U.S. patent application number 16/452593 was filed with the patent office on 2020-01-02 for gas sensor.
The applicant listed for this patent is NGK INSULATORS, LTD.. Invention is credited to Nobukazu IKOMA, Kunihiko NAKAGAKI, Osamu NAKASONE, Taku OKAMOTO.
Application Number | 20200003726 16/452593 |
Document ID | / |
Family ID | 68886116 |
Filed Date | 2020-01-02 |
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United States Patent
Application |
20200003726 |
Kind Code |
A1 |
NAKAGAKI; Kunihiko ; et
al. |
January 2, 2020 |
GAS SENSOR
Abstract
A gas sensor includes an element body, an adjustment pump cell,
a preliminary pump cell, a measurement electrode, a reference
electrode, a specific gas concentration detection device. The
element body provides a measurement-object gas flow section to
allow a measurement-object gas to be introduced into and flow
through the measurement-object gas flow section. The adjustment
pump cell adjusts an oxygen concentration of an oxygen
concentration adjustment chamber in the measurement-object gas flow
section. The preliminary pump cell pumps oxygen into a preliminary
chamber to prevent the measurement-object gas in a low-oxygen
atmosphere from reaching the oxygen concentration adjustment
chamber, the preliminary chamber being provided upstream of the
oxygen concentration adjustment chamber in the measurement-object
gas flow section. The measurement electrode is disposed on an inner
peripheral surface of a measurement chamber provided downstream of
the oxygen concentration adjustment chamber in the
measurement-object gas flow section.
Inventors: |
NAKAGAKI; Kunihiko; (Nagoya,
JP) ; OKAMOTO; Taku; (Nagoya, JP) ; NAKASONE;
Osamu; (Inabe, JP) ; IKOMA; Nobukazu; (Nagoya,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NGK INSULATORS, LTD. |
Nagoya |
|
JP |
|
|
Family ID: |
68886116 |
Appl. No.: |
16/452593 |
Filed: |
June 26, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01M 15/102 20130101;
G01N 27/4071 20130101; G01N 33/0037 20130101; G01M 15/104 20130101;
G01N 27/41 20130101; G01N 27/419 20130101; F01N 11/00 20130101;
F01N 2560/026 20130101 |
International
Class: |
G01N 27/41 20060101
G01N027/41; G01M 15/10 20060101 G01M015/10; F01N 11/00 20060101
F01N011/00; G01N 27/407 20060101 G01N027/407 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 2, 2018 |
JP |
2018-126301 |
Mar 27, 2019 |
JP |
2019-059954 |
Claims
1. A gas sensor comprising: an element body including an
oxygen-ion-conductive solid electrolyte layer, a measurement-object
gas flow section being provided within the element body to allow a
measurement-object gas to be introduced into the measurement-object
gas flow section and flow through the measurement-object gas flow
section; an adjustment pump cell that adjusts an oxygen
concentration of an oxygen concentration adjustment chamber, the
oxygen concentration adjustment chamber being provided in the
measurement-object gas flow section; a preliminary pump cell that
pumps oxygen into a preliminary chamber to prevent the
measurement-object gas from reaching the oxygen concentration
adjustment chamber in a state in which the measurement-object gas
is a low-oxygen atmosphere, the preliminary chamber being provided
upstream of the oxygen concentration adjustment chamber in the
measurement-object gas flow section; a measurement electrode
disposed on an inner peripheral surface of a measurement chamber,
the measurement chamber being provided downstream of the oxygen
concentration adjustment chamber in the measurement-object gas flow
section; a reference electrode that is disposed within the element
body and to which a reference gas is to be introduced, the
reference gas serving as a reference for detecting a specific gas
concentration in the measurement-object gas; a measurement voltage
detection device that detects a measurement voltage present between
the reference electrode and the measurement electrode; and a
specific gas concentration detection device that obtains, based on
the measurement voltage, a detection value according to oxygen
produced in the measurement chamber and, based on the detection
value, detects the specific gas concentration in the
measurement-object gas, the oxygen being oxygen derived from the
specific gas.
2. The gas sensor according to claim 1, further comprising a
preliminary pump control device that controls the preliminary pump
cell in a manner such that a constant preliminary pump current
flows through the preliminary pump cell.
3. The gas sensor according to claim 1, further comprising a
storage device that stores information related to a relationship
formula representing a relationship between the detection value and
the specific gas concentration, wherein regardless of whether a
measurement-object gas that is outside of the element body is a
low-oxygen atmosphere, the specific gas concentration detection
device detects the specific gas concentration by using the same
relationship formula stored in the storage device.
4. The gas sensor according to claim 1, wherein the specific gas
concentration detection device detects the specific gas
concentration, the specific gas concentration being a concentration
corrected based on an oxygen concentration of the
measurement-object gas that is outside of the element body.
5. The gas sensor according to claim 4, further comprising a
preliminary pump control device that controls the preliminary pump
cell in a manner such that a constant preliminary pump current
flows through the preliminary pump cell, and an oxygen
concentration detection device that detects the oxygen
concentration of the measurement-object gas that is outside of the
element body, the oxygen concentration being detected based on the
constant preliminary pump current, an adjustment pump current that
flows when the adjustment pump cell pumps oxygen from the oxygen
concentration adjustment chamber in a manner such that the oxygen
concentration of the oxygen concentration adjustment chamber
reaches a target concentration, and the target concentration,
wherein the specific gas concentration detection device corrects
the specific gas concentration by using the oxygen concentration
detected by the oxygen concentration detection device.
6. The gas sensor according to claim 1, further comprising a
measurement-object gas-side electrode disposed at a portion that is
to be exposed to the measurement-object gas that is outside of the
element body, wherein the preliminary pump cell pumps oxygen into
the preliminary chamber from a vicinity of the measurement-object
gas-side electrode.
7. The gas sensor according to claim 1, wherein the
measurement-object gas is an exhaust gas from an internal
combustion engine, the reference gas is air, and the preliminary
pump cell pumps oxygen into the preliminary chamber from a vicinity
of the reference electrode.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application claims priority on the basis of the
Japanese Patent Application No. 2018-126301 filed on Jul. 2, 2018,
and the Japanese Patent Application No. 2019-059954 filed on Mar.
27, 2019, the entire contents of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0002] The present invention relates to a gas sensor.
2. Description of the Related Art
[0003] In the related art, a gas sensor is known for detecting a
specific gas concentration, such as NOx, in a measurement-object
gas, such as automobile exhaust gases. For example, Patent
Literature 1 describes a gas sensor including a layered body that
includes a plurality of oxygen-ion-conductive solid electrolyte
layers and electrodes provided on the solid electrolyte layers.
When the concentration of NOx is to be detected by using this gas
sensor, first, oxygen is pumped to the inside or outside by a
measurement-object gas flow section, which is within the sensor
element, and a portion outside of the sensor element, thereby
adjusting the oxygen concentration of the measurement-object gas
flow section. Subsequently, NOx in the measurement-object gas,
which is a gas after the oxygen concentration is adjusted, is
reduced, and the concentration of NOx in the measurement-object gas
is detected based on a current that flows at an electrode
(measurement electrode) within the sensor element in accordance
with the oxygen concentration after the reduction. Furthermore,
Patent Literature 2 describes a gas sensor for detecting the
concentration of ammonia present in a measurement-object gas. The
gas sensor detects the concentration of ammonia as follows. Ammonia
is converted to NOx by being oxidized with oxygen present in the
measurement-object gas, and the concentration of NOx derived from
the ammonia is detected by using a method similar to that of Patent
Literature 1.
CITATION LIST
Patent Literature
[0004] PTL 1: JP 2014-190940 A
[0005] PTL 2: JP 2011-039041 A
SUMMARY OF THE INVENTION
[0006] To date, not many studies have been conducted on using, as a
measurement-object gas, a low-oxygen atmosphere gas (including
cases in which the measurement-object gas is a rich atmosphere gas
containing unburned fuel). Recently, the present inventors measured
a specific gas concentration in a measurement-object gas that was
in a state of a low-oxygen atmosphere and found that a measurement
accuracy decreased.
[0007] The present invention has been made to solve such a problem,
and a principal object of the present invention is to suppress a
specific gas concentration measurement accuracy from decreasing in
a case where the measurement-object gas is a low-oxygen
atmosphere.
[0008] To achieve the above-described principal object, the
following configuration is employed in the present invention.
[0009] The gas sensor of the present invention includes an element
body including an oxygen-ion-conductive solid electrolyte layer, a
measurement-object gas flow section being provided within the
element body to allow a measurement-object gas to be introduced
into the measurement-object gas flow section and flow through the
measurement-object gas flow section; an adjustment pump cell that
adjusts an oxygen concentration of an oxygen concentration
adjustment chamber, the oxygen concentration adjustment chamber
being provided in the measurement-object gas flow section; a
preliminary pump cell that pumps oxygen into a preliminary chamber
to prevent the measurement-object gas from reaching the oxygen
concentration adjustment chamber in a state in which the
measurement-object gas is a low-oxygen atmosphere, the preliminary
chamber being provided upstream of the oxygen concentration
adjustment chamber in the measurement-object gas flow section; a
measurement electrode disposed on an inner peripheral surface of a
measurement chamber, the measurement chamber being provided
downstream of the oxygen concentration adjustment chamber in the
measurement-object gas flow section; a reference electrode that is
disposed within the element body and to which a reference gas is to
be introduced, the reference gas serving as a reference for
detecting a specific gas concentration in the measurement-object
gas; a measurement voltage detection device that detects a
measurement voltage present between the reference electrode and the
measurement electrode; and a specific gas concentration detection
device that obtains, based on the measurement voltage, a detection
value according to oxygen produced in the measurement chamber and,
based on the detection value, detects the specific gas
concentration in the measurement-object gas, the oxygen being
oxygen derived from the specific gas.
[0010] In this gas sensor, a measurement-object gas is introduced
into the measurement-object gas flow section, and then the oxygen
concentration of the measurement-object gas is adjusted by the
adjustment pump cell in the oxygen concentration adjustment
chamber, and, after the adjustment, the measurement-object gas
reaches the measurement chamber. Further, the gas sensor obtains,
based on the measurement voltage, a detection value that
corresponds to oxygen produced in the measurement chamber, the
oxygen being oxygen derived from the specific gas, and based on the
obtained detection value, the gas sensor detects the specific gas
concentration in the measurement-object gas. Furthermore, to
prevent a measurement-object gas from reaching the oxygen
concentration adjustment chamber in a state in which the
measurement-object gas is a low-oxygen atmosphere, the preliminary
pump cell pumps oxygen into the preliminary chamber provided
upstream of the oxygen concentration adjustment chamber. In the gas
sensor of the present invention, since the preliminary pump cell
supplies oxygen to a measurement-object gas prior to adjustment of
the oxygen concentration as just described, it is unlikely that the
measurement-object gas will be introduced into the oxygen
concentration adjustment chamber in a state in which the
measurement-object gas is a low-oxygen atmosphere, even in a case
where the measurement-object gas is a low-oxygen atmosphere before
being introduced into the measurement-object gas flow section.
Hence, a decrease in measurement accuracy that occurs in a case
where the measurement-object gas is a low-oxygen atmosphere is
suppressed.
[0011] It is to be noted that, in the case where the specific gas
is an oxide, the phrase "oxygen produced in the measurement
chamber, the oxygen being oxygen derived from the specific gas" may
refer to oxygen produced when the specific gas itself is reduced in
the measurement chamber. In the case where the specific gas is a
non-oxide gas, the phrase "oxygen produced in the measurement
chamber, the oxygen being oxygen derived from the specific gas" may
refer to oxygen produced when the specific gas is converted into an
oxide and the resulting gas is reduced in the measurement chamber.
Furthermore, the specific gas concentration detection device may
obtain the detection value as follows. Based on the measurement
voltage, oxygen produced in the measurement chamber may be pumped
from the measurement chamber to the outside, the oxygen being
oxygen derived from the specific gas, so that the oxygen
concentration in the measurement chamber can reach a predetermined
low concentration. When the pumping is performed, a measurement
pump current flows. The measurement pump current may be the
detection value. The element body may be a layered body including a
plurality of stacked oxygen-ion-conductive solid electrolyte
layers.
[0012] The gas sensor of the present invention may further include
a preliminary pump control device. The preliminary pump control
device may control the preliminary pump cell in a manner such that
a constant preliminary pump current flows through the preliminary
pump cell. With this configuration, oxygen can be supplied, by
performing a relatively simple control, to a measurement-object gas
that is a low-oxygen atmosphere in the preliminary chamber.
[0013] The gas sensor of the present invention may further include
a storage device. The storage device may store information related
to a relationship formula representing a relationship between the
detection value and the specific gas concentration. Regardless of
whether a measurement-object gas that is outside of the element
body is a low-oxygen atmosphere, the specific gas concentration
detection device may detect the specific gas concentration by using
the relationship formula stored in the storage device, the
relationship formula being a common formula. In this manner, the
gas sensor of the present invention can detect the specific gas
concentration accurately without using different relationship
formulas for the case in which the measurement-object gas is a
low-oxygen atmosphere and for the case in which the
measurement-object gas is not a low-oxygen atmosphere. Hence, the
gas sensor can detect the specific gas concentration readily and
accurately.
[0014] In the gas sensor of the present invention, the specific gas
concentration detection device detects the specific gas
concentration, and the specific gas concentration may be a
concentration corrected based on an oxygen concentration of the
measurement-object gas that is outside of the element body. It is
to be noted that, even in the case where the actual specific gas
concentration (real concentration) in a measurement-object gas is
uniform, the detection value may change with the oxygen
concentration of a measurement-object gas that is outside of the
element body, and in this case, the specific gas concentration
measured based on the detection value also changes. Accordingly, by
detecting the specific gas concentration by involving the
oxygen-concentration-based correction, a specific gas concentration
measurement accuracy is improved. The phrase "detect the specific
gas concentration, and the specific gas concentration is a
concentration corrected based on an oxygen concentration of the
measurement-object gas" encompasses the following: cases in which
the specific gas concentration is detected based on a detection
value obtained after the oxygen-concentration-based correction; and
cases in which, when detecting the specific gas concentration based
on the detection value, the oxygen-concentration-based correction
is performed, and the corrected specific gas concentration is
detected.
[0015] In this case, the gas sensor of the present invention may
further include a preliminary pump control device and an oxygen
concentration detection device. The preliminary pump control device
may control the preliminary pump cell in a manner such that a
constant preliminary pump current flows through the preliminary
pump cell. The oxygen concentration detection device may detect the
oxygen concentration of the measurement-object gas that is outside
of the element body, the oxygen concentration being detected based
on the constant preliminary pump current, an adjustment pump
current that flows when the adjustment pump cell pumps oxygen from
the oxygen concentration adjustment chamber in a manner such that
the oxygen concentration of the oxygen concentration adjustment
chamber reaches a target concentration, and the target
concentration. The specific gas concentration detection device may
correct the specific gas concentration by using the oxygen
concentration detected by the oxygen concentration detection
device. It is to be noted that the constant preliminary pump
current that flows through the preliminary pump cell corresponds to
the flow rate of oxygen pumped into the measurement-object gas flow
section by the preliminary pump cell. Furthermore, the adjustment
pump current corresponds to the flow rate of oxygen pumped from the
oxygen concentration adjustment chamber. Hence, the oxygen
concentration of a measurement-object gas outside of the element
body can be detected based on the currents and the target
concentration. That is, the oxygen concentration necessary for
correction can be detected by the gas sensor of the present
invention.
[0016] The gas sensor of the present invention may further include
a measurement-object gas-side electrode disposed at a portion that
is to be exposed to the measurement-object gas that is outside of
the element body. The preliminary pump cell may pump oxygen into
the preliminary chamber from a vicinity of the measurement-object
gas-side electrode. With this configuration, the following is
possible. In comparison with, for example, a case in which oxygen
is pumped into the preliminary chamber from a vicinity of the
reference electrode, a decrease in measurement accuracy that may
occur when the potential of the reference electrode changes as a
result of a voltage drop due to the current during pumping is
suppressed.
[0017] In the gas sensor of the present invention, the
measurement-object gas may be an exhaust gas from an internal
combustion engine, the reference gas may be air, and the
preliminary pump cell may pump oxygen into the preliminary chamber
from a vicinity of the reference electrode. With this
configuration, the following is possible. In comparison with, for
example, a case in which oxygen is pumped to the inside from
exhaust gases that are outside of the element body, oxygen can be
pumped into the preliminary chamber at a low applied voltage
because air has a higher oxygen concentration than exhaust
gases.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic cross-sectional view of a gas sensor
100.
[0019] FIG. 2 is a block diagram illustrating an electrical
connection relationship between a controller 90 and individual
cells.
[0020] FIG. 3 is a graph illustrating a relationship between the
oxygen concentration of a measurement-object gas and a pump current
Ip0.
[0021] FIG. 4 is a graph illustrating a relationship between the
oxygen concentration of a measurement-object gas and a pump current
Ip2.
[0022] FIG. 5 is a graph illustrating in enlarged view a region of
FIG. 4 corresponding to oxygen concentrations of 10 vol % or
less.
[0023] FIG. 6 is a graph illustrating temporal changes in the pump
current in a case where a target value Ip0s* is 0 mA.
[0024] FIG. 7 is a graph illustrating temporal changes in the pump
current in a case where the target value Ip0s* is 1 mA.
[0025] FIG. 8 is a schematic cross-sectional view of a sensor
element 201.
[0026] FIG. 9 is a graph illustrating a relationship between the
A/F ratio of a measurement-object gas and a pump current Ip2.
[0027] FIG. 10 is a graph illustrating temporal changes in a
sensitivity change ratio in a case where the target value Ip0s* is
0 mA.
[0028] FIG. 11 is a graph illustrating temporal changes in the
sensitivity change ratio in a case where the target value Ip0s* is
1 mA.
DETAILED DESCRIPTION OF THE INVENTION
[0029] An embodiment of the present invention will now be described
with reference to the drawings. FIG. 1 is a schematic
cross-sectional view schematically illustrating an example of a
configuration of a gas sensor 100, which is an embodiment of the
present invention. FIG. 2 is a block diagram illustrating an
electrical connection relationship between a controller 90 and
individual cells. The gas sensor 100 is attached to, for example, a
pipe such as an exhaust gas pipe of an internal combustion engine,
examples of which include gasoline engines and diesel engines. The
gas sensor 100 detects the specific gas concentration in a
measurement-object gas, which is exhaust gases from an internal
combustion engine. Examples of the specific gas include NOx and
ammonia. In the present embodiment, the specific gas concentration
measured by the gas sensor 100 is the concentration of NOx. The gas
sensor 100 includes a sensor element 101, cells 15, 21, 41, 50, and
80 to 83, and a controller 90. The sensor element 101 has an
elongated parallelepiped shape. Each of the cells 15, 21, 41, 50,
and 80 to 83 includes a portion of the sensor element 101. The
controller 90 controls the gas sensor 100 as a whole.
[0030] The sensor element 101 is an element including a layered
body in which six layers, namely a first substrate layer 1, a
second substrate layer 2, a third substrate layer 3, a first solid
electrolyte layer 4, a spacer layer 5, and a second solid
electrolyte layer 6, are layered in this order from the bottom
side, as viewed in the drawing. Each of the six layers is formed of
an oxygen-ion-conductive solid electrolyte layer containing, for
example, zirconia (ZrO.sub.2). Furthermore, the solid electrolyte
forming each of the six layers is dense and gas-tight. The sensor
element 101 is produced as follows, for example. Ceramic green
sheets corresponding to the respective layers are subjected to, for
example, a predetermined process and circuit pattern printing. The
resulting sheets are then layered together and subjected to firing
to be unified.
[0031] On the front side (left side of FIG. 1) of the sensor
element 101, a gas inlet port 10, a first diffusion-rate-limiting
portion 11, a buffer space 12, a second diffusion-rate-limiting
portion 13, a first internal space 20, a third
diffusion-rate-limiting portion 30, a second internal space 40, a
fourth diffusion-rate-limiting portion 60, and a third internal
space 61 are formed adjacent to one another in such a manner as to
be in communication with one another in this order, between the
lower surface of the second solid electrolyte layer 6 and the upper
surface of the first solid electrolyte layer 4.
[0032] The gas inlet port 10, the buffer space 12, the first
internal space 20, the second internal space 40, and the third
internal space 61 constitute a space within the sensor element 101.
The space is provided in such a manner that a portion of the spacer
layer 5 is hollowed out. The top of the space is defined by the
lower surface of the second solid electrolyte layer 6, the bottom
of the space is defined by the upper surface of the first solid
electrolyte layer 4, and sides of the space are defined by side
surfaces of the spacer layer 5.
[0033] The first diffusion-rate-limiting portion 11, the second
diffusion-rate-limiting portion 13, and the third
diffusion-rate-limiting portion 30 are each provided as two
horizontally extending slits (whose openings have a longitudinal
direction in a direction perpendicular to the drawing).
Furthermore, the fourth diffusion-rate-limiting portion 60 is
provided as one horizontally extending slit (whose opening has a
longitudinal direction in a direction perpendicular to the
drawing), which is formed as a gap with respect to the lower
surface of the second solid electrolyte layer 6. Note that the
region extending from the gas inlet port 10 to the third internal
space 61 is also referred to as a "measurement-object gas flow
section".
[0034] Furthermore, a reference gas introduction space 43 is
provided at a position farther from the front side than is the
measurement-object gas flow section. The position is between the
upper surface of the third substrate layer 3 and the lower surface
of the spacer layer 5, and a side of the reference gas introduction
space 43 is defined by a side surface of the first solid
electrolyte layer 4. A reference gas for measuring the
concentration of NOx is introduced into the reference gas
introduction space 43. Examples of the reference gas include
air.
[0035] An air introduction layer 48 is a layer formed of a porous
ceramic material. A reference gas can be introduced into the air
introduction layer 48 through the reference gas introduction space
43. Furthermore, the air introduction layer 48 is formed to cover
the reference electrode 42.
[0036] The reference electrode 42 is an electrode formed in such a
manner as to be sandwiched between the upper surface of the third
substrate layer 3 and the first solid electrolyte layer 4. As
described above, the air introduction layer 48, which is coupled to
the reference gas introduction space 43, is provided in a vicinity
of the reference electrode 42. Furthermore, as will be described
later, the reference electrode 42 can be used to measure the oxygen
concentrations (oxygen partial pressures) of the first internal
space 20, the second internal space 40, and the third internal
space 61. The reference electrode 42 is formed as a porous cermet
electrode (e.g., cermet electrode containing Pt and ZrO.sub.2).
[0037] In the measurement-object gas flow section, the gas inlet
port 10 is a portion open to an external space. A
measurement-object gas can be drawn into the sensor element 101
through the gas inlet port 10 from an external space. The first
diffusion-rate-limiting portion 11 is a portion that imparts a
predetermined diffusion resistance to the measurement-object gas
drawn in through the gas inlet port 10. The buffer space 12 is a
space provided to guide the measurement-object gas introduced from
the first diffusion-rate-limiting portion 11 to the second
diffusion-rate-limiting portion 13. The buffer space 12 also serves
as a space (preliminary chamber) for pumping oxygen into the
measurement-object gas introduced through the first
diffusion-rate-limiting portion 11. Pumping of oxygen into the
buffer space 12 is carried out by the operation of a preliminary
pump cell 15. The second diffusion-rate-limiting portion 13 is a
portion that imparts a predetermined diffusion resistance to the
measurement-object gas, which is introduced into the first internal
space 20 from the buffer space 12. The measurement-object gas is
introduced from outside of the sensor element 101 into the first
internal space 20 as follows. Upon pressure fluctuations of a
measurement-object gas in an external space (exhaust gas pressure
pulsations in the case where the measurement-object gas is an
automobile exhaust gas), the measurement-object gas is rapidly
drawn into the sensor element 101 through the gas inlet port 10.
Then, the measurement-object gas is not directly introduced into
the first internal space 20 but introduced into the first internal
space 20 after concentration variations of the measurement-object
gas are eliminated by the first diffusion-rate-limiting portion 11,
the buffer space 12, and the second diffusion-rate-limiting portion
13. As a result, concentration variations of the measurement-object
gas, when being introduced into the first internal space 20, are
substantially negligible. The first internal space 20 is provided
as a space for adjusting the partial pressure of oxygen present in
the measurement-object gas, which is introduced through the second
diffusion-rate-limiting portion 13. The oxygen partial pressure is
adjusted by the operation of a main pump cell 21.
[0038] The preliminary pump cell 15 is an electrochemical pump cell
including a preliminary pump electrode 16, an outer pump electrode
23, and the second solid electrolyte layer 6, which is sandwiched
between the electrodes. The preliminary pump electrode 16 is
provided on substantially the entire surface of a portion of the
lower surface of the second solid electrolyte layer 6, the portion
facing the buffer space 12. The outer pump electrode 23 is disposed
on a portion that is to be exposed to a measurement-object gas that
is outside of the sensor element 101. Of a plurality of electrodes
in the measurement-object gas flow section, the preliminary pump
electrode 16 is an electrode disposed most upstream. A pump voltage
Vp0s can be applied by a variable power supply 17, which is
provided between the preliminary pump electrode 16 and the outer
pump electrode 23, thereby passing a pump current Ip0s between the
preliminary pump electrode 16 and the outer pump electrode 23. This
enables the preliminary pump cell 15 to pump oxygen from an
external space into the buffer space 12.
[0039] The main pump cell 21 is an electrochemical pump cell
including an inner pump electrode 22, the outer pump electrode 23,
and the second solid electrolyte layer 6, which is sandwiched
between the electrodes. The inner pump electrode 22 includes a
ceiling electrode portion 22a, which is provided on substantially
the entire surface of a portion of the lower surface of the second
solid electrolyte layer 6, the portion facing the first internal
space 20. The outer pump electrode 23 is provided on a region of
the upper surface of the second solid electrolyte layer 6, the
region corresponding to the ceiling electrode portion 22a. The
outer pump electrode 23 is provided in such a manner as to be
exposed to an external space.
[0040] The inner pump electrode 22 is formed to extend along
portions of the upper and lower solid electrolyte layers (second
solid electrolyte layer 6 and first solid electrolyte layer 4),
which define the first internal space 20, and along portions of the
spacer layer 5, which serve as side walls. Specifically, the
ceiling electrode portion 22a is formed on a portion of the lower
surface of the second solid electrolyte layer 6, the portion
serving as a ceiling surface of the first internal space 20; a
bottom electrode portion 22b is formed on a portion of the upper
surface of the first solid electrolyte layer 4, the portion serving
as a bottom surface of the first internal space 20; side electrode
portions (not illustrated) are formed on portions of the side wall
surfaces (inner surfaces) of the spacer layer 5, the portions
forming respective side wall portions of the first internal space
20, the side electrode portions connecting the ceiling electrode
portion 22a to the bottom electrode portion 22b; and thus, in the
region where the side electrode portions are disposed, the
structure has a shape of a tunnel.
[0041] The inner pump electrode 22 and the outer pump electrode 23
are each formed as a porous cermet electrode (e.g., cermet
electrode containing Pt and ZrO.sub.2 and containing 1% Au). Note
that the inner pump electrode 22, which comes into contact with a
measurement-object gas, is formed by using a material in which a
reduction ability that is exhibited to a NOx component present in a
measurement-object gas is decreased.
[0042] The main pump cell 21 can pump oxygen from the first
internal space 20 to an external space and can pump oxygen from an
external space into the first internal space 20. This can be
carried out by applying a desired pump voltage Vp0 between the
inner pump electrode 22 and the outer pump electrode 23, thereby
passing a pump current Ip0 in the positive or negative direction
between the inner pump electrode 22 and the outer pump electrode
23.
[0043] Furthermore, an electrochemical sensor cell, namely, an
oxygen partial pressure detection sensor cell 80 for controlling
the main pump is configured to detect the oxygen concentration
(oxygen partial pressure) of the atmosphere in the first internal
space 20. The electrochemical sensor cell includes the inner pump
electrode 22, the second solid electrolyte layer 6, the spacer
layer 5, the first solid electrolyte layer 4, the third substrate
layer 3, and the reference electrode 42.
[0044] The oxygen concentration (oxygen partial pressure) in the
first internal space 20 can be determined by measuring an
electromotive force V0 of the oxygen partial pressure detection
sensor cell 80 for controlling the main pump. In addition, the pump
voltage Vp0 of a variable power supply 24 is feedback-controlled in
a manner such that the electromotive force V0 becomes a constant
electromotive force, thereby controlling the pump current Ip0. As a
result, the oxygen concentration in the first internal space 20 can
be maintained at a predetermined constant value.
[0045] The third diffusion-rate-limiting portion 30 is a portion
that imparts a predetermined diffusion resistance to the
measurement-object gas, which has an oxygen concentration (oxygen
partial pressure) controlled in the first internal space 20 by the
operation of the main pump cell 21, and guides the
measurement-object gas to the second internal space 40.
[0046] The second internal space 40 is provided as a space for
further adjusting, by using an auxiliary pump cell 50, the oxygen
partial pressure of the measurement-object gas, which is introduced
into the second internal space 40 through the third
diffusion-rate-limiting portion 30 after the oxygen concentration
(oxygen partial pressure) is adjusted in advance in the first
internal space 20. With this configuration, the oxygen
concentration in the second internal space 40 is maintained at a
constant concentration precisely; hence, with the gas sensor 100,
the concentration of NOx can be measured accurately.
[0047] The auxiliary pump cell 50 is an auxiliary electrochemical
pump cell including an auxiliary pump electrode 51, an outer pump
electrode 23, and the second solid electrolyte layer 6 (the outer
pump electrode 23 here is not limited to the outer pump electrode
23 described above, and it is sufficient that the electrode be a
suitable electrode positioned outside of the sensor element 101).
The auxiliary pump electrode 51 includes a ceiling electrode
portion 51a, which is provided on substantially the entire surface
of a portion of the lower surface of the second solid electrolyte
layer 6, the portion facing the second internal space 40.
[0048] The auxiliary pump electrode 51 is disposed in the second
internal space 40, and the structure of the auxiliary pump
electrode 51 has a shape of a tunnel similar to that of the inner
pump electrode 22, which is disposed in the first internal space 20
as described above. That is, the ceiling electrode portion 51a is
formed on a portion of the second solid electrolyte layer 6, the
portion serving as a ceiling surface of the second internal space
40; a bottom electrode portion 51b is formed on a portion of the
first solid electrolyte layer 4, the portion serving as a bottom
surface of the second internal space 40; side electrode portions
(not illustrated) are formed on portions of the respective wall
surfaces of the spacer layer 5, the portions serving as side walls
of the second internal space 40, the side electrode portions
coupling the ceiling electrode portion 51a to the bottom electrode
portion 51b; and thus, the structure has a shape of a tunnel. Note
that, as with the inner pump electrode 22, the auxiliary pump
electrode 51, too, is formed by using a material in which a
reduction ability that is exhibited to a NOx component present in a
measurement-object gas is decreased.
[0049] The auxiliary pump cell 50 can pump oxygen present in the
atmosphere of the second internal space 40 to an external space and
can pump oxygen from an external space into the second internal
space 40. This can be carried out by applying a desired voltage Vp1
between the auxiliary pump electrode 51 and the outer pump
electrode 23.
[0050] Furthermore, an electrochemical sensor cell, namely, an
oxygen partial pressure detection sensor cell 81 for controlling
the auxiliary pump is configured to control the oxygen partial
pressure of the atmosphere in the second internal space 40. The
electrochemical sensor cell includes the auxiliary pump electrode
51, the reference electrode 42, the second solid electrolyte layer
6, the spacer layer 5, the first solid electrolyte layer 4, and the
third substrate layer 3.
[0051] Note that the auxiliary pump cell 50 performs pumping with a
variable power supply 52, the voltage of the power supply 52 being
controlled based on an electromotive force V1, which is detected by
the oxygen partial pressure detection sensor cell 81 for
controlling the auxiliary pump. With this configuration, the
partial pressure of oxygen present in the atmosphere of the second
internal space 40 can be controlled to a low partial pressure that
has substantially no influence on the measurement of NOx.
[0052] Furthermore, in addition to this, a pump current Ip1 is used
to control the electromotive force of the oxygen partial pressure
detection sensor cell 80 for controlling the main pump.
Specifically, the pump current Ip1, which is a control signal, is
input into the oxygen partial pressure detection sensor cell 80 for
controlling the main pump, thereby controlling the electromotive
force V0, and accordingly, the gradient of the partial pressure of
oxygen present in the measurement-object gas, which is introduced
into the second internal space 40 through the third
diffusion-rate-limiting portion 30, is controlled to be
consistently a constant gradient. In the case where the gas sensor
is used as a NOx sensor, the oxygen concentration in the second
internal space 40 is maintained at a constant value of
approximately 0.001 ppm by the operation of the main pump cell 21
and the auxiliary pump cell 50.
[0053] The fourth diffusion-rate-limiting portion 60 is a portion
that imparts a predetermined diffusion resistance to the
measurement-object gas, which has an oxygen concentration (oxygen
partial pressure) controlled in the second internal space 40 by the
operation of the auxiliary pump cell 50, and guides the
measurement-object gas to the third internal space 61. The fourth
diffusion-rate-limiting portion 60 has a function of limiting the
amount of NOx flowing into the third internal space 61.
[0054] The third internal space 61 is provided as a space for
performing a process related to the measurement of the
concentration of nitrogen oxide (NOx) present in the
measurement-object gas, the measurement-object gas being introduced
into the third internal space 61 through the fourth
diffusion-rate-limiting portion 60 after the oxygen concentration
(oxygen partial pressure) is adjusted in advance in the second
internal space 40. The measurement of the concentration of NOx is
carried out primarily by the operation of a measurement pump cell
41 in the third internal space 61.
[0055] The measurement pump cell 41 measures the concentration of
NOx present in the measurement-object gas in the third internal
space 61. The measurement pump cell 41 is an electrochemical pump
cell including a measurement electrode 44, the outer pump electrode
23, the second solid electrolyte layer 6, the spacer layer 5, and
the first solid electrolyte layer 4. The measurement electrode 44
is provided on a portion of the upper surface of the first solid
electrolyte layer 4, the portion facing the third internal space
61. The measurement electrode 44 is a porous cermet electrode
formed of a material in which a reduction ability that is exhibited
to a NOx component present in a measurement-object gas is increased
compared with the material of the inner pump electrode 22. The
measurement electrode 44 also functions as a NOx reduction catalyst
that reduces NOx present in the atmosphere of the third internal
space 61.
[0056] The measurement pump cell 41 can detect a pump current Ip2
by pumping out oxygen produced by decomposition of nitrogen oxide
in the atmosphere of a vicinity of the measurement electrode 44 and
determining the pump current Ip2 as the amount of the oxygen
produced.
[0057] Furthermore, an electrochemical sensor cell, namely, an
oxygen partial pressure detection sensor cell 82 for controlling
the measurement pump is configured to detect the oxygen partial
pressure of a vicinity of the measurement electrode 44. The
electrochemical sensor cell includes the first solid electrolyte
layer 4, the third substrate layer 3, the measurement electrode 44,
and the reference electrode 42. A variable power supply 46 is
controlled based on an electromotive force V2, which is detected by
the oxygen partial pressure detection sensor cell 82 for
controlling the measurement pump.
[0058] After being guided into the second internal space 40, the
measurement-object gas flows through the fourth
diffusion-rate-limiting portion 60 in a situation in which the
oxygen partial pressure is controlled and reaches the measurement
electrode 44, which is within the third internal space 61. Nitrogen
oxide in the measurement-object gas in a vicinity of the
measurement electrode 44 is reduced (2NO.fwdarw.N.sub.2+O.sub.2) to
produce oxygen. The produced oxygen is then pumped by the
measurement pump cell 41. At that time, a voltage Vp2 of the
variable power supply 46 is controlled in a manner such that the
electromotive force V2, which is detected by the oxygen partial
pressure detection sensor cell 82 for controlling the measurement
pump, becomes a constant electromotive force. The amount of oxygen
produced in a vicinity of the measurement electrode 44 is
proportional to the concentration of nitrogen oxide present in the
measurement-object gas. Accordingly, the concentration of nitrogen
oxide present in the measurement-object gas is calculated by using
the pump current Ip2 of the measurement pump cell 41.
[0059] Furthermore, an electrochemical sensor cell 83, which
includes the second solid electrolyte layer 6, the spacer layer 5,
the first solid electrolyte layer 4, the third substrate layer 3,
the outer pump electrode 23, and the reference electrode 42, is
configured. The partial pressure of oxygen present in a
measurement-object gas outside of the sensor can be detected based
on an electromotive force Vref, which is obtained by the sensor
cell 83.
[0060] In the gas sensor 100, which is configured as described
above, as a result of the operation of the main pump cell 21 and
the auxiliary pump cell 50, the measurement-object gas has an
oxygen partial pressure consistently maintained at a constant low
value (value that has substantially no influence on the measurement
of NOx), and, in this state, the measurement-object gas is provided
to the measurement pump cell 41. Accordingly, the concentration of
NOx present in the measurement-object gas can be determined based
on the pump current Ip2, which flows when oxygen that is produced,
by reduction of NOx, substantially proportionally to the
concentration of NOx in the measurement-object gas, is pumped to
the outside by the measurement pump cell 41.
[0061] In addition, to enhance the oxygen ion conductivity of the
solid electrolyte, the sensor element 101 includes a heater unit
70, which serves to perform temperature adjustment for heating the
sensor element 101 and maintaining the temperature. The heater unit
70 includes a heater connector electrode 71, a heater 72, a through
hole 73, a heater insulating layer 74, and a pressure release hole
75.
[0062] The heater connector electrode 71 is an electrode formed in
such a manner as to be in contact with the lower surface of the
first substrate layer 1. Power can be supplied to the heater unit
70 from outside by connecting the hater connector electrode 71 to
an external power source.
[0063] The heater 72 is an electrical resistor formed in such a
manner as to be sandwiched by the second substrate layer 2 and the
third substrate layer 3 from above and below. The heater 72 is
connected to the heater connector electrode 71 via the through hole
73 and generates heat upon receiving power from outside via the
heater connector electrode 71, thereby heating the solid
electrolyte forming the sensor element 101 and maintaining the
temperature.
[0064] Furthermore, the heater 72 is embedded over an entire area
extending from the first internal space 20 to the third internal
space 61 and therefore can adjust the temperature of the sensor
element 101 as a whole to a temperature at which the solid
electrolyte becomes active.
[0065] The heater insulating layer 74 is an insulating layer
disposed adjacent to upper and lower surfaces of the heater 72 and
formed of an insulating material, such as alumina. The heater
insulating layer 74 is formed to provide electrical insulation
between the second substrate layer 2 and the heater 72 and
electrical insulation between the third substrate layer 3 and the
heater 72.
[0066] The pressure release hole 75 is a portion provided to extend
through the third substrate layer 3 and the air introduction layer
48 and to be in communication with the reference gas introduction
space 43. The pressure release hole 75 is formed to mitigate an
increase in internal pressure due to a temperature increase within
the heater insulating layer 74.
[0067] The controller 90 is a microprocessor including a CPU 92, a
memory 94, and the like. The controller 90 inputs the electromotive
force V0, the electromotive force V1, the electromotive force V2,
the electromotive force Vref, the pump current Ip0s, the pump
current Ip0, the pump current Ip1, and the pump current Ip2. The
electromotive force V0 is detected by the oxygen partial pressure
detection sensor cell 80 for controlling the main pump. The
electromotive force V1 is detected by the oxygen partial pressure
detection sensor cell 81 for controlling the auxiliary pump. The
electromotive force V2 is detected by the oxygen partial pressure
detection sensor cell 82 for controlling the measurement pump. The
electromotive force Vref is detected by the sensor cell 83. The
pump current Ip0s is detected by the preliminary pump cell 15. The
pump current Ip0 is detected by the main pump cell 21. The pump
current Ip1 is detected by the auxiliary pump cell 50. The pump
current Ip2 is detected by the measurement pump cell 41.
Furthermore, the controller 90 outputs control signals to the
variable power supply 17 of the preliminary pump cell 15, the
variable power supply 24 of the main pump cell 21, the variable
power supply 52 of the auxiliary pump cell 50, and the variable
power supply 46 of the measurement pump cell 41.
[0068] The controller 90 feedback-controls the voltage Vp0s of the
variable power supply 17 in a manner such that the pump current
Ip0s of the preliminary pump cell 15 reaches a target value Ip0s*.
The controller 90 controls the voltage Vp0s in a manner such that
oxygen is pumped into the buffer space 12 and does not control the
voltage Vp0s in a manner such that oxygen is pumped from the buffer
space 12. Furthermore, in the present embodiment, for the
controller 90, the target value Ip0s* is set to a constant value.
The target value Ip0s* is set to a value such that, even in a case
where a measurement-object gas outside of the sensor element 101 is
a low-oxygen atmosphere (e.g., atmosphere having an oxygen
concentration less than or equal to 0.1 vol %, less than 0.2 vol %,
less than 1 vol %, or the like), the measurement-object gas after
oxygen is pumped to the inside by the preliminary pump cell 15
(i.e., the measurement-object gas to be introduced into the first
internal space 20) is not a low-oxygen atmosphere. It is to be
noted that, in a case where the air-fuel ratio of the
measurement-object gas is lower than the stoichiometric air-fuel
ratio, that is, the measurement-object gas is a rich atmosphere,
unburned fuel is included in the measurement-object gas, and
therefore, the oxygen concentration can be determined by the amount
of oxygen necessary to sufficiently combust the fuel. In this case,
the oxygen concentration is expressed as a negative value.
Accordingly, the target value Ip0s* is set in the following manner,
for example. First, a minimum oxygen concentration of exhaust gases
from an internal combustion engine that uses the gas sensor 100 is
investigated in advance. The minimum oxygen concentration is a
minimum among oxygen concentrations in various operation conditions
(in some cases, the oxygen concentration may decrease to a negative
value).
[0069] Subsequently, the target value Ip0s* is set based on the
amount of oxygen necessary to increase the minimum oxygen
concentration of the measurement-object gas to an oxygen
concentration higher than the oxygen concentration of a low-oxygen
atmosphere (e.g. oxygen concentration greater than 0.1 vol %,
greater than or equal to 0.2 vol %, greater than or equal to 1 vol
%, or the like). Since the target value Ip0s* is set to a constant
value, the controller 90 controls the preliminary pump cell 15 in a
manner such that a constant flow rate of oxygen is pumped into the
buffer space 12. The value of the target value Ip0s* may be
appropriately set based on an experiment as described above. For
example, the target value Ip0s* may be 0.5 mA or greater and 3 mA
or less.
[0070] The controller 90 feedback-controls the pump voltage Vp0 of
the variable power supply 24 in a manner such that the
electromotive force V0 reaches a target value (referred to as a
"target value V0*") (i.e., in a manner such that the oxygen
concentration in the first internal space 20 becomes a constant
target concentration).
[0071] Accordingly, the pump current Ip0 changes in accordance with
the concentration of oxygen present in the measurement-object gas
and the flow rate of oxygen being pumped to the inside by the
preliminary pump cell 15.
[0072] Furthermore, the controller 90 feedback-controls the voltage
Vp1 of the variable power supply 52 in a manner such that the
electromotive force V1 reaches a constant value (referred to as
"target value V1*") (i.e., in a manner such that the oxygen
concentration in the second internal space 40 becomes a
predetermined low-oxygen concentration that has substantially no
influence on the measurement of NOx). In addition, the controller
90 sets (feedback-controls) the target value V0* of the
electromotive force V0 based on the pump current Ip1 in a manner
such that the pump current Ip1, which flows due to the voltage Vp1,
reaches a constant value (referred to as a "target value Ip1*"). As
a result, the gradient of the partial pressure of oxygen present in
the measurement-object gas to be introduced into the second
internal space 40 through the third diffusion-rate-limiting portion
30 is consistently a constant gradient. Furthermore, the partial
pressure of oxygen present in the atmosphere of the second internal
space 40 is controlled to a low partial pressure that has
substantially no influence on the measurement of NOx.
[0073] Further, the controller 90 feedback-controls the voltage Vp2
of the variable power supply 46 in a manner such that the
electromotive force V2 reaches a constant value (referred to as a
"target value V2*") (i.e., in a manner such that the oxygen
concentration in the third internal space 61 becomes a
predetermined low concentration). Accordingly, oxygen is pumped
from the third internal space 61 in a manner such that the amount
of oxygen, which is produced when NOx in the measurement-object gas
is reduced in the third internal space 61, becomes substantially
zero. Subsequently, the controller 90 obtains the pump current Ip2
and, based on the pump current Ip2, calculates the concentration of
NOx present in the measurement-object gas. The pump current Ip2 is
a detection value corresponding to the oxygen produced in the third
internal space 61, the oxygen being oxygen derived from a specific
gas (in this case, NOx).
[0074] A relationship formula representing a relationship between
the pump current Ip2 and the concentration of NOx is stored in the
memory 94. The relationship formula may be, for example, a linear
function formula. The relationship formula can be determined by
experimentation in advance.
[0075] An example of usage of the gas sensor 100, which is
configured as described above, will be described below. Assume that
the CPU 92 of the controller 90 is in a state in which the CPU 92
is controlling the above-described pump cells 15, 21, 41, and 50
and obtaining the voltages V0, V1, V2, and Vref from the
above-described sensor cells 80 to 83. In this state, when the
measurement-object gas is introduced through the gas inlet port 10,
the measurement-object gas, first, passes through the first
diffusion-rate-limiting portion 11 and is then introduced into the
buffer space 12, and, in the buffer space 12, oxygen is pumped into
the measurement-object gas by the preliminary pump cell 15.
Thereafter, the measurement-object gas, which contains the pumped
oxygen, reaches the first internal space 20. Next, in the first
internal space 20 and the second internal space 40, the oxygen
concentration of the measurement-object gas is adjusted by the main
pump cell 21 and the auxiliary pump cell 50, and the adjusted
measurement-object gas reaches the third internal space 61.
Subsequently, based on the obtained pump current Ip2 and the
relationship formula stored in the memory 94, the CPU 92 detects
the concentration of NOx present in the measurement-object gas.
[0076] Thus, oxygen is pumped into the buffer space 12 by the
preliminary pump cell 15 for the purpose of, as described above,
suppressing the measurement-object gas from being introduced into
the first internal space 20 in a state in which the
measurement-object gas is a low-oxygen atmosphere. Reasons for
doing this will be described. The present inventors investigated
pump currents Ip0 and pump currents Ip2 obtained by varying the
value of the oxygen concentration of the measurement-object gas and
the value of the target value Ip0s*. The measurement-object gas was
a gas before being introduced into the gas inlet port 10. The
measurement-object gas used was an adjusted model gas. In the model
gas, the base gas was nitrogen, the specific gas component was 500
ppm NO, and the fuel gas was 1000 ppm carbon monoxide gas and 1000
ppm ethylene gas. The model gas was adjusted such that the water
concentration was 5 vol % and the oxygen concentration was 0.005 to
20 vol %. The temperature of the model gas was 250.degree. C., and
the model gas was flowed through a pipe having a diameter of 20 mm
at a flow rate of 50 L/min.
[0077] FIG. 3 is a graph illustrating a relationship between the
oxygen concentration of a measurement-object gas and the pump
current Ip0. FIG. 3 illustrates cases in which the target value
Ip0s* was 0 mA, the target value Ip0s* was 1 mA, and the target
value Ip0s* was 2 mA. The left graph of FIG. 3 is an enlarged view
of the portion of the right graph of FIG. 3 encircled by a dashed
line (in the left graph, the horizontal axis is on a log scale).
FIG. 4 is a graph illustrating a relationship between the oxygen
concentration of a measurement-object gas and the pump current Ip2.
FIG. 4 shows the same cases as those in FIG. 3. FIG. 5 is a graph
illustrating in enlarged view a region of FIG. 4 corresponding to
oxygen concentrations of 10 vol % or less. In FIG. 5, the
horizontal axis is on a log scale. The oxygen concentration on the
horizontal axis is the oxygen concentration of the adjusted model
gas, that is, the oxygen concentration of a measurement-object gas
that is outside of the sensor element 101. Furthermore, the A/F of
the model gas is also shown in parenthesis on the horizontal axis
of FIG. 5. The A/F is a value measured by using a MEXA-730X, which
is manufactured by HORIBA, Ltd.
[0078] As can be seen from FIGS. 4 and 5, in the case where the
oxygen concentration of the model gas was greater than or equal to
1 vol %, the values of pump currents Ip2 corresponding to the same
oxygen concentration were substantially the same in all of the
cases in which the target value Ip0s* was 0 mA, the target value
Ip0s* was 1 mA, and the target value Ip0s* was 2 mA. In contrast,
in the case where the oxygen concentration of the model gas was
less than or equal to 0.1 vol %, a pump current Ip2 generated in a
case where the target value Ip0s* was 0 mA, that is, the
preliminary pump cell 15 did not pump oxygen to the inside at all,
had a value smaller than a pump current Ip2 generated in a case
where the preliminary pump cell 15 pumped oxygen to the inside.
That is, the sensitivity of the pump current Ip2 to the
concentration of NOx was decreased.
[0079] FIG. 3 confirms that, even when the values of the oxygen
concentrations of the model gases are the same, the greater the
target value Ip0s*, the greater the pump current Ip0. However, with
regard to the amount of increase in the pump current Ip0, which was
determined in comparison with the pump current Ip0 obtained in a
case where the target value Ip0s* was 0 mA, the pump current Ip0
obtained in a case where the target value Ip0s* was 2 mA was not
equal to twice the pump current Ip0 obtained in a case where the
target value Ip0s* was 1 mA. That is, the amount of increase in the
pump current Ip0 was not directly proportional to the target value
Ip0s*. The reason for this is believed to be that, even when the
target value Ip0s* is large, some of the oxygen pumped into the
buffer space 12 escapes through the gas inlet port 10 to the
outside as a result of diffusion, and therefore not all of the
pumped oxygen reaches the first internal space 20. Furthermore,
FIG. 3 shows that the pump current Ip0 had a negative value only in
cases where Ip0s* is 0 mA and the concentration of the model gas
was less than or equal to 0.1 vol % (cases shown on the left side
of FIG. 3, in which the oxygen concentration was 0.005 vol %, 0.01
vol %, or 0.1 vol %). Thus, FIGS. 3 to 5 confirm that the
sensitivity of the pump current Ip2 was low when the pump current
Ip0 had a negative value. Negative values of the pump current Ip0
mean that the main pump cell 21 is pumping oxygen into the first
internal space 20 (pumping oxygen in a manner such that the oxygen
partial pressure in the first internal space 20 reaches the target
value V0*), not pumping oxygen from the first internal space 20.
That is, negative values of the pump current Ip0 mean that the
oxygen concentration of the measurement-object gas to be introduced
into the first internal space 20 is lower than the oxygen
concentration represented by the target value V0*.
[0080] The results described above demonstrate that a specific gas
measurement accuracy decreases in a case where the oxygen
concentration of the measurement-object gas to be introduced into
the first internal space 20 is low. In contrast, in the gas sensor
100 of the present embodiment, the measurement-object gas, after
oxygen is supplied by the preliminary pump cell 15, is introduced
into the first internal space 20 as described above, and
consequently, as illustrated in FIG. 3, the value of Ip0 can be
increased (i.e., the oxygen concentration of the measurement-object
gas to be introduced into the first internal space 20 can be
increased). As a result, it is unlikely that the measurement-object
gas will reach the first internal space 20 in a state in which the
measurement-object gas is a low-oxygen atmosphere, and
consequently, a decrease in measurement accuracy that may occur in
the case where the measurement-object gas is a low-oxygen
atmosphere is suppressed. From the results in FIGS. 3 to 5, it is
believed that, when the preliminary pump cell 15 pumps oxygen into
the buffer space 12 in a manner such that a measurement-object gas
having an oxygen concentration less than or equal to 0.1 vol % does
not reach the first internal space 20, that is, the oxygen
concentration of the measurement-object gas reaching the first
internal space 20 is greater than 0.1 vol %, a decrease in
measurement accuracy can be suppressed. Furthermore, it is believed
that the preliminary pump cell 15 is to be operated in a manner
such that the oxygen concentration of the measurement-object gas
reaching the first internal space 20 is preferably greater than or
equal to 0.2 vol % and is more preferably greater than or equal to
1 vol %.
[0081] In the case where the preliminary pump cell 15 does not pump
oxygen to the inside, a measurement accuracy decreases when the
measurement-object gas is a low-oxygen atmosphere. The reason for
this is unknown, but, for example, may be as follows. One possible
reason is that, when a measurement-object gas that is a low-oxygen
atmosphere is introduced into the first internal space 20, the
inner pump electrode 22 acts as a catalyst to cause NOx to be
reduced in the first internal space 20 before the
measurement-object gas reaches the third internal space 61. Another
possible reason is as follows. In the case where the
measurement-object gas is a rich atmosphere, unburned components
such as a hydrocarbon (HC) and carbon monoxide exist in the
measurement-object gas. NOx can react with the components, and
therefore NOx tends to be reduced in the first internal space 20.
For example, in the instance of a gasoline engine, the air-fuel
ratio of the measurement-object gas remains at or near the
stoichiometric air-fuel ratio in many cases, and therefore the
measurement-object gas may be consistently a low-oxygen atmosphere.
Even in such a case, the gas sensor 100 of the present embodiment
can detect the specific gas concentration accurately. Furthermore,
in the present embodiment, the target value V0* is
feedback-controlled in a manner such that the pump current Ip1
reaches a constant value. Possibly, this may also be related to a
decrease in measurement accuracy that may occur when the
measurement-object gas is a low-oxygen atmosphere. For example, in
a case where the oxygen concentration of the measurement-object gas
to be introduced into the first internal space 20 is temporarily
decreased, there is a time lag before the second internal space 40
is affected by the influence. Accordingly, there is a time lag
before the target value V0* is changed based on the pump current
Ip1 to an appropriate value, and as a result, a phenomenon in
which, temporarily, oxygen in the first internal space 20 is pumped
to the outside excessively may occur. Thus, it is possible that, in
the case where the oxygen concentration in the first internal space
20 decreases excessively as a result of the phenomenon, reduction
of NOx may occur in the first internal space 20. In contrast, in
the gas sensor 100 of the present embodiment, it is believed that a
decrease in measurement accuracy is suppressed for the following
reason: since oxygen is supplied by the preliminary pump cell 15,
the oxygen concentration in the first internal space 20 does not
decrease to such an extent that NOx is reduced in the first
internal space 20, even when a temporary decrease in the oxygen
concentration of the measurement-object gas, as described above,
occurs.
[0082] Furthermore, in the gas sensor 100 of the present
embodiment, spike noise in the pump current Ip1 and the pump
current Ip2, which occurs when the atmosphere of the
measurement-object gas suddenly changes from a rich atmosphere to a
lean atmosphere, or vice versa, can be suppressed. The present
inventors investigated the behavior of pump currents Ip0, Ip1, and
Ip2, which were generated when the measurement-object gas to be
introduced into the gas inlet port 10 was suddenly changed from a
rich atmosphere to a lean atmosphere. The measurement-object gases
used were adjusted model gases. The following model gases were
prepared: a rich-atmosphere gas having an oxygen concentration of
0.05 vol % and a lean-atmosphere gas having an oxygen concentration
of 0.65 vol %. When 30 seconds had passed after the rich-atmosphere
gas started to flow through a pipe, the rich-atmosphere gas was
replaced with the lean-atmosphere gas. The conditions for the model
gases except for the oxygen concentration were the same as those
for the model gas used in the measurement associated with FIGS. 3
to 5. Note that the model gas contains fuel gases (1000 ppm carbon
monoxide gas and 1000 ppm ethylene gas) as described above, and
therefore a model gas having an oxygen concentration of 0.05 vol %
is a rich atmosphere. FIG. 6 is a graph illustrating temporal
changes in the pump currents Ip0, Ip1, and Ip2 in a case where the
target value Ip0s* is 0 mA. FIG. 7 is a graph illustrating temporal
changes in the pump currents Ip0, Ip1, and Ip2 in a case where the
target value Ip0s* is 1 mA.
[0083] As can be seen from FIGS. 6 and 7, in the case where the
target value Ip0s* was 1 mA (FIG. 7), unlike the case of FIG. 6,
the pump current Ip0 did not decrease to a negative value and
consistently had a positive value even in the time period during
which the measurement-object gas was a rich atmosphere (elapsed
time was 0 to 30 seconds). Furthermore, in FIG. 7, the spike noise
in the pump currents Ip1 and Ip2, which occurred when the rich
atmosphere was changed to the lean atmosphere, was reduced compared
with FIG. 6. This is believed to be because spike noise tends to
occur in the pump currents Ip1 and Ip2 when the pump current Ip0
changes from positive to negative or vice versa. For example, in
the instance of a gasoline engine, the air-fuel ratio of the
measurement-object gas remains at or near the stoichiometric
air-fuel ratio in many cases, and therefore, if the preliminary
pump cell 15 does not pump oxygen into the buffer space 12, the
pump current Ip0 may frequently change between positive and
negative, and consequently spike noise may frequently occur. In the
gas sensor 100 of the present embodiment, such changes in the pump
current Ip0 between positive and negative can be suppressed from
occurring.
[0084] Here, correspondence relationships between constituent
elements of the present embodiment and constituent elements of the
present invention will be clarified. The layered body of the
present embodiment in which six layers, namely the first substrate
layer 1, the second substrate layer 2, the third substrate layer 3,
the first solid electrolyte layer 4, the spacer layer 5, and the
second solid electrolyte layer 6 are layered in this order
corresponds to the element body of the present invention. The
buffer space 12 corresponds to the preliminary chamber. The
preliminary pump cell 15 corresponds to the preliminary pump cell.
The first internal space 20 corresponds to the oxygen concentration
adjustment chamber. The main pump cell 21 corresponds to the
adjustment pump cell. The third internal space 61 corresponds to
the measurement chamber. The measurement electrode 44 corresponds
to the measurement electrode. The reference electrode 42
corresponds to the reference electrode. The oxygen partial pressure
detection sensor cell 82 for controlling the measurement pump
corresponds to the measurement voltage detection device. The pump
current Ip2 corresponds to the detection value. The CPU 92 of the
controller 90 corresponds to the specific gas concentration
detection device. Furthermore, the pump current Ip0s corresponds to
the preliminary pump current. The CPU 92 corresponds to the
preliminary pump control device. The memory 94 corresponds to the
storage device. The pump current Ip0 corresponds to the adjustment
pump current. The CPU 92 corresponds to the oxygen concentration
detection device. The outer pump electrode 23 corresponds to the
measurement-object gas-side electrode.
[0085] In the gas sensor 100 of the present embodiment described
above, since the preliminary pump cell 15 supplies oxygen to the
measurement-object gas before the oxygen concentration is adjusted
by the main pump cell 21, it is unlikely that the
measurement-object gas will be introduced into the first internal
space 20 in a state in which the measurement-object gas is a
low-oxygen atmosphere, even in the case where the
measurement-object gas is a low-oxygen atmosphere before being
introduced into the measurement-object gas flow section. Hence, a
decrease in measurement accuracy that occurs in a case where the
measurement-object gas is a low-oxygen atmosphere is
suppressed.
[0086] Furthermore, the CPU 92 controls the preliminary pump cell
15 in a manner such that a constant preliminary pump current
(target value Ip0s*) flows, and therefore, with a relatively simple
control, oxygen can be supplied to a measurement-object gas that is
a low-oxygen atmosphere, in the buffer space 12.
[0087] In addition, the CPU 92 detects the specific gas
concentration by using a common relationship formula stored in the
memory 94 regardless of whether a measurement-object gas outside of
the element body is a low-oxygen atmosphere. As described with
reference to FIGS. 4 and 5, in the gas sensor 100 of the present
embodiment, the sensitivity of the pump current Ip2 tends not to
decrease even in the case where the measurement-object gas is a
low-oxygen atmosphere. Hence, the gas sensor 100 can detect the
specific gas concentration accurately without using different
relationship formulas for the case in which the measurement-object
gas is a low-oxygen atmosphere and for the case in which the
measurement-object gas is not a low-oxygen atmosphere. Hence, the
gas sensor 100 can detect the specific gas concentration readily
and accurately.
[0088] In addition, the preliminary pump cell 15 pumps oxygen into
the buffer space 12 from a vicinity of the outer pump electrode 23.
With this configuration, the following is possible. In comparison
with, for example, a case in which oxygen is pumped into the buffer
space 12 from a vicinity of the reference electrode 42, a decrease
in measurement accuracy that may occur when the potential of the
reference electrode 42 changes as a result of a voltage drop due to
the current during pumping is suppressed.
[0089] Note that the present invention is in no way limited to the
embodiment described above and may be implemented in a variety of
embodiments that fall within the technical scope of the present
invention.
[0090] For example, in the embodiment described above, the CPU 92
detects the specific gas concentration based on the pump current
Ip2 and a relationship formula representing a relationship between
the pump current Ip2 and the concentration of NOx stored in the
memory 94, but this configuration is non-limiting. For example, the
CPU 92 may detect the specific gas concentration corrected based on
the oxygen concentration of a measurement-object gas that is
outside of the sensor element 101. For example, referring to FIG.
5, according to the data in which the target value Ip0s* is 1 mA or
2 mA, in the case where the oxygen concentration of the
measurement-object gas is consistently less than or equal to 5%,
the value of the pump current Ip2 does not significantly change
even when the oxygen concentration changes, provided that the
actual specific gas concentration (real concentration) is uniform
(see FIG. 5). On the other hand, in the case where the oxygen
concentration of the measurement-object gas may change over a
larger range, the pump current Ip2 may change with the oxygen
concentration, as illustrated in FIG. 4. In a case where the pump
current Ip2 changes with the oxygen concentration relatively
significantly as just described, the CPU 92 may detect the specific
gas concentration with a correction based on the oxygen
concentration. This improves a specific gas concentration
measurement accuracy. For example, referring to FIG. 4, according
to the data in which the target value Ip0s* is 1 mA or 2 mA, the
pump current Ip2 linearly changes with the oxygen concentration
when the specific gas concentration is uniform, and therefore the
relationship between the oxygen concentration and the pump current
Ip2 can be approximated with a linear function. Accordingly, by
using the linear function formula (relationship formula for
correction), the CPU 92 may derive a corrected pump current by
excluding an influence of the oxygen concentration from the pump
current Ip2, which is obtained from the measurement pump cell 41,
and, based on the corrected pump current and the relationship
formula stored in the memory 94 in the embodiment described above,
the CPU 92 may detect the specific gas concentration. In this case,
the memory 94 may also store the relationship formula for
correction. Alternatively, in place of the relationship formula
stored in the memory 94 in the embodiment described above, a
relationship formula in which the relationship formula for
correction is taken into account, that is, a relationship formula
representing a relationship between the pump current Ip2, the
specific gas concentration, and the oxygen concentration of a
measurement-object gas outside of the sensor element 101 may be
stored, and, by using this relationship formula, the CPU 92 may
detect a corrected specific gas concentration. As with the
relationship formula stored in the memory 94 in the embodiment
described above, with regard to the relationship formula for
correction and the relationship formula in which the relationship
formula for correction is taken into account, too, the specific gas
concentration can be detected accurately by using the formula,
which is a common formula, regardless of whether a
measurement-object gas outside of the sensor element 101 is a
low-oxygen atmosphere.
[0091] In the case where the CPU 92 performs correction as
described above, the CPU 92 may detect the oxygen concentration of
the measurement-object gas that is outside of the sensor element
101. It is to be noted that the constant pump current Ip0s (i.e.,
target value Ip0s*) corresponds to the flow rate of oxygen pumped
into the buffer space 12 by the preliminary pump cell 15.
[0092] Furthermore, the pump current Ip0 corresponds to the flow
rate of oxygen pumped from the first internal space 20. Hence,
based on the pump current Ip0s, the pump current Ip0, and the
target concentration of the oxygen concentration in the first
internal space 20, the CPU 92 can detect the oxygen concentration
of a measurement-object gas that is a gas before the preliminary
pump cell 15 pumps oxygen to the inside and the main pump cell 21
pumps oxygen to the outside. That is, the CPU 92 can detect the
oxygen concentration of the measurement-object gas that is outside
of the sensor element 101. Accordingly, the oxygen concentration
necessary for the correction can be detected by the gas sensor 100.
In addition, the CPU 92 can also detect the oxygen concentration of
the measurement-object gas that is outside of the sensor element
101 based on, for example, a voltage Vref between the reference
electrode 42 and the outer pump electrode 23. Alternatively, the
CPU 92 may obtain the oxygen concentration of the
measurement-object gas that is outside of the sensor element 101
from a device other than the gas sensor 100, such as a different
sensor or the ECU of the engine, and may use the oxygen
concentration for the correction.
[0093] In the embodiment described above, the preliminary pump cell
15 pumps oxygen into the buffer space 12 from a vicinity of the
outer pump electrode 23, but this configuration is non-limiting.
For example, oxygen may be pumped into the buffer space 12 from a
vicinity of the reference electrode 42. With this configuration,
the following is possible. In comparison with, for example, a case
in which oxygen is pumped to the inside from an external
measurement-object gas, oxygen can be pumped into the buffer space
12 at a low applied voltage because the reference gas (in this
case, air) has a higher oxygen concentration than the
measurement-object gas. In contrast, in the case where oxygen is
pumped into the buffer space 12 from the vicinity of the outer pump
electrode 23, the voltage Vp0s of the variable power supply 17
needs to be relatively high because, in particular, if a vicinity
of the outer pump electrode 23 is a low-oxygen atmosphere, it is
necessary to produce oxygen ions by reducing carbon monoxide,
water, or the like present in the measurement-object gas.
[0094] In the embodiment described above, the second
diffusion-rate-limiting portion 13 is present between the buffer
space 12 and the first internal space 20, but this configuration is
non-limiting. For example, the second diffusion-rate-limiting
portion 13 may be omitted, and the buffer space 12 and the first
internal space 20 may constitute a single space.
[0095] In the embodiment described above, the specific gas
concentration detected by the gas sensor 100 is the concentration
of NOx, but this configuration is non-limiting, and the specific
gas concentration may be the concentration of a different oxide. In
the case where the specific gas is an oxide, oxygen is produced
when the specific gas itself is reduced in the third internal space
61 as in the embodiment described above, and accordingly, the CPU
92 can detect the specific gas concentration by obtaining a
detection value corresponding to the oxygen. Furthermore, the
specific gas may be a non-oxide gas, such as ammonia. In the case
where the specific gas is a non-oxide gas, the specific gas may be
converted into an oxide (e.g., in the case of ammonia, converted
into NO). When the converted gas is reduced in the third internal
space 61, oxygen is produced, and accordingly, the CPU 92 can
detect the specific gas concentration by obtaining a detection
value corresponding to the oxygen. For example, in a case where the
preliminary pump electrode 16 contains a metal having a catalytic
function for promoting oxidation of ammonia, the specific gas can
be converted into an oxide in the buffer space 12 via the catalytic
function of the preliminary pump electrode 16. A similar
configuration is possible for the inner pump electrode 22. Ammonia
is converted into an oxide, which is NO, and therefore the
measurement of the concentration of ammonia is performed basically
by using the same principle as that for the measurement of the
concentration of NOx.
[0096] In the embodiment described above, the CPU 92 controls the
preliminary pump cell 15 in a manner such that a constant
preliminary pump current (target value Ip0s*) flows, but this
configuration is non-limiting. For example, the CPU 92 may
feedback-control the voltage Vp0s in a manner such that the oxygen
concentration in the buffer space 12, which is detected based on
the voltage between the preliminary pump electrode 16 and the
reference electrode 42, reaches a target value. Alternatively, the
CPU 92 may control the voltage Vp0s in a manner such that the lower
the oxygen concentration of an outside of the sensor element 101,
the greater the amount of oxygen to be pumped into the buffer space
12. In this case, the CPU 92 may detect the oxygen concentration of
an outside of the sensor element 101 by using the method described
above or obtain the oxygen concentration from a device other than
the gas sensor 100. Furthermore, the CPU 92 may control the voltage
Vp0s to be a constant voltage.
[0097] In the embodiment described above, the target value Ip0s* is
set based on the amount of oxygen necessary to increase the minimum
oxygen concentration of the measurement-object gas, which is the
minimum among oxygen concentrations in various operation conditions
of an internal combustion engine, to an oxygen concentration higher
than the oxygen concentration of a low-oxygen atmosphere (e.g.
oxygen concentration greater than 0.1 vol %, greater than or equal
to 0.2 vol %, greater than or equal to 1 vol %, or the like).
However, this configuration is non-limiting. For example, the
target value Ip0s* may be set to a value such that the pump current
Ip0 does not decrease to a negative value even in a case where a
measurement-object gas having a minimum oxygen concentration, which
is the minimum among oxygen concentrations in various operation
conditions of an internal combustion engine, is introduced into the
measurement-object gas flow section of the sensor element 101. That
is, the preliminary pump cell 15 may "prevent the
measurement-object gas from reaching the first internal space 20 in
a state in which the measurement-object gas is a low-oxygen
atmosphere" by ensuring that "the pump current Ip0 does not
decrease to a negative value". In any case, the amount of oxygen to
be pumped into the buffer space 12 by the preliminary pump cell 15
may be set by experimentation in a manner such that, in accordance
with a range of fluctuations a component of the measurement-object
gas may experience, a decrease in measurement accuracy is
suppressed within the range of fluctuations (e.g., in a manner such
that, as illustrated in FIGS. 4 and 5, a state in which the
sensitivity of the pump current Ip2 to the concentration of NOx
decreases does not easily occur.
[0098] In the embodiment described above, the sensor element 101 of
the gas sensor 100 includes the first internal space 20, the second
internal space 40, and the third internal space 61, but this
configuration is non-limiting. For example, the third internal
space 61 may not be included, as in a sensor element 201, which is
illustrated in FIG. 8. In the sensor element 201, which is a
modified example and illustrated in FIG. 8, the gas inlet port 10,
the first diffusion-rate-limiting portion 11, the buffer space 12,
the second diffusion-rate-limiting portion 13, the first internal
space 20, the third diffusion-rate-limiting portion 30, and the
second internal space 40 are formed adjacent to one another in such
a manner as to be in communication with one another in this order,
between the lower surface of the second solid electrolyte layer 6
and the upper surface of the first solid electrolyte layer 4.
Furthermore, the measurement electrode 44 is disposed on the upper
surface of the first solid electrolyte layer 4 within the second
internal space 40. The measurement electrode 44 is covered with a
fourth diffusion-rate-limiting portion 45. The fourth
diffusion-rate-limiting portion 45 is a film formed of a ceramic
porous member containing, for example, alumina (Al.sub.2O.sub.3).
As with the fourth diffusion-rate-limiting portion 60 of the
embodiment described above, the fourth diffusion-rate-limiting
portion 45 serves to limit the amount of NOx flowing into the
measurement electrode 44. Furthermore, the fourth
diffusion-rate-limiting portion 45 functions as a protective film
for the measurement electrode 44. A ceiling electrode portion 51a
of the auxiliary pump electrode 51 is formed to extend to a
position immediately above the measurement electrode 44. The sensor
element 201, configured as described above, can also detect the
concentration of NOx based on, for example, the pump current Ip2 as
with the embodiment described above. In this case, a vicinity of
the measurement electrode 44 functions as a measurement
chamber.
[0099] In the embodiment described above, the outer pump electrode
23 serves as the following electrodes: the measurement-object
gas-side electrode (outer preliminary pump electrode) of the
preliminary pump cell 15, an outer main pump electrode of the main
pump cell 21, an outer auxiliary pump electrode of the auxiliary
pump cell 50, and an outer measurement electrode of the measurement
pump cell 41. However, this configuration is non-limiting. One or
more of the outer preliminary pump electrode, the outer main pump
electrode, the outer auxiliary pump electrode, and the outer
measurement electrode may be an additional electrode, other than
the outer pump electrode 23. The additional electrode may be
provided outside of the element body and be in contact with a
measurement-object gas.
[0100] In the embodiment described above, the element body of the
sensor element 101 is a layered body including a plurality of solid
electrolyte layers (layers 1 to 6), but this configuration is
non-limiting. It is sufficient that the element body of the sensor
element 101 include at least one oxygen-ion-conductive solid
electrolyte layer and a measurement-object gas flow section be
provided in the interior. For example, referring to FIG. 1, each of
the layers 1 to 5, other than the second solid electrolyte layer 6,
may be a layer formed of a material other than a solid electrolyte
(e.g., a layer formed of alumina). In this case, the electrodes to
be included in the sensor element 101 may be disposed on the second
solid electrolyte layer 6. For example, the measurement electrode
44 of FIG. 1 may be disposed on the lower surface of the second
solid electrolyte layer 6. Furthermore, the reference gas
introduction space 43 may be provided in the spacer layer 5 instead
of the first solid electrolyte layer 4; the air introduction layer
48 may be provided between the second solid electrolyte layer 6 and
the spacer layer 5 instead of being provided between the first
solid electrolyte layer 4 and the third substrate layer 3; and the
reference electrode 42 may be provided behind the third internal
space 61 and on the lower surface of the second solid electrolyte
layer 6.
[0101] In the embodiment described above, the controller 90 sets
(feedback-controls) the target value V0* of the electromotive force
V0 based on the pump current Ip1 in a manner such that the pump
current Ip1 reaches a target value Ip1*, and the controller 90
feedback-controls the pump voltage Vp0 in a manner such that the
electromotive force V0 reaches the target value V0*. However, the
controller 90 may perform a different control. For example, the
controller 90 may feedback-control the pump voltage Vp0 based on
the pump current Ip1 in a manner such that the pump current Ip1
reaches a target value Ip1*. That is, the controller 90 may not
obtain the electromotive force V0 from the oxygen partial pressure
detection sensor cell 80 for controlling the main pump or set the
target value V0*; the controller 90 may directly control the pump
voltage Vp0 (therefore, control the pump current Ip0) based on the
pump current Ip1.
[0102] In the embodiment described above, the inner pump electrode
22 is a cermet electrode containing Pt and ZrO.sub.2 and containing
1% Au. However, this configuration is non-limiting. It is
sufficient that the inner pump electrode 22 contain a noble metal
having catalytic activity (e.g., at least one of Pt, Rh, Ir, Ru,
and Pd) and a noble metal having a catalytic activity suppression
ability (e.g., Au). The catalytic activity suppression ability is
an ability to suppress the catalytic activity of the noble metal
having catalytic activity from being exhibited to the specific gas.
Also, as with the inner pump electrode 22, it is sufficient that
the auxiliary pump electrode 51 and the preliminary pump electrode
16 each contain a noble metal having catalytic activity and a noble
metal having a catalytic activity suppression ability, which is an
ability to suppress the catalytic activity of the noble metal
having catalytic activity from being exhibited to the specific gas.
It is sufficient that the outer pump electrode 23, the reference
electrode 42, and the measurement electrode 44 each contain a noble
metal having catalytic activity as described above. It is
preferable that each of the electrodes 16, 22, 23, 42, 44, and 51
be a cermet electrode containing a noble metal and an oxide having
oxygen ion conductivity (e.g., ZrO.sub.2)However, one or more of
these electrodes may not be a cermet electrode. It is preferable
that each of the electrodes 16, 22, 23, 42, 44, and 51 be a porous
member. However, one or more of these electrodes may not be a
porous member.
[0103] The "minimum oxygen concentration, which is the minimum
among oxygen concentrations in various operation conditions of an
internal combustion engine", described above, may be, for example,
-11 vol % (value of 11 in terms of air-fuel ratio for gasoline
engines). For example, when setting the target value Ip0s* in the
manner described in the above embodiment, the following is
possible: in a case where a measurement-object gas having an oxygen
concentration of -11 vol % flows into the buffer space 12, an
amount of oxygen is necessary to increase the oxygen concentration
of the measurement-object gas to an oxygen concentration higher
than that of a low-oxygen atmosphere (the higher oxygen
concentration may be greater than 0.1 vol %, preferably greater
than or equal to 0.2 vol %, and more preferably greater than or
equal to 1 vol %), and the target value Ip0s* may be set based on
the amount of oxygen. Similarly, in the case where "the CPU 92
controls the voltage Vp0s to be a constant voltage", as described
in the above modified example, the target value (constant value) of
the voltage Vp0s may be set in a manner such that, with a pump
current Ip0s that flows in a state in which the voltage Vp0s is
controlled at a constant value, the oxygen concentration of -11 vol
% of a measurement-object gas can be increased to an oxygen
concentration higher than that of a low-oxygen atmosphere in the
buffer space 12. In the case where "the CPU 92 feedback-controls
the voltage Vp0s in a manner such that the oxygen concentration in
the buffer space 12 reaches a target value", as described in the
above modified example, the target value of the oxygen
concentration in the buffer space 12 may be a value that is in a
state in which the oxygen concentration is higher than that of a
low-oxygen atmosphere. In the case where "the CPU 92 controls the
voltage Vp0s in a manner such that the lower the oxygen
concentration of an outside of the sensor element 101, the greater
the amount of oxygen to be pumped into the buffer space 12", as
described in the above modified example, the following is possible:
the correspondence relationship between the oxygen concentration of
an outside of the sensor element 101 and the target value of the
voltage Vp0s may be set in advance in a manner such that, in a case
where a measurement-object gas having an oxygen concentration of
-11 vol % flows into the buffer space 12, the oxygen concentration
of the measurement-object gas can be increased to an oxygen
concentration higher than that of a low-oxygen atmosphere, and
accordingly, the CPU 92 may control the voltage Vp0s based on the
correspondence relationship. Similarly, the preliminary pump cell
15 may pump oxygen into the buffer space 12 in a manner such that,
even in a case where a measurement-object gas having an oxygen
concentration of -11 vol % flows into the buffer space 12, the
measurement-object gas does not reach the first internal space 20
in a state in which the measurement-object gas is a low-oxygen
atmosphere. Furthermore, the CPU 92 may control the preliminary
pump cell 15 in a manner such that oxygen is pumped to the inside
as just described.
[0104] It is to be noted that, in a case where the
measurement-object gas is a low-oxygen atmosphere and the amount of
oxygen that is pumped to the inside by the preliminary pump cell 15
is too small, the preliminary pump electrode 16 may cause the
specific gas to be reduced because the preliminary pump electrode
16 contains a noble metal having catalytic activity. Furthermore,
in some cases, the catalytic activity of the preliminary pump
electrode 16 is highest at or near the stoichiometric air-fuel
ratio (oxygen concentration is 0 vol %, A/F=14.7). In such cases,
the following may occur: the preliminary pump electrode 16 reduces
a greater amount of the specific gas when the oxygen concentration
of the measurement-object gas flowing into the buffer space 12 is
at or near the stoichiometric air-fuel ratio than when the oxygen
concentration is -11 vol % (see also FIG. 9, which will be
described later). Accordingly, it is preferable that the
preliminary pump cell 15 pump oxygen into the buffer space 12 in a
manner such that, even in a case where a measurement-object gas
having any oxygen concentration that is within a range of -11 vol %
or greater and 0.1 vol % or less flows into the buffer space 12,
the measurement-object gas reaching the first internal space 20 has
an oxygen concentration of greater than 0.1 vol %. It is more
preferable that the preliminary pump cell 15 pump oxygen into the
buffer space 12 in a manner such that, even in a case where a
measurement-object gas having any oxygen concentration that is
within a range of -11 vol % or greater and less than 0.2 vol %
flows into the buffer space 12, the measurement-object gas reaching
the first internal space 20 has an oxygen concentration of 0.2 vol
% or greater. It is even more preferable that the preliminary pump
cell 15 pump oxygen into the buffer space 12 in a manner such that,
even in a case where a measurement-object gas having any oxygen
concentration that is within a range of -11 vol % or greater and
less than 1 vol % flows into the buffer space 12, the
measurement-object gas reaching the first internal space 20 has an
oxygen concentration of 1 vol % or greater. Furthermore, it is
preferable that the CPU 92 control the preliminary pump cell 15 in
a manner such that oxygen is pumped to the inside in any of the
ways just described. For example, when setting the target value
Ip0s* in the manner described in the above embodiment, the target
value Ip0s* may be set to a value such that, even in a case where a
measurement-object gas having any oxygen concentration that is
within the range of -11 vol % or greater and 0.1 vol % or less
flows into the buffer space 12, the oxygen concentration of the
measurement-object gas can be increased to greater than 0.1 vol %.
The same applies to the cases described above in the modified
examples: the case in which "the CPU 92 controls the voltage Vp0s
to be a constant voltage", the case in which the CPU 92
feedback-controls the voltage Vp0s in a manner such that the oxygen
concentration in the buffer space 12 reaches a target value", and
the case in which "the CPU 92 controls the voltage Vp0s in a manner
such that the lower the oxygen concentration of an outside of the
sensor element 101, the greater the amount of oxygen to be pumped
into the buffer space 12".
[0105] [Investigation of Pump Current Ip2 in Highly Rich
Atmosphere]
[0106] With regard to FIGS. 4 and 5, described above, the
relationship between the oxygen concentration and the pump current
Ip2 in a range in which the oxygen concentration of the
measurement-object gas was greater than 0 vol % was investigated.
In addition, the relationship between the A/F ratio of the
measurement-object gas and the pump current Ip2 in a case where the
measurement-object gas was an even richer atmosphere (highly rich
atmosphere) was investigated. The measurement-object gas used was
an adjusted model gas. In the model gas, the base gas was nitrogen,
the specific gas component was 500 ppm NO, and the fuel gas
(unburned component) was ethylene gas. The model gas had a water
concentration of 3 vol % and an oxygen concentration of 0 vol %.
Further, the A/F ratio of the model gas was adjusted by changing
the concentration of the ethylene gas. The A/F ratio was measured
by using a MEXA-730X, which is manufactured by HORIBA, Ltd. The
temperature of the model gas was 250.degree. C., and the model gas
was flowed through a pipe having a diameter of 20 mm at a flow rate
of 100 L/min. Subsequently, similarly to the cases of FIGS. 4 and
5, the relationship between the A/F ratio of the measurement-object
gas and the pump current Ip2 in cases where the target value Ip0s*
was 0 mA and the target value Ip0s* was 1 mA in the gas sensor 100
was investigated. The result is shown in FIG. 9.
[0107] As can be seen from FIG. 9, in the case where the
measurement-object gas had the stoichiometric air-fuel ratio or was
a rich atmosphere, that is, the A/F ratio was less than or equal to
14.7, including a case in which the measurement-object gas was a
highly rich atmosphere, the pump current Ip2 generated in the case
where the target value Ip0s* was 0 mA, that is, no oxygen was
pumped to the inside by the preliminary pump cell 15 had a value
smaller than that of the pump current Ip2 generated in the case
where oxygen was pumped to the inside by the preliminary pump cell
15. That is, in the case where the target value Ip0s* was 0 mA, the
sensitivity of the pump current Ip2 to the concentration of NOx was
decreased. Reasons for this may be as follows. First, the
preliminary pump electrode 16 is, for example, a cermet electrode
containing Pt and ZrO.sub.2 and containing 1% Au, as with the inner
pump electrode 22. Thus, one possible reason is that in the case
where the target value Ip0s* is 0 mA, when a measurement-object gas
that is a low-oxygen atmosphere is introduced into the buffer space
12 and the first internal space 20, the preliminary pump electrode
16 and the inner pump electrode 22 act as catalysts to cause NOx to
be reduced in the buffer space 12 and the first internal space 20
before the measurement-object gas reaches the third internal space
61. Another possible reason is that the preliminary pump electrode
16 and the inner pump electrode 22 may act as catalysts to cause a
reaction between the ethylene gas and NOx present in the model gas.
Note that the preliminary pump electrode 16 and the inner pump
electrode 22 serve as so-called three-way catalysts for NOx and
hydrocarbons and that the catalytic activity of the electrodes is
high at or near the stoichiometric air-fuel ratio (A/F=14.7). As
such, in the case where the target value Ip0s* is 0 mA, it is
predicted that the sensitivity of the pump current Ip2 to the NOx
concentration will likely be lowest at or near the stoichiometric
air-fuel ratio. However, in the case where the target value Ip0s*
is 0 mA, when a measurement-object gas that is a low-oxygen
atmosphere reaches the first internal space 20, the main pump cell
21 pumps oxygen into the first internal space 20, and therefore, it
is believed that the A/F ratio of the measurement-object gas at
which the sensitivity of the pump current Ip2 to the concentration
of NOx is likely to be lowest shifts to the rich side by a
corresponding amount. Accordingly, although no measurement was
made, it is also predicted that, in the case where the target value
Ip0s* is 0 mA, the pump current Ip2 will have an even smaller value
when the measurement-object gas is a slightly rich atmosphere (A/F
ratio is approximately 14.4), as shown by the dash-dot line in FIG.
9.
[0108] In contrast, in the case where the target value Ip0s* is 1
mA, the oxygen concentration in the buffer space 12 can be
increased because the preliminary pump cell 15 pumps oxygen to the
inside. As a result, it is unlikely that the preliminary pump
electrode 16 will cause NOx to be reduced or will cause a reaction
between NOx and a hydrocarbon. Furthermore, almost no reduction of
NOx or reaction between NOx and a hydrocarbon due to the inner pump
electrode 22 will occur. Accordingly, referring to FIG. 9, it is
believed that in the case where the target value Ip0s* is 1 mA, the
pump current Ip2 is suppressed from decreasing even when the
measurement-object gas is a low-oxygen atmosphere.
[0109] Note that in the embodiment described above, with regard to
FIG. 5, the pump current Ip2 decreased in the case where the target
value Ip0s* was 0 mA, and, as stated above, a reason for this is
that the inner pump electrode 22 acted as a catalyst. However, as
described above with regard to FIG. 9, in the case where the target
value Ip0s* was 0 mA in FIG. 5, too, it is believed that the pump
current Ip2 decreased because not only the inner pump electrode 22
but also the preliminary pump electrode 16 acted as a catalyst.
[0110] [Investigation of Durability of Gas Sensor]
[0111] The durability of the gas sensor 100 was investigated in the
following manner, for a case in which the target value Ip0s* was 1
mA and a case in which the target value Ip0s* was 0 mA. First,
three types of model gases were prepared. The A/F ratios of the
model gases were 12.6, 14.5, and 16.6. The model gases were
obtained by adjusting the ethylene gas concentration of a model gas
having a similar composition to that of the model gas used for the
above-described measurement regarding FIG. 9. Subsequently, a gas
sensor 100 in which the target value Ip0s* was set to 0 mA was
prepared, and in a condition at the time of the start of the test
(durability period=0 h), the pump current Ip2 was measured for each
of the three types of model gases used as the measurement-object
gas. Each of the measured values was used as a reference value for
a sensitivity change ratio [%]. Next, the sensor element 101 of the
gas sensor 100 was exposed to exhaust gases from a gasoline engine
for 100 hours in a state in which the gas sensor 100 was operated
(state in which the gas sensor 100 was measuring the concentration
of NOx). The gasoline engine was a V-type 8-cylinder engine having
a displacement of 4.6 L with the air intake mode being natural
aspiration (NA). One of the cylinder banks of the engine was used.
An operation of the engine was a cycle operation at the air-fuel
ratio .lamda.1 (A/F ratio within a range of 14.3 to 15.1 and
exhaust gas temperature within a range of 400.degree. C. to
800.degree. C.) The sensor element 101 was exposed to the resulting
exhaust gases. After 100 hours had passed (durability period=100
h), the gas sensor 100 was taken out, and in a manner similar to
that for the measurement at the time of the start of the test, the
pump current Ip2 was measured for each of the three types of model
gases used as the measurement-object gas. The rate of change in the
pump current Ip2, which is a change from the pump current Ip2 at
the start of the test to the measured pump current Ip2, was
derived, and this was taken as a sensitivity change ratio [%] at a
durability period of 100 h. The exposure of the sensor element 101
for 100 hours and the measurement of the sensitivity change ratio
were repeated in a similar manner, and thus the sensitivity change
ratio was measured until the durability period reached 500 h. Also,
for a gas sensor 100 in which the target value Ip0s* was set to 1
mA, the sensitivity change ratio was measured in a similar manner
until the durability period reached 500 h. The results are shown in
FIGS. 10 and 11.
[0112] As can be seen from FIG. 11, in the case where the
preliminary pump cell 15 pumped oxygen to the inside continuously
with the target value Ip0s* being 1 mA, the sensitivity change
ratio was maintained at or near 0% for all of the three types of
model gases even after elapse of time. That is, almost no decrease
was observed in the NOx concentration detection accuracy of the gas
sensor 100 after the durability test. In contrast, as can be seen
from FIG. 10, in the case where the preliminary pump cell 15 did
not pump oxygen to the inside, with the target value Ip0s* being 0
mA, the following tendencies were observed: for the model gas
having an A/F ratio of 16.6, the sensitivity change ratio was
maintained at or near 0% even after elapse of time, but for the
model gas having an A/F ratio of 12.6 and the model gas having an
A/F ratio of 14.5, which were rich atmospheres, the sensitivity
change ratio deviated from 0% with time. In particular, for the
model gas having an A/F ratio of 12.6, which was a highly rich
atmosphere, the sensitivity change ratio significantly deviated
from 0% with time and eventually reached a negative value having a
large absolute value. This result confirmed that, in the case where
the preliminary pump cell 15 pumped oxygen to the inside, the gas
sensor 100 had higher durability, particularly in the case where
the gas sensor 100 was exposed to a measurement-object gas that was
a rich atmosphere.
[0113] Reasons for this are unknown but may be as follows, for
example. First, the following can be assumed. In the case where the
preliminary pump cell 15 does not pump oxygen to the inside, an
unburned component (ethylene, in the above-described test) in
exhaust gases is adsorbed onto the measurement electrode 44, which
results in a decrease in active sites of the measurement electrode
44, and as a result, the sensitivity of the pump current Ip2 in the
gas sensor 100 decreases with time. In contrast, it can be assumed
that, in the case where the preliminary pump cell 15 pumps oxygen
to the inside, the unburned component can be easily converted into,
for example, CO.sub.2 and H.sub.2O by being oxidized by the pumped
oxygen, and therefore adsorption of the unburned component onto the
measurement electrode 44 does not easily occur.
* * * * *