Variable compression ratio mechanism for reciprocating internal combustion engine

A variable compression ratio mechanism for a reciprocating engine includes upper and lower links linking a piston pin to a crankpin, an eccentric cam equipped control shaft and a control link cooperating with each other to vary the attitude of the upper and lower links. A control-shaft actuator is provided to vary a compression ratio. The actuator includes a reciprocating block slider linked at a front end to the control shaft, and a rotary member being in meshed-engagement with the rear end of the slider by a meshing pair of screw-threaded portions. A hydraulic modulator has a hydraulic pressure chamber facing the rear end face of the slider, so that working-fluid pressure in the pressure chamber forces the slider in the same axial direction as the direction of action of reciprocating load acting on the slider owing to combustion load.