Spark plug and method of manufacturing spark plug

1. A spark plug comprising:

a ground electrode having a recess with a bottom surface formed therein;

a noble metal tip having an outer circumferential surface; and

a holder having an inner circumferential surface defining a through hole, said through hole dimensioned to receive and surround the noble metal tip, the recess in said ground electrode being dimensioned to receive the noble metal tip and surrounding holder therein, and the holder being joined to a body of the ground electrode by laser welding with said noble metal tip held between a rearward end surface of the holder and the bottom surface of the recess; and

a center electrode for forming a gap in cooperation with the noble metal tip, wherein Gf<Tr, Gf<Gr, and Tf<Tr

where a forward side is a side toward the gap as viewed from the noble metal tip,

Gf is an inside diameter of the holder at a forward end surface of the holder,

Gr is an inside diameter of the holder at the rearward end surface of the holder,

Tf is an outside diameter of the noble metal tip at a forward end surface of the noble metal tip, and

Tr is an outside diameter of the noble metal tip at a rearward end surface of the noble metal tip;

at least one of a) the inner circumferential surface of the holder which forms the through hole, and b) the outer circumferential surface of the noble metal tip which is disposed in the through hole, continuously reduces in diameter toward the forward side; and

the forward end surface of the noble metal tip is located on the forward side with respect to the forward end surface of the holder.

2. A spark plug according to claim 1 , wherein

the inner circumferential surface of the holder has a first taper surface which continuously reduces in diameter toward the forward side, and

the outer circumferential surface of the noble metal tip has a second taper surface which continuously reduces in diameter toward the forward side.

3. A spark plug according to claim 2 , wherein

in a section of the noble metal tip which contains a center axis of the noble metal tip, a difference dAg obtained by subtracting a first angle Ag1 from a second angle Ag2 is from −10 degrees to +10 degrees, where the first angle Ag1 is an acute angle between the first taper surface and the center axis, and the second angle Ag2 is an acute angle between the second taper surface and the center axis.

4. A spark plug according to any one of claims 1 to 3 , wherein

the ground electrode further has a first fusion zone which joins at least the noble metal tip and the holder.

5. A spark plug according to claim 4 , wherein

the ground electrode has a plurality of the first fusion zones, and

the first fusion zones are disposed at such positions as not to be directly opposite one another with respect to the center axis of the noble metal tip.

6. A spark plug according to claim 5 , further comprising

an insulator which holds the center electrode, and

a metallic shell disposed radially around the insulator,

wherein the body has a proximal end connected to the metallic shell, and

at least one first fusion zone is located toward the proximal end with respect to the center axis of the noble metal tip.

7. A spark plug according to claim 6 , wherein

in a view from a direction parallel to the center axis of the noble metal tip, at least one first fusion zone is superposed on a longitudinal axis of the body while being located toward the proximal end with respect to the center axis.

8. A spark plug according to claim 5 , wherein

the first fusion zone has an exposed surface which is exposed at a surface of the body.

9. A spark plug according to claim 1 , wherein

the ground electrode has a second fusion zone which joins the holder and the body, and

the second fusion zone is away from the noble metal tip.

10. A spark plug according to claim 9 , wherein

the ground electrode further has a first fusion zone which joins at least the noble metal tip and the holder;

the body has a proximal end connected to the metallic shell;

the entire first fusion zone is located toward the proximal end with respect to the center axis of the noble metal tip; and

at least a portion of the second fusion zone is located opposite the proximal end with respect to the center axis of the noble metal tip.

11. A spark plug according to claim 9 or 10 , wherein

the noble metal tip has a protrusion which is connected to a rearward end of a portion disposed within the through hole and which protrudes radially outward from an edge of the through hole at the rearward end surface of the holder.

12. A spark plug according to claim 11 , wherein

the protrusion has a thickness of 0.2 mm or more along a direction parallel to the center axis of the noble metal tip.

13. A spark plug according to claim 12 , wherein

a length, along a radial direction of a circle centered on the center axis of the noble metal tip, between a rearward end of an outer circumferential surface of a portion of the noble metal tip disposed within the through hole and an outer circumferential end of the protrusion is from 0.05 mm to 0.25 mm.

14. A method of manufacturing a spark plug according to claim 1 , comprising:

a disposition step of disposing the noble metal tip in the through hole of the holder; and

a step of applying load to the holder from a radial direction of the holder after the disposition step.

15. A method of manufacturing a spark plug which has

a ground electrode having a noble metal tip, a holder having a through hole for disposing therein the noble metal tip, and a body to which the holder is joined, and

a center electrode for forming a gap in cooperation with the noble metal tip, comprising:

a disposition step of disposing the noble metal tip within the through hole of the holder; and

a joining step of joining the holder to the body while the noble metal tip is disposed within the through hole of the holder, wherein

Gf<Tr, Gf<Gr, and Tf<Tr

where a forward side is a side toward the gap as viewed from the noble metal tip,

Gf is an inside diameter of the holder at a forward end surface of the holder,

Gr is an inside diameter of the holder at a rearward end surface of the holder,

Tf is an outside diameter of the noble metal tip at a forward end surface of the noble metal tip, and

Tr is an outside diameter of the noble metal tip at a rearward end surface of the noble metal tip;

at least one of that inner circumferential surface of the holder which forms the through hole, and that outer circumferential surface of the noble metal tip which is disposed in the through hole, continuously reduces in diameter toward the forward side; and

the forward end surface of the noble metal tip is located on the forward side with respect to the forward end surface of the holder in a state in which the noble metal tip is disposed in the through hole of the holder.

16. A method of manufacturing a spark plug according to claim 15 , wherein

the inner circumferential surface of the holder has a first taper surface which continuously reduces in diameter toward the forward side, and

the outer circumferential surface of the noble metal tip has a second taper surface which continuously reduces in diameter toward the forward side.

17. A method of manufacturing a spark plug according to claim 16 , wherein

in that section of the holder and the noble metal tip disposed within the through hole of the holder which contains the center axis of the noble metal tip, a difference dAg obtained by subtracting a second angle Ag2 from a first angle Ag1 is from −10 degrees to +10 degrees, where the first angle Ag1 is an acute angle between the first taper surface and the center axis, and the second angle Ag2 is an acute angle between the second taper surface and the center axis.

18. A method of manufacturing a spark plug according to any one of claims 15 to 17 , further comprising

a forming step of forming a first fusion zone which joins the noble metal tip and the holder.

19. A method of manufacturing a spark plug according to claim 18 , wherein

the forming step includes a step of forming a plurality of the first fusion zones which are disposed at such positions as not to be directly opposite one another with respect to the center axis of the noble metal tip.