Hydraulic control system for an infinitely variable belt-drive transmission

Claim 1

In a system for controlling the transmission ratio of an infinitely variable transmission for transmitting the power of an internal combustion engine comprising a primary pulley having a hydraulically shiftable disc and a hydraulic cylinder for shifting the disc, a secondary pulley having a hydraulically shiftable disc and a hydraulic cylinder for operating the latter disc, a belt engaged with both pulleys, and a pressure oil circuit having a pump for supplying pressurized oil, a tranmission ratio control valve having a spool responsive to engine speed for controlling the pressurized oil so as to move the disc of the primary pulley to change the transmission ratio, the improvement comprising: a hydraulic circuit for applying control oil to one end of the spool of the transmission ratio control valve so as to axially move the spool for the controlling of the pressurized oil; an electronic control circuit responsive to ignition pulses of said engine for producing an output signal dependent on the engine speed; a modulator valve provided in the hydraulic circuit for producing a constant pressure of the control oil; an electromagnetic valve provided in the hydraulic circuit and having a coil responsive to said output signal for controlling the constant pressure of the control oil applied to the spool of the transmission ratio control valve, thereby to control the transmission ratio.