Gas sensor

Details Gas sensor Gas sensor

1998-05-15

2001-09-11

Warden, Sr., Robert J. (Department: 1744)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S408000, C204S426000, C205S781000, C219S209000, C219S543000

Reexamination Certificate

active

06287439

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gas sensor for measuring gas components such as NO, NO
2
, SO
2
, CO
2
, and H
2
O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles.
2. Description of the Related Art
Exhaust gas, which is discharged from vehicles or automobiles such as gasoline-fueled automobiles and diesel powered automobiles, contains nitrogen oxides (NOx) such as nitrogen monoxide (NO) and nitrogen dioxide (NO
2
), as well as carbon monoxide (CO), hydrocarbon (CH), hydrogen (H
2
), oxygen (O
2
) and so on. In such exhaust gas, about 80% of the entire NOx is occupied by NO, and about 95% of the entire NOx is occupied by NO and NO
2
.
The three way catalyst, which is used to clean HC, CO, and NOx contained in the exhaust gas, exhibits its maximum cleaning efficiency in the vicinity of the theoretical air fuel ratio (A/F=14.6). If A/F is controlled to be not less than 16, the amount of produced NOx is decreased. However, the cleaning efficiency of the catalyst is lowered, and consequently the amount of discharged NOx is apt to increase.
Recently, in order to effectively utilize fossil fuel and avoid global warming, the market demand increases, for example, in that the discharge amount of CO
2
should be suppressed. In order to respond to such a demand, it becomes more necessary to improve the fuel efficiency. In response to such a demand, for example, the lean burn engine and the catalyst for cleaning NOx are being researched. Especially, the need for a NOx sensor increases.
A conventional NOx analyzer has been hitherto known in order to detect NOx as described above. The conventional NOx analyzer is operated to measure a characteristic inherent in NOx, based on the use of chemical luminous analysis. However, the conventional NOx analyzer is inconvenient in that the instrument itself is extremely large and expensive. The conventional NOx analyzer requires frequent maintenance because optical parts are used to detect NOx. Further, when the conventional NOx analyzer is used, any sampling operation should be performed for measurement of NOx, wherein it is impossible to directly insert a detecting element itself into a fluid. Therefore, the conventional NOx analyzer is not suitable for analyzing transient phenomena such as those occur in the exhaust gas discharged from an automobile, in which the condition frequently varies.
In order to dissolve the inconveniences as described above, there has been suggested and practically used a sensor for measuring a desired gas component in exhaust gas by using a substrate composed of an oxygen ion-conductive solid electrolyte.
FIG. 10
shows a cross-sectional arrangement of a gas sensor
10
disclosed in Japanese Laid-Open Patent Publication No. 8-271476.
The gas sensor
10
is operated as follows. That is, a measurement gas is introduced into a first hollow space
14
via a first diffusion rate-determining section
12
. A first oxygen pumping means
22
, which comprises an inner pumping electrode
16
, a solid electrolyte
18
, and an outer pumping electrode
20
, is used to pump in or pump out oxygen contained in the measurement gas, into or from the first hollow space
14
to an extent that the measurement gas is not decomposed.
Subsequently, the measurement gas is introduced into a second hollow space
26
via a second diffusion rate-determining section
24
. A second oxygen pumping means
36
, which comprises a measurement gas-decomposing electrode
28
disposed in the second hollow space
26
, a solid electrolyte
30
, and a reference electrode
34
disposed in a reference gas-introducing space
32
, is used to pump out oxygen produced by decomposition and electrolysis caused by the applied voltage or the catalytic action effected by the measurement gas-decomposing electrode
28
.
The value of the current, which is required to pump out oxygen by the second oxygen pumping means
36
, is measured to measure the predetermined gas component contained in the measurement gas, on the basis of the current value.
Those to which the gas sensor
10
is applicable include, for example, NOx sensors, H
2
O sensors and CO
2
sensors for measuring NOx, H
2
O, and CO
2
in which the predetermined gas component has bound oxygen.
In the case of the use as a NOx sensor, NOx is catalytically decomposed by using, for example, Rh or Pt for the measurement gas-decomposing electrode
28
. The oxygen produced during the decomposition can be detected as a pumping current, or it can be detected as a change in voltage of an oxygen concentration cell.
As shown in
FIG. 11
, another gas sensor
10
A has been suggested (see, for example, Japanese Laid-Open Patent Publication No. 9-113484), in which the oxygen dependency of the gas sensor
10
described above is improved when the gas sensor
10
is used as a NOx sensor.
The gas sensor
10
A comprises an auxiliary pumping electrode
38
disposed at a second hollow space
26
. A third oxygen pumping means, i.e., an auxiliary pumping means
40
is constructed by the auxiliary pumping electrode
38
, solid electrolytes (including
18
and
30
), and a reference electrode
34
. The oxygen, which diffuses and enters from a first hollow space
14
in a minute amount, is pumped out again by using the auxiliary pumping means
40
. Accordingly, it is possible to greatly improve the measurement accuracy (especially the dependency on oxygen concentration).
However, the gas sensors
10
,
10
A are in the following actual state of affairs. That is, even when the oxygen concentration in the measurement gas is controlled to be, for example, not more than 1 ppm in the previous stage of NOx measurement by using the first oxygen pumping means
22
, or by using the first oxygen pumping means
22
and the auxiliary pumping means
40
, the pumping current value at NOx=0 (hereinafter referred to as “offset value”) is a value corresponding to 100 ppm which is much higher than a value corresponding to 1 ppm.
If the offset value is always constant in all environments in which the gas sensors
10
,
10
A are used, no problem arises. However, it is feared that a large measurement error may be caused, because the offset value varies depending on the change in temperature of exhaust gas.
In order to decrease the offset value, it is conceived that strict control is performed by using the first oxygen pumping means
22
, and strict control is performed by using the auxiliary pumping means
40
, for example, so that the oxygen concentration to be controlled is further decreased. However, if such strict control is performed, a problem arises in that NOx is decomposed by the pumping process effected thereby.
FIG. 12
shows a situation of the problem described above. When the oxygen concentration in the first hollow space
14
of the gas sensor
10
shown in
FIG. 10
is controlled to be 10
−7
atm (about 300 mV as a voltage detected by an oxygen concentration detector), the offset value is 1.0 &mgr;A.
Theoretically and essentially, the offset value should be a value corresponding to a residual oxygen concentration in the first hollow space
14
, i.e., 0.1 ppm (or corresponding to 0.2 ppm after conversion into a value of NO). However, the offset value is actually 1 &mgr;A (or a value of NO corresponding to 200 ppm obtained by conversion). The NO sensitivity is calculated in accordance with 5 &mgr;A/1000 ppm.
Therefore, if the offset value slightly changes depending on, for example, the temperature of the sensor element, and it is changed in a degree of 10%, then the resultant change corresponds to 20 ppm, which causes a serious problem when NOx is measured at a low concentration in a degree of several hundreds of ppm.
FIG. 13
shows such a situation. For example, when the temperature of the measurement gas (gas temperature) is changed by about 150° C. from 650° C. to 800° C., the offset value is changed in an amount of about 1.5 &mgr;A, i.e., 300 ppm. This causes a serious problem when NOx is measured at a low concentration of se

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