Patent ID: 7573185

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, and further including a rounded transition and space there from by a transition length L(transition) a filleted transition, both said rounded and filleted transitions located longitudinally between the disparate diameters of said large and small shoulders; 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; and and 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 .