A spark plug includes: a center electrode; a ground electrode that is provided such that a gap for spark discharge is formed between the center electrode and the ground electrode; and a plug cover covering the center electrode and the ground electrode from a front side to form an auxiliary chamber, the plug cover being provided with a through hole, wherein 60% or more of the auxiliary chamber is present in a sphere. The sphere t has a center at a midpoint of a line segment connecting the center electrode and the ground electrode at a shortest distance on an axial line of the center electrode and is in contact with a point located closest from the center on an inner open end of the through hole is imagined.

Disclosed are a flame-ejecting spark plug, and an internal combustion engine and an automobile having same. On the basis of a conventional spark plug, a space near to an electrode is closed to form a cavity (10), an end face is provided with at least one first hole (11), and a side face is provided with at least one second hole (13). A mixture of air and fuel enters the cavity (10) through the first hole (11) and the second hole (13). Electric discharge between the electrodes produces a spark igniting the combustible gas within the cavity (10), and the flame extends in the cavity (10) and the temperature and pressure rise. The flame is ejected from the first hole (11) and the second hole (13) to form a plurality of columnar flames, and the flame penetrates the combustible gas in a combustion chamber and a cylinder to realize stereoscopic ignition, large-area ignition and high-energy ignition of the combustible gas in the combustion chamber.

A spark plug having an insulator with a leg portion whose outer circumferential surface has an arithmetic mean roughness Ra of 0.5 m or less. The leg portion has a first section, a second section, and a third section. The first section has an outer diameter that decreases toward the forward end of the first section. The second section is located forward of and adjacent to the first section and its outer diameter decreases toward the forward end of the second section. The outer circumferential surface of the second section is located inward of a first straight line passing through the base and forward ends of the outer circumferential surface of the first section. The third section is located forward of and adjacent to the second section. The outer diameter of the third section is equal to or less than a forward end outer diameter Ds of the second section over the entire third section.

A spark plug includes: a center electrode; a ground electrode that is provided such that a gap for spark discharge is formed between the center electrode and the ground electrode; and a plug cover covering the center electrode and the ground electrode from a front side to form an auxiliary chamber, the plug cover being provided with a through hole, wherein 60% or more of the auxiliary chamber is present in a sphere. The sphere t has a center at a midpoint of a line segment connecting the center electrode and the ground electrode at a shortest distance on an axial line of the center electrode and is in contact with a point located closest from the center on an inner open end of the through hole is imagined.

A spark plug includes: a center electrode; a ground electrode that is provided such that a gap for spark discharge is formed between the center electrode and the ground electrode; and a plug cover covering the center electrode and the ground electrode from a front side to form an auxiliary chamber, the plug cover being provided with a through hole, wherein 60% or more of the auxiliary chamber is present in a sphere. The sphere t has a center at a midpoint of a line segment connecting the center electrode and the ground electrode at a shortest distance on an axial line of the center electrode and is in contact with a point located closest from the center on an inner open end of the through hole is imagined.

A spark plug wherein the occurrence of cracks at a joint portion between a cover portion and a metal shell in the spark plug is prevented. The spark plug includes a cylindrical metal shell that accommodates an insulator therein, and a cover portion that covers, from a front end side of the spark plug, a front end portion of a center electrode and a facing portion of a ground electrode to form a pre-chamber space. The cover portion is joined to a front end side of the metal shell and has injection holes that are through-holes. A first coefficient of thermal expansion A (10.sup.5/K) of the material forming the cover portion at normal temperature and a second coefficient of thermal expansion B (10.sup.5/K) of the metal shell at normal temperature satisfy a formula (1): A<B.

A spark plug wherein the occurrence of cracks at a joint portion between a cover portion and a metal shell in the spark plug is prevented. The spark plug includes a cylindrical metal shell that accommodates an insulator therein, and a cover portion that covers, from a front end side of the spark plug, a front end portion of a center electrode and a facing portion of a ground electrode to form a pre-chamber space. The cover portion is joined to a front end side of the metal shell and has injection holes that are through-holes. A first coefficient of thermal expansion A (10.sup.5/K) of the material forming the cover portion at normal temperature and a second coefficient of thermal expansion B (10.sup.5/K) of the metal shell at normal temperature satisfy a formula (1): A<B.

In a spark plug for an internal combustion engine, a discharge gap is formed between a central electrode and a ground electrode in an axial direction of the spark plug. An insulator of a cylindrical shape has an axial hole and an insulator front end part. The axial hole is formed in the insulator, into which the central electrode is arranged and supported. The insulator front end part projects further toward a front side of the insulator in the axial direction of the spark plug than a location of an opening part formed at a front end side of the axial hole. The discharge gasp is formed in an auxiliary combustion chamber. The outer peripheral side of the auxiliary combustion chamber is covered with the insulator front end part.

In a spark plug for an internal combustion engine, a discharge gap is formed between a central electrode and a ground electrode in an axial direction of the spark plug. An insulator of a cylindrical shape has an axial hole and an insulator front end part. The axial hole is formed in the insulator, into which the central electrode is arranged and supported. The insulator front end part projects further toward a front side of the insulator in the axial direction of the spark plug than a location of an opening part formed at a front end side of the axial hole. The discharge gasp is formed in an auxiliary combustion chamber. The outer peripheral side of the auxiliary combustion chamber is covered with the insulator front end part.

A spark plug includes: a center electrode; a ground electrode that is provided such that a gap for spark discharge is formed between the center electrode and the ground electrode; and a plug cover covering the center electrode and the ground electrode from a front side. The plug cover has a through hole, wherein the plug cover includes a diameter reduction portion formed in a range of 0.1 mm or less from an outer open end of the through hole in a direction along a central axis of the through hole and having a diameter gradually decreasing from the outer open end toward an inner open end of the through hole. A relationship of 0 mm<xy<0.2 mm is satisfied, where x is a diameter at the outer open end, and y is a diameter at an inner end of the diameter reduction portion.