A spark plug including a center electrode that has a large-diameter portion having an outside diameter that is largest in the center electrode. The large-diameter portion is retained at a rear facing surface in an axial hole of an insulator. A metal shell has a diameter increased portion on a rear end side with respect to the center electrode, the diameter increased portion having an inside diameter that is increased toward the rear end side. The insulator has a first portion which is a portion of the insulator in a region from a rear end of the rear facing surface to a front end of the diameter increased portion. The first portion has a thickness that is the largest in the region, and is disposed at least on an outer periphery of the large-diameter portion.

A spark plug including a housing, an insulator, a center electrode, and a ground electrode situated at a combustion chamber-side end of the housing. The insulator has an insulator collar, an insulator base, and a transition area, which rests on a projection of the housing. A breathing space is configured at the combustion chamber-side end of the spark plug and is delimited by a section of the inner side of the housing and a section of the insulator base. The section of the insulator base has a rounding including a first leg length and a second leg length angled with respect to the first leg length, the first leg length extending between the intersecting point of the leg lengths with one another and a first end point of the rounding, and the second leg length extending between the intersecting point and a second end point of the rounding.

A spark plug including a housing, an insulator, a center electrode, and a ground electrode situated at a combustion chamber-side end of the housing. The insulator has an insulator collar, an insulator base, and a transition area, which rests on a projection of the housing. A breathing space is configured at the combustion chamber-side end of the spark plug and is delimited by a section of the inner side of the housing and a section of the insulator base. The section of the insulator base has a rounding including a first leg length and a second leg length angled with respect to the first leg length, the first leg length extending between the intersecting point of the leg lengths with one another and a first end point of the rounding, and the second leg length extending between the intersecting point and a second end point of the rounding.

A spark plug includes a tubular metal shell; an insulator including a locking portion locked onto the metal shell; and a cap disposed at a front end side of the metal shell, the cap having a plurality of orifices. The plurality of orifices include orifices with different cross-sectional areas. A sum of the number of one or more largest orifices and the number of one or more large orifices that have a cross-sectional area of larger than or equal to 90% of the one or more largest orifices is smaller than the number of orifices that are other than the one or more largest orifices and the one or more large orifices. A length of the front end portion in an axial direction between a front end of the insulator and a front end of the locking portion is smaller than or equal to 12 mm.

A spark plug includes a tubular metal shell; an insulator including a locking portion locked onto the metal shell; and a cap disposed at a front end side of the metal shell, the cap having a plurality of orifices. The plurality of orifices include orifices with different cross-sectional areas. A sum of the number of one or more largest orifices and the number of one or more large orifices that have a cross-sectional area of larger than or equal to 90% of the one or more largest orifices is smaller than the number of orifices that are other than the one or more largest orifices and the one or more large orifices. A length of the front end portion in an axial direction between a front end of the insulator and a front end of the locking portion is smaller than or equal to 12 mm.

A spark plug including a housing, an insulator, a center electrode, and a ground electrode situated at a combustion chamber-side end of the housing. The insulator has an insulator collar, an insulator base, and a transition area, which rests on a projection of the housing. A breathing space is configured at the combustion chamber-side end of the spark plug and is delimited by a section of the inner side of the housing and a section of the insulator base. The section of the insulator base has a rounding including a first leg length and a second leg length angled with respect to the first leg length, the first leg length extending between the intersecting point of the leg lengths with one another and a first end point of the rounding, and the second leg length extending between the intersecting point and a second end point of the rounding.

A spark plug including a housing, an insulator, a center electrode, and a ground electrode situated at a combustion chamber-side end of the housing. The insulator has an insulator collar, an insulator base, and a transition area, which rests on a projection of the housing. A breathing space is configured at the combustion chamber-side end of the spark plug and is delimited by a section of the inner side of the housing and a section of the insulator base. The section of the insulator base has a rounding including a first leg length and a second leg length angled with respect to the first leg length, the first leg length extending between the intersecting point of the leg lengths with one another and a first end point of the rounding, and the second leg length extending between the intersecting point and a second end point of the rounding.

A spark plug 100 of the present disclosure includes a center electrode 20; a metallic member provided to form a tubular shape around an axis AX of the spark plug, holding the center electrode 20 therein in an insulated state, and having a hole 55 formed in a side wall thereof and extending in a radial direction; and a ground electrode 30 supported in the hole 55 and extending from the hole 55 toward the axis AX. The ground electrode 30 has a fixing portion formed of a metal and fixed to the hole 55, and an ignition portion 39 containing a noble metal, disposed on a side toward the axis AX in relation to the fixing portion, and having a discharge surface for forming a gap G between the ignition portion and the center electrode 20. The absolute value of the difference in coefficient of thermal expansion between the metallic member and the fixing portion is smaller than the absolute value of the difference in coefficient of thermal expansion between the metallic member and the ignition portion 39.

A spark plug 100 of the present disclosure includes a center electrode 20; a metallic member provided to form a tubular shape around an axis AX of the spark plug, holding the center electrode 20 therein in an insulated state, and having a hole 55 formed in a side wall thereof and extending in a radial direction; and a ground electrode 30 supported in the hole 55 and extending from the hole 55 toward the axis AX. The ground electrode 30 has a fixing portion formed of a metal and fixed to the hole 55, and an ignition portion 39 containing a noble metal, disposed on a side toward the axis AX in relation to the fixing portion, and having a discharge surface for forming a gap G between the ignition portion and the center electrode 20. The absolute value of the difference in coefficient of thermal expansion between the metallic member and the fixing portion is smaller than the absolute value of the difference in coefficient of thermal expansion between the metallic member and the ignition portion 39.

An insulator for a spark plug including a first portion; a second portion located on the rear side of the first portion, an outer diameter F at a front end of the second portion being smaller than an outer diameter of the first portion; a third portion located on the front side of the first portion, an outer diameter D at a rear end of the third portion being smaller than the outer diameter of the first portion and the outer diameter F at the front end of the second portion; a first slope portion connecting the first portion and the second portion; and a second slope portion connecting the first portion and the third portion. A length A of the second slope portion and a length B of the first slope portion satisfy 2.0≤A/B≤3.9, and 0.50≤D/F≤0.88 is satisfied.