JP2007321587A - Control device for exhaust gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a control device for an exhaust gas sensor improved so as to surely detect deterioration of the exhaust gas sensor even in an environment in which exhaust gas is changed, while avoiding a fuel quantity increase control in a deterioration determination of the exhaust gas sensor. <P>SOLUTION: The control device for an exhaust gas sensor has a sensor element getting into an activated state when reaching an activation temperature, and determines the deterioration of the exhaust gas sensor mounted in an exhaust gas passage of an internal combustion engine. In the control device, a temperature of the sensor element is detected, and start of activation of the sensor element is detected. When the start of the activation of the sensor element is detected, whether or not the temperature of the sensor element is not more than a reference temperature, and whether or not a target air-fuel ratio of the internal combustion engine is set within a range of a reference air-fuel ratio, is determined. If it is determined that the sensor element temperature is not more than the reference temperature and the target air-fuel ratio is set within the range of the reference air-fuel ratio, output deviation with respect to reference output of output of the exhaust gas sensor is detected, and the deterioration of the exhaust gas sensor is determined based on the output deviation. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust gas sensor control apparatus. More specifically, the present invention relates to an exhaust gas sensor control device suitable as a control device for detecting deterioration of an exhaust gas sensor mounted in an exhaust passage of an internal combustion engine.

  Japanese Patent Application Laid-Open No. 4-330348 discloses an air-fuel ratio sensor control device disposed in an exhaust gas path of an internal combustion engine. This device detects the air-fuel ratio of the internal combustion engine by detecting a limit current flowing through the air-fuel ratio sensor. This air-fuel ratio sensor shows an output of almost 0 mA when the air-fuel ratio is controlled in the vicinity of the theoretical air-fuel ratio. In addition, even when no current flows through the air-fuel ratio sensor due to the failure of the air-fuel ratio sensor, the output is 0 mA. For this reason, when the output of the air-fuel ratio sensor indicates 0 mA, the above-described prior art control device detects whether the output is a normal output of the air-fuel ratio sensor or a failure of the air-fuel ratio sensor. It has a detection function.

  Specifically, when the failure detection function is activated, it is confirmed that when the output of the air-fuel ratio sensor indicates 0 mA, the air-fuel ratio is controlled near the stoichiometric air-fuel ratio, not during acceleration / deceleration of the internal combustion engine. After that, the fuel amount increase control is performed. When the output of the air-fuel ratio sensor does not change corresponding to the increase in the fuel amount, it is determined that the air-fuel ratio sensor has failed. Thus, according to the above prior art, when the air-fuel ratio sensor indicates 0 mA, it is determined whether the output is a normal output or an output due to a sensor failure, and an abnormal air-fuel ratio sensor output is determined. The air-fuel ratio control based on this can be avoided.

JP-A-4-330348 JP 2005-76612 A

  According to the above prior art, when the output of the air-fuel ratio sensor indicates 0 mA, it is possible to determine whether the output of the air-fuel ratio sensor is normal or abnormal. In this failure detection, it is a condition that the internal combustion engine is warmed up, not under acceleration / deceleration, and the air-fuel ratio is controlled in the vicinity of the theoretical air-fuel ratio. However, even under such conditions, the ratio of each component gas contained in the exhaust gas changes under the influence of various factors, so the output of the air-fuel ratio sensor also changes accordingly. For this reason, when the output of the air-fuel ratio is detected simply by increasing the amount of fuel as in the above prior art, the influence of changes due to various factors of the exhaust gas cannot be eliminated, and the output of the air-fuel ratio sensor A shift in output that is unrelated to deterioration of the output is included. Therefore, it is difficult to accurately determine whether the air-fuel ratio sensor has deteriorated based on the output of the air-fuel ratio sensor.

  Further, in the above prior art, in order to detect an abnormality in the air-fuel ratio sensor, the fuel amount is temporarily increased to control the air-fuel ratio to be rich. However, such control for enriching the fuel for determination is not preferable from the viewpoint of improving fuel consumption and emission characteristics.

  The present invention has been made to solve the above-described problems, and reliably prevents the exhaust gas sensor from deteriorating even in an environment where the exhaust gas changes while avoiding the fuel increase control at the time of determining the deterioration of the exhaust gas sensor. It is an object of the present invention to provide an exhaust gas sensor control apparatus improved so that it can be detected in a short time.

In order to achieve the above object, a first invention is an exhaust gas sensor control apparatus for controlling an exhaust gas sensor mounted in an exhaust passage of an internal combustion engine,
The exhaust gas sensor includes a sensor element that is activated when the activation temperature is reached,
Element temperature detecting means for detecting the temperature of the sensor element;
Activation start detecting means for detecting the activation start of the sensor element;
Element temperature determination means for determining whether or not the element temperature of the sensor element is equal to or lower than a reference temperature when activation of the sensor element is detected;
Air / fuel ratio determining means for determining whether or not the target air / fuel ratio of the internal combustion engine is set within a range of a reference air / fuel ratio when it is determined that the element temperature is equal to or lower than the reference temperature;
An output deviation detecting means for detecting an output deviation of the output of the exhaust gas sensor with respect to a reference output when it is determined that the target air-fuel ratio is set within the range of the reference air-fuel ratio;
Deterioration determining means for determining deterioration of the exhaust gas sensor based on the output deviation;
It is characterized by providing.

According to a second invention, in the first invention,
Active temperature determining means for determining whether the element temperature is equal to or lower than the active temperature;
An integrated value calculating means for calculating an integrated value of the output deviation while the element temperature is determined to be equal to or lower than the activation temperature;
With
The deterioration determining means determines that the exhaust gas sensor has deteriorated when the integrated value of the output deviation is larger than the deterioration determining value.

According to a third invention, in the first invention,
Temperature determining means for determining whether the element temperature is 500 ° C. or higher and 700 ° C. or lower;
An integrated value calculating means for calculating an integrated value of the output deviation while the element temperature is determined to be 500 ° C. or higher and 700 ° C. or lower;
The deterioration determining means determines that the exhaust gas sensor has deteriorated when the integrated value of the output deviation is larger than the deterioration determining value.

4th invention is 2nd or 3rd invention,
An elapsed time detecting means for detecting an elapsed time from the previous stop of the internal combustion engine to the start of activation of the element temperature;
A deterioration determination value setting means for setting the deterioration determination value according to the elapsed time;
It is characterized by providing.

According to a fifth invention, in any one of the first to fourth inventions,
The reference air-fuel ratio is 14.5 or more and 16.0 or less.

A sixth invention is any one of the first to fifth inventions,
Water temperature detecting means for detecting the temperature of the cooling water of the internal combustion engine;
Water temperature determining means for determining whether the water temperature is equal to or lower than a reference water temperature;
With
The output deviation detecting means detects the output deviation when it is determined that the water temperature is equal to or lower than the reference water temperature.

  According to the first invention, at the start of activation of the sensor element of the exhaust gas sensor, the element temperature of the sensor element is equal to or lower than the reference temperature, and the target air-fuel ratio of the internal combustion engine is set within the reference air-fuel ratio range. In this case, the output deviation of the output of the exhaust gas sensor with respect to the reference output is detected, and based on this, deterioration of the exhaust gas sensor is determined. When the warm-up is started after the sensor element is cooled, the output deviation with respect to the output corresponding to the air-fuel ratio during the warm-up of the deteriorated sensor becomes large. Therefore, when the sensor element detects an output shift between the reference temperature and lower, deterioration of the exhaust gas sensor can be detected even in a state where the exhaust gas concentration or the like changes.

  By the way, such output deviation due to sensor deterioration during sensor warm-up appears remarkably before the sensor reaches the activation temperature. Therefore, according to the second invention, while the element temperature is equal to or lower than the activation temperature, the integrated value of the output deviation is calculated, and the deterioration determination is performed based on this. Thereby, deterioration of the exhaust gas sensor can be detected more reliably.

  Further, the output shift due to sensor deterioration during sensor warm-up becomes more prominent when the element temperature is 500 ° C. or higher and 700 ° C. or lower. Therefore, according to the third invention, the deterioration determination is performed based on the integrated value of the output deviation when the element temperature is between 500 ° C. and 700 ° C. Thereby, deterioration of the exhaust gas sensor can be detected more reliably.

  Also, the magnitude of the output deviation varies depending on the elapsed time from the previous stop of the internal combustion engine to the start of activation of the element temperature. Therefore, according to the fourth invention, the deterioration determination value in the deterioration determination is set according to the elapsed time from the previous stop of the internal combustion engine to the start of activation of the element temperature. Thereby, deterioration of the exhaust gas sensor can be detected more reliably.

  Further, the output shift due to the deterioration of the exhaust gas sensor is detected as a shift amount toward the rich side. Therefore, according to the fifth aspect of the invention, the reference air-fuel ratio is set in the range of 14.5 to 16.0. Thereby, the output shift to the rich side can be detected more reliably, and deterioration of the exhaust gas sensor can be determined.

  Further, the output deviation due to the deterioration of the exhaust gas sensor becomes more prominent when the sensor element is sufficiently cooled after the previous stop of the internal combustion engine. Therefore, according to the sixth aspect of the invention, when the coolant temperature of the internal combustion engine is equal to or lower than the reference water temperature, output deviation is detected and deterioration of the exhaust gas sensor is determined. Thus, after the internal combustion engine is stopped, the output deviation can be detected in a state where the internal combustion engine is sufficiently cooled and the sensor element is sufficiently cooled to a low temperature, and deterioration of the exhaust gas sensor can be more reliably determined. it can.

  Embodiments of the present invention will be described below with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and the description thereof is simplified or omitted.

Embodiment.
[Configuration of Air-Fuel Ratio Sensor of Embodiment]
FIG. 1 is a diagram for explaining the configuration of an air-fuel ratio sensor according to an embodiment of the present invention. An air-fuel ratio sensor 10 (exhaust gas sensor) shown in FIG. 1 is a sensor that is disposed, for example, in an exhaust passage of an internal combustion engine and used to detect an air-fuel ratio of exhaust gas. The air-fuel ratio sensor 10 includes a cover 12 and is assembled in the exhaust passage so that the cover 12 is exposed to the exhaust gas. The cover 12 is provided with a hole (not shown) for introducing exhaust gas therein.

A sensor element 14 is disposed inside the cover 12. The sensor element 14 has an annular structure in which one end (the lower end in FIG. 1) is closed. The sensor element 14 includes a solid electrolyte layer 16, an exhaust side electrode 18 formed along the outer peripheral surface, and an atmosphere side electrode 20 formed along the inner peripheral surface. The solid electrolyte layer 16 is a sintered body containing ZrO ₂ or the like and has a characteristic of conducting oxygen ions. The exhaust side electrode 18 and the atmosphere side electrode 20 are made of, for example, platinum (Pt). These electrodes 18 and 20 are connected to leads (not shown) through output metal terminals (not shown) that are in contact with the surface of the solid electrolyte layer 16.

A protective coating 22 is formed on the outer surface of the exhaust side electrode 18. The protective film 22 is made of, for example, porous ceramics such as spinel (MgO.Al ₂ O ₃ ), and is formed by plasma spraying using this powder as a spray material. The protective coating 22 protects the surface of the exhaust-side electrode 18 from harmful substances in the exhaust.

A catalyst layer 24 is formed on the outer surface of the protective coating 22 of the air-fuel ratio sensor 10. The catalyst layer 24 is formed, for example, by supporting a catalyst metal (Pt, Pt—Rh, etc.) on alumina (γ-Al ₂ O ₃ or θ-Al ₂ O ₃ ). The catalyst carrying amount of the catalyst layer 24 is adjusted so as to limit the diffusion of hydrogen (H ₂ ) within a range in which the responsiveness of the air-fuel ratio sensor 10 is not lowered. The catalyst layer 24 oxidizes the hydrogen contained in the exhaust gas before reaching the protective film 22, thereby preventing a sensor output shift due to the influence of hydrogen. That is, when both hydrogen and oxygen (O ₂ ) are present in the exhaust gas, the hydrogen diffusion rate is faster than oxygen. For this reason, when the protective coating 22 is exposed at the outermost periphery and exposed to the exhaust gas, hydrogen passes through the protective coating 22 earlier than oxygen and reaches the surface of the exhaust-side electrode 18. As a result, the atmosphere near the surface of the exhaust-side electrode 18 becomes richer than actual exhaust gas due to hydrogen. As a result, the output of the air-fuel ratio sensor 10 is shifted to the rich side. In the air-fuel ratio sensor 10, a catalyst layer 24 that controls the diffusion of hydrogen is disposed on the outer surface of the protective coating 22 in order to suppress such sensor output deviation due to hydrogen.

  An air chamber 26 is formed inside the sensor element 14. The atmosphere chamber 26 is open to the atmosphere, and the atmosphere-side electrode 20 surface is exposed to the atmosphere. A heater 28 is disposed in the atmospheric chamber 26. The heater 28 is electrically connected to a heater control circuit, which will be described later, and is controlled by this control circuit to heat and maintain the sensor element 14 at an appropriate temperature. The sensor element 14 exhibits stable output characteristics when heated to an activation temperature (for example, about 700 ° C.).

[Control device for air-fuel ratio sensor]
FIG. 2 is a block diagram of a control device for the air-fuel ratio sensor 10. As shown in FIG. 2, the sensor element 14 can be equivalently expressed using a resistance component and an electromotive force component. The heater 28 can be equivalently expressed using a resistance component. A sensor element drive circuit 30 is connected to the sensor element 14 via a lead or the like. The sensor element drive circuit 30 includes a bias control circuit for applying a desired bias voltage to the sensor element 14 and a sensor current detection circuit for detecting a current flowing through the sensor element 14. .

  A microcomputer (hereinafter referred to as “microcomputer”) 32 is connected to the bias control circuit included in the sensor element drive circuit 30 via a low-pass filter (LPF), a D / A converter, and the like. The microcomputer 32 can command a voltage to be applied to the sensor element 14 to the bias control circuit via these elements. A microcomputer 32 is connected to the sensor current detection circuit via a D / A converter. The microcomputer 32 can read the sensor current detected by the sensor current detection circuit via the D / A converter.

  The bias control circuit can apply an air-fuel ratio detection bias voltage and an impedance detection voltage to the sensor element 14 in accordance with a command from the microcomputer 32. The sensor element 14 circulates a sensor current corresponding to the air-fuel ratio of the exhaust gas when the air-fuel ratio detection bias voltage is applied. Using this, the microcomputer 32 can calculate the air-fuel ratio of the exhaust gas based on the sensor current generated under the condition where the air-fuel ratio detection voltage is applied to the sensor element 14.

  When the bias voltage for the sensor element 14 is changed from the air-fuel ratio detection bias voltage to the impedance detection voltage, a change occurs in the sensor current corresponding to the change in the applied voltage. At this time, the ratio between the change amount of the applied voltage and the change amount of the sensor current is a value corresponding to the element impedance of the sensor element 14. Using this, the microcomputer 32 can calculate the element impedance of the sensor element 14 based on the sensor current generated in the situation where the impedance detection voltage is applied.

  A heater control circuit 34 is connected to the heater 28. A microcomputer 32 is connected to the heater control circuit 34. The heater control circuit 34 can receive a command supplied from the microcomputer 32 and supply a drive signal corresponding to the command to the heater 28 to cause the heater 28 to generate a desired amount of heat. Further, the microcomputer 32 can measure the amount of electric power supplied to the heater 28 and measure the heater energization integrated electric energy to the heater 28.

  Here, the element temperature of the sensor element 14 and the element impedance have a certain correlation. In the present embodiment, the microcomputer 32 stores an impedance corresponding to the element temperature. Therefore, the microcomputer 32 can detect the element temperature of the sensor element 14 based on the element impedance. For example, when the sensor element 14 is warmed up, the power supplied to the heater 28 is controlled via the heater control circuit 34 so that the element impedance matches the target impedance. As a result, the sensor element 14 can be accurately controlled near the target activation temperature.

  During the operation of the internal combustion engine, the microcomputer 32 applies a positive voltage and a voltage for measuring the element impedance to the sensor element 14 while repeatedly switching them. In parallel with the detection of the air-fuel ratio A / F, the element impedance for element temperature detection can be measured.

  The microcomputer 32 is connected to an engine control ECU (Electronic Control Unit) 36. Between the microcomputer 32 and the ECU 36, sensor information (such as the output of the air-fuel ratio sensor 10) and information related to the operating state of the internal combustion engine (such as the intake air amount and water temperature) are exchanged. The ECU 36 executes, for example, air-fuel ratio feedback control of the fuel injection amount using the received sensor information.

[Deterioration judgment control of air-fuel ratio sensor]
As described above, in the internal combustion engine, air-fuel ratio feedback for increasing or decreasing the fuel injection amount is executed so that the detected air-fuel ratio A / F matches the target air-fuel ratio. Therefore, when an abnormality occurs in the output of the air-fuel ratio sensor 10, air-fuel ratio feedback is executed based on the abnormal output, which greatly affects the emission characteristics. Therefore, when an abnormality occurs in the air-fuel ratio sensor 10, it is preferable to immediately detect the abnormality.

  However, during normal operation of the internal combustion engine, the concentration of exhaust gas discharged from the internal combustion engine is not constant because it is affected by various factors in the operation state. Furthermore, there are variations in the concentrations of various gas components in the exhaust gas. That is, there are variations that change in a fluid manner in the situation such as the concentration of exhaust gas discharged during the operation of the internal combustion engine, and it is difficult to specify it. In such an environment where the exhaust gas varies, it is difficult to immediately detect an abnormality in the output of the air-fuel ratio sensor 10. That is, even when the output of the air-fuel ratio sensor 10 is greatly deviated from the value corresponding to the air-fuel ratio control value, the output deviation is caused by the variation of the exhaust gas or the output due to the abnormality of the air-fuel ratio sensor 10 It is difficult to accurately determine whether or not the deviation is due to the deterioration of the air-fuel ratio sensor 10.

  In particular, in the air-fuel ratio sensor 10 having the catalyst layer 24 for limiting the diffusion rate of hydrogen on the exhaust side electrode 18 side, the catalyst layer 24 is likely to deteriorate over time. When the catalyst layer 24 deteriorates, the function of limiting the rate of hydrogen is not exhibited, so that the sensor output shifts. This output deviation is caused by hydrogen in the exhaust gas, but it is difficult to specify the hydrogen concentration in the exhaust gas due to the variation in the exhaust gas as described above. Therefore, even when the output of the air-fuel ratio sensor 10 is shifted due to deterioration of the catalyst layer 24, it is difficult to determine whether the output is normal or abnormal during normal operation of the internal combustion engine. It is.

  By the way, immediately after the start of the internal combustion engine, that is, during the warm-up of the air-fuel ratio sensor until the element temperature of the sensor element 14 reaches the activation temperature, a sensor in which the catalyst layer 24 is not deteriorated (hereinafter referred to as “catalyst layer normal sensor”). And the sensor in which the catalyst layer 24 has deteriorated (hereinafter referred to as “catalyst layer deterioration sensor”), the following tendency is observed. FIG. 3 is a diagram for explaining changes in sensor outputs of the catalyst layer normal sensor and the catalyst layer deterioration sensor after the start of the internal combustion engine. In FIG. 3, the horizontal axis represents elapsed time, and the vertical axis represents the output of the air-fuel ratio sensor. In FIG. 3, the dotted line (a) is the air-fuel ratio A / F detected by the output of the catalyst layer normal sensor, the solid line (b) is the air-fuel ratio A / F detected by the output of the catalyst layer deterioration sensor, and the thin line ( c) represents the air-fuel ratio control value. In the example shown in FIG. 3, the air-fuel ratio is controlled to a constant value near the stoichiometric air-fuel ratio, as shown by the thin line (c).

  As shown in FIG. 3, during the warm-up process of the sensor element 14, the rich shift of the output detected by the catalyst layer normal sensor with respect to the air-fuel ratio control value is slight (dotted line (a)). On the other hand, the rich shift with respect to the air-fuel ratio control value detected by the catalyst layer deterioration sensor is large (solid line (b)). Such a rich shift in output during the warm-up process of the sensor element 14 tends to increase as the deterioration of the catalyst layer 24 of the sensor element 14 proceeds. Further, the tendency shown in FIG. 3 is a tendency to appear before the sensor element 14 reaches the activation temperature, that is, before the sensor output is stabilized at the output corresponding to the air-fuel ratio, and is strongly influenced by the concentration of each component in the exhaust gas. The same tendency of output deviation appears without receiving it. According to the inventor's knowledge, this means that, as will be described below, the amount of oxygen adsorbed on the sensor element 14 when the sensor element 14 is cooled depends on the catalyst layer deterioration sensor and the catalyst layer normal sensor. It is thought that this is because of differences.

FIG. 4 is a schematic diagram for explaining the state of the sensor element 14 after the internal combustion engine is stopped and when the internal combustion engine is started. Specifically, FIG. 4A shows an exhaust gas in the sensor element 14 after the internal combustion engine is stopped. FIG. 4B shows a state in which the adsorbed components are desorbed after starting the internal combustion engine. The sensor element 14 is disposed in the exhaust gas path of the internal combustion engine as described above, and is used in a state where the exhaust side electrode 18 side is exposed to the exhaust gas. The exhaust gas contains various components such as water vapor (H ₂ O), carbon dioxide (CO ₂ ), and oxygen (O ₂ ). When the sensor element 14 is in an active state, these components do not adsorb near the exhaust side electrode 18, pass through the catalyst layer 24 and the protective film 22, and reach the exhaust side electrode 18. However, it is considered that these components are chemically adsorbed on the exhaust side electrode 18 side in the process in which the temperature of the sensor element 14 decreases after the internal combustion engine is stopped.

  Such an adsorption reaction is likely to occur in an adsorption temperature range in which the temperature of the sensor element 14 is lowered to some extent. After the internal combustion engine is stopped, energization to the heater 28 is stopped, so that the temperature of the sensor element 14 inevitably decreases to an adsorption temperature range where adsorption is started. Therefore, as shown in FIG. 4A, after the internal combustion engine is stopped, the component 50 in the exhaust gas is inevitably adsorbed in the vicinity of the exhaust-side electrode 18.

  As described above, the component 50 adsorbed in the vicinity of the exhaust-side electrode 18 includes oxygen 52, and it is considered that the oxygen 52 is mainly adsorbed on the catalyst layer 24. Furthermore, the amount of adsorption of oxygen 52 to the catalyst layer 24 is large when the sensor element 14 is new, and the amount of adsorption is considered to decrease as the catalyst layer 24 deteriorates.

  Here, as shown in FIG. 4B, when the warming-up of the sensor element 14 is started from the state where the element temperature of the sensor element 14 is lowered, the temperature of the element becomes the lower limit of the adsorption temperature range during the warm-up process. The component 50 that has been adsorbed starts to desorb. Here, when the oxygen 52 adsorbed on the catalyst layer 24 is sufficiently large, the oxygen 52 and the component that causes the rich shift at the time of warm-up cancel each other to some extent, so that the output of the air-fuel ratio sensor 10 is rich. Deviation is kept small. Therefore, when the air-fuel ratio A / F is controlled near the stoichiometric air-fuel ratio, for example, the air-fuel ratio sensor 10 is considered to represent a value near the stoichiometric air-fuel ratio (see the dotted line (a) in FIG. 3). On the other hand, when the deterioration of the catalyst layer 24 of the sensor element 14 progresses and the adsorption amount of the oxygen 52 at the time of cooling the sensor is small, the action of canceling out the component that causes the rich shift at the time of warming up becomes small. . As a result, the vicinity of the exhaust-side electrode 18 becomes a rich atmosphere, and the rich deviation of the output during the warm-up process of the air-fuel ratio sensor 10 increases (see the solid line (b) in FIG. 3).

By utilizing such a phenomenon, the deterioration of the catalyst layer 24 can be determined. That is, when a rich shift in output is detected during the warm-up process of the sensor element 14 and the shift amount exceeds an allowable range, the air-fuel ratio sensor 10 may be determined to deteriorate. Specifically, after the internal combustion engine is started, the rich shift RS of the output of the air-fuel ratio sensor 10 is calculated as in the following equation (1) during the deterioration determination in the warm-up process of the sensor element 14.
Rich shift RS = | Actual sensor output-Theoretical air-fuel ratio output | (1)

  In Expression (1), the actual sensor output is an output value detected by the air-fuel ratio sensor 10 during the deterioration determination. The theoretical air-fuel ratio output is a normal sensor output value corresponding to the case where the air-fuel ratio is the stoichiometric air-fuel ratio, and is stored in the microcomputer 32 in advance. Although the air-fuel ratio is not necessarily controlled to the stoichiometric air-fuel ratio, the rich deviation RS is a value for calculating an index for determining the deviation of the sensor output, and therefore does not necessarily correspond to the actual air-fuel ratio. The output value to be used may not be used. Therefore, since the air-fuel ratio control is performed in a predetermined range in the vicinity of the theoretical air-fuel ratio during the deterioration determination, the difference between the output value corresponding to the theoretical air-fuel ratio and the actual sensor output is used here as a rich deviation.

  Next, the sum of the rich deviation RS calculated by the equation (1) is obtained, and the total value is used as the rich deviation index SUMRS. Here, when the rich deviation index SUMRS is large, it is considered that the oxygen adsorption amount during cooling before warming up the sensor element 14 was small. That is, it is considered that the catalyst layer 24 of the sensor element 14 is deteriorated. Therefore, when the rich deviation index SUMRS is larger than the predetermined deterioration determination value WRS, it is determined that the catalyst layer 24 has deteriorated, and an abnormality of the air-fuel ratio sensor 10 is detected. On the other hand, when the rich deviation index SUMRS is smaller than the deterioration determination value WRS, it is considered that a large amount of oxygen 52 is adsorbed on the sensor element 14 and the influence of the component that causes the rich deviation is offset. For this reason, it is determined that the air-fuel ratio sensor 10 is normal.

  The deterioration determination value WRS may be set by, for example, obtaining a lower limit value of a rich deviation index obtained in accordance with an output from an air-fuel ratio sensor in which the catalyst layer 24 is recognized to be deteriorated, by an experiment or the like. Further, such deterioration determination value WRS is affected by the elapsed time from the previous stop of the internal combustion engine. FIG. 5 is a diagram for explaining the relationship between the elapsed time and the deterioration determination value. In FIG. 5, the horizontal axis represents the elapsed time, and the vertical axis represents the deterioration determination value.

  The adsorption amount of the component 50 adsorbed in the vicinity of the exhaust side electrode increases as the elapsed time from the previous stop of the internal combustion engine becomes longer until the adsorption becomes saturated. As the amount of adsorption increases, the influence of these components 50 on the sensor output increases, so it is considered that the rich shift increases. Therefore, as shown in FIG. 5, the deterioration determination value WRS is set to increase as the elapsed time becomes longer. The microcomputer 32 stores in advance a map that defines the relationship between the deterioration determination value WRS and the elapsed time based on such a relationship. When determining the deterioration of the sensor element 14, the deterioration determination value WRS corresponding to the elapsed time is set according to this map.

  In this determination, it is necessary to detect a rich shift caused by the desorption of the component 50. That is, it is preferable that the above-described rich deviation index SUMRS can be detected by identifying what is attributed to the desorption under the condition where desorption of the adsorbed component 50 containing oxygen 52 occurs. Accordingly, the deterioration determination is performed only when the following conditions A to D are satisfied. Since the following conditions A to D affect the rich displacement amount of the sensor, the conditions A to D are also assumed to be satisfied in the map setting of the deterioration determination value WRS.

<Condition A> The operation is stopped after the normal operation of the internal combustion engine, and then the internal combustion engine and the sensor element 14 are both sufficiently cooled.
Specifically, the temperature of the cooling water of the internal combustion engine at start-up (start-up water temperature TW) is in the range of about 0 to 30 ° C., and the element impedance Rst of the sensor element 14 immediately after start-up is the reference impedance Rbase. It is determined by the following.

  That is, the condition is that the component 50 in the exhaust gas is sufficiently adsorbed by the sensor element 14 and is in a saturated state or a state close thereto. That is, in this embodiment, the deterioration determination based on the rich shift amount is performed using the difference in the adsorption amount of the oxygen 52. Therefore, in order to make a more accurate deterioration determination, it is preferable that the rich deviation is conspicuous. Therefore, it is preferable that more components 50 in the exhaust gas including oxygen 52 are adsorbed. . Therefore, in the present embodiment, it is a condition that the sensor element 14 and the internal combustion engine are sufficiently cooled so that the adsorption amount of the component 50 is increased. That is, it is a condition that the sensor element 14 is sufficiently cooled while the exhaust side electrode 18 side is exposed to the exhaust gas discharged in the previous normal operation, and specifically, the condition A is satisfied. Is done.

  In addition, for example, in the case of starting at an extremely low temperature, the energization control of the heater 28 is different from normal for reasons such as prevention of water exposure of the air-fuel ratio sensor 10, and the element temperature rises more slowly than usual. It may be controlled to do. In this case, since the behavior of the output of the air-fuel ratio sensor 10 is not stable, the deterioration determination cannot be performed. Therefore, the case of extremely low temperature is excluded, and it is a condition that the water temperature at start-up is 0 ° C. or higher.

<Condition B> At the time of deterioration determination, the target air-fuel ratio after starting the internal combustion engine is controlled within the range of A / F 14.5 to 16.0.
The system according to the embodiment makes use of the difference in the amount of adsorption of oxygen 52 to perform the deterioration determination based on a slight rich shift of the output when the sensor is warmed up. Therefore, in order to more reliably detect the occurrence of the rich shift, it is necessary to provide an environment in which the output shift can be reliably confirmed. Therefore, it is desirable to control the air-fuel ratio A / F of the exhaust gas within the range of the lean air-fuel ratio from the vicinity of the stoichiometric air-fuel ratio so that the rich shift becomes remarkable. Therefore, the condition B must be satisfied.

<Condition C> The exhaust gas flow rate must be stable and repeatable.
Specifically, this is determined when the engine speed and the intake air amount during the deterioration determination are below the reference value, and the integrated value of the intake air amount is below the reference value. In other words, because the deterioration judgment is performed based on the magnitude of the accumulated value SUMRS of the rich deviation RS, the detection environment for the rich deviation RS may be in a stable state so that other factors do not significantly affect the sensor output result. desired. In addition, since the integrated value of rich deviation is calculated and the deterioration determination is performed by comparison with the deterioration determination value, the environment for detecting the rich deviation needs to be an environment that is repeatedly reproduced. Therefore, the condition C regarding the operating state determined by the engine speed, the intake air amount, the integrated intake air amount, etc. is satisfied to satisfy such a condition.

<Condition D> The element temperature of the sensor element 14 is 500 ° C. to 750 ° C.
Specifically, this is determined when the detected element impedance Rt is within the range of element impedance values R500 to R700 corresponding to 500 to 750 ° C. Even during the warming-up process of the sensor element 14, since the desorption amount of the component 50 is relatively small during the period from the state immediately after starting to about 500 ° C., the influence on the sensor output is small. Further, when the sensor element 14 reaches an activation temperature of about 750 ° C., the desorption of the component 50 is finished, and the output deviation due to the component 50 does not occur. That is, the output shift due to the deterioration of the sensor is most noticeable immediately after the desorption of the component 50 is started until the end of the desorption. Therefore, in the present embodiment, only the range in which the output deviation is large is selected and the sensor output is detected, thereby performing deterioration determination due to rich deviation with higher accuracy. Therefore, the condition D is satisfied.

[Control Routine of Embodiment]
FIG. 6 is a flowchart for illustrating a control routine executed by the system in the embodiment of the present invention. The routine shown in FIG. 6 is a routine that is repeatedly executed when the internal combustion engine is started. In the routine shown in FIG. 6, first, in step S102, it is determined whether or not the deterioration determining flag is ON. The deterioration determining flag is a flag that is turned ON when a condition A for starting deterioration determination of the catalyst layer 24 of the air-fuel ratio sensor 10 is satisfied in a process described later. If it is not determined in step S102 that the deterioration determination flag = ON is established, it is next determined whether or not the start of the internal combustion engine is requested (step S104). If the start of the internal combustion engine is not recognized, this process is temporarily terminated.

  On the other hand, if a request for starting the internal combustion engine is accepted in step S104, the starting water temperature TWst is detected (step S106). The starting water temperature TWst is detected according to the output of a water temperature sensor (not shown) arranged in the internal combustion engine. Next, it is determined whether the starting water temperature TWst is 0 ° C. or higher and 30 ° C. or lower (step S108). That is, here, as described in the condition A, it is determined whether or not the internal combustion engine has been sufficiently cooled from the previous stop and is not started at an extremely low temperature. In step S108, if 0 ≦ start-up water temperature TWst ≦ 30 is not established, it is considered that the component 50 in the exhaust gas is not sufficiently adsorbed or the sensor output cannot be detected in a stable state. Therefore, the determination of deterioration of the sensor element 14 is not started, and this process is temporarily ended.

  On the other hand, if the establishment of 0 ≦ starting water temperature TWst ≦ 30 is recognized in step S108, then the starting element impedance Rst is detected (step S110). The element impedance Rst at the time of starting is calculated based on a change in sensor current detected when a voltage for detecting element impedance is applied to the sensor element 14. This element impedance is a value corresponding to the element temperature when the sensor element 14 is started. Next, it is determined whether or not the detected element impedance Rst is less than or equal to the reference impedance Rbase (step S112). The reference impedance Rbase here is a value corresponding to the upper limit temperature of the element temperature at which it can be determined that the element temperature is sufficiently low. Therefore, when it is not recognized that the starting element impedance Rst ≦ the reference impedance Rbase, it is determined that the element temperature has not sufficiently decreased, and this process is once completed.

  On the other hand, when the element impedance Rst ≦ reference impedance Rbase is found to be established in step S112, the element temperature is sufficiently lowered, that is, the sensor element 14 is sufficiently cooled and the component 50 in the exhaust gas is sufficiently adsorbed. It is determined that In this case, when the conditions in steps S108 and S112 are satisfied, the condition A is satisfied and the deterioration determination flag is set to ON (step S114).

  When it is determined that the deterioration determination flag = ON is established in step S102, or when the deterioration determination flag = ON is determined in step S114, information regarding the current operating state of the internal combustion engine is detected (step S116). Specifically, for example, the current engine speed and the intake air amount, the integrated value of the intake air amount from the start to the present, the current target air-fuel ratio, the current element impedance Rt, and the coolant temperature TW, The elapsed time from the previous stop of the internal combustion engine to the current start is detected.

  Next, it is determined whether or not the current operating condition satisfies the condition B (step S118). That is, the target air-fuel ratio is set between 14.5 and 16.0, and it is determined whether or not the air-fuel ratio is controlled with this control target value. If it is determined that the condition B is satisfied, it is next determined whether or not the condition C is satisfied (step S120). That is, whether or not the exhaust gas flow rate is stable and reproducibility is expected. Specifically, the engine speed is equal to or less than the reference speed, the intake air amount is equal to or less than the reference value, and the integrated intake air amount is the reference value. It is determined that the value is less than or equal to the value. If the condition B is not satisfied in step S118, or if the condition C is not satisfied in step S120, the condition for performing the deterioration determination is not satisfied, so the deterioration determination flag is set to OFF ( Step S122). Next, the rich deviation index SUMRS = 0 is set (step S124), and then the current deterioration determination process is terminated.

  On the other hand, if it is determined in Steps S118 and S120 that the conditions B and C are satisfied, it is next determined whether or not the element temperature of the sensor element 14 is 500 ° C. or higher (Step S126). Specifically, when the detected element impedance Rt of the sensor element 14 is an impedance R500 or less which is a value corresponding to the element temperature of 500 ° C., it is determined that the element temperature is 500 ° C. or more. If it is not confirmed in step S126 that the element temperature ≧ 500 ° C., the process is temporarily terminated. Thereafter, when this routine is repeatedly executed, while it is recognized that the deterioration determination flag = ON is established in step S102, steps S116 to S124 are performed until it is recognized in step S126 that the element temperature is 500 ° C. or higher. According to the above, the above processing is performed.

  On the other hand, if it is found in step S126 that the element temperature of the sensor element ≧ 500 ° C. is established, it is next determined whether the element temperature is 750 ° C. or less (step S128). Specifically, when the detected element impedance Rt of the sensor element 14 is equal to or higher than the impedance R750 which is a value corresponding to the element temperature of 750 ° C., the formation of the element temperature ≦ 750 ° C. is recognized.

  If it is determined in step S128 that the element temperature ≦ 750 ° C., then the rich shift RS is calculated (step S130). The rich shift RS is calculated as an absolute value of a value obtained by subtracting an output corresponding to the theoretical air-fuel ratio from the current output of the air-fuel ratio sensor 10 according to the above equation (1). Next, a rich deviation index SUMRS is calculated (step S132). The rich deviation index SUMRS is calculated by adding the rich deviation RS calculated in step S130 this time to the rich deviation index SUMRS that is the integrated value of the rich deviation RS calculated up to the previous time. Note that the accumulated value SUMRS of the rich deviation RS until the previous time is SUMRS = 0 in the initial value, and SUMRS = RS + 0 in the first calculation of the rich deviation index SUMRS. Next, the rich deviation index SUMRS calculated in step S130 is stored (step S134). Thereafter, this process is temporarily terminated.

  When the internal combustion engine is started, this routine is repeatedly executed. While the conditions A to C are satisfied, the element temperature ≧ 500 ° C. is recognized in step S126, and the element temperature ≦ 750 ° C. is recognized in step S128. Meanwhile, the rich deviation index SUMRS is repeatedly updated and stored based on the rich deviation RS according to steps S130 to S134.

  On the other hand, if the element temperature ≦ 750 ° C. is not recognized in step S128 while the routine is repeatedly executed, the previously stored rich deviation index SUMRS is read in step S136. Next, a deterioration determination value WRS is set (step S138). The deterioration determination value WRS is set according to a map stored in advance in the microcomputer 32 according to the elapsed time from when the internal combustion engine stopped last time until the current start is started.

  Next, it is determined whether or not the rich deviation index SUMRS is equal to or less than the deterioration determination value WRS (step S140). If it is determined that the rich deviation index SUMRS is equal to or less than the deterioration determination value WRS, it is determined that the air-fuel ratio sensor 10 is normal (step S142). On the other hand, when it is not recognized that the rich deviation index SUMRS ≦ deterioration determination value WRS, deterioration of the catalyst layer 24 is recognized and deterioration of the air-fuel ratio sensor 10 is determined (step S144). Thereafter, the deterioration determination flag is set to OFF (step S146), and the rich deviation index SUMRS = 0 is set (step S148). Thereafter, this process ends.

  As described above, according to the present embodiment, after the sensor element 14 is cooled, the deterioration of the catalyst layer 24 is determined based on the rich deviation of the sensor output when the warm-up is started again. Therefore, even in the air-fuel ratio sensor 10 in which the catalyst layer 24 for suppressing the output deviation due to hydrogen gas is provided on the exhaust side electrode 18 side of the sensor element 14, the abnormality of the air-fuel ratio sensor due to the deterioration of the catalyst layer 24 can be reliably ensured. Can be determined. Further, according to such control, it is possible to avoid the control for increasing the amount of fuel in the deterioration determination.

  In the present embodiment, a case has been described in which deterioration of the sensor element 14 is determined on the condition that the starting water temperature TWst is 0 to 30 ° C. This is for determining that the internal combustion engine is sufficiently cooled, and for avoiding the start of the deterioration determination in the case of starting in a special environment such as an extremely low temperature. However, in the present invention, the range of the starting water temperature is not limited to this, and may be other conditions as long as the cooling of the internal combustion engine can be confirmed. In the present invention, it is sufficient that at least the sensor element 14 is cooled. Therefore, the determination of the water temperature of the internal combustion engine may not be performed.

  Further, in the present embodiment, the case where the condition B is that the air-fuel ratio is A / F 14.5 to 16.0 has been described as a condition for determining the deterioration of the sensor element 14. This is because the output deviation of the sensor to the rich side can be detected more accurately in the vicinity of the theoretical air-fuel ratio to the lean atmosphere as described above. However, in the present invention, the control value of the air-fuel ratio is not necessarily limited to this range, and may be, for example, about A / F 14.0 to 16.0. Moreover, it is not restricted to this range. However, it is desirable that the output deviation toward the rich side is within the range from the vicinity of the theoretical air-fuel ratio to the lean air-fuel ratio.

  In the present embodiment, the case where the output deviation is determined when the temperature of the sensor element 14 is 500 ° C. or higher and 750 ° C. or lower has been described. This is because in the case of the air-fuel ratio sensor 10, the output deviation becomes significant when the temperature of the sensor element 14 is 500 ° C. to 750 ° C., and by detecting the sensor output in this range selectively, the air is more accurately detected. This is because the deterioration of the fuel ratio sensor 10 can be determined. However, in the present invention, the temperature of the sensor element 14 at the time of deterioration determination is not limited to this range. For example, the rich deviation RS and the rich deviation index are based on the sensor output immediately after the start to the activation temperature. SUMRS may be calculated to determine deterioration.

  In the present embodiment, the case where the air-fuel ratio sensor 10 shown in FIG. 1 is used has been described. However, the present invention is not limited to the air-fuel ratio sensor 10 and can be applied to other exhaust gas sensors. When applied to other exhaust gas sensors, the desorption start temperature, desorption end temperature, and activation temperature of the adsorbed species vary depending on the sensor used. For example, in the case of an oxygen sensor, the activation temperature is about 300 ° C. Reach. Therefore, the temperature setting in the condition D may be set for each sensor to which the present invention is applied so that the sensor output is detected from the desorption of the adsorbed component to the end of the desorption.

  In the present embodiment, the difference between the output of the air-fuel ratio sensor and the output at the stoichiometric air-fuel ratio is obtained as the rich deviation RS, and the integrated value of the rich deviation RS is used as the rich deviation index SUMRS, and this rich deviation index SUMRS. A case has been described in which the deterioration of the catalyst layer 24 is determined when is smaller than the deterioration determination value WRS. However, in the present invention, the deterioration determination method is not limited to this. For example, the deterioration determination may be performed by obtaining a rich deviation RS and calculating a value serving as a determination index based on the RS. Good. Further, for example, the rich shift RS may be obtained as a difference between the actual output and the output corresponding to the control target air-fuel ratio, instead of the difference between the actual output and the output with respect to the theoretical air-fuel ratio. Further, the output of the air-fuel ratio sensor 10 is not limited to the one used as a determination parameter. For example, the air-fuel ratio may be calculated based on the output and used as a parameter for determining deterioration.

  In the embodiment, the case has been described in which the deterioration determination value WRS is set as a value corresponding to the elapsed time. However, the present invention is not limited to this, and may be stored in advance as a constant value, for example.

  Further, the case where the detection and determination of the element temperature of the sensor element 14 is performed based on the element impedance has been described. However, in the present invention, the method for detecting and determining the element temperature of the sensor element 14 is not limited to this. For example, the element temperature of the sensor element 14 is detected directly or estimated from the energization amount to the heater 28. For example, it may be detected by other methods.

  Further, the configurations of the catalyst layer 24 and the protective film 22 of the sensor element 14 have been specifically described. However, the catalyst layer 24 is not necessarily limited to such a configuration, and may have a function of limiting the rate of hydrogen diffusion. Any other configuration may be used.

  For example, in the embodiment, the “element temperature detecting means” of the present invention is realized by executing step S110, and the “activation start detecting means” is realized by executing step S104. By executing S112, an “element temperature determination means” is realized, by executing step S118, an “air-fuel ratio determination means” is realized, and by executing step S130, an “output deviation detection means” is realized. By implementing steps S140 to S144, a “deterioration determination unit” is realized.

  In addition, for example, the “activity determination unit” of the present invention is realized by executing step S128 in the embodiment, and the “temperature determination unit” is realized by executing steps S126 and S128. Is executed, "integrated value calculating means" is realized, step S138 is executed, "deterioration determination value setting means" is realized, and step S106 is executed, "water temperature detecting means" is realized. In addition, the “water temperature determination unit” is realized by executing step S108.

It is a schematic diagram for demonstrating the air fuel ratio sensor in embodiment of this invention. It is a schematic diagram for demonstrating the control apparatus of the air fuel ratio sensor in embodiment of this invention. It is a figure for demonstrating the output of an air fuel ratio sensor at the time of sensor element warming-up. It is a figure for demonstrating the state at the time of sensor element cooling and sensor element warming-up in embodiment of this invention. It is a figure for demonstrating the relationship between the elapsed time after an internal combustion engine stop, and a deterioration determination value in embodiment of this invention. It is a flowchart for demonstrating the control routine which a system performs in Embodiment 1 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Air fuel ratio sensor 12 Cover 14 Sensor element 16 Solid electrolyte layer 18 Exhaust side electrode 20 Atmosphere side electrode 22 Protective film 24 Catalyst layer 26 Atmosphere chamber 28 Heater 30 Sensor element drive circuit 32 Microcomputer 34 Heater control circuit 36 ECU
50 Oxygen 52 Adsorbed species

Claims (6)

A control device for controlling an exhaust gas sensor mounted in an exhaust passage of an internal combustion engine,
The exhaust gas sensor includes a sensor element that is activated when the activation temperature is reached,
Element temperature detecting means for detecting the temperature of the sensor element;
Activation start detecting means for detecting the activation start of the sensor element;
Element temperature determination means for determining whether or not the element temperature of the sensor element is equal to or lower than a reference temperature when activation of the sensor element is detected;
Air / fuel ratio determining means for determining whether or not the target air / fuel ratio of the internal combustion engine is set within a range of a reference air / fuel ratio when it is determined that the element temperature is equal to or lower than the reference temperature;
An output deviation detecting means for detecting an output deviation of the output of the exhaust gas sensor with respect to a reference output when it is determined that the target air-fuel ratio is set within the range of the reference air-fuel ratio;
Deterioration determining means for determining deterioration of the exhaust gas sensor based on the output deviation;
An exhaust gas sensor control device comprising: Active temperature determining means for determining whether the element temperature is equal to or lower than the active temperature;
An integrated value calculating means for calculating an integrated value of the output deviation while the element temperature is determined to be equal to or lower than the activation temperature;
With
2. The exhaust gas sensor control according to claim 1, wherein the deterioration determination unit determines that the exhaust gas sensor is deteriorated when an integrated value of the output deviation is larger than a deterioration determination value. apparatus. Temperature determining means for determining whether the element temperature is 500 ° C. or higher and 700 ° C. or lower;
An integrated value calculating means for calculating an integrated value of the output deviation while the element temperature is determined to be 500 ° C. or higher and 700 ° C. or lower;
2. The exhaust gas sensor control according to claim 1, wherein the deterioration determination unit determines that the exhaust gas sensor is deteriorated when an integrated value of the output deviation is larger than a deterioration determination value. apparatus. An elapsed time detecting means for detecting an elapsed time from the previous stop of the internal combustion engine to the start of activation of the element temperature;
A deterioration determination value setting means for setting the deterioration determination value according to the elapsed time;
The exhaust gas sensor control device according to claim 2, comprising:   The exhaust gas sensor control device according to any one of claims 1 to 4, wherein the reference air-fuel ratio is 14.5 or more and 16.0 or less. Water temperature detecting means for detecting the temperature of the cooling water of the internal combustion engine;
Water temperature determining means for determining whether the water temperature is equal to or lower than a reference water temperature;
With
The exhaust gas sensor control according to any one of claims 1 to 5, wherein the output deviation detecting means detects the output deviation when it is determined that the water temperature is equal to or lower than the reference water temperature. apparatus.

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