INTRODUCTION AND BACKGROUND

This invention relates to a drive system. More particularly, the invention relates to a hybrid drive system wherein a plurality of energy paths between a plurality of energy storage devices and energy sources, are utilised to optimise drive efficiency and performance of the hybrid drive system.

Of recent, global legislative restrictions related to carbon emissions and fuel consumption, coupled with dwindling natural resources, have been key driving forces behind a continued search for improved efficiency in the drive systems of, amongst others, vehicles. A number of hybrid drive systems have to date been proposed and developed with varying levels of success.

In answer to the aforementioned, one area that has seen a significant amount of development is that of electric alternatives supplanting fossil fuel drive circuits. Some of the obvious disadvantages associated with these electrical systems relate to the cost and weight of suitable batteries to store energy on board the vehicle, the life expectancy and cost of replacing said batteries, the time associated with the charging and recharging of the batteries, all of which affect the amount of energy available and the number of available charge/recharge cycles (often referred to as the Peukert effect), the availability of recharge stations, especially in rural areas, and, ultimately, the range of the vehicle.

As an interim solution, while electrical and other complete replacements for fossil fuel drive systems are being developed to achieve commercial viability, other avenues along which carbon emissions of current fossil fuel drive systems can be reduced is investigated. In this regard, some well- known alternative systems relate to energy harvesting by means of regenerative breaking and engine optimisation.

In particular, engine optimisation systems relate to the concept of utilising electronic engine management systems to regulate internal combustion engine operation to achieve higher efficiencies. Engines are managed to operate closer to their most efficient lines, sometimes at the temporary cost of performance. "Stop-start" technology also falls within the category of engine optimisation. The latter concept refers to engines automatically switching off when the vehicle comes to a halt (at a traffic light for instance) and switching back on again when the accelerator pedal is depressed.

Regenerative braking refers to the concept of converting kinetic energy associated with the momentum of a travelling vehicle into stored potential energy for later use, while the vehicle is being slowed down or stopped. In other words, when a travelling vehicle has to be slowed down by means of its brakes, a regenerative braking system harvests and stores energy instead of dissipating it in the form of heat, as in conventional braking systems.

Hybrid drive systems incorporating a combination of fossil fuel and electrical motors have also been used with promising results, and have of late, been utilised in passenger and even high-performance sports vehicles. The combination of fossil fuel and electrical motors overcomes the above difficulties with range experienced by purely electrical vehicles, while it allows the provision of torque at low engine speeds and facilitates regenerative braking, collectively resulting in overall efficiency increases. Battery banks can furthermore be charged by the fuel-powered motor.

In WO2009/057082 there is disclosed a hybrid drive system incorporating, in addition to a fossil fuel motor, and instead of an electrical motor and battery system, a hydraulic pump and/or motor system with an accumulator as an energy storage device.

However, it is not easy to retrofit the aforementioned hybrid system to existing vehicles. A need therefore exists to provide a hybrid drive system that can be incorporated into current drive systems without the need for major system changes. OBJECT OF THE INVENTION

Accordingly, it is an object of the present invention to provide a hybrid drive system with which the applicant believes the aforementioned disadvantages may at least be alleviated or which may provide a useful alternative for the known drive systems.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a hybrid drive system comprising:

- a power-splitting arrangement coupled to a first drive means;

- a hydraulic mechanism selectively configurable as one of a pump and a motor, coupled to the power-splitting arrangement and having a first port in fluid flow communication with a hydraulic accumulator; and

- an output drive coupled to the first drive means,

wherein the system is selectively configurable in one of: i) a first configuration wherein the output drive is driven by at least one of the first drive means and the hydraulic mechanism configured as a motor; and ii) a second configuration wherein the hydraulic mechanism configured as a pump is driven by at least one of the first drive means and the output drive. The output drive may be coupled to the first drive means through a shaft or gearbox associated with the first drive means. Alternatively, the output may be coupled directly to the first power-splitting arrangement.

According to a second aspect of the invention there is provided a hybrid drive system comprising:

- a first drive means linked to an output drive via a first linking arrangement;

- a hydraulic mechanism selectively configurable as one of a pump and a motor, coupled to the first linking arrangement, and having a first port in fluid flow communication with an accumulator; and

- a first energy storage arrangement coupled to the first linking arrangement,

the system being selectively configurable in one of: i) a first configuration wherein at least one of the accumulator and the first energy storage arrangement takes up energy from at least one of the first drive means and the output drive; and ii) a second configuration wherein the output drive is driven by at least one of the first drive means, the hydraulic mechanism configured as a motor and the first energy storage arrangement.

The first energy storage arrangement may comprise an electrical machine selectively configurable as one of a motor and generator, coupled to the first linking arrangement, and electrically connected to a charge receiving means. Alternatively, the first energy storage arrangement may comprise a flywheel coupled to the first linking arrangement via a continuously variable transmission (CVT).

When the system is configured in the first configuration, the electrical machine may act as an electric generator or alternator to convert kinetic energy from the first linking arrangement into electric potential energy to be stored by the charge storage means. When the system is configured in the second configuration, the electrical machine may act as a motor to convert electric potential energy from the charge storage device into kinetic energy for driving the first linking arrangement.

Similarly, when the system is configured in the first configuration, the hydraulic mechanism may act as a hydraulic pump to utilize kinetic energy from the first linking arrangement to provide pressurized hydraulic fluid to the hydraulic accumulator, thereby storing potential energy in the accumulator. When the system is configured in the second configuration, the hydraulic mechanism may act as a hydraulic motor to convert the stored potential energy from the accumulator into kinetic energy for driving the first linking arrangement. At least two of the first drive means, the hydraulic mechanism and the first energy storage arrangement may drive the output drive in a torque- summing configuration, when the system is configured in the second configuration.

The electrical charge receiving means may be one of a battery, capacitor and an electrical grid.

The hydraulic mechanism may be selectively configurable as one of a pump and a motor, in any of a clockwise and counterclockwise rotational direction.

The first linking arrangement may comprise a first shaft. According to any of the first and second aspects, the hybrid drive system may comprise a control valve arrangement in fluid flow communication with the accumulator and the hydraulic mechanism. The control valve may regulate the flow of hydraulic fluid between the hydraulic mechanism and the accumulator.

The control valve arrangement may comprise a directional control valve arranged in parallel with a check valve. Alternatively, the control valve arrangement may comprise a pilot operated check valve. The hybrid drive system may furthermore comprise a control system to control interactions, configurations and settings of different components of the hybrid drive system.

The output drive may comprise a drive shaft or a drivetrain of one of a vehicle, a machine, a turbine and a generator.

The first drive means may comprise an electrical motor, an internal combustion motor, a wind turbine, a water turbine or a flywheel.

The hydraulic mechanism may comprise an open-loop, over-center, variable displacement hydraulic mechanism, in the form of a plurality of axially reciprocating pistons and an associated manipulatable swash-plate arrangement which is controllable to move over-center. Alternatively, the hydraulic mechanism may be in the form of a bent axis hydraulic device configurable to move over-center.

According to a third aspect of the invention there is provided a method of operating a hybrid drive system, the method comprising one of: i) driving an output drive by at least one of a first drive means and a hydraulic mechanism selectively configured as a motor and coupled via a power- splitting arrangement to the first drive means; and ii) configuring the first hydraulic mechanism as a pump and driving the first hydraulic mechanism via at least one of the first drive means and the output drive.

According to a fourth aspect of the invention there is provided method of operating a hybrid drive system, the method comprising one of: i) storing potential energy in at least one of an accumulator connected to a hydraulic mechanism configured as a pump and coupled to a first linking arrangement, and a first energy storage arrangement, by driving the first linking arrangement via at least one of a first drive means coupled to the first linking arrangement and an output drive coupled to the first linking arrangement; and ii) driving the output drive by at least one of the first drive means, the hydraulic mechanism configured as a motor, and the first energy storage arrangement. BRIEF DESCRIPTION OF THE ACCOMPANYING DIAGRAMS

The invention will now further be described, by way of example only, with reference to the accompanying diagrams wherein: figure 1 is a diagrammatic representation of a first hybrid drive system for driving an output drive; figure 2 is a diagrammatic representation of a second hybrid drive system for driving an output drive; figure 3 is an exploded view of an example open-loop, over-centre variable displacement hydraulic mechanism selectively operable as one of a pump and a motor, comprising a plurality of reciprocating pistons with an associated manipulatable swash plate arrangement; figure 4 is a side section view of the open-loop, over-centre variable displacement hydraulic mechanism of figure 3; and figure 5 is a side section view an alternative example hydraulic mechanism, in the form of a bent axis open-loop, over-centre variable displacement hydraulic mechanism selectively operable as one of a pump and a motor.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

A first hybrid drive system is generally indicated by reference numeral 10 in figure 1 . The hybrid drive system is utilized to drive an output drive 12.

The hybrid drive system 10 comprises a power-splitting arrangement 14 coupled to a first drive means 16. A hydraulic mechanism 18 selectively operable as one of a pump and a motor is mechanically coupled to the power-splitting arrangement 14. The hydraulic mechanism 18 has a first port 20 in fluid flow communication with a hydraulic accumulator 22.

The output drive 12 is mechanically coupled to the first drive means 16 by means of a first shaft 24. The system 10 is selectively configurable in at least a first configuration and a second configuration. When the system 10 is configured in the first configuration, the output drive 12 is driven by at least one of the first drive means 16 and the hydraulic mechanism 18 configured as a motor. When the system is configured in the second configuration, the hydraulic mechanism 18, configured as a pump, is driven by at least one of the first drive means 16 and the output drive 12.

The hydraulic mechanism 18 is coupled to the power-splitting arrangement 14 via a second shaft 26. The hydraulic mechanism 18 furthermore comprises a second port 28 which is provided in fluid flow communication with a hydraulic reservoir 30. Hydraulic fluid ejected from the second port 28 is allowed to drain into the reservoir 30.

As described in more detail below, the hydraulic mechanism 18 is an open-loop, over-centre, variable displacement hydraulic mechanism 100.

The hydraulic mechanism 100 comprises a plurality of axially reciprocating pistons 1 18 and an associated manipulatable and variable swash-plate arrangement 102 which is controllable to move over-center. Alternatively, the hydraulic mechanism 18 may be a bent axis hydraulic mechanism, as shown in figure 5, which is configurable to move over-center.

The hydraulic mechanism 18 is configurable as either a pump or a motor. For example, when the hydraulic mechanism 18 is configured as a motor, the second shaft 26 is an output shaft, driven by the hydraulic mechanism (motor) 18, whereas, when the second hydraulic mechanism 18 is configured as a pump, the second shaft 26 is an input shaft for driving the hydraulic mechanism (pump) 18. The configuration of the hydraulic mechanism 18 changes from a pump to a motor when the swash plate

102 (or the bent axis, as the case may be) moves over-centre (for a given direction of rotation).

In certain cases, such as when the hybrid drive system 10 is utilised in normal road going passenger vehicles, or vehicles that are not conventionally fitted with power take-offs (PTOs), the power-splitting arrangement 14 may be a suitable coupling or linking arrangement, that is coupled or linked directly to the vehicle's engine via the engine's output shaft, or linked indirectly to the vehicle's engine via the gearbox or propshaft associated with the engine.

In such the case, the linking arrangement may be in any suitable form, including a chain and sprocket arrangement, a belt and pulley arrangement, direct coupling between spur, bevel or other types of gears, etc.

In other cases, such as when the hybrid drive system 10 is utilised in heavy commercial vehicles, the power-splitting arrangement 14 may be in the form of a conventional PTO. The PTO may relate to either engine speed, or a speed of a gearbox, any shaft or gear within the gearbox or a drivetrain or output shaft of the vehicle. The PTO may typically form part of a conventional gearbox (having multiple speed ratios). In such a case, retrofitting the system 10 to an existing drive train may be feasible.

A control valve arrangement 32 is provided between the accumulator 22 and the hydraulic mechanism 18 to regulate the flow of hydraulic fluid from the accumulator 22. The control valve arrangement 32 is therefore provided in fluid flow communication with both the accumulator 22 and the hydraulic mechanism 18.

The control valve arrangement 32 is required to regulate flow from the accumulator 22, especially during changes in the configuration of the components of the system. The hydraulic mechanism 18 is configurable to prevent flow of hydraulic fluid (for instance, in the case of an open-loop over center variable displacement hydraulic mechanism 100, as described in more detail below, by rotating the swash-plate 102 to substantially zero degrees). However, configuration changes of the hydraulic mechanism 18 from a pump to a motor configuration (or vice versa) is not instantaneous, which might lead to unwanted torque supplied to the second shaft 26. The control valve arrangement 32 may, depending on design requirements, take one of two forms: firstly (as shown at "A" in figure 1 and 2), the control valve arrangement 32 may comprise a directional control valve 32.1 arranged in parallel with a check valve 32.2. Secondly (as shown at "B" in figure 1 ), the control valve arrangement 32 may comprise a pilot operated check valve 32.3, without the use of a directional control valve.

The directional control valve 32.1 is configurable between an open and a closed configuration. The check valve 32.2 allows fluid flow in one direction only. Since the check valve 32.2 is arranged in parallel with the directional control valve 32.1 , hydraulic fluid is allowed to flow into the accumulator 22, even in cases where the directional control valve 32.1 is closed (and provided the fluid pressure at the first port exceeds the pressure within the accumulator). However, the check valve 32.2 does not allow flow from the accumulator 22 towards the first port 20, and therefore, if the directional control valve 32.1 is closed, hydraulic fluid can flow into the accumulator 22 but not from the accumulator 22. When the directional control valve 32.1 is open, hydraulic fluid can flow into and from the accumulator 22 uninhibited.

The pilot operated check valve 32.3 differs from the check valve 32.2 in that a pilot signal can open the pilot operated check valve to allow fluid flow towards and from the accumulator 22.

The accumulator 22 acts as an energy storage device. As is described in more detail below, energy may be stored in the accumulator 22 by the hydraulic mechanism 18 when it is configured as a pump.

A control system (not shown) is used to control the configuration of the different components of the hybrid drive system 10 to result in a number of different system configurations.

For the purpose of exemplifying the system configurations below, the output drive 12 will be taken to form part of a drivetrain of a vehicle, while the first drive means 16 is an internal combustion (IC) engine. In a first example configuration, torque supplied by the IC engine 16 and the hydraulic mechanism 18 configured as a motor is summed an provided to the output drive 12. In a second example configuration, the IC engine 16 may be idling or even switched off, while the hydraulic mechanism 18 configured as a motor drives the output drive 12. In a third example configuration, the IC engine 16 drives the output drive

12 while simultaneously driving the hydraulic mechanism 18 which is configured as a pump. The hydraulic mechanism 18 therefore absorbs a surplus portion of the energy supplied by the IC engine 16, storing it as potential energy in the accumulator for later use.

In a fourth example configuration the output drive 12 is stationary and disconnected from the first drive means (by way of a clutch provided in the first shaft 24, or by utilizing a neutral gear selection in the gearbox 34, as will be discussed in more detail below), while the IC engine 16 drives the hydraulic mechanism 18 which is configured as a pump, to store energy in the accumulator 22 for later use.

In a fifth example configuration, termed "regenerative braking", the hydraulic mechanism 18 configured as a pump is used to slow down or brake the vehicle. Thus, the output drive 12 drives the hydraulic mechanism 18, which converts the kinetic energy of the vehicle into potential energy, in the form of pressurized hydraulic fluid, storing the potential energy in the accumulator for later use. Since the displacement of the hydraulic mechanism 18 is variable, the rate at which the vehicle is braked, and thus the rate at which energy is absorbed and stored in the accumulator 22 can be varied. In a sixth example configuration, the system is again configured in a regenerative braking configuration as described above, with the addition that the IC engine 16 is used to supplement the power supplied to the hydraulic mechanism 18 configured as a pump. This increases the rate at which energy is stored in the accumulator.

The hybrid drive system 10 may also be configured to be bypassed, so that the IC engine 16 drives the output drive 12 directly through a conventional gearbox 34 (having multiple speed ratios) and via the first shaft 24. In the bypass configuration, the hydraulic mechanism is disconnected from the power splitting arrangement (by way of a clutch (not shown) in the power-splitting arrangement 14 or the shaft 26 or by way of a one-way sprag clutch (not shown)). Alternatively, the swash plate may be angled at substantially zero degrees so that the hydraulic mechanism acts neither as a pump nor a motor. Therefore, no hydraulic fluid will flow between the hydraulic mechanism 18 and the accumulator 22.

It should be noted that the power-splitting arrangement 14 may typically be coupled between the first drive means 16 and the gearbox 34, on a lay shaft (not shown) of the gearbox 34 or any specific gear (not shown) fitted to the main shaft as is common practice in many gearboxes. Preferably, the power-splitting arrangement 14 is coupled between the first drive means 16 and the gearbox 34 so that the various torque/speed ratios of the gearbox 34, and the resulting mechanical advantages, are retained.

Therefore, in the preferred embodiment, the rotational speed of the power- splitting arrangement 14 relates to the rotational speed of the first drive means 16 (even though the relation need not be 1 :1 ). Furthermore, in order to facilitate the configurations as mentioned above, any of the power-splitting arrangement 14, the second shaft 26, the gearbox 34 and the first shaft 24 may be provided with clutches (not shown). By utilizing the configurations as described above, the IC engine 16 can operate as close as possible to its most efficient line, even under braking. Surplus energy produced by the engine is stored for later use, while shortages in energy supplied by the IC engine to the output drive is supplemented by energy channeled from the accumulator 22.

In general, the output drive driven by the hybrid drive system may be one of a drivetrain of a vehicle, a turbine, a machine and a generator. The first drive means may be one of an electrical motor and internal combustion motor. Alternatively, the first drive means may be one of a wind turbine, a water turbine and a flywheel. A second hybrid drive system for driving an output drive 52, is generally indicated by reference numeral 50 in figure 2. The hybrid drive system 50 comprises a first linking arrangement 54 which links the output drive 52 to a first drive means 56. A hydraulic mechanism 58 selectively configurable as one of a pump and a motor is mechanically coupled to the first linking arrangement 54. A rotation ratio between the hydraulic mechanism 58 and the first linking arrangement will be determined by the type and characteristics of the coupling therebetween and need not be 1 :1 . Again a suitable linking arrangement as described above may be utilized. The hydraulic mechanism 58 has a first port 60 in fluid flow communication with a hydraulic accumulator 62 and a second port 74 in fluid flow communication with a reservoir.

A first energy storage arrangement 64 is coupled to the first linking arrangement 54. A rotation ratio between the first energy storage arrangement 64 and the first linking arrangement will also be determined by the type and characteristics of the coupling therebetween and need not be 1 :1 . Again a suitable linking arrangement as described above may be utilized. The first linking arrangement may be in the form of a first shaft which in use may drive the output drive 52.

The system 50 is selectively configurable in one of a first configuration and a second configuration. When the system 50 is configured in the first configuration, at least one of the accumulator 62 and the first energy storage arrangement 64 takes up energy from at least one of the first drive means 56 and the output drive 52. When the system is configured in the second configuration, the output drive 52 is driven by at least one of the first drive means 56, the hydraulic mechanism 60, configured as a motor and the first energy storage arrangement 64.

Preferably the first energy storage arrangement 64 comprises an electrical machine 66 which is selectively configurable as one of motor and a generator/alternator and mechanically coupled to the first shaft 54. The electrical machine 66 is electrically connected to a charge storage or receiving means 68.

Alternatively, the first energy storage arrangement 64 may comprise a flywheel 70 coupled mechanically to the first shaft 54 via a continuously variable transmission (CVT) 72. Since the rotational speed of the flywheel 70 is a function of the energy stored therein, in most cases the rotational speed of the first shaft 54 will not match the rotational speed of the flywheel 70. Since the connection between the first shaft 54 and the flywheel 70 results in a torque-summing arrangement, the rotational speed of the flywheel 70 needs to be matched to the rotational speed of the first shaft 54. This can be achieved by means of the CVT 72.

When the system 50 is configured in the first configuration, either or both of the first energy storage arrangement 64 and the hydraulic mechanism 58 is used to convert kinetic energy from the first shaft 54 into potential energy, and to store the potential energy for later use.

Thus, when the system 50 is configured in the first configuration the electrical machine 66 may act as an electric generator or alternator to convert kinetic energy from the first shaft 54 into electric potential energy to be stored by the charge storage means 68, while the hydraulic mechanism 58 may be configured as a hydraulic pump to convert kinetic energy from the first shaft 54 into potential energy in the form of pressurized hydraulic fluid stored by the hydraulic accumulator 62.

The first configuration is therefore used to store surplus energy for later use. This configuration is used when the first drive means 56, operating at its most efficient line, produces more energy than is required by the output drive 52. Therefore, the power produced by the first drive means is split between the output drive 52, and one or both of the storage devices (i.e. the accumulator 62 and the first energy storage arrangement). Also, as described above, energy may be absorbed by the storage devices during a regenerative braking cycle. When the system 50 is configured in the second configuration, either or both of the first energy storage arrangement 64 and the hydraulic mechanism 58 is used to convert potential energy previously stored into kinetic energy, that is supplied to the first shaft 54. The second configuration may therefore be a torque summing configuration between any two or more of the first drive means, the first storage arrangement 64 and the first hydraulic mechanism 58 configured as a motor. This is used to supplement the power provided to the first shaft 54 by the first drive means 56. In some cases, the first drive means may be uncoupled from the first shaft, while one or both of the first energy storage arrangement 64 and hydraulic mechanism 58 drives the output drive 52.

Thus, when the system 50 is configured in the first configuration the electrical machine 66 acts as an electric motor to convert electric potential energy from the charge storage device into kinetic energy for driving the first shaft, while the hydraulic mechanism acts as a hydraulic motor to convert pressurized hydraulic fluid from the accumulator into kinetic energy for driving the first shaft. It will be appreciated that many combinations of the aforementioned configurations are possible.

The electrical supply/charge storage or receiving means may be one of a battery bank, capacitor and an electrical grid.

A control valve arrangement 78 comprising one of a directional control valve 78.1 arranged in parallel with a check valve 78.2, or a pilot operated check valve 78.3 (all of which are similar to the directional control valve 32.1 , the check valve 32.2 and the pilot operated check valve 32.3 described above in relation to the hybrid drive system 10) is provided between the accumulator 62 and the hydraulic mechanism 58.

Furthermore, the first drive means 56 may be similar to the first drive means 16 described above, the accumulator 62 may be similar to accumulator 22 described above and the output drive 52 may be similar to the output drive 12 described above.

The hydraulic mechanism 58 may be similar to the hydraulic mechanism 18 described above, with the exception that the hydraulic mechanism 58 has the ability to selectively act as a pump or a motor in both a forward and reverse rotational direction. The configuration of the hydraulic mechanism depends on input variables such as the angle of the swash- plate or the angle of the axis of the bent axis hydraulic mechanism, the rotational speed and rotational direction of the shafts and the hydraulic pressure. In a first example embodiment as is shown in figures 2 and 3, each of the first and second hydraulic mechanisms (22, 46) comprises an open loop over-center variable displacement hydraulic device 100 that can be operated as a pump or as a motor, such as an axial piston-type hydraulic device utilizing a variable swash plate 102 wherein the angle of the swash plate 102 may be varied to so that the swash plate may move over-center.

The hydraulic mechanism 100 comprises a swash plate 102 having a face 104. The swash plate 102 is arranged to pivot about pivot point 106, so that the angle 108 between the face 104 and a reference line 1 10 can be adjusted. In the current example, the reference line 1 10 lies in a vertical plane.

The hydraulic mechanism 100 furthermore comprises a rotatable, cylindrical barrel 1 12, having a first face 1 12.1 and a second face 1 12.2. The second face 1 12.2 of the barrel 1 12 is connected to a shaft 1 14, which can serve either to drive the barrel 1 12, in the case of the hydraulic mechanism 100 operating as a pump, or alternatively to be driven by the rotatable barrel 1 12 in the case of the hydraulic mechanism 100 operating as a motor. The arrangement is such that the angle 108 is zero when the face 104 of the swash plate 102 and the first face 1 12.1 of the barrel 1 12 are parallel. The angle 108 may vary through a range of both positive and negative angles.

The barrel 1 12 comprises a plurality of radially and equidistantly spaced cylinder bores 1 16 extending from the first face 1 12.1 towards the second face 1 12.2 and thus through the barrel 1 12. The bores 1 16 extend parallel to a centerline 128 of the barrel 1 12. Each bore 1 16 is associated with a respective piston 1 18 which is allowed to reciprocate within the bore 1 16.

The pistons 1 18 extend from the bores 1 16 beyond the first face 1 12.1 , towards the swash plate 102 and terminate against a bearing surface 120 on the face 104. Each piston is connected to the bearing surface 120 by a slipper (not shown).

In use the barrel 1 12 rotates relative to the swash plate 102. Because of the angle 108, the swash plate 102 causes the pistons 1 18 to reciprocate within the cylinder bores 1 16 between a top dead center (TDC) and a bottom dead center (BDC). The TDC represents a situation where the piston 1 18 is closest to the second face 1 12.2, while the BDC represents a situation where the piston 1 18 is furthest away from the second face 1 12.2. The order in which adjacent pistons 1 18 reach either the TDC or BDC is determined by the direction in which the barrel 1 12 is rotating. The hydraulic mechanism 100 further comprises a porting plate 130 having a first face 132 and second face 134. The porting plate 130 has a diameter similar to the barrel 1 12. The first face 132 of the porting plate 130 abuts against the second face 1 12.2 of the barrel 1 12, so that a substantially fluid tight seal forms between the porting plate 130 and the second face 1 12.2 of the barrel 1 12 (a degree of leakage is present, which results in minor losses). The porting plate has an aperture 136 through which the shaft 1 14 protrudes. The porting plate 130 is fixed in position so that, in use, the barrel 1 12 and shaft 1 14 rotates relative to the porting plate.

The porting plate comprises a first fluid channel 138 and a second fluid channel 140 that extends from the first face 132 to the second face 134. The first fluid channel 138 is associated with a high-pressure fluid line 54 whereas the second fluid channel 140 is associated with a low-pressure fluid line 144. Hydraulic fluid is therefore transferred between the bores 1 16 and the high-pressure fluid line 54, through the first fluid channel 138, while hydraulic fluid is transferred between the bores 1 16 and the low- pressure fluid line 144, through the second fluid channel 140.

The operation of the hydraulic mechanism 100 will now be described from the viewpoint of one of the cylinder bores 1 16 and its concomitant piston 1 18. It will be understood that all of the cylinder bores 1 16 and pistons 1 18 progress in similar fashion, albeit out of phase with the one described, resulting in a smooth motion of the barrel 1 12. When the hydraulic mechanism 100 is operating as a pump, high pressure hydraulic fluid flows along the high-pressure line 54 towards the mechanism 100 and through the first fluid channel 138. The swash plate 102 is angled such that, taking into account the desired direction of rotation of the barrel 1 12, the pistons 1 18 in fluid flow communication with the first channel 138 has at least reached or passed the TDC. High pressure hydraulic fluid thus exerts a force on the pistons 1 18 in fluid flow communication with the first fluid channel 138, forcing the pistons 1 18 in a direction towards the first face 1 12.1 of the barrel 1 12. The piston 1 18 therefore transfers an axial force on the face 104 of the swash plate 102. The angle of the swash plate 102, which is not at zero, results in a transverse component of the axial force, which translates into a torque causing the barrel 1 12 to rotate. The rotation of the barrel 1 12 in turn rotates the shaft 1 14. At the time a piston 1 18 reaches the BDC, a maximum volume of high pressure hydraulic fluid is thus contained within the bore 1 16. The barrel 1 12 has now rotated to a point where it is no longer in fluid flow communication with the first fluid channel 138. When the barrel 1 12 rotates further, the bore 1 16 comes into fluid flow communication with the second fluid channel 140 which is left at a relatively low pressure (atmospheric pressure or slightly above this). Further rotation of the barrel 1 12, together with the interaction of the piston 1 18 with the swash plate 102 causes the piston 1 18 to start moving back towards the TDC, which causes the volume of hydraulic fluid contained within the bore 1 16 to be deposited through the second fluid channel 140 into the low pressure hydraulic fluid line 144. A single revolution of the barrel 1 12 has thus been completed. One revolution of the barrel 1 12 thus results in the bore 1 16 being in alternating fluid communication with the first and second fluid channels (138, 140) respectively.

When the hydraulic mechanism operates as a pump, the piston 1 18 has at least reached TDC by the time it comes into fluid flow communication with the second fluid channel 140. Unlike when the mechanism 100 is configured as a motor, the barrel 1 12 is now driven by the shaft 1 14. The rotation of the barrel 1 12, and the interaction of the piston 1 18 with the swash plate 102 causes the piston 1 18 to start moving towards the BDC, which causes the piston to create low pressure (or suction) within the bore 1 16. Hydraulic fluid from the low-pressure line 144 thus enters the bore 1 16 through the second fluid channel 140. By the time the piston reaches the BDC, a maximum volume of hydraulic fluid has thus entered the bore

1 16. Further rotation of the barrel 1 12 caused by the shaft 1 14, and the interaction of the barrel 1 12 with the porting plate 130 terminates the fluid flow communication between the bore 1 16 and the second fluid channel 140. Further rotation of the barrel 1 12 causes bore 1 16 to come into fluid flow communication with the first fluid channel 138, while interaction of the piston 1 18 with the swash plate causes the piston 1 18 to start moving towards the TDC. This causes the piston 1 18 to exert a force on the hydraulic fluid contained in the bore 1 16, which is deposited under pressure, through the first fluid channel 138 into the high-pressure fluid line 54. When the piston 1 18 reaches the TDC, a full revolution of the barrel 1 12 has been completed. Thus, when acting as a motor, high-pressure hydraulic fluid kept in a high- pressure source is used to cause the shaft 1 14 to rotate, while, when acting as a pump, rotation of the shaft 1 14 is used to provide high pressure hydraulic fluid to an actuator or sink. By having a variable swash plate 102, the load created the hydraulic mechanism 100 when operating as a pump may be finely controlled from zero load (when the angle 108 is zero) to a maximum load through an infinite number of steps. Similarly, the power delivered through the shaft 1 14 (and thus the output torque and speed of the shaft 1 14) may be controlled from zero to a maximum available power through an infinite number of steps. When the angle 108 is substantially zero, no torque will be transferred to or from the shaft 1 14, and effectively, no flow of hydraulic fluid into or from the bores 1 16 will occur. The angle 108 of the swash plate 102 at rest may be off-set from the zero position so that pressure is generated as the pump starts rotating. This pressure may be used as a control pressure, negating the need for a conventional charge pump used for controlling the configuration of the hydraulic unit 100. A pressure reducing valve (not shown) may be used to set the control pressure typically to between 10 and 30 bar.

In a second example embodiment as is shown in figure 4, each of the first and second hydraulic mechanisms (22, 46) comprises an open loop over- center variable displacement hydraulic device that can be operated as a pump or as a motor, in any one of a forward and reverse direction, which hydraulic mechanism 200 is in the form of a variable displacement, variable axis, bent axis hydraulic mechanism 200.

The mechanism 200 operates on substantially the same principle as the mechanism 100 described above, in that a rotatable cylindrical barrel 202 houses a plurality of pistons 204, within a concomitant number of cylindrical bores 206, formed within the cylindrical barrel 202. The pistons are pivotably fixed to a holder 208 that is fixed to a shaft 210. When the mechanism 200 is configured as a pump, the shaft 210 is an input shaft, while, when the mechanism 200 is configured as a motor, the shaft 210 is an output shaft. The shaft 210 has a central axis 212, while the cylindrical barrel also has a central axis 214.

The barrel 202 is pivotable such that the central axis 214 of the barrel 202 is pivotable relative to the central axis 212 of the shaft 210. By pivoting the central axis 214 of the barrel through an angle 216 relative to the central axis 212 of the shaft (to constitute a "bent axis"), the pistons 204 reciprocate within the cylindrical bores 206 when the shaft 210 is rotated. Therefore, when the central axis 214 of the barrel and the central axis 212 of the shaft are substantially in line, the pistons 204 will not reciprocate within the barrel 202, and the mechanism 200 will act as neither a pump nor a motor.

A porting plate (not shown) performing a similar function as the porting plate 130 described above, is provided in fluid flow communication with high and low-pressure lines respectively. The pistons reciprocating within the barrel causes hydraulic fluid to be expelled and received in the cylindrical bores in similar fashion as described above in relation to the hydraulic mechanism 100. An actuator (not shown) is used to pivot the barrel 202. By pivoting the barrel 202 over-centre (in other words, pivoting the central axis 214 of the barrel up to, and beyond the central axis 212 of the shaft) the configuration of the mechanism 200 is changed from a pump to a motor, or vice versa, for a specific rotational direction of the shaft 210. However, this will not cause the high-pressure and low-pressure lines to change around.

The bent-axis mechanism 200 has a number of advantages over the swashplate mechanism 100.

Firstly, the bent axis mechanism 200 is inherently more powerful than a swashplate mechanism 100 of a comparable size, as the angle 216 of the bent-axis mechanism 200 may inherently be greater than the swashplate angle 108. Consequently, the stroke of the pistons of the bent axis mechanism 200 is larger than that of a swashplate mechanism 100 of comparable size, resulting in a larger volume of hydraulic fluid that can be displaced by the pistons per revolution of the shaft 210. The angle 216 may typically reach a maximum of around 40 to 45 degrees, while the angle 108 of the swashplate is typically restricted to 22 to 23 degrees.

Secondly, the increased piston stroke, and the concomitant increased length of the pistons 204, results in an increased leakage path length. This in turn results in a lower rate of leakage, and increased efficiency, even though drag and viscous losses may be slightly higher.

Furthermore, the holder 208 is more robust than the slippers of the swashplate mechanism 100. This means that the rotational speed at which the pistons 204 start to pull out of, or get dislodged from the holder 208, is a lot higher than the rotational speed at which the pistons 1 18 pull out of, or get dislodged from, the slippers of the mechanism 100. Consequently, a bent axis mechanism 200 is capable of producing more power than a swashplate hydraulic mechanism 100 of a comparable size.

The accumulator (22, 62) and flywheel 70 is of the known kind.

By cycling energy between different sources and storage devices, the first drive means (16, 56) can be operated as close as possible to a most efficient line from a fuel economy point of view. Ultimately, the drive provided to any component may be a summation of drives from a number of system components. This might lead to fuel savings and provide a means to boost performance of the output drive (12, 52).

Because of the inherent capability of the hydraulic mechanisms (18, 58) and accumulators (22, 62) to absorb energy at high rates especially compared to electric charge storage devices, the hydraulic mechanisms and accumulators may be used to reduce loads on the charge storage devices, leading to concomitant reductions in system component sizes. Through the hydraulic mechanisms, energy received under regenerative braking can be stored at high rates, leading to increased system efficiency and a reduction in the stress on batteries.

Furthermore, the combination of the charge storage means or the flywheel and the accumulators can be used to store energy received under regenerative braking, which again increases the efficiency of the regenerative braking cycle, while reducing the loads on the different components of the system.

It should also be noted that the braking power effected by the hydraulic mechanism acting as a pump is easily variable by varying the angle of the swash plate or angle of the bent axis and the displacement of the hydraulic mechanism.

It will be appreciated that, throughout this document, any reference made to conditions of no-flow or zero-flow of hydraulic fluid, does not exclude the possibility that minor losses, and leakage of hydraulic fluid may be present. It will be appreciated that a "forward and reverse" direction, when used in relation to the hydraulic mechanism, refers to a clockwise and anticlockwise rotational direction of a shaft of the mechanism (the shaft is an input shaft when the mechanism is configured as a pump, and an output shaft when the mechanism is configured as a motor).

It will be appreciated by those skilled in the art that the invention is not limited to the precise details as described herein and that many variations are possible without departing from the spirit and scope of the claimed invention.