Hydraulic regenerative braking system for a vehicle

A hydraulic regenerative braking system and method for a vehicle include a variable displacement hydraulic machine operable as a pump or a motor (22). The hydraulic machine facilitates connections between some of its piston cylinders (50, 52) and a low pressure source (32), and some other of its piston cylinders (50, 52) and a high pressure source (28). The cylinders are alternately connected to the high and low pressure sources such that a pressure transition occurs during each piston stroke. By controlling where in a piston stroke the transitions occur, the power of the hydraulic machine can be modulated. The hydraulic machine is configured to effect the pressure transitions in only a portion of the cylinders at one time. This can be accomplished by asymmetrically configuring cam lobes (86, 88, 90) and fluid ports (74, 76, 78, 80, 82, 84). Staggering the pressure transitions helps to inhibit flow disturbances in the machine when it is operating at less than full displacement.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hydraulic regenerative braking system and method for a vehicle.

2. Background Art

It is well known that hydraulic regenerative systems promise improved efficiency over electric regenerative systems incorporating a battery. Hydraulic regeneration involves using a pump connected in the vehicle drive train as a retarding device, and then storing the resulting high pressure fluid in an accumulator. On subsequent vehicle acceleration, the high pressure fluid from the accumulator is routed to a hydraulic motor and the stored energy is recovered in the form of mechanical work which drives the vehicle forward. A low pressure accumulator acts as a reservoir to make up for fluid volume variations within the high pressure accumulator, and also provides a charge pressure to the inlet side of the pump.

One method of modulating braking and driving forces in hydraulic regenerative systems is to incorporate a variable displacement device to operate in concert with the fixed pressure accumulator. Conventional variable displacement hydraulic machines may vary the piston strokes to achieve the desired power modulation. Such devices can be bulky, heavy and expensive. Moreover, they do not package easily in automotive passenger vehicles, especially in the front of a vehicle, where space is limited.

One way to overcome the limitations associated with conventional variable displacement hydraulic machines is to use a fixed displacement machine. Such a machine is generally smaller and lighter than its variable displacement counterpart, but it does not allow the power modulation required in most applications. One solution to this problem is to use a fixed displacement hydraulic machine in conjunction with a variable ratio hydraulic transformer to facilitate the desired power modulation. One such system is described in U.S. patent application Ser. No. 10/535,354, entitled “Hydraulic Regenerative Braking System for a Vehicle,” filed on May 18, 2005, which is hereby incorporated herein by reference. As an alternative, it would be desirable to have a system that included a relatively compact variable displacement hydraulic machine, thus eliminating the requirement of a separate variable ratio transformer.

Therefore, a need exists for a hydraulic regenerative braking system and method for a vehicle that uses a variable displacement hydraulic machine to provide control of power modulation, without consuming too much space in the vehicle powertrain.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a hydraulic regenerative braking system and method that conserves space by using one or more compact variable displacement hydraulic machines.

The invention also provides a hydraulic regenerative braking system and method that uses a hydraulic machine that effects modulation of its power by varying the amount of fluid pumped out of, or received by, the hydraulic machine.

The invention further provides a hydraulic regenerative braking system and method that staggers pressure changes in a variable displacement hydraulic machine to reduce flow disturbances.

The invention also provides a hydraulic regenerative braking system for a vehicle having a shaft connected to at least one wheel. The system includes a hydraulic machine operable as a pump configured to be driven by the shaft, thereby increasing the pressure of fluid flowing through the hydraulic machine. The hydraulic machine is further operable as a motor configured to be driven by pressurized fluid, thereby providing torque to the shaft. The hydraulic machine includes a housing having a high pressure fluid port and a low pressure fluid port, and also includes a plurality of radial pistons. Each of the pistons is configured to reciprocate within a corresponding cylinder in the housing, and has a corresponding piston stroke. The pistons pump fluid when the hydraulic machine is operating as a pump, and provide torque when the hydraulic machine is operating as a motor. Each of the pistons includes a corresponding cam follower. A cam is disposed within the housing, and has a plurality of lobes configured to cooperate with the cam followers to translate rotational motion of the cam into linear motion of the pistons when the hydraulic machine is operating as a pump, and to translate linear motion of the pistons into rotational motion of the cam when the hydraulic machine is operating as a motor. A valve plate includes a plurality of apertures therethrough, at least one of which communicates with the high pressure fluid port and at least one of which communicates with the low pressure fluid port. The valve plate is configured to connect at least one of the cylinders with the high pressure fluid port and at least one other of the cylinders with the low pressure fluid port. The valve plate is movable relative to the housing to effect a first transition to disconnect the at least one cylinder from the high pressure fluid port and connect it with the low pressure fluid port, and to effect a second transition to disconnect the at least one other cylinder from the low pressure fluid port and connect it with the high pressure fluid port. The valve plate is movable such that the first and second transitions can be effected at a plurality of piston positions within a corresponding piston stroke, thereby facilitating variable displacement operation of the hydraulic machine.

The invention further provides a hydraulic regenerative braking system for a vehicle that includes a first accumulator configured to receive fluid and store the fluid under pressure, and a second accumulator configured to store the fluid at a pressure lower than the pressure of the fluid in the first accumulator. A hydraulic machine is operable as a pump configured to be driven by energy received from at least one vehicle wheel when the vehicle is braking, thereby facilitating storage of vehicle braking energy. The hydraulic machine is further operable as a motor configured to be driven by stored braking energy, thereby providing torque to the at least one wheel. The hydraulic machine is configured to pump fluid into the first accumulator when operating as a pump, and to receive fluid from the first accumulator when operating as a motor. The hydraulic machine is further configured such that the amount of fluid pumped into the first accumulator during each cycle of the hydraulic machine can be varied, and the amount of fluid received from the first accumulator during each cycle of the hydraulic machine can be varied, thereby facilitating operation of the hydraulic machine as a variable displacement pump and as a variable displacement motor, respectively. The hydraulic regenerative braking system also includes a control system having at least one control module. The control system is configured to receive inputs related to the operation of the vehicle, and to control operation of the hydraulic machine.

The invention also provides a method for hydraulic regenerative braking of a vehicle. The vehicle includes a shaft connected to at least one wheel, first and second accumulators for storing and providing pressurized fluid, and a hydraulic machine. The hydraulic machine is operable as a pump configured to receive energy from the at least one wheel and pump fluid into the first accumulator, and further operable as a motor to receive pressurized fluid from the first accumulator and provide energy to the at least one wheel. The method includes operating the hydraulic machine as a pump during a vehicle braking event. During the braking event, the hydraulic machine is driven by the shaft, thereby providing pressurized fluid to at least the first accumulator to store the pressurized fluid. The hydraulic machine is operated to control the amount of fluid provided to the first accumulator during each cycle of the hydraulic machine during the braking event, thereby controlling the displacement of the hydraulic machine when operating as a pump. The hydraulic machine is operated as a motor during a vehicle driving event. During the vehicle driving event, the hydraulic machine is driven by pressurized fluid provided from at least the first accumulator, and thereby provides torque to the shaft. The hydraulic machine is operated to control the amount of fluid received from the first accumulator during each cycle of the hydraulic machine during the driving event. This controls the displacement of the hydraulic machine when operating as a motor.

Embodiments of the invention include a high pressure hydraulic pump/motor capable of operating at high speeds and with a variable output flow capability. Embodiments of the pump/motor provide the advantages of radial pistons in a compact design, the capability of disengaging the pistons to increase longevity, the capability to mount the pump/motor on an existing shaft with through-drive capability, and the capability to internally shift from pump function to motor function by re-indexing a rotating valve plate relative to a cam that either drives, or is driven by, the pistons.

Embodiments of the invention include a hydraulic machine that can be scaled to larger vehicles such as buses, medium-to-large trucks such as garbage trucks, and combat vehicles such as HumVee's. For these vehicles, it is desirable to mount a single hydraulic pump/motor on the propeller drive shaft, meaning that the unit will rotate at speeds approximately 3 to 4 times that of an axle mounted units. The invention modulates the power at the source, rather than using a separate transformer, thereby decreasing cost, reducing package size, and increasing efficiency.

To increase the operating speed, the number of cam lobes may be kept to a low number, for example, two, and a relatively short piston stroke can be maintained. Packaging in current vehicles, without major redesign and retooling of the major components, is considered a high priority. Thus, the present invention also allows for retrofit of existing vehicles. To accommodate larger vehicles, the size of the hydraulic machine can be increased; however, for propeller shaft mounting, or mounting on the transmission or transfer case, increasing the length of the hydraulic machine may be more desirable than increasing the girth (diameter). For this reason, additional banks of cylinders can be added.

In one embodiment, the hydraulic machine includes two banks of eight piston/cylinder combinations, with the second bank rotated 22.5 degrees to mount half way between the original 8 cylinders. Thus, with 16 equally spaced cylinders around the circle, angular spacing is 360/16 degrees—i.e., the pistons are spaced at 22.5 degree intervals. A cam for this machine may have two lobes, leading to 16 double power pulses per revolution (16×2/2). The duration of each intake stroke is one-half the cam increment; the other half of the duration is the exhaust stroke. The cam increment for a two lobe cam is 180 degrees, so the intake stroke duration is 90 degrees. With 16 pairs of power pulses per revolution, there are two new intake strokes beginning every 22.5 degrees, and lasting 90 degrees.

A rotating valve plate in the hydraulic machine cooperates with the cam to effect transitions from high pressure to low pressure, and from low pressure to high pressure. Although embodiments of the hydraulic machine described herein effect the pressure transitions by rotating the valve plate, other embodiments may effect the pressure transitions by other valve plate movements, such as translation, or a combination of rotation and translation. These pressure transitions can be considered as events, so that in the embodiment described above, there would be 4 events happening 16 times per revolution. The present invention includes a hydraulic machine having a means of varying the power output, effectively making it a variable displacement machine. For example, when the machine is operating as a pump, the rotating valve plate can be indexed slightly, so that the pressure transitions occur somewhere in mid-stroke, instead of at top dead center (TDC) or bottom dead center (BDC). This allows some of the high pressure fluid to be recirculated back into the cylinder before the pressure transition occurs, rather than having all of it expelled as when the transition to low pressure occurs at TDC.

Indexing meets the variable displacement requirement, but can cause a problem if there are four simultaneous pressure transitions, two high-to-low pressure, and two low-to-high pressure, with pistons in mid-stroke at some velocity, rather than when the piston velocity is approximately zero at top and bottom dead center. Each of the four events causes an interruption in the continuous flow every 22.5 degrees of rotation, and the four simultaneous interruptions are additive. To mitigate the disturbance, the cam and the valve plate can be modified slightly to re-index the four events so they do not occur simultaneously, but rather, occur at 22.5/4 degree increments, or every 5.625 degrees.

A two lobe cam typically consists of two increasing radius profiles of 90 degree duration each, and two decreasing radius profiles of 90 degree duration. To accomplish the event separation described above, and to space the events evenly, the four cam profiles are shortened by 5.625 degrees each, and a constant radius segment of 22.5 degrees is inserted in the cam to complete the circle. Three of the corresponding rotating valve ports are shortened by the same 5.625 degree increment, and the fourth port which synchronizes with the constant radius portion and an adjacent ramp, is increased by 16.875 degrees. With 16 equally spaced cylinders, two cam lobes with modified spacing, and pressure transitions occurring at the beginning and ending of each stroke, 64 events can now occur at even increments of 5.625 degrees.

For full displacement, the rotating valve plate, turning synchronously with the cam, is indexed to cause pressure transitions at top center of the cam profile and at bottom center of the cam profile. Indexing the rotating valve relative to the cam switches the machine from a pump to a motor, and vice versa when indexing in the opposite direction. With the cam lobe spacing described above, indexing to change the machine from a pump to a motor is 84.325 degrees, instead of 90 degrees for a machine having four synchronous events. To accomplish modulated performance when operating as either a pump or motor, the indexing of the rotating valve ports is at some lesser value—i.e., less than 84.325 degrees. The theoretical mode crossover from pump to motor occurs at 42.1625 degrees of indexing from top dead center of the cam, at which point all fluid is recirculated within the pump/motor, and there is no external flow.

The example described above with a hydraulic machine having 16 pistons and a two lobe cam is just one combination of elements in a hydraulic machine contemplated by the present invention. As described in more detail below, a hydraulic machine having two banks of seven pistons each, working in concert with a three lobe cam, is also an effective arrangement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1shows a schematic representation of one embodiment of a hydraulic regenerative braking system10for a vehicle11in accordance with the present invention. The vehicle11includes an engine12, a transmission13, and four wheels14,16,18,20. The regenerative braking system10includes a hydraulic machine, or pump/motor22, connected to a front drive shaft24. The present invention also contemplates the use of a hydraulic machine attached to a rear drive shaft, rather than a front drive shaft. Moreover, in other embodiments of the present invention, more than one pump/motor may be used, for example, a pump/motor attached to each of the front and rear drive shafts, or a respective pump/motor attached to front and/or rear half axle shafts.

The pump/motor22is operable to pump fluid into a first, or high pressure accumulator28, where the high pressure fluid is stored for later use. The pump/motor22is also operable as a motor, driven by fluid from the high pressure accumulator28. Thus, the braking energy stored in the high pressure accumulator28during a braking event is used to operate the pump/motor22as a motor to provide torque to the wheels14,16during a driving event.

The regenerative braking system10also includes a second, or low pressure accumulator30. The low pressure accumulator30provides a charge pressure—i.e., a relatively low pressure—to the pump/motor22to help ensure that there is always some liquid supplied to the pump/motor22, thereby avoiding cavitation. The low pressure accumulator30includes two parts: a liquid/gas container32, and a gas only container34. Similarly, the high pressure accumulator28includes two parts: a liquid/gas container36, and a gas only container38. Configuring each of the accumulators28,30with two containers facilitates packaging by reducing the size of each liquid/gas container32,36. Of course, high and low pressure accumulators, such as the high and low pressure accumulators28,30, may include a single liquid/gas container, rather than the two-part configuration shown inFIG. 1.

The two container arrangement takes advantage of residual volume for gas in the accumulator which is available after the accumulator piston (not shown) is at the end of its stroke. The gas only containers34,38may be approximately 30% of the total respective accumulator volume, though different sizes of gas only containers may be used. To increase efficiency of the accumulators28,30, the gas side of each liquid/gas container32,36, and the gas only containers34,38, may be filled with an open cell foam, such as polyester, to help to ensure that compression and expansion of the gas occurs at constant temperature.

The regenerative braking system10also includes a control system, shown inFIG. 1as a control module40. The control module40receives inputs related to operation of the vehicle, and uses these inputs to control operation of the pump/motor22. Such inputs may include driver initiated acceleration requests and braking requests, which may be input directly into the control module40, or may be input from another controller, such as a vehicle system controller. In addition to electronic inputs, the control module40may also receive a number of hydraulic inputs (removed inFIG. 1for clarity) to detect various fluid pressures in the system10, and to help control operation of the pump/motor22.

When the control module40is signaled to use regenerative braking during a braking event, it sends a control pressure to the pump/motor22to ensure that the pump/motor22operates as a pump. As explained more fully below, this involves appropriate positioning of components of the pump/motor22to ensure that fluid is pumped into the high pressure accumulator28as the pump/motor22is driven by the front drive shaft24. Conversely, when the control module40is signaled to provide torque to the wheels14,16during a driving event, it sends a control pressure to the pump/motor22to ensure that the pump/motor22operates as a motor. In this mode, fluid from the high pressure accumulator28drives the pump/motor22such that torque is provided to the wheels14,16.

FIGS. 2A and 2Bshow sectional views of the pump/motor22. As shown inFIG. 2B, the pump/motor22includes two banks42,44of piston/cylinder combinations. As discussed above, hydraulic machines in accordance with the present invention can be configured with different numbers of piston/cylinder combinations as desired. As shown inFIG. 2A, the first bank42includes seven pistons46radially oriented around the pump/motor22. Although only one piston48is shown in the second bank44inFIG. 2B, it is understood that the second bank42also includes seven of the pistons48radially oriented around the pump/motor22. Also shown inFIG. 2B, each of the pistons46has a corresponding cylinder50, and each of the pistons48has a corresponding cylinder52.

The pump/motor22also includes a cam54, having an aperture56configured to be keyed to the drive shaft24. The cam54is configured to cooperate with cam followers58on each of the pistons46,48. Thus, the drive shaft24turns the cam54which operates the pistons46,48to pump fluid to the high pressure accumulator28when the pump/motor22is operating as a pump—i.e., during vehicle braking. Conversely, when the pump/motor22is operating as a motor, the high pressure accumulator28provides fluid to the pump/motor22to operate the pistons46,48, which in turn rotate the cam54to provide torque to the drive shaft24, and thus, the vehicle wheels14,16.

The pump/motor22includes a cylinder block, or housing60, which includes a high pressure fluid port62, and a low pressure fluid port64. As shown inFIG. 2B, the housing60includes one portion65that contains the cylinders50,52, and another portion67that substantially surrounds the one portion65and includes a tapered bore69. AlthoughFIG. 2Bshows the high pressure fluid port62connected only to the cylinders50in the first bank42, and the low pressure fluid port64is shown inFIG. 2Bconnected only to the cylinders52in the second bank44, it is understood that both the high and low pressure fluid ports62,64are connected to the cylinders50,52in each of the banks42,44. The high pressure fluid port62is connected to the high pressure accumulator28, while the low pressure fluid port64is connected to the low pressure accumulator30.

In order to facilitate a connection between the cylinders50,52and the high and low pressure fluid ports62,64—and thus the high and low pressure accumulators28,30—the pump/motor22includes a valve plate66. The valve plate66is also attached to the drive shaft24, and rotates with it, making it movable relative to the housing60. It is worth noting that in other embodiments, a cam and valve plate, such as the cam54and the valve plate66, may not rotate with a driving shaft; rather, a pump/motor housing can rotate with the shaft, while the respective cam and valve plate are stationary—in such a case, the valve plate is still movable relative to the housing, though it is the housing that moves. Regardless of whether a cam and valve plate rotate with a driving shaft, they will rotate relative to each other. This provides a number advantages. First, it allows the pump/motor22to switch from a pump to a motor, and vice versa. Second, as explained more fully below, it allows pressure transitions within the cylinders50,52to take place at different points in a respective piston stroke, thereby effecting operation of the pump/motor22as a variable displacement machine.

In order to effect movement of the valve plate66relative to the cam54, the pump/motor22includes an axial piston68. The piston68may be keyed or splined to the drive shaft24. The piston68drives the valve plate66via one or more links70, which are shown in phantom inFIG. 2Bin three different positions. The links70translate the linear movement of the axial piston68into rotational movement of the valve plate66. Movement of the axial piston68in one direction is effected by fluid entering the mode port72. A spring (not shown) is provided to return the axial piston68to its previous position when the fluid pressure from the mode port72is exhausted.

In order to facilitate a connection between the accumulators28,30and the cylinders50,52via the high and low pressure ports62,64, the valve plate66includes a number of apertures, or ports74,76,78,80,82,84—seeFIG. 3. In the embodiment shown inFIGS. 2 and 3, the valve plate66has six ports74,76,78,80,82,84, which is two ports for each of three lobes86,88,90on the cam54—seeFIG. 4. Returning toFIG. 3, the valve plate66is shown juxtaposed over seven apertures92and seven other apertures94in the housing60—see alsoFIG. 2B. Each of the apertures92is configured to facilitate fluid flow to and from the cylinders50in the first bank42, while each of the apertures94is configured to facilitate fluid flow to and from the cylinders52in the second bank44.

At any given moment while the pump/motor22is operating, half of the ports—for example, the ports74,78,82—are connected to the high pressure fluid port62, and thus the high pressure accumulator28. At the same time, the other half of the ports—i.e., the ports76,80,84—are connected to the low pressure fluid port64, and thus the low pressure accumulator30. Whether the pump/motor22is operating as a pump or a motor depends on where in the piston strokes the connection is made to the high and low pressure fluid ports62,64. For example, if the cylinders50,52are connected to the high pressure fluid port62when the respective pistons46,48are at TDC, and the cylinders50,52are connected to the low pressure fluid port64when the respective pistons46,48are at BDC, the pump/motor22will operate as a full displacement motor. Conversely, if the cylinders50,52are connected to the high pressure fluid port62when the respective pistons46,48are at BDC, and the cylinders50,52are connected to the low pressure fluid port64when the respective pistons46,48are at TDC, the pump/motor22will operate as a full displacement pump.

As discussed above, indexing the valve plate66relative to the cam54such that the transition from high pressure to low pressure takes place within the cylinders50,52when the respective pistons46,48are moving with some velocity—i.e., not at TDC or BDC—effects operation of the pump/motor22as a reduced displacement machine. To effect the indexing, the position of the valve plate66, and thus the fluid ports74,76,78,80,82,84, is controlled relative to the position of the cam54.FIGS. 5A-5Dshow the relative positions of the valve plate66and the cam54for four different operating modes. In each of these views, one of the pistons48is shown with its associated cam follower58engaging the cam54, and each view shows the point at which the respective cylinder50(not shown) is connected to the high pressure fluid port62.

InFIG. 5A, the valve plate66and cam54are aligned for operation of the pump/motor22as a full displacement pump. The cam54is at its lowest point, and the piston46is at BDC. At this point, the valve plate66connects the respective cylinder50with the high pressure fluid port62. It is understood that each of the other six pistons46and each of the other seven pistons48will have their respective cylinders50,52connected with the high pressure fluid port62when the respective pistons46,48are at BDC.

FIG. 5Bshows the piston46in the first half of its exhaust stroke—i.e., the profile of the cam54is increasing. Connecting the respective cylinder50to the high pressure fluid port62at this point also facilitates operation of the pump/motor22as a pump; however, this connection is made for only a portion of the exhaust stroke of the piston46, and in particular for more than one-half the exhaust stroke, and so the output of the pump/motor22is reduced as compared to the operation shown inFIG. 5A. It is readily understood that the switch from the high pressure fluid port62to the low pressure fluid port64will occur when the piston46is at a corresponding point in its intake stroke.

FIG. 5Cshows the piston46in the second half of its exhaust stroke, and like the other figures in this set of illustrations, it is assumed that the valve plate66connects the respective cylinder50to the high pressure fluid port62at this point in the piston stroke. This means that inFIG. 5C, the piston46will be connected to the high pressure fluid port62for only a portion, but for more than one-half, of its intake stroke, which occurs while the profile of the cam54is decreasing. Thus, inFIG. 5C, the pump/motor22is operating as a motor, but at reduced output, since the respective cylinder50was not connected to the high pressure fluid port62for the entire intake stroke of the piston46. Finally,FIG. 5Dshows the arrangement of the valve plate66and the cam54for operation of the pump/motor22as a full displacement motor. The piston46is shown inFIG. 5Dat TDC, and the switch to high pressure in the respective cylinder50will allow the piston46to be connected to the high pressure fluid port62for the full length of its intake stroke.

As described above, the valve plate66is indexed relative to the cam54to switch the operation of the pump/motor22from a pump to a motor—and vice versa.FIG. 4shows the details of the cam profile, which includes the three lobes86,88,90. With each cam of the cam lobes86,88,90having two segments, a working portion and a return portion, there are84events per revolution, or cycle, of the pump/motor22(14 pistons×3 lobes×2 lobe portions). When the pump/motor22is operating at full displacement, all pressure transitions (events) occur at TDC and at BDC. To have events equally spaced, there needs to be 360/84=4.29 degrees between events.

If a cam, such as the cam54, was configured with six symmetric sections of 60 degrees each—three working and three return—and the six cam sections were matched up with six corresponding fluid ports of 60 degrees in a corresponding valve plate, and further matched up with fourteen equally spaced ports in the housing, two of the events would always occur simultaneously. To stagger the events, the cam54and the valve plate66have been configured asymmetrically. In particular, two of the cam segments are shortened from60degrees to 55.71 degrees, and two constant radius (dead spots) are added—one at TDC and one 180 degrees away at BDC—seeFIG. 4. The two shortened cam segments straddle one of the constant radius segments, for example, the segment at BDC. On the valve plate66, the port durations are modified to communicate with the cam; they are changed from six at 60 degrees, to: four at 60 degrees, one at 64.29 degrees, and one opposite at 55.71 degrees—seeFIGS. 6A and 6B.

As described above, for a full displacement pump or motor, all pressure transitions occur at TDC and BDC when the piston velocity is approximately zero. For partial pump or motor displacement, however, the valve is indexed somewhere between TDC and BDC, so that pressure transitions do not occur at zero piston velocity. The total external flow (fluid velocity) is the algebraic sum of all piston velocities, positive and negative, indexed to all of the connected ports (high pressure or low pressure). As the cam rotates past the piston rollers (cam followers) one event at a time, the ports in the rotating valve plate do the same, and the maximum instantaneous flow change will be on the order of 17% (⅙) because half of the cylinders (seven) are connected to each of the external ports at any one time, but some of the seven are operating at less than full velocity. Small decreases from maximum flow will result in much less interruption in flow velocity than the 17% maximum because the interrupted port will be corresponding with a low piston velocity cylinder at the time.

An example is now provided to describe a transition from a full displacement pump—seeFIGS. 4,5A and6A—to a full displacement motor—seeFIGS. 4,5D and6B. Cam segments are numbered 1 through 6, and valve ports in the valve plate are labeled A through F. A single direction of rotation is assumed when describing “increasing” and “decreasing” cam segments.

Pump Full Displacement

Motor Full Displacement

Cam SegmentValve Portconst rad TDC4.29°1decreasing60°Bhigh pressure60°2increasing60°Clow pressure60°3decreasing55.71°Dhigh pressure55.71°const rad BDC4.29°4increasing55.71°Elow pressure60°5decreasing60°Fhigh pressure60°6increasing60°Alow pressure64.29°
It can be seen that the short valve port always lines up with a short cam segment, the standard valve ports always line up with a standard cam segment or a short segment and a constant radius segment, and the long valve port always lines up with a standard cam segment and constant radius segment.

To further illustrate the different modes of operation of the pump/motor22,FIGS. 7A-7Dshow velocity profiles for the cam54and the valve plate66for four different operating modes. As shown inFIGS. 7A and 7D, the transitions between high and low pressure occur at TDC and BDC when the pump/motor22is operating at full displacement. Conversely, the transitions occur in mid-stroke when the pump/motor22is operating at reduced displacement. In addition,FIG. 7Aclearly illustrates how the various fluid ports74,76,78,80,82,84line up with corresponding cam segments, such that an increasing or decreasing cam segment by itself, or combined with a constant radius segment, lines up with a fluid port of appropriate length.