Electric heating – Heating devices – Resistive element: igniter type
Reexamination Certificate
2002-03-15
2004-12-14
Jeffery, John A. (Department: 3742)
Electric heating
Heating devices
Resistive element: igniter type
C123S14500A
Reexamination Certificate
active
06831253
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ceramic glow plug used for accelerating start-up of a diesel engine or a like application.
2. Description of the Related Art
FIG. 9
shows a ceramic glow plug
1
for use in a diesel engine. The ceramic glow plug
1
is configured in the following manner: a round-bar-like (columnar) ceramic heater
2
is fixedly fitted into the interior (cylindrical bore) of a straight tubular, metallic, cylindrical member
3
such that a heater end
2
a
projects from a front end
3
a
of the metallic, cylindrical member (hereinafter may be referred to as merely a cylindrical member)
3
; and the resultant assembly consisting of the ceramic heater
2
and the cylindrical member
3
is fixedly fitted into (united with) a metallic, cylindrical plug body (hereinafter also referred to as merely a plug body or body)
4
such that the front end
3
a
of the cylindrical member
3
projects from a front end
4
a
of the body
4
. The cylindrical member
3
used in this structure serves as a cover for protecting the ceramic heater
2
from various external forces (impact induced by dropping, a bending force applied in the course of mounting on an engine, etc.) that may be imposed on the ceramic heater
2
in the course of manufacture of the glow plug
1
and handling of the manufactured glow plug
1
.
Such a ceramic glow plug
1
is usually assembled in the following manner. First, the ceramic heater
2
is loosely fitted into the cylindrical member
3
, and a molten brazing metal (not shown) is poured into the gap therebetween. Thermal expansion of the cylindrical member
3
induced by the poured, molten brazing metal and contraction of the cylindrical member
3
induced by cooling are utilized for hermetically fixing the ceramic heater
2
in the cylindrical member
3
through squeezing action of the cylindrical member
3
. Similarly, the assembly consisting of the ceramic heater
2
and the cylindrical member
3
, together with other necessary components, is loosely fitted into the body
4
, and a molten brazing metal is poured into the gap therebetween (brazing), thereby completing assembly. Instead of employing such brazing, the assembly consisting of the ceramic heater
2
and the cylindrical member
3
may be press-fitted into the body
4
. However, this method is not put into practical use, because of a drawback described below.
As shown in
FIGS. 9 and 10
, the thus-assembled glow plug
1
is mounted onto a cylinder head
101
of a diesel engine via a threaded portion
6
formed on the outer circumferential surface of the body
4
. Specifically, as shown in
FIGS. 9 and 10
, a mounting bore
104
for mounting the glow plug
1
, the mounting bore
104
communicating with a prechamber
103
, is provided on the cylinder head
101
. The glow plug
1
is inserted from its end into the mounting bore
104
. The threaded portion
6
of the body
4
is engaged with a threaded portion
105
formed in the mounting bore
104
such that the heater end
2
a
partially projects into the prechamber
103
.
The mounting bore
104
is configured in the following manner. In order to mount the plug body
4
while a seal is established at the front end
4
a
of a cylindrical portion
7
of the plug body
4
, a cylindrical bore
106
formed at a deep location of the mounting bore
104
allows the cylindrical portion
7
to be loosely fitted thereinto. Additionally, the front end
4
a
of the cylindrical portion
7
is located at a deep location of the cylindrical bore
106
and pressed against a dead end portion
107
, whose bore diameter is smaller than the thread diameter. A cylindrical, small bore
108
, whose diameter is equal to the bore diameter of the dead end portion
107
, is formed at a location deeper than the dead end portion
107
. The cylindrical member
3
is disposed in the cylindrical, small bore
108
in a loosely fit condition.
3. Problems to be Solved by the Invention
When current is applied to the thus-mounted glow plug
1
via electricity supply leads
15
and
16
, a heating member embedded in the ceramic heater
2
generates heat through resistance to thereby accelerate start-up of an engine. The ceramic heater
2
is exposed to significant temperature variations (thermal shock) and blast waves during combustion of the engine. In the course of use, vibration and temperature rise may cause loosening at a joint between the cylindrical member
3
and the body
4
. Such loosening potentially causes the cylindrical member
3
united with the ceramic heater
2
to drop off the inner circumferential surface of the body
4
into the prechamber
103
.
Such a problem may arise even when the cylindrical member
3
united with the ceramic heater
2
is press-fitted into the body
4
. When the cylindrical member
3
and the body
4
are to be joined together by press fitting, the cost of a glow plug can be lowered as compared with the case where the cylindrical member
3
and the body
4
are joined by brazing. This is because placement of a brazing metal and design to endure brazing temperature are not involved. In order to prevent breakage of the ceramic heater
2
, which may result from stress concentration at the time of press fit, the tolerance of press fit must be reduced. However, a reduction in the tolerance of press fit increases the possibility of drop-off, and this is the reason why the press fit method is not put into practical use.
SUMMARY OF THE INVENTION
The present invention has been accomplished in view of the above-described problems involved in a conventional glow plug. It is therefore an object of the present invention to realize a structure for mounting a glow plug on a cylinder head, the structure preventing an assembly consisting of a cylindrical member and a ceramic heater from dropping off a body and entering an engine (prechamber), regardless of the means employed for joining the body and the cylindrical member, even when looseness arises in a joint between the body and the cylindrical member in the course of use of a glow plug mounted on the cylinder head.
The above object has been achieved in a first aspect of the invention by providing a ceramic glow plug in which a ceramic heater is fixedly disposed in a metallic, cylindrical member such that a front end thereof projects from a front end of the metallic, cylindrical member, and a resultant assembly consisting of the ceramic heater and the metallic, cylindrical member is fixedly disposed in a metallic, cylindrical plug body such that the front end of the metallic, cylindrical member projects from a front end of the metallic, cylindrical plug body.
The ceramic glow plug is characterized in that a portion of the metallic, cylindrical member which projects from the front end of the metallic, cylindrical plug body and extends over a predetermined range from the front end of the metallic, cylindrical member toward a rear end of the metallic, cylindrical member is smaller in outside diameter than a portion of the metallic, cylindrical member which is fixedly disposed within the metallic, cylindrical plug body, and that a front-end-oriented end face is formed in a diameter transition region of the metallic, cylindrical member where the outside diameter changes between those of the two portions.
According to the present invention, the above-described front-end-oriented end face is formed on the cylindrical member, thereby preventing entry of the cylindrical member into the cylinder head (prechamber) when the ceramic glow plug is mounted in a glow plug mounting bore (hereinafter also referred to merely as a mounting bore) formed in the cylinder head. Specifically, as described above, the ceramic glow plug is mounted onto the cylinder head via a threaded portion formed on an outer circumferential surface of the plug body so as to be screwed into a threaded portion of a ceramic glow plug mounting bore formed in the cylinder head until the front end of the plug body is pressed against a dead end portion (seat face) located at a deep location of the
Otani Takayuki
Taniguchi Masato
Jeffery John A.
NGK Spark Plug Co. Ltd.
Sughrue & Mion, PLLC
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