Abstract:
A pressure regulator for a power source may include a housing in fluid communication with a pressurized fluid line and a reservoir, and a pressure relief valve having an open position that creates a flow path between the pressurized fluid line and the reservoir, and a closed position. The pressure relief valve may open when an opening force exerted by pressurized fluid entering the housing overcomes a closing force acting on the pressure relief valve, allowing pressurized fluid to flow from the pressurized fluid line to the reservoir. The pressure regulator may also include a governor coupled to the pressure relief valve, wherein the governor is configured to regulate the pressure in the pressurized fluid line by selectively adjusting the closing force exerted on the pressure relief valve based on engine speed.

Description:
TECHNICAL FIELD 
     The present disclosure relates to a pressure regulator, and, more particularly, to an engine speed sensitive oil pressure regulator for an internal combustion engine. 
     BACKGROUND 
     Internal combustion engines such as, for example, gasoline engines, diesel engines, and gaseous fuel powered engines, contain internal moving parts that rely on a pressurized fluid for proper lubrication. The fluid is often a high-viscosity oil, which is introduced between moving parts to create a thin, protective layer of oil, allowing the parts to be separated, thereby reducing friction and wear. A pump draws the fluid from a reservoir and pressurizes it, causing it to flow through passageways in the engine to moving parts that require lubrication. The pressure of the fluid is regulated by a pressure relief valve consisting of a spring-loaded spool, which relieves the fluid pressure by moving the spool against the spring to create a relief passage for the fluid when the pressure reaches a predetermined level. 
     For a rotating part of an engine that transports oil from its surface to its central axis of rotation, such as, for example, a crank journal that receives oil from an engine block and transports the oil to one or more rod journals, the oil pressure required for proper lubrication increases as the speed of the engine increases. That is, as the rotating speed of moving parts in the engine increases, the pressure required to effectively lubricate moving parts by creating a thin layer of oil between the parts increases. The increase allows the oil to overcome its own inertia to make its way to the center of a crankshaft of the engine. It may be beneficial to operate an engine with the lowest effective oil pressure, to reduce inefficiencies due to pumping losses, and life-reducing heat-cycling of the oil that occurs when dumping high pressure oil to the reservoir through the relief valve. To achieve the lowest effective pressure, the maximum allowable pressure is set to correspond to the required pressure at the maximum operating speed of the engine. Setting the maximum pressure entails adjusting a pre-load force of the spring against the spool in the relief valve. 
     Due to the location of the pressure regulator on the engine, manually adjusting the maximum allowable oil pressure during operation may not be practical. Though the maximum allowable pressure is generally set to correspond to the pressure required at the maximum operating speed of the engine, modern engines often operate in overspeed conditions that may exceed the maximum engine speed. An overspeed condition of an engine is common on engines that are used in mechanical drive applications. An overspeed condition may result from an operation such as engine braking on a steep grade. Because the engine speed during an overspeed condition may be much higher than at the maximum engine operating speed, the maximum allowable pressure setting of the pressure regulator may not be high enough to provide sufficient lubrication at overspeed conditions, if the maximum pressure was set to correspond to the maximum pressure required at a lower operating speed. 
     One attempt to vary the oil pressure with the engine speed is described in U.S. Pat. No. 6,488,479 B1 (the &#39;479 patent), issued to Berger on Dec. 3, 2002. The &#39;479 patent discloses a system that includes a variable pressure oil pump. The system also includes a controller (ECU) and various sensors, including an oil pressure sensor, an oil temperature sensor, an engine load sensor, an engine speed sensor, a coolant temperature sensor, and an oil viscosity sensor. The oil pump includes an adjustable pressure regulator that uses a solenoid to move a plunger to selectively allow passage of oil through a bypass when the pressure of the oil is too high. The ECU moves the solenoid to regulate the oil pressure based on inputs from the various sensors, and the ECU may allow a higher maximum oil pressure as the engine speed increases. 
     Although the system disclosed in the &#39;479 patent may allow a higher maximum oil pressure as engine speeds increase, it may be complex and costly. Specifically, the &#39;479 system requires not only a mechanism to vary the oil pressure, it also requires an ECU and multiple sensors. The additional components increase the control difficulty and expense of the system. The additional components also preclude the retrofit of an oil pressure regulation system on engines that do not include an ECU. 
     The disclosed pressure regulator is directed to overcoming one or more of the problems set forth above. 
     SUMMARY OF THE DISCLOSURE 
     In one aspect, the presently disclosed embodiments may be directed to a pressure regulator for a power source. The pressure regulator may include a housing in fluid communication with a pressurized fluid line and a reservoir. The pressure regulator may also include a pressure relief valve in the housing having an open position that creates a flow path between the pressurized fluid line and the reservoir, and a closed position. The pressure relief valve may be configured to open when an opening force exerted on the pressure relief valve by pressurized fluid entering the housing overcomes a closing force acting on the pressure relief valve, allowing pressurized fluid to flow from the pressurized fluid line to the reservoir to reduce pressure in the pressurized fluid line. The pressure regulator may further include a governor coupled to the pressure relief valve. The governor may be configured to regulate the pressure in the pressurized fluid line by selectively adjusting the closing force exerted on the pressure relief valve based on engine speed. 
     In another aspect, the presently disclosed embodiments may be directed to a method of regulating pressure in a pressurized fluid line of an engine assembly. The method may include coupling a pressure relief valve to the pressurized fluid line. The pressure relief valve may have an open position for reducing pressure in the pressurized fuel line, and a closed position. The method may also include exerting a closing force on the pressure relief valve using a biasing mechanism. The method may further include exerting an opening force on the pressure relief valve using the pressure in the pressurized fluid line. The pressure relief valve may open when the opening force exceeds the closing force. The method may further include regulating the pressure in the pressurized fluid line by selectively adjusting with a governor the amount of opening force required to open the pressure relief valve based on engine speed. 
     In yet another aspect, the presently disclosed embodiments may be directed to a power system including an engine assembly. The power system may also include a pressure regulator for the engine assembly. The pressure regulator may include a housing having a first opening fluidly coupled to a pressurized fluid line, a second opening fluidly coupled to the pressurized fluid line, and a third opening fluidly coupled to a reservoir. The pressure regulator may also include a pressure relief valve in the housing having an open position that creates a flow path between the first opening and the third opening, and a closed position. The pressure relief valve may be configured to open when an opening force exerted on the pressure relief valve by pressurized fluid entering the second opening overcomes a closing force acting on the pressure relief valve, allowing pressurized fluid to flow from the pressurized fluid line to the reservoir to reduce pressure in the pressurized fluid line. The pressure regulator may also include a governor coupled to the pressure relief valve. The governor may be configured to regulate the pressure in the pressurized fluid line by selectively adjusting the closing force exerted on the pressure relief valve based on engine speed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an exemplary disclosed power system; and 
         FIG. 2  is an illustration of a pressure regulator for use with the exemplary disclosed power system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary embodiment of a power source  10 . For the purposes of this disclosure, power source  10  is depicted and described as a four-stroke engine. One skilled in the art will recognize that power source  10  may embody any type of internal combustion engine such as, for example, a heavy fuel engine, a diesel engine, a gasoline engine, a gaseous fuel-powered engine, or any other suitable engine. Power source  10  may be configured to cooperate with a drive train (not shown) to provide motive power to a machine or vehicle (not shown). Power source  10  may further be configured to cooperate with the drive train to provide compression braking to slow the machine or vehicle. Power source  10  may have maximum operating speed, or redline speed, which is the maximum sustained speed at which power source  10  may safely and reliably operate to provide power or compression braking. Power source  10  may also be capable of operating in a overspeed condition for short periods of time. An overspeed condition may be a speed in excess of the maximum speed, and it may provide for extra motive power or extra compression braking as needed. Power source  10  may include an engine block  12  that defines a plurality of cylinders  14 , and a lubrication system  16  that provides a pressurized lubricating fluid to engine block  12 . 
     A piston  18  and a cylinder head  20  may be associated with each cylinder  14  to form a combustion chamber  22 . Specifically, piston  18  may be slidably disposed within each cylinder  14  to reciprocate between a top-dead-center position and a bottom-dead-center position. Cylinder head  20  may be positioned to cap off an end of cylinder  14 , the space within cylinder  14  between piston  18  and cylinder head  20  being the combustion chamber  22 . In the illustrated embodiment, power source  10  includes six combustion chambers  22 . However, it is contemplated that power source  10  may include a greater or lesser number of combustion chambers  22  and that combustion chambers  22  may be disposed in an “in-line” configuration, a “V” configuration, or in any other suitable configuration. 
     Power source  10  may also include a crankshaft  24  rotatably disposed within engine block  12 . A connecting rod  26  may connect each piston  18  to crankshaft  24  so that a sliding motion of piston  18  between the top-dead-center and bottom-dead-center positions within each respective cylinder  14  results in a rotation of crankshaft  24 . Similarly, a rotation of crankshaft  24  may result in a sliding motion of piston  18  between the top-dead-center and bottom-dead-center positions. Main gear  28  may be attached to crankshaft  24  on the outside of the engine block  12 . Main gear  28  may be used to drive auxiliary gear  30  or any other gear of a gear train (not shown), which in turn may drive a variety of auxiliary systems and components (not shown). Power source  10  may contain other rotating and reciprocating components known in the art that require and receive lubrication from a source of pressurized fluid. 
     Crankshaft  24  may be supported by and rotate within one or more journal bearings  32 . The journal bearings  32  may rely on hydrodynamic effects to create a thin layer of lubricating fluid between crankshaft  24  and journal bearings  32 . The thin layer separates crankshaft  24  and journal bearing  32 , preventing the two from coming into contact, and allowing crankshaft  24  to rotate freely within journal bearings  32 . In cases where lubricating fluid may be transported from the surface of crankshaft  24  to the central axis of rotation of crankshaft  24 , as the rotational speed of crankshaft  24  increases, the pressure of the lubricating fluid may also increase so that the lubricating fluid may be capable of overcoming its own inertia and make its way to the center of crankshaft  24 . In cases where the lubricating fluid may be transported from one or more lubricating fluid passages (not shown) within crankshaft  24  to a lubricating surface of crankshaft  24 , increased lubricating fluid pressure may serve the purpose of helping the lubricating fluid overcome internal inertia force in the one or more lubricating fluid feed passages, thus allowing the lubricating fluid to make its way toward the lubricating surface of crankshaft  24 . 
     Lubrication system  16  may contain a pump  34 , a pressure regulator  36 , a reservoir or sump  38 , and passageway or pressurized fluid line  40 . Lubrication system  16  may provide a source of pressurized fluid to power source  10  through passageway  40 , to lubricate reciprocating and rotating components, and to prevent metal-on-metal contact resulting from the motion thereof. Passageway  40  may connect the various components of lubrication system  16 , and provide pressurized fluid to internal passages (not shown) of crankshaft  24 , for lubrication of crankshaft  24  and journal bearings  32 . Lubrication system  16  may contain other components commonly known in the art, such as, for example, a filter (not shown) configured to remove particles from the lubricating fluid. 
     Pump  34  may be driven by main gear  28  or any other gear of the power source  10  gear train, to provide pressurized lubricating fluid to power source  10  through passageway  40 . Pump  34  may be a gear pump, a piston type pump, an impeller type pump, or any other type of pump known in the art. Pump  34  may draw fluid from sump  38 , pressurize the fluid, and discharge the pressurized fluid into passageway  40 . Pump  34  may repeat this cycle, continuously pumping lubricating fluid through passageway  40  during the operation of power source  10 . 
     Pressure regulator  36  may relieve the pressure of the fluid in passageway  40  when the pressure exceeds a predetermined value. Pressure regulator  36  may be a spring-loaded spool-type valve as described in further detail below. Pressure regulator  36  may be located in passageway  40  downstream of pump  34 , and may limit the maximum pressure of lubricating fluid entering engine block  12 . 
     Sump  38  may be any metallic or polymeric chamber known in the art for holding a lubrication fluid. For example, sump  38  may be located at a bottom portion of power source  10 , and function as a reservoir for lubrication system  16 . Lubrication system may both begin and end at sump  38 . Lubrication fluid may be drawn from sump  38  at a beginning of lubrication system  16 , and trickle down through engine block  12  under gravity to be collected in sump  38  at an end of lubrication system  16 . 
       FIG. 2  illustrates an exemplary embodiment of pressure regulator  36  for use with power source  10 . Pressure regulator  36  may include a pressure relief valve  42  and a governor  44 . Valve  42  may be arranged to receive pressurized fluid from pump  34  at inlet  52 , and to direct fluid to sump  38  through outlet  54  when the pressure of the fluid exceeds a predetermined level. Governor  44  may be configured to adjust the predetermined pressure at which valve  42  will direct fluid to sump  38 , based on a rotational speed of power source  10 . 
     Valve  42  may include a housing  46 , a biasing mechanism such as spring  48 , and a spool  50 . Housing  46  may include a cover  47 , and housing  46  may enclose both valve  42  and governor  44 . Housing  46  may be situated adjacent pump  34  and/or engine block  12 , and may include inlet  52  connected to passageway  40 , and outlet  54  connected to sump  38 . Housing  46  may also include a pressure port  56  connected to passageway  40 , and drain  57  connected to sump  38 . Pressure port  56  may communicate to spool  50  a pressure of the fluid in passageway  40 . Drain  57  may drain to sump  38  fluid that leaks past spool  50 . Housing  46  may have a circular internal chamber within which spool  50  may be situated. Housing  46  may allow valve  42  to relieve the pressure of fluid pumped by pump  34 . Valve  42  may also include additional elements, such as seals (not shown), to prevent fluid from leaking from housing  46 , and/or into the area of housing  46  that contains governor  44 . Alternatively, leakage past spool  50  may be controlled by tight tolerances between an inner diameter of housing  46 , and an outer diameter of spool  50 . Such tolerances may be on the order of, for example, one thousandth of an inch. 
     Spool  50  may be an elongated, cylindrical element located and configured for movement within housing  46 , and may contain at one end a recess  51  for aligning spring  48 , and a groove  58  to selectively allow fluid flow from inlet  52  to outlet  54 . Groove  58  may be, for example, an annulus or any other suitably shaped groove known in the art. As the pressure of the fluid at pressure port  56  increases, the pressure will impart a force on an end face of spool  50 , moving spool  50  toward spring  48 . When spool  50  moves a predetermined distance toward spring  48 , groove  58  may connect inlet  52  and outlet  54 , thereby allowing fluid flow from inlet  52  to outlet  54 , and on to sump  38 . Fluid flow from inlet  52  to outlet  54  may result in a decrease in pressure of the fluid in passageway  40 . When the pressure of the fluid at pressure port  56  decreases, the force on the end face of spool  50  may decrease, causing spring  48  to move spool  50  such that groove  58  not longer provides a flow path from inlet  52  to outlet  54 . The outer diameter of spool  50  may be closely matched to an inner diameter of housing  46 , leaving only a small clearance that prevents most fluid flow past spool  50  into the cavity housing governor  44 , with minimal fluid leakage. 
     Spring  48  may be a compression spring, or any other similar type of elastic element known in the art that behaves as a linear spring. Spring  48  may be disposed axially with spool  50  in recess  51  to provide a varying resistance to the movement of spool  50  within housing  46 . The varying resistance of spring  48  may result from a change in length of spring  48 . Spring  48  may have a nearly linear spring rate, such that as spring  48  is compressed, the force required for additional compression of spring  48  increases linearly. 
     The linear forces on spool  50  may be balanced. That is, the force on spool  50  resultant from the pressure of fluid at pressure port  56 , i.e. the pressure force, may be balanced by the force from spring  48 , i.e. the spring force. As the pressure force increases, spool  50  may move toward spring  48 , causing spring  48  to compress, thereby causing the spring force to increase to balance the pressure force. Though the forces on spool  50  may be balanced, the location of spool  50  within housing  46  may change depending upon the magnitude of the forces. The amount spool  50  moves may be controlled by the characteristics of spring  48 , such as the initial length and the spring rate of spring  48 . One may determine, based on the spring characteristics and the required spring force to balance the pressure force, the amount of compression of spring  48  that may occur when the forces are balanced. In this way, one may predict the movement of spool  50  within housing  46 , and, consequently, the pressure force needed to move spool  50  the predetermined distance to cause groove  58  to create a passageway for flow from inlet  52  to outlet  54 . 
     Governor  44  may be located in housing  46 , adjacent valve  42 , and may be configured to adjust the length of spring  48  by applying a force to spring  48 . Governor  44  may include a shaft  60 , a shaft bearing  62 , a drive gear  64 , a clevis  66 , two or more flyweights  68 , and a bearing assembly including a thrust bearing  70 , a plain bearing  71 , a first plate  72  located on the clevis side of thrust bearing  70 , and a second plate  74  located on the spring side of thrust bearing  70 . 
     Shaft  60  may be positioned within housing  46  by shaft bearing  62  pressed into cover  47 . Shaft  60  may extend through spring  48 , while stopping short of spool  50 . The overall length of shaft  60  may be such that spool  50  may move within housing  46  to create a flow path from inlet  52 , through groove  58 , to outlet  54 , without interference from shaft  60 . Shaft  60  may be of sufficient length to allow plain bearing  71  sufficient movement to compress spring  48 . Shaft  60  may alternatively or additionally be of sufficient length to act as a hard stop for spool  50 , by preventing movement of spool  50  beyond a predetermined distance. 
     Shaft bearing  62  may be located in cover  47  and may support shaft  60 . Shaft bearing  62  may be a plain or journal bearing commonly known in the art that allows shaft  60  to rotate. Drive gear  64  may be arranged on shaft  60 , and disposed adjacent to auxiliary gear  30  such that auxiliary gear  30  and drive gear  64  drivingly mesh to cause a rotation of shaft  60  with a rotational speed proportional to the rotational speed of crankshaft  24 . One having ordinary skill in the art will recognize that drive gear  64  may alternatively include any other means commonly known in the art for transferring rotational motion from crankshaft  24  to shaft  60 . 
     Clevis  66  may be fixedly attached to shaft  60  to rotate with shaft  60 , and may support two or more flyweights  68 , arranged in radial symmetry around shaft  60 . Clevis  66  may be located on shaft  60  such that it touches first plate  72  when in a static condition, that is, when shaft  60  is not rotating and flyweights  68  are not pivoted due to centrifugal force. In this manner, clevis  66  may act as a positive stop for spring  48  and spool  50 . 
     Flyweights  68  may be pivotally attached to clevis  66 . Flyweights  68  may be shaped and arranged such that as the rotational speed of shaft  60  increases, flyweights  68  pivot, and the distance between a center of gravity of each flyweight  68  and the axis of shaft  60  increases. This increasing distance may be due to an increased centrifugal force on flyweights  68  as the rotational speed of shaft  60  increases. As the distance increases, flyweights  68  may be configured to apply to first plate  72  a linear force proportional to the centrifugal force, thereby causing a linear movement of thrust bearing  70 , and a compression of spring  48 . Flyweights  68  may be arranged such that they do not apply a force to first plate  72  until a predetermined rotational speed of shaft  60  is attained. This predetermined rotational speed may be adjusted by controlling the distance between clevis  66  and thrust bearing  70 . The predetermined rotational speed may also or alternatively be adjusted through selective design of the geometry and mass of flyweights  68 , and/or a selection of spring characteristics of spring  48 . 
     Plain bearing  71  may fit closely on the diameter of shaft  60 , and allow for low-friction axial movement along shaft  60 , and low-friction rotation against shaft  60 . The length of plain bearing  71  may be long enough to prevent binding of plain bearing  71  on shaft  60  during axial movement. Thrust bearing  70  may be pressed tightly onto plain bearing  71  and be disposed between clevis  66  and spring  48 . The fit between thrust bearing  70  and plain bearing  71  may be such that a first half of thrust bearing  70  facing plate  74  is held stationary and prevented from rotation relative to plain bearing  71 . Thrust bearing  70  may be a commonly known thrust bearing. First plate  72  and second plate  74  may be located on either side of thrust bearing  70 , and may provide a support surface against thrust bearing  70  for flyweights  68  and spring  48 , respectively. First plate  72  may be pressed tightly against the rotating second half of thrust bearing  70  by flyweights  68  when in a pivoted configuration. Second plate  74  may be pressed tightly against the first non-rotating half of thrust bearing  70  by spring  48 . First plate  72  and second plate  74  may be made from a metal or any other suitable material commonly known in the art. First plate  72  and second plate  74  may fit loosely on plain bearing  71 . 
     Valve  42  and governor  44  may be configured such that an initial position of spool  50  positions groove  58  such that flow of fluid from inlet  52  to outlet  54  is prevented. For example, in an initial condition with no rotation of shaft  60  and no fluid pressure at pressure port  56 , an end of spool  50  may rest on the housing in the area adjacent pressure port  56 . First plate  72  may be touching clevis  66 , and spring  48  may be in a compressed state. Valve  42  and governor  44  may further be configured such that when the pressure of the fluid at pressure port  56  reaches a predetermined level or threshold value, spool  50  may move the predetermined distance within housing  46  that positions groove  58  to allow fluid to flow from inlet  52  to outlet  54 . This predetermined pressure level or threshold value may be the relief pressure of the lubricating fluid, i.e. the maximum allowable pressure of the lubricating fluid. 
     One having ordinary skill in the art will recognize that adjusting the characteristics of spring  48 , such as the initial length and/or spring rate, may change the predetermined pressure required for spool  50  to move the predetermined distance. Valve  42  and governor  44  may additionally be configured such that flyweights  68  do not begin increasing the maximum allowable fluid pressure until shaft  60  is rotating at a predetermined speed that is indicative of power source  10  operating in an overspeed condition. That is, valve  42  and governor  44  may maintain a constant maximum pressure for all power source  10  operating speeds up to an overspeed condition, at which speed governor  44  may begin increasing a maximum allowable pressure proportionally with increasing power source  10  operating speed. 
     The operation of the exemplary embodiment shown in  FIG. 2  will be described in detail below. 
     INDUSTRIAL APPLICABILITY 
     The disclosed pressure regulator may be applicable to any power source that includes a pressurized lubrication system. The disclosed pressure regulator may provide a system for altering the trigger pressure of a pressure regulator based on a speed of a component. For example, the pressure regulator may increase a maximum allowable oil pressure as a rotational speed of a power source increases. This may allow the maximum allowable oil pressure to be set lower during normal operating conditions, while allowing a higher maximum pressure during overspeed operating conditions. In this manner, inefficiencies due to pumping losses and unnecessary heating of the oil at lower power source operating speeds may be avoided, and proper lubrication of the power source may be obtained during overspeed operating conditions. The operation of the system shown in  FIGS. 1 and 2  will now be explained. 
     Power source  10  may be operated to perform a variety of tasks, such as, for example, mechanically driving a vehicle. Operation of power source  10  may cause a rotation of crankshaft  24 . Rotation of crankshaft  24  may cause a corresponding rotation of main gear  28 , which in turn may cause a rotation of auxiliary gear  30 . 
     Rotation of auxiliary gear  30  may cause rotation of drive gear  64 , which may cause shaft  60  to rotate at a speed proportional to crankshaft  24 . As the operating speed of power source  10  increases, the rotational speed of shaft  60  may increase. As shaft  60  rotates, clevis  66  also rotates, along with flyweights  68 . Rotation of flyweights  68  may cause a distance between a center of gravity of flyweights  68  and the axis of shaft  60  to increase, due to an increasing centrifugal force acting on flyweights  68 . 
     When shaft  60  rotates at operating speeds below an overspeed condition of power source  10 , flyweights  68  may pivotally move and touch first plate  72 , but they may not generate a centrifugal force sufficient to apply a linear force to first plate  72  that in turn compresses spring  48 . That is, at speeds below an overspeed condition of power source  10 , the force applied by flyweights  68  to first plate  72  may be below the force required to compress spring  48 . Alternatively, flyweights  68  may be configured such that they do not touch first plate  72  until shaft  60  rotates at a speed corresponding to an overspeed condition of power source  10 . 
     When shaft  60  rotates at a predetermined speed equal to an overspeed condition of power source  10 , flyweights  68  may begin to apply a linear force, proportional to the centrifugal force, to first plate  72  and to thrust bearing  70  that is high enough to compress spring  48 . As the operating speed of power source  10  increases beyond the threshold overspeed condition, shaft  60  may rotate faster, thereby increasing the force with which flyweights  68  push on first plate  72 , and in turn increasing the compression of spring  48 . As spring  48  is compressed in this manner, the maximum allowable pressure of the fluid in passageway  40  is proportionally increased. 
     Operation of power source  10  may also cause fluid to be drawn from sump  38  into pump  34 , where the fluid is pressurized to flow through passageway  40 , into power source  10  to lubricate various parts, and then returned to sump  38 . The fluid may also flow into pressure port  56  of pressure regulator  36 . The pressure of the fluid against spool  50  may generate a force to move spool  50  against spring  48 . Because, during overspeed operation of power source  10 , governor  44  may generate a force against spring  48  that increases as the rotational speed of power source  10  increases, the pressure of fluid required to move spool  50  may also increase as the rotational speed increases. In this way, the maximum allowable pressure of the fluid may increase as the rotational speed of power source  10  increases. 
     When the pressure of the fluid in passageway  40  reaches a maximum allowable pressure, the force of spring  48  on spool  50  and the force from the fluid on spool  50  may be balanced such that spool  50  moves a predetermined distance in housing  46 , and allows the fluid to flow from inlet  52 , through groove  58 , to outlet  54 , and on to sump  38 . Spool  50  may remain in a position to allow flow from inlet  52  to outlet  54  for as long as the pressure of the fluid is high enough to generate a force on spool  50  to allow spool  50  to maintain a position that allows flow. When the pressure of the fluid falls below the maximum allowable pressure, spool  50  may move such that flow is no longer permitted from inlet  52  to outlet  54 . When spool  50  is in a position to allow flow from inlet  52  to outlet  54 , the pressure of the fluid may be prevented from increasing beyond the pressure at which spool  50  begins to allow flow across valve  42 . 
     The pressure regulator disclosed in  FIG. 2  may allow an increase in a maximum allowable pressure proportional to the rotational speed of a power source as the rotational speed of the power source increases. Such an increase in a maximum allowable pressure may allow the maximum allowable pressure to be set at a lower level for normal power source operations, while allowing a higher maximum allowable pressure during overspeed operation of the power source. The pressure regulator may also be retrofitted to power sources which lack sophisticated electronics, thereby providing a low cost, reliable means for increasing a maximum allowable oil pressure proportional to the operating speed of the power source. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed pressure regulator without departing from the scope of the disclosure. Other embodiments of the pressure regulator will be apparent to those skilled in the art from consideration of the specification and practice of the pressure regulator disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.