Electric heating – Heating devices – Resistive element: igniter type
Reexamination Certificate
1999-09-30
2001-02-06
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
Resistive element: igniter type
C123S14500A, C123S14500A
Reexamination Certificate
active
06184497
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to ceramic heater elements. In particular, this invention relates to ceramic heater elements, and methods of manufacture therefor, such as ceramic heaters used in high-temperature a glow plugs for diesel engines.
BACKGROUND OF THE INVENTION
It is well known to manufacture ceramic glow plugs having a multi-layered construction. Examples of such conventional glow plugs are described in U.S. Pat. Nos. 4,742,209, 5,304,778 and 5,519,187. In general, these glow plugs have a ceramic heater with a conductive core enclosed by insulative and resistive ceramic layers, respectively. The layers are separately cast and fitted together. The resulting green body is then sintered to form a ceramic heater. Such ceramic heaters suffer several drawbacks. Used in a glow plug, they experience cyclic heating and cooling, which results in high internal stresses at the interfacial junction between the ceramic layers, promoting eventual failure of the glow plugs. To reduce this failure rate, such ceramic heaters tend to be cycled at lower temperatures than would be optimal in a diesel engine.
The internal stresses of a layered glow plug are mainly the result of differences in the coefficients of thermal expansion between the differently composed layers. The different layers of the glow plug expand and contract at different rates. Further, residual stresses are the result of manufacture, particularly from uneven contraction in the cooling period which occurs below the plastic deformation state of the ceramic composition, and from non-uniform attachment between the layers.
A ceramic heater that has reduced internal stress is described in U.S. patent application Ser. No. 08/882,306, filed Jun. 25, 1997. This application discloses a ceramic heater that is slip cast as a unitary body with a graduated composition in the interfacial boundary zones. While the ceramic heater described in this application has reduced internal stresses, it has been found to be difficult to manufacture to the stringent standards required of such heaters. In particular, the layer thicknesses are difficult to control precisely, and even minor discrepancies can lead to widely varying heat output in the final heater. Precise control of heating characteristics, and limiting heating losses in the base portion of the heater element, is important if the ceramic heaters are to be mass produced for vehicle and engine manufacturers.
It is, therefore, desirable to provide a ceramic heater element that overcomes the disadvantages of the prior art. In particular, it is desirable to provide a ceramic heater element that has low internal thermal stresses, and precisely controllable and reproducible heating characteristics that are focussed mainly to the heating tip of the element.
SUMMARY OF INVENTION
The disadvantages of the prior art may be overcome by providing a novel ceramic heater element, particularly for a glow plug, wherein the ceramic heater element has at least five layers, including an inner conductive core and an outer conductive layer that do not extend into the heater tip.
Generally, the present invention provides a ceramic heater element and a glow plug incorporating the novel heater element. The heater element has a base portion and a heater portion. Conductive, insulative and resistive layers extend through both the base and heater portions. An outer conductive layer is applied to the outside of the base portion to provide a highly conductive return path. This tends to limit the heating of the resistive layer in the base portion and results in better and more reliable heat concentration in the heater portion. The heater element can be assembled to form a glow plug for a diesel engine.
In a preferred embodiment of the present invention, the ceramic heater includes a base portion with a heater portion formed at one end. The heater portion has a lesser diameter than the base portion. The base portion and heater portion each having a conductive ceramic layer and a resistive ceramic layer, which are separated by an insulative ceramic layer except at a tip of the heater portion where they are electrically connected. The base portion further has an outer conductive ceramic layer in electrical contact with the resistive ceramic layer. An optional central conductive core can be included in this heater, which extends substantially the length of the base portion.
In a further embodiment of the present invention, there is provided a glow plug for a diesel engine, employing the above-described heater element. The glow plug has a metallic housing, including a barrel and a tapered sleeve. A ceramic heater element, having a base portion tapered to wedgingly fit within the sleeve, is mounted within the housing. The heater element has a heater portion formed at an end of the base portion. The heater portion has a lesser diameter than the base portion, and generally extends beyond the housing. The base portion and heater portion each having a conductive ceramic layer and a resistive ceramic layer, which are separated by an insulative ceramic layer except at a tip of the heater portion where they are electrically connected. The base portion further has an outer conductive ceramic layer in electrical contact with the resistive ceramic layer. An optional central conductive core can be included in this heater, which extends substantially the length of the base portion.
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Dahbour Fadi H.
Gowling Lafleur Henderson LLP
Le-Mark International Ltd.
Walberg Teresa
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