Patent Grant
9041273
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, and a method of forming the igniter.
2. Description of the Prior Art
Corona discharge ignition systems include an igniter with a central 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 a 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. Preferably, the electric field is controlled so that the fuel-air mixture does not lose all dielectric properties, which would create a thermal plasma and an electric arc between the electrode and grounded cylinder walls, piston, or other portion of the igniter. An example of a corona discharge ignition system is disclosed in U.S. Pat. No. 6,883,507 to Freen.
The corona igniter typically includes the central electrode formed of an electrically conductive material for receiving the high radio frequency voltage and emitting the radio frequency electric field to ionize the fuel-air mixture and provide the corona discharge. The igniter also includes a shell formed of a metal material receiving the central electrode and extending longitudinally from an upper shell end to a lower shell end. An insulator formed of an electrically insulating material is disposed in the shell and surrounds the central electrode. The igniter of the corona discharge ignition system does not include any grounded electrode element intentionally placed in close proximity to a firing end of the central electrode. Rather, the ground is preferably provided by cylinder walls or a piston of the ignition system. An example of a corona igniter is disclosed in U.S. Patent Application Publication No. 2010/0083942 to Lykowski and Hampton.
During operation of the corona igniter, when the central electrode is at a maximum possible positive voltage, such as a 100% voltage, and the shell is grounded at the lowest possible voltage, such as a 0% voltage, an ionized gas is formed in a gap between the insulator and the shell. Under certain conditions, a very high electric field strength exists in the gap. Negative ions of the ionized gas typically follow a voltage potential gradient and electric field over the surface of the insulator to the central electrode, forming a conductive path from the shell to the central electrode. The ionized gas is also formed in a gap between the central electrode and insulator, and an identical situation exists, except with the charges, voltages, and currents reversed. The conductive path between the central electrode and shell can create undesirable power-arcing and deplete the remaining corona discharge, which can degrade the quality of ignition.
One aspect of the invention provides a corona igniter for emitting a radio frequency electric field to ionize a fuel-air mixture and provide a corona discharge. The corona igniter comprises a central electrode formed of an electrically conductive material for receiving the high radio frequency voltage and emitting the radio frequency electric field to ionize the fuel-air mixture and provide the corona discharge. A shell formed of a metal material extends along the central electrode and longitudinally from an upper shell end to a lower shell end. An insulator formed of an electrically insulating material is disposed between the central electrode and the shell. The insulator includes an insulator outer surface facing away from the central electrode and extending longitudinally from an insulator upper end to an insulator nose end. The insulator outer surface presents an abruption extending radially outward relative to the central electrode.
Another aspect of the invention provides a method of forming a corona igniter. The method includes the step of providing an insulator formed of an electrically insulating material, which includes an insulator inner surface presenting an insulator bore and an oppositely facing insulator outer surface, each extending longitudinally from an insulator upper end to an insulator nose end. The insulator is also provided to include an insulator nose region adjacent the insulator nose end, and the insulator outer surface of the insulator nose region presents an abruption extending radially outward relative to the insulator bore. The method next includes disposing a central electrode formed of an electrically conductive material in the insulator bore. The method further includes providing a shell formed of a metal material and including an inner shell surface presenting a shell bore extending longitudinally form a lower shell end to an upper shell end, and disposing the insulator in the shell bore.
During operation of the corona igniter of the present invention, an ionized gas with a high electric field strength is formed in a gap between the insulator and the shell, and the negative ions may begin to travel along the insulator. However, before the negative ions reach the central electrode, the abruption reverses the electric field and voltage potential gradient along the insulator outer surface and repels the negative ions. The negative ions do not travel to an area along the insulator having a decreasing voltage, which would be along the abruption and past the abruption. Rather, the repelled negative ions may combine with positive ions in the air surrounding the insulator. Thus, the abruption prevents the negative ions from reaching the central electrode and forming a conductive path from the shell to the central electrode, which typically creates undesirable power-arcing and depletes the corona discharge being emitted from the electrode into the combustion chamber. The abruption also creates a blockage of the electrical path along the insulator outer surface between the shell and the central electrode. The abruption may also prevent power-arcing by repelling positive ions traveling along the insulator from the central electrode to the shell, in the same manner as the negative ions. The abruption of the insulator preserves a robust corona discharge and provides a higher quality ignition, compared to igniters without the abruption.
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:
One aspect of the invention provides a corona igniter 20 for a corona discharge ignition system. The igniter 20 includes a central electrode 22 for receiving a high radio frequency voltage and emitting a radio frequency electric field to ionize a portion of a fuel-air mixture and provide a corona discharge 24 in a combustion chamber 26 of an internal combustion engine. The corona igniter 20 includes an insulator 28 receiving the central electrode 22 and surrounded by a metal shell 30. The insulator 28 includes an insulator outer surface 32 presenting an abruption 34 extending radially outward relative to the central electrode 22. The abruption 34 is an increase in a local thickness t of the insulator 28 in a direction moving from the shell 30 toward an insulator nose end 54, which is typically provided by a notch or a protrusion. The abruption 34 repels positive and negative ions away from the insulator 28, between the shell 30 and the central electrode 22. The abruption 34 also creates a blockage of the electrical path along the insulator outer surface 32 between the shell 30 and the central electrode 22 to sustain the corona discharge 24 and prevent power-arcing between the shell 30 and the central electrode 22.
In one embodiment, as shown in
The central electrode 22 of the corona igniter 20 has an electrode center axis ae extending longitudinally from an electrode terminal end 42 for receiving the high radio frequency voltage to an electrode firing end 44. The central electrode 22 includes an electrode body portion 46 formed of a first electrically conductive material, such as nickel or nickel alloy, extending longitudinally from the electrode terminal end 42 along the electrode center axis ae to the electrode firing end 44. During operation of the igniter 20 when the central electrode 22 receives the high radio frequency voltage, the central electrode 22 has a high voltage, typically 1,000 to 100,000 volts.
As shown in
The insulator 28 of the corona igniter 20 is disposed annularly around and longitudinally along the electrode body portion 46 and extends from an insulator upper end 52 to an insulator nose end 54. The insulator nose end 54 is adjacent the electrode firing end 44 and abuts the firing tip 50. The insulator 28 includes an insulator inner surface 56 presenting an insulator bore extending longitudinally along the electrode center axis ae from the insulator upper end 52 to the insulator nose end 54. The insulator inner surface 56 faces the central electrode 22 and the insulator bore receives the central electrode 22. As shown in
The insulator 28 includes a matrix 62 of electrically insulating material extending continuously from the insulator inner surface 56 to the insulator outer surface 32. The electrically insulating material has a relative permittivity greater than the relative permittivity of air, in other words greater than 1. In one embodiment, the electrically insulating material is alumina and has a relative permittivity of about 9. In another embodiment, the electrically insulating material is boron nitride and has a relative permittivity of about 3.5. In yet another embodiment, the insulating material is silicon nitride and has a relative permittivity of about 6.0
As shown in
The insulator 28 also includes an insulator second region 70 adjacent the insulator middle region 66 extending toward the insulator nose end 54. The insulator 28 includes an insulator lower shoulder 72 extending radially inwardly from the insulator middle region 66 to the insulator second region 70. The insulator second region 70 presents an insulator second diameter D2 extending generally perpendicular to the longitudinal electrode body portion 46, which is typically equal to the insulator first diameter D1 and less than the insulator middle diameter Dm.
The insulator 28 includes an insulator nose region 74 extending from the insulator second region 70 to the insulator nose end 54. The insulator nose region 74 presents an insulator nose diameter Dn extending generally perpendicular to the longitudinal electrode body portion 46 and tapering to the insulator nose end 54. As shown in
The insulator outer surface 32 of the insulator nose region 74 presents the abruption 34, which prevents the undesirable arc discharge and sustains a robust corona discharge 24. The abruption 34 extends radially outwardly away from the central electrode 22 and is an increase in the local thickness t of the insulator 28 in a direction moving from the shell 30 toward the insulator nose end 54. The local thickness t of the insulator 28 is equal to the distance between the insulator inner surface 56 and the insulator outer surface 32 at one point along the insulator 28. The abruption 34 is typically provided by a flank 82, face, or surface facing toward the shell 30. As shown in
The abruption 34 is provided by an increase in the local thickness t of the insulator, which typically is an increase in the insulator nose diameter Dn over the nose length el of the insulator 28 in a direction moving from the shell 30 toward an insulator nose end 54. In one embodiment, the abruption 34 is provided by an increase of at least 15% in the insulator local thickness t, wherein the increase occurs over less than 25% of the nose length el. An example of the increase in local thickness t of the insulator 28 is shown in
The abruption 34 may be provided by one face or flank 82 of a notch, as shown in
In another embodiment, the abruption 34 is provided by one face or flank 82 of a protrusion extending radially outwardly away from the central electrode 22 and into the combustion chamber 26, as shown in
The abruption 34 can comprise a various designs, for example the designs shown in
In other embodiments, the insulator outer surface 32 includes a sharp edge 80 providing the abruption 34. For example, the sharp edge 80 can be adjacent the abruption 34, along the abruption 34, or between the abruption 34 and the adjacent areas of the insulator outer surface 32. In the embodiments of
In one embodiment, the abruption 34 is the flank 82 along the insulator outer surface 32. The flank 82 faces generally toward the lower shell end 76 and is an increase of at least 15% in the local thickness t of the insulator 28 over less than 25% of the nose length el. The flank 82 presents a flank angle α that is preferably greater than a line of equipotential at the flank 82. Examples of the flank 82 presenting the flank angle α are shown in
In one embodiment, the abruption 34 is disposed closer to the shell 30 than the insulator nose end 54. In another embodiment, the abruption 34 is disposed closer to the insulator nose end 54 than the shell 30. In yet another embodiment, the abruption 34 is spaced equally from the shell 30 and the insulator nose end 54. The insulator nose region 74 typically decreases gradually from the abruption 34 to the insulator nose end 54.
In one embodiment, the insulator nose diameter Dn including the abruption 34 is less than a shell bore diameter Ds of the shell 30. This allows the igniter 20 to be formed by inserting the insulator nose end 54 through the shell 30, and then clamping the shell 30 about the insulator shoulders 68, 72. In another embodiment, the insulator nose diameter Dn including the abruption 34 is greater than or equal to the shell bore diameter Ds, and the igniter 20 can be formed by inserting the insulator upper end 52 through the shell bore diameter D.
As shown in
As shown in
The shell 30 is formed of a metal material, such as steel. The shell 30 extends longitudinally along the insulator 28 from an upper shell end 100 to a lower shell end 76. The lower shell end 76 is disposed at a border of the insulator second region 70 and the insulator nose region 74, such that the insulator nose region 74 projects outwardly of the lower shell end 76. The inner shell surface 92 faces the insulator 28 and presents a shell bore extending longitudinally along the electrode center axis ae from the upper shell end 100 to the lower shell end 76 for receiving the insulator 28. The shell bore presents a shell bore diameter Ds extending generally perpendicular to the longitudinal electrode body portion 46. In one preferred embodiment, the shell bore diameter Ds is greater than the insulator nose diameter Dn, as shown in
During operation of the igniter 20 in the internal combustion engine application, the high radio frequency voltage is provided to the central electrode 22, so that the central electrode 22 has a first voltage, typically 100 to 100,000 volts. The metal shell 30 is grounded and has a second voltage less than the first voltage, typically 0 volts. Thus, the shell gap 104 is filled with an ionized gas, including ions having positive and negative electric charges. The electrode gap 60 is also filled with the ionized gas during operation. Thus, an electric field and a voltage potential gradient forms along the insulator outer surface 32 and through the matrix 62 to the central electrode 22.
During operation, for example during a moment in the electric cycle where the central electrode 22 is at a maximum possible positive voltage, such as a 100% voltage, and the shell 30 is grounded at the lowest possible voltage, such as a 0% voltage, the positive ions in the shell gap 104 can pass easily to the grounded shell 30. A portion of the negative ions of the shell gap 104 may combine with positive ions of the surrounding air of the combustion chamber 26. However, another portion of the negative ions in the shell gap 104 follow the voltage potential gradient over the insulator outer surface 32 toward the electrode firing end 44 of the central electrode 22. Before the negative ions reach the central electrode 22, the abruption 34 repels the negative ions away from the insulator 28 and allows them to combine with positive ions in the air surrounding the insulator 28. The negative ions do not travel to an area along the insulator nose region 74 having a reducing voltage, which would be along the abruption 34 and past the abruption 34. Thus, the abruption 34 prevents the negative ions from reaching the central electrode 22 and forming a conductive path from the shell 30 to the central electrode 22, which typically creates undesirable power-arcing and depletes the corona discharge 24 at the electrode firing end 44. The abruption 34 of the insulator 28 preserves a robust corona discharge 24 and provides a higher quality ignition compared to igniters without the abruption 34.
The voltage of the insulator 28 presents a voltage potential gradient aligned in the first direction over the insulator outer surface 32 longitudinally from adjacent the lower shell end 76 toward the insulator nose end 54, until reaching the abruption 34. The abruption 34 reverses the voltage potential gradient. The voltage potential gradient is aligned in a second direction, reverse of the first direction, at the abruption 34.
While the high radio frequency voltage is provided to the central electrode 22, the insulator 28 also has an electric field. The electric field is aligned in a first direction radially from the insulator outer surface 32 through the matrix 62 and toward the central electrode 22, and longitudinally over the insulator outer surface 32 from adjacent the lower shell end 76 toward the insulator nose end 54. When the electric field of the insulator outer surface 32 reaches the abruption 34, the abruption 34 reverses the electric field. The electric field then becomes aligned in a second direction, reverse of the first direction, at the abruption 34.
Likewise, the positive ions in the electrode gap 60 follow the voltage potential gradient over the insulator outer surface 32 and through the matrix 62 toward the shell 30, with the charges, voltages, and currents reversed. The abruption 34 also repels the positive ions away from the insulator 28 and allows them to combine with negative ions in the air surrounding the insulator 28. The positive ions do not travel to an area along the insulator nose region 74 having a higher voltage, which would be along the abruption 34 and past the abruption 34. The abruption 34 prevents the positive ions from reaching the shell 30 and forming a conductive path from the central electrode 22 to the shell 30, which typically creates undesirable power-arcing and depletes the corona discharge 24 at the electrode firing end 44. Thus, the abruption 34 of the insulator 28 preserves a robust corona discharge 24 and provides a higher quality ignition compared to igniters without the abruption 34.
For comparison,
Unlike the present invention, at least a portion of the negative ions of the shell gap follow the voltage potential gradient and electric field over the insulator outer surface and reach the central electrode. The negative ions form a conductive path from the shell to the central electrode and create undesirable power-arcing and deplete the corona discharge at the electrode firing end. Therefore, the insulator of the prior art does not preserve a robust corona discharge and provide a quality ignition to the extent provided by the subject invention.
Another aspect of the invention provides a method of forming the corona igniter 20. The method includes providing the insulator 28 formed of the electrically insulating material. The insulator 28 includes the insulator inner surface 56 presenting the insulator bore and the oppositely facing insulator outer surface 32 each extending longitudinally from the insulator upper end 52 to the insulator nose end 54. The method also includes providing the abruption 34 extending radially relative to the insulator bore in the insulator nose region 74, or forming the abruption 34 along the insulator nose region 74.
The method also includes providing the central electrode 22 formed of the electrically conductive material and the shell 30 formed of the metal material and including the inner shell surface 92 presenting the shell bore extending longitudinally from the lower shell end 76 to the upper shell end 100.
The method next includes disposing the central electrode 22 formed of the electrically conductive material in the insulator bore along the insulator inner surface 56. Next, the insulator 28 is disposed in the shell bore. In one embodiment, the step of disposing the insulator 28 in the shell bore includes inserting the insulator 28 through the shell bore at the upper shell end 100 and sliding the insulator 28 through the shell bore until the insulator nose region 74 passes by the lower shell end 76 and is disposed outwardly of the lower shell end 76. The method next includes forming the shell 30 about the insulator shoulders 68, 72 after disposing the insulator 28 in the shell bore. The forming step typically includes deforming and clamping the upper shell end 100 about the insulator upper should 68, so that the shell 30 rests on the insulator upper shoulder 68, as shown in
In another embodiment, the step of disposing the insulator 28 in the shell bore includes inserting the insulator 28 through the shell bore at the lower shell end 76 and sliding the insulator 28 through the shell bore. Alternatively, other methods can be used to form the igniter 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 appended claims. In addition, the reference numerals in the claims are merely for convenience and are not to be read in any way as limiting.
This application claims the benefit of application U.S. Provisional Application Ser. No. 61/422,833, filed Dec. 14, 2010.
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