JP2010174790A - Control device of air-fuel ratio sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To carry out malfunction diagnosis of an air-fuel ratio sensor with high accuracy for a control device of an air-fuel sensor in which the output current value varies depending on an oxygen concentration of the exhaust gas and an applied voltage. <P>SOLUTION: When an applied voltage value corresponding to the output current value of the air-fuel ratio sensor previously stored corresponding to the operation conditions and an actual applied voltage value are compared with each other during the fuel-cut or idling operation of an engine. A judgment that voltage-current characteristics are not appropriately obtained due to the deterioration of the sensor and the like is made, and an air-fuel sensor 1 is determined to have an abnormality. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a control device for an air-fuel ratio sensor that is disposed in an exhaust system or the like of an internal combustion engine and detects an air-fuel ratio of exhaust gas. In particular, the present invention relates to abnormality diagnosis of an air-fuel ratio sensor.

  Conventionally, in a general internal combustion engine, a mixture of air and fuel is combusted in a combustion chamber, and exhaust gas after combustion is discharged outside through an exhaust passage. This exhaust passage is provided with an air-fuel ratio sensor for detecting the air-fuel ratio of the air-fuel mixture from the oxygen concentration of the exhaust gas. In an internal combustion engine, the amount of fuel in the air-fuel mixture (the amount of fuel injected from the injector) is fed back so that the air-fuel ratio detected by the air-fuel ratio sensor becomes a predetermined air-fuel ratio (usually the theoretical air-fuel ratio). Be controlled.

  As one of the air-fuel ratio sensors, for example, a limit current type sensor as disclosed in Patent Documents 1 to 3 is known.

  FIG. 4 shows voltage-current characteristics in this air-fuel ratio sensor. The solid line, the alternate long and short dash line, and the alternate long and two short dashes line in the figure are characteristics when the air-fuel ratio is stoichiometric (theoretical air-fuel ratio), rich, and lean, respectively. Is shown.

  As shown in FIG. 4, in this type of air-fuel ratio sensor, the output current (hereinafter referred to as current or output current value) is applied even if the applied voltage (hereinafter may be simply referred to as voltage) is changed as voltage-current characteristics. There is a region in which a constant current (hereinafter referred to as “limit current value”) is maintained with little change. Hereinafter, this region (the voltage range in which the current is maintained constant) is referred to as a “limit current region”. In FIG. 4, Vst is a limiting current region when the air-fuel ratio is stoichiometric, Vri is a limiting current region when the air-fuel ratio is rich, and Vle is a limiting current region when the air-fuel ratio is lean. is there.

  Further, when the voltage becomes higher than the voltage in the limit current region, the current increases in proportion to the increase in the voltage. Conversely, when the voltage becomes lower than the voltage in the limit current region, the current decreases in proportion to the decrease in the voltage. For this reason, in the limit current type air-fuel ratio sensor, it is possible to accurately detect the air-fuel ratio based on the output current of the sensor by adjusting the applied voltage within the limit current range.

  The limit current region shifts to the higher voltage side as the oxygen concentration increases. Therefore, in order to accurately detect the air-fuel ratio, it is necessary to correctly control the applied voltage to the air-fuel ratio sensor in the limit current region. Generally, a target applied voltage line indicated by a broken line in the figure is set. The applied voltage is determined on the target applied voltage line in accordance with the current value each time (every time the current value changes as the air-fuel ratio changes). Specifically, a voltage value (voltage value on the target applied voltage line) as a median value of the limit current region is obtained for each current value that is the output of the air-fuel ratio sensor, and the applied voltage is determined according to the current value. It is adjusted to be the median value of the basin. As a result, the applied voltage is prevented from deviating from the limit current region, and thereby the oxygen concentration detection accuracy can be increased.

Japanese Patent Laid-Open No. 1-211638 Japanese Patent Laid-Open No. 10-62376 JP 2002-243694 A

  By the way, in this type of air-fuel ratio sensor, when the sensor element is cracked or platinum on the electrode is agglomerated, the voltage-current characteristic cannot be obtained properly, and the oxygen concentration detection accuracy is not obtained. May get worse.

  Therefore, it is necessary to periodically perform abnormality diagnosis of the air-fuel ratio sensor. For example, in Patent Document 1, it is determined that a sensor failure has occurred when an applied voltage continues a value outside a predetermined range for a predetermined time or longer when the air-fuel ratio sensor is activated.

  However, the abnormality diagnosis operation disclosed in Patent Document 1 cannot be said to have sufficient reliability. This is because the current value that is the output of the air-fuel ratio sensor changes depending on the air-fuel ratio of the exhaust gas and the state of the applied voltage. Therefore, in order to perform abnormality diagnosis with high accuracy, the air-fuel ratio of the exhaust gas at the time of abnormality diagnosis Must be recognized correctly. However, in Patent Document 1, there is a possibility that abnormality diagnosis of the air-fuel ratio sensor may be performed in a situation where the air-fuel ratio is unstable, and thus it cannot be said that the reliability of the diagnosis is sufficiently ensured.

  The present invention has been made in view of such a point, and an object of the present invention is to provide an air-fuel ratio control apparatus for an air-fuel ratio sensor in which an output current changes according to an oxygen concentration of an exhaust gas and an applied voltage. This is to perform sensor abnormality diagnosis with high accuracy.

-Principle of solving the problem-
The solution principle of the present invention taken to achieve the above object is that when the oxygen concentration of the exhaust gas is recognizable in advance, specifically, the fuel supply stop state and the idling operation state are entered. In this case, abnormality diagnosis is performed according to the voltage applied to the air-fuel ratio sensor.

-Solution-
Specifically, the present invention is provided in the exhaust passage of the internal combustion engine, and the output current value when energized changes according to the applied voltage and the oxygen concentration of the exhaust gas. A control device for an air-fuel ratio sensor that detects the oxygen concentration of exhaust gas based on the above is assumed. An abnormality diagnosis execution means for executing an abnormality diagnosis of the air-fuel ratio sensor based on the value of the applied voltage when the operation state of the internal combustion engine is in a fuel supply stop state or an idling operation state. Is provided.

  Specific examples of the abnormality diagnosis operation of the air-fuel ratio sensor by the abnormality diagnosis execution means include the following.

  First, when the operation state of the internal combustion engine is in a fuel supply stop state or an idling operation state, the value of the applied voltage is an appropriate applied voltage that is preset with respect to an output current value corresponding to the operation state of the internal combustion engine. The abnormality diagnosis execution means is configured to diagnose that the air-fuel ratio sensor is abnormal when out of the range.

  That is, when the fuel supply of the internal combustion engine is stopped, almost no fuel flows in the exhaust system, and the output current of the air-fuel ratio sensor has the value that is the leanest (for example, the output current is maximum). Further, during idling operation of the internal combustion engine, generally, the air-fuel ratio is stably maintained at the theoretical air-fuel ratio, and the output current value of the air-fuel ratio sensor is “0”. In this solution, the abnormality diagnosis of the air-fuel ratio sensor is performed in such a state that the air-fuel ratio is recognized and stabilized in advance. Then, the actual applied voltage value deviates from an appropriate applied voltage range (for example, a predetermined range in the above-described limit current range) with respect to the output current value that is substantially uniquely determined according to the air-fuel ratio. If so, the air-fuel ratio sensor is diagnosed as abnormal. That is, when no abnormality has occurred in the air-fuel ratio sensor, the actual applied voltage should be within the appropriate range of the applied voltage, so that it is recognized that the applied voltage is outside this range. Thus, the abnormality of the air-fuel ratio sensor can be accurately diagnosed.

  As another solution, the air-fuel ratio sensor has a voltage-current characteristic with a limit current region in which the output current hardly changes even when the applied voltage is changed, and the operating state of the internal combustion engine is While in the fuel supply stop state, the applied voltage is gradually changed, and the applied voltage value when the output current value starts to change is previously set to the output current value corresponding to the fuel supply stop state. The abnormality diagnosis execution means is configured to diagnose that the air-fuel ratio sensor is abnormal when it deviates from the set current change start voltage value.

  In this case, if there is no abnormality in the air-fuel ratio sensor, when the applied voltage is gradually increased, the output is performed from the time when the maximum voltage in the limit current region (corresponding to the current change start voltage value) is reached. The current value changes (the output current value increases). In addition, when the applied voltage is gradually lowered, the output current value changes (output current value decreases) from the point when the minimum voltage (corresponding to the current change start voltage value) in the above limit current region is reached. It will follow. On the other hand, if an abnormality occurs in the air-fuel ratio sensor, the applied voltage value at which the change in the output current value is started is different from that described above, and the applied voltage value (the change in the output current value starts). Applied voltage value) deviates from the current change start voltage value. Thereby, the abnormality of the air-fuel ratio sensor can be accurately diagnosed.

  Furthermore, as another solution, the air-fuel ratio sensor has a voltage-current characteristic having a limit current region in which the output current value hardly changes even when the applied voltage is changed. When the fuel supply is stopped, the applied voltage is gradually changed, and when the output current value changes, the change amount of the output current value with respect to the unit change amount of the applied voltage is a predetermined abnormality determination change. The abnormality diagnosis execution means is configured to diagnose that the air-fuel ratio sensor is abnormal when the amount is smaller than the amount.

  In general, when an abnormality such as deterioration occurs in the air-fuel ratio sensor, the unit change amount of the applied voltage in a region outside the limit current region (region where the output current value changes proportionally with the change in the applied voltage) The amount of change in the output current value with respect to is small. That is, the change rate of the output current value is small. In the present solution, the abnormality of the air-fuel ratio sensor can be accurately diagnosed by making it possible to recognize this while gradually changing the applied voltage.

  As described above, the following methods are available as a countermeasure when the amount of change in the current value with respect to the unit change amount of the applied voltage becomes small. First, the applied voltage is adjusted so that the applied voltage becomes substantially the central voltage value in the limit current region in accordance with the change in the output current value of the air-fuel ratio sensor accompanying the change in the oxygen concentration of the exhaust gas. On the other hand, when it is diagnosed that the air-fuel ratio sensor is abnormal, the center voltage value of the limit current region in this abnormal state is set in accordance with the amount of change in the output current value with respect to the unit change amount of the actual applied voltage. Thus, an applied voltage adjusting means for adjusting the applied voltage to the air-fuel ratio sensor is provided.

  According to this, even if an abnormality occurs in the air-fuel ratio sensor, the output current value corresponding to the air-fuel ratio of the exhaust gas can be obtained by adjusting the applied voltage accordingly, and the air-fuel ratio sensor is continued. Can be used.

  Further, when the air-fuel ratio sensor is diagnosed as abnormal, the detection of the oxygen concentration of the exhaust gas based on the output current value corresponding to the voltage range where the application is impossible is limited, and the application becomes possible. Only the detection of the oxygen concentration of the exhaust gas based on the output current value corresponding to the voltage range is executed.

  This also makes it possible to continue using the air-fuel ratio sensor although the range of use is limited. Further, when the oxygen concentration of the exhaust gas is a concentration (for example, leaner than stoichiometric) that can obtain an output current value corresponding to the voltage range in which the application is possible, the air-fuel ratio sensor is set to a normal sensor. Can be used as well.

  In the present invention, the oxygen concentration is recognized in advance as the operating state of the internal combustion engine for the air-fuel ratio sensor that can detect the oxygen concentration of the exhaust gas by changing the output current value according to the applied voltage and the oxygen concentration of the exhaust gas. When a possible operating state is reached, abnormality diagnosis is performed according to the voltage applied to the air-fuel ratio sensor. This makes it possible to perform abnormality diagnosis of the air-fuel ratio sensor with high accuracy.

It is a figure which shows schematic structure of the engine which concerns on embodiment, and its intake / exhaust system. It is sectional drawing which shows schematic structure of an air fuel ratio sensor. It is a circuit diagram which shows the control circuit of an air fuel ratio sensor. It is a figure which shows the voltage-current characteristic of an air fuel ratio sensor. It is a flowchart figure which shows the operation | movement procedure of the air fuel ratio sensor abnormality diagnosis which concerns on 1st Embodiment. It is a voltage-current characteristic view for explaining an air-fuel ratio sensor abnormality diagnosis operation at the time of fuel cut. FIG. 6 is a voltage-current characteristic diagram for explaining an air-fuel ratio sensor abnormality diagnosis operation during idling operation. It is a voltage-current characteristic view for explaining an air-fuel ratio sensor abnormality diagnosis operation according to the second embodiment. It is a flowchart figure which shows the operation | movement procedure of the air-fuel ratio sensor abnormality diagnosis which concerns on 3rd Embodiment, and the coping operation | movement procedure at the time of sensor abnormality. It is a voltage-current characteristic diagram for demonstrating the air fuel ratio sensor abnormality diagnosis operation | movement which concerns on 3rd Embodiment, and the coping operation | movement at the time of sensor abnormality.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In this embodiment, the case where the present invention is applied to an air-fuel ratio sensor disposed in an exhaust system of a four-cylinder gasoline engine (internal combustion engine) will be described.

-Engine-
FIG. 1 is a schematic configuration diagram showing an example of an engine 101 to which the present invention is applied. FIG. 1 shows only the configuration of one cylinder of the engine 101.

  The engine 101 according to the present embodiment is, for example, a four-cylinder gasoline engine, and includes a piston 101b that forms a combustion chamber 101a and a crankshaft 115 that is an output shaft. The piston 101b is connected to the crankshaft 115 via a connecting rod 116, and the reciprocating motion of the piston 101b is converted into rotation of the crankshaft 115 by the connecting rod 116.

  A signal rotor 117 having a plurality of protrusions (teeth) 117a, 117a,... Is attached to the crankshaft 115 on the outer peripheral surface. A crank position sensor (engine speed sensor) 124 is disposed near the side of the signal rotor 117. The crank position sensor 124 is an electromagnetic pickup, for example, and generates a pulsed signal (output pulse) corresponding to the protrusion 117a of the signal rotor 117 when the crankshaft 115 rotates.

  The cylinder block 101c of the engine 101 is provided with a water temperature sensor 121 that detects the engine water temperature (cooling water temperature). A spark plug 103 is disposed in the combustion chamber 101 a of the engine 101. The ignition timing of the spark plug 103 is adjusted by the igniter 104. The igniter 104 is controlled by an ECU (Electronic Control Unit) 200.

  An intake passage 111 and an exhaust passage 112 are connected to the combustion chamber 101 a of the engine 101. An intake valve 113 is provided between the intake passage 111 and the combustion chamber 101a. By opening and closing the intake valve 113, the intake passage 111 and the combustion chamber 101a are communicated or blocked. Further, an exhaust valve 114 is provided between the exhaust passage 112 and the combustion chamber 101a. By opening and closing the exhaust valve 114, the exhaust passage 112 and the combustion chamber 101a are communicated or blocked. The opening / closing drive of the intake valve 113 and the exhaust valve 114 is performed by each rotation of the intake camshaft and the exhaust camshaft to which the rotation of the crankshaft 115 is transmitted.

  An air cleaner 107, a hot-wire air flow meter 122, an intake air temperature sensor 123 (built in the air flow meter 122), and an electronically controlled throttle valve 105 for adjusting the intake air amount of the engine 101 are disposed in the intake passage 111. Has been. The throttle valve 105 is driven by a throttle motor 106. The opening degree of the throttle valve 105 is detected by a throttle opening degree sensor 125.

  In the exhaust passage 112 of the engine 101, an air-fuel ratio sensor (exhaust gas sensor) 1 for detecting the oxygen concentration in the exhaust gas, a three-way catalyst 108, and the like are arranged. Details of the air-fuel ratio sensor 1 will be described later.

  An injector (fuel injection valve) 102 for fuel injection is disposed in the intake passage 111. Fuel of a predetermined pressure is supplied from the fuel tank to the injector 102 by a fuel pump, and the fuel is injected into the intake passage 111. This injected fuel is mixed with the intake air to be mixed into the combustion chamber 101a of the engine 101. The air-fuel mixture (fuel + air) introduced into the combustion chamber 101a is ignited by the spark plug 103 and combusted / exploded. The piston 101b reciprocates due to the combustion / explosion of the air-fuel mixture in the combustion chamber 101a, and the crankshaft 115 rotates. The operation state of the engine 101 is controlled by the ECU 200.

-ECU-
The ECU 200 includes a CPU 201 (see FIG. 3), a ROM, a RAM, a backup RAM, and the like. The ROM stores various control programs, maps that are referred to when the various control programs are executed, and the like. The CPU 201 executes arithmetic processing based on various control programs and maps stored in the ROM. The RAM is a memory for temporarily storing calculation results from the CPU, data input from each sensor, and the like. The backup RAM is a non-volatile memory for storing data to be saved when the engine 101 is stopped. is there.

  As shown in FIG. 1, various sensors such as a water temperature sensor 121, an air flow meter 122, an intake air temperature sensor 123, a crank position sensor 124, a throttle opening sensor 125, and an air-fuel ratio sensor 1 are connected to the ECU 200. . The ECU 200 is connected to an injector 102, an igniter 104 of a spark plug 103, a throttle motor 106 of a throttle valve 105, and the like.

  The ECU 200 executes various controls of the engine 101 including the fuel injection amount feedback control and the ignition timing control of the spark plug 103 based on the detection signals of the various sensors described above. Further, the ECU 200 executes control of the air-fuel ratio sensor 1 and energization control of the heater 2 which will be described later.

-Air-fuel ratio sensor-
Next, the structure of the air-fuel ratio sensor 1 will be described.

  The air-fuel ratio sensor 1 according to the present embodiment is a stacked air-fuel ratio sensor that outputs a signal corresponding to the oxygen concentration in exhaust gas. As shown in FIG. 2, the sensor element 10 has air permeability. A cover 16 and an outer cover 17 are provided. Further, the air-fuel ratio sensor 1 incorporates a heater 2 for heating the sensor element 10.

  The sensor element 10 includes a plate-shaped solid electrolyte layer (for example, partially stabilized zirconia) 11, an atmosphere side electrode (for example, a platinum electrode) 12 formed on one surface of the solid electrolyte layer 11, and the other of the solid electrolyte layer 11. The exhaust-side electrode (for example, platinum electrode) 13 and the diffusion resistance layer (for example, porous ceramic) 14 formed on the surface are formed.

  The atmosphere side electrode 12 of the sensor element 10 is disposed in the atmosphere duct 15. The atmosphere duct 15 is open to the atmosphere, and the atmosphere flowing into the atmosphere duct 15 contacts the atmosphere side electrode 12.

  The surface of the exhaust side electrode 13 is covered with the diffusion resistance layer 14, and a part of the exhaust gas flowing through the exhaust passage 112 contacts the exhaust side electrode 13 while being diffused by the diffusion resistance layer 14. The exhaust gas passes through the small hole 17a of the outer cover 17 and the small hole 16a of the inner cover 16, and reaches the sensor element 10 (exhaust side electrode 13).

  The heater 2 is composed of a linear heating element that generates heat by energization from the on-vehicle battery power supply VB, and heats the entire sensor element 10 by the heat generated by the heating element.

  In the air-fuel ratio sensor 1 having the above structure, a detection voltage VR (= [VAF +] − [VAF−]), which will be described later, is applied between the atmosphere side electrode 12 and the exhaust side electrode 13. A current corresponding to the oxygen concentration in the exhaust gas flows through the fuel ratio sensor 1.

  More specifically, when the air-fuel ratio of the exhaust gas is lean, surplus oxygen in the exhaust gas is ionized by receiving electrons by an electrode reaction at the exhaust-side electrode 13. When the oxygen ions move in the solid electrolyte layer 11 in the direction of the exhaust side electrode 13 → the atmosphere side electrode 12 and reach the atmosphere side electrode 12, electrons are desorbed there, return to oxygen, and discharged to the atmosphere duct 15. . By such movement of oxygen ions, a current (positive current) flows in the direction from the atmosphere-side electrode 12 to the exhaust-side electrode 13.

On the other hand, when the air-fuel ratio of the exhaust gas is rich, on the contrary to the case of lean described above, oxygen in the atmospheric duct 15 receives electrons by the electrode reaction at the atmospheric side electrode 12 and is ionized. After the oxygen ions move in the solid electrolyte layer 11 from the atmosphere side electrode 12 to the exhaust side electrode 13, catalytic oxygen reacts with unburned components HC, CO, H ₂ existing in the diffusion resistance layer 14. Carbon dioxide CO ₂ and water H ₂ O are generated. By such movement of oxygen ions, a current (negative current) flows in the direction from the exhaust side electrode 13 to the atmosphere side electrode 12.

  Since the value of the current flowing through the air-fuel ratio sensor 1 (sensor current Is) changes according to the air-fuel ratio of the exhaust gas, the relationship between the value of the sensor current Is and the exhaust gas air-fuel ratio is obtained by experiments and calculations. If so, the air-fuel ratio of the exhaust gas can be calculated by detecting the value of the sensor current Is. The increase / decrease of the sensor current Is corresponds to the increase / decrease of the air / fuel ratio (lean / rich degree). The sensor current Is increases as the air / fuel ratio of the exhaust gas becomes leaner, and the air / fuel ratio becomes richer. The sensor current Is decreases.

-Control circuit-
Next, the configuration of the control circuit 20 that controls the air-fuel ratio sensor 1 will be described with reference to FIG. The control circuit 20 of the air-fuel ratio sensor 1 is incorporated in the ECU 200.

  The control circuit 20 of the air-fuel ratio sensor 1 according to this embodiment includes operational amplifiers OP1 to OP4, a shunt resistor R1, resistors R2 to R7, a reference voltage generation circuit 21, a pulse application circuit 22, a transistor 23, and the like.

  The reference voltage generation circuit 21 includes three resistors R11, R12, and R13 connected in series between the power supply VC and the ground, and a voltage VAF + (3.3 V) is generated between the resistors R11 and R12. The voltage VFA− (2.9V) is generated between the resistors R12 and R13.

  The voltage VAF + generated between the resistor R11 and the resistor R12 is applied to the positive terminal of the operational amplifier OP1 through the operational amplifier OP3 and the pulse application circuit 22. Further, the voltage VAF− generated between the resistors R12 and R13 is directly applied to the plus side terminal of the operational amplifier OP2.

  The output terminal of the operational amplifier OP1 is connected to the positive side input terminal (terminal on the atmosphere side electrode 12) 1a of the air-fuel ratio sensor 1 through the shunt resistor R1, and the negative side of the operational amplifier P1 through the resistors R1 and R2. Connected to the terminal.

  The output terminal of the operational amplifier OP2 is connected to the negative side input terminal (terminal on the exhaust side electrode 13) 1b of the air-fuel ratio sensor 1 through resistors R3 and R5, and the negative terminal of the operational amplifier OP2 through resistors R3 and R4. Connected to the side terminal.

  A pulse applying circuit 22 is provided between the output terminal of the operational amplifier OP3 and the positive terminal of the operational amplifier OP1. The pulse application circuit 22 changes the voltage applied to the plus side terminal of the operational amplifier OP1, that is, the voltage VAF + (3.3V) applied to the plus side input terminal 1a (atmosphere side electrode 12) of the air-fuel ratio sensor 1 with a predetermined change. This is a circuit that changes a positive side and a negative side in a predetermined cycle with a width ΔVAF (for example, ΔVAF = 0.2 V). Thereby, the voltage applied to the air-fuel ratio sensor 1 can be adjusted. Various configurations can be applied as a configuration for changing the voltage applied to the air-fuel ratio sensor 1 as a known configuration.

  One end of the heater 2 is connected to a battery power source VB. A transistor 23 is connected to the other end of the heater 2. The base of the transistor 23 is connected to the CPU 201, and energization of the heater 2 is controlled by turning on / off the transistor 23 in accordance with a heater control signal from the CPU 201.

  In the above circuit configuration, the same current as the current flowing through the air-fuel ratio sensor 1 (sensor current Is) flows through the shunt resistor R1 connected between the operational amplifier OP1 and the air-fuel ratio sensor 1, so that the shunt resistance R1 The both-end potential difference (V1-V2) is a value proportional to the sensor current Is. Therefore, the sensor current Is of the air-fuel ratio sensor 1 can be detected by calculating the potential difference (V1−V2) across the shunt resistor R1.

  Therefore, in this embodiment, the voltage V2 on the air-fuel ratio sensor 1 side of the shunt resistor R1 is guided to the negative terminal of the operational amplifier OP4 via the resistor R7, and the voltage V1 on the opposite side of the shunt resistor R1 from the air-fuel ratio sensor 1 side. Is led to the positive terminal of the operational amplifier OP4 via a resistor R8, and the difference (V1−V2) between these voltages is calculated and amplified.

  The output signal V3 of the operational amplifier OP4 is digitally converted by an analog-digital converter (ADC) 24 for detecting a sensor current and then input to the CPU 201. The CPU 201 calculates the sensor current Is from the output signal V3 (after digital conversion) of the operational amplifier OP4, and calculates the air-fuel ratio of the exhaust gas based on the sensor current Is.

  Further, the voltage V1 and the voltage V2 at both ends of the shunt resistor R1 are input to the CPU 201 through analog / digital converters (ADC) 25 and 26 for impedance detection, respectively. In the CPU 201, the element impedance (AC impedance) of the air-fuel ratio sensor 1 is based on these input voltages V1, V2 and the change width ΔVAF of the voltage VAF + applied to the plus-side input terminal 1a (atmosphere-side electrode 12) of the air-fuel ratio sensor 1. ) Is calculated.

  Then, ECU 200 feedback-controls the amount of fuel injected from injector 102 to intake passage 111 so that the actual air-fuel ratio calculated in the above processing matches the target air-fuel ratio.

  The ECU 200 obtains the temperature of the sensor element 10 based on the element impedance of the air-fuel ratio sensor 1 calculated by the above processing, and the actual element temperature coincides with the target value 750 ° C. (sensor element activation temperature). As described above, the duty ratio of the heater control signal to the transistor 23 is calculated to control the energization of the heater 2.

  The ECU 200 including the air-fuel ratio sensor 1 and the control circuit 20 described above constitutes a sensor device.

-Characteristics of the air-fuel ratio sensor 1-
Next, voltage-current characteristics of the air-fuel ratio sensor 1 will be described.

  FIG. 4 is a graph showing the relationship between the voltage applied to the air-fuel ratio sensor 1 and the current value that is the sensor output. The solid line, the alternate long and short dash line, and the alternate long and two short dashes line in FIG. (Fuel ratio), rich, and lean.

  As shown in FIG. 4, regardless of whether the air-fuel ratio is stoichiometric, rich or lean, the voltage-current characteristic of the air-fuel ratio sensor 1 is that the current value hardly changes even if the applied voltage is changed. There is a limit current region where the limit current value is constant. When the applied voltage becomes higher than the maximum voltage in the limit current region (hereinafter referred to as “upper limit voltage”), the current increases in proportion to the increase in the applied voltage. Conversely, when the applied voltage becomes lower than the minimum voltage in the limit current region (hereinafter referred to as “lower limit voltage”), the current decreases in proportion to the decrease in the applied voltage. For example, when the air-fuel ratio is in a stoichiometric state, the limit current region is “0.1 V” to “0.9 V”, and the magnitude of the limit current value is approximately “0 mA”. In this case, the upper limit voltage corresponds to “0.9 V”, and the lower limit voltage corresponds to “0.1 V”.

  The limit current region shifts to the higher voltage side as the oxygen concentration increases. For this reason, in order to accurately detect the air-fuel ratio, it is necessary to correctly control the applied voltage to the air-fuel ratio sensor 1 in the limit current region, and a target applied voltage line indicated by a broken line in the figure is set in advance. The applied voltage is determined according to the current value on each target applied voltage line. Specifically, a voltage value as the median value of the limit current region is obtained for each current value that is the output of the air-fuel ratio sensor 1, and the applied voltage becomes the median value of the limit current region according to this current value. It is adjusted to. Thereby, it is avoided that the applied voltage deviates from the limit current region, and detection of the oxygen concentration according to the current value is obtained with high accuracy.

-Air-fuel ratio sensor abnormality diagnosis-
Next, a plurality of embodiments for abnormality diagnosis of the air-fuel ratio sensor 1 that are operations characteristic of the present embodiment will be described.

(First embodiment)
First, the first embodiment will be described. In the present embodiment, the following two abnormality diagnoses are performed.
(1) When the applied voltage is outside the predetermined range, it is determined that an abnormality has occurred in the air-fuel ratio sensor 1.
(2) When the fuel injection of the engine 101 is stopped (when fuel supply is stopped in the present invention: hereinafter referred to as fuel cut) or during idling operation, if the applied voltage is outside the predetermined range, the air-fuel ratio sensor 1 is It is determined that an abnormality has occurred.

  Hereinafter, the abnormality diagnosis of the present embodiment will be specifically described with reference to the flowchart of FIG. 5 and the voltage-current characteristic diagrams of FIGS. 6 and 7. The routine shown in FIG. 5 is executed every predetermined time or every predetermined angle rotation of the crankshaft 115.

  First, in step ST1, it is determined whether or not the voltage applied to the air-fuel ratio sensor 1 is within a predetermined range. This determination is made every predetermined time or every predetermined angle rotation of the crankshaft 115 regardless of whether the fuel cut or idling operation is performed.

  If the applied voltage is outside the predetermined range and NO is determined in step ST1, the process proceeds to step ST7, where it is determined that an abnormality has occurred in the air-fuel ratio sensor 1.

  The predetermined range here is a predetermined range within the above-mentioned limit current region, and in this embodiment, 60% region (in the limit current region) of the central portion of the entire limit current region. 60% of the voltage value range). That is, although control as described above is performed to adjust the voltage value to the target voltage value (control so that the applied voltage becomes a value on the target applied voltage line that is the median value of the limit current region) It is determined that an abnormality has occurred in the air-fuel ratio sensor 1 when the voltage value deviates by 30% or more on the positive side or the negative side with respect to the median value of the limit current region (voltage value at the center of the limit current region). (Step ST7).

  On the other hand, if the applied voltage is within the predetermined range and YES is determined in step ST1, the process proceeds to step ST2, and it is determined whether or not the current operating state of the engine 101 is fuel cut (F / C). judge. For example, if the engine speed is equal to or higher than a predetermined value (fuel cut speed) and the accelerator is turned off (fuel cut condition) is established and fuel cut is in progress, YES is determined in step ST2. Then, the process proceeds to step ST3.

  On the other hand, if the current operating state of the engine 101 is not during fuel cut, a NO determination is made in step ST2, and this routine is temporarily terminated.

  In step ST3, it is determined whether or not the voltage applied to the air-fuel ratio sensor 1 is within a predetermined range. That is, it is determined whether or not the applied voltage is within a predetermined range in a situation where the fuel cut is being performed and most of the exhaust passage 112 is air and there is almost no fuel (abnormality diagnosis execution means). (Abnormality diagnosis operation of the air-fuel ratio sensor 1).

  The predetermined range here refers to the above limit current region when no fuel is present in the exhaust passage 112 (when the air-fuel ratio is the maximum lean), that is, when the output current value of the air-fuel ratio sensor 1 is the highest. (In the range Vle in FIG. 4), in the present embodiment, with respect to the entire limit current region, about 30% of the central portion (the voltage value in the limit current region) The range is set to about 30% of the range (corresponding to the appropriate applied voltage range in the present invention).

  In other words, despite the control for adjusting the voltage value as described above to the target voltage value, the voltage value is positive with respect to the median value of the limit current region (the voltage value at the center of the limit current region). Alternatively, when there is a deviation of 15% or more on the negative side, it is determined that an abnormality has occurred in the air-fuel ratio sensor 1 (step ST7). The above numerical values are not limited to this, and are appropriately set according to the characteristics of the air-fuel ratio sensor 1 and the like.

  FIG. 6 shows voltage-current characteristics when no abnormality occurs in the air-fuel ratio sensor 1 during the fuel cut. As described above, the abnormality determination lines L1 and L2 are set at positions shifted by 15% on the positive side and the negative side with respect to the central voltage value (voltage value on the target applied voltage line) in the limit current region. When the voltage value exceeds the abnormality determination lines L1 and L2, it is determined that an abnormality has occurred in the air-fuel ratio sensor 1.

  On the other hand, if the applied voltage is within the predetermined range and YES is determined in step ST3, the process proceeds to step ST4 to determine whether or not the current operating state of the engine 101 is idling. In general, during the idling operation, the engine 101 is in a substantially no-load state, and the stoichiometric air-fuel ratio (stoichiometric) is maintained as the air-fuel ratio. Whether or not the idling operation is being performed is determined based on the engine speed and the accelerator operation amount. For example, when the engine speed is a predetermined idling speed and the accelerator operation amount is “0”. In step ST4, YES is determined, and the process proceeds to step ST5.

  On the other hand, if the current operating state of the engine 101 is not idling, NO is determined in step ST4, and this routine is temporarily terminated.

  In step ST5, it is determined whether the voltage applied to the air-fuel ratio sensor 1 is within a predetermined range. That is, it is determined whether or not the applied voltage is within a predetermined range in a situation where the idling operation is being performed and the air-fuel ratio (exhaust air-fuel ratio) in the exhaust passage 112 is the stoichiometric air-fuel ratio (by the abnormality diagnosis executing means). Abnormality diagnosis operation of the air-fuel ratio sensor 1).

  The predetermined range here refers to the limit current region when the exhaust passage 112 has a stoichiometric air-fuel ratio (stoichiometric), that is, when the output current value of the air-fuel ratio sensor 1 is “0” in the limit current region. In this embodiment, in the present embodiment, about 50% of the central portion of the entire limit current region (about 50% of the voltage value range in the limit current region). Range: equivalent to the appropriate applied voltage range in the present invention).

  In other words, despite the control for adjusting the voltage value as described above to the target voltage value, the voltage value is positive with respect to the median value of the limit current region (the voltage value at the center of the limit current region). Alternatively, when there is a deviation of 25% or more on the negative side, it is determined that an abnormality has occurred in the air-fuel ratio sensor 1 (step ST7). The above numerical values are not limited to this, and are appropriately set according to the characteristics of the air-fuel ratio sensor 1 and the like.

  FIG. 7 shows voltage-current characteristics when no abnormality occurs in the air-fuel ratio sensor 1 during the idling operation. As described above, the abnormality determination lines L3 and L4 are set at positions shifted by 25% on the positive side and the negative side with respect to the central voltage value (voltage value on the target applied voltage line) in the limit current region. When the voltage value exceeds the abnormality determination lines L3 and L4, it is determined that an abnormality has occurred in the air-fuel ratio sensor 1.

  If the applied voltage to the air-fuel ratio sensor 1 is within the predetermined range and YES is determined in step ST5, the routine proceeds to step ST6, and this routine is terminated as it is, assuming that no abnormality has occurred in the air-fuel ratio sensor 1.

  On the other hand, if NO is determined in step ST1, step ST3, or step ST5, the process proceeds to step ST7, where it is determined that an abnormality has occurred in the air-fuel ratio sensor 1, and for example, MIL ( The warning light is turned on to alert the driver, and abnormal information is written in the diagnosis provided in the ECU 200.

  As described above, according to the present embodiment, the abnormality diagnosis of the air-fuel ratio sensor 1 is performed in a situation where the air-fuel ratio is recognized in advance and is stable. An appropriate applied voltage range set in advance for the current value corresponding to the air-fuel ratio (as described above, a range of about 30% with respect to the entire limit current region when the fuel cut is in progress, idling operation) If the actual applied voltage value deviates from the range of about 50% with respect to the entire limit current region in the case of medium, the air-fuel ratio sensor 1 is diagnosed as abnormal. Thereby, the abnormality of the air-fuel ratio sensor can be accurately diagnosed.

  In the above-described embodiment, the fuel cut state has been described with respect to the case where the fuel injection is stopped while the engine 101 is operating. Is possible. For example, an abnormality diagnosis operation of the air-fuel ratio sensor 1 may be performed when the engine is stopped in a car or a hybrid car that performs idling stop control.

(Second Embodiment)
Next, a second embodiment will be described. In the present embodiment, the applied voltage is gradually changed at the time of fuel cut of the engine 101, the applied voltage value when the current value changes accordingly is detected, and the air-fuel ratio is based on the detected applied voltage value. An abnormality diagnosis of the sensor 1 is performed. This will be specifically described below.

  The solid line in FIG. 8 shows the voltage-current characteristics when no abnormality occurs in the air-fuel ratio sensor 1 when the engine 101 is fuel cut (when the air-fuel ratio is the maximum lean). On the other hand, the broken line shows an example of the voltage-current characteristic in the case where an abnormality occurs in the air-fuel ratio sensor 1 at the same time during fuel cut. Thus, when an abnormality occurs in the air-fuel ratio sensor 1, the limit current region shifts to the high voltage side. This shift amount (voltage-current characteristic deviation amount) increases as the degree of deterioration of the air-fuel ratio sensor 1 progresses.

  For this reason, when the applied voltage is gradually changed at the time of the fuel cut of the engine 101, the applied voltage value at which the change of the current value is started is detected, and the voltage value is different from the normal value. Determines that an abnormality has occurred in the air-fuel ratio sensor 1.

  That is, a gradual change operation in which the applied voltage is gradually increased is performed, and the current value changes (the current value is reduced) from the time when the upper limit voltage (corresponding to the current change start voltage value in the present invention) is reached. When the air-fuel ratio sensor 1 is abnormal, it is determined according to the upper limit voltage.

  When no abnormality has occurred in the air-fuel ratio sensor 1, the voltage value increases from the time when the applied voltage exceeds the applied voltage Va in FIG. On the other hand, if an abnormality occurs in the air-fuel ratio sensor 1, the applied voltage value at which the change in the current value is started is different from that described above. That is, even if the applied voltage is increased to Va in the figure, the current value does not change. For example, the current value increases from the time when the applied voltage exceeds the applied voltage Vb in FIG.

  As described above, in this embodiment, the applied voltage is gradually changed, and the abnormality diagnosis of the air-fuel ratio sensor 1 is performed by detecting the applied voltage value when the current value changes accordingly. . Thereby, the abnormality of the air-fuel ratio sensor 1 can be accurately diagnosed.

  In the present embodiment, the case where the applied voltage is gradually increased as the gradually changing operation of the applied voltage has been described. However, a gradually changing operation in which the applied voltage is gradually decreased may be performed. Also in this case, when an abnormality occurs in the air-fuel ratio sensor 1, the applied voltage value at which the change in current value is started is set to a higher value than when no abnormality occurs in the air-fuel ratio sensor 1. (See applied voltages Va ′ and Vb ′ in FIG. 8).

(Third embodiment)
Next, a third embodiment will be described. In the present embodiment, the applied voltage is gradually changed at the time of fuel cut of the engine 101, and the change amount of the current value with respect to the unit change amount of the applied voltage when the current value changes is a predetermined abnormality determination change amount. If the air-fuel ratio sensor is smaller, the air-fuel ratio sensor is diagnosed as abnormal. That is, the air-fuel ratio sensor is diagnosed as abnormal when the slope of the change in the current value outside the limit current region is smaller than the proper slope.

  Hereinafter, the abnormality diagnosis of this embodiment will be described in detail with reference to the flowchart of FIG. 9 and the voltage-current characteristic diagram of FIG. Note that the solid line in FIG. 10 indicates the voltage-current characteristics at each air-fuel ratio when no abnormality has occurred in the air-fuel ratio sensor 1. On the other hand, the broken line shows an example of the voltage-current characteristic at each air-fuel ratio when an abnormality occurs in the air-fuel ratio sensor 1.

  The routine shown in FIG. 9 is executed every predetermined time or every predetermined angle rotation of the crankshaft 115.

  First, in step ST11, it is determined whether the current operating state of the engine 101 is during fuel cut (F / C) or when the engine is stopped. For example, when the engine speed is equal to or higher than a predetermined value (fuel cut speed) and the accelerator off condition (fuel cut condition) is satisfied, the fuel cut is in progress, or the operation of the engine 101 is stopped. If YES, YES is determined in step ST11 and the process proceeds to step ST12.

  On the other hand, if the current operating state of the engine 101 is neither the fuel cut nor the engine stopped, a NO determination is made in step ST11, and this routine is temporarily terminated.

  In step ST12, an applied voltage gradual change operation that gradually increases the applied voltage from the current applied voltage is started.

  Thereafter, the process proceeds to step ST13, where the applied voltage when the current value changes with the applied voltage gradual change operation (the current value at the start of change: Ia in FIG. 10). And the applied voltage when the current value (Ib in FIG. 10) is higher than the current value at the start of change by a predetermined amount. At this time, if no abnormality has occurred in the air-fuel ratio sensor 1, the applied voltage corresponding to the current value Ia is Va in the figure, and the applied voltage corresponding to the current value Ib is Vb in the figure. On the other hand, when an abnormality occurs in the air-fuel ratio sensor 1, the applied voltage corresponding to the current value Ia is Va ′ in the figure, and the applied voltage corresponding to the current value Ib is Vb ′ in the figure. . That is, when an abnormality occurs in the air-fuel ratio sensor 1, the difference between both applied voltages becomes large.

  Further, in step ST14, the change amount of the current value with respect to the unit change amount of the applied voltage (current value change ratio (inclination of current change): R) is calculated from the above values by the following equation (1).

R = (Ib−Ia) / (Vb−Va) (1)
Here, as specific values of Va and Vb in the above formula (1), Va and Vb in FIG. 10 are substituted when there is no abnormality in the air-fuel ratio sensor 1. On the other hand, when an abnormality occurs in the air-fuel ratio sensor 1, Va ′ and Vb ′ in FIG. 10 are substituted.

  Thereafter, the process proceeds to step ST15, where it is determined whether or not the current value change rate R is smaller than a preset determination threshold Ka. As will be described later, this determination threshold value Ka has a relatively small degree of deterioration of the air-fuel ratio sensor 1, for example, whether or not the lean-side air-fuel ratio detection is of such a degree that the air-fuel ratio sensor 1 can be used continuously. It is set as a value for determining whether or not.

  If the change rate R of the current value is equal to or greater than the determination threshold Ka and is within the normal range, NO is determined in step ST15, the process proceeds to step ST16, and no abnormality occurs in the air-fuel ratio sensor 1. Then, this routine is finished as it is.

  On the other hand, when the change rate R of the current value is less than the determination threshold value Ka and the determination in step ST15 is YES, the process proceeds to step ST17. In this step ST17, it is determined whether or not the change rate R of the current value is smaller than a preset determination threshold value Kb. The determination threshold value Kb is a value smaller than the determination threshold value Ka, and when the change rate R is smaller than the determination threshold value Kb, the degree of deterioration of the air-fuel ratio sensor 1 is so large that it cannot be used. It is set to a value determined as being. That is, the determination threshold value Kb is set as a value corresponding to the abnormality determination change amount in the present invention.

  If the change rate R is smaller than the determination threshold value Kb, YES is determined in step ST17, the process proceeds to step ST18, where it is determined that the air-fuel ratio sensor 1 is in an unusable state. The upper MIL (warning light) is turned on to urge the driver to warn, and abnormality information is written in the diagnosis provided in the ECU 200.

  On the other hand, if the change rate R is greater than the determination threshold value Kb, that is, if the change rate R is smaller than the determination threshold value Ka and greater than or equal to the determination threshold value Kb, NO is determined in step ST17, and step ST19 is reached. Move.

  In this step ST19, an abnormality has occurred in the air-fuel ratio sensor 1, but the adverse effect due to the abnormality is limited to a part of the region where the air-fuel ratio is rich. When the air-fuel ratio is stoichiometric or lean, the air-fuel ratio sensor 1 is continuously used. This will be specifically described below.

  In the case of step ST19, as described above, since the change rate R is smaller than that in the normal case, the target applied voltage line is changed to correspond to the change in the voltage-current characteristic. That is, the inclination angle of the target applied voltage line is reduced so that the normal target applied voltage line indicated by the alternate long and short dash line in FIG. 10 is changed to the abnormal target applied voltage line indicated by the alternate long and two short dashes line. The target applied voltage line is changed so that the voltage value at the center of the limit current region of the air-fuel ratio sensor 1 is set to the target voltage value (applied voltage adjusting operation by the applied voltage adjusting means).

  Therefore, when the air-fuel ratio sensor 1 is normal, the air-fuel ratio sensor 1 can be used until the output current value becomes Ic in FIG. 10, but the air-fuel ratio sensor 1 has deteriorated. In this case, the air-fuel ratio sensor 1 can be used only until the output current value becomes Id in FIG. For this reason, the detection of the air-fuel ratio corresponding to the output current value equal to or smaller than the output current value Id is limited, and the detection of the air-fuel ratio corresponding to the output current value higher than that is continuously performed.

  As a result, the air-fuel ratio sensor 1 can be continuously used. In addition, even when the oxygen concentration of the exhaust gas is on the lean side or on the rich side, the air-fuel ratio sensor 1 is detected in the same manner as a normal sensor for detecting the air-fuel ratio corresponding to the output current value equal to or greater than the output current value Id Can be used.

-Other embodiments-
In each of the above-described embodiments, the case where the present invention is applied to the air-fuel ratio sensor 1 disposed in the exhaust system of the four-cylinder gasoline engine has been described. The present invention is not limited to this, and can be applied to an air-fuel ratio sensor disposed in an exhaust system of a multi-cylinder gasoline engine having any other number of cylinders such as a cylinder 6-cylinder gasoline engine. The present invention can also be applied to an air-fuel ratio sensor mounted on a V-type multi-cylinder gasoline engine or a vertical multi-cylinder gasoline engine.

  Further, the present invention is not limited to a port injection type gasoline engine, but can be applied to an air-fuel ratio sensor mounted on a direct injection type gasoline engine. Furthermore, the present invention is not limited to a gasoline engine, and can also be applied to an air-fuel ratio sensor mounted on a diesel engine.

  The present invention can be applied to abnormality diagnosis of a limit current type air-fuel ratio sensor provided in an exhaust system of an automobile engine.

1 Air-fuel ratio sensor 101 Engine (internal combustion engine)
102 Injector 112 Exhaust passage

Claims (6)

Provided in the exhaust passage of the internal combustion engine, the output current value when energized changes according to the applied voltage and the oxygen concentration of the exhaust gas, and the oxygen concentration of the exhaust gas is determined based on the output current value when a predetermined voltage is applied. In the control device of the air-fuel ratio sensor to detect,
An abnormality diagnosis execution means is provided for executing an abnormality diagnosis of the air-fuel ratio sensor based on the value of the applied voltage when the operation state of the internal combustion engine is in a fuel supply stop state or an idling operation state. Control device for the fuel ratio sensor. In the control apparatus for an air-fuel ratio sensor according to claim 1,
The abnormality diagnosis execution means sets the value of the applied voltage in advance with respect to an output current value corresponding to the operation state of the internal combustion engine when the operation state of the internal combustion engine is in a fuel supply stop state or an idling operation state. A control apparatus for an air-fuel ratio sensor, wherein the air-fuel ratio sensor is diagnosed as abnormal when the applied voltage is outside the appropriate range. In the control apparatus for an air-fuel ratio sensor according to claim 1,
The air-fuel ratio sensor has a voltage-current characteristic having a limit current region in which the output current value hardly changes even when the applied voltage is changed,
The abnormality diagnosis execution means gradually changes the applied voltage when the operating state of the internal combustion engine is in a fuel supply stop state, and the applied voltage value when the output current value starts to change is The air-fuel ratio is configured to diagnose that the air-fuel ratio sensor is abnormal when the output current value corresponding to the fuel supply stop state deviates from a preset current change starting voltage value. Sensor control device. In the control apparatus for an air-fuel ratio sensor according to claim 1,
The air-fuel ratio sensor has a voltage-current characteristic having a limit current region in which the output current value hardly changes even when the applied voltage is changed,
The abnormality diagnosis execution means gradually changes the applied voltage when the operation state of the internal combustion engine is in a fuel supply stop state, and the unit for the unit change amount of the applied voltage when the output current value changes. A control apparatus for an air-fuel ratio sensor configured to diagnose that the air-fuel ratio sensor is abnormal when the change amount of the output current value is smaller than a predetermined abnormality determination change amount. In the control device for an air-fuel ratio sensor according to claim 4,
In accordance with the change in the output current value of the air-fuel ratio sensor accompanying the change in oxygen concentration of the exhaust gas, the applied voltage is adjusted so that the applied voltage becomes substantially the center voltage value in the limit current region
When it is diagnosed that the air-fuel ratio sensor is abnormal, the empty air-fuel ratio sensor is emptied so that it becomes approximately the center voltage value in the limit current region in this abnormal state according to the amount of change in the output current value with respect to the unit change amount of the actual applied voltage. An air-fuel ratio sensor control apparatus comprising an applied voltage adjusting means for adjusting an applied voltage to the fuel ratio sensor. In the control device for an air-fuel ratio sensor according to claim 4,
When the air-fuel ratio sensor is diagnosed as abnormal, it can be applied by limiting the detection of the oxygen concentration of the exhaust gas based on the output current value corresponding to the voltage range where the application is impossible. A control device for an air-fuel ratio sensor, characterized in that only the detection of the oxygen concentration of exhaust gas based on the output current value corresponding to the voltage range is executed.

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