Stable combustion becomes possible even in a super-lean combustion engine or an engine that carries out a large amount of EGR. In a spark discharge ignition promoting apparatus, non-thermal plasma is generated in a cylinder by a non-thermal plasma generating unit. The non-thermal plasma generating unit is provided at a location where a processed air-fuel mixture by the non-thermal plasma reaches a spark plug after an in-cylinder flow or where the air-fuel mixture exists around an electrode of the spark plug in a time when the air-fuel mixture keeps an easy combustion state. The spark plug ignites the processed air-fuel mixture by discharge of the spark plug at timing when the processed air-fuel mixture by the non-thermal plasma of an easy combustion state reaches the spark plug after an in-cylinder flow or timing when the air-fuel mixture of the easy combustion state exists around an electrode of the spark plug in the time when the air-fuel mixture keeps the easy combustion state.

A spark plug having an insulating body defining a longitudinal axis and including a base portion and an obstruction portion, a first electrode including a proximal portion, a sheathed portion and a distal portion, the sheathed portion of the first electrode extending through the base portion of the insulating body, and a second electrode including a proximal portion, a sheathed portion and a distal portion, the sheathed portion of the second electrode extending through the base portion and the obstruction portion of the insulating body, wherein the obstruction portion axially extends beyond the distal portion of the first electrode.

A spark plug having an insulating body defining a longitudinal axis and including a base portion and an obstruction portion, a first electrode including a proximal portion, a sheathed portion and a distal portion, the sheathed portion of the first electrode extending through the base portion of the insulating body, and a second electrode including a proximal portion, a sheathed portion and a distal portion, the sheathed portion of the second electrode extending through the base portion and the obstruction portion of the insulating body, wherein the obstruction portion axially extends beyond the distal portion of the first electrode.

A spark plug for an internal combustion engine includes a cylindrical housing, a cylindrical insulator, a center electrode, and a ground electrode. The ground electrode includes an upright part and an inclined part. The upright part is a portion vertically provided on a tip end part of the housing to a tip end side. The inclined part is a portion that is bent from the tip of the upright part toward the center electrode side, and is extended toward an oblique tip end side. The inclined part has a ground end face, an opposed face, and a corner curved face. The corner curved face is a face that smoothly connects the ground end face and the opposed face. The corner curved face has a curved shape. The curvature radius R of the corner curved face satisfies 0.3 mmR0.7 mm. The inclination angle of the inclined part with respect to a plug axial direction (Z) satisfies 3060.

A spark plug having an insulating body defining a longitudinal axis and including a base portion and an obstruction portion, a first electrode including a proximal portion, a sheathed portion and a distal portion, the sheathed portion of the first electrode extending through the base portion of the insulating body, and a second electrode including a proximal portion, a sheathed portion and a distal portion, the sheathed portion of the second electrode extending through the base portion and the obstruction portion of the insulating body, wherein the obstruction portion axially extends beyond the distal portion of the first electrode.

A spark plug having an insulating body defining a longitudinal axis and including a base portion and an obstruction portion, a first electrode including a proximal portion, a sheathed portion and a distal portion, the sheathed portion of the first electrode extending through the base portion of the insulating body, and a second electrode including a proximal portion, a sheathed portion and a distal portion, the sheathed portion of the second electrode extending through the base portion and the obstruction portion of the insulating body, wherein the obstruction portion axially extends beyond the distal portion of the first electrode.

An igniter having at least two electrodes spaced from each other by an insulating member having a substantially continuous surface along a path between the electrodes. The electrodes extend substantially parallel to each other for a distance both above and below said surface. The insulating member has a channel (recess) for receiving at least a portion of a length of at least one of said electrodes below and to said surface of the insulating member. The surface of the insulating member may preferably be augmented with a conductivity enhancing agent. The insulating member and electrodes are configured so that an electric field between the electrodes at said surface does not have abrupt field intensity changes, whereby when a potential is applied to the electrodes sufficient to cause breakdown to occur between the electrodes, discharge occurs at said surface of the insulating member to define a plasma initiation region.

An igniter having at least two electrodes spaced from each other by an insulating member having a substantially continuous surface along a path between the electrodes. The electrodes extend substantially parallel to each other for a distance both above and below said surface. The insulating member has a channel (recess) for receiving at least a portion of a length of at least one of said electrodes below and to said surface of the insulating member. The surface of the insulating member may preferably be augmented with a conductivity enhancing agent. The insulating member and electrodes are configured so that an electric field between the electrodes at said surface does not have abrupt field intensity changes, whereby when a potential is applied to the electrodes sufficient to cause breakdown to occur between the electrodes, discharge occurs at said surface of the insulating member to define a plasma initiation region.

A spark plug 1 of the present invention includes an insulator 2 made from an alumina-based sintered body and including: a leg portion 22 provided on a front end side; a middle trunk portion 23 provided on a rear end side of the leg portion 22 and having a larger diameter than the leg portion 22; and a flange portion 24 provided on the rear end side of the middle trunk portion 23 and having a larger diameter than the middle trunk portion 23. In the spark plug 1, in a state where a cut surface 23b obtained by cutting the middle trunk portion 23 in a direction perpendicular to an axial line AX direction at an arbitrary position in the axial line AX direction is mirror-polished, when 20 observation regions X each being 185 ?m?250 ?m are set so as not to overlap each other, and with respect to each of a plurality of pores included in the 20 observation regions X, a square value P.sup.2 [m.sup.2] of a length P [?m] of a circumference of the pore is obtained, an average value of the square values P.sup.2 [?m.sup.2] of top 20 pores having the largest square values P.sup.2 [?m.sup.2] is not greater than 2200 ?m.sup.2, and a proportion T [%] of a total area of all pores included in the 20 observation regions X relative to a total area (100%) of the 20 observation regions X is not greater than 5%.

A spark plug 1 of the present invention includes an insulator 2 made from an alumina-based sintered body and including: a leg portion 22 provided on a front end side; a middle trunk portion 23 provided on a rear end side of the leg portion 22 and having a larger diameter than the leg portion 22; and a flange portion 24 provided on the rear end side of the middle trunk portion 23 and having a larger diameter than the middle trunk portion 23. In the spark plug 1, in a state where a cut surface 23b obtained by cutting the middle trunk portion 23 in a direction perpendicular to an axial line AX direction at an arbitrary position in the axial line AX direction is mirror-polished, when 20 observation regions X each being 185 ?m?250 ?m are set so as not to overlap each other, and with respect to each of a plurality of pores included in the 20 observation regions X, a square value P.sup.2 [m.sup.2] of a length P [?m] of a circumference of the pore is obtained, an average value of the square values P.sup.2 [?m.sup.2] of top 20 pores having the largest square values P.sup.2 [?m.sup.2] is not greater than 2200 ?m.sup.2, and a proportion T [%] of a total area of all pores included in the 20 observation regions X relative to a total area (100%) of the 20 observation regions X is not greater than 5%.