Patent Publication Number: US-10790640-B2

Title: Spark plug for internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is the U.S. national phase of International Application No. PCT/JP2016/076608 filed Sep. 9, 2016, which designated the U.S. and claims priority to Japanese Patent Application No. 2015/203,199 filed on Oct. 14, 2015, the entire contents of each of which are hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a spark plug for an internal combustion engine. 
     BACKGROUND ART 
     A spark plug is conventionally used as an ignition device for an internal combustion engine. In a high discharge voltage environment, the spark plug is liable to suffer from what is called flashover, i.e., creepage insulation breakdown that occurs between a proximal end of an insulator and a terminal fitting of a plug head or between a proximal end of a housing and the insulator. If a flashover occurs, spark discharge does not occur at the distal end of the plug, inhibiting fuel gas from being ignited. 
     Flashover at the spark plug occurs in the following manner. When a high voltage is applied to a center electrode, the electric field is concentrated at an air space formed in a gap between the terminal fitting and the proximal end of the insulator, causing negative corona discharge. Similarly, the electric field is concentrated at an air space formed in a gap between the proximal end of the housing and the insulator, causing positive corona discharge. After that, if the application of high voltage is further continued, the positive corona discharge becomes creeping streamers and moves to the negative side. The creeping streamers then reach the negative corona discharge, thereby causing a short circuit and creeping discharge, namely, flashover. 
     As a conventional technique for suppressing the occurrence of such flashover, PTL 1 discloses a configuration for reducing the eccentricity or bending of a terminal fitting of a plug head by inclining an abutment surface of the terminal fitting in accordance with the inclination of a proximal end of an insulator. Consequently, the creeping distance between the center electrode and the terminal fitting along the surface of the insulator is extended, so that the creeping streamers have difficulty reaching the negative corona discharge, and the occurrence of flashover is suppressed. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] JP 2003-45609 A 
     SUMMARY OF THE INVENTION 
     In the configuration disclosed in PTL 1, when axial pressure is applied to the terminal fitting for fusing the insulator and the terminal fitting together, radially-extending force is liable to be exerted on the proximal end of the insulator to cause the breakage of the insulator. Therefore, there is room for improvement. 
     An object of the present disclosure is to provide a spark plug for an internal combustion engine that suppresses the occurrence of flashover and prevents the breakage of an insulator. 
     A spark plug for an internal combustion engine according to an aspect of the present disclosure includes: 
     a housing having a cylindrical shape; an insulator having a cylindrical shape and held inside the housing such that a proximal end projects in an axial direction; 
     a center electrode held inside the insulator such that a distal end projects in the axial direction; 
     a terminal fitting connected to the proximal end of the insulator and provided such that electricity is conducted between the center electrode and the terminal fitting; and 
     a ground electrode fixed to a distal end of the housing and forming a spark discharge gap between the distal end of the center electrode and the ground electrode, and 
     a first gap is formed between the proximal end of the insulator and the terminal fitting, a second gap is formed between a proximal end of the housing and the insulator, and at least one of the first gap and the second gap is filled with a filler. 
     In the spark plug for an internal combustion engine, at least one of the first gap and the second gap is sealed with the filler, preventing the formation of an air space in the gap. As a result, ionization of the air space due to the concentration of the electric field at the gap is suppressed when a high voltage is applied to the center electrode, and the occurrence of corona discharge is suppressed. Therefore, the occurrence of flashover resulting from the occurrence of corona discharge is also suppressed. In addition, since this configuration eliminates the need to incline the abutment surface of the terminal fitting in accordance with the inclination of the proximal end of the insulator, the insulator is prevented from being broken when the insulator and the terminal fitting are fused together. 
     As described above, the present disclosure can provide the spark plug for an internal combustion engine that suppresses the occurrence of flashover and prevents the breakage of the insulator. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other objects, characteristics, and advantages of the present disclosure will be further clarified in the following detailed description with reference to the accompanying drawings, in which: 
         FIG. 1  is a front partial cross-sectional view of a spark plug according to a first embodiment; 
         FIG. 2  is a cross-sectional partial enlarged view of the area near a first gap of the spark plug according to the first embodiment; 
         FIG. 3  is a cross-sectional partial enlarged view of the area near a second gap of the spark plug according to the first embodiment; 
         FIG. 4  is a cross-sectional partial enlarged view of the area near the first gap of the spark plug according to a first modification; 
         FIG. 5  is a cross-sectional partial enlarged view of the area near the second gap of the spark plug according to a second modification; and 
         FIG. 6  is a diagram illustrating results of measuring flashover voltages in Examples 1 to 3 and Comparative Example. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     First Embodiment 
     An embodiment of a spark plug for an internal combustion engine will be described using  FIGS. 1 to 3 . 
     The spark plug  1  for an internal combustion engine according to the present embodiment includes a housing  10 , an insulator  20 , a center electrode  30 , a terminal fitting  40 , and a ground electrode  50 . 
     The housing  10  has a cylindrical shape. 
     The insulator  20  has a cylindrical shape and held inside the housing  10  such that a proximal end  21  of the insulator  20  projects. 
     The center electrode  30  is held inside the insulator  20  such that a distal end  32  of the center electrode  30  projects. 
     The terminal fitting  40  is connected to the proximal end  21  of the insulator  20  and provided such that electricity is conducted between the center electrode  30  and the terminal fitting  40 . 
     The ground electrode  50  is fixed to a distal end  12  of the housing  10  and forms a spark discharge gap G 0  between the distal end  32  of the center electrode  30  and the ground electrode  50 . 
     A first gap G 1  is formed between the proximal end  21  of the insulator  20  and the terminal fitting  40 , a second gap G 2  is formed between a proximal end  11  of the housing  10  and the insulator  20 , and at least one of the first gap G 1  and the second gap G 2  is filled with a filler  60 . 
     Hereinafter, the spark plug  1  for an internal combustion engine according to the present embodiment will be described in detail. Note that the spark plug  1  for an internal combustion engine is hereinafter also referred to as the “spark plug  1 ”. 
     The spark plug  1  can be used as an ignition means for an internal combustion engine provided in a car or the like. One side of the spark plug  1  which is inserted into a combustion chamber (not illustrated) is referred to as a distal end side, and the end of the distal end side is referred to as a distal end. Similarly, the side opposite to the distal end side is referred to as a proximal end side, and the end of the proximal end side is referred to as a proximal end. In the present description, a plug axial direction Y means the axial direction of the spark plug  1 . In the plug axial direction Y, a direction from the distal end toward the proximal end is referred to as a proximal end direction Y 1 , and a direction from the proximal end toward the distal end is referred to as a distal end direction Y 2 . 
     As illustrated in  FIG. 1 , the housing  10  is made of metal and has a cylindrical shape extending in the plug axial direction Y. An attachment screw  13  is formed on an outer peripheral surface of the housing  10  so as to be screwed with an internal combustion engine (not illustrated). The spark plug  1  is attached to the internal combustion engine via the attachment screw  13 . The insulator  20  is inserted into and held inside the housing  10 . 
     As illustrated in  FIG. 1 , the insulator  20  has a cylindrical shape extending in the plug axial direction Y. The proximal end  21  of the insulator  20  projects from the proximal end  11  of the housing  10 . An inclined surface  21   a  is formed inside the proximal end  21  of the insulator  20 . As illustrated in  FIG. 3 , the proximal end  11  of the housing  10  is swaged via talc  14  and an O-ring  15  substantially in the middle of the plug axial direction Y, whereby the insulator  20  is held by the housing  10 . 
     As illustrated in  FIG. 1 , the center electrode  30  is inserted into and held inside the insulator  20 . The center electrode  30  has a rod shape extending in the plug axial direction Y. A center electrode side metal tip  33  is attached to the distal end  32  of the center electrode  30  and projects from a distal end  22  of the insulator  20 . 
     As illustrated in  FIG. 1 , the terminal fitting  40  is provided at the proximal end  21  of the insulator  20 . The terminal fitting  40  is electrically connected to a proximal end  31  of the center electrode  30  inserted into and held inside the insulator  20 , and configured such that electricity is conducted between the center electrode  30  and the terminal fitting  40 . The terminal fitting  40  is fused with and fixed to the proximal end  21  of the insulator  20 . A proximal end  41  of the terminal fitting  40  is electrically connected to the secondary side of an ignition coil of an ignition device (not illustrated). 
     As illustrated in  FIG. 1 , the ground electrode  50  extends from the distal end  12  of the housing  10  in the plug axial direction Y and is bent to cross an axial center  30   a  of the center electrode  30 . The ground electrode  50  is provided with a ground electrode side metal tip  53  at a position facing the center electrode side metal tip  33 . The center electrode side metal tip  33  and the ground electrode side metal tip  53  are spaced apart from each other by a predetermined distance, so that the spark discharge gap G 0  is formed. 
     As illustrated in  FIG. 2 , the terminal fitting  40  has a facing part  42  that faces the proximal end  21  of the insulator  20 . The proximal end  21  of the insulator  20  and the facing part  42  of the terminal fitting  40  are in contact with each other in a region P extending in the entire circumferential direction. In a region extending outward from the region P in a radial direction X, the first gap G 1  is formed between the proximal end  21  of the insulator  20  and the facing part  42  of the terminal fitting  40 . 
     The first gap G 1  is filled with a first filler  61 . As illustrated in  FIG. 1 , the entire circumferential area of the proximal end  21  of the insulator  20  is filled with the first filler  61 . In the present embodiment, in order for the first gap G 1  to be securely sealed, the first filler  61  is put in the entire area of the first gap G 1  and also provided to form a small bulge on the first gap G 1 . The material for the first filler  61  is not particularly limited as long as the first gap G 1  can be sealed therewith, and preferable examples thereof include insulating materials such as silicone resin, fluororesin, and epoxy resin. In the present embodiment, silicone resin is employed. 
     As illustrated in  FIG. 3 , the second gap G 2  is formed between the proximal end  11  of the housing  10  and the insulator  20 . The second gap G 2  is a space formed in a region between an end surface  110  of the proximal end  11  of the housing  10  and a side surface  23  of the insulator  20 , and in particular in a region Q extending in the plug axial direction Y from a proximal end side corner  111 , i.e., the edge of the end surface  110  of the proximal end  11  on the proximal end side Y 1 , to a distal end side corner  112 , i.e., the edge of the end surface  110  on the distal end side Y 2 . 
     In the present embodiment, the second gap G 2  is filled with a second filler  62 . The entire circumferential area of the proximal end  11  of the housing  10  is filled with the second filler  62 . In this example, as illustrated in  FIG. 3 , the second filler  62  is provided on the proximal end side Y 1  of the second gap G 2  as well as in the second gap G 2 , so that the proximal end side corner  111  of the proximal end  11  is covered therewith. 
     As illustrated in  FIG. 1 , the spark plug  1  according to the present embodiment includes the above-mentioned first filler  61  and second filler  62  as the filler  60 . 
     Next, the effect of the spark plug  1  according to the present embodiment will be described in detail. 
     In the spark plug  1 , the first gap G 1  and the second gap G 2  are sealed with the first filler  61  and the second filler  62  serving as the filler  60 , preventing the formation of air spaces in the first gap G 1  and the second gap G 2 . As a result, ionization of the air spaces due to the concentration of the electric field at the first gap G 1  and the second gap G 2  is suppressed when a high voltage is applied to the center electrode  30 , and the occurrence of corona discharge is suppressed. Therefore, the occurrence of flashover resulting from the occurrence of corona discharge is also suppressed. In addition, since this configuration eliminates the need to incline the facing part  42  of the terminal fitting  40  in accordance with the inclined surface  21   a  of the proximal end  21  of the insulator  20 , the insulator  20  is prevented from being broken when the insulator  20  and the terminal fitting  40  are fused together. 
     In the present embodiment, the filler  60  is made of an insulating resin. Therefore, insulation is secured in the first gap G 1  and the second gap G 2 , whereby the occurrence of flashover is further suppressed. 
     In the present embodiment, both the first gap G 1  and the second gap G 2  are filled with the filler  60 . Consequently, the occurrence of flashover can be effectively prevented. 
     Note that at least the second gap G 2  may be filled with the second filler  62  serving as the filler  60 . In this case, since the occurrence of positive corona discharge is suppressed, the occurrence of creeping streamers is suppressed. Therefore, the effect of preventing the occurrence of flashover can be ensured. 
     In the present embodiment, the second filler  62  serving as the filler  60  covers the proximal end side corner  111  of the housing  10 . Consequently, the occurrence of positive corona discharge is suppressed at the part between the proximal end side corner  111  and the side surface  23  of the insulator  20  as well as at the second gap G 2 , and the occurrence of creeping streamers is further suppressed. Therefore, the occurrence of flashover is further prevented. 
     Note that at least the first gap G 1  may be filled with the first filler  61  serving as the filler  60 . In this case, since the occurrence of negative corona discharge is suppressed at the first gap G 1 , the effect of suppressing the occurrence of flashover can be achieved. 
     As can be seen in  FIG. 4  illustrating the spark plug  1  according to a first modification, the first filler  61  put in the first gap G 1  may further cover a part of a distal end side surface  43  of the terminal fitting  40 . In this case, the occurrence of negative corona discharge can be further suppressed at the part between the distal end side surface  43  of the terminal fitting  40  and the proximal end  21  of the insulator  20 , and the occurrence of flashover can be further suppressed. 
     As can be seen in  FIG. 5  illustrating the spark plug  1  according to a second modification, the proximal end  11  of the housing  10  may be swaged such that it is folded toward the distal end side Y 2 , that is, toward the O-ring  15 . In addition, as illustrated in  FIG. 5 , the second filler  62  serving as the filler  60  is provided to cover not only the second gap G 2  but also a proximal end side endmost part  113  of the proximal end  11  located on the proximal end side Y 1  relative to the proximal end side corner  111  in the plug axial direction Y. 
     Consequently, even though the proximal end  11  is formed in a folded manner as described above, the occurrence of positive corona discharge is further suppressed, and the occurrence of flashover can be further suppressed. 
     The present disclosure is not limited to the above-mentioned embodiment and modifications and can be applied to various embodiments without departing from the gist of the present disclosure. For example, the formation mode of the second filler  62  according to the first embodiment may be combined with the formation mode of the first filler  61  according to the first modification, or the formation mode of the first filler  61  according to the first embodiment may be combined with the formation mode of the second filler  62  according to the first modification. Alternatively, only one of either the first filler  61  or the second filler  62  may be provided. 
     (Evaluation Test) 
     In spark plugs for internal combustion engines according to the present disclosure, evaluation tests were conducted on Examples 1 to 3, in terms of the occurrence of flashover. 
     With regard to the spark plug of Example 1, the first gap G 1  illustrated in  FIG. 1  was filled with the first filler  61  serving as the filler  60 , and the second gap G 2  was not filled with the filler  60 . 
     With regard to the spark plug of Example 2, the first gap G 1  illustrated in  FIG. 1  was not filled with the filler  60 , and the second gap G 2  was filled with the second filler  62  serving as the filler  60 . 
     The spark plug of Example 3 had the same configuration as the spark plug of the above first embodiment, so that the first gap G 1  and the second gap G 2  were respectively filled with the first filler  61  and the second filler  62  serving as the filler  60  as illustrated in  FIG. 1 . 
     With regard to the spark plug for use as Comparative Example, neither the first gap G 1  nor the second gap G 2  was filled with the filler. 
     Note that the other configurations in Examples 1 to 3 and Comparative Example are equivalent to those in the above first embodiment. 
     The evaluation tests were conducted in the following manner. First, the distal end of each spark plug including the spark discharge gap G 0  illustrated in  FIG. 1  was immersed in insulating oil with the proximal end thereof exposed to the atmosphere, so that no discharge occurred at the spark discharge gap G 0 . Then, a high voltage was applied from an ignition coil (not illustrated) connected to the terminal fitting  40  at an applied frequency of 30 Hz. The applied voltage was gradually raised from 20 kV and measured a flashover voltage, the applied voltage at the time that a flashover occurred between the first gap G 1  and the second gap G 2 . 
     As shown in  FIG. 6 , the flashover voltage of Comparative Example was 25 kV, whereas the flashover voltages of Examples 1 and 2 were 28 kV and 28.5 kV, respectively, which were higher than the flashover voltage of Comparative Example. Furthermore, the flashover voltage of Example 3 was 30.5 kV, which was higher than the flashover voltage of Comparative Example and also higher than the flashover voltages of Examples 1 and 2. 
     From the results of measurement mentioned above, it was confirmed that the flashover voltage, for the case where at least one of the first gap G 1  and the second gap G 2  was filled with the filler  60  as in Examples 1 to 3, was higher than that, for the case where neither the first gap G 1  nor the second gap G 2  was filled with the filler  60  as in Comparative Example, and the occurrence of flashover was more suppressed in the former case than in the latter case. It was also confirmed that the flashover voltage, for the case where both the first gap G 1  and the second gap G 2  were filled with the filler  60  as in Example 3, was even higher than that, for the case where only one of either the first gap G 1  or the second gap G 2  was filled with the filler  60  as in Examples 1 and 2, and the occurrence of flashover was even more suppressed in the former case than in the latter case. 
     It was also confirmed that the flashover voltage for the case where the second gap G 2  was filled with the second filler  62  as in Example 2 was slightly higher than that for the case where the first gap G 1  was filled with the first filler  61  as in Example 1, and the occurrence of flashover was slightly more suppressed in the former case than in the latter case.