GB2370611A

GB2370611A - Hydraulic Accumulators

Info

Publication number: GB2370611A
Application number: GB0026423A
Authority: GB
Inventors: David Anthony Harries
Original assignee: LuK Lamellen und Kupplungsbau GmbH
Current assignee: LuK Lamellen und Kupplungsbau GmbH
Priority date: 2000-10-28
Filing date: 2000-10-28
Publication date: 2002-07-03
Also published as: GB2370318A; DE10148082A1; GB0026423D0; GB0125038D0; GB2370318B; ITMI20012264A1

Abstract

A hydraulic accumulator (275) includes a housing (300) defining a cylinder (302), a piston (304) is slidably mounted in the cylinder (302), the piston (304) being sealed with respect to the wall of the cylinder (302), a stem (306) extends from the piston (304) to one side thereof, the stem (306) extending coaxially of the cylinder (302) beyond one end of the cylinder (302) and a leaf spring (322) acting on an end of the stem (306) remote from the piston (304), the leaf spring (322) biasing the piston (304) towards the other end of the cylinder (302), said other end of the cylinder (302) being closed and a port (308) opening to the cylinder (302) adjacent said other end.

Description

237061 1

HYDRAULIC ACCUMULATORS

The present invention relates to hydraulic accumulators and in particular to hydraulic accumulators for use in automobile transmission systems.

Hydraulic accumulators in accordance with the present invention may however also be used in other applications.

In automated transmission systems of, for example, the type disclosed in WO97/05410 or W097/40300, whose content is expressly incorporated in the disclosure content of the present application, fluid actuators are

used to control actuation of a clutch actuator mechanism and/or a gear engaging mechanism.

Such systems conventionally utilise gas accumulators, in which a chamber containing gas is separated from a hydraulic fluid chamber by a flexible membrane, whereby fluid may be pumped into the accumulator, compressing the gas, so that the fluid is stored under pressure. Gas accumulators of this type have the advantage that they follow the gas laws, so that pressure is proportional to volume. Consequently, the gas accumulator may be designed to store a relatively large volume of fluid without a large pressure change. Gas accumulators however have the disadvantages that: a) temperature variation causes changes in the stored pressure/volumes; b) the permeability of the membrane causes loss of gas pressure, requiring periodic replacement of the diaphragm and/or regular re-charging of the accumulator; c) the gas, membrane and fluid must be selected for chemical compatibility; d) the container needs to be strong (and heavy) and has to contain both the fluid and gas under pressure; and

e) the compliance of the system is not consistent and consequently it is difficult to control pressure in the system.

in more recent hydraulic actuation systems for controlling motor vehicle transmission systems, for example as disclosed in UK Patent Applications GB0024999.5 and GB0025000.1, whose content is expressly incorporated in the disclosure content of the present application, it has

been proposed to use spring accumulators in place of a gas accumulator.

Spring accumulators which comprise a piston mounted in a cylinder, the piston sealingly engaging the walls of the cylinder and being urged towards one end thereof by spring means; have the advantage that the spring rate and consequently the pressure of fluid stored in the accumulator, does not vary significantly with temperature. Moreover, the use of a piston in place of a diaphragm solves the permeability problem and simplifies the compatibility problem, allowing many more seal and fluid combinations to be used. Spring accumulators also provide consistent compliance which will simplify control of pressure in the system. Such spring accumulators normally use a coiled compression spring.

Because with coiled compression springs, the force/deflection relationship is linear, in order to achieve a high load but with a low spring rate, a long spring with a large number of turns is required. This will add considerably to the weight and size of the accumulator. As weight and spatial restraints are critical in the design of hydraulic actuation systems for motor vehicle transmission systems, this represents a significant drawback to the use of spring accumulators.

The present invention provides an improvement in the design of spring accumulators which will mitigate this problem.

According to one aspect of the present invention, a hydraulic accumulator comprises a cylinder, a piston slidably mounted in the cylinder, the piston being sealed with respect to the wall of the cylinder, a stem extending from the piston to one side of the piston, the stem extending coaxially of the cylinder beyond one end of the cylinder and a leaf spring acting on the end of the stem remote from the piston, the leaf spring Liaising the piston towards the other end of the cylinder, said other end of the cylinder being closed and a port being provided to the cylinder adjacent said other end thereof. The use of a leaf spring in the manner described above avoids the disadvantages of a coil compression spring, providing a low spring rate without significantly increasing the overall size and weight of the accumulator. The spring force of the leaf spring can be varied by modifications of the blade design, for example the blade width, thickness, tapering of the blade or use of multi-blades, as well as the material from which it is made. The leaf spring may also be designed to provide a variation in spring rate as it is deflected, or may be provided with means for adjusting the spring rate.

According to a preferred embodiment of the invention, the leaf spring may be formed as an integral part of a mounting bracket for the accumulator.

Furthermore, a pump and pump motor may be located in a common housing with the accumulator.

The invention is now described by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows diagrammatically a semi-automated transmission system utilising a hydraulic actuation system in accordance with the present invention;

Figure 2 shows a gear selector mechanism and associated selector gate of the transmission system illustrated in Fig. 1; Figure 3 illustrates diagrammatically the hydraulic actuation system of the transmission system illustrated in Fig. 1; Figure 4 shows a sectional diagrammatic illustration of the main control valve of the hydraulic actuation system illustrated in Fig. 3, in an energised second position; Figure 5 shows a view similar to Fig. 4 of the main control valve in an energised third position; Figure 6 shows a view similar to Fig. 4 of the main control valve in an energised fourth position; Figure 7 shows a sectional diagrammatic illustration of the gear shift control valve of the hydraulic actuation system illustrated in Fig. 3, in an energised null position; Figure 8 shows a view similar to Fig. 7 with the gear shift control valve in an energised third position; Figure 9 shows a view similar to Fig. 7 of the gear shift control valve in an energised fourth position; and Figure 10 shows a sectional side elevation of a spring accumulator, pump and pump motor unit used in the system illustrated in Figs. 1 to 9.

Figure 1 of the accompanying drawings shows an engine 10 with a starter and associated starter circuit 1 Oa which is coupled through the main drive friction clutch 14 to a multi-speed synchromeshed lay shaft-type gearbox

12, via a gearbox input shaft 15. Fuel is supplied to the engine by a throttle 16 which includes a throttle valve 18, operated by accelerator pedal 19. The invention is equally applicable to electronic or mechanical fuel injection petrol or diesel engine.

The clutch 14 is actuated by a release fork 20 which is operated by a hydraulic slave cylinder 22, under the control of a clutch actuator control means 38.

A gear selector lever 24 operates in a gate 50 having two limbs 51 and 52 joined by a cross track 53 extending between the end of limb 52 and intermediate of the ends of limb 51. The gate 50 defines five positions; "R" at the end of limb 52; UN" intermediate of the ends of the cross track 53; "S" at the junction of limb 51 with the cross track 53; and +,, and "-" at the extremities of limb 51. In limb 51 the lever 24 is biased to the central "S" position. The "N" position of the selector lever 24 corresponds to neutral; "R" corresponds to selection of reverse gear; "S" corresponds to selection of a forward drive mode; momentary movement of the lever to the " + " position provides a command to cause the gearbox to shift up one gear ratio; and momentary movement of the gear lever 24 to the _n position provides a command to cause the gearbox to shift down one gear ratio.

The positions of the lever 24 are sensed by a series of sensors, for example micro switches or optical sensors, positioned around the gate 50.

Signals from the sensors are fed to an electronic control unit 36. An output from the control unit 36 controls a gear engaging mechanism 25, which engages the gear ratios of the gearbox 12, in accordance with movement of the selector lever 24 by the vehicle operator.

In addition to signals from the gear selector lever 24, the control unit 36 receives signals from:

sensor 19a indicative of the degree of depression of the accelerator pedal 19; sensor 30 indicative of the degree of opening of the throttle control valve 18; sensor 26 indicative of the engine speed; sensor 42 indicative of the speed of the clutch driven plate; and sensor 34 indicative of the clutch slave cylinder position.

The control unit 36 utilises the signals from these sensors to control actuation of the clutch 14 during take-up from rest and gear changes, for example as described in patent specifications EP0038113, EP0043660,

EP0059035, EP0101220 and W092/13208 whose content is expressly incorporated in the disclosure content of the present application.

In addition to the above mentioned sensors, control unit 36 also receives signals from a vehicle speed sensor 52, ignition switch 54 and brake switch 56 associated with the main braking system, for example the footbrake 58 of the vehicle.

A buzzer 50 is connected to the control unit 36 to warn/indicate to the vehicle operator as certain operating conditions occur. In addition or in place of the buzzer 50 a flashing warning light or other indicating means may be used. A gear indicator 60 is also provided to indicate the gear ratio selected.

As illustrated in Figure 2, the gear engagement mechanism 25 comprises three shift rails 111,112,113 mounted parallel to one another for movement in an axial direction. Each shift rail 111,112,113 is associated with two of the gear ratios of the gearbox 12, via a selector fork and synchromesh unit in conventional manner, so that movement of the shift rails 111,112,113 in one axial direction will cause engagement of one of the associated gear ratios and axial movement of the shift rail

1 1 1,1 12,1 13 in the opposite axial direction will cause engagement of the other associated gear ratio.

Typically; first and second gear ratios are associated with shift rail 111, so that axial movement of the shift rail 111 in a first direction will engage first gear or axial movement of shift rail 111 in a second direction will engage second gear; third and fourth gear ratios are associated with shift rail 1 12, so that axial movement of shift rail 1 12 in the first direction will engage third gear or axial movement of shift 1 12 in a second direction will engage fourth gear; and fifth and reverse gear ratios are associated with shift rail 1 13, so that axial movement of shift rail 1 13 in the first direction will engage fifth gear while axial movement of shift rail 113 in the second direction will engage reverse gear.

A selector member 1 10 is mounted for movement in a select direction X transverse to the axes of the shift rails 1 1 1,1 12, 1 13 and in a shift direction Y. for movement axially of the shift rails 1 1 1,1 12 and 1 13. The selector member 1 10 may thus be moved in direction X along a neutral plane A-B, so that it may be indexed with and engaged a selected one of the shift rails 1 1 1,1 12 and 1 1 3. The selector member 1 10 may then be moved in direction Y to move the engaged shift rail 1 1 1,1 1 2,1 1 3 axially in either direction to engage one of the gear ratios associated therewith.

As illustrated in Fig. 3, selector member 1 10 is movable in the select direction X by means of a fluid pressure operated select actuator 1 14, along the neutral plane A-B of the gate illustrated in Fig. 2, to align the select member 1 10 with one of the shift rails 1 1 1,1 12, 1 13, and thereby select a pair of gears associated with that shift rail. The selector member 1 10 may then be moved in the shift direction Y by means of a fluid pressure operated shift actuator 1 1 5, to move the shift rail 1 1 1,1 1 2,1 13 axially in either direction to engage one of the gear ratios associated therewith.

The actuators 114 and 115 each comprise a double-acting ram having pistons 116,117 respectively, which divide the actuators 114,115 into two working chambers 118,119, the working chambers 118,119 being disposed on opposite sides of each of the pistons 116,117. Operating rods 114a,115a extend from one side of the pistons 116,117 respectively and are operatively connected with the selector member 110 for movement thereof in the select and shift directions X and Y respectively.

As a consequence of the connection of operating rods 114a,115a to the pistons 116,117, the working area of pistons 116,117 exposed to working chamber 118 is smaller than the working area of pistons 116,117 exposed to working chamber 119.

A solenoid operated main control valve 120 comprises a housing 122, defining a bore 124. A spool 126 is slidably located in the bore 124, the spool 126 having three axially spaced circumferential lands 128,130,132 which sealingly engage the bore 124. A solenoid 134 acts on one end of the spool 126, so that upon energisation of the solenoid 134, the spool 126 is moved axially of the bore 124 against a load applied by a compression spring 136, acting on the opposite end of the spool 126.

An inlet 138 to the bore 124 of valve 120 is connected to a spring accumulator 275. An electrically-driven pump 223 is provided to charge the accumulator 275 via a non-return valve 276. An outlet 140 from the bore 124 is connected to a reservoir 278. A first port 142 from bore 124 is connected to working chambers 118 of the select and shift actuators 114,115 and selectively to working chambers 119 via select and shift valves 144,146 and a second port 148 is connected to the clutch slave cylinder 22. A pressure relief valve 280 is provided between the outlet of the pump 223 and the reservoir 278, to ensure that the pressure supplied by the pump 223 does not exceed a minimum predetermined value.

The shift and select valves 144,146 are both solenoid operated valves having a housing 150 defining a bore 151 with a spool 152 slidably mounted in the bore 151. The spool 152 has three axially spaced circumferential lands 154,156,158, the lands sealingly engaging the bore 151. An axial bore 160 opens to end 162 of the spool 152 and connects to a cross-bore 164, the cross-bore 164 opening between lands 154 and 156 of the spool 152. A solenoid 166 acts on end 168 of spool 152 remote from the end 162, so that upon energisation of the solenoid 166, the spool 152 will move axially of the bore 151 against a load applied by a compression spring 170 acting on end 162 of the spool 152.

An inlet 172 to the bore 151 is connected to port 142 of the main control valve 120. An outlet 174 from the bore 151 is connected to the reservoir 278. Port 178 of the select valve 144 is connected to the second working chamber 119 of the select actuator 114 and port 178 of shift valve 146 is connected to the second working chamber 119 of shift actuator 115.

The construction and operation of the valves 144 and 146 and actuators 114 and 115 are identical as illustrated in figures 7 to 9.

When the transmission is in gear and the clutch 14 engaged, the solenoids 134 and 166 will be de-energised and valves 120,144 and 146 will be in the rest positions illustrated in Fig. 3. In this position, the clutch slave cylinder 22 is connected via port 148 and outlet 140 of the main control valve 120 to the reservoir 278; the working chambers 118 of the select and shift actuators 114,115 will be connected to the reservoir 278 via inlet 172, passageways 164,160 and outlet 174 of the select and shift valves 144,146; and working chambers 119 of the select and shift actuators 114,115 will be connected to the reservoir 278 via port 178 and outlet 174 of the select and shift valves 144,146. There will

consequently be no movement of the clutch slave cylinder 22 or select and shift actuators 114,115.

When a gear change is initiated by, for example, the driver of the vehicle moving the gear selector lever 24 momentarily to the '+' position, or by automatic initiation, solenoid 134 is energised to move the spool 126 of main control valve 120 to a second position, as illustrated in Figure 4. In this second position the working chambers 118 of both the select and shift actuators 114,115, and inlets 172 of the select and shift valves 144,146 are connected to the spring accumulator 275, via port 142 and inlet. In this second position the clutch slave cylinder 22 remains connected to the reservoir 278.

Simultaneously, with energisation of solenoid 134 to move the main control valve 120 to the second position illustrated in figure 4, solenoids 166 of the select and shift control valves 144,146 are energised to move the spool 152 to a null position as illustrated in Fig. 7. In this position, the land 158 of spool 152 closes port 178 thereby closing working chamber 119 and creating a hydraulic lock preventing movement of the select and shift actuators 114 and 115, even though working chambers 118 thereof are connected to the spring accumulator 275 by the 144,146 and the main control valve 120. The connection of port 172 to the outlet 174 via bores 160 and 164 of the select and shift valves 144,146, is also closed. Further energisation of the solenoid 134 to the third position illustrated in figure 5 will then close the connection between the clutch slave cylinder 22 and the reservoir 278 and open the connection between the clutch slave cylinder 22 and the spring accumulator 275, actuating the release fork 20 to disengage the clutch 14.

Upon disengagement of the clutch 14, solenoid 134 of the main control valve 120 may be energised to move the main control valve back to a fourth position, as illustrated in figure 6. In this fourth position, the port 148 is isolated from the inlet 138 and the outlet 140, so that the clutch 14 will be clamped in the disengaged position. The solenoids 166 of the select and shift valves 144,146 may then be selectively energised, moving the select and shift valves 144,146 between third and fourth positions, in order to disengage the currently selected gear and engage a new gear.

Energisation of solenoid 166 to move the select or shift valve 144,146 to the third position illustrated in Fig. 8, in which working chamber 119 is connected to reservoir 278, while working chamber 118 is connected to the accumulator 275, will create a pressure differential across the pistons 116 and 117, causing the operating rod 114a,115a to extend.

Energisation of Solenoid 166 to move the select or shift valve 144,146 to the fourth position illustrated in Fig. 9, in which both working chambers 118 and 119 are connected to the accumulator 275, will cause the operating rods 114a,115a to retract, due to the differential working areas of the pistons 116 and 117. Consequently, by appropriate control solenoids 166 of the select and shift valves 144,146, the selector member 110 may be moved to engage the desired gear.

Potentiometers 226 and 227 are connected to the operating rods 114a,115a respectively, to provide signals indicative of the position of the associated operating rods. Signals from the potentiometers 226,227 are fed to the control unit 36 to provide an indication of the position of the operating rods 114a,115a, for each of the gear ratios of the gear box 12 and also to indicate the position of the operating rod 115a, when the select member 110 is in the neutral plane A-B of Fig. 2. The transmission system may thus be calibrated, so that predetermined position signals

from the potentiometers 226 and 227 correspond to engagement of each of the gear ratios of the gear box 12.

Measurements from the potentiometers 226 and 227 may thus be used by a closed loop control system to control valves 144 and 146, to move the operating rods 114a and 115a, to the predetermined positions to engage the desired gear ratio.

When the desired gear ratio has been engaged, the solenoids 166 of the select and shift valves 144,146 are energised to move the valves 144,146 back to their null positions, closing the ports 178 and creating a hydraulic lock preventing movement of the actuators 114,115.

Solenoid 134 of the main control valve 120 may then be energised to move the main control valve 120 from its fourth to its second position, thereby allowing fluid from the clutch slave cylinder 22 to be returned to the reservoir 278, permitting re-engagement of the clutch 14. The main control valve 120 may be switched between the third and second positions, so that the clutch 14 is re-engaged in controlled manner, for example as disclosed in EP0038113; EP0043660; EP0059035; EP0101220 or W092/13208.

When the clutch 14 has been re-engaged, solenoid 134 of the master control valve 120 may be de-energised, so that it returns to the rest position illustrated in Fig. 3. Similarly the solenoids 166 of the shift and select valves 144,146 may be de-energised. Movement of the select and shift valves 144,146 to the rest position illustrated in Fig. 3 will open working chamber 119 to reservoir 278, thereby releasing pressure therein.

During the gear selection with the main control valve 120 in the fourth position, illustrated in figure 6, the force applied by the select and shift actuators 114,115 may be controlled by controlling the pressure of fluid

applied by the pump 223, which is in turn controlled by the pump motor current, using pulse width modulation. Alternatively, the pressure in the system may be controlled by switching the pump 223 on and off at predetermined times or by suitable manipulation of the proportional flow control valves 144 and 146. Consequently the pressure may be varied depending on the type of change required, for example a fast change or slow change; the gear step concerned; and whether it is a change-up or change-down. Furthermore the pressure, particularly to the shift actuator 115 may be varied during a gear change, for example to vary the force applied during the run-up; synchronization; shift-through; and dog-in phases of the gear change. In particular the loads applied during the synchronization phase may be reduced in order to reduce ware on the synchromeshes. As illustrated in page 10, the spring accumulator 275 comprises a housing 300. The housing 300 defines a closed bore 302 and a piston 304 is slidably located in the closed bore 302, the piston 304 being sealed with respect to the cylindrical wall of bore 302.

A stem 306 mounted with respect to piston 304 extends to one side of the piston 304 coaxially of the bore 302, towards the open end of the bore 302. The stem 306 is dimensioned to extend beyond the open end of bore 302 when the piston 304 is at the end of its extremity of movement towards the closed end of bore 302. A port 308 opens to the closed end of bore 302.

The housing 300 also defines a body 310 of the pump 223, which is located in juxtaposed position to the bore 302 of the accumulator 275. A pump motor 312 is attached to the pump body 310 to drive the pump 223. An inlet 314 is provided for connection of the pump 223 to the reservoir 278. The inlet from the pump 223 which is connected to the bore 302 adjacent the closed end thereof, is formed by passageways (not

shown) defined in the housing 300, these passageways defining a housing for the non return valve 276. The housing 300 may also define a housing and suitable connections for the pressure release valve 280.

A bracket formation 320 is secured to the housing 300 by which it may be mounted to a support structure, in suitable manner. An extension 322 of the bracket formation 320 is made of spring material and bears against the free end 324 of stem 306, the extension 322 thereby acting as a leaf spring, urging the piston 304 towards the closed end of bore 302. As fluid is pumped by pump 223 into the bore 302, the piston 304 will be moved towards the open end of bore 302, thereby deflecting the extension 322 of bracket formation 320. The fluid stored in the bore 302 will thereby be pressurised as the extension 322 is deflected.

The pressure of fluid in the accumulator will depend on the spring rate of the extension 322 of bracket 320. This may be designed as required, by suitable selection of material from which it is made and the width and thickness of the extension 322. Moreover, the extension 322 may be tapered in width or thickness, or made of multiple layers of material, in order to provide the spring rate required. Furthermore, the extension 322 may be designed to provide a varying spring rate as it is deflected and/or means may be provided for adjustment of the spring rate.

While in the above embodiment the leaf spring is defined by an integral extension 322 of the bracket 320, the leaf spring may alternatively be formed separately and secured to, for example, the housing 300 or bracket 320, in suitable manner.

Furthermore, the accumulator, pump and associated components may alternatively be provided with separate housings which are interconnected by suitable pipelines.

In accordance with a further modification of the invention, the open end of bore 302 may be closed and vented to atmosphere to provide the reservoir 278 for the system. With this construction, the vented chamber on one side of the piston 304 would preferably be connected to the inlet side of the pump 223, by passageways defined in the housing 300.

According to another embodiment of the invention, the bores 124 and 151 of the main control valve 120 and the select and shift valves 144,146 and also of the select and shift actuators 114,115 may be defined by a common housing, the bores 124,151 of the various components being appropriately inter-connected by passages through the common housing. The valve/actuator pack so formed would be mounted on or adjacent the gearbox 12. This valve/actuator pack may also include the accumulator/pump pack described above.

Various modifications may be made without departing from the invention.

For example, while in the above embodiment the hydraulic circuit has been described with reference to a semi-automated transmission system, the invention is equally applicable to fully-automated transmission systems or to automated manual transmission systems.

While in the above embodiment the pressure in the system may be controlled by switching of the pump 223 in predetermined manner or by the pump motor current, a pressure transducer 282 as illustrated in broken line in figure 3 may optionally be included in the gear engagement system (114,115,144,146), said pressure transducer 282 being used, in a closed loop feedback control system, to provide accurate control of the pressure. Moreover while in the embodiment described above the clutch slave cylinder 22 is connected directly to the main control valve 120, a remote displacement valve with position sensing means of the type disclosed in EP 0702760 whose content is expressly incorporated in the disclosure

1 6 content of the present application, may be interposed between the main control valve 120 and clutch slave cylinder 22.

The patent claims submitted with the application are proposed formulations without prejudice to the achievement of further patent protection. The applicant reserves the right to submit claims for further combinations of characteristics, previously only disclosed in the description and/or drawings.

References back used in sub-claims refer to the further development of the subject of the main claim by the characteristics of the respective sub-

claim; they are not to be understood as a waiver with regard to achieving independent item protection for the combination of characteristics in the related sub-claims.

Since the subject of the sub-claims can form separate and independent inventions with reference to the prior art on the priority date, the applicant

reserves the right to make them the subject of independent claims or of division declarations. Furthermore, they may also contain independent inventions which demonstrate a design which is independent of one of the objects of the preceding sub-claims.

The embodiments are not to be considered a restriction of the invention.

Rather, a wide range of amendments and modifications is possible withithe scope of the current disclosure, especially those variations, elements

and combinations and/or materials which, for example, the expert can learn by combining individual ones together with those in the general description and embodiments in addition to characteristics and/or

elements or process stages described in the claims and contained in the drawings with the aim of solving a task thus leading to a new object or new process stages or sequences of process stages via combinable characteristics, even where they concern manufacturing, testing and work

1 7 processes.

Claims

1 8 CLAIMS

1. A hydraulic accumulator comprising; a cylinder; a piston slidably mounted in the cylinder, the piston sealed with respect to the wall of the cylinder, a stem extending from the piston to one side thereof, the stem extending coaxially of the cylinder beyond one end of the cylinder and a leaf spring acting on an end of the stem remote from the piston, the leaf spring biasing the piston towards the other end of the cylinder, said other end of the cylinder being closed and a port opening to the cylinder adjacent said other end.

2. A hydraulic accumulator according to claim 1 in which the spring rate of the leaf spring is variable.

3. A hydraulic accumulator according to claim 1 or 2 in which the spring rate of the leaf spring is adjustable.

4. A hydraulic accumulator according to any one of the preceding claims in which the cylinder of the accumulator is defined by a housing, the housing also defining the body of a positive displacement pump and an electric motor for driving the positive displacement pump being mounted on the housing.

5. A hydraulic accumulator according to claim 4 in which a connection between the positive displacement pump and the closed end of the cylinder of the accumulator is provided by an internal passageway formed in the housing.

6. A hydraulic accumulator according to claim 5 in which the internal passageway defines a housing for a non-return valve which prevents fluid flowing from the accumulator to the pump.

7. A hydraulic accumulator according to any one of the preceding claims in which said one end of the cylinder is also closed, the chamber defined between that end of the cylinder and the piston being vented to atmosphere, said chamber defining a reservoir for fluid at atmospheric pressure.

8. A hydraulic accumulator according to claim 7 when taken with any one of claims 4 to 6 in which internal passageways in the housing connect the reservoir to the pump.

9. A hydraulic accumulator according to claim 8 in which the housing defines a housing for a pressure relief valve between the outlet from the pump and the reservoir.

10. A hydraulic accumulator according to any one of the preceding claims in which a housing defining the cylinder of the accumulator has a bracket formation for securing the housing to a support structure, an integral extension of the bracket defining the leaf spring.

11. A hydraulic accumulator substantially as described herein with reference to and as shown in Figures 1 to 10 of the accompanying drawings.

12. A hydraulic actuation system for a motor vehicle transmission system including a hydraulic accumulator as claimed in any one of claims 1 to 11.