Oxygen concentration detector

Chemistry: electrical and wave energy – Apparatus – Electrolytic

Reexamination Certificate

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Details

C204S408000, C205S784000

Reexamination Certificate

active

06432289

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an oxygen concentration detector which is used for air/fuel ratio control, etc. in an automobile engine.
2. Description of the Related Art
Oxygen concentration detectors have conventionally been provided with protecting covers to protect their sensing elements. That is, an oxygen concentration detector has a solid electrolyte, a sensing element which consists of the solid electrolyte coated on the surface with an outer electrode, a heater provided inside the above-mentioned solid electrolyte, and a protecting cover which protects the above-mentioned sensing element. In addition, two levels of openings are provided in the above-mentioned protecting cover, through which a gas to be measured is introduced into the gas-measuring chamber.
The oxygen concentration detector described above is sometimes mounted onto the exhaust pipe, etc. and used as a part of the engine's combustion control system, in order to make the automobile engine burn fuel at the theoretical air/fuel ratio. The exhaust gas cleaning efficiency is highest when the engine burns fuel at the theoretical air/fuel ratio.
However, in order to further increase the exhaust gas cleaning efficiency, it has been attempted, in recent years, to raise the sensitivity of the oxygen concentration detectors used for detecting the air/fuel ratio.
The sensing elements of such oxygen concentration detectors, however, have unstable characteristics until heated above the activation temperature. As a result, when the temperature of the sensing element is low, for example, when the engine is started, the oxygen concentration sensitivity is also low.
Consequently, in order to increase the exhaust gas cleaning efficiency, it is necessary to heat the sensing element to the above-mentioned activation temperature within a short time after starting the engine.
To meet this requirement, it has been proposed to heat the sensing element by increasing the power fed to the heater to increase the heater temperature.
However, in such cases there is a risk that the temperature of the heat-generating part inside the heater may increase to an abnormally high temperature which is considerably higher than the temperature necessary for oxygen concentration detection, and may even rise above the heat-resistant temperature of the ceramic or other material of which the heater is constructed. This results in problems such as damage to the heater and the shortening of its useable life.
In light of these problems, it is an object of the present invention to provide an oxygen concentration sensing element which allows the sensing element to be heated to the activation temperature more rapidly, without raising the temperature of the heat-generating part above the heat-resistant temperature of the heater.
SUMMARY OF THE INVENTION
The present invention is an oxygen concentration detector which comprises a sensing element which includes a solid electrolyte coated on the surface thereof with an outer electrode, a heater provided inside or near the above-mentioned solid electrolyte, and a protecting cover which protects the above-mentioned sensing element, characterized in that
the protecting cover has two levels of openings, and
the area of the outer electrode which contributes to the exchange (conduction) of oxygen ions is constructed within the range defined by the length of the heat-generating part of the heater, and the section of the protecting cover adjacent (nearest) to the sensing element is provided with two levels of openings in the axial direction outside the range corresponding to the range in which the outer electrode contributes to exchange of oxygen ions, wherein the relationship between the length L
1
of the heat-generating part and the distance between the openings L
2
of the two levels of openings in the axial direction is such that L
1
/L
2
=0.9-1.3.
The above-mentioned two levels of openings may be coaxial with respect to the central axis of the oxygen concentration detector, or they may be non-coaxial, at spirally offset positions. The distance between the two levels of openings is the distance from the perimeter line including the lower edge of the openings at the proximal end of the sensing element to the perimeter line including the upper edge of the openings at the distal end of the sensing element (see FIGS.
7
(A), (B)).
The heater consists of a heat-generating part which increases in temperature upon electrification to heat the sensing element, a lead part which supplies power to the heat-generating part, and a ceramic body which houses the heat-generating part and the lead part. The heat-generating part and the lead part may take any of a variety of shapes (see
FIG. 8
, FIGS.
15
-
18
), and the construction of the present invention may be applied to heat-generating parts and lead parts of all such shapes.
The heat-generating part is made of a paste consisting of, for example, platinum, tungsten, molybdenum or the like. Furthermore, the heater may either be provided inside the sensing element as a separate heater, or it may be formed integrally with the solid electrolyte of the sensing element (which results in a layered sensing element).
The relationship between the length L
1
of the heat-generating part and the distance between the openings L
2
is such that L
1
/L
2
=0.9-1.3. If the value of L
1
/L
2
is less than 0.9, then there is a possibility that the temperature of the heat-generating part may rise above the heat-resistant temperature of the heater when the sensing element is heated to the activation temperature. This may result in damage to the heater and the shortening of its useable life.
On the other hand, if the value exceeds 1.3 the power consumption of the heat-generating part may increase, thus lowering efficiency.
A lower power consumption is also preferred since the heater has a positive resistance temperature coefficient, and thus in cases where the heater resistance is high, sufficient power sometimes may not be provided by the voltage of the battery mounted in the automobile.
The length of the heat-generating part is preferably between 8 mm and 16 mm. If the length is less than 8 mm, then the temperature of the heat-generating part may rise above the heat-resistant temperature of the heater when the sensing element is heated to the activation temperature. Conversely, a length exceeding 16 mm will increase power consumption by the heat-generating part and may result in the same type of problem as when the value of L
1
/L
2
is greater than 1.3.
The distance between the openings L
2
is preferably between 9 mm and 16 mm. If the length is less than 9 mm, then the shorter distance between the outer electrode of the sensing element and the openings may result in a greater tendency toward deterioration of the outer electrode due to contamination in the gas to be measured. Conversely, if the length exceeds 16 mm then the longer distance between the outer electrode of the sensing element and the openings may result in a poor response.
The axial offset &Dgr;L between the center position of the heat-generating part of the heater and the center position of the distance between the openings L
2
. is preferably no more than L
2
/4.
If this &Dgr;L is longer than L
2
/4, as explained below, the sensing element will be cooled by the low-temperature gas to be measured which is introduced through the openings, and this may make it impossible to rapidly heat the sensing element to the activation temperature.
The area of the individual openings in the openings is preferably between 0.75 and 3.5 mm
2
, and the total area counting both levels of openings is preferably between 10 and 23 mm
2
. If the total area of both levels of openings is less than 10 mm
2
, then it may become impossible to introduce a sufficient amount of the gas to be measured into the gas-measuring chamber to allow detection of the oxygen concentration, and if the area of each individual opening is less than 0.75 mm
2
, then working of the openings becomes pr

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