Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-08-05
2002-02-12
Tung, T. (Department: 1743)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S426000
Reexamination Certificate
active
06346179
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gas sensor, such as an oxygen sensor, an HC sensor, or an NO
x
sensor, for detecting a component of a gas to be measured.
2. Description of the Related Art
A known gas sensor of the type described above has a rodlike or cylindrical detection element having a detection portion for detecting a component of interest. The detection portion is formed at the front end of the detection element, which is housed in a metallic casing. This gas sensor has a protector that covers the detection portion placed within an environment to be measured. Gas passage holes are formed in the sidewall of the protector. A gas such as exhaust gas to be measured is guided into the protector via the gas passage holes and brought into contact with the detection portion.
Many of various gas sensors recently developed for use in automobiles use a double-structure protector consisting of inner and outer cylindrical portions to provide enhanced protection for the detection portion from water droplets, oil droplets, and contaminants. The conventional double-structure protector indicated by
106
in
FIG. 9
has the inner and outer cylindrical portions
106
a
and
106
b.
Gas inlets
163
and
161
are formed in the sidewalls of the cylindrical portions, respectively. Gas under measurement first passes through the gas inlets
163
in the outer cylindrical portion
106
a
and then passes through the gas inlets
161
in the inner cylindrical portion
106
b
to the detection portion
102
.
In the protector of the double structure described above, the protecting performance of the detection portion is enhanced. However, the resistance to the passing gag is increased because of the double wall structure. For example, the rate at which the gas under measurement is exchanged is often low in the space between the outside and inside of the protector. Therefore, the structure has the problem that the response tends to be delayed if the concentration of the component under measurement within the measured ambient varies rapidly.
If the detection portion
102
shown in
FIG. 9
has a gas-sensitive surface DP on only one side of a lamination, the following problem takes place. If gas EG to be measured such as exhaust gas flows into the protector
106
from the side of the gas-sensitive surface DP, the gas stream arrives at the gas-sensitive surface DP relatively directly. Therefore, where the concentration of the component under measurement changes, the detection response is relatively good. However, where the gas flows from the opposite side, the gas stream strikes the surface of the detection portion
102
on the opposite side of the sensitive surface DP. This tends to delay the response of the detection. In this way, the sensor has the drawback that the response and output characteristics are easily affected according to the direction of the gas stream to be measured relative to the protector.
If the protector is built as a single structure, the rate at which the gas is exchanged between the inside and outside of the protector is enhanced so that the response of the sensor is improved. Of course, however, the function of protecting the detection portion deteriorates. If the gas flow rate increases rapidly, or if the gas temperature drops quickly, the temperature of the detection portion drops, thus deactivating an oxygen concentration cell (also known as a differential aeration cell). This leads to a deterioration of the detection sensitivity, or the output signal from the cell is interrupted. The gas exchange rate may also be increased by increasing the dimensions of the gas inlets in the double-structure protector. This structure inevitably produces problems similar to those with the aforementioned single-structure protector, though the results may be more or less serious. Hence, it has been difficult to achieve good response and high protecting performance simultaneously.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a gas sensor which is provided with a multiple-structure protector and thus protects a detection element well and which has sensor response characteristics depending to a lesser extent on the direction of the stream of a gas under measurement than conventional and, therefore, produces uniform response or output characteristics at an appropriate level.
The above object of the present invention has been achieved by providing a gas sensor which comprises a detection element having a front-end portion, a detection portion formed on or in the front-end portion of the detection element, and a protector that covers the detection portion. The protector comprises a first cylindrical portion and a second cylindrical portion disposed outside said first cylindrical portion. A tapering reduced portion is formed at an axially front-end side of a sidewall portion of said first cylindrical portion. A second side gas inlet is formed in a sidewall portion of said second cylindrical portion and corresponds in position to said reduced portion.
In the above-described gas sensor in accordance with the present invention, the protector is of the multiple structure, i.e., at least the double structure having the inner first cylindrical portion and the outer second cylindrical portion. This suppresses intrusion of water droplets, oil droplets, and so on into the protector. Hence, the instrument is excellent in protecting the detection portion.
Furthermore, a tapering reduced portion is formed at an axially front side of the sidewall portion of the first cylindrical portion. Second side gas inlets are formed in the sidewall portion of the second cylindrical portion and correspond in position to the reduced portion. Gas stream to be measured strikes the reduced portion and flows along the outer surface of the sidewall portion of the reduced portion. This produces a negative pressure at the side of the first side gas outlet, decreasing the pressure inside the first cylindrical portion. The gas under measurement is quickly drawn in from the first gas inlets spaced circumferentially. In consequence, sufficient response can be secured in spite of the multiple structure of the protector.
Where the first cylindrical portion is cut along a plane including the axis, the cross section through the tapering portion may be straight or curved outwardly or inwardly. Setting the diameter of the front end of the reduced portion smaller than that of the base end is preferable for production of a negative pressure in the first side gas outlet.
More specifically, the first cylindrical portion has the tapering portion that is a truncated cone coupled to the front end of a cylindrical body. For example, once the total length of the first cylindrical portion is set, the tilt angle of the outer surface of the tapering portion can be easily adjusted to a value adapted for production of a negative pressure in the first side gas outlet by adjusting the length of the cylindrical body formed at the side of the base end.
Accordingly, a specific embodiment of the present invention comprises:
a detection element for detecting a detected constituent of a gas under measurement by a detection portion formed in a front-end portion;
a cylindrical element container that covers the detection element while permitting the detection portion to protrude; and
a protector coupled to an opening end portion in the element container through which the detection portion protrudes, the protector covering the detection portion while permitting passage of the gas to be measured;
said protector comprising
a first cylindrical portion circumferentially surrounding the detection portion around the axis of the detection element, the first cylindrical portion having a sidewall portion provided with plural first side gas inlets circumferentially spaced from each other at regular intervals, a tapering reduced portion being formed at an axially front end side of the sidewall portion, the reduced portion having a front-end surface provided with a first gas outlet,
Awano Shinya
Makino Keisuke
Nakao Takashi
Oi Mitsunori
Okawa Teppei
NGK Spark Plug Co. Ltd.
Tung T.
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