JP7261184B2 - gas sensor - Google Patents

Description

The present disclosure relates to gas sensors.

Conventionally, as a gas sensor capable of detecting the concentration of ammonia in exhaust gas, there is a gas sensor described in Patent Document 1 below. The gas sensor disclosed in Patent Literature 1 includes a sensor element and a control device.
The sensor element has a mixed potential cell. A mixed potential cell is composed of a solid electrolyte body, a sensing electrode arranged on one surface of the solid electrolyte body, and a reference electrode arranged on the other surface of the solid electrolyte body. In a mixed potential cell, electrochemical reactions such as oxidation of ammonia and ionization of oxygen in the exhaust occur at the three-phase interface between the sensing electrode, the solid electrolyte, and the exhaust, resulting in a mixed potential at the sensing electrode. Therefore, an electromotive force corresponding to the concentrations of ammonia and oxygen in the exhaust gas is generated between the detection electrode and the reference electrode.

The control device includes an electromotive force acquisition section, an oxygen concentration acquisition section, and an ammonia concentration derivation section. The electromotive force acquisition unit acquires an electromotive force generated in the mixed potential cell. The oxygen concentration acquisition unit acquires the oxygen concentration in exhaust gas. The ammonia concentration derivation unit derives the concentration of ammonia in the exhaust based on the following equation f1 from the electromotive force obtained by the electromotive force obtaining unit and the oxygen concentration in the exhaust obtained by the oxygen concentration obtaining unit. Note that in formula f1, “EMF” is the electromotive force of the mixed potential cell, “α”, “β”, and “B” are constants, “a” and “b” are arbitrary bases, “p _NH3 ” is the concentration of ammonia in the exhaust, and “p ₀₂ ” is the concentration of oxygen in the exhaust.

EMF=αlog _a (p _NH3 )−βlog _b (p _O2 )+B (f1)

JP 2018-173397 A

By the way, according to the experiments and simulations conducted by the inventors, it has been confirmed that when the concentration of ammonia in the exhaust gas is low, the accuracy of calculation of the concentration of ammonia deteriorates when the concentration of ammonia is calculated based on the above equation f1. The reason for this is thought to be that part of the ammonia disappears due to thermal decomposition and oxidation of the ammonia on the surface of the sensing electrode. More specifically, if some of the ammonia disappears on the surface of the sensing electrode, there will be an error between the ammonia concentration detected by the mixed potential cell and the actual ammonia concentration in the exhaust gas. This error affects the ammonia detection accuracy of the gas sensor, and the lower the concentration of ammonia in the exhaust gas, the greater the effect. Therefore, when the above equation (1) is used, especially when the concentration of ammonia in the exhaust gas is low, there is a possibility that the accuracy of calculation of the concentration of ammonia cannot be ensured.

This problem is not limited to gas sensors that detect ammonia in exhaust gas, but is common to gas sensors that detect the concentration of a target gas component contained in a gas to be measured.
The present disclosure has been made in view of such circumstances, and an object thereof is to provide a gas sensor capable of detecting with higher accuracy the concentration of a detection target gas component contained in a gas to be measured. .

A gas sensor that solves the above problems comprises an oxygen concentration detector (41), a mixed potential cell (44), and a gas concentration calculator (226). The oxygen concentration detector detects the concentration of oxygen contained in the gas to be measured. The mixed potential cell includes a detection electrode (65) that detects a gas component to be detected and oxygen contained in the gas to be measured and generates a mixed potential that is a potential corresponding to their concentrations, and a reference that is provided so as to be in contact with the atmosphere. An electromotive force is generated between the detection electrode and the reference electrode according to the concentration of the gas component to be detected and the concentration of oxygen in the gas to be measured. The gas concentration calculator indicates the relationship between the concentration of the gas component to be detected and the electromotive force of the mixed potential cell corresponding to when the concentration of the gas component to be detected is equal to or higher than a predetermined concentration, and calculates a plurality of second values corresponding to the oxygen concentration. 1 calculation formula and the relationship between the concentration of the detection target gas component and the electromotive force of the mixed potential cell corresponding to when the concentration of the detection target gas component is less than the predetermined concentration, and a correlation different from the first calculation formula is shown. and a plurality of second arithmetic expressions corresponding to the oxygen concentration, and from the plurality of first arithmetic expressions and the plurality of second arithmetic expressions, a first first arithmetic expression corresponding to the oxygen concentration detected by the oxygen concentration detection unit An arithmetic expression and a second arithmetic expression are selected, and the concentration of the gas component to be detected in the gas to be measured is calculated based on the electromotive force of the mixed potential cell from the selected first arithmetic expression and second arithmetic expression .
Another gas sensor that solves the above problem comprises an oxygen concentration detector (41), a mixed potential cell (44), and a gas concentration calculator (226). The oxygen concentration detector detects the concentration of oxygen contained in the gas to be measured. The mixed potential cell includes a detection electrode (65) that detects a gas component to be detected and oxygen contained in the gas to be measured and generates a mixed potential that is a potential corresponding to their concentrations, and a reference that is provided so as to be in contact with the atmosphere. An electromotive force is generated between the detection electrode and the reference electrode according to the concentration of the gas component to be detected and the concentration of oxygen in the gas to be measured. The gas concentration calculator indicates the relationship between the concentration of the gas component to be detected and the electromotive force of the mixed potential cell corresponding to when the concentration of the gas component to be detected is equal to or higher than a predetermined concentration, and calculates a plurality of second values corresponding to the oxygen concentration. 1 map and the relationship between the concentration of the detection target gas component and the electromotive force of the mixed potential cell corresponding to when the concentration of the detection target gas component is less than the predetermined concentration, and has a different correlation from the first map. and a plurality of second maps corresponding to the oxygen concentration, the first map and the second map corresponding to the oxygen concentration detected by the oxygen concentration detection unit from the plurality of first maps and the plurality of second maps is selected, and the concentration of the gas component to be detected in the gas to be measured is calculated based on the electromotive force of the mixed potential cell from the selected first map and second map.

According to this configuration, compared to the case of calculating the concentration of the detection target gas component using only the first arithmetic expression or the first map used when the concentration of the detection target gas component is high, the concentration of the detection target gas component is low, the concentration can be calculated with higher accuracy.
It should be noted that the means described above and the reference numerals in parentheses described in the claims are examples showing the corresponding relationship with specific means described in the embodiments described later.

According to the gas sensor of the present disclosure, the concentration of the detection target gas component contained in the gas under measurement can be detected with higher accuracy.

FIG. 1 is a block diagram showing a schematic configuration of an exhaust purification system for a vehicle according to an embodiment. FIG. 2 is a cross-sectional view showing the cross-sectional structure of the sensor element of the gas sensor of the embodiment. FIG. 3 is a cross-sectional view showing a cross-sectional structure taken along line III--III in FIG. FIG. 4 is a block diagram showing a schematic configuration of the SCU of the gas sensor of the embodiment. FIG. 5 is a graph showing the relationship between the mixed potential cell electromotive force V _NH3 and the ammonia concentration p _NH3 in a gas sensor without a protective layer. FIG. 6 is a graph showing the relationship between the ammonia concentration calculated by a gas sensor that does not have a protective layer and the actual ammonia concentration. FIG. 7 is a graph showing the relationship between the mixed potential cell electromotive force V _NH3 and the ammonia concentration p _NH3 in a gas sensor having a protective layer. FIG. 8 is a graph showing the relationship between the ammonia concentration calculated by the gas sensor having the protective layer and the actual ammonia concentration. FIG. 9 is a graph showing an arithmetic expression for obtaining the ammonia concentration p _NH3 from the electromotive force V _NH3 of the mixed potential cell, which is used by the gas sensor of the embodiment.

An embodiment of the gas sensor will be described below with reference to the drawings. In order to facilitate understanding of the description, the same constituent elements in each drawing are denoted by the same reference numerals as much as possible, and overlapping descriptions are omitted.
First, an outline of an exhaust purification system for a vehicle in which the gas sensor of the embodiment is mounted will be described.

As shown in FIG. 1, a vehicle exhaust purification system 1 of the present embodiment is a system for purifying exhaust from an engine 10, which is a diesel engine. An exhaust pipe 11 forming an exhaust passage is connected to the engine 10. The exhaust pipe 11 is connected to an oxidation catalytic converter 12 and a selective reduction catalytic converter (hereinafter referred to as "SCR catalytic converter") in order from the engine 10 side. 13 are provided. The oxidation catalyst converter 12 has a diesel oxidation catalyst 14 and a DPF (Diesel Particulate Filter) 15 . The SCR catalytic converter 13 has an SCR catalyst 16 as a selective reduction type catalyst. A urea water addition valve 17 is provided between the oxidation catalyst converter 12 and the SCR catalyst converter 13 in the exhaust pipe 11 for adding and supplying urea water (urea aqueous solution) as a reducing agent into the exhaust pipe 11 . ing.

The diesel oxidation catalyst 14 is mainly composed of a ceramic carrier, an oxide mixture containing aluminum oxide, cerium dioxide and zirconium dioxide as components, and a noble metal catalyst such as platinum, palladium and rhodium. The diesel oxidation catalyst 14 oxidizes and purifies hydrocarbons, carbon monoxide, nitrogen oxides, etc. contained in the exhaust.

The DPF 15 is formed of a honeycomb structure, and is configured by supporting a platinum group catalyst such as platinum or palladium on a porous ceramic. The DPF 15 collects particulate matter contained in the exhaust gas by depositing it on the partition walls of the honeycomb structure. Accumulated particulate matter is oxidized and purified by combustion. This combustion utilizes the temperature rise in the diesel oxidation catalyst 14 and the decrease in the combustion temperature of the particulate matter due to the additive.

The SCR catalytic converter 13 is a device for reducing NOx to nitrogen and water as a post-treatment device for the oxidation catalytic converter 12 . As the SCR catalyst 16, for example, a catalyst in which a noble metal such as platinum is supported on the surface of a base material such as zeolite or alumina is used. The SCR catalyst 16 reduces and purifies NOx by adding urea as a reducing agent when the catalyst temperature is in the activation temperature range.

The exhaust purification system 1 includes a gas sensor 20 for detecting NOx concentration and ammonia (NH ₃ ) concentration in the exhaust. The gas sensor 20 has a sensor element 21 and an SCU (Sensor Control Unit) 22 . The sensor element 21 is arranged downstream of the SCR catalytic converter 13 in the exhaust pipe 11 . The sensor element 21 detects NOx concentration and ammonia concentration in the exhaust gas at the detection position. An SCU 22 is connected to the sensor element 21 . A detection signal from the sensor element 21 is input to the SCU 22 . The SCU 22 is an electronic control unit having a microcomputer having a CPU and various memories and its peripheral circuits. In this embodiment, exhaust corresponds to the gas to be measured.

The SCU 22 is connected to various ECUs such as an engine ECU 30 via a communication line 31 such as a CAN bus. The SCU 22 and the engine ECU 30 can exchange information with each other using a communication line 31 . Information on, for example, oxygen concentration, NOx concentration, and ammonia concentration in the exhaust gas is transmitted from the SCU 22 to the engine ECU 30 . The engine ECU 30 is an electronic control device having a CPU, a microcomputer having various memories, and peripheral circuits thereof, and controls the engine 10 and various devices in the exhaust system. The engine ECU 30 performs fuel injection control and the like based on, for example, the accelerator opening and the engine rotation speed. Further, the engine ECU 30 controls addition of urea water by the urea water addition valve 17 based on the NOx concentration and the ammonia concentration detected by the gas sensor 20 .

Next, the configuration of the gas sensor 20 will be described. 2 and 3 are diagrams showing the internal structure of the sensor element 21 of the gas sensor 20. FIG. The horizontal direction of the drawing is the longitudinal direction of the sensor element 21, and the left side of the drawing is the element tip side.
The sensor element 21 includes a pump cell 41, a sensor cell 42, a monitor cell 43, a mixed potential cell 44, a first solid electrolyte body 51, a second solid electrolyte body 52, a substrate portion 53, a diffusion resistor 54, A heater 59 and a protective layer 70 are provided.

The first solid electrolyte body 51, the second solid electrolyte body 52, and the substrate portion 53 are formed in a plate shape and arranged side by side in this order with a predetermined gap in the thickness direction. A gap formed between the substrate portion 53 and the second solid electrolyte body 52 forms a first reference gas chamber 64 . A gap formed between the first solid electrolyte body 51 and the second solid electrolyte body 52 forms a measurement gas chamber 61 and a second reference gas chamber 62 . The measurement gas chamber 61 and the second reference gas chamber 62 are partitioned as independent spaces by partition walls 63 . The atmosphere as a reference oxygen concentration gas is introduced into the first reference gas chamber 64 and the second reference gas chamber 62 .

One side of the first solid electrolyte body 51 is open, and a diffusion resistor 54 is arranged on the open side. Exhaust gas flowing through the exhaust pipe 11 is introduced into the measurement gas chamber 61 through the protective layer 70 and the diffusion resistor 54 . The diffusion resistor 54 is made of a porous member such as alumina or a member having pores. A diffusion resistor 54 is provided to limit the amount of exhaust introduced into the measurement gas chamber 61 .

The pump cell 41 is arranged closer to the diffusion resistor 54 than the sensor cell 42 and monitor cell 43 . The pump cell 41 removes oxygen from the exhaust introduced from the diffusion resistor 54 .
The pump cell 41 includes a second solid electrolyte body 52, a pump electrode 55 arranged on the surface of the second solid electrolyte body 52 on the side of the measurement gas chamber 61, and an electrode on the side of the first reference gas chamber 64 in the second solid electrolyte body 52. and a common electrode 58 arranged on the surface. The pump electrode 55 is a NOx-inactive electrode that hardly decomposes NOx, such as an electrode made of a Pt--Au (platinum--gold) alloy. The common electrode 58 is arranged to extend to regions corresponding to the sensor cells 42 and the pump cells 41 . A pump voltage Vp is applied between the pump electrode 55 and the common electrode 58 by the SCU 22 .

The exhaust gas introduced into the measuring gas chamber 61 through the diffusion resistor 54 contacts the pump electrode 55 . When oxygen in the exhaust contacts the pump electrode 55 , oxygen ions are generated at the pump electrode 55 . The oxygen ions flow through the second solid electrolyte body 52 toward the common electrode 58 and release charges at the common electrode 58 to become oxygen. This oxygen is released to the atmosphere from the first reference gas chamber 64 . A pump current Ip flows between the pump electrode 55 and the common electrode 58 according to the charge flow at this time. Therefore, the pump current Ip indicates a value corresponding to the amount of oxygen removed in the pump cell 41, in other words, the oxygen concentration in the exhaust. In this embodiment, the pump cell 41 corresponds to an oxygen concentration detector that detects the concentration of oxygen contained in the gas to be measured.

As shown in FIG. 3 , the sensor cell 42 is located farther from the diffused resistor 54 than the pump cell 41 is. The sensor cell 42 detects the NOx concentration and residual oxygen concentration in the exhaust that has passed through the pump cell 41 .
As shown in FIG. 2, the sensor cell 42 includes a second solid electrolyte body 52, a sensor electrode 56 arranged on the surface of the second solid electrolyte body 52 on the side of the measurement gas chamber 61, and a common electrode 58. ing. The sensor electrode 56 is a NOx active electrode that easily decomposes NOx, such as an electrode made of a Pt--Rh (platinum-rhodium) alloy. A sensor voltage Vs is applied between the sensor electrode 56 and the common electrode 58 by the SCU 22 .

The sensor electrode 56 is contacted by the exhaust gas that has passed through the pump electrode 55, that is, the exhaust gas from which oxygen has been removed. When NOx in the exhaust comes into contact with the sensor electrode 56 , the NOx is decomposed into nitrogen and oxygen at the sensor electrode 56 . Also, if there is residual oxygen in the exhaust that has not been removed by the pump electrode 55 , this residual oxygen also contacts the sensor electrode 56 . Oxygen ions are generated at the sensor electrode 56 when the oxygen decomposed at the sensor electrode and the residual oxygen in the exhaust come into contact with the sensor electrode 56 . The oxygen ions flow through the second solid electrolyte body 52 toward the common electrode 58 and release charges at the common electrode 58 to become oxygen. This oxygen is released to the atmosphere from the first reference gas chamber 64 . A sensor current Is flows between the sensor electrode 56 and the common electrode 58 according to the charge flow at this time. Therefore, the sensor current Is indicates a value corresponding to the concentration of NOx and residual oxygen in the exhaust gas.

As shown in FIG. 3 , the monitor cell 43 is arranged side by side with the sensor cell 42 . A monitor cell 43 detects the concentration of residual oxygen in the exhaust that has passed through the pump cell 41 .
As shown in FIG. 2, the monitor cell 43 includes a second solid electrolyte body 52, a monitor electrode 57 arranged on the surface of the second solid electrolyte body 52 on the side of the measurement gas chamber 61, and a common electrode 58. ing. The monitor electrode 57 is a NOx inert electrode that is difficult to decompose NOx, such as an electrode made of a Pt--Au (platinum--gold) alloy. A monitor voltage Vm is applied between the monitor electrode 57 and the common electrode 58 by the SCU 22 .

Exhaust gas from which oxygen has been removed by the pump electrode 55 contacts the monitor electrode 57 . If residual oxygen is present in the exhaust gas, oxygen ions are generated at the monitor electrode 57 due to contact of the residual oxygen with the monitor electrode 57 . The oxygen ions flow through the second solid electrolyte body 52 toward the common electrode 58 and release charges at the common electrode 58 to become oxygen. This oxygen is released to the atmosphere from the first reference gas chamber 64 . A monitor current Im flows between the monitor electrode 57 and the common electrode 58 according to the charge flow at this time. Therefore, the monitor current Im indicates a value corresponding to the concentration of residual oxygen in the exhaust gas.

The heater 59 is provided inside the substrate portion 53 . The heater 59 heats the solid electrolyte bodies 51 and 52 by generating heat when energized, and maintains the temperatures of the solid electrolyte bodies 51 and 52 at the activation temperature.
The mixed potential cell 44 is arranged in a portion of the sensor element 21 where the second reference gas chamber 62 is provided. The mixed potential cell 44 detects oxygen and ammonia present in the exhaust outside the sensor element 21 .

The mixed potential cell 44 includes the first solid electrolyte body 51, the detection electrode 65 arranged on the outer surface 510 of the first solid electrolyte body 51, and the inner surface of the first solid electrolyte body 51 on the second reference gas chamber 62 side. and a reference electrode 66 that The detection electrode 65 is made of an electrode whose main component is noble metal such as platinum, gold, palladium, or the like.

The exhaust gas present near the outer surface 510 of the first solid electrolyte body 51 contacts the sensing electrode 65 . In the sensing electrode 65, the ammonia and oxygen in the exhaust gas existing outside the sensor element 21 come into contact with each other, whereby an anodic reaction represented by (f2) below and a cathodic reaction represented by the following formula (f3) occur. occur.

2NH ₃ +3O ²⁻ →N ₂ +3H ₂ O+6e ⁻ (f2)
O ₂ +4e ⁻ →2O ²⁻ (f3)
The anodic reaction represented by the formula f2 and the cathodic reaction represented by the formula f3 occur simultaneously at the three-phase interface of one detection electrode 65, so that a mixed potential, which is a potential corresponding to the concentrations of ammonia and oxygen contained in the exhaust gas, is generated. It occurs at the sensing electrode 65 . As a result, an electromotive force _VNH3 corresponding to the concentrations of ammonia and oxygen in the exhaust gas is generated between the detection electrode 65 and the reference electrode 66 provided so as to be in contact with the atmosphere. The electromotive force V _NH3 is the potential difference between the sensing electrode 65 and the reference electrode 66 . In this embodiment, ammonia corresponds to the detection target gas component contained in the measurement target gas.

The protective layer 70 is provided so as to cover the outer surface of the sensor element 21 . Protective layer 70 also covers sensing electrode 65 of mixed potential cell 44 . The protective layer 70 is made of a porous body containing Al ₂ O ₃ as a main component and having a porosity in the range of “20[%] to 60[%]”. The thickness of the protective layer 70 is about 50 [μm]. The protective layer 70 protects the sensor element 21 from the external environment while allowing exhaust gas outside the sensor element 21 to enter the measuring gas chamber 61 and the sensing electrode 65 .

As shown in FIG. 4, the SCU 22 includes a heater control section 220, a pump current detection section 221, a sensor current detection section 222, a monitor current detection section 223, a mixed potential detection section 224, and a voltage adjustment section 225. , a gas concentration calculation unit 226 and a communication unit 227 .
The heater control unit 220 controls the amount of heat generated by the heater 59 by controlling the voltage applied to the heater 59 .

Pump current detector 221 detects pump current Ip flowing between pump electrode 55 and common electrode 58 .
The sensor current detector 222 detects a sensor current Is flowing between the sensor electrode 56 and the common electrode 58 .

A monitor current detector 223 detects a monitor current Im flowing between the monitor electrode 57 and the common electrode 58 .
The mixed potential detector 224 detects the electromotive force _VNH3 of the mixed potential cell 44 generated between the detection electrode 65 and the reference electrode 66 of the mixed potential cell 44 .

The voltage adjuster 225 applies a pump voltage Vp between the pump electrode 55 and the common electrode 58, applies a sensor voltage Vs between the sensor electrode 56 and the common electrode 58, and further applies a sensor voltage Vs between the monitor electrode 57 and the common electrode. 58, a monitor voltage Vm is applied. Note that the voltage adjustment unit 225 does not apply a voltage between the detection electrode 65 and the reference electrode 66 of the mixed potential cell 44 .

The communication unit 227 is a part that performs various communications between the SCU 22 and the engine ECU 30 via the communication line 31 .
The gas concentration calculation unit 226 detects the pump current Ip detected by the pump current detection unit 221, the sensor current Is detected by the sensor current detection unit 222, and the monitor current Im detected by the monitor current detection unit 223. The oxygen concentration detection value and the NOx concentration detection value are calculated. Specifically, the pump current Ip has a correlation with the oxygen concentration in the exhaust. Also, the sensor current Is has a correlation with the NOx concentration and residual oxygen in the exhaust. Furthermore, the monitor current Im is correlated with the residual oxygen in the exhaust. Using these relationships, the gas concentration calculator 226 calculates the oxygen concentration detection value in the exhaust gas based on the pump current Ip using an arithmetic expression, map, or the like. Further, the gas concentration calculation unit 226 subtracts the monitor current Im from the sensor current Is to obtain a current value “Is-Im” that correlates with the NOx concentration, and uses the calculated current value “Is-Im” to calculate , a map, or the like is used to calculate the NOx concentration detection value in the exhaust gas.

Further, the gas concentration calculation unit 226 calculates the ammonia concentration detection value in the exhaust based on the electromotive force _VNH3 of the mixed potential cell 44 detected by the mixed potential detection unit 224 . Specifically, the electromotive force _VNH3 of the mixed potential cell 44 is correlated with the oxygen concentration and ammonia concentration in the exhaust. Using this, the gas concentration calculation unit 226 calculates the concentration of ammonia in the exhaust gas from the oxygen concentration detection value calculated based on the pump current Ip and the electromotive force _VNH3 of the mixed potential cell 44 using a calculation formula, map, or the like. Calculate the detected value _pNH3 . In this embodiment, the ammonia concentration p _NH3 corresponds to the concentration p _Gas of the gas component to be detected. Further, the electromotive force _VNH3 corresponds to the electromotive force _VGas generated in the mixed potential cell 44 according to the concentration of the gas component to be detected.

The gas concentration calculation unit 226 transmits information on the calculated oxygen concentration detection value, NOx concentration detection value, and ammonia concentration detection value to the engine ECU 30 via the communication unit 227 .
Next, a procedure for calculating the concentration of ammonia by the gas concentration calculating section 226 will be specifically described.

The inventors experimentally determined the relationship between the electromotive force _VNH3 of the mixed potential cell 44 and the concentration of ammonia in the exhaust. FIG. 5 is a logarithmic graph showing experimentally obtained data of the relationship between the electromotive force V _NH3 and the ammonia concentration p _NH3 of the mixed potential cell 44 when the protective layer 70 is not provided on the sensor element 21 . It has become.

As shown in FIG. 5, when the protective layer 70 is not provided, the high-concentration region A11 where the ammonia concentration _pNH3 is equal to or higher than the predetermined concentration p10 and the low-concentration region A11 where the ammonia concentration _pNH3 is less than the predetermined concentration p10 The change tendency of the electromotive force _VNH3 of the mixed potential cell 44 with respect to the unit change amount of the ammonia concentration _pNH3 differs between the region A10 and the region A10. Specifically, the change amount of the electromotive force _VNH3 of the mixed potential cell 44 with respect to the unit change amount of the ammonia concentration _pNH3 is smaller in the low concentration region A10 than in the high concentration region A11.

Based on the graph shown in FIG. 5, the relationship between the electromotive force V _NH3 of the mixed potential cell 44 and the ammonia concentration p _NH3 can be expressed by the approximate expression shown in the following formula f4 in the low concentration region A10. The density region A11 can be represented by the following approximation formula f5. In formulas f4 and f5, "A ₁ ", "B ₁ ", "C ₁ ", and "D ₁ " are predetermined constants. A relationship of “A ₁ <C ₁ ” is established between the constants A ₁ and C ₁ , and a relationship of “B ₁ >D ₁ ” is established between the constants B ₁ and D ₁ .

_VNH3 = _A1 *ln( _pNH3 )+ _B1 (f4)
V _NH3 =C ₁ ×ln(p _NH3 )+D ₁ (f5)
The reason why the change tendency of the electromotive force V _NH3 of the mixed potential cell 44 with respect to the change of the ammonia concentration p _NH3 differs between the region A10 where _the ammonia concentration p NH3 is low and the region A11 where the ammonia concentration p NH3 is high is as follows. it is conceivable that.

At the sensing electrode 65 of the mixed potential cell 44, not only the above-described reactions f2 and f3 occur, but also a disappearance phenomenon due to thermal decomposition of ammonia actually occurs on the surface of the sensing electrode 65. FIG. Specifically, ammonia is thermally decomposed at the sensing electrode 65 of the mixed potential cell 44 as shown in the following formula f6.

_2NH3 → _N2 + _3H2 (f6)
When the phenomenon shown in formula f6 occurs and ammonia is thermally decomposed, the concentration of ammonia detected by the detection electrode 65 of the mixed potential cell 44 decreases accordingly. This causes the actual electromotive force _VNH3 of the mixed potential cell 44 to deviate from the theoretical value of the electromotive force that should be generated in the mixed potential cell 44 with respect to the ammonia concentration in the actual exhaust gas. The deviation between the theoretical value and the actual value of the electromotive force _VNH3 of the mixed potential cell 44 due to the thermal decomposition of ammonia becomes more pronounced as the concentration of ammonia in the exhaust gas becomes lower. Therefore, if the relationship between the electromotive force V _NH3 of the mixed potential cell 44 and the ammonia concentration p _NH3 is approximated by the above equation f1 as in the gas sensor described in Patent Document 1, the concentration of ammonia in the exhaust gas is low. Under such circumstances, the difference between the ammonia concentration _pNH3 calculated by the formula f1 and the actual ammonia concentration becomes large, and the ammonia concentration _pNH3 cannot be detected with high accuracy.

FIG. 6 shows the correlation between the calculated value of the ammonia concentration p _NH3 by the gas sensor 20 that does not have the protective layer 70 and the actual ammonia concentration, using the above two arithmetic expressions f4 and f5 _. is compared with the case where the ammonia concentration _pNH3 is calculated using only the above equation f5. In FIG. 6, the correlation when the ammonia concentration p _NH3 is calculated using two calculation formulas is indicated by a solid line, and the correlation when the ammonia concentration p _NH3 is calculated using only the above formula f5 is two points. indicated by dashed lines.

As shown in FIG. 6, when the ammonia concentration _pNH3 is calculated using only the above equation f5, the calculated value of the ammonia concentration _pNH3 approximately matches the actual ammonia concentration in the high concentration region A11. However, in the low concentration region A10, the difference between the calculated value of the ammonia concentration _pNH3 and the actual ammonia concentration becomes large. That is, the calculation accuracy of the ammonia concentration _pNH3 is lowered.

On the other hand, when the ammonia concentration p _NH3 is calculated using the above two equations f4 and f5, the calculated value of the ammonia concentration p _NH3 is the actual ammonia concentration not only in the high concentration region A11 but also in the low concentration region A10. approximately match. Therefore, it is possible to ensure the calculation accuracy of the ammonia concentration _pNH3 .

On the other hand, FIG. 7 shows, using a logarithmic graph, experimentally determined data of the relationship between the electromotive force V _NH3 and the ammonia concentration p _NH3 of the mixed potential cell 44 when the protective layer 70 is provided on the sensor element 21. is a graph. As shown in FIG. 7, even when the protective layer 70 is provided, the electromotive force of the mixed potential cell 44 with respect to the unit change amount of the ammonia concentration p _NH3 is similarly in the low-concentration region A10 and the high-concentration region A11. The change trend of _VNH3 is different. Based on the graph shown in FIG. 7, the relationship between the electromotive force _VNH3 of the mixed potential cell 44 and the ammonia concentration _pNH3 can be expressed by the approximate expression shown in the following equation f7 in the low-concentration region A10. The density region A11 can be represented by the following approximation formula f8. In formulas f7 and f8, " _A2 ", " _B2 ", " _C2 ", and " _D2 " are predetermined constants. A relationship of “A ₂ <C ₂ ” is established between the constants A ₂ and C ₂ , and a relationship of “B ₂ >D ₂ ” is established between the constants B ₂ and D ₂ .

V _NH3 = A ₂ × ln(p _NH3 ) + B ₂ (f7)
_VNH3 = _C2 *ln( _pNH3 )+ _D2 (f8)
5 and 7, when the protective layer 70 is provided, the unit of the ammonia concentration p _NH3 in the low-concentration region A10 is higher than when the protective layer 70 is not provided. The amount of change in the electromotive force _VNH3 of the mixed potential cell 44 with respect to the amount of change is reduced. The reason why the change tendency of the electromotive force _VNH3 of the mixed potential cell 44 differs depending on the presence or absence of the protective layer 70 is considered as follows.

When the protective layer 70 is provided, in addition to the phenomenon of thermal decomposition of ammonia shown in the above formula f6, the ammonia adsorbed on the protective layer 70 reacts with oxygen and is oxidized, thereby increasing the amount of ammonia. disappears. Specifically, in the protective layer 70, oxidation reactions of ammonia occur as represented by the following formulas f9 to f11.

_4NH3 + _5O2 →4NO+ _6H2O (f9)
_4NH3 +6NO→ _5N2 + _6H2O (f10)
_4NH3 +4NO+ _O2 → _4N2 + _6H2O (f11)
Due to the oxidation reactions of ammonia occurring in the protective layer 70 as shown by the formulas f9 to f11, more ammonia is lost compared to the case where the protective layer 70 is not provided. This causes the theoretical value and the actual value of the electromotive force _VNH3 of the mixed potential cell 44 to diverge further.

FIG. 8 shows the correlation between the ammonia concentration p _NH3 calculated by the gas sensor 20 having the protective layer 70 and the actual ammonia concentration, using the above two calculation formulas f7 and f8 _. is compared with the case where the ammonia concentration _pNH3 is calculated using only the above equation f8. In FIG. 8, the solid line indicates the correlation when the ammonia concentration p _NH3 is calculated using the two equations f7 and f8 above, and the ammonia concentration p _NH3 is calculated using only the above equation f8. The correlation between the cases is indicated by a two-dot chain line.

As shown in FIG. 8, when the gas sensor 20 is provided with the protective layer 70, if the ammonia concentration p _NH3 is calculated using only the above equation f8, the calculated value of the ammonia concentration p _NH3 in the low concentration region A10 and The deviation from the actual ammonia concentration becomes very large. That is, the calculation accuracy of the ammonia concentration _pNH3 is greatly reduced.

On the other hand, when the ammonia concentration p _NH3 is calculated using the two equations f7 and f8 above, the calculated value of the ammonia concentration p _NH3 is the actual ammonia concentration not only in the high concentration region A11 but also in the low concentration region A10. approximately match. Therefore, it is possible to ensure the calculation accuracy of the ammonia concentration _pNH3 .

As described above, irrespective of the presence or absence of the protective layer 70, if two computational equations are used as computational equations showing the relationship between the electromotive force _VNH3 of the mixed potential cell 44 and the ammonia concentration _pNH3 , the ammonia concentration _pNH3 Arithmetic accuracy can be ensured. Specifically, the two types of arithmetic expressions are a first arithmetic expression corresponding to the case where the ammonia concentration _pNH3 is equal to or greater than the predetermined concentration p10, and a second arithmetic expression corresponding to the case where the ammonia concentration _pNH3 is less than the predetermined concentration p10. 2 is an arithmetic expression. The first arithmetic expression and the second arithmetic expression have different correlations.

Based on the above, the gas concentration calculator 226 of the present embodiment uses an arithmetic expression as shown in FIG. 9 as an arithmetic expression representing the relationship between the electromotive force V _NH3 of the mixed potential cell 44 and the ammonia concentration p _NH3 . As shown in FIG. 9, the gas concentration calculation unit 226 calculates the oxygen concentration _pO2 for each of "5 [%]", "10 [%]", "15 [%]", and "20 [%]". It has arithmetic expressions m1 to m4 corresponding to the case of . Each of the arithmetic expressions m1 to m4 includes low-concentration approximate expressions m11, m21, m31, and m41 corresponding to the case where the ammonia concentration _pNH3 is less than the predetermined concentration p10, and m11, m21, m31, and m41 for the case where the ammonia concentration _pNH3 is the predetermined concentration p10 or more. It is composed of corresponding high-density approximation formulas m12, m22, m32, and m42.

Note that the high-concentration approximation formula m12 is an arithmetic expression in which the value of the electromotive force V _NH3 with respect to the ammonia concentration p _NH3 is larger than the low-concentration approximation formula m11 when the ammonia concentration p _NH3 is equal to or higher than the predetermined concentration p10. consists of On the other hand, in the low-concentration approximation formula m11, when the ammonia concentration p _NH3 is less than the predetermined concentration p10, the value of the electromotive force V _NH3 with respect to the ammonia concentration p _NH3 is calculated to be larger than the high-concentration approximation formula m12. Consists of arithmetic expressions. The same applies to the relationships between other low-density approximation formulas m21, m31, m41 and high-density approximation formulas m22, m32, m42.

As shown in FIG. 2, since the gas sensor 20 of the present embodiment has the protective layer 70, the approximate expressions m11, m21, m31, and m41 for low concentration can be obtained from the arithmetic expression shown in the above expression f7. Thus, the high-concentration approximation formulas m12, m22, m32, and m42 are calculated as shown in the above formula f8. In the present embodiment, the high-concentration approximation formulas m12, m22, m32, and m42 correspond to the first calculation formula, and the low-concentration approximation formulas m11, m21, m31, and m41 correspond to the second calculation formula.

The inventors obtained each of the arithmetic expressions m1 to m4 by the following method. First, the _{electromotive} force V of the mixed potential cell 44 is The relationship between _NH3 and ammonia concentration _pNH3 was experimentally determined. The relationship between them, ammonia concentration p _NH3 is "1 [ppm]", "10 [ppm]", "50 [ppm]", "100 [ppm]", "200 [ppm]", "500 [ppm] ” was measured in each case.

Then, among the data showing the relationship between the electromotive force V _NH3 of the mixed potential cell 44 and the ammonia concentration p _NH3 measured when the oxygen concentration p _O2 is “5 [%]”, “1 [ppm]”, A low-concentration approximation formula m11 was obtained by obtaining a logarithmic approximation line for the data of "10 [ppm]" and "50 [ppm]". Also, a high-concentration approximation formula m12 was obtained by obtaining a logarithmic approximation line for the data of "100 [ppm]", "200 [ppm]", and "500 [ppm]". Then, out of the obtained low-concentration approximation formula m11 and high-concentration approximation formula m12, by using the approximation formula with the larger value of the electromotive force V _NH3 with respect to the ammonia concentration p _NH3 , the oxygen concentration p _O2 is "5 [ %]” was created. For example, when the ammonia concentration p _NH3 is less than the predetermined concentration p10, the value of the electromotive force V _NH3 with respect to the ammonia concentration p _NH3 is greater in the low-concentration approximation formula m11 than in the high-concentration approximation formula m12. Therefore, when the ammonia concentration _pNH3 is less than the predetermined concentration p10, the low-concentration approximation formula m11 is selected as the calculation formula m1. Further, when the ammonia concentration p _NH3 is equal to or higher than the predetermined concentration p10, the value of the electromotive force V _NH3 with respect to the ammonia concentration p _NH3 is larger in the high concentration approximation formula m12 than in the low concentration approximation formula m11. Therefore, when the ammonia concentration _pNH3 is equal to or higher than the predetermined concentration p10, the high-concentration approximation formula m12 is selected as the calculation formula m1. As described above, the creation of the computational expression m1 corresponding to the case where the oxygen concentration _pO2 is "5 [%]" is completed. Using a similar method, calculation formulas m2 to m4 corresponding to the cases where the oxygen concentration p _O2 is "10 [%]", "15 [%]", and "20 [%]" were also created.

The arithmetic expressions m1 to m4 obtained in this manner are stored in the memory of the SCU 22 in advance. The gas concentration calculation unit 226 selects, from the calculation formulas m1 to m4, the calculation formula corresponding to the oxygen concentration detection value _pO2 calculated based on the pump current Ip. Further, the gas concentration calculation unit 226 calculates the ammonia concentration detection value p _NH3 in the exhaust from the electromotive force V _NH3 of the mixed potential cell 44 detected by the mixed potential detection unit 224 based on the selected calculation formula.

At this time, when the oxygen concentration detection value p _O2 is an intermediate value among "5 [%]", "10 [%]", "15 [%]", and "20 [%]", There is no arithmetic expression corresponding to the detected oxygen concentration value _pO2 . In this case, the gas concentration calculator 226 creates an arithmetic expression corresponding to the detected oxygen concentration value p _O2 from the arithmetic expressions m1 to m4. For example, when the oxygen concentration detection value p _O2 is "17.5 [%]", the gas concentration calculation unit 226 calculates the calculation formula m3 corresponding to "15 [%]" and the calculation formula m3 corresponding to "20 [%]". By synthesizing the arithmetic expression m4 and the arithmetic expression m4, an arithmetic expression corresponding to the case where the detected oxygen concentration value _pO2 is "17.5 [%]" is obtained. Specifically, the gas concentration calculator 226 synthesizes the low-concentration arithmetic expressions m31 and m41 to create a low-concentration synthetic arithmetic expression, and synthesizes the high-concentration arithmetic expressions m32 and m42 to create a high-concentration arithmetic expression Create a compositing expression. In this case, the high-concentration synthesis arithmetic expression corresponds to the first synthesis arithmetic expression, and the low-concentration synthesis arithmetic expression corresponds to the second synthesis arithmetic expression. The gas concentration calculation unit 226 calculates the detected ammonia concentration value p _NH3 from the electromotive force V _NH3 of the mixed potential cell 44 using these combined calculation expressions.

According to the gas sensor 20 of the present embodiment described above, it is possible to obtain the actions and effects shown in (1) to (6) below.
(1) When the ammonia concentration p _NH3 is equal to or higher than the predetermined concentration p10, the gas concentration calculation unit 226 calculates the ammonia concentration from the electromotive force V _NH3 of the mixed potential cell 44 using the high-concentration approximate expressions m12, m22, m32, and m42. Calculate _pNH3 . When the ammonia concentration _pNH3 is less than the predetermined concentration p10, the gas concentration calculator 226 calculates low-concentration approximation formulas m11, m21, m31, and m41 that have different correlations from the high-concentration approximation formulas m12, m22, m32, and m42. is used to calculate the ammonia concentration p _NH3 from the electromotive force V _NH3 of the mixed potential cell 44 . According to such a configuration, when the ammonia concentration p NH3 is low, the ammonia concentration p _NH3 can be calculated with higher accuracy than when the ammonia concentration p _NH3 is calculated using only the high concentration approximation formulas m12, m22, m32, and m42. The concentration _pNH3 can be calculated.

(2) The gas concentration calculator 226 uses the above formula f7 as the low-concentration approximation formulas m11, m21, m31, and m41. According to this configuration, the ammonia concentration p _NH3 can be calculated with high accuracy when the ammonia concentration p _NH3 is less than the predetermined concentration p10.

(3) The gas concentration calculator 226 has a plurality of low-concentration approximation formulas m11, m21, m31, m41 and high-concentration approximation formulas m12, m22, m32, m42 corresponding to the oxygen concentration _pO2 . The gas concentration calculation unit 226 selects a low-concentration approximation formula and a high-concentration approximation formula corresponding to the oxygen concentration _pO2 detected by the pump cell 41 from those calculation formulas, and calculates the selected low-concentration approximation The ammonia concentration p _NH3 is calculated from the electromotive force V _NH3 of the mixed potential cell 44 using the formula and the approximate formula for high concentration. If there are no low-concentration approximation formulas and high-concentration approximation formulas corresponding to the oxygen concentration _pO2 , the gas concentration calculation unit 226 calculates the low-concentration approximation formulas m11, m21, m31, and m41 and the high-concentration approximation formulas m12, A synthetic arithmetic expression is prepared by synthesizing m22, m32, and m42, and the ammonia concentration _pNH3 is calculated from the electromotive force _VNH3 of the mixed potential cell 44 using the prepared synthetic arithmetic expression. According to such a configuration, even if the oxygen concentration p _O2 is not "5 [%]", "10 [%]", "15 [%]", or "20 [%]", the ammonia concentration It becomes possible to calculate _pNH3 with high accuracy.

(4) The gas to be detected by the gas sensor 20 of this embodiment is ammonia. As described above, since ammonia is a gas that is particularly susceptible to thermal decomposition and oxidation, it is of great significance to employ the configuration of this embodiment.
(5) The detection electrode 65 of the mixed potential cell 44 is mainly composed of noble metal. According to such a configuration, the chemical reaction of ammonia is promoted by the catalytic action of the noble metal, so that the detection accuracy of ammonia can be greatly improved.

(6) The gas sensor 20 further includes a protective layer 70 that protects the sensing electrode 65 of the mixed potential cell 44 . As described above, when the gas sensor 20 is provided with the protective layer 70, ammonia is likely to be oxidized and disappear.
The above embodiment can also be implemented in the following forms.

The gas concentration calculation unit 226 may use a plurality of calculation formulas as the low concentration approximation formula m11, or may use a plurality of calculation formulas as the high concentration approximation formula m12. The same applies to other low-density approximation formulas m21, m31, m41 and high-density approximation formulas m22, m32, m42.

The gas concentration calculation unit 226 replaces the low-concentration approximation formulas m11, m21, m31, m41 and the high-concentration approximation formulas m12, m22, m32, m42 with a first map corresponding to the high-concentration approximation formulas, A second map corresponding to the low-density approximation formula may also be used.
- The protective layer 70 may not be provided on the gas sensor 20 .

The mixed-potential cell 44 of the gas sensor 20 is not limited to using ammonia as a detection target gas component, and may use, for example, hydrocarbon (HC), which is a combustible gas, as a detection target gas component.
- The structure of the gas sensor 20 can be changed as appropriate. The configuration of the gas sensor 20 of the above embodiment is applicable to any gas sensor having a mixed potential cell 44 .

- The SCU 22 and its control method described in this disclosure are provided by configuring a processor and memory programmed to perform one or more functions embodied by a computer program. It may also be implemented by a dedicated computer. The SCU 22 and its control methods described in this disclosure may be implemented by a dedicated computer provided by configuring a processor that includes one or more dedicated hardware logic circuits. The SCU 22 and its control method described in this disclosure are configured by a combination of a processor and memory programmed to perform one or more functions and a processor including one or more hardware logic circuits. It may be implemented by one or more special purpose computers. The computer program may be stored as computer-executable instructions on a computer-readable non-transitional tangible storage medium. Dedicated hardware logic circuits and hardware logic circuits may be implemented by digital circuits containing multiple logic circuits or by analog circuits.

- The present disclosure is not limited to the above specific examples. Appropriate design changes made by those skilled in the art to the above specific examples are also included in the scope of the present disclosure as long as they have the features of the present disclosure. Each element included in each specific example described above, and its arrangement, conditions, shape, etc., are not limited to those illustrated and can be changed as appropriate. As long as there is no technical contradiction, the combination of the elements included in the specific examples described above can be changed as appropriate.

20: Gas sensor 41: Pump cell (oxygen concentration detector)
44: mixed potential cell 70: protective layer 65: detection electrode 66: reference electrode 226: gas concentration calculator

Claims (8)

an oxygen concentration detector (41) for detecting the concentration of oxygen contained in the gas to be measured;
A detection electrode (65) for detecting a detection target gas component and oxygen contained in the gas to be measured and generating a mixed potential, which is a potential corresponding to their concentrations, and a reference electrode (66) provided so as to be in contact with the atmosphere. and a mixed potential cell (44) in which an electromotive force corresponding to the concentration of the gas component to be detected and the concentration of oxygen in the gas to be measured is generated between the detection electrode and the reference electrode;
showing a relationship between the concentration of the detection target gas component and the electromotive force of the mixed potential cell corresponding to when the concentration of the detection target gas component is equal to or higher than a predetermined concentration, and a plurality of first calculations corresponding to the oxygen concentration; and the relationship between the concentration of the gas component to be detected and the electromotive force of the mixed potential cell corresponding to when the concentration of the gas component to be detected is less than the predetermined concentration. having different correlations and having a plurality of second arithmetic expressions corresponding to the oxygen concentration, and detected by the oxygen concentration detection unit from the plurality of the first arithmetic expressions and the plurality of the second arithmetic expressions The first arithmetic expression and the second arithmetic expression corresponding to the oxygen concentration are selected, and from the selected first arithmetic expression and the second arithmetic expression, based on the electromotive force of the mixed potential cell, and a gas concentration calculation unit (226) for calculating the concentration of the detection target gas component of
gas sensor. When the electromotive force of the mixed potential cell is "V _Gas ", the concentration of the detection target gas component is "p _Gas ", and predetermined constants are "A" and "B", As the second arithmetic expression, the following formula V _Gas =A x ln (p _Gas ) + B
The gas sensor according to claim 1. According to the first arithmetic expression, when the concentration of the gas component to be detected is equal to or higher than the predetermined concentration, the value of the electromotive force of the mixed potential cell with respect to the concentration of the gas component to be detected is larger than that of the second arithmetic expression. consists of an arithmetic expression to be operated on,
The second calculation formula is such that when the concentration of the detection target gas component is less than the predetermined concentration, the value of the electromotive force of the mixed potential cell with respect to the concentration of the detection target gas component is larger than that of the first calculation formula. The gas sensor according to claim 1 or 2, comprising an arithmetic expression to be calculated. The gas concentration calculation unit
If the first arithmetic expression and the second arithmetic expression corresponding to the oxygen concentration detected by the oxygen concentration detection unit do not exist among the plurality of first arithmetic expressions and the plurality of second arithmetic expressions, the plurality of the Synthesizing a first arithmetic expression to create a first synthetic arithmetic expression corresponding to the oxygen concentration detected by the oxygen concentration detection unit, and synthesizing a plurality of the second arithmetic expressions to form the oxygen concentration detection unit A second synthetic arithmetic expression corresponding to the oxygen concentration detected by is created, and the concentration of the detection target gas component is calculated from the electromotive force of the mixed potential cell using the first synthetic arithmetic expression and the second synthetic arithmetic expression The gas sensor according to any one of claims 1 to 3, which calculates. The gas sensor according to any one of claims 1 to 4, wherein the detection target gas component is ammonia. The gas sensor according to any one of claims 1 to 5, wherein the detection electrode is mainly composed of a noble metal. The gas sensor according to any one of claims 1 to 6, further comprising a protective layer (70) that protects the sensing electrode. an oxygen concentration detector (41) for detecting the concentration of oxygen contained in the gas to be measured;
A detection electrode (65) for detecting a detection target gas component and oxygen contained in the gas to be measured and generating a mixed potential, which is a potential corresponding to their concentrations, and a reference electrode (66) provided so as to be in contact with the atmosphere. and a mixed potential cell (44) in which an electromotive force corresponding to the concentration of the gas component to be detected and the concentration of oxygen in the gas to be measured is generated between the detection electrode and the reference electrode;
a plurality of first maps showing the relationship between the concentration of the detection target gas component and the electromotive force of the mixed potential cell corresponding to when the concentration of the detection target gas component is equal to or higher than a predetermined concentration, and corresponding to the oxygen concentration; and shows the relationship between the concentration of the detection target gas component and the electromotive force of the mixed potential cell corresponding to when the concentration of the detection target gas component is less than the predetermined concentration, and a correlation different from the first map and a plurality of second maps corresponding to the oxygen concentration, and corresponding to the oxygen concentration detected by the oxygen concentration detection unit from the plurality of first maps and the plurality of second maps selecting the first map and the second map, and determining the concentration of the detection target gas component in the gas under measurement based on the electromotive force of the mixed potential cell from the selected first map and the second map; a gas concentration calculation unit (226) that calculates
gas sensor.

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