Piston of internal combustion engine

A piston of an internal combustion engine comprising:

• a piston head including a piston crown surface facing a combustion chamber;
• a pair of cylindrical piston pin boss portions positioned on an opposite side of the piston head with respect to the piston crown surface, the pair of cylindrical piston pin boss portions each including a piston pin hole into which a piston pin is inserted;
• a pair of skirt portions positioned on an opposite side of the piston head with respect to the piston crown surface, the pair of skirt portions being positioned on both sides of the pair of piston pin boss portions in a direction of a third axis;
• and
• four apron portions connecting the pair of piston pin boss portions and the pair of skirt portions in the direction of the third axis, the four apron portions including a first apron portion, a second apron portion, a third apron portion, and a fourth apron portion, the first apron portion connecting a first piston pin boss portion included in the pair of piston pin boss portions and a first skirt portion included in the pair of skirt portions, the second apron portion connecting the first piston pin boss portion and a second skirt portion included in the pair of skirt portions, the third apron portion connecting a second piston pin boss portion included in the pair of piston pin boss portions and the first skirt portion, the fourth apron portion connecting the second piston pin boss portion and the second skirt portion,
• wherein the third axis is perpendicular to both a first axis and a second axis,
• wherein the first axis extends along a movement direction of the piston in a cylinder of the internal combustion engine and passes through a center of a cross section of the piston head that extends perpendicularly to the movement direction of the piston,
• wherein the second axis extends along a longitudinal direction of the piston pin hole and passes through a center of a cross section of the piston pin hole that extends perpendicularly to the longitudinal direction of the piston pine hole,
• wherein each of the four apron portions includes a bent portion, a boss portion-side portion positioned on a piston pin boss portion pair side where the pair of piston pin boss portions is located with respect to the bent portion, and a skirt portion-side portion positioned on an opposite side from the pair of piston pin boss portions with respect to the bent portion, and
• wherein, in a cross section perpendicular to the first axis, a minor angle among angles defined by each of the skirt portion-side portions and each of the skirt portions included in the pair of skirt portions that are located adjacent to each of the skirt portion-side portions is an obtuse angle, and a distance between each of the boss portion-side portions and the third axis is increasing according to an increase in a distance from the second axis.