Patent Publication Number: US-7224109-B2

Title: Spark plug

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based upon and claims the benefit of priority from earlier Japanese Patent Application No. 2004-154388 filed on May 25, 2004 so that the description of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a spark plug for an internal combustion engine. 
   In general, a spark plug includes a metal housing having a fixing screw portion provided on an outer surface thereof so as to be installed to an engine via this fixing screw portion. An insulator, fixed in the metal housing, has one end portion protruding from one end portion of the metal housing. A center electrode, fixed in an axial hole of the insulator, has one end portion protruding from one end portion of the insulator. A ground electrode has one end portion fixed to the metal housing, a bent portion provided at an intermediate portion thereof, and the other end portion positioned in a confronting relationship with one end portion of the center electrode to form a spark discharge gap. 
   For example, the Japanese patent application laid-open No. 2002-343533 corresponding to the U.S. Pat. No. 6,794,803 discloses a conventional spark plug capable of securing good heat-resisting properties of a ground electrode based on improvement in the relationship between a surface area and a volume of this ground electrode. 
   Recent advanced engines are generally required to have low fuel consumption and high power output. To assure stable ignition with lean fuel mixture, the flow velocity of fuel mixture tends to be increased at a spark discharging portion of the spark plug. 
   SUMMARY OF THE INVENTION 
   In general, improvement of ignitability depends on growth of the flame kernel formed after spark discharge. However, if the mixture flow velocity is fast, the flame kernel will be shifted toward a ground electrode and will be brought into contact with the ground electrode. As soon as the flame kernel contacts with the ground electrode, the flame kernel loses its thermal energy. This phenomenon is referred to as quenching effects. The quenching effects lessen the ignitability. 
   To reduce quenching effects, it is effective to reduce the thickness of the ground electrode. However, reducing the thickness of the ground electrode will reduce the thermal capacity of the ground electrode. Accordingly, the ground electrode will have insufficient heat-resisting properties. 
   Considering these problems, it is prospective to provide a spark plug capable of securing sufficient heat-resisting properties based on improvement in the composition of the ground electrode even when the thickness of this ground electrode is reduced. 
   More specifically, according to the inventor of this patent application, it is preferable that the ground electrode contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al. 
   In high-temperature engine operating conditions, additive elements having relatively small standard formation free energies are easily oxidized than the main components having relatively large standard formation free energies. Accordingly, the additive elements shift toward the surface of the ground electrode and form surface oxides there. 
   Namely, adding Cr or Al as additive elements to the ground electrode enables the ground electrode to form a stable surface oxide layer (i.e. a coating layer) of the additive elements on the surface, as each of Cr or Al has a standard formation free energy smaller than those of the main components. As the above-described surface oxide coating is stably formed on the surface of the ground electrode, the oxidation phenomenon does not advance into the inside of the ground electrode. Thus, it becomes possible to secure excellent heat-resisting and oxidation-resisting properties for the ground electrode. 
   However, if the thickness is reduced, the ground electrode may be broken. For example, the ground electrode is subjected to severe vibrations in severe engine operating conditions. In this respect, the ground electrode must have sufficient breakage-resisting properties. 
   In view of the above problems, the present invention has an object to provide a spark plug capable of securing high ignitability even when the mixture flow velocity is high and also capable of assuring satisfactory heat-resisting properties and breakage-resisting properties when the ground electrode is thinned. 
   According to the inventor of this application, the state of crystal grains has important role in securing satisfactory breakage-resisting properties for a thinned ground electrode. It is needless to say that the strength of the ground electrode should be in a predetermined level. 
   In general, when recrystallization of the ground electrode occurs in high-temperature conditions, the crystal grain diameters of the ground electrode will increase and the strength of the ground electrode will reduce. Accordingly, increasing the crystal grain diameters is disadvantageous in assuring the breakage-resisting properties. 
   However, through enthusiastic efforts and activities in the research and development, the inventor of this application has obtained a result that, in a case that the ground electrode has excellent heat-resisting properties (i.e. when the ground electrode contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al), the ground electrode having small crystal grain diameters tends to be easily broken in high-temperature conditions. 
   Large engine vibrations and combustion pressures are main causes of breakages. The breakages chiefly occur at the bent portion of the ground electrode where a largest external force is applied. Upon investigation on breakages at the bent portion, the inventor has confirmed that breakages occur at the grain boundaries. 
   In general, in an ordinary temperature level sufficiently lower than the above high-temperature conditions, the intergranular strength is superior to the transgranular strength. When the grain diameters are small, a large number of grain boundaries are presents. Thus, when the crystal grain diameters are small, the ground electrode is strong enough and robust against breakages. 
   However, the bent portion of the ground electrode is subjected to relatively high temperatures during operations of the engine. Under such severe temperature conditions, the transgranular strength is superior to the intergranular strength. Thus, it can be concluded that having larger grain diameters is advantageous to have excellent breakage-resisting properties in high-temperature conditions. 
   Namely, according to the inventor of this application, having a large crystal structure at the bent portion is a key to assure excellent breakage-resisting properties in the high-temperature conditions. From the above inventor&#39;s knowledge and based on various analyses and evaluations, the present invention provides the following first to third spark plugs. 
   More specifically, the first spark plug of the present invention includes a metal housing, an insulator, a center electrode, and a ground electrode. The metal housing has a fixing screw portion provided on an outer surface thereof, so that the metal housing can be installed to an engine via the fixing screw portion. The insulator, fixed in the metal housing, has one end portion protruding from one end portion of the metal housing. The center electrode, fixed in an axial hole of the insulator, has one end portion protruding from one end portion of the insulator. The ground electrode has one end portion fixed to the metal housing, a bent portion provided at an intermediate portion thereof, and the other end portion positioned in a confronting relationship with one end portion of the center electrode to form a spark discharge gap. 
   The first spark plug of the present invention is characterized in that the ground electrode contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al, a cross-sectional area of the ground electrode is not less than 2 mm 2  and not greater than 3 mm 2 , and an average value of crystal grain diameters in a thickness direction is not less than 100 μm at least at the bent portion. 
   The first spark plug of the present invention can secure excellent heat-resisting properties for the ground electrode, as the ground electrode contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al. 
   Furthermore, as the cross-sectional area of the ground electrode is not less than 2 mm 2  and not greater than 3 mm 2 , the first spark plug of the present invention can secure high ignitability by reducing the cooling loss due to quenching effects at high mixture flow velocities and also can prevent the temperature from increasing steeply in the ground electrode. 
   Furthermore, as the average value of crystal grain diameters in the thickness direction is not less than 100 μm at least at the bent portion of the ground electrode, the first spark plug of the present invention can suppress breakages of the ground electrode even in severe temperature and vibration conditions during engine operations. 
   As described above, the first spark plug of the present invention can secure satisfactory heat-resisting properties and breakage-resisting properties for the ground electrode even when the ground electrode is thinned to secure high ignitability against high mixture flow velocities. 
   Furthermore, the second spark plug of the present invention includes a metal housing, an insulator, a center electrode, and a ground electrode. The metal housing has a fixing screw portion provided on an outer surface thereof, so that the metal housing can be installed to an engine via the fixing screw portion. The insulator, fixed in the metal housing, has one end portion protruding from one end portion of the metal housing. The center electrode, fixed in an axial hole of the insulator, has one end portion protruding from one end portion of the insulator. The ground electrode has one end portion fixed to the metal housing, a bent portion provided at an intermediate portion thereof, and the other end portion positioned in a confronting relationship with one end portion of the center electrode to form a spark discharge gap. 
   The second spark plug of the present invention is characterized in that the ground electrode contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al, a cross-sectional area of the ground electrode is not less than 2 mm 2  and not greater than 3 mm 2 , an average value of crystal grain diameters in the thickness direction is not greater than 50 μm, and a height of the bent portion from one end portion of the metal housing is not less than 4 mm and not greater than 6.5 mm. 
   According to the second spark plug of the present invention, the ground electrode contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al, and the cross-sectional area of the ground electrode is not less than 2 mm 2  and not greater than 3 mm 2 . Therefore, the second spark plug of the present invention brings the functions and effects substantially identical with those of the above-described first spark plug of the present invention. 
   Furthermore, according to the second spark plug of the present invention, although the average value of crystal grain diameters in the thickness direction at the ground electrode is not greater than 50 μm, the height of the bent portion from one end portion of the metal housing is not less than 4 mm and not greater than 6.5 mm. 
   The above-described range of the height is based on the result of analysis conducted by the inventor. Setting the above-described height brings the effect of suppressing the temperature increase of the ground electrode within a practical range. Furthermore, it becomes possible to increase the temperature of the bent portion during operations of the spark plug to a desirable temperature level so that the average value of crystal grain diameters in the thickness direction becomes equal to or greater than 100 μm. 
   Namely, according to the second spark plug of the present invention which regulates the height as described above, even if initial crystal grain diameters are small, recrystallization occurs at the bent portion when the spark plug is used in high-temperature engine operating conditions, and accordingly the crystal grain diameters become sufficiently larger to secure excellent breakage-resisting properties. Thus, the second spark plug of the present invention can secure sufficient breakage-resisting properties. 
   As described above, the second spark plug of the present invention can secure satisfactory heat-resisting properties and breakage-resisting properties for the ground electrode even when the ground electrode is thinned to secure high ignitability against high mixture flow velocities. 
   More specifically, the third spark plug of the present invention includes a metal housing, an insulator, a center electrode, and a ground electrode. The metal housing has a fixing screw portion provided on an outer surface thereof, so that the metal housing can be installed to an engine via the fixing screw portion. The insulator, fixed in the metal housing, has one end portion protruding from one end portion of the metal housing. The center electrode, fixed in an axial hole of the insulator, has one end portion protruding from one end portion of the insulator. The ground electrode has one end portion fixed to the metal housing, a bent portion provided at the intermediate portion thereof, and the other end portion positioned in a confronting relationship with one end portion of the center electrode to form a spark discharge gap. 
   The third spark plug of the present invention is characterized in that the ground electrode contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al, a cross-sectional area of the ground electrode is not less than 2 mm 2  and not greater than 3 mm 2 , an average value of crystal grain diameters in the thickness direction is not greater than 50 μm, and the average value of crystal grain diameters in the thickness direction becomes equal to or greater than 100 μm at least at the bent portion of the ground electrode when the spark plug is used for 10 hours or more in an engine of 2000 cc under a condition that the rotational speed is 5600 rpm and the throttle is fully opened. 
   According to the third spark plug of the present invention, the ground electrode contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al, and the cross-sectional area of the ground electrode is not less than 2 mm 2  and not greater than 3 mm 2 . Therefore, the third spark plug of the present invention brings the functions and effects substantially identical with those of the above-described first spark plug of the present invention. 
   Furthermore, according to the third spark plug of the present invention, although the average value of crystal grain diameters in the thickness direction at the ground electrode is not greater than 50 μm, the average value of crystal grain diameters in the thickness direction becomes equal to or greater than 100 μm at least at the bent portion of the ground electrode when the spark plug is used for 10 hours or more in an engine of 2000 cc under a condition that the rotational speed is 5600 rpm and the throttle is fully opened. 
   Namely, according to the third spark plug of the present invention, when the engine is operating in the severe thermal load conditions which may cause breakage of the ground electrode, recrystallization occurs at the bent portion of the ground electrode and accordingly the crystal grain diameters become sufficiently larger to secure excellent breakage-resisting properties. Thus, the third spark plug of the present invention can secure sufficient breakage-resisting properties. 
   As described above, the third spark plug of the present invention can secure satisfactory heat-resisting properties and breakage-resisting properties for the ground electrode even when the ground electrode is thinned to secure high ignitability against high mixture flow velocities. 
   According to the above-described first to third spark plugs of the present invention, it is preferable that the ground electrode contains Al by an amount not less than 0.5 weight % and not greater than 2 weight %, and also contains Cr by an amount not less than 18 weight % and not greater than 25 weight %. 
   Alternatively, according to the above-described first to third spark plugs of the present invention, it is preferable that the ground electrode contains Al by an amount not less than 2 weight % and not greater than 5 weight % and also contains Cr by an amount not less than 10 weight % and not greater than 18 weight % 
   Furthermore, according to the above-described first to third spark plugs of the present invention, it is preferable that the ground electrode contains a rare earth element. Adding the rare earth element, such as lanthanoids, to the ground electrode is effective in improving the heat-resisting properties of the ground electrode. 
   Furthermore, according to the above-described first to third spark plugs of the present invention, it is preferable that a bending angle of the bent portion is equal to or less than 100°. 
   The present invention is based on the result of analysis conducted by the inventor. When the bending angle is large, the flame kernel may contact with the ground electrode and cause quenching effects. As a result, the ignitability is lessened. However, setting the bending angle to be equal to or less than 100° makes it possible to secure adequate ignitability. 
   Furthermore, according to the above-described first to third spark plugs of the present invention, it is preferable that the fixing screw portion is M 10  or less according to the Japanese Industrial Standard. 
   According to the spark plug having the fixing screw portion of M 10  or less, the ground electrode has a sufficiently thin thickness. Hence, it becomes possible to obtain appropriate effects by employing the above-described arrangements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description which is to be read in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a half cross-sectional view showing an overall arrangement of a spark plug in accordance with a first embodiment of the present invention; 
       FIG. 2A  is an enlarged side view showing an igniting portion of the spark plug shown in  FIG. 1 ; 
       FIG. 2B  is a cross-sectional view showing the spark plug, taken along a line A—A of  FIG. 2A ; 
       FIG. 3  is a view explaining dimensions L and H of a ground electrode of the spark plug in accordance with the first embodiment of the present invention; 
       FIG. 4  is an enlarged cross-sectional view showing a bent portion of the ground electrode of the spark plug in accordance with the first embodiment of the present invention; 
       FIG. 5  is a view schematically explaining a relationship between mixture flow velocity V and flame kernel K in a spark plug; 
       FIG. 6  is a graph showing a relationship between the mixture flow velocity V and a cooling energy; 
       FIG. 7  is a graph showing a relationship between a cross-sectional area S of the ground electrode and the cooling energy; 
       FIG. 8  is a graph showing a relationship between the cross-sectional area S of the ground electrode and a front end temperature of the ground electrode; 
       FIG. 9  is a view explaining a method for evaluating breakage-resisting properties; 
       FIG. 10  is a graph showing a relationship between a bending angle θ of the bent portion and the cooling energy; 
       FIG. 11  is a graph showing a relationship between the height H and the front end temperature of the ground electrode; and 
       FIG. 12  is a graph showing a relationship between the height H and a bent portion temperature of the ground electrode. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, preferred embodiments of the present invention will be explained with reference to attached drawings. In the drawings, the same or equivalent portions or members are denoted by the same reference numerals. 
   First Embodiment 
     FIG. 1  is a half cross-sectional view showing an overall arrangement of a spark plug S 1  in accordance with a first embodiment of the present invention.  FIG. 2A  is an enlarged side view showing an igniting portion of the spark plug S 1  shown in  FIG. 1 .  FIG. 2B  is a cross-sectional view showing the spark plug S 1 , taken along a line A—A of  FIG. 2A . 
   Arrangement of Spark Plug 
   The spark plug S 1  is preferably applicable to an automotive engine as an ignition plug which is inserted and fixed in a screw hole provided in an engine head (not shown) defining therein a combustion chamber. 
   The spark plug S 1  includes a cylindrical metal housing  10  made of an electrically conductive steel plate (e.g. low-carbon steel). A fixing screw portion  11  is provided on an outer cylindrical surface of the metal housing  10 , so that the metal housing  10  can be fixed to an engine block (not shown). According to this embodiment, the fixing screw portion  11  is M 10  or less according to JIS (i.e. Japanese Industrial Standard). 
   An insulator  20 , made of almina ceramic (Al 2 O 3 ) or the like, is accommodated and fixed in the metal housing  10 . The insulator  20  has one end portion  20   a  protruding from one end portion  10   a  of the metal housing  10 . 
   A center electrode  30  is fixed in an axial hole  21  of the insulator  20 , so that the center electrode  30  can be held in an insulated condition relative to the metal housing  10 . 
   For example, the center electrode  30  is a columnar member consisting of an inner material arranged by Cu or other metallic material having excellent thermal conductivity and an outer material arranged by a Ni-based alloy or other metallic material having excellent heat-resisting and ant-corrosion properties. 
   As shown in  FIG. 1 , the center electrode  30  has one end portion  30   a  protruding from one end portion  20   a  of the insulator  20 . Thus, the center electrode  30  is held in an insulated condition relative to the metal housing  10 , with one end portion  30   a  protruding from one end portion  10   a  of the metal housing  10 . 
   Furthermore, the ground electrode  40  is a pillar member containing either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al. 
   According to this embodiment, to secure heat-resisting properties for the ground electrode  40 , the ground electrode  40  contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al. 
   As described above, in high-temperature engine operating conditions, additive elements having relatively small standard formation free energies are easily oxidized than the main components having relatively large standard formation free energies. Accordingly, the additive elements shift toward the surface of the ground electrode  40  and form surface oxides there. 
   Namely, adding Cr or Al as additive elements to the ground electrode enables the ground electrode to form a stable surface oxide layer (i.e. coating layer) of the additive elements on the surface, as each of Cr or Al has a standard formation free energy smaller than those of the main components. As the above-described surface oxide coating is stably formed on the surface of the ground electrode, the oxidation phenomenon does not advance into the inside of the ground electrode. Thus, it becomes possible to secure excellent heat-resisting and oxidation-resisting properties for the ground electrode  40 . 
   More specifically, the ground electrode  40  contains Al by an amount not less than 0.5 weight % and not greater than 2 weight %, and also contains Cr by an amount not less than 18 weight % and not greater than 25 weight %. Alternatively, the ground electrode  40  contains Al by an amount not less than 2 weight % and not greater than 5 weight % and also contains Cr by an amount not less than 10 weight % and not greater than 18 weight % 
   Furthermore, the ground electrode  40  of this embodiment has one end portion  41  being fixed by welding to one end portion  10   a  of the metal housing  10 , a bent portion  42  being bent into an L-shaped configuration at the intermediate portion, and the other end portion  43  positioned in a confronting relationship with one end portion  30   a  of the center electrode  30  to form a spark discharge gap  50 . 
   According to this embodiment, a noble metallic tip  35  is bonded on one end portion  30   a  of the center electrode  30  by laser welding or resistance welding. The noble metallic tip  35  serves as a spark discharging member. For example, the noble metallic tip  35  is a columnar member made of a Pt alloy or an Ir alloy. The spark discharge gap  50  is a clearance (i.e. gap) between a distal end portion of the noble metallic tip  35  and the other end portion  43  of the ground electrode  40  (more specifically, a side surface of the end portion  43  facing to the spark discharge gap  50 ). 
   Dimensions of the ground electrode  40  will be explained with reference to  FIGS. 2A ,  2 B and  3 .  FIG. 3  is a view explaining dimensions L and H of the ground electrode  40  of the spark plug S 1 . 
   According to this embodiment, an area S (i.e. a cross-sectional area S) of a cross section  40   a  of the ground electrode  40  shown in  FIG. 2B  is not less than 2 mm 2  and not greater than 3 mm 2 . According to the example shown in  FIG. 2B , the ground electrode  40  is a square pole having a rectangular cross section. For example, the ground electrode  40  has thickness ‘d’ of 1.15 mm and width ‘w’ of 2.2 mm. 
   Furthermore,  FIG. 3  shows a height H of the bent portion  42  of the ground electrode  40  from one end portion  10   a  (i.e. front end surface  10   a ) of the metal housing  10  and a length L from the bent portion  42  to the front end surface of the other end portion  43 . 
   As shown in  FIG. 3 , a circle A 1  is a virtual circle tangent to each of the front end surface of the other end portion  43  of the ground electrode  40 , the side surface of the other end portion  43  facing to the spark discharge gap  50 , and the opposite side surface of the other end portion  43  of the spark discharge gap  50 . A circle A 2  is a virtual circle tangent to the circle A 1  and both of the side surfaces of the other end portion  43 . A virtual line L 1  passes the centers of these circles A 1  and A 2 . 
   On the other hand, as shown in  FIG. 3 , a circle B 1  is a virtual circle tangent to a boundary surface between the ground electrode  40  and the metal housing  10 , a side surface of one end portion  41  facing to the spark discharge gap  50 , and the opposite side surface of one end portion  41 . A circle B 2  is a virtual circle tangent to the circle B 1  and both of the side surfaces of one end portion  41 . A virtual line L 2  passes the centers of these circles B 1  and B 2 . 
   Two virtual lines L 1  and L 2  intersect at a point P. The angle formed between these virtual lines L 1  and L 2  is defined as a bending angle θ of the bent portion  42 . It is desirable that the bending angle θ is equal to or less than 100°. 
   Furthermore, the height H of the bent portion  42  of the ground electrode  40  from the front end surface  10   a  of the metal housing  10  is defined as representing a distance from the intersecting point P to the front end surface  10   a  of the metal housing  10 . Furthermore, the length L from the bent portion  42  of the ground electrode  40  to the front end surface of the other end portion  43  is defined as representing a distance from the intersecting point P to the front end surface of the other end portion  43 . The length L is in a range from 3 mm to 5 mm. 
   Furthermore,  FIG. 4  is an enlarged cross-sectional view showing the bent portion  42  of the ground electrode  40  in accordance with this embodiment. The microscope observation applied to a cut surface reveals the crystal structure of metals arranging the ground electrode  40 . 
   According to this embodiment, an average value D of crystal grain diameters in the thickness direction is not less than 100 μm at least at the bent portion  42  of the ground electrode  40 . More specifically, it is possible to observe numerous crystal grains arrayed along a place indicated by the line B—B in  FIG. 4 . These numerous crystal grains have grain diameters, and the average value D of the grain diameters is equal to or greater than 100 μm. The line B—B represents a line extending from the intersecting point P and dividing the bending angle θ into the same angles. 
   For example, the average grain diameter D can be obtained by using the following method. First of all, the ground electrode  40  is cut along the longitudinal axis so that a bare cut surface is formed as shown in  FIG. 4 . 
   Then, the bare cut surface of the ground electrode  40  is treated by etching fluid such as oxalic acid to visualize grain boundaries. Then, with respect to the crystal grains arrayed along the line B—B on the treated cut surface, grain diameters are measured through the microscope observation. Then, the average value D of crystal grain diameters is calculated based on the measured grain diameters of these crystal grains. 
   For example, applying a heat treatment to the ground electrode  40  to cause recrystallization makes it possible to obtain crystal grain diameters having the average value D not less than 100 μm in the thickness direction at least at the bent portion  42  of the ground electrode  40 . When the heat treatment temperature is high, the average value D of crystal grain diameters in the thickness direction tends to become larger. 
   Furthermore, as shown in  FIG. 1 , according to the spark plug S 1  of this embodiment, the insulator  20  is inserted in the metal housing  10 . The metal housing  10  has a caulking portion  12  formed at the other end portion  10   b . The insulator  20  and the metal housing  10  are fixed together by deforming the caulking portion  12 . 
   Furthermore, at this caulking portion  12 , two metallic members  60  and talc  61  are positioned between the metal housing  10  and the insulator  20  so as to cooperatively seal the clearance between the metal housing  10  and the insulator  20 . The talc  61  interposes between two metallic rings  60 . 
   Furthermore, as shown in  FIG. 1 , the insulator  20  has a barrel portion  22  where the diameter of the insulator  20  is maximized. Namely, the barrel portion  22  of the insulator  20  is located in the metal housing  10  as a stepped portion having the maximum diameter. 
   Providing such a stepped portion (i.e. barrel portion  22 ) is effective in realizing the caulking operation since the metallic members  60  and talc  61  can be surely held between the caulking portion  12  and the barrel portion  22 . 
   Furthermore, the insulator  20  has a sublevel portion  23  extending from the barrel portion  22  toward one end portion  20   a  in the metal housing  10 . The sublevel portion  23  has a diameter smaller than that of the barrel portion  22 . In other words, there is a step (i.e. a radial difference) between the barrel portion  22  and the sublevel portion  23 . 
   As described above, the insulator  20  has the barrel portion  22  for ensuring the caulking fixation of metal housing  10  and for stably holding the seal members. Furthermore, the diameter of insulator  20  is thinned at the sublevel portion  23  positioned next to the barrel portion  22  which extends toward one end portion  20   a  of the insulator  20  (i.e. toward the spark discharging portion). 
   Furthermore, as shown in  FIG. 1 , in the axial hole  21  of insulator  20 , the other end portion  30   b  of the center electrode  30  is electrically connected to a resistance element  75  via an electrically conductive glass seal member  70 . 
   Furthermore, as shown in  FIG. 1 , the resistance element  75  is electrically connected to one end portion  80   a  of a terminal electrode (i.e. stem)  80  via an electrically conductive glass seal member  70  in the axial hole  21  positioned closely to the other end portion  20   b  of insulator  20 . 
   The other end portion  80   b  of the terminal electrode  80  protrudes out of the other end portion  20   b  of insulator  20 . An ignition coil (not shown) is connected to the other end portion  80   b  of terminal electrode  80 . 
   Dimensional Relationships 
   According to this embodiment, the cross-sectional area S of the ground electrode  40  is in a range not less than 2 mm 2  and not greater than 3 mm 2 , and the average value D of crystal grain diameters in the thickness direction is not less than 100 μm at least at the bent portion  42  of the ground electrode  40 . The dimensional relationship of the above-described embodiment is based on inventor&#39;s analysis and experimental results. 
   First of all, the inventor has obtained a relationship between mixture flow velocity and ignitability.  FIG. 5  is a view schematically explaining a relationship between mixture flow velocity V and flame kernel K in a spark plug. When the mixture flow velocity V is high, the flame kernel K possibly contacts with the ground electrode  40  and causes quenching effects. The ignitability is lessened. 
   In view of the above, the inventor has analyzed the relationship among the mixture flow velocity V, the cross-sectional area S (refer to  FIG. 2B ) of ground electrode  40 , and the ignitability. In this case, the cross-sectional area S of ground electrode  40  is a cross-sectional area of the ground electrode  40  at distance LA (refer to  FIG. 2A ) of 2 mm from the front end surface of the other end portion  43   
   The inventor has analyzed a cooling energy Q at the ground electrode  40  (i.e. energy loss caused when the flame kernel K is cooled by the ground electrode  40 ), assuming a mixture flow velocity V at which the ignitability is most worsened, i.e. the flow velocity V at which the flame kernel K is brought into contact with the ground electrode  40 .  FIGS. 6 and 7  show the result of this analysis. 
     FIG. 6  is a graph showing a relationship between the mixture flow velocity V and the cooling energy.  FIG. 7  is a graph showing a relationship between a cross-sectional area S of ground electrode  40  and the cooling energy. In these  FIGS. 6 and 7 , the cooling energy is expressed in terms of relative ratio, i.e. cooling energy ratio. 
   As shown in  FIG. 6 , when the mixture flow velocity V is large, the flame kernel K tends to contact with the ground electrode  40  and accordingly the cooling energy Q becomes larger. In other words, the effect of cooling loss becomes larger and the ignitability is lessened when the mixture flow velocity V increases. 
   According to conventional engines, the mixture flow velocity V is approximately 5 m/s. On the other hand, recent advanced engines have the mixture flow velocity V of approximately 15 m/s. The cooling energy is 1.5 times the conventional value. 
   Furthermore, as shown in  FIG. 7 , the cooling energy becomes larger with increasing cross-sectional area S of the ground electrode  40 . When the cross-sectional area S of ground electrode  40  is less than 3 mm 2 , the effect of cooling loss becomes smaller and accordingly the ignitability can be secured adequately. 
   Furthermore, if the ground electrode  40  is too much thin, the temperature will increase greatly at the front end portion of ground electrode  40 , i.e. at the other end portion  43 , up to approximately 1100° C. 
   If the temperature is increased up to 1100° C., irregular ignition phenomenon (i.e. so-called preignition) will occur at the front end portion of ground electrode  40  prior to the regular ignition occurring in the spark discharge gap  50 . Such irregular ignitions possibly damage the engine. 
   According to conventional spark plugs, the cross-sectional area S of ground electrode  40  is approximately 4.4 mm 2 . In this case, the front end temperature will increase up to 1000° C. at maximum in ordinary engine operating conditions. Regarding the dimensions of ground electrode  40 , the thickness ‘d’ is 1.6 mm and the width ‘w’ is 2.8 mm (refer to  FIG. 2B ). 
   The inventor has obtained a relationship between the cross-sectional area S of ground electrode  40  and the front end temperature of ground electrode  40  according to a temperature analysis using the finite-element method.  FIG. 8  shows the result of this analysis. As shown in  FIG. 8 , when the cross-sectional area S of ground electrode  40  is less than 2 mm 2 , the front end temperature of ground electrode  40  increases steeply. 
   From the result of analyses shown in  FIG. 6  to  FIG. 8 , when the cross-sectional area S of the ground electrode  40  is not less than 2 mm and not greater than 3 mm 2 , it is understood that the effect of cooling loss due to quenching effects at high mixture flow velocities can be reduced and accordingly high ignitability can be secured without causing steep temperature increase at the ground electrode  40 . This is the reason why the cross-sectional area S of the ground electrode  40  is set to be not less than 2 mm 2  and not greater than 3 mm 2  in this embodiment. 
   Furthermore, as described above, to secure heat-resisting properties for the ground electrode  40  in high-temperature conditions, e.g. at the temperature level exceeding 1000° C., it is effective that the ground electrode  40  contains Ni or Fe as a main component and Cr or Al as an additive. 
   Hence, the inventor has conducted evaluations with respect to breakage-resisting properties of the ground electrode  40  which has the cross-sectional area S of 2.5 mm 2  (equivalent to thickness d=1.15 mm and width w=2.2 mm) and uses a material of Ni-15 wt % Cr-2.5 wt % Al. 
     FIG. 9  is a view explaining a method for evaluating breakage-resisting properties of the ground electrode  40 . As shown in  FIG. 9 , as an acceleration evaluation, a large force is applied to the bent portion  42  by using a vibrator F (having a testing power of 10 G). The length L of the tested ground electrode is set to 100 mm, although the length L of a practical ground electrode is usually in a range from 3 mm to 5 mm. Furthermore, the tested ground electrode has the height H of 6 mm. 
   Furthermore, the temperature of the ground electrode  40 , at the region from the front end portion  43  to the bent portion  42 , is increased up to 900° C. by using a gas burner to realize engine operating conditions. 
   Then, as shown in  FIG. 9 , one end portion  41  of ground electrode  40  is fixed to the vibrator F. By adjusting the frequency (e.g. 60 Hz) of the vibrator F, the front end portion  43  is vibrated to forcibly cause breakages. 
   Then, the inventor has evaluated the breakage-resisting properties of the ground electrode  40  by changing the average value D of crystal grain diameters in the thickness direction at the bent portion  42 . For example, the crystal grain diameters can be changed by adjusting heat treatment conditions to evaluation the breakage-resisting properties. 
   More specifically, evaluations of the breakage-resisting properties of the ground electrode  40  were conducted at 30 μm, 75 μm, 100 μm, and 160 μm of the average value D of crystal grain diameters. The average value D of crystal grain diameters of the ground electrode  40  is initially 30 μM, and becomes 75 μm when the ground electrode  40  is subjected to a 30-minute heat treatment at 850° C. Furthermore, the average value D of crystal grain diameters of the ground electrode  40  becomes 100 μm when subjected to a 30-minute heat treatment at 900° C., and becomes 160 μm when subjected to a 30-minute heat treatment at 1000° C. 
   The following Table 1 shows the evaluation result with respect to the breakage-resisting properties of the ground electrode  40 . More specifically, Table 1 shows the presence of breakages in relation to the average value D of crystal grain diameters in the thickness direction at the bent portion  42  of ground electrode  40 . Regarding the “presence of breakage” in Table 1, the sign X represents occurrence of breakages and the sign ◯ represents no occurrence of breakages. 
   
     
       
         
             
             
             
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
           
          
             
                 
               D 
               30 
               75 
               100 
               160 
             
             
                 
               Presence of Breakages 
               X 
               X 
               ◯ 
               ◯ 
             
             
                 
                 
             
          
         
       
     
   
   As shown in Table 1, when the average value D of crystal grain diameters in the thickness direction at the bent portion  42  of the ground electrode  40  is not less than 100 μm, no breakage occurs. 
   From the above evaluation result, this embodiment applies a heat treatment to increase the temperature of the ground electrode  40  up to 900° C. or more, so that the average value D of crystal grain diameters in the thickness direction becomes equal to or greater than 100 μm at least at the bent portion  42  of the ground electrode  40 . 
   According to this embodiment, it is required to satisfy the condition that the average value D of crystal grain diameters in the thickness direction is not less than 100 μm, at least at the bent portion  42  of the ground electrode  40  which is relatively weak and tends to cause breakages. It is however preferable that the above condition (D≧100 μm) is satisfied in the entire region of the ground electrode  40 . It is of course acceptable that the above condition (D≧100 μm) is satisfied only at the bent portion  42 . 
   Effects 
   The spark plug S 1  of this embodiment includes the metal housing  10  having the fixing screw portion  11  provided on the outer surface thereof so as to be installed to an engine via the fixing screw portion  11 . The insulator  20 , fixed in the metal housing  10 , has one end portion  20   a  protruding from one end portion  10   a  of the metal housing  10 . The center electrode  30 , fixed in the axial hole  21  of the insulator  20 , has one end portion  30   a  protruding from one end portion  20   a  of the insulator  20 . The ground electrode  40  has one end portion  41  fixed to the metal housing  10 , the bent portion  42  provided at the intermediate portion thereof, and the other end portion  43  positioned in a confronting relationship with one end portion  30   a  of the center electrode  30  to form the spark discharge gap  50 . 
   The spark plug S 1  of this embodiment is characterized in that the ground electrode  40  contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al. The cross-sectional area S of the ground electrode  40  is not less than 2 mm 2  and not greater than 3 mm 2 . And, the average value D of crystal grain diameters in the thickness direction is not less than 100 μm at least at the bent portion  42 . 
   Thus, the spark plug S 1  of this embodiment can secure excellent heat-resisting properties for the ground electrode  40  and also can secure adequate strength for the ground electrode  40 , as the ground electrode  40  contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al. 
   Furthermore, as the cross-sectional area S of the ground electrode  40  is not less than 2 mm 2  and not greater than 3 mm 2 , the spark plug S 1  of this embodiment can secure high ignitability by reducing the cooling loss due to quenching effects at high mixture flow velocities and also can prevent the temperature from increasing steeply in the ground electrode  40 . 
   Furthermore, as the average value D of crystal grain diameters in the thickness direction is not less than 100 μm at least at the bent portion  42  of the ground electrode  40 , the spark plug S 1  of this embodiment can suppress breakages of the ground electrode  40  even in severe temperature and vibration conditions during engine operations. 
   As described above, the spark plug S 1  of this embodiment can secure satisfactory heat-resisting properties and breakage-resisting properties for the ground electrode  40  even when the ground electrode  40  is thinned to secure high ignitability against high mixture flow velocities. 
   Furthermore, according to this embodiment, the ground electrode  40  contains Al by an amount not less than 0.5 weight % and not greater than 2 weight %, and also contains Cr by an amount not less than 18 weight % and not greater than 25 weight %. It is also preferable that the ground electrode  40  contains Al by an amount not less than 2 weight % and not greater than 5 weight % and also contains Cr by an amount not less than 10 weight % and not greater than 18 weight % 
   As described above, adding both of Cr and Al is effective to secure sufficient heat-resisting properties. However, excessively adding these elements will worsen workability of the ground electrode  40 , for example, in forming a gap. Especially, the amount of added Al gives large effect on the workability. Hence, it is preferable that the ground electrode  40  has the above-described composition, when the heat-resisting properties and the workability are taken into consideration. 
   Furthermore, according to this embodiment, it is preferable that the ground electrode  40  contains rare earth elements, such as Sc, Y, and lanthanoids. When the ground electrode  40  contains a small amount of (e.g. 0.5 weight %) lanthanoids or other rare earth elements, the heat-resisting properties of ground electrode  40  can be improved. 
   Furthermore, according to the above-described embodiment, it is preferable that the bending angle θ (refer to  FIG. 3 ) of the bent portion  42  of ground electrode  40  is equal to or less than 100°. 
   This is based on the result of analysis conducted by the inventor. When the bending angle θ is large, the flame kernel K tends to contact with the ground electrode  40  and causes quenching effects. The ignitability will be lessened. 
   The inventor has conducted the above analysis based on the ground electrode  40  having the cross-sectional area S of 2.5 mm 2  (corresponding to a ground electrode having thickness d=1.15 mm and width w=2.2 mm) at the mixture flow velocity V of 15 m/s.  FIG. 10  shows the result of this analysis. 
     FIG. 10  is a graph showing a relationship between the above-described bending angle θ and the cooling energy. In  FIG. 10 , the cooling energy is expressed in terms of relative ratio, i.e. cooling energy ratio. 
   As shown in  FIG. 10 , when the bending angle θ exceeds 100°, the cooling energy Q becomes larger greatly. Hence, it is preferable that the bending angle θ is equal to or less than 100°. Setting the bending angle θ to be equal to or less than 100° makes it possible to secure adequate ignitability. 
   If the gap formation of the ground electrode  40  is performed to excessively reduce the bending angle θ, the bent portion  42  may cause cracks due to large deformation. Hence, it is desirable that the bending angle θ is equal to or greater than 80°. 
   Second Embodiment 
   A spark plug in accordance with a second embodiment of the present invention will be explained with reference to the drawings used for explaining the first embodiment. Hereinafter, differences between the first and second embodiments will be explained in detail. 
   The spark plug of the second embodiment includes the metal housing  10  having the fixing screw portion  11  provided on the outer surface thereof so as to be installed to an engine via the fixing screw portion  11 . The insulator  20 , fixed in the metal housing  10 , has one end portion  20   a  protruding from one end portion  10   a  of the metal housing  10 . The center electrode  30 , fixed in an axial hole  21  of the insulator  20 , has one end portion  30   a  protruding from one end portion  20   a  of the insulator  20 . The ground electrode  40  has one end portion  41  fixed to the metal housing  10 , the bent portion  42  provided at the intermediate portion thereof, and the other end portion  43  positioned in a confronting relationship with one end portion  30   a  of the center electrode  30  to form the spark discharge gap  50 . 
   The spark plug of the second embodiment is characterized in that the ground electrode  40  contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al. The cross-sectional area S of the ground electrode  40  is not less than 2 mm 2  and not greater than 3 mm 2 . The average value D of crystal grain diameters in the thickness direction is not greater than 50 μm. And, the height H of the bent portion  42  from one end portion  10   a  of the metal housing  10  is not less than 4 mm and not greater than 6.5 mm. 
   As the ground electrode  40  contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al, and the cross-sectional area S of the ground electrode  40  is not less than 2 mm 2  and not greater than 3 mm 2 , the spark plug of the second embodiment brings the functions and effects substantially identical with those of the spark plug S 1  of the first embodiment. 
   Furthermore, according to the spark plug of the second embodiment, the ground electrode  40  satisfies the condition that the height H of the bent portion  42  from one end portion  10   a  of the metal housing  10  is not less than 4 mm and not greater than 6.5 mm, although the average value D of crystal grain diameters in the thickness direction is not greater than 50 μm. 
   The above-described range of the height H is based on the result of analysis conducted by the inventor. 
   Regarding the crystal grain diameters of the ground electrode  40  in the thickness direction, a smaller average value D will be acceptable in an initial condition, if the average value D can be later increased to a required value, i.e. 100 μm or more, due to recrystallization at the bent portion  42  which is heated during engine operations. In fact, breakages of the ground electrode  40  occur at high-speed/high-load engine operating conditions. In such severe conditions, the ground electrode  40  has high temperatures. 
   Hence, the inventor has conducted the following analysis according to the finite-element method with the above-described height H as a variable parameter. 
   As described above, to suppress occurrence of preignition, it is necessary to prevent the temperature of the front end portion  43  of ground electrode  40  (i.e. front end temperature) from exceeding 1100° C. In general, when the ground electrode  40  has a longer length L or when the ground electrode  40  has a smaller cross-sectional area S, the ground electrode  40  has poor heat-dissipating properties and accordingly the front end temperature tends to increase. 
   Hence, the inventor has conducted the analysis based on a spark plug of this embodiment which has the length L of 5 mm and the cross-sectional area S of 2 mm 2  (as representative dimensions causing severe temperature increase at the front end of the ground electrode).  FIG. 11  shows the result of this analysis. 
     FIG. 11  is a graph showing a relationship between the height H and the front end temperature of the ground electrode. As shown in  FIG. 11 , it is understood that the front end temperature of the ground electrode increases when the height H becomes larger. When the height H is equal to or less than 6.5 mm, the front end temperature does not exceed 1100° C. 
   On the other hand, when the temperature of the bent portion  42  (i.e. bent portion temperature) is equal to or higher than 900° C., recrystallization occurs at the bent portion  42  of ground electrode  40  and accordingly it becomes possible to set the average value D of crystal grain diameters in the thickness direction to be not less than 100 μm at the bent portion  42 . When length L is shorter, or when cross-sectional area S is larger, the ground electrode  40  has excellent heat-dissipating properties and accordingly the bent portion temperature does not increase so much. 
   Hence, the inventor has conducted the analysis based on a spark plug of this embodiment which has the length L of 3 mm and the cross-sectional area S of 3 mm 2  (as representative dimensions causing no severe temperature increase at the front end of the ground electrode).  FIG. 12  shows the result of this analysis. 
     FIG. 12  is a graph showing a relationship between the height H and the bent portion temperature of the ground electrode. As shown in  FIG. 12 , it is understood that the bent portion temperature of the ground electrode decreases when the height H becomes smaller. When the height H is equal to or greater than 4 mm, the bent portion temperature exceeds 900° C. 
   From the results shown in these  FIGS. 11 and 12 , this embodiment sets the height H of the bent portion  42  from one end portion  10   a  of the metal housing  10  to be not less than 4 mm and not greater than 6.5 mm. 
   Regulating the height H within the above-described range makes it possible to adequately suppress the temperature increase in the ground electrode  40  and also makes it possible to increase the average value D of crystal grain diameters in the thickness direction to 100 μm or more during the engine operations. 
   Namely, according to the spark plug of the second embodiment which regulates the height H as described above, even if initial crystal grain diameters are small, recrystallization occurs at the bent portion  42  when the spark plug is used in high-temperature engine operating conditions, and accordingly the crystal grain diameters become sufficiently larger to secure excellent breakage-resisting properties. Thus, the spark plug of the second embodiment can secure sufficient breakage-resisting properties. 
   As described above, the second embodiment can provide a spark plug capable of securing satisfactory heat-resisting properties and breakage-resisting properties for the ground electrode  40  even when the ground electrode  40  is thinned to secure high ignitability against high mixture flow velocities. 
   According to the second embodiment, initial crystal grain diameters of the ground electrode  40  are not larger than 50 μm. As temperatures at the region ranging from the bent portion  42  of the ground electrode  40  to the metal housing  10  are relatively low, it is desirable to have smaller crystal grain diameters at this region from the viewpoint of strength. 
   Third Embodiment 
   A spark plug in accordance with a third embodiment of the present invention will be explained with reference to the drawings used for explaining the first embodiment. Hereinafter, differences between the first and third embodiments will be explained in detail. 
   The spark plug of the third embodiment includes the metal housing  10  having the fixing screw portion  11  provided on the outer surface thereof so as to be installed to an engine via the fixing screw portion  11 . The insulator  20 , fixed in the metal housing  10 , has one end portion  20   a  protruding from one end portion  10   a  of the metal housing  10 . The center electrode  30 , fixed in an axial hole  21  of the insulator  20 , has one end portion  30   a  protruding from one end portion  20   a  of the insulator  20 . The ground electrode  40  has one end portion  41  fixed to the metal housing  10 , the bent portion  42  provided at the intermediate portion thereof, and the other end portion  43  positioned in a confronting relationship with one end portion  30   a  of the center electrode  30  to form a spark discharge gap  50 . 
   The spark plug of the third embodiment is characterized in that the ground electrode  40  contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al. The cross-sectional area S of the ground electrode  40  is not less than 2 mm 2  and not greater than 3 mm 2 . And, the average value D of crystal grain diameters in the thickness direction is not greater than 50 μm 
   Furthermore, the spark plug of the third embodiment is characterized the average value D of crystal grain diameters in the thickness direction becomes equal to or greater than 100 μm at least at the bent portion  42  of the ground electrode  40 , when this spark plug is used for 10 hours or more in an engine of 2000 cc under a condition that the rotational speed is 5600 rpm and the throttle is fully opened. 
   As the ground electrode  40  contains either Ni or Fe as a main component and at least one additive selected from the group consisting of Cr and Al, and the cross-sectional area S of the ground electrode  40  is not less than 2 mm 2  and not greater than 3 mm 2 , the spark plug of the third embodiment brings the functions and effects substantially identical with those of the spark plug S 1  of the first embodiment. 
   Furthermore, according to the spark plug of third embodiment, although the average value D of crystal grain diameters in the thickness direction is not greater than 50 μm, the ground electrode  40  satisfies the condition that the average value D of crystal grain diameters in the thickness direction becomes equal to or greater than 100 μm at least at the bent portion  42  of the ground electrode  40 , when the spark plug is used for 10 hours or more in an engine of 2000 cc under a condition that the rotational speed is 5600 rpm and the throttle is fully opened. 
   This arrangement can be realized, for example, by employing the above-described arrangement of the second embodiment which regulates the height H. Furthermore, the above-described used conditions of this embodiment can be realized after shipping the spark plug since this spark plug is anyhow used in a practical engine. Alternatively, it will be possible to realize the above-described used conditions before shipping the spark plug if a sufficient facility is prepared for realizing such conditions in the manufacturer of this spark plug. 
   In short, according to the third embodiment, the crystal grain diameters of the bent portion  42  become sufficiently larger due to recrystallization when the practical engine operating conditions become severe thermal load conditions causing breakages in the ground electrode. Accordingly, the spark plug of the third embodiment can secure excellent breakage-resisting properties. 
   As described above, the third embodiment can provide a spark plug capable of securing satisfactory heat-resisting properties and breakage-resisting properties for the ground electrode  40  even when the ground electrode  40  is thinned to secure high ignitability against high mixture flow velocities. 
   Other Embodiments 
   According to the present invention, it is preferable that the ground electrode  40  of the spark plug has a core material of Cu to improve its heat-dissipating properties. 
   Although the above-described embodiments are related to the spark plug having the ground electrode  40  being thinned to secure high ignitability against high mixture flow velocities, the fixing screw portion  11  of the spark plug has a thinned size equivalent to M 10  or less so that the thinned ground electrode can be preferably used. 
   However, it is needless to say that the spark plug of the present invention can have the fixing screw portion  11  being larger than M 10 .