JP2018017535A - Gas concentration detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas concentration detector with which it is possible to maintain the high responsiveness of air-fuel ratio detection and use a gas sensor for separate purposes as an air-fuel ratio sensor and as an oxygen sensor in accordance with the air-fuel ratio of exhaust gas.SOLUTION: A gas concentration detector 1 comprises a gas sensor 2 and a controller 5, the controller 5 having an air-fuel ratio detection unit 51 and a rich-lean detection unit 52. The air-fuel ratio detection unit 51 detects an air-fuel ratio on the basis of the magnitude and forward/reverse directions of a current flowing in the gas sensor 2 when the air-fuel ratio of an exhaust gas G discharged from an internal combustion engine lies within a stoichiometric range between a rich-side lower limit and a lean-side upper limit in the vicinity of a logical air-fuel ratio. The rich-lean detection unit 52 detects whether the air-fuel ratio is closer to the rich side or the lean side than is the logical air-fuel ratio, on the basis of the forward/reverse directions of a current flowing in the gas sensor 2 when the air-fuel ratio of the exhaust gas G lies out of the stoichiometric range.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a gas concentration detection device that detects an air-fuel ratio of exhaust gas.

  Gas sensors include applications that detect the oxygen concentration of exhaust gas exhausted from an internal combustion engine, applications that determine the air-fuel ratio of exhaust gas (the air-fuel ratio during combustion determined from exhaust gas), and applications that detect specific gas components such as NOx. is there. The air-fuel ratio sensor that detects the air-fuel ratio is in accordance with the ratio of air to unburned gas in the exhaust gas in substantially the entire air-fuel ratio, for example, in a wide range where the air-fuel ratio A / F is 10 to 20 or more (atmosphere). It can be detected by linear. On the other hand, the oxygen sensor that detects the oxygen concentration detects whether the air-fuel ratio compared to the stoichiometric air-fuel ratio is on the rich side that is the fuel-rich side or the lean side that is the air-rich side.

  In the air-fuel ratio sensor, the surface of the detection electrode provided on the solid electrolyte body and exposed to the exhaust gas is covered with a diffusion resistance layer made of porous ceramics. When a voltage is applied between the reference electrode and the detection electrode exposed to the atmosphere, the flow of exhaust gas to the detection electrode is controlled by the diffusion resistance layer, the current flowing between the reference electrode and the detection electrode is the limiting current characteristic. Saturates at a constant value. In the rich region, a voltage is applied to pump oxygen from the reference electrode to the detection electrode. In the lean region, a voltage is applied to pump oxygen from the detection electrode to the reference electrode. There is a positive correlation between the saturation current flowing between the detection electrode and the reference electrode and the air-fuel ratio, and the air-fuel ratio in the rich region and the lean region is linearly changed based on the magnitude and direction of the current. Detected.

  For example, in the air-fuel ratio sensor of Patent Document 1, in order to set the output current value between the reference electrode and the detection electrode to an appropriate value from the viewpoint of detection accuracy and responsiveness, a diffusion resistance layer covering the detection electrode is used. It is disclosed that the thickness is 200 to 800 μm and the porosity is 3 to 5%.

  Further, in the oxygen sensor, when the air-fuel ratio changes from the lean side to the rich side, or when it is on the rich side, carbon monoxide or carbonization in the exhaust gas reaching the detection electrode is caused by catalytic action of platinum or the like in the detection electrode. Unburned gas components such as hydrogen are converted into carbon dioxide, water, and the like. At this time, it is detected whether the air-fuel ratio is on the rich side or the lean side by utilizing the fact that a large electromotive force is generated between the reference electrode and the detection electrode.

Japanese Unexamined Patent Publication No. 7-198673

In the air-fuel ratio sensor, the thickness of the diffusion resistance layer is increased in order to set the air-fuel ratio detection range in a wide range, assuming that the air-fuel ratio varies greatly according to the combustion state of the internal combustion engine. The reason for increasing the thickness of the diffusion resistance layer is to limit the amount of exhaust gas that reaches the detection electrode, and in particular to ensure detection accuracy when the air-fuel ratio fluctuates greatly to the rich side.
However, if the thickness of the diffusion resistance layer is large, the time until the exhaust gas passes through the diffusion resistance layer and reaches the detection electrode becomes long, and the detection response of the air-fuel ratio sensor deteriorates.

In addition, when the thickness of the diffusion resistance layer is small, more exhaust gas passes through the diffusion resistance layer in a short time and reaches the detection electrode. Therefore, when the air-fuel ratio fluctuates greatly toward the rich side, a large amount of unburned gas reaches the detection electrode and is detected from the reference electrode to oxidize the unburned gas and convert it into carbon dioxide, water, etc. A large amount of oxygen needs to be rapidly supplied to the electrode. On the other hand, when the air-fuel ratio fluctuates greatly toward the lean side, a large amount of oxygen reaches the detection electrode, and it is necessary to rapidly discharge this oxygen from the detection electrode to the reference electrode.
There is a limit to the oxygen transfer flow rate, and the oxygen transfer flow rate is limited by the clearance between the solid electrolyte body and the heater disposed inside the solid electrolyte body. Therefore, there is a limit to improve the response of detection by the air-fuel ratio sensor.

On the other hand, in the oxygen sensor, it is not necessary to detect the air-fuel ratio linearly according to the magnitude of the current. Therefore, there is almost no need to limit the amount of exhaust gas reaching the detection electrode by increasing the thickness of the diffusion resistance layer.
The present inventors have arranged the necessary conditions for the air-fuel ratio sensor and the oxygen sensor described above, and have come to find a completely new type of gas sensor in which the response of the conventional air-fuel ratio sensor is improved.

  The present invention has been made in view of such a problem, and can maintain a high responsiveness in detecting the air-fuel ratio. According to the air-fuel ratio of the exhaust gas, the gas sensor is used as an air-fuel ratio sensor and an oxygen sensor. It was obtained in an attempt to provide a gas concentration detection device that can be used properly.

One aspect of the present invention includes a solid electrolyte body (21) having oxygen ion conductivity, a detection electrode (22) provided on a surface of the solid electrolyte body exposed to a detection gas, and a reference gas in the solid electrolyte body A gas sensor (2) having a reference electrode (23) provided on the surface exposed to the surface and a diffusion resistance layer (24) made of porous ceramics covering the detection electrode;
A control device (5) electrically connected to the gas sensor and controlling the operation of the gas sensor,
The controller is
The lower limit value (R1) on the rich side when the air-fuel ratio of the exhaust gas (G) as the detection gas exhausted from the internal combustion engine is close to the stoichiometric air-fuel ratio, and the upper limit value (R2) on the lean side near the stoichiometric air-fuel ratio. An air-fuel ratio detection unit (51) for detecting the air-fuel ratio when it is within a stoichiometric range (R) between;
A rich / lean detector (52) for detecting whether the air-fuel ratio is on the rich side or the lean side of the stoichiometric air-fuel ratio when the air-fuel ratio of the exhaust gas is outside the stoichiometric range, It is in the gas concentration detector (1).

  In the control device for the gas concentration detection device, an air-fuel ratio detection unit and a rich / lean detection unit are constructed. When the air-fuel ratio of the exhaust gas is within the stoichiometric range between the rich-side lower limit value near the stoichiometric air-fuel ratio and the lean-side upper limit value near the stoichiometric air-fuel ratio, the air-fuel ratio detection unit When the air-fuel ratio of the exhaust gas is outside the stoichiometric range, the rich / lean detection unit is operated.

  As a result, when the air-fuel ratio is in the vicinity of the stoichiometric air-fuel ratio and there is no possibility that a large amount of unburned gas or exhaust gas containing oxygen will reach the detection electrode, the air-fuel ratio detection unit uses the gas sensor as the air-fuel ratio sensor. It can be operated. When the air-fuel ratio is far from the stoichiometric air-fuel ratio and there is a possibility that a large amount of unburned gas or exhaust gas containing oxygen may reach the detection electrode, the rich / lean detection unit operates the gas sensor as an oxygen sensor. Can be made.

  Therefore, the thickness of the diffusion resistance layer covering the detection electrode of the gas sensor can be reduced, and the time for exhaust gas to pass through the diffusion resistance layer and reach the detection electrode can be shortened. The air-fuel ratio detection unit can maintain high air-fuel ratio responsiveness when the gas sensor is operated as an air-fuel ratio sensor.

Therefore, according to the gas concentration detection device, the responsiveness of air-fuel ratio detection can be maintained high, and the gas sensor is selectively used for the use of the air-fuel ratio sensor and the use of the oxygen sensor according to the air-fuel ratio of the exhaust gas. be able to.
The “air-fuel ratio of exhaust gas” means the air-fuel ratio at the time of combustion of the air-fuel mixture in the internal combustion engine that exhausted the exhaust gas.

  Note that the reference numerals in parentheses of the constituent elements shown in one embodiment of the present invention indicate the correspondence with the reference numerals in the drawings in the embodiment, but the constituent elements are not limited only to the contents of the embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram showing a gas concentration detection device according to a first embodiment. The graph which shows the relationship between the applied voltage and sensor output current concerning Embodiment 1. FIG. The graph which shows the relationship between the air fuel ratio and purification rate concerning Embodiment 1. FIG. FIG. 3 is an explanatory diagram showing a gas sensor according to the first embodiment. 3 is a graph showing the relationship between the thickness of the diffusion resistance layer and the detectable range of the air-fuel ratio according to the first embodiment. 6 is a graph showing the relationship between the thickness of the diffusion resistance layer and the response time for rich / lean detection according to the first embodiment. 3 is a graph showing the relationship between the thickness of the diffusion resistance layer and the response time of air-fuel ratio detection according to the first embodiment. 6 is a graph showing the responsiveness of air-fuel ratio control by the conventional air-fuel ratio sensor according to the first embodiment. 3 is a graph showing the responsiveness of air-fuel ratio control by a gas sensor according to the first embodiment. The graph which shows the relationship between the applied voltage and sensor output current concerning Embodiment 1 by using a porosity as a parameter. 3 is a flowchart showing an operation of switching between air-fuel ratio detection and rich / lean detection according to the first embodiment. 9 is a flowchart showing an operation of switching between air-fuel ratio detection and rich / lean detection according to the second embodiment. 10 is a flowchart showing an operation of switching between air-fuel ratio detection and rich / lean detection according to the third embodiment.

A preferred embodiment of the above-described gas concentration detection device will be described with reference to the drawings.
As shown in FIG. 1, the gas concentration detection device 1 of this embodiment includes a gas sensor 2 and a control device 5, and the control device 5 includes an air-fuel ratio detection unit 51 and a rich / lean detection unit 52.
The gas sensor 2 includes a solid electrolyte body 21 having oxygen ion conductivity, a detection electrode 22 provided on the surface of the solid electrolyte body 21 that is exposed to the detection gas, and the atmosphere as the reference gas A in the solid electrolyte body 21. A reference electrode 23 provided on the exposed surface and a diffusion resistance layer 24 made of porous ceramics covering the detection electrode 22 are provided. The control device 5 is electrically connected to the gas sensor 2 and controls the operation of the gas sensor 2.

  As shown in FIG. 2, the air-fuel ratio detection unit 51 has an air-fuel ratio of the exhaust gas G as a detection gas exhausted from the internal combustion engine in the vicinity of the lower limit value R1 on the rich side near the stoichiometric air-fuel ratio and the stoichiometric air-fuel ratio. When it is within the stoichiometric range R between the lean upper limit R2, the air-fuel ratio is detected based on the magnitude and forward / reverse direction of the current flowing between the detection electrode 22 and the reference electrode 23. When the air-fuel ratio of the exhaust gas G is outside the stoichiometric range R, the rich / lean detection unit 52 determines that the air-fuel ratio is the stoichiometric air-fuel ratio based on the forward and reverse directions of the current flowing between the detection electrode 22 and the reference electrode 23. It detects whether it is on the rich side or the lean side.

Hereinafter, the gas concentration detection apparatus 1 of this embodiment will be described in detail.
The gas concentration detection device 1 detects an air-fuel ratio of exhaust gas G flowing through an exhaust pipe of an internal combustion engine (engine) of a vehicle. The gas concentration detection device 1 is used for setting the air-fuel ratio in an internal combustion engine in the vicinity of a purification window in which the catalytic activity of a three-way catalyst disposed in an exhaust pipe is effectively maintained. As shown in FIG. 3, the purification window indicates an air-fuel ratio range in which the purification rate of carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx) is high. The purification window of this embodiment is shown in the vicinity of the theoretical air-fuel ratio, specifically, when the theoretical air-fuel ratio is 14.5, the air-fuel ratio (A / F) is in the range of 14.3 to 14.7. The purification window of this embodiment is assumed to be the same as the stoichiometric range R.
The gas sensor 2 is disposed in the exhaust pipe, and can be disposed either upstream or downstream of the three-way catalyst.

  As shown in FIGS. 1 and 4, in the gas sensor 2, a sensor element 20 is formed by a solid electrolyte body 21 provided with electrodes 22 and 23 and a diffusion resistance layer 24. In addition to the sensor element 20, the gas sensor 2 includes a heater 31 disposed inside the sensor element 20, a housing 32 that holds the sensor element 20, a protective cover 33 that is attached to the housing 32 and covers the sensor element 20, each electrode 22, 23, a lead wire 34 connected to the lead portion 23 and the energization portion of the heater 31, a holding cover 35 attached to the housing 32 and holding the lead wire 34 via a bush 36, and the like.

  As shown in FIG. 1, the solid electrolyte body 21 is formed in a bottomed cylindrical shape. The detection electrode 22 is provided on the outer surface 211 of the solid electrolyte body 21, and the reference electrode 23 is provided on the inner surface 212 of the solid electrolyte body 21. The solid electrolyte body 21 is formed of a solid electrolyte mainly composed of zirconia. The solid electrolyte body 21 of this embodiment is made of yttria stabilized zirconia or yttria partially stabilized zirconia. The detection electrode 22 and the reference electrode 23 contain platinum as a noble metal component and a solid electrolyte that is the same as the solid electrolyte body 21. In the detection electrode 22, a three-phase interface of the exhaust gas G, the noble metal component, and the solid electrolyte is formed.

  The heater 31 has a ceramic substrate and a heating element provided on the substrate, and is disposed inside the solid electrolyte body 21. When the control device 5 energizes the heating element of the heater 31, the heating element generates heat, and the solid electrolyte body 21 and the electrodes 22 and 23 are heated to an activation temperature that expresses oxygen ion conductivity.

  As shown in FIG. 1, the diffusion resistance layer 24 is formed with alumina as a main component, and has a property of transmitting the exhaust gas G under a predetermined diffusion resistance. The thickness t of the diffusion resistance layer 24 is in the range of 100 to 200 μm. The diffusion resistance layer 24 is formed by plasma spraying a ceramic slurry such as alumina on the surface of the detection electrode 22 of the solid electrolyte body 21 and drying and sintering the slurry. The diffusion resistance layer 24 is formed so that its thickness t is uniform. However, when the thickness t of the diffusion resistance layer 24 is partially changed, the average thickness of the diffusion resistance layer 24 is set in the range of 100 to 200 μm. The average thickness is obtained as an average value obtained by measuring the thickness t at a plurality of locations of the diffusion resistance layer 24.

  The thickness t of the diffusion resistance layer 24 refers to the thickness in the direction along the thickness direction T of the solid electrolyte body 21 and the electrodes 22 and 23. The thickness direction T of the solid electrolyte body 21 and the electrodes 22 and 23 refers to a direction in which the detection electrode 22 and the reference electrode 23 face each other with the solid electrolyte body 21 interposed therebetween.

When the thickness t of the diffusion resistance layer 24 is less than 100 μm, the range in which the air-fuel ratio can be detected by the air-fuel ratio detection unit 51 may become too narrow.
FIG. 5 shows a range (A / F) in which the thickness t (μm) of the diffusion resistance layer 24 and the air-fuel ratio can be detected linearly according to the magnitude of the current flowing between the detection electrode 22 and the reference electrode 23. Shows the relationship. When the thickness t of the diffusion resistance layer 24 is 100 μm, the detectable range of the air-fuel ratio (A / F), which is the ratio of air to fuel, is in the range of 14.3 to 14.7. When the thickness t of the diffusion resistance layer 24 is less than 100 μm, the detectable range of the air-fuel ratio becomes even narrower and only near the vicinity of 14.5 which is the stoichiometric air-fuel ratio (stoichiometric).

The detectable range of the air-fuel ratio is a range for determining the stoichiometric range R that is a range in the vicinity of the theoretical air-fuel ratio for detecting the air-fuel ratio by the air-fuel ratio detector 51. In order to make the stoichiometric range R wider than the range of 14.3 to 14.7, the thickness t of the diffusion resistance layer 24 is set to 100 μm or more. The stoichiometric range R may be 14.2 to 14.8 as the air-fuel ratio range when the thickness t of the diffusion resistance layer 24 is 200 μm.
The stoichiometric range R is a range for determining a purification window for maintaining a high purification rate by the three-way catalyst.

When the thickness t of the diffusion resistance layer 24 exceeds 200 μm, there is a possibility that the response time of rich / lean detection by the rich / lean detection unit 52 for detecting whether the air-fuel ratio is on the rich side or the lean side is long. is there. The response time of the rich / lean detection is caused by the time until the exhaust gas G passes through the diffusion resistance layer 24 and reaches the detection electrode 22.
FIG. 6 shows the relationship between the thickness t (μm) of the diffusion resistance layer 24 and the response time (ms) for rich / lean detection. When the thickness t of the diffusion resistance layer 24 is 200 μm, the response time for rich / lean detection is 200 ms. This response time is a time for the exhaust gas G on the surface of the diffusion resistance layer 24 to reach the detection electrode 22 through the diffusion resistance layer 24. When the gas sensor 2 is used as an oxygen sensor that performs rich / lean detection, the response time required for the gas sensor 2 is 200 ms or less. Therefore, in order to ensure the responsiveness of the gas sensor 2, the thickness t of the diffusion resistance layer 24 is set to 200 μm or less.

The thickness t of the diffusion resistance layer 24 of this embodiment is extremely small compared to the thickness of the diffusion resistance layer in the conventional air-fuel ratio sensor.
FIG. 7 shows the relationship between the thickness (μm) of the diffusion resistance layer and the response time (ms) for air-fuel ratio detection. The diffusion resistance layer in the conventional air-fuel ratio sensor has a thickness of about 650 to 800 μm in order to set the air-fuel ratio detection range in a wide range. At this time, the response time when detecting the air-fuel ratio is about 630 to 770 ms. This response time is a time for the exhaust gas G on the surface of the diffusion resistance layer 24 to reach the detection electrode 22 through the diffusion resistance layer 24. On the other hand, the thickness of the diffusion resistance layer 24 in the gas sensor 2 of the present embodiment is extremely small compared to the conventional case, and is 100 to 200 μm. At this time, the response time when detecting the air-fuel ratio is 100 to 200 ms.

  In the conventional air-fuel ratio sensor, the response time of air-fuel ratio detection is long, and as shown in FIG. 8, when the air-fuel ratio is controlled by the engine control unit 5B, it is long until the air-fuel ratio becomes the target air-fuel ratio. It took time. On the other hand, in the gas sensor 2 of the present embodiment, the response time of air-fuel ratio detection is shortened, and when the air-fuel ratio is controlled by the engine control unit 5B as shown in FIG. 9, the air-fuel ratio becomes the target air-fuel ratio. Can be shortened. Thereby, when detecting the air fuel ratio by the air fuel ratio detection part 51 of the gas concentration detection apparatus 1 of this form, it can contribute to reduction of an emission (air pollutant).

The porosity of the diffusion resistance layer 24 is in the range of 4 to 9%. The porosity of the diffusion resistance layer 24 is the ratio of the volume of the pores to the overall volume of the diffusion resistance layer 24. The volume of the entire outer shape of the diffusion resistance layer 24 includes the volume of pores. The pores include open pores that appear on the surface of the diffusion resistance layer 24 and closed pores that are disposed inside the diffusion resistance layer 24.
The porosity can be obtained as a mercury porosimeter based on the mass change between before the pores are filled with a liquid such as mercury and after the pores are filled with a liquid such as mercury. The porosity can also be obtained by observing a plurality of cross sections of the diffusion resistance layer 24 using a scanning electron microscope (SEM).

When the porosity of the diffusion resistance layer 24 is less than 4%, the exhaust gas G becomes too difficult to pass through the diffusion resistance layer 24, and a sensor output current that is a current flowing between the detection electrode 22 and the reference electrode 23 is There is a possibility that the detection accuracy of the air-fuel ratio may be reduced.
FIG. 10 shows the relationship between the voltage (V) applied between the detection electrode 22 and the reference electrode 23 and the sensor output current (mA) when the air-fuel ratio is 14.7. In order to maintain the detection accuracy of the gas sensor 2, the sensor output current is preferably 0.5 mA or more. As the sensor output current decreases, the ratio of current detection error increases and the air-fuel ratio detection accuracy decreases. The sensor output current is 0.5 mA when the porosity of the diffusion resistance layer 24 is 4%, and the sensor output current is further reduced when the porosity is less than 4%. Accordingly, the porosity is set to 4% or more in order to secure a sensor output current having a necessary magnitude.

When the porosity of the diffusion resistance layer 24 exceeds 9%, the exhaust gas G becomes too easy to pass through the diffusion resistance layer 24, and the voltage applied between the detection electrode 22 and the reference electrode 23 changes. It may be difficult to obtain a limit current characteristic in which the sensor output current does not change, and the air-fuel ratio detection accuracy may be reduced.
The limiting current characteristic is that when the voltage is applied between the detection electrode 22 and the reference electrode 23, the flow rate of the exhaust gas G reaching the detection electrode 22 is determined by the rate limiting of the flow rate of the exhaust gas G by the diffusion resistance layer 24. Therefore, the sensor output current does not change even when the voltage changes. However, if the porosity increases until it exceeds 9%, the diffusion resistance layer 24 may not function as a rate-limiting function of the flow rate of the exhaust gas G, and the limit current characteristics may not be obtained. In this case, the sensor output current is likely to vary, and the air-fuel ratio detection accuracy may be reduced.

  In FIG. 10, when the porosity is 9% or less, the limit current characteristic is obtained. On the other hand, when the porosity exceeds 9%, for example, 10%, the limit current characteristic may not be obtained. Accordingly, the porosity is set to 9% or less in order to obtain the limit current characteristics.

  As shown in FIG. 1, the control device 5 includes a voltage application unit 61 that applies a voltage between the detection electrode 22 and the reference electrode 23, and a current detection that detects a current that flows between the detection electrode 22 and the reference electrode 23. Means 62, energizing means 63 for energizing the heating element of the heater 31, and the like are provided. These means 61, 62 and 63 are formed in a sensor control unit (SCU) 5 </ b> A as the control device 5.

The voltage application means 61 applies a voltage so that the reference electrode 23 becomes a positive electrode so that oxygen ions move from the detection electrode 22 to the reference electrode 23.
When the air-fuel ratio detected by the air-fuel ratio detection unit 51 is in the lean region, current flows from the reference electrode 23 to the detection electrode 22. On the other hand, when the air-fuel ratio detected by the air-fuel ratio detection unit 51 is in the rich region, the air-fuel ratio is in the lean region due to the electromotive force generated between the detection electrode 22 and the reference electrode 23 via the solid electrolyte body 21. The current flows from the detection electrode 22 to the reference electrode 23 in the opposite direction to that in the case of.

As shown in FIG. 1, the air-fuel ratio detection unit 51 and the rich / lean detection unit 52 are constructed in a computer of an engine control unit (ECU) 5B serving as a control device 5 higher than the sensor control unit 5A. .
As shown in FIG. 2, the air-fuel ratio detection unit 51 stores the relationship between the applied voltage (V) in the forward direction and the reverse direction and the sensor output current (mA). Then, the air-fuel ratio detection unit 51 detects the air-fuel ratio in the weak rich region and the weak lean region in the vicinity of the theoretical air-fuel ratio based on the change in the magnitude of the sensor output current and the forward / reverse direction of the sensor output current.

The voltage application unit 61 of this embodiment continuously applies a voltage between the detection electrode 22 and the reference electrode 23 regardless of which of the air-fuel ratio detection unit 51 and the rich / lean detection unit 52 operates.
The rich / lean detection unit 52 of the present embodiment confirms whether the air-fuel ratio is on the rich side of the stoichiometric air-fuel ratio by checking the forward / reverse direction of the current flowing between the detection electrode 22 and the reference electrode 23. Whether it is on the lean side can be detected.

  In the control device 5, the sensor output current detected by the current detection means 62 is between the lower limit threshold value corresponding to the lower limit value R1 of the stoichiometric range R (purification window range) and the upper limit threshold value corresponding to the upper limit value R2. It has the determination part 53 which determines whether it is. Then, by the operation of the determination unit 53, the gas concentration detection device 1 switches between air-fuel ratio detection and rich / lean detection.

FIG. 11 is a flowchart showing an operation of switching between air-fuel ratio detection and rich / lean detection by the control device 5 of the gas concentration detection device 1 of the present embodiment.
The control device 5 detects the sensor output current flowing between the detection electrode 22 and the reference electrode 23 at a predetermined measurement time interval by the current detection means 62 (step S11). Next, the determination unit 53 of the control device 5 determines whether or not the sensor output current is between the lower limit threshold and the upper limit threshold (S12). If the sensor output current is between the lower limit threshold and the upper limit threshold, the air / fuel ratio is detected by the air / fuel ratio detector 51 of the control device 5 (S13). On the other hand, if the sensor output current is not between the lower limit threshold value and the upper limit threshold value, rich / lean detection is performed by the rich / lean detection unit 52 of the control device 5 (S14).

  When the vehicle on which the gas concentration detection device 1 is mounted is in steady running other than during acceleration and deceleration, the engine control unit 5B maintains the air-fuel ratio of the engine within the purification window range near the stoichiometric air-fuel ratio. To be controlled. At this time, the control device 5 operates the air-fuel ratio detection unit 51 to accurately detect the air-fuel ratio, and feeds back the detected air-fuel ratio to the engine control unit 5B.

  When the vehicle is in acceleration, the air / fuel ratio is controlled to be richer than the stoichiometric air / fuel ratio. At this time, the air-fuel ratio of the engine deviates from the purification window range to the rich side. Further, the control device 5 operates the rich / lean detection unit 52, and the rich / lean detection unit 52 detects the reverse current in which the sensor output current flows from the reference electrode 23 to the detection electrode 22. Then, the control device 5 feeds back to the engine control unit 5B that the detected air-fuel ratio is on the rich side.

  When the vehicle is decelerating, the air-fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio. At this time, the air-fuel ratio of the engine deviates from the purification window range to the lean side. Further, the control device 5 operates the rich / lean detection unit 52, and the rich / lean detection unit 52 detects a positive current in which the sensor output current flows from the detection electrode 22 to the reference electrode 23. Then, the control device 5 feeds back to the engine control unit 5B that the detected air-fuel ratio is on the lean side.

In the control device 5 of the present embodiment, the air-fuel ratio of the exhaust gas G is within the stoichiometric range R between the rich-side lower limit value R1 near the stoichiometric air-fuel ratio and the lean-side upper limit value R2 near the stoichiometric air-fuel ratio. If the air-fuel ratio of the exhaust gas G is outside the stoichiometric range R, the rich / lean detector 52 is operated.
As a result, the air-fuel ratio is in the range of 14.3 to 14.7, which is close to the stoichiometric air-fuel ratio, and there is no possibility that the exhaust gas G containing a large amount of unburned gas or oxygen will reach the detection electrode 22. The air-fuel ratio detection unit 51 can operate the gas sensor 2 as an air-fuel ratio sensor. Further, in a state where the air-fuel ratio is far from the vicinity of the stoichiometric air-fuel ratio and there is a possibility that the exhaust gas G containing a large amount of unburned gas in the rich region or a large amount of air in the lean region may reach the detection electrode 22, rich lean The detector 52 can operate the gas sensor 2 as an oxygen sensor.

  Accordingly, the thickness t of the diffusion resistance layer 24 covering the detection electrode 22 of the gas sensor 2 can be reduced within the range of 100 to 200 μm, and the exhaust gas G reaches the detection electrode 22 through the diffusion resistance layer 24. Time can be shortened. The air-fuel ratio detection unit 51 can maintain high responsiveness in detecting the air-fuel ratio when the gas sensor 2 is operated as an air-fuel ratio sensor.

  Therefore, according to the gas concentration detection device 1 of the present embodiment, the responsiveness of air-fuel ratio detection can be maintained high, and the gas sensor 2 can be used as an air-fuel ratio sensor and an oxygen sensor according to the air-fuel ratio of the exhaust gas G. Can be used for different purposes.

(Embodiment 2)
The voltage application means 61 in the control device 5 of the gas concentration detection apparatus 1 according to the present embodiment applies a voltage between the detection electrode 22 and the reference electrode 23 when the air-fuel ratio detection unit 51 operates, thereby performing rich / lean detection. When the unit 52 operates, no voltage is applied between the detection electrode 22 and the reference electrode 23.
As shown in FIG. 1, the engine control unit 5 </ b> B as the control device 5 of the present embodiment can form a state in which the voltage is applied by the voltage application unit 61 and a state in which the voltage application unit 61 stops applying the voltage. A switching unit 54 is provided. The switching unit 54 is configured to turn on / off voltage application.

The operation by the air-fuel ratio detection unit 51 is the same as that in the first embodiment. The rich / lean detection unit 52 of the present embodiment detects an electromotive force generated between the detection electrode 22 and the reference electrode 23 in a state where no voltage is applied by the voltage application unit 61, and the air-fuel ratio is greater than the stoichiometric air-fuel ratio. Is also detected on the rich side or the lean side.
Also in the gas concentration detection apparatus 1 of this embodiment, the other configurations are the same as those in the first embodiment.

FIG. 12 is a flowchart showing an operation of switching between air-fuel ratio detection and rich / lean detection by the control device 5 of the gas concentration detection device 1 of the present embodiment.
The control device 5 detects the sensor output current flowing between the detection electrode 22 and the reference electrode 23 at a predetermined measurement time interval by the current detection means 62 (step S21). Next, the determination unit 53 of the control device 5 determines whether or not the sensor output current is between the lower limit threshold and the upper limit threshold (S22). If the sensor output current is between the lower limit threshold and the upper limit threshold, the air / fuel ratio is detected by the air / fuel ratio detector 51 of the control device 5 (S23). At this time, the voltage application unit 61 continues to apply a voltage between the detection electrode 22 and the reference electrode 23.

  On the other hand, when the sensor output current is not between the lower limit threshold and the upper limit threshold, the control device 5 stops the application of the voltage between the detection electrode 22 and the reference electrode 23 by the voltage application unit 61 (S24). ). Next, rich / lean detection is performed by the rich / lean detection unit 52 of the control device 5 (S25). Thereafter, the application of voltage between the detection electrode 22 and the reference electrode 23 by the voltage application means 61 is resumed by the control device 5 (S26).

  Also in this embodiment, the constituent elements and the like indicated by the same reference numerals as those in the first embodiment are the same as the constituent elements and the like in the first embodiment. Also in this embodiment, the same effect as that of the first embodiment can be obtained.

(Embodiment 3)
The control device 5 of the gas concentration detection device 1 of this embodiment has a travel determination unit that detects whether or not the vehicle is in steady travel, instead of the determination unit 53. The gas concentration detection device 1 may switch between air-fuel ratio detection and rich / lean detection by the operation of the travel determination unit. In this case, the vehicle is provided with an acceleration sensor that detects whether the vehicle is accelerating or decelerating.

FIG. 13 is a flowchart showing an operation of switching between air-fuel ratio detection and rich / lean detection by the travel detection unit of the control device 5.
The control device 5 detects the acceleration or deceleration of the vehicle at predetermined measurement time intervals using the acceleration sensor (step S31). Next, the travel determination unit of the control device 5 determines whether the vehicle is in acceleration / deceleration (acceleration, deceleration) or other steady travel (step S32). When the vehicle is in steady running, the air-fuel ratio is detected by the air-fuel ratio detector 51 of the control device 5 (S33). On the other hand, when the vehicle is in acceleration / deceleration, rich / lean detection is performed by the rich / lean detection unit 52 of the control device 5 (S34). In this case, the same effect as that obtained when the determination unit 53 is used can be obtained.

  Also in this embodiment, the constituent elements and the like indicated by the same reference numerals as those in the first embodiment are the same as the constituent elements and the like in the first embodiment. Also in this embodiment, the same effect as that of the first embodiment can be obtained.

  In the first and second embodiments, the gas sensor 2 constitutes an air-fuel ratio sensor, and the usage method of the air-fuel ratio sensor can be expanded to be used as an oxygen sensor. In this case, the air-fuel ratio sensor is used to perform air-fuel ratio detection in the stoichiometric range R (purification window range) during steady state, and is used to perform rich / lean detection only when the stoichiometric range R is exceeded. The An unprecedented air-fuel ratio sensor in which the thickness t of the diffusion resistance layer 24 is as small as 100 to 200 μm can be configured.

  The present invention is not limited to each embodiment, and can be applied to further different embodiments without departing from the scope of the invention.

DESCRIPTION OF SYMBOLS 1 Gas concentration detection apparatus 2 Gas sensor 21 Solid electrolyte body 22 Detection electrode 23 Reference electrode 24 Diffusion resistance layer 5 Control apparatus 51 Air-fuel ratio detection part 52 Rich / lean detection part R Stoke range

Claims (7)

A solid electrolyte body (21) having oxygen ion conductivity; a detection electrode (22) provided on a surface of the solid electrolyte body exposed to a detection gas; and a surface of the solid electrolyte body exposed to a reference gas. A gas sensor (2) having a diffusion resistance layer (24) made of porous ceramics covering the reference electrode (23) and the detection electrode;
A control device (5) electrically connected to the gas sensor and controlling the operation of the gas sensor,
The controller is
The lower limit value (R1) on the rich side when the air-fuel ratio of the exhaust gas (G) as the detection gas exhausted from the internal combustion engine is close to the stoichiometric air-fuel ratio, and the upper limit value (R2) on the lean side near the stoichiometric air-fuel ratio. An air-fuel ratio detection unit (51) for detecting the air-fuel ratio when it is within a stoichiometric range (R) between;
A rich / lean detector (52) for detecting whether the air-fuel ratio is on the rich side or the lean side of the stoichiometric air-fuel ratio when the air-fuel ratio of the exhaust gas is outside the stoichiometric range, Gas concentration detector (1). The air-fuel ratio detection unit is configured to detect the air-fuel ratio based on the magnitude and forward / reverse direction of the current flowing between the detection electrode and the reference electrode,
The rich / lean detection unit detects whether the air-fuel ratio is on the rich side or the lean side of the stoichiometric air-fuel ratio based on a current or an electromotive force generated between the detection electrode and the reference electrode. The gas concentration detection device according to claim 1, configured as described above. The control device includes a voltage applying unit (61) for applying a voltage between the detection electrode and the reference electrode, regardless of which of the air-fuel ratio detection unit and the rich / lean detection unit operates. ,
The rich / lean detection unit detects whether the air-fuel ratio is on the rich side or on the lean side of the stoichiometric air-fuel ratio based on the forward and reverse directions of the current flowing between the detection electrode and the reference electrode. The gas concentration detection device according to claim 2. The control device includes a voltage application unit (61) for applying a voltage between the detection electrode and the reference electrode when the air-fuel ratio detection unit operates.
The rich / lean detection unit detects an electromotive force generated between the detection electrode and the reference electrode, and detects whether the air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio. The gas concentration detection apparatus according to claim 2.   5. The gas concentration detection device according to claim 1, wherein a thickness (t) of the diffusion resistance layer is in a range of 100 to 200 μm.   The gas concentration detection device according to claim 5, wherein the porosity of the diffusion resistance layer is in a range of 4 to 9%. The solid electrolyte body is formed in a bottomed cylindrical shape,
The detection electrode is provided on an outer surface (211) of the solid electrolyte body, and the reference electrode is provided on an inner surface (212) of the solid electrolyte body. The gas concentration detection device according to item 1.

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