A spark plug is provided which includes a cylindrical metal shell, a cylindrical porcelain insulator retained in the metal shell, a center electrode, and a ground electrode. The center electrode is disposed in the insulator with a head protruding outside a front end surface of the insulator. The ground electrode is joined to the metal shell and includes a gap-defining portion which faces a side surface of a side surface of the center electrode to define a spark gap and also faces the front end surface of the porcelain insulator to define a semi-surface discharge gap for burning off, for example, carbon deposits on the front end surface of the insulator.

A spark plug is provided which includes a cylindrical metal shell, a cylindrical porcelain insulator retained in the metal shell, a center electrode, and a ground electrode. The center electrode is disposed in the insulator with a head protruding outside a front end surface of the insulator. The ground electrode is joined to the metal shell and includes a gap-defining portion which faces a side surface of a side surface of the center electrode to define a spark gap and also faces the front end surface of the porcelain insulator to define a semi-surface discharge gap for burning off, for example, carbon deposits on the front end surface of the insulator.

A spark plug for internal combustion engines is provided which includes a porcelain insulator which is disposed inside a housing and extends outside a front end of the porcelain insulator. An annular ground electrode is secured to a front end portion of the housing. A pocket is created between a nose of the porcelain insulator and the housing. The housing has formed therein a plurality of vent holes which are located outside the ground electrode in a radial direction of the housing and communicate between the pocket and a combustion chamber of the engine. The vent holes include a first vent hole and a second vent hole which are different in opening area from each other, thereby enhancing scavenging of the pocket to ensure the stability of operation of the spark plug.

Provided is a spark plug capable of ensuring fouling resistance with a simple configuration. An insulator includes a step portion formed on an outer circumferential surface thereof. A center electrode is arranged in an axial hole of the insulator. A cylindrical metal shell having a shelf portion formed on an inner circumferential surface thereof is arranged radially outside the insulator. The insulator further includes a front end portion located frontward of the step portion. An outer circumferential surface of the front end portion has an arithmetic average roughness of 0.5 m or smaller in a circumferential direction. A recess is formed with a depth of 3 to 20 m in at least part of an end surface and the outer circumferential surface of the front end portion so as to extend from the front toward the rear.

An ignition control apparatus applied to an internal combustion engine including a spark plug includes an in-cylinder pressure acquisition section, a frequency signal transmitting section which transmits a frequency signal having a predetermined frequency to a switching element, and a weak discharge generating section which causes the frequency signal to be transmitted during an intake stroke and controls the frequency signal such that a weak discharge is generated at the spark plug a plurality of times. The weak discharge generating section controls the frequency signal so as to cause a duty ratio of the switching element to be changed in accordance with the in-cylinder pressure, such that the frequency of generating weak discharges during a time period in which the frequency signal is transmitted becomes higher than a predetermined frequency.

An ignition control apparatus applied to an internal combustion engine including a spark plug includes an in-cylinder pressure acquisition section, a frequency signal transmitting section which transmits a frequency signal having a predetermined frequency to a switching element, and a weak discharge generating section which causes the frequency signal to be transmitted during an intake stroke and controls the frequency signal such that a weak discharge is generated at the spark plug a plurality of times. The weak discharge generating section controls the frequency signal so as to cause a duty ratio of the switching element to be changed in accordance with the in-cylinder pressure, such that the frequency of generating weak discharges during a time period in which the frequency signal is transmitted becomes higher than a predetermined frequency.

A spark plug is provided which includes a cylindrical metal shell, a cylindrical porcelain insulator retained in the metal shell, a center electrode, and a ground electrode. The center electrode is disposed in the insulator with a head protruding outside a front end surface of the insulator. The ground electrode is joined to the metal shell and includes a gap-defining portion which faces a side surface of a side surface of the center electrode to define a spark gap and also faces the front end surface of the porcelain insulator to define a semi-surface discharge gap for burning off, for example, carbon deposits on the front end surface of the insulator.

A spark plug is provided which includes a cylindrical metal shell, a cylindrical porcelain insulator retained in the metal shell, a center electrode, and a ground electrode. The center electrode is disposed in the insulator with a head protruding outside a front end surface of the insulator. The ground electrode is joined to the metal shell and includes a gap-defining portion which faces a side surface of a side surface of the center electrode to define a spark gap and also faces the front end surface of the porcelain insulator to define a semi-surface discharge gap for burning off, for example, carbon deposits on the front end surface of the insulator.

An igniter plug for an aircraft turbomachine includes an external electrode, an internal electrode, as well as a semiconducting body arranged between the external electrode and the internal electrode and set back from these electrodes so as to define an electrical arc forming cavity, the bottom of which is formed by an axial end surface of the semiconducting body. The free end of the external electrode is equipped with at least one scoop for discharging possible fuel residues present in the cavity.

In an ignition device having an ignition plug for igniting a fuel mixture gas introduced in a combustion chamber, a super hydrophilic membrane is formed on a surface at the combustion chamber side of a plug forming member of the ignition plug. The super hydrophilic membrane contains super hydrophilic particles and thermal excitation catalyst particles, and satisfies a relationship of .sub.W2<.sub.W1, where .sub.W1 indicates a water contact angle between water and the plug forming member on which no super hydrophilic membrane is formed, and .sub.W2 indicates a water contact angle between water and the plug forming member on which the super hydrophilic membrane is formed.