FLUID WORKING MACHINES AND METHODS

1 . A fluid working machine comprising a controller and a working chamber of cyclically varying volume, the working chamber having a high pressure valve associated therewith to control the connection of the working chamber to a high pressure manifold, and an electronically controllable primary low pressure valve to control the connection of the working chamber to a low pressure manifold, the controller being operable to actively control at least the primary low pressure valve, in phased relationship to cycles of working chamber volume, to determine the net displacement of fluid by the working chamber on a cycle by cycle basis, the fluid working machine being operable to carry out a motoring cycle under at least some circumstances, characterised in that the fluid working machine is adapted to release pressurised fluid from the working chamber prior to the opening of the primary low pressure valve, during a motoring cycle.

2 . A fluid working machine according to claim 1 , further comprising depressurisation means which are operable to release pressurised fluid from the working chamber prior to the opening of the primary low pressure valve, during a motoring cycle, to facilitate the opening of the primary low pressure valve.

3 . A fluid working machine according to claim 1 , wherein the working chamber has a secondary low pressure port associated therewith, which is openable and closable in phased relationship to the cycles of working chamber volume to release pressurised fluid from the working chamber, prior to the opening of the primary low pressure valve, during a motoring cycle, to reduce the pressure within the working chamber and thereby facilitate the opening of the primary low pressure valve.

4 . A fluid working machine according to claim 3 , wherein the secondary low pressure port is openable and closable in phased relationship to the cycles of working chamber volume to release pressurised fluid from the working chamber, by way of a mechanical arrangement operatively linked to the expansion and contraction cycle of the working chamber.

5 . A fluid working machine according to claim 4 , wherein the secondary low pressure port comprises one or more apertures in the working chamber and the fluid working machine comprises one or more fluid conducting conduits, the fluid working machine being operable to periodically bring the one or more fluid conducting conduits into alignment with the one or more apertures to thereby bring the working chamber into fluid communication with a low pressure manifold for a period of time, in phased relation to cycles of working chamber volume.

6 . A fluid working machine according to claim 5 , wherein the fluid working machine is a radial piston motor, in which the working chamber has a volume defined by a cylinder and reciprocating piston, the piston having a base in sliding contact with an eccentric attached to a rotatable crankshaft, the secondary low pressure port comprising an aperture in the base of the cylinder, wherein the eccentric comprises one or more fluid conducting conduits adapted to periodically bring the aperture into fluid communication with a low pressure manifold in phased relation to cycles of working chamber volume.

7 . A fluid working machine according to claim 1 , wherein the fluid working machine is a radial piston motor, in which the working chamber has a volume defined by a cylinder and reciprocating piston, wherein either or both of the cylinder and the reciprocating piston comprises an aperture which is periodically unobscured in phased relation to cycles of working chamber volume to form a secondary low pressure port through which pressurised fluid is released.

8 . A fluid working machine according to claim 1 , wherein the primary low pressure valve and the secondary low pressure port are each openable to bring the working chamber into and out of fluid communication with different low pressure manifolds.

9 . A fluid working machine according to claim 1 , wherein the pressure differential between the working chamber and the low pressure manifold into which the secondary low pressure port releases pressurised fluid exceeds the pressure differential against which the primary low pressure valve can open by a factor of at least 10.

10 . A fluid working machine according to claim 1 , wherein the secondary low pressure port remains open in use until the point in the subsequent contraction stroke where the rate of decrease of working chamber volume is greatest, to facilitate the flow of fluid out of the working chamber to one or more low pressure manifolds.

11 . A method of operating a fluid working machine working chamber of cyclically varying volume, during a motoring cycle of the working chamber, comprising opening an electronically controllable primary low pressure valve, in phased relation to cycles of working chamber volume, to bring the working chamber into fluid communication with a low pressure manifold under the active control of a controller on a cycle by cycle basis, characterised in that the method further comprises releasing pressurised fluid from the working chamber prior to the opening of the primary low pressure valve, during the expansion stroke of a said motoring cycle.

12 . A method according to claim 11 , wherein pressurised fluid is released from the working chamber by depressurising means.

13 . A method according to claim 11 , wherein the pressure is released within the working chamber prior to the opening of the primary low pressure valve by opening a secondary low pressure port, through which fluid can vent from the working chamber.

14 . A method according to claim 13 , wherein the secondary low pressure port is opened by a mechanical arrangement which is operatively linked to cycles of working chamber volume.

15 . Program code which, when executed on a fluid working machine controller, causes the fluid working machine to function as a fluid working machine according to claim 1 .

16 . Program code which, when executed on a fluid working machine controller, causes the fluid working machine to operate according to the method of claim 11 .