Corona ignition device and assembly method

1. A corona igniter for emitting a radio frequency electric field to ionize a fuel-air mixture and provide a corona discharge, comprising:

a central electrode formed of an electrically conductive material for receiving a high radio frequency voltage and emitting the radio frequency electric field;

an insulator formed of an electrically insulating material surrounding said central electrode and extending longitudinally from an insulator upper end to an insulator nose end;

said insulator including an insulator outer surface extending from said insulator upper end to said insulator nose end;

said insulator outer surface presenting an insulator outer diameter;

said insulator outer surface including an insulator lower shoulder extending outwardly and located between said insulator upper end and said insulator nose end;

said insulator lower shoulder presenting an increase in said insulator outer diameter;

a shell surrounding at least a portion of said insulator and extending from a shell upper end to a shell firing end;

said shell presenting a shell inner surface facing and extending along said insulator outer surface from said shell upper end to said shell firing end;

said shell inner surface presenting a shell inner diameter;

said shell inner diameter of at least one location of said shell being less than said insulator outer diameter at said insulator lower shoulder;

an intermediate part formed of an electrically conductive material disposed between said insulator outer surface and said shell inner surface and between said insulator upper end and said insulator lower shoulder.

2. The corona igniter of claim 1 , wherein said insulator is supported only along said intermediate part so that said insulator is not in tension.

3. The corona igniter of claim 1 , wherein said shell inner diameter at said shell firing end is less than said insulator outer diameter at said insulator lower shoulder.

4. The corona igniter of claim 1 , wherein said intermediate part is a layer of metal securing said insulator outer surface to said shell inner surface.

5. The corona igniter of claim 4 , wherein the layer of metal brazes the insulator outer surface to the shell inner surface.

6. The corona igniter of claim 1 , wherein said intermediate part is a sleeve of metal extending circumferentially around said insulator.

7. The corona igniter of claim 6 , wherein the intermediate part includes a layer of metal securing said sleeve of metal to said insulator outer surface and said shell inner surface.

8. The corona igniter of claim 1 , wherein said insulator outer diameter decreases to present a middle ledge spaced from the increase in said insulator outer diameter at said insulator lower shoulder, said insulator includes a groove between said middle ledge and said insulator lower shoulder, and said intermediate part is disposed in said groove.

9. The corona igniter of claim 8 , wherein said shell includes a lower turnover flange at said shell firing end, said lower turnover flange extends radially inwardly and into said groove of said insulator, and said intermediate part is disposed in said groove between said lower turnover flange and said insulator outer surface.

10. The corona igniter of claim 9 , wherein said lower turnover flange is bent around said middle ledge.

11. The corona igniter of claim 1 , wherein said intermediate part is fixed to said insulator outer surface and said shell inner surface.

12. The corona igniter of claim 1 , wherein said intermediate part is a layer of metal, and said insulator outer surface presents a plurality of ribs with depressions therebetween along said intermediate part.

13. The corona igniter of claim 1 , wherein said intermediate part is spaced from said insulator lower shoulder.

14. The corona igniter of claim 1 , wherein said central electrode includes a corona enhancing tip disposed outwardly of said insulator nose end and including a plurality of prongs extending radially outwardly.

15. The corona igniter of claim 1 , wherein said insulator extends longitudinally from said insulator upper end to an insulator upper shoulder and from said insulator upper shoulder to said insulator lower shoulder;

said insulator upper shoulder presents an increase in said insulator outer diameter;

said insulator outer diameter is constant from said insulator upper end to said insulator upper shoulder;

said insulator outer diameter is greater at said insulator upper shoulder than at said insulator upper end;

said insulator outer diameter is greater at said insulator lower shoulder than said insulator upper shoulder;

said insulator outer diameter decreases from said insulator lower shoulder to said insulator nose end;

said insulator is supported only along said intermediate part so that said insulator is not in tension and not in compression;

said shell firing end engages said insulator lower shoulder;

said shell inner diameter at said shell firing end is less than said insulator outer diameter at said insulator lower shoulder;

said intermediate part is a layer of metal which secures said insulator to said metal shell, said metal contains one or more of nickel, cobalt, iron, copper, tin, zinc, silver, and gold;

said central electrode includes a corona enhancing tip disposed outwardly of said insulator nose end and including a plurality of prongs extending radially outwardly.

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

providing an insulator formed of an electrically insulating material extending from an insulator upper end to and insulator nose end,

the insulator including an insulator outer surface extending from the insulator upper end to the insulator nose end and presenting an insulator outer diameter, the insulator outer surface presenting an insulator lower shoulder extending outwardly and located between the insulator upper end and the insulator nose end, the insulator lower shoulder presenting an increase in the insulator outer diameter;

providing a shell extending from a shell upper end to a shell firing end and including a shell inner surface presenting a shell bore, the shell inner surface presenting a shell inner diameter, the shell inner diameter of at least one location of the shell being less than the insulator outer diameter at the insulator lower shoulder;

inserting the insulator upper end into the shell bore through the shell firing end; and

disposing an intermediate part formed of an electrically conductive material between the insulator outer surface and the shell inner surface.

17. The method of claim 16 , including supporting the insulator only along the intermediate part so the insulator is not in tension.

18. The method of claim 16 , wherein the step of disposing the intermediate part between the insulator outer surface and the shell inner surface includes brazing the insulator outer surface to the shell inner surface.

19. The method of claim 16 , wherein the step of disposing the intermediate part between the insulator outer surface and the shell inner surface includes disposing a solid piece of metal around the insulator, and brazing the solid piece of metal to the insulator outer surface and to the shell inner surface.

20. The method of claim 16 including engaging the shell firing end with the insulator lower shoulder.

21. The method of claim 16 , wherein the insulator outer diameter decreases to present a middle ledge spaced from the insulator lower shoulder, the insulator includes a groove between the middle ledge and the insulator lower shoulder, and the step of disposing the intermediate part between the insulator outer surface and shell inner surface includes disposing the intermediate part in the groove.

22. The method of claim 20 , wherein the shell includes a lower turnover flange at the shell firing end, and bending the lower turnover flange into the groove.