FIELD OF THE INVENTION

[0001] The present invention relates to hydraulic pump/motors, and more particularly to hydraulic pump/motors for use in vehicle transmissions, mobile hydraulic applications, and industrial hydraulic applications.

BACKGROUND OF THE FNVENTION

[0002] Multi-cylinder hydraulic pump/motors are typically utilized in tandem, for example, in a vehicle hydrostatic transmission. A first of the pump/motors is connected to a prime mover (e.g., an engine), while the second pump/motor is connected to the driveline of the vehicle. The first pump/motor is powered by the engine to operate as a pump to supply pressurized hydraulic fluid to the second pump/motor, which operates as a motor to power the driveline. When the second pump/motor is operating as a motor at less than full capacity or displacement (i.e., at low flow fractions), low frequency variations in torque at the output shaft of the second pump/motor often result. Such variations may lead to lugging of a drive train coupled to the output shaft, or undesirable noise, vibration, and harshness generated by the second pump/motor when operating as a motor.

SUMMARY OF THE INVENTION

[0003] The present invention provides, in one aspect, a method of operating a pump/motor in a system including a high-pressure manifold and a low-pressure manifold. The pump/motor includes an output shaft, a plurality of piston/cylinder assemblies, and a cam coupled to the output shaft and disposed between the output shaft and the piston/cylinder assemblies. Each piston/cylinder assembly includes a cylinder and a piston at least partially disposed in the cylinder and engaged with the cam. The method includes displacing the pistons of the respective piston/cylinder assemblies from a bottom dead center position to a top dead center position, and then back to the bottom dead center position, within each revolution of the cam and the output shaft, pumping working fluid into the high-pressure manifold with a first piston/cylinder assembly while the piston in the first piston/cylinder assembly is displaced from the bottom dead center position to the top dead center position, and transferring working fluid from the high-pressure manifold to the cylinder of a second piston/cy linder assembly to displace the piston of the second piston/cylinder assembly from the top dead center position to the bottom dead center position, thereby imparting torque on the cam and the output shaft, within the same complete revolution of the cam and the output shaft in which working fluid is pumped into the high-pressure manifold by the first piston/cy linder assembly.

[0004] The present invention provides, in another aspect, a method of operating a pump/motor in a system including a high-pressure manifold and a low-pressure manifold. The pump/motor includes an output shaft, a plurality of piston/cy linder assemblies, and a cam coupled to the output shaft and disposed between the output shaft and the piston/cy linder assemblies. Each piston/cy linder assembly includes a cylinder and a piston at least partially disposed in the cylinder and engaged with the cam, a first valve selectively fluidly communicating the high-pressure manifold and the cylinder, and a second valve selectively fluidly communicating the low-pressure manifold and the cylinder. The method includes opening the first valve of a first group of piston/cy linder assemblies to at least partially fill each of the cylinders within the first group with high-pressure working fluid, thereby displacing the pistons within the respective cylinders in the first group from a top dead center position to a bottom dead center position, rotating the output shaft and the cam with the pistons in the first group, opening the second valve of a second group of piston/cy linder assemblies, driving each of the pistons within the second group, with the rotating cam, from the bottom dead center position to the top dead center position to at least partially exhaust working fluid from each of the cylinders within the second group to the low-pressure manifold, opening the first valve of a first piston/cy linder assembly not in either of the first and second groups while the respective first valves in the first group are opened, and driving the piston in the first piston/cy linder assembly, with the rotating cam, from the bottom dead center position toward the top dead center position to pump working fluid into the high- pressure manifold while the first valve of the first piston/cy linder assembly is opened.

[0005] The present invention provides, in yet another aspect, a method of operating a pump/motor in a system including a high-pressure manifold and a low-pressure manifold. The pump/motor includes an output shaft, a plurality of piston/cy linder assemblies, and a cam coupled to the output shaft and disposed between the output shaft and the piston/cy linder assemblies. Each piston/cy linder assembly includes a cylinder and a piston at least partially disposed in the cylinder and engaged with the cam, a first valve selectively fluidly communicating the high-pressure manifold and the cylinder, and a second valve selectively fluidly communicating the low-pressure manifold and the cylinder. The method includes opening the first valve of a first group of piston/cylinder assemblies to fluidly communicate the cylinders in the first group with the high-pressure manifold, rotating the output shaft and the cam, thereby displacing the pistons within the respective cylinders in the first group from a bottom dead center position to a top dead center position to pump working fluid in the respective cylinders in the first group into the high-pressure manifold, opening the second valve of a second group of piston/cylinder assemblies, at least partially filling the respective cylinders within the second group with working fluid from the low-pressure manifold, thereby displacing the pistons within the respective cylinders in the second group from the top dead center position to the bottom dead center position, opening the first valve of a first piston/cylinder assembly not in either of the first and second groups, while the respective first valves in the first group are opened, to at least partially fill the cylinder of the first piston/cylinder assembly with high-pressure working fluid, thereby displacing the piston in the first piston/cylinder assembly from the top dead center position to the bottom dead center position, and imparting a torque on the cam and the output shaft with the piston in the first piston/cylinder assembly as the piston in the first piston/cylinder assembly moves from the top dead center position to the bottom dead center position.

[0006] Other features and aspects of the invention will become apparent by consideration of the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 illustrates a prior-art multi-cylinder hydraulic pump/motor in which the method of the present invention may be implemented.

[0008] FIG. 2 is a schematic of the pump/motor of FIG. 1, illustrating a prior-art method of operating the pump/motor.

[0009] FIG. 3 is a schematic of the pump/motor of FIG. 1, illustrating another prior- art method of operating the pump/motor.

[0010] FIG. 4 is a schematic of the pump/motor of FIG. 1, illustrating a method of operating the pump/motor according to one embodiment of the invention. [0011] FIG. 5 is a schematic of the pump/motor of FIG. 1, illustrating a method of operating the pump/motor according to another embodiment of the invention.

[0012] FIG. 6 is a graph illustrating torque versus cam shaft angle of a pump/motor configured similarly as that shown in FIG. 1, utilizing the prior-art method of operating the pump/motor shown in FIG. 2.

[0013] FIG. 7 is a graph illustrating torque versus cam shaft angle of a pump/motor configured similarly as that shown in FIG. 1, utilizing the method of operating the pump/motor shown in FIG. 4.

[0014] FIG. 8 is a graph illustrating torque versus cam shaft angle of a pump/motor configured similarly as that shown in FIG. 1, utilizing the prior-art method of operating the pump/motor shown in FIG. 3.

[0015] FIG. 9 is a graph illustrating torque versus cam shaft angle of a pump/motor configured similarly as that shown in FIG. 1, utilizing the method of operating the pump/motor shown in FIG. 5.

[0016] Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

DETAILED DESCRIPTION

[0017] FIG. 1 illustrates a system 10 including a multi-cylinder hydraulic

pump/motor 14 connected to a high-pressure manifold 18 and a low-pressure manifold 22. Both the high-pressure and low-pressure manifolds 18, 22 contain working fluid (e.g., hydraulic fluid), however, the working fluid in the high-pressure manifold 18 is maintained at a higher pressure than the working fluid in the low-pressure manifold 22. Although not shown, an accumulator may be fluidly connected to each of the manifolds 18, 22 to maintain the working fluid in the manifolds 18, 22 pressurized. [0018] The pump/motor 14 includes an output shaft 26, a plurality of piston/cylinder assemblies 30, and a cam 34 coupled to the output shaft 26 and disposed between the output shaft 26 and the piston/cylinder assemblies 30. Each piston/cylinder assembly 30 includes a cylinder 38 and a piston 42 at least partially disposed in the cylinder 38 and engaged with the cam 34. In operation, each of the pistons 42 is displaced from a bottom dead center position (see piston 42b) to a top dead center position (see piston 42a), and then back to the bottom dead center position, within each revolution of the cam 34 and the output shaft 26. Although only two piston/cylinder assemblies 30 are shown in FIG. 1, the pump/motor 14 may include any of a number of different piston/cylinder assemblies 30 (e.g., 24; see FIGS. 2-5).

[0019] With reference to FIG. 1, each piston/cylinder assembly 30 of the pump/motor

14 includes a low-pressure valve 46 operable to selectively fluidly communicate the cylinder 38 with the low-pressure manifold 22. The low-pressure valve 46 is seated inside the cylinder 38, such that the low-pressure valve 46 must be moved in a direction toward the interior of the cylinder 38 to unseat or open the low-pressure valve 46 to fluidly communicate the cylinder 38 with the low-pressure manifold 22. In the illustrated construction of the pump/motor 14, the low-pressure valve 46 is actuated to an unseated position by an electromagnetic coil 48, and is biased toward a seated position by a biasing element (e.g., a spring; not shown). Alternatively, other actuators and/or biasing elements may be utilized to move the low-pressure valve 46 between the seated and unseated positions.

[0020] Each piston/cylinder assembly 30 of the pump/motor 14 also includes a high- pressure valve 50 operable to selectively fluidly communicate the cylinder 38 with the high- pressure manifold 18. The high-pressure valve 50 is seated outside the cylinder 38, such that the high-pressure valve 50 must be moved in a direction away from the cylinder 38 to unseat or open the high-pressure valve 50 to fluidly communicate the cylinder 38 with the high- pressure manifold 18. In the illustrated construction of the pump/motor 14, the high-pressure valve 50 is actuated to an unseated position by an electromagnetic coil 54, and is biased toward a seated position by a biasing element (e.g., a spring; not shown). Alternatively, other actuators and/or biasing elements may be utilized to move the high-pressure valve 50 between the seated and unseated positions.

[0021] The system 10 also includes a controller 58 in communication with each of the actuators (i.e., the electromagnetic coils 48, 54) of the low-pressure valves 46 and the high- pressure valves 50 to control the opening and closing of the valves 46, 50. The controller 58 may communicate with each of the coils 48, 54 of the low-pressure and high-pressure valves 46, 50 using electrical wires 60. Alternatively, any of a number of different wireless protocols may be employed. The system 10 also includes an encoder 62 in communication with the controller 58 to monitor the rotational position of the cam 34 over time (and therefore the rotational speed of the cam 34 and the output shaft 26). Alternatively, other components or devices may be used to permit the controller 58 to monitor the rotational position of the cam 34 during operation of the system 10.

[0022] The system 10 may be incorporated, for example, in a vehicle hydrostatic transmission in which the output shaft 26 is coupled to a driveline of the vehicle. Such a vehicle hydrostatic transmission would also include a second pump/motor (not shown) driven by a prime mover (e.g., an engine; also not shown). In operation of the hydrostatic transmission, the engine would drive the second pump/motor as a pump to provide high- pressure working fluid to the high-pressure manifold 18, which would be used to operate the pump/motor 14 shown in FIG. 1 as a motor to drive or deliver torque to the vehicle driveline.

[0023] To deliver or impart torque to the cam 34 and the output shaft 26, each of the piston/cylinder assemblies 30 in the pump/motor 14 is actuated through a cycle in which the pistons 42 of the respective assemblies 30 are displaced from the top dead center position to the bottom dead center position, and then back to the top dead center position. Particularly, starting at the top dead center position of the piston 42a, the controller 58 activates the coil 54 of the high-pressure valve 50 to open the valve 50 for a period of time to fluidly

communicate the cylinder 38 and the high-pressure manifold 18, causing a transfer of high- pressure working fluid from the high-pressure manifold 18 to the substantially empty cylinder 38 in which the piston 42a is located. The transfer of high-pressure working fluid into the cylinder 38 subsequently displaces the piston 42a toward the cam 34. The cam 34 includes an inclined cam surface 66 which converts the axial motion of the piston 42a to rotational motion of the cam 34 and the output shaft 26. As a result, the linear force exerted on the piston 42a by the high-pressure working fluid transferred into the cylinder 38 is converted to torque on the cam 34 and the output shaft 26 about a rotational axis 70 of the cam 34 and the output shaft 26.

[0024] The piston 42a will continue to impart torque to the cam 34 and the output shaft 26 until the piston 42a reaches its bottom dead center position (i.e., the position of the piston 42b in FIG. 1). For a configuration of the pump/motor 14 including a plurality of piston/cylinder assemblies 30 oriented symmetrically about the rotational axis 70, this portion of the cycle (i.e., the piston 42 moving from the top dead center position to the bottom dead center position) may be initiated sequentially such that at least one-half of the piston/cylinder assemblies 30 in the pump/motor 14 are imparting torque to the cam 34 and the output shaft at 26 any given time throughout a complete revolution of the cam 34 and the output shaft 26.

[0025] Shortly before the piston 42a reaches the bottom dead center position, the controller 58 closes the high-pressure valve 50 to cease fluid communication between the cylinder 38 and the high-pressure manifold 18. Continued movement of the piston 42a toward the bottom dead center position reduces the pressure in the cylinder 38 until it is substantially equal to the pressure of the working fluid in the low-pressure manifold 22. After a brief period of time during which the piston 42a dwells near the bottom dead center position, the controller 58 then activates the coil 48 of the low-pressure valve 46 to open the valve 46 to fluidly communicate the cylinder 38 and the low-pressure manifold 22. The rotating cam 34 then drives the piston 42 (e.g., piston 42b) from the bottom dead center position to the top dead center position, during which time the working fluid in the cylinder 38 is exhausted past the low-pressure valve 46 and into the low-pressure manifold 22. The controller 58 closes the low-pressure valve 46 shortly before the piston 42b reaches the top dead center position to permit the remaining working fluid in the cylinder 38 to be pressurized to a value that is substantially equal to the pressure of the working fluid in the high-pressure manifold 18 to permit the high-pressure valve 50 to open for the subsequent cycle of transferring high-pressure working fluid into the cylinder 38.

[0026] This process is schematically illustrated in FIG. 2, in which a profile 74 of the cam surface 66 is shown in two dimensions relative to the piston/cylinder assemblies 30 of the pump/motor 14. The span of the cam profile 74 is representative of a single complete revolution of the cam 34 in which each piston 42 is displaced from the top dead center position (TDC") to the bottom dead center position ("BDC"), and then back to the top dead center position. Arrow A indicates the direction of movement of the cam profile 74 relative to the respective piston/cylinder assemblies 30, which remain stationary on the page of FIG. 2 as the cam profile 74 passes underneath the piston/cylinder assemblies 30. As such, the pistons 42 of the respective piston/cylinder assemblies 30 engaged with the left side of the cam profile 74 from the point of view of FIG. 2 are displaced from the bottom dead center position to the top dead center position, while the pistons 42 of the respective piston/cylinder assemblies 30 engaged with the right side of the cam profile 74 from the point of view of FIG. 2 are displaced from the top dead center position to the bottom dead center position.

[0027] The piston/cylinder assemblies 30 identified with an "M" are those undergoing the cycle described above in which working fluid from the high-pressure manifold 18 is transferred into the piston/cylinder assemblies 30 to perform work on the cam 34 and the output shaft 26 (i.e., by imparting torque to the cam 34 and the output shaft 26), and subsequently exhausted to the low-pressure manifold 22. This cycle is hereinafter referred to as "motoring," in which the pump/motor 14 is used as a motor to power the vehicle drive line or other mechanism. The piston/cylinder assemblies 30 used in the motoring cycle (i.e., those marked with an "M") that are further identified with an "X" as "active" are those in which high-pressure working fluid from the high-pressure manifold 18 is being injected or transferred as described above to impart torque to the cam 34 and the output shaft 26 to rotate the cam 34 and the output shaft 26. The piston/cylinder assemblies 30 used in the motoring cycle in which an "X" is omitted are those in which the pistons 42 are being driven by the cam 34 to exhaust working fluid to the low-pressure manifold 22 as described above.

[0028] As shown in FIG. 2, the pump/motor 14 includes 24 piston/cylinder assemblies 30. However, not all 24 of the pump/motor assemblies 30 are shown being used in the motoring cycle. Rather, only 15 of the piston/cylinder assemblies 30 in the

pump/motor 14 are being used in the motoring cycle. Those piston/cylinder assemblies 30 that are inactive (i.e., with no "M" or "X" identifier) do not contribute to the work performed on the cam 34 to rotate the cam 34. Rather, the respective valves 46, 50 of the inactive piston/cylinder assemblies 30 may remain closed such that working fluid is inhibited from entering the cylinders 38 of the inactive piston/cylinder assemblies 30. Consequently, the piston 42 in the inactive piston/cylinder assemblies 30 would remain stationary within the housing of the pump/motor 14 after being displaced to the top dead center position, and would not reciprocate between the top dead center position and the bottom dead center position. Alternatively, the low-pressure valves 46 of the inactive piston/cylinder assemblies 30 may remain open, such that low-pressure working fluid is allowed to flow into and out of the cylinders 38 of the inactive piston/cylinder assemblies 30 as the respective pistons 42 reciprocate between the top dead center position and the bottom dead center position, without substantially contributing to the work performed on the cam 34 to rotate the cam 34. [0029] Fewer than the total number of available piston/cylinder assemblies 30 may be utilized at any time when the full capacity or the displacement of the pump/motor 14 (when operating as a motor) is not needed. For example, the pump/motor 14, when operating as a motor, may be operated at less than full displacement when the desired speed of the vehicle driveline (and therefore the speed of the cam 34 and the output shaft 26) is relatively low. To operate the pump/motor 14 at a reduced displacement, a select number of piston/cylinder assemblies 30 in the pump/motor 14 are made inactive. Those inactive piston/cylinder assemblies 30 are interspersed amongst the piston/cylinder assemblies 30 that are motoring. As shown in FIG. 2, this yields several gaps along the cam profile 74 within which work is not performed on the cam 34 to rotate the cam 34. If enough of the piston/cylinder assemblies 30 are made inactive, the gaps along the cam profile 74 within which work is not performed on the cam 34 to rotate the cam 34 may cause the torque output of the output shaft 26 to fluctuate which, in turn, may yield undesirable noise and roughness from the pump/motor 14.

[0030] With reference to FIG. 1, it should also be understood that the pump/motor 14 may be operated as a pump to recover energy from the vehicle driveline (e.g., to slow the rotation of the driveline). When operating as a pump, the output shaft 26 and the cam 34 are driven by the vehicle driveline to displace the respective pistons 42 in the piston/cylinder assemblies 30 from the bottom dead center position to the top dead center position, and then back to the bottom dead center position.

[0031] Particularly, starting at the top dead center position of the piston 42a, the controller 58 activates the coil 48 of the low-pressure valve 46 to open the valve 46 for a period of time to fluidly communicate the cylinder 38 and the low-pressure manifold 22, causing a transfer of low-pressure working fluid from the low-pressure manifold 22 to the substantially empty cylinder 38 in which the piston 42a is located. The transfer of low- pressure working fluid into the cylinder 38 subsequently displaces the piston 42a toward the cam 34. As the pressure of the working fluid in the low-pressure manifold 22 is substantially less than the pressure of the working fluid in the high-pressure manifold 18, any torque imparted on the cam 34 by the piston 42a as it is displaced from the top dead center position to the bottom dead center position is negligible.

[0032] The piston 42a will continue to be displaced toward the cam 34 until the piston

42a reaches the bottom dead center position (i.e., the position of the piston 42b in FIG. 1). Shortly before or when the piston 42b reaches the bottom dead center position, the controller 58 closes the low-pressure valve 46 to cease fluid communication between the cylinder 38 and the low-pressure manifold 22. The piston 42b is then driven by the rotating cam 34 upward toward the top dead center position until the pressure of the working fluid in the cylinder 38 is increased to a value that is substantially equal to the pressure of the working fluid in the high-pressure manifold 18, after which time the controller 58 opens the high- pressure valve 50 to fluidly communicate the cylinder 38 and the high-pressure manifold 18. The resultant high-pressure working fluid in the cylinder 38 is then pumped into the high- pressure manifold 18 for later use by the pump/motor 14 during a motoring cycle. The controller 58 closes the high-pressure valve 50 when a substantial amount of high-pressure working fluid in the cylinder 38 is pumped into the high-pressure manifold 18 and the piston 42b is near or at the top dead center position. The pressure of the working fluid left over in the cylinder 38 then decreases to a value that is substantially equal to the pressure of the working fluid in the low-pressure manifold 22 to permit the low-pressure valve 46 to open for the subsequent cycle of transferring low-pressure working fluid into the cylinder 38 for pumping into the high-pressure manifold 18.

[0033] This process is schematically illustrated in FIG. 3 in a similar manner as the schematic of FIG. 2. The piston/cylinder assemblies 30 identified with a "P" are those undergoing the cycle described above in which the cam 34 is performing work on the pistons 42 to pump working fluid from the low-pressure manifold 22 into the high-pressure manifold 18. This cycle is hereinafter referred to as "pumping," in which the pump/motor 14 is used as a pump to recover energy from the vehicle drive line or other mechanism. The piston/cylinder assemblies 30 used in the pumping cycle (i.e., those marked with a "P") that are further identified with an "X" as "active" are those in which working fluid from the low-pressure manifold 22 is being pumped into the high-pressure manifold 18 as described above for later use by the pump/motor 14 when operating as a motor. The piston/cylinder assemblies 30 used in the pumping cycle in which an "X" is omitted are those in which working fluid from the low-pressure manifold 22 is being introduced into the cylinders 38 as described above when the piston 42 is moving from the top dead center position to the bottom dead center position.

[0034] As shown in FIG. 3, the pump/motor 14 includes 24 piston/cylinder assemblies 30. However, not all 24 of the pump/motor assemblies 30 are shown being used in the pumping cycle. Rather, only 6 of the piston/cylinder assemblies 30 in the pump/motor 14 are being used in the pumping cycle. Those piston/cylinder assemblies 30 that are inactive (i.e., with no "P" or "X" identifier) do not contribute to recovering energy from the vehicle driveline. Rather, the respective valves 46, 50 of the inactive piston/cylinder assemblies 30 may remain closed such that working fluid is inhibited from entering the cylinders 38 of the inactive piston/cylinder assemblies 30. Consequently, the piston 42 in the inactive piston/cylinder assemblies 30 would remain stationary within the housing of the pump/motor 14 after being displaced to the top dead center position, and would not reciprocate between the top dead center position and the bottom dead center position. Alternatively, the low- pressure valves 46 of the inactive piston/cylinder assemblies 30 may remain open, such that low-pressure working fluid is allowed to flow into and out of the cylinders 38 of the inactive piston/cylinder assemblies 30 as the respective pistons 42 reciprocate between the top dead center position and the bottom dead center position.

[0035] Fewer than the total number of available pump/motor assemblies 30 may be utilized at any time when the full capacity or the displacement of the pump/motor 14 (when operating as a pump) is not needed. For example, the pump/motor 14, when operating as a pump, may be operated at less than full displacement when the speed of the vehicle driveline (and therefore the speed of the cam 34 and the output shaft 26) is relatively low. To operate the pump/motor 14 at a reduced displacement, a select number of piston/cylinder assemblies 30 in the pump/motor 14 are made inactive. Those inactive piston/cylinder assemblies 30 are interspersed amongst the piston/cylinder assemblies 30 that are pumping. As shown in FIG. 3, this yields several gaps along the cam profile 74 within which a reaction force on the cam 34 is absent. If enough of the piston/cylinder assemblies 30 are made inactive, the gaps along the cam profile 74 within which a reaction force on the cam 34 is absent may cause fluctuation of the output shaft 26 (and therefore lugging of the vehicle driveline) which, in turn, may yield undesirable noise and roughness from the pump/motor 14.

[0036] FIG. 4 is a schematic of the pump/motor 14 of FIG. 1, illustrating a method of operating the pump/motor 14 according to one embodiment of the invention in which at least one of the piston/cylinder assemblies 30 in the pump/motor 14 is pumping while a plurality or a group of the piston/cylinder assemblies 30 are motoring during each complete revolution of the cam 34 and the output shaft 26. The inventors have found that the method of FIG. 4 is particularly improved over the prior-art method of FIG. 2 when fewer than the total number of available piston/cylinder assemblies 30 are utilized when the full capacity or displacement of the pump/motor 14 is not needed. FIG. 4 illustrates 18 piston/cylinder assemblies 30 motoring and 3 piston/cylinder assemblies 30 pumping during each complete revolution of the cam 34 and the output shaft 26, yielding a net displacement of 15 piston/cylinder assemblies 30 (the same number of motoring pump/motor assemblies 30 in the prior art method of FIG. 2) from a total of 24 total piston/cylinder assemblies 30. Alternatively, the method of operating the pump/motor 14 illustrated in FIG. 4 may use any of a number of different combinations of piston/cylinder assemblies 30 that are motoring or pumping.

However, when the pump/motor 14 is being used as a motor, the number of piston/cylinder assemblies 30 that are motoring must exceed the number of piston/cylinder assemblies 30 that are pumping to yield a net motoring effect.

[0037] With respect to the particular manner of operation of the pump/motor shown in FIG. 4, at least two piston/cylinder assemblies (e.g., the assemblies 30a, 30b) are pumping working fluid into the high-pressure manifold 18 at some time during each complete revolution of the cam 34 and the output shaft 26. Particularly, as the active motoring piston/cylinder assemblies 30 (those identified with an "M" and an "X") are consuming high- pressure working fluid from the high-pressure manifold 18 to perform work on the cam 34 to rotate the cam 34 and the output shaft 26 (in the direction of arrow A), the active pumping piston/cylinder assemblies 30 (those identified with a "P" and an "X") are diverting some of that energy from the output shaft 26 and the vehicle driveline by pumping high-pressure working fluid back into the high-pressure manifold 18.

[0038] FIGS. 6 and 7 illustrate graphs of torque (measured at the output shaft 26) versus the rotational angle of the cam 34, corresponding with the displacement of one of the pistons 42 of the piston/cylinder assemblies 30 from the top dead center position to the bottom dead center position. As one of ordinary skill in the art would expect, the average torque output T2 of the output shaft 26 when using the method of FIG. 4 is less than the average torque output Tl of the output shaft 26 when using the prior-art method of FIG. 2 because some of the torque imparted to the cam 34 by the active motoring piston/cylinder assemblies 30 is diverted to the active pumping piston/cylinder assemblies 30 to pump high- pressure working fluid into the high-pressure manifold 18. With reference to the particular data underlying the graphs in FIGS. 6 and 7, the average torque output T2 of the output shaft 26 using the method of FIG. 4 is about 2.3% less than the average torque output Tl of the output shaft using the prior-art method of FIG. 2.

[0039] However, the inventors have unexpectedly discovered that the method of FIG.

4 also reduced the range of torque output values measured at the output shaft 26. As used herein, the "range" of torque output values measured at the output shaft 26 is the difference between the highest torque output value and the lowest torque output value measured at the output shaft 26 over a particular amount of rotation of the cam 34 (e.g., one-half a revolution of the cam 34, a complete revolution of the cam 34, etc.). With reference to the particular data underlying the graphs in FIGS. 6 and 7, the range R2 of torque output values measured at the output shaft 26 using the method of FIG. 4 is about 11% less than the range Rl of torque output values measured at the output shaft 26 using the prior-art method of FIG. 2. Consequently, in exchange for a relatively small decrease in average torque output at the output shaft 26, a relatively large improvement is achieved in reducing the range of torque output values at the output shaft 26. The inventors have found that such an improvement in the reduction of the range of torque output values at the output shaft 26 also reduces the noise, vibration, and harshness associated with operating the pump/motor 14 as a motor at less than full capacity or displacement. The inventors have also found that the reduction in the fluctuation of the torque output at the output shaft 26 improves the capability of the pump/motor 14 when used as a motor to deliver a relatively consistent torque output to the vehicle drivetrain at relatively low rotational speeds.

[0040] FIG. 5 is a schematic of the pump/motor 14 of FIG. 1, illustrating a method of operating the pump/motor 14 according to another embodiment of the invention in which at least one of the piston/cylinder assemblies 30 in the pump/motor 14 is motoring while a plurality or a group of the piston/cylinder assemblies 30 are pumping during each complete revolution of the cam 34 and the output shaft 26. The inventors have found that the method of FIG. 5 is particularly improved over the prior-art method of FIG. 3 when fewer than the total number of available pump/motor assemblies 30 are utilized when the full capacity or displacement of the pump/motor 14 is not needed. FIG. 5 illustrates 9 piston/cylinder assemblies 30 pumping and 3 piston/cylinder assemblies 30 motoring during each complete revolution of the cam 34 and the output shaft 26, yielding a net displacement of 6

piston/cylinder assemblies 30 that are pumping (the same number of pumping piston/cylinder assemblies 30 in the prior art method of FIG. 3) from a total of 24 piston/cylinder assemblies. Alternatively, the method of operating the pump/motor 14 illustrated in FIG. 5 may use any of a number of different combinations of piston/cylinder assemblies 30 that are motoring or pumping. However, when the pump/motor 14 is being used as a pump, the number of piston/cylinder assemblies 30 that are pumping must exceed the number of piston/cylinder assemblies 30 that are motoring to yield a net pumping effect.

[0041] With respect to the particular manner of operation of the pump/motor 14 shown in FIG. 5, at least two piston/cylinder assemblies 30 (e.g., the assemblies 30c, 30d) are consuming high-pressure working fluid from the high-pressure manifold 18 at some time during each complete revolution of the cam 34 and the output shaft 26. Particularly, as the active pumping piston/cylinder assemblies 30 (those identified with a "P" and an "X") are pumping high-pressure working fluid into the high-pressure manifold 18 for later use by the pump/motor 14 when operating as a motor, the active motoring piston/cylinder assemblies 30 (those identified with an "M" and an "X") are consuming some of the high-pressure working fluid from the high-pressure manifold 18 to impart torque to the cam 34 and the output shaft 26.

[0042] FIGS. 8 and 9 illustrate graphs of torque (measured at the output shaft 26) versus the rotational angle of the cam 34, corresponding with the displacement of one of the pistons 42 of the piston/cylinder assemblies 30 from the top dead center position to the bottom dead center position, and then back to the top dead center position, using the prior-art method of FIG. 3 and the method of FIG. 5, respectively. Particularly, the waveforms having negative torque values relate to the individual piston/cylinder assemblies 30 that are pumping, while the waveforms having positive torque values relate to the individual piston/cylinder assemblies 30 that are motoring.

[0043] The inventors have unexpectedly discovered that the method of FIG. 5 reduces the range of reaction torque values measured at the output shaft 26. With reference to the particular data underlying the graphs in FIGS. 8 and 9, the range R4 of reaction torque values measured at the output shaft 26 using the method of FIG. 5 is about 75% less than the range R3 of reaction torque values measured at the output shaft 26 using the prior-art method of FIG. 3. The inventors have found that such an improvement in the reduction of the range of reaction torque values at the output shaft 26 increases the consistency of the flow rate of the working fluid output from the pump/motor 14 when operating as a pump. The inventors have also found that this improvement n the reduction of the range of reaction torque values at the output shaft 26 reduces the noise, vibration, and harshness associated with operating the pump/motor 14 as a pump at less than full capacity or displacement.

[0044] Various features of the invention are set forth in the following claims.