JP2002014079A - Sensor element temperature detector for air-fuel ratio sensor - Google Patents

Abstract

(57) Abstract: In an air-fuel ratio sensor provided with a heater for heating a sensor element, the impedance of the sensor element is accurately measured by removing the influence of an energized state to the heater for heating the sensor element. . A voltage applied to a heater for heating a sensor element is kept constant (for example, heater duty (HDUTY) = 0%) during impedance measurement of a sensor element of an air-fuel ratio sensor.
Thus, the instantaneous fluctuation of the sensor element temperature is prevented, and the impedance is measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an element temperature detecting device for detecting a sensor element temperature based on a sensor element impedance of an air-fuel ratio sensor mounted on an exhaust system of an internal combustion engine.

[0002]

2. Description of the Related Art As an air-fuel ratio control device for an internal combustion engine, an air-fuel ratio which feedback-controls an amount of fuel supplied to the engine based on an oxygen concentration in exhaust gas by an air-fuel ratio sensor so as to approach an actual air-fuel ratio. Feedback control is known.

In the air-fuel ratio feedback control, it is a precondition that the air-fuel ratio sensor is activated. Since the air-fuel ratio sensor is activated when the element temperature reaches a predetermined activation temperature, it has been conventionally performed to determine the activation state of the sensor element and detect the element temperature for controlling the heater for heating the sensor element. ing.

For example, in Japanese Patent Application Laid-Open No. 61-12556, the impedance of a sensor element depends on the element temperature in order to control a heater for heating a sensor element provided in an air-fuel ratio sensor based on the element temperature. Therefore, the sensor element temperature is detected using the impedance of the sensor element.

Specifically, a high-frequency AC voltage is applied to the sensor element of the air-fuel ratio sensor, the impedance of the sensor element is measured from the value of the current flowing through the sensor element, and the element temperature is detected based on the measured impedance. Like that.

Further, as disclosed in Japanese Patent Application Laid-Open No. H11-344466, the impedance or the element temperature of the sensor element is obtained, and the heater power supply is feedback-controlled so that the impedance or the element temperature of the sensor element becomes a target value. .

[0007]

However, in recent years,
The following problems have arisen when measuring the impedance of the sensor element.

That is, the sensor element is activated early,
In addition, the capacity of the heater for heating the sensor element has been increased and the size of the sensor element has been reduced in order to reliably maintain the activation state, and the temperature followability of the sensor element with respect to the energization of the heater has been improved. Relatively small). For this reason, a change in the amount of current supplied to the heater quickly affects the temperature of the sensor element, and the fluctuation of the sensor element temperature with respect to the state of current supplied to the heater has become more remarkable than in the past.

In particular, in the case of controlling the amount of current supplied to the heater by duty-controlling ON / OFF of the power supply to the heater, the temperature of the sensor element fluctuates instantaneously due to the ON / OFF of the heater. Since the impedance also fluctuates, an error in impedance measurement easily occurs.

As a result, the detection accuracy of the sensor element temperature (the activated state of the sensor) is reduced, and furthermore, the heater control based on the element temperature is not stable, the impedance measurement error increases, and the air-fuel ratio feedback control is not performed. A vicious circle will be caused.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide an element temperature detecting device capable of accurately detecting a sensor element temperature of an air-fuel ratio sensor.

[0012]

Therefore, according to the present invention, as shown in FIG. 1, a heater for heating a sensor element and an impedance of the sensor element are mounted on an exhaust system of an internal combustion engine. Impedance measuring means, an element temperature detecting means for detecting a sensor element temperature based on the measured impedance, and a heater control means for controlling a sensor element temperature by controlling an amount of current supplied to the heater. An element temperature detection device for an air-fuel ratio sensor,
During the impedance measurement of the sensor element by the impedance measuring means, a heater applied voltage control means for maintaining a constant applied voltage to the heater is provided.

According to a second aspect of the present invention, the air-fuel ratio sensor includes a Nernst cell unit for generating a voltage corresponding to a lean-rich air-fuel ratio, and a lean-rich air-fuel ratio detected by the Nernst cell unit. A predetermined voltage is applied in a corresponding direction, and a current value continuously changes in accordance with the air-fuel ratio.The impedance measuring unit is configured to apply an AC voltage to the Nernst cell. The impedance of the Nernst cell section is measured from a current value flowing in the Nernst cell section.

According to a third aspect of the present invention, the heater power supply control means controls the heater power supply by duty-controlling ON / OFF of the heater power supply.
The heater application voltage control means maintains the heater energization OFF.

According to a fourth aspect of the present invention, the heater power supply control means controls the heater power supply by duty-controlling ON / OFF of the heater power supply.
The heater application voltage control means maintains heater energization ON.

The invention according to claim 5 is characterized in that the heater applied voltage control means maintains a voltage value set based on the rotation speed of the engine and the fuel injection amount.

[0017]

According to the first aspect of the present invention, while the impedance of the sensor element is measured, the voltage applied to the heater for heating the sensor element is kept constant. The impedance can be measured accurately by preventing temperature fluctuation. As a result, the detection accuracy of the sensor element temperature is also improved.

According to the second aspect of the present invention, when the air-fuel ratio sensor includes a Nernst cell section and a pump cell section, an AC voltage is applied to the Nernst cell section, and a current flowing through the Nernst cell section is applied. By measuring the value, the impedance can be measured without affecting the air-fuel ratio detection in the pump cell section.

According to the third aspect of the present invention, when the heater energization amount is controlled by duty control of ON / OFF of the heater energization, the heater energization is turned off during impedance measurement of the sensor element. Since the power supply to the heater is stopped by maintaining the temperature, the instantaneous fluctuation of the sensor element temperature due to ON / OFF is prevented, and the element temperature is measured at almost the same temperature as the exhaust temperature. Can be measured.

According to the fourth aspect of the present invention, while the impedance of the sensor element is being measured, the heater energization is kept ON to prevent the sensor element temperature from instantaneously fluctuating due to the ON / OFF operation, thereby achieving stable operation. The impedance can be measured in the state.

According to the fifth aspect of the present invention, the heater applied voltage control means estimates the temperature substantially equal to the element temperature by maintaining the voltage value set based on the rotation speed of the engine and the fuel injection amount. Since the voltage value can be set according to the state of the exhaust gas temperature, the change in the element temperature during impedance measurement and before and after the impedance measurement can be minimized, and the impedance measurement accuracy can be further improved.

[0022]

Embodiments of the present invention will be described below. FIG. 2 shows a system configuration diagram of the internal combustion engine in one embodiment of the present invention. 2, an intake passage 2 of the internal combustion engine 1 is provided with an air flow meter 3 for detecting an intake air amount Qa and a throttle valve 4 for controlling the intake air amount Qa.

A fuel injection valve 5 provided for each cylinder is driven to open by an injection pulse signal from an ECM (engine control module) 6 containing a microcomputer, and is pressure-fed from a fuel pump (not shown) by a pressure regulator. The fuel controlled to a predetermined pressure is injected and supplied.

The exhaust passage 7 is provided with a wide-range air-fuel ratio sensor 8 for linearly detecting the air-fuel ratio according to the oxygen concentration in the exhaust gas. Further, a crank angle sensor 9 for outputting a crank angle signal for each predetermined crank angle of the engine 1 or the engine 1
Is provided with a water temperature sensor 10 for detecting a cooling water temperature Tw in the cooling jacket.

The ECM 6 is, for example, an intake air amount Qa
And a basic fuel injection amount Tp = K × Qa × Ne (K is a constant) corresponding to stoichiometric (λ = 1) from the engine speed Ne detected based on the signal from the crank angle sensor 9 and
This is corrected by a target air-fuel ratio tλ and an air-fuel ratio feedback correction coefficient α based on a signal from the air-fuel ratio sensor 8 to obtain a fuel injection amount Ti = Tp × (1 / tλ) × α.
The injection pulse corresponding to i is synchronized with the engine rotation cycle,
Output to the fuel injection valve 5.

FIG. 3 shows the structure of the air-fuel ratio sensor 8. In FIG. 3, the sensor element body 20 is formed in a porous layer of a solid electrolyte material such as zirconia having oxygen ion conductivity, and a heater 21, an air chamber 22, A gas diffusion chamber 23 is provided.

The heater 21 can heat the sensor element by energizing it. The atmosphere chamber 22 is formed outside the exhaust passage so as to communicate with the atmosphere, which is a reference gas.

The gas diffusion chamber 23 has an exhaust introduction hole 24 formed from the upper surface side of the main body 20 in FIG.
It is formed so as to communicate with the exhaust gas through the air outlet 2. Here, the Nernst cell part 26 is constituted by the electrode 26A provided on the upper wall of the atmosphere chamber 22 and the electrode 26B provided on the lower wall of the gas diffusion chamber 23.

The pump cell section 27 is composed of an electrode 27A provided on the upper wall of the gas diffusion chamber 23 and an electrode 27B provided on the upper wall of the main body 20 and covered with a protective layer 28. The Nernst cell section 26 generates a voltage according to the oxygen partial pressure between the Nernst cell section electrodes 26A and 26B which is affected by the oxygen ion concentration (oxygen partial pressure) in the gas diffusion chamber 23. , By detecting the voltage,
It is possible to detect whether the air-fuel ratio is lean or rich with respect to stoichiometry (λ = 1).

When a predetermined voltage is applied to the pump cell section 27, oxygen ions in the gas diffusion chamber 23 move, and a current flows between the pump cell section electrodes 27A and 27B. And the pump cell part electrode 27A,
The current value (limit current value) Ip flowing when a predetermined voltage is applied between 27B is affected by the oxygen ion concentration in the gas diffusion chamber 23. Therefore, by detecting the current value Ip, the exhaust air The fuel ratio can be detected.

That is, as shown in FIG. 4 (A), the voltage-current characteristic of the pump cell section 27 changes according to the air-fuel ratio λ, and the exhaust air is evacuated by the current value Ip when a predetermined voltage Vp is applied. The fuel ratio λ can be detected.

In the Nernst cell section 27, the lean
By inverting the voltage application direction to the pump cell unit 27 based on the rich output, the air flows through the pump cell unit 27 in the air-fuel ratio region in both the lean region and the rich region as shown in FIG. Based on the current value Ip, it is possible to detect a wide range of the air-fuel ratio λ.

FIG. 5 shows a control circuit for the air-fuel ratio sensor and the heater for heating the sensor element. Nernst cell part 2
6, an AC voltage is applied by an AC power supply 31 under the control of a microcomputer 30 for impedance measurement, whereby the current value Is flowing through the Nernst cell unit 26 is changed by a current detection resistor 32 and a detection amplifier 33. Voltage conversion and detection.

The signal from the detection amplifier 33 is input to an impedance detection circuit 34 composed of, for example, a high-pass filter and an integrator, thereby extracting only the AC component and detecting the impedance Ri from the amplitude. Thereby, the impedance Ri of the Nernst cell unit 26 can be measured.

The signal from the detection amplifier 33 is input to a low-pass filter 35 to extract only a DC component and detect a voltage generated in the Nernst cell unit 26 in accordance with the oxygen concentration. As a result, the oxygen concentration is
Rich can be detected.

A predetermined voltage Vp is applied to the pump cell unit 27 by the DC power supply 35 under the control of the microcomputer 30, and the application direction is reversed according to the lean / rich oxygen concentration detected by the Nernst cell unit 26. As a result, the current Ip flowing through the pump cell unit 27 is converted into a voltage by the current detection resistor 36 and the detection amplifier 37 and detected.
Thereby, the air-fuel ratio λ is detected.

The battery voltage VB is applied to the heater 21 from the battery.
9, the switching element 3 is normally provided.
The amount of power to the heater 21 can be controlled by duty-controlling the ON / OFF of 9 by the microcomputer 30.

FIG. 6 is a flowchart showing the element temperature detection of the air-fuel ratio sensor executed by the microcomputer 30 every predetermined time. In step 1 (referred to as S1 in the figure, the same applies hereinafter), various operating conditions are read.

In step 2, it is determined whether or not the impedance measurement permission condition is satisfied. Here, the impedance measurement permission condition is satisfied when the influence of heat removal due to a change in the exhaust gas flow rate is small.
That is, the operating state of the engine is within the predetermined rotation speed Ne and the predetermined fuel injection amount Tp.

If the impedance measurement permission condition is not satisfied, the process returns to step 1. If the impedance measurement permission condition is satisfied, the process proceeds to step 3, where the heater duty (HDUTY) is set to 0 (%), and the power supply to the sensor element heating heater is stopped.
Proceed to.

In step 4, the impedance of the sensor element (Nernst cell) is measured. A predetermined AC voltage is applied to the Nernst cell unit 26 by the AC power supply 31,
The terminal voltage of the current detection resistor 32 at that time is read, and the impedance of the Nernst cell unit 26 is measured based on the terminal voltage.

That is, the normal duty control by the heater control means is performed as shown in FIG. 7 (A).
The state is maintained as FF. This corresponds to heater applied voltage control means.

Then, the process proceeds to a step 5, wherein the element temperature is detected based on the impedance measured in the step 4 using a table of a preset element temperature and a theoretical impedance value.

After the completion of the impedance measurement, the normal duty control by the heater control means is resumed. As described above, during the measurement of the impedance of the sensor element, the current supply to the heater for heating the sensor element is stopped (the applied voltage is kept constant at 0 V), thereby preventing instantaneous fluctuations in the sensor element temperature. Since the element temperature can measure the impedance at substantially the same temperature as the exhaust temperature at that time, the impedance measurement accuracy of the sensor element is improved, and the detection accuracy of the sensor element temperature is improved.

Step 3 in the above embodiment
As shown in FIG. 7B, the heater duty (HDUTY) = 100 (%) is set during the impedance measurement of the sensor element, and the heater power is always turned on, as shown in FIG. Also, the applied voltage is kept constant as the set maximum value, and the instantaneous fluctuation of the sensor element temperature is prevented, so that the impedance can be measured in a stable state.

Next, a second embodiment of the present invention will be described. FIG. 8A is a control circuit diagram for the heater of the air-fuel ratio sensor according to the second embodiment. In a normal state, the switching element 39 is provided in the same manner as in the first embodiment.
The microcomputer 30 controls the duty of the ON / OFF control of the microcomputer 30 to control the amount of current to the heater 21, and applies a preset voltage value to the heater 21 by adjusting the resistance of the switch 40 during impedance measurement. It was done.

The switch 40 sets a voltage value applied to the heater by arbitrarily selecting a plurality of resistors as shown in FIG. 8B, for example. FIG. 9 is a flowchart showing element temperature detection of the air-fuel ratio sensor according to the second embodiment.

Steps 11 and 12 are the same as in the first embodiment, and a description thereof will be omitted. If it is determined in step 12 that the impedance measurement permission is satisfied, the process proceeds to step 1
Proceed to 3.

In step 13, the target heater voltage during the impedance measurement is determined by a preset map or the like based on the read engine speed Ne and fuel injection amount Tp. This makes it possible to determine an appropriate applied voltage according to the state of the exhaust gas temperature that is estimated to be substantially the same as the element temperature.

Next, at step 14, the switching device 4
After adjusting the resistance of 0 to set the target heater voltage value determined in step 13, the process proceeds to step 15 where the impedance is measured and the element temperature is detected in the same manner as in the first embodiment.

As described above, during the impedance measurement, the voltage value set based on the engine rotation speed Ne and the fuel injection amount Tp is applied to the heater, so that the element temperature is prevented from fluctuating and the element before and after the impedance measurement is performed. The change in temperature can be minimized, and the impedance can be measured more accurately.

It should be noted that the present invention is not limited to the case where the heater control in the normal state is performed by the duty control as in the first and second embodiments. The same effect can be obtained even if the voltage applied to the device is controlled.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of the present invention.

FIG. 2 is a system configuration diagram showing an embodiment of the present invention.

FIG. 3 is a diagram showing a sensor element structure of an air-fuel ratio sensor.

FIG. 4 is a characteristic diagram of a sensor element of the air-fuel ratio sensor.

FIG. 5 is a control circuit diagram for an air-fuel ratio sensor and a heater.

FIG. 6 is a flowchart of sensor element temperature detection according to the first embodiment of the present invention.

FIG. 7 is a diagram showing duty control during impedance measurement.

FIG. 8 is another control circuit diagram for the heater of the air-fuel ratio sensor.

FIG. 9 is a flowchart of sensor element temperature detection according to the second embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 6 ... ECM 9 ... Air-fuel ratio sensor 20 ... Sensor element main body 21 ... Heater 22 ... Atmosphere chamber 23 ... Gas diffusion chamber 24 ... Exhaust introduction hole 25 ... Protective layer 26 ... Nernst cell part 27 ... Pump cell part 28 ... Protection Layer 30 Microcomputer 31 AC power supply 32 DC power supply 39 Switching element

Claims (5)

[Claims] 1. A heater for heating a sensor element, mounted on an exhaust system of an internal combustion engine, impedance measuring means for measuring an impedance of the sensor element, and an element temperature detection for detecting a sensor element temperature based on the measured impedance. And a heater control means for controlling the sensor element temperature by controlling the amount of current supplied to the heater, wherein the impedance measuring means measures impedance of the sensor element by the impedance measuring means. An element temperature detecting device for an air-fuel ratio sensor, further comprising a heater applied voltage control means for maintaining a constant applied voltage to the heater. 2. An air-fuel ratio sensor comprising: a Nernst cell for generating a voltage corresponding to a lean / rich air-fuel ratio; and a predetermined direction in a direction corresponding to a lean / rich air-fuel ratio detected by the Nernst cell. A pump cell unit to which a voltage is applied and a current value continuously changes in accordance with an air-fuel ratio; and wherein the impedance measuring means flows into the Nernst cell unit when an AC voltage is applied to the Nernst cell unit. 2. The device temperature detecting device for an air-fuel ratio sensor according to claim 1, wherein an impedance of the Nernst cell section is measured from a current value. 3. The heater control means according to claim 1, wherein said heater control means turns on heater power.
3. The heater according to claim 1, wherein the heater energization amount is controlled by duty control of OFF, wherein the heater application voltage control means keeps the heater energization OFF. Element temperature detector for fuel ratio sensor. 4. The heater control means according to claim 1, further comprising:
3. The heater according to claim 1, wherein the heater energization amount is controlled by duty control of OFF, and the heater application voltage control means maintains the heater energization on. Element temperature detector for fuel ratio sensor. 5. The air-fuel ratio sensor according to claim 1, wherein said heater applied voltage control means maintains a voltage value set based on a rotation speed of the engine and a fuel injection amount. Device temperature detector.