Continuously variable transmission and method of operation thereof

A method of operating a continuously variable transmission comprising a transmission input, a transmission output, and a continuously variable ratio unit arranged to be coupled between the transmission input and output by means of least one clutch thereby to enable transfer of drive between the input and the output at a continuously variable transmission ratio, the variator being connected to a hydraulic control circuit and being constructed and arranged such as to adjust its ratio to provide a variator reaction torque which corresponds to a control pressure from the control circuit, the method being characterised in that it comprises establishing a desired control pressure and setting the control circuit in dependence thereupon, the control pressure being also influenced by changes in variator ratio, and adjusting the torque capacity of the clutch while it is engaged such that excursions of the control pressure from the desired level due to changes in variator loading are controlled by virtue of slipping of the clutch.

A hydraulic arrangement for controlling a clutch of a continuously variable transmission having a continuously variable ratio unit which can be coupled to a transmission output through the clutch, the arrangement comprising first supply means for supplying hydraulic fluid at high pressure, second supply means for supplying hydraulic fluid at controlled pressure, and being characterised by provision of a valve arrangement controlling connection of the first and second supply means to the clutch, the valve arrangement receiving opposed pilot pressure signals from connections to the clutch and to the second supply means and being arranged to effect engagement of the clutch by connecting the high pressure supply means to the clutch, to subsequently disconnect the first supply means from the clutch when the pilot pressure signal from the clutch overcomes the pilot pressure signal from the second supply means, and to connect the second supply means to the clutch to maintain the clutch in its engaged state.