Multi-link piston-crank mechanism for internal combustion engine

A multi-link piston-crank mechanism for an internal combustion engine, comprising:

• an upper link connected through a piston pin with a piston;
• a lower link swingably connected through a first connecting pin with the upper link;
• a control link including one end swingably connected through a second connecting pin with the lower link;
• and
• a control shaft rotatably attached to a cylinder block, the control shaft including an eccentric shaft that swingably supports another end of the control link, wherein
• the lower link is formed with a crankpin through-hole through which a crankpin is passing, and
• the lower link is rotatably attached to the crankpin through a cylindrical bearing metal held in the crankpin through-hole, wherein
• one end portion of the lower link is formed in a bifurcated shape to insert one end of the upper link into the bifurcated shape,
• inner diameters of both end portions of the crankpin through-hole are larger than an inner diameter of a center portion of the crankpin through-hole,
• the lower link is formed such that a rigidity of outer circumferential portions of the both end portions of the crankpin through-hole is higher than a rigidity of an outer circumferential portion of the center portion of the crankpin through-hole,
• an inner circumferential surface of the crankpin through-hole is curved along an axial direction of the crankshaft at a curvature according to a rigidity of an outer circumferential portion of the crankpin through-hole, and
• the curvature is set such that a part of the outer circumferential portions of the both end portions of the crankpin through-hole which has a high rigidity has a curvature level larger than that of a part of the outer circumferential portion of the center portion of the crankpin through-hole which has a low rigidity.