JP2020020676A - Temperature control device of air-fuel ratio sensor - Google Patents

Abstract

To provide a temperature control device for preventing the temperature of a detection element from greatly deviating from a target temperature, during a fine energization duty control for controlling the energization duty of a heater of an air-fuel ratio sensor so that the temperature of the detection element of the air-fuel ratio sensor becomes a target temperature lower than an activation temperature.SOLUTION: A temperature control device includes: an exhaust temperature sensor; an air-fuel ratio sensor having a detection element and a heater; and an ECU (electronic control device). When the ECU executes "fine energization duty control" of controlling the energization duty of the heater so that the temperature (=element temperature Ta/f) of the detection element becomes a fine energization target temperature lower than the target activation temperature, an element temperature Ta/f is calculated using a heat model formula. This heat model formula defines the relationship among a heat-receiving amount Q received by the detection element from the exhaust gas, a heat transfer coefficient α between the exhaust gas and the detection element, an exhaust temperature Tgas, and a detection element temperature Ta/f.SELECTED DRAWING: Figure 8

Description

  The present disclosure relates to a temperature control device for an air-fuel ratio sensor.

  2. Description of the Related Art Conventionally, an air-fuel ratio sensor that detects an air-fuel ratio (A / F) in exhaust gas is often arranged in an exhaust passage of an engine. In order for the air-fuel ratio sensor to accurately detect the air-fuel ratio, it is desirable to maintain the detection element of the air-fuel ratio sensor at an appropriate temperature (active temperature).

  Japanese Patent Laying-Open No. 2015-215212 (Patent Document 1) discloses a temperature control device for an air-fuel ratio sensor including a detection element and a heater. In view of the fact that there is a predetermined correlation between the temperature of the detection element of the air-fuel ratio sensor and the electric resistance (impedance), the temperature control device estimates the temperature of the detection element from the electric resistance of the detection element and estimates the temperature. The amount of power supplied to the heater of the air-fuel ratio sensor is feedback-controlled so that the temperature of the detected element becomes the activation temperature.

  Further, in the temperature control device disclosed in Patent Document 1, the temperature of the detection element of the air-fuel ratio sensor is low for a predetermined period after the engine is started, and the temperature of the detection element is detected in a state where moisture is attached to the surface of the detection element. The temperature of the detection element is lower than the activation temperature based on the exhaust temperature detected by the exhaust temperature sensor in consideration of the possibility that the detection element may be damaged when the element rapidly rises from a low temperature to a high temperature. The energization duty of the heater (the ratio between the energization time and the non-energization time within one cycle when the energization and non-energization of the heater are alternately and periodically repeated) is controlled so as to reach the target temperature. Thereby, it is avoided that the detection element rapidly rises from a low temperature to a high temperature, so that the detection element is prevented from being cracked by water.

JP 2015-215212 A

  In the case where the temperature of the detection element of the air-fuel ratio sensor is low, simply controlling the energization duty of the heater based on the temperature of the exhaust gas as in Patent Literature 1 causes the exhaust gas in a transient state where the temperature of the exhaust gas fluctuates rapidly. There is a concern that the energization duty cannot follow the fluctuation of the temperature, and the temperature of the detection element greatly deviates from the target temperature. If the temperature of the detection element is much higher than the target temperature, the above-mentioned water-break may occur. In addition, when the temperature of the detection element is much lower than the target temperature, the moisture on the surface of the detection element remains without evaporating and freezes (the detection element is damaged by the freezing of the moisture on the surface of the detection element). Phenomenon).

  The present disclosure has been made in order to solve the above-described problem, and has as its object to turn on the heater of the air-fuel ratio sensor so that the temperature of the detection element of the air-fuel ratio sensor becomes a target temperature lower than the activation temperature. In controlling the duty, it is to prevent the temperature of the detection element from largely deviating from the target temperature.

  (1) A temperature control device according to an embodiment of the present disclosure includes an exhaust gas temperature sensor that detects a temperature of an exhaust gas of an engine, an air-fuel ratio sensor including a detection element that detects an air-fuel ratio of the exhaust gas, and a heater that heats the detection element. A control device for controlling energization of the heater of the sensor. When the detected value of the exhaust gas temperature sensor is lower than the predetermined temperature, the control device calculates the temperature of the detected device using a predetermined model formula representing a heat balance between the exhaust gas and the detected device, and calculates the temperature of the detected device. The energization duty of the heater is controlled so that the temperature becomes a target temperature lower than the activation temperature of the detection element. The model formula is a formula that defines the relationship among the amount of heat received by the detection element from the exhaust gas, the heat transfer coefficient between the exhaust gas and the detection element, the temperature of the exhaust gas, and the temperature of the detection element.

  According to the above system, when the detection value of the exhaust gas temperature sensor is lower than the predetermined temperature, the temperature of the detection device exposed to the exhaust gas is also low, and there is a high possibility that moisture is attached to the surface of the detection device (that is, the activation temperature). In view of the fact that the temperature of the detection element is likely to be increased, the energization duty of the heater is controlled so that the temperature of the detection element becomes the target temperature lower than the activation temperature. At this time, the temperature of the detection element is calculated using a model equation representing a heat balance between the exhaust gas and the detection element. In the model formula, not only the temperature of the exhaust gas, but also the amount of heat received by the detecting element from the exhaust gas and the heat transfer coefficient between the exhaust gas and the detecting element are considered. Therefore, the temperature of the detection element can be made to quickly follow the target temperature as compared with a case where only the temperature of the exhaust gas is simply considered. Thus, even in a transient state in which the temperature of the exhaust gas fluctuates rapidly, it is possible to prevent the temperature of the detection element from largely deviating from the target temperature.

  (2) In one embodiment, the temperature control device includes an air flow meter that detects an intake air amount of the engine, an intake air amount, an exhaust gas temperature, and a fitting constant that is a ratio between a heat reception amount and a heat transfer coefficient. A storage unit that stores the correspondence in advance. The control device refers to the correspondence relation stored in the storage unit, calculates an adaptation constant corresponding to the detection value of the air flow meter and the detection value of the exhaust gas temperature sensor, and calculates the calculated adaptation constant and the detection value of the exhaust gas temperature sensor. The temperature of the detection element is calculated using the value and the model formula.

  The amount of heat received by the detection element from the exhaust gas can vary depending not only on the temperature of the exhaust gas but also on the flow rate of the exhaust gas. In view of this point, in the above embodiment, the adaptation constant, which is the ratio between the amount of heat received and the heat transfer coefficient, is not only the value detected by the exhaust gas temperature sensor but also the value detected by the air flow meter (a value corresponding to the exhaust gas flow rate). It is calculated using The temperature of the detection element is calculated by using the thus calculated adaptation constant. Therefore, the temperature of the detection element can be accurately calculated.

  (3) In one embodiment, when the detected value of the exhaust gas temperature sensor exceeds a predetermined temperature, the control device calculates the temperature of the detection element from the impedance of the detection element, and sets the heater so that the temperature of the detection element becomes the activation temperature. Is controlled.

  In the above embodiment, when the detection value of the exhaust gas temperature sensor exceeds the predetermined temperature, the temperature of the detection device exposed to the exhaust gas is also high, and the possibility that moisture is attached to the surface of the detection device is low (that is, the temperature rises to the activation temperature). In view of this, it is difficult for water breaks to occur), so that the amount of electricity supplied to the heater is controlled so that the temperature of the detection element becomes the activation temperature. Thus, the temperature of the detection element is maintained at the activation temperature, so that the air-fuel ratio sensor can accurately detect the air-fuel ratio.

  According to the present disclosure, when controlling the energization duty of the heater of the air-fuel ratio sensor so that the temperature of the detection element of the air-fuel ratio sensor becomes the target temperature lower than the activation temperature, the temperature of the detection element greatly deviates from the target temperature. Can be prevented.

It is a figure showing an example of the whole composition of an engine system. It is a figure showing an example of an internal configuration of an air-fuel ratio sensor. FIG. 10 is a diagram illustrating an example (comparative example) of a relationship between the exhaust gas temperature Tgas and the element temperature Ta / f when the heater is slightly energized in duty control using a one-dimensional map of the exhaust gas temperature in a steady state. FIG. 9 is a diagram illustrating an example (comparative example) of changes in the exhaust gas temperature Tgas and the element temperature Ta / f when the heater is slightly energized in duty control using a one-dimensional map of the exhaust gas temperature in a transient state. It is a figure showing the image of an adaptation constant map. FIG. 4 is a diagram illustrating an example of a map used for calculating a correction amount ΔD of a heater energization duty. FIG. 9 is a diagram illustrating an example of changes in the exhaust gas temperature Tgas and the element temperature Ta / f when the minute energization duty control of the heater is performed using a thermal model equation in a transient state. 4 is a flowchart illustrating an outline of processing by an ECU.

  Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the drawings, the same or corresponding portions have the same reference characters allotted, and description thereof will not be repeated.

<Structure>
FIG. 1 is a diagram illustrating an example of an overall configuration of an engine system including the temperature control device according to the present embodiment. The engine system includes an engine 10, an air flow meter 40, an engine rotation speed sensor 50, an exhaust gas temperature sensor 60, an air-fuel ratio sensor 70, and an electronic control unit (ECU) 100. Including.

  The engine 10 is a general diesel engine. Note that the engine 10 may be a gasoline engine. Each cylinder of the engine 10 is provided with a fuel injection valve 20. Each fuel injection valve 20 is supplied with fuel from a fuel tank by a fuel pump (not shown). Each fuel injection valve 20 operates (opens) according to a control signal from the ECU 100, and injects fuel into each cylinder.

  Engine rotation speed sensor 50 detects a rotation speed of a crankshaft of engine 10 (hereinafter, also referred to as “engine rotation speed Ne”), and outputs a detection result to ECU 100.

  An intake passage 11 and an exhaust passage 12 are connected to the engine 10. The air flow meter 40 is provided in the intake passage 11. The air flow meter 40 detects an intake air amount Ga flowing through the intake passage 11 and outputs a detection result to the ECU 100.

  The exhaust gas temperature sensor 60 is provided in the exhaust passage 12. The exhaust gas temperature sensor 60 detects the temperature of the exhaust gas flowing through the exhaust passage 12 (hereinafter, also referred to as “exhaust gas temperature Tgas”), and outputs the detection result to the ECU 100.

  The air-fuel ratio sensor 70 is provided at a portion of the exhaust passage 12 downstream of the exhaust gas temperature sensor 60 at a predetermined distance from the exhaust gas temperature sensor 60. The air-fuel ratio sensor 70 detects an air-fuel ratio (ratio of fuel amount to air amount) A / F in the exhaust gas of the engine 10 and outputs a detection result to the ECU 100.

  FIG. 2 is a diagram showing an example of the internal configuration of the air-fuel ratio sensor 70. The air-fuel ratio sensor 70 includes a detection element (sensor element) 71 and a heater 72. The detection element 71 is an element for detecting the air-fuel ratio A / F. The heater 72 is a heating wire embedded inside the detection element 71. The heater 72 generates heat when a current flows therein to heat the detection element 71. The energizing timing and energizing amount of the heater 72 are controlled by the ECU 100.

  Returning to FIG. 1, the ECU 100 includes a CPU (Central Processing Unit) (not shown) and a memory 101. The ECU 100 executes various controls related to the engine 10 based on information stored in the memory 101, information from various sensors, and the like.

<Normal energization control of the heater of the air-fuel ratio sensor>
The detection element 71 of the air-fuel ratio sensor 70 can accurately detect the air-fuel ratio A / F when the temperature of the detection element 71 (hereinafter also referred to as “element temperature Ta / f”) is a predetermined activation temperature. It has the property of being able to. Therefore, in order for the detection element 71 to accurately detect the air-fuel ratio A / F, it is desirable that the element temperature Ta / f be the activation temperature or a value close to the activation temperature.

  Further, when the element temperature Ta / f is included in the high temperature region including the activation temperature, the element temperature Ta / f has a characteristic that it is correlated with the electric resistance of the detection element 71 (hereinafter, also referred to as “element impedance Z”). .

  In view of this, the ECU 100 calculates the element impedance Z, calculates the element temperature Ta / f from the calculated element impedance Z, and sets the heater 72 so that the calculated element temperature Ta / f becomes the target activation temperature Ttag2. The feedback control of the amount of current is performed. Target activation temperature Ttag2 is set to a value corresponding to the activation temperature of detection element 71.

  The element impedance Z can be calculated from the relationship between the voltage and the current when a voltage is applied to the detection element 71. For example, the ECU 100 applies a voltage to an electrode (not shown) provided on the detection element 71, and measures a change amount ΔI of a current of the detection element 71 when the voltage is changed by a predetermined change amount ΔV. , A value obtained by dividing the voltage change amount ΔV by the current change amount ΔI (= ΔV / ΔI) can be calculated as the element impedance Z.

  As a method of calculating the element temperature Ta / f from the element impedance Z, a map (hereinafter, also referred to as a “Z-Ta / f map”) showing the correspondence between the element impedance Z and the element temperature Ta / f is previously tested. A method of referring to the Z-Ta / f map can be adopted.

  Hereinafter, the process of performing feedback control of the amount of current supplied to the heater 72 so that the element temperature Ta / f calculated from the element impedance Z becomes the target activation temperature Ttag2 is also referred to as “normal energization control”.

<Slight energization duty control of heater of air-fuel ratio sensor>
As described above, in order for the detection element 71 to accurately detect the air-fuel ratio A / F, it is desirable that the element temperature Ta / f be the activation temperature or a value close to the activation temperature. Therefore, when element temperature Ta / f is included in a low-temperature area lower than a high-temperature area including the activation temperature, it is desirable to raise element temperature Ta / f early toward the activation temperature (target activation temperature Ttag2). . However, if the element temperature Ta / f is rapidly increased to the target activation temperature Ttag2 by the above-described normal energization control during a cold time when the element temperature Ta / f is included in the low temperature region, the following problem may occur.

  First, when cold, moisture (condensed water) may adhere to the surface of the detection element 71. If the element temperature Ta / f is rapidly raised to the activation temperature in a state where moisture is attached to the surface of the detection element 71, a phenomenon that the detection element 71 is damaged (hereinafter, also referred to as “water crack”) may occur. . Therefore, in the cold state, it is desirable to increase the element temperature Ta / f, but to suppress the element temperature Ta / f to a temperature at which water-breaking does not occur (a temperature lower than the activation temperature).

  On the other hand, if the element temperature Ta / f is too low, the surface of the detection element 71 may not be sufficiently dried and water may remain. When the engine is stopped in a state where moisture on the surface of the detection element 71 remains, the phenomenon that the moisture remaining on the surface of the detection element 71 freezes to break the detection element 71 (hereinafter also referred to as “freezing crack”) ) Can occur. Therefore, in the cold state, it is desirable to set the element temperature Ta / f to a temperature at which water-decomposition cracks do not occur and moisture on the surface of the detection element 71 can evaporate (freezing cracks do not occur).

  Further, in a cold state in which the element temperature Ta / f is included in the low temperature region, the correlation between the element temperature Ta / f and the element impedance Z is broken, so that the element temperature Ta / f is calculated from the element impedance Z. There may be a problem that the calculation accuracy of the element temperature Ta / f is reduced.

  In view of the above, when the exhaust gas temperature Tgas is lower than the threshold value Tth, the ECU 100 determines that the temperature of the detection element 71 exposed to the exhaust gas is low and the element temperature Ta / f is in a low temperature range. Therefore, "small energization duty control" is executed instead of "normal energization control" described above.

  Here, the "slight energization duty control" means that the energization duty DUTY of the heater 72 is set so that the element temperature Ta / f becomes the slight energization target temperature Ttag1 capable of avoiding water-split cracks and freezing cracks. Is a process of controlling The slight energization target temperature Ttag1 is set to a value lower than the target activation temperature Ttag2. The energization duty DUTY is the ratio of the energization time and the non-energization time within one cycle when energization and non-energization of the heater 72 are alternately and periodically repeated.

  Since the slight energization duty control is performed during a cold time when the correlation between the element temperature Ta / f and the element impedance Z is broken, the element temperature Ta / f should be calculated from the element impedance Z during the slight energization duty control. Can not. Therefore, during the fine energization duty control, there is a problem how to grasp the element temperature Ta / f and reflect the element temperature Ta / f on the energization duty DUTY.

  For example, in view of the fact that the element temperature Ta / f correlates with the exhaust gas temperature Tgas, a one-dimensional map of the exhaust gas temperature Tgas is simply obtained during the fine energization duty control (map defining the correspondence between the exhaust gas temperature Tgas and the energization duty DUTY). May be used to control the energization duty DUTY. However, this method is effective in a steady state in which the exhaust gas temperature Tgas does not fluctuate so rapidly, but in a transient state in which the exhaust gas temperature Tgas fluctuates rapidly, there is a case where the temperature greatly deviates from the slight energization target temperature Ttag1. is there.

  FIG. 3 is a diagram illustrating an example of the relationship between the exhaust gas temperature Tgas and the element temperature Ta / f when the minute energization duty control of the heater 72 is performed using a one-dimensional map of the exhaust gas temperature Tgas in a steady state. An example of a one-dimensional map of the exhaust gas temperature Tgas is shown in the lower part of FIG. The upper part of FIG. 3 shows a relationship between the exhaust gas temperature Tgas and the element temperature Ta / f in a steady state. FIG. 3 shows a comparative example for the present disclosure.

  In the one-dimensional map shown in the lower part of FIG. 3, the lower the exhaust gas temperature Tgas, the higher the energization duty DUTY. As a result, even when the exhaust gas temperature Tgas is low, the element temperature Ta / f is quickly raised to the slight energization target temperature Ttag. As a result, in the steady state, as shown in the upper part of FIG. 3, the element temperature Ta / f is maintained at the slight energization target temperature Ttag regardless of whether the exhaust gas temperature Tgas is low or high.

  However, in a transient state in which the exhaust gas temperature Tgas fluctuates rapidly, the element temperature Ta / f cannot fully follow the fluctuation of the exhaust gas temperature Tgas, and the element temperature Ta / f greatly deviates from the slight energization target temperature Ttag1. There is.

  FIG. 4 is a diagram illustrating an example of changes in the exhaust gas temperature Tgas and the element temperature Ta / f when the fine energization duty control of the heater 72 is performed using a one-dimensional map of the exhaust gas temperature Tgas in a transient state. In FIG. 4, the horizontal axis represents time, and the vertical axis represents temperature. In FIG. 4, the solid line indicates the exhaust gas temperature Tgas, and the broken line indicates the element temperature Ta / f. FIG. 4 also shows a comparative example of the present disclosure, similarly to FIG.

  When the slight energization duty control is performed using the one-dimensional map of the exhaust gas temperature Tgas in a transient state in which the exhaust gas temperature Tgas rapidly increases and decreases, as shown in FIG. 4, the element temperature Ta / f changes with the fluctuation of the exhaust gas temperature Tgas. Without being able to follow, overshoot occurs in which the element temperature Ta / f exceeds the upper limit of the target temperature control range including the slight energization target temperature Ttag1, or undershoot occurs in which the element temperature Ta / f falls below the lower limit of the target temperature control range. Or

  If the element temperature Ta / f exceeds the upper limit of the target temperature control range, the element temperature Ta / f may exceed a limit temperature at which water-penetrating cracks can be avoided, and there is a concern that water-penetrating cracks may occur. Further, when the element temperature Ta / f is lower than the upper limit of the target temperature control range, there is a concern that moisture on the surface of the detection element 71 remains without evaporating, and then freezing cracks may occur.

  In order to solve the above problem, the ECU 100 according to the present embodiment does not use a one-dimensional map of the exhaust gas temperature Tgas in the slight energization duty control, but expresses a heat balance between the exhaust gas and the detection element 71. The element temperature Ta / f is calculated using a thermal model equation. Then, ECU 100 controls energization duty DUTY of heater 72 such that element temperature Ta / f calculated using the thermal model equation becomes slight energization target temperature Ttag1. Hereinafter, this point will be described in detail.

The heat balance between the exhaust and the detection element 71 can be defined as in the following equation (1).
Q = α × S × (Tgas−Ta / f) (1)
In the equation (1), “Q” represents the amount of heat received from the exhaust gas by the detection element 71 (unit: W), and “α” represents the heat transfer coefficient between the exhaust gas and the detection element 71 (unit: W / (m ² K)) indicates, "S" is the surface area of the portion touching the exhaust gas in the detection element 71 (unit: it indicates ^{m 2).} The units of the exhaust gas temperature Tgas and the element temperature Ta / f are both “K” (Kelvin). When the heat reception amount Q is a negative value, the heat reception amount Q is substantially a value indicating the amount of heat released from the detection element 71 to the exhaust gas.

Equation (1) can be modified as in the following equation (2).
Ta / f = Tgas− (Q / α) / S (2)
The above equation (1) or equation (2) is a thermal model equation representing a heat balance between the exhaust gas and the detection element 71.

  In equation (2), the exhaust gas temperature Tgas can be detected by the exhaust gas temperature sensor 60, and the surface area S is a fixed value determined in advance by design specifications. Therefore, in the present embodiment, “Q / α”, which is the ratio between the amount of heat received Q and the heat transfer coefficient α, is used as an adaptation constant. And the like, and the result is stored in the memory 101 as a matching constant map.

  FIG. 5 is a diagram showing an image of the matching constant map. As shown in FIG. 5, the value of the adaptation constant (= Q / α) is defined in advance in the adaptation constant map using the intake air amount Ga and the exhaust gas temperature Tgas as parameters. This matching constant map is obtained in advance by experiments or the like and stored in the memory 101.

  The ECU 100 refers to the adaptation constant map as shown in FIG. 5 and adapts the intake air amount Ga detected by the air flow meter 40 and the exhaust gas temperature Tgas detected by the exhaust gas temperature sensor 60 to the adaptation constant (= Q / α). ) Is calculated.

  Then, the ECU 100 substitutes the adaptive constant (= Q / α) calculated using the adaptive constant map and the exhaust gas temperature Tgas detected by the exhaust gas temperature sensor 60 into the above equation (2), The element temperature Ta / f is calculated.

  Then, ECU 100 calculates a correction amount ΔD of energization duty DUTY of heater 72 based on the difference between calculated element temperature Ta / f and target slight energization temperature Ttag1 (= Ta / f−Ttag1).

  FIG. 6 is a diagram showing an example of a map used for calculating the correction amount ΔD of the energization duty DUTY of the heater 72. In FIG. 6, the horizontal axis represents the difference between the element temperature Ta / f and the slight energization target temperature Ttag1 (= Ta / f-Ttag1), and the vertical axis represents the correction amount ΔD of the energization duty DUTY.

  In the example shown in FIG. 6, when the element temperature Ta / f is higher than the slight energization target temperature Ttag1 (when Ta / f-Ttag1> 0), the correction amount ΔD is set to a negative value, and As the magnitude (absolute value) of the difference between the temperature Ta / f and the slight energization target temperature Ttag1 increases, the magnitude (absolute value) of the correction amount ΔD increases. When the element temperature Ta / f is lower than the slight energization target temperature Ttag1 (when Ta / f-Ttag1 <0), the correction amount ΔD is set to a positive value, and the element temperature Ta / f The magnitude (absolute value) of the correction amount ΔD increases as the magnitude (absolute value) of the difference from the slight energization target temperature Ttag1 increases.

  Then, the ECU 100 calculates the energization duty DUTY of the heater 72 by the following equation (3).

DUTY current value = DUTY previous value + correction amount ΔD (3)
In Equation (3), “DUTY current value” indicates the energization duty DUTY calculated in the current operation cycle, and “DUTY last time value” indicates the energization duty DUTY calculated in the previous operation cycle. That is, the ECU 100 calculates a value obtained by adding the correction amount ΔD to the previous duty value as the current duty value. Then, the ECU 100 sets the energization duty DUTY of the heater 72 to the DUTY current value.

  FIG. 7 shows changes in the exhaust gas temperature Tgas and the element temperature Ta / f when the slight energization duty control of the heater 72 is performed using the above-mentioned thermal model equation (Equation (1) or Equation (2)) in a transient state. It is a figure showing an example.

  In the upper part of FIG. 7, the solid line indicates the exhaust gas temperature Tgas, and the dashed line indicates the element temperature Ta / f when the thermal model equation is used (this embodiment). Note that the broken line shown in the upper part of FIG. 7 shows the element temperature Ta / f when a one-dimensional map of the exhaust gas temperature Tgas is used (comparative example). The solid line and the broken line shown in the upper part of FIG. 7 have the same waveform as the solid line and the broken line shown in FIG.

  In the lower part of FIG. 7, the dashed line indicates the energization duty DUTY when the thermal model equation is used (this embodiment). In the lower part of FIG. 7, the broken line indicates the energization duty DUTY when a one-dimensional map of the exhaust gas temperature Tgas is used (comparative example).

  In the transient state, if the fine energization duty control is performed using a one-dimensional map of the exhaust gas temperature Tgas, the energization duty DUTY cannot follow the fluctuation of the exhaust gas temperature Tgas as shown by the broken line, so that the element temperature Ta / f Exceeds the upper limit of the target temperature control range, or the element temperature Ta / f falls below the lower limit of the target temperature control range.

  On the other hand, when the above-described thermal model equation is used as in the present embodiment, the energization duty DUTY follows the fluctuation of the exhaust gas temperature Tgas, as indicated by the dashed line. Specifically, at locations where the element temperature Ta / f is higher or lower than the slight energization target temperature Ttag1, the followability of the energization duty DUTY to the slight energization target temperature Ttag1 is improved. As a result, since the element temperature Ta / f is suppressed from deviating from the target temperature control range, it is possible to prevent water cracking or freezing cracking.

<Air-fuel ratio sensor heater control flow>
FIG. 8 is a flowchart showing an outline of processing executed when ECU 100 according to the present embodiment controls heater 72 of air-fuel ratio sensor 70. This flowchart is repeatedly executed, for example, every time a predetermined condition is satisfied while the engine 10 is operating.

  The ECU 100 determines whether the exhaust gas temperature Tgas is lower than the threshold value Tth (Step S10).

  When exhaust gas temperature Tgas is lower than threshold value Tth (YES in step S10), it is considered that element temperature Ta / f is included in the low temperature region, and ECU 100 executes the above-described slight energization duty control (step S20). To S28).

  Specifically, first, the ECU 100 refers to the above-mentioned adaptation constant map (FIG. 5) and corresponds to the intake air amount Ga detected by the air flow meter 40 and the exhaust gas temperature Tgas detected by the exhaust gas temperature sensor 60. An adaptation constant (= Q / α) is calculated (step S20).

  Next, the ECU 100 uses the adaptation constant (= Q / α) calculated in step S20, the exhaust gas temperature Tgas detected by the exhaust gas temperature sensor 60, and the above-described thermal model equation (2) to obtain the element temperature Ta. / F is calculated (step S22).

  Next, the ECU 100 refers to the map shown in FIG. 6 to calculate the correction amount ΔD corresponding to the difference (= Ta / f−Ttag1) between the element temperature Ta / f calculated in step S22 and the slight energization target temperature Ttag1. It is calculated (step S24).

  Next, the ECU 100 calculates the current value of the energization duty DUTY using the above equation (3) (step S26). That is, the ECU 100 calculates, as the DUTY current value, a value obtained by adding the correction amount ΔD calculated in step S24 to the previous DUTY value (= the previous DUTY value + the correction amount ΔD).

  Next, the ECU 100 sets the energization duty DUTY of the heater 72 to the DUTY current value calculated in step S26 (step S28).

  On the other hand, when exhaust gas temperature Tgas is higher than threshold value Tth (NO in step S10), it is considered that element temperature Ta / f is included in the high temperature region, and ECU 100 executes the above-described normal energization control (step S10). S30 to S34). That is, the ECU 100 calculates the element impedance Z (= ΔV / ΔI) from the relationship between the voltage change ΔV of the detection element 71 and the current change ΔI (step S30). Next, referring to the Z-Ta / f map stored in the memory 101, the ECU 100 calculates an element temperature Ta / f corresponding to the element impedance Z calculated in step S30 (step S32). Then, the ECU 100 performs feedback control of the amount of current supplied to the heater 72 so that the element temperature Ta / f calculated in step S32 becomes equal to the target activation temperature Ttag2 (step S34).

  As described above, when the exhaust gas temperature Tgas is lower than the threshold value Tth, the ECU 100 according to the present embodiment also has a low temperature of the detection element 71 exposed to the exhaust gas, and moisture adheres to the surface of the detection element 71. In consideration of the possibility that the element temperature Ta / f is raised to the target activation temperature Ttag2, water cracking is likely to occur, the slight energization target temperature at which the element temperature Ta / f is lower than the target activation temperature Ttag2. “Small energization duty control” for controlling the energization duty of the heater 72 to be Ttag1 is executed.

  In this slight energization duty control, the ECU 100 calculates the element temperature Ta / f using a heat model equation (the above equation (1) or equation (2)) representing the heat balance between the exhaust gas and the detection element 71. I do. In this thermal model equation, not only the exhaust gas temperature Tgas, but also the amount of heat Q received by the detection element 71 from the exhaust gas and the heat transfer coefficient α between the exhaust gas and the detection element 71 are considered. Therefore, the element temperature Ta / f is calculated with higher accuracy than when only the exhaust gas temperature Tgas is simply considered. This allows the element temperature Ta / f to quickly follow the slight energization target temperature Ttag1. As a result, even in a transient state in which the exhaust gas temperature Tgas fluctuates rapidly, it is possible to prevent the element temperature Ta / f from largely deviating from the slight energization target temperature Ttag1.

  Further, in the present embodiment, in view of the fact that heat receiving amount Q received by detecting element 71 from exhaust gas can vary not only with exhaust gas temperature Tgas but also with exhaust gas flow rate, ECU 100 sets heat receiving amount Q Is calculated using not only the exhaust gas temperature Tgas but also the intake air amount Ga (a value corresponding to the exhaust gas flow rate). Then, the ECU 100 calculates the element temperature Ta / f by using the adaptive constant calculated as described above. Therefore, the element temperature Ta / f can be accurately calculated.

  Further, when exhaust gas temperature Tgas is higher than threshold value Tth, ECU 100 according to the present embodiment may have a high temperature of detection element 71 exposed to the exhaust gas and moisture may adhere to the surface of detection element 71. In view of the fact that the element temperature Ta / f rises to the target activation temperature Ttag2, it is difficult for water breakage to occur. Therefore, the amount of electricity supplied to the heater 72 is set so that the element temperature Ta / f becomes the target activation temperature Ttag2. The “normal energization control” to be controlled is executed. As a result, the element temperature Ta / f is maintained at the target activation temperature Ttag2, so that the air-fuel ratio sensor 70 can accurately detect the air-fuel ratio A / F.

  The embodiments disclosed this time are to be considered in all respects as illustrative and not restrictive. The scope of the present disclosure is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  Reference Signs List 10 engine, 11 intake passage, 12 exhaust passage, 20 fuel injection valve, 40 air flow meter, 50 engine rotation speed sensor, 60 exhaust temperature sensor, 70 air-fuel ratio sensor, 71 detection element, 72 heater, 100 ECU, 101 memory.

Claims (3)

An exhaust temperature sensor that detects the temperature of the exhaust of the engine;
An air-fuel ratio sensor including a detection element for detecting an air-fuel ratio of the exhaust gas and a heater for heating the detection element,
A control device for controlling the energization of the heater of the air-fuel ratio sensor,
The control device calculates a temperature of the detection element using a predetermined model formula representing a heat balance between the exhaust gas and the detection element when a detection value of the exhaust temperature sensor is lower than a predetermined temperature. Controlling the energization duty of the heater such that the temperature of the detection element is a target temperature lower than the activation temperature of the detection element,
The model equation is an equation that defines the relationship between the amount of heat received by the detection element from the exhaust gas, the heat transfer coefficient between the exhaust gas and the detection element, the temperature of the exhaust gas, and the temperature of the detection element. A temperature control device for an air-fuel ratio sensor. The temperature control device,
An air flow meter for detecting an intake air amount of the engine,
A storage unit that stores in advance a correspondence relationship between the intake air amount, the temperature of the exhaust gas, and a matching constant that is a ratio between the heat reception amount and the heat transfer coefficient,
The control device includes:
With reference to the correspondence stored in the storage unit, calculate the adaptation constant corresponding to the detection value of the air flow meter and the detection value of the exhaust gas temperature sensor,
2. The temperature control device for an air-fuel ratio sensor according to claim 1, wherein the temperature of the detection element is calculated using the calculated adaptation constant, the detection value of the exhaust gas temperature sensor, and the model formula. 3.   The control device calculates the temperature of the detection element from the impedance of the detection element when the detection value of the exhaust gas temperature sensor exceeds the predetermined temperature, and sets the heater so that the temperature of the detection element becomes the activation temperature. The temperature control device for an air-fuel ratio sensor according to claim 1, wherein the amount of current is controlled.

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