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 corona igniter.
2. Related Art
Corona discharge ignition systems provide an alternating voltage and current, reversing high and low potential electrodes in rapid succession which makes arc formation difficult and enhances the formation of corona discharge. The system includes a corona igniter with a central electrode charged to a high radio frequency voltage potential and 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 controlled so that the fuel-air mixture maintains dielectric properties and corona discharge occurs at the electrode firing end, 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 concentrated at the electrode firing end and 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 central electrode of the corona igniter is formed of an electrically conductive material and receives the high radio frequency voltage and emits the radio frequency electric field into the combustion chamber to ionize the fuel-air mixture and provide the corona discharge. An insulator formed of an electrically insulating material surrounds the central electrode and is received in a metal shell. 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 use of the corona igniter, when energy is supplied to the central electrode, the electrical potential and the voltage can drop significantly between the central electrode and the metal shell due to the low relative permittivity of air between those components. The high voltage drop and a corresponding spike in electric field strength tends to ionize the air between the central electrode and the shell, leading to significant energy loss at the electrode firing end. In addition, the ionized air adjacent the shell is prone to migrating toward the electrode firing end, or vice versa, forming a conductive path across the insulator between the central electrode and the shell, and reducing the effectiveness of the corona discharge at the electrode firing end. The conductive path between the central electrode and shell may lead to arc discharge between those components, which is oftentimes undesired and reduces the quality of ignition at the electrode firing end.