Corona ignition with hermetic combustion seal

1. A corona igniter, comprising:

a central electrode for receiving a radio frequency voltage and emitting a radio frequency electric field to ionize a fuel-air mixture and provide a corona discharge;

a shell formed of metal surrounding said central electrode and extending longitudinally along a center axis from a shell upper end to a shell lower end;

said shell including a shell inner surface facing said insulator and presenting a shell inner diameter extending across and perpendicular to said center axis, said shell inner diameter increasing adjacent said shell lower end;

an insulator disposed between said central electrode and said shell;

said insulator extending longitudinally along said center axis and including an insulator nose region extending outwardly of said shell lower end;

said insulator and said shell presenting a gap therebetween, said gap extending longitudinally along said center axis; and

a ceramic combustion seal sealing said gap between said shell and said insulator, wherein said ceramic combustion seal is disposed in said gap along said increased shell inner diameter adjacent said shell lower end.

2. The corona igniter of claim 1 , wherein said ceramic combustion seal is formed of sintered ceramic material.

3. The corona igniter of claim 2 , wherein said sintered ceramic material of said ceramic combustion seal includes alumina.

4. The corona igniter of claim 1 , wherein said ceramic combustion seal extends continuously from said shell lower end and/or a shell inner surface adjacent said shell lower end across said gap to said insulator; and said ceramic combustion seal hermetically seals said gap.

5. The corona igniter of claim 1 , wherein said ceramic combustion seal includes a cylinder disposed in said gap along said increased shell inner diameter.

6. The corona igniter of claim 5 , wherein said ceramic combustion seal is a bushing including said cylinder disposed in said gap along said increased shell inner diameter; and said bushing includes a flange extending outwardly from said cylinder along said shell lower end.

7. The corona igniter of claim 1 , wherein said ceramic combustion seal comprises a ring disposed along said shell lower end.

8. The corona igniter of claim 1 , wherein a glass material or a mixture of glass and ceramic adheres said ceramic combustion seal to at least one of said insulator and said shell.

9. The corona igniter of claim 1 , wherein said ceramic combustion seal is brazed to said shell.

10. The corona igniter of claim 1 , wherein said insulator presents an insulator outer surface having an insulator outer diameter extending across and perpendicular to said center axis;

said insulator outer surface extends longitudinally from an insulator upper end to an insulator nose end;

said insulator outer diameter decreases along a portion of said insulator moving toward said insulator nose end to present an insulator lower shoulder;

said insulator outer diameter decreases along a portion of said insulator moving toward said insulator upper end at a location spaced from said insulator lower shoulder to present an insulator upper shoulder;

said insulator outer diameter tapers along said insulator nose region to said insulator nose end;

said insulator outer diameter between said insulator lower shoulder and said insulator nose end is less than said insulator outer diameter between said insulator lower shoulder and said insulator upper shoulder;

said shell upper end is disposed on said insulator upper shoulder; and

said ceramic combustion seal is disposed along said portion of said insulator outer surface between said insulator lower shoulder and said insulator nose region.

11. The corona igniter of claim 1 including at least one conformal element disposed in said gap and compressed between said insulator and said shell; and said conformal element is spaced longitudinally from said ceramic combustion seal.

12. The corona igniter of claim 1 including a filler material filling at least a portion of said gap between said insulator and said shell.

13. The corona igniter of claim 1 , wherein said central electrode is disposed on and extends longitudinally along said center axis from a terminal end to a firing end;

said firing end of said central electrode includes a corona enhancing tip disposed axially outwardly of said insulator nose region and has a plurality of prongs each extending radially outwardly from said center axis;

said insulator extends longitudinally from an insulator upper end to an insulator nose end, wherein said insulator nose region is adjacent said insulator nose end;

said insulator presents an insulator inner surface surrounding a bore receiving said central electrode and extending longitudinally from said insulator upper end to said insulator nose end;

said insulator presents an insulator outer surface having an insulator outer diameter extending across and perpendicular to said center axis;

said insulator outer surface extends longitudinally from said insulator upper end to said insulator nose end;

said insulator outer diameter decreases along a portion of said insulator moving toward said insulator nose end to present an insulator lower shoulder;

said insulator outer diameter decreases along a portion of said insulator moving toward said insulator upper end at a location spaced from said insulator lower shoulder to present an insulator upper shoulder;

said insulator outer diameter tapers along said insulator nose region to said insulator nose end;

said insulator outer diameter between said insulator lower shoulder and said insulator nose end is less than said insulator outer diameter between said insulator lower shoulder and said insulator upper shoulder;

said insulator upper shoulder engages said shell upper end;

said insulator is formed of alumina;

said shell inner surface surrounds said insulator outer surface and a shell outer surface facing away from said insulator outer surface;

said shell inner surface and said shell outer surface extend longitudinally along said center axis from said shell upper end to said shell lower end;

said shell inner surface presents a bore receiving said insulator;

said shell inner diameter is greater than said insulator outer diameter along the length of said shell;

said gap between said insulator and said shell extends radially relative to said center axis from said insulator outer surface to said shell inner surface;

said gap between said insulator and said shell extends longitudinally along said center axis from said insulator lower shoulder to said shell lower end;

at least one conformal element is disposed in said gap and compressed between said insulator outer surface and said shell inner surface and is spaced longitudinally from said ceramic combustion seal;

said at least one conformal element seals said gap at a location spaced longitudinally from said ceramic combustion seal;

one of said at least one conformal element is disposed between said insulator lower shoulder and said shell inner surface;

one of said at least one conformal element is a gasket formed of metal, a rubber material, or a plastic material;

a filler material is disposed in said gap between said ceramic combustion seal and said conformal element;

said ceramic combustion seal extends continuously from said shell lower end and/or said shell inner surface adjacent said shell lower end across said gap to said insulator outer surface and hermetically seals said gap;

said ceramic combustion seal is formed of sintered ceramic material;

said sintered ceramic material of said ceramic combustion seal includes alumina;

said ceramic combustion seal is provided as a bushing, a cylinder, or a ring; and

a glass material and/or a mixture of glass and ceramic adheres said ceramic combustion seal to at least one of said insulator outer surface and said shell inner surface.

14. The corona igniter of claim 13 , wherein said ceramic combustion seal comprises a cylinder, a bushing, or a ring disposed along or adjacent said lower shell end.

15. The corona igniter of claim 13 , wherein one of said at least one conformal element is compressed between said insulator upper shoulder and said shell inner surface.

16. A method of forming a corona igniter, comprising the steps of:

providing a central electrode for receiving a radio frequency voltage and emitting a radio frequency electric field to ionize a fuel-air mixture and provide a corona discharge;

disposing the central electrode in a bore of an insulator, wherein the insulator extends longitudinally along a center axis and includes an insulator nose region;

surrounding the insulator with a shell formed of metal and extending longitudinally from a shell upper end to a shell lower end such that the insulator nose region extends outwardly of the shell lower end and the insulator and shell form a gap therebetween, wherein the gap extends longitudinally along the center axis, the shell including a shell inner surface facing the insulator and presenting a shell inner diameter extending across and perpendicular to the center axis, and the shell inner diameter increasing adjacent the shell lower end; and

sealing the gap by disposing a ceramic combustion seal between the insulator and the shell, wherein said ceramic combustion seal is disposed in said gap along said increased shell inner diameter adjacent said shell lower end.

17. The method of claim 16 including inserting the insulator nose region into a bore of the shell through the shell upper end.

18. The method of claim 16 , wherein the sealing step includes adhering the ceramic combustion seal to at least one of the insulator and the shell with a glass material and/or a mixture of glass and ceramic.

19. The method of claim 16 , wherein the sealing step includes brazing the ceramic combustion seal to the shell.