Spark plug ensuring enhanced ignitability of fuel

A spark plug for an internal combustion engine which has a length with a top and a base comprising:

• a metal shell;
• a porcelain insulator retained in said metal shell;
• a center electrode installed in said porcelain insulator;
• a main ground electrode including an upright portion and a center electrode-facing portion, the upright portion extending from said metal shell in a longitudinal direction of the spark plug, the center electrode-facing portion extending from the upright portion inwardly of the spark plug in a lateral direction substantially perpendicular to the longitudinal direction to have a center electrode-facing surface facing said center electrode to define a spark gap between itself and a top end of said center electrode;
• and
• an auxiliary ground electrode including an upright portion and an inward-facing portion, the upright portion extending from said metal shell in the longitudinal direction of the spark plug, the inward-facing portion extending from the upright portion inwardly of the spark plug in the lateral direction to have an inner end facing the spark gap,
• wherein the inner end of the inward-facing portion has a top edge and a base edge located outward and inward, respectively, as viewed in the longitudinal direction of the spark plug, the top edge being located one of closer to the top of the spark plug than the center electrode-facing surface of said main ground electrode and flush with the center electrode-facing surface, and
• wherein a distance A between the base edge of the inner end of the inward-facing portion of said auxiliary ground electrode and the center electrode-facing surface of said main ground electrode, as defined in the longitudinal direction of the spark plug, and a distance Gm of the spark gap, as defined in the longitudinal direction of the spark plug, are selected to meet a relation of A≧Gm/3.