Spark plug and the method of manufacturing the same

Abstract

A spark plug for internal combustion engines provided with a noble metal tip on the pointed top of a spark plug center electrode. The noble metal tip is welded to the center electrode top end by heating and pressing the portion to be welded. After the welding, the center electrode is shaped to provide the pointed top. The noble metal tip is a platinum alloy consisting of platinum of 85 through 70 in weight percentage and iridium of 15 through 30 in weight percentage.

What is claimed is: 
     
       1. A spark plug for internal combustion engines comprising: a ground electrode made of an electrically conductive base metal;   a center electrode made of an electrically conductive base metal and provided with a noble metal tip only on a flat end of a tapered end portion thereof, said noble metal tip facing said ground electrode through an air gap said noble metal tip being completely resistance welded under mechanical pressure to said electrically conductive base metal to provide a strong bond therebetween; and   an insulator securely encasing said center electrode and electrically insulating said center electrode from said ground electrode, wherein said noble metal tip is made of a platinum alloy consisting essentially of 70-80 percent platinum, by weight, and 30-20 percent iridium, by weight, and wherein said center electrode is manufactured by the steps of: preparing the base metal of said central electrode having a uniform diameter;   welding said noble metal tip to the top end surface of said center electrode; and   shaping said center electrode to provide said flat end of said tapered end portion to be smaller in diameter than said center electrode axially beyond said tapered end portion.     
     
     
       2. A spark plug according to claim 1, wherein said welding step is performed by heating and pressing the portion to be welded. 
     
     
       3. A spark plug according to claim 1, wherein said center electrode is provided by said shaping step with a tapered end portion which gradually reduces the diameter thereof toward said flat end. 
     
     
       4. A spark plug according to claim 3, wherein said ground electrode is provided with a further noble tip metal facing said noble metal tip of said center electrode. 
     
     
       5. A spark plug according to claim 4, wherein said noble metal tips are less than 0.5 mm in thickness and consist of platinum of 80 through 70 in weight percentage and iridium of 20 through 30 in weight percentage. 
     
     
       6. A spark plug for internal combustion engines comprising: a ground electrode made of an electrically conductive base metal;   a center electrode made of an electrically conductive base metal and provided with a flat end of a tapered end portion which is smaller in diameter than said center electrode axially beyond said tapered end portion and which flat end faces said ground electrode;   an insulator securely encasing said center electrode and electrically insulating the same from said ground electrode; and   a platinum alloy tip welded only to said flat end of said center electrode tapered end portion and consisting essentially of 70-80 percent platinum, by weight, and 30-20 percent iridium.   
     
     
       7. A spark plug according to claim 6, wherein said center electrode is provided with a tapered end portion which gradually reduces the diameter thereof toward said flat end, and wherein said platinum alloy tip is less than 0.5 mm in thickness. 
     
     
       8. A spark plug according to claim 6 or 7, further comprising: a platinum alloy tip welded to said ground electrode to face said platinum alloy tip on said center electrode flat end, said platinum alloy tip on said ground electrode including iridium of at least 15 percent by weight but less that 30 percent by weight and less than 0.5 mm in thickness.   
     
     
       9. A spark plug according to claim 8, wherein said platinum alloy tip on said ground electrode includes iridium of 20 through 30 in weight percentage. 
     
     
       10. A spark plug according to claim 8, wherein said platinum alloy tip on said ground electrode consists of platinum of 85 through 70 in weight percentage and iridium of 15 through 30 in weight percentage. 
     
     
       11. A spark plug according to claim 8, wherein said platinum alloy tip on said ground electrode consists of platinum of 80 through 70 in weight percentage and iridium of 20 through 30 in weight percentage. .Iadd. 
     
     
       12.  A spark plug according to claim 1, wherein a diameter of the top end surface of the center electrode is larger than the diameter of the noble metal tip so that after the welding step, the noble metal tip is buried inside the flat top end surface of the center electrode. .Iaddend..Iadd.13. A spark plug according to claim 1, wherein the noble metal tip has a first surface and a second surface, the first surface of the noble metal tip is welded to the top end surface of the center electrode and the second surface of the noble metal tip faces the ground electrode so that a spark gap is formed between the second surface of the noble metal tip and the ground electrode. .Iaddend..Iadd.14. A spark plug according to claim 6, wherein a diameter of the top end surface of the center electrode is larger than the diameter of the platinum alloy tip so that after the welding step, he platinum alloy tip is buried inside the flat top end surface of the center electrode. .Iaddend..Iadd.15. A spark plug according to claim 6, wherein the platinum alloy tip has a first surface and a second surface, the first surface of the platinum alloy tip is welded to the end of the center electrode and the second surface of the platinum alloy tip faces the ground electrode so that a spark gap is formed between the second surface of the platinum alloy tip and the ground electrode. .Iaddend.