Abstract: Two synchronous hydraulic polyphase motor-generators with axial cylinders are connected by rigid or flexible pipes. One functions as a generator, the other as a motor to transmit torque and power. Sinusoidal piston motion is developed without the use of cams. The pistons cause columns of fluid to alternate between motor and generator, producing synchronous rotation. Apparatus for synchronizing, coupling and decoupling the motive force are disclosed.