Patent Publication Number: US-2012034115-A1

Title: Method of operating a pump/motor

Description:
FIELD OF THE INVENTION 
     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 INVENTION 
     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 
     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/cylinder 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/cylinder assembly. 
     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/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, 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 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/cylinder 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/cylinder 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/cylinder 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/cylinder assembly is opened. 
     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/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, 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. 
     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 
         FIG. 1  illustrates a prior-art multi-cylinder hydraulic pump/motor in which the method of the present invention may be implemented. 
         FIG. 2  is a schematic of the pump/motor of  FIG. 1 , illustrating a prior-art method of operating the pump/motor. 
         FIG. 3  is a schematic of the pump/motor of  FIG. 1 , illustrating another prior-art method of operating the pump/motor. 
         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. 
         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. 
         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 . 
         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 . 
         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 . 
         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 . 
     
    
    
     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 
       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. 
     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  42   b ) to a top dead center position (see piston  42   a ), 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 ) 
     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. 
     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. 
     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 . 
     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. 
     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  42   a , 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  42   a  is located. The transfer of high-pressure working fluid into the cylinder  38  subsequently displaces the piston  42   a  toward the cam  34 . The cam  34  includes an inclined cam surface  66  which converts the axial motion of the piston  42   a  to rotational motion of the cam  34  and the output shaft  26 . As a result, the linear force exerted on the piston  42   a  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 . 
     The piston  42   a  will continue to impart torque to the cam  34  and the output shaft  26  until the piston  42   a  reaches its bottom dead center position (i.e., the position of the piston  42   b  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 . 
     Shortly before the piston  42   a  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  42   a  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  42   a  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  42   b ) 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  42   b  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 . 
     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. 
     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 driveline 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. 
     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 . 
     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 . 
     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. 
     Particularly, starting at the top dead center position of the piston  42   a , 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  42   a  is located. The transfer of low-pressure working fluid into the cylinder  38  subsequently displaces the piston  42   a  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  42   a  as it is displaced from the top dead center position to the bottom dead center position is negligible. 
     The piston  42   a  will continue to be displaced toward the cam  34  until the piston  42   a  reaches the bottom dead center position (i.e., the position of the piston  42   b  in  FIG. 1 ). Shortly before or when the piston  42   b  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  42   b  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  42   b  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 . 
     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 driveline 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. 
     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. 
     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 . 
       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. 
     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  30   a ,  30   b ) 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 . 
       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 T 2  of the output shaft  26  when using the method of  FIG. 4  is less than the average torque output T 1  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 T 2  of the output shaft  26  using the method of  FIG. 4  is about 2.3% less than the average torque output T 1  of the output shaft using the prior-art method of  FIG. 2 . 
     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 R 2  of torque output values measured at the output shaft  26  using the method of  FIG. 4  is about 11% less than the range R 1  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. 
       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. 
     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  30   c ,  30   d ) 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 . 
       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. 
     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 R 4  of reaction torque values measured at the output shaft  26  using the method of  FIG. 5  is about 75% less than the range R 3  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. 
     Various features of the invention are set forth in the following claims.