Corona igniter including ignition coil with improved isolation

A corona igniter (20) for providing a radio frequency electric field to ionize a portion of a fuel-air mixture and provide a corona discharge (24) in a combustion chamber (22), comprising:

• a housing (34) having interior walls (40, 42, 44) presenting a total housing volume therebetween,
• a coil (26) disposed in said housing (34) for receiving energy at a first voltage and transmitting the energy at a second voltage being at least 15 times higher than the first voltage,
• said coil (26) extending longitudinally along a coil center axis (ac) and having a length (l) extending from a coil low voltage end (28) receiving the energy at the first voltage to a coil high voltage end (30) transmitting the energy at the second voltage,
• said coil (26) having an inductance of at least 500 micro henries,
• said coil (26) including a plurality of windings (54) horizontally aligned with one another and extending longitudinally along said coil center axis (ac) and a winding gap disposed around each of said windings (54),
• said windings (54) presenting a perimeter around said coil center axis (ac) and having a winding diameter (d) extending across said coil center axis (ac),
• a coil former (62) formed of electrically insulating resin material disposed along said coil center axis (ac) and spacing said windings (54) from said coil center axis (ac),
• a coil filler (36) formed of electrically insulating resin material different from said coil former (62) disposed in said housing (34) at said coil high voltage end (30) and disposed in said winding gap around windings (54),
• said coil filler (36) having a dielectric strength of at least 3 kV/mm, a thermal conductivity of at least 0.125 W/m.K, and a relative permittivity of at less than 6,
• said coil filler (36) having a filler volume of 10 to 70% of said total housing volume,
• an electrode electrically coupled to said coil (26) for receiving the energy from said coil (26),
• a gap region disposed between said coil (26) and said interior walls (40, 42, 44) of said housing (34),
• a capacitance reducing component (38) having a relative permittivity of less than 6 and a component volume filing said gap region of said housing (34),
• said capacitance reducing component (38) extending continuously around said coil (26) and along said interior walls (40, 42, 44) of said housing (34),
• said capacitance reducing component (38) disposed along at least 50% of said length (l) of said coil (26),
• said component volume being greater than said filler volume,
• said component volume being 20 to 90% of said total housing volume,
• said capacitance reducing component (38) having a dielectric strength of at least 3 kV/mm, and
• said capacitance reducing component (38) including at least one of a gas, an oil having a dielectric strength of at least 3 kV/mm, a liquid having a dielectric strength of at least 10 kV/mm, and a solid having a permittivity of less than 6.