Patent Publication Number: US-10777976-B1

Title: Spark plug

Description:
FIELD OF THE INVENTION 
     The present invention relates to a spark plug. 
     BACKGROUND OF THE INVENTION 
     Spark plugs including an ignition chamber have been developed. For example, a pre-chamber ignition plug according to Japanese Unexamined Patent Application Publication No. 2012-199236 (“PTL 1”) includes a cylindrical metal housing, and an ignition chamber cap that surrounds a center electrode and a ground electrode to form an ignition chamber. The ignition chamber cap has multiple orifices that allow an air-fuel mixture to flow into the ignition chamber from a combustion chamber. This ignition plug ignites in the ignition chamber, and injects torch-shaped flames into the combustion chamber through the orifices to burn an air-fuel mixture in the combustion chamber. 
     The ignition plug disclosed in PTL 1, however, has a structure where the ignition chamber is closed except for the orifices. Thus, the temperature inside the ignition chamber tends to rise at the ignition, which may cause pre-ignition. On the other hand, in this ignition plug, an amount of combustion gas that enters the ignition chamber is small, and cooling around the ignition chamber progresses due to, for example, heat conduction to the cylinder head, which may cause misfires. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-described circumstances, and aims to suppress occurrence of pre-ignition and misfires in a spark plug including a cover portion that forms a pre-chamber. The present invention can be embodied in the following forms. 
     (1) A spark plug includes a center electrode, a ground electrode that includes a facing portion facing a front end portion of the center electrode and forms a discharge gap between the facing portion and the front end portion of the center electrode, a cylindrical insulator that accommodates the center electrode therein with the front end portion of the center electrode being exposed from a front end of the insulator, a metal shell that accommodates the insulator therein, and a cover portion that covers, from a front end side of the spark plug, the front end portion of the center electrode and the facing portion of the ground electrode to form a pre-chamber, the cover portion including injection holes that are through-holes. A total area A (mm 2 ) of inner peripheral surfaces of the injection holes and a thermal conductivity B (W/mK) of a material of the cover portion satisfy a formula (1):
 
10&lt;A×B&lt;4000  formula (1).
 
     In a spark plug according to an aspect of the present invention, as the total area A (mm 2 ) of inner peripheral surfaces of the injection holes increases, heat in the pre-chamber is more likely to be transferred from the cover portion toward the metal shell side. As the thermal conductivity B (W/mK) of the material of the cover portion increases, heat in the pre-chamber is more likely to be transferred from the cover portion toward the metal shell side. Therefore, when A×B is set to be smaller than 4000, heat is not excessively transferred from the cover portion toward the metal shell side, so that misfires due to lowering of temperature of the cover portion can be prevented. In addition, when A×B is set to be larger than 10, heat transfer from the cover portion toward the metal shell side is facilitated, so that pre-ignition can be prevented. 
     (2) In the spark plug described in (1), the total area A (mm 2 ) and the thermal conductivity B (W/mK) satisfy a formula (2):
 
20&lt;A×B&lt;2400  formula (2).
 
     In this spark plug, when the product of A×B is set to be larger than 20, where A is the total area (mm 2 ) of the inner peripheral surfaces of the injection holes and B is the thermal conductivity (W/mK) of the material of the cover portion, heat transfer from the cover portion toward the metal shell side is further facilitated, so that pre-ignition can be more efficiently prevented. 
     (3) In the spark plug described in (1) or (2), when the inner peripheral surface of at least one of the injection holes having a center axial line inclined with respect to an axial line of the spark plug is cut by a plane P, a portion inside the injection hole on the front end side with respect to the plane P has a smaller surface area than a portion inside the injection hole on a rear end side with respect to the plane P, where the plane P is a plane that passes the center axial line of the injection hole and is orthogonal to a plane including the axial line of the spark plug and the center axial line of the injection hole. 
     In this spark plug, heat is more likely to be guided to be dissipated from the front end side of the cover portion to the rear end side in an environment where pre-ignition easily occurs. Therefore, temperature does not rise excessively, so that pre-ignition can be prevented. 
     (4) In the spark plug described in (1) or (2), when the inner peripheral surface of at least one of the injection holes having a center axial line inclined with respect to an axial line of the spark plug is cut by a plane P, a portion inside the injection hole on the front end side with respect to the plane P has a larger surface area than a portion inside the injection hole on a rear end side with respect to the plane P, where the plane P is a plane that passes the center axial line of the injection hole and orthogonal to a plane including the axial line of the spark plug and the center axial line of the injection hole. 
     In this spark plug, heat is more likely to be guided to be collected to the front end side of the cover portion in an environment where misfires easily occur. Therefore, temperature is not easily lowered, so that misfires can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a structure of a spark plug according to a first embodiment. 
         FIG. 2  is a partially-enlarged cross-sectional view of the spark plug according to a first embodiment. 
         FIG. 3  is a partially-enlarged cross-sectional view of a spark plug according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     Hereinafter, a first embodiment of a spark plug  100  will be described in detail with reference to the drawings. In the following description, the lower side in  FIG. 1  is referred to as a front end side (front side) of the spark plug  100 , and the upper side in  FIG. 1  is referred to as a rear end side of the spark plug  100 . 
       FIG. 1  is a cross-sectional view of a schematic structure of the spark plug  100  according to the first embodiment, in  FIG. 1 , a center axial line CX of the spark plug  100  (an axial line of the spark plug) is drawn with a dot-and-dash line. 
     The spark plug  100  is mounted on an internal combustion engine and used to ignite an air-fuel mixture in a combustion chamber. When mounted on the internal combustion engine, the front end side of the spark plug  100  (lower side in the drawing) is disposed inside the combustion chamber of the internal combustion engine, and the rear end side (upper side in the drawing) is disposed outside the combustion chamber. The spark plug  100  includes a center electrode  10 , a ground electrode  13 , an insulator  20 , a terminal electrode  30 , and a metal shell  40 . 
     The center electrode  10  is constituted by a shaft-shaped electrode member and disposed in such a manner that a center axis thereof is coincident with the center axial line CX of the spark plug  100 . The center electrode  10  is held by the metal shell  40  with the insulator  20  interposed therebetween in such a manner that a front end portion  11  is positioned on the rear end side (upper side in the drawing) with respect to a front-end-side opening portion  40 A of the metal shell  40 . The center electrode  10  is electrically connected to an external power source via the terminal electrode  30  disposed on the rear end side. 
     The ground electrode  13  is a rod-shaped electrode extending from a position slightly on the rear end side (upper side in the drawing) with respect to the front-end-side opening portion  40 A of the metal shell  40  toward a position slightly on the front end side (lower side in the drawing) with respect to the front end portion  11  of the center electrode  10 . Specifically, the ground electrode  13  is connected to the metal shell  40  at a position slightly on the rear end side (upper side in the drawing) with respect to the front-end-side opening portion  40 A. The ground electrode  13  extends up to the front of the front end portion  11  of the center electrode  10 . As illustrated in  FIG. 2 , the ground electrode  13  includes a facing portion  13 A facing the front end portion  11  of the center electrode  10 . A discharge gap SG is formed between the facing portion  13 A of the ground electrode  13  and the front end portion  11  of the center electrode  10 . 
     The insulator  20  is a cylindrical member including an axial hole  21  penetrating through the center thereof. The insulator  20  is constituted by, for example, a ceramic sintered body made of alumina or aluminum nitride. On the front end side of the axial hole  21  of the insulator  20 , the center electrode  10  is accommodated with the front end portion  11  thereof being exposed. On the rear end side of the axial hole  21 , the terminal electrode  30 , which is a shaft-shaped electrode member, is held. A rear end portion  31  of the terminal electrode  30  extends out from a rear end opening portion  22  of the insulator  20  so as to be connectable with the external power source. The center electrode  10  and the terminal electrode  30  are electrically connected to each other via a resistor  35  that is held between glass sealing materials in order to suppress generation of radio interference noise when a spark discharge occurs. The center axis of the insulator  20  is coincident with the center axial line CX of the spark plug  100 . 
     The metal shell  40  is a substantially cylindrical metal member including a cylinder hole  41  at the center thereof. The metal shell  40  is constituted of, for example, carbon steel. The center axis of the metal shell  40  is coincident with the center axial line CX of the spark plug  100 . As described above, the ground electrode  13  is attached near the front-end-side opening portion  404  of the metal shell  40 . A packing  43  is disposed between a diameter reduced portion inside the metal shell  40  and the insulator  20 . The packing  43  is constituted by, for example, a metal material softer than a metal material constituting the metal shell  40 . 
     The spark plug  100  includes a cover portion  50 . The cover portion  50  has a dome shape. The cover portion  50  is constituted of, for example, stainless steel, nickel-based alloy, or copper-based alloy. The cover portion  50  is annularly joined to the front end of the molal shell  40  (more specifically, the front-end-side opening portion  40 A). As illustrated in  FIG. 2 , the cover portion  50  covers the front end portion  11  of the center electrode  10  and the facing portion  13 A of the ground electrode  13  from the front side. The space surrounded by the cover portion  50  is a pre-chamber space (pre-chamber)  63 . The cover portion  50  has its thickness gradually decreasing from the rear end side toward an apex  51 A. 
     As illustrated in  FIG. 2 , the cover portion  50  has multiple injection holes  61  on the rear end side of the apex  51 A. The cover portion  50  has, for example, four injection holes  61 . Each of the injection holes  61  is a substantially cylindrical through-hole. Each of the injection holes  61  has its center axial line AX inclined with respect to the center axial line CX of the spark plug  100 . The multiple injection holes  61  are positioned on a virtual circumference centered on the center axial line CX of the spark plug  100 . The multiple injection holes  61  are arranged at equal intervals on the virtual circumference. 
     The pre-chamber space  63 , which is a space covered with the cover portion  50 , functions as an ignition chamber, and communicates with the combustion chamber via the injection holes  61 . When the inner peripheral surface of each of the four injection holes  61  of the cover portion  50  is cut by a plane P, the portion inside the injection hole  61  on the front end side with respect to the plane P has a smaller surface area than the portion inside the injection hole  61  on the rear end side. Here, the plane P is a plane that passes the center axial line AX of the injection hole  61  and is orthogonal to a plane including the center axial line CX of the spark plug  100  and the center axial line AX of the injection hole  61  (cross section of the spark plug  100  illustrated in  FIG. 2 ). In other words, when the inner peripheral surface of the injection hole  61  is cut by the plane including the center axial line CX of the spark plug  100  and the center axial line AX of the injection hole  61  (cross section of the spark plug  100  illustrated in  FIG. 2 ), the front-end-side cross-sectional edge of the inner peripheral surface of the injection hole  61  has a length L 1 , which is smaller than a length L 2  of the rear-end-side cross-sectional edge. Thus, in the cover portion  50 , a portion  50 A on the front end side with respect to the injection holes  61  is thinner than a portion  50 B on the rear end side with respect to the injection holes  61 . In the spark plug  100  with this structure, heat is more likely to be guided to be dissipated from the front end side of the cover portion  50  to the rear end side in an environment where pre-ignition easily occurs. Therefore, temperature does not rise excessively, so that pre-ignition can be prevented. 
     In the spark plug  100  according to the first embodiment, the total area A (mm 2 ) of the inner peripheral surfaces of the four injection holes  61  and the thermal conductivity B (W/mK) of the material of the cover portion  50  satisfy the following formulae (1), (3), and (4):
 
10&lt;A×B&lt;4000  formula (1),
 
0.7≤A≤18.5  formula (3), and
 
13≤B≤372  formula (4).
 
     In this spark plug  100 , as the total area A (mm 2 ) of the inner peripheral surfaces of the four injection holes  61  increases, heat in the pre-chamber space  63  is more likely to be transferred from the cover portion  50  toward the metal shell  40  side. As the thermal conductivity B (W/mK) of the material of the cover portion  50  increases, heat in the pre-chamber space  63  is more likely to be transferred from the cover portion  50  toward the metal shell  40  side. Therefore, when A×B is set to be smaller than 4000, heat is not excessively transferred from the cover portion  50  toward the metal shell  40  side, so that misfires due to lowering of temperature of the cover portion  50  can be prevented. In addition, when Ax B is set to be larger than 10, heat transfer from the cover portion  50  toward the metal shell  40  side is facilitated, so that pre-ignition can be prevented. 
     In the spark plug  100  according to the first embodiment, preferably, the total area A (mm 2 ) of the inner peripheral surfaces of the four injection holes  61  and the thermal conductivity B (W/mK) of the material of the cover portion  50  satisfy the following formula (2):
 
20&lt;A×B&lt;2400  formula (2).
 
     In this spark plug  100 , when the product of A×B is set to be larger than 20, where A is the total area (mm 2 ) of the inner peripheral surfaces of the four injection holes  61  and B is the thermal conductivity (W/mK) of the material of the cover portion, heat transfer from the cover portion  50  toward the metal shell  40  side is further facilitated, so that pre-ignition can be more efficiently prevented. 
     Second Embodiment 
     A spark plug  200  according to a second embodiment will now be described with reference to  FIG. 3 . The spark plug  200  according to the second embodiment differs from the spark plug  100  according to the first embodiment in terms of the structure of a cover portion  250 . The other configurations are substantially the same as those in the spark plug  100  according to the first embodiment. Components having substantially the same configurations are thus driven the same reference signs, and description of structures, actions, and effects thereof is omitted. 
     As illustrated in  FIG. 3 , the cover portion  250  has a dome shape. The cover portion  250  is annularly joined to the front end of the metal shell  40  (more specifically, the front-end-side opening portion  40 A). The cover portion  250  covers the front end portion  11  of the center electrode  10  and the facing portion  13 A of the ground electrode  13  from the front side. The space surrounded by the cover portion  250  is a pre-chamber space  263 . The cover portion  250  has its thickness gradually increasing from the rear end side toward an apex  251 A. 
     As illustrated in  FIG. 3 , the cover portion  250  has multiple injection holes  261  on the rear end side of the apex  251 A. The cover portion  250  has, for example, four injection holes. Each of the injection holes  261  is a substantially cylindrical through-hole. Each of the injection holes  261  has its center axial line AX inclined with respect to the center axial line CX of the spark plug  200 . The multiple injection holes  261  are positioned on a virtual circumference centered on the center axial line CX of the spark plug  200 . The multiple injection holes  261  are arranged at equal intervals on the virtual circumference. 
     The pre-chamber space  263 , which is a space covered with the cover portion  250 , communicates with the combustion chamber through the injection holes  261 . When the inner peripheral surface of one injection hole  261  of the cover portion  250  is cut by a plane P, the portion inside the injection hole  261  on the front end side with respect to the plane P has a larger surface area than the portion inside the injection hole  261  on the rear end side with respect to the plane P. Here, the plane P is a plane that passes the center axial line AX of the injection hole  261  and is orthogonal to the plane including the center axial line CX of the spark plug  200  and the center axial line AX of the injection hole  261  (cross section of the spark plug  200  illustrated in  FIG. 3 ). In other words, as illustrated in  FIG. 3 , when the inner peripheral surface of the injection hole  261  is cut by the plane including the center axial line CX of the spark plug  200  and the center axial line AX of the injection hole  261  (cross section of the spark plug  200  illustrated in  FIG. 3 ), the front-end-side cross-sectional edge of the inner peripheral surface of the injection hole  261  has a length L 3 , which is larger than a length L 4  of the rear-end-side cross-sectional edge. Thus, in the cover portion  250 , a portion  250 A on the front end side with respect to the injection holes  261  is thicker than a portion  250 B on the rear end side with respect to the injection holes  261 . In the spark plug  200  with this structure, heat is more likely to be guided to be collected to the front end side of the cover portion  250  in an environment where misfires easily occur, and thus temperature is not easily lowered, so that misfires can be prevented. 
     In the spark plug  200  according to the second embodiment, as in the case of the spark plug  100  according to the first embodiment, the total area. A (mm 2 ) of the inner peripheral surfaces of the four injection holes  261  and the thermal conductivity B (W/mK) of the material of the cover portion  250  satisfy the above formula (1) (10&lt;A×B&lt;4000). Thus, the spark plug  200  achieves the same effects as the spark plug  100  according to the first embodiment. 
     As in the case of the spark plug  100  according to the first embodiment, in the spark plug  200  according to the second embodiment, preferably, the total area A. (mm 2 ) of the inner peripheral surfaces of the four injection holes  261  and the thermal conductivity B (W/mK) of the material of the cover portion  250  satisfy the formula (2) (20&lt;A×B&lt;2400). Thus, the spark plug  200  achieves the same effects as the spark plug  100  according to the first embodiment. 
     EXAMPLES 
     The present invention will be more specifically described below using examples. 
     1. Experiment (Experiment Corresponding to First Embodiment) 
     (1) Method of Experiment 
     (1.1) Examples 
     Samples of the spark plug  100  illustrated in  FIGS. 1 and 2  were used herein. Table 1, below, shows the detailed conditions. The spark plug  100  satisfies the requirements of the first embodiment. In Table 1, each experiment example is denoted with “No.”. Nos. 2 to 28 in Table 1 are examples. 
     (1.2) Comparative Examples 
     Samples of a spark plug having a structure different from that of the spark plug  100  illustrated in  FIGS. 1 and 2  (different in total area A (mm 2 ) of the inner peripheral surfaces of the injection holes or thermal conductivity B (W/mK) of the material of the cover portion) were used herein. Table 1, below, shows the detailed conditions. This spark plug does not satisfy the requirements of the first embodiment. Numbers marked with an asterisk “*”, like “1*” in Table 1, denote that they are comparative examples. Specifically, Nos. 1, 29, and 30 in Table 1 are comparative examples. 
     (2) Method for Evaluation 
     (2.1) Measurement of Total Area a (mm 2 ) of Inner Peripheral Surfaces of Injection Holes 
     Using an X ray computed tomography (CT) scanner, the cover portion of each sample was scanned under the conditions of a tube voltage of 120 kV and a tube current of 140 μA. A three-dimensional image was manufactured from the scanning result for each cover portion, and the total area A (mm 2 ) of the inner peripheral surfaces of the four injection holes was measured. 
     (2.2) Pre-Ignition Resistance Evaluation Test 
     Each sample underwent a pre-ignition resistance evaluation test. The summary of the pre-ignition resistance evaluation test is as follows. Each sample was mounted on an in-line four-cylinder naturally aspirated engine with a displacement of 1.3 L, and the engine was operated 1000 cycles of a series of processes on full throttle (6000 rpm) at an ignition angle (crank angle) of a predetermined initial value. During the engine operation, whether pre-ignition occurs was checked. When pre-ignition occurred, the ignition angle at that time was specified as a pre-ignition occurrence angle. When no pre-ignition occurred, the ignition angle was advanced by one degree, and the engine was activated again on full throttle to check whether pre-ignition occurs. This operation was performed repeatedly until pre-ignition occurs to specify the pre-ignition occurrence angle of each sample. Similarly, the pre-ignition occurrence angle of a reference spark plug (a genuine spark plug installed on a test engine) was specified. Then, the difference between the pre-ignition occurrence angle of the reference spark plug and the pre-ignition occurrence angle of each sample was calculated. When the pre-ignition occurrence angle is on more advanced side with respect to the reference spark plug, the spark plug is evaluated as having higher pre-ignition resistance. The pre-ignition occurrence angle of each sample with respect to that of the reference spark plug was evaluated based on the following standards, and each experiment example was given an evaluation score. The results are shown in the column “pre-ignition resistance” in Table 1. 
     &lt;Evaluation of Pre-Ignition Resistance&gt; 
     Each sample was evaluated with the following three grades. Higher evaluation scores represent higher pre-ignition resistance. 
     Evaluation Score:
         3: Advanced by 5° C.A or more with respect to the reference spark plug   1: Advanced by 2° C.A or more and less than 5° C.A with respect to the reference spark plug   0: Lagged or advanced by less than 2° C.A with respect to the reference spark plug       

     (2.3) Misfire Resistance Test 
     Each sample underwent a misfire resistance evaluation test. The summary of the misfire resistance evaluation test is as follows. Each sample was mounted on an in-line four-cylinder direct-injection turbocharger engine with a displacement of 1.6 L, and the engine was operated 1000 cycles under the conditions of 2000 rpm and an intake pressure of 1000 kPa to measure the misfire rate. Spark plugs having a smaller misfire rate are evaluated as having higher misfire resistance (ignitability). The misfire rate of each sample was evaluated based on the following standards, and each experiment example was given an evaluation score. The results are shown in the column “misfire resistance” in Table 1. 
     &lt;Evaluation of Misfire Resistance&gt; 
     Each sample was evaluated with the following three grades. Higher evaluation scores represent higher misfire resistance. 
     Evaluation Score:
         3: Misfire rate of lower than 1%   1: Misfire rate of 1% or higher and lower than 3%   0: Misfire rate of 3% or higher       

     (2.4) Overall Evaluation 
     Based on the total score of the evaluation score for the pre-ignition resistance and the evaluation score for the misfire resistance, overall evaluation was made for each sample. Higher total scores are evaluated as being more preferable in both pre-ignition resistance and misfire resistance. The overall evaluation of a sample with the total score of 6 is denoted with “Excellent”, the overall evaluation of a sample with the total score of 4 is denoted with “Good”, and the overall evaluation of a sample with the total score of 3 is denoted with “Poor”. The results are shown in the column “overall evaluation” in Table 1, 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 A: Total area of inner 
                 B: Thermal 
                   
                 Pre- 
                   
                   
               
               
                   
                 peripheral surfaces of 
                 conductivity of cover 
                   
                 ignition 
                 Misfire 
                 Overall 
               
               
                 No. 
                 injection holes (mm 2 ) 
                 portion (W/mK) 
                 A × B 
                 resistance 
                 resistance 
                 evaluation 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                  1* 
                 0.7 
                 13 
                 9.1 
                 0 
                 3 
                 3 
                 Poor 
               
               
                 2 
                 1.5 
                 13 
                 19.5 
                 1 
                 3 
                 4 
                 Good 
               
               
                 3 
                 2.2 
                 13 
                 28.6 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 4 
                 4.4 
                 13 
                 57.2 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 5 
                 11.2 
                 13 
                 145.6 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 6 
                 18.5 
                 13 
                 240.5 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 7 
                 0.7 
                 26 
                 18.2 
                 1 
                 3 
                 4 
                 Good 
               
               
                 8 
                 1.5 
                 26 
                 39.0 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 9 
                 2.2 
                 26 
                 57.2 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 10 
                 4.4 
                 26 
                 114.4 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 11 
                 11.2 
                 26 
                 291.2 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 12 
                 18.5 
                 26 
                 481.0 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 13 
                 0.7 
                 53 
                 37.1 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 14 
                 1.5 
                 53 
                 79.5 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 15 
                 2.2 
                 53 
                 116.6 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 16 
                 4.4 
                 53 
                 233.2 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 17 
                 11.2 
                 53 
                 593.6 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 18 
                 18.5 
                 53 
                 980.5 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 19 
                 0.7 
                 130 
                 91.0 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 20 
                 1.5 
                 130 
                 195.0 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 21 
                 2.2 
                 130 
                 286.0 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 22 
                 4.4 
                 130 
                 572.0 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 23 
                 11.2 
                 130 
                 1456.0 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 24 
                 18.5 
                 130 
                 2405.0 
                 3 
                 1 
                 4 
                 Good 
               
               
                 25 
                 0.7 
                 372 
                 260.4 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 26 
                 1.5 
                 372 
                 558.0 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 27 
                 2.2 
                 372 
                 818.4 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                 28 
                 4.4 
                 372 
                 1636.8 
                 3 
                 3 
                 6 
                 Excellent 
               
               
                  29* 
                 11.2 
                 372 
                 4166.4 
                 3 
                 0 
                 3 
                 Poor 
               
               
                  30* 
                 18.5 
                 372 
                 6882.0 
                 3 
                 0 
                 3 
                 Poor 
               
               
                   
               
               
                 (3) Evaluation Results 
               
            
           
         
       
     
     The experiment example 1 (comparative example) was rated 3 in overall score, with a product A×B of 9.1, where A is the total area (mm 2 ) of the inner peripheral surfaces of the injection holes and B is the thermal conductivity (W/mK) of the material of the cover portion. The experiment example 29 (comparative example) was rated 3 in overall score, with a product A×B of 4166.4. The experiment example 30 (comparative example) was rated 3 in overall score, with a product A×B of 6882.0. On the other hand, the experiment examples 2 to 28 (examples) were rated 4 or 6 in overall scores with a product satisfying 10&lt;A×B&lt;4000. Thus, the examples satisfying the formula (1) (10&lt;A×B&lt;4000) had suppressed both pre-ignition and misfires as compared with the comparative examples. 
     The experiment example 1 (comparative example) had a product A×B of 9.1, and rated 0 in pre-ignition resistance evaluation score. The experiment example 2 (example) had a product A×B of 19.5, and rated 1 in pre-ignition resistance evaluation score. The experiment example 7 (example) had a product A×B of 18.2, and rated 1 in pre-ignition resistance evaluation score. On the other hand, the experiment examples 3 to 6, 8 to 23, and 25 to 28 (examples) had a product satisfying 20&lt;A×B&lt;2400, and rated 3 in pre-ignition resistance evaluation score. Thus, the experiment examples 3 to 6, 8 to 23, and 25 to 28 satisfying the formula (2) (20&lt;A×B&lt;2400) had further suppressed pre-ignition. 
     Other Embodiments (Modifications) 
     The present invention is not limited to the above embodiments, and may be embodied in various different forms within the scope not departing from the gist of the invention. 
     (1) In the above embodiments, the cover portion has a specific shape, but the shape is changeable as appropriate. The cover portion may have, for example, a circular cylindrical shape, a quadrangular box shape, or a conical shape. 
     (2) In the above embodiments, a spark plug having a specific number of injection holes is described as an example, but the number of injection holes is not limited to a specific one and changeable as appropriate. The arrangement of the injection holes and the penetrating direction of the injection hole are also changeable as appropriate.