Spark plug with center electrode having variable diameter portion retracted from front end on insulator

A spark plug for an internal combustion engine comprising:

• a cylindrical metal shell having an annular projection extending inwardly from a front end of the metal shell;
• an insulator having an axial bore and placed in a cylindrical metal shell with a front end of the insulator extending beyond the metal shell;
• a center electrode placed within the axial bore of the insulator so that a front end of the center electrode extends by 1.2 mm or more beyond the insulator;
• at least one ground electrode secured to a front end of the metal shell so that a front end of the ground electrode faces an elevational side of the center electrode to form a semi-creeping spark discharge gap therebetween along a front end surface of the insulator;
• a parallel ground electrode secured to a front end of the metal shell so that an inner side of the parallel ground electrode faces a front end surface of the center electrode to form an air-gap therebetween;
• and
• a front end portion of the center electrode having an electrode base in the axial bore and an electrode front which is diametrically smaller than the electrode base and connected thereto by a portion of variable diameter;
• wherein a diametrical difference between an initial point of the variable diameter portion of the center electrode and the axial bore of the insulator is 1 mm or less, and the initial point of the variable diameter portion of the center electrode is retracted by 0.0 mm-1.0 mm from the front end surface of the insulator.

A spark plug for use in an internal combustion engine comprising:

• an insulator having an axial bore and placed in a cylindrical metal shell with a front end of the insulator extending beyond the metal shell;
• a center electrode placed within the axial bore of the insulator so that a front end of the center electrode extends beyond the insulator;
• at least one ground electrode secured to a front end of the metal shell;
• and
• a front end portion of the center electrode having an electrode base in the axial bore and an electrode front which is diametrically smaller than the electrode base and, connected thereto by a portion of a variable diameter portion;
• wherein a diametrical difference between an initial point of the variable diameter portion of the center electrode and the axial bore of the insulator is 1 mm or less, and the initial point of the variable diameter portion of the center electrode is retracted by 0.0 mm-1.0 mm from a front end surface of the insulator, said front end surface of the insulator being substantially transverse to a longitudinal axis of the axial bore of the insulator.

A spark plug for use in an internal combustion engine comprising:

• a cylindrical metal shell;
• an insulator having an axial bore and placed in said cylindrical metal shell with a front end of the insulator extending beyond the metal shell;
• a center electrode placed within the axial bore of the insulator so that a front end of the center electrode extends beyond the insulator;
• at least one ground electrode secured to a front end of the metal shell;
• and
• a front end portion of the center electrode having an electrode base in the axial bore and an electrode front which is diametrically smaller than the electrode base and connected thereto by a portion of variable diameter;
• wherein a diametrical difference between an initial point of the variable diameter portion of the center electrode and the axial bore of the insulator is 1 mm or less, and the initial point of the variable diameter portion of the center electrode is retracted by 0.0 mm-1.0 mm from a front end surface of the insulator;
• and
• wherein the cylindrical metal shell has an annular projection extending inwardly from a front end of the metal shell such that the center electrode forms an auxiliary spark gap with the annular projection of the metal shell.