Ignition system

An ignition system for an internal combustion engine comprising:

• a substantially shaft-like center electrode that has a length with a base end and a tip end in both ends thereof;
• a substantially bottomed cylindrical center dielectric disposed coaxially with the center electrode;
• a substantially cylindrical ground electrode disposed coaxially with the center dielectric with a discharge gap having a predetermined distance therebetween;
• a power source that applies a high voltage between the center electrode and the ground electrode;
• a discharge portion of the center electrode in which a part thereof is surrounded by a bottom portion of the center dielectric;
• a tubular tip portion of the center dielectric;
• the part of the discharge portion and the tubular tip portion are projected into a combustion chamber of the internal combustion engine from a substantially annular tip portion of the ground electrode that opens to the combustion chamber at a distal end of the ground electrode;
• a diameter-changing portion formed by reducing a diameter of a part of the tubular tip portion in a radial direction gradually as approaching toward a tip thereof;
• and
• a thin-walled portion formed by reducing a thickness of the tip of the tubular tip portion.