Abstract:
A hydrostatically driven vehicle includes an engine operating at a first speed and operably connected to a variable displacement pump in fluid communication with a hydraulic circuit. The pump includes a rotating swashplate being adapted to operate at selective angles, which dictate pump displacement ranging from zero to a maximum displacement. The pump is capable of providing a pump flow rate at the first speed when the pump swashplate is set to the maximum displacement, wherein the pump flow rate is greater than the maximum flow rate that may be received by the hydraulic circuit.

Description:
TECHNICAL FIELD 
       [0001]    This patent disclosure relates generally to hydrostatically driven vehicles and, more particularly, to a combination of components sized to provide operational efficiency. 
       BACKGROUND 
       [0002]    Hydrostatically driven vehicles typically include a hydraulic pump driven by an engine or motor. The hydraulic pump propels a flow of fluid to one or more actuators, typically hydraulic motors, connected to wheels or other driving features of the vehicle. The flow of fluid from the pump passes through each actuator, causing the vehicle to move along at a travel speed. An operator adjusting a control input, for example, a lever, pedal, or any other appropriate device controls motion of the vehicle. When the operator displaces the control input, a signal is generated by a displacement sensor integrated with the control input or, alternatively, by displacement of a mechanical linkage. The signal is conveyed to a controller associated with the vehicle where it is interpreted and an appropriate command is issued to an actuator associated with the hydraulic pump, the actuator being arranged to move a control aim of the pump operating to change the displacement of the pump. Alternatively, the control input may be mechanically connected to the pump, for example, by cable, which causes the control arm of the pump to move in response to displacement of the control input. 
         [0003]    Displacement of the control arm of the pump causes a change in the pump&#39;s displacement by changing the angle of operation of a swashplate within the pump and, accordingly, a change in the pressure and flow rate of fluid propelled through the pump. Modulation of the flow rate of fluid also modulates the rate of rotation of hydraulic motors driving the wheels of the vehicle and, therefore, the travel speed of the vehicle. Additional systems may be available for control of the travel speed of the vehicle, for example, braking systems or transmissions may be used to decelerate the vehicle when the operator so desires. 
         [0004]    Even though these types of control are generally effective in controlling the vehicle, such hydrostatically driven vehicles generally do not operate efficiently with regard to fuel consumption most of the time. The engine for a typical vehicle, for example, a soil compactor, is arranged for steady state operation at or about 2300 revolutions per minute (RPM). When the vehicle is operating at full power, the pump is set to its highest setting and inefficiencies of the pump cause an increase in fuel consumption. Accordingly, it is desirable to provide an arrangement that overcomes or minimizes one or more of these shortcomings. 
       SUMMARY 
       [0005]    A hydrostatically driven vehicle includes an engine operating at a first speed and operably connected to a variable displacement pump. The pump includes a rotating swashplate being adapted to operate at selective angles, which dictate pump displacement. Pump displacement ranges from zero to a maximum displacement. A hydraulic circuit is adapted to receive a flow of fluid from the pump, the flow of fluid circulating at a flow rate through the hydraulic circuit. The hydraulic circuit is capable of operating at or below a maximum hydraulic circuit flow rate. The pump is capable of pumping the maximum hydraulic circuit flow rate of fluid into the hydraulic circuit while the engine operates at the first speed and while the swashplate is disposed at an operating angle corresponding to an operating displacement that is less than the maximum displacement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is an outline view of a soil compactor as one example for a hydrostatically driven vehicle in accordance with the disclosure. 
           [0007]      FIG. 2  is a schematic diagram of a hydraulic system in accordance with the disclosure. 
           [0008]      FIG. 3  is a schematic cross section of a simplified variable displacement pump. 
           [0009]      FIG. 4  is a graph qualitatively plotting flow rate versus outlet pressure for a variable displacement pump. 
           [0010]      FIG. 5  is a comparison of two graphs, each corresponding to a variable displacement pump, in accordance with the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    This disclosure relates to hydrostatically operated machines. The examples presented for illustration relate to a hydrostatically driven vehicle and, more specifically, to a combination of components of the vehicle that yield a reduced engine operating speed for optimization of most operating conditions. The present disclosure is applicable to any type of machine having a hydraulic system associated therewith. In the exemplary vehicle presented, the engine can operate at a lower engine speed and torque output when the demand of the vehicle is less than a maximum demand and the flow of fluid is at a maximum flow. According to the disclosure, this reduction in engine speed is accomplished by use of a pump having a larger maximum displacement than pumps used in the past, even though the pump may be so sized that it will never operate at a maximum displacement setting because the hydraulic system of the vehicle is incapable of accepting such a flow. In this manner, the operation of the vehicle and, accordingly, any other hydraulically operated machine, may be optimized. 
         [0012]      FIG. 1  shows an outline view of a vehicle  100  as one example of a hydrostatically driven vehicle. Although a soil compactor is illustrated in  FIG. 1 , the term “vehicle” may refer to any hydrostatic machine that performs some type of operation associated with an industry such as mining, paving, construction, fanning, transportation, or any other industry known in the art. For example, the vehicle  100  may be an earth-moving machine, such as a wheel or track loader, excavator, dump truck, backhoe, motor-grader, material handler or the like. 
         [0013]    The vehicle  100  includes an engine frame portion  102  and a non-engine frame portion  104 . An articulated joint  106  that includes a hinge  108 , which allows the vehicle  100  to steer during operation, connects the two portions of the frame  102  and  104 . The engine portion  102  of the frame includes an engine  110  and a set of wheels  112  (only one wheel visible). The engine  110  can be an internal combustion engine, for example, a compression ignition engine, but, in general, the engine  110  can be any prime mover that provides power to various systems of the vehicle by consuming fuel. 
         [0014]    In the exemplary vehicle  100  presented herein, the non-engine frame  104  accommodates a drum  114  that rotates about a centerline thereof while the vehicle  100  is in motion. An operator occupying a cab  116  typically operates the vehicle  100 . The cab  116  may include a seat  118 , a steering mechanism  120 , a speed-throttle or control lever  122 , and a console  124 . An operator occupying the cab  116  can control the various functions and motion of the vehicle  100 , for example, by using the steering mechanism  120  to set a direction of travel for the vehicle  100  or using the control lever  122  to set the travel speed of the vehicle. As can be appreciated, the representations of the various control mechanisms presented herein are generic and are meant to encompass all possible mechanisms or devices used to convey an operator&#39;s commands to a vehicle. 
         [0015]    A simplified circuit diagram for a hydraulic system  200  including electrical controls is shown in  FIG. 2 . The system  200 , shown simplified for purposes of illustration, includes a portion of the drive circuit for driving the drum  114  of the vehicle  100 . As can be appreciated, hydraulic components and connections to drive the wheels  112 , or vibrators (not shown) within the drum  114  are not shown for the sake of simplicity. Similar hydraulic components and connections may be provided in alternate hydrostatically driven vehicles to perform operations such as, by way of example only, lifting and/or tilting of attached implements. 
         [0016]    The hydraulic circuit  200  includes a variable displacement pump  202  connected to a prime mover, in this case, the engine  204  of the vehicle. The pump  202  has an inlet conduit  206  connected to a vented reservoir or drain  208 . When the engine  204  (such as engine  110 ) is operating, the pump  202  draws a flow of fluid from the reservoir  208  that it pressurizes before sending it to a four-port two-way (4-2) valve  210  via a supply line or conduit  212 . A drain port of the valve  210  is connected via a drain passage  213 , which drains to the reservoir  208 . A control lever  214  is connected to a swashplate (not shown) internal to the pump  202  and arranged to change the angle of the swashplate in response to motion of control lever  214 . Motion of the control lever  214  is accomplished by an actuator  216  connected to the control lever  214 . The displacement or angle of the control lever  214 , which is equivalent to the angle of the swashplate of the pump  202 , may be sensed or measured with a sensor  218 . The sensor  218  may be, for example, an analog or digital sensor measuring the angle (or, equivalently, the displacement) of the control lever  214  and, hence, the position of the swashplate within the pump  202 . 
         [0017]    In use, the pump  202  functions to propel a flow of fluid through the supply line  212  when the engine  204  operates. Depending on the position of the 4-2 valve  210 , the flow of fluid from the supply line  212  is routed into one of two conduits, a first conduit  220  and a second conduit  222 , which are respectively connected to either side of a hydraulic motor  224 . The position of the 4-2 valve  210  is controlled by a valve actuator  226  disposed to reciprocally move the 4-2 valve  210  between two positions causing the motor  224  to move in the desired direction. In an alternate embodiment, the 4-2 valve may be replaced by a bidirectional variable displacement pump (not shown) capable of routing fluid to the motor  224  in both directions. 
         [0018]    The motor  224  is connected to a wheel or drum  227  of the vehicle (such as wheel  112  or drum  114 ) and arranged to rotate the wheel or drum  227  when the vehicle is travelling. A brake  228 , shown schematically, is arranged to arrest or stall motion of the drum  227  when actuated by an actuator  230 . The brake actuator  230  shown in this embodiment is electronic and actuates the brake  228  causing function to arrest motion of the drum  227 , but other configurations may be used. For example, a pin may be inserted into an opening of a rotating disk connected to the drum  227  such that motion of the disk and drum  227  with respect to the pin is stalled, and so forth. Further, the brake  228  is shown external to the drum  227  for illustration, but more conventional designs such as those having the brake  228  protected within the drum  227  may be utilized. 
         [0019]    An electronic controller  232  is connected to the vehicle and arranged to receive information from various sensors on the vehicle, process that information, and issue commands to various actuators within the system during operation. Connections pertinent to the present description are shown but, as can be appreciated, a great number of other connections may be present relative to the controller  232 . In this embodiment, the controller  232  is connected to a control input  234  (such as control lever  122 ) via a control signal line  236 . The control input  234 , shown schematically, may be, for example, a lever moveable by the operator of the vehicle used to set a desired speed setting for the vehicle. The position of the control input  234  may be translated to a command signal through a sensor  238  associated with the control input  234 . The control signal relayed to the controller  232  may be used in a calculation, along with other parameters, for example, the speed of the engine  204 , the temperature of fluid within the reservoir  208 , and so forth, to yield a desired angle for the swashplate that causes the vehicle to move at the desired speed. 
         [0020]    When the operator commands motion of the vehicle by displacing the control input  234 , a command signal is relayed to the controller  232  via the command input line  236 . This signal, as is described in further detail below, causes the pump actuator  216  to move the control lever  214  by an appropriate extent to achieve a desired angle. The desired angle of the control lever  214 , which translates into a desired setting for the swashplate of the pump  202 , causes an appropriate flow of motive fluid through the hydraulic motor  224 , which results in rotation of the drum  227  achieving the desired travel speed of the vehicle. 
         [0021]    The various fluid conduits and actuators, for example, the hydraulic motor  224 , belonging to the hydraulic system  200  are sized relatively to a maximum flow rate of fluid through the system  200 . For example, a calculation for the maximum flow of the system  200  by a designer may account for various parameters, such as, the weight of the vehicle, the maximum travel speed, any grades the vehicle should be capable of traversing during operation, and so forth. 
         [0022]    A cross section of a typical arrangement for a variable displacement pump  300  (such as pump  202 ) is shown in  FIG. 3 . The variable displacement pump  300  includes a housing  302  forming a plurality of cylindrical bores  304 , which are radially arranged parallel to each other within the housing  302 . Each bore  304  sealably and reciprocally accepts a plunger  306 . Each plunger  306 , shown simplified, forms an actuation linkage  308  extending from the plunger  306  and contacting an angled rotating plate or swashplate  310 . The swashplate  310  is connected to a rotating shaft  312  and is capable of rotating at an angle  314  with respect to the rotating shaft  312 . The angle  314  can be adjusted such that the stroke of each plunger  306  can be altered, thus altering the displacement of the variable displacement pump  300 . In a typical arrangement, the shaft  312  rotates under action of a rotating machine, for example, the engine or transmission of a vehicle. Motion of the plungers  306  caused by rotation of the swashplate  310  acts to compress a fluid within a plurality of compression volumes  316  defined between each respective bore  304  and plunger  306 . The volume of each compression volume depends on the angle  314  of the swashplate  310 . 
         [0023]    A qualitative efficiency chart for an exemplary variable displacement pump is shown in  FIG. 4 . The chart of  FIG. 4  is a graphical illustration of parameters plotted against a vertical axis  402 , representing a rate of flow for fluid being compressed in a variable displacement pump, and a horizontal axis  404 , representing an outlet pressure of the pump. The graph illustrates the relationship between outlet flow versus pressure of the pump as well as the corresponding pumping efficiencies of the pump for various angles or settings of the swashplate, tested under a steady state rate of rotation of an input shaft of the pump, for example, 2300 RPM. 
         [0024]    A series of flow curves  406  illustrate the negative correlation between flow rate of the pump and the outlet pressure. The curvature of each flow curve  406  can change depending on the angle setting of the pump. For example, a flow curve  408  corresponding to a low angle setting of the pump has a concave curvature, indicating that flow rates decrease faster as pressure increases from a low pressure condition than they decrease at higher pressure conditions. In contrast, a flow curve  410  corresponding to a high or steep angle setting of the pump has a convex curvature, indicating that the flow rate may decrease faster as pressure increases from a higher pressure condition than it does at a lower pressure. The shape of each flow curve  406  corresponding to an angle setting of the pump is indicative of the efficiency of the pump, with higher efficiencies exhibited for angles corresponding to flow curves  406  having concave shapes. One can surmise that a flow curve  411  corresponding to an intermediate angle will have a generally straight shape at the transition between the concave and convex shaped flow curves  406 . 
         [0025]    The efficiency of the pump, which can be determined as a ratio between the hydraulic power at the pump outlet and mechanical power at the driving shaft at nominal pressure, angular velocity, and fluid viscosity, is represented on the graph by a plurality of efficiency curves  412 , each corresponding to a respective angle setting of the pump. Each efficiency curve  412  has an inflection point indicative of optimum pump performance for each angle setting. It can be appreciated that the relative drop in efficiency when, for example, the pressure deviates from the optimum performance, will increase as the angle settings of the pump increase. It can also be appreciated that low or declining efficiencies of the pump during operation cause a waste of energy and an increase in fuel consumption of the vehicle. 
         [0026]    For this and other reasons, issues of increased fuel consumption may advantageously be avoided by incorporation of a larger pump into a vehicle that would have been typically incorporated a smaller pump. By increasing the size of the pump, even to the extent that the pump may never be used at its maximum angle setting, one can advantageously operate the larger pump at a higher efficiency and at a lower shaft speed, thus reducing the fuel consumption of the vehicle while still operating at a high efficiency. 
         [0027]    Two qualitative charts indicative of the flow and pressure characteristics of two exemplary pumps are shown for comparison in  FIG. 5 . The first graph  500  includes flow curves  502  and efficiency curves  504  plotted for data indicative of performance of a first pump  506 , a smaller frame pump, while the second graph  501 , shown below the first graph  500 , includes flow curves  503  and efficiency curves  505  plotted for data generated by a second pump  507 , a larger frame pump. The first pump  506  may operate at a steady shaft speed of 2300 RPM, and the second pump  507  may operate at a steady shaft speed of 1600 RPM. The data shown in the first and second graphs  500  and  501  is qualitative and does not represent actual data. Two operating points have been selected for illustration of the operating conditions of each pump under the assumption that they are each used in the same or similar hydraulic systems. 
         [0028]    In both charts, a first operating point, O 1 , corresponds to a pressure, P 1 , at the outlet of each pump and to a flow rate, F 1 . Similarly, a second operating point, O 2 , corresponds to a pressure, P 2 , at the outlet of each pump and to a flow rate, F 2 . Dashed lines are used to identify each operating point on both graphs  500  and  501 . 
         [0029]    Regarding the first pump  506 , the operating point O 1  can be attained by setting the first pump  506  to a second angle setting, A 2 . The operating point O 2 , however, can only be approached, but not attained, by setting the pump at a maximum angle setting, Amax, representing a maximum displacement setting for the pump  506 . Operation of the first pump  506  at the maximum angle setting Amax occurs at a very low efficiency, E 1 . Based on the description above, the combination of the high angle setting Amax, along with operation at the high pressure P 2  yields the very low pump efficiency E 1  because the first pump  506  is operating at a high angle setting and a high pressure. This condition can be readily seen in the first graph  500  where the low efficiency E 1  lies beyond the maximum efficiency Emax for the corresponding angle setting Amax. 
         [0030]    Regarding the second pump  507 , the operating point O 1  can be attained by setting the second pump  507  to a first angle setting, A 1 , which is relatively less than the second angle setting A 2  used on the first pump  506 . In contrast with the first pump  506 , the second pump  507  can easily attain the second operating point O 2  by setting the second pump  507  at a third angle setting, A 3 , which, for this pump, is also advantageously less than the maximum setting. Operation of the second pump  507  at either the first or second angle settings A 1  and A 3  can occur at relatively high efficiencies and at a lower shaft speed. If the second operating point O 2  is assumed to be representative of the maximum flow rate that a hydraulic system can accept, it can be appreciated that the second pump  507  is capable of pumping the maximum flow rate of fluid into the hydraulic circuit, while operating at 1600 RPM, and while the swashplate is disposed at an operating angle corresponding to an operating displacement that is less than the maximum displacement. In comparing the displacement of the second pump  507  with that of the first pump  506 , while both pumps are operating at the operating point O 2  at their respective speeds, it can further be appreciated that the displacement of the second pump  507  at the second operating point O 2  is at least less than 70% of the maximum displacement. 
       INDUSTRIAL APPLICABILITY 
       [0031]    The present disclosure is applicable to hydrostatically driven vehicles having an engine or motor driving a variable displacement pump. Typical vehicles use a maximum displacement condition for the pump to size the pump such that the maximum flow rate can be pushed into the hydraulic system of the vehicle when the engine operates at its maximum useable RPM. As described above, this mode of matching a specific pump size to an engine can often lead to operation of the vehicle that is both wasteful of fuel, due to the engine&#39;s operation at high speeds, as well as detrimental to the efficiency of the system. The present disclosure, in one aspect, describes using a larger pump paired with the engine that, even if the full displacement of the pump may never be used, allows the system to operate at a high efficiency state. Moreover, the engine operates at a lower RPM during most operating conditions. The advantages of this configuration can be readily appreciated as fuel economy and noise are reduced during operation, and the efficiency of the system is increased. 
         [0032]    It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
         [0033]    Accordingly, this disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.