Patent Publication Number: US-7905209-B2

Title: Glow plug with combustion pressure sensor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to Japanese Patent Application No. 2007-224595, filed on Aug. 30, 2007, the content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to combustion pressure sensors for use in internal combustion engines and, more particularly, to a glow plug with combustion pressure sensor for detecting a pressure in a combustion chamber formed in an engine head to allow an engine to be controlled based on a detected pressure to achieve an optimized combustion state. 
     2. Description of the Related Art 
     There has heretofore been generally known a glow plug with combustion pressure sensor composed of a glow plug, preheating a combustion chamber when starting up an engine, and a combustion chamber which are integrally structured for detecting a pressure inside the combustion chamber. Japanese Patent Application Publication No. 2005-90954 discloses one example of such a structure, which is shown in  FIG. 3 .  FIG. 3  is a typical view showing a glow plug with combustion pressure sensor  300  of the related art that is mounted on an engine head  301 . 
     Hereunder, for the sake of convenience of illustration, an upper area and a lower area in  FIG. 3  are referred to as a base end or base end portion and leading end or leading end portion, respectively. 
     The glow plug with combustion pressure sensor  300  has a heating rod  31 , having a leading end exposed to a combustion chamber  302 , which has a base end portion connected to an intermediate shaft  37  made of metal to act as an electrode. The intermediate shaft  37  and a heating member are electrically connected to each other. The intermediate shaft  37  protrudes from a housing  30  and fixedly retained with a contact tube  34  via an O-ring  38 . 
     With such a structure, the heating rod  31  is displaced toward the base end of the glow plug with combustion pressure sensor  300  in response to fluctuation in combustion pressure inside the combustion chamber  302 . This causes the contact tube  34 , fixed to the heating rod  31 , to be displaced toward the base end. With such displacement, a diaphragm  35 , fixed to the engine head  301  via the housing  30 , has one portion, fixedly secured to a base end of the contact tube  34 , which is displaced toward the base end relative to another portion fixed to the housing  30 . This causes strain to occur on the diaphragm  35 . A combustion pressure sensor  36 , placed on the diaphragm  35  at a base end thereof, detects a pressure inside the combustion chamber  302  based on such strain. 
     With the structure of the related art shown in  FIG. 3 , the combustion pressure sensor  36  takes the form of a structure exposed to outside air. With such a structure, the combustion pressure sensor  36  directly receives an effect of outside air prevailing at a base end portion of the cylinder head  301 . Thus, the combustion pressure sensor  36  detects the combustion chamber with degraded precision. In particular, with the combustion pressure sensor  36  arranged to detect the combustion pressure based on small changes in strain resulting from fluctuation in combustion pressure, a pyroelectric effect occurs due to moisture contained in outside air. This causes the combustion pressure sensor  36  to generate an output signal with variation caused by the pyroelectric effect, resulting in the detection of the combustion pressure with degraded precision. 
     With such a structure of the related art set forth above, an attempt may be made to provide a package member to cover the combustion pressure sensor  36 . For the combustion pressure sensor  36  to be completely shut off from outside air, the package member and the intermediate shaft  37 , made of metal, need to be hermetically sealed by welding. When this takes place, the package member and the intermediate shaft  37  are fixed to each other with accompanying difficulty of causing the heating rod  31  and the contact tube  34  to be displaced in an axial direction. Thus, the combustion pressure sensor  36  cannot take a structure needed for detecting the combustion pressure. 
     To address such an issue, the package member may be arranged to retain the intermediate shaft  37  via, for instance, an O-ring. Even under such an arrangement, a drag occurs on a contact portion between the package member and the O-ring due to sliding resistance occurring thereon during axial displacement of the intermediate shaft  37 . This results in an effect of suppressing displacement of the heating rod  31 , causing the combustion pressure sensor  36  to have difficulty in detecting the combustion pressure with high precision. In addition, the O-ring has an area, held in contact with the intermediate shaft  37 , which is progressively worn away in operation of the combustion pressure sensor  36 . Thus, the O-ring encounters a difficulty of ensuring a hermetic sealing effect, causing the combustion pressure sensor  36  to have a risk with the occurrence of pyroelectric effect. 
     SUMMARY OF THE INVENTION 
     The present invention has been completed with the above view in mind and has an object to provide a glow plug with combustion pressure sensor for detecting a pressure of a combustion chamber with high precision. 
     To achieve the above object, a first aspect of the present invention provides a glow plug with combustion pressure sensor comprising a heating member adapted to be placed in one end of a plughole to raise a temperature of a combustion chamber, a cylindrical member fixedly secured to an outer circumferential wall of the heating member, a housing adapted to be fixedly secured to the plughole and holding an outer circumferential wall of the cylindrical member for an axial displacement capability, a diaphragm fixedly supported with the housing and the cylindrical member, a combustion pressure sensor mounted on the diaphragm and responsive to strain occurring in the diaphragm due to axial displacement of the cylindrical member for detecting a combustion pressure of the combustion chamber, a cover associated with the housing to define a closed air space to hermetically accommodate the combustion pressure sensor and having an insertion bore, and a lead wire, having flexibility and fixedly connected to the heating member to supply electric power thereto, which extends through the insertion bore and is hermetically bonded to an inner circumferential wall of the insertion bore. 
     The present invention contemplates the provision of the glow plug with combustion pressure sensor having a structure including the lead wire provided in place of the metallic intermediate shaft employed in the structure of the related art. That is, the lead wire, having flexibility, serves as a member connected to the heating member for supplying electric power to the heating member. In addition, a hermetic sealing structure is provided to hermetically accommodate the combustion sensor. 
     With such a structure, the combustion sensor can be hermetically accommodated in a closed space between the housing and the cover. This prevents the occurrence of a pyroelectric effect on the combustion sensor, enabling the combustion sensor to detect the combustion pressure with high precision. 
     Even if the heating member is axially displaced in response to fluctuation in combustion chamber, further, the lead wire fixed to the heating member can be flexed due to own flexibility. This avoids a joint portion between the lead wire and the insertion bore of the cover from suffering the occurrence of a drag disturbing fine displacement of the heating element. Accordingly, the combustion sensor has no hindrance in detecting the combustion pressure with high precision. 
     With the glow plug with combustion pressure sensor of the present embodiment, the lead wire may preferably include a conductive wire and a shielding layer, made of insulating material and covered on an outer circumferential periphery of the conductive wire, which has flexibility. 
     With such a structure, the insulation of the lead wire can be ensured, enabling the combustion pressure sensor to detect the combustion pressure with high precision. 
     With the glow plug with combustion pressure sensor of the present embodiment, the combustion pressure sensor may preferably include one of a piezoelectric element and a strain gauge. 
     Such a structure allows the heating element to be axially displaced in response to fluctuation in combustion pressure, with accompanying capability of detecting strain of the diaphragm with high precision. 
     With the glow plug with combustion pressure sensor of the present embodiment, a clearance may be preferably provided between an outer circumferential wall of the lead wire and an inner circumferential wall of the cylindrical member. 
     The lead wire is liable to vibrate at its own natural frequency due to vibration exerted on the glow plug with combustion pressure sensor from an external source. If the lead wire is brought into contact with an inner periphery of the cylindrical member, the combustion chamber generates an output signal overlapped with noise in the presence of such a natural frequency, causing degradation in precision of detecting the combustion pressure. To address such an adverse affect, the clearance is provided between the outer circumferential wall of the lead wire and the inner circumferential wall of the cylindrical member to avoid the occurrence of abutting contact between the lead wire and the cylindrical member, resulting in an effect of suppressing the occurrence of noise. 
     With the glow plug with combustion pressure sensor of the present embodiment, the clearance may be preferably spaced in an extent not to cause the outer circumferential wall of the lead wire and the inner circumferential wall of the cylindrical member to be brought into contact with each other when the lead wire flexes greatest due to an axial displacement of the heating member caused by fluctuation in combustion pressure. 
     With such a structure, even if the heating member is axially displaced at a maximum extent to cause the lead wire to flex greatest, no risk occurs for the outer circumferential wall of the lead wire and the inner circumferential wall of the cylindrical member to be brought into contact with each other. This prevents the combustion pressure sensor from having degraded detecting precision resulting from the combustion pressure sensor generating the output signal overlapped with noise. 
     With the present embodiment, the glow plug with combustion pressure sensor may preferably further comprise an antivibration member disposed in the clearance between the outer circumferential wall of the lead wire and the inner circumferential wall of the cylindrical member. 
     As a result of repetition in natural oscillation of the lead wire due to vibration exerted on the glow plug with combustion pressure sensor, there is a risk of fatigue occurring in the lead wire in breakdown. Therefore, placing the antivibration member in the clearance between the outer circumferential wall of the lead wire and the inner circumferential wall of the cylindrical member enables the damping of natural oscillation of the lead wire. In addition, the antivibration member prevents the occurrence of a contact between the outer circumferential wall of the lead wire and the inner circumferential wall of the cylindrical member, thereby preventing noise from overlapping on the output signal of the combustion pressure sensor. 
     With the glow plug with combustion pressure sensor of the present embodiment, the antivibration member may be preferably made of resilient material. With the antivibration member made of resilient material, it becomes possible to prevent vibration of the antivibration member vibrating at a natural frequency from being transferred to the cylindrical member. 
     With the glow plug with combustion pressure sensor of the present embodiment, the heating member may preferably include a ceramic heater. Such a structure enables the provision of a glow plug with combustion pressure sensor having excellent durability in power supply with a capability of rapidly increasing a temperature of a combustion chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal cross sectional view showing a glow plug with combustion pressure sensor of one embodiment according to the present invention. 
         FIG. 2  is a cross sectional view showing an essential part of a glow plug with combustion pressure sensor of another embodiment according to the present invention. 
         FIG. 3  is a cross sectional view showing an essential part of a glow plug with combustion pressure sensor of the related art. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Now, glow plugs with combustion pressure sensors of various embodiments according to the present invention are described below in detail with reference to the accompanying drawings. However, the present invention is construed not to be limited to such embodiments described below and technical concepts of the present invention may be implemented in combination with other known technologies or other technology having functions equivalent to such known technologies. 
     Referring now to  FIG. 1 , there is shown a glow plug with combustion pressure sensor  100  of one embodiment according to the present invention. The glow plug with combustion pressure sensor  100  is mounted on an engine head  1  of an internal combustion engine such as a diesel engine of a motor vehicle. The glow plug  100  is arranged to increase a temperature of a combustion chamber  2  during an ignition and startup of the internal combustion engine while detecting a combustion pressure of the combustion chamber  2  for generating an output signal representing a combustion state during the ignition and startup of the engine. This output signal is fed back to an electronic control unit (not shown) for engine control to be performed. Hereunder, a fundamental structure of the glow plug with combustion pressure sensor  100  is described below in detail. 
     In the following description, for the sake of convenience of illustration, the term “distal end portion” refers to a lower portion of the structure shown in  FIG. 1  and the term “base end portion” refers to an upper portion of the structure shown in  FIG. 1 . 
     (Fundamental Structure) 
     The glow plug with combustion pressure sensor  100  includes a housing  10 , made of metallic material such as stainless steel or the like, which has an outer profile formed in a nearly stepped cylindrical shape composed of a small diameter portion  10   a  formed at the distal end portion and a large diameter portion  10   b  formed at the base end portion. The housing  10  is mounted on the engine head  1  such that the small diameter portion  10   a  is disposed in a plughole  1   b  formed in the engine head  1  and the large diameter portion  10   b  is located in an area outside of the engine head  1 . The housing  10  has a threaded mounting portion  10   c , formed on the small diameter portion  10   a , which is held in screwing engagement with a female-threaded portion  1   d  formed on the plughole  1   b . With such an arrangement, the housing  10  is held in a fixed place with the small diameter portion  10   a  having a leading end  10   aa  held in abutting engagement with a tapered restricting shoulder  1   a  formed in the engine head  1  at a leading end of the plughole  1   b . The large diameter portion  10   b  has an upper base end  10   d  to which metallic cover  19  is joined to cover the upper base end  10   d.    
     A heating member  11  extends through the housing  10  and has a leading end  11   a , a base end portion  11   b  and an intermediate portion  11   c . The leading end portion  11   a  of the heating member  11  is exposed to the combustion chamber  2  to directly receive a combustion pressure. The heating member  11  is a ceramic heater comprised of a ceramic compact body and a resistance heating element buried in the ceramic compact body. The base end portion  11   b  and the intermediate portion  11   c  of the heating member  11  are inserted to and fitted to a cylindrical fixing sleeve  12  by brazing for fixing the heating member  11 . Also, the fixing sleeve  12  is made of metallic material such as stainless steel or the like. 
     The base end portion  11   b  of the heating member  11  is electrically connected to a lead wire  17 . The lead wire  17  is comprised of a conductive wire  17   a  and a shielding layer  17   b , made of insulating material, which is provided on an outer periphery of the conductive wire  17   a . The lead wire  17  has a leading end portion fixedly connected to a base end portion of the resistance heating element via a conducting member (not shown) for capability of supplying electric power to the heating member  11  via the conductive wire  17   a . The lead wire  17  has a base end portion, inserted through an insertion bore  119  provided in the cover  19  at a center thereof, which protrudes outward from a base-end end face of the cover  19  for electrical connection to an external power source (not shown). 
     An annular hermetic sealing member  13  is disposed between the leading end  10   aa  of the housing  10  and the tapered restricting shoulder  1   a  of the engine head  1 . The annular hermetic sealing member  13  has an outer circumferential periphery that is fixedly attached to the leading end  10   aa  of the housing  10  by welding all around. The annular hermetic sealing member  13  has an inner peripheral wall  13   a  that is fixedly connected to an outer periphery of the fixing sleeve  12  by welding all around. In addition, the sealing member  13  is made of metallic material having small spring constant. Thus, the outer periphery of the fixing sleeve  12  is fixedly supported on the housing  10  by means of the sealing member  13 , which does not prevent the heating member  11  from synchronizing in axial displacement upon direct receipt of a combustion pressure. 
     That is, when the heating member  11  and the fixing sleeve  12  axially displaced toward a base end of the glow plug  100 , the sealing member  13  is also displaced toward the base end of the glow plug  100  in synchronisation with the axial displacements of the heating member  11  and the fixing sleeve  12 . Therefore, even with the heating member  11  and the fixing sleeve  12  held on the housing  10 , the heating member  11  and the fixing sleeve  12  can be axially displaced toward the base end of the glow plug  100 . In addition, the sealing member  13  can prevent gasses from flowing from the combustion chamber  2  into the housing  10  via the leading end thereof. 
     The fixing sleeve  12  has a base end portion  12   a  having an upper end face welded to and fixedly connected to an end face of a leading end portion  14   a  of a cylindrical transfer sleeve  14 . The cylindrical transfer sleeve  14  is made of metallic material such as stainless steel and has the same inner and outer diameters as those of the fixing tube  12 . In addition, the fixing sleeve  12  and the cylindrical transfer sleeve  14  refers to cylindrical members in claims, respectively. 
     The large diameter portion  10   b  of the housing  10  accommodates therein a diaphragm  15 . The diaphragm  15  has a cylindrical outer sleeve portion  15   a  located in the outermost position, a cylindrical inner sleeve portion  15   b  axially extending from the cylindrical outer sleeve portion  15   a  at a central portion thereof, and a flange-like bridging portion  15   c  through which the diaphragm  15  and the cylindrical inner sleeve portion  15   b  are integrally connected to each other. The cylindrical outer sleeve portion  15   a  has an outer circumferential periphery held in abutting contact with an inner circumferential periphery of the large diameter portion  10   b  of the housing  10  to be fixedly retained therein. The cylindrical inner sleeve portion  15   b  has a leading end fixedly connected to an end face of a base end  14   b  of the transfer sleeve  14  by welding or the like. Further, with the diaphragm  15 , the bridging portion  15   c  has a smaller thickness than those of the cylindrical outer sleeve portion  15   a  and the cylindrical inner sleeve portion  15   b . Here, like the fixing sleeve  12  and the transfer sleeve  14 , the diaphragm  15  is made of metallic material such as stainless steel or the like. 
     Hereunder, the structure of the present embodiment will be described below in detail with a focus on how the combustion pressure, occurring due to explosion in the combustion chamber  2 , is transferred and a principle of detecting the combustion pressure. 
     When the combustion pressure occurs in the combustion chamber  2 , the heating element  11  and the fixing sleeve  12  are axially displaced, with accompanying displacement of the transfer sleeve  14  bonded to the fixing sleeve  12  toward the base end portion of the glow plug  100  in an axial direction thereof (as indicated by an arrow A in  FIG. 1 ). 
     Since the diaphragm  15  is substantially fixed to the engine head  1  by means of the housing  10 , the displacement of the transfer sleeve  14  is transferred to the diaphragm  15 . In this moment, the cylindrical inner sleeve portion  15   b  is displaced toward the base end of the glow plug  100  with respect to the cylindrical outer sleeve portion  15   a . This causes the bridging portion  15   c  to bear strain. 
     The bridging portion  15   c  has an upper end face, facing the base end of the glow plug  100 , to which an annular piezoelectric element  16  is coaxially bonded. With the occurrence of strain on the bridging portion  15   c , the annular piezoelectric element  16  responds to such strain to generate electrical charges in varying rate depending on a piezoelectric characteristic of the piezoelectric element  16  per se. The resulting electrical charges of the piezoelectric element  16  are converted to a voltage signal, which is amplified to provide amplified voltage signal to be output to an on-vehicle ECU (not shown). Thus, the combustion pressure is fed back to perform a combustion control. Here, the piezoelectric element  16  corresponds to a combustion pressure sensor defined in the claims. In addition, the piezoelectric element  16  is comprised of a strain-detecting element such as a piezoelectric or quartz crystal oscillator or the like. 
     With the present embodiment, further, the glow plug  100  may take the form of a structure employing a stain gauge in place of the piezoelectric element  16  to allow the stain gauge to provide a strain characteristic based on which a combustion pressure is detected. In addition, the piezoelectric element  16  may include, for instance, a plurality of piezoelectric segments in place of the piezoelectric element  16  provided that the piezoelectric segments can detect the existence of average strain on the disc-like bridging portion  15   c  in an unbiased fashion. The piezoelectric segments are placed on the upper wall of the bridging portion  15   c  at circumferentially and equidistantly spaced positions. 
     In the foregoing, the fundamental structure of the glow plug with combustion pressure sensor  100  has been described. The glow plug with combustion pressure sensor  100  has characteristic structures as will be described below. 
     (First Characteristic Structure) 
     As shown in  FIG. 1 , the cover  19  is associated with the housing  10  to provide a closed inner space B that hermetically accommodate therein the piezoelectric element  16  and the diaphragm  15 . With the present embodiment, the cover  19  is comprised of, for instance, a hermetic seal whose large portion is made of metallic material with a partial area having an insulating layer. 
     The cover  19  has the insertion bore  119  formed in a metallic layer  119   a  made of metallic material such as stainless steel or the like. The metallic layer  119   a  has an outer circumferential periphery fitted to an insulating layer  119   b , which is placed radially inward of an annular metallic layer  119   c  made of metallic material such as stainless steel or the like. The shielding layer  17   b  is peeled off at a base end portion of the lead wire  17  to expose the conductive wire  17   a . The conductive wire  17   a  has an outer circumferential periphery to which terminal portions  17   c , made of metallic material such as stainless steel or the like, are fixed secured in axially spaced relationship by caulking or the like. The conductive wire  17   a  has an intermediate portion  17   d , corresponding to the base end portion of the lead wire  17  and intervening between the terminal portions  17   c , which has an outer circumferential wall bonded to the metallic layer  119   a  by welding all around. The intermediate portion  17   d  may be welded to a wall of the insertion bore  119  of the metallic layer  119   a  by arc welding or resistance welding, etc. 
     With such a structure set forth above, the welded portion formed around the insertion bore  119  prevents ambient air surrounding around the cover  19  from intruding the closed interspace in which the piezoelectric element  16  is accommodated. In addition, the presence of the insulating layer  119   b  avoids the conductive wire  17   a  of the lead wire  17  from being short-circuited to the housing  10  via the cover  19 . 
     With the cover  19  set forth above, no probability takes place for the piezoelectric element  16  to be brought into contact with moisture contained in atmospheric air to prevent the occurrence of a pyroelectric effect. The piezoelectric element  16  can detect the combustion pressure based on strain of the diaphragm  15  with high precision. 
     Further, the cover  19  is not limited to the hermetic seal. Also, no shape of the cover  19  is limited provided that the cover  19  has the insertion bore  19  and the insulating layer  119   b  to obtain the same effects as those mentioned above. For instance, the cover  19  may be integrally formed with the housing  10  with a partial area formed with the insulating layer  119   b  to hermetically accommodate the piezoelectric element  16 . 
     (Second Characteristic Structure) 
     The lead wire  17  needs to have flexibility available to absorb the displacement of the heating member  11  due to fluctuation in combustion pressure. To this end, with the present embodiment, the lead wire  17  is comprised of the conductive wire  17   a , made of copper alloy, which is covered with the shielding layer  17   b  made of fluorine resin. 
     As set forth above, the lead wire  17  is fixedly attached to the heating member  11  and the cover  19 . Therefore, with an axial displacement of the heating member  11  due to fluctuation of the combustion chamber, an intermediate portion  17   e  of the lead wire  17 , extending in an area between the end face of the base end portion  11   b  of the heating member  11  and an end face of the cover  19 , tends to be displaced in the same extent as that in which the heating member  11  is displaced. However, since the lead wire  17  undergoes a deflection by itself to absorb a displacement component of the heating member  11 , a joint portion between the lead wire  17  and the cover  19  encounters no drag to block the axial displacement of the heating member  11 . 
     Therefore, the whole of the displacement component of the heating member  11  resulting from the combustion pressure occurred in the combustion chamber  2  is present in the form of the diaphragm  15  via the fixing sleeve  12  and the transfer sleeve  14 . That is, the diaphragm  15  undergoes strain in conformity to the combustion pressure, so that the piezoelectric element  16  generates an output signal with high precision in accord with the combustion pressure. 
     Further, a formation material of the lead wire  17  has a quality that is not particularly limited provided that the formation material is composed of material with excellent flexibility and heat resistance. In addition, the conductive wire  17   a  of the lead wire  17  may be comprised of a single wire. In another alternative, the conductive wire  17   a  of the lead wire  17  may include a twisted wire composed of a plurality of thin copper wires. 
     (Third Characteristic Structure) 
     With the glow plug with combustion pressure sensor  100  mounted to the plughole  1   b , the lead wire  17  oscillates at a natural frequency with a fixed portion between the heating member  11  and the cover  19  acting as a fixing end upon receipt of an oscillation exerted from the outside. With such an oscillation repeatedly exerted, the conductive wire  17   a  of the lead wire  17  undergoes fatigue with the accompanying possibility of fatigue burnout. 
     To avoid such a defect, an air space  20  is defined between an outer circumferential wall of the lead wire  17  and an inner circumferential wall of the transfer sleeve  14 . The air space  20  accommodates therein three cylindrical antivibration members  18 , each composed of resilient material such as fluorine rubber or the like, which are coaxially placed inside the air space  20  at axially spaced positions. With the present embodiment, particularly, the antivibration members  18  have outer circumferential peripheries fixedly held in contact with the inner circumferential wall of the transfer sleeve  14  and inner circumferential peripheries radially spaced from the outer circumferential wall of the lead wire  17  by open space portions  20   a . This does not block the flexing of the lead wire  17 . In addition, the inner circumferential peripheries of the antivibration members  18  may be fixed to the outer circumferential wall of the lead wire  17  so as to provide the open space portions between the outer circumferential wall of the antivibration members  18  and the inner circumferential wall of the transfer sleeve  14 . 
     With such a structure, when the lead wire  17  flexes, the lead wire  17  is brought into contact with one or more of the antivibration members  18  to damp the natural frequency of the lead wire  17 , thereby avoiding the disconnection of the conductive wire  17   a . Further, the antivibration members  18  prevents the outer circumferential wall of the lead wire  17  from being brought into contact with the inner circumferential wall of the transfer sleeve  14  when subjected to the natural frequency of the wire lead  17 . This prevents noise, occurring due to a contact between the outer circumferential wall of the lead wire  17  and the inner circumferential wall of the transfer sleeve  14 , from being superimposed on the output signal generated by the piezoelectric element  16 . This further prevents not only the occurrence of a drop in SN ratio but also the occurrence of the natural frequency of the lead wire  17  being transferred to the transfer sleeve  14 . 
     Further, the antivibration members  18  may be preferably placed in areas corresponding to peak portions of vibration amplitudes during oscillation of the lead wire  17  at the natural frequency. Furthermore, the open space  20  is preferably determined to have an adequate radial space, i.e. for instance 0.1 mm or more such that when the lead wire  17  is caused to flex with most displacement in a radial direction, no outer circumferential wall of the lead wire  17  is brought into contact with the inner circumferential wall of the transfer sleeve  14 . 
     Another Embodiment 
     While the present invention has been described above with reference to various embodiments in which the heating element  11  is comprised of the ceramic heater, it will be appreciated that it may suffice to use a heater formed in a metallic cylinder body accommodating therein a heating coil. 
     With the present embodiment, although the antivibration members  18  have been described above as having cylindrical structures in shape, the antivibration members  18  may take annular shapes. In addition, the number of the antivibration members  18  to be provided is not limited. Further, the antivibration members  18  may be replaced by an antivibration material  18 A filled in the open space  20  between the outer circumferential wall of the lead wire  17  and the inner circumferential wall of the transfer sleeve  14  as shown in  FIG. 2 . In particular, the antivibration material  18 A is comprised of a liquid sealant such as a potting material, composed of silicone rubber, or the like, providing the same advantageous effects as those of the antivibration members  18 . The liquid sealant has adequately small Young&#39;s modulus with no occurrence of an effect of blocking the flexure of the lead wire  17 . Furthermore, no antivibration member may be disposed provided that the outer circumferential wall of the lead wire  17  is radially spaced from the inner circumferential wall of the transfer sleeve  14  by a distance of, for instance, 0.1 mm or more. 
     With the present embodiment, further, the lead wire  17  is radially spaced from the inner circumferential wall of the transfer sleeve  14  by the open space portions  20   a . However, there may be no open space portions  20   a . That is, the antivibration members  18  may be arranged in structure to be brought into contact with both the lead wire  17  and the transfer sleeve  14  provided that each of the antivibration members  18  has small Young&#39;s modulus with no hindrance to the flexure of the lead wire  17 . 
     While the specific embodiments of the present invention have been described in detail, the present invention is not limited to the particularly illustrated structures of the glow plug of the various embodiment set forth above. It will be appreciated by those skilled in the art that various other modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure.