Niterra Co., Ltd.

Tomoya Kukino

Hiroki Shimada

Takuto Koba

Noriyuki Tamura

Kengo Fujimura

Shoma Tsumagari

A spark plugof the present invention includes: an insulatormade from an alumina-based sintered body; a center electrode being a bar-like electrode inserted in the insulatorand having, on a rear end side of the center electrode, a diameter-enlarged portion enlarged in a radial direction and engaged with an inner wall of the insulator; and a conductive sealing material provided on the rear end side of the center electrodeinside the insulator. In a mirror-polished surface obtained by mirror-polishing a cut surface obtained by cutting the insulator in a direction perpendicular to the axial line direction, at a position 2 mm from a portion having a maximum diameter of the diameter-enlarged portion to the rear end side along the axial line direction, when 20 observation regions each being 192 μm×255 μm are set so as to each overlap a reference position being a center position between an inner peripheral surface and an outer peripheral surface of the insulator and so as not to overlap each other, an average of a proportion (porosity) of pores included in each observation region is not greater than 3.5% and, with respect to a variation in the proportion (porosity), when a standard deviation is defined as σ, σ is not greater than 0.36.

Spark plug

In a spark plugof the present invention, in a mirror-polished surfaceobtained by mirror-polishing a cut surfaceobtained by cutting an insulatorin a direction perpendicular to an axial line AX direction, at a position separated by 2 mm from a portion having a maximum diameter of a diameter-enlarged portionto a rear end side along the axial line AX direction, when 20 first observation regions X each being 192 μm×255 μm are set so as to each overlap a reference position mbeing a position 0.2 mm in a radial direction from an inner peripheral surfaceside of the insulatorand so as not to overlap each other, a proportion (porosity) of an area of all of pores included in the 20 first observation regions X relative to a total area (100%) of the 20 first observation regions X is not greater than 3.5%, and in a thermally etched surfaceobtained by subjecting the mirror-polished surfaceto thermal etching, when 20 second observation regions Y each being 32 μm×43 μm are set so as to each overlap the reference position mand so as not to overlap each other, a particle size distribution of alumina particles included in the 20 second observation regions Y is regarded as a normal distribution, an average particle diameter of the alumina particles is defined as A, and a standard deviation of a particle diameter of the alumina particles is defined as σ, A is not less than 1.9 μm and not greater than 2.8 μm, and (A+3σ) is not greater than 3.0 μm.

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Aichi, Japan

Tomoya Kukino