JP3734685B2 - Sensor element temperature detection device for air-fuel ratio sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an element temperature detection device that detects 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]
[Prior art]
As an air-fuel ratio control device for an internal combustion engine, air-fuel ratio feedback control is known in which an air-fuel ratio sensor controls the amount of fuel supplied to the engine so that the actual air-fuel ratio approaches the target air-fuel ratio based on the oxygen concentration in the exhaust gas. ing.
[0003]
In such 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 detect the element temperature for determining the active state of the sensor element and controlling the heater for heating the sensor element. ing.
[0004]
For example , for the purpose of controlling a heater for heating a sensor element provided in an air- fuel ratio sensor based on the element temperature, the impedance of the sensor element depends on the element temperature. Some devices detect the element temperature .
[0005]
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 current value flowing through the sensor element, and the element temperature is detected based on the measured impedance. Yes.
[0006]
Further, as disclosed in Japanese Patent Application Laid-Open No. 11-344466, the impedance or element temperature of the sensor element is obtained, and the heater energization amount is feedback-controlled so that the impedance or element temperature of the sensor element becomes a target value.
[0007]
[Problems to be solved by the invention]
However, in recent years, the following problems have arisen when measuring the impedance of a sensor element.
[0008]
That is, in order to activate the sensor element at an early stage and to reliably maintain the activated state, the capacity of the heater for heating the sensor element is increased and the sensor element is miniaturized, and the temperature followability of the sensor element with respect to heater energization (The heat capacity of the sensor element has become relatively small). For this reason, a change in the energization amount to the heater quickly affects the temperature of the sensor element, and the variation of the sensor element temperature with respect to the energization state of the heater has become more prominent than in the past.
[0009]
In particular, in the case of controlling the energization amount to the heater by duty control of heater energization ON / OFF, the sensor element temperature fluctuates instantaneously due to the ON / OFF, and the element impedance also varies accordingly. Therefore, impedance measurement errors are likely to occur.
[0010]
As a result, the detection accuracy of the sensor element temperature (sensor activation state) also decreases, heater control based on the element temperature is not stable, impedance measurement error increases, and air-fuel ratio feedback control is affected. A vicious circle will occur.
[0011]
The present invention has been made in view of the above problems, and an object of the present invention is to provide an element temperature detection device that can accurately detect the sensor element temperature of an air-fuel ratio sensor.
[0012]
[Means for Solving the Problems]
For this reason, as shown in FIG. 1, the invention according to claim 1 is mounted on an exhaust system of an internal combustion engine, and is measured with a heater for heating the sensor element, and an impedance measuring means for measuring the impedance of the sensor element. An element temperature detecting device for an air-fuel ratio sensor comprising element temperature detecting means for detecting a sensor element temperature based on impedance and heater control means for controlling the sensor element temperature by controlling an energization amount of the heater. There,
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.
[0013]
According to a second aspect of the present invention, the air-fuel ratio sensor includes a Nernst cell unit that generates a voltage corresponding to the lean / rich air / fuel ratio, and a direction corresponding to the lean / rich air / fuel ratio detected by the Nernst cell unit. A predetermined voltage is applied to the pump cell unit, and the current value continuously changes according to the air-fuel ratio.
The impedance measuring means is for measuring the impedance of the Nernst cell part from a current value flowing in the Nernst cell part when an AC voltage is applied to the Nernst cell part.
[0014]
According to a third aspect of the present invention, the heater energization control means controls the heater energization amount by duty-controlling ON / OFF of the heater energization, and the heater application voltage control means turns off the heater energization. It is characterized by maintaining to.
[0015]
According to a fourth aspect of the present invention, the heater energization control means controls the heater energization amount by duty-controlling ON / OFF of the heater energization, and the heater application voltage control means turns on the heater energization. It is characterized by maintaining to.
[0016]
The invention according to claim 5 is characterized in that the heater applied voltage control means maintains a voltage value set based on the rotational speed of the engine and the fuel injection amount.
[0017]
【The invention's effect】
According to the first aspect of the present invention, the voltage applied to the heater for heating the sensor element is kept constant during the measurement of the impedance of the sensor element, thereby preventing fluctuations in the sensor element temperature caused by fluctuations in the applied voltage due to heater control. Thus, impedance can be measured with high accuracy. As a result, the detection accuracy of the sensor element temperature is also improved.
[0018]
According to the second aspect of the present invention, when the air-fuel ratio sensor includes a Nernst cell part and a pump cell part, an AC voltage is applied to the Nernst cell part, and a current value flowing through the Nernst cell part is measured. By doing so, the impedance can be measured without affecting the air-fuel ratio detection in the pump cell section.
[0019]
According to the invention of claim 3, in the case of controlling the heater energization amount by duty control of heater energization ON / OFF, the heater energization is maintained OFF during the impedance measurement of the sensor element. Since the heater is deenergized, instantaneous fluctuations in the sensor element temperature due to ON / OFF are prevented, and the element temperature is measured at substantially the same temperature as the exhaust temperature, so that the impedance can be accurately measured.
[0020]
According to the invention of claim 4, during the measurement of the impedance of the sensor element, the heater energization is kept ON to prevent instantaneous fluctuations in the sensor element temperature due to ON / OFF, and the impedance in a stable state. Can be measured.
[0021]
According to the fifth aspect of the present invention, the heater applied voltage control means maintains the voltage value set based on the engine speed and the fuel injection amount, so that the exhaust temperature estimated to be substantially the same as the element temperature. Since the voltage value according to the temperature state can be set, changes in the element temperature during impedance measurement and before and after impedance measurement can be minimized, and impedance measurement accuracy can be further improved.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
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. In FIG. 2, the intake passage 2 of the internal combustion engine 1 is provided with an air flow meter 3 for detecting the intake air amount Qa and a throttle valve 4 for controlling the intake air amount Qa.
[0023]
The fuel injection valve 5 provided for each cylinder is driven to open by an injection pulse signal from an ECM (engine control module) 6 having a built-in microcomputer, and is pumped from a fuel pump (not shown) to a predetermined pressure by a pressure regulator. The controlled fuel is injected and supplied.
[0024]
The exhaust passage 7 is provided with a wide-range air-fuel ratio sensor 8 that linearly detects the air-fuel ratio according to the oxygen concentration in the exhaust gas.
Further, a crank angle sensor 9 that outputs a crank angle signal for each predetermined crank angle of the engine 1 and a water temperature sensor 10 that detects the cooling water temperature Tw in the cooling jacket of the engine 1 are provided.
[0025]
The ECM 6 is, for example, a basic fuel injection amount Tp = K × Qa × Ne (K) corresponding to the stoichiometric (λ = 1) from the engine speed Ne detected based on the intake air amount Qa and a signal from the crank angle sensor 9. Is calculated 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. An injection pulse corresponding to Ti is output to the fuel injection valve 5 in synchronization with the engine rotation period.
[0026]
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 the heater 21, the atmospheric chamber 22, A gas diffusion chamber 23 is provided.
[0027]
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 as the reference gas.
[0028]
The gas diffusion chamber 23 is formed so as to communicate with the exhaust through an exhaust introduction hole 24 formed from the upper surface side in the figure of the main body 20 and a protective layer 22 such as γ alumina.
Here, the Nernst cell portion 26 is configured by the electrode 26 </ b> A provided on the upper wall of the atmospheric chamber 22 and the electrode 26 </ b> B provided on the lower wall of the gas diffusion chamber 23.
[0029]
The pump cell unit 27 is configured by the electrode 27A provided on the upper wall of the gas diffusion chamber 23 and the electrode 27B provided on the upper wall of the main body 20 and covered with the 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 influenced 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).
[0030]
In the pump cell unit 27, when a predetermined voltage is applied thereto, oxygen ions in the gas diffusion chamber 23 move, and a current flows between the pump cell unit electrodes 27A and 27B. Since the current value (limit current value) Ip that flows when a predetermined voltage is applied between the pump cell electrodes 27A and 27B is affected by the oxygen ion concentration in the gas diffusion chamber 23, the current value Ip is detected. By doing so, the air-fuel ratio of the exhaust can be detected.
[0031]
That is, as shown in FIG. 4A, the voltage-current characteristic of the pump cell unit 27 changes in accordance with the air-fuel ratio λ, and the air-fuel ratio λ of the exhaust is determined by the current value Ip when a predetermined voltage Vp is applied. Can be detected.
[0032]
Further, by reversing the direction in which the voltage is applied to the pump cell unit 27 based on the lean / rich output from the Nernst cell unit 27, the air-fuel ratio region in both the lean region and the rich region in FIG. As shown in FIG. 4, a wide range of air-fuel ratio λ can be detected based on the current value Ip flowing through the pump cell unit 27.
[0033]
FIG. 5 shows a control circuit for the air-fuel ratio sensor and the sensor element heater.
An AC voltage is applied to the Nernst cell unit 26 by an AC power source 31 under the control of the microcomputer 30 for impedance measurement, whereby the current value Is flowing through the Nernst cell unit 26 is detected by a current detection resistor 32 and a detection amplifier. 33 to detect the voltage.
[0034]
The signal from the detection amplifier 33 is input to an impedance detection circuit 34 including, for example, a high-pass filter and an integrator, so that only the AC component is extracted and the impedance Ri is detected from the amplitude. Thereby, the impedance Ri of the Nernst cell part 26 can be measured.
[0035]
Further, the signal from the detection amplifier 33 is input to the low-pass filter 35, so that only the DC component is extracted, and the voltage generated in the Nernst cell unit 26 is detected according to the oxygen concentration. Thereby, lean / rich oxygen concentration can be detected.
[0036]
A predetermined voltage Vp is applied to the pump cell unit 27 by the DC power source 35 under the control of the microcomputer 30, but the application direction is reversed according to the lean / rich oxygen concentration detected by the Nernst cell unit 26. Thus, the current Ip flowing through the pump cell unit 27 is detected by converting the voltage with the current detection resistor 36 and the detection amplifier 37. Thereby, the air-fuel ratio λ is detected.
[0037]
A battery voltage VB is applied to the heater 21 from the battery, but since the switching element 39 is provided in the energization circuit, the heater 30 is normally controlled by the microcomputer 30 to perform duty control on and off of the switching element 39. The energization amount to 21 can be controlled.
[0038]
FIG. 6 is a flowchart showing element temperature detection of the air-fuel ratio sensor executed by the microcomputer 30 every predetermined time.
In step 1 (denoted as S1 in the figure, the same applies hereinafter), various operating conditions are read.
[0039]
In step 2, it is determined whether or not an impedance measurement permission condition is satisfied. Here, the impedance measurement permission condition is satisfied when there is little influence of heat absorption due to a change in the exhaust flow rate. For example, the operating state of the engine is within a predetermined rotational speed Ne and a predetermined fuel injection amount Tp. It is to be.
[0040]
If the impedance measurement permission condition is not satisfied, the process returns to step 1.
When the impedance measurement permission condition is satisfied, the process proceeds to step 3, the heater duty (HDUTY) is set to 0 (%), the energization to the sensor element heating heater is stopped, and then the process proceeds to step 4.
[0041]
In step 4, the impedance of the sensor element (Nernst cell part) is measured. A predetermined AC voltage is applied to the Nernst cell unit 26 by the AC power source 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 this.
[0042]
That is, during normal impedance control by the heater control means, as shown in FIG. 7A, during the impedance measurement, the power supply to the heater for heating the sensor element is turned off and the state is maintained. This corresponds to the heater applied voltage control means.
[0043]
Thereafter, the process proceeds to step 5 where the element temperature is detected based on the impedance measured in step 4 based on a preset element temperature and impedance theoretical value table.
[0044]
Note that energization of the heater resumes normal duty control by the heater control means after the impedance measurement is completed.
As described above, while measuring the impedance of the sensor element, the energization to the heater for heating the sensor element is stopped (the applied voltage is kept constant at 0 V) to prevent an instantaneous change in the sensor element temperature, Since the impedance can be measured at substantially the same temperature as the exhaust temperature at that time, the impedance measurement accuracy of the sensor element is improved, and consequently the detection accuracy of the sensor element temperature is improved.
[0045]
In step 3 in the above-described embodiment, as shown in FIG. 7B, the heater energization amount is set to heater duty (HDUTY) = 100 (%) during the impedance measurement of the sensor element, to the heater. Even when the current is always turned on, the applied voltage is kept constant as the set maximum value, the instantaneous fluctuation of the sensor element temperature is prevented, and the impedance can be measured in a stable state.
[0046]
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 amount of current supplied to the heater 21 is controlled by duty control of the switching element 39 by the microcomputer 30 as in the first embodiment, and the resistance of the switcher 40 is measured during impedance measurement. A voltage value set in advance is applied to the heater 21 by adjustment.
[0047]
For example, the switcher 40 sets a voltage value to be applied to the heater by arbitrarily selecting a plurality of resistors as shown in FIG.
FIG. 9 is a flowchart showing element temperature detection of the air-fuel ratio sensor in the second embodiment.
[0048]
Steps 11 and 12 are the same as those in the first embodiment, and a description thereof will be omitted. If it is determined in step 12 that the impedance measurement permission determination is established, the process proceeds to step 13.
[0049]
In step 13, the target heater voltage during impedance measurement is determined by a map or the like set in advance based on the read engine rotational speed Ne and fuel injection amount Tp. Thereby, an appropriate applied voltage can be determined according to the state of the exhaust temperature estimated to be substantially the same as the element temperature.
[0050]
Next, in step 14, after adjusting the resistance of the switch 40 and setting it to the target heater voltage value determined in step 13, the process proceeds to step 15 where the impedance is measured in the same manner as in the first embodiment. The element temperature is detected.
[0051]
As described above, during impedance measurement, a voltage value set based on the engine rotational speed Ne and the fuel injection amount Tp is applied to the heater, so that fluctuations in element temperature are prevented and changes in element temperature before and after impedance measurement are performed. Can be minimized, and impedance can be measured with higher accuracy.
[0052]
Note that the present invention is not limited to the heater control that is normally performed by duty control as in the first and second embodiments, but is controlled by other methods, for example, application to the heater. Even if the voltage value is controlled, the same effect can be obtained.
[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 an 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]
1 ... Internal combustion engine 6 ... ECM
DESCRIPTION OF SYMBOLS 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 ... Protective layer 30 ... Microcomputer 31 ... AC Power supply 32 ... DC power supply 39 ... Switching element

Claims (5)

A heater for heating the sensor element, mounted on an exhaust system of the internal combustion engine, an impedance measuring means for measuring the impedance of the sensor element, an element temperature detecting means for detecting the sensor element temperature based on the measured impedance, and the heater An element temperature detecting device for an air-fuel ratio sensor, comprising a heater control means for controlling a sensor element temperature by controlling an energization amount of
An element temperature detecting device for an air-fuel ratio sensor, comprising: heater applied voltage control means for maintaining a constant applied voltage to the heater during impedance measurement of the sensor element by the impedance measuring means. The air-fuel ratio sensor includes a Nernst cell unit that generates a voltage corresponding to the lean / rich of the air-fuel ratio, and a predetermined voltage applied in a direction corresponding to the lean / rich of the air-fuel ratio detected by the Nernst cell unit. A pump cell unit in which the current value continuously changes according to the air-fuel ratio,
The impedance measuring means measures impedance of the Nernst cell part from a current value flowing through the Nernst cell part when an AC voltage is applied to the Nernst cell part. Element temperature detection device for air-fuel ratio sensor. The heater control means controls the heater energization amount by duty-controlling ON / OFF of the heater energization, and the heater application voltage control means maintains the heater energization OFF. The element temperature detection device for an air-fuel ratio sensor according to claim 1 or 2. The heater control means controls the heater energization amount by duty-controlling ON / OFF of heater energization, and the heater application voltage control means maintains the heater energization ON. The element temperature detection device for an air-fuel ratio sensor according to claim 1 or 2. 3. The element temperature detecting device for an air-fuel ratio sensor according to claim 1, wherein the heater applied voltage control means maintains a voltage value set based on an engine speed and a fuel injection amount. .

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