Fluid-working machine

1. A hydraulic or pneumatic motor or pump, comprising:

a high-pressure line;

a working chamber having a piston that is movable to cyclically vary a volume of the working chamber; and

a valve arrangement disposed between the high-pressure line and the working chamber for controlling fluid flow from the high-pressure line to the working chamber, the valve arrangement comprising

a main valve having a valve element arranged to provide sealing contact with a valve seat, and

an auxiliary valve comprising an electromagnetically-movable valve member for equalising pressure on either side of the main valve by placing the working chamber in direct fluid communication with the high-pressure line,

wherein

the valve element has an internal conduit communicating with an opening defined by the valve seat, and

the electromagnetically-movable valve member is adapted to place the internal conduit in direct fluid communication with a space surrounding said valve element such that the internal conduit connects the working chamber with the high pressure line,

wherein

the valve element is a poppet valve having a poppet shank, and

the electromagnetically-moveable member is a moving pole ring slidable on the poppet shank, and

wherein

a pole compression spring is configured to act on the pole ring and the poppet shank.

2. A valve arrangement for controlling fluid flow from a high-pressure line to a working chamber of cyclically varying volume of a hydraulic or pneumatic motor or pump, the working chamber having a piston that is movable to cyclically vary a volume of the working chamber, the valve arrangement comprising:

a main valve having a valve element arranged to provide sealing contact with a valve seat, and

an auxiliary valve comprising an electromagnetically-movable valve member for equalising pressure on either side of the main valve by placing the working chamber in direct fluid communication with the high pressure line,

wherein

the valve element has an internal conduit communicating with an opening defined by the valve seat, and

the electromagnetically-movable valve member is adapted to place the internal conduit in direct fluid communication with a space surrounding said valve element such that the internal conduit connects the working chamber with the high pressure line,

wherein

the valve element is a poppet valve having a poppet shank, and

the electromagnetically-moveable member is a moving pole ring slidable on the poppet shank, and

wherein

a pole compression spring is configured to act on the pole ring and the poppet shank.

3. A hydraulic or pneumatic motor or pump, comprising:

a high-pressure line;

a working chamber having a piston that is movable to cyclically vary a volume of the working chamber; and

a valve arrangement disposed between the high-pressure line and the working chamber for controlling fluid flow from the high-pressure line to the working chamber, the valve arrangement comprising

a main valve having a valve element arranged to provide sealing contact with a valve seat, and

an auxiliary valve comprising an electromagnetically-movable valve member for equalising pressure on either side of the main valve by placing the working chamber in direct fluid communication with the high-pressure line, the main and auxiliary valves being integrated,

wherein

the valve element has an internal conduit communicating with an opening defined by the valve seat,

the electromagnetically-movable valve member is adapted to place the internal conduit in direct fluid communication with a space surrounding said valve element such that the internal conduit connects the working chamber with the high pressure line, and

the valve arrangement further comprises

a main biasing element for biasing the valve element towards the valve seat, and

an auxiliary biasing element for biasing the electromagnetically-movable valve member to a position in which the internal conduit is isolated from the space surrounding said valve element.

4. A hydraulic or pneumatic motor or pump according to claim 1 , wherein the electromagnetically-movable valve member comprises a spool valve member.

5. A hydraulic or pneumatic motor or pump according to claim 1 , wherein the electromagnetically-movable member comprises a poppet valve member located at least partly within the internal conduit.

6. A hydraulic or pneumatic motor or pump according to claim 1 , wherein the main and/or auxiliary biasing element comprises a helical spring.

7. A hydraulic or pneumatic motor or pump according to claim 1 , wherein the auxiliary biasing element is configured to exert a greater force than the main biasing element.

8. A hydraulic or pneumatic motor or pump according to claim 1 , wherein the valve element is a poppet valve having a poppet shank, and the electromagnetically-moveable member is a moving pole ring slidable on the poppet shank, and wherein a pole compression spring is configured to act on the pole ring and the poppet shank.

9. A hydraulic or pneumatic motor or pump according to claim 1 , wherein the hydraulic or pneumatic motor or pump is a hydraulic pump or motor.

10. A hydraulic or pneumatic motor or pump according to claim 1 , wherein the valve arrangement is operable to undergo

two-stage operation at a first cycling rate of the working chamber's volume in which the electromagnetically-movable valve member is movable to equalise the pressure on either side of the main valve while the valve element is held against the valve seat, the valve element being movable away from the valve seat after the pressure on either side of the main valve has been equalised, and

single-stage operation at a second cycling rate of the working chamber's volume in which the electromagnetically-movable valve member and the valve element are movable in unison, the first cycling rate being lower than the second cycling rate.

11. A hydraulic or pneumatic motor or pump according to claim 1 ,

wherein the electromagnetically-movable valve member covers an entry to the internal conduit when the electromagnetically-movable valve member is adjacent to the valve element, and

wherein the electromagnetically-movable valve member opens the entry to the internal conduit when the electromagnetically-movable valve member is away from the valve element.

12. A valve arrangement for controlling fluid flow from a high-pressure line to a working chamber of cyclically varying volume of a hydraulic or pneumatic motor or pump, the working chamber having a piston that is movable to cyclically vary a volume of the working chamber, the valve arrangement comprising:

a main valve having a valve element arranged to provide sealing contact with a valve seat,

an auxiliary valve comprising an electromagnetically-movable valve member for equalising pressure on either side of the main valve by placing the working chamber in direct fluid communication with the high pressure line, the main and auxiliary valves being integrated,

wherein

the valve element has an internal conduit communicating with an opening defined by the valve seat,

the electromagnetically-movable valve member is adapted to place the internal conduit in direct fluid communication with a space surrounding said valve element such that the internal conduit connects the working chamber with the high pressure line, and

the valve arrangement further comprises

a main biasing element for biasing the valve element towards the valve seat, and

an auxiliary biasing element for biasing the electromagnetically-movable valve member to a position in which the internal conduit is isolated from the space surrounding said valve element.

13. A valve arrangement according to claim 12 wherein the electromagnetically-movable member comprises a spool valve member.

14. A valve arrangement according to claim 12 , wherein the electromagnetically-movable member comprises a poppet valve member located at least partly within the internal conduit.

15. A valve arrangement according to claim 12 , wherein the main and/or auxiliary biasing element comprises a helical spring.

16. A valve arrangement according to claim 12 , wherein the auxiliary biasing element is configured to exert a greater force than the main biasing element.

17. A valve arrangement according to claim 12 , wherein the valve element is a poppet valve having a poppet shank, and the electromagnetically-moveable member is a moving pole ring slidable on the poppet shank, and wherein a pole compression spring is configured to act on the pole ring and the poppet shank.

18. A valve arrangement according to claim 12 , wherein the valve arrangement is disposed between the high-pressure line and the working chamber for controlling fluid flow from the high-pressure line to the working chamber.

19. A valve arrangement according to claim 12 , wherein the valve arrangement is operable to undergo

two-stage operation at a first cycling rate of the working chamber's volume in which the electromagnetically-movable valve member is movable to equalise the pressure on either side of the main valve while the valve element is held against the valve seat, the valve element being movable away from the valve seat after the pressure on either side of the main valve has been equalised, and

single-stage operation at a second cycling rate of the working chamber's volume in which the electromagnetically-movable valve member and the valve element are movable in unison, the first cycling rate being lower than the second cycling rate.

20. A valve arrangement according to claim 12 ,

wherein the electromagnetically-movable valve member covers an entry to the internal conduit when the electromagnetically-movable valve member is adjacent to the valve element and

wherein the electromagnetically-movable valve member opens the entry to the internal conduit when the electromagnetically-movable valve member is away from the valve element.