Patent Publication Number: US-10333281-B2

Title: Spark plug

Description:
CROSS REFERENCE TO RELATED DOCUMENT 
     The present application claims the benefit of priority of Japanese Patent Application No. 2017-135475 filed on Jul. 11, 2017, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     This disclosure relates generally to a spark plug designed to have an enhanced self-cleaning feature. 
     2. Background Art 
     Japanese Patent No. 3272615 teaches a spark plug which has a center electrode and a ground electrode. The center electrode is firmly installed in an axial hole of a porcelain insulator with a head thereof protruding outside an end of the porcelain insulator. The ground electrode is disposed on an end of a metal shell and has a surface of a head directly facing a side surface of the head of the center electrode. The spark plug has a spark gap between the surface of the head of the ground electrode and the side surface of the head of the center electrode. The spark plug usually works to create a spark between the surface of the head of the ground electrode and the side surface of the head of the center electrode. When fouled, the spark plug creates a semi-surface discharge between the ground electrode and the side surface of the head of the center electrode along the surface of the head of the porcelain insulator. 
     The above spark plug, however, has a short spark path of the semi-surface discharge along the surface of the head of the porcelain insulator when the spark plug is fouled, thus having a limited ability to burn off carbon deposits on the surface of the head of the porcelain insulator. 
     SUMMARY 
     It is therefore an object of this disclosure to provide a spark plug which has an enhance ability to turn off deposits of, for example, carbon on an end surface of a porcelain insulator. 
     According to one aspect of the invention, there is provided a spark plug which comprises: (a) a cylindrical metal shell; (b) a cylindrical porcelain insulator which is retained inside the metal shell; (c) a center electrode which is disposed inside the porcelain insulator to have a head protruding outside a front end surface of the porcelain insulator; and (d) a ground electrode which is joined to the metal shell and has a gap-defining portion. The gap-defining portion extends along a side surface of the center electrode and the front end surface of the porcelain insulator. 
     In operation, the spark plug works to produce a spark between the ground electrode joined to the metal shell and the center electrode disposed in the porcelain insulator to ignite an air-fuel mixture. 
     The head of the center electrode, as described above, protrudes outside the front end surface of the porcelain insulator. The ground electrode has the gap-defining portion which extends along the side surface of the center electrode and the front end surface of the porcelain insulator, in other words, which faces the side surface of the center electrode and the front end surface of the porcelain insulator. This creates a first spark gap between the side surface of the center electrode and the gap-defining portion of the ground electrode for producing a spark in a normal operation mode of the spark plug and also creates a second spark gap between the front end surface of the porcelain insulator and the gap-defining portion of the ground electrode for achieving semi-surface discharge in a fouling mode of the spark plug. In the normal operating mode where a small amount of deposit of, example, carbon accumulates on the front end surface of the porcelain insulator, the spark plug works to produce a spark in the first spark gap to ignite an air-fuel mixture 
     In the fouling mode where a large amount of deposit of, for example, carbon, accumulates on the front end surface of the porcelain insulator, the spark plug works to achieve the semi-surface discharge in the second spark gap on the front end surface of the porcelain insulator to burn off the deposit on the front end surface of the porcelain insulator. The gap-defining portion extends over the front end surface of the porcelain insulator, thereby resulting in an increased region where the front end surface of the porcelain insulator faces the gap-defining portion, which leads to an increased length of a path in which the spark is developed along the front end surface of the porcelain insulator in the fouling mode, thereby enhancing the ability of the spark plug to burn off the deposit on the front end surface of the porcelain insulator. 
     If the ground electrode extends straight from the circumference of the metal shell toward the center electrode, the ground electrode has a portion which faces the front end surface of the porcelain insulator and has a length which is less than or equal to the width of the front end surface in a radial direction of the porcelain insulator. This results in a decreased length of a path along which the semi-surface discharge is developed along the front end surface of the porcelain insulator in the fouling mode, which leads to an insufficient amount of deposit burned off on the front end surface of the porcelain insulator. 
     In the preferred mode of this disclosure, the gap-defining portion may have a length which extends over the front end surface of the porcelain insulator and is selected to be greater than a width of the front end surface of the porcelain insulator in a radial direction of the porcelain insulator. This results in an increased length of the path in which the semi-surface discharge is developed in the fouling mode, thereby enhancing the ability of the spark plug to burn off the deposits on the front end surface of the porcelain insulator. 
     The spark plug may alternatively be designed to have the gap-defining portion which has a length extending over the front end surface of the porcelain insulator and selected to be greater than half an outer diameter of the porcelain insulator. This results in an increased length of the path along which the semi-surface discharge is developed in the fouling mode, thereby enhancing the ability of the spark plug to burn off the deposits on the front end surface of the porcelain insulator. 
     The spark plug may be designed to have the gap-defining portion which defines a first gap that is a minimum gap between the gap-defining portion and the side surface of the center electrode. The first gap is selected to be more than or equal to 0.4 mm and less than or equal to 0.8 mm. The defining portion may also define a second gap that is a minimum gap between the gap-defining portion and the front end surface of the porcelain insulator. The second gap is selected to be more than or equal to 0.2 mm and less than or equal to the first gap. This ensures the stability in producing sparks between the side surface of the center electrode and the gap-defining portion in the normal operating mode. 
     The second gap that is a minimum air gap between the front end surface of the porcelain insulator and the gap-defining portion is selected to be 0.2 mm or more. This eliminates a probability that the semi-surface discharge is undesirably developed in the second gap along the front end surface of the porcelain insulator when the insulation resistance of the porcelain insulator is decreased with an increase in temperature thereof. The second gap is smaller in size than the first gap, thereby causing the semi-surface discharge to be initiated in the second gap when the fouling mode is entered without generating sparks in the first gap. 
     The spark plug may be designed to have a ground electrode which is shaped to straight extend. This facilitates the ease with which the ground electrode is joined to the metal shell in a production process of the spark plug. 
     The spark plug may also include a second ground electrode extending parallel to a first ground electrode that is the above ground electrode on opposite sides of the center electrode. The first and second ground electrodes may extend parallel to each other. This results in an increased region where the front end surface of the porcelain insulator faces the first and second ground electrodes as compared with when the spark plug is equipped with only one ground electrode, thereby improving the ability of the spark plug to burn off, for example, carbon deposits on the front end surface of the porcelain insulator. 
     The first and second ground electrodes may extend from portions of the metal shell which are diametrically opposed to each other. This facilitates the ease with which the ground electrodes are joined to the metal shell in the production process of the spark plug as compared with when the ground electrodes extend from portions of the metal shell which are located closer to each other in the circumferential direction of the metal shell. 
     According to another aspect of the invention, there is provided a spark plug which comprises: (a) a cylindrical metal shell; (b) a cylindrical porcelain insulator which is retained inside the metal shell; (c) a center electrode which is disposed inside the porcelain insulator to have a head protruding outside a front end surface of the porcelain insulator; and (d) a ground electrode which extends at a given angle to a line, as defined to extend from a front circumference of the metal shell to the center electrode, when tips of the metal shell and the center electrode are viewed in a longitudinal direction of the spark plug. The given angle is selected to be greater than 0° and less than 45°. The ground electrode is oriented to face a side surface of the center electrode and the front end surface of the porcelain insulator. 
     If the ground electrode is designed to extend straight from the circumference of the metal shell toward the center electrode, the ground electrode has a portion which faces the front end surface of the porcelain insulator and has a length which is less than or equal to the width of the front end surface in a radial direction of the porcelain insulator. 
     In the above structure according to the second aspect of this disclosure, when the tips of the metal shell and the center electrode are viewed in the longitudinal direction of the spark plug, the ground electrode extends at the angle of 0° to 45° to a line defined to extend from the circumference of the metal shell to the center electrode. The ground electrode is oriented to face the side surface of the center electrode in the radial direction of the spark plug and also face the front end surface of the porcelain insulator in the longitudinal direction of the spark plug. These arrangements result in an increase in length of the ground electrode which faces or overlaps the front end surface of the porcelain insulator in the longitudinal direction of the spark plug as compared with an example wherein the ground electrode extends from the end of the metal shell straight toward the center electrode. The structure of the spark plug, therefore, has the long path in which the semi-surface discharge is developed over the front end surface of the porcelain insulator when the front end surface is fouled with, for example, carbon deposits, thereby enhancing the ability of the spark plug to burn off the carbon deposits on the front end surface of the porcelain insulator. 
     The ground electrode may have a length which extends over the front end surface of the porcelain insulator and is selected to be greater than a width of the front end surface of the porcelain insulator in a radial direction of the porcelain insulator. This results in an increased length of the path in which the semi-surface discharge is developed in the fouling mode, thereby enhancing the ability of the spark plug to burn off the carbon deposits on the front end surface of the porcelain insulator. 
     The spark plug may alternatively be designed to have the ground electrode which has a length extending over the front end surface of the porcelain insulator and selected to be greater than half an outer diameter of the porcelain insulator. This results in an increased length of the path in which the semi-surface discharge is developed in the fouling mode, thereby enhancing the ability of the spark plug to burn off the deposits on the front end surface of the porcelain insulator. 
     The spark plug may be designed to have the ground electrode which is shaped to straight extend. This facilitates the ease with which the ground electrode is joined to the metal shell in a production process of the spark plug. 
     The spark plug may also include a second ground electrode extending parallel to a first ground electrode that is the above ground electrode on opposite sides of the center electrode. The first and second ground electrodes may extend parallel to each other. This results in an increased region where the front end surface of the porcelain insulator faces the first and second ground electrodes as compared with when the spark plug is equipped with only one ground electrode, thereby improving the ability of the spark plug to burn off, for example, carbon deposits on the front end surface of the porcelain insulator. 
     The first and second ground electrodes may extend from portions of the metal shell which are diametrically opposed to each other. This facilitates the ease with which the ground electrodes are joined to the metal shell in the production process of the spark plug as compared with when the ground electrodes extend from portions of the metal shell which are located closer to each other in the circumferential direction of the metal shell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
       In the drawings: 
         FIG. 1  is a plan view which shows a head of a spark plug; 
         FIG. 2  is a sectional view taken along the line II-II in  FIG. 1 ; 
         FIG. 3  is a plan view which illustrates paths of semi-surface discharge developed on the spark plug of  FIG. 1 ; 
         FIG. 4  is a sectional view taken along the line IV-IV in  FIG. 3 ; 
         FIG. 5  is a plan view which illustrates a head of a comparative example of a spark plug; 
         FIG. 6  is a sectional view taken along the line VI-VI in  FIG. 5 ; 
         FIG. 7  is a plan view which illustrates paths of semi-surface discharge developed on the spark plug of  FIG. 5 ; 
         FIG. 8  is a sectional view taken along the line VIII-VIII in  FIG. 6 ; 
         FIG. 9  is a plan view which illustrates a modification of a spark plug; 
         FIG. 10  is a sectional view taken along the line X-X in  FIG. 9 ; 
         FIG. 11  is a sectional view which illustrates a modification of a spark plug; 
         FIG. 12  is a plan view which illustrates a modification of a spark plug; 
         FIG. 13  is a plan view which illustrates a modification of a spark plug; 
         FIG. 14  is a plan view which illustrates a modification of a spark plug; and 
         FIG. 15  is a sectional view which illustrates a modification of a spark plug. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, particularly to  FIGS. 1 and 2 , the spark plug  10  for use in a gasoline internal combustion engine according to an embodiment is shown. The spark plug  10  includes the metal shell  11 , the porcelain insulator  20 , the center electrode  30 , and the ground electrodes  15 . 
     The metal shell  11  is made of a metallic hollow cylinder. 
     The porcelain insulator  20  is made of an electrically insulating hollow cylinder. The porcelain insulator  20  is firmly installed in the metal shell  11 . The porcelain insulator  20  has a tapered head whose diameter decreases toward the tip thereof. The porcelain insulator  20  has the axial hole  21  which has a length extending in the axial direction of the porcelain insulator  20 . 
     The center electrode  30  is made of a heat-resistant nickel alloy or copper in the shape of a circular cylinder. The center electrode  30  is inserted into the axial hole  21  of the porcelain insulator  20  and firmly retained therein. The center electrode  30  has the head  30   a  (i.e., a tip) protruding outside both the front end surface  20   a  of the porcelain insulator  20  and the end surface  11   a  of the metal shell  11  in a lengthwise direction of the spark plug  10 . 
     The metal shell  11  has two ground electrodes  15  welded to an end of the head thereof. The ground electrodes  15  are each made of a heat-resistant nickel alloy in the shape of a square pole extending straight. Each of the ground electrodes  15  is of a square or rectangular shape in cross section. The joints of the ground electrodes  15  to the metal shell  11  are diametrically opposed to each other. The ground electrodes  15  extend substantially perpendicular to the length of the metal shell  11  parallel to each other on opposite sides of the center electrode  30 . 
     Specifically, in  FIG. 1  where the tips of the metal shell  11  and the center electrode  30  are viewed in the longitudinal direction of the spark plug  10 , each of the ground electrodes  15  extends at an angle θ 1  to a line dr 1 , as defined to extend from the front circumference (i.e., the front end surface  11   a   9  of the metal shell  11  to the center electrode  15 , more specifically, pass through the center of the joint of the ground electrode  15  to the end surface  11   a  of the metal shell  11  and the center axis of the center electrode  30 . In other words, the angle θ 1  is an angle which the line dr 1  makes with the longitudinal center line (i.e., the length) of the ground electrode  15  on a plane extending perpendicular to the length of the spark plug  10 . The angle θ 1  is selected to be greater than 0° and less than 45°. Each of the ground electrodes  15  is oriented to face the side surface  30   b  of the center electrode  30  in the radial direction of the spark plug  10  and also face the front end surface  20   a  of the porcelain insulator  20  in the longitudinal direction of the spark plug  10 . In other words, each of the ground electrodes  15  has the gap-defining portion  15   a  extending along the side surface  30   b  of the center electrode  30  and the front end surface  20   a  of the porcelain insulator  20 . 
       FIG. 5  is a plan view which illustrates the tip (i.e., the ends of the metal shell  11  and the center electrode  30 ) of a comparative example of a spark plug  90 .  FIG. 6  is a cross section taken along the line VI-VI in  FIG. 5 . The same reference numbers as describing the spark plug  10  refer to the same parts, and explanation thereof in detail will be omitted here. 
     The ground electrodes  95 , as illustrated in  FIG. 5 , extend straight from the end of the metal shell  11  toward the center electrode  30 . Each of the ground electrodes  95  has a portion which faces the front end surface  20   a  of the porcelain insulator  20  and has a length L 2 . The length L 2  is less than or equal to the width W 1  of the front end surface  20   a  in the radial direction of the porcelain insulator  20 . 
     The spark plug  10  of this embodiment, as clearly illustrated in  FIG. 1 , has the gap-defining portion  15   a  of each of the ground electrodes  15 . The gap-defining portion  15   a  has a length L 1  which extends along the front end surface  20   a  of the porcelain insulator  20 , in other words, faces the front end surface  20   a  in the longitudinal direction of the porcelain insulator  20 . The length L 1  is selected to be greater than the width W 1  of the front end surface  20   a  in the radial direction of the porcelain insulator  20 . Specifically, the length L 1  of the gap-defining portion  15   a  which extends along or faces the front end surface  20   a  of the porcelain insulator  20  is selected to be longer than a half r 1  of an outer diameter of the porcelain insulator  20 . 
     A sparking air gap G 1  (which will also be referred to as a spark gap or a first gap) is created between the side surface  30   b  of the center electrode  30  and the gap-defining portion  15   a  of each of the ground electrodes  15 . The spark gap G 1  is a minimum interval between the side surface  30   b  of the center electrode  30  and the gap-defining portion  15   a . The spark gap G 1  is selected lie in a range of 0.4 mm to 0.8 mm (i.e., 0.4 mm≤G 1 ≤0.8). In other words, the spark gap G 1  is selected to be a typical air gap whose size is large enough to properly create a sequence of sparks between the side surface  30   b  of the center electrode  30  and the gap-defining portion  15   a  in a normal operating mode of the spark plug  10 . 
     A semi-surface discharge gap G 2  (which will also be referred to as a spark gap or a second gap) is created between the front end surface  20   a  of the porcelain insulator  20  and the gap-defining portion  15   a  of each of the ground electrodes  15 . The spark gap G 2  is a minimum interval between the front end surface  20   a  of the porcelain insulator  20  and the gap-defining portion  15   a . The spark gap G 2  is selected lie in a range of 0.2 mm to the spark gap G 1  (i.e., 0.2 mm≤G 2 ≤G 1 ). The insulation resistance of the porcelain insulator  20  usually drops with a rise in temperature thereof. The spark gap G 2  is engineered to have a size large enough to stop the semi-surface discharge from being developed through the spark gap G 2  on the front end surface  20   a  of the porcelain insulator  20  when the insulation resistance of the porcelain insulator  20  is decreased with an increase in temperature thereof. 
       FIG. 3  is a plan view which demonstrates paths of the semi-surface discharge.  FIG. 4  is a sectional view taken long the line IV-IV in  FIG. 3 . 
     In the normal operating mode where the amount of carbon deposits accumulated on the front end surface  20   a  of the porcelain insulator  20  is small, a spark is created in the spark gap G 1  between the side surface  30   b  of the center electrode  30  and the gap-defining portion  15   a  of each of the ground electrodes  15 . This ignites an air-fuel mixture (i.e. a mixture of gasoline and air) in the engine. 
     In a fouling mode where the amount of carbon deposits on the front end surface  20   a  of the porcelain insulator  20  is great, the semi-surface discharge is created, as indicated by arrows, through the spark gap G 2  on the front end surface  20   a  of the porcelain insulator  20  between the side surface  30   b  of the center electrode  30  and the gap-defining portion  15   a  of each of the ground electrodes  15 . The spark gap G 2  is, as described above, smaller in size than the spark gap G 1 , thereby causing sparks to be created in the spark gap G 2  along the front end surface  20   a  of the porcelain insulator  20  in the semi-surface discharge mode (i.e., the fouling mode) without generating sparks within the spark gap G 1  in the normal operating mode. This burns off the carbon deposits on the front end surface  20   a  of the porcelain insulator  20 . 
     The gap-defining portion  15   a , as described above, extends a distance equivalent to the length L 1  over the front end surface  20   a  of the porcelain insulator  20 . In other words, the gap-defining portion  15   a  is exposed to the front end surface  20   a  in a wide range equivalent to the length L 1 . This results in, as indicated arrows in  FIG. 3 , increased lengths of the paths along which the semi-surface discharge is created on the front end surface  20   a  of the porcelain insulator  20  in the fouling mode. This enables the carbon deposits on the front end surface  20   a  of the porcelain insulator  20  to be burned out in a wide range R 1 , as indicated by a broken line in  FIG. 3 , 
       FIG. 7  is a plan view which demonstrates a comparative example of paths of sparks in the semi-surface discharge mode.  FIG. 8  is a sectional view taken along the line VIII-VIII in  FIG. 7 . 
     In the comparative example, in the normal operating mode where a small amount of carbon deposit is on the front end surface  20   a  of the porcelain insulator  20 , sparks are produced in the gap G 1  between the side surface  30   b  of the center electrode  30  and the gap-defining portion  15   a , thereby igniting the air-fuel mixture in the engine. 
     In the fouling mode where there is a large amount of carbon deposit on the front end surface  20   a  of the porcelain insulator  20 , the semi-surface discharge is created, as indicated by arrows, through the spark gap G 2  on the front end surface  20   a  of the porcelain insulator  20  between the side surface  30   b  of the center electrode  30  and the gap-defining portion  15   a  of each of the ground electrodes  15 . In the comparative example, the spark gap G 2  is smaller in size than the spark gap G 1 , thereby causing sparks to be created in the spark gap G 2  along the front end surface  20   a  of the porcelain insulator  20  in the semi-surface discharge mode (i.e., the fouling mode) without generating sparks within the spark gap G 1 . 
     However, in the comparative example, the ground electrodes  95  extend from the end of the metal shell  11  straight to the center electrode  30 , so that the interval between the front end surface  20   a  of the porcelain insulator  20  and the ground electrodes  95  is smaller than that in the spark plug  10  in this embodiment. This causes sparks to be developed with shorter lengths of the paths, as indicated by arrows in  FIG. 7 , than in  FIG. 3 , so that the carbon deposits on the front end surface  20   a  of the porcelain insulator  20  are burned out only in narrow ranges R 2 , as indicated by broken lines in  FIG. 7 , which leads to an insufficient amount of carbon deposit burned off on the front end surface  20   a  of the porcelain insulator  20 . 
     The structure of the spark plug  10  in the embodiment offers the following beneficial advantages. 
     The head  30   a  of the center electrode  30 , as described above, protrudes outside the front end surface  20   a  of the porcelain insulator  20  in the lengthwise direction of the spark plug  10 . Each of the ground electrodes  15  has the gap-defining portion  15   a  which extends along the side surface  30   b  of the center electrode  30  and the front end surface  20   a  of the porcelain insulator  20 , in other words, which faces the side surface  30   b  of the center electrode  30  in the radial direction of the spark plug  10  and also faces the front end surface  20   a  of the porcelain insulator  20  in the lengthwise direction of the spark plug  10 . This creates the spark gap G 1  between the side surface  30   b  of the center electrode  30  and the gap-defining portion  15   a  of each of the ground electrodes  15  and also creates the spark gap G 2  between the front end surface  20   a  of the porcelain insulator  20  and the gap-defining portion  15   a  of each of the ground electrodes  15 , thereby causing sparks to be developed in the spark gaps G 1  to ignite the air-fuel mixture in the engine in the normal operating mode where a small amount of carbon deposit is on the front end surface  20   a  of the porcelain insulator  20 . 
     When the amount of carbon deposit on the front end surface  20   a  of the porcelain insulator  20  becomes large, the fouling mode is entered. The spark plug  10  then works to create sparks in the spark gaps G 2  along the front end surface  20   a  of the porcelain insulator  20  in the semi-surface discharge mode to burn off the carbon deposits on the front end surface  20   a  of the porcelain insulator  20 . Each of the gap-defining portions  15   a  extends over the front end surface  20   a  of the porcelain insulator  20  in a direction substantially perpendicular to the center axis of the porcelain insulator  20 , thereby resulting in an increased region where the front end surface  20   a  of the porcelain insulator  20  directly faces the gap-defining portion  15   a , which leads to increased lengths of the paths in which sparks are developed along the front end surface  20   a  of the porcelain insulator  20  in the fouling mode, thereby enhancing the ability of the spark plug  10  to burn off carbon deposits on the front end surface  20   a  of the porcelain insulator  20 . 
     In  FIG. 1  where the tips of the metal shell  11  and the center electrode  30  are viewed in the longitudinal direction of the spark plug  10 , each of the ground electrodes  15  extends at the angle θ 1  to the line dr 1 , as defined to pass through the center of the joint of the ground electrode  15  to the end surface  11   a  of the metal shell  11  and the center of the center electrode  30 . The angle θ 1  is selected to be greater than 0° and less than 45°. Each of the ground electrodes  15  is oriented to face the side surface  30   b  of the center electrode  30  in the radial direction of the spark plug  10  and also face the front end surface  20   a  of the porcelain insulator  20  in the longitudinal direction of the spark plug  10 . These arrangements result in an increase in the length L 1  of each of the ground electrodes  15  which faces or overlaps the front end surface  20   a  of the porcelain insulator  20  in the longitudinal direction of the spark plug  10  as compared with the example in  FIG. 5  wherein the ground electrodes  95  extend from the end of the metal shell  11  straight toward the center electrode  30  in opposite radial directions of the spark plug  90 . The structure of the spark plug  10  in the embodiment, therefore, has the long paths in which the semi-surface discharge is developed over the front end surface  20   a  of the porcelain insulator  20  when the front end surface  20   a  is fouled, thereby enhancing the ability of the spark plug  10  to burn off carbon deposits on the front end surface  20   a  of the porcelain insulator  20 . 
     The length L 1  of each of the gap-defining portions  15   a  which directly faces the front end surface  20   a  of the porcelain insulator  20  is, as described above, selected to be greater than the width W 1  of the front end surface  20   a  of the porcelain insulator  20  in the radial direction of the porcelain insulator  20 . This results in increased lengths of the paths of the semi-surface discharge along the front end surface  20   a  of the porcelain insulator  20  in the fouling mode, thereby improving the ability of the spark plug  10  to burn off carbon deposits on the front end surface  20   a  of the porcelain insulator  20 . 
     The length L 1  of each of the gap-defining portions  15   a  extending over or facing the front end surface  20   a  of the porcelain insulator  20  in the longitudinal direction of the spark plug  10  is greater than the half r 1  of the outer diameter of the porcelain insulator  20 , thereby resulting in increased lengths of the paths in which sparks are discharged in the semi-surface discharge mode along the front end surface  20   a  of the porcelain insulator  20  This improves the ability of the spark plug  10  to burn off carbon deposits on the front end surface  20   a  of the porcelain insulator  20 . 
     The spark gap G 1  that is a minimum air gap between the side surface  30   b  of the center electrode  30  and the gap-defining portion  15   a  of each of the ground electrodes  15  is greater than or equal to 0.4 mm and less than or equal to 0.8 mm. This ensures the stability in creating sparks between the side surface  30   b  of the center electrode  30  and the gap-defining portion  15   a  in the normal operating mode of the spark plug  10 . 
     The spark gap G 2  that is a minimum air gap between the front end surface  20   a  of the porcelain insulator  20  and the gap-defining portion  15   a  of each of the ground electrodes  15  is selected to be 0.2 mm or more. This eliminates a probability that the semi-surface discharge is undesirably developed in the spark gaps G 2  along the front end surface  20   a  of the porcelain insulator  20  when the insulation resistance of the porcelain insulator  20  is decreased with an increase in temperature thereof. The spark gaps G 2  are smaller in size than the spark gaps G 1 , thereby causing the semi-surface discharge to be initiated in the spark gaps G 2  when the fouling mode is entered without generating sparks in the spark gaps G 1 . 
     Each of the ground electrodes  15  is shaped to extend straight, thus facilitating the ease with which the ground electrodes  15  are produced. 
     The spark plug  10  is, as described above, equipped with the two ground electrodes  15  which extend parallel on opposite sides of the center electrode  30 . This results in an increased region in which the ground electrodes  15  face the front end surface  20   a  of the porcelain insulator  20  as compared with when the spark plug  10  is designed to have only one ground electrode  15 , thereby enhancing the ability of the spark plug  10  to burn off carbon accumulated on the front end surface  20   a  of the porcelain insulator  20 . 
     The two ground electrodes  15  extend from portions of the circumference (i.e., a circumferential edge) of the metal shell  11  which are diametrically opposed to each other across the longitudinal center of the spark plug  10  (i.e., the metal shell  11 ). In other words, the joints of the ground electrodes  15  to the metal shell  11  are located farther away from each other than when the ground electrodes  15  extend from portions of the circumference of the metal shell  11  which are not diametrically opposed to each other. This facilitates the ease with which the ground electrodes  15  are joined to the metal shell  11  in the production process of the spark plug  10  as compared with when the ground electrodes  15  extend from portions of the metal shell which are located closer to each other in the circumferential direction of the metal shell. 
     The structure of the spark plug  10  may be modified in the following ways. In the following discussion, the same reference numbers as employed in the above embodiment will refer to the same parts, and explanation thereof in detail will be omitted here. 
     The spark plug  10  may be, as illustrated in  FIGS. 9 and 10 , designed to have a single ground electrode  15 . The gap-defining portion  15   a , like in the above embodiment, extends along the front end surface  20   a  of the porcelain insulator  20 , thereby resulting in an increased region where the front end surface  20   a  of the porcelain insulator  20  directly faces the gap-defining portion  15   a . The center electrode  30  may, as shown in  FIG. 10 , have the small-diameter portion  30   c  which has a diameter smaller than that of a major body of the center electrode  30 . The small-diameter portion  30   c  defines an air gap between an outer periphery thereof and an inner periphery of the porcelain insulator  20 . 
     The spark plug  10  may alternatively be, as illustrated in  FIG. 11 , designed to have two ground electrodes  115  each of which is made of a straight extending cylindrical bar. Specifically, each of the ground electrodes  115  is circular in cross section. The gap-defining portion  115   a  extends along or faces the front end surface  120   a  of the porcelain insulator  20 , thereby resulting in an increased region where the front end surface  120   a  of the porcelain insulator  20  directly faces the gap-defining portion  115   a . Each of the ground electrodes  115  may alternatively be made to have a groove formed therein. This results in an increase in number of edges of a cross section of the ground electrode  115 , which leads to an increase in portions of the ground electrode  115  on which an electric field concentrates, thus facilitating the discharge from the spark plug  10 . The front end surface  120   a  of the porcelain insulator  20  may, as illustrated in  FIG. 11 , have an annular tapered portion which is inclined at a given angle relative to a plane extending perpendicular to the center axis of the porcelain insulator  20  (i.e., the center electrode  30 ). 
     The spark plug  10  may alternatively be, as illustrated in  FIG. 12 , designed to have two ground electrodes  15  which extend from portions of the end of the metal shell  11  which are not diametrically opposed to each other, but located adjacent each other in the circumferential direction of the metal shell  11 . The ground electrodes  15  extend substantially parallel to each other on opposite sides of the center electrode  30 . In other words, the ground electrodes  15  face each other across the center electrode  30  in the radial direction of the spark plug  10 . Such arrangements of the ground electrodes  15 , like in the above embodiment, result in an increased region where the front end surface  20   a  of the porcelain insulator  20  directly faces the ground electrodes  15  as compared with when the spark plug  10  has only one ground electrode  15 . 
     The spark plug  10  may be, as illustrated in  FIG. 13 , designed to have three ground electrodes  15  straight extending from the circumference end of the metal shell  11  toward the center electrode  30 . Joints of the ground electrodes  15  to the metal shell  11  may be located at a given angular interval away from each other in the circumferential direction of the metal shell  15 . The use of the three ground electrodes  15  results in an increased region the front end surface  20   a  of the porcelain insulator  20  faces the ground electrodes  15  as compared with use of two ground electrodes  15 . The spark plug  10  may alternatively be equipped with four or more ground electrodes  15  extending from the end of the metal shell  11  to the center electrode  30 . 
     The spark plug  10  may be, as illustrated in  FIG. 14 , equipped with the ground electrode  215  with the J-shaped gap-defining portion  215   a . The gap-defining portion  215   a  is curved in the form of an arc surrounding the center electrode  30 . In other words, the gap-defining portion  215   a  has an inner surface directly facing the outer periphery of the center electrode  30  and a lower surface directly facing the front end surface  20   a  of the porcelain insulator  20 . Such a configuration of the gap-defining portion  215   a  results in an increased region where the front end surface  20   a  of the porcelain insulator  20  faces the gap-defining portion  215   a  of the ground electrode  215 . 
     The spark plug  10  may be, as illustrated in  FIG. 15 , designed to have the center electrode  30  whose head  30   a  protrudes from the front end surface  20   a  of the porcelain insulator  20 , but is located inside the end surface  111   a  of the metal shell  111  in the longitudinal direction of the spark plug  10 . In other words, the head  30   a  does not have to protrude outside the end surface  111   a  of the metal shell  111  in the longitudinal direction of the spark plug  10 .