JP2022085177A - Air-fuel ratio sensor failure detection device - Google Patents

Abstract

To detect a crack in a sold electrolyte layer as a fault mode of an air-fuel ratio sensor.SOLUTION: A fault detector 1 of an air-fuel ratio sensor 10 comprises: a voltage application circuit 3 for applying a voltage to a sensor element 11; a current detection circuit 4 for detecting the output current of the sensor element; and a fault assessment unit 6 for detecting a fault of the air-fuel ration sensor. When the engine revolution speed of an internal combustion engine drops to a prescribed value from a higher value than the prescribed value and is thereafter maintained at the prescribed value or below, the fault assessment unit acquires a first current value detected by the current detection circuit at a first timing within a period in which the engine revolution speed is maintained at the prescribed value or below and a second current value detected by the current detection circuit at a second timing after a lapse of a prescribed time from the first timing within the period in which the engine revolution speed is maintained at the prescribed value or below, and assesses that there is occurrence of crack in a solid electrolyte layer when the second current value is larger than the first current value and a difference or a ratio between the second and first current values is greater than or equal to a threshold.SELECTED DRAWING: Figure 8

Description

The present invention relates to a failure detection device for an air-fuel ratio sensor.

Conventionally, it is known that an air-fuel ratio sensor for detecting the air-fuel ratio of exhaust gas is arranged in an exhaust passage of an internal combustion engine, and the air-fuel ratio of the air-fuel mixture is feedback-controlled based on the output of the air-fuel ratio detection sensor. By such feedback control, the air-fuel ratio of the exhaust gas can be controlled to an appropriate value, and the exhaust emission can be reduced.

However, if a failure such as element cracking occurs in the air-fuel ratio sensor, the detection accuracy of the air-fuel ratio deteriorates, and the exhaust emission may deteriorate. On the other hand, Patent Document 1 describes the outside of the air-fuel ratio sensor based on the output current of the air-fuel ratio sensor detected when a reverse voltage is applied to the air-fuel ratio sensor under different operating conditions of the internal combustion engine. It is described to detect the occurrence of cracks penetrating the air chamber of the air-fuel ratio sensor.

Japanese Unexamined Patent Publication No. 2007-232709

However, in the air-fuel ratio sensor, cracks in the solid electrolyte layer may occur in addition to the cracks described in Patent Document 1. Although the air-fuel ratio sensors in which these different failure modes occur exhibit different output abnormalities, Patent Document 1 does not mention any method for detecting cracks in the solid electrolyte layer.

Therefore, an object of the present invention is to detect cracks in the solid electrolyte layer as a failure mode of the air-fuel ratio sensor.

In order to solve the above problems, in the present invention, the solid electrolyte layer having oxide ion conductivity is arranged on one side surface of the solid electrolyte layer so as to be exposed to the exhaust gas flowing through the exhaust passage of the internal combustion engine. An air-fuel ratio sensor that detects a failure of an air-fuel ratio sensor comprising a sensor element having an exhaust-side electrode and an air-side electrode arranged on the other side surface of the solid electrolyte layer so as to be exposed to the atmosphere. In a failure detection device, when a voltage application circuit that applies a current to the sensor element so that the potential of the atmosphere side electrode is higher than the potential of the exhaust side electrode, and a voltage is applied to the sensor element. A current detection circuit that detects the output current of the sensor element and a failure determination unit that determines a failure of the air-fuel ratio sensor are provided. When the value drops from a high value to the predetermined value and then is maintained below the predetermined value, it is detected by the current detection circuit at the first timing within the period during which the engine rotation speed is maintained below the predetermined value. The obtained first current value and the second current value detected by the current detection circuit at the second timing in which a predetermined time has elapsed from the first timing within the period are acquired, and the second current value is obtained. An air-fuel ratio sensor that determines that a crack has occurred in the solid electrolyte layer when it is larger than the first current value and the difference or ratio between the second current value and the first current value is equal to or larger than the threshold value. Failure detection device is provided.

According to the present invention, cracks in the solid electrolyte layer can be detected as a failure mode of the air-fuel ratio sensor.

FIG. 1 is a diagram schematically showing an internal combustion engine or the like to which a failure detection device for an air-fuel ratio sensor according to the first embodiment of the present invention is applied. FIG. 2 is a partial cross-sectional view of the air-fuel ratio sensor of FIG. FIG. 3 is a diagram showing the relationship between the air-fuel ratio of exhaust gas and the output current of the sensor element when a positive voltage is applied to the sensor element. FIG. 4 is a diagram schematically showing the configuration of the failure detection device according to the first embodiment of the present invention. FIG. 5 is a partial cross-sectional view of the air-fuel ratio sensor in which a crack has occurred. FIG. 6 is a diagram showing the relationship between the applied voltage of the sensor element and the output current when a crack occurs in the solid electrolyte layer. FIG. 7 is a time chart of the engine speed and the output current of the sensor element when the internal combustion engine is stopped. FIG. 8 is a flowchart showing a control routine for failure determination processing according to the first embodiment of the present invention. FIG. 9 is a flowchart showing a control routine for failure determination processing according to the second embodiment of the present invention. FIG. 10 is a diagram schematically showing an internal combustion engine or the like to which the failure detection device of the air-fuel ratio sensor according to the third embodiment of the present invention is applied. FIG. 11 is a flowchart showing a control routine for failure determination processing according to the third embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following description, similar components are given the same reference numbers.

<First Embodiment>
First, the first embodiment of the present invention will be described with reference to FIGS. 1 to 8.

<Explanation of the entire internal combustion engine>
FIG. 1 is a diagram schematically showing an internal combustion engine or the like to which a failure detection device for an air-fuel ratio sensor according to the first embodiment of the present invention is applied. The internal combustion engine shown in FIG. 1 is a spark-ignition type internal combustion engine. The internal combustion engine is mounted on the vehicle.

Referring to FIG. 1, 42 is a cylinder block, 43 is a piston that reciprocates in the cylinder block 42, 44 is a cylinder head fixed on the cylinder block 42, and 45 is formed between the piston 43 and the cylinder head 44. A combustion chamber, 46 is an intake valve, 47 is an intake port, 48 is an exhaust valve, and 49 is an exhaust port. The intake valve 46 opens and closes the intake port 47, and the exhaust valve 48 opens and closes the exhaust port 49.

As shown in FIG. 1, the spark plug 50 is arranged in the central portion of the inner wall surface of the cylinder head 44, and the fuel injection valve 51 is arranged in the peripheral portion of the inner wall surface of the cylinder head 44. The spark plug 50 is configured to generate sparks in response to an ignition signal. Further, the fuel injection valve 51 injects a predetermined amount of fuel into the combustion chamber 45 in response to the injection signal. In this embodiment, gasoline having a stoichiometric air-fuel ratio of 14.6 is used as the fuel.

The intake port 47 of each cylinder is connected to the surge tank 54 via the corresponding intake branch pipe 53, and the surge tank 54 is connected to the air cleaner 56 via the intake pipe 55. The intake port 47, the intake branch pipe 53, the surge tank 54, the intake pipe 55, and the like form an intake passage that guides air to the combustion chamber 45. Further, a throttle valve 58 driven by the throttle valve drive actuator 57 is arranged in the intake pipe 55. The throttle valve 58 can be rotated by the throttle valve drive actuator 57 to change the opening area of the intake passage.

On the other hand, the exhaust port 49 of each cylinder is connected to the exhaust manifold 59. The exhaust manifold 59 has a plurality of branches connected to each exhaust port 49, and an aggregate portion in which these branches are aggregated. The collecting portion of the exhaust manifold 59 is connected to the casing 61 containing the catalyst 60. The casing 61 is connected to the exhaust pipe 62. The exhaust port 49, the exhaust manifold 59, the casing 61, the exhaust pipe 62, and the like form an exhaust passage for discharging the exhaust gas generated by the combustion of the air-fuel mixture in the combustion chamber 45. The catalyst 60 is, for example, a three-way catalyst capable of simultaneously purifying hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx).

Various controls of the internal combustion engine are executed by the electronic control unit (ECU) 31 mounted on the vehicle. That is, the ECU 31 functions as a control device for the internal combustion engine. The outputs of various sensors provided in the internal combustion engine and the like are input to the ECU 31, and the ECU 31 controls various actuators based on the outputs of the various sensors and the like.

The ECU 31 is composed of a digital computer, and has a RAM (random access memory) 33, a ROM (read-only memory) 34, a CPU (microprocessor) 35, an input port 36, and an output port connected to each other via a bidirectional bus 32. 37 is provided. In this embodiment, one ECU 31 is provided, but a plurality of ECUs may be provided for each function.

An air flow meter 70 for detecting the flow rate of air flowing in the intake pipe 55 (intake air flow rate) is arranged in the intake pipe 55, and the output of the air flow meter 70 is input to the input port 36 via the corresponding AD converter 38. Will be done. Therefore, the output of the air flow meter 70 is transmitted to the ECU 31, and the ECU 31 acquires the output of the air flow meter 70.

Further, in the exhaust passage (aggregate portion of the exhaust manifold 59) on the upstream side of the catalyst 60, an air-fuel ratio sensor 10 for detecting the air-fuel ratio of the exhaust gas discharged from the combustion chamber 45 of the internal combustion engine and flowing into the catalyst 60 is arranged. Will be done. Details of the air-fuel ratio sensor 10 will be described later.

Further, a load sensor 72 that generates an output voltage proportional to the amount of depression of the accelerator pedal 71 is connected to the accelerator pedal 71 provided in the vehicle equipped with the internal combustion engine, and the output voltage of the load sensor 72 is converted to the corresponding AD. It is input to the input port 36 via the device 38. Therefore, the output of the load sensor 72 is transmitted to the ECU 31, and the ECU 31 acquires the output of the load sensor 72. The ECU 31 calculates the engine load based on the output of the load sensor 72.

Further, a crank angle sensor 73 that generates an output pulse each time the crankshaft rotates by a predetermined angle (for example, 10 °) is connected to the input port 36, and this output pulse is input to the input port 36. Therefore, the output of the crank angle sensor 73 is transmitted to the ECU 31, and the ECU 31 acquires the output of the crank angle sensor 73. The ECU 31 calculates the engine speed based on the output of the crank angle sensor 73.

On the other hand, the output port 37 is connected to various actuators of the internal combustion engine via the corresponding drive circuit 39. In the present embodiment, the output port 37 is connected to the spark plug 50, the fuel injection valve 51, and the throttle valve drive actuator 57, and the ECU 31 controls them. Specifically, the ECU 31 controls the ignition timing of the spark plug 50, the injection timing and injection amount of the fuel injection valve 51, and the opening degree of the throttle valve 58.

The above-mentioned internal combustion engine is a non-supercharged internal combustion engine that uses gasoline as fuel, but the configuration of the internal combustion engine is not limited to the above configuration. Therefore, even if the specific configuration of the internal combustion engine such as the cylinder arrangement, fuel injection mode, intake / exhaust system configuration, valve mechanism configuration, and presence / absence of a supercharger is different from the configuration shown in FIG. good. For example, the fuel injection valve 51 may be arranged so as to inject fuel into the intake port 47.

<Structure of air-fuel ratio sensor>
Hereinafter, the configuration of the air-fuel ratio sensor 10 will be described in detail. FIG. 2 is a partial cross-sectional view of the air-fuel ratio sensor 10 of FIG. As shown in FIG. 2, the air-fuel ratio sensor 10 includes a sensor element 11 and a heater 20.

In the present embodiment, the air-fuel ratio sensor 10 is a laminated air-fuel ratio sensor configured by laminating a plurality of layers. The sensor element 11 has a solid electrolyte layer 12, a diffusion rate controlling layer 13, a first impermeable layer 14, a second impermeable layer 15, an exhaust side electrode 16, and an atmospheric side electrode 17. The solid electrolyte layer 12, the exhaust side electrode 16 and the atmosphere side electrode 17 constitute a sensor cell which is an electrochemical cell.

Each layer of the sensor element 11 is laminated in the order of the first impermeable layer 14, the solid electrolyte layer 12, the diffusion rate controlling layer 13, and the second impermeable layer 15 from the lower part of FIG. A gas chamber 18 to be measured is formed between the solid electrolyte layer 12 and the diffusion rate-determining layer 13, and an air chamber 19 is formed between the solid electrolyte layer 12 and the first impermeable layer 14. That is, the side gas chamber 18 is defined by the solid electrolyte layer 12 and the diffusion rate controlling layer 13, and the atmosphere chamber 19 is defined by the solid electrolyte layer 12 and the first impermeable layer 14.

The gas chamber 18 to be measured communicates with the exhaust passage of the internal combustion engine via the diffusion rate control layer 13, and the exhaust gas flowing through the exhaust passage of the internal combustion engine is introduced into the gas chamber 18 to be measured as the gas to be measured. On the other hand, the atmosphere chamber 19 is open to the atmosphere, and the atmosphere is introduced into the atmosphere chamber 19.

The solid electrolyte layer 12 is a thin plate having oxide ion conductivity. The solid electrolyte layer 12 is, for example, a sintered body obtained by adding CaO, MgO, Y ₂ O ₃ , Yb ₂ O ₃ , etc. to ZrO ₂ (zirconia), HfO ₂ , ThO ₂ , Bi ₂ O ₃ , etc. as stabilizers. be. The diffusion rate-determining layer 13 is a thin plate having gas permeability. The diffusion rate-determining layer 13 is composed of, for example, a porous ceramic such as alumina, magnesia, silica stone, spinel, and mullite. The first impermeable layer 14 and the second impermeable layer 15 are gas impermeable thin plates, and are composed of, for example, alumina.

The exhaust side electrode 16 is on the side surface of one of the solid electrolyte layers 12 (on the side of the gas chamber 18) so as to be exposed to the gas to be measured in the gas chamber 18 to be measured, that is, the exhaust gas flowing through the exhaust passage of the internal combustion engine. Is located in. On the other hand, the atmospheric side electrode 17 is arranged on the side surface of the other side (atmospheric chamber 19 side) of the solid electrolyte layer 12 so as to be exposed to the atmosphere in the atmospheric chamber 19. The exhaust side electrode 16 and the atmosphere side electrode 17 are arranged so as to face each other with the solid electrolyte layer 12 interposed therebetween. The exhaust side electrode 16 and the atmosphere side electrode 17 are each made of a noble metal having high catalytic activity such as platinum (Pt). For example, the exhaust side electrode 16 and the atmosphere side electrode 17 are porous cermet electrodes containing Pt as a main component.

The heater 20 is arranged in the sensor element 11 and heats the sensor element 11. In this embodiment, the heater 20 is embedded in the first impermeable layer 14. The heater 20 is a cermet thin plate containing, for example, platinum (Pt) and ceramic (for example, alumina), and is a heating element that generates heat when energized. The heater 20 is electrically connected to the ECU 31 and is controlled by the ECU 31. The ECU 31 controls the temperature of the sensor element 11, particularly the temperature of the solid electrolyte layer 12, by the heater 20.

As shown in FIG. 2, an electric circuit 2 is connected to the exhaust side electrode 16 and the atmosphere side electrode 17 of the sensor element 11. The electric circuit 2 has a voltage application circuit 3 and a current detection circuit 4.

The voltage application circuit 3 is connected to the sensor element 11 and applies a voltage to the sensor element 11. Specifically, the voltage application circuit 3 applies a voltage to the sensor element 11 so that the potential of the atmospheric side electrode 17 is higher than the potential of the exhaust side electrode 16. Therefore, the exhaust side electrode 16 functions as a negative electrode, and the atmospheric side electrode 17 functions as a positive electrode. The voltage application circuit 3 is electrically connected to the ECU 31, and the ECU 31 controls the voltage applied to the sensor element 11 via the voltage application circuit 3.

The current detection circuit 4 is connected to the sensor element 11, and when a voltage is applied to the sensor element 11, a current flowing between the exhaust side electrode 16 and the atmosphere side electrode 17, that is, a voltage is applied to the sensor element 11. The output current of the sensor element 11 when the sensor element 11 is used is detected. The current detection circuit 4 is electrically connected to the ECU 31, and the output of the current detection circuit 4 is transmitted to the ECU 31.

When a voltage is applied to the sensor element 11, oxide ions move between the exhaust side electrode 16 and the atmosphere side electrode 17 according to the air-fuel ratio of the exhaust gas on the exhaust side electrode 16, and as a result, the exhaust gas. The output current of the sensor element 11 changes according to the air-fuel ratio of. FIG. 3 is a diagram showing the relationship between the air-fuel ratio of the exhaust gas when a voltage is applied to the sensor element 11 and the output current I of the sensor element 11. In the example of FIG. 3, as the voltage applied to the sensor element 11, a voltage within the limit current region in which the output current hardly changes even if the applied voltage changes, specifically 0.45 V is used.

As can be seen from FIG. 3, when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio, the output current I becomes zero. Further, in the air-fuel ratio sensor 10, the higher the oxygen concentration of the exhaust gas, that is, the leaner the air-fuel ratio of the exhaust gas, the larger the output current I. Therefore, the air-fuel ratio sensor 10 can continuously (linearly) detect the air-fuel ratio of the exhaust gas. The ECU 31 feedback-controls the air-fuel ratio of the air-fuel mixture to an appropriate value based on the air-fuel ratio detected by the air-fuel ratio sensor 10. This makes it possible to reduce exhaust emissions in the internal combustion engine.

<Failure detection device for air-fuel ratio sensor>
However, if a failure such as element cracking occurs in the air-fuel ratio sensor 10, the detection accuracy of the air-fuel ratio deteriorates, and the exhaust emission may deteriorate. Therefore, in the present embodiment, the failure of the air-fuel ratio sensor 10 is detected by using the failure detection device of the air-fuel ratio sensor (hereinafter, simply referred to as “fault detection device”).

FIG. 4 is a diagram schematically showing the configuration of the failure detection device 1 according to the first embodiment of the present invention. The failure detection device 1 includes a voltage application circuit 3, a current detection circuit 4, a voltage control unit 5, and a failure determination unit 6. In this embodiment, the ECU 31 functions as a voltage control unit 5 and a failure determination unit 6. The voltage control unit 5 and the failure determination unit 6 are functional modules realized by the CPU 35 of the ECU 31 executing the program stored in the ROM 34 of the ECU 31.

The voltage control unit 5 applies a voltage to the sensor element 11 via the voltage application circuit 3. The failure determination unit 6 determines the failure of the air-fuel ratio sensor 10. In the present embodiment, the failure determination unit 6 detects a crack in the solid electrolyte layer 12 as a failure mode of the air-fuel ratio sensor 10.

FIG. 5 is a partial cross-sectional view of the air-fuel ratio sensor 10 in which a crack has occurred. The crack 12a of the solid electrolyte layer 12 extends between the gas chamber 18 and the atmosphere chamber 19.

In the normal air-fuel ratio sensor 10, the air chamber 19 is closed by the first impermeable layer 14 and the solid electrolyte layer 12, and the communication between the air chamber 19 and the gas chamber 18 is cut off. However, as shown in FIG. 5, when the crack 12a communicating the gas chamber 18 and the atmosphere chamber 19 is generated in the solid electrolyte layer 12, the exhaust gas of the gas chamber 18 and the atmosphere of the atmosphere chamber 19 are separated from each other. Be mixed. As a result, the oxygen concentration in the exhaust gas of the gas chamber 18 to be measured becomes high, and the oxygen concentration in the atmosphere of the air chamber 19 decreases. Therefore, the occurrence of the crack 12a causes an abnormality in the output current of the sensor element 11.

FIG. 6 is a diagram showing the relationship between the applied voltage and the output current of the sensor element 11 when the crack 12a is generated in the solid electrolyte layer 12. The solid line, one-point chain line, and broken line in FIG. 6 are the normal sensor element 11 (without crack 12a) when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio (14.6), and the abnormality when the intake air flow rate is large, respectively. The voltage and current characteristics of the sensor element 11 (with crack 12a) and the abnormal sensor element 11 (with crack 12a) when the intake air flow rate is small are shown.

The output current of the sensor element 11 when a voltage is applied to the sensor element 11 changes according to the air-fuel ratio of the exhaust gas as shown in FIG. As described above, when the crack 12a is generated in the solid electrolyte layer 12, the oxygen concentration in the exhaust gas of the gas chamber 18 to be measured becomes high. Therefore, as shown in FIG. 6, when the crack 12a occurs in the solid electrolyte layer 12, the output current of the sensor element 11 shifts to the lean side (plus side).

Further, when the intake air flow rate is high, the pressure of the exhaust gas flowing into the test gas chamber 18 becomes higher than when the intake air flow rate is low, and the exhaust gas in the test gas chamber 18 and the atmosphere in the atmosphere chamber 19 Mixing is less likely to occur. In other words, when the intake air flow rate is low, the pressure of the exhaust gas flowing into the test gas chamber 18 is lower than when the intake air flow rate is high, and the exhaust gas in the test gas chamber 18 and the atmosphere in the atmosphere chamber 19 are Mixing with and is likely to occur. Further, the amount of increase in the oxygen concentration in the exhaust gas of the gas chamber 18 to be measured increases in proportion to the amount of the atmosphere mixed with the exhaust gas. Therefore, as shown in FIG. 6, the output current of the sensor element 11 when the intake air flow rate is small shifts to the lean side (plus side) with respect to the output current of the sensor element 11 when the intake air flow rate is large.

On the other hand, when the crack 14a of the first impermeable layer 14 as shown in FIG. 5 occurs, the exhaust gas flows into the atmosphere chamber 19 from the outside of the air-fuel ratio sensor 10 communicating with the exhaust passage, and the exhaust gas flows into the atmosphere chamber 19. As a result, the oxygen concentration in the atmosphere of the air chamber 19 decreases. However, when the intake air flow rate is low, the pressure of the exhaust gas is lower than when the intake air flow rate is high, and the amount of exhaust gas flowing into the atmosphere chamber 19 from the exhaust passage through the crack 14a is small. Therefore, when the intake air flow rate is small, the abnormality of the output current of the sensor element 11 due to the crack 14a of the first impermeable layer 14 hardly occurs.

Therefore, when the intake air flow rate is small, the abnormality of the output current of the sensor element 11 due to the crack 12a of the solid electrolyte layer 12 becomes more remarkable than the crack 14a of the first impermeable layer 14. Based on this finding, in the present embodiment, the presence or absence of the crack 12a in the solid electrolyte layer 12 is determined based on the time change of the output current of the sensor element 11 detected when the intake air flow rate is small.

Basically, as the engine speed of the internal combustion engine decreases, the intake air flow rate decreases. Therefore, in the failure determination unit 6, when the engine rotation speed of the internal combustion engine decreases from a value higher than the predetermined value to a predetermined value and then maintained at the predetermined value or less, the engine rotation speed becomes the predetermined value or less. The first current value detected by the current detection circuit 4 at the first timing within the maintained period, and the second time elapsed from the first timing within the period during which the engine speed is maintained below the predetermined value. The second current value detected by the current detection circuit 4 at the timing of is acquired, and the failure of the air-fuel ratio sensor 10 is determined based on these current values. The predetermined value is predetermined and is set to, for example, 0 rpm to 1000 rpm.

For example, when the internal combustion engine is stopped, the engine speed drops to zero and the intake air flow rate becomes zero. Therefore, in the present embodiment, when the internal combustion engine is stopped, the failure determination unit 6 includes the first current value detected by the current detection circuit 4 at the first timing in the stopped state of the internal combustion engine and the internal combustion engine. The second current value detected by the current detection circuit 4 at the second timing when a predetermined time has elapsed from the first timing in the stopped state is acquired. The internal combustion engine is stopped, for example, when the idling stop function is activated while the vehicle is temporarily stopped, or when the power of the vehicle is output only by the motor in the hybrid vehicle. Even after the internal combustion engine is stopped, voltage application to the sensor element 11 and temperature control of the sensor element 11 by the heater 20 are continued.

FIG. 7 is a time chart of the engine speed and the output current of the sensor element 11 when the internal combustion engine is stopped. In FIG. 7, the output current for the normal air-fuel ratio sensor 10 in which the solid electrolyte layer 12 is not cracked 12a is shown by a solid line, and the output for the abnormal air-fuel ratio sensor 10 in which the solid electrolyte layer 12 is cracked is shown by a solid line. The current is shown by a broken line.

In the example of FIG. 7, the internal combustion engine is stopped at time t1, and fuel injection and the like by the fuel injection valve 51 are stopped. As a result, after the time t1, the engine speed reaches zero at the time t2. At this time, in the normal air-fuel ratio sensor 10, after the internal combustion engine is stopped, the output current of the sensor element 11 converges to a value corresponding to the air-fuel ratio of the residual gas left in the gas chamber 18 to be measured. On the other hand, in the abnormal air-fuel ratio sensor 10, after the internal combustion engine is stopped, the atmosphere in the atmosphere chamber 19 mixes with the residual gas in the gas chamber 18 to be measured, and as a result, the output current of the sensor element 11 gradually becomes lean side (plus). It shifts to the side). Therefore, in the abnormal air-fuel ratio sensor 10, when the internal combustion engine is stopped, the second current value detected at the second timing after the first timing is detected at the first timing. It becomes larger than the current value.

Therefore, in the failure determination unit 6, when the second current value is larger than the first current value and the difference or ratio between the second current value and the first current value is equal to or larger than the threshold value, the air-fuel ratio sensor 10 is abnormal. Therefore, it is determined that the solid electrolyte layer 12 is cracked. Therefore, in the present embodiment, the failure detection device 1 can be used to detect the crack 12a of the solid electrolyte layer 12 as the failure mode of the air-fuel ratio sensor 10.

<Failure judgment processing>
Hereinafter, the control for detecting the failure of the air-fuel ratio sensor 10 will be described in detail with reference to the flowchart of FIG. FIG. 8 is a flowchart showing a control routine for failure determination processing according to the first embodiment of the present invention. This control routine is repeatedly executed by the ECU 31 at predetermined time intervals after the ignition switch of the vehicle equipped with the internal combustion engine is turned on.

First, in step S101, the voltage control unit 5 applies a predetermined voltage to the sensor element 11 via the voltage application circuit 3. The predetermined voltage is predetermined and is set to, for example, a voltage (0.15V to 0.7V) within the limit current region. In this embodiment, the predetermined voltage is set to 0.45V.

Next, in step S102, the failure determination unit 6 determines whether or not the failure determination condition is satisfied. The failure determination condition is satisfied, for example, when the temperature of the sensor element 11 of the air-fuel ratio sensor 10 is equal to or higher than a predetermined active temperature. The temperature of the sensor element 11 is calculated based on, for example, the impedance of the sensor element 11. The failure determination conditions are that the temperature of the catalyst 60 is equal to or higher than the predetermined active temperature, that a predetermined time has elapsed after the start of the internal combustion engine, and that the air-fuel ratio sensor 10 has been turned on after the ignition switch of the vehicle is turned on. It may include that the failure determination has not been performed yet.

If it is determined in step S102 that the failure determination condition is not satisfied, this control routine ends. On the other hand, if it is determined in step S102 that the failure determination condition is satisfied, the control routine proceeds to step S103.

In step S103, the failure determination unit 6 determines whether or not the internal combustion engine has stopped. If it is determined in step S103 that the internal combustion engine has not stopped, this control routine ends.

On the other hand, if it is determined in step S103 that the internal combustion engine has stopped, the control routine proceeds to step S104. In step S104, the failure determination unit 6 acquires the first current value I1 detected by the current detection circuit 4 at the first timing. For example, the failure determination unit 6 acquires the current value detected by the current detection circuit 4 when the engine speed reaches zero as the first current value I1. In this case, the engine speed is calculated based on, for example, the output of the crank angle sensor 73.

Next, in step S105, the failure determination unit 6 determines whether or not the stopped state of the internal combustion engine has continued for a predetermined time from the first timing. The predetermined time is predetermined and is set to, for example, 3 seconds to 10 seconds, preferably 5 seconds. If it is determined that the internal combustion engine has not been stopped for a predetermined time, this control routine ends.

On the other hand, if it is determined in step S105 that the internal combustion engine has been stopped for a predetermined time, the control routine proceeds to step S106. In step S106, the failure determination unit 6 acquires the second current value I2 detected by the current detection circuit 4 at the second timing in which a predetermined time has elapsed from the first timing in which the first current value I1 is detected. ..

Next, in step S107, the failure determination unit 6 determines whether or not the value obtained by subtracting the first current value I1 from the second current value I2 is equal to or greater than the threshold value K. The threshold value K is predetermined and set to a positive value.

If it is determined in step S107 that the value obtained by subtracting the first current value I1 from the second current value I2 is equal to or greater than the threshold value K, the control routine proceeds to step S108. In step S108, the failure determination unit 6 determines that the air-fuel ratio sensor 10 is abnormal and the solid electrolyte layer 12 is cracked. At this time, the failure determination unit 6 may turn on the warning light provided in the vehicle. After step S108, this control routine ends.

On the other hand, if it is determined in step S107 that the value obtained by subtracting the first current value I1 from the second current value I2 is less than the threshold value K, the control routine proceeds to step S109. In step S109, the failure determination unit 6 determines that the air-fuel ratio sensor 10 is normal and that the solid electrolyte layer 12 is not cracked. After step S109, this control routine ends.

In step S107, the failure determination unit 6 may determine whether or not the value obtained by dividing the second current value I2 by the first current value I1 is equal to or greater than the threshold value K. That is, the failure determination unit 6 may determine whether or not the following equation (1) is satisfied.
I2 / I1 ≧ K… (1)
In this case, the threshold value K is set to a value larger than 1.

Also, when the ignition switch of the vehicle is turned off, the internal combustion engine is stopped. Therefore, the failure determination of the air-fuel ratio sensor 10 may be performed after the ignition switch of the vehicle is turned off. In this case, even after the ignition switch of the vehicle is turned off, a voltage is applied to the sensor element 11 and the temperature of the sensor element 11 is controlled by the heater 20 until the first current value I1 and the second current value I2 are detected. Etc. will continue.

<Second embodiment>
The failure detection device according to the second embodiment is basically the same as the configuration and control of the failure detection device according to the first embodiment, except for the points described below. Therefore, hereinafter, the second embodiment of the present invention will be described focusing on the parts different from the first embodiment.

When the operating state of the internal combustion engine becomes the idle operating state, the engine speed drops to the idle speed, and the intake air flow rate decreases. Therefore, in the second embodiment, the failure determination of the air-fuel ratio sensor 10 is performed in the idle operation state of the internal combustion engine.

Specifically, the failure determination unit 6 includes the first current value detected by the current detection circuit 4 at the first timing in the idle operation state and the idle state when the operation state of the internal combustion engine becomes the idle operation state. The second current value detected by the current detection circuit 4 at the second timing after a predetermined time has elapsed from the first timing in the operating state is acquired, and the failure of the air-fuel ratio sensor 10 is determined based on these current values. In the idle operation state, when the accelerator opening is zero (engine load is zero) (for example, when the vehicle is temporarily stopped), the engine speed is a predetermined idle speed due to the combustion of the air-fuel mixture in the combustion chamber 45 (for example, when the vehicle is temporarily stopped). For example, it means an operating state maintained at 400 to 1000 rpm).

FIG. 9 is a flowchart showing a control routine for failure determination processing according to the second embodiment of the present invention. This control routine is repeatedly executed by the ECU 31 at predetermined time intervals after the ignition switch of the vehicle equipped with the internal combustion engine is turned on.

Steps S201 and S202 are executed in the same manner as steps S101 and S102 in FIG. If it is determined in step S202 that the failure determination condition is satisfied, the control routine proceeds to step S203.

In step S203, the failure determination unit 6 determines whether or not the operating state of the internal combustion engine has become the idle operating state. If it is determined that the operating state of the internal combustion engine is not in the idle operating state, this control routine ends. On the other hand, if it is determined that the operating state of the internal combustion engine is in the idle operating state, the control routine proceeds to step S204.

In step S204, the failure determination unit 6 stops the air-fuel ratio feedback control using the air-fuel ratio sensor 10 and starts open control (open loop control). In the open control, the fuel injection amount FA of the fuel injection valve 51 is calculated by the following equation (2) based on the target air-fuel ratio TAF and the intake air amount IA so that the air-fuel ratio of the air-fuel mixture matches the target air-fuel ratio. .. The intake air amount IA is calculated based on the output of the air flow meter 70.
FA = IA / TAF ... (2)

In this embodiment, the target air-fuel ratio TAF in open control is set to the theoretical air-fuel ratio (14.6). This makes it possible to prevent the exhaust emissions from deteriorating during the failure determination.

Next, in step S205, the failure determination unit 6 acquires the first current value I1 detected by the current detection circuit 4 at the first timing. For example, the failure determination unit 6 acquires, as the first current value I1, the current value detected by the current detection circuit 4 at the first timing when a predetermined time has elapsed from the start of the open control in the idle operation state. The predetermined time is set in advance, for example, the time required for the gas in the cylinder of the internal combustion engine to be replaced.

Next, in step S206, the failure determination unit 6 determines whether or not the idle operation state has continued for a predetermined time from the first timing. The predetermined time is predetermined and is set to, for example, 3 seconds to 10 seconds, preferably 5 seconds. If it is determined that the idle operation state has not continued for a predetermined time, this control routine ends.

On the other hand, if it is determined in step S206 that the idle operation state has continued for a predetermined time, the control routine proceeds to step S207. In step S207, the failure determination unit 6 acquires the second current value I2 detected by the current detection circuit 4 at the second timing in which a predetermined time has elapsed from the first timing in which the first current value I1 is detected. ..

Steps S208 to S210 are executed in the same manner as steps S107 to S109 of FIG. The control routine can be modified in the same manner as the control routine shown in FIG.

<Third embodiment>
The failure detection device according to the third embodiment is basically the same as the configuration and control of the failure detection device according to the first embodiment, except for the points described below. Therefore, the third embodiment of the present invention will be described below focusing on the parts different from the first embodiment.

FIG. 10 is a diagram schematically showing an internal combustion engine or the like to which the failure detection device of the air-fuel ratio sensor according to the third embodiment of the present invention is applied. In the third embodiment, the vehicle equipped with the internal combustion engine is provided with the atmospheric pressure sensor 74 for detecting the atmospheric pressure. The output of the atmospheric pressure sensor 74 is input to the input port 36 via the corresponding AD converter 38. Therefore, the output of the atmospheric pressure sensor 74 is transmitted to the ECU 31, and the ECU 31 acquires the output of the atmospheric pressure sensor 74.

As described above, the atmosphere is introduced into the atmosphere chamber 19 of the air-fuel ratio sensor 10, but the atmospheric pressure changes according to the traveling environment of the vehicle equipped with the internal combustion engine. For example, the atmospheric pressure decreases as the altitude of the road on which the vehicle travels increases. When the atmospheric pressure becomes low, the amount of increase in the oxygen concentration in the exhaust gas when the atmosphere in the atmosphere chamber 19 mixes with the exhaust gas in the gas chamber 18 decreases, and as a result, the exhaust side electrode 16 to the atmosphere side electrode 17 The amount of oxide ion that moves to is reduced. That is, when the atmospheric pressure becomes low, the amount of the output current of the sensor element 11 shifting to the lean side due to the influence of the crack 12a of the solid electrolyte layer 12 becomes small.

Therefore, in the third embodiment, the failure determination unit 6 corrects the value obtained by subtracting the first current value from the second current value based on the atmospheric pressure detected by the atmospheric pressure sensor 74, and the corrected value. When is equal to or greater than the threshold value, it is determined that the solid electrolyte layer 12 is cracked. As a result, it is possible to reduce erroneous detection of cracks due to fluctuations in atmospheric pressure, and it is possible to improve the detection accuracy of failure of the air-fuel ratio sensor 10.

FIG. 11 is a flowchart showing a control routine for failure determination processing according to the third embodiment of the present invention. This control routine is repeatedly executed by the ECU 31 at predetermined time intervals after the ignition switch of the vehicle equipped with the internal combustion engine is turned on.

Steps S301 to S303 are executed in the same manner as steps S101 to S103 in FIG. If it is determined in step S303 that the internal combustion engine has stopped, the control routine proceeds to step S304.

In step S304, the failure determination unit 6 acquires the first current value I1 detected by the current detection circuit 4 at the first timing and the atmospheric pressure P detected by the atmospheric pressure sensor 74 at the first timing. .. For example, the failure determination unit 6 acquires the first current value I1 and the atmospheric pressure P detected at the first timing when the engine speed reaches zero.

Steps S305 and S306 are executed in the same manner as steps S105 and S106 of FIG. After step S306, in step S307, the failure determination unit 6 uses a map or a calculation formula to subtract the first current value I1 from the second current value I2 based on the atmospheric pressure P (output difference (I2). -I1)) is corrected. The map or calculation formula is created so that the lower the atmospheric pressure P, the larger the output difference.

Next, in step S308, the failure determination unit 6 determines whether or not the corrected output difference is equal to or greater than the threshold value K. The threshold value K is predetermined and set to a positive value.

If it is determined in step S308 that the corrected output difference is equal to or greater than the threshold value K, the control routine proceeds to step S309. In step S309, similarly to step S108 of FIG. 8, the failure determination unit 6 determines that the air-fuel ratio sensor 10 is abnormal and the solid electrolyte layer 12 is cracked. After step S309, this control routine ends.

On the other hand, if it is determined in step S308 that the corrected output difference is less than the threshold value K, the control routine proceeds to step S310. In step S310, similarly to step S109 of FIG. 8, the failure determination unit 6 determines that the air-fuel ratio sensor 10 is normal and that the solid electrolyte layer 12 is not cracked. After step S310, this control routine ends.

The control routine can be modified in the same manner as the control routine shown in FIG. For example, in step S307, the failure determination unit 6 divides the second current value I2 by the first current value I1 based on the atmospheric pressure P using a map or a calculation formula (output ratio (I2 / I1). ) May be corrected, and in step S308, it may be determined whether or not the corrected output ratio is equal to or higher than the threshold value K. In this case, the threshold value K is set to a value larger than 1. Further, the map or the calculation formula is created so that the lower the atmospheric pressure P, the larger the output ratio.

Further, in step S307, the failure determination unit 6 may correct the threshold value K based on the atmospheric pressure P using a map or a calculation formula, and the corrected threshold value K may be used in step S308. In this case, the map or calculation formula is created so that the lower the atmospheric pressure P, the smaller the threshold value K.

Further, the atmospheric pressure P mainly affects the oxygen concentration of the exhaust gas after mixing with the atmosphere, that is, the second current value I2. Therefore, in step S307, the failure determination unit 6 corrects the second current value I2 based on the atmospheric pressure P using a map or a calculation formula, and even if the corrected second current value I2 is used in step S308. good. In this case, the map or calculation formula is created so that the lower the atmospheric pressure P, the larger the second current value I2.

The atmospheric pressure P used for correction can be detected at any timing from the first timing at which the first current value I1 is detected to the second timing at which the second current value I2 is detected. Further, the atmospheric pressure P used for the correction may be the average value of the atmospheric pressure detected between the first timing and the second timing.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and modifications can be made within the scope of the claims. For example, the internal combustion engine to which the failure detection device of the air-fuel ratio sensor is applied may be a compression self-ignition type internal combustion engine (diesel engine).

Further, the failure detection device 1 may be used to determine the failure of the air-fuel ratio sensor arranged on the downstream side of the catalyst 60. Further, the air-fuel ratio sensor for which the failure determination is performed by the failure detection device 1 may be arranged on the upstream side and the downstream side of the catalyst 60. In this case, when the failure determination of the downstream air-fuel ratio sensor is performed, only the air-fuel ratio control using the downstream air-fuel ratio sensor (for example, switching of the target air-fuel ratio) is stopped in step S204 of FIG. 9, and the upstream side is stopped. The air-fuel ratio feedback control using the air-fuel ratio sensor may be continued.

Further, the above-described embodiments can be implemented in any combination. When the second embodiment and the third embodiment are combined, steps S203 and S204 of FIG. 9 are executed instead of step S303 in the control routine of the failure determination process of FIG.

1 Air fuel ratio sensor failure detection device 3 Voltage application circuit 4 Current detection circuit 6 Failure determination unit 10 Air fuel ratio sensor 11 Sensor element 12 Solid electrolyte layer 16 Exhaust side electrode 17 Atmosphere side electrode

Claims (1)

The solid electrolyte layer having oxide ion conductivity and the exhaust side electrode arranged on one side surface of the solid electrolyte layer so as to be exposed to the exhaust gas flowing through the exhaust passage of the internal combustion engine are exposed to the atmosphere. A failure detection device for an air fuel ratio sensor, which detects a failure of an air fuel ratio sensor including a sensor element having an atmosphere side electrode arranged on the other side surface of the solid electrolyte layer.
A voltage application circuit that applies a voltage to the sensor element so that the potential of the atmosphere-side electrode is higher than the potential of the exhaust-side electrode.
A current detection circuit that detects the output current of the sensor element when a voltage is applied to the sensor element, and
It is equipped with a failure determination unit that determines the failure of the air-fuel ratio sensor.
In the failure determination unit, when the engine rotation speed of the internal combustion engine decreases from a value higher than the predetermined value to the predetermined value and then maintained at the predetermined value or less, the engine rotation speed becomes the predetermined value. The first current value detected by the current detection circuit at the first timing within the period maintained below, and the current detection circuit at the second timing in which a predetermined time has elapsed from the first timing within the period. When the second current value detected by is larger than the first current value and the difference or ratio between the second current value and the first current value is equal to or more than the threshold value. In addition, a failure detection device for an air-fuel ratio sensor that determines that a crack has occurred in the solid electrolyte layer.

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