Electricity: measuring and testing – Using ionization effects – For analysis of gas – vapor – or particles of matter
Reexamination Certificate
2000-04-28
2002-07-30
Le, N. (Department: 2858)
Electricity: measuring and testing
Using ionization effects
For analysis of gas, vapor, or particles of matter
C123S438000, C204S406000, C204S424000
Reexamination Certificate
active
06426631
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an oxygen sensor device used for controlling an air-fuel ratio in an internal combustion engine such as of automobiles. More specifically, the invention relates to an oxygen sensor device incorporating a heater therein and is capable of detecting the concentration of oxygen within a very short period of time.
2. Description of the Prior Art
Modern internal combustion engines of automobiles are employing a method of detecting the concentration of oxygen in the exhaust gas, and controlling the amounts of the air and fuel supplied into the internal combustion engine based on the detected value, in order to decrease harmful substances such as CO, HC and NOX emitted from the internal combustion engine.
As a device for detecting the concentration of oxygen, there has heretofore been known a cylindrical oxygen sensor device of the indirectly heated type having a structure as shown in, for example, FIG.
18
. The oxygen sensor device is constituted by a cylindrical tube
31
made of a solid electrolyte such as zirconia having oxygen ion conducting property with its one end being closed. On the inner surface of the cylindrical tube
31
is provided a reference electrode
32
that comes in contact with a reference gas such as the air. On the outer surface of the cylindrical tube
31
is formed a measuring electrode
33
that comes in contact with a gas to be measured such as exhaust gas. Further, on the surface of the measuring electrode
33
is formed various porous ceramic layers
34
depending upon the use of the oxygen sensor device.
In, for example, a so-called stoichiometric air-fuel sensor (&lgr; sensor) used for controlling the air-fuel ratio (A/F ratio) near 1, the porous ceramic layer
34
formed on the surface of the measuring electrode
33
works as a protection layer, and a difference in the oxygen concentration between the inner surface and the outer surface of the cylindrical tube
31
is detected at a predetermined temperature to control the A/F ratio in the engine intake system.
On the other hand, in a so-called wide-range air-fuel ratio sensor (AF sensor) used for controlling the A/F ratio over a wide range, the porous ceramic layer formed on the surface of the measuring electrode
33
contains fine pores and works as a gas diffusion rate-determining layer, and in which a voltage is applied to the cylindrical tube
31
of a solid electrolyte through a pair of electrodes
32
and
33
, and a limit current that is obtained is measured to control the A/F ratio in a lean burn region.
In either the above-mentioned &lgr; sensor or AF sensor, the sensing portion (where the reference electrode
32
and the measuring electrode
33
are provided) must be heated up to an operation temperature of about 700° C. For this purpose as shown in
FIG. 18
, a rod-like heater
35
is inserted in the inner space of the cylindrical tube
31
to heat the sensing portion up to the operation temperature (activating temperature).
In recent years, strict regulations have been enforced against the exhaust gases and, hence, it has been urged to detect CO, HC and NOx from immediately after the start of the engine. With the cylindrical oxygen sensor device of the indirectly heating type in which the above-mentioned heater
35
is inserted in the cylindrical tube
31
, however, an extended period of time (activating time) is required before the sensing portion is heated up to the activating temperature leaving a problem in that regulations against the exhaust gases cannot be coped with to a sufficient degree.
To solve this problem, Japanese Unexamined Utility Model Publication (Kokai) No. 199666/1986 proposes a flat plate-type oxygen sensor device incorporating a heater as shown in FIG.
19
. In this oxygen sensor device, space for a reference gas is formed in a solid electrolyte
39
of a flat plate, a measuring electrode
37
and a reference electrode
38
are formed on the outer surface and on the inner surface of a flat wall
36
of the solid electrolyte
39
, and a heater
42
is integrally laminated on the solid electrolyte
39
. The heater
42
is constituted by a ceramic insulating board
40
of a flat plate in which a heat-generating member
41
is buried.
Further, Japanese Unexamined Patent Publication (Kokai) No. 206380/1998 proposes a cylindrical oxygen sensor device incorporating a heater therein. In this oxygen sensor device like the oxygen sensor device of
FIG. 18
, a reference electrode and a measuring electrode are provided on the inner surface and on the outer surface of the cylindrical tube of a solid electrolyte, but having a gas-permeable porous insulating layer formed on the surface of the measuring electrode and a platinum heat-generating member provided in the insulating layer where the gas-permeability is low.
Unlike those of the conventional indirectly heated type, the above-mentioned flat-plate or cylindrical oxygen sensor incorporating a heater can be quickly heated owing to its direct-heating system, and the sensing portion can be quickly activated.
However, the oxygen sensor device incorporating a heater shown in
FIG. 19
has poor durability and heat resistance due to its flat-plate shape, and is liable to be broken during the operation.
The oxygen sensor device incorporating a heater proposed in Japanese Unexamined Patent Publication (Kokai) No. 206380/1998 is manufactured by forming a cylindrical solid electrolytic portion by firing, forming the electrodes by plating or sputtering, and forming the insulating layer by plasma melt-injection method. In other words, this oxygen sensor device is produced by a complex method through an increased number of steps accompanied by such problems as poor yield and increased manufacturing cost. Besides, since a porous insulating layer is formed on the whole surface of the measuring electrode while burying the heat-generating member in the insulating layer, the junction strength of the heater portion is small lacking durability and heat resistance.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an oxygen sensor device incorporating a heater therein, which can be activated within a short period of time, exhibits excellent durability and heat resistance, and can be easily produced.
According to the present invention, there is provided an oxygen sensor device comprising:
a cylindrical tube of a ceramic solid electrolyte having an oxygen ion conducting property and with its one end being closed;
a reference electrode formed on an inner surface of said cylindrical tube; and
a measuring electrode formed on an outer surface of said cylindrical tube at a position at least opposed to said reference electrode; wherein
a ceramic layer is formed on the outer surface of said cylindrical tube, said ceramic layer having an opening portion and incorporating a heat-generating member therein;
said opening portion in said ceramic layer is formed at such a position that said measuring electrode is at least partly exposed therein; and
said heat-generating member is buried in said ceramic layer at a position at least near said measuring electrode.
In the oxygen sensor device of the invention, the outer surface of the cylindrical tube of the solid electrolyte is covered with the ceramic layer incorporating a heat-generating member therein and, besides, the heat-generating member is arranged near the measuring electrode (e.g., around the opening portion that works as a sensing portion). According to the present invention, therefore, the sensing portion is very efficiently heated by the heat-generating member and is quickly heated making it possible to shorten the time (activating time) until the activating temperature is reached. Besides, even compared with the conventional flat-plate type oxygen sensor device incorporating a heater shown in
FIG. 19
, the oxygen sensor device of the invention greatly shortens the activating time and exhibits excellent sensor response, since the heat-generating member has been arranged near the se
Akiyama Masahide
Matsunosako Hitoshi
Ono Hiroshi
Hogan & Hartson L.L.P.
Kerveros James
Kyocera Corporation
Le N.
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