Electric heating – Heating devices – Resistive element: igniter type
Reexamination Certificate
1999-09-09
2001-03-20
Jeffery, John A. (Department: 3742)
Electric heating
Heating devices
Resistive element: igniter type
C123S14500A
Reexamination Certificate
active
06204481
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ceramic heater that can be used to help start diesel engines, wherein the heater includes a ceramic heating member.
2. Description of the Related Art
FIG. 7A
shows a conventionally known ceramic heater
100
, which can aid in the starting of a diesel engine. As shown in
FIG. 7A
, the conventional ceramic heater
100
includes a metallic cylindrical member
101
and a ceramic heating element
102
, which is held at an end portion of the cylindrical member
101
. The ceramic heating member
102
includes an insulating ceramic body
103
having a bar shape; a conductive ceramic element
104
having a U-shape, which is embedded in an end portion of the insulating ceramic body
103
; and electrodes
105
, which are embedded and thus connected to the respective end portions of the conductive ceramic element
104
. Upon being supplied with electricity by means of the electrodes
105
, the conductive ceramic element
104
generates heat through electrical resistance.
In the above-described ceramic heater
100
, the cylindrical member
101
repeatedly expands and contracts from the heat generated by resistive heating of the ceramic heating element
102
as well as from repeated heating and cooling during combustion of the engine. As a result, a compressive stress is repeatedly exerted on the ceramic heating element
102
. This compressive stress tends to become excessively large at an end portion
101
a
of the cylindrical member
101
, since the end portion is more likely to be subjected to heat generated by the conductive ceramic element
104
and heat radiated from the engine. End portions
104
a
of the conductive ceramic element
104
, where the respective electrodes
105
are embedded, are located within the end portion
101
a
. As shown in
FIG. 7B
, due to a difference in thermal expansion coefficient between the electrode
105
and the conductive ceramic element
104
, a fine defect, such as a gap
105
a
, may form in the boundary there-between during cooling such as would occur after firing. When the above-mentioned compressive force is exerted on such a defective region, a crack may develop in the conductive ceramic element
104
, potentially shortening the life of the conductive ceramic element
104
.
At the same time, in order to meet the recent tightening of exhaust gas regulations and to improve fuel consumption ratio, employment of a direct injection system in a diesel engine is rapidly becoming common practice. Thus, there is also a need for increasing the distance between the end of a seat surface and the end of a ceramic heating member by at least 5 mm longer compared with a ceramic heating member used in a swirl-chamber type diesel engine. However, the longer projection of the ceramic heating member into a combustion chamber inevitably leads to thermally induced stress and thus cracking, which may not be sufficiently suppressed simply by disposing within the cylindrical member
101
the boundary between the electrode
105
and the conductive ceramic element
104
.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a ceramic heater whose conductive ceramic element exhibits excellent durability. To achieve this object, the present invention provides a ceramic heater comprising a metallic shell which is attached to a structural body such that a seat surface located on an end portion thereof abuts the structural body. The present ceramic heater also comprises a ceramic heating member which is disposed within the metallic shell such that an end portion thereof is projected from an end face of the metallic shell.
The present ceramic heating member, therefore, comprises a ceramic body, a conductive ceramic element, and two electrodes. The conductive ceramic element is embedded in a portion of the ceramic body corresponding to the end portion of the ceramic heating member. The two electrodes are connected to the conductive ceramic element such that one end of one electrode is embedded in one end of the conductive ceramic element, and one end of the other electrode is embedded in the other end of the conductive ceramic element. Electricity is applied to the conductive ceramic element by means of the electrodes so that the conductive ceramic element generates heat through electrical resistance. The conductive ceramic element may include a U-shaped portion for carrying electrical current. This portion, which is referred to as a direction-changing portion, extends from one base end thereof and changes directions to extend to the other base end thereof and contains two straight portions, which extend in the same direction from the corresponding ends of the direction-changing portion. The conductive ceramic element is disposed such that the direction-changing portion corresponds to the end portion of the ceramic heating member.
The distance L between the ends of the electrodes embedded in the conductive ceramic element and the end of the seat surface of the metallic shell is set so as to satisfy the expression L≦1 mm, where the distance L is considered negative when the ends of the electrodes are located within the metallic shell. More preferably, the distance L is set so as to satisfy the expression L≦0 mm.
By maintaining the distance L as described above, resistive-induced heat, i.e., heat that is generated in an interface portion between the electrode and the conductive ceramic from the electricity that is applied to the conductive ceramic element, can be released effectively to the structural body. As a result, cracking in the conductive ceramic element which would otherwise result from the aforementioned compressive stress is effectively prevented or suppressed.
Preferably, the ceramic heater further comprises a cylindrical member which is interposed between the ceramic heating member and the metallic shell and is projected from the end of the seat surface of the metallic shell. As a result, the interface portion between the electrode and the conductive ceramic element is located apart from an end portion of the cylindrical member, which can expand and contract due to the increased temperatures resulting from resistive heat and heat radiated from the engine. Accordingly, the compressive stress induced by expansion/contraction of the cylindrical member is hardly exerted on the interface portion.
The effect of the present invention becomes remarkable when the end of the ceramic heating member is located at least 20 mm apart from the end of the seat surface of the metallic shell. In this case, heat generated by electrical resistance in the ceramic heating member and radiated from the engine becomes more difficult to release to the structural body through the cylindrical member.
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Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Jeffery John A.
NGK Spark Plug Co. Ltd.
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