A spark-plug wire includes a boot of electrically insulative material defining a central bore configured to receive a spark plug, an electrical connector disposed in the central bore and configured to connect with the spark plug, and a cylindrical heat shield circumscribing the boot. The heat shield has retaining features extending past a distal end of the boot and configured to radially flex to engage with a hex of the spark plug to increase a retaining force of the spark-plug wire to the spark plug.

A spark-plug wire includes a boot of electrically insulative material defining a central bore configured to receive a spark plug, an electrical connector disposed in the central bore and configured to connect with the spark plug, and a cylindrical heat shield circumscribing the boot. The heat shield has retaining features extending past a distal end of the boot and configured to radially flex to engage with a hex of the spark plug to increase a retaining force of the spark-plug wire to the spark plug.

A plasma jet plug includes a rod-shaped central electrode, an insulator having an axial hole, a metal shell disposed around the insulator, an orifice electrode electrically connected to the metal shell and located at a distal end side of the insulator, and a gasket that contacts an outer surface of the insulator and an inner surface of the metal shell. The insulator includes a first member, a second member and an inorganic seal layer. The first member has the axial hole and includes a large-diameter portion and a small-diameter portion. The second member has a through hole into which the small-diameter portion is inserted. The second member has an outer surface that contacts the gasket. The inorganic seal layer seals a gap between the first member and the second member. In a section including the axis, the sum of seal lengths is greater than or equal to 3 mm.

A plasma jet plug includes a rod-shaped central electrode, an insulator having an axial hole, a metal shell disposed around the insulator, an orifice electrode electrically connected to the metal shell and located at a distal end side of the insulator, and a gasket that contacts an outer surface of the insulator and an inner surface of the metal shell. The insulator includes a first member, a second member and an inorganic seal layer. The first member has the axial hole and includes a large-diameter portion and a small-diameter portion. The second member has a through hole into which the small-diameter portion is inserted. The second member has an outer surface that contacts the gasket. The inorganic seal layer seals a gap between the first member and the second member. In a section including the axis, the sum of seal lengths is greater than or equal to 3 mm.

An ignition system (10) comprises a high voltage transformer (12) comprising a primary winding (12.1) and a secondary winding (12.2). A primary resonant circuit (26) is formed by the primary winding (12.1) and a primary circuit capacitance (24). A secondary resonant circuit (16) is formed by an ignition plug (14), as a load, the secondary winding (12.2); the ignition plug (14) being represented by a secondary circuit capacitance (18) and a secondary circuit load resistance (Rp) put in parallel. Said load resistance value varies during an ignition cycle. The primary resonant circuit (26) and the secondary resonant circuit (16) have a common mode resonance frequency (f.sub.c) and a differential mode resonance frequency (f.sub.d). A controller (28) is configured to cause a drive circuit (22) to drive the primary winding at a frequency, which is either the common-mode resonance frequency (f.sub.c) or the differential mode resonance frequency (f.sub.d) and is connected to a feed-back circuit (50) to adapt the frequency of the primary winding to the variable load resistance.

An ignition system (10) comprises a high voltage transformer (12) comprising a primary winding (12.1) and a secondary winding (12.2). A primary resonant circuit (26) is formed by the primary winding (12.1) and a primary circuit capacitance (24). A secondary resonant circuit (16) is formed by an ignition plug (14), as a load, the secondary winding (12.2); the ignition plug (14) being represented by a secondary circuit capacitance (18) and a secondary circuit load resistance (Rp) put in parallel. Said load resistance value varies during an ignition cycle. The primary resonant circuit (26) and the secondary resonant circuit (16) have a common mode resonance frequency (f.sub.c) and a differential mode resonance frequency (f.sub.d). A controller (28) is configured to cause a drive circuit (22) to drive the primary winding at a frequency, which is either the common-mode resonance frequency (f.sub.c) or the differential mode resonance frequency (f.sub.d) and is connected to a feed-back circuit (50) to adapt the frequency of the primary winding to the variable load resistance.

An extender for connecting a high-voltage source to a spark plug has a conductive member, a tube having an interior passageway in which the conductive member is positioned therein, and a boot affixed over an exterior of the tube. The boot is formed of a material having a rigidity less than a rigidity of a material of the tube. The boot has a first end adapted to connect with the high-voltage source and a second end adapted to be connected to the spark plug such that the conductive member is in electrical connection with the spark plug and the high-voltage source. The conductive member includes a spring that is adapted to electrically connect with the high-voltage source and the spark plug.

An ignition system for an internal combustion engine has a power source, a transformer having first and second primary windings and a secondary winding, a connector extending from the secondary winding and adapted so as to connect with a terminal of the spark plug of the internal combustion engine, and electronic spark timing circuit cooperative with the transformer so as to activate and deactivate voltage to the first and second primary windings. The first and second primary windings are connected to the power source such that the transformer produces an alternating voltage output from the secondary winding of between 1 kHz and 100 kHz and a voltage of at least 20 kV. A forced push-pull inverter is cooperative with the electronic spark timing circuit so as to fix a frequency of voltage to the first and second primary windings.

An ignition system for an internal combustion engine has a power source, a transformer having first and second primary windings and a secondary winding, a connector extending from the secondary winding and adapted so as to connect with a terminal of the spark plug of the internal combustion engine, and electronic spark timing circuit cooperative with the transformer so as to activate and deactivate voltage to the first and second primary windings. The first and second primary windings are connected to the power source such that the transformer produces an alternating voltage output from the secondary winding of between 1 kHz and 100 kHz and a voltage of at least 20 kV. A forced push-pull inverter is cooperative with the electronic spark timing circuit so as to fix a frequency of voltage to the first and second primary windings.

A spark plug includes a housing, an insulator inside the housing, a center electrode inside the insulator, and a connecting pin inside the insulator. At least one fillet is formed at a change of cross section at the housing, at the insulator, at the center electrode, and/or at the connecting pin. The fillet, viewed in cross section, includes a first leg and a second leg at an angle to the first leg. The length of the first leg is greater than the length of the second leg.