Patent Publication Number: US-2023163575-A1

Title: Spark plug

Description:
CROSS REFERENCE TO RELATED DOCUMENT 
     The present application claims the benefit of priority of Japanese Patent Application No. 2021-190680 filed on Nov. 25, 2021, the disclosure of which is incorporated in its entirety herein by reference. 
     BACKGROUND 
     1 Technical Field 
     This disclosure relates generally to a spark plug. 
     2 Background Art 
     Internal combustion engines mounted in automotive vehicles are usually equipped with a spark plug to ignite fuel. The spark plug includes a center electrode and a ground electrode and works to produce a sequence of electrical sparks between the center and ground electrodes to ignite the fuel. Japanese Patent No. 4761401 discloses a spark plug designed to have a portion where electrical sparks are produced and which is made from a metallic material mainly containing iridium or platinum. 
     Usually, when the internal combustion engine is operating, thermal energy arising from the combustion of an air-fuel mixture is developed cyclically, thus causing a portion(s) of the spark plug where electrical sparks occur to be subjected to severe heat/cooling cycles. This may lead to a risk that each of the center and ground electrodes may experience separation of a portion of a metallic surface therefrom and that the separated portion may be melted and then adhered to the surface of the center or ground electrodes again. Such metal re-attachment is also referred to as redeposition. 
     The occurrence of the redeposition at portion (a) of the spark plug (which will also be referred to as a discharge portion) where the sparks occur may result in a reduction in size of the spark gap, which leads to a short-circuit between the center and the ground electrodes. Particularly, spark plugs which are designed for small-sized gas engines in which the size of the spark gap is approximately 0.2 mm usually encounter an increased risk of a short-circuit between the center and ground electrodes due to redeposition. 
     In order to eliminate the above drawback, the spark plug disclosed in the above patent publication is designed to have a noble metal chip which is used with an electrode and made from material whose content of oxygen is in a given range to alleviate a risk of partial separation of the surface from the noble metal chip or the redeposition on the noble metal chip. 
     In recent years, internal combustion engines have been highly required to enhance the efficiency in operation or output power therefrom. Such a requirement is, however, difficult to meet in the spark plug taught in the above patent publication. 
     SUMMARY 
     It is an object of this disclosure to provide a spark plug which is capable of minimizing separation of part from a discharge portion of the spark plug or redeposition of the part on the discharge portion. 
     According to one aspect of this disclosure, there is provided a spark plug which comprises: (a) a first discharge chip; (b) a second discharge portion which faces the first discharge portion; and (c) a spark gap which is defined between the first discharge portion and has a size of 0.23 mm or more and 0.6 mm or less. At least one of the first discharge portion and the second discharge portion is made from material mainly containing iridium or platinum and an additive of tantalum of 0.5 Wt % to 5.0 WT %. 
     The inventor has knowledge that a risk of the separation of part from a discharge portion(s) of a spark plug or the redeposition of the part on the discharge portion is minimized by making the discharge portion from material mainly containing iridium or platinum and an additive of tantalum of 0.5 Wt % to 5.0 Wt % with the aid of solution hardening (also called solid solution strengthening) in production of the discharge portion. The spark plug is, therefore, designed to have the first discharge portion and the second discharge portion at least one of which is made from the above-described material to eliminate the risk of the separation of part from the first and/or second discharge portions or the redeposition of the part on the first and/or second discharge portions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
       In the drawings: 
         FIG.  1    is a partially longitudinal sectional view which illustrates a structure of a spark plug according to an embodiment in this disclosure. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The structure of the spark plug  10  according to the embodiment will first be described with reference to  FIG.  1   .  FIG.  1    illustrates a longitudinal sectional view of the spark plug  10 , as cut along the center axis CX, on the left side of the drawing, but shows the external appearances of the center electrode  30  and the metal terminal  40  as they are on the left side of the drawing. 
     The spark plug  10  is mounted in each cylinder of an internal combustion engine, not shown, in use and works to ignite fuel in a combustion chamber in the cylinder. The internal combustion engine may be of any type, however, the spark plug  10  in this embodiment is designed for use in a gas engine or a gasoline engine. The spark plug  10  includes the porcelain insulator  20 , the center electrode  30 , the metal terminal  40 , the metal shell  50 , and the ground electrode  60 . 
     The porcelain insulator  20  is of a hollow cylindrical shape and made from insulating material, such as alumina. The porcelain insulator  20  has formed therein the axial hole  200  which extends through the center axis of the porcelain insulator  20 . The axial hole  200  has the center axis coinciding with that of the porcelain insulator  20 . The center axis (i.e., a longitudinal center line) of the axial hole  200  will also be referred to as the center axis CX. The axial hole  200  is shaped to have a circular transverse section, as viewed in a cross section of the porcelain insulator  20  taken perpendicular to the center axis CX. 
     The axial hole  200  has a given length with a first end portion (i.e., a lower end portion, as viewed in the drawing) and a second end portion (i.e., an upper end portion, as viewed in the drawing). The first end portion will also be referred to as a head end portion or a head end side, while the second end portion will also be referred to as a rear end portion or a rear end side. The center electrode  30  is made of a metallic material and retained by the porcelain insulator  20  in the first end portion (i.e., the head end portion) of the axial hole  200 . The center electrode  30  is of a bar-shape and mostly disposed inside the axial hole  200 . The center electrode  30  has a head end portion which protrudes from the axial hole  200  outside the porcelain insulator  20  and has the discharge chip  31  secured to the tip thereof. The porcelain insulator  20  also serves as a member which retains an outer periphery of the center electrode  30 . 
     The metal terminal  40  is made of metallic material and arranged in the second end portion (i.e., the rear end portion) of the axial hole  200 . The metal terminal  40  extends along the center axis CX and is retained by the porcelain insulator  20 . The metal terminal  40  is of a bar-shape and mostly disposed inside the axial hole  200 . The metal terminal  40  has a portion which protrudes from the axial hole  200  outside the porcelain insulator  20  and functions as an electrode terminal to which voltage is applied from an external power supply, not shown. 
     The porcelain insulator  20  has a portion in which the center electrode  30  extends along the center axis CX and which will also be referred to below as a head end portion or a head end side. The porcelain insulator  20  has also a portion in which the metal terminal  40  extends along the center axis CX and which will also be referred to as a rear end portion or a rear end side. 
     The axial hole  200  has the resistor  71  disposed therein between the metal terminal  40  and the center electrode  30 . The resistor  71  is an electrical component working to control or regulate an electrical resistance of an electrical circuit extending from the metal terminal  40  to the center electrode  30 . The resistor  71  is made from material of glass and zirconia powder with a selected amount of additive of carbon powder. The degree of electrical resistance implemented by the resistor  71  is determined by the added amount of carbon powder. The resistor  71  arranged in the electrical circuit between the metal terminal  40  and the center electrode  30  serves to minimize electromagnetic noises arising from spark discharge from the spark plug  10 . The resistor  71  and the center electrode  30  are electrically connected together through the conductive sealing layer  72 . Similarly, the metal terminal  40  and the resistor  71  are electrically connected together through the conductive sealing layer  73 . Each of the conductive sealing layers  72  and  73  is made from glass powder with an additive of copper powder in the form of a conductive layer. 
     The metal shell  50  is of a hollow cylindrical shape and surrounds a portion of the porcelain insulator  20  from outside. The whole of the metal shell  50  is made from metallic material. The metal shell  50  is mechanically crimped to make a firm joint to the porcelain insulator  20 , so that the metal shell  50  firmly retains the porcelain insulator  20 . The metal shell  50  includes the gripping portion  52 , the flange  55 , and the insertion portion  56 . 
     The gripping portion  52  is used to provide a mechanical grip to a tool, such as a plug wrench, to attach the spark plug  10  to the internal combustion engine. The gripping portion  52  is of, for example, a hexagonal shape, as viewed along the center axis CX. 
     The flange  55  is arranged in contact with the outer surface of the internal combustion engine through the gasket GK when the spark plug  10  is mounted in the internal combustion engine. The flange  55  is located closer to the head of the spark plug  10  (i.e., the metal shell  50 ) than the gripping portion  52  is and protrudes radially outward. 
     The insertion portion  56  is a portion of the metal shell  50  which is located closer to the head of the spark plug  10  than the flange  55  is and inserted into a hole, not shown, formed in the internal combustion engine. The insertion portion  56  has the male thread  561  formed on an outer periphery thereof and is rotated around the center axis CX by mechanical pressure exerted by a tool on the gripping portion  52  when the spark plug  10  is mounted in the internal combustion engine. This achieves mechanical engagement between a female thread formed in the hole of the internal combustion engine and the male thread  561  of the insertion portion  56 , thereby tightly fastening the spark plug  10  to the internal combustion engine. After the spark plug  10  is installed in the internal combustion engine, the electrical potential at the metal shell  50  will be at the same ground potential as the body of the internal combustion engine. 
     The ground electrode  60  is made of a metallic member extending from an end surface S of the head of the metal shell  50 . The end surface S extends perpendicular to the center axis CX. The ground electrode  60  is bent to have a portion (which will also be referred to as a head portion) which faces the discharge chip  31  of the center electrode  30  in the lengthwise direction of the center axis CX. The ground electrode  60  has a length with a first end and a second end opposed to the first end. The ground electrode  60  is joined at the first end to the end surface S of the metal shell  50  and has the second end (i.e., the head portion) facing the center electrode  30 . The head portion of the ground electrode  60  which faces the discharge chip  31  has the discharge chip  61  secured thereto. The discharge chip  61  and the discharge chip  31  are, therefore, aligned with each other along the center axis CX. The discharge chip  61  and the discharge chip  31  face each other through the spark gap GP in which the electrical spark is created. The spark gap GP is schematically illustrated in  FIG.  1   , however, an actual size of the spark gap GP is selected to lie in a small range of 0.23 mm or more and 0.6 mm or less. The size of the spark gap GP, as referred to herein, represents a minimum distance between the discharge chip  31  and the discharge chip  61 . 
     In operation of the internal combustion engine, a high voltage is applied in the form of a pulse between the metal terminal  40  of the spark plug  10  and the body of the internal combustion engine. The high-voltage is then applied between the discharge chip  61  and the discharge chip  31  which face each other, thereby creating a spark discharge in the spark gap GP. 
     The discharge chip  31  is welded to a joint counterpart, i.e., the center electrode  30 . Similarly, the discharge chip  61  is welded to a joint counterpart, i.e., the ground electrode  60 . The discharge chip  31  will also be referred to as a first discharge portion. The discharge chip  61  will also be referred to as a second discharge portion. Each of the discharge chip  31  and the discharge chip  61  is, as will be described later in detail, designed in the form of a noble metal chip made from material mainly containing iridium (Ir) or platinum (Pt). 
     Usually, when the internal combustion engine is operating, thermal energy arising from the combustion of an air-fuel mixture is developed cyclically, thus causing a portion of the spark plug  10  where electrical sparks occur to be subjected to severe heat/cooling cycles. This may lead to a risk that each of the discharge chip  31  and the discharge chip  61  may experience separation of a portion of a metallic surface therefrom and that the separated portion may be melted and then adhered to the surface of the discharge chip  31  or  61  again. Such metal re-attachment is also referred to as redeposition. 
     The occurrence of the redeposition at the discharge chip  31  or the discharge chip  61  may result in a reduction in size of the spark gap GP, which leads to a short-circuit between the center electrode  31  and the ground electrode  60 . The spark plug  10  which is designed for small-sized gas engines in which the size of a spark gap is approximately 0.2 mm usually encounters an increased risk of a short-circuit between the center electrode  31  and the ground electrode  60  due to the redeposition. 
     In order to alleviate the above problem, the spark plug  10  is designed to have the discharge chip  31  whose material is selected to minimize the separation of the surface from the discharge chip  31  and/or the redeposition on the discharge chip  31 . The material of the discharge chip  31  is a metallic material which contains iridium (Ir) as a main composition and tantalum (Ta) whose amount is in a range of 0.5 percent by weight (Wt %) to 5 Wt %. The main component or the phrase “mainly contain”, as referred to herein, means that the ratio of mass of a substance of material to a total mass of the material is 50% or more. 
     In order to evaluate advantages offered by the discharge chip  31  made of the above material, the inventor of this application conducted the following tests. First, eight types of alloys were prepared which mainly contain iridium. Specifically, the alloys were made by adding a given quantity of tantalum (Ta) to a metallic material containing iridium of 90 Wt % and rhodium (Rh) of 10 Wt % and then melting it. The alloys were different in content of tantalum from each other. Subsequently, each of the alloys was forged, extended using pressure into a cylindrical shape of 1 mm in diameter, and then cut into 1 mm in length. In this way, eight types of specimens of the discharge chip  31  for different contents of tantalum. The discharge chip  61  was also prepared which was made from an alloy containing iridium of 90 Wt %, rhodium (Rh) of 10 Wt %, and tantalum of 0 Wt %, in other words, containing only iridium and rhodium. 
     Afterwards, specimens of the spark plug  10  were made which are equipped with the above different types of specimens of the discharge chip  31  and the above type of discharge chip  61 . Each specimen of the spark plug  10  was installed in the internal combustion engine. After the internal combustion engine was driven for a given period of time, we checked each specimen of the spark plug  10  for the degree of redeposition on each of the discharge chips  31  and  61 . The degree of redeposition was evaluated using the largest diameter (which will also be referred to as a redeposited particle diameter) among particles deposited on each of the discharge chips  31  and  61 . 
     Each specimen of the spark plug  10  had the spark gap GP whose initial size is 0.23 mm. In the tests, the internal combustion engine was driven at 750 rpm and 8,500 kW for 2,000 hours. 
     Results of the tests are shown in table 1 below. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Added Ta amount Wt % 
                 0 
                 0.2 
                 0.5 
                 1 
                 2 
                 3 
                 4 
                 5 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 center electrode redeposited 
                 0.09 
                 0.07 
                 0.03 
                 0.03 
                 0.03 
                 0.02 
                 0.02 
                 0.02 
               
               
                 particle diameter mm (Ta) 
               
               
                 ground electrode redeposited 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 particle diameter mm (No Ta) 
                   
               
               
                 TOTAL mm 
                 0.19 
                 0.17 
                 0.13 
                 0.13 
                 0.13 
                 0.12 
                 0.12 
                 0.12 
               
               
                   
               
            
           
         
       
     
     In table 1, the first row “added Ta amount” lists Wt % of tantalum contained in the eight types of specimens of the discharge chip  31 . Specifically, the eight types of specimens of the discharge chip  31  contain 0 Wt %, 0.2 Wt %, 0.5 Wt %, 1.0 Wt %, 2.0 Wt %, 3.0 Wt %, 4 Wt %, and 5.0 Wt % of tantalum, respectively. The reason why the content of tantalum in the discharge chip  31  is a maximum of 5.0 Wt % is because 5.0 Wt % or more of tantalum usually results in an increase in hardness of the material of the discharge chip  31 , which leads to a difficulty in machining the discharge chip  31 . 
     In table 1, the second row “center electrode redeposited particle diameter” lists diameters of particles adhered to the specimens of the discharge chip  31 . The third row “ground electrode redeposited particle diameter” lists diameters of particles adhered to the specimens of the discharge chip  61  made from material excluding tantalum. The fourth row “total” lists totals of the diameters of particles adhered to the specimens of the discharge chips  31  and  61 . 
     The sum of the center electrode redeposited particle diameter and the ground electrode redeposited particle diameter represents a decrease in size of the spark gap GP from an initial size thereof. It is advisable that typical spark plugs for gas engines have the spark gap whose size is 0.1 mm or more in order to ensure the stability in igniting the air-fuel mixture. Accordingly, in the above tests where the initial size of the spark gap GP is selected to be 0.23 mm, the sum of the center electrode redeposited particle diameter and the ground electrode redeposited particle diameter needs to be 0.13 mm or less. 
     Table 1 shows that the larger the content of tantalum, the small the center electrode redeposited particle diameter and the smaller the sum of the center electrode and ground electrode redeposited particle diameters. Table 1 also shows that when the content of tantalum is 0.5 Wt % or more, the sum of the center electrode and ground electrode redeposited particle diameters is 0.13 mm or less, in other words, the size of the spark gap GP is kept higher than or equal to 0.1 mm, meaning that the stability in igniting operation of the spark plug  10  is ensured even after the spark plug  10  is operated for 2,000 hours that is usually a target time when typical spark plugs are required to be replaced. We have, therefore, found from the above tests that the content of tantalum in the discharge chip  31  is preferably selected to be 0.5 Wt % or more and 5.0 Wt % or less because the fact that the addition of tantalum to the material of the discharge chip  31  minimizes the separation of the surface from the discharge chip  31  and/or the redeposition on the discharge chip  31  is thought of as resulting from reduction in growth of metallic particles in the discharge chip  31  rather than the solution hardening (also called solid solution strengthening) in production of the discharge chip  31 . 
     The spark plug  10  in this embodiment may be designed to have the discharge chip  31  and the discharge chip  61  at least one which is made from material mainly containing iridium and an additive of tantalum of 0.5 Wt % or more to 5.0 Wt % or less. The discharge chip  61  made from the above material offers the same beneficial advantages as those of the discharge chip  31 . 
     The inventor of this application also conducted tests which will be described below. 
     First, specimens of the spark plug  10  were prepared which are equipped with the discharge chip  61  made from the same material as that of the discharge chip  31 . The tests were conducted on such specimens and evaluated in the same way as described above. Specifically, we prepared specimens of each of the discharge chip  31  and the discharge chip  61  which are made from eight types of materials different in content of tantalum from each other. 
     Results of the tests are shown in table 2 below. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Added Ta amount Wt % 
                 0 
                 0.2 
                 0.5 
                 1 
                 2 
                 3 
                 4 
                 5 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 center electrode redeposited 
                 0.09 
                 0.07 
                 0.03 
                 0.03 
                 0.03 
                 0.02 
                 0.02 
                 0.02 
               
               
                 particle diameter mm (Ta) 
               
               
                 ground electrode redeposited 
                 0.1 
                 0.09 
                 0.05 
                 0.05 
                 0.04 
                 0.03 
                 0.03 
                 0.03 
               
               
                 particle diameter mm (No Ta) 
                   
               
               
                 TOTAL mm 
                 0.19 
                 0.16 
                 0.08 
                 0.08 
                 0.07 
                 0.05 
                 0.05 
                 0.05 
               
               
                   
               
            
           
         
       
     
     In table 2, the first row “added Ta amount” lists Wt % of tantalum contained in the eight types of specimens of each of the discharge chip  31  and the discharge chip  61 . The second row “center electrode redeposited particle diameter” lists diameters of particles adhered to the specimens of the discharge chip  31 . The third row “ground electrode redeposited particle diameter” lists diameters of particles adhered to the specimens of the discharge chip  61 . The fourth row “total” lists the sums of the diameters of particles adhered to the specimens of the discharge chips  31  and  61 . 
     Table 2 shows that the larger the content of tantalum, the smaller the center electrode redeposited particle diameter and the smaller the sum of the center electrode and ground electrode redeposited particle diameters. Table 2 also shows that when the content of tantalum is 0.5 Wt % or more, the sum of the center electrode and ground electrode redeposited particle diameters is 0.13 mm or less, in other words, the size of the spark gap GP is kept higher than or equal to 0.1 mm. We have found from the above tests that the content of tantalum in the noble metal material of each of the discharge chip  31  and the discharge chip  61  is preferably selected to be 0.5 Wt % or more and 5.0 Wt % or less. 
     As apparent from the above discussion, the spark plug  10  may be designed to have the discharge chip  31  (i.e., the first discharge portion) and the discharge chip  61  (i.e., the second discharge portion) either or both of which are made from material containing iridium as a main composition and an additive of tantalum of 0.5 Wt % or more to 5.0 Wt % or less. 
     Although results of tests are omitted, the inventor of this application has found that substantially the same beneficial advantages as those described above are obtained in a case where at least one of the discharge chips  31  and  61  is made from a noble metal material containing platinum as a main composition instead of iridium and an additive of tantalum of 0.5 Wt % or more to 5.0 Wt % or less. 
     The spark plug  10  is, as described above, equipped with the discharge chips  31  and  61  at least one of which is made from material containing iridium or platinum as a main composition and an additive of tantalum of 0.5 Wt % to 5.0 Wt %, but however, may alternatively be designed to have only portions which face each other to define the spark gap GP and are made of the above-described material. For instance, at least one of the center electrode  30  and the ground electrode  60  is made from the above-described material without use of noble metal chips (i.e., the discharge chips  31  and  61 ). In this case, either of the center electrode  30  or the ground electrode  60  serves as the first discharge portion, while the other may serve as the second discharge portion. 
     The component parts described in the above embodiment are not necessarily essential unless otherwise specified or viewed to be essential in principle. When the number of the component parts, a numerical number, a volume, or a range is referred to in the above discussion, this disclosure is not limited to it unless otherwise specified or viewed to be essential in principle. Similarly, when the shape of, the orientation of, or the positional relation among the component parts is referred to in the above discussion, this disclosure is not limited to it unless otherwise specified or viewed to be essential in principle.