Corona igniter including ignition coil with improved isolation

1. 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 ) including a plurality of 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 higher than the first voltage,

an electrode electrically coupled to said coil ( 26 ) for receiving the energy and providing the radio frequency electric field,

a coil filler ( 36 ) formed of a resin material disposed on said coil ( 26 ),

said coil filler ( 36 ) having a filler volume being a portion of said total housing volume,

a capacitance reducing component ( 38 ) having a relative permittivity of less than 6 disposed in said housing ( 34 ),

said capacitance reducing component ( 38 ) having a component volume being a portion of said total housing volume and being greater than said filler volume.

2. The corona igniter ( 20 ) of claim 1 wherein said capacitance reducing component ( 38 ) is at least 20% of said total housing volume and said filler volume is at least 10% of said total housing volume.

3. The corona igniter ( 20 ) of claim 1 wherein said filler volume is 10 to 70% of said total housing volume.

4. The corona igniter ( 20 ) of claim 1 wherein said component volume is 20 to 90% of said total housing volume.

5. The corona igniter ( 20 ) of claim 1 wherein said component volume is at least two times greater than said filler volume.

6. The corona igniter ( 20 ) of claim 1 wherein said capacitance reducing component ( 38 ) extends continuously around said coil ( 26 ).

7. The corona igniter ( 20 ) of claim 1 wherein said coil ( 26 ) includes a plurality of windings ( 54 ) and a winding gap around said windings ( 54 ) and wherein said coil filler ( 36 ) is disposed in said winding gaps.

8. The corona igniter ( 20 ) of claim 7 wherein said windings ( 54 ) extend circumferentially around a coil center axis (a c ) and said capacitance reducing component ( 38 ) extends continuously around said windings ( 54 ) along said housing ( 34 ).

9. The corona igniter ( 20 ) of claim 1 wherein said coil ( 26 ) has a length (I) extending from a coil low voltage end ( 28 ) receiving the energy to a coil high voltage end ( 30 ) transmitting the energy and said capacitance reducing component ( 38 ) extends along at least 50% of said length (l).

10. The corona igniter ( 20 ) of claim 9 wherein said coil filler ( 36 ) is disposed at said coil high voltage end ( 30 ).

11. The corona igniter ( 20 ) of claim 9 including a retainer ( 84 ) formed of an electrically insulating material separate from said coil filler ( 36 ) coupling said coil low voltage end ( 30 ) to said housing ( 34 ).

12. The corona igniter ( 20 ) of claim 1 wherein said capacitance reducing component ( 38 ) has a dielectric strength of at least 3 kV/mm.

13. The corona igniter ( 20 ) of claim 1 wherein said capacitance reducing component ( 38 ) includes 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 less than 6.

14. The corona igniter ( 20 ) of claim 13 wherein said gas includes at least one of ambient air and a gas having a pressure of not greater than 10 bar.

15. The corona igniter ( 20 ) of claim 1 wherein said coil filler ( 36 ) has a dielectric strength of at least 10 kV/mm and a relative permittivity of less than 8.

16. 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 (a c ) 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 (a c ) and a winding gap disposed around each of said windings ( 54 ),

said windings ( 54 ) presenting a perimeter around said coil center axis (a c ) and having a winding diameter (d) extending across said coil center axis (a c ),

a coil former ( 62 ) formed of electrically insulating resin material disposed along said coil center axis (a c ) and spacing said windings ( 54 ) from said coil center axis (a c ),

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.

17. A method of forming 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 the steps of:

providing a coil filler ( 36 ) attached to a coil ( 26 ), wherein the coil filler ( 36 ) includes a resin and has a filler volume and the coil ( 26 ) has an inductance of at least 500 micro henries,

disposing the coil ( 26 ) and the attached coil filler ( 36 ) in a housing ( 34 ), and

filling the housing ( 34 ) with a capacitance reducing component ( 38 ) having a permittivity of less than 6 and having a component volume being greater than the filler volume.

18. The method of claim 17 wherein the step of filling the housing ( 34 ) with the capacitance reducing component ( 38 ) includes filling at least 20% of a total housing volume.

19. The method of claim 17 wherein the step of providing the coil filler ( 36 ) attached to the coil ( 26 ) includes disposing the resin on the coil ( 26 ) and curing the resin.