Oxygen sensor with a heater

Details Oxygen sensor with a heater Oxygen sensor with a heater

1999-06-01

2001-07-31

Tung, T. (Department: 1743)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S427000, C204S428000, C205S785000

Reexamination Certificate

active

06267857

ABSTRACT:

This application claims the benefit of Japanese Patent Application No. Hei. 10-151481, filed in Japan on Jun. 1, 1998, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an oxygen sensor for sensing the oxygen concentration in exhaust gases from, for example, an internal combustion engine or to an oxygen sensor for sensing the oxygen concentration in a specific gas. More particularly, the present invention relates to an oxygen sensor with a heater which can quickly heat the oxygen sensor up to its required activation temperature.
2. Description of the Related Art
Recently, demand for purifying exhaust gases from an internal combustion engine of an automobile, for example, has been increasing. In these circumstances, an oxygen sensor with a heater has been developed which is able to accurately determine the oxygen concentration in the exhaust gases from the internal combustion engine even under a condition that the exhaust gas temperature is low, for example, at engine startup or while the engine is idling. Japanese Patent Application Laid-Open (kokai) No. 4-157358, for example, discloses an oxygen sensor which includes an oxygen sensing element of a hollow shaft-like member having a closed end and an electrode layer on the inner wall thereof, and a shaft-like heating member, which is disposed within the oxygen sensing element, for heating the oxygen sensing element. In the oxygen sensor, the shaft (bar)-like heating member (heater) is coaxially inserted into the inner space of the test-tube like oxygen sensing element, which is made of an oxygen-ion conductive solid electrolyte, until the extreme end thereof reaches or approaches the inner surface of the extreme end of the oxygen sensing element.
In the oxygen sensor described above, when the oxygen sensing element is heated in a nonuniform manner, sufficiently-heated and activated portions and insufficiently-heated and high-resistance portions coexist in the oxygen sensing element. The electrical resistance of the entire oxygen sensing element is frequently determined by the high-resistance portions. This results in prolonging the time until the resistance of the oxygen sensing element becomes satisfactorily low and is thus activated, viz., a rise time of the sensor. In the conventional sensor construction, the heating member is disposed coaxially with the oxygen sensing element, so that the oxygen sensing element is uniformly heated with respect to the circumferential direction and therefore uniformly activated with respect to the same direction. The extreme end of the heating member is in contact with or in proximity to the inner surface of the extreme end of the oxygen sensing element. Therefore, heat transfer from the extreme end of the heating member to the oxygen sensing element will be in a satisfactory level. In this respect, the goal of reducing the rise time of the sensor will be achieved to some extent.
However, conventional sensors have the following problems to be solved. When the bar-like heating member and/or the hollow oxygen sensing element is thermally expanded, the extreme end of the heating member may be spaced apart from the inner surface of the extreme end of the oxygen sensing element, thereby deteriorating heat transfer efficiency. Alternatively, the extreme end of the heating member may be pressed against the inner surface of the extreme end of the oxygen sensing element. In this case, a stress is generated, thus adversely reducing the durability of the device. Thus, the oxygen sensor is greatly influenced by the thermal expansion. This leads to nonuniformity of the heating state of the oxygen sensing element and a variation of the characteristics among individual oxygen sensor products. A possible approach to solve this problem is to form a relatively large space between the heating member and the oxygen sensing element. However, the approach fails to solve the problem because the heat transfer efficiency is lowered, and the sensor rise time is long.
Accordingly, when a reduction in the element rise time is desired, contact between the heating element and the oxygen sensing element becomes mandatory. In terms of a reduction in the sensor rise time, those skilled in the art consider it essential to dispose the oxygen sensing element and the heating element concentrically, for the purpose of attaining uniform heating of the oxygen sensing element So long as sensor design is bound by this concept, there is no alternative but to bring the extreme end of the heating element and the inner surface of the extreme end of the oxygen sensing element into contact. As a result, the conventional sensor construction, in which the extreme end of the heating member is in contact with or in proximity to the inner surface of the extreme end of the oxygen sensing element, involves a variation of characteristics among individual oxygen sensor products in return for a reduction in the sensor rise time. Therefore, the conventional sensor construction is not necessarily the best in terms of quick, effective heating of the oxygen sensing element.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an oxygen sensor with a heater which can accurately sense the oxygen concentration in exhaust gases from an internal combustion engine, even under conditions where the exhaust gas temperature is low; for example, at engine startup or while the engine is idling, by quickly and efficiently heating an oxygen sensing element of a hollowed shaft-like member by a heating member contained in the sensing element, and which can suppress a variation of the characteristics of individual oxygen sensors to a minimum.
To achieve the object described above, an oxygen sensor with a heater (oxygen sensor) according to an embodiment of the invention includes: an oxygen sensing element having a hollow shaft-like member, which is closed at the extreme end and has electrode layers on the inner and outer sides thereof; a shaft-like heating member disposed within a hollow portion of the oxygen sensing element and adapted to heat the oxygen sensing element; and a fixture member comprising a fixture portion circumferentially surrounding the heating member and directly or indirectly (i.e., via another member) maintaining contact with the inner wall of the oxygen sensing element, and a heating-member holding portion being coupled with the fixture portion while being located at least at one side of the fixture portion as viewed in the axial direction of the heating member and being adapted to hold the heating member, thereby fixing the heating member within the oxygen sensing element by means of the fixture portion. A positioning projection is formed on at least either the fixture portion or the heating-member holding portion in such a manner as to project inward and abut the peripheral surface of the heating member. Through employment of such an abutting feature, the heating member is positioned such that the center line of the heating member is disposed eccentric (offset) to the center line of the hollow portion of the oxygen sensing element in the vicinity of a heating portion of the heating member. Preferably, as the result of such an eccentricity (offset), the surface of the heating portion of the heating member abuts the inner wall of the hollow portion of the oxygen sensing element. In other words, as a result of the fact that the center line of the heating member is eccentric (offset) to the center line of the hollow portion of the oxygen sensing element in the vicinity of the heating portion of the heating member, the surface of the heating portion of the heating member abuts the inner wall of the hollow portion of the oxygen sensing element.
When the center line of the heating member is eccentric (offset) to the center line of the hollow portion of the oxygen sensing element, a portion of the oxygen sensing element closer to the heating member is heated to a greater extent, and the heat distribution over the circumference of the oxygen sensing eleme

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