Corona suppression at the high voltage joint through introduction of a semi-conductive sleeve between the central electrode and the dissimilar insulating materials

A corona ignition assembly comprising a plurality of different insulators disposed between an ignition coil assembly and firing end assembly is provided. A high voltage center electrode extends longitudinally between an igniter central electrode and the ignition coil assembly. A high voltage insulator formed of a fluoropolymer surrounds the high voltage center electrode, and a firing end insulator firing of alumina surrounds the igniter central electrode. A sleeve formed of a semi-conductive and complaint material, such as silicone rubber with conductive filler, is disposed radially between the electrodes and adjacent insulators. The sleeve fills air gaps and minimizes the peak electric field within the corona igniter assembly. The sleeve is able to prevent unwanted corona discharge, and thus extends the life of the materials and directs energy to the firing end.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to corona ignition assemblies, and methods of manufacturing the corona ignition assemblies.

2. Related Art

Corona igniter assemblies for use in corona discharge ignition systems typically include an ignition coil assembly attached to a firing end assembly as a single component. The firing end assembly includes a center electrode charged to a high radio frequency voltage potential, creating a strong radio frequency electric field in a combustion chamber. The electric field causes a portion of a mixture of fuel and air in the combustion chamber to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture. The electric field is preferably controlled so that the fuel-air mixture maintains dielectric properties and corona discharge occurs, also referred to as non-thermal plasma. The ionized portion of the fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining portion of the fuel-air mixture. The electric field is also preferably controlled so that the fuel-air mixture does not lose all dielectric properties, which would create thermal plasma and an electric arc between the electrode and grounded cylinder walls, piston, or other portion of the igniter.

Ideally, the electric field is also controlled so that the corona discharge only forms at the firing end and not along other portions of the corona igniter assembly. However, such control is oftentimes difficult to achieve due to air gaps located between the components of the corona igniter assembly where unwanted corona discharge tends to form. For example, although the use of multiple insulators formed of different materials provides improved efficiency, robustness, and overall performance, the metallic shielding and the different electrical properties between the insulator materials leads to an uneven electrical field and air gaps at the interfaces. The dissimilar coefficients of thermal expansion and creep between the insulator materials can also lead to air gaps at the interfaces when operating in the −40° C. to 150° C. temperature range. During use of the corona igniter, the electrical field tends to concentrate in those air gaps. The high voltage and frequency applied to the corona igniter assembly ionizes the trapped air causes unwanted corona discharge. Such corona discharge can cause material degradation and hinder the performance of the corona igniter assembly.

In addition, the different materials disposed radially across the assembly can lead to an uneven distribution of electrical field strength between those materials. While moving from the coil to the firing end, the electrical field loads and unloads the capacitance in a direction moving radially between the electrode and external shield. The electrical field concentrated at the interfaces between the different electrode and insulator materials, and in any cavities or air voids between the materials, is typically high. Oftentimes, this voltage is higher than the voltage of corona inception, which could contribute to the unwanted corona discharge along the interfaces, cavities, or air voids.

SUMMARY OF THE INVENTION

One aspect of the invention provides a corona igniter assembly comprising an ignition coil assembly and a firing end assembly capable of maintaining the peak electric field below the voltage of corona inception. The firing end assembly includes an igniter central electrode surrounded by a ceramic insulator. A high voltage center electrode is coupled to the igniter central electrode. A high voltage insulator formed of a material different from the ceramic insulator surrounds the high voltage center electrode. A semi-conductive sleeve is disposed radially between the high voltage center electrode and the insulators and extends axially along an interface between the adjacent insulators. A dielectric compliant insulator is optionally disposed between the high voltage insulator and the ceramic insulator of firing end assembly. If the optional dielectric complaint insulator is present, then the semi-conductive sleeve is also disposed radially between the high voltage center electrode and the dielectric complaint insulator and extends axially along the interfaces between the dielectric compliant insulator and the adjacent insulators.

Another aspect of the invention provides a method of manufacturing the corona igniter assembly by disposing the semi-conductive sleeve radially between the high voltage center electrode and the different insulator.

The semi-conductive sleeve relieves stress and stabilizes the electrical field between the different materials disposed radially across the corona igniter assembly, where more air gaps or changes in geometry leading to increases in electric field typically exist. More specifically, the semi-conductive sleeve minimizes the peak electric field within the corona igniter assembly by contrasting the electric charge concentration in any air gaps located along the high voltage center electrode or ceramic insulator. The voltage drop through the semi-conductive sleeve is significant, and thus the voltage peak at the interface between the semi-conductive sleeve and the adjacent materials is lower than the voltage peak between the high voltage center electrode and the ceramic insulator would be without the semi-conductive sleeve. Studies show that the semi-conductive sleeve performs like an actual conductor, with limited loss of power, when fed with a high frequency and high voltage (HV-HF).

The semi-conductive sleeve also conducts charge away and relieves any cavities from static electrical charge that could generate unwanted corona discharge. Furthermore, the semi-conductive sleeve is typically formed of a compliant material, and thus minimizes the amount or volume of air gaps along the interfaces between the high voltage center electrode and the ceramic insulator. In summary, by preventing the unwanted corona discharge, the life of the materials can be extended and the energy can be directed to the corona discharge formed at the firing end, which in turn improves the performance of the corona igniter assembly.

DESCRIPTION OF EXAMPLE EMBODIMENTS

A corona igniter assembly20for receiving a high radio frequency voltage and distributing a radio frequency electric field in a combustion chamber containing a mixture of fuel and gas to provide a corona discharge is generally shown inFIG. 1. The corona igniter assembly20includes an ignition coil assembly22, a firing end assembly24, and a metal tube26surrounding and coupling the ignition coil assembly22to the firing end assembly24. The corona igniter assembly20also includes a high voltage insulator28and an optional dielectric compliant insulator30each disposed between the ignition coil assembly22and a ceramic insulator32of the firing end assembly24, inside of the metal tube26. A high voltage center electrode62connects the ignition coil assembly22to the firing end assembly24. A semi-conductive sleeve76extends continuously along the interfaces between the different insulators28,30,32. The semi-conductive sleeve76dampens the peak electric field and fills air gaps located along the high voltage center electrode62and adjacent insulators28,30,32, which in turn prevents unwanted corona discharge.

The ignition coil assembly22includes a plurality of windings (not shown) receiving energy from a power source (not shown) and generating the high radio frequency and high voltage electric field. The ignition coil assembly22extends along a center axis A and includes a coil output member36for transferring energy toward the firing end assembly24. In the exemplary embodiment, the coil output member36is formed of plastic material. As shown inFIG. 3, the coil output member36presents an output side wall38which tapers toward the center axis A to an output end wall40. The output side wall38has a conical shape, and the output end wall40extends perpendicular to the center axis A. In addition, a coil connector86typically extends outwardly of the coil output member36and abuts the high voltage center electrode62.

The firing end assembly24includes a corona igniter42, as shown inFIGS. 1-3, for receiving the energy from the ignition coil assembly22and distributing the radio frequency electric field in the combustion chamber to ignite the mixture of fuel and air. The corona igniter42includes an igniter center electrode44, a metal shell46, and the ceramic insulator32. The ceramic insulator32includes an insulator bore receiving the igniter center electrode44and spacing the igniter center electrode44from the metal shell46.

The igniter center electrode44of the firing end assembly24extends longitudinally along the center axis A from a terminal end48to a firing end50. In the exemplary embodiment, the igniter center electrode44has a thickness in the range of 0.8 mm to 3.0 mm. In the preferred embodiment, an electrical terminal52is disposed on the terminal end48, and a crown54is disposed on the firing end50of the igniter center electrode44. The crown54includes a plurality of branches extending radially outwardly relative to the center axis A for distributing the radio frequency electric field and forming a robust corona discharge.

The ceramic insulator32, also referred to as the firing end insulator32, includes a bore receiving the igniter center electrode44and can be formed of various different ceramic materials which are capable of withstanding the operating conditions in the combustion chamber. In one exemplary embodiment, the ceramic insulator32is formed of alumina. The material used to form the ceramic insulator32also has a high capacitance which drives the power requirements for the corona igniter assembly20and therefore should be kept as small as possible. The ceramic insulator32extends along the center axis A from a ceramic end wall56to a ceramic firing end58adjacent the firing end50of the igniter center electrode44. The ceramic end wall56is typically flat and extends perpendicular to the center axis A, as shown inFIGS. 2-4. In another embodiment, the ceramic insulator32includes a ceramic side wall60having a conical shape and extending to the ceramic end wall56, as shown inFIGS. 13-15. In this embodiment, the igniter center electrode44is wider but is still within the range of 0.8 to 3.0 mm. The metal shell46surrounds the ceramic insulator32, and the crown54is typically disposed outwardly of the ceramic firing end58.

The high voltage center electrode62is received in the bore of the ceramic insulator32and extends to the coil output member36, as shown inFIGS. 2 and 3. The high voltage center electrode62is formed of a conductive metal, such as brass. As shown inFIG. 4, the high voltage center electrode62presents an electrode outer diameter D1extending perpendicular to the center axis A, and which can be constant or vary along the center axis A. In the exemplary embodiment, the electrode outer diameter D1stays constant. Preferably, a brass pack64is disposed in the bore of the ceramic insulator32to electrically connect the high voltage center electrode62and the electrical terminal52. In addition, the high voltage center electrode62is preferably able to float along the bore of the high voltage insulator28. Thus, a spring66or another axially complaint member is disposed between the brass pack64and the high voltage center electrode62. Alternatively, although not shown, the spring66could be located between the high voltage center electrode62and the coil output member36.

In the exemplary embodiment ofFIGS. 2-4, the high voltage insulator28extends between an HV insulator upper wall68coupled to the coil output member36and an HV insulator lower wall70coupled to the dielectric compliant insulator30. The HV insulator lower wall70could alternatively be coupled to the ceramic insulator32. The high voltage insulator28preferably fills the length and volume of the metal tube26located between the ceramic insulator32or the optional dielectric compliant insulator30and the ignition coil assembly22. In the exemplary embodiment shown inFIGS. 2-4, the high voltage insulator28also includes an HV insulator side wall72adjacent the HV insulator end wall74which mirrors the size and shape of the coil output member36.

In the exemplary embodiment ofFIGS. 2-4, the HV insulator lower wall70and the ceramic end wall56are both flat. In the embodiments ofFIGS. 14 and 15, however, the HV insulator lower wall70has a conical shape which mirrors the conical shape of the ceramic end wall56. This conical connection provides a better escape for any air present between the components during the assembly process. However, the flat connection provides for a more even distribution of the forces on the dielectric compliant insulator30and thus provides for a better seal.

The high voltage insulator28is formed of an insulating material which is different from the ceramic insulator32of the firing end assembly24and different from the optional dielectric compliant insulator30. Typically, the high voltage insulator28has a coefficient of thermal expansion (CLTE) which is greater than the coefficient of thermal expansion (CLTE) of the ceramic insulator32. This insulating material has electrical properties which keeps capacitance low and provides good efficiency. Table 1 lists preferred dielectric strength, dielectric constant, and dissipation factor ranges for the high voltage insulator28; and Table 2 lists preferred thermal conductivity and coefficient of thermal expansion (CLTE) ranges for the high voltage insulator28. In the exemplary embodiment, the high voltage insulator28is formed of a fluoropolymer, such as polytetrafluoroethylene (PTFE). The outer surface of the fluoropolymer is chemically etched prior to applying the glue34since no material can stick to the unprocessed fluoropolymer. The high voltage insulator28could alternatively be formed of other materials having electrical properties within the ranges of Table 1 and thermal properties within the ranges of Table 2.

In the exemplary embodiments shown inFIGS. 2-15, the dielectric compliant insulator30is compressed between the high voltage insulator28and the ceramic insulator32. The dielectric compliant insulator30provides an axial compliance which compensates for the differences in coefficients of thermal expansion between the high voltage insulator28and the ceramic insulator32. Preferably, the hardness of the dielectric compliant insulator30ranges from 40 to 80 (shore A). The compression force applied to the dielectric compliant insulator30is set to be within the elastic range of the complaint material. Typically, the dielectric compliant insulator30is formed of rubber or a silicon compound, but could also be formed of silicon paste or injection molded silicon.

In the embodiment shown inFIGS. 2-4, when the HV insulator lower wall70and the ceramic end wall56are both flat, the surfaces of the dielectric compliant insulator30are also flat. In the alternate embodiment shown inFIGS. 14 and 15, the dielectric compliant insulator30conforms to the conical shapes of the HV insulator lower wall70and the ceramic end wall56. The flat dielectric compliant insulator30, however, is thicker and thus provides for improved axial compliance.

In another embodiment, shown inFIGS. 16-20, the corona igniter assembly20is formed without the dielectric compliant insulator30. In yet another embodiment, shown inFIGS. 21-23, the dielectric compliant insulator30is moved toward the ignition coil assembly22. In this embodiment, the dielectric compliant insulator30is sandwiched between the coil output member36and the HV insulator upper wall68, which is a cooler area of the corona igniter assembly20. Moving the dielectric compliant insulator30to this cooler area of the corona igniter assembly20can also improve robustness. In yet another embodiment, the corona igniter assembly20includes the dielectric compliant insulator30in both locations.

The metal tube26of the corona igniter assembly20surrounds the insulators28,30,32and the high voltage center electrode62and couples the ignition coil assembly22to the firing end assembly24. In the exemplary embodiment, the metal tube26extends between a coil end78attached to the ignition coil assembly22and a tube firing end80attached to the metal shell46. The metal tube26typically surrounds and extends along the entire length of the high voltage insulator28and the semi-conductive sleeve76. The metal tube26also surrounds at least a portion of the coil output member36and at least a portion of the high voltage center electrode62. The metal tube26can also surround the optional dielectric compliant insulator30and/or a portion of the ceramic insulator32. As best shown inFIG. 4, the metal tube presents a tube inner diameter D2extending perpendicular to the center axis A, and which can be constant or vary along the center axis A. In the exemplary embodiment, the tube inner diameter D2stays constant between the coil end78and the tube firing end80.

The metal tube26is typically formed of aluminum or an aluminum alloy, but may be formed of other metal materials. The metal tube26can also include at least one exhaust hole82, as shown inFIGS. 24-26, for allowing air and excess glue34to escape from the interior of the metal tube26during the manufacturing process. In addition, the coil end78and/or the tube firing end80of the metal tube26can be tapered.

As stated above, the electric field concentrated at the interface of the different insulators28,30,32and the high voltage center electrode62is high, and typically higher than the voltage required for inception of corona discharge. Thus, the corona igniter assembly20includes the semi-conductive sleeve76surrounding a portion of the high voltage center electrode62to dampen the peak electric field and fill air gaps along the high voltage center electrode62and adjacent insulators28,30,32. The semi-conductive sleeve76preferably extends continuously, uninterrupted, along the interfaces between the different insulators28,30,32. In the exemplary embodiment, the semi-conductive sleeve76extends continuously, uninterrupted, from adjacent the coil output member36to the brass pack64.

As best shown inFIGS. 2-4, the semi-conductive sleeve76is disposed radially between the high voltage center electrode62and the insulators28,30,32and extends axially along an interface between the adjacent insulators28,30,32. If the optional dielectric complaint insulator30is not present, then the semi-conductive sleeve76is only disposed along the interface between the high voltage insulator28and the ceramic insulator32. As shown inFIGS. 3 and 4, the conductive sleeve76extends from an upper sleeve end88to a lower sleeve end90. The upper sleeve end88is located along the high voltage insulator28and is typically close to the coil connector86. The lower sleeve end90is located along the ceramic insulator32and typically rests on the brass pack64.

The semi-conductive sleeve76is formed from a semi-conductive and compliant material, which is different from the other semi-conductive and complaint materials used in the corona igniter assembly20. The complaint nature of the semi-conductive sleeve76allows the semi-conductive sleeve76to fill the air gaps along the high voltage center electrode62and the insulators28,30,32. In the exemplary embodiment, the semi-conductive sleeve76is formed of a semi-conductive rubber material, for example a silicone rubber. The semi-conductive sleeve76includes some conductive material, for example a conductive filler, to achieve the partially conductive properties. In one embodiment, the conductive filler is graphite or a carbon-based material, but other conductive or partially conductive materials could be used. The material used to form the semi-conductive sleeve76can also be referred to as partially conductive, weakly-conductive, or partially resistive. The high voltage and high frequency (HV-HF) nature of the semi-conductive sleeve behaves like a conductor. The resistivity or DC conductivity of the semi-conductive sleeve76can vary from 0.5 Ohm/mm to 100 Ohm/mm, without sensibly changing the behavior of the corona igniter assembly20. In the exemplary embodiment, the semi-conductive sleeve76has a DC conductivity of 1 Ohm/mm. The peak electrical field within the assembly20can be minimized by the conductive nature at high voltage and high frequency (HV-HF) of the semi-conductive sleeve76placed between the high voltage center electrode62and the insulators28,30,32. The semi-conductive sleeve76ensures that all cavities and irregularities within the assembly20at the interfaces are not filled with electrical charge. The stress-relieving function of the semi-conductive sleeve76also prevents the joint from failing.

The semi-conductive sleeve76includes a sleeve outer surface92and a sleeve inner surface94each presenting a cylindrical shape. The high voltage center electrode62and spring66are received along the sleeve inner surface94, and the sleeve outer surface92engages the insulators28,30,32. The semi-conductive sleeve76can be formed of a single piece of material, or multiple pieces which can have the same or different composition. The sleeve outer surface92also presents a sleeve outer diameter D3extending perpendicular to the center axis A. The sleeve outer diameter D3can be constant or vary along the center axis A between the sleeve upper end88and the sleeve lower end90. In the exemplary embodiment, the semi-conductive sleeve76is formed of two pieces of material, wherein an upper piece96is received in a lower piece98, as best shown inFIG. 4. In this embodiment, the sleeve outer diameter D3is greater along the lower piece98than the upper piece96. However, the sleeve inner surface94presents a constant inner diameter along both pieces96,98, which is equal to the electrode outer diameter D1.

The main constraints that control the design of the corona igniter assembly29are the maximum voltage across the insulators28,30,32and the distance between the high voltage center electrode62and the external metal tube26. These parameters are typically fixed by the overall geometry and performance requirements, and thus the ratios between the diameters of the high voltage center electrode D1, the metal tube D2, and the semi-conductive sleeve D3, are tuned to control the distribution of the electrical field within the corona igniter assembly20. The design goal is the keep the electric field peaks as low as possible and generally below the corona inception voltage. There is a range of diameters that allow this goal to be achieved, for example diameters that fall within the ratio limits provided below. However, new geometry constraints or other factors may force the design to adapt different ratios.

In the exemplary embodiment, the following ratios were used to keep the electric field peaks as low as possible and generally below the corona inception voltage:

Table 3 provides examples of the electric field reduction and the interfaces with various different diameter ratios.

As discussed above, the semi-conductive sleeve76relieves stress and stabilizes the electrical field between the different materials disposed radially across the corona igniter assembly20, where more air gaps or changes in geometry leading to increases in electric field typically exist. More specifically, the semi-conductive sleeve76minimizes the peak electric field within the corona igniter assembly20by contrasting the electric charge concentration in any air gaps located along the high voltage center electrode62or ceramic insulator32. The voltage drop through the semi-conductive sleeve76is significant, and thus the voltage peak at the interface between the semi-conductive sleeve76and the adjacent materials is lower than the voltage peak between the high voltage center electrode62and the ceramic insulator32would be without the semi-conductive sleeve76. The semi-conductive sleeve76also relieves any cavities from static electrical charge that could generate unwanted corona discharge.

The semi-conductive sleeve76is typically formed of a compliant material, and thus minimizes the amount or volume of air gaps along the interfaces between the high voltage center electrode62and the ceramic insulator32. In summary, by preventing the unwanted corona discharge, the life of the materials can be extended and the energy can be directed to the corona discharge formed at the firing end50, which in turn improves the performance of the corona igniter assembly20.FIG. 27includes results of a FEA study of the electrical field distribution of the corona igniter assembly20ofFIG. 1with the semi-conductive sleeve76, andFIG. 28includes results of a comparative FEA study of the electrical field distribution of the same corona igniter assembly except without the semi-conductive sleeve76.FIG. 29is a graph illustrating results of a test conducted to compare the electrical field of the semi-conductive sleeve76to the electrical field of a conductive brass material of the same diameter. The test results illustrate that the high voltage and high frequency (HV-HF) nature of the semi-conductive sleeve76behaves like a conductor.

In one embodiment, in addition to the semi-conductive sleeve, a glue34is used to further improve the high voltage seal between the high voltage center electrode62and adjacent insulators28,30,32. The glue34, also referred to as an adhesive sealant, is disposed along interfaces between the insulators28,30,32, as shown inFIGS. 2-8. The glue34helps ensure that the adjacent insulators28,30,32stick together and maintain even contact. The glue34also eliminates air gaps or voids at the interfaces which, if left unfilled, could lead to the formation of the unwanted corona discharge.

In the exemplary embodiment, the glue34is applied to a plurality of interfaces between the ceramic end wall56of the ceramic insulator32and the HV insulator lower wall70of the high voltage insulator28. The glue34functions as an overmaterial and is applied in liquid form so that it flows into all of the crevices and air gaps left between the insulators28,30,32and metal shell46or metal tube26, and/or between the insulators28,30,32and high voltage center electrode62. The glue34is cured during the manufacturing process and thus is solid or semi-solid (non-liquid) to provide some compliance along the interfaces in the finished corona igniter assembly20.

The glue34is formed of an electrically insulating material and thus is able to withstand some corona formation. The glue34is also capable of surviving the ionized ambient generated by the high frequency, high voltage field during use of the corona igniter assembly20in an internal combustion engine. Also, when the glue34is applied between the ceramic insulator32and the high voltage insulator28, it adheres the ceramic insulator32and to the high voltage insulator28. In the exemplary embodiment, the glue34is formed of silicon and has the properties listed in Table 3. However, other materials having properties similar to those of Table 4 could be used to form the glue34.

In the embodiments shown inFIGS. 2-9, the glue34is applied to the HV insulator lower wall70of the high voltage insulator28, the ceramic end wall56of the ceramic insulator32, and all of the surfaces of the dielectric compliant insulator30. Bonding of the HV insulator lower wall70and the ceramic end wall56to the dielectric compliant insulator30is especially important. The glue34could also be applied along other surfaces of the high voltage insulator28and/or other surfaces of the ceramic insulator32. The glue34could further be applied to surfaces of the high voltage center electrode62and/or surfaces of the semi-conductive sleeve76. In this embodiment, the glue34is preferably applied to a thickness in the range of 0.05 millimeters to 4 millimeters.

Alternate embodiments of the corona igniter assembly20are shown inFIGS. 16-23, wherein the corona igniter assembly20does not include the dielectric compliant insulator30; the dielectric compliant insulator30is disposed adjacent the ignition coil assembly22; and/or the glue34is applied as a layer sandwiched between the HV insulator lower wall70and the ceramic end wall56. When the glue34is applied between the HV insulator lower wall70and the ceramic end wall56, the glue34is preferably applied to a greater thickness. For example, the glue34could have a thickness of 1 millimeter to 6 millimeters, or greater.

Another aspect of the invention provides a method of manufacturing the corona igniter assembly20including the ignition coil assembly22, the firing end assembly24, the metal tube26, the insulators28,30,32, the high voltage center electrode62, and the semi-conductive sleeve76. The method first includes preparing the components of the corona igniter assembly20.

When the glue34is used in the corona igniter assembly20, the preparation step includes preparing the surfaces of the insulators28,30,32for application of the glue34. In the exemplary embodiment, each of the insulators28,30,32is prepared by degreasing the surfaces with acetone or alcohol and then drying for approximately 2 hours at 100° C. When the high voltage insulator28is formed of the fluoropolymer, the method can include etching the surfaces of the fluoropolymer so that the glue34will stick. The high voltage insulator28is first machined to its final dimension and then immersed in solution. Once the surface is clean, the surfaces to which the glue34will be applied are etched or hatched for about 1 to 5 minutes, typically 2 minutes. The etched high voltage insulator28is then washed with filtered water and is ready for application of the glue34. Cleanliness and monitoring of the chemical processes is recommended to ensure proper bonding of the surfaces.

When the glue34is used, the method next includes applying the glue34to the surfaces of the ceramic insulator32, the high voltage insulator28, and the semi-conductive sleeve76to be joined. The method can also include applying the glue34to the optional dielectric compliant insulator30. Once the glue34is applied, these components are joined together as shown in the Figures. In the exemplary embodiment shown inFIGS. 2-4, the glue34is applied to the ceramic end wall56, the HV insulator lower wall70, and all of the surfaces of the dielectric compliant insulator30. In another embodiment, the glue34is also applied to the inner surface of the metal tube26, and/or the inner surface of the metal shell46.

The high voltage insulator28, dielectric compliant insulator30, semi-conductive sleeve76, and high voltage center electrode62are typically disposed in the metal tube26, as shown inFIG. 6, before being coupled to the firing end assembly24. The dielectric compliant insulator30is then coupled to the ceramic insulator32of the firing end assembly24via the glue34; and the metal tube26is coupled to the metal shell46of the firing end assembly24via the threaded fastener84. Once assembled, the dielectric compliant insulator30is sandwiched between the ceramic end wall56and the HV insulator lower wall70with the glue34optionally disposed along the interfaces. Preferably, any excess glue34is able to escape through the exhaust holes82in the metal tube26. The semi-conductive sleeve76is also pressed between the corona igniter assembly20and the ignition coil assembly22to fill any air gaps along the insulators28,30,32.

In the embodiments that employ the glue34, the method also includes curing the joined components to increase the bond strength of the glue34. This curing step includes heating the components in a climatic chamber at a temperature of approximately 30° C. and 75% relative humidity for 50 hours. The curing step also includes applying a pressure of 0.01 to 5 N/mm2to the joined components while heating the components in the climatic chamber.

A variety of different techniques can be used to attach the metal tube26to the ignition coil assembly22and the firing end assembly24. In the exemplary embodiment, a separate threaded fastener84attaches the tube firing end80to the metal shell46. The inner surface of the metal tube26presents a tube volume between the coil end78and the tube firing end80which could contain air gaps. However, the semi-conductive sleeve76and glue34can fill those air gaps, especially the air gaps along the interfaces of the insulators28,30,32contained within the tube volume, and thus prevents unwanted corona discharge which could otherwise form in those air gaps during use of the corona igniter assembly20.