Abstract:
An improved structure of a spark plug is provided for improving heat dissipation from an Ir-alloy chip attached to a ground electrode. The Ir-alloy chip works to a sequence of sparks between itself and the tip of a center electrode mounted in a metal shell and is embedded in a center electrode-facing surface of the ground electrode, thereby enhancing the transmission of heat produced in the Ir-alloy chip to the metal shell through the ground electrode.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field of the Invention  
           [0002]    The present invention relates generally to a spark plug which may be employed in automotive vehicles, gas pumps and cogeneration systems, and more particularly to a spark plug with a ground electrode having installed therein an Ir-alloy chip.  
           [0003]    2. Background Art  
           [0004]    Japanese Patent First Publication No.  8-298178  discloses a spark plug equipped with an Ir-alloy chip. The spark plug includes a center electrode and a ground electrode. The center electrode is disposed within a metal shell through a porcelain insulator and has a tip exposed outside an end of the metal shell. The ground electrode is joined to the end of the metal shell and has a spark discharging surface formed on an end thereof which defines an air gap (also called a spark plug gap) between itself and the tip of the center electrode. The Ir-alloy chip is installed on the spark discharging surface of the ground electrode for producing a sequential of sparks between itself and the end of the center electrode.  
           [0005]    When the spark plug is used in an internal combustion engine, the Ir-alloy chip is subjected to intense heat. The heat principally dissipates from the Ir-alloy chip to the ground electrode and to the metal shell and the atmosphere. The Ir-alloy chip is bonded to the surface of the ground electrode through a corrosion resisting non-noble metallic member. Specifically, the whole of the Ir-alloy chip lies over the surface of the ground electrode. This structure, therefore, arrests the transmission of heat from the Ir-alloy chip to the ground electrode, so that the Ir-alloy chip is exposed to intense heat for a long time, resulting in acceleration of oxidation and wear of the Ir-alloy chip.  
         SUMMARY OF THE INVENTION  
         [0006]    It is therefore a principal object of the invention to avoid the disadvantages of the prior art.  
           [0007]    It is another object of the invention to provide a spark plug with an Ir-alloy chip joined to a ground electrode which is designed to provide a desired amount of heat dissipation from the Ir-alloy chip.  
           [0008]    According to one aspect of the invention, there is provided a spark plug which may be employed in automotive vehicles, gas pumps and cogeneration systems. The spark plug comprises: (a) metal shell; (b) a center electrode retained within the metal shell to be insulated from the metal shell; (c) a ground electrode joined to the metal plug, the ground electrode having a center electrode-facing surface opposed to a tip of the center electrode through a spark plug gap; and (d) an Ir-alloy chip working to produce a spark between itself and the tip of the center electrode, the Ir-alloy chip being embedded in the center electrode-facing surface of the ground electrode with a portion thereof exposed outside the center electrode-facing surface of the ground electrode.  
           [0009]    In the preferred mode of the invention, the Ir-alloy chip other than the exposed portion thereof is installed inside the ground electrode.  
           [0010]    The Ir-alloy chip may alternatively have at least one surface which lies flush with a side surface of the ground electrode continuing from a peripheral edge of the center electrode-facing surface.  
           [0011]    The exposed portion of the Ir-alloy chip projects from the center electrode-facing surface of the ground electrode toward the center electrode.  
           [0012]    The Ir-alloy chip is joined to the ground electrode through at least one fused portion in which materials of the Ir-alloy chip and the ground electrode are melted together. The fused portion may be formed by laser welding.  
           [0013]    The shortest distance between the fused portion and the center electrode is more than or equal to the sum of an interval between the tip of the center electrode and the Ir-alloy chip through the spark plug gap and 0.3 mm.  
           [0014]    The ground electrode has a recess formed in the center electrode-facing surface. The Ir-alloy chip is fitted within the recess. The fused portion extends continuously from an outer side wall of the ground electrode inside the Ir-alloy chip through an outer side wall of the Ir-alloy chip.  
           [0015]    The ground electrode has a second surface opposed to the center electrode-facing surface. The tip of the fused portion lies within the Ir-alloy chip closer to the center electrode-facing surface than the second surface. The distance between the tip of the fused portion and the bottom of the Ir-alloy chip lying inside the ground electrode is greater than or equal to 0.1 mm.  
           [0016]    The length of a part of the fused portion lying within the Ir-alloy chip is greater than or equal to 0.2 mm.  
           [0017]    The distance between the tip of the fused portion and the center electrode-facing surface of the ground electrode is greater than or equal to 0.2 mm.  
           [0018]    The distance between the outer side wall of the Ir-alloy chip and the outer side wall of the ground electrode is greater than or equal to 0.25 mm.  
           [0019]    The fused portion may lie close to a joint of the ground electrode and the metal shell from a center line of the Ir-alloy chip extending toward the center electrode through the spark plug gap.  
           [0020]    The distance between an end of the exposed portion of the Ir-alloy chip oriented toward the center electrode and the center electrode-facing surface of the ground electrode lies within a range of 0. mm to 1.0 mm.  
           [0021]    The Ir-alloy chip is made from material containing a main component of Ir (Iridium) and an additive of at least one of Rh (rhodium), Pt (platinum), Ru (ruthenium), Pd (palladium), and W (tungsten). The Ir-alloy chip may contain 70 to 99 Wt % of Ir.  
           [0022]    The Ir-alloy chip may be joined to the ground electrode through a plurality of fused portions in which materials of the Ir-alloy chip and the ground electrode are melted together. In this case, at least one of the fused portion lies preferably close to a joint of the ground electrode and the metal shell from a center line of the Ir-alloy chip extending toward the center electrode through the spark plug gap for increasing the degree of joining of the Ir-alloy chip to the ground electrode. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    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.  
         [0024]    In the drawings:  
         [0025]    [0025]FIG. 1 is a partially sectional view which shows a spark plug with an Ir-alloy chip according to the first embodiment of the invention;  
         [0026]    [0026]FIG. 2( a ) is a partially enlarged sectional view which shows a joint structure for an Ir-alloy chip in the first embodiment of the invention;  
         [0027]    [0027]FIG. 2( b ) is a sectional view taken along the line A-A in FIG. 2( a );  
         [0028]    [0028]FIG. 2( c ) is an illustration which shows an Ir-alloy chip as viewed from a center electrode;  
         [0029]    [0029]FIG. 3 is a partially enlarged sectional view which shows a joint structure of a spark plug sample used in durability tests;  
         [0030]    [0030]FIG. 4 is a graph which indicates a relation between the length L 1  (mm) of a projecting portion of an Ir-alloy chip and the temperature (° C.) of the Ir-alloy chip;  
         [0031]    [0031]FIG. 5 is a graph which indicates a relation between the length L 1  (mm) and the worn volume (mm 3 ) of an Ir-alloy chip;  
         [0032]    [0032]FIG. 6 is a graph which indicates the relation between the shortest length L 2  in FIG. 2( b ) and the number of sparks flying at fused portions forming joints of an Ir-alloy chip and a ground electrode;  
         [0033]    [0033]FIG. 7( a ) is a sectional view which shows a modification of the first embodiment;  
         [0034]    [0034]FIG. 7( b ) is a sectional view taken along the line B-B in FIG. 7( a );  
         [0035]    [0035]FIG. 8( a ) is a partial view which shows another modification of the first embodiment;  
         [0036]    [0036]FIG. 8( b ) is a partial illustration as viewed from a direction C in FIG. 8( a );  
         [0037]    [0037]FIG. 9( a ) is a partially sectional view which shows a spark plug according to the second embodiment of the invention;  
         [0038]    [0038]FIG. 9( b ) is a partially sectional view taken along the line D-D in FIG. 9( a );  
         [0039]    [0039]FIG. 10 is a partially plan view which shows an Ir-alloy chip and a ground electrode of a spark plug according to the third embodiment of the invention;  
         [0040]    [0040]FIG. 11 is a partially plan view which shows an Ir-alloy chip and a ground electrode of a spark plug according to the fourth embodiment of the invention;  
         [0041]    [0041]FIG. 12 is a partially plan view which shows an Ir-alloy chip and a ground electrode of a spark plug according to the fifth embodiment of the invention;  
         [0042]    [0042]FIG. 13( a ) is a partially plan view which shows an Ir-alloy chip and a ground electrode of a spark plug according to the sixth embodiment of the invention;  
         [0043]    [0043]FIG. 13( b ) is a sectional view taken along the line E-E in FIG. 13( a );  
         [0044]    [0044]FIG. 14 is a partially vertical sectional view which shows an Ir-alloy chip and a ground electrode of a spark plug according to the seventh embodiment of the invention;  
         [0045]    [0045]FIG. 15 is a partially vertical sectional view which shows an Ir-alloy chip and a ground electrode of a spark plug according to the eighth embodiment of the invention;  
         [0046]    [0046]FIG. 16 is a partially vertical sectional view which shows an Ir-alloy chip and a ground electrode of a spark plug according to the ninth embodiment of the invention;  
         [0047]    [0047]FIG. 17 is a partially vertical sectional view which shows an Ir-alloy chip and a ground electrode of a spark plug according to the tenth embodiment of the invention;  
         [0048]    [0048]FIG. 18( a ) is a partially horizontal sectional view which shows an Ir-alloy chip and a ground electrode of a spark plug according to the eleventh embodiment of the invention;  
         [0049]    [0049]FIG. 18( b ) is a sectional view taken along the line F-Fin FIG. 18( a );  
         [0050]    [0050]FIG. 18( c ) is a sectional view taken along the line H-H in FIG.  18 ( a ); and  
         [0051]    [0051]FIG. 19 is a partially vertical view which shows an Ir-alloy chip and a ground electrode of a spark plug according to the twelfth embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0052]    Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to FIG. 1, there is shown a spark plug  100  which may be used in a gas engine of a generator in a cogeneration system.  
         [0053]    The spark plug  100  includes a cylindrical metal shell  10 , a porcelain insulator  20 , a center electrode  30 , and a ground electrode  40 . The metal shell  10  has cut therein a thread  11  for mounting the spark plug  100  in an engine block (not shown). The porcelain insulator  20  made of an alumina ceramic (Al 2 O 3 ) is retained within the metal shell  10  and has a tip  21  exposed outside an end  12  of the metal shell  10 .  
         [0054]    The center electrode  30  is secured in a central chamber  22  of the porcelain insulator  20  and insulated electrically from the metal shell  10 . The center electrode  30  has a tip  31  projecting from the tip  21  of the porcelain insulator  20  outside the end  12  of the metal shell  10 . The center electrode  30 , as shown in FIG. 2( a ), consists of a body  32  and an Ir-alloy chip  31   a . The body  32  is made of a cylindrical member which consists of a core portion made of a metallic material such as Cu having a higher thermal conductivity and an external portion made of a metallic material such as an Ni-based alloy having higher thermal and corrosion resistances. The Ir-alloy chip  31   a  is welded to an end of the body  32  to define the tip  31 .  
         [0055]    The ground electrode  40  made of an Ni-alloy bar or an Fe-alloy bar is welded to the end  12  of the metal shell  10  through an intermediate block  40   a . The intermediate block  40   a , is made of an Ni-alloy or an Fe-alloy. The ground electrode  40 , as clearly shown in FIG. 2( a ), has an end  41  which faces at a side surface  42  thereof the tip  31  of the center electrode  30  through a spark plug gap  50 . The side surface  42  forms a spark discharging surface. A second Ir-alloy chip  43  is embedded in the spark discharging surface  42  which works to produce a sequence of sparks between itself and the tip  31  of the center electrode  30 .  
         [0056]    The second Ir-alloy chip  43 , as can be seen from FIG. 2( a ), projects partially from the spark discharging surface  42  of the ground electrode  40 . A peripheral wall of the Ir-alloy chip  43  may either coincide partially with or be all located inside an edge of the spark discharging surface  42 . In this embodiment, the whole of the Ir-alloy chip  43  is, as shown in FIG. 2( c ), located inside the periphery of the spark discharging surface  42 .  
         [0057]    The attachment of the Ir-alloy chip  43  to the ground electrode  40  is accomplished in the following manner. First, the Ir-alloy chip  43  is put on the spark discharging surface  42  and forced thereinto to form a recess  44  which has substantially the same area as that of the Ir-alloy chip  43 . The laser beams are, as shown in FIG. 2( b ), applied to each outer side wall of the recess  44  to form fused portions  45  where materials of the ground electrode  40  and the Ir-alloy chip  43  are melted together, thereby producing joints of the Ir-alloy chip  43  and the ground electrode  40 . The recess  44  may alternatively be formed using cutting or cold forging techniques.  
         [0058]    The Ir-alloy chip  43 , as can be seen from FIG. 2( a ), projects partially from the spark discharging surface  42  toward the tip  31  of the center electrode  30  to define the spark plug gap  50 , as described above.  
         [0059]    Each of the Ir-alloy chips  31   a  and  43  is made from material containing a main component of Ir (Iridium) and an additive of at least one of Rh (rhodium), Pt (platinum), Ru (ruthenium), Pd (palladium), and W (tungsten). In this embodiment, the Ir-alloy chips  31   a  and  43  each contain 90 Wt % of Ir and 10 Wt % of Rh (referred to as an Ir-10 Rh below).  
         [0060]    The Ir-alloy chip  43  is, as discussed above, located inside the outer periphery of the spark discharging surface  42 . Specifically, most of the Ir-alloy chip  43  is surrounded by the ground electrode  40  in contact therewith. Therefore, when a spark discharge is taken place between the Ir-alloy chips  43  and  31   a , the heat produced in the Ir-alloy chip  43  flows to the metal shell  10  through the ground electrode  40  effectively, thus resulting in an increased degree of dissipation of heat from the Ir-alloy chip  43  as compared with the conventional spark plug as discussed in the introductory part of this application.  
         [0061]    We researched a suitable length L 1 , as shown in FIG. 3, of a portion of the Ir-alloy chip  43  projecting from the end  41  of the ground electrode  40  in terms of the degrees of dissipation of heat from the Ir-alloy chip  43  and spark-caused wear of the Ir-alloy chip  43 . We first performed durability tests of the spark plug  100  for different lengths L 1  of 4 mm to −2 mm. The spark plug  100  was installed in a 6-cylinder gas cogeneration engine and run for 500 hours under a condition of a rated engine output. A thermocouple thermometer was used to measure the temperature of the Ir-alloy chip  43 . After the durability tests, a worn volume of the Ir-alloy chip  43  was measured.  
         [0062]    [0062]FIG. 4 indicates a relation between the length L 1  (mm) of the projecting portion of the Ir-alloy chip  43  and the temperature (° C.) of the Ir-alloy chip  43 . FIG. 5 indicates a relation between the length L 1  (mm) and the worn volume (mm 3 ) of the Ir-alloy chip  43 . The graph of FIG. 4 shows that the temperature of the Ir-alloy chip  43  is lowered most when the length L 1  is less than 0 mm. Similarly, the graph of FIG. 5 shows that the worn volume of the Ir-alloy chip  43  is minimized when the length L 1  is less than 0 mm. This is because when the length L 1  is decreased below Omm, the oxidation-caused wear of the Ir-alloy chip  43  is suppressed to increase the spark wear resistance thereof. Note that length L 1  m=0 mm indicates the case where the end of the Ir-alloy chip  43  lies flush with the outer periphery of the spark discharging surface  42 .  
         [0063]    As apparent from the above discussion, most of the Ir-alloy chip  43  is embedded in the spark discharging surface  42 , thus resulting in an increased degree of heat dissipation from the Ir-alloy chip  43 . The Ir-alloy chip  43  has one surface exposed outside the spark discharging surface  42  toward the tip  31  of the center electrode  30 , thereby enabling the spark plug gap  50  to be defined allowing for the amount of spark-caused wear of the Ir-alloy chip  43 , which results in an increase in service left of the spark plug  100 . A sequence of sparks are produced mainly between the tip  31  of the center electrode  30  and the Ir-alloy chip  43 , thus minimizing the amount of wear of the spark discharging surface  42  of the ground the spark plug  100 .  
         [0064]    The Ir-alloy chip  43  is, as described above, laser-welded to the ground electrode  40  to form the fused portions  45 . If the shortest distance, as shown in FIG. 2( a ), between the tip  31  of the center electrode  30  and each of the fused portions  45  is defined as L 2 , it is advisable that L 2  be longer than the sum of the distance G between the Ir-alloy chips  31  and  43  through the spark plug gap  50  and 0.3 mm. This value is found based on results of tests, as discussed below, performed by the inventors of this application in terms of the relation between the shortest distance L 2  and sparks landing on the fused portions  45 .  
         [0065]    In the tests, spark plugs with the Ir-alloy chip  43  whose gap  50  (i.e., the distance G) lies within a range of 0.3 mm to 0.8 mm and which have different length L 2  were prepared. The spark plugs were installed in a test chamber under a gauge pressure of 0.6 Mpa. The voltage was applied to each of the spark plugs to produce a sequence of sparks to measure the number of sparks flying at the fused portions  45 . FIG. 6 indicates the relation between the shortest length L 2  and the number of sparks flying at the fused portions  45  and shows that all the sparks fly within the spark plug gap  50  when the distance G is 0.3 mm, as indicated by black circles, and the shortest length L 2  is more than 0.5 mm or more, when the distance G is 0.5 mm, as indicated by black triangles, and the shortest length L 2  is 0.8 mm or more, and when the distance G is 0.8 mm, as indicated by black squares, and the shortest length L 2  is more than 1.15 mm or more. Specifically, when length L 2  ≧G+0.3 mm, the possibility that sparks occur between the fused portions  45  and the tip  31  of the center electrode  30  will be zero (0), thus minimizing the spark-caused wear of the fused portions  45 .  
         [0066]    The Ir-alloy chip  43  is welded to the inner wall of the recess  44  in the ground electrode  40  by irradiating laser beams to the outer wall of the recess  44 , so that the fused portions  45  which contain less Ir than the Ir-alloy chip  43  and are inferior in spark wear resistance are formed outside a spark discharging portion of the ground electrode  40 , thereby minimizing the spark-caused wear of the fused portions  45 .  
         [0067]    The surface of the Ir-alloy chip  43  exposed to the spark plug gap  50  is, as shown in FIG. 2( c ), rectangular, however, may alternatively be, as shown in FIG. 7( a ), circular. Specifically, the Ir-alloy chip  43  may be made of an Ir-alloy disc. FIG. 7( b ) shows a vertical cross section taken along the line B-B in FIG. 7( a ).  
         [0068]    The Ir-alloy chip  43  may alternatively be embedded in the ground electrode  40  in the manner as illustrated in FIGS.  8 ( a ) and  8 ( b ). FIG. 8( b ) shows the surface of the Ir-alloy chip  43  as viewed from a direction C in FIG. 8( a ). Specifically, the ground electrode  40  has a C-shaped opening  60  formed in the end thereof by cutting or forging. The Ir-alloy chip  43  is fitted in and laser-welded to the C-shaped chamber  60  in the same manner as described above. The laser beams may alternatively be irradiated to an interface between an inner wall of the opening  60  and an outer wall of the Ir-alloy chip  43  to weld the Ir-alloy chip  43  to the ground electrode  40 . The ground electrode  40  may be installed, as shown in FIG. 8( a ), directly on the end of the metal shell  10 .  
         [0069]    FIGS.  9 ( a ) and  9 ( b ) show the second embodiment of the invention.  
         [0070]    Usually, the thermal stress arising from burning of the engine may cause cracks to be formed between the Ir-alloy chip  43  and the fused portions  45  which lead to dislodgement of the Ir-alloy chip  43  from the ground electrode  40 . Particularly, when used in a gas cogeneration engine operated continuously under high loads, spark plugs are exposed at electrodes to intense heat, thus having a high possibility of formation of such cracks.  
         [0071]    The second embodiment aims at forming the fused portions  45  under optimum conditions in order to avoid the dislodgement of the Ir-alloy  43  from the ground electrode  40 . FIG. 9( a ) illustrates the Ir-alloy chip  43  embedded in the ground electrode  40 , as viewed from the side of the center electrode  30 . FIG. 9( b ) is a sectional view taken along the line D-D in FIG. 9( a ).  
         [0072]    The Ir-alloy chip  43  is made of a disc member. The Ir-alloy chip  43  is fitted in the recess  44  of the ground electrode  40  and laser-welded to form, as clearly shown in FIG. 9( a ), five fused portions  45 . The fused portions  45  each extend continuously from the outer side surface  46  of the ground electrode  40  to a central portion of the Ir-alloy chip  43  through an outer side wall  47  of the Ir-alloy chip  43 . The Ir-alloy chip  43  is, like the first embodiment, exposed partially outside the surface of the ground electrode  40  toward the center electrode  30  through the spark plug gap  50 . The ground electrode  40  is joined at the right side thereof, as viewed in the drawings, to the metal shell  10 .  
         [0073]    We made a study of optimum conditions for forming the fused portions  45 , which will be discussed below in detail.  
         [0074]    The tip of each of the fused portions  45  is, as can be seen from FIG. 9( b ), located closer to the spark discharging surface  42  of the ground electrode  40  than the bottom  48  of the Ir-alloy chip  43 . Durability tests were performed for different values of distance L 3  between the bottom  48  of the Ir-alloy chip  43  and the tip of each of the fused portions  45  using spark plug samples prepared in three sets of four. The three sets have L 3 =0 mm, L 3 =0.1 mm, L 3 =0.2 mm, respectively. In each sample, the length L 4  of a tip of each of the fused portions  45  entering the Ir-alloy chip  43  was 0.5 mm.  
         [0075]    The spark plug samples were exposed to air at 1000° C. for six minutes, after which they were left in air at 25° C. for six minutes. This thermal shock test were repeated cyclically (i.e., a thermal cycle test). The spark plug samples having L 3 =0 mm all experienced dislodgement of the Ir-alloy chip  43  from the ground electrode  40  before 100 cycles of the thermal shock tests. The spark plug samples having L 3 =0. 1 mm and L 3 =0.2 mm all did not experience dislodgement of the Ir-alloy chip  43  from the ground electrode  40  even after 800 cycles of the thermal shock tests. It is, therefore, found that the distance L 3  between the tip of the fused portions  45  and the bottom  48  of the Ir-alloy chip  43  is preferably greater than or equal to 0.1 mm (i.e., L 3 ≧0.1 mm) in order to avoid the dislodgement of the Ir-alloy chip  43  from the ground electrode  40 .  
         [0076]    Each of the fused portions  45 , as can be seen from FIG. 9( b ), extends perpendicular to a direction in which the Ir-alloy chip  43  peels off the ground electrode  40 . Thus, when the distance L 3 is set more than 0.1 m, a relatively thick bottom wall  70  is defined beneath the fused portions  45 , thereby keeping tight engagement of the Ir-alloy chip  43  with the inner wall of the recess  44  even if cracks occur between the Ir-alloy chip  43  and the fused portions  45 .  
         [0077]    Additionally, similar thermal shock tests were also performed for different values of length L 4  of the tip of each of the fused portions  45  entering the Ir-alloy chip  43  using spark plug samples prepared in three sets of four. The three sets have L 4 =0.2 mm, L 4 =0.5 mm, L 4 =0.8 mm, respectively. In each sample, the distance L 3  between the tip of each of the fused portions  45  and the bottom  48  of the Ir-alloy chip  43  was 0.2. All the spark plug samples do not experience the dislodgement of the Ir-alloy chip  43  from the ground electrode  40  even after 800 cycles of the thermal shock tests. It is, thus, found that when the distance L 3  is more than or equal to 0.1, and the length L 4  is more than or equal to 0.2, it enhances the avoidance of dislodgement of the Ir-alloy chip  43  from the ground electrode  40 .  
         [0078]    We also made a study of suitable values of distance L 5  between the tip of each of the fused portions  45  and the spark discharging surface  42  of the ground electrode  40  and found that the distance L 5  of more than or equal to 0.2 is required for forming the fused portions  45  desirably.  
         [0079]    The above thermal shock tests also showed that when the distance L 6  between the outer side wall  47  of the Ir-alloy chip  43  (i.e., a line tangent to the outer side wall  47  of the Ir-alloy chip  43 ) and the outer side wall  46  of the ground electrode  40  is less than 0.25 mm, it may cause cracks to be formed in the ground electrode  40  before dislodgement of the Ir-alloy chip  43 . It is, thus, advisable that distance L 6  be more than or equal to 0.25 mm.  
         [0080]    We further made a study of suitable values of length L 7  of a portion of the Ir-alloy chip  43  exposed outside the spark discharging surface  42  toward the center electrode  30  and found that when the length L 7  is set more than or equal to 0.1, it enables a sequence of sparks to be produced between the center electrode  30  and the Ir-alloy chip  43  and also serves to prevent sparks from flying directly at the ground electrode  40 , and that when the length L 7  is more than 1.0 mm, the temperature of the Ir-alloy chip  43  is elevated undesirably by the heat of burning of the engine, which will result in an increase in wear of the Ir-alloy chip  43 . Therefore, it is advisable that the length L 7  meet a relation of 0.1≦L 7 ≦1 .0 mm.  
         [0081]    In order to enhance the heat flow from the Ir-alloy chip  43 , at least one of the fused portions  45  is preferably formed close to the joint of the ground electrode  40  and the metal shell  10  (i.e., the right side of the drawings) from a vertical center line  80  of the Ir-alloy chip  43 . In this embodiment, two of the fused portions  45  are located on the right side of the vertical center line  80 .  
         [0082]    The shortest distances L 6  between the outer side wall  46  of the ground electrode  40  and the outer side wall  47  of the Ir-alloy chip  43  are preferably equal to each other because it makes it possible to form the fused portions  45  in the same welding condition, thereby facilitating ease of a welding operation or resulting in a decrease in manufacturing process.  
         [0083]    [0083]FIG. 10 shows the Ir-alloy chip  43  embedded in the ground electrode  40  according to the third embodiment of the invention, as viewed from the side of the center electrode  30 .  
         [0084]    Two corners of the tip of the ground electrode  40  are cut to form surfaces  85  tapering off to the tip. The fused portion  45  is formed in each of the tapered surfaces  85 . It is advisable that the distances L 6  be equal to each other for facilitating ease of the welding operation to join the Ir-alloy chip  43  to the ground electrode  40 . Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.  
         [0085]    [0085]FIG. 11 shows the fourth embodiment of the invention which is different from the one shown in FIG. 10 in that two tapered surfaces  86  are formed on the tip of the ground electrode  40  which traverse each other to define a sharp tip and which have the fused portions  45  formed therein. It is advisable that the distances L 6  between the outer side wall of the Ir-alloy chip  43  and portions of the outer side wall of the ground electrode  40  in which the fused portions  45  are to be formed be equal to each other for facilitating ease of the welding operation to join the Ir-alloy chip  43  to the ground electrode  40 . Other arrangements are identical with those in the third embodiment, and explanation thereof in detail will be omitted here.  
         [0086]    [0086]FIG. 12 shows the fifth embodiment of the invention which is different from the ones shown in FIGS. 10 and 11 in that the ground electrode  40  has a round tip in which the fused portions  45  are formed at constant angular intervals. It is advisable that the distance L 6  between the outer side wall of the Ir-alloy chip  43  and the outer side wall of the round tip of the ground electrode  40  be constant for facilitating ease of the welding operation to join the Ir-alloy chip  43  to the ground electrode  40 . Other arrangements are identical with those in the third and fourth embodiments, and explanation thereof in detail will be omitted here.  
         [0087]    FIGS.  13 ( a ) and  13 ( b ) show the sixth embodiment of the invention which is a modification of the one shown in FIG. 10.  
         [0088]    Seven fused portions  45  are formed in the outer side wall of the ground electrode  40 , while two fused portions  45  are also formed in the bottom  49  of the ground electrode  40  (i.e., the surface of the ground electrode  40  opposite the center electrode  30 ) and extend inside the Ir-alloy chip  43 .  
         [0089]    [0089]FIG. 14 shows the seventh embodiment of the invention which is different from the above embodiments only in that a single fused portion  45  is formed in the outer side wall  46  of the ground electrode  40  to define a wider bottom wall  70  which establishes tight engagement with the inner wall of the recess  44 . This structure also provides substantially the same effects as those in the above embodiments.  
         [0090]    [0090]FIG. 15 shows the eighth embodiment of the invention.  
         [0091]    The joining of the Ir-alloy chip  43  to the ground electrode  4  is achieved by at least one fused portion  45  extending from the outer side wall  46  of the ground electrode  40  inside the Ir-alloy chip  43  and a plurality of fused portions  45  extending downward, as viewed in the drawing, from the surface of the ground electrode  40  exposed outside the spark discharging surface  42  of the ground electrode  40 . The vertical fused portions  45  extend through the outer side wall  47  of the Ir-alloy chip  43 , that is, they extend through an interface between the outer side wall  47  of the Ir-alloy chip  43  and the inner wall of the recess  44 .  
         [0092]    [0092]FIG. 16 shows the ninth embodiment of the invention.  
         [0093]    A plurality of fused portions  45  are formed in a corner defined between the outer side wall of the Ir-alloy chip  47  and the spark discharging surface  42 . Specifically, the fused portions  45  extend from the outer side wall  47  of the Ir-alloy chip  43  and the spark discharging surface  42  of the ground electrode  40  diagonally toward the vertical center line  80  of the Ir-alloy chip  43  so as to define the bottom wall  70  of a given thickness beneath the fused portions  45  which establishes tight engagement with the inner wall of the recess  44 .  
         [0094]    [0094]FIG. 17 shows the tenth embodiment of the invention.  
         [0095]    The Ir-alloy chip  43  is made of a cylindrical member consisting of a small-diameter portion  92  and a large-diameter portion  95 . The small-diameter portion  92  is fitted within the recess  44  of the ground electrode  40 , while the large-diameter portion is placed on the spark discharging surface  42  of the ground electrode  40 . The fused portions  45  are formed around the outer side wall  47  of the small-diameter portion  92 . This structure provides a relatively wider spark-discharging surface to the Ir-alloy chip  43  without sacrificing the distances L 6  between the outer side wall  47  of the small-diameter portion  92  of the Ir-alloy chip  43  and portions of the outer side wall  46  of the ground electrode  40  in which the fused portions  45  are to be formed.  
         [0096]    FIGS.  18 ( a ),  18 ( b ), and  18 ( c ) show the eleventh embodiment of the invention. FIG. 18( a ) is a sectional view which illustrates the Ir-alloy chip  43  embedded in the ground electrode  40  as viewed from the center electrode  30 . FIG. 18( b ) is a sectional view taken along the line F-Fin FIG. 18( a ). FIG. 18( c ) is a sectional view taken along the line H-H in FIG. 18( a ).  
         [0097]    The Ir-alloy chip  43  is made of a square block (i.e., a prism) and has a side surface exposed, as clearly shown in FIG. 18( c ), outside the end  46  of the ground electrode  40 . Three fused portions  45  are formed in each side wall of the ground electrode  40 . Other arrangements are identical with those in the above embodiments, and explanation thereof in detail will be omitted here.  
         [0098]    [0098]FIG. 19 shows the twelfth embodiment of the invention.  
         [0099]    The ground electrode  40  has a chamber  44  formed in an end portion thereof which opens into the spark discharging surface  42  and the bottom  49 . The Ir-alloy chip  43  is fitted within the opening  44 . This structure provides for ease of machining of the chamber  44 .  
         [0100]    While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims. For example, the joining of Ir-alloy chip  43  to the ground electrode  40  may be achieved with resistance welding or plasma arc welding. The invention may also be used with a spark plugs of the type, as taught in U.S. Pat. No. 6,225,752, in which a sequence of sparks are produced between a side peripheral wall of a center electrode and an end of a ground electrode. In this case, the Ir-alloy chip  43  is installed in the end of the ground electrode. The Ir-alloy chips  31   a  and  43  are each made from material containing 90 Wt % of Ir, but may be made from material containing 70 to 99 Wt % of Ir.