Patent Publication Number: US-9850894-B2

Title: Self priming hydraulic pump and circuit

Description:
TECHNICAL FIELD 
     The present disclosure relates to the field of reciprocating hydraulic devices. 
     BACKGROUND 
     Today, hydraulic systems are widely used by manufacturing, construction, power generation, mining and transportation industries. Over the years, systems for the harnessing and distribution of power have become increasingly sophisticated, their applications more numerous and their operating conditions more demanding. Hydraulic systems are particularly advantageous in that they allow for actuation of large surfaces under heavy loads with minimal input, due to the fact that hydraulic fluid resists compression (i.e. incompressible) and therefore facilitates the direct transfer of applied work to the actuated surfaces. Hydraulic systems also offer an advantage of being more powerful than an electrical system of the same size, particularly in heavy load applications. 
     Hydraulic systems involving reciprocating piston pumps and motors can provide efficient power transfer mechanisms in alternative energy conversion systems, however hydraulic fluid movement between the hydraulic pump and motor can be problematic during system start-up and other non-steady state operating conditions. Priming of a reciprocating piston hydraulic pump in a open loop can be very difficult, as the ability for the pump piston to draw its own hydraulic fluid into the respective bore inlet is limited due the force required to overcome inertia in circulation of the hydraulic fluid between the pump and reservoir. This problem in draw ability of the pump is analogous to the age-old problem of pushing a rope, as the pump must overcome inertia of the hydraulic fluid in the system in order to begin operation. 
     The priming (and other non-steady state conditions) problem is exacerbated if the hydraulic reciprocating piston pump is located at a head height above the reciprocating piston hydraulic motor and reservoir tank in the open loop system, as the piston must draw the hydraulic fluid under further influence of gravity. This problem of inertia is further exaggerated by the pump size (i.e. bore/stroke), inlet hydraulic fluid volume (i.e. bore inlet diameter) and/or separation between the pump and motor increase(s) in magnitude, or if the pump piston (s) decouples from the actuator driving the pump. 
     The current solution employed for the aforementioned problems is the provision of an open loop hydraulic system, in which a reservoir is positioned at higher altitude than the pump and the pump pistons are fixed to the actuator (do not decouple) or in some applications, a supplemental pump or other priming device is used to supply hydraulic fluid to the main, more efficient pump. However, the open loop system is undesirable, as the supplemental pump increases the complexity and can decrease the efficiency of the hydraulic system. Also, in applications where space and accessibility to the hydraulic pump(s) are limited or difficult, the use of an open loop storage reservoir system is unworkable and undesirable. 
     One exemplary application of pump-motor hydraulic systems is for a hydraulic wind turbine, where blade rotation is converted to hydraulic flow using a hydraulic pump, pressure of hydraulic flow is generated by an opposing load or resistance, and the hydraulic pressure is converted to electrical energy using a hydraulic motor coupled to an electrical generator. 
     SUMMARY 
     It is an object of the present invention to provide a self-priming, self-sustaining hydraulic system that obviates or mitigates at least one of the above presented disadvantages. 
     In one embodiment, the self-priming hydraulic system comprises a hydraulic pump, the hydraulic pump comprising a first piston operable to reciprocate within a first piston bore, the first piston bore co-operating with a surface of the first piston to define a first fluid cavity, the first piston bore providing a first fluid inlet and a first fluid outlet disposed on the first piston bore and fluidly coupled to the first fluid cavity, and a second piston operable to reciprocate within a second piston bore, the second piston bore co-operating with a top surface of the second piston to define a second fluid cavity, the second piston bore providing a second fluid inlet and a second fluid outlet disposed on the second piston bore and fluidly coupled to the second fluid cavity. The self-priming hydraulic system also comprises a low pressure loop defined by a low pressure accumulator fluidly coupled to the first fluid outlet and the second fluid inlet of the hydraulic pump, and a high pressure loop defined by a high pressure accumulator fluidly coupled to the second fluid outlet and the first fluid inlet of the hydraulic pump. 
     According to another aspect, the low pressure loop of the self priming hydraulic system comprises a resistive element disposed between and fluidly coupled to the low pressure accumulator and the first fluid outlet of the hydraulic pump. 
     According to another aspect, the resistive element of the self-priming hydraulic system is a hydraulic motor. 
     According to another aspect, the hydraulic motor of the self-priming hydraulic system is a reciprocating hydraulic motor. 
     According to another aspect, the first piston and the second piston of the self-priming hydraulic system are arranged in a stacked configuration such that the first piston is coupled to the second piston by a stem. 
     According to another aspect, a relief line fluidly couples the high pressure accumulator to the low pressure accumulator in the self-priming hydraulic system. 
     According to another aspect, the low pressure loop further comprises a heat exchanger disposed between and fluidly coupled to the resistive element and the low pressure accumulator. 
     According to another aspect, a reciprocating hydraulic motor of the self-priming hydraulic system comprises a capillary tube fluidly coupled to a compressible fluid storage tank. 
     According to another aspect, a dual fluid accumulator of the self-priming hydraulic system is fluidly coupled to the low pressure accumulator to provide a relief for the low pressure accumulator. 
     According to another aspect, a first fluid support fluidly couples the high pressure loop to the low pressure loop upstream of the resistive element to direct non-compressible fluid from the high pressure loop to the low pressure loop, and a second fluid support fluidly couples the low pressure loop downstream of the resistive element to the high pressure loop to direct non-compressible fluid from the low pressure loop to the high pressure loop. 
     According to another aspect, the self-priming hydraulic system comprises a hydraulic pump, the hydraulic pump comprising a first piston operable to reciprocate within a first piston bore, the first piston bore co-operating with a surface of the first piston to define a first fluid cavity, the first piston bore providing a first fluid inlet and a first fluid outlet disposed on the first piston bore and fluidly coupled to the first fluid cavity, and a second piston operable to reciprocate within a second piston bore, the second piston bore co-operating with a top surface of the second piston to define a second fluid cavity, the second piston bore providing a second fluid inlet and a second fluid outlet disposed on the second piston bore and fluidly coupled to the second fluid cavity. The self-priming hydraulic system also comprises a low pressure loop defined by a low pressure accumulator fluidly coupled to the first fluid inlet and the first fluid outlet of the hydraulic pump, and a high pressure loop defined by a high pressure accumulator fluidly coupled to the second fluid inlet and the second fluid outlet of the hydraulic pump. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present disclosure will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
         FIGS. 1A and 1B  show cross-section views of a double-decker embodiment of a self-priming pump and circuit where a first piston and a second piston are shown in their top dead center (TDC) and bottom dead center (BDC) positions. 
         FIGS. 2A and 2B  show a cross-section view of an alternate double-decker embodiment of a self-priming pump and circuit where a first piston and a second piston are shown at their TDC and BDC positions and the circuit includes a heat exchanger. 
         FIGS. 3A and 3B  show a cross-section view of an alternate double-decker embodiment of a self-priming pump and circuit where a first piston and a second piston are shown at TDC and BDC positions and the circuit includes a heat exchanger and a motor. 
         FIGS. 4A and 4B  show a cross-section view of an alternate double-decker embodiment of a self-priming pump and circuit where a first piston and a second piston are shown at TDC and BDC and the circuit includes a motor, a heat exchanger, a control valve in the effluent stream from the high pressure accumulator, a first fluid support fluidly couples the high pressure loop to the low pressure loop upstream of the resistive element to direct non-compressible fluid from the high pressure loop to the low pressure loop, and a second fluid support fluidly couples the low pressure loop downstream of the resistive element to the high pressure loop to direct non-compressible fluid from the low pressure loop to the high pressure loop. 
         FIG. 5A  shows a cross-section view of a side-by-side embodiment of a pair of self-priming pumps where a first piston is shown at its TDC position and a second piston is shown at its BDC position. 
         FIG. 5B  shows a cross-section view of a side-by-side embodiment of a pair of self-priming pumps and a circuit where a first piston and a second piston are shown at their BDC positions and the circuit includes a fluid resistive element and a heat exchanger. 
         FIG. 6  shows a cross-section view of a double-decker embodiment of a self-priming pump and circuit where a first piston and a second piston are shown in their BDC position and the circuit includes a motor and a control system to facilitate pressure control within a low pressure loop and a high pressure loop. 
         FIGS. 7A, 7B and 7C  are cross-section views of portions of  FIG. 6  illustrating the control system. 
     
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DETAILED DESCRIPTION 
     Embodiments are described below, by way of example, with reference to  FIGS. 1 to 7 . The embodiments described and depicted herein provide a self-priming hydraulic system. 
     It will be understood that the terms “top” and “bottom” referred to herein are used in the context of the attached Figures. The terms are not necessarily reflective of the orientation of reciprocating hydraulic pump  100  in actual use and are therefore not meant to be limiting in their use herein. 
     Described herein are various embodiments for a self-priming hydraulic system that provides for the manipulation of a non-compressible fluid to generate and perform work through the use of inherent properties of a non-compressible fluid as further described below. 
     The self-priming hydraulic system includes the reciprocating hydraulic pump with a first piston operable to reciprocate within a first piston bore, the first piston bore co-operating with a top surface of the first piston to define a first fluid cavity, the first piston bore providing a first fluid inlet and a first fluid outlet disposed on the first piston bore and fluidly coupled to the first fluid cavity. A surface of the piston bore (e.g. opposing bore wall to the top surface of the first piston) can be variable in position during the operating cycle of the hydraulic pump, thus providing for an increase or decrease in bore volume of the first fluid cavity as experienced by the first piston during travel between TDC and BDC. Position of the variable position surface (e.g. a piston also referred to as a floating piston) can be controlled by a resilient element (e.g. compressible fluid) chamber positioned behind the variable position surface, such that the variable position surface is located between the resilient element chamber and the first fluid cavity. For example, as volume of the resilient element (e.g. compressed fluid) is decreased in the resilient element chamber (e.g. via ejection of compressible fluid therefrom), the position of the variable position surface will become biased away from the first piston and thus provide for an increased volume of the first fluid cavity experienced by the first piston. Alternatively, as volume of the resilient element (e.g. compressed fluid) is increased in the resilient element chamber (e.g. via injection of compressible fluid therein), the position of the variable position surface will become biased towards the first piston and thus provide for a decreased volume of the first fluid cavity experienced by the first piston. Control in position of the variable position surface can be provided for (e.g. electronic control of injection/ejection of fluid with respect to the resilient element chamber in response to sensed operating conditions such as pressure, speed, and/or position experienced by the pump pistons, and/or in response to sensed operating conditions such as pressure, speed, and/or position experienced by the motor piston) during travel of the first piston between TDC and BDC while the pump is in operation. 
     The hydraulic system also has a second piston operable to reciprocate within a second piston bore, the second piston bore co-operating with a top surface of the second piston to define a second fluid cavity, the second piston bore providing a second fluid inlet and a second fluid outlet disposed on the second piston bore and fluidly coupled to the second fluid cavity. Reciprocation of the second piston is coupled to reciprocation of the first piston, as herein described by numerous examples, in order to synchronize positioning of the first piston within the first piston bore to positioning of the second piston within the second piston bore. For example, the piston coupling mechanism (e.g. couple  103 , 503  in  FIGS. 1A and 5B , respectively) can be used to synchronize travel of the first piston towards BDC within the first fluid cavity at the same time as travel of the second piston towards BDC within the second fluid cavity. For example, the piston coupling mechanism (e.g. couple  103 , 503  in  FIGS. 1A and 5B , respectively) can be used to synchronize travel of the first piston towards TDC within the first fluid cavity at the same time as travel of the second piston towards TDC within the second fluid cavity. Alternatively, in some configurations as described, the piston coupling mechanism (e.g. couple  103 , 503  in  FIGS. 1A and 5B , respectively) can be used to synchronize travel of the first piston towards TDC within the first fluid cavity at the same time as travel of the second piston towards BDC within the second fluid cavity. Alternatively, in some configurations as described, the piston coupling mechanism (e.g. couple  103 , 503  in  FIGS. 1A and 5B , respectively) can be used to synchronize travel of the first piston towards BDC within the first fluid cavity at the same time as travel of the second piston towards TDC within the second fluid cavity. 
     The hydraulic system also has a first loop (e.g. a low pressure loop) defined by a low pressure accumulator fluidly coupled to the first fluid outlet and the second fluid inlet of the hydraulic pump. The first loop is a closed loop for the hydraulic fluid circulating between the first fluid outlet and the second fluid inlet. The hydraulic system also has a second loop (e.g. a high pressure loop) defined by a high pressure accumulator fluidly coupled to the second fluid outlet and the first fluid inlet of the hydraulic pump. The second loop is a closed loop for the hydraulic fluid circulating between the second fluid outlet and the first fluid inlet of the hydraulic pump. It is also recognised that the high pressure accumulator contains hydraulic fluid at a pressure higher than the pressure of the hydraulic fluid contained in the low pressure accumulator. Examples relative pressures of the high and low pressure accumulators can be 500 psi and 100 psi, respectively. 
     It should be understood that the self-priming hydraulic system can be described in a number of different configurations with a number of different components. For instance, the self-priming hydraulic system can comprise one single piston reciprocating pump, more than one single piston reciprocating pump, one reciprocating piston pump comprising more than one piston, or more than one reciprocating piston pump comprising more than one piston. 
     In an exemplary configuration, the self-priming hydraulic system comprises a single self-priming pump comprising a pair of reciprocating pistons that are configured in a “double-decker” configuration such that the pair of reciprocating pistons are vertically aligned and coupled to each other via a piston coupling mechanism (e.g. stem). This configuration provides a consolidated footprint to the self-priming pump and circuit. One of the pair of reciprocating pistons, the lower or second piston, directly engages an actuator while the other piston, the upper or first piston, engages the actuator through the lower piston and a couple. Actuation of the lower piston by the actuator results in movement of the upper piston because the couple synchronizes movement of the upper piston and the lower piston. In this configuration, each piston actuates within an independent piston bore, both bores are contained in a single piston housing. It is understood that both bores could also be housed in individual housing. 
     In another exemplary configuration, the self-priming hydraulic system comprises a pair of self-priming pumps configured in a “side-by-side” configuration, each pump with a single reciprocating piston. The pumps can be either actuated by a single actuator engaged to both pistons through a piston coupling mechanism (e.g. connecting rod) or each pump can be actuated by a separate actuator. In this configuration, each piston is free to move from its BDC position to its TDC position either independently or in unison with the other piston. 
     In both of the exemplary configurations described above, a circuit is also provided comprising a pair of closed loops, a low pressure loop and a high pressure loop, to circulate non-compressible fluid from one of the pumps to the other. 
     In this same configuration, a low pressure loop is defined by fluid supports (e.g. conduits) fluidly connecting an outlet of the first piston bore to a resistive element, the resistive element to a low pressure accumulator and the low pressure accumulator to an Inlet of the second piston bore. In this configuration, upon actuation of the first piston from its BDC position to its TDC position, non-compressible fluid is driven out of a first fluid cavity, through a first fluid outlet, through fluid supports to a resistive element, to the low pressure accumulator and subsequently to the second piston cavity through the second piston bore inlet. 
     In an exemplary closed loop configuration, the high pressure loop is defined by fluid supports fluidly connecting an outlet of the second piston bore to a high pressure accumulator and an inlet of a first piston bore to the high pressure accumulator. In this configuration, upon actuation of the second piston from its BDC position to its TDC position, non-compressible fluid is driven out of a second fluid cavity, through a second fluid outlet, through fluid supports to the high pressure accumulator and subsequently into the first piston bore through the first piston bore inlet. 
     These exemplary configurations and various embodiments are described in further detail below. 
     Self-Priming Hydraulic System 
     In general, a self-priming hydraulic system as described herein comprises at least one reciprocating piston pump integrated into a closed loop system further comprising a high pressure loop and a low pressure loop. The reciprocating piston pump comprises at least one piston, each of the at least one pistons engaged to actuator to drive it to its respective TDC position. 
     Turning to the Figures, reciprocating hydraulic pump  100  and circuit  102  are described in further detail. 
       FIG. 1A  shows a cross-section view of the main components of one embodiment the self-priming hydraulic system comprising self-priming reciprocating hydraulic pump  100  and circuit  102 . In this embodiment, reciprocating hydraulic pump  100  comprises housing  101  within which first piston  104  and a second piston  106  are contained. First piston  104  is received within first piston bore  105  and second piston  106  is received within second piston bore  107 . 
     In this embodiment, first piston  104  and second piston  106  are connected by couple  103  such that movement of second piston  106  along a vertical axis A causes movement of first piston  104 . Movement of first piston  104  and second piston  106  is caused by the actuation of actuator  130  which directly engages second piston  106 . As can be seen in  FIG. 1 , an axis A runs lengthwise through first piston  104  and second piston  106 . It is recognised that, shown by example in  FIG. 1A , both first piston  104  and second piston  106  are concentric about axis A. However, it is recognised that first piston  104  and second piston  106  can be non-concentric about the axis A, as desired. Further, first piston  104  and second piston  106  are axially aligned with axis A. 
     First piston  104  and second piston  106  can be any contemplated shape provided that the contour of first piston  104  is similar to the contour of first piston bore  105  and the contour of second piston  106  is similar to the contour of second piston bore  107 . Other configurations can therefore be utilized while operating in a similar manner as described herein. It is understood that second piston  106  can have a slightly larger cross-sectional area than first piston  104  to compensate for the space lost in second fluid cavity  124  due to the presence of couple  103  that is not present in first fluid cavity  120 . 
     First piston bore  105  co-operates with a top surface  122  of first piston  104  to define a first fluid cavity  120 , the first piston bore  105  providing a first fluid inlet  110  and a first fluid outlet  114  disposed on the first piston bore  105  and fluidly coupled to the first fluid cavity  120 . A surface  121  of the piston bore  105  (e.g. opposing bore wall to the top surface  122  of the first piston  104 ) can be variable in position during the operating cycle of the hydraulic pump  100 , thus providing for an increase or decrease in bore volume of the first fluid cavity  120  as experienced by the first piston  104  during travel between TDC and BDC. Position of the variable position surface  121  (e.g. a piston also referred to as a floating piston) can be controlled by a resilient element (e.g. compressible fluid) chamber (not shown) positioned behind the variable position surface  121 , such that the variable position surface  121  is located between the resilient element chamber (not shown) and the first fluid cavity  120 . For example, as volume of the resilient element (e.g. compressed fluid) is decreased in the resilient element chamber (e.g. via ejection of compressible fluid therefrom), the position of the variable position surface  121  will become biased away from the first piston  104  and thus provide for an increased volume of the first fluid cavity  120  experienced by the first piston  104 . Alternatively, as volume of the resilient element (e.g. compressed fluid) is increased in the resilient element chamber (e.g. via injection of compressible fluid therein), the position of the variable position surface  121  will become biased towards the first piston  104  and thus provide for a decreased volume of the first fluid cavity  120  experienced by the first piston  104 . Control in position of the variable position surface  121  can be provided for (e.g. electronic control of injection/ejection of fluid with respect to the resilient element chamber in response to sensed operating conditions such as pressure, speed, and/or position experienced by the pump pistons, and/or in response to sensed operating conditions such as pressure, speed, and/or position experienced by the motor piston) during travel of the first piston  104  between TDC and BDC while the pump  100  is in operation. 
     The hydraulic system also has a second piston  106  operable to reciprocate within a second piston bore  107 , the second piston bore  107  co-operating with a top surface  126  of the second piston  106  to define a second fluid cavity  124 , the second piston bore  107  providing a second fluid inlet  112  and a second fluid outlet  116  disposed on the second piston bore  107  and fluidly coupled to the second fluid cavity  124 . Reciprocation of the second piston  106  is coupled to reciprocation of the first piston  104 , as herein described by numerous examples, in order to synchronize positioning of the first piston  104  within the first piston bore  105  to positioning of the second piston  106  within the second piston bore  107 . For example, the piston coupling mechanism (e.g. couple  103 , 503  in  FIGS. 1A and 5B , respectively) can be used to synchronize travel of the first piston  104  towards BDC within the first fluid cavity  120  at the same time as travel of the second piston  106  towards BDC within the second fluid cavity  124 . For example, the piston coupling mechanism (e.g. couple  103 , 503  in  FIGS. 1A and 5B , respectively) can be used to synchronize travel of the first piston  104  towards TDC within the first fluid cavity  120  at the same time as travel of the second piston  106  towards TDC within the second fluid cavity  124 . Alternatively, in some configurations as described, the piston coupling mechanism  103  can be used to synchronize travel of the first piston  104  towards TDC within the first fluid cavity  120  at the same time as travel of the second piston  106  towards BDC within the second fluid cavity  124 . Alternatively, in some configurations as described, the piston coupling mechanism  103  can be used to synchronize travel of the first piston  104  towards BDC within the first fluid cavity  120  at the same time as travel of the second piston  106  towards TDC within the second fluid cavity  124 . It should be understood that first piston  104 , second piston  106  and coupling mechanism  103  can be two or more attachable pieces to facilitate assembly. 
     The self-priming hydraulic system described in  FIGS. 1A and 1B  further comprise a pair of loops to circulate non-compressible fluid: a low pressure loop  118  and a high pressure loop  119 . Low pressure loop  118  is defined by a low pressure accumulator  160  fluidly connected to first outlet  114  through a resistive element  159 , and fluidly connected to second inlet  112 . High pressure loop  119  is defined by high pressure accumulator  162  fluidly connected to second outlet  116  and first inlet  110 . It is desirable to have the effluent non-compressible fluid from the resistive element  159  exhaust into the low pressure accumulator  160  to maximize the efficiency of work performed by the resistive element  159 . 
     First fluid cavity  120  is supplied with non-compressible fluid by first inlet  110  when first piston  104  moves to its BDC position. Similarly, second fluid cavity  124  is supplied with non-compressible fluid by second inlet  112  when the second piston  106  moves to its BDC position. Non-compressible fluid exits first fluid cavity  120  through first outlet  114 . Non-compressible fluid exits second fluid cavity  124  through second outlet  116 . 
     First inlet  110  contains check valve  172  through which non-compressible fluid passes as it enters first fluid cavity  120 . Similarly, first outlet  114  contains check valve  150  through which non-compressible fluid passes as it exits first fluid cavity  120  and enters fluid support  152  of circuit  102 . Check valve  172  inhibits flow of non-compressible fluid out of first cavity  120  and back into fluid support  164  from which it entered the self-priming reciprocating hydraulic pump  100 . Check valve  150  inhibits flow of non-compressible fluid into first cavity  120  from fluid support  152  after the ejection phase of piston  104 . Fluid support  152  delivers non-compressible fluid exiting first fluid cavity  120  to resistive element  159 . 
     Second inlet  112  contains check valve  170  through which non-compressible fluid passes as it enters second fluid cavity  124 . Similarly, second outlet  116  contains check valve  154  through which non-compressible fluid passes as it exits second fluid cavity  124  and enters fluid support  156  of circuit  102 . Check valve  170  inhibits the flow of non-compressible fluid out of second cavity  124  and back into fluid support  168  from which it entered self-priming reciprocating hydraulic pump  100 . Check valve  154  inhibits flow of non-compressible fluid into second cavity  124  from fluid support  156  after the ejection phase of piston  106 . Fluid support  156  delivers non-compressible fluid exiting second fluid cavity  124  to high pressure accumulator  162 . 
     Low pressure accumulator  160  is a storage reservoir in which non-compressible hydraulic fluid is held under pressure. High pressure accumulator  162  is also a storage reservoir in which non-compressible hydraulic fluid is held under pressure. In one embodiment, compressed gas is used to maintain desired pressures in low pressure accumulator  160  and high pressure accumulator  162 . Typically, low pressure accumulator  160  will store hydraulic fluid at a lower pressure than high pressure accumulator  162 . It should be noted that pistons  104  and  106  can be decoupled from actuator  130  at their TDC positions. Low pressure accumulator  160  and high pressure accumulator  162  can provide non-compressible fluid to second fluid cavity  124  and first fluid cavity  120 , respectively, to restart the hydraulic system and push piston  104 ,  106  back towards BDC to re-engage actuator  130 . 
     Gate valve  174  controls injection of non-compressible fluid into second fluid cavity  124 . Gate valve  174  can be an on-off valve as flow of non-compressible fluid into second fluid cavity  124  does not need to be closely monitored. Pressure control valve  178  controls infection of non-compressible fluid into first fluid cavity  120 . Injection pressure P 1in  of the non-compressible fluid at first inlet  110  can be influenced by pressure control valve  178 . Pressure control valve  178  can be an electronically controlled valve. Pressure reducing valve  180  controls the pressure P HPAcc  in high pressure accumulator  162  by controlling the flow of non-compressible fluid from high pressure accumulator  162  to low pressure accumulator  160  through fluid support  166 . Pressure reducing valve  180  can be a self-monitoring valve (e.g. spring loaded) such that when pressure P HPAcc  becomes higher than the calibrated tolerance of pressure reducing valve  180 , non-compressible fluid passes through pressure reducing valve  180  and reduces P HPAcc  in high pressure accumulator  162 . 
     It should be understood that in this configuration first fluid cavity  120  ejects non-compressible fluid at P 1out , which is higher than the pressure P 2out  at which second fluid cavity  124  ejects non-compressible fluid. This is because in some embodiments described below non-compressible fluid ejected from first fluid cavity  120  will enter resistive element  159  and is used to perform work. It is desirable to have effluent non-compressible fluid from resistive element  159  exhaust into low pressure accumulator  160  at P LPAcc  to maximize the pressure difference between the inlet and outlet of resistive element  159 . 
     Fluid support  152  delivers non-compressible fluid from first outlet  114  to resistive element  159 . Resistive element  159  uses non-compressible fluid provided from first fluid cavity  120  at P 1out  to perform work. Fluid support  155  delivers effluent non-compressible fluid from resistive element  159  to low pressure accumulator  160 . Fluid support  155  can contain check valve  154  between resistive element  159  and low pressure accumulator  160 . 
     Low pressure accumulator  160  of low pressure loop  118  stores returning volumes of non-compressible hydraulic fluid that exits resistive element  159  at pressure P LAcc . It should be understood that low pressure accumulator  160  can be charged with a compressed gaseous fluid such as air or nitrogen. Low pressure accumulator  160  addresses thermal expansion of the non-compressible fluid as it exits resistive element  159 . Non-compressible fluid exiting resistive element  159  increases in temperature. By entering low pressure accumulator  160  after exiting resistive element  159 , non-compressible fluid can rest in the low pressure accumulator  160  before entering second fluid cavity  124  of reciprocating hydraulic pump  100  at second inlet  112 . 
     Fluid support  156  delivers non-compressible fluid from second outlet  116  to high pressure accumulator  162  of high pressure loop  119 . 
     High pressure accumulator  162  is used to store returning volumes of non-compressible fluid that exits second cavity  124  at P 2out . High pressure accumulator  162  can also be charged with a gaseous fluid such as air or nitrogen. Maintaining pressure P HAcc  within high pressure accumulator  162  allows the closed loop system to inject non-compressible fluid into first fluid cavity  120  at a high pressure. 
     Fluid support  168  delivers non-compressible fluid from low pressure accumulator  160  to second inlet  112  through check valve  170 . Fluid support  168  can contain gate valve  174 . Fluid support  164  delivers non-compressible fluid from high pressure accumulator  162  to first inlet  110  through check valve  172 . Fluid support  164  can contain pressure control valve  178  to control the pressure of non-compressible fluid first inlet  110 . 
     Fluid support  166  provides a relief to high pressure accumulator  162  by delivering non-compressible fluid from the high pressure accumulator  162  to the low pressure accumulator  160 . Pressure release valve  180  controls the flow of non-compressible fluid from high pressure accumulator  162  to low pressure accumulator  160  and therefore can be used to control pressure P HAcc  in high pressure accumulator  162 . 
       FIG. 1B  shows a cross-section of the same system as was described in  FIG. 1A , however, first piston  104  and second piston  106  are at their respective BDC positions. This configuration marks the beginning of the ejection phase of reciprocating hydraulic pump  100 , which is described in greater detail below. 
       FIG. 2A  shows the same system as described in  FIGS. 1A and 1B , however, resistive element  159  is shown as pressure control valve  201 . Pressure control valve  201  is used to choke the flow of non-compressible fluid out of first fluid cavity  120  via first outlet  114  to enable the non-compressible fluid to perform work. Pressure control valve  201  is followed by heat exchanger  202  to collect heat from the non-compressible fluid exiting pressure control valve  201 . In this embodiment, work is generated and the temperature of the non-compressible fluid rises as a result. Pressure control valve  201  and heat exchanger  202  can be provided anywhere within low pressure loop  118  provided that non-compressible fluid exiting first fluid cavity  120  passes through pressure control valve  201  and heat exchanger  202 , respectively, before entering low pressure accumulator  160 . Low pressure accumulator  160  of low pressure loop  118  is provided downstream from both pressure control valve  201  and heat exchanger  202 . As previously described, placing a resistive element such as pressure control valve  201  in the effluent from first fluid cavity  120  before low pressure accumulator  160  allows the non-compressible fluid from pressure control valve  201  to exhaust into low pressure accumulator  160 . This configuration produces a large pressure difference between the inlet and outlet of pressure control valve  201  and maximizes the efficiency of work performed by pressure control valve  201 . 
       FIG. 2B  shows a cross-section of the system as described above in  FIG. 2A , however, first piston  104  and second piston  106  are at their respective BDC positions. 
       FIG. 3A  shows the same system as described in  FIGS. 2A and 2B  with resistive element  159  of  FIGS. 1A and 1B  as a hydraulic motor  301  (e.g. reciprocating). Motor  301  is able to perform work using non-compressible fluid ejected from first fluid cavity  120 . It should be understood that motor  301  illustrated in  FIGS. 3A and 3B  is a single reciprocating piston motor, however, any motor configuration, including more complex motor configurations, can be used as a fluid resistive element. In one non-limiting example, motor  301  can be a double-acting piston motor. 
     Motor  301  comprises a motor inlet  303  and a motor outlet  304  fluidly connected to a motor fluid cavity  307  defined by opposite surfaces of motor bore  308 , a top surface  312  of motor piston  305  and a top surface  310  of motor bore  307  opposed to top surface  312  of motor piston  305 . It should be understood that top surface  310  of motor bore  307  can be a rigid surface of bore  307  or a movable surface of another structure within bore  307 , with structure and function as previously described for surface  121  of piston  104 . Motor piston  305  is coupled to load  315  to drive motor piston to TDC upon being driven to BDC by the non-compressible fluid from first fluid cavity  120 . Upon movement to its TDC position, non-compressible fluid is driven out of motor fluid cavity  307  through motor outlet  304 . Motor inlet  303  can comprise motor check valve  320  and motor outlet  304  can comprise flow control valve  321 . Check valve  320  can also be an electronic solenoid valve. During an injection phase of motor  301 , flow control valve  320  is open and flow control valve  321  is closed. During an ejection phase, flow control valve  320  is closed and flow control valve  321  is open. 
     Motor  301  can comprise capillary tube  306  to facilitate passage of a compressible fluid in and out of lower cavity  330  of motor bore  308 , where lower cavity  330  is defined by opposite and adjacent sides of motor bore  308 , an underside surface  332  of motor piston  305  and a lower surface of motor bore  331 . 
     As previously described, to facilitate efficiency of motor  301 , a large pressure change (e.g. Delta, Δ) is desired between the pressure P 1out  of non-compressible fluid and low pressure accumulator  160 . As shown in  FIG. 3A , motor  301  is fluidly connected to heat exchanger  202 . It should be recognized that when motor  301  is present as a resistive element, the heat exchanger  202  in loop  118  is an optional component and motor  301  can also be directly fluidly connected to low pressure accumulator  160  of low pressure loop  118  through fluid support  155 . 
       FIG. 3B  shows the same system as described in  FIG. 3A , however, first piston  104  and second piston  106  are at their respective BDC positions. 
       FIG. 4A  shows the same system as described in  FIGS. 3A and 3B , with pressure control valve  178  shown as a three-way control valve  478  to control a volume of non-compressible fluid entering first fluid cavity  120  from high pressure accumulator  162  and the volume of non-compressible fluid by-passed to the low pressure accumulator  160  from high pressure accumulator  162  via fluid support  166 . 
       FIG. 4A  also shows fluid support  496  comprising valve  497  (e.g gate), fluid support  498  comprising valve  499  (e.g. gate), and valve  492  (e.g. gate). Valves  492  and  497  can be used in combination to facilitate directing non-compressible fluid ejected from second fluid cavity  124  to join non-compressible fluid ejected from first fluid cavity  120  towards resistive element  159 . Resistive element  159  is shown as motor  301  in  FIGS. 4A and 4B . Valve  499  facilitates directing a portion of non-compressible fluid ejected from motor  301  to high pressure loop  119 . 
     Fluid support  496  is upstream of motor  301  and can optionally be upstream of high pressure accumulator  162 . The term upstream can be defined as direction of fluid flow experienced by (i.e. away from) a position on a flow pathway (i.e. loop or fluid support) relative to the direction experienced by (i.e. towards) another position on the same flow pathway (i.e. loop or fluid support). For example, a location A of a flow pathway (e.g. high pressure loop  119  or low pressure loop  118 ) is considered upstream of a relative location B of the same flow pathway if, at location A, fluid is flowing away from location A and towards location B. For example, in  FIG. 4A  non-compressible fluid enters low pressure loop  118  from fluid support  496  and flows from fluid support  496  towards motor  301 . Therefore, fluid support  496  is upstream of motor  301 . 
     Valve  492  is downstream of fluid support  496 . The term downstream can be defined as direction of fluid flow experienced by (i.e. towards) a position on a flow pathway (i.e. loop or fluid support) relative to the direction experienced by (i.e. away from) another position on the same flow pathway (i.e. loop or fluid support). For example, a location A of a flow pathway (e.g. high pressure loop  119  or low pressure loop  118 ) is considered downstream of a relative location B of the same flow pathway if, at location A, fluid is flowing towards location A from location B. For example, in  FIG. 4A , when non-compressible fluid flows through valve  492  fluid is flowing towards valve  492  from fluid support  496 . Therefore, valve  492  is downstream of fluid support  496 . 
     In  FIG. 4A , fluid support  496  is shown as being fluidly coupled to fluid supports  152  and  156  adjacent to first fluid outlet  114  and second fluid outlet  116 . Although shown as two independent valves in  FIG. 4A , valves  492  and  497  can be combined into a single three-way valve (not shown). 
     Fluid support  498  is downstream of motor  301 . Fluid support  498  can be upstream or downstream of high pressure accumulator  162 . Fluid support  498  is shown in  FIG. 4A  as being fluidly coupled to fluid support  164  and  168  adjacent to first fluid inlet  110  and second fluid inlet  112 . When fluid support  498  is located as shown in  FIG. 4A , non-compressible fluid ejected from second fluid cavity  124  into high pressure loop  119  temporarily bypasses high pressure accumulator  162 . When valves  492  and  497  are used to join non-compressible fluid ejected from first and second fluid cavities  120 , 124 , respectively, valve  499  can be used downstream of motor  301  in combination with valves  492  and  497 , which are upstream of motor  301 , to direct a portion of non-compressible fluid ejected from motor  301  back into high pressure loop  199  upstream of first fluid inlet  110 . It can be desirable to direct non-compressible fluid into high pressure loop  119  upstream of first fluid inlet  110  to inhibit first fluid cavity  120  from developing a vacuum, which could occur if an insufficient volume of non-compressible fluid was present during a downstroke of first piston  104 . 
     It should be noted that although fluid supports  496  and  498  and valves  492 ,  497  and  499  are shown in the configuration shown in  FIGS. 4A and 4B , these components can be included as described in any of the variations and embodiments discussed herein including but not limited to the side-by-side configuration shown in  FIGS. 5A and 5B . 
       FIG. 4B  shows the same system as described in  FIG. 4A , however, first piston  104 , second piston  106  and motor piston  305  are at their respective BDC positions. 
       FIGS. 5A and 5B  show a cross-section view of a second embodiment of a self-priming hydraulic system.  FIG. 5A  illustrates a pair of reciprocating piston pumps in a side-by-side configuration and the connectability of their respective inlet and outlet fluid supports. Figure SB shows the entire side-by-side configuration of a self-priming hydraulic system comprising two reciprocating piston pumps in a side-by-side configuration. 
     In this embodiment, pumps  500 A and  500 B comprise first housing  501 A and second housing  501 B, respectively. First piston  504  is contained in first housing  501 A and second piston  506  is contained in second housing  501 B. First piston  504  is received within first piston bore  505  and second piston  506  is received within second piston bore  507 . 
     In this embodiment as shown, first piston  504  and second piston  506  both engage actuator  530  through couple  503  (e.g. a cam shaft). It should be understood that first piston  504  and second piston  506  can also directly engage actuator  530  directly without couple  503  or can each engage separate actuators  530  and  530 B (not shown). 
     In the embodiment shown in  FIG. 5B , movement of first piston  504  and second piston  506  along axes AA and BB towards their respective TDC positions, respectively, is caused by the actuation of actuator  530 . Actuation of actuator  530  can transmit motion through couple  503  to both first piston  504  and second piston  506  such that movement of first piston  504  and second piston  506  towards their respective TDC positions is reasonably synchronized. Movement of first piston  504  and second piston  506  towards their respective TDC and BDC positions can also be opposite each other, such that as first piston  504  moves towards TDC, second piston  506  moves towards BDC and vice versa by offsetting the reciprocation of one of pistons  504 ,  506  by 180 degrees. 
     It is recognised that, shown by example in  FIGS. 5A and 5B , both first piston  504  and second piston  506  are concentric about an axes AA and BB, respectively, running through first piston  504  and second piston  506 . However, it is recognised that first piston  504  and second piston  506  can be non-concentric about the axes AA and BB as desired. First piston  504  and second piston  506  can be any contemplated shape provided that the contour of first piston  504  is similar to the contour of first piston bore  505  and the contour of second piston  506  is similar to the contour of second piston bore  507 . Other configurations can therefore be utilized while operating in a similar manner as described herein. It is understood that in this embodiment second piston  506  and first piston  504  will have the same size and shape. 
     First piston bore  505  co-operates with a top surface  522  of first piston  504  to define a first fluid cavity  520 , the first piston bore  505  providing a first fluid inlet  510  and a first fluid outlet  514  disposed on the first piston bore  505  and fluidly coupled to the first fluid cavity  520 . A surface  521  of the piston bore  505  (e.g. opposing bore wall to the top surface of the first piston) can be variable in position during the operating cycle of the hydraulic pump  500 A, thus providing for an increase or decrease in bore volume of the first fluid cavity  520  as experienced by the first piston  504  during travel between TDC and BDC. Position of the variable position surface  521  (e.g. a piston also referred to as a floating piston) can be controlled by a resilient element (e.g. compressible fluid) chamber (not shown) positioned behind the variable position surface  521 , such that the variable position surface  521  is located between the resilient element chamber (not shown) and the first fluid cavity  520 . For example, as volume of the resilient element (e.g. compressed fluid) is decreased in the resilient element chamber (e.g. via ejection of compressible fluid therefrom), the position of the variable position surface  521  will become biased away from the first piston  504  and thus provide for an increased volume of the first fluid cavity  520  experienced by the first piston  504 . Alternatively, as volume of the resilient element (e.g. compressed fluid) is increased in the resilient element chamber (e.g. via injection of compressible fluid therein), the position of the variable position surface  521  will become biased towards the first piston  504  and thus provide for a decreased volume of the first fluid cavity  520  experienced by the first piston  504 . Control in position of the variable position surface  521  can be provided for (e.g. electronic control of injection/ejection of fluid with respect to the resilient element chamber in response to sensed operating conditions such as pressure, speed, and/or position experienced by the pump pistons, and/or in response to sensed operating conditions such as pressure, speed, and/or position experienced by the motor piston) during travel of the first piston  504  between TDC and BDC while the pump  500 A is in operation. 
     The hydraulic system also has a second piston  506  operable to reciprocate within a second piston bore  507 , the second piston bore  507  co-operating with a top surface  528  of the second piston  506  to define a second fluid cavity  524 , the second piston bore  507  providing a second fluid inlet  512  and a second fluid outlet  516  disposed on the second piston bore  507  and fluidly coupled to the second fluid cavity  524 . Reciprocation of the second piston  506  is coupled to reciprocation of the first piston  504 , as herein described by numerous examples, in order to synchronize positioning of the first piston  504  within the first piston bore  505  to positioning of the second piston  506  within the second piston bore  507 . For example, the piston coupling mechanism can be used to synchronize travel of the first piston  504  towards BDC within the first fluid cavity  520  at the same time as travel of the second piston  506  towards BDC within the second fluid cavity  524 . For example, the piston coupling mechanism  503  can be used to synchronize travel of the first piston  504  towards TDC within the first fluid cavity  520  at the same time as travel of the second piston  506  towards TDC within the second fluid cavity  524 . Alternatively, in some configurations as described, the piston coupling mechanism  503  can be used to synchronize travel of the first piston  504  towards TDC within the first fluid cavity  520  at the same time as travel of the second piston  506  towards BDC within the second fluid cavity  524 . Alternatively, in some configurations as described, the piston coupling mechanism  503  can be used to synchronize travel of the first piston  504  towards BDC within the first fluid cavity  520  at the same time as travel of the second piston  506  towards TDC within the second fluid cavity  524 . 
     The self-priming hydraulic system described in  FIGS. 5A and 5B  further comprise a pair of loops to circulate non-compressible fluid: a low pressure loop  518  and a high pressure loop  519 . Low pressure loop  518  is defined by a low pressure accumulator  560  fluidly connected to first outlet  514  through a resistive element  590 , and fluidly connected to second inlet  512 . High pressure loop  519  is defined by high pressure accumulator  582  fluidly connected to second outlet  516  and first inlet  510 . It is desirable to have the effluent non-compressible fluid from the resistive element  590  exhaust into the low pressure accumulator  560  to maximize the efficiency of work performed by the resistive element  590 . 
     Low pressure accumulator  560  is a storage reservoir in which non-compressible hydraulic fluid is held under pressure. High pressure accumulator  562  is also a storage reservoir in which non-compressible hydraulic fluid is held under pressure. In one embodiment, compressed gas is used to maintain desired pressures in low pressure accumulator  560  and high pressure accumulator  562 . Typically, low pressure accumulator  560  will store hydraulic fluid at a lower pressure than high pressure accumulator  562 . It should be noted that pistons  504  and  506  can be decoupled from actuator  530  at their TDC positions. Low pressure accumulator  560  and high pressure accumulator  562  can provide non-compressible fluid to second fluid cavity  524  and first fluid cavity  520 , respectively, to restart the hydraulic system and push pistons  504 ,  506  back towards BDC to re-engage actuator  530 . 
     First fluid cavity  520  is supplied with non-compressible fluid by first inlet  510  when first piston  504  moves to its BDC position. Similarly, second fluid cavity  524  is supplied with non-compressible fluid by second inlet  512  when the second piston  506  moves to its BDC position. Non-compressible fluid exits first fluid cavity  520  through first outlet  514 . Non-compressible fluid exits second fluid cavity  524  through second outlet  516 . 
     First inlet  510  contains check valve  572  through which non-compressible fluid passes as it enters first fluid cavity  520 . First outlet  514  contains check valve  550  through with non-compressible fluid passes as it exits first fluid cavity  520  and enters fluid support  552  of low pressure loop  518 . Check valve  572  inhibits flow of non-compressible fluid out of first cavity  520  and back into fluid support  564  from which it entered self-priming pump  500 A. Check valve  550  inhibits flow of non-compressible fluid into first fluid cavity  520  from fluid support  552  after the ejection phase of piston  504 . Fluid support  552  delivers non-compressible fluid exiting first fluid cavity  520  to resistive element  590 . 
     Second inlet  512  contains check valve  570  through with non-compressible fluid passes as it enters second fluid cavity  524 . Similarly, second outlet  516  contains check valve  554  through which non-compressible fluid passes as it exits second fluid cavity  524  and enters fluid support  556  of high pressure loop  519 . Check valve  570  inhibits the flow of non-compressible fluid out of second cavity  524  and back into fluid support  568  from which it entered self-priming pump  500 B. Fluid support  556  delivers non-compressible fluid exiting second fluid cavity  524  to high pressure accumulator  562 . 
     Pressure control valve  574  controls injection of non-compressible fluid into second fluid cavity  524 . Pressure control valve  574  can also be an on-off valve. Pressure control valve  578  controls infection of non-compressible fluid into first fluid cavity  520 . Injection pressure P 1in  of the non-compressible fluid at first inlet  510  can be influenced by pressure control valve  578 . Pressure-control valve  578  can be an electronically controlled valve. Pressure reducing valve  580  controls the pressure P HPAcc  in high pressure accumulator  582  by controlling the flow of non-compressible fluid from high pressure accumulator  562  to low pressure accumulator  560  through fluid support  566 . Pressure reducing valve  580  can be a self-monitoring valve (e.g. spring loaded) such that when pressure P HPAcc  becomes higher than a calibrated tolerance of pressure reducing valve  580 , non-compressible fluid passes through pressure reducing valve  580  and reduces P HPAcc  in high pressure accumulator  562 . Non-compressible fluid from high pressure accumulator  562  can enter low pressure accumulator  580  through pressure reducing valve  580 . Pressure reducing valve  580  can also direct non-compressible fluid from either low pressure accumulator  560  or high pressure accumulator  562  to another accumulator vessel (not shown) through fluid support  582 . 
     It should be understood that in this configuration first fluid cavity  520  ejects non-compressible fluid at P 1out , which is higher than the pressure P 2out  at which second fluid cavity  524  ejects non-compressible fluid. This is because in some embodiments non-compressible fluid ejected from first fluid cavity  520  will enter resistive element  590  and used to perform work. It is desirable to have effluent non-compressible fluid from resistive element  590  exhaust into low pressure accumulator  560  (via heat exchanger  591 ) at P LPAcc  to maximize the pressure difference between the Inlet and outlet of resistive element  590 . 
     Fluid support  552  delivers non-compressible fluid from first outlet  514  to resistive element  590 . Resistive element  590  uses non-compressible fluid provided from first fluid cavity  520  at P 1out  to perform work. Fluid support  555  delivers effluent non-compressible fluid from resistive element  590  to low pressure accumulator  560  (via heat exchanger  591 ). Fluid support  555  can contain check valve  554  between heat exchanger  591  and low pressure accumulator  560 . 
     Low pressure accumulator  560  of low pressure loop  518  stores returning volumes of non-compressible hydraulic fluid that exit resistive element  590 . Low pressure accumulator  560  has a pressure P LAcc . It should be understood that low pressure accumulator  560  can be charged with a compressed gaseous fluid such as air or nitrogen. Low pressure accumulator  560  addresses thermal expansion of the non-compressible fluid as it exits resistive element  590 . Non-compressible fluid exiting resistive element  590  increases in temperature and volume. Low pressure accumulator  560  is therefore provided after resistive element  590  to offer space for this additional volume of non-compressible fluid before re-entering second fluid cavity  524  of reciprocating hydraulic pump  500 B at second inlet  512 . 
     Fluid support  556  delivers non-compressible fluid from second outlet  516  to high pressure accumulator  562  of high pressure loop  519 . 
     High pressure accumulator  562  is used to store returning volumes of non-compressible fluid that exits second cavity  524  at P 2out . High pressure accumulator  562  can also be charged with a gaseous fluid such as air or nitrogen. Maintaining pressure P HAcc  within high pressure accumulator  562  allows the closed loop system to inject non-compressible fluid into first fluid cavity  520  at a high pressure. 
     Fluid support  568  delivers non-compressible fluid from low pressure accumulator  560  to second inlet  512  through check valve  570 . Fluid support  568  can contain pressure control valve  574 . Fluid support  564  delivers non-compressible fluid from high pressure accumulator  562  to first inlet  510  through check valve  572 . Fluid support  564  can contain pressure control valve  578  to control the pressure of non-compressible fluid leaving high pressure accumulator  562  and entering first inlet  510 . 
     Fluid support  566  provides a relief to high pressure accumulator  562  by delivering non-compressible fluid from the high pressure accumulator  562  to the low pressure accumulator  560 . Pressure reducing valve  580  controls the flow of non-compressible fluid from high pressure accumulator  562  to low pressure accumulator  560  and therefore can be used to control pressure P HAcc  in high pressure accumulator  562 . 
     In  FIGS. 6 and 7 , hydraulic motor  301  is provided as previously described in  FIGS. 3 and 4 . In  FIGS. 6 and 7 , hydraulic motor  301  comprises capillary tube  606  which acts is a feed line to lower cavity  330  of motor  301  from ambient. Lower cavity  330  is defined by opposed and adjacent sides of motor bore  308 , a lower surface  331  of piston  305  and a lower surface  332  of piston bore  308 . As previously described for  FIGS. 3A and 3B , motor  301  is hydraulic and non-compressible fluid is injected into motor cavity  307  from first outlet  114  at pressure P 1out  during the ejection phase of first piston  104 . The remaining structures of motor  301  are as previously described. 
     Control valve  608  is provided to control the flow of compressible fluid (e.g. air) from atmosphere through capillary tube  606  to lower cavity  330 . Conversely, control valve  607  is provided to control the flow of compressible fluid out of lower cavity  330  and into reservoir  609 . Compressible fluid is stored in reservoir  609  at some nominal pressure P AAcc . Reservoir  609  is shown in greater detail in  FIG. 7C . As shown in  FIG. 7A , a pressure release valve  615  can be provided to allow compressible fluid stored in reservoir  609  to be returned to atmosphere if P AAcc  becomes higher than any predefined value. In one non-limiting embodiment, P AAcc  is set at 100 psi. 
     The structure of fluid support  155 , valve  605  and heat exchanger  602  are as previously described for corresponding elements in  FIGS. 3A and 3B . Heat exchanger  602  is fluidly connected to low pressure accumulator  660  by fluid support  672 , so non-compressible fluid exiting heat exchanger  602  travels through check valve  604  and fluid support  672  before entering low pressure accumulator  660 . Pressure release valve  680  is provided to control non-compressible fluid flow out of low pressure accumulator  660  and to maintain low pressure accumulator  660  at a pre-determined pressure P LPAcc . Pressure release valve  680  is pressure actuated and facilitates flow of non-compressible fluid along fluid support  668  to dual fluid accumulator  620  (e.g. compressible fluid over non-compressible fluid), which is shown in greater detail in  FIG. 7B . 
     Dual fluid accumulator  620  comprises (as shown in  FIG. 7B ) a non-compressible fluid side  701  and a compressible fluid side  702 , divided by double-sealing piston  703 . Compressible fluid side  702  further comprises return spring  704  to encourage double-sealing piston  703  to resist flow of incoming non-compressible fluid. Compressible fluid side  702  of dual fluid accumulator  620  is fluidly connected to fluid support  619 , which in turn is fluidly connected to electronic control valve  616 . To return non-compressible fluid to low pressure accumulator  660  in the event of a drop in pressure therein, a pressure increase in compressible fluid side  702  of dual fluid accumulator  620  is provided. This pressure increase is generated by actuating electronic control valve  616 , which releases compressible fluid stored in reservoir  609  and directs it to fluid supports  612 ,  614 ,  617  and  619  to the compressible fluid contacting side of dual fluid accumulator  620 . 
     Electronic control valve  616  fluidly connects to dual fluid accumulator  620  through fluid supports  617 ,  619  and reservoir  609  through fluid supports  612 , 614 . Electronic control valve is configured to control the circulation of compressible fluid between reservoir  609 , dual fluid accumulator  620  and atmosphere to maintain desired pressures in low pressure accumulator  660  and reservoir  609 . The mechanism of action of electronic control valve  616  is described in more detail below. 
     Operation 
     In the system shown in  FIG. 1 , opening gate valve  174  allows pressurized non-compressible fluid to be released from low pressure accumulator  160 , travel along fluid support  168  and enter second piston cavity  124  of reciprocating hydraulic pump  100  through check valve  170  and second inlet  112 . Movement of non-compressible fluid into second cavity  124  drives second piston  106  downward. 
     Opening pressure control valve  178  also allows a controlled injection of pressurized non-compressible fluid to be released from high pressure accumulator  162 , travel along fluid support  164  and enter first fluid cavity  120  through check valve  172  and first inlet  110 . Movement of non-compressible fluid into first cavity  120  drives first piston  104  downward. 
     As second piston  106  moves downward, first piston  104  also moves downward in synchronized motion because first piston  104  and second piston  106  are coupled by couple  103 , allowing first fluid cavity  120  to substantially simultaneously fill with non-compressible fluid from high pressure accumulator  162  as second fluid cavity  124  fills with non-compressible fluid from low pressure accumulator  160 . 
     As non-compressible fluid fills first fluid cavity  120 , first piston  104  moves downward towards its BDC position. As previously described, in this embodiment, second piston  106  also moves towards its BDC position as first piston  104  moves towards its BDC position. Once first and second pistons  104 ,  106  are at their BDC positions, an ejection phase is completed and actuator  130  drives first and second pistons back towards their respective TDC positions. Non-compressible fluid is pushed out of first outlet  114  into high pressure loop  118 , and out of second outlet  116  into low pressure loop  119 . Specifically, non-compressible fluid ejected from first cavity  120  passes through first outlet  114  and enters fluid support  152 . Similarly, non-compressible fluid ejected from second cavity  124  passes through second outlet  116  and enters fluid support  156 . Fluid support  152  carries ejected non-compressible fluid at P 1out  towards resistive element  159  and subsequently, through fluid support  155  to low pressure accumulator  160 . Fluid support  156  carries ejected non-compressible fluid at P 2out  towards high pressure accumulator  162 . 
     In high pressure loop  119 , non-compressible fluid travels along fluid support  156  towards high pressure accumulator  162 . Upon arriving at high pressure accumulator  162 , non-compressible fluid can either continue into high pressure accumulator  162  for temporary storage, travel along fluid support  164  through pressure control valve  178  towards first fluid cavity  120 , or travel along fluid support  166  through pressure release valve  180  towards low pressure accumulator  160 . 
     Non-compressible fluid entering fluid support  164  passes through pressure control valve  178  and enters first inlet  110  through check valve  172 . Pressure control valve  178  is used to control the pressure P 1in  of non-compressible fluid into first fluid cavity  120 . 
     In low pressure loop  118 , non-compressible fluid travels along fluid support  152  towards resistive element  159 . Non-compressible fluid first enters resistive element  159  at P 1out . Resistive element  159  effluent enters fluid support  155  and continues towards low pressure accumulator  160  at P LPAcc . As previously described, maximum change in pressures between P 1out  and P LPAcc  is desired to maximize the efficiency of resistive element  159  to perform work. When non-compressible fluid reaches low pressure accumulator  160 , non-compressible fluid can either enter low pressure accumulator  160  or travel along fluid support  168  towards second fluid cavity  124 . 
       FIG. 2A  shows the same system as described in  FIGS. 1A and 1B , however, fluid support  152  contains pressure control valve  201 . 
     In the embodiment illustrated in  FIGS. 2A and 2B , the same operation of reciprocating hydraulic pump  100  occurs as was previously described. Pressure control valve  201  is used to control the flow of non-compressible fluid in fluid support  152 . The flow of non-compressible fluid through reciprocating hydraulic pump  100  and circuit  102  remains the same as previously described for  FIGS. 1A and 1B , however, heat is generated at pressure control valve  201  and a cooling system, such as heat exchanger  202 , can be provided to maintain system operating temperatures. 
       FIG. 2B  shows a cross-section of the system as described above in  FIG. 2A , however, first piston  104  and second piston  106  are at their respective BDC positions. Non-compressible fluid exiting first fluid cavity  120  moves through first outlet  114  and check valve  150 , through pressure control valve  201  and towards heat exchanger  202 . It is recognized that any fluid resistive element could be placed in the position of pressure control valve  201 , including a motor as described in  FIGS. 3 and 4 . As non-compressible fluid exits heat exchanger  202  it continues along fluid support  152  to low pressure accumulator  160 . 
       FIG. 3A  shows the same system as described in  FIGS. 2A and 2B , however, pressure control valve  201  has been replaced by motor  301 . 
     In  FIG. 3A , an intake stroke of motor piston  305  is initiated by injecting a pressurized volume of non-compressible fluid released from first outlet  114  of first cavity  120  into low pressure loop  118 . Flow control valve  321  is closed to inhibit ejection of non-compressible fluid from motor  301  and facilitate the non-compressible fluid to drive piston  305  to its BDC position. Upon reaching its BDC position, load  315  actuates piston  305  back to its TDC position exhausting the non-compressible fluid filling motor cavity  307 . Flow control valve  320  (e.g. on-off valve) is closed during this exhaust stroke. Flow control valves  320 ,  321  can be electronically controlled.  FIG. 3B  shows the same system as described in  FIG. 3A , however, first piston  104 , second piston  106  and motor piston  305  are at their BDC positions. It should be understood that during normal operating conditions, pistons  104 ,  106  of pump  100  and motor piston  305  can be at different phases of their respective stroke. For example, the stroke of pistons  104 ,  106  of pump  100  and motor piston  305  can be offset 180 degrees. 
       FIG. 4A  shows the same system as described in  FIGS. 3A and 3B , however, pressure control valve  178  is a pressure control valve  478  to control an amount of non-compressible fluid entering first fluid cavity  120  from high pressure accumulator  162 . In this embodiment, intake and exhaust of non-compressible fluid from piston bores  105 ,  107  are the same as described previously in  FIGS. 3A and 3B . 
     As non-compressible fluid is ejected from second fluid cavity  124 , it travels along fluid support  158  towards high pressure accumulator  162 . As previously described, fluid support  496  can be used to join non-compressible fluid ejected from second fluid cavity  124  with non-compressible fluid ejected from first fluid cavity  120  upstream of motor  301 . This configuration can be used in applications where an increase in power output from motor  301  is desired such as but not limited to during acceleration of a vehicle. 
     By closing gate valve  492  and opening gate valve  497 , non-compressible fluid ejected from second fluid cavity  124  can be taken out of high pressure loop  119  to join non-compressible fluid in low pressure loop  118  upstream of motor  301 . Closing gate valve  492  inhibits flow of non-compressible from second fluid cavity  124  towards high pressure accumulator  162 . Opening gate valve  497  facilitates joining non-compressible fluid ejected from second fluid cavity  124  with non-compressible fluid ejected from first fluid cavity  120  heading to motor  301 . Non-compressible fluid ejected from second fluid cavity  124  can therefore temporarily bypass a portion of high pressure loop  119  when valve  492  is closed and valve  497  is opened. As previously noted, valves  492  and  497  can be combined into a single three-way valve (not shown). 
     When non-compressible fluid ejected from second fluid cavity is directed to join non-compressible fluid ejected from first fluid cavity  120  heading to motor  301 , any non-compressible fluid ejected from motor  301  can be put back into high pressure loop  119  downstream of valve  492  (or downstream of a three-way valve if used in place of valves  492 ,  497 ) with fluid support  498  and valve  499 . In  FIG. 4A , fluid support  498  fluidly couples fluid support  168  and fluid support  164 . Valve  499  can be opened in combination with opening valve  497  and closing valve  492  as previously described. Opening valve  499  directs a portion of non-compressible fluid ejected from motor  301  into low pressure loop  118  downstream of valve  492 . In  FIG. 4A , the position of fluid support  498  creates a temporary bypass of high pressure accumulator  162  as non-compressible fluid is directed into low pressure loop  118  from high pressure loop  119  downstream of high pressure accumulator  162 . As previously described, it is desirable to direct a portion of non-compressible fluid ejected from motor  301  into high pressure loop  119  downstream of valve  492  and upstream of first fluid inlet  110  to inhibit first fluid cavity  120  from developing a vacuum, which could occur if an insufficient volume of non-compressible fluid was present in first fluid cavity  120  during a downstroke of first piston  104 . 
       FIG. 4A  also shows pressure control valve  478  which can control the flow of non-compressible fluid injected into first fluid cavity  120  from second fluid outlet  116  and the pressure of high pressure accumulator  162  as previously described. 
       FIG. 4B  shows the same system as described in  FIG. 4A , however, first piston  104 , second piston  106  and motor piston  305  are at their BDC positions. 
       FIG. 5  shows another exemplary configuration of a self-priming hydraulic system. In this side-by-side configuration, reciprocating hydraulic pumps  500 A and  500 B comprise first housing  501  and second housing  501 B, respectively. First piston bore  505  is disposed in first housing  501  and second piston bore  507  is disposed in second housing  501 B. Circuit  502  is not illustrated in  FIG. 5A , however, it is understood that first inlet  510  is fluidly connected to second outlet  516  and second inlet  512  is fluidly connected to first outlet  514 , as previously described. 
       FIG. 5B  shows a cross-section view of a side-by-side embodiment of a self-priming hydraulic system where a first piston  504  and a second piston  506  are shown at their BDC positions and low pressure loop  518  includes a fluid resistive element  590  and a heat exchanger  591 . Operation of the side-by-side embodiment shown in  FIGS. 5A and 5B  is substantially similar to as previously described for  FIGS. 1A and 1B , except that first piston  504  and second piston  506  can be configured such that they approach their respective TDC and BDC positions at substantially the same time or substantially opposite times. For instance, when piston  504  approaches TDC, piston  506  can be configured to approach either its TDC or BDC positions. 
       FIG. 6  shows a cross-section view of a double-decker embodiment of a self-priming hydraulic system where a first piston  104  and a second piston  106  are shown in their BDC position and a motor  301  and a control system  650  facilitate pressure control in low pressure accumulator  660  and high pressure accumulator  662 . Portions A, B and C illustrated on  FIG. 6  are shown in more detail in  FIGS. 7A, 7B and 7C , respectively. 
     Referring to  FIG. 7A , capillary tube  606  is a feed line to lower side of piston  305  of motor  301 . As previously described for  FIGS. 3A and 3B , motor  301  is hydraulic and non-compressible fluid is injected into motor cavity  307  from first outlet  114  during the ejection phase of first piston  104 . 
     Compressible fluid is drawn into a lower cavity  330  through capillary tube  606  and control valve  608 . In one embodiment, control valve  608  can lead to atmosphere. Compressible fluid is used to fill lower cavity  330  as motor piston  305  moves from its BDC position to its TDC position. 
     During a downstroke of motor piston  305  from TDC to BDC, compressible fluid in lower cavity  330  is forced out of lower cavity  330  through capillary tube  606  and control valve  607  and into reservoir  609 . Compressible fluid is stored in reservoir  609  at some predefined pressure P AAcc . Reservoir  609  is shown in greater detail in  FIG. 7C . Pressure release valve  615  is dedicated to reservoir  609  to provide pressure control for reservoir  609 . If pressure P AAcc  in reservoir  609  rises above a predefined value, the excess fluid pressure opens pressure release valve  615  and compressible fluid is exhausted to ambient. Compressible fluid (e.g. air) drawn from ambient into lower cavity  330  passes through check valve  608 . Compressible fluid forced out of lower cavity  330  into reservoir  609  passes through check valve  607 . 
     Non-compressible fluid in fluid cavity  307  of motor  301  is ejected through motor outlet  304  during an upstroke of motor piston  305  to its TDC position from its BDC position. After passing through motor outlet  304 , non-compressible fluid travels through fluid support  155  and valve  605  to heat exchanger  602 . Heat exchanger  602  is fluidly connected to low pressure accumulator  660  by fluid support  672 , so when non-compressible fluid exits heat exchanger  602  it travels through control valve  604  and fluid support  672  before entering low pressure accumulator  660 . To allow low pressure accumulator  660  to be maintained at a pre-determined pressure, pressure release valve  680  is provided. When pressure release valve  680  is forced open by excessive pressure, non-compressible fluid can flow along fluid support  666  to dual fluid accumulator  620 , which is shown in greater detail in  FIG. 7B . 
     Dual fluid accumulator  620  (as shown in  FIG. 7B ) comprises a non-compressible fluid side  701  and a compressible fluid side  702 , divided by double-sealing piston  703 . Compressible fluid side  702  further comprises return spring  704  to encourage double-sealing piston  703  to tend to resist flow of incoming non-compressible fluid into non-compressible fluid side  701 . Compressible fluid side  702  of dual fluid accumulator  620  is fluidly connected to fluid support  619 , which in turn is fluidly connected to electronic control valve  616 . To return non-compressible fluid to low pressure loop  118  (see  FIG. 6 ) in the event of a drop in pressure in low pressure loop  118 , a pressure increase is provided in the compressible fluid side  702  of dual fluid accumulator  620 . An increase in compressible fluid pressure is generated by actuating electronic control valve  616 , which releases compressible fluid stored in reservoir  609  and directs it along fluid supports  612 ,  614 ,  617  and  619  to compressible fluid side  702  of dual fluid accumulator  620 . 
     Upon a drop in pressure in low pressure loop  118  (see  FIG. 6 ) or low pressure accumulator  660  below a pre-determined pressure, non-compressible fluid is released from dual fluid accumulator  620  by opening valve  622  and closing valve  616 . Compressible fluid enters compressible fluid side  702  of dual fluid accumulator  620  and actuates double-sealing piston  703  such that non-compressible fluid in non-compressible fluid side  701  is forced out of dual fluid accumulator  620  and into fluid support  621 . Non-compressible fluid passes through valve  622  and enters low pressure accumulator  660 . It should be noted that valve  622  is typically closed. It should also be understood that non-compressible fluid forced from dual fluid accumulator  620  can also enter fluid support  168  through valve  622  at a position between gate valve  174  and second inlet  112  (not shown). In this embodiment, non-compressible fluid can be injected into low pressure loop  118  even if gate valve  174  is closed. 
     It should be understood that dual fluid accumulator  620  is not intended to maintain a pressurized supply of compressible fluid in compressible fluid side  702 . While non-compressible fluid is being stored in non-compressible fluid side  701 , rather, dual fluid accumulator  620  is intended to receive compressible fluid from reservoir  609  when delivery of non-compressible fluid from non-compressible fluid side  701  to low pressure accumulator  660  is desired. Dual fluid accumulator  620  therefore acts as a temporary storage unit for both high pressure accumulator  662  and low pressure accumulator  660  to allow each to continuously operate at predefined settings despite thermal expansion of non-compressible fluid therein. Dual fluid accumulator  620  facilitates the continuous operation of low pressure accumulator  660  and high pressure accumulator  662  by accepting and storing increased volumes of non-compressible fluid from thermal expansion. 
     Electronic control valve  616  is configured to communicate with pressure transducer  640  at low pressure accumulator  660  such that when P LPAcc  in low pressure accumulator  660  either drops below or raises above a predefined pressure, an electronic signal is sent to electronic control valve  616  to change electronic control valve  616  between an on and an off position. In one embodiment, the desired pressure P LPAcc  is 100 psi. 
     Electronic control valve  616  is configured such that when it is in the off position, compressible fluid temporarily injected in compressible fluid side  702  of dual fluid accumulator  620  can pass through fluid supports  618 ,  619  to atmosphere. If P LPAcc  raises above a predefined pressure, pressure release valve  680  is opened by the non-compressible fluid and non-compressible fluid flows from low pressure accumulator  660  through fluid support  666  to dual fluid accumulator  620  to relieve the pressure in low pressure accumulator  660 . Conversely, compressible fluid stored in reservoir  609  can flow to electronic control valve  616  and dual fluid accumulator  620  via fluid supports  612 ,  614  when electronic control valve  616  is switched on, increasing the pressure P LPAcc  by forcing non-compressible fluid to flow from dual fluid accumulator  620  through fluid support  621  and valve  622  to low pressure loop  118 . 
     Pressure transducers  640  and  661  are provided to monitor pressures in low pressure accumulator  660  and high pressure accumulator  662 , respectively. If pressure P LPAcc  rises above its predefined value, pressure release valve  680  is opened by non-compressible fluid and excess non-compressible fluid flows from low pressure loop  118  to non-compressible fluid side  701  of dual fluid accumulator  620 . Likewise, if pressure P HPAcc  rises above its predefined value, pressure release valve  670  is opened by non-compressible fluid and excess non-compressible fluid flows from high pressure loop  119  to non-compressible fluid side  701  of dual fluid accumulator  620 . 
     If pressure P LPAcc  in low pressure accumulator  660  is too low, pressure transducer  640  signals electronic control valve  616  and control valve  622  to open, thereby facilitating non-compressible fluid flow out of non-compressible fluid side  701  of dual fluid accumulator  620  through fluid support  621  and valve  622  to low pressure loop  118 . Similarly, if pressure P HPAcc  in high pressure accumulator  662  is too low, pressure transducer  640  signals electronic control valve  616  and control valve  622  to open, thereby allowing non-compressible fluid to flow out of non-compressible fluid side  701  of dual fluid accumulator  620  through fluid support  621  and valve  622  to low pressure loop  118 . 
     In another embodiment not shown, compressible fluid can be supplied to electronic control valve  616  directly from an external fluid source. In this configuration, an external fluid source is fluidly coupled to electronic control valve  616  directly through fluid support  614 . This configuration can be used when motor  301  does not comprise capillary tube  606  (e.g. motor  301  is a double-acting motor), or capillary tube  606  is not fluidly connected to electronic control valve  616 . In such a configuration, check valves  607 ,  608 , reservoir  609  and pressure release valve  615  can be removed from the system.