FLUID-WORKING MACHINE

Background to the Invention

[0001] This invention relates to a hydraulic or pneumatic motor or pump (a "fluid-working machine").

[0002JEP-B-0494236 introduces a mechanism for actuating electromagnetic poppet valves in a fluid-working machine, using the pre-compression and pre- expansion of the contents of a working chamber to create a motoring cycle of the machine. Working chambers of the machine are selected in real time so that fluid power is controllably converted to shaft power and vice versa, thus allowing a controllable bi-directional energy flow. The nature of poppet valves is that the fluid pressure acts over the seating area creating a large closing force. The motoring cycle of EP-B-0494236 relies on the energy from an already spinning shaft to allow the opening of the solenoid-operated poppet valve against the pressure in the high-pressure manifold. This requirement limits the use of this technique to conditions where the machine is already operating above a threshold shaft speed. For applications with the requirement of starting from zero speed, the motoring cycle must be initiated by different means.

[0003JUS-A-5735582 describes an electromagnetic valve assembly comprising a master valve and an auxiliary valve, the auxiliary valve serving to open the master valve against a high pressure. In order for the master valve to open, fluid must flow along a capillary between the master valve body and the bore until the pressure is equalised. This means that actuation of the valve assembly requires 200 to 300 ms which is too slow for use in a fluid- working machine.

Summary of the Invention

[0004] It is an aim of the invention to provide a face-seating valve, interposed between the working chamber and the high-pressure manifold of a fluid-

working machine, which can open whilst the latter is pressurised and the output shaft of the machine is stationary.

[0005] Accordingly, the present invention provides a fluid-working machine according to claim 1. Optional features of the invention are set out in the dependent claims.

[0006] The valve arrangement of the invention is capable of functioning because the working chamber is essentially a contained volume and the leakage flow is limited to the sum of the volume change due to compression and fluid leakage from the working chamber.

[0007] In operation of the machine of the invention, once the pressure between the high-pressure line and the working chamber has equalised it is possible, with a force of modest magnitude, to move the main valve element into the open position in order to complete the second stage of operation.

[0008] It is also possible, and advantageous, for a machine of this type with the integral main and auxiliary valves to function in the same way as in EP-B- 0494236. The valve begins with two-stage operation at start-up, continues to use it during low-speed operation and automatically changes to single-stage operation with the pre-compression cycle as speed increases.

Brief Description of the Drawings

[0009] The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:

[0010] Figure 1 is a schematic diagram showing the general principle of the invention;

[0011] Figure 2 is a schematic sectional view of a valve arrangement according to an embodiment of the invention is a closed position;

[0012] Figure 3 shows the valve arrangement of Figure 2 in the first stage of operation;

[0013] Figure 4 shows the valve arrangement of Figure 2 in the second stage of operation; and

[0014] Figure 5 is a schematic sectional view of a valve arrangement according to an alternative embodiment of the invention.

Detailed Description of Particular Embodiments

[0015] Figure 1 shows how a fluid-working machine according to the invention operates in a motoring cycle. A solenoid-operated, normally-open, face-seating low-pressure line valve 1 is interposed between the line 7 and a working chamber 2 of cyclically varying volume. A solenoid-operated, normally-closed, face-seating high-pressure line valve 4, interposed between the working chamber and the high-pressure line 6, is connected in parallel with a much smaller pressure-equalising valve 5. The low-pressure line valve 1 has its poppet facing the working chamber 2, while the high-pressure line valve 4 has its poppet facing the high-pressure manifold.

[0016] The following is a description of the complete motoring cycle utilising the two-stage high-pressure valve operation.

[0017] At the beginning of the motoring cycle the low-pressure line valve 1 is in the open position, and the high-pressure line valve 4 and the pressure equalising valve 5 are in the closed position. The pressure in the high-pressure line 6 is above the pressure of the low-pressure line 7. The pressure in the working chamber 2 equals the pressure of the low-pressure line 7.

[0018] The motoring cycle starts with the closing of low-pressure line valve 1 and the piston moving, possibly from rest, towards bottom-dead centre. Next the pressure-equalising valve 5 is commanded to open. With the valves I ₁ 5 in these

positions, the pressure in the high-pressure line 6 and the working chamber 2 will equalise. Once the pressure has equalised, high-pressure line valve 4 can be commanded to open. The working chamber 2 is now connected to the high- pressure line 6 and disconnected from the low-pressure line 7 so that the high- pressure fluid can act on the piston 3 and produce torque on the crankshaft of the machine, through the downstroke. The pressure-equalising valve 5 can be closed as soon as high-pressure line valve 4 has opened or might be left open for the ^v duration of the motoring stroke. Both valve 4 and valve 5 have to be closed before the piston 3 reaches bottom-dead-centre. The remaining expansion of the working space, as the piston 3 reaches its limit condition, will depressurise it and allow low-pressure line valve 1 to open. With this valve in the open position and valves 4 and 5 in the closed position it is now possible to displace the fluid into the low-pressure line 7 as the piston 3 moves toward its top-dead-centre position.

[0019]Figure 2 shows the pressure equalising valve in the form of a sliding spool valve integrated into the main solenoid operated poppet valve. A fluid chamber 8 is connected to the high-pressure line. A passage 9 is connected to the working chamber. A ferromagnetic moving pole ring 10 can slide along the base of a poppet 11. A pole compression spring 12 forces the moving pole towards the head of the poppet 11. The poppet 10 has a flow channel 13 connecting the fluid chamber 8 and the working chamber passage 9. The moving pole 10 covers the entry to the flow channel when the moving pole 10 is close to head of the poppet 11 and opens the flow channel 13 when it has moved away from the head of the poppet, as shown in Figure 3. The poppet 11 can slide along a guide 14. A poppet compression spring 15 is employed to force the poppet toward its seat 16. The preload on spring 15 is lower than the preload on pole compression spring 12. A solenoid coil 17 is arranged to create magnetic flux to attract the moving pole 10 towards a static pole 18.

[0020] The pole ring 10 is displaced axially when the coil 17 is energised. This pole ring acts directly on pole spring 12 and, if there is significant pressure

gradient between the high-pressure chamber 8 and the passage 9 so that the poppet 11 is held against the seat 16, then the pole ring will slide on the poppet shank and expose the holes 13, as shown in Figure 3. This connects the two spaces and equalises the pressure across the valve arrangement. Once the pressure is equalised, so long as the force of poppet spring 15 is less than that of pole spring 12 across the full range of displacement, the axial force of pole spring 12 on the poppet shank will pull the poppet 11 away from the seat 16 and open the main fluid passageway, as shown in Figure 4.

[0021] During high cycling rates of the working chamber there will not be sufficient time for this two-stage operation to take place. In this case the solenoid coil of the low-pressure normally-open valve 1 will be energised just before the working chamber reaches its minimum volume condition so that the pressure in the working chamber rises in the closed volume and equals that of the high- pressure manifold and so that, when the coil of the normally-closed valve is energised, the moving pole ring 10 moves in unison with the poppet 11 and opens the valve in a single stage.

[0022] Figure 5 shows an alternative pressure equalising valve 5 in the form of an auxiliary poppet type valve 19 integrated into the main solenoid operated poppet valve 11'.

[0023] The basic operation is the same as described above. The main difference is the use of auxiliary pressure-equalising poppet valve 19 rather than a sliding of the moving pole 10 to uncover the flow channel 13'.

[0024] Again the higher pressure in fluid chamber 8 than that in passage 9 at the beginning of the motoring cycle prevents the poppet 11' from moving towards the static pole 18. The moving pole 10' starts to compress pole spring 12' and poppet spring 15 as it slides away from the head of the poppet 11', removing the auxiliary poppet valve 19 from its seat 20 and opening the flow channel 13'. When the pressure is equalised, main poppet 11' can open under the force of

pole spring 12' which is grater than that of poppet spring 15. As before, the main and auxiliary valves move together "automatically" when the motor has sufficient momentum.

[0025] AU forms of the verb "to comprise" used in this specification should be understood as forms of the verbs "to consist of" and/ or "to include".