Abstract: A power transmission apparatus includes a friction engagement device in which a piston receiving a hydraulic pressure in an oil chamber moves to a side in which friction plates and plates are to be pressed. When a piston stroke amount is equal to or smaller than a specified amount, a cylindrical surface and the piston cylindrical surface oppose each other in a radial direction, and a first pressure-receiving surface receives the hydraulic pressure of the oil chamber. When the piston stroke amount is larger than the specified amount, the cylindrical surface and the piston cylindrical surface do not oppose each other in the radial direction, and the first pressure-receiving surface and second pressure-receiving surface receive the hydraulic pressure of the oil chamber.

Abstract: A piston of an internal combustion engine, includes an intake-side recess and an exhaust-side recess. At a peripheral edge of at least one recess, of the intake-side recess and the exhaust-side recess, a curvature radius of a cross-section along the defined direction of a portion where the recess is connected to a portion of the piston top surface that is on one side of the recess in the defined direction, is smaller than a curvature radius of a cross-section along the defined direction of a portion where the recess is connected to a portion of the piston top surface that is on the other side of the recess in the defined direction.