Apparatus and method for recirculating hydraulic fluid

Abstract

An apparatus and method for recirculating hydraulic fluid includes commanding a fluid switching valve in a first actuated position in which pressurized hydraulic fluid moves a piston; detecting when the piston completes the first stroke; the delay command fluid switching valve is in the second actuation position, the delay command bypass valve is in the bypass position, or the delay command hydraulic pump stops pressurizing the hydraulic fluid, the piston bypass valve is open such that the hydraulic fluid flows through the inlet into the sub-circuit and through the piston bypass valve, where the hydraulic pump is in the first actuation position, the delay command fluid switching valve is in the second actuation position, the delay command bypass valve is in the bypass position, or the hydraulic pump stops pressurizing the hydraulic fluid. Flushing an amount of hydraulic fluid from the sub-circuit through an outlet; and commanding the fluid switching valve to be in the second actuated position, commanding the bypass valve to be in the bypass position, or commanding the hydraulic pump to stop pressurizing the hydraulic fluid.

Description

Technical Field

[0001] The present application relates to apparatus and methods for recirculating hydraulic fluid. Background Art

[0002] Hydraulic fluid systems use pressurized hydraulic fluid as a medium to transfer energy through the system so that work and movement can be performed. Referring to FIG. 1 , a typical hydraulic system 10 includes a pump 30 for pressurizing hydraulic fluid from a reservoir 20 to circulate around the system. Immediately after the pump 30, preferably before the pressurized hydraulic fluid is delivered to various components in the system through a hydraulic line 50, a filter 40 removes contaminants from the hydraulic fluid. As used herein, a hydraulic line is a conduit for circulating hydraulic fluid. These systems may include a pressure sensor 60 and a temperature sensor 70, which are used to sense pressure parameters and temperature parameters of the hydraulic fluid, respectively, to verify whether these parameters are within the rated specifications of the hydraulic system 10. In the sensors 60, 70, the hydraulic fluid system typically includes a pressure sensor and may not include a temperature sensor. Instead of the pressure sensor, or in addition to the pressure sensor, there may be a pressure reducing valve 80 as a safety measure to limit the maximum hydraulic fluid pressure in the system. The pressurized hydraulic fluid is delivered to end-use hydraulic components, such as a selector valve and / or actuation unit, generally indicated by reference numeral 100, and returned to the reservoir 20 via the hydraulic return line 55 through the heat exchanger 90, which may use various heat exchange fluids, such as engine coolant or air, to cool the hydraulic fluid during operation, or in some cases, such as when the hydraulic system 10 is started up in cold ambient temperatures, to heat the hydraulic fluid. A controller 110, preferably an electronic controller, may send command signals to and / or receive status signals from the pump 30, the sensors 60, 70, and in some cases, may send command signals to and / or receive status signals from the selector valve and / or actuation unit 100. In other embodiments, the pump 30 may be driven directly by the engine and operated without a clutch, such that the controller 110 cannot shut down the pump, and this type of pump operation is contemplated for the embodiments disclosed herein. The durability of the components in the hydraulic fluid system 10 is directly related to the cleanliness of the hydraulic fluid. For some components, cleanliness has a large impact on their performance, life, and the level and frequency of maintenance they require.

[0003] Referring now to FIG. 2 , a hydraulic system 11 is shown which is a specific embodiment of the hydraulic system 10 in which like components have like reference numerals and only the differences are discussed. The end-use hydraulic component 101 includes a hydraulic motor 120 which is a double-acting, reciprocating piston type actuating unit which is selectively actuated by a controller 111 to effect reciprocating motion of a piston rod 190. A piston 170 is connected to the piston rod 190 and reciprocates within a cylinder 175, between a cylinder head 180 and a cylinder head 185 when pressurized hydraulic fluid is alternately applied to the hydraulic chambers 160 and 165. In FIG. 2 , the fluid switching valve 130 and the bypass valve 140 are shown in a first actuated position for the hydraulic motor 120. In the first actuated position, pressurized hydraulic fluid from hydraulic line 50 is delivered to hydraulic chamber 165 through hydraulic line 155, causing piston 170 to move toward cylinder head 180 so that piston rod 190 can perform work, whereby hydraulic fluid in hydraulic chamber 160 is discharged into hydraulic line 150, through fluid switching valve 130, and into hydraulic return line 55. Referring to FIG3 , fluid switching valve 130 and bypass valve 140 are shown in a second actuated position for hydraulic motor 120. In the second actuated position, pressurized hydraulic fluid is delivered to hydraulic chamber 160 through hydraulic line 150, causing piston 170 to move toward cylinder head 185 so that piston rod 190 can perform work, whereby hydraulic fluid in hydraulic chamber 165 is discharged into hydraulic line 155, through fluid switching valve 130, and into hydraulic return line 55. In the first actuation position and the second actuation position, the bypass valve 140 is in the blocking position, whereby the bypass valve 140 blocks the flow of hydraulic fluid from the hydraulic line 50 to the hydraulic line 55. Referring now to FIG. 4 , the bypass valve 140 is shown in the bypass position, whereby the hydraulic fluid in the hydraulic line 50 is delivered to the hydraulic return line 55 through the bypass valve 140, thereby bypassing the hydraulic motor 120. The piston 170 typically employs one or more sealing rings (not shown) to seal the pressurized hydraulic fluid to prevent the pressurized hydraulic fluid from leaking between the chambers 160 and 165 between the piston 170 and the cylinder 175. These sealing rings are typically made of non-metallic materials such as polymers, which can be damaged by contaminants in the hydraulic fluid, causing their sealing effectiveness to decrease, which reduces the volumetric efficiency of the hydraulic motor 120 and increases parasitic losses and maintenance requirements. It is advantageous to improve the cleanliness of the hydraulic fluid to reduce wear of these and other components in the hydraulic system 10, 11.

[0004] The hydraulic fluid reciprocates in a plurality of cycles of the piston 170 between the first actuation position and the second actuation position of the hydraulic motor 120 before the hydraulic fluid is returned to the reservoir 20 through the hydraulic return line 55. The reason for this is that in the first actuation position, at the end of the piston stroke (when the piston 170 is adjacent to the cylinder head 180), the hydraulic fluid remains in the hydraulic line 150, and in the second actuation position, at the end of the piston stroke (when the piston 170 is adjacent to the cylinder head 185), the hydraulic fluid remains in the hydraulic line 155, so that the hydraulic fluid is not flushed into the hydraulic return line 55 before the fluid switching valve 130 is changed to the second actuation position and the first actuation position, respectively. In the first actuation position and the second actuation position, at the end of the piston stroke, the hydraulic fluid (old hydraulic fluid) remaining in the hydraulic line 150 and the hydraulic line 155 is returned to the chamber 160 and the chamber 165, respectively, during subsequent piston cycles. In the example shown in FIGS. 2 and 3 , the maximum volume of chamber 160 (when piston 170 is located at cylinder head 185) is greater than the volume of hydraulic line 150, and the maximum volume of chamber 165 (when piston 170 is located at cylinder head 180) is greater than the volume of hydraulic line 155, but this is not a necessary condition and can be the opposite: that is, the maximum volume of chamber 160 (when piston 170 is located at cylinder head 185) is less than the volume of hydraulic line 150, and the maximum volume of chamber 165 (when piston 170 is located at cylinder head 180) is less than the volume of hydraulic line 155. In the second actuation position shown in FIG. 3 , during the piston stroke, the hydraulic fluid (new hydraulic fluid) from hydraulic line 50 entering hydraulic line 150 can be mixed to some extent with the old hydraulic fluid from hydraulic line 150, especially in chamber 160. However, it should be noted that, depending on the viscosity of the hydraulic fluid, there may be only a small amount of mixing or no mixing at all. Similarly, in the first actuated position shown in FIG. 2 , during the piston stroke, hydraulic fluid from hydraulic line 50 that enters hydraulic line 155 (new hydraulic fluid) mixes to some extent with old hydraulic fluid from hydraulic line 155, particularly in chamber 165. The mixing of old and new hydraulic fluid is the reason that the old hydraulic fluid is eventually flushed out into hydraulic return line 55 after a number of piston cycles have occurred. Once hydraulic fluid from hydraulic line 50 enters sub-circuit 151, the hydraulic fluid remains in the sub-circuit for a number of cycles of reciprocation of piston 170 between cylinder head 180 and cylinder head 185 before the hydraulic fluid is flushed into hydraulic return line 55. Again, it is important to note that without mixing, the old hydraulic fluid would be trapped in sub-circuit 151. A portion of sub-circuit 151, in which the hydraulic fluid reciprocates back and forth, is defined by hydraulic line 150, hydraulic motor 120, and hydraulic line 155.Sub-circuit 151 may also be considered to include fluid switching valve 130, since fluid switching valve directs pressurized hydraulic fluid into the sub-circuit, and more specifically, fluid switching valve 130 directs pressurized hydraulic fluid into hydraulic line 150 or hydraulic line 155. During each piston cycle, old hydraulic fluid collects and / or generates debris as it contacts surfaces in hydraulic lines 150 and 155 and hydraulic motor 120, thereby being more contaminated with debris than new hydraulic fluid from hydraulic line 50 (which has recently been filtered through filter 40). Any particles or debris that may be generated will move around sub-circuit 151 many times before the fluid in the sub-circuit is replaced, and may cause increased wear through continued wear. The more contaminated (i.e., the dirtier) the hydraulic fluid in hydraulic motor 120 is, the greater the wear experienced by components within hydraulic motor 120, particularly seal rings (not shown), and the more the life of these components and the hydraulic motor is reduced.

[0005] In applications, the viscosity of the hydraulic fluid can vary widely over a typical hydraulic fluid operating temperature range. For example, when the hydraulic system is used in a mobile engine application such as a heavy truck application, the temperature of the hydraulic fluid can vary from the ambient temperature at the start of the heavy truck, which can vary between -50 degrees Celsius (°C) and +50°C, for example, to the desired operating temperature of the hydraulic fluid, which is typically between 70°C and 80°C, but in other applications, the desired operating temperature range can vary between 50°C and 90°C. The kinematic viscosity of an exemplary hydraulic fluid used in some heavy truck applications is approximately 345 centistokes (cSt) at -25°C and 18 cSt at 75°C, i.e., the viscosity difference between possible hydraulic fluid operating temperatures exceeds an order of magnitude. During a cold start, the hydraulic fluid is recirculated and heated by the heat exchanger 90 shown in Figures 1, 2, and 3 to raise the temperature of the hydraulic fluid to its operating temperature and form a temperature-uniform hydraulic fluid mixture within the hydraulic system 10, 11. As described above, due to the entrapment of hydraulic fluid within the hydraulic motor 120 over multiple cycles of the piston 170 , it takes significantly longer to form a temperature uniform hydraulic fluid mixture in the hydraulic system 11 shown in FIGS. 2 and 3 .

[0006] The prior art lacks a technique for recirculating hydraulic fluid. The present apparatus and method provide a technique for recirculating hydraulic fluid. Summary of the invention

[0007] An improved device for recirculating hydraulic fluid in a hydraulic system. The hydraulic system may include a reservoir of hydraulic fluid, a hydraulic pump, a bypass valve, a fluid switching valve, a sub-circuit, and a controller. The hydraulic pump pressurizes hydraulic fluid from the hydraulic fluid reservoir. The sub-circuit includes a hydraulic motor, and a first hydraulic line and a second hydraulic line connecting the hydraulic motor to the fluid switching valve. The hydraulic fluid enters the sub-circuit through an inlet and leaves the sub-circuit through an outlet. The hydraulic motor includes a piston that divides a cylinder and is located between a first chamber and a second chamber. The first chamber is located between the piston and the first cylinder head and is connected to the first hydraulic line. The second chamber is located between the piston and the second cylinder head and is connected to the second hydraulic line. The piston can reciprocate between the first cylinder head and the second cylinder head. The piston bypass valve allows the hydraulic fluid to selectively communicate fluidically between the first chamber and the second chamber. The controller is operably connected to the fluid switching valve and is programmed to command a first actuation position and a second actuation position. In the first actuation position, the fluid switching valve supplies pressurized hydraulic fluid to the second chamber through the second hydraulic line. In the second actuation position, the fluid switching valve supplies pressurized hydraulic fluid to the first chamber through the first hydraulic line. The controller is operably connected to the bypass valve and is programmed to command the bypass valve to be in the bypass position, in which the pressurized hydraulic fluid bypasses the sub-circuit. The improved device includes a controller, which is programmed to: command the fluid switching valve to be in the first actuation position, and in the first actuation position, the piston bypass valve is closed, so that the pressurized hydraulic fluid moves the piston toward the first cylinder head; detect when the piston completes the first stroke and is adjacent to the first cylinder head; delay commanding (i) the fluid switching valve to be in the second actuation position, (ii) the bypass valve to be in the bypass position, or (iii) the hydraulic pump to stop pressurizing the hydraulic fluid, the piston bypass valve is open, so that the hydraulic fluid circulates through the inlet to the sub-circuit and flows through the piston bypass valve, wherein a certain amount of hydraulic fluid is flushed out of the sub-circuit through the outlet; and command (i) the fluid switching valve to be in the second actuation position, (ii) the bypass valve to be in the bypass position, or (iii) the hydraulic pump to stop pressurizing the hydraulic fluid.

[0008] The piston bypass valve may be a shuttle valve configured in the piston, wherein in the first actuation position, the pressurized hydraulic fluid closes the shuttle valve at the beginning of the first stroke, and wherein the shuttle valve opens when the piston completes the first stroke. Alternatively, the controller may be operably connected to the piston bypass valve, and the controller may be programmed to command the piston bypass valve to close at the beginning of the first stroke, and to command the piston bypass valve to open when the first stroke is completed, and the controller may also command the piston bypass valve to close when the controller commands the fluid switching valve to be in the second actuation position. In other embodiments, between the detection step and the delay step, the controller may also be programmed to stop the hydraulic pump, and then restart the hydraulic pump after a time interval, or, between the detection step and the delay step, the controller may also be programmed to command the bypass valve to be in the bypass position, and then command the bypass valve to be in the non-bypass position after a time interval.

[0009] In another embodiment, when the fluid switching valve is commanded to be in the second actuation position, the controller can also be programmed to: detect when the piston completes the second stroke and is adjacent to the second cylinder head; delay commanding (i) the fluid switching valve to be in the first actuation position, (ii) the bypass valve to be in the bypass position, or (iii) the hydraulic pump to stop pressurizing the hydraulic fluid, the piston bypass valve is open, so that the hydraulic fluid flows through the inlet to the sub-circuit and flows through the piston bypass valve, wherein a certain amount of hydraulic fluid is flushed out of the sub-circuit through the outlet; and commanding (i) the fluid switching valve to be in the first actuation position, (ii) the bypass valve to be in the bypass position, or (iii) the hydraulic pump to stop pressurizing the hydraulic fluid.

[0010] The bypass valve may be part of a fluid switching valve, whereby the fluid switching valve may be actuated to a first actuation position, a second actuation position and a bypass position. The amount of hydraulic fluid flushed from the sub-circuit may be greater than, equal to or less than the fluid volume of the sub-circuit. The amount of hydraulic fluid flushed from the sub-circuit may be the amount of hydraulic fluid flushed when the temperature of the hydraulic fluid is within a desired operating temperature range, and preferably, the desired operating temperature range is between 50 degrees Celsius and 90 degrees Celsius. The amount of hydraulic fluid flushed from the sub-circuit may be the amount of hydraulic fluid flushed after a flushing time interval. The amount of hydraulic fluid flushed from the sub-circuit may be the amount of hydraulic fluid flushed when the pressure of the hydraulic fluid drops below a desired value.

[0011] An improved method for recirculating hydraulic fluid in a hydraulic system, comprising: supplying pressurized hydraulic fluid to a second chamber and closing a piston bypass valve to move a piston toward a first cylinder head; detecting when the piston has completed a first stroke and is adjacent to the first cylinder head; delaying (i) supplying pressurized hydraulic fluid to the first chamber and closing the piston bypass valve to move the piston toward the second cylinder head, (ii) bypassing the pressurized hydraulic fluid around a subcircuit, or (iii) stopping a hydraulic pump that pressurizes hydraulic fluid through the piston bypass valve, the piston bypass valve being open so that hydraulic fluid flows through an inlet into the subcircuit and through the piston bypass valve, wherein a quantity of hydraulic fluid is flushed from the subcircuit through an outlet; and after flushing a quantity of hydraulic fluid from the subcircuit, performing the following steps: (i) supplying pressurized hydraulic fluid to the first chamber and closing the piston bypass valve to move the piston toward the second cylinder head, (ii) bypassing the pressurized hydraulic fluid around the subcircuit, or (iii) stopping the hydraulic pump from pressurizing the hydraulic fluid.

[0012] In further embodiments, between the detecting step and the delaying step, the method may include stopping the hydraulic pump and then, after a time interval, restarting the hydraulic pump, which pressurizes the hydraulic fluid and supplies the hydraulic fluid to the sub-circuit, or, between the detecting step and the delaying step, the method may include bypassing the hydraulic fluid around the sub-circuit and then, after a time interval, supplying the hydraulic fluid to the sub-circuit.

[0013] In yet other embodiments, when pressurized hydraulic fluid is supplied to the first chamber and the piston bypass valve is closed to move the piston toward the second cylinder head, the method may include: detecting when the piston has completed the second stroke and is adjacent to the second cylinder head; delaying (i) supplying pressurized hydraulic fluid to the second chamber and closing the piston bypass valve to move the piston toward the first cylinder head, (ii) stopping a hydraulic pump that pressurizes hydraulic fluid passing through the piston bypass valve, or (iii) bypassing the sub-circuit with the pressurized hydraulic fluid bypassing the sub-circuit, the piston bypass valve being open so that hydraulic fluid flows through the inlet into the sub-circuit and flows through the piston bypass valve, wherein a certain amount of hydraulic fluid is flushed out of the sub-circuit through the outlet; and after the certain amount of hydraulic fluid is flushed out of the sub-circuit, performing the following steps: (i) supplying pressurized hydraulic fluid to the second chamber and closing the piston bypass valve to move the piston toward the first cylinder head,

[0014] (ii) stopping the hydraulic pump from pressurizing the hydraulic fluid, or (iii) bypassing the pressurized hydraulic fluid through the sub-circuit.

[0015] The present invention is not limited to the above summary and includes other features disclosed in the embodiments in the written description of exemplary embodiments herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate specific embodiments of the apparatus, system and method and, together with the above general description and detailed description of the specific embodiments, serve to explain the principles of the apparatus, system and method.

[0017] FIG. 1 is a schematic diagram of a hydraulic system in the prior art.

[0018] FIG. 2 is a schematic diagram of another hydraulic system in the prior art, the hydraulic system being shown in a first actuated position.

[0019] 3 is a schematic diagram of the hydraulic system of FIG. 2 , the hydraulic system being shown in a second actuated position.

[0020] 4 is a schematic diagram of the hydraulic system of FIG. 2 , the hydraulic system being shown in a bypass position.

[0021] Figure 5 is a schematic diagram of a hydraulic system according to an embodiment, the hydraulic system being shown in a first actuated position.

[0022] Figure 6 yes Figure 5 Detailed view of a portion of a hydraulic system in which the shuttle valve is shown in a first closed position.

[0023] Figure 7 yes Figure 5 Detailed view of a portion of a hydraulic system in which the shuttle valve is shown in a first open position.

[0024] Figure 8 yes Figure 5 Schematic diagram of a hydraulic system of the embodiment of the present invention, which is shown in a second actuated position.

[0025] Fig. 9 yes Figure 8 Detailed view of a portion of a hydraulic system in which the shuttle valve is shown in a second closed position.

[0026] Fig.10 yes Figure 8 Detailed view of a portion of a hydraulic system in which the shuttle valve is shown in a second open position.

[0027] Fig.11 is a schematic diagram of a hydraulic system according to another embodiment, the hydraulic system being shown in a first actuated position.

[0028] Fig.12 yes Fig.11 Schematic diagram of a hydraulic system of the embodiment of the present invention, which is shown in a second actuated position.

[0029] Fig.13 According to an embodiment, a method for operating Figure 5 and Fig.11 Flowchart of the algorithm of the hydraulic system.

[0030] Fig.14 According to another embodiment, a method for operating Figure 5 and Fig.11 Flowchart of the algorithm of the hydraulic system.

[0031] Fig.15 According to yet another embodiment, a method for operating Figure 5 and Fig.11 Flowchart of the algorithm of the hydraulic system.

[0032] Fig.16 According to an embodiment, a method for operating Figure 8 and Fig.12 Flowchart of the algorithm of the hydraulic system. DETAILED DESCRIPTION

[0033] Reference Figure 5 and Figure 8 , shows a hydraulic system 12 according to an embodiment having similar components to the hydraulic systems 10 and 11, wherein in this and other embodiments, similar components have similar reference numerals and only the differences are discussed. The hydraulic motor 122 in the terminal hydraulic component 102 is a double-acting, reciprocating piston type hydraulic motor, which includes a piston bypass valve 200, which in the illustrated embodiment is a shuttle valve configured in the piston 172. As used herein, a double-acting, reciprocating piston type hydraulic motor means that the pressurized hydraulic fluid selectively acts in the chamber 160 to move the piston 172 toward the cylinder head 185, and selectively acts in the chamber 165 to move the piston 172 toward the cylinder head 180. The hydraulic motor 122 can be part of a reciprocating piston pump, wherein the piston rod 190 actuates a pumping piston (not shown) to reciprocate in a pumping cylinder (not shown). Examples of reciprocating piston pumps are cryogenic pumps for pressurizing cryogenic fluids, and gas compressors for pressurizing, for example, gaseous fuels, wherein the cryogenic pumps may include single-acting pistons or double-acting pistons. In exemplary embodiments, the cryogenic fluids and gaseous fuels may be hydrogen, methane, propane, natural gas, and mixtures thereof. The cryogenic pumps may also be used to pressurize propane that is stored in liquefied form but above cryogenic temperatures, which are generally defined as temperatures from -150°C to absolute zero (-273°C). Back Figure 5 and Figure 8, pressurized hydraulic fluid enters sub-circuit 152 through inlet 132, and hydraulic fluid leaves sub-circuit 152 through outlet 134. The fluid volume of sub-circuit 152 is at least the sum of the fluid volumes of hydraulic lines 150 and 155 and hydraulic motor 122. The fluid volume of hydraulic motor 122 is the sum of the fluid volumes of chambers 160 and 165 and cavity 220 minus the volume of valve member 210 ( Figure 6 and Fig. 9 152). As used herein, a sub-circuit is a portion of a hydraulic system. In other embodiments, the hydraulic system may include one or more sub-circuits fluidically connected in parallel or in series, wherein each sub-circuit may be similar or different than sub-circuit 152, and wherein each sub-circuit has the problem of trapped hydraulic fluid due to the reciprocating cycle. The function of piston bypass valve 200 is to selectively allow fluid flow through piston 172, which will be described in more detail below. Piston bypass valve 200 is provided in Figure 5 and Figure 6 is shown in a first closed position, and in Figure 7 1 is shown in a first open position, wherein the fluid switching valve 130 and the bypass valve 140 are shown in a first actuated position for the hydraulic motor 122. The piston bypass valve 200 is shown in a first open position. Figure 8 and Fig. 9 is shown in a second closed position, and in Fig.10 1 is shown in a second open position, wherein the fluid switching valve 130 and the bypass valve 140 are shown in a second actuated position for the hydraulic motor 122. Figure 7 In the first open position of , the volume of chamber 160 is shown as zero, but this is not a requirement and in other embodiments, chamber 160 may have a non-zero minimum volume, also referred to as residual volume, when piston 172 is at the end of its stroke adjacent cylinder head 180. Similarly, in Fig.10 In the second open position of the embodiment, the volume of chamber 165 is shown as zero, which is not a requirement and in other embodiments, chamber 165 may have a non-zero minimum volume or residual volume when piston 172 is at the end of its stroke adjacent cylinder head 185. In other embodiments, bypass valve 140 may be part of fluid switching valve 130 such that the fluid switching valve may be actuated to a first actuated position, a second actuated position, and a bypass position. Figure 6 to Figure 7 as well as Figures 9 and 10, the valve member 210 is movable between the valve seat 230 and the valve seat 240 in the valve cavity or valve bore 220. The valve cavity 220 includes an opening 250 in the valve seat 230 through which a protruding portion 260 of the valve member 210 can protrude into the chamber 160. Similarly, the valve cavity 220 includes an opening 270 in the valve seat 240 through which a protruding portion 280 of the valve member 210 can protrude into the chamber 165. The valve member 210 and the cavity 220 can be part of an assembly that includes a cylindrical body (not shown) in which the cavity is located, and wherein the assembly is inserted into a hole (not shown) in the piston 172. Alternatively, the piston 172 can be divided into a first portion and a second portion, wherein the second portion is connected to the first portion (longitudinally) to capture the valve member 210 in the cavity 220. The pressurized hydraulic fluid in chamber 165 (during the first actuated position) causes the sealing surface 290 ( Figure 7 ) against the valve seat 230 ( Figure 6 200) to form a fluid tight seal when the piston bypass valve 200 is in the first closed position, which greatly reduces and preferably prevents the flow of hydraulic fluid from chamber 165 through the piston bypass valve 200 into chamber 160. When the piston bypass valve 200 is in the first closed position, the protrusion 260 protrudes into the chamber 160. The pressurized hydraulic fluid in the chamber 160 (during the second actuated position) causes the sealing surface 295 ( Fig.10 ) against the valve seat 240 ( Fig. 9 ), thereby forming a fluid tight seal when piston bypass valve 200 is in the second closed position, which greatly reduces and preferably prevents the flow of hydraulic fluid from chamber 160 through piston bypass valve 200 into chamber 165. When piston bypass valve 200 is in the second closed position, projection 280 projects into chamber 165.

[0034] In operation, when the piston 172 is near the cylinder head 180 during the first actuation position, as shown in FIG. Figure 6 As shown, the piston bypass valve 200 is in the first closed position. Eventually, the protrusion 260 abuts against the cylinder head 180, and then the piston lifts the valve member 210 off the valve seat 230, thereby allowing the pressurized hydraulic fluid to pass through. Figure 7The piston bypass valve 200 shown in the first open position causes flow past the piston 172, which results in a drop in the hydraulic fluid pressure in the hydraulic line 50 as sensed by the pressure sensor 60. The drop in pressure is detected by the controller 112, which then actuates the end-use component 102 to switch to the second actuation position, in which the pressurized hydraulic fluid begins to enter the chamber 160 and begins to act on the valve member 210 to move toward the valve seat 240 until the piston bypass valve 200 is in the second closed position, at which point the pressurized hydraulic fluid moves the piston 172 toward the cylinder head 185. When the piston 172 is proximate to the cylinder head 185 during the second actuation position, as shown in FIG. Fig. 9 As shown, the piston bypass valve 200 is in the second closed position. Eventually, the protrusion 280 abuts against the cylinder head 185, and the piston then lifts the valve member 210 off the valve seat 240, thereby allowing the pressurized hydraulic fluid to pass through. Fig.10 The piston bypass valve 200 shown in the second open position flows past the piston 172, which causes a drop in hydraulic fluid pressure in the hydraulic line 50 as sensed by the pressure sensor 60. The controller 112 detects the drop in hydraulic fluid pressure and can actuate the end-use component 102 to switch to the first actuation position to begin repeating the cycle when this is the desired operation. This operation of the piston bypass valve 200 in the hydraulic motor 122 is described in more detail in the applicant's U.S. Patent No. 7,739,941 issued on June 22, 2010 and U.S. Patent No. 10,385,890 issued on August 20, 2019. In other embodiments, other techniques may be used to detect the end of the piston stroke that do not include detecting the drop in hydraulic fluid pressure after the piston bypass valve 200 is opened when the valve member 210 abuts the cylinder head 180 or cylinder head 185. For example, proximity sensors, linear displacement sensors, and linear position sensors may be employed to detect the position of piston 172. Alternatively or additionally, the volumetric flow rate of hydraulic fluid entering chambers 160 and 165 and the elapsed time may be employed to calculate the displacement of piston 172. The volumetric flow rate may be estimated based on the operation of hydraulic fluid pump 30, and alternatively or additionally, a volumetric flow sensor or mass flow sensor (as well as measurements of hydraulic fluid pressure and hydraulic fluid temperature) may be employed to determine the volumetric flow rate of the hydraulic fluid.

[0035] Now refer to Fig.11 and Fig.12, shows a hydraulic system 13 according to another embodiment, which includes a piston bypass valve 300 that can be actuated by the controller 113 to a blocking position or a piston bypass position. In an exemplary embodiment, the piston bypass valve 300 (or any other valve that can be actuated by the controller 112 or the controller 113 herein) is a solenoid valve that is electromagnetically actuated to move a valve member (not shown) that opens or closes the valve, and in other embodiments, other types of valves may be used, such as hydraulically actuated valves. In the blocking position, the valve 300 is in a closed position (i.e., the valve 300 is closed), and preferably, no hydraulic fluid can flow through the valve 300. In the piston bypass position, the valve 300 is in an open position (i.e., the valve 300 is open), which allows pressurized hydraulic fluid to flow between chamber 160 and chamber 165 in either direction depending on the pressure differential between the hydraulic fluid pressures in chamber 160 and chamber 165. As shown Fig.11 As shown, when the piston bypass valve 300 is in the piston bypass position and the fluid switching valve 130 is in the first actuated position, pressurized hydraulic fluid flows from chamber 165 to chamber 160. Fig.12 As shown, when the piston bypass valve 300 is in the piston bypass position and the fluid switching valve 130 is in the second actuation position, pressurized hydraulic fluid flows from chamber 160 to chamber 165. In operation, the controller 113 uses any conventional technique - such as the techniques discussed herein before - to detect the end of the piston stroke (in the first actuation position or the second actuation position) and commands the valve 300 to the open position to allow the hydraulic fluid to continue to flow, thereby flushing the hydraulic fluid from the sub-circuit 153 of the end-use hydraulic component 103. In the illustrated embodiment, the fluid volume of the sub-circuit 153 is the sum of the fluid volumes of the hydraulic line 150, the hydraulic line 155, the hydraulic line 303 and the hydraulic line 304, as well as the valve 300 and the hydraulic motor 123. Although the piston 170 of the hydraulic motor 123 does not include a shuttle valve, in other embodiments, instead of the piston 170, a shuttle valve may be used. Figure 5 The piston 172 of the shuttle valve 200 shown in FIG. 1 can actuate the valve 300 to increase the flow area between the chamber 160 and the chamber 165. Return to Fig.11 and Fig.12 , cylinder 176 includes port 301 in cylinder head 180 and port 302 in cylinder head 185, but in other embodiments, port 301 and port 302 may be located at other locations along cylinder 175 between cylinder head 180 and cylinder head 185. Hydraulic line 303 connects port 301 to a first side of valve 300, and hydraulic line 304 connects port 302 to a second side of valve 300.

[0036] Now refer to Fig.13, a flowchart is shown, which shows an algorithm 400 for operating the hydraulic system 12 or the hydraulic system 13 according to one embodiment. Figures 5 to 10 The algorithm 400 is described with reference to the hydraulic system 12 shown in FIG. In step 410, the controller 112 commands the fluid switching valve 130 and the bypass valve 140 to be in the Figure 5 When executing step 410, the piston 172 is generally adjacent to or near the cylinder head 185, and the piston bypass valve 200 is in the second open position (eg, Fig.10 155); however, this is not a requirement, and the piston 172 may be located at other positions within the cylinder 175, and the piston bypass valve 200 may be in the second closed position. When the hydraulic system 12 is in the first actuated position, if the piston bypass valve 200 is open, the pressurized hydraulic fluid from the hydraulic pump 30 through the hydraulic line 155 causes the piston bypass valve 200 ( Fig.10 ) is closed to a first closed position ( Figure 6 ), or the piston bypass valve 200 is moved from the second closed position ( Fig. 9 ) changes to a first closed position ( Figure 6 ). Figure 6 , with the piston bypass valve 200 closed, as the pressurized hydraulic fluid acts on the piston in chamber 165, the piston 172 begins to move toward the cylinder head 180. In step 420, the controller 112 detects when the piston 172 completes the first stroke and is adjacent to or near the cylinder head 180, whereby the piston bypass valve 200 automatically opens ( Figure 714. As shown in FIG. 14 ). Upon detecting that the first stroke is completed, in step 430, the controller 112 delays commanding the fluid switching valve 130 to be in the second actuation position, delays commanding the bypass valve 140 to be in the bypass position, or delays commanding the hydraulic pump 30 to stop pressurizing the hydraulic fluid, so that a certain amount of hydraulic fluid is flushed out of the sub-circuit 152 through the outlet 134. That is, the pressurized hydraulic fluid that was filtered by the filter 40 a short time ago circulates into the hydraulic line 155 through the inlet 132, thereby transferring the hydraulic fluid in the chamber 165 into the hydraulic line 150 through the piston bypass valve 200 and the chamber 160, thereby transferring the hydraulic fluid in the hydraulic line 150 into the hydraulic return line 55 through the outlet 134. The filtered hydraulic fluid from the hydraulic line 50 that enters the sub-circuit 152 transfers the old hydraulic fluid (i.e., the hydraulic fluid that was previously in the sub-circuit 152) from the sub-circuit 152 into the hydraulic return line 55. In step 440, after a certain amount of hydraulic fluid is flushed out of the sub-circuit 152, the controller 112 commands the fluid switching valve 130 to be in the second actuation position, commands the bypass valve 140 to be in the bypass position, or commands the hydraulic pump 30 to stop pressurizing the hydraulic fluid. The algorithm 400 may correspond to an extension stroke / compression stroke in a single-acting cryogenic pump, wherein the cryogenic fluid in the pump is pressurized.

[0037] Now refer to Fig.11 and Fig.13 , the algorithm 400 is now discussed with respect to the hydraulic system 13. In step 410, the controller 113 commands the fluid switching valve 130 and the bypass valve 140 to be in Fig.11180. When step 410 is performed, the piston 170 is typically adjacent to the cylinder head 185 and the piston bypass valve 300 is in an open position; however, this is not a requirement and the piston 170 may be located elsewhere within the cylinder 176 and the piston bypass valve may be in a closed position. In step 410, if the piston bypass valve 300 is not already in a closed position, the controller 113 commands the piston bypass valve 300 to close. With the piston bypass valve 300 closed, the piston 170 begins to move toward the cylinder head 180 as pressurized hydraulic fluid acts on the piston within chamber 165. In step 420, the controller 113 detects when the piston 170 completes the first stroke and is adjacent to or near the cylinder head 180 and commands the piston bypass valve 300 to open. Upon detecting the completion of the first stroke, in step 430, the controller 113 delays commanding the fluid switching valve 130 to be in the second actuation position, delays commanding the bypass valve 140 to be in the bypass position, or delays commanding the hydraulic pump 30 to stop pressurizing the hydraulic fluid, so that a certain amount of hydraulic fluid is flushed out of the sub-circuit 153 through the outlet 134. That is, the pressurized hydraulic fluid filtered by the filter 40 a short time ago circulates into the hydraulic line 155 through the inlet 132, thereby transferring the hydraulic fluid in the chamber 165 into the hydraulic line 150 through the piston bypass valve 300 and the chamber 160, thereby transferring the hydraulic fluid in the hydraulic line 150 into the hydraulic return line 55 through the outlet 134. The filtered hydraulic fluid from the hydraulic line 50 entering the sub-circuit 153 transfers the old hydraulic fluid (i.e., the hydraulic fluid previously in the sub-circuit 153) from the sub-circuit 153 into the hydraulic return line 55. In step 440, after a certain amount of hydraulic fluid is flushed out of the sub-circuit 153, the controller 113 commands the fluid switching valve 130 to be in the second actuation position, commands the bypass valve 140 to be in the bypass position, or commands the hydraulic pump 30 to stop pressurizing the hydraulic fluid. The algorithm 400 may correspond to an extension stroke / compression stroke in a single-acting cryogenic pump, wherein the cryogenic fluid in the pump is pressurized.

[0038] The amount of hydraulic fluid flushed from the sub-circuits 152, 153 may be greater than, equal to, or less than the fluid volume of the sub-circuits 152, 153. Alternatively or additionally, the amount of hydraulic fluid flushed from the sub-circuits 152, 153 may be the amount of hydraulic fluid flushed from the sub-circuits 152, 153 when the hydraulic fluid pressure drops below a desired value, or to produce a desired pressure drop in hydraulic fluid pressure that occurs during startup of the hydraulic system 12, 13. That is, during startup, the hydraulic fluid is more viscous due to its temperature (which may be equal to ambient temperature if the hydraulic system has been idle for a period of time) compared to the viscosity of the hydraulic fluid during operation, and therefore, significant back pressure may occur in the hydraulic system 12, 13 due to the viscosity increasing the hydraulic fluid pressure. After the system recirculates the hydraulic fluid for a period of time, the hydraulic fluid temperature increases, the viscosity decreases, the back pressure decreases, the flow increases, and this back pressure reduction may be detected. Still further, the amount of hydraulic fluid flushed from the sub-circuits 152, 153 may be the amount of hydraulic fluid flushed when the temperature of the hydraulic fluid is within a desired operating temperature range. In an exemplary embodiment, the desired operating temperature range is between 50 degrees Celsius and 90 degrees Celsius. Additionally, the amount of hydraulic fluid flushed from the sub-circuits 152, 153 may be the amount of hydraulic fluid flushed after a desired, delayed (in step 430) flushing time interval has elapsed; and the desired time interval may be empirically associated with the time required to complete the above disclosed flushing of a determined amount of hydraulic fluid from the sub-circuits 152, 153. Alternatively or additionally, the amount of hydraulic fluid flushed from the sub-circuits 152, 153 may be the amount of hydraulic fluid flushed after a desired level or amount of filtering of the hydraulic fluid in the hydraulic systems 12, 13 has occurred, such as by a desired volume or mass of hydraulic fluid flowing through the filter 40. When the hydraulic pump 30 is driven directly by the engine, the flow rate of the hydraulic fluid may be directly proportional to the engine speed, and in this case, the engine speed may be used to determine the time interval required to circulate a desired volume of hydraulic fluid.

[0039] Now refer to Fig.14 , shows an alternative flowchart, which shows a method for operating the hydraulic system 12 or the hydraulic system 13 (respectively at Figure 6 and Fig.11) and only the differences are discussed. Between step 420 (where the end of the piston stroke is detected) and step 430 (where there is a delay to allow hydraulic fluid to flow through the piston bypass valves 200, 300), in step 510, the hydraulic pump 30 is stopped, and then after a first time interval, in step 520, the hydraulic pump 30 is started again. The purpose of stopping and then starting the hydraulic pump 30 can be for various reasons, such as to deactivate the hydraulic motors 122, 123 for a first time interval, or when the hydraulic system 12, 13 is part of an internal combustion engine system (not shown), the stopping and then starting of the hydraulic pump 30 can be associated with an engine shutdown event and a subsequent engine start event. During the engine start event, the hydraulic fluid can be recirculated through the piston bypass valves 200, 300 to improve and speed up the heating and homogenization of the hydraulic fluid within the hydraulic system 12, 13.

[0040] Now refer to Fig.15 , shows an alternative flowchart, which shows a method for operating the hydraulic system 12 or the hydraulic system 13 (respectively at Figure 6 and Fig.11 ) and only the differences are discussed. Between step 420 (where the piston stroke end is detected) and step 430 (where there is a delay to allow hydraulic fluid to flow through the piston bypass valves 200, 300), in step 610, the bypass valve 140 is commanded to the bypass position, and then after the second time interval, in step 620, the bypass valve 140 and the fluid switching valve 130 are commanded to the first actuated position. The purpose of the pressurized hydraulic fluid bypassing the sub-circuits 152, 153 can be for various reasons, such as, in particular, to deactivate the hydraulic motors 122, 123 for the second time interval.

[0041] Now refer to Fig.16 , a flow chart is shown, which shows a method for operating the hydraulic system 12 or the hydraulic system 13 (respectively in Figure 8 and Fig.12 Compared with algorithm 400, algorithm 700 is a mirror image algorithm because algorithm 700 reflects the second stroke in the opposite direction of the first stroke. Figures 5 to 10 The hydraulic system 12 shown in FIG. 1 depicts the algorithm 700. In step 710, the controller 112 commands the fluid switching valve 130 and the bypass valve 140 to be in the Figure 8When step 710 is performed, piston 172 is generally adjacent to or near cylinder head 180, and piston bypass valve 200 is in the first open position; however, this is not a requirement, and piston 172 may be located at other positions within cylinder 175, and piston bypass valve 200 may be in the first closed position. In the second actuated position, if piston bypass valve 200 is open ( Fig.10 ), the pressurized hydraulic fluid from the hydraulic pump 30 through the hydraulic line 150 closes the piston bypass valve 200 to the second closed position, or closes the piston bypass valve 200 from Figure 5 The first closed position shown changes to Figure 8 With the piston bypass valve 200 closed, as the pressurized hydraulic fluid acts on the piston in the chamber 160, the piston 172 begins to move toward the cylinder head 185. In step 720, the controller 112 detects when the piston 172 completes the second stroke and is adjacent to or near the cylinder head 185, whereupon the piston bypass valve 200 opens to the second open position (eg, Fig.10 ). Upon detecting the completion of the second stroke, the controller 112 delays (in step 730) commanding the fluid switching valve 130 to be in the first actuation position, delays commanding the bypass valve 140 to be in the bypass position, or delays commanding the hydraulic pump 30 to stop pressurizing the hydraulic fluid, so that the second amount of hydraulic fluid is flushed out of the sub-circuit 152 through the outlet 134. That is, the pressurized hydraulic fluid that was filtered by the filter 40 a short time ago circulates into the hydraulic line 150 through the inlet 132, thereby transferring the hydraulic fluid in the chamber 160 to the hydraulic line 155 through the piston bypass valve 200 and the chamber 165, thereby transferring the hydraulic fluid in the hydraulic line 155 to the hydraulic return line 55 through the outlet 134. The filtered hydraulic fluid from the hydraulic line 50 entering the sub-circuit 152 transfers the old hydraulic fluid (i.e., the hydraulic fluid that was previously in the sub-circuit 152) from the sub-circuit 152 to the hydraulic return line 55. In step 740, after flushing the second amount of hydraulic fluid from the sub-circuit 152, the controller 112 commands the fluid switching valve 130 to the second actuation position, commands the bypass valve 140 to the bypass position, or commands the hydraulic pump 30 to stop pressurizing the hydraulic fluid. The algorithm 700 may correspond to a retraction stroke / intake stroke in a single-acting cryogenic pump, wherein cryogenic fluid flows into the cryogenic pump prior to pressurization.

[0042] Now refer to Fig.12 and Fig.16 , the algorithm 700 is now discussed with respect to the hydraulic system 13. In step 710, the controller 113 commands the fluid switching valve 130 and the bypass valve 140 to be in Fig.12185. When step 710 is performed, the piston 170 is typically adjacent to or near the cylinder head 180 and the piston bypass valve 300 is in the open position; however, this is not a requirement and the piston 170 may be located at other locations within the cylinder 176 and the piston bypass valve may be in the closed position. In step 710, if the piston bypass valve 300 is not already in the closed position, the controller 113 commands the piston bypass valve 300 to close. With the piston bypass valve 300 closed, the piston 170 begins to move toward the cylinder head 185 as the pressurized hydraulic fluid in the chamber 160 acts on the piston. In step 720, the controller 113 detects when the piston 170 has completed the second stroke and is adjacent to or near the cylinder head 185 and commands the piston bypass valve 300 to be in the open position. Upon detecting the completion of the second stroke, the controller 113 delays commanding the fluid switching valve 130 to be in the first actuation position, delays commanding the bypass valve 140 to be in the bypass position, or delays commanding the hydraulic pump 30 to stop pressurizing the hydraulic fluid, so that the second amount of hydraulic fluid is flushed out of the sub-circuit 153 through the outlet 134. That is, the pressurized hydraulic fluid that was filtered by the filter 40 a short time ago circulates into the hydraulic line 150 through the inlet 132, thereby transferring the hydraulic fluid in the chamber 160 into the hydraulic line 155 through the piston bypass valve 200 and the chamber 165, thereby transferring the hydraulic fluid in the hydraulic line 155 into the hydraulic return line 55 through the outlet 134. The filtered hydraulic fluid from the hydraulic line 50 entering the sub-circuit 153 transfers the old hydraulic fluid (i.e., the hydraulic fluid previously in the sub-circuit 153) from the sub-circuit 153 to the hydraulic return line 55. In step 740, after flushing the second amount of hydraulic fluid from the sub-circuit 153, the controller 113 commands the fluid switching valve 130 to the second actuation position, commands the bypass valve 140 to the bypass position, or commands the hydraulic pump 30 to stop pressurizing the hydraulic fluid. The algorithm 700 may correspond to a retraction stroke or a suction stroke in a single-acting cryogenic pump, wherein cryogenic fluid flows into the cryogenic pump prior to pressurization.

[0043] The second amount of hydraulic fluid may be Fig.13 Although the algorithm 400 and the algorithm 700 may be executed sequentially from a piston stroke to a subsequent piston stroke, there are usually two or more piston strokes between the execution of the algorithm 400 and the algorithm 700; that is, the two or more piston strokes do not involve the subcircuit 152, the subcircuit 153 (respectively in Figures 5 to 10 As shown in Figure 11 to Figure 12 Algorithm 700 may be adjusted similarly to algorithm 400 to include the steps shown in algorithm 500 or algorithm 600 in other embodiments.

[0044] The technique of clearing or flushing hydraulic fluid from sub-circuits 152, 153 according to algorithms 400, 500, 600 and 700 has a positive effect not only on the cleanliness of the hydraulic fluid in the sub-circuits, but also on the fluid viscosity and fluid temperature. The fluid volume in the sub-circuits 152, 153 and the instantaneous hydraulic fluid flow rate are known in the hydraulic system 12, 13, which allows the calculation of the duration for which the fluid flow must be maintained to replace the entire fluid volume. In internal combustion engine applications, the instantaneous hydraulic fluid flow rate can be a function of the engine speed. The duration of free flow required to clear the "used" or "dirty" hydraulic fluid (i.e., old hydraulic fluid) in the sub-circuits 152, 153 can be adjusted according to the engine speed (i.e., hydraulic flow rate) to reduce parasitic loads. A shorter duration is required at higher flow rates. The timing of entering the cleaning mode is any time when a low probability of a pump stroke can be predicted in the future long enough to be able to complete the fluid replacement. In addition to keeping the sub-circuits 152 , 153 clean, there may be other reasons to enter the cleaning mode, such as as part of a warm-up procedure after a cold start, or perhaps during a high load drive cycle to mitigate local heating effects.

[0045] While particular elements, embodiments, and applications of the present invention have been shown and described, it should be understood that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the above teachings, without departing from the scope of the present disclosure.

Claims

1. A device for recirculating hydraulic fluid in a hydraulic system, the hydraulic system comprising a hydraulic pump, a bypass valve, a sub-circuit and a controller, the hydraulic pump pressurizing the hydraulic fluid to circulate through the hydraulic system, the sub-circuit comprising a fluid switching valve, a piston bypass valve, a cylinder and a piston, the piston reciprocating in the cylinder between a first cylinder head and a second cylinder head, the piston dividing the cylinder between a first chamber adjacent to the first cylinder head and a second chamber adjacent to the second cylinder head, a first hydraulic line and a second hydraulic line connecting the first chamber and the second chamber to the fluid switching valve, respectively, the hydraulic fluid entering the sub-circuit through an inlet on the fluid switching valve and leaving the sub-circuit through an outlet on the fluid switching valve, the piston bypass valve selectively allowing the first chamber and the second chamber to be reciprocated. The first chamber and the second chamber are fluidly connected, the controller is operably connected to the fluid switching valve, and the controller is programmed to command a first actuation position and a second actuation position, in which the inlet is fluidly connected to the second chamber through the second hydraulic line and the outlet is fluidly connected to the first chamber through the first hydraulic line, and in the second actuation position, the inlet is fluidly connected to the first chamber through the first hydraulic line and the outlet is fluidly connected to the second chamber through the second hydraulic line, the controller is operably connected to the piston bypass valve, and the controller is programmed to command the piston bypass valve to be in a bypass position, in which the pressurized hydraulic fluid bypasses the sub-circuit, the device includes the controller, and the controller is programmed to: commanding the fluid switching valve to the first actuated position with the piston bypass valve closed so that pressurized hydraulic fluid moves the piston toward the first cylinder head; detecting when the piston has completed a first stroke and is adjacent to the first cylinder head; Delay Command: The fluid switching valve is in the second actuation position, The piston bypass valve is in the bypass position, or The hydraulic pump stops pressurizing the hydraulic fluid, The piston bypass valve is open, allowing hydraulic fluid to flow through the inlet into the sub-circuit and through the piston bypass valve, wherein, flushing a quantity of hydraulic fluid from the sub-circuit through the outlet; and Order: The fluid switching valve is in the second actuation position, The piston bypass valve is in the bypass position, or The hydraulic pump stops pressurizing the hydraulic fluid.

2. The device according to claim 1, wherein: The piston bypass valve is a shuttle valve disposed in the piston, and in the first actuated position, the pressurized hydraulic fluid closes the shuttle valve at the beginning of the first stroke, and the shuttle valve opens when the piston completes the first stroke.

3. The device according to claim 1, wherein: The controller is operably connected to the piston bypass valve, and the controller is programmed to command the piston bypass valve to close at the beginning of the first stroke and to command the piston bypass valve to open at the completion of the first stroke.

4. The device according to claim 1, wherein: When the controller commands the fluid switching valve to be in the second actuated position, the controller is further programmed to also command the piston bypass valve to close.

5. The device according to claim 1, wherein: Between the detecting step and the delaying step, the controller is further programmed to stop the hydraulic pump and then restart the hydraulic pump after a time interval.

6. The device according to claim 1, wherein: The controller is further programmed to command the piston bypass valve to the bypass position between the detecting step and the delaying step, and then, after a time interval, command the piston bypass valve to the non-bypass position.

7. The device according to claim 1, wherein: When the fluid switching valve is commanded to the second actuated position and the piston bypass valve is closed, the controller is further programmed to: detecting when the piston has completed a second stroke and is adjacent to the second cylinder head; Delay Command: The fluid switching valve is in the first actuation position, The piston bypass valve is in the bypass position, or The hydraulic pump stops pressurizing the hydraulic fluid, The piston bypass valve is open so that hydraulic fluid circulates into the sub-circuit through the inlet and flows through the piston bypass valve, wherein a certain amount of hydraulic fluid is flushed out of the sub-circuit through the outlet; and Order: The fluid switching valve is in the first actuation position, The piston bypass valve is in the bypass position, or The hydraulic pump stops pressurizing the hydraulic fluid.

8. The device according to claim 1, wherein: The piston bypass valve is a part of the fluid switching valve, whereby the fluid switching valve is actuatable to the first actuation position, the second actuation position and the bypass position.

9. The device according to claim 1, wherein: The amount of hydraulic fluid flushed from the sub-circuit is greater than, equal to, or less than a fluid volume of the sub-circuit.

10. The device according to claim 1, wherein: The amount of hydraulic fluid flushed from the sub-circuit is the amount of hydraulic fluid flushed when the temperature of the hydraulic fluid is within a desired operating temperature range.

11. The device according to claim 1 or 10, wherein: The expected operating temperature range is between 50 degrees Celsius and 90 degrees Celsius.

12. The device according to claim 1, wherein: The amount of hydraulic fluid flushed out of the sub-circuit is the amount of hydraulic fluid flushed out after a flushing time interval.

13. The device according to claim 1, wherein: The amount of hydraulic fluid flushed out of the sub-circuit is the amount of hydraulic fluid flushed out when the pressure of the hydraulic fluid drops below a desired value.

14. A method for recirculating hydraulic fluid in a hydraulic system, the hydraulic system comprising a hydraulic pump, a piston bypass valve, a sub-circuit and a controller, the hydraulic pump pressurizing the hydraulic fluid to circulate through the hydraulic system, the sub-circuit comprising a fluid switching valve, a piston bypass valve, a cylinder and a piston, the piston reciprocating in the cylinder between a first cylinder head and a second cylinder head, the piston dividing the cylinder between a first chamber adjacent to the first cylinder head and a second chamber adjacent to the second cylinder head, a first hydraulic line and a second hydraulic line connecting the first chamber and the second chamber to the fluid switching valve, respectively, the hydraulic fluid entering the sub-circuit through an inlet on the fluid switching valve and leaving the sub-circuit through an outlet on the fluid switching valve, the piston bypass valve a valve selectively fluidly connecting the first chamber and the second chamber, the controller being operably connected to the fluid switching valve and being programmed to command a first actuation position and a second actuation position, in which the inlet is fluidly connected to the second chamber through the second hydraulic line and the outlet is fluidly connected to the first chamber through the first hydraulic line, and in which the inlet is fluidly connected to the first chamber through the first hydraulic line and the outlet is fluidly connected to the second chamber through the second hydraulic line, the controller being operably connected to the piston bypass valve and being programmed to command the piston bypass valve to a bypass position in which pressurized hydraulic fluid bypasses the sub-circuit, the method comprising: supplying pressurized hydraulic fluid to the second chamber and closing the piston bypass valve to move the piston toward the first cylinder head; detecting when the piston has completed a first stroke and is adjacent to the first cylinder head; Delay: supplying pressurized hydraulic fluid to the first chamber and closing the piston bypass valve to move the piston toward the second cylinder head, causing the pressurized hydraulic fluid to bypass the sub-circuit, or stopping the hydraulic pump that pressurizes hydraulic fluid passing through the piston bypass valve, The piston bypass valve is open so that hydraulic fluid circulates into the sub-circuit through the inlet and flows through the piston bypass valve, wherein a certain amount of hydraulic fluid is flushed out of the sub-circuit through the outlet; and After flushing the amount of hydraulic fluid from the sub-circuit, the following steps are performed: supplying pressurized hydraulic fluid to the first chamber and closing the piston bypass valve to move the piston toward the second cylinder head, causing the pressurized hydraulic fluid to bypass the sub-circuit, or The hydraulic pump is stopped to pressurize the hydraulic fluid.

15. The method according to claim 14, wherein: Between the detecting step and the delaying step, the method further comprises stopping a hydraulic pump and then, after a time interval, restarting the hydraulic pump, the hydraulic pump pressurizing the hydraulic fluid and supplying the hydraulic fluid to the sub-circuit.

16. The method according to claim 14, wherein: Between the detecting step and the delaying step, the method further includes bypassing the sub-circuit with hydraulic fluid and then, after a time interval, supplying hydraulic fluid to the sub-circuit.

17. The method of recirculating hydraulic fluid according to claim 14, wherein: When pressurized hydraulic fluid is supplied to the first chamber and the piston bypass valve is closed to move the piston toward the second cylinder head: detecting when the piston has completed a second stroke and is adjacent to the second cylinder head, opening the piston bypass valve so that pressurized hydraulic fluid flows through the piston bypass valve; Delay: supplying pressurized hydraulic fluid to the second chamber and closing the piston bypass valve to move the piston toward the first cylinder head, causing the pressurized hydraulic fluid to bypass the sub-circuit, or stopping the hydraulic pump that pressurizes hydraulic fluid passing through the piston bypass valve, The piston bypass valve is open so that hydraulic fluid circulates into the sub-circuit through the inlet and flows through the piston bypass valve, wherein a certain amount of hydraulic fluid is flushed out of the sub-circuit through the outlet; and After flushing the amount of hydraulic fluid from the sub-circuit, the following steps are performed: supplying pressurized hydraulic fluid to the second chamber and closing the piston bypass valve to move the piston toward the first cylinder head, causing the pressurized hydraulic fluid to bypass the sub-circuit, or The hydraulic pump is stopped to pressurize the hydraulic fluid.

18. The method according to claim 14, wherein: The amount of hydraulic fluid flushed from the sub-circuit is greater than, equal to, or less than a fluid volume of the sub-circuit.

19. The method according to claim 14, wherein: The amount of hydraulic fluid flushed out of the sub-circuit is the amount of hydraulic fluid flushed out when the temperature of the hydraulic fluid is within a desired operating temperature range, or when the pressure of the hydraulic fluid drops below a desired value.

20. The method according to claim 14, wherein: The amount of hydraulic fluid flushed out of the sub-circuit is the amount of hydraulic fluid flushed out after a flushing time interval.

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