Patent ID: 7508121

Claim:
A spark plug for a spark-ignited combustion event, said spark plug comprising: an elongated ceramic insulator having an upper terminal end, a lower nose end, and a central passage extending longitudinally between said terminal and nose ends; said insulator including an exterior surface presenting a generally circular large shoulder proximate said terminal end and a generally circular small shoulder proximate said nose end, said large shoulder having a diameter greater than the diameter of said small shoulder; a conductive shell surrounding at least a portion of said insulator, said shell including at least one ground electrode; a conductive center electrode disposed in said central passage and having an exposed sparking tip proximate said ground electrode; said insulator having a nose region extending from said nose end, said nose region having a maximum outer diameter d(base) measured adjacent said small shoulder and a minimum outer diameter d(tip) measured adjacent said sparking tip of said center electrode; and said shell including an inner bore diameter ID (shell) surrounding said nose region of said insulator, and wherein a spatial relationship is established according to the formula: 0.05 ≤ ( d ( base ) + d ⁢ ⁢ ( tip ) 2 ) ÷ ID ( shell ) ≤ 0.7 said insulator including a rounded transition and a filleted transition, said rounded transition spaced from said filleted transition by a transition length (L), both said rounded and filleted transitions located longitudinally between the disparate diameters of said large and small shoulders, said rounded transition having a major diameter D 2 and said filleted transition having a minor diameter D 1 , and wherein a spatial relationship is established according to the formula: 0.5 ≤ ( D ⁢ ⁢ 2 - D ⁢ ⁢ 1 ) L ( transition ) ≤ 3.5 .