting current after the convergence is predicted by the detection current in the middle of the convergence after the positive voltage is applied and the oxygen concentration is determined based on the predicted limiting current, the positive voltage is applied. There is an excellent effect that the time from when the oxygen concentration is determined to the determination of the oxygen concentration can be shortened, and the time when the oxygen concentration cannot be determined can be significantly reduced.

[Brief description of drawings]

FIG. 1 is a diagram corresponding to the description in the claims.

FIG. 2 is a block circuit diagram showing a first embodiment of the present invention.

3A is an enlarged cross-sectional view of a sensor body of the oxygen sensor of FIG. 2, and FIG. 3B is a graph showing a limiting current-voltage characteristic of the oxygen sensor with temperature as a parameter.

4 is a flowchart showing the operation of the microcomputer of FIG.

FIG. 5 is a graph showing the relationship between the oxygen concentration of an oxygen sensor and the limiting current.

FIG. 6 is a time chart showing waveforms of a voltage applied to the sensor main body and a current flowing through the sensor main body when a negative bias and a positive bias are applied.

FIG. 7 is a flowchart showing details of a saturation current estimation step in the above embodiment.

FIG. 8 is a flowchart showing a detail 1 of a limiting current estimation step in the above embodiment.

FIG. 9 is a flowchart showing the detail 2 of the limiting current estimation step in the above embodiment.

FIG. 10 is a flowchart showing details of a temperature detection cycle variable setting step in the above embodiment.

FIG. 11 is a diagram showing a detection cycle map 1 for a warm-up state and a detection cycle map 2 for an operating state.

12 is a time chart used to explain the operation of FIG.

FIG. 13 is a diagram showing a limiting current deviation i map with respect to an absolute value change amount of a detected current.

[Explanation of symbols]

 S Oxygen sensor 20 Sensor body 40 Bias control circuit 50 Current detection circuit 70 Microcomputer

Claims (7)

[Claims] 1. A limiting current type oxygen sensor, voltage applying means for applying a positive voltage to the oxygen sensor and switching to a negative voltage for a predetermined period, and detecting a current flowing through the oxygen sensor by applying the voltage. Current detection means, impedance detection means for detecting a DC impedance of the oxygen sensor based on the detected current when the negative voltage is applied to the oxygen sensor for a predetermined period, and the positive voltage is applied to the oxygen sensor. An oxygen concentration determination device comprising: a limiting current predicting unit that predicts a limiting current after convergence based on the detected current that is being converged afterward; and an oxygen concentration determining unit that determines an oxygen concentration based on the predicted limiting current. 2. A limit current detection unit that detects the detected current after convergence as a limit current after applying the positive voltage to the oxygen sensor, and limits the detected current after convergence by the limit current detection unit. The oxygen concentration determination device according to claim 1, wherein after being detected as a current, the oxygen concentration determination unit determines the oxygen concentration based on the detected current after the convergence instead of the predicted limit current. 3. The limiting current detecting means sets the detected current after a lapse of a predetermined time sufficient for the detected current to converge after applying the positive voltage to the oxygen sensor, as the limited current after the convergence. The oxygen concentration determination device according to claim 2, further comprising means for repeatedly detecting while the positive voltage is being applied. 4. The limiting current detecting means detects the detected current when the detected current is substantially unchanged after the voltage is applied to the oxygen sensor, as the limited current after the convergence. The oxygen concentration determination device according to claim 2, further comprising: 5. A temperature / current predicting unit that predicts a current flowing through the oxygen sensor after convergence by the detected current that is being converged after the negative voltage is applied to the oxygen sensor, and the impedance detecting unit includes the impedance detecting unit. The oxygen concentration determination device according to claim 1, further comprising means for detecting the DC impedance based on the current predicted by the temperature / current prediction means. 6. A heater for heating the oxygen sensor, and a heating control unit for controlling heating of the heater based on the DC impedance detected by the impedance detection unit. The oxygen concentration determination device described in 1. 7. The oxygen concentration according to any one of claims 1 to 6, further comprising: an applied voltage varying unit that varies a voltage applied to the oxygen sensor based on the DC impedance detected by the impedance detecting unit. Judgment device.