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.
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.
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 the 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 high voltage center electrode 62 is received in the bore of the ceramic insulator 32 and extends 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 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 and the semi-conductive sleeve 76. 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 and/or a portion of the ceramic insulator 32. As best shown in
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
As stated above, 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 includes the semi-conductive sleeve 76 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 and adjacent insulators 28, 30, 32. The semi-conductive sleeve 76 preferably extends continuously, uninterrupted, along the interfaces between the different insulators 28, 30, 32. In the exemplary embodiment, the semi-conductive sleeve 76 extends continuously, uninterrupted, from adjacent the coil output member 36 to the brass pack 64.
As best shown in
The semi-conductive sleeve 76 is formed from a semi-conductive and compliant material, which is different from the other semi-conductive and complaint materials used in the corona igniter assembly 20. The complaint nature of the semi-conductive sleeve 76 allows the semi-conductive sleeve 76 to fill the air gaps along the high voltage center electrode 62 and the insulators 28, 30, 32. In the exemplary embodiment, the semi-conductive sleeve 76 is formed of a semi-conductive rubber material, for example a silicone rubber. The semi-conductive sleeve 76 includes 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 sleeve 76 can 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 sleeve 76 can vary from 0.5 Ohm/mm to 100 Ohm/mm, without sensibly changing the behavior of the corona igniter assembly 20. In the exemplary embodiment, the semi-conductive sleeve 76 has a DC conductivity of 1 Ohm/mm. The peak electrical field within the assembly 20 can be minimized by the conductive nature at high voltage and high frequency (HV-HF) of the semi-conductive sleeve 76 placed between the high voltage center electrode 62 and the insulators 28, 30, 32. The semi-conductive sleeve 76 ensures that all cavities and irregularities within the assembly 20 at the interfaces are not filled with electrical charge. The stress-relieving function of the semi-conductive sleeve 76 also prevents the joint from failing.
The semi-conductive sleeve 76 includes a sleeve outer surface 92 and a sleeve inner surface 94 each presenting a cylindrical shape. The high voltage center electrode 62 and spring 66 are received along the sleeve inner surface 94, and the sleeve outer surface 92 engages the insulators 28, 30, 32. The semi-conductive sleeve 76 can be formed of a single piece of material, or multiple pieces which can have the same or different composition. The sleeve outer surface 92 also presents a sleeve outer diameter D3 extending perpendicular to the center axis A. The sleeve outer diameter D3 can be constant or vary along the center axis A between the sleeve upper end 88 and the sleeve lower end 90. In the exemplary embodiment, the semi-conductive sleeve 76 is formed of two pieces of material, wherein an upper piece 96 is received in a lower piece 98, as best shown in
The main constraints that control the design of the corona igniter assembly 29 are the maximum voltage across the insulators 28, 30, 32 and the distance between the high voltage center electrode 62 and the external metal tube 26. 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 assembly 20. 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.
D1:D2=0.036-0.215
D3:D2=0.107-0.357
D1:D3:=0.1-2.0
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:
D1:D2=0.071
D3 (upper piece):D2=0.180
D3 (lower piece):D2=0.286
D1:D3 (upper piece):=0.400
D1:D3 (lower piece):=0.250
Table 3 provides examples of the electric field reduction and the interfaces with various different diameter ratios.
As discussed above, the semi-conductive sleeve 76 relieves stress and stabilizes the electrical field between the different materials disposed radially across the corona igniter assembly 20, where more air gaps or changes in geometry leading to increases in electric field typically exist. More specifically, the semi-conductive sleeve 76 minimizes the peak electric field within the corona igniter assembly 20 by contrasting the electric charge concentration in any air gaps located along the high voltage center electrode 62 or ceramic insulator 32. The voltage drop through the semi-conductive sleeve 76 is significant, and thus the voltage peak at the interface between the semi-conductive sleeve 76 and the adjacent materials is lower than the voltage peak between the high voltage center electrode 62 and the ceramic insulator 32 would be without the semi-conductive sleeve 76. The semi-conductive sleeve 76 also relieves any cavities from static electrical charge that could generate unwanted corona discharge.
The semi-conductive sleeve 76 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 62 and the ceramic insulator 32. 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 50, which in turn improves the performance of the corona igniter assembly 20.
In one embodiment, in addition to the semi-conductive sleeve, a glue 34 is used to further improve the high voltage seal between the high voltage center electrode 62 and adjacent insulators 28, 30, 32. The glue 34, also referred to as an adhesive sealant, is disposed along interfaces between the insulators 28, 30, 32, as shown in
In the exemplary embodiment, 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. Also, when the glue 34 is applied between the ceramic insulator 32 and the high voltage insulator 28, it adheres the ceramic insulator 32 and to the high voltage insulator 28. 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 4 could be used to form the glue 34.
In the embodiments shown in
Alternate embodiments of the corona igniter assembly 20 are shown in
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, the high voltage center electrode 62, and the semi-conductive sleeve 76. The method first includes preparing the components of the corona igniter assembly 20.
When the glue 34 is used in the corona igniter assembly 20, the preparation step 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 prepared by degreasing the surfaces with acetone or alcohol and then drying for approximately 2 hours at 100° C. When the high voltage insulator 28 is formed of the fluoropolymer, the method can include 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.
When the glue 34 is used, the method next includes applying the glue 34 to the surfaces of the ceramic insulator 32, the high voltage insulator 28, and the semi-conductive sleeve 76 to be joined. The method can also include applying the glue 34 to the optional dielectric compliant insulator 30. Once the glue 34 is applied, these components are joined together as shown in the Figures. In the exemplary embodiment shown in
The high voltage insulator 28, dielectric compliant insulator 30, semi-conductive sleeve 76, and high voltage center electrode 62 are typically disposed in the metal tube 26, as shown in
In the embodiments that employ the glue 34, the method also includes curing the joined components to increase the bond strength of the glue 34. 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/mm2 to the joined components while heating the components in the climatic chamber.
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 semi-conductive sleeve 76 and glue 34 can fill 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.
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 while within the scope of the following claims.
This U.S. patent application claims the benefit of U.S. Provisional Patent Application No. 62/138,642, filed Mar. 26, 2015, which is incorporated herein by reference in its entirety.
Number | Date | Country | |
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62138642 | Mar 2015 | US |