Patent ID: 7204220

Claim:
A device for igniting an air-fuel mixture in an internal combustion engine using a high-frequency electrical power source, comprising: a coaxial waveguide structure into which the high-frequency electrical power source is coupled, the coaxial waveguide structure having an inner conductor and outer conductor; wherein the inner conductor projects from one end of the coaxial waveguide structure by a predefined amount and protrudes into an individual combustion chamber of a cylinder of the internal combustion engine, a microwave plasma being generated at the one end of the coaxial waveguide structure by a high voltage potential, and wherein the one end of the coaxial waveguide structure is configured as an igniter such that when a voltage potential is applied, a free-standing plasma is generated in the air-fuel mixture around the inner conductor of the coaxial waveguide structure that projects from the one end of the coaxial waveguide structure, by field lines protruding into the combustion chamber; wherein the coaxial waveguide structure is configured such that, for a predefined effective wavelength of a high-frequency oscillation that is coupled in, a cavity resonator results at least to (predefined effective wavelength)/4; wherein the one end of the coaxial waveguide structure having the inner conductor protruding into the combustion chamber includes a seal made of dielectric material between the outer conductor and the inner conductor, the seal having at least one of an abrupt change and a smooth change in radial diameter along the axial direction, whereby an optimal configuration of the field lines is provided for generating the free-standing plasma.