GB2459520A

GB2459520A - Fluid machine with secondary low pressure port opening before controlled primary valve

Info

Publication number: GB2459520A
Application number: GB0811448A
Authority: GB
Inventors: Uwe Bernhard Pascal Stein; Michael Richard Fielding; Stephen Michael Laird; Alasdair Ian Fletcher Robertson
Original assignee: Artemis Intelligent Power Ltd
Current assignee: Artemis Intelligent Power Ltd
Priority date: 2008-06-20
Filing date: 2008-06-23
Publication date: 2009-10-28
Anticipated expiration: 2028-06-23
Also published as: GB2459520B; GB0811448D0

Abstract

The machine comprises a controller, a cyclically varying volume working chamber 100, an electronically controllable primary low pressure valve 106 and a secondary low pressure port 120, each opening to a low pressure manifold 108, 122 in phased relation to the working cycle such that the machine releases working chamber fluid during a motoring stroke prior to opening of the primary low pressure valve to facilitate primary valve opening. In a radial piston machine, the secondary port may comprise an aperture 118 at the piston end of a cylinder which is brought periodically into alignment with a peripheral crankshaft slot 120. The primary valve is actively controlled to determine the net cycle displacement. The low pressure valve and the secondary port are openable concurrently during a portion of at least some working cycles to enable concurrent fluid flow into or out of the working chamber through both. The low pressure valve may close after the secondary port closes.

Description

1 Fluid Working Machines and Methods

3 Field of the Invention

The invention relates to the field of fluid working machines, particularly fluid working 6 machines which comprise at least one working chamber of cyclically varying volume, 7 in which the net displacement of fluid through the or each working chamber is 8 regulated by at least one electronically controllable valve, on a cycle by cycle basis, 9 to determine the net throughput of fluid through the or each working chamber.

11 Some embodiments of the invention relate to a method of supplying fluid to, or 12 receiving fluid from, a working chamber of a fluid working machine. Some 13 embodiments of the invention aim to facilitate the opening of an electronically 14 controllable valve during a motoring cycle of a fluid working machine.

16 Background to the Invention

18 Fluid working machines include fluid-driven and/or fluid-driving machines, such as 19 pumps, motors, and machines which can function as either a pump or as a motor in different operating modes.

22 When a fluid working machine operates as a pump, a low pressure manifold typically 23 acts as a net source of fluid and a high pressure manifold typically acts as a net sink 24 for fluid. When a fluid working machine operates as a motor, a high pressure 1 manifold typically acts as a net source of fluid and a low pressure manifold typically 2 acts as a net sink for fluid. Within this description and the appended claims, the 3 terms "high pressure manifold" and "low pressure manifold" refer to manifolds with 4 higher and lower pressures relative to each other. The pressure difference between the high and low pressure manifolds, and the absolute values of the pressure in the 6 high and low pressure manifolds will depend on the application. For example, the 7 pressure difference may be higher in the case of a pump which is optimised for a high 8 power pumping application than in the case of a pump which is optimised to precisely 9 determine the net displacement of fluid, for example, a pump for dispensing a metered amount of fluid (e.g. a liquid fuel), which may have only a minimal pressure 11 difference between high and low pressure manifolds. A fluid working machine may 12 have more than one low pressure manifold.

14 Although the invention will be illustrated with reference to applications in which the fluid is a liquid, such as a generally incompressible hydraulic liquid, the fluid could 16 alternatively be a gas.

18 Fluid working machines are known which comprise a plurality of working chambers of 19 cyclically varying volume, in which the displacement of fluid through the working chambers is regulated by electronically controllable valves, on a cycle by cycle basis 21 and in phased relationship to cycles of working chamber volume, to determine the net 22 throughput of fluid through the machine. For example, EP 0 361 927 disclosed a 23 method of controlling the net throughput of fluid through a multi-chamber pump by 24 opening and/or closing electronically controllable poppet valves, in phased relationship to cycles of working chamber volume, to regulate fluid communication 26 between individual working chambers of the pump and a low pressure manifold. As a 27 result, individual chambers are selectable by a controller, on a cycle by cycle basis, to 28 either displace a predetermined fixed volume of fluid or to undergo an idle cycle with 29 no net displacement of fluid, thereby enabling the net throughput of the pump to be matched dynamically to demand.

32 EP 0 494 236 developed this principle and included electronically controllable poppet 33 valves which regulate fluid communication between individual working chambers and 34 a high pressure manifold, thereby facilitating the provision of a fluid working machine functioning as either a pump or a motor in alternative operating modes.

36 EP 1 537 333 introduced the possibility of part cycles, allowing individual cycles of 1 individual working chambers to displace any of a plurality of different volumes of fluid 2 to better match demand.

4 Key factors which determine the performance of fluid working machines of this type include the performance characteristics of the electronically controllable valves.

6 These valves are typically electromagnetically actuated poppet valves, although other 7 valves types could conceivably be employed. Relevant performance characteristics 8 include the speed at which the electronically controllable valves open and close, the 9 pressure difference against which they can open, their operational lifetime and the cross-section of the flow path through the valve whilst open, which limits the 11 throughput of fluid and influences the flow characteristics of fluid into and out of the 12 working chambers. Accordingly, the electronically controllable valves are an 13 expensive and performance limiting component of such fluid working machines and it 14 would be desirable to reduce one or more of the demands made on the electronically controllable valves.

17 In particular, a significant technical problem, which determines the specification of 18 electronically controllable valves for a particular application, arises when fluid flows 19 into a working chamber of a pump from a low pressure manifold during an expansion stroke of a working chamber. The rate of fluid flow is limited by the cross-section and 21 geometry of the flow path through the poppet valve and the properties of the working 22 fluid. Where the fluid flowing into the working chamber is a liquid, it is subject to 23 cavitation, which increases noise, reduces efficiency by requiring a pressure 24 difference across the poppet valve, and leads to damage to the machine. A different problem applies during the contraction stoke of a working chamber in a motor, when 26 fluid flows out to a low pressure manifold, where an increased pressure drop causes 27 inefficiency, and where the poppet valve may be inadvertently closed causing 28 possible damage to the valve and inadvertent pumping.

This problem has typically been solved by specifying larger electronically controllable 31 valves for higher throughput applications, or applications where superior fluid flow 32 characteristics are required. However, larger electronically controllable valves are 33 more expensive and there can be a trade off in performance characteristics. For 34 example, larger electronically controllable valves may open and close more slowly than smaller valves or use more electrical power, forcing compromises to be made.

1 Accordingly, some aspects of the invention aim to reduce the performance demands 2 on the electronically controllable valves, to facilitate improved performance or to 3 enable smaller and/or reduced specification electronically controllable valves to be 4 employed than would otherwise be the case to obtain a fluid working machine with specified performance characteristics. Some aspects of the invention also aim to 6 reduce the build up of hot fluid that can occur in the crankcase in radial piston pumps 7 and/or motors.

9 Further aspects of the invention address problems associated with opening the low pressure valve, which connects a working chamber to a low pressure manifold, in a 11 fluid working motor (such as a fluid working machine which can function only as a 12 motor, or a fluid working machine which can function either as a motor or a pump, in 13 different operating modes). In a motoring cycle, a high pressure valve associated 14 with the working chamber is closed, under the active control of the controller, shortly before the end of the expansion stroke. As the working chamber continues to 16 expand, the pressure of the fluid trapped within the working chamber drops.

17 Typically, the pressure of the fluid trapped within the working chamber will need to 18 drop to close to the low pressure manifold pressure before the low pressure valve can 19 open. However, it can take a significant period of time for the pressure of the fluid trapped within the working chamber to drop to a sufficiently low value, for several 21 reasons. Firstly, the rate of change of working chamber volume decreases towards 22 the end of the expansion stroke in most fluid working machines. Secondly, the 23 variation in pressure of the fluid trapped within the working chamber is not a linear 24 function of the volume of the working chamber, in the case of many commonly used hydraulic fluids. Furthermore, gases which are dissolved within the hydraulic fluid 26 may evaporate, which has the effect of reducing the expected rate of decrease of 27 pressure within the working chamber. This delay can reduce the efficiency of the 28 fluid working motor. Indeed, malfunctions can arise if the pressure within the working 29 chamber does not drop to a sufficiently low value to enable the opening of the low pressure valve, for example on start-up, or when operating in especially high or low 31 temperature conditions.

33 Accordingly, some aspects of the invention aim to facilitate the opening of a low 34 pressure valve, which regulates communication between the interior of a working chamber and a low pressure manifold, during a motoring cycle of a fluid working 36 machine.

1 Summary of the Invention

3 According to a first aspect of the present invention there is provided a fluid working 4 machine comprising a controller and a working chamber of cyclically varying volume, the working chamber having an electronically controllable primary low pressure valve 6 associated therewith to control the connection of the working chamber to a low 7 pressure manifold, the controller being operable to actively control at least the 8 primary low pressure valve, in phased relationship to cycles of working chamber 9 volume, to determine the net displacement of fluid by the working chamber on a cycle by cycle basis, characterised in that the working chamber further comprises a 11 secondary low pressure port which is openable and closable in phased relationship to 12 the cycles of working chamber volume to connect the working chamber to a low 13 pressure manifold, to enable fluid to flow into or out of the working chamber 14 concurrently through both the primary low pressure valve and the secondary low pressure port, during a portion of at least some cycles of working chamber volume.

17 By determining the net displacement of fluid by the working chamber on a cycle by 18 cycle basis, we refer to determining the net displacement of fluid by the working 19 chamber, during individual cycles of working chamber volume, from amongst a plurality of possible net displacements of fluid (which may be discrete net 21 displacements and/or selected from a continuous range of net displacements). In 22 order to determine the net displacement of fluid by the working chamber, the 23 controller may actively control a plurality of electronically controllable valves.

The fluid working machine may comprise a plurality of said working chambers. In this 26 case, the controller may be operable to actively control a plurality of electronically 27 controllable valves, comprising at least the primary low pressure valve associated 28 with each of the plurality of said working chambers, in phased relationship to cycles 29 of working chamber volume, to determine the net displacement of each of the said plurality of working chambers on a cycle by cycle basis. Typically, this determines 31 the net throughput of fluid through the fluid working machine as a whole. The 32 controller may be operable to determine the net displacement of fluid by individual 33 working chambers, or groups of working chambers, during individual cycles of 34 working chamber volume.

36 By "actively control" we refer to enabling the controller to affect the state of an 37 electronically controllable valve, in at least some circumstances, by a control 1 mechanism which consumes power and is not exclusively a passive response, for 2 example, the opening or closing of a valve responsive solely to the pressure 3 difference across a valve. Related terms such as "active control" should be 4 construed accordingly. Nevertheless, the primary low pressure valve, and one or more other electronically controllable valves, where present, are preferably also 6 operable to open or close by passive means. The primary low pressure valve 7 typically opens passively due to the drop in pressure within the working chamber, 8 such as during an intake stroke. For example, the primary low pressure valve, or one 9 or more other electronically controllable valves, where present, may, during at least some cycles, open passively due to a pressure difference and be selectively closable 11 under the active control of the controller during a portion of the cycle.

13 By "actively control" (and related terms such as "active control") we include the 14 possibilities that the controller is operable to selectively cause an electronically controllable valve to do one or more of open, close, remain open and/or remain 16 closed. The controller may only be able to affect the state of an electronically 17 controllable valve during a portion of a working cycle. For example, the controller 18 may be unable to open the primary low pressure valve against a pressure difference 19 during the majority of a working cycle when pressure within the working chamber is substantial. Typically, the controller actively controls the electronically controllable 21 primary low pressure valve, and one or more other electronically controllable valves 22 where present, by transmitting a control signal either directly to an electronically 23 controllable valve or to an electronically controllable valve driver, such as a 24 semiconductor switch. By transmitting a control signal, we include transmitting a signal which denotes the intended state of an electronically controllable valve (e.g. 26 open or closed) or a pulse which denotes that the state of an electronically 27 controllable valve should be changed (e.g. that the valve should be opened or 28 closed), or a pulse which denotes that the state of an electronically controllable valve 29 should be maintained. The controller may transmit a signal on a continuous basis and stop or change the signal to cause a change in the state of an electronically 31 controllable valve, for example, the electronically controllable primary low pressure 32 valve, or one or more other electronically controllable valves where present, may 33 comprise a normally closed solenoid opened valve which is held open by provision of 34 an electric current and actively closed by switching off the current.

36 By "in phased relationship to cycles of working chamber volume" we mean that the 37 timing of active control by the controller of the primary low pressure valve, and one or 1 more other electronically controllable valves, where present, is determined with 2 reference to the phase of the volume cycles of the working chamber. Accordingly, 3 the fluid working machine typically comprises working chamber phase determining 4 means, such as a position sensor. For example, where the cycles of working chamber volume are mechanically linked to the rotation of a shaft, the fluid working 6 machine preferably comprises a shaft position sensor, and optionally a shaft speed 7 sensor, and the controller is operable to receive a shaft position signal from the shaft 8 position sensor, and optionally a shaft speed signal from a said shaft speed sensor.

9 In embodiments which comprise a plurality of working chambers, with a phase difference between the volume cycles of different working chambers, the controller 11 will typically be operable to determine the phase of individual working chambers.

13 In this way, the primary low pressure valve and secondary low pressure port work 14 together to supply fluid into or out of the working chamber, from at least one low pressure manifold, during a portion of at least some cycles of working chamber 16 volume. As a result, the fill or exhaust characteristics of the working chamber are 17 better than would be the case if the working chamber could be brought into fluid 18 connection with one or more low pressure manifolds only by way of the primary low 19 pressure valve. For example, the force acting against the expansion or contraction of the working chamber, due to the pressure difference between the working chamber 21 and the or each low pressure manifold, may be reduced. Where the fluid is a liquid, 22 the improved flow characteristics with the secondary low pressure port can eliminate 23 cavitation while using an electronically controllable primary low pressure valve that 24 would otherwise have had a too small cross-sectional area. This may have the effect of reducing noise and/or improving the efficiency of the fluid working machine and/or 26 increasing the operating life of the machine. The provision of a secondary flow path 27 for fluid during an expansion stroke can particularly improve the performance of the 28 pump at start-up, or in cold conditions, when the hydraulic fluid is at a relatively low 29 temperature and so has a relatively high viscosity.

31 Preferably, the secondary low pressure port is closed for at least part of each cycle of 32 working chamber volume. Preferably, the primary low pressure valve and the 33 secondary low pressure port are closed concurrently only during selected cycles of 34 working chamber volume which are determined by the controller. For example, the primary low pressure valve may remain open throughout selected cycles of working 36 chamber volume where determined by the controller. Preferably, the primary low 37 pressure valve and the secondary low pressure port are closed concurrently between 1 instances when the primary low pressure valve is open. Typically, at least under 2 some operating conditions, the primary low pressure valve and the secondary low 3 pressure port are closed concurrently between consecutive periods where the 4 primary low pressure valve and the secondary low pressure port are open concurrently.

7 The at least one working chamber may have a commutator associated therewith to 8 alternately attach the electronically controllable primary low pressure valve to (i) the 9 said low pressure manifold and (ii) a high pressure manifold, for example as disclosed in EP 1 738 077). However, the working chamber typically comprises a 11 high pressure valve to control the connection of the working chamber to a high 12 pressure manifold. The high pressure valve may comprise a pressure operated 13 check valve (e.g. in the case of a pump) or a further electronically controllable valve 14 (e.g. in the case of a motor, or a fluid working machine operable to function either as a pump or a motor), which is preferably under the control of the controller.

17 Preferably, the controller is operable, in respect of at least some cycles of working 18 chamber volume in which both the primary low pressure valve and the secondary low 19 pressure port are open concurrently, to cause the primary low pressure valve to close under the active control of the controller, to bring the working chamber out of 21 communication with the or each said low pressure manifold, a period of time after the 22 secondary low pressure port closes. In these circumstances, the secondary low 23 pressure port is already closed when the controller may cause the primary low 24 pressure valve to close to bring the working chamber out of communication with the or each said low pressure manifold, and so the end of a period during which the 26 working chamber is in fluid communication with one, or optionally two or more, low 27 pressure manifolds, remains under the control of the controller. This enables the 28 controller to select the net displacement of fluid through the working chamber on a 29 cycle by cycle basis, for example, by selecting the timing of the closure of the primary low pressure valve relative to the phase of cycles of working chamber volume or, for 31 example, by optionally selecting an idle cycle of the working chamber in which there 32 is no net displacement of fluid through the working chamber, perhaps by holding the 33 primary low pressure valve open throughout a cycle (e.g. as disclosed in EP 0 361 34 927) or keeping the working chamber out of fluid communication with any low pressure manifold throughout a cycle (e.g. as disclosed in WO 2007/088380).

36 Furthermore, the controller can more precisely define the end of the period during 37 which the working chamber is in fluid communication with one, or optionally two or 1 more, low pressure manifolds, than would be the case using a non-electronically 2 controllable valve.

4 Accordingly, the primary low pressure valve does not require as large a flow path cross-section as would be the case if the secondary low pressure port was not 6 provided. This may allow an electronically controllable valve with a smaller flow path 7 cross-section to be employed than would otherwise be the case to obtain desired 8 performance characteristics. Accordingly, the primary low pressure valve may be 9 selected with increased emphasis on its performance in defining the end of the period during which a working chamber is in fluid communication with one, or optionally two 11 or more, low pressure manifolds, for example, because of its speed of closing, its 12 ability to open against a pressure gradient, its power consumption, or its reliability, 13 than would be the case if the flow path cross-section of the primary low pressure 14 valve was a higher priority.

16 The primary low pressure valve and secondary low pressure port may each be 17 openable to bring the working chamber into and out of fluid communication with the 18 same low pressure manifold. Alternatively, the primary low pressure valve and 19 secondary low pressure port may each be openable to bring the working chamber into and out of fluid communication with a different low pressure manifold. In this 21 case, the two low pressure manifolds would typically have similar pressures.

23 It may be that the primary low pressure valve and the secondary low pressure port 24 are only open concurrently during an expansion stroke of the working chamber, for example, where the fluid working machine is operating as a pump. The secondary 26 low pressure port may be openable only during an expansion stroke of the working 27 chamber, but the primary low pressure valve may be optionally closed under the 28 active control of the controller within or just before the beginning of the contraction 29 stroke (bottom dead centre in a piston machine) and openable at the end of the contraction stroke (top dead centre in a piston machine) of the working chamber.

32 It may be that the primary low pressure valve and the secondary low pressure port 33 are only open concurrently during a contraction stroke of the working chamber, for 34 example, in the case of a fluid working machine operating as a motor, such as a fluid working machine in which the high pressure valve comprises an electronically 36 controllable valve under the active control of the controller. The secondary low 37 pressure port may be openable only during a contraction stroke of the working 1 chamber, but the electronically controllable low pressure valve may be optionally 2 closed under the active control of the controller before the end of the contraction 3 stroke (top dead centre in a piston machine) and openable at or after the end of the 4 contraction stroke (top dead centre in a piston machine).

6 Preferably, the primary low pressure valve and the secondary low pressure port are 7 both open in use, during at least some cycles of working chamber volume, at the 8 point in an expansion or contraction stroke, as appropriate, where the rate of change 9 of the volume of the working chamber is greatest, as this is the time when the greatest rate of fluid intake or discharge respectively is required. Indeed, as the 11 pressure difference across the primary low pressure valve is proportional to the 12 square of the rate of fluid flow through the primary low pressure valve, it may be 13 sufficient for the primary low pressure valve and the secondary low pressure port to 14 both be open in use during a limited portion of an expansion or contraction stroke, as appropriate. Said limited portion of an expansion or contraction stroke is preferably 16 less than 50%, of the duration of an expansion or contraction stroke, as appropriate, 17 including the point in an expansion or contraction stroke, as appropriate, where the 18 rate of change of the volume of the working chamber is greatest.

The period of time during which both the secondary low pressure port and the primary 21 low pressure valve are open concurrently during selected cycles is preferably less 22 than 90%, and preferably more than 30%, of the duration of a contraction stroke or 23 expansion stroke, as appropriate. This allows scope for variation in the period of time 24 which elapses between closure of the secondary low pressure port and closure of the primary low pressure valve from cycle to cycle, to select different net displacements 26 of fluid during individual cycles of working chamber volume whilst enabling the 27 secondary low pressure port to supply or receive additional fluid for a significant 28 portion of the contraction stroke or expansion stroke.

It may be that the primary low pressure valve opens after the secondary low pressure 31 port during at least some cycles of working chamber volume. It may be that the 32 primary low pressure valve opens before the secondary low pressure port during at 33 least some cycles of working chamber volume. In some embodiments, the controller 34 is operable to determine whether the primary low pressure valve opens before or after the secondary low pressure port on a cycle by cycle basis.

1 Preferably, whichever of the primary low pressure valve and the secondary low 2 pressure port opens first during the said some cycles of working chamber volume 3 opens at a time during the volume cycle of the working chamber when the pressure 4 difference between the working chamber and the low pressure manifold is minimal, for example less than 5% of the maximum design pressure of the working chamber.

7 The opening and/or closing of the secondary low pressure port may, or may not, be 8 controlled by the controller. The secondary low pressure port may be openable 9 passively, for example, responsive to the pressure in the working chamber being at least a predetermined amount below the pressure in the respective low pressure 11 manifold. Accordingly, the secondary low pressure port may be a pressure operated 12 valve.

14 In some embodiments, the secondary low pressure port is openable or closable by a secondary electronically controllable valve, the opening or closing or both opening 16 and closing of which is under the active control of the controller, to bring the working 17 chamber into or out of fluid communication with a low pressure manifold by way of the 18 secondary low pressure port. The secondary low pressure port may be openable and 19 closable by a secondary electronically controllable valve which opens passively in use, in response to the pressure in the working chamber being below the pressure in 21 the low pressure manifold. The secondary low pressure port may be openable or 22 closable by a secondary electronically controllable valve which closes passively in 23 use, in response to the pressure in the working chamber being above the pressure in 24 the respective low pressure manifold.

26 Where the secondary low pressure port is openable or closable by means of a 27 secondary electronically controllable low pressure valve, the primary low pressure 28 valve and the secondary electronically controllable low pressure valve may be 29 selected to each have operating characteristics which are better suited to the roles of last closing and first opening the connection between the working chamber and the or 31 each low pressure manifold, respectively.

33 The secondary low pressure port may be openable other than by an electronically 34 controllable valve. For example, the secondary low pressure port may be normally-closed but openable responsive to the pressure within the working chamber being a 36 predetermined amount less than the pressure in the low pressure manifold 1 communicating with the secondary low pressure port. Thus, the secondary low 2 pressure port may comprise a normally-closed pressure-openable check valve.

4 The phase of the opening and closing of the secondary low pressure port may be invariable relative to cycles of working chamber volume, that is to say, the opening 6 and closing of the secondary low pressure port may be phase locked. In the case of 7 a fluid working machine which is operable to function as either a pump or a motor in 8 different operating modes, the opening and closing of the secondary low pressure 9 port is preferably not phase locked. This is because the secondary low pressure port is typically openable during the expansion stroke for a pumping cycle and the 11 contraction stroke for a motoring cycle, but not both.

13 Where the opening and closing of each secondary low pressure port is phase locked 14 to the expansion and contraction cycle of the working chamber, each secondary low pressure port may be opened and closed by a mechanical arrangement operatively 16 linked to the expansion and contraction cycle of the working chamber.

18 Where the fluid working machine comprises a rotatable shaft, such as a crankshaft, 19 the opening and closing of the secondary low pressure port may be operatively linked by a mechanical arrangement to the angle of the rotatable shaft. Accordingly, the 21 primary low pressure valve may be openable on a cycle by cycle basis under the 22 active control of the controller, but the opening and closing of the secondary low 23 pressure port may not be variable on a cycle by cycle basis, and may be fixedly 24 phase locked to the expansion and contraction cycle of the working chamber, e.g. by virtue of a mechanical arrangement operatively linked to the angle of a rotatable 26 shaft, where present. The secondary low pressure port may comprise a mechanically 27 actuated valve operated by a pushrod mechanically linked to the expansion and 28 contraction cycles of the working chamber.

The secondary low pressure port may comprise one or more apertures in the working 31 chamber, for example, where the working chamber comprises a hollow piston, the 32 secondary low pressure port may comprise an aperture in the hollow piston, such as 33 an aperture in the base of the hollow piston. The fluid working machine may be 34 operable to bring one or more fluid conducting conduits periodically into alignment with the said one or more apertures to thereby bring the working chamber into fluid 36 communication with a manifold for a period of time, typically in phased relation with, 37 and preferably phase locked to, cycles of working chamber volume. Where the fluid 1 working machine comprises a plurality of said working chambers, a single fluid 2 conducting conduit may periodically align with the apertures associated with a 3 plurality of said working chambers in turn. Typically, the or each fluid conducting 4 conduit is formed in a rotatable member, such as a rotatable shaft, or a rotatable eccentric or shaft having a plurality of lobes, such as a ring cam.

7 For example, the fluid working machine may be a piston pump, with the working 8 chamber having a volume defined by a cylinder and reciprocating piston, for example, 9 a hollow piston. The fluid working machine may be a radial piston pump in which a cylinder has a base in sliding contact with an eccentric attached to (typically 11 integrated into the surface of) a rotatable crankshaft. Where the fluid working 12 machine comprises a plurality of said working chambers defined by cylinders, each of 13 which has a base in sliding contact with the same eccentric, the eccentric may 14 include one or more fluid conducting conduits adapted to periodically bring an aperture in the base of each cylinder which is in sliding contact with the eccentric into 16 fluid communication with a low pressure manifold in turn, thereby opening the 17 secondary low pressure port associated with each working chamber in turn in phased 18 relation to cycles of working chamber volume to bring each working chamber into, 19 and subsequently out of, fluid communication with the said low pressure manifold.

The said low pressure manifold may comprise the crankshaft case of a radial piston 21 pump. The one or more fluid conducting conduits may comprise one or more 22 peripheral slots extending around part of the circumference of the eccentric. Thus, 23 the or each peripheral slot may periodically bring the interior of pistons into fluid 24 communication with fluid within the surrounding crankshaft case in phased relation to cycles of working chamber volume.

27 Alternatively, the fluid working machine may be an axial piston pump in which the 28 working chamber has a volume defined by a cylinder and reciprocating piston, for 29 example, a hollow piston, driven by and in communication with a wobble plate, wherein the working chamber comprises an aperture which functions as the 31 secondary low pressure port and the wobble plate comprises one or more fluid 32 conducting conduits adapted to periodically bring the said aperture in the base of the 33 cylinder into fluid communication with a low pressure manifold, thereby periodically 34 opening the secondary low pressure port of the working chamber. Where a plurality of said working chambers are provided, more than one of which has a volume defined 36 by a cylinder and reciprocating piston in communication with the same wobble plate, 37 the one or more fluid conducting conduits are preferably arranged to periodically 1 bring the aperture in the base of each said working chamber into fluid communication 2 with a low pressure manifold to thereby open the secondary low pressure port of each 3 said working chamber in turn. The low pressure manifold in communication with the 4 one or more fluid conducting conduits may comprise the crankshaft case of an axial piston pump. The one or more fluid conducting conduits may comprise one or more 6 slots in the surface of the wobble plate arranged to periodically bring the interior of 7 the piston, or each of the said plurality of pistons in turn, into, and subsequently, out 8 of, fluid communication with fluid within the surrounding crankshaft case in phased 9 relation to cycles of working chamber volume.

11 The working chamber is preferably elongate at its maximum extent and the primary 12 low pressure valve and secondary low pressure port may be provided spaced apart 13 along the length of the working chamber, for example, at or proximate to opposite 14 ends of the working chamber. By "spaced apart along the length" we mean that a vector extending from the primary low pressure valve to the secondary low pressure 16 port has a component parallel to the length of the working chamber and do not mean 17 to imply a limitation that the said vector is necessarily parallel to the axis of the 18 working chamber.

By providing paths for fluid to enter the working chamber at two different locations 21 which are spaced apart along the length of the working chamber, the flow 22 characteristics of fluid flowing into or out of the working chamber are better than 23 would be the case if the primary low pressure valve and the secondary low pressure 24 port were adjacent. Where the working chamber is elongate whilst at maximum extent, the primary low pressure valve and the secondary low pressure port are 26 preferably provided at opposite ends of the working chamber to maximise this effect.

28 Where the working chamber is a piston-cylinder having a generally fixed end and a 29 moving end (for example, in the case of a radial or axial piston machine), the primary low pressure valve is preferably provided at the fixed end of the cylinder, to minimise 31 movement of the primary low pressure valve. The primary low pressure valve may be 32 coaxial with the cylinder or extend radially from the cylinder at the fixed end of the 33 cylinder. The high pressure valve is typically also provided at the fixed end of the 34 cylinder, typically either coaxially with or extending radially from the low pressure valve. In these arrangements, the secondary low pressure port is preferably provided 36 at the opposite end of the cylinder. This has the advantage of causing an exchange 37 of fluid in all parts of the cylinder on each cycle, reducing hot spots in the fluid around 1 the base of the cylinder. For example, the secondary low pressure port may be 2 coaxial with or extend radially from the cylinder, at the moving end of the cylinder.

4 The controller is operable to control the opening and/or closing of the primary low pressure valve. Where the high pressure valve comprises an electronically 6 controllable valve, the controller is preferably operable to control the opening and/or 7 closing of the said electronically controllable valve. Where the secondary low 8 pressure port is openable and/or closable by a secondary electronically controllable 9 low pressure valve, the controller is preferably operable to control the opening and/or closing of the secondary electronically controllable low pressure valve.

12 The controller is preferably operable to control the opening and/or closing of the at 13 least one electronically controllable valve (comprising at least the primary low 14 pressure valve) on a cycle by cycle basis by either, or preferably both, of determining whether or not to open and/or close a specific electronically controllable valve during 16 a specific cycle, and determining the phase of the opening and/or closing of a specific 17 electronically controllable valve relative to a cycle of the volume of the working 18 chamber. By controlling the opening and/or closing of the at least one electronically 19 controllable valve we include the possibility of holding a valve open or closed.

21 Typically, by controlling the opening and/or closing phase of the at least one 22 electronically controllable valve (comprising at least the primary low pressure valve) 23 on a cycle by cycle basis, the controller is operable to cause the working chamber to 24 displace a volume of fluid selected from a plurality of different selectable volumes, on a cycle by cycle basis. Typically, the plurality of different selectable volumes includes 26 the maximum volume displaceable by an individual working chamber, and no net 27 displacement. No net displacement may be achieved by an idle cycle in which the 28 electronically controllable low pressure valve remains open throughout a cycle of 29 working chamber volume or by sealing the working chamber throughout a cycle of working chamber volume, for example as described in WO 2007/088380. By 31 displacement we refer to the net movement of fluid from the or each low pressure 32 manifold to the (or each) high pressure manifold, or vice versa, and do not refer to 33 any net movement of fluid between low pressure manifolds, or high pressure 34 manifolds, which may occur. The plurality of different selectable volumes preferably also includes at least one volume, and preferably a plurality of volumes (for example, 36 a continuous range of volumes) between no net displacement and the maximum 37 volume displaceable by the working chamber. However, where a plurality of working 1 chambers are provided, the controller may also control groups of working chambers 2 in this manner. The controller typically balances the time averaged net throughput of 3 fluid of one or more working chambers against a received demand signal which may 4 be constant or variable. The fluid working machine is typically used in combination with high and/or low pressure accumulators in communication with the high and/or 6 low pressure manifolds respectively to smooth the pressure or flow of the input and/or 7 output fluid.

9 The one or more electronically controllable valves (including the electronically controllable primary low pressure valve, and the high pressure valve and/or the 11 secondary electronically controllable valve where provided) are typically face-sealing 12 valves. The one or more electronically controllable valves (including the 13 electronically controllable primary low pressure valve, and the high pressure valve 14 and/or the secondary electronically controllable valve where provided) are typically poppet valves. The one or more electronically controllable valves (including the 16 electronically controllable primary low pressure valve, and the electronically 17 controllable high pressure valve and/or the secondary electronically controllable valve 18 where provided) may be electromagnetically actuated poppet valves. The one or 19 more electronically controllable valves (including the electronically controllable primary low pressure valve, and the electronically controllable high pressure valve 21 and/or the secondary electronically controllable valve where provided) may be 22 solenoid operated poppet valves.

24 The primary low pressure valve is typically inward opening, toward the working chamber. The high pressure valve is typically outward opening, away from the 26 working chamber.

28 The fluid working machine may be a pump. The fluid working machine may be a 29 motor. The fluid working machine may be operable to function as either a pump or a motor in alternative operating modes. The fluid working machine may further 31 comprise one or more manifolds in communication with the primary low pressure 32 valve, secondary low pressure port and/or high pressure valve.

34 In embodiments in which the fluid working machine comprises a plurality of said working chambers, the optional and preferred features discussed herein typically 36 apply to each said working chamber and the primary low pressure valve, secondary 37 low pressure port and, where relevant, high pressure valve associated with each said 1 working chamber, as appropriate. Typically, the or each low and high pressure 2 manifold is in communication with more than one (for example, each) of the plurality 3 of said working chambers.

According to a second aspect of the present invention there is provided a method of 6 supplying fluid to or receiving fluid from a fluid working machine working chamber of 7 cyclically varying volume, during an intake or discharge stroke of the working 8 chamber respectively, comprising opening an electronically controllable primary low 9 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 11 active control of a controller on a cycle by cycle basis, characterised in that the 12 method further comprises opening a secondary low pressure port, in phased relation 13 to cycles of working chamber volume, to bring the working chamber into fluid 14 communication with a low pressure manifold by a second path, such that, during a portion of at least some cycles of working chamber volume, the primary low pressure 16 valve and secondary low pressure port are open concurrently such that fluid flows 17 into or out of the working chamber, as appropriate, through both the primary low 18 pressure valve and the secondary low pressure port.

Preferably, during at least some cycles of working chamber volume in which both the 21 said primary low pressure valve and the said secondary low pressure port are open 22 concurrently, the controller is operable to close the primary low pressure valve a 23 period of time after the secondary low pressure port closes.

Further optional features of the second aspect of the invention correspond to those 26 discussed in relation to the first aspect of the invention above.

28 According to a third aspect of the present invention there is provided a fluid working 29 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 31 connection of the working chamber to a high pressure manifold, and an electronically 32 controllable primary low pressure valve to control the connection of the working 33 chamber to a low pressure manifold, the controller being operable to actively control 34 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 36 on a cycle by cycle basis, the fluid working machine being operable to carry out a 37 motoring cycle under at least some circumstances, characterised in that the fluid 1 working machine is adapted to release pressurised fluid from the working chamber 2 prior to the opening of the primary low pressure valve, during a motoring cycle.

4 The resulting release of pressurised fluid preferably facilitates the opening of the primary low pressure valve. Preferably, the high pressure valve is also electronically 6 controllable and the at least one valve actively controlled by the controller typically 7 also comprises the high pressure valve.

9 The fluid working machine may comprise depressurisation means which are operable to release pressurised fluid from the working chamber prior to the opening of the 11 primary low pressure valve, during a motoring cycle, to facilitate the opening of the 12 primary low pressure valve.

14 Preferably, the working chamber has a secondary low pressure port associated therewith, which is openable and closable in phased relationship to the cycles of 16 working chamber volume to release pressurised fluid from the working chamber, for 17 example, by connecting the working chamber to a low pressure manifold, prior to the 18 opening of the primary low pressure valve, during a motoring cycle, to reduce the 19 pressure within the working chamber and thereby facilitate the opening of the primary low pressure valve.

22 Thus, by releasing pressurised fluid from the working chamber, prior to the opening of 23 the low pressure valve, during a motoring cycle, the pressure within the working 24 chamber drops more quickly than would otherwise be the case, or to a lower value than would otherwise be the case, facilitating the opening of the low pressure valve.

26 Indeed, the opening of the secondary low pressure port may trigger the opening of 27 the primary low pressure valve.

29 By releasing pressurised fluid from the working chamber prior to the opening of the primary low pressure valve we refer to releasing pressurised fluid from the working 31 chamber prior to the opening of the primary low pressure valve during a given 32 motoring cycle. Typically, the pressurised fluid is released during the second half of 33 an expansion stroke. Typically, the pressurised fluid is released after the high 34 pressure valve closes. Typically, the pressurised fluid is released between the time when the high pressure valve closes and the time when the primary low pressure 36 valve opens.

1 Preferably, the secondary low pressure port is openable and closable in phased 2 relationship to the cycles of working chamber to release pressurised fluid from the 3 working chamber, by way of a mechanical arrangement operatively linked to the 4 expansion and contraction cycles of the working chamber. Advantageously, a mechanical arrangement can be provided which can open against a significant 6 pressure differential, which substantially exceeds the pressure differential against 7 which the low pressure valve can open.

9 The timing of the opening and closing of the secondary low pressure port is selected depending on the intended application of the fluid working machine. For example, 11 where the fluid working machine comprises a rotatable shaft (e.g. in a rotary piston 12 machine) and the fluid working machine is adapted so that the rotatable shaft rotates 13 always or primarily in one direction, the period of time between the opening of the 14 secondary low pressure port and bottom dead centre may be different to the period of time between bottom dead centre and the closing of the secondary low pressure port.

16 Where the fluid working machine operates always or primarily as a motor, the 17 secondary low pressure port may be opened slightly before, at, or slightly after 18 bottom dead centre, and the secondary low pressure port may close significantly after 19 bottom dead centre, and preferably at or after the point of maximum rate of change of working chamber volume intermediate bottom dead centre and top dead centre.

21 Where the fluid working machine operates primarily as a pump, the secondary low 22 pressure port may close slightly before, or at, bottom dead centre.

24 Where the secondary low port associated with the working chamber is openable and closable in phased relationship to the cycles of working chamber volume to connect 26 the working chamber to a low pressure manifold, prior to the opening of the low 27 pressure valve, during a motoring cycle, to release pressurised fluid and thereby 28 reduce the pressure within the working chamber and facilitate the opening of the low 29 pressure valve, it may be that the secondary low pressure port remains open until at least the point in the subsequent contraction stroke where the rate of decrease of 31 working chamber volume is greatest, to facilitate the flow of fluid out of the working 32 chamber to one or more low pressure manifolds. However, it may be that the 33 secondary low pressure port closes shortly after the low pressure valve has opened.

34 It may be that the secondary low pressure port closes before the low pressure valve opens.

1 The fluid working machine may comprise a rotatable shaft, such as a crankshaft. In 2 this case, the opening and closing of the secondary low pressure port may be 3 operatively linked by a mechanical arrangement to the angle of the rotatable shaft.

4 Accordingly, the primary low pressure valve may be openable on a cycle by cycle basis under the active control of the controller, but the opening and closing of the 6 secondary low pressure port may not be variable on a cycle by cycle basis, and may 7 be fixedly phase locked to the expansion and contraction cycle of the working 8 chamber, e.g. by virtue of a mechanical arrangement operatively linked to the angle 9 of a rotatable shaft, where present. The secondary low pressure port may comprise a mechanically actuated valve operated by a pushrod mechanically linked to the 11 expansion and contraction cycles of the working chamber.

13 The secondary low pressure port may comprise one or more apertures in the working 14 chamber, for example, where the working chamber comprises a hollow piston, the secondary low pressure port may comprise an aperture in the hollow piston, such as 16 an aperture in the base of the hollow piston. The fluid working machine may be 17 operable to bring one or more fluid conducting conduits periodically into alignment 18 with the said one or more apertures to thereby bring the working chamber into fluid 19 communication with a manifold for a period of time, typically in phased relation with, and preferably phase locked to, cycles of working chamber volume. Where the fluid 21 working machine comprises a plurality of said working chambers, a single fluid 22 conducting conduit may periodically align with the apertures associated with a 23 plurality of said working chambers in turn. Typically, the or each fluid conducting 24 conduit is formed in a rotatable member, such as a rotatable shaft, or a rotatable eccentric or shaft having a plurality of lobes, such as a ring cam.

27 For example, the fluid working machine may be a piston pump, with the working 28 chamber having a volume defined by a cylinder and reciprocating piston, for example, 29 a hollow piston. The fluid working machine may be a radial piston pump in which a cylinder has a base in sliding contact with an eccentric attached to (typically 31 integrated into the surface of) a rotatable crankshaft. Where the fluid working 32 machine comprises a plurality of said working chambers defined by cylinders, each of 33 which has a base in sliding contact with the same eccentric, the eccentric may 34 include one or more fluid conducting conduits adapted to periodically bring an aperture in the base of each cylinder which is in sliding contact with the eccentric into 36 fluid communication with a low pressure manifold in turn, thereby opening the 37 secondary low pressure port associated with each working chamber in turn in phased 1 relation to cycles of working chamber volume to bring each working chamber into, 2 and subsequently out of, fluid communication with the said low pressure manifold.

3 The said low pressure manifold may comprise the crankshaft case of a radial piston 4 pump. The one or more fluid conducting conduits may comprise one or more peripheral slots extending around part of the circumference of the eccentric. Thus, 6 the or each peripheral slot may periodically bring the interior of pistons into fluid 7 communication with fluid within the surrounding crankshaft case in phased relation to 8 cycles of working chamber volume.

Alternatively, the fluid working machine may be an axial piston pump in which the 11 working chamber has a volume defined by a cylinder and reciprocating piston, for 12 example, a hollow piston, driven by and in communication with a wobble plate, 13 wherein the working chamber comprises an aperture which functions as the 14 secondary low pressure port and the wobble plate comprises one or more fluid conducting conduits adapted to periodically bring the said aperture in the base of the 16 cylinder into fluid communication with a low pressure manifold, thereby periodically 17 opening the secondary low pressure port of the working chamber. Where a plurality 18 of said working chambers are provided, more than one of which has a volume defined 19 by a cylinder and reciprocating piston in communication with the same wobble plate, the one or more fluid conducting conduits are preferably arranged to periodically 21 bring the aperture in the base of each said working chamber into fluid communication 22 with a low pressure manifold to thereby open the secondary low pressure port of each 23 said working chamber in turn. The low pressure manifold in communication with the 24 one or more fluid conducting conduits may comprise the crankshaft case of an axial piston pump. The one or more fluid conducting conduits may comprise one or more 26 slots in the surface of the wobble plate arranged to periodically bring the interior of 27 the piston, or each of the said plurality of pistons in turn, into, and subsequently, out 28 of, fluid communication with fluid within the surrounding crankshaft case in phased 29 relation to cycles of working chamber volume.

31 Thus, the secondary low pressure port may comprise one or more apertures in the 32 working chamber which are periodically revealed, or brought into alignment with a 33 fluid conduit, for example, a groove inlaid into the surface of a rotatable crankshaft.

34 Where the working chamber comprises a hollow piston which reciprocates within a cylinder, the secondary low pressure port may comprise an aperture in either or both 36 of the hollow piston, or the cylinder, which aperture is revealed, or which apertures 37 are aligned, during a motoring cycle, towards the end of the expansion stroke to 1 release pressurised fluid from the working chamber, reducing the pressure within the 2 working chamber, and thereby facilitating the opening of the low pressure valve.

4 Preferably, the pressure differential between the working chamber and the low pressure manifold into which the secondary low pressure port releases pressurised 6 fluid exceeds the pressure differential against which the primary low pressure valve 7 can open by a factor of at least 10, and typically at least 100 or at least 1,000.

9 The fluid working machine may be a motor, in which case it may be operable to carry out only motoring cycles. However, the fluid working machine may be operable to 11 function as either a motor or a pump in different operating modes, in which case it will 12 only carry out motoring cycles in circumstances where it is operating as a motor.

14 The fluid working machine typically comprises a plurality of said working chambers.

Pressurised fluid may be released from individual said working chambers, or 16 individual groups of said working chambers, at different times within cycles of the 17 volume of the respective working chambers, for example, individual said working 18 chambers, or individual groups of said working chambers, may release pressurised 19 fluid by way of a secondary low pressure port at different times in cycles of the volume of the respective working chambers. Thus, individual working chambers, or 21 individual groups of working chambers, may be optimised for different purposes.

23 The fluid working machine may also comprise one or more working chambers which 24 are not operable to release pressurised fluid from the working chamber prior to the opening of the primary low pressure valve.

27 The fluid working machine may comprise a rotatable crankshaft having a plurality of 28 working chambers arranged either individually, or in groups, at axially spaced apart 29 locations along the length of the rotatable crankshaft, each axially spaced apart location having a peripheral slot in the rotatable crankshaft through which pressurised 31 fluid can be released from the respective working chambers, wherein at least two 32 peripheral slots are located at different angles around the axis of the crankshaft so 33 that pressurised fluid cannot be retained simultaneously within all of the working 34 chambers located on one side of the crankshaft, thereby reducing the maximum potential resultant force on the crankshaft. In this case, at least two peripheral slots 36 are typically located on separate axially spaced eccentric cams, and it may be that 37 the at least two said axially spaced eccentric cams are located at different angles 1 around the axis of the crankshaft, with the respective peripheral slots each being 2 located at a similar orientation relative to the eccentric cam on which they are 3 located.

Further features of the third aspect of the invention may correspond to the features 6 discussed above in connection with the first and second aspects of the invention.

8 According to a fourth aspect of the present invention there is provided a method of 9 operating a fluid working machine working chamber of cyclically varying volume, during a motoring cycle of the working chamber, comprising opening an electronically 11 controllable primary low pressure valve, in phased relation to cycles of working 12 chamber volume, to bring the working chamber into fluid communication with a low 13 pressure manifold under the active control of a controller on a cycle by cycle basis, 14 characterised in that the method further comprises releasing pressure within the working chamber prior to the opening of the primary low pressure valve, during the 16 expansion stroke of a said motoring cycle.

18 The resulting release of pressurised fluid preferably facilitates the opening of the 19 primary low pressure valve. Preferably, pressure is released within the working chamber prior to the opening of the primary low pressure valve by opening a 21 secondary low pressure port, through which fluid can be released from the working 22 chamber.

24 Preferably also, the secondary low pressure port is opened by a mechanical arrangement which is operatively linked to cycles of working chamber volume.

26 Typically, the fluid working machine comprises a rotatable shaft, and the opening of 27 the secondary low pressure port is mechanically linked to the rotatable shaft.

29 Preferred and optional features of the method correspond to those features discussed in relation to the first three aspects of the invention.

32 The invention also extends in a fifth aspect to program code which, when executed 33 on a fluid working machine controller, causes the fluid working machine to function as 34 a fluid working machine according to the first aspect of the invention, or to cause the fluid working machine to function as a fluid working machine according to the third 36 aspect of the invention, or to carry out a method according to the second aspect of 37 the invention, or to carry out a method according to the fourth aspect of the invention.

2 The program code may take the form of source code, object code, a code 3 intermediate source, such as in partially compiled form, or any other form suitable for 4 use in the implementation of the methods of the invention. The program code may be stored on or in a carrier, which is typically a computer readable carrier such as a 6 ROM, for example a CD ROM or a semiconductor ROM, or a magnetic recording 7 medium, for example a floppy disc or hard disc. Furthermore, the carrier may be a 8 transmissible carrier such as an electrical or optical signal which may be conveyed 9 via electrical or optical cable or by radio or other means. When a program is embodied in a signal which may be conveyed directly by cable, the carrier may be 11 constituted by such cable or other device or means.

13 Description of the Drawings

An example embodiment of the present invention will now be illustrated with 16 reference to the following Figures in which: 18 Figure 1 is a schematic diagram of an individual working chamber of a fluid working 19 machine; 21 Figure 2 is a timing diagram illustrating the status of the primary low pressure valve, 22 the secondary low pressure port, and the high pressure valve, as well as the pressure 23 within a working chamber during a pumping cycle; Figure 3 is a schematic diagram of fluid flow into a working chamber of a hydraulic 26 radial piston pump, during an expansion stroke; 28 Figure 4 is a schematic diagram of fluid flow out of a working chamber of the 29 hydraulic radial piston pump of Figure 3, during a contraction stroke; 31 Figure 5 is a timing diagram illustrating the status of the primary low pressure valve, 32 the secondary low pressure port, and the high pressure valve, as well as the pressure 33 within a working chamber during a motoring cycle; Figure 6 is a timing diagram for a hydraulic motor, or hydraulic pump/motor, having a 36 depressurising port, illustrating the status of the primary low pressure valve, 1 depressurising port, and a high pressure valve, as well as the pressure within a 2 working chamber, and the crank shaft torque, during a motoring cycle; 4 Figure 7 is a schematic diagram of fluid flow out of a working chamber of a hydraulic motor, or hydraulic pump/motor having a depressurising port; 7 Figure 8 is a schematic diagram of fluid flow out of a working chamber of an 8 alternative embodiment of a hydraulic motor, or a hydraulic pump/motor, with a 9 depressurising port; 11 Figure 9 is a schematic diagram of fluid flow out of the working chamber of a further 12 example of a hydraulic motor, or hydraulic pump/motor with a depressurising port; 13 and Figure 10 is a schematic diagram showing the reduction in resultant forces on a 16 crankshaft from the release of pressurised fluid from two banks of pistons.

18 Detailed Description of Example Embodiments

Example One

22 In a first example, a fluid working machine in the form of a hydraulic pump includes a 23 plurality of working chambers. Figure 1 illustrates an individual working chamber 2 24 which has a volume defined by the interior surface of a cylinder 4 and a piston 6 which is driven from a crankshaft 8 by a crank mechanism 9 and which reciprocates 26 within the cylinder to cyclically vary the volume of the working chamber. A shaft 27 position and speed sensor 10 determines the instantaneous angular position and 28 speed of rotation of the shaft, and transmits shaft position and speed signals to a 29 controller 12, which enables the controller to determine the instantaneous phase of the cycles of each individual working chamber. The controller is typically a 31 microprocessor or microcontroller which executes a stored program in use.

33 The working chamber comprises a primary low pressure valve in the form of an 34 electronically actuatable face-sealing poppet valve 14, which faces inwards toward the working chamber and is operable to selectively seal off a channel extending from 36 the working chamber to a first low pressure manifold 16, which functions generally as 37 a net source of fluid in use. The primary low pressure valve is a normally open 1 solenoid closed valve which opens passively when the pressure within the working 2 chamber is less than the pressure within the first low pressure manifold, during an 3 intake stroke, to bring the working chamber into fluid communication with the first low 4 pressure manifold, but is selectively closable under the active control of the controller to bring the working chamber out of fluid communication with the first low pressure 6 manifold. One skilled in the art will appreciate that alternative electronically 7 controllable valves may be employed, such as normally closed solenoid opened 8 valves.

The working chamber further comprises a high pressure valve 18 in the form of a 11 pressure actuated delivery valve. The high pressure valve faces outwards from the 12 working chamber and is operable to seal off a channel extending from the working 13 chamber to a high pressure manifold 20, which functions as a net sink of fluid in use.

14 The high pressure valve functions as a normally-closed pressuring-opening check valve which opens passively when the pressure within the working chamber exceeds 16 the pressure within the high pressure manifold.

18 A secondary low pressure port 22 is openable and closable by means of a secondary 19 low pressure valve 24 which, when open, brings the working chamber into fluid communication with a second low pressure manifold 26, which also functions as a net 21 source of fluid in use. In this example, the primary low pressure valve and the 22 secondary low pressure port are connected to two distinct low pressure manifolds of 23 similar pressure. However, they may alternatively be connected to the same low 24 pressure manifold. The opening and closing of the secondary low pressure port may be phase locked to the working cycle of the working chamber, for example, by virtue 26 of a mechanical linkage 28 between the crankshaft and the secondary low pressure 27 valve. Alternatively, the opening or closing of the secondary low pressure valve may 28 be actively controlled by the controller, by virtue of an electronic connection 30.

29 Alternatively, the secondary low pressure valve may be a normally-closed pressure-openable check valve which opens responsive to a drop in the pressure of the 31 working chamber relative to the second low pressure manifold in which case neither 32 the mechanical linkage nor the electronic connection need to be present.

34 Figure 2 is a timing diagram illustrating the status of the primary low pressure valve, the secondary low pressure port, and the high pressure valve, as well as the pressure 36 within the working chamber during a pumping cycle. The primary low pressure valve 37 opens at or around top dead centre due to the pressure difference between the first 1 low pressure manifold and the working chamber which allows fluid to flow into the 2 working chamber from the first low pressure manifold to begin an intake stroke. The 3 increasing velocity of fluid past the primary valve causes the working chamber 4 pressure to fall until, for a period of time during the intake stroke, the secondary low pressure valve opens. Opening of the secondary low pressure port may be 6 mechanically phase locked to the position of the crankshaft and occur a period of 7 time after the opening of the primary low pressure valve. Alternatively, the opening of 8 the secondary low pressure valve may be caused by the increasing pressure 9 difference between the low pressure manifold and the working chamber. The secondary low pressure port is open at the point in the pumping cycle when the rate 11 of change of working cylinder volume is greatest and the additional fluid flow is of 12 greatest benefit.

14 Once the secondary low pressure valve has opened, hydraulic fluid enters the working chamber from the low pressure manifold via both the primary low pressure 16 valve and the secondary low pressure port. After a period of time, the secondary low 17 pressure valve closes so that fluid once again enters the working chamber from the 18 low pressure manifold only through the primary low pressure valve.

The controller determines the phase of the working chamber pumping cycle using the 21 received shaft position and speed signals and, at or around bottom dead centre, 22 makes a decision as to whether to select a pumping cycle or an idle cycle. In the 23 example illustrated in Figure 2, the controller selects a pumping cycle and sends a 24 signal causing the primary low pressure valve to close. The primary low pressure valve closes a period of time after the closure of the secondary low pressure port.

26 Once the primary low pressure valve closes, the working chamber is isolated from the 27 low pressure manifolds, the pressure in the working chamber increases and the high 28 pressure valve opens to receive a defined volume of fluid into the high pressure 29 manifold. During other cycles, the controller may alternatively cause the primary low pressure valve to remain open so that low pressure fluid received from both low 31 pressure manifolds is vented back to the first low pressure manifold with no net 32 displacement of fluid from the low pressure manifolds to the high pressure manifolds.

34 By providing a secondary low pressure port, the flow characteristics of the hydraulic fluid entering the working chamber during an intake stroke are better than would be 36 the case if only the primary low pressure valve was provided. For example, less 37 cavitation occurs and less drag is exerted to resist expansion of the working chamber 1 than would otherwise be the case. However, because the opening and closing of the 2 secondary low pressure port is phased away from the opening and closing of the 3 primary low pressure valve, the electronically controllable primary low pressure valve 4 controls the timing of the communication between the working chamber and the first low pressure manifold to start and finish the intake stroke. Thus, the primary low 6 pressure valve may have a smaller fluid flow cross-section than would be the case if 7 all of the fluid entered the working chamber through the primary low pressure valve.

9 Importantly, as well as determining whether or not to close or hold open the primary low pressure valve on a cycle by cycle basis, the controller is operable to vary the 11 precise phasing of the closure of the primary low pressure valve with respect to the 12 varying working chamber volume to determine the net displacement of fluid from the 13 low pressure manifolds to the high pressure manifold during a pumping cycle. As 14 described above, by keeping the primary low pressure valve open throughout a cycle an idle stroke can occur in which, although fluid flows into the working chamber from 16 the low pressure manifolds and flows out to the first low pressure manifold there is no 17 net displacement from the low pressure manifolds to the high pressure manifold.

18 (There may be net displacement from the second low pressure manifold to the first 19 low pressure manifold, but this is not considered to be net displacement by the pump). A partial stroke which displaces a volume of fluid equal to a proportion 21 (usually a relatively small proportion) of the capacity of the working chamber may be 22 implemented by delaying closure of the primary low pressure valve and opening of 23 the high pressure valve until just before top dead centre, and the precise volume 24 which is displaced may be selected by the precise timing of these events. The precise timing of the opening and/or closing of the primary low pressure valve and the 26 high pressure valve may also be varied in specific circumstances, such as start-up, 27 operation while still relatively cold, and shut down of the device. Further details of 28 these timing options are disclosed in EP 0 361 927, EP 0 494 236 and EP 1 537 333, 29 the contents of which are incorporated herein by virtue of this reference.

31 Fluid discharged through the high pressure manifold is typically delivered to a 32 pressure accumulator to smooth the output pressure and the time averaged 33 throughput is varied by the controller on the basis of a demand signal received by the

34 controller in the manner of the prior art.

1 Example Two

3 In a second example, the fluid working machine is operable to function as either a 4 motor or a pump. The structure of the second example fluid working machine also corresponds to the structure illustrated in Figure 1. In this embodiment, the primary 6 low pressure valve functions as a net source of fluid or a net sink in pumping or 7 motoring mode respectively. The secondary low pressure port also functions as 8 either a net source of fluid or net sink respectively, and the high pressure valve 9 functions as either a net sink of fluid or net source respectively. A single low pressure manifold functions as either a net source of fluid, in pumping mode, or as a sink of 11 fluid, in motoring mode, and the high pressure manifold functions as either a sink of 12 fluid, in pumping mode, or as a source of fluid, in motoring mode. During idle strokes 13 in which a working chamber is kept in fluid communication with the low pressure 14 manifold, neither manifold functions as a net source or sink of fluid.

16 As with the first example, the primary low pressure port is an inward facing 17 electronically controllable poppet valve. However, in this example, the secondary low 18 pressure port and the high pressure valve also comprise electronically actuatable 19 poppet valves which face inwards and outwards respectively and which are actively controllable by the controller on a cycle by cycle basis through electronic connections 21 30 and 32. In pumping mode, the timings of the secondary low pressure port and the 22 high pressure valve are the same as in the first example. In motoring mode, fluid is 23 received through the high pressure valve during working chamber expansion strokes 24 to drive the crankshaft and output through the primary low pressure valve during working chamber contraction strokes. The secondary low pressure port opens for a 26 portion of the contraction stroke to provide an additional path for fluid to be displaced 27 from the working chamber.

29 By using an electronically controllable valve to regulate the secondary low pressure port, rather than a mechanical arrangement driven from the crankshaft, the controller 31 can open the secondary low pressure port during expansion strokes when the fluid 32 working machine is operating as a pump and during contraction strokes when the 33 fluid working machine is operating as a motor.

In an alternative implementation of this second example embodiment, the secondary 36 low pressure port may be closed by means of a pressure-operated check valve not 37 under the control of the controller. The pressure-operated check valve allows fluid to 1 be received into the cylinder from the low pressure manifold on the expansion stroke 2 when the primary low pressure valve is open. By using a pressure-operated check 3 valve to provide a second path for fluid to enter the working chamber, the working 4 chamber is more easily able to receive fluid from the low pressure manifold and can thus avoid cavitation. The pressure-operated check valve will be closed on the 6 contraction stroke either when exhausting to the low pressure manifold in an idle or 7 motor exhaust stroke, and closed on the expansion stroke during a motor stroke.

9 Example Three

11 In a third example embodiment a fluid working machine in the form of a hydraulic 12 radial piston pump uses a slotted crankshaft to provide a secondary low pressure 13 port. Figure 3 illustrates fluid flow through an individual working chamber 100, 14 defined by the interior surface of a cylinder 102 and reciprocating hollow piston 104, part way through an expansion stroke.

17 The working chamber has a primary low pressure valve 106, in the form of an 18 electronically controllable poppet valve, which is openable and closable to bring the 19 working chamber into and out of fluid communication with a first low pressure manifold 108. A high pressure valve in the form of a pressure-operable discharge 21 valve 110 is openable and closable to bring the working chamber into and out of fluid 22 communication with a high pressure manifold 112. The base 114 of the piston is in 23 sliding contact with a crankshaft eccentric 116. An aperture 118 in the base of the 24 piston functions as a secondary low pressure port which is open when a slot 120, which extends around a portion of periphery of the eccentric, extends across either 26 side of the piston wall to bring the interior of the working chamber into fluid 27 communication with hydraulic fluid within the crankshaft case 122, which functions as 28 a second low pressure manifold. Accordingly, for a portion of the expansion stroke, 29 fluid will flow into the working chamber both (i) through the primary low pressure valve and (ii) through the crankshaft slot and the aperture in the base of the piston.

32 As before, the secondary low pressure port opens a period of time after the primary 33 low pressure valve opens due to the pressure in the working chamber 100 falling to a 34 level where it is no longer held closed, and the secondary low pressure port closes a period of time before the controller may optionally send a signal to cause the primary 36 low pressure valve to close so as initiate the pumping mode on the contraction stroke.

1 Figure 4 illustrates the fluid flow during the subsequent contraction stroke, where the 2 primary low pressure valve and secondary low pressure port are both closed, by the 3 electronically controllable poppet valve and the body of the crankshaft eccentric 4 respectively, and fluid is displaced to the high pressure manifold through the high pressure discharge valve. The opening and closing of the secondary low pressure 6 port is phase locked to the cycles of working chamber volume, as defined by the 7 location of the slot on the crankshaft eccentric. The variation in working chamber 8 pressure during the expansion and contraction strokes corresponds to that illustrated 9 in Figure 2.

11 This arrangement has several advantages. Firstly, by supplying fluid concurrently 12 from either end of the elongate working chambers during the part of the expansion 13 stroke where the volume of the working chambers is most rapidly increasing, fluid 14 need not flow as quickly and so the flow characteristics of fluid entering the working chambers are improved. Secondly, there is not a pool of fluid at the moving end of 16 each working chamber which can remain in place from one cycle to the next. A fresh 17 supply of fluid enters the aperture in the base of each piston during each cycle, 18 cooling the base of each piston. Furthermore, centrifugal forces act in the same 19 direction as net fluid flow from the crankshaft to the high pressure outlet, increasing the overall efficiency of the pump.

22 Example Four

24 The arrangement of Figure 3 and 4 can operate as a hydraulic radial piston motor by the use of an active high pressure valve and by changing the location of the slot on 26 the crankshaft to amend the phase of the opening of the secondary low pressure port 27 so that the secondary low pressure port opens during the contraction stroke rather 28 than the expansion stroke. Figure 5 illustrates the opening and closing of the phase- 29 locked secondary low pressure port during the motoring cycle. In this case the pressure in the working chamber rises during the exhaust of fluid to the low pressure 31 manifold through the primary low pressure port, until the opening of the phase-locked 32 secondary low pressure port provides an alternative flow path and reduces the 33 working chamber pressure.

1 Example Five

3 A fifth example embodiment addresses technical problems related to the opening of a 4 low pressure valve during a motoring cycle of a fluid working motor, or a fluid working machine which can operate as either a motor or a pump, in different operating 6 modes.

8 This embodiment corresponds to the hydraulic radial piston motor of Example Four, 9 except that the location of the slot on the crankshaft is positioned so that the secondary low pressure port opens shortly before the end of the expansion stroke, 11 after the high pressure valve has closed, is phase locked to cycles of working 12 chamber volume.

14 The effect of this arrangement on the operation of the fluid working machine is illustrated in Figure 6. The operation of the fluid working motor is conventional during 16 the first part of the expansion stroke. Pressurised fluid is received from the high 17 pressure manifold into the working chamber, through an active high pressure valve.

18 Once the high pressure valve is closed, the pressure within the working chamber 19 begins to decrease, however, the fluid within the working chamber remains pressurised. After the closure of the high pressure valve, but before bottom dead 21 centre, the slot aligns with the base of the working chamber piston forming a 22 secondary low pressure port. Pressurised fluid vents from the interior of the working 23 chamber into the crankshaft case via the crankshaft slot. Accordingly, the pressure 24 within the working chamber drops rapidly to close to the pressure of the low pressure manifold. The low pressure valve, which is gently biased to an open position by a 26 weak spring, therefore opens passively against only a minimal pressure differential.

27 Shortly after bottom dead centre, the slot no longer aligns with the base of the piston 28 and so the secondary low pressure port closes. The low pressure valve may 29 alternatively be dragged open when the pressure within the working chamber is sufficiently low.

32 Because the slot is integral to the crankshaft, it can open despite the substantial 33 pressure differential between the working chamber and the surrounding crankshaft 34 case. An electronically controllable low pressure valve which could open against the substantial pressure differentials which occur at this point in a motoring cycle in many 36 practical applications would require considerable power and/or open more slowly.

37 Furthermore, the provision of a secondary low pressure port, or other depressurising 1 means, enables the time which elapses between the closure of the high pressure 2 valve and the opening of the low pressure valve to be less than would otherwise be 3 the case, allowing the high pressure valve to close later and/or the low pressure valve 4 to open earlier than would otherwise be the case and thereby minimising the amount of time that the working chamber is not either receiving high pressure fluid or 6 releasing fluid to the low pressure manifold, and thereby increasing the energy 7 efficiency of the fluid working machine. In the example illustrated in Figure 6, were it 8 not for the release of pressurised fluid using the secondary low pressure port, the 9 pressure within the working chamber would follow the path illustrated with a dashed line, in which case the low pressure valve would not open.

12 It is also envisaged that the secondary low pressure port could remain open until at 13 least the point in the contraction stroke where the volume of the working chamber is 14 most rapidly changing, to enable fluid to flow out of the working chamber to the low pressure manifold through both the primary low pressure valve and the secondary 16 low pressure port concurrently.

18 Example Six

In further example embodiment, illustrated in Figure 8, an aperture 123, is provided 21 towards the radially outwards end of a piston. The portion of the piston which 22 includes the aperture extends out of the cylinder, forming a secondary low pressure 23 port through which hydraulic fluid can be released to the crankshaft case, from shortly 24 before to shortly after bottom dead centre. In an alternative embodiment, illustrated in Figure 9, an aperture 124 is instead provided towards the radially inwards end of 26 the cylinder, forming a secondary low pressure port through which pressurised 27 hydraulic fluid can be released to the crankshaft case, from shortly before to shortly 28 after bottom dead centre. In a further embodiment, apertures can be provided in 29 each of the piston and the cylinder which overlap for a period of time from shortly before to shortly after bottom dead centre.

32 One skilled in art will appreciate that secondary low pressure ports which open to 33 vent fluid from the working chamber of a fluid working machine, during a motoring 34 stroke, to facilitate the opening of a primary low pressure valve, could be implemented in numerous ways. Mechanically linking the opening and closing of the 36 secondary low pressure port to cycles of working chamber volume has the advantage 1 that the secondary low pressure port can be opened against a substantial pressure 2 differential.

4 With reference to Figure 10, one possible implementation of the invention is in a fluid working machine, which includes a crankshaft, with a plurality of banks of working 6 chambers (130a to 130f, and 132a to 132f) arranged at axially spaced apart locations 7 along the crankshaft, each bank having an eccentric cam 116. Preferably, the 8 eccentric cams are arranged in different phases with respect to each other. In this 9 case, a peripheral slot in each crankshaft eccentric (122a and 122b) can be provided in respect of each bank of working chambers, and the peripheral slots in each 11 crankshaft eccentric can be arranged at similar orientations with respect to the 12 eccentric in which they lie, so that it is not possible to retain pressurised fluid 13 simultaneously within all working chambers on any one side of the crankshaft.

14 Because pressurised working chambers apply forces orthogonal to the axis of the crankshaft to said crankshaft, this reduces the maximum potential resultant force on 16 the crankshaft, in a plane orthogonal to the axis of the crankshaft, reducing the net 17 forces on the crankshaft, potentially increasing operating lifetime, and reducing 18 vibration.

Further variations and modifications may be made within the scope of the invention 21 herein disclosed.