Spark plug with high capability to ignite air-fuel mixture

A spark plug comprising:

• a tubular metal shell having an end;
• an insulator having a length and a first end and a second end that are opposite to each other in a lengthwise direction of said insulator, said insulator also having a bore that extends in the lengthwise direction of said insulator, said insulator being fixed in said metal shell such that the first end thereof protrudes from the end of said metal shell;
• a center electrode secured in the bore of said insulator, said center electrode having an end that protrudes from the first end of said insulator;
• and
• a ground electrode having a base end joined to the end of said metal shell and a tip portion that faces the end of said center electrode in the lengthwise direction of said insulator through a spark gap,
• wherein
• said insulator increases in outer diameter from an edge of an inner surface of said insulator at the first end of said insulator to a first reference plane that is defined to extend perpendicular to the lengthwise direction of said insulator and away 0.1 mm from the edge of the inner surface of said insulator toward the second end of said insulator in the lengthwise direction, and
• wherein a first volume V1, which is a volume of a portion of said insulator between the edge of the inner surface of said insulator and the first reference plane, is in a range of 0.15 to 0.38 mm3.