1. Field of the Invention
The present invention relates to glow plugs used in assisting startup of a diesel engine.
2. Description of the Related Art
Conventionally, glow plugs used in assisting startup of a diesel engine include a tubular metallic shell and a heater mounted at the front end of an axial hole of the shell, and are configured such that a front end portion of the heater projects into the engine. Further, a bar-shaped center shaft, formed of metal, is inserted into the axial hole of the metallic shell. The center shaft is insulated from the metallic shell and is mounted such that one end portion of the center shaft projects from the rear end of the metallic shell. Two electrodes extend from the heater so as to supply electricity to the heater and are electrically connected to the metallic shell and the center shaft, respectively.
In a glow plug having such a structure, in order to maintain the axial hole of the metallic shell airtight, an O-ring is disposed between the wall surface of the axial hole and the center shaft at the rear end of the axial hole. Further, an insulating member is disposed between the wall surface of the axial hole and the center shaft, and the O-ring is pressed from the rear end side by an end surface of the insulating member. Thus, the O-ring is brought into close contact with the end surface of the insulating member, the wall surface of the axial hole, and the outer circumferential surface of the center shaft, so as to seal the interior of the axial hole. Reference is made, for example, to Japanese Patent Application Laid-Open (kokai) No. 2005-315474.
One problem that has occurred with current glow plug constructions is that glow plugs have been reduced in diameter so as to meet the recent demand for downsizing diesel engines, and the clearance space between the wall surface of the axial hole and the center shaft of each glow plug has decreased. Therefore, disposing an O-ring in the clearance space at the time of assembly of a glow plug is difficult. In addition, because the cross section of an O-ring perpendicular to the circumferential direction thereof is of a circular shape, when such an O-ring is disposed in the narrow clearance space between the wall surface of the axial hole and the center shaft, a portion thereof undergoes substantial local deformation, because the O-ring is sandwiched between the wall surface of the axial hole and the outer circumferential surface of the center shaft and is pressed by the insulating member. If the internal stress of the O-ring increases due to that deformation, the elastic force of the O-ring decreases, which reduces the degree of contact between the wall surface of the axial hole and the outer circumferential surface of the center shaft, and makes it difficult to maintain the axial hole airtight.