An erosion of an electrode is prevented in a spark plug that performs an ignition by use of a spark discharge and an electromagnetic wave. The spark plug that a pulse voltage for a spark discharge and an electromagnetic wave supplied into the spark discharge as an energy are applied to a center electrode, the spark plug comprises the center electrode, an insulator including an axis hole into which the center electrode is engaged, a ground electrode configured to form a space between the ground electrode and the center electrode in which the spark discharge is generated, and a cylindrical conductor that is arranged at an outer circumference of the insulator, and an annular member is formed at an inner circumference of the cylindrical conductor and at a position where a distance from a distal end surface of the cylindrical conductor is within a range between wavelength and wavelength of the electromagnetic wave.

An igniter that has a large ignition power and an electromagnetic wave resonance structure with a small reflected power is provided. An igniter comprises a first rectangular substrate and a second rectangular substrate each having a longitudinal side, and at least one intermediate substrate arranged between the first substrate and the second substrate and having a longitudinal side which is shorter than each longitudinal side of the first substrate and the second substrate, the first substrate has an input part configured to receive an input of an electromagnetic wave from an outside, a first electrode, and an electromagnetic wave transmission line that connects the input part to the first electrode, each of the first electrode and the electromagnetic wave transmission line being provided at a surface of the first substrate on a side of the at least one intermediate substrate, the second substrate has an electromagnetic wave resonator and a second electrode that is electrically connected to the electromagnetic wave resonator, each of the electromagnetic wave resonator and a second electrode being provided at a surface of the second substrate on a side of the at least one intermediate substrate, and a space is formed between the first substrate and the second substrate at a position at which the at least one intermediate substrate does not exist therebetween, such that the first electrode and the second electrode are faced each other and located away from each other across the space and a part of the electromagnetic wave transmission line and a part of the resonator are faced each other and located away from each other across the space.

The igniter can have a first conductor having a tip extending along an axis and forming a positive terminal; a second conductor having a tip surrounding the tip of the first conductor around the axis and forming a negative terminal; an insulator filling a space between the first conductor and the second conductor, a spark path extending radially, relative the axis, between the tip of the first conductor and the tip of the second conductor, from the positive terminal to the negative terminal; at least one magnet operable to generate a magnetic field B, the magnetic field B having a magnetic field orientation extending circumferentially around the axis, the at least one magnet disposed radially, relative the axis, between the first conductor and the second conductor, the at least one magnet embedded within the insulator.

The igniter can have a first conductor having a tip extending along an axis and forming a positive terminal; a second conductor having a tip surrounding the tip of the first conductor around the axis and forming a negative terminal; an insulator filling a space between the first conductor and the second conductor, a spark path extending radially, relative the axis, between the tip of the first conductor and the tip of the second conductor, from the positive terminal to the negative terminal; at least one magnet operable to generate a magnetic field B, the magnetic field B having a magnetic field orientation extending circumferentially around the axis, the at least one magnet disposed radially, relative the axis, between the first conductor and the second conductor, the at least one magnet embedded within the insulator.

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%.

An igniter for a spark ignition system includes a support body fabricated from an electrically insulating material and a metal casing disposed outwardly of and at least partially surrounding the support body. The metal casing has a structure for connecting the metal casing to ground. At least two rod-shaped first electrodes are supported one relative to another by the support body and are electrically isolated one from the other by the support body. Each first electrode has a first end that protrudes from a first end of the support body at a spark forming end of the igniter. The support body is disposed at least partly within an axial channel of a generally cylindrically-shaped second electrode. The second electrode projects past the support body at the spark forming end of the igniter and cooperates with the first ends of the at least two first electrodes to define at least two spark gaps. The second electrode is electrically isolated from the metal casing by an air gap. During use the first electrodes receive a first voltage that is higher than ground and the second electrode receives a second voltage that is lower than the first voltage. A first spark is formed within a first one of the at least two spark gaps and a second spark is formed within a second one of the at least two spark gaps.

An igniter for a spark ignition system includes a support body fabricated from an electrically insulating material and a metal casing disposed outwardly of and at least partially surrounding the support body. The metal casing has a structure for connecting the metal casing to ground. At least two rod-shaped first electrodes are supported one relative to another by the support body and are electrically isolated one from the other by the support body. Each first electrode has a first end that protrudes from a first end of the support body at a spark forming end of the igniter. The support body is disposed at least partly within an axial channel of a generally cylindrically-shaped second electrode. The second electrode projects past the support body at the spark forming end of the igniter and cooperates with the first ends of the at least two first electrodes to define at least two spark gaps. The second electrode is electrically isolated from the metal casing by an air gap. During use the first electrodes receive a first voltage that is higher than ground and the second electrode receives a second voltage that is lower than the first voltage. A first spark is formed within a first one of the at least two spark gaps and a second spark is formed within a second one of the at least two spark gaps.

A multipoint spark plug for igniting an air-fuel mixture in a combustion chamber of an engine includes a main body portion that is formed in a flattened shape and inserted into an insertion hole of the engine such that a tip end portion thereof opposes the combustion chamber, a pair of side electrodes provided via a gap in a lengthwise direction of the tip end portion, and at least one intermediate electrode provided in the gap between the pair of side electrodes such that a plurality of ignition gaps are formed in the lengthwise direction of the tip end portion.

A multipoint spark plug for igniting an air-fuel mixture in a combustion chamber of an engine includes a main body portion that is formed in a flattened shape and inserted into an insertion hole of the engine such that a tip end portion thereof opposes the combustion chamber, a pair of side electrodes provided via a gap in a lengthwise direction of the tip end portion, and at least one intermediate electrode provided in the gap between the pair of side electrodes such that a plurality of ignition gaps are formed in the lengthwise direction of the tip end portion.