JP2023145123A - NOx sensor - Google Patents

Abstract

To provide a NOx sensor that can improve the detection accuracy of NOx concentration by suppressing an influence such as a change in an ambient environment and more accurately performing correction based on moisture content in gas to be measured.SOLUTION: A NOx sensor S comprises: a sensor element 1 that has a first cell 1p which outputs a first signal Ip having a correlation to a H2O concentration and a second cell 1s which outputs a second signal Is in accordance with a NOx concentration and a H2O concentration; a heater H; and a sensor control section 10. The sensor control section 10 includes: a NOx concentration reference value calculation section 101 that calculates a NOx concentration reference value C1 based on the second signal Is; a H2O concentration information detection section 102 that detects H2O concentration information Ti based on the first signal Ip or external information; a second cell temperature information detection section 103 that detects second cell temperature information Ti; and a NOx concentration correction section 104 that corrects the NOx concentration reference value C1 using an offset correction value C2 based on the H2O concentration information Ci and the second cell temperature information Ti.SELECTED DRAWING: Figure 1

Description

The present invention relates to a NOx sensor for detecting NOx concentration in a gas to be measured.

NOx sensors for vehicles are equipped with a sensor element that uses a solid electrolyte body, and exhaust gas, which is the gas to be measured, is introduced into the sensor element, and the solid electrolyte body is dissolved due to the decomposition of nitrogen oxides (NOx). The NOx concentration is detected by measuring the current flowing through the sensor. Generally, a sensor element has a plurality of cells in which electrodes are formed on the surface of a solid electrolyte body, and for example, oxygen (O ₂ ) in the gas to be measured is pumped by a pump cell provided on the upstream side of the gas flow. After adjusting the oxygen concentration to a predetermined low oxygen concentration, a sensor cell provided on the downstream side detects NOx contained in the gas to be measured.

On the other hand, in the detection of NOx concentration, moisture (H ₂ O) contained in the gas to be measured may cause an error. As a countermeasure, Patent Document 1 describes a method of estimating the amount of moisture in exhaust gas from the excess air ratio or A/F value determined by the engine operating conditions, and correcting the gas concentration detection signal according to the estimated amount of moisture. is proposed. In addition, in a gas sensor that uses oxygen pumping, a second cell decomposes NOx by utilizing the correlation between the signal obtained from the first cell that pumps out oxygen and the oxygen concentration and water content in the exhaust gas. A method has been proposed for correcting the signal obtained from a map using a map or on an analog circuit.

Patent No. 3372186

The gas sensor of Patent Document 1 estimates the amount of moisture in exhaust gas or obtains a value corresponding to the amount of moisture from engine information or sensor information, and uses a correction amount corresponding to the amount of moisture to output NOx gas concentration detection output. is being corrected. The premise is that there is a linear relationship between the NOx gas concentration detection output and the NOx gas concentration, and if no correction is made based on moisture content, the offset will change, although the sensitivity will not be affected. There is. However, it has been found that when the temperature conditions vary over a wider range depending on the load conditions, the offset based on the moisture content does not necessarily become constant. This is presumed to be because the relationship between moisture content and offset is affected by the temperature environment around the sensor and sensor control, which vary depending on the load conditions, and errors caused by moisture content are not properly corrected and NOx gas concentration increases. There was a risk that detection accuracy would decrease.

The present invention has been made in view of such problems, and improves the detection accuracy of NOx concentration by suppressing the influence of changes in the surrounding environment, performing correction based on the amount of water contained in the gas to be measured with higher accuracy. The present invention aims to provide a possible NOx sensor.

One aspect of the present invention is
A sensor element (1) that detects NOx contained in the gas to be measured (G), a heater (H) that heats the sensor element, and controls the operation of the sensor element and the heater, and A NOx sensor (S) comprising a sensor control unit (10) that calculates a NOx concentration based on a signal,
The above sensor element is
A gas to be measured chamber (2) into which the gas to be measured is introduced via a gas introduction part (3) and has a solid electrolyte body (11) as a chamber wall;
The gas chamber to be measured has a first cell electrode (21) disposed upstream of the gas flow to be introduced, and a first signal (Ip) having a correlation with the H ₂ O concentration in the gas to be measured. A first cell ( _1p ) that outputs a second cell (1s) that outputs a second signal (Is),
The above sensor control section is
a NOx concentration reference value calculation unit (101) that calculates a NOx concentration reference value (C1) based on the second signal;
an H ₂ O _{concentration} information detection unit (102) that detects H 2 O concentration or H ₂ O concentration information (Ci) including concentration information having a correlation with the H ₂ O concentration, based on the first signal or external information; ,
a second cell temperature information detection unit (103) that detects second cell temperature information (Ti) having a correlation with the temperature (Ts) of the second cell;
The NOx sensor includes a NOx concentration correction section (104) that corrects the NOx concentration reference value using an offset correction value (C2) based on the _H2O concentration information and the second cell temperature information.

In the NOx sensor configured as described above, the sensor control section controls the operation of the sensor element and the heater, and calculates the NOx concentration based on the signal from the sensor element. When the second signal is input from the second cell, the NOx concentration reference value calculation section calculates the NOx concentration reference value based on a previously known relationship between the second signal and the NOx concentration. The H ₂ O concentration information detection section detects H ₂ O concentration information based on the first signal input from the first cell or external information of the sensor element. Further, the second cell temperature information detection section detects the temperature of the corresponding second cell based on the input second cell temperature information. The NOx concentration correction section calculates an offset correction value based on the H ₂ O concentration information and the second cell temperature information, and corrects the NOx concentration reference value.

Here, the surrounding environment of the sensor element changes depending on the discharge environment of the gas to be measured, the amount of water contained in the gas to be measured changes, and the temperature and flow rate of the gas to be measured also change depending on the load conditions. Therefore, even if the sensor control unit performs temperature control using the heater, the temperature of the second cell where the second signal is output may deviate from the temperature controlled by the heater, causing an offset. There was found. Even in such a case, an offset correction value is calculated in the NOx concentration correction section based on the H ₂ O concentration information and the second cell temperature information, and the NOx Since the concentration reference value is corrected, the NOx concentration can be calculated with higher accuracy.

As described above, according to the above aspect, the NOx sensor can improve the detection accuracy of NOx concentration by suppressing the influence of changes in the surrounding environment, performing correction based on the amount of water contained in the gas to be measured with higher accuracy. can be provided.
Note that the numerals in parentheses described in the claims and means for solving the problem indicate correspondence with specific means described in the embodiments described later, and do not limit the technical scope of the present invention. It's not a thing.

1 is an overall schematic configuration diagram of a NOx sensor in Embodiment 1. FIG. 1 is a longitudinal cross-sectional view and a cross-sectional view taken along the line II, showing a schematic configuration of a sensor element, which is a main part of a NOx sensor, in Embodiment 1. FIG. FIG. 2 is a block diagram showing the configuration of main parts of a sensor control section of a NOx sensor in Embodiment 1. FIG. FIG. 2 is an overall configuration diagram and a partially enlarged cross-sectional view showing a state in which a NOx sensor is attached in Embodiment 1. FIG. 3 is a diagram showing the relationship between the pump cell current of the sensor element and the O ₂ concentration and H ₂ O concentration in Embodiment 1. FIG. FIG. 3 is a diagram showing the relationship between the NOx output and the H ₂ O concentration before offset correction of the sensor element in Embodiment 1; 3 is a diagram showing the relationship between NOx output and H ₂ O concentration after offset correction of a sensor element in Embodiment 1. FIG. 6 is a diagram showing the relationship between the electrode material of the sensor element and the adsorption energy of gas species in Embodiment 1. FIG. 3 is a diagram showing the relationship between the pump cell current of the sensor element and the O ₂ concentration and H ₂ O concentration in Embodiment 1. FIG. FIG. 2 is a block diagram showing the main configuration of a NOx concentration calculation section of a sensor control section in Embodiment 1. FIG. 3 is a diagram showing the relationship between the offset correction value of the sensor element and the H ₂ O concentration in Embodiment 1. FIG. 3 is a diagram showing the relationship between the offset correction value of the sensor element and the O ₂ concentration in Embodiment 1. FIG. 1 is a diagram showing a schematic configuration diagram of a test device for obtaining an offset correction value calculation map and a gas composition formula before and after combustion in Embodiment 1. FIG. FIG. 2 is a diagram for explaining energization control by a heater control section of a sensor control section in Embodiment 1, and is a sectional view of main parts of a sensor element. 2 is a diagram showing the relationship between O ₂ concentration and H ₂ O concentration in exhaust gas in Embodiment 1. FIG. 2 is a diagram showing the influence of fuel properties on the relationship between O ₂ concentration and H ₂ O concentration in exhaust gas in Embodiment 1. FIG. 7 is a flowchart showing a calculation processing procedure by a NOx concentration calculation section of a sensor control section in Embodiment 2. FIG. 7 is a time chart diagram showing the relationship between the voltage applied by the detection control unit of the sensor control unit and the current output in Embodiment 2. FIG. 7 is a schematic diagram showing the relationship between the voltage applied to the sensor element and the output current in Embodiment 2. FIG. 7 is a diagram showing the relationship between sensor cell temperature and cell impedance in Embodiment 2. FIG. FIG. 3 is a block diagram showing the main part configuration of a NOx concentration calculation section of a sensor control section in Embodiment 3. FIG. FIG. 7 is a schematic diagram for explaining the principle of detecting sensor cell temperature information of a sensor element in Embodiment 3. 7 is a diagram showing the relationship between sensor cell temperature and monitor cell current in Embodiment 3. FIG. 7 is a diagram showing the relationship between the voltage applied to the sensor cell and the NOx output in Embodiment 3. FIG. 7 is a diagram showing the relationship between sensor cell temperature and heater resistance in Embodiment 3. FIG. 7 is a diagram showing the relationship between heater resistance and heater temperature, and between heater temperature and sensor cell temperature in Embodiment 3. FIG. FIG. 7 is a block diagram showing the main part configuration of a NOx concentration calculation section of a sensor control section in Embodiment 4. FIG. FIG. 7 is an O ₂ concentration conversion map diagram used in the NOx concentration calculation section of the sensor control section in Embodiment 4. FIG. 4 is a diagram showing a H ₂ O concentration conversion map and an atmospheric H ₂ O concentration conversion map used in a NOx concentration calculation unit of a sensor control unit in Embodiment 4; FIG. 7 is a cross-sectional view showing a sensor element mounting structure in Embodiment 5. 7 is a diagram showing the relationship between the sensor cell temperature of the sensor element, the exhaust gas temperature, and the exhaust gas flow rate in Embodiment 5. FIG. 7 is a diagram showing the relationship between the chamber structure of the sensor element and the NOx output offset in Embodiment 5. FIG.

(Embodiment 1)
Embodiment 1 of the NOx sensor will be described with reference to the drawings.
As shown in FIG. 1, the NOx sensor S of this embodiment includes a sensor element 1, a heater H that heats the sensor element 1, and a sensor control section 10. As shown in the overall view (right figure) of FIG. 4, the sensor main body 1A in which the sensor element 1 is housed is installed, for example, in the exhaust gas passage EX of an internal combustion engine such as a vehicle engine, and the sensor element 1 is used to Nitrogen oxides (ie, NOx) contained in exhaust gas G, which is a gas to be measured, is detected. The sensor control unit 10 is installed outside the exhaust gas passage EX, controls the detection operation by the sensor element 1 and the heating operation by the heater H, and calculates the NOx concentration based on the output from the sensor element 1.

As shown in FIGS. 1 and 2, the sensor element 1 is configured, for example, as a limiting current type sensor using a solid electrolyte body 11, and includes a pump cell 1p as a first cell and a sensor cell 1s as a second cell. It has . In the sensor element 1, a gas chamber 2 to be measured having a solid electrolyte body 11 as a chamber wall is formed inside one end side (for example, the lower end side in the right diagram of FIG. 4) that serves as a gas detection section. Exhaust gas G is introduced from the outside of the element 1 into the gas chamber 2 to be measured via the gas introduction section 3 . The sensor element 1 can also include a monitor cell 1m as a third cell arranged in parallel with the sensor cell 1s.

As shown in the left diagram of FIG. 4, the exhaust gas G is introduced into the gas chamber 2 to be measured from the gas introduction part 3 provided at the tip of the sensor element 1, with the longitudinal direction of the sensor element 1 being the gas flow direction X. In the gas chamber 2 to be measured, the pump cell 1p is arranged on the upstream side of the gas flow, and while adjusting the oxygen concentration in the exhaust gas G, generates a signal corresponding to the oxygen concentration in the exhaust gas G, which is correlated with the H ₂ O concentration. The first signal is configured to output a first signal having a first signal. The sensor cell 1s is arranged downstream of the pump cell 1p, and outputs a second signal corresponding to the NOx concentration and _H2O concentration in the exhaust gas G whose oxygen concentration has been adjusted. A reference gas chamber 4 is arranged on the opposite side of the gas chamber 2 to be measured with the solid electrolyte body 11 in between. A porous protective layer 5 is formed on the outer surface of the sensor element 1, covering the entire gas detection section.

As shown in FIG. 2, specifically, the pump cell 1p has a pump electrode 21, which is a first cell electrode, on the surface of the solid electrolyte body 11 facing the gas chamber 2 to be measured, and performs oxygen pumping. , adjust the oxygen concentration in the exhaust gas G to a predetermined low concentration. At this time, due to the catalytic action of the pump electrode 21, oxygen (i.e., O ₂ ) contained in the exhaust gas G is reduced to become oxide ions (i.e., O ^2- ), which are conducted inside the solid electrolyte body 11 . , is discharged to the reference gas chamber 4 side. At this time, the current Ip flowing through the pump cell 1p (hereinafter referred to as pump cell current) corresponds to the oxygen concentration in the exhaust gas G before adjustment (hereinafter referred to as O ₂ concentration as appropriate), and the H ₂ O concentration and It is output to the sensor control unit 10 as a first signal having a correlation.

The sensor cell 1s has a sensor electrode 22, which is a second cell electrode, on the surface of the solid electrolyte body 11 facing the gas chamber 2 to be measured, and detects NOx contained in the exhaust gas G whose O ₂ concentration has been adjusted. At this time, if the exhaust gas G contains H ₂ O, the catalytic action of the sensor electrode 22 decomposes NOx and H ₂ O, and oxide ions resulting from NOx and H ₂ O are decomposed. , conducts inside the solid electrolyte body 11 and is discharged to the reference gas chamber 4 side. At this time, the current Is flowing through the sensor cell 1s (hereinafter referred to as sensor cell current) corresponds to the NOx concentration and H ₂ O concentration in the exhaust gas G, and is outputted to the sensor control unit 10 as a second signal.

The NOx sensor S is used, for example, in a selective catalytic reduction (SCR) system for purifying NOx, detects the NOx concentration downstream of the catalyst, and directs the reducing agent to the upstream side of the catalyst. Used for supply amount control, etc. In that case, if NOx and moisture (that is, H ₂ O) are mixed in the exhaust gas G, the output from the sensor element 1 will be the output based on H ₂ O added to the original output based on NOx. . H ₂ O in the exhaust gas G is mainly caused by combustion of fuel in the engine, and has a correlation with O ₂ in the exhaust gas G. Therefore, it is possible to correct the NOx concentration by estimating the offset amount of NOx output using the H ₂ O concentration or O ₂ concentration, and furthermore, it is possible to correct the NOx concentration by taking into account the temperature characteristics of the output based on H ₂ O. By doing so, offset correction can be appropriately performed.

As shown in FIGS. 1 and 3, the sensor control section 10 is configured to be able to communicate with the sensor element 1 or an external device, and functionally includes a NOx concentration calculation section 100, a detection control section 200, and a heater. A control unit 300 is provided. The NOx concentration calculation unit 100 corrects an error in NOx output caused by H ₂ O (hereinafter referred to as NOx output error) based on the first signal, the second signal output from the sensor element 1, and external information. Then, calculate the NOx concentration. The detection control unit 200 controls the detection operation by the sensor element 1 to be performed appropriately, and the heater control unit 300 controls the heating by the heater H so that the sensor element 1 has a temperature suitable for detection. Control behavior. Details of the control by the detection control section 200 and the heater control section 300 will be described later.

The NOx concentration calculation section 100 includes a NOx concentration reference value calculation section 101, an H ₂ O concentration information detection section 102, a sensor cell temperature information detection section 103 which is a second cell temperature information detection section, and a NOx concentration correction section 104. has. The sensor cell current Is, which is a second signal, is input to the NOx concentration reference value calculation unit 101, and the NOx concentration reference value C1 is calculated based on the sensor cell current Is. The pump cell current Ip as a first signal or external information is input to the H ₂ O concentration information detection unit 102, and the H ₂ O concentration or a concentration having a correlation with the H ₂ O concentration is detected based on the pump cell current Ip or the external information. H ₂ O concentration information Ci containing information is detected. The sensor cell temperature information detection unit 103 detects sensor cell temperature information Ti having a correlation with the temperature of the sensor cell 1s (hereinafter referred to as sensor cell temperature) Ts as second cell temperature information.

The NOx concentration correction unit 104 calculates an offset correction value C2 based on the detected H ₂ O concentration information Ci and sensor cell temperature information Ti, and uses this offset correction value C2 to correct the NOx concentration reference value C1. Specifically, the NOx concentration correction unit 104 includes an offset correction value calculation unit 104A, and calculates the value based on the relationship between the offset amount of the sensor cell current Is acquired in advance, the H ₂ O concentration information Ci, and the sensor cell temperature information Ti. A correction map, correction formula, etc. for calculating the offset correction value C2 are stored.

In this way, the NOx concentration calculation unit 100 calculates the NOx concentration reference value C1 based on the sensor cell current Is that is the output from the sensor cell 1s, while calculating the H ₂ O concentration information Ci based on the pump cell current Ip or external information. To detect. As shown in FIG. 5, the O ₂ concentration known from the pump cell current Ip has a correlation with the H ₂ O concentration in the exhaust gas G, and there is a roughly inverse proportional relationship in which the O ₂ concentration decreases as the H ₂ O concentration increases. It is in. Using this relationship, the O ₂ concentration can be converted into the H ₂ O concentration, or the H ₂ O concentration can be obtained based on external information to obtain the H ₂ O concentration information Ci. Further, sensor cell temperature information Ti having a correlation with the sensor cell temperature Ts is detected, and an offset correction value C2 is calculated using this detected information.

As shown in FIG. 6, the NOx concentration (unit: ppm) based on the sensor cell current Is shows an offset due to moisture in the atmosphere, and even if the NOx concentration in the model gas is substantially zero, H ₂ As the O concentration (unit: %) increases, the sensor output increases. This NOx output error further varies depending on the ambient temperature, and exhibits a temperature characteristic in which the sensor output at the same H ₂ O concentration increases as the temperature of the model gas increases from 70° C. to 200° C. to 500° C. Therefore, in a usage environment where the temperature of the introduced exhaust gas G changes significantly, such as with NOx sensor S for vehicles, there is a risk that sufficient correction accuracy cannot be obtained by only output correction based on H ₂ O concentration. .

Here, the sensor control unit 10 heats and controls one end of the sensor element 1, which serves as a gas detection unit, to a predetermined temperature using a built-in heater H. However, as shown in FIG. 6, it was found that the sensor cell temperature Ts fluctuated toward a higher temperature side (for example, in the range of 600 to 650 degrees Celsius) with respect to the control temperature of the pump cell 1p (for example, 600 degrees Celsius). did. Due to this temperature fluctuation, for example, under low load conditions such as idling, the sensor cell temperature Ts can be controlled to the same temperature as the pump cell 1p, but under higher load operating conditions, it is difficult to make it follow the control temperature. Become.

In FIG. 1, the heater H faces the solid electrolyte body 11 that forms the chamber wall of the gas chamber to be measured 2 with the reference gas chamber 4 in between, and heats the area where the pump cell 1p and the sensor cell 1s are arranged by heat transfer. It is possible. However, it is difficult to uniformly heat the entire solid electrolyte body 11, and in a configuration in which the pump cell 1p and the sensor cell 1s are arranged in tandem in the gas flow direction Since there is a difference in the temperature, the temperature becomes more susceptible to the influence of the temperature of the exhaust gas G. Therefore, when the temperature of the exhaust gas G increases, the sensor cell temperature Ts no longer matches the control temperature of the pump cell 1p, and the high temperature of the sensor cell 1s accelerates the decomposition of H ₂ O, and the sensor cell current Is is expected to increase.

On the other hand, as shown in FIG. 7, when the NOx concentration correction unit 104 performs offset correction based on the H ₂ O concentration information Ci and the sensor cell temperature information Ti, the H ₂ O concentration and the sensor cell temperature Ts Regardless, the sensor cell current Is substantially matches the NOx concentration of the model gas. In this way, when correcting the sensor output, focusing on the difference between the temperature controlled by the heater H and the temperature of the sensor cell 1s where the NOx concentration is detected, and using the offset correction value C2 that takes the sensor cell temperature Ts into consideration, NOx The concentration can be calculated with higher accuracy.

The H ₂ O concentration information Ci detected by the H ₂ O concentration information detection unit 102 may include the H ₂ O concentration or concentration information having a correlation with the H ₂ O concentration, for example, information on the O ₂ concentration. good. When information on the O ₂ concentration is acquired _based on the pump cell current Ip input from the sensor element 1, the air-fuel ratio (A/F ), excess air ratio (λ=air-fuel ratio/stoichiometric air-fuel ratio), equivalence ratio (φ=1/λ), etc.

Further, the H ₂ O concentration information Ci is not limited to such internal information of the sensor element 1, but may be external information acquired outside the sensor element 1. The external information includes information such as engine operating information and mounting environment, and the H 2 O concentration may be calculated in the H ₂ O concentration information detection unit 102 based on this external information, or the H ₂ O concentration may be calculated based on this external information. The H ₂ O concentration or the estimated value of the O ₂ concentration estimated based on the above may be acquired as external information.

The sensor cell temperature information Ti detected by the sensor cell temperature information detection unit 103 may include information having a correlation with the sensor cell temperature Ts. Examples of such information include impedance information, resistance value information, etc. acquired inside the sensor element 1. The impedance information may be, for example, a detected value of the cell impedance Zac based on the sensor cell current Is input from the sensor cell 1s or the monitor cell current Im as a third signal input from the monitor cell 1m.

Further, the information is not limited to internal information of the sensor element 1, but may be external information acquired outside the sensor element 1. The external information includes, for example, information such as engine operating information and exhaust gas information around the sensor element 1, and the sensor cell temperature Ts is calculated in the sensor cell temperature information detection unit 103 based on this external information. Alternatively, an estimated value of the sensor cell temperature Ts estimated based on these external information may be acquired as the external information.

In the H ₂ O concentration information detection unit 102 or the sensor cell temperature information detection unit 103, the external information is, for example, information from a vehicle engine control unit (not shown) (hereinafter referred to as ECU), which is used for sensor control. 10. The sensor control unit 10 is configured as a sensor control unit that includes a communication unit, a microcomputer (hereinafter referred to as a microcomputer), various terminals connected to the sensor element 1 and the ECU, and receives internal information from the sensor element 1 or The NOx concentration is calculated in the NOx concentration calculating section 100 based on external information from the ECU.

Note that the ECU receives inputs such as intake air amount detected by an air flow meter (not shown), detection signals from an engine rotation speed sensor, an accelerator opening sensor, etc., and operates the engine E based on these input information. It knows the status and controls the entire vehicle.

In this way, in the sensor control unit 10, the NOx concentration calculation unit 100 is equipped with the NOx concentration correction unit 104 that performs offset correction in consideration of the sensor cell temperature Ts, so that the NOx output error caused by the moisture in the exhaust gas G can be appropriately corrected. It is possible to calculate the NOx concentration with high accuracy by correcting it.
Hereinafter, a configuration example of the NOx sensor S and details of control by the sensor control unit 10 will be described.

(Overall configuration of NOx sensor S)
In FIG. 4, in a sensor body 1A of a NOx sensor S, a sensor element 1 is inserted and held inside a cylindrical housing S1, and an outer peripheral threaded portion of the housing S1 is fixed to a passage wall EX1 of an exhaust gas passage EX. The sensor main body 1A is housed in an element cover S2 with the distal end of the sensor element 1 protruding into the exhaust gas passage EX, and housed in the element cover S2 with the proximal end of the sensor element 1 protruding outside the exhaust gas passage EX. It is accommodated in S3. The sensor control unit 10 is electrically connected to the sensor element 1 via a lead wire S4 drawn out to the base end side of the sensor main body 1A.

The element cover S2 has a double cylinder structure with a bottom surface, and a plurality of gas flow holes (not shown) are provided on the side surfaces and bottom surfaces of the outer cover and the inner cover. Thereby, the exhaust gas G flowing through the exhaust gas passage EX is taken into the inside of the element cover S2, and the tip side of the sensor element 1, which serves as the gas detection section, is exposed to the exhaust gas G, which is the gas to be measured. Further, a plurality of gas flow holes (not shown) are provided on the side surface of the cylindrical atmospheric cover S3. Thereby, the atmosphere A, which is the reference gas, is taken into the atmosphere cover S3 and can reach the base end side of the sensor element 1.

As shown in an enlarged view in FIG. 4, the exhaust gas G that has reached the distal end of the sensor element 1 is taken into the gas chamber 2 to be measured from the gas introduction section 3 and moves toward the proximal end. In the gas chamber 2 to be measured, a pump cell 1p is arranged on the upstream side in the gas flow direction X, and a sensor cell 1s and a monitor cell 1m are arranged on the downstream side. Detect NOx.

(Configuration of sensor element 1)
In FIG. 2, the sensor element 1 is a stacked element with a three-cell structure, and the stacking direction is a direction perpendicular to the gas flow direction X (that is, the vertical direction in FIG. 2). Electrolyte body 11 and heater insulating layer 61 are arranged in this order. A spacer layer 12 and a shielding layer 13 are arranged on one surface side of the solid electrolyte body 11 to form a space that will become the gas chamber 2 to be measured. Further, on the other surface side of the solid electrolyte body 11, a space serving as the reference gas chamber 4 is formed between the solid electrolyte body 11 and the heater insulating layer 61 constituting the heater H.

The gas chamber 2 to be measured is connected to the inside of the element cover S2 (see, for example, FIG. 4) where the exhaust gas G exists via the diffusion resistance layer 31 embedded in the front end surface (i.e., the left end surface in FIG. 2) of the spacer layer 12. It communicates with the space of The diffusion resistance layer 31, together with a part of the porous protective layer 5 arranged on the outside thereof, constitutes a gas introduction section 3 into the gas chamber 2 to be measured, so that the exhaust gas G is introduced under a predetermined diffusion resistance. has been adjusted to. The porous protective layer 5 is formed to cover the entire surface of the tip side of the sensor element 1, which is the gas detection section, and protects the sensor element 1 from poisonous substances and the like.

The gas detection section of the sensor element 1 is provided with a pump cell 1p, a sensor cell 1s, and a monitor cell 1m, and each cell has a pair of electrodes facing each other with a solid electrolyte body 11 in between. In the gas chamber 2 to be measured, the pump electrode 21 of the pump cell 1p is arranged on the surface of the solid electrolyte body 11 on the upstream side in the gas flow direction X, and the sensor electrode 22 of the sensor cell 1s and the monitor cell 1m are arranged on the downstream side thereof. A monitor electrode 23 is arranged. In the reference gas chamber 4, a reference electrode 41 serving as a common electrode is provided on the surface of the solid electrolyte body 11 at a position facing these electrodes 21, 22, and 23.

The reference gas chamber 4 in which the common reference electrode 41 is arranged extends to the base end side (for example, the right end side in FIG. 2) of the sensor element 1, and through a reference gas inlet opening at an end surface (not shown), It communicates with the space within the atmosphere cover S3 (see, for example, FIG. 4) where the atmosphere A exists. A heating element 62 that generates heat when energized is buried inside a heater insulating layer 61 that forms the bottom wall of the reference gas chamber 4, and constitutes a heater H. The heating element 62 is disposed corresponding to the portion where the electrodes of the pump cell 1p, the sensor cell 1s, and the monitor cell 1s are formed, and is capable of heating the entire tip side of the element, which becomes the gas detection section.

As a result, one electrode 21, 22, 23 of each cell is exposed to the exhaust gas G introduced into the gas chamber 2 to be measured, and the other electrode 41 is exposed to the exhaust gas G introduced into the reference gas chamber 4. exposed to atmosphere A. At this time, by applying a voltage between a pair of electrodes in each cell, oxygen contained in the exhaust gas G is pumped out to the reference gas chamber 4 side via the solid electrolyte body 11 having oxide ion conductivity or the reference gas Oxygen pumping from the chamber 4 side becomes possible. Further, by operating the heater H, each cell is heated to a temperature higher than the temperature at which it becomes active, and oxygen pumping can be performed stably.

In the gas chamber 2 to be measured, the pump electrode 21 of the pump cell 1p is formed on the upstream side in the gas flow direction X with a large area. When the exhaust gas G that has passed through the diffusion resistance layer 31 comes into contact with the pump electrode 21, oxygen is decomposed by its catalytic action and is discharged to the reference electrode 41 side. The exhaust gas G adjusted to have a low oxygen concentration by the pump cell 1p reaches the sensor electrode 22 and monitor electrode 23 on the downstream side, and at the sensor electrode 22, oxygen caused by NOx is discharged together with residual oxygen by oxygen pumping. Further, residual oxygen at the monitor electrode 23 is exhausted by oxygen pumping.

The sensor electrode 22 and the monitor electrode 23 are arranged in parallel at the same position in the gas flow direction X, and are formed in the same shape with a smaller area than the pump electrode 21. As a result, the sensor electrode 22 and the monitor electrode 23 have the same conditions with respect to the flow of exhaust gas G, and by comparing the outputs of the sensor cell 1s and the monitor cell 1m, it is possible to remove the influence of residual oxygen contained in the NOx output. .

The solid electrolyte body 11 is made of a zirconia-based solid electrolyte material having oxide ion conductivity. As the zirconia-based solid electrolyte material, for example, partially stabilized zirconia or stabilized zirconia containing a stabilizer such as yttria can be used. Further, the diffusion resistance layer 31 and the porous protective layer 5 are made of a ceramic material such as alumina, for example. The heater insulating layer 61, the shielding layer 13, and the spacer layer 12 can be constructed using an insulating ceramic material such as alumina.

The electrodes of the pump cell 1p, the sensor cell 1s, and the monitor cell 1m may be porous cermet electrodes containing a noble metal or noble metal alloy material and a zirconia-based solid electrolyte. The sensor electrode 22 of the sensor cell 1s is constructed using an electrode material that has decomposition activity for NOx to be detected. As such an electrode, for example, an electrode containing platinum and rhodium (hereinafter referred to as a Pt-Rh electrode) can be used.

As shown in FIG. 8, the noble metal elements constituting the electrodes of each cell exhibit different adsorption energies (decomposition properties) for the gas species contained in the exhaust gas G, and in descending order of gas adsorption properties, Rh>Pt >Au. Furthermore, when viewed from the viewpoint of gas species, the adsorption energy (degradability) for noble metal elements is in the order of O ₂ >NO > H ₂ O. Therefore, by changing the combination and metal ratio of these noble metal elements and adjusting the voltage applied to each cell, it becomes possible to selectively decompose a desired gas in each cell.

The pump electrode 21 of the pump cell 1p is configured using an electrode material that has oxygen decomposition activity and low NOx decomposition activity. As such an electrode, for example, an electrode containing platinum and gold (hereinafter referred to as a Pt-Au electrode) can be used. The monitor electrode 23 of the monitor cell 1m is also constructed using the same electrode material as the pump electrode 21. Further, the reference electrode 41 can be configured as, for example, an electrode containing platinum (hereinafter, appropriately referred to as a Pt electrode).

As a result, NOx in the exhaust gas G is adjusted to a predetermined low oxygen concentration in the pump cell 1p by heating the sensor element 1 to a predetermined temperature by the heater H and applying a predetermined voltage between the electrodes of each cell. can be detected by the sensor cell 1s. Furthermore, by utilizing the difference in gas adsorption between the Pt-Rh electrode used for the sensor electrode 22 and the Pt-Au electrode used for the monitor electrode 23, the difference value between the currents output from the sensor cell 1s and the monitor cell 1m is calculated. By setting the output to NOx, the influence of oxygen remaining in the exhaust gas G can be canceled.

However, since Rh contained in the Pt-Rh electrode constituting the sensor electrode 22 has decomposition activity against H ₂ O in addition to NOx and O ₂ , H If ₂ O is included, an output error will occur due to these decompositions. This output error is corrected by the NOx concentration calculation unit 100 of the sensor control unit 10 using offset correction information acquired in advance. Control of each section and output correction by the sensor control section 10 will be described next.

(Configuration of sensor control unit 10)
In FIG. 1, the sensor control unit 10 includes a current detection unit 10A that detects the output current from each cell of the sensor element 1, a pump cell current Ip, a sensor cell current Is, and a monitor cell current Im detected by the current detection unit 10A. and a NOx concentration calculation unit 100 that calculates the NOx concentration based on the NOx concentration. The sensor control unit 10 also includes a detection control unit 200 that controls NOx detection by the sensor element 1, and a heater control unit 300 that controls heating of the sensor element 1 by the heater H.

Each cell of the sensor element 1 is connected to the sensor control unit 10 via a detection terminal P- and a detection terminal S-, M-, and both ends of the heating element 62 of the heater H are connected to a pair of heater terminals H+, H-. It is connected to the. The sensor control unit 10 is connected to an ECU (not shown) via a pair of communication terminals CAN+ and CAN-, and is capable of receiving commands from the ECU, performing NOx detection processing, and outputting the detection results. There is. Further, the sensor control unit 10 is connected to a vehicle battery and ground (not shown) via a power supply terminal VB and a ground terminal GND, respectively, and is connected to various circuits of the sensor control unit 10, a heater H of the sensor element 1, etc. power supply is possible.

The NOx concentration calculation unit 100 is composed of a microcomputer or the like having a storage unit and a calculation unit, and executes a pre-stored program using internal information of the sensor element 1 input from the current detection unit 10A. A predetermined calculation is performed to calculate the NOx concentration. The current detection unit 10A is connected to the pump electrode 21 of the pump cell 1p via the detection terminal P-, and is connected to the sensor electrode 22 of the sensor cell 1s and the monitor electrode 23 of the monitor cell 1m via detection terminals S- and M-. It is connected to the. The current detection unit 10A includes, for example, a detection circuit using a resistor for current detection, and is configured to be able to detect the output current of each cell from the potential difference between both ends of the resistor. The detected output current is converted into a digital signal in an analog/digital conversion circuit and input to each part of the NOx concentration calculation section 100.

The detection control unit 200 controls the detection operations of the pump cell 1p, the sensor cell 1s, and the monitor cell 1m, respectively. The detection control unit 200 includes, for example, a voltage application circuit for applying a voltage between a pair of electrodes of each cell, generates a predetermined voltage signal based on a control command, and generates a predetermined voltage signal via a common terminal COM+. It can be applied to the common reference electrode 41 side. The applied voltage is adjusted within a range in which the current flowing through each cell exhibits a limiting current characteristic so that the O ₂ concentration in the gas chamber 2 to be measured becomes a predetermined low concentration by, for example, oxygen pumping in the pump cell 1p.

The heater control unit 300 controls energization of the heater H built into the sensor element 1 . The heater control unit 300 includes, for example, a switch circuit connected to a power supply terminal VB, and the switch circuit is turned on and off based on a control command, thereby supplying power to the heater H via the heater terminals H+ and H-. The power supply can be controlled. Thereby, each cell of the sensor element 1 heated by the heater H is maintained at a temperature suitable for NOx detection.

In FIG. 3, the NOx concentration calculation section 100 includes a NOx concentration reference value calculation section 101, a H ₂ O concentration information detection section 102, a sensor cell temperature information detection section 103, and a NOx concentration correction section 104. NOx concentration calculation section 100 calculates NOx concentration reference value C1 in NOx concentration reference value calculation section 101 based on sensor cell current Is and monitor cell current Im detected by current detection section 10A. Specifically, the difference value between the sensor cell current Is and the monitor cell current Im is calculated to calculate the NOx output with the influence of residual oxygen removed, and the relationship between the NOx output and the NOx concentration obtained by conducting tests etc. in advance is calculated. Based on this, the corresponding NOx concentration reference value C1 is calculated.

As shown by the solid line in FIG. 9, the difference value [Is-Im] which is the NOx output from the sensor element 1 and the actual NOx concentration (measured value) measured by an analyzer etc. are different from each other. For example, the relationship is expressed by a straight line passing through the zero point. On the other hand, as shown by the dotted line in FIG. 8, if the NOx output includes an error caused by H ₂ O, the difference value (detected value) based on the actually detected output current will be different from the measured value. In contrast, the output has an error added to it. Therefore, the NOx concentration reference value calculation unit 101 maps or stores the correspondence represented by the solid line as a calculation formula in advance, and corrects the NOx output error by using the value corresponding to the difference value (detected value). It is acquired as the previous NOx concentration reference value C1.

The NOx concentration correction unit 104 corrects the error included in the NOx concentration reference value C1 to calculate the original NOx concentration. At this time, as described above, since the error included in the NOx concentration reference value C1 increases or decreases depending on the H ₂ O concentration and has temperature characteristics, the NOx concentration correction unit 104 has a function based on this information. An offset correction value calculation unit 104A is provided that calculates an offset correction value C2 based on the offset correction value C2.

Specifically, as shown in FIG. 10, H ₂ O concentration information Ci is detected by the H ₂ O concentration information detection unit 102, and sensor cell temperature information Ti is detected by the sensor cell temperature information detection unit 103. . The H ₂ O concentration information detection unit 102 detects H 2 O concentration information Ci based on, for example, the pump cell current Ip input from the pump cell 1p, and the sensor cell temperature information detection unit 103 detects H ₂ O concentration information Ci based on the pump cell current Ip input from the monitor cell 1m, for example. Sensor cell temperature information Ti is detected based on the monitor cell current Im.

The H ₂ O concentration information Ci is, for example, the H ₂ O concentration known from the pump cell current Ip, and the sensor cell temperature information Ti can be, for example, the cell impedance Zac of the monitor cell 1m known from the monitor cell current Im. These pieces of information Ci and Ti are input to the offset correction value calculation unit 104A and are used to calculate an offset correction value C2 for performing offset correction.

As shown in FIG. 11, the offset correction value calculation section 104A stores an offset correction value calculation map obtained through tests and the like in advance. The map shows the offset corresponding to the H ₂ O concentration range (for example, 0% to about 13%) in the exhaust gas G as an example of the relationship between the H ₂ O concentration information Ci, the sensor cell temperature information Ti, and the offset correction value C2. The correction value C2 is shown for each temperature in the mounting environment of the sensor element 1. As shown in the figure, the offset correction value C2 increases as the H ₂ O concentration increases and as the mounting environment temperature increases.

By using such a calculation map, it is possible to obtain the offset correction value C2 according to the H ₂ O concentration information Ci and the sensor cell temperature information Ti, and as described above, it becomes possible to appropriately correct the NOx output error. (See, for example, Figure 5). For convenience, here, we will show the relationship between low and high temperature ranges corresponding to the upper and lower limits of the temperature range that the sensor cell temperature Ts can take in the mounting environment, and a medium temperature range between them. Of course, the correction may be performed using a map value corresponding to the entire mounting environment temperature range.

As shown in FIG. 12, the calculation map calculates the offset correction value C2 corresponding to the O ₂ concentration range (for example, 0% to about 21%) from the relationship between the H ₂ O concentration and O ₂ concentration in the exhaust gas G. , may be shown for each temperature in the mounting environment of the sensor element 1. In that case, contrary to the relationship shown in FIG. 11 described above, the offset correction value C2 becomes a smaller value as the O ₂ concentration increases, and becomes a larger value as the mounting environment temperature becomes higher. The relationship between the H ₂ O concentration and O ₂ concentration in the exhaust gas G will be described later.

The calculation map can be acquired using, for example, a test device 400 shown in FIG. 13. The test device 400 includes a gas mixing section 401 that generates a model gas G1 imitating the exhaust gas G, and an exhaust gas pipe 402 connected to the gas mixing section 401. A supply pipe 403 for various gases such as N ₂ gas, O ₂ gas, and CO ₂ gas is connected to the gas mixing section 401, and by adjusting the opening degree of the valve 403A, the gas mixture ratio in the model gas G1 and the like are controlled. Gas flow rate can be adjusted. In the exhaust gas pipe 402, a heater 404 is arranged around the outer periphery of the connection part with the gas mixing section 401, so that the inflowing model gas G1 can be heated to a predetermined temperature. The sensor body 1A of the gas sensor S is attached to the sensor body 402, and the temperature of the model gas G1 and the concentrations of various gases can be measured by a thermocouple 405 and a gas concentration measuring section 406 arranged downstream thereof.

In such a test apparatus 400, the NOx concentration was measured by the gas sensor S under the following conditions, and the calculation map shown in FIG. 10 was obtained by comparing the NOx output with the measured value of the model gas G1.
H ₂ O concentration: variable (0% to about 13%)
Gas temperature: variable (70℃, 200℃, 500℃)
Gas flow rate: Set value Control temperature: 600℃ (Pump cell temperature Tp)
By changing the gas mixture ratio, the model gas G1 is adjusted to a gas temperature such that the H ₂ O concentration is in the range of 0% to about 13% (that is, the O ₂ concentration is in the range of about 21% to 0%). The temperature was changed to three stages within the range of 70°C to 500°C. The gas flow rate was adjusted to be the same regardless of the H ₂ O concentration by adjusting the flow rates of various gases in the model gas G1.

Further, the gas sensor S was connected to a sensor control section 10 (not shown), and was controlled by a heater control section 300 so that the pump cell temperature Tp serving as the control temperature was constant. When this control temperature was set to 600° C. and the gas temperature was changed in the range of 70° C. to 500° C., the sensor cell temperature Ts of the sensor element 1 changed as follows.
Gas temperature: 70℃→Sensor cell temperature Ts: 600℃
Gas temperature: 200℃→Sensor cell temperature Ts: 620℃
Gas temperature: 500℃→Sensor cell temperature Ts: 650℃

As shown in FIG. 14, the sensor control unit 10 can detect the pump cell temperature Tp based on, for example, the cell impedance Zp of the pump cell 1p. For example, the heater control unit 300 compares the detected pump cell temperature Tp with a predetermined control temperature, sets a duty ratio for heater energization, and drives the heater H at a duty ratio. At this time, while the temperature of the pump cell 1p is controlled, the temperature of the sensor cell 1s is controlled as usual. The factor (1) is heat shrinkage to the passage wall EX1 (for example, see FIG. 4) of the exhaust gas passage EX to which the sensor element 1 is attached, and the factor (2) is heat sinking around the sensor element 1. There is heat exchange with the exhaust gas G.

In the gas chamber 2 to be measured, the sensor cell 1s is located on the downstream side of the gas flow and on the base end side of the sensor element 1 than the pump cell 1p, so the effects of these factors (1) and (2) are is different. Further, the pump cell 1p and the sensor cell 1s are not necessarily heated uniformly by the heating element 62 of the heater H. For this reason, for example, when the gas temperature of the exhaust gas G is relatively high, if the temperature is controlled based on the pump cell 1p, which is exposed to the exhaust gas G and easily exchanges heat, the heat sink from the sensor cell 1s will not proceed, and the temperature difference will be may be more likely to occur.

Even in such a case, the NOx concentration correction unit 104 can perform offset correction effectively using the offset correction value calculation map described above by considering the temperature characteristics of the NO output error based on the H ₂ O concentration. I can do it. In addition, since the pump cell temperature Tp can be controlled with good responsiveness using the cell impedance Zp of the pump cell 1p, oxygen can be efficiently discharged by oxygen pumping in the pump cell 1p, and the pump cell current Ip corresponding to the H ₂ O concentration information Ci can be adjusted. It can be detected with high accuracy. In this way, the NOx concentration can be calculated with high accuracy by offset correction based on the H ₂ O concentration information Ci and the sensor cell temperature information Ti.

Here, as shown in equation (1) in FIG. 13, the unburnt mixture contains hydrocarbon fuel (C _m H _n O _o ) and air in an amount corresponding to the excess air ratio λ. Air contains an amount of water vapor (hereinafter referred to as atmospheric water vapor) corresponding to the atmospheric water vapor concentration x. In addition, as shown in equation (2) in FIG. 13, in the exhaust gas G, which is a burnt gas, oxygen is consumed by the combustion of fuel, and surplus oxygen remains according to the excess air ratio λ, and carbon dioxide ( That is, CO ₂ ) and water vapor (hereinafter referred to as fuel-derived water vapor) are generated. The water vapor contained in the exhaust gas G (hereinafter referred to as water vapor in exhaust gas) consists of fuel-derived water vapor and water vapor based on atmospheric water vapor (hereinafter referred to as air-derived water vapor), and the air-derived water vapor has an air excess ratio λ. It is determined according to the atmospheric water vapor concentration x.

From this relationship, as the excess air ratio λ increases beyond 1, the O ₂ concentration in the exhaust gas G increases and the H ₂ O concentration decreases relatively. Alternatively, as the excess air ratio λ becomes smaller, the O ₂ concentration decreases and the H ₂ O concentration increases relatively. Therefore, by detecting the pump cell current Ip accompanying the discharge of oxygen in the pump cell 1p, it is possible to know the O ₂ concentration and further the H ₂ O concentration in the exhaust gas G. At this time, it is desirable to consider the atmospheric water vapor concentration x, and an actual measured value or an estimated value may be used, but this is not necessarily easy. On the other hand, since the influence of the atmospheric water vapor concentration , it is preferable to set a certain reference value (for example, 2%) based on the surrounding environment.

As shown in FIG. 15, the relationship between the O ₂ concentration in the exhaust gas G and the H ₂ O concentration in the exhaust gas G is expressed by the above equations (1) and (2) when the atmospheric water vapor concentration x is 0% or 2%. It was calculated based on the relationship between the above and compared with the evaluation results from actual vehicle tests. In the vehicle test, the H/C ratio of the fuel (n/m ratio in C _m H _n O _o ) was set to 1.886, and the O ₂ concentration in the exhaust gas G was changed by changing the excess air ratio λ. The actual concentration of H ₂ O contained in the exhaust gas G was measured. As a result, the relationship almost coincided with the relationship shown by the characteristic line when the atmospheric water vapor concentration x was 2%. In this way, by setting the reference value for the atmospheric water vapor concentration x, the H ₂ O concentration can be easily calculated. Note that this reference value can be arbitrarily set depending on the environment in which the gas sensor S is installed, taking into account, for example, variations in the atmospheric water vapor concentration x that differ depending on region and climate.

Further, as shown in FIG. 16, when the properties of the fuel used in the engine differ, the water vapor in the exhaust gas also changes depending on the H/C ratio, and therefore the relationship between the O ₂ concentration and the H ₂ O concentration changes. In equation (2) in FIG. 13, fuel-derived water vapor is expressed as [n/2], and the fuel-derived water vapor increases as the fuel contains less oxygen and the H/C ratio increases. Therefore, the slope of the characteristic line in FIG. 16 changes depending on the H/C ratio (for example, 1.8 to 2.2), and as indicated by the arrow in the figure, the atmospheric water vapor concentration By increasing from % to 4%, the characteristic line shifts in the increasing direction. Therefore, it is preferable to calculate the H ₂ O concentration in the exhaust gas G by considering the fuel properties. Specifically, in the relationship between the reference O ₂ concentration and H ₂ O concentration, the air-derived water vapor based on the following calculation formula is added from the air-derived water vapor term in the above equation (2) to calculate the amount of air-derived water vapor in the exhaust gas. Water vapor can be calculated.
Air-derived water vapor = [m+(n/4)-(O/2)]×[x/(100-x)/0.209]

As described above, according to the present embodiment, by using the H ₂ O concentration information Ci and the sensor cell temperature information Ti, the pump cell 1p is controlled to a state suitable for oxygen pumping, while the sensor cell 1s is Appropriate offset correction can be made in response to temperature fluctuations. Thereby, under a wide range of operating conditions from low load conditions to high load conditions, the temperature characteristics of the output due to moisture in the exhaust gas G can be corrected, and the NOx concentration can be calculated with high accuracy.

(Embodiment 2)
A second embodiment of the NOx sensor will be described with reference to the drawings. In this embodiment, an example of a specific procedure for detection control and NOx concentration calculation by the sensor control unit 10 described above will be shown. FIG. 17 is a flowchart showing the procedure of offset correction performed by the NOx concentration calculation unit 100, and FIG. 18 is a time chart of applied voltage control performed by the detection control unit 200 for NOx detection. shows. In the NOx sensor S of this embodiment, the basic configuration of the sensor element 1 and the basic control of the sensor control unit 10 are the same as those in the first embodiment, and the differences will be mainly described below.
Note that among the symbols used in the second embodiment and subsequent embodiments, the same symbols as those used in the previously described embodiments represent the same components as those in the previously described embodiments, unless otherwise specified.

Prior to implementing the flowchart in FIG. 17, the sensor control section 10 drives the heater control section 300 shown in FIG. 1 above to energize the heater H of the sensor element 1, and starts feedback control based on the pump cell temperature Tp. . Further, the detection control unit 200 is driven to generate a predetermined voltage signal and apply it to the common electrode 41 side of the sensor element 1 . Note that in the flowchart of FIG. 17, step S1 and step S2 correspond to the H ₂ O concentration information detection section 102 and the sensor cell temperature information detection section 103, respectively. Furthermore, step S3 corresponds to the offset correction value calculation section 104A, step S4 and step S5 correspond to the NOx concentration reference value calculation section 101, and step S6 corresponds to the NOx concentration correction section 104.

When the flowchart of FIG. 17 starts, in step S1, the pump cell current Ip of the pump cell 1p is detected as H ₂ O concentration information Ci. When a predetermined DC voltage is applied by the detection control unit 200, oxygen pumping is performed in the pump cell 1p to exhaust oxygen from the gas chamber 2 to be measured, and the concentration is adjusted to a predetermined low concentration. The pump cell current Ip _flowing at this time has a certain relationship with the O ₂ concentration and H ₂ O concentration in the exhaust gas G, as shown in FIG. Alternatively, it can be used as H ₂ O concentration information Ci to calculate the offset correction value without converting it to H ₂ O concentration.

As shown in the upper part of FIG. 18, the detection control unit 200 applies a predetermined DC voltage for oxygen pumping between a pair of electrodes of each cell, and detects the cell impedance Zac of the monitor cell 1m at regular intervals. Apply a predetermined AC voltage. In this way, by periodically switching the applied voltage and detecting the corresponding output current at predetermined detection timings t1 and t2, as shown in the lower part of FIG. 18, H ₂ O concentration information for offset correction is obtained. Ci and sensor cell temperature information Ti can be acquired without significantly changing the sensor configuration.

t1 is the Zac detection timing for detecting the cell impedance Zac, and from the ratio of the change Δv (unit: V) in the applied AC voltage to the corresponding change Δi (unit: A) in the output current, the cell impedance Zac ( =Δv/Δi) can be calculated. Further, t2 is a concentration detection timing for detecting a current according to the gas concentration, and it is possible to detect the current generated as oxygen is discharged to the reference electrode 41 side by applying a DC voltage. can. At this time, in the monitor cell 1m, a current is generated in the monitor electrode 23 due to the decomposition of residual oxygen. Further, in the sensor cell 1s, NOx and residual oxygen are decomposed in the sensor electrode 22, and a current is generated due to the decomposition of _H2O .

Here, as shown in FIG. 19, in each cell of the sensor element 1, the resistance (R) component and the capacitance (C) component of the solid electrolyte body 11 made of a zirconia-based material are electrically connected between a pair of electrodes. Represented by a model diagram connected in parallel. Among these, the resistance component (R) of the solid electrolyte body 11 is known to have a correlation with its temperature, and as shown in FIG. By detecting the current (Δi), it becomes possible to measure the AC resistance (R) inside the element.

In step S2, the cell impedance Zac of the monitor cell 1m is detected as sensor cell temperature information Ti. This detection is performed by the detection control unit 200 at Zac detection timing t1 when a predetermined AC voltage for impedance detection is applied. As shown in FIG. 20, the cell impedance Zac of the monitor cell 1m almost matches the cell impedance Zac of the sensor cell 1s, and as the sensor cell temperature Ts (for example, 500° C. to 700° C.) rises, the cell impedance Zac (for example, 200Ω to 2500Ω), the rate of decrease becomes more gradual.

Therefore, the cell impedance Zac of the monitor cell 1m can be suitably used as the sensor cell temperature information Ti corresponding to the sensor cell temperature Ts. Note that from the relationship shown in FIG. 20, the cell impedance Zac of the sensor cell 1s can be used instead of the cell impedance Zac of the monitor cell 1m, and in that case, the temperature of the sensor cell 1s corresponding to the NOx output can be directly detected. There are advantages. However, as shown in Figure 18, the current detection range when detecting gas concentration is on the order of nA, whereas the current detection range required for impedance detection is on the order of μA to mA. If this is done, the current detection range necessary for detecting the output current will increase, and there is a risk that the detection accuracy of the current on the low current side will decrease.

To add more information about the current detection range, the relationship between the maximum current detection width and resolution is as follows, and depends on the number of bits of the AD converter of the detection circuit.
Maximum current detection width = minimum current detection value (resolution) x AD (number of bits)
That is, if the resolution is made finer, the maximum current detection width becomes smaller. For example, in a 12-bit AD converter with a resolution of 1 nA, the maximum current detection width becomes 4096 nA (=1 nA×2 ¹² ). On the other hand, cell impedance Zac detection is an output on the order of μA to mA (10 ^-3 to 10 ^-6 A), so if you try to detect an output current of nA with the sensor cell 1s, it will be difficult to handle it while maintaining the resolution. I can't.

Therefore, by preferably using the cell impedance Zac of the monitor cell 1m, the sensor cell 1s can have a configuration suitable for current detection on the order of nA, and there is no need to expand the current detection range, so the influence on NOx detection is reduced. can be suppressed.

Next, in step S3, an offset correction value C2 is calculated based on the H ₂ O concentration information Ci and the sensor cell temperature information Ti. The pump cell current Ip detected in step S1 is used as the H ₂ O concentration information Ci, and the cell impedance Zac of the monitor cell 1m detected in step S2 is used as the sensor cell temperature information Ti, and these pieces of information and the offset correction value are used. The relationship with C2 can be stored in advance as a calculation map, calculation formula, or the like.

In step S4, the sensor cell current Is of the sensor cell 1s and the monitor cell current Im of the monitor cell 1m are detected as sensor cell temperature information Ti. This detection is performed by the detection control unit 200 at concentration detection timing t2 when a predetermined DC voltage is applied. Thereby, in the subsequent step S5, the NOx concentration reference value C1 corresponding to the difference value [Is-Im] between the sensor cell current Is and the monitor cell current Im is calculated based on the relationship shown in FIG. 9 above.

Thereafter, in step S6, the offset correction value C2 calculated in step S3 is subtracted from the NOx concentration reference value C1 calculated in step S5 from the NOx concentration reference value C1 calculated in step S5. Once the NOx concentration is calculated, this process is once terminated.

Through these steps, the NOx concentration correction unit 104 calculates the offset correction value C2 using the H ₂ O concentration information Ci based on the input signal from the sensor element 1 and the sensor cell temperature information Ti, and sets the NOx concentration reference value. The output error included in C1 can be corrected. Furthermore, when the sensor element 1 has a three-cell structure, by detecting the cell impedance Zac using the output current from the monitor cell 1m, efficient current detection can be performed without interfering with NOx detection in the sensor cell 1s. By doing this and optimizing the circuit configuration, it becomes possible to detect the NOx concentration with high accuracy.

At this time, as shown in FIG. 18, the detection control unit 200 applies a predetermined DC voltage to the monitor cell 1m in order to monitor the residual oxygen, and periodically applies a predetermined AC voltage to the monitor cell 1m. Then, a monitor cell current Im for impedance detection is output. The NOx concentration calculation unit 100 acquires the monitor cell current Im generated in response to the applied DC voltage at a predetermined detection timing t1, and the sensor cell temperature information detection unit 103 acquires the applied AC voltage at a predetermined detection timing t2. It is possible to obtain the monitor cell current Im generated in response to .

The NOx concentration correction section 104 calculates the NOx concentration reference value C1 input from the NOx concentration reference value calculation section 101, the O 2 concentration input from the H ₂ O concentration information detection section 102, and the O ₂ concentration input from the sensor cell temperature information detection section 103. The NOx concentration is calculated using the cell impedance Zac of the monitor cell 1m.

(Embodiment 3)
A third embodiment of the NOx sensor will be described with reference to the drawings. As shown in the block diagram in FIG. 21, this embodiment is another example of offset correction performed in the NOx concentration calculation unit 100 of the sensor control unit 10 described above, and is output from the monitor cell 1m as sensor cell temperature information Ti. The monitor cell current Im is used. The other basic configuration of the sensor element 1 and the basic control of the sensor control unit 10 are the same as those of the second embodiment, and the differences will be mainly described below.

In FIG. 21, the NOx concentration calculation unit 100 detects the pump cell current Ip input to the H ₂ O concentration information detection unit 102 as H ₂ O concentration information Ci. As shown in FIG. 5 above, the pump cell current Ip corresponds to the O ₂ concentration in the exhaust gas G introduced into the gas chamber 2 to be measured, and has a correlation with the H ₂ O concentration, so the H ₂ O concentration information It is used as Ci and is output to the offset correction value calculation section 104A of the NOx concentration correction section 104.

Further, the NOx concentration calculation unit 100 detects the monitor cell current Im input to the sensor cell temperature information detection unit 103 as sensor cell temperature information Ti, and outputs it to the offset correction value calculation unit 104A. As shown in FIG. 4, the monitor cell 1m and the sensor cell 1s are located at the same position with respect to the gas flow in the gas chamber 2 to be measured, and are heated to the same temperature by the heater H. Therefore, the temperature of the monitor cell 1m substantially corresponds to the sensor cell temperature Ts. In the above embodiment, the cell impedance Zac is used as a parameter corresponding to temperature, but instead of this, the monitor cell current Im can be used.

As shown in FIG. 22, in the sensor element 1, when a predetermined pump voltage (for example, about 0.4 V) for oxygen pumping is applied between the pair of electrodes 21 and 41 of the pump cell 1p, the sensor cell 1s and A similar voltage is also applied between a pair of electrodes in the monitor cell 1m. As a result, in the sensor cell 1s and the monitor cell 1m, free electrons (i.e., e ^- in the figure) move from the sensor electrode 22 and monitor electrode 23 side to the reference electrode 41 side, and the accompanying current flow (i.e., (indicated by the dotted arrow inside) occurs in the opposite direction. Since this current flow is considered to have temperature dependence, it can be used as sensor cell temperature information Ts.

As shown in FIG. 23, the monitor cell current Im has a correlation with the temperature of the sensor cell 1s, and the movement of free electrons is promoted in proportion to the temperature rise, and the current increases. Furthermore, when the H ₂ O concentration is high, the current increases and the slope of the characteristic line changes. For example, in the temperature range from around 600°C to around 700°C, the monitor cell current Im is around 30 nA ( This shows an increase of about 50 nA (H ₂ O concentration 13%) from 0% (H ₂ O concentration). Therefore, the monitor cell current Im is also correlated with the H ₂ O concentration, and it is presumed that the current increases due to the movement of free electrons and the decomposition of H ₂ O.

Here, as shown in FIG. 8 above, the Pt-Au electrode used as the monitor electrode 23 of the monitor cell 1m is configured as an electrode with low H ₂ O decomposition activity, and under conditions where the H ₂ O concentration is low. , H ₂ O decomposition causes almost no effect. However, as the cell temperature or H ₂ O concentration increases, the output due to decomposition of H ₂ O increases, so it is preferable to perform a correction that takes the H ₂ O concentration into account to accurately detect the sensor cell temperature Ts. can. In that case, the H ₂ O concentration information Ci (for example, pump cell current Ip) detected by the H ₂ O concentration information detection unit 102 can be used for correction.

In FIG. 21, the NOx concentration correction unit 104 uses the pump cell current Ip and the monitor cell current Im input to the offset correction value calculation unit 104A to calculate the relationship between the previously acquired H ₂ O concentration information Ci and the sensor cell temperature information Ti. The offset correction value C2 is calculated based on the calculation map shown. The monitor cell current Im, which is the sensor cell temperature information Ti, may be corrected in advance according to the H ₂ O concentration, or a calculation map prepared for each H ₂ O concentration may be used.

In this way, the sensor cell temperature information Ti may be used as the monitor cell current Im, and by offset-correcting the NOx concentration reference value C1 using the acquired offset correction value C2, the NOx concentration can be calculated with high accuracy. Note that, like the monitor cell current Im, the sensor cell current Is is also considered to have temperature dependence, but since the sensor electrode 22 contains Rh, its decomposition activity for H ₂ O is higher than that of the monitor electrode 23, and the H ₂ O It is difficult to separate the current increase due to temperature from the current increase due to temperature. On the other hand, since the monitor electrode 23 decomposes less H ₂ O, it can be used as sensor cell temperature information Ti.

Here, as shown in FIG. 24, in the sensor element 1, when the voltage (Vs; unit V) applied between the pair of electrodes 22 and 41 of the sensor cell 1s is varied, the NOx output changes depending on the H ₂ O concentration. Increase accordingly. For example, when the H ₂ O concentration is 13%, when the applied voltage becomes 0.35 V or more, the NOx output increases rapidly. When the H ₂ O concentration is 0%, the NOx output due to the applied voltage hardly changes, so when H ₂ O is present, the decomposition of H ₂ O is promoted by increasing the applied voltage. It is thought that Therefore, when the sensor electrode 22 is a Pt-Rh electrode, it is necessary to make adjustments such as setting the applied voltage to a voltage range in which H ₂ O is not decomposed, or changing the electrode material or alloy ratio.

Even in such a case, as described above, by performing offset correction using the H ₂ O concentration information Ci and sensor cell temperature information Ti in the NOx concentration correction _section 104, Even if the sensor cell temperature Ts fluctuates, suitable correction can be performed. In that case, the sensor electrode 22 is an electrode containing Rh, and the applied voltage is preferably set to 0.35 V or more, so that offset correction can be performed effectively. Further, by applying a voltage of 0.35 V or more to the sensor electrode 22 containing Rh, it is expected that the life of the electrode will be improved. The applied voltage can be suitably set within a range of about 0.45 V or less, making it possible to realize a highly accurate and long-life NOx sensor S.

Furthermore, as shown in FIG. 25, the heater resistance of the heater H that heats the sensor cell 1s can also be used as the sensor cell temperature information Ti. In the sensor element 1, the temperature of the heater H is controlled by energizing the heating element 62, and the sensor cell is heated by heat transfer via the heater insulating layer 61 and the solid electrolyte body 11 (for example, see FIG. 1). There is a certain relationship with the temperature of 1s. Specifically, as shown in the upper diagram of FIG. 26, when the heater temperature is controlled in the range from room temperature (20°C) to the maximum temperature Ta (unit:°C), the heater resistance range is, for example, 2Ω to As shown in the lower diagram of FIG. 26, the sensor cell temperature Ts is, for example, in the range from room temperature to the maximum temperature Tb (unit: °C; Tb<Ta) for the heater temperature range (room temperature to Ta). , the temperature increases in proportion to the heater temperature.

From this relationship, the relationship between the heater resistance and the sensor cell temperature Ts shown in FIG. 25 is obtained, and the sensor cell temperature Ts in the range of, for example, room temperature to about 700° C. can be detected depending on the heater resistance. In addition, since the temperature information of the heater resistor is constantly acquired for heater control, there is no need to change the circuit configuration to acquire the sensor cell temperature information Ti, and the sensor cell can be adjusted using the conventional configuration. Offset correction can be easily performed taking temperature information Ti into consideration. In this way, the sensor cell temperature information Ti is not limited to the monitor cell current Im or the cell impedance Zac, but may be any information that has a correlation with the sensor cell temperature Ts and allows the sensor cell temperature Ts to be estimated.

(Embodiment 4)
A fourth embodiment of the NOx sensor will be described with reference to the drawings.
In the above embodiment, the NOx concentration calculation unit 100 uses internal information such as the detection signal from the sensor element 1 to obtain the H ₂ O concentration information Ci. It is also possible to estimate the H ₂ O concentration in the exhaust gas G (hereinafter referred to as an estimated H ₂ O concentration value) using the external information No. 1 and use it as the H ₂ O concentration information Ci. Specifically, the external information includes driving information based on detection signals input from each part of the engine to the ECU, fuel information such as fuel properties used for engine combustion, environmental information based on the external environment of the gas sensor S, etc. Information that affects the H ₂ O concentration contained in G can be used. In the NOx sensor S of this embodiment, the basic configuration of the sensor element 1 and the basic control of the sensor control unit 10 are the same as those in the first embodiment, and the differences will be mainly described below.

The block diagram in FIG. 27 shows steps [A] to [C] performed in the H ₂ O concentration information detection unit 102 of the NOx concentration calculation unit 100 to obtain H ₂ O concentration information Ci. . In step [A], the O ₂ concentration of the unburned mixture in the combustion chamber of the engine is calculated based on the engine operating information, and in step [B], the H ₂ O concentration resulting from combustion of the unburned mixture is calculated. Further, in step [C], the H ₂ O concentration in the exhaust gas G is calculated. In procedure [A], the O ₂ concentration may be calculated by procedure [a] as a modified example. Each procedure will be explained below.

In step [A], the H ₂ O concentration information detection unit 102 uses the torque and rotation speed acquired by the ECU as engine operating information, and uses the O ₂ concentration conversion map 111 to determine the O ₂ concentration. calculate. As shown in FIG. 28, the O ₂ concentration conversion map 111 is a map of relationships obtained through tests and the like in advance. In the figure, regions where the O ₂ concentration is 5%, 10%, and 15% are shown as examples, and the relationship is such that the larger the torque or the lower the rotation speed, the lower the O ₂ concentration. The obtained O ₂ concentration is used in procedure [B].

When procedure [a] is used instead of procedure [A], the intake air amount and fuel injection amount are used as the engine operating information, and the engine uses the exhaust gas recirculation (EGR) system. If adopted, the EGR gas amount is added to the intake air amount. Using this information, the calculation unit 112 performs calculations based on a previously known relationship to calculate the A/F value. Next, using the O ₂ concentration conversion map 113, the A/F value is converted to O ₂ concentration. The obtained O ₂ concentration is used in procedure [B].

In step [B], the H ₂ O concentration conversion map 114 is used to calculate the combustion of the fuel from the O ₂ concentration obtained in step [A] or step [a] and the H/C ratio of the fuel used in the engine. Calculate the H ₂ O concentration estimated to have occurred. The H ₂ O concentration in the atmosphere is added to the calculated H ₂ O concentration in the calculation unit 116 of step [C], and the H 2 O concentration in the exhaust gas G is calculated, and the H ₂ O concentration is calculated as an estimated H ₂ O concentration value. Output. The atmospheric H ₂ O concentration is acquired by the atmospheric H ₂ O calculation unit 115 .

As shown in the upper diagram of FIG. 29, in the H ₂ O concentration conversion map 114, the relationship between O ₂ concentration and H ₂ O concentration differs depending on the fuel used, and here, the following two types of fuel are exemplified. .
HVO (hydrogen biodiesel fuel) C/H=2.151
Light oil (quality standard EN590) C/H=1.886
As described above, depending on the fuel properties, if the H/C ratio of the fuel differs, the amount of water vapor generated by combustion also differs (for example, see FIG. 16). In that case, based on the formula shown in FIG. 13 above, for example, if light oil with C/H=1.886 is completely combusted, the H ₂ O concentration will be 12.5%, and C/H= When 2.151 HVO is completely combusted, the H ₂ O concentration becomes 13.6%. As described above, since the correlation between O ₂ concentration and H ₂ O concentration changes slightly depending on the H/C ratio, it is preferable to use a map based on the relationship obtained in advance depending on the fuel used to It is desirable to calculate the H ₂ O concentration in consideration of the properties.

Furthermore, the atmospheric H ₂ O calculation unit 115 calculates the atmospheric H ₂ O concentration using temperature, atmospheric pressure, and humidity from the environmental information of the NOx sensor S acquired by the ECU. As shown in the lower diagram of FIG. 29, the maximum water vapor concentration in the atmosphere is determined by temperature and atmospheric pressure, and for example, when the pressure is atmospheric, the relationship shown in the figure is established. At this time, the maximum water vapor concentration has a relationship that the higher the temperature, the higher the maximum water vapor concentration, and further, by using the humidity information, the H ₂ O concentration in the atmosphere can be calculated. For example, as shown in the figure, when the atmospheric pressure is 28° C. and the humidity is 50%, the H ₂ O concentration in the atmosphere is about 2%, which corresponds to 50% of the maximum water vapor concentration.

In this way, the estimated H ₂ O concentration value is calculated by steps [A] to [C] and output to the NOx concentration correction section 104 as H ₂ O concentration information Ci. The NOx concentration correction unit 104 performs offset correction on the NOx concentration reference value C1 using the H ₂ O concentration information Ci and the sensor cell temperature information Ti in the same manner as in the above embodiment. In this way, when calculating the H ₂ O concentration, which is the H ₂ O concentration information Ci, based on external information, it is possible to calculate high The H ₂ O concentration in the exhaust gas G can be estimated with accuracy.

(Embodiment 5)
A fifth embodiment of the NOx sensor will be described with reference to the drawings.
In the above embodiment, the NOx concentration calculation unit 100 uses internal information such as the detection signal from the sensor element 1 to obtain the sensor cell temperature information Ti, but uses external information from the sensor element 1 to calculate the sensor cell temperature Ts. It can also be estimated (hereinafter referred to as sensor cell temperature estimate value) and used as sensor cell temperature information Ti. As the external information, various information such as engine operation information and environmental information can be used. In the NOx sensor S of this embodiment, the basic configuration of the sensor element 1 and the basic control of the sensor control unit 10 are the same as those in the first embodiment, and the differences will be mainly described below.

Specifically, in this embodiment, the sensor cell temperature Ts can be estimated based on exhaust gas information that is gas information to be measured outside the sensor element 1. The exhaust gas information includes information regarding engine operation, such as the gas temperature and gas flow rate of the exhaust gas G near the mounting position of the sensor element 1. These gas temperatures and gas flow rates are input to the sensor cell temperature information detection section 103 of the sensor control section 10 as information from the ECU, for example.

As shown in FIG. 30, in the sensor body 1A of the NOx sensor S, the housing S1 is fixed to the passage wall EX1 of the exhaust gas passage EX, and the sensor element 1 housed in the housing S1 is located within the exhaust gas passage EX. are doing. As mentioned above, the temperature of each part of the sensor element 1 is not necessarily the same temperature, and the factor (1) is heat shrinkage from the housing S1 to the passage wall EX1, and the factor (2) is the exhaust gas An example of this is the exchange of heat with G. At this time, the heat exchange between the exhaust gas G and the exhaust gas G changes depending on the gas temperature and gas flow rate of the exhaust gas G. For example, the gas temperature and gas of the exhaust gas G introduced from the element cover S2 and reaching the tip of the sensor element 1 change. The winnings change. As a result, when the amount of power input to the heater H by the heater control unit 300 in order to maintain the controlled temperature of the sensor element 1 changes, the balance between heat loss and heat transfer is likely to be lost.

In that case, as shown in 14 above, the temperature of the sensor cell 1s located downstream of the pump cell 1p inside the sensor element 1 is affected by the gas temperature and gas flow rate of the exhaust gas G, and the temperature of the sensor cell 1s of the heater H is influenced by the gas temperature and gas flow rate of the exhaust gas G. It becomes easy to deviate from the controlled temperature. Therefore, as shown in FIG. 31, by conducting tests in advance to obtain the relationship between the exhaust gas temperature and exhaust gas flow rate at the mounting position of the sensor element 1 and the sensor cell temperature Ts and creating a map, exhaust gas information can be obtained. Based on the sensor cell temperature estimate can be obtained.

The estimated sensor cell temperature value acquired in this manner is output to the NOx concentration correction section 104 as sensor cell temperature information Ti. The NOx concentration correction unit 104 performs offset correction on the NOx concentration reference value C1 using the H ₂ O concentration information Ci and the sensor cell temperature information Ti in the same manner as in the above embodiment. In this way, the sensor cell temperature information Ti can also be acquired based on external information, and in the NOx sensor S whose heater is controlled based on the temperature of the pump cell 1p, the temperature of the sensor cell 1s located downstream of the pump cell 1p. can be estimated with high accuracy.

Here, as shown in FIG. 32, the relationship between the internal structure of the sensor element 1 and the NOx output offset was compared by changing the engine operating conditions. As shown in FIG. 4, one chamber in the figure is a sensor element 1 in which a pump cell 1p, a sensor cell 1s, and a monitor cell 1m are arranged in a gas chamber 2 to be measured consisting of a single space. NOx output offset changed significantly. The plurality of chambers in the figure is the sensor element 1 in which the gas chamber 2 to be measured is divided into a plurality of chambers, and the pump cell 1p, the sensor cell 1s, and the monitor cell 1m are arranged in separate chambers, and the change in NOx output offset is smaller.

As for the driving conditions, in a driving test in WLTC mode, sensor output was measured under a low load condition corresponding to an idling state and a high load condition corresponding to a 130 km/h running state. Furthermore, under each condition, the O ₂ concentration in the exhaust gas G was measured using an analyzer, and the results were as follows.
Low load conditions: _O2 concentration 10%, exhaust gas temperature 130℃
High load conditions: _O2 concentration 9%, exhaust gas temperature 350℃

Since the measured O ₂ concentrations are almost the same under each condition, it is considered that the change in the NOx output offset essentially indicates that the H ₂ O decomposition reaction in the sensor cell 1s has temperature characteristics. . The reason for this is not necessarily clear, but in the one-chamber sensor element 1, the temperature of the sensor cell 1s tends to rise due to the above-mentioned exchange of heat with the exhaust gas G and the control of the heater H, especially on the high load side. It is assumed that the NOx output offset increases. Even in such a case, by performing offset correction in the NOx concentration calculation unit 100 of the sensor control unit 10 as in each of the above embodiments, it is possible to accurately calculate the NOx concentration from low load conditions to high load conditions. I can do it.

The present invention is not limited to the above-mentioned embodiments, but can be applied to various embodiments without departing from the gist thereof. Further, the configurations in each embodiment may be combined or partially changed. For example, the H ₂ O concentration information Ci of the above-mentioned embodiment 4 and the sensor cell temperature information Ti of the above-mentioned embodiment 5 may be combined. good. Further, the NOx sensor S can be applied to NOx detection using not only exhaust gas G from a vehicle engine but also gas discharged from various internal combustion engines and the like as the gas to be measured.

1 sensor element 1p pump cell (first cell)
1s sensor cell (second cell)
101 NOx concentration reference value calculation unit 102 H ₂ O concentration information detection unit 103 Sensor cell temperature information detection unit 104 NOx concentration correction unit 2 Gas chamber to be measured Ip Pump cell current (first signal)
Is sensor cell current (second signal)

Claims (8)

A sensor element (1) that detects NOx contained in the gas to be measured (G), a heater (H) that heats the sensor element, and controls the operation of the sensor element and the heater, and A NOx sensor (S) comprising a sensor control unit (10) that calculates a NOx concentration based on a signal,
The above sensor element is
A gas to be measured chamber (2) into which the gas to be measured is introduced via a gas introduction part (3) and has a solid electrolyte body (11) as a chamber wall;
The gas chamber to be measured has a first cell electrode (21) disposed upstream of the gas flow to be introduced, and a first signal (Ip) having a correlation with the H ₂ O concentration in the gas to be measured. A first cell ( _1p ) that outputs a second cell (1s) that outputs a second signal (Is),
The above sensor control section is
a NOx concentration reference value calculation unit (101) that calculates a NOx concentration reference value (C1) based on the second signal;
an H ₂ O concentration information detection unit (102) that detects H 2 O concentration or H ₂ O concentration information (Ci _{) including concentration information having a correlation with the H 2 O} concentration, based on the first signal or external information; ,
a second cell temperature information detection unit (103) that detects second cell temperature information (Ti) having a correlation with the temperature (Ts) of the second cell;
A NOx sensor, comprising: a NOx concentration correction section (104) that corrects the NOx concentration reference value using an offset correction value (C2) based on the _H2O concentration information and the second cell temperature information. In the gas chamber to be measured, the first cell outputs the first signal as oxygen contained in the gas to be measured is discharged through the solid electrolyte body;
A third cell that outputs a third signal (Im) corresponding to the residual oxygen concentration remaining in the gas to be measured, downstream of the first cell with respect to the gas flow introduced into the gas to be measured chamber. The NOx sensor according to claim 1, wherein (1 m) is arranged in parallel with the second cell. The NOx sensor according to claim 2, wherein the second cell temperature information is cell impedance (Zac) of the second cell or the third cell. The NOx sensor according to claim 2, wherein the second cell temperature information is the third signal output from the third cell. The NOx sensor according to claim 1 or 2, wherein the second cell temperature information is heater resistance of the heater. The NOx sensor according to claim 1 or 2, wherein the second cell temperature information is acquired based on the flow rate and temperature of the gas to be measured outside the sensor element. The sensor control unit includes a detection control unit (200) that controls voltages applied to the first cell and the second cell, and a heater that controls the temperature (Tp) of the first cell by energizing the heater. The NOx sensor according to any one of claims 1 to 6, further comprising a control section (300). The second cell electrode is made of an electrode material containing Rh,
The NOx sensor according to claim 7, wherein the detection control section controls the voltage applied to the second cell to be 0.35V or higher.

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