The invention provides a free piston type internal combustion engine, an internal combustion engine generator and an internal combustion compressor and relates to the technical field of internal combustion engines, wherein the compression ratio can be effectively controlled so as to further ensure that the free piston type internal combustion engine which operates in different working conditions has a good operating property. The free piston type internal combustion engine comprises an air cylinder body provided with an air cylinder and a connecting rod supporting body located in the bottom of the air cylinder body; a power transmission main oil channel and a control oil channel which communicate with each other are arranged in the connecting rod supporting body; a piston is arranged in the air cylinder, and a piston rod of the piston extends out of the corresponding air cylinder, extends into the power transmission main oil channel and reciprocates in the power transmission main oil duct; the free piston type internal combustion engine further comprises a control mechanism for changing the volume of hydraulic oil filling the control oil channel. The free piston type internal combustion engine is used for guaranteeing the good operating property when the free piston type internal combustion engine operates in different working conditions.