1. Field of the Invention
This invention relates generally to a corona igniter for emitting a radio frequency electric field to ionize a fuel-air mixture and provide a corona discharge, a corona discharge ignition system, and methods of manufacturing the same.
2. Related Art
A corona igniter of a corona discharge ignition system receives a voltage from a power source and emits an electrical field that forms a corona to ionize a mixture of fuel and air of an internal combustion engine. The igniter includes an electrode extending longitudinally form an electrode terminal end to an electrode firing end. An insulator is disposed along the center electrode, and a shell is disposed along the insulator.
The electrode terminal end receives the voltage from the power source and the electrode firing end emits the electrical field that forms the corona. The electrode of the corona igniter may also include a crown at the firing end for emitting the electrical field. The electrical field includes at least one streamer, and typically a plurality of streamers forming the corona. The mixture of air and fuel is ignited along the entire length of the high electrical field generated from the electrode firing end. An example of a corona igniter is disclosed in U.S. Patent Application Publication No. US 2010/0083942 to Lykowski et al.
In an ideal corona ignition system, the corrosion and/or erosion of the metallic parts of the corona igniter in the combustion chamber is low since a corona discharge does not have the high current and high temperatures associated with the discharge of a conventional spark. Although the corona igniter does not include any grounded electrode element in close proximity to the firing tips of the crown, in some applications, there are grounded engine components that come close to the firing tips. Accordingly, it is not always possible to avoid an arc formation, also referred to as arcing, between the corona igniter and grounded component. If an arc forms, the high current and temperatures associated with the arc formation could cause some erosion and/or corrosion damage to the firing tips of the crown. Overtime, the erosion and/or corrosion damage could decrease the quality of corona formation and combustion.
One aspect of the invention provides a corona igniter comprising an electrode extending along a central axis for emitting an electrical field that forms a corona, an insulator formed of an electrically insulating material disposed around the electrode and extending along the central axis to an insulator firing end, and a shell formed of a metal material disposed around the insulator. The electrode includes a central extended member extending longitudinally along the central axis to a central firing end. The electrode also includes a crown disposed outwardly of the insulator firing end. The crown includes at least one branch extending radially outwardly of the central extended member. The crown also extends along the central axis from a top surface to at least one firing tip. The crown presents a crown length between the top surface and the at least one firing tip, and the central extended member presents an extended length extending from the top surface of the crown to the central firing end. The crown length and the extended length are parallel to the central axis. The extended length presented by the central extended member is greater than the crown length presented by the crown.
Another aspect of the invention provides a corona discharge ignition system including the corona igniter with the extended length greater than the crown length. The system includes a cylinder head presenting an opening for receiving the corona igniter, a piston disposed opposite the cylinder head and presenting a space therebetween, and a cylinder block connected to the cylinder head and surrounding the piston. The cylinder head, cylinder block, and piston present a combustion chamber therebetween. The corona igniter is position in the opening of the cylinder head such that the central firing end of the central extended member and the crown are disposed in the combustion chamber.
Yet another aspect of the invention provides a method of manufacturing the corona igniter for use in the corona discharge system including the step of providing the central extended member so that the extended length is greater than the crown length.
The corona igniter including the central extended member with the extended length greater than the crown length provides several advantages over comparative corona igniters without the central extended member. When a grounded component, such as the piston, comes close to the central firing end of the central extended member and the firing tips of the crown, if any arc forms it will preferentially form between the piston and central firing end of the central extended member due to the extended length of the central extended member, its proximity to the grounded component, and hence its higher field strength compared to the firing tips of the crown. Therefore, if arcing does occur, corrosion and erosion damage to the firing tips of the crown is reduced.
Furthermore, in situations where the grounded components are far from the corona igniter, the central extended member tends to repel the corona streamers as they form, thereby providing a wider volume of corona discharge and reducing the tendency of the corona discharge to approach the piston and form an arc.
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:
Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, a corona igniter 20 including a central extended member 22 which is capable of providing improved corona discharge 24 and improved combustion performance is generally shown.
As shown in
The crown 34 of the electrode is disposed outwardly of the insulator firing end 30. The crown 34 surrounds the central axis A and the central extended member 22. The crown 34 of the electrode also includes at least one branch 36 extending radially outwardly of the central extended member 22, but typically includes a plurality of branches 36 each extending radially outwardly from the central axis A and radially outwardly of the central extended member 22. In an exemplary embodiment, the crown 34 includes four branches 36 spaced an equal distance from one another around the central axis A, as shown in
Also shown in
Each branch 36 of the crown 34 also presents at least one first spherical radius r1 located at or adjacent to the associated firing tip 38.
The crown 34 can be formed of various different metal materials. In one exemplary embodiment, the crown 34 is formed of nickel, nickel alloy, or a precious metal, such as platinum or iridium. Due to the central extended member 22 of the electrode, the material of the crown 34 can be formed of a less wear resistant material and experiences less corrosion and erosion if arcing occurs during operation of the corona igniter 20.
The central extended member 22 of the electrode extends longitudinally along the central axis A to a central firing end 46. The central extended member 22 presents an extended length le extending from the top surface 40 of the crown 34 to the central firing end 46, as best shown in
The central extended member 22 presents at least one second spherical radius r2 located at or adjacent to the central firing end 46.
Also shown in
Various different materials can be used to form the central extended member 22, such as nickel, copper, precious metals, or alloys thereof. Portions of the central extended member 22 can also be formed of an insulating material. The central extended member 22 is typically formed of a first material and the crown 34 is typically formed of a second material different from the first material. The first material used to form the central extended member 22 is typically more resistant to erosion and corrosion than the second material used to form the crown 34, since the central extended member 22 is more likely to be in contact with high current and temperature of the arc, if arcing does occur. Alternatively, the second material used to form the crown 34 can be more resistant to erosion and corrosion than the first material used to form the central extended member 22.
The central extended member 22 is oftentimes formed of a plurality of separate pieces joined together, such as a body portion 52 and a wear element 54, as shown in
In each embodiment, the wear element 54 presents the central firing end 46. Thus, the wear element 54 is typically formed of a material having good thermal characteristics and being more resistant to wear than the material of the body portion 52. In one embodiment, the wear element 54 is formed of a nickel-based alloy, a noble metal, or a precious metal, such as platinum, tungsten, or iridium. In another embodiment, the wear element 54 is formed of an electrically insulating material preferably having a relative permittivity of greater than 2, and more preferably greater than 8, for example an alumina-based material. The wear element 54 can also comprise a coating of metal material or a coating of electrically insulating material.
The wear element 54 may be applied to the body portion 52 of the central extended member 22 by any suitable means, for example PVD, co-extrusion, or co-sintering. Alternatively, the wear element 54 may be attached by brazing or a similar process. When the wear element 54 is a coating, the coating can be applied by plating, spraying, sintering, or another suitable method. The material of the body portion 52 and the material of the wear element 54 should be selected and joined to provide good bonding, no small gaps, good thermal contact, and to avoid problems with differential thermal expansion, for example.
In the embodiment of
Another aspect of the invention provides a corona discharge ignition system 60 including the corona igniter 20 with the central extended member 22 to reduce corrosion and erosion at the firing tips 38, as shown in
The cylinder head 62 presents an opening 68 for receiving the corona igniter 20. The shell 32 of the corona igniter 20 is typically coupled to the cylinder head 62, for example threaded into the opening 68 of the cylinder head 62, as shown in
During operation, power is supplied to the corona igniter 20, the fuel is sprayed toward the corona igniter 20, and the piston 50 reciprocates with the cylinder block 64, moving towards and away from the cylinder head 62 and the corona igniter 20, as in a conventional corona ignition system. In
In
As mentioned above, the electrode of the corona igniter 20 of the present invention can also increase the size of the corona discharge 24 during operation.
Another aspect of the invention provides a method of manufacturing the corona igniter 20 for use in the corona discharge ignition system 60, which includes providing the central extended member 22 so that the extended length le of the central extended member 22 is greater than the crown length lc.
Various techniques can be used to determine the appropriate extended length le of the central extended member 22 in order to provide the preferred performance. In one embodiment, the method first includes (a) identifying the firing tip 38 of the crown 34 which will be closest to the cylinder block 64 when the corona igniter 20 is received in the cylinder head 62. Next, the method includes (b) determining a point during movement of the piston 50 where a distance from the firing tip 38 identified in step (a) to the cylinder block 64 is equal to a distance from the firing tip 38 identified in step (a) to the piston 50. When the piston 50 is located at this point, or closer to the firing tips 38, there is a possibility of arcing between the firing tips 38 and piston 50, but this possibility is mitigated by the central extended member 22.
The method next includes (c) selecting the extended length le of the central extended member 22 such that when power is provided to the electrode and when the firing tip 38 identified in step (a) is at the point identified in step (b), the peak electric field at the central firing end 46 of the central extended member 22 is equal to or greater than the peak electric field at the firing tip 38 identified in step (a). The peak electric field at the central firing end 46 of the central extended member 22 depends on the distance between the central firing end 46 and the piston 50, and the distance between the central firing end 46 and the cylinder block 64. The method can also include adjusting the extended length le of the central extended member 22 to space the central firing end 46 of the central extended member 22 farther from the cylinder block 64 and/or the piston 50 during operation.
The method also typically includes step (d): selecting the first spherical radii r1 of the firing tips 38 and the second spherical radii r2 of the central firing end 46 such that during operation, corona discharge will preferentially form from the firing tips 38, and arcing, if any occurs, will preferentially form between the piston 50 and the central firing end 46 of the central extended member 22. The step of selecting the spherical radii r1, r2 can be conducted before or after selecting the extended length le. The step of selecting the spherical radii r1, r2 includes selecting the first spherical radii r1 for each of the firing tips 38 of the crown 34 and selecting the second spherical radii r2 for the central firing end 46 of the central extended member 22 such that each of the first spherical radii r1 at the firing tips 38 of the crown 34 are smaller than the second spherical radii r2 of the central extended member 22.
The spherical radii r1, r2 are preferably selected so that when power is provided to the electrode, and the at least one firing tip 38 of the crown 34 and the central firing end 46 of the central extended member 22 are spaced from the cylinder block 64 and the piston 50, and a corona discharge 24 is provided from the firing tips 38, the peak electric field at the firing tip 38 closest to ground is at least 25% higher than the peak electric field at the central firing end 46 of the central extended member 22. This may be achieved, for example, by using data of the form shown in
Once the distance is identified in step (b), and the spherical radii r1, r2 are selected in step (d), the method typically includes (e) determining the peak electric field of the firing tip 38 identified in step (a) at the distance identified in step (b). As an example again, the data of
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 appended claims.
This application claims the benefit of U.S. Provisional Patent Application No. 61/799,117, filed Mar. 15, 2013, the entire contents of which is incorporated herein by reference in its entirety.
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