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. During use of the corona igniter, the electrical field tends to concentrate in those air gaps, leading to unwanted corona discharge. Such corona discharge can cause material degradation and hinder the performance of the corona igniter assembly.
One aspect of the invention provides a corona igniter assembly comprising an ignition coil assembly and a firing end assembly. The firing end assembly includes an igniter central electrode surrounded by a ceramic insulator. A high voltage center electrode is surrounded by a high voltage insulator which is formed of a material different from the ceramic insulator. According one embodiment, a dielectric compliant insulator is disposed between the high voltage insulator and the ceramic insulator of firing end assembly, and/or between the high voltage insulator and the ignition coil assembly. Glue is disposed between at least two of the different insulators to provide a sealed, even contact along the insulator interfaces.
Another aspect of the invention provides a method of manufacturing the corona igniter assembly by joining surfaces of the ceramic insulator, the high voltage insulator, and/or the dielectric compliant insulator with the glue.
The glue eliminates any air gaps or voids along the insulator interfaces which could allow for the formation of unwanted corona discharge when a high voltage and frequency electrical field ionizes the trapped air. Without the glue, such air gaps can be present in the corona igniter assembly due to geometrical tolerances and process constraints, or may develop when compression of the components is voided by the thermal expansion and creep of the different materials used in the corona igniter assembly. Using the glue to prevent unwanted corona discharge in the air gaps extends the life of the materials and allows the energy to be directed to the corona discharge formed at the firing end, which in turn improves the performance of the corona igniter assembly.
Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
A corona igniter assembly 20 for 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 in
The ignition coil assembly 22 includes 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 assembly 22 extends along a center axis A and includes a coil output member 36 for transferring energy toward the firing end assembly 24. In the exemplary embodiment, the coil output member 36 is formed of plastic material. As shown in
The firing end assembly 24 includes a corona igniter 42, as shown in
The igniter center electrode 44 of the firing end assembly 24 extends longitudinally along the center axis A from a terminal end 48 to a firing end 50. In the exemplary embodiment, the igniter center electrode 44 has a thickness in the range of 0.8 mm to 3.0 mm. In the preferred embodiment, an electrical terminal 52 is disposed on the terminal end 48, and a crown 54 is disposed on the firing end 50 of the igniter center electrode 44. The crown 54 includes 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 insulator 32, also referred to as a firing end insulator 32, includes a bore receiving the igniter center electrode 44 and 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 insulator 32 is formed of alumina. The material used to form the ceramic insulator 32 also has a high capacitance which drives the power requirements for the corona igniter assembly 20 and therefore should be kept as small as possible. The ceramic insulator 32 extends along the center axis A from a ceramic end wall 56 to a ceramic firing end 58 adjacent the firing end 50 of the igniter center electrode 44. The ceramic end wall 56 is typically flat and extends perpendicular to the center axis A, as shown in
The corona igniter assembly 20 also includes a high voltage center electrode 62 received in the bore of the ceramic insulator 32 and extending to the coil output member 36, as shown in
In the exemplary embodiment of
In the exemplary embodiment of
The high voltage insulator 28 is formed of an insulating material which is different from the ceramic insulator 32 of the firing end assembly 24 and different from the optional dielectric compliant insulator 30. Typically, the high voltage insulator 28 has a coefficient of thermal expansion (CLTE) which is greater than the coefficient of thermal expansion (CLTE) of the ceramic insulator 32. 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 insulator 28; and Table 2 lists preferred thermal conductivity and coefficient of thermal expansion (CLTE) ranges for the high voltage insulator 28. In the exemplary embodiment, the high voltage insulator 28 is formed of a fluoropolymer, such as polytetrafluoroethylene (PTFE). The outer surface of the fluoropolymer is chemically etched prior to applying the glue 34 since no material can stick to the unprocessed fluoropolymer. The high voltage insulator 28 could 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 in
In the embodiment shown in
In another embodiment, shown in
The electric field concentrated at the interface of the different insulators 28, 30, 32 and the high voltage center electrode 62 is high, and typically higher than the voltage required for inception of corona discharge. Thus, the corona igniter assembly 20 can optionally include a semi-conductive sleeve 76a, 76b surrounding a portion of the high voltage center electrode 62 to dampen the peak electric field and fill air gaps along the high voltage center electrode 62. In the exemplary embodiment, the semi-conductive sleeve 76a, 76b includes an inner 76a sleeve portion and an outer sleeve portion 76b and extends from adjacent the coil output member 36 to the brass pack 64. The semi-conductive sleeve 76a, 76b also preferably extends continuously, uninterrupted, along the interfaces between the different insulators 28, 30, 32. In an example embodiment, the semi-conductive sleeve 76a, 76b is formed of a rubber material with a conductive filler, such as graphite or another carbon-based material. It has been found that the semi-conductive sleeve 76a, 76b behaves like a conductor at high voltage and high frequency (HV-HF). In one embodiment, the semi-conductive sleeve 76a, 76b has a resistivity of 0.5 Ohm/mm to 100 Ohm/mm;
The glue 34 is applied to a plurality of interfaces between the ceramic end wall 56 of the ceramic insulator 32 and the HV insulator lower wall 70 of the high voltage insulator 28. The glue 34 functions as an overmaterial and is applied in liquid form so that it flows into all of the crevices and air gaps left between the insulators 28, 30, 32 and metal shell 46 or metal tube 26, and/or between the insulators 28, 30, 32 and high voltage center electrode 62. The glue 34 is 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 assembly 20.
The glue 34 is formed of an electrically insulating material and thus is able to withstand some corona formation. The glue 34 is also capable of surviving the ionized ambient generated by the high frequency, high voltage field during use of the corona igniter assembly 20 in an internal combustion engine. In the exemplary embodiment, the glue 34 is formed of silicon and has the properties listed in Table 3. However, other materials having properties similar to those of Table 3 could be used to form the glue 34.
In the embodiments shown in
In the embodiments shown in
The metal tube 26 of the corona igniter assembly 20 surrounds the insulators 28, 30, 32 and the high voltage center electrode 62 and couples the ignition coil assembly 22 to the firing end assembly 24. In the exemplary embodiment, the metal tube 26 extends between a coil end 78 attached to the ignition coil assembly 22 and a tube firing end 80 attached to the metal shell 46. The metal tube 26 typically surrounds and extends along the entire length of the high voltage insulator 28. The metal tube 26 also surrounds at least a portion of the coil output member 36 and at least a portion of the high voltage center electrode 62. The metal tube 26 can also surround the optional dielectric compliant insulator 30, the optional semi-conductive sleeve 76a, 76b, and/or a portion of the ceramic insulator 32. The metal tube 26 is typically formed of aluminum or an aluminum alloy, but may be formed of other metal materials. The metal tube 26 can also include at least one exhaust hole 82, as shown in
A variety of different techniques can be used to attach the metal tube 26 to the ignition coil assembly 22 and the firing end assembly 24. In the exemplary embodiment, a separate threaded fastener 84 attaches the tube firing end 80 to the metal shell 46. The inner surface of the metal tube 26 presents a tube volume between the coil end 78 and the tube firing end 80 which could contain air gaps. However, the glue 34 fills those air gaps, especially the air gaps along the interfaces of the insulators 28, 30, 32 contained within the tube volume, and thus prevents unwanted corona discharge which could otherwise form in those air gaps during use of the corona igniter assembly 20.
Another aspect of the invention provides a method of manufacturing the corona igniter assembly 20 including the ignition coil assembly 22, the firing end assembly 24, the metal tube 26, the insulators 28, 30, 32, and the glue 34 filling the air gaps or crevices. The method first includes preparing the components of the corona igniter assembly 20. The preparation step typically includes preparing the surfaces of the insulators 28, 30, 32 for application of the glue 34. In the exemplary embodiment, each of the insulators 28, 30, 32 is degreased with acetone or alcohol and dried for approximately 2 hours at 100° C. When the high voltage insulator 28 is formed of the fluoropolymer, the method includes etching the surfaces of the fluoropolymer so that the glue 34 will stick. The high voltage insulator 28 is first machined to its final dimension and then immersed in solution. Once the surface is clean, the surfaces to which the glue 34 will be applied are etched or hatched for about 1 to 5 minutes, typically 2 minutes. The etched high voltage insulator 28 is then washed with filtered water and is ready for application of the glue 34. Cleanliness and monitoring of the chemical processes is recommended to ensure proper bonding of the surfaces.
The method next includes applying the glue 34 to the surfaces of the ceramic insulator 32 and the high voltage insulator 28 to be joined. The method can also include applying the glue 34 to the optional dielectric compliant insulator 30 and the optional semi-conductive sleeve 76a, 76b. Once the glue 34 is applied, these components are joined together. In the exemplary embodiment shown in
The high voltage insulator 28, dielectric compliant insulator 30, semi-conductive sleeve 76a, 76b, and high voltage center electrode 62 are typically disposed in the metal tube 26, as shown in
The method also includes curing the joined components to increase the bond strength of the glue 34. In the exemplary embodiment, the 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/mm2 to the joined components while heating the components in the climatic chamber.
Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described.
This U.S. Patent Application claims the benefit of U.S. Provisional Patent Application No. 62/138,638, filed Mar. 26, 2015, which is incorporated herein by reference in its entirety.
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