Patent Publication Number: US-2009235994-A1

Title: Coaxial pressure retention and relief mechanism

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/070205, filed on Mar. 19, 2008. 
    
    
     FIELD 
     The present disclosure relates to a coaxial pressure retention and relief mechanism, which may be used in a vehicle fuel system. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. Conventional vehicular fuel systems, such as those installed in automobiles, may employ a “return fuel system” whereby a fuel supply line is utilized to supply fuel to an engine and a fuel return line is utilized to return, hence “return fuel system,” unused fuel to a fuel tank. More modern fuel systems typically employ either a mechanical returnless fuel system (“MRFS”) or an electronic returnless fuel system (“ERFS”). In an ERFS only a fuel supply line from a fuel pump module in a fuel tank to an engine is utilized since the speed of the fuel pump may be varied electronically; therefore, no separate return fuel line from the engine to the fuel tank is necessary. As a result, in an ERFS only the exact volume of fuel required by an engine is delivered to the engine, regardless of the varying degree of the volume of fuel required by the engine. In an MRFS, a fuel supply line from a fuel pump module in the fuel tank to the engine is utilized. However, with an MRFS a pressure regulator is usually required to regulate the pressure and volume of fuel supplied to the engine. 
     While current returnless fuel systems have generally proven to be satisfactory for their applications, each is associated with its share of limitations. One limitation of current returnless fuel systems is maintaining fuel pressure in as much of the fuel line as possible in order to accomplish engine starting and restarting as quickly as possible with no interruptions of fuel supply to the engine. Another limitation of current returnless fuel systems is maintaining the prime condition of the fuel line to prevent “depriming” of the fuel line. An adequate prime condition will permit an adequate fuel supply to reach the engine during engine starting. Another limitation is maintaining a high flow rate and high fuel system pressure during engine operation and a high fuel system pressure when the engine is off. 
     In still yet another limitation pertaining to current returnless fuel system is relieving fuel line pressure during periods of “dead soak,” to lessen any adverse effects of excessive pressure buildup in the fuel line. Additionally, concerning pressure related valves, valve placement may not be advantageous for ease of assembly or for best utilizing space along the fuel system route. Additionally, placement of such pressure relief and/or check valves may not be optimally advantageous for maintaining adequate fuel volumes and pressures in the fuel line. 
     What is needed then is a device that does not suffer from the above limitations. This, in turn, will provide a co-axial pressure relief mechanism capable of installation in a fuel line and capable of relieving pressure in more than one direction. 
     SUMMARY 
     In one example, a pressure relief mechanism may employ an outer valve that may employ an outer valve carriage, an outer valve body, and an outer valve spring that biases the outer valve body against the casing, the outer valve permitting fuel flow in a first direction. An inner valve may employ an inner valve carriage, an inner valve body, and an inner valve spring that biases the inner valve body against the outer valve body, the inner valve permitting fuel flow in a direction opposite to the first direction. The outer valve body may define a cavity and the inner valve may be partially or completely contained within the cavity. The pressure relief mechanism may employ a casing such that the inner valve and the outer valve may be contained within the casing. The outer valve and the inner valve may have centerlines that coincide or are coincident. Furthermore, the pressure relief mechanism may employ a pressure relief valve inlet and a pressure relief valve outlet. The outer valve centerline and the inner valve centerline may coincide with the pressure relief valve inlet and the pressure relief valve outlet. Additionally, a separate and additional side relief valve may be resident in the casing. 
     In another example, a pressure relief mechanism may utilize a casing that defines an internal cavity, an internal cavity fluid inlet, and an internal cavity fluid outlet. The first valve permits fluid flow in a first direction and may reside within the casing internal cavity and employ a first valve carriage and a first valve spring that forces the first valve carriage against a first end of the casing internal cavity. The first valve spring may reside against a second end of the casing cavity. 
     A second valve may reside within the internal cavity and employ a second valve carriage, a second valve body, and a second valve spring that biases the second valve body against the first valve body. The second valve may permit fuel flow opposite to the first direction of the first valve. The casing defines a casing centerline, the first valve defines a first valve centerline, and the second valve defines a second valve centerline, and the casing centerline, the first valve centerline and the second valve centerline are coincident. The first valve carriage defines a first valve carriage cavity within which the second valve is disposed. The first valve carriage further entails an inner circumferential structure, and an outer circumferential structure such that the inner circumferential structure is covered by the first valve spring and the outer circumferential structure slidably contacts a wall of the internal cavity of the casing when biased by the first valve spring or the force of fluid pressure entering the casing internal cavity through the internal cavity fluid inlet. The inner circumferential structure and the outer circumferential structure may define a gap therebetween through which fluid may flow from the internal cavity fluid inlet to the internal cavity fluid outlet. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a perspective view of a vehicle depicting a fuel system in phantom; 
         FIG. 2  is a perspective view of a fuel pump module; 
         FIG. 3  is a side view of a vehicle fuel tank depicting a representative location of a fuel pump module; 
         FIG. 4  is a diagram of a vehicle fuel supply system depicting a representative location of a co-axial pressure relief mechanism; 
         FIG. 5  is an enlarged view of a co-axial pressure relief mechanism in a first position; 
         FIG. 6  is an enlarged view of a co-axial pressure relief mechanism in a second position; 
         FIG. 7  is an enlarged view of a co-axial pressure relief mechanism depicting an auxiliary relief valve; 
         FIG. 8  is an enlarged view of a co-axial pressure relief mechanism in a second position; and 
         FIG. 9  is an enlarged view of a co-axial pressure relief mechanism in a second position. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. With reference to  FIGS. 1-9 , description of a fuel pump module for a returnless fuel system, such as an electronic returnless fuel system (“ERFS”) or a mechanical returnless fuel system (“MRFS”), will be described. 
       FIG. 1  depicts a vehicle such as an automobile  10  having an engine  12 , a fuel supply line  14 , a fuel tank  16 , and a fuel pump module  18 . The fuel pump module  18  fits within the fuel tank  16  and is normally submerged in or surrounded by varying amounts of liquid fuel within the fuel tank  16  when the fuel tank  16  possesses liquid fuel. A fuel pump  20  within the fuel pump module  18  pumps fuel to the engine  12  through the fuel supply line  14 .  FIG. 2  depicts a representative fuel pump module  18  while  FIG. 3  depicts the fuel pump module  18  in its typical environment within the fuel tank  16 . Continuing with  FIGS. 1-3 , the fuel pump module  18  and a portion of its component parts will be presented. Such component parts may consist of a reservoir  22 , a fuel pump module flange  24 , also known simply as a flange  24 , a fuel exit line  26 , a first strut  28 , a second strut  30 , and a spring  32  that surrounds the first strut  28 . 
     When installed and secured within a typical environment of a fuel tank  16 , the flange  24  of the module  18  rests upon a top surface  34  of the fuel tank  16 . Although the flange  24  ultimately abuts the top surface  34  of the fuel tank  16 , the flange  24  must be forced downwardly into the fuel tank  16  during installation of the module  18 , in order to sufficiently compress the spring  32 , which resides around the first strut  28 , to bias the spring  32  and cause the reservoir  22  to be held against the fuel tank floor  36 , or bottom interior surface of the fuel tank, by the force of the spring  32 . A second strut  30  assists in securing the reservoir  22 , and although not depicted, a spring may be secured around the second strut  30  to supply additional securing force to secure the module against the fuel tank floor  36 . Upon compression of the spring  32 , the flange  24  is secured to the top of the fuel tank  16  by a locking ring (not shown) or similar device. While the flange  24  creates a seal around the periphery of a hole in the top surface of the fuel tank  16 , the reservoir  38  is securely held against the bottom floor of the fuel tank  16 .  FIG. 3  depicts the fuel pump  20  within the reservoir  22  with a fuel filter  38 , also called a suction filter  38  or sock filter  38 , attached at an end of the fuel pump  20  from which fuel is drawn. 
     Continuing,  FIG. 4  further depicts a fuel filter  40 , different from the suction filter  38 , a co-axial pressure relief mechanism  42 , a fuel rail  45 , and fuel injectors  46 .  FIG. 4  is a schematic representation of a fuel system in which the co-axial pressure relief mechanism  42 , which will be explained in further detail later, may be installed. For instance, the fuel filter  40 , although depicted away from the fuel pump  20  in  FIG. 4 , is typically located around the fuel pump  20  in practical use. Nevertheless,  FIG. 4  depicts the fuel filter  40  as being in the fuel path, in addition to the suction filter  38 , to provide an additional fuel filtering to fuel that is pumped to the common fuel rail  45  and numerous fuel injectors  46  attached to the engine  12 . It should be noted that the injection system may also be a port fuel injection system or a direct fuel injection system that utilizes a high pressure pump. 
     Turning now primarily to  FIG. 5 , a more detailed explanation of the co-axial pressure relief valve  42  will be provided.  FIG. 5  depicts the co-axial pressure relief valve  42 , which is enclosed by a casing  44 . The casing  44  may be manufactured from a plastic material that is capable of withstanding the operational pressure range of the fuel system and capable of being in contact with liquid fuel, such as gasoline and diesel fuel. Continuing with  FIG. 5 , the co-axial pressure relief valve  42  has a first end  46  and a second end  48 , each equipped with a number of steps or barbs  50  to facilitate the installation of a fuel line, such as a rubber tube, over the barbs  50 . In place of barbs  50 , other connection systems or methods may be used, such as SAE quick connect connectors. Such a fuel line  14 , upon being installed over the barbs  50 , may then be further secured with a clamp, such as a hose clamp. Within the casing  44  of the co-axial pressure relief valve  42 , is a pair of coaxial valves, an inner valve  52  and an outer valve  54 . The inner valve  52  utilizes an inner valve body  56  that is biased with an inner valve spring  58  against an inner valve carriage  60 . Continuing, the inner valve body  56 , inner valve spring  58  and inner valve carriage  60  all may reside within an outer valve body  62 . The outer valve body  62  is biased by an outer valve spring  64  that biases against an outer valve carriage  66 . 
     Continuing with reference to  FIGS. 1-5 , operation of the co-axial pressure relief mechanism  42  will be explained. Fuel is drawn from the reservoir  22 , and through the suction filter  38  by the fuel pump  20  within the fuel pump module  18 . The liquid fuel then exits the fuel pump  20  via tube  21  and subsequently the fuel pump module  18  via the fuel exit line  26  on the fuel pump module flange  24 . The fuel then flows through the fuel supply line  14  en route to the engine  12 .  FIG. 5  depicts the flow of liquid fuel  68  into the co-axial pressure relief mechanism  42 . As an example, the fuel pump  20  may pump fuel at a variety of pressures, such as 40-95 Kpa, and even from 350-600 Kpa depending upon the application (injection system) and fuel type (such as gasoline or diesel). The pump must be capable of supplying what the engine demands in terms of fuel pressure to properly operate the engine; however, this pressure may vary depending upon the engine and other fuel system requirements. Therefore, the outer valve  54  of co-axial pressure relief valve  42  may be set to open at a pressure of 20 Kpa, which ensures that the outer valve  54  will permit fuel to flow uninterrupted to the engine  12 . Regardless of the fuel pressure necessary to operate the engine  12 , the outer valve  54  will open at a pressure less than the fuel pressure necessary to operate the engine  12 . The outer valve  54  is able to open based upon selection of the outer spring  64 . That is, the outer spring  64  may have a spring constant such that the force of the fuel flow  68  is able to bias the outer spring  64  and compress it. Such a configuration will ensure that the engine  12  receives the fuel it needs at the requisite fuel pressure. 
     As the fuel flow  68  enters the co-axial pressure relief valve  42  the outer spring  64  compresses due to the force of the fuel flow  68  against the outer valve head  70  and the force of the fuel flow  68  against the inner valve head  72  of the inner valve body  56 . When the force of the fuel flow  68  strikes the inner valve head  72 , it moves the outer valve body  62  to permit the flow of fuel between the outer valve head  70  and the valve casing  44  and through the co-axial pressure relief valve  42  and onward toward the engine  12 . As the outer valve body  62  moves (to the right when viewing  FIG. 5 ), it biases against and compresses the outer valve spring  64 . No only does the outer valve body  62  move, but the inner valve body  56 , inner valve spring  58 , and the inner valve carriage  60  translate together with the outer valve body  62 . Continuing, the outer valve spring  64  biases against the outer valve carriage  66 , which also provides guidance and stability to the outer valve body  62  as it moves, that is, opens to permit the flow of fuel through the casing  44 . Thus, when the fuel flow  68  is flowing to the engine  12 , that is from left to right when viewing  FIG. 5 , the fuel pressure is consistent within, and on each side of the co-axial pressure relief valve  42 . One explanation as to why the pressure is consistent through the co-axial pressure relief valve  42  is because the fuel flow  68  is permitted to largely flow in a straight line of flow through the casing  44 . More specifically, other than the angle formed between the outer valve head  70  and the inside wall  74  upon which the outer valve head  70  contacts, the fuel flow  68  does not change directions, thus resulting in reduced pressure drops or eliminating pressure drops. That is, because the outer valve head  70  is angled and mates with the angled casing inside wall  74 , fuel may maintain a largely unaltered fuel path while contact sufficient to form a seal when fuel flow is necessary in the opposite direction (from the engine to the fuel pump module  18 , as will be explained. 
       FIG. 6  depicts a fuel flow scenario such that the fuel flow  74 , and associated fuel pressure, flows from the engine side  76  of the co-axial pressure relief mechanism  42  to the fuel pump module side  78 . Such a fuel flow  74  is in response to a pressure differential which will now be explained. When an engine  12  is operating, fuel flows in accordance with the description provided above with respect to  FIG. 5 . Upon turning off or stopping the engine  12 , the outer valve  54  will close and more specifically, the outer valve head  70  will be forced against the inside wall  74  of the casing  44 , because the pressure on the fuel pump side  78  of the co-axial pressure relief mechanism  42  becomes insufficient to compress the outer valve spring  64 . With the engine  12  not operating, the fuel line  14  on the engine side of the co-axial pressure relief mechanism  42  may experience an increase in temperature and thus fuel pressure within the fuel line  14 . Such an increase in temperature and fuel pressure on the engine side  76  of the co-axial pressure relief mechanism  42  may occur due to heat from the engine  12  that remains in the engine compartment after the engine is turned off. The temperature and pressure in the fuel line  14  may increase on hot, sunny summer days, such as when the ambient temperature is greater than the fuel line temperature. Such temperature and pressure may result or be further increase if the vehicle  10  is parked on a hot surface, whether or not subjected to direct sunlight, such as a macadam parking lot, from which heat is radiating upwards. 
     Continuing with  FIG. 6 , when the pressure on the engine side  76  of the co-axial pressure relief mechanism  42  is 400 Kpa, for example, due to conditions prevalent after stopping the engine as described above, the fuel pressure will rise and be great enough to begin to compress the inner valve spring  58  as the force due to pressure is applied to the inner valve head  72 . The inner valve spring  58  biases against the inner valve carriage  60 , which is secured to the outer valve body  62 . At the same time that the inner valve  52  is being forced open so that fuel flow  74  may pass to the fuel pump side  78  of the co-axial pressure relief mechanism  42 , the fuel pressure from the fuel of fuel flow  74  also is forced against the shoulder  80  of the outer valve head  70 , which seals the outer valve head  70  against the inside wall of the casing  44 . Upon fuel passing from the engine side  76  of the co-axial pressure relief mechanism  42  to the fuel pump module side  78 , the excess fuel and pressure will flow back into the fuel tank  16 , which is equipped to handle excess fuel vapor, such as through a vapor vent valve and/or a charcoal canister. By relieving the fuel pressure above that at which the inner valve  52  is set, fuel system components on the engine side  76  of the co-axial pressure relief mechanism  42  may be maintained or preserved, that is not subject to damage by excess pressure. 
     Turning now to  FIG. 7 , another embodiment of a co-axial pressure relief mechanism  82  will be presented in which a side relief valve  84  is resident in the casing  86 . The inner valve  52  and outer valve  54  operate on the same principles as presented in the discussions of  FIGS. 5 and 6 . The side relief valve  84  may be used to relieve fuel pressure that is above the pressure at which the outer valve  54  operates. That is, during engine operation when the outer valve  54  is open and the outer valve spring  64  is being compressed to permit fuel flow  68  to pass to the engine  12 , the side relief valve  84  will normally remain closed. However, if the fuel pump  20  or fuel pumps, in the case of more than one fuel pump, begin to pump fuel at a higher pressure than normally designed, such as in the case of an ERFS that is electronically controlled and not functioning properly, then the side relief valve  84  will open and relief the excess fuel pressure while the engine  12  continues to operate. The side relief valve  84  operates on the same principles as the inner valve  52  and has a valve body  88 , a valve spring  90 , and a valve carriage  92 . When the fuel pressure caused by the fuel flow  68  is able to provide enough force on the valve body  88  to cause compression of the valve spring  90 , the valve permits fuel flow  94  to escape from the casing  86 . By utilizing the side valve  84  in the casing, fuel system components from the fuel pump  20 , or multiple fuel pumps, to the engine, including the fuel rail  45 , the fuel injectors  46 , the fuel line  14 , the co-axial pressure relief mechanism  82 , and any component subjected to the fuel pressure created by the fuel flow  68 . Thus, the side relief valve  84  relieves fuel pressure caused by the fuel pump  20  or pumps, as the case may be. 
     Turning now to  FIG. 8 , another embodiment of a coaxial pressure relief mechanism  100  will be presented. The coaxial pressure relief mechanism  100  has a casing  102 , within which a outer carriage  104  may slide in a longitudinal direction or path. The outer carriage  104  has an outer circumferential structure  106  that makes contact with an interior wall surface  108  of the casing  102 . More specifically, the outer circumferential structure  106  slides against the interior wall surface  108  as the fuel flow  68  applies a force against the end face  110  of the outer carriage  104 . The outer carriage  104  slides along the interior wall  108  until the end  106  of the outer circumferential structure  106  strikes the wall  112 . As the outer circumferential structure  106  begins to slide, from left to right in  FIG. 8 , and the outer carriage  104  moves away from the inlet orifice  114 , the inlet orifice  114  permits the fuel flow  68  to enter the interior of the casing  102  and flow to the outer circumferential structure  106  where it is able to pass through orifices  116  in the outer circumferential structure  106 . Upon passing through the orifices  116  of the outer circumferential structure  106 , the fuel flow proceeds around an inner circumferential structure  118  of the outer carriage  104  and out through an exit orifice  120  of the casing  102 . 
     Continuing with  FIG. 8 , as the carriage moves due to the force of the fuel flow  68 , the outer valve spring  122  begins to compress against the casing  102  at an interior end  124  opposite that from which the outer carriage  104  originated. The interior end  124  has a protuberance  126  to retain the spring  122  in its interior position longitudinally at one end of the spring, while the other end of the spring resides around the inner circumferential structure  118  of the outer carriage  104  and abuts at the juncture of the inner circumferential structure  118  and the outer circumferential structure  106 . Thus the outer valve spring  122  may be adjusted with regards to spring constant to adjust the force at which the outer carriage  104  will begin to move. 
     Continuing with  FIG. 8 , and to assist the outer carriage  104  in moving within the casing  102 , the outer carriage  104  has an outer carriage inlet orifice  128  or hole and an outer carriage outlet orifice  130  or hole. When the engine is operating and the fuel flow  68  creates a pressure on the fuel pump side  78  of the co-axial pressure relief mechanism  100  that is greater than the pressure on the engine side  76  of the co-axial pressure relief mechanism  100 , the fuel flow  68  enters the outer carriage inlet orifice  128  and applies a force against the inner valve body  132 , which is the same direction that the inner valve spring  134  exerts a force against the inner valve body, to provide the external force necessary to move the outer carriage  104  against the force of the outer valve spring  122  and thus permit the fuel flow  68  into the casing  102  and flow to the engine  12 . In so doing the fuel pressure is regulated by the outer valve spring  122 .  FIG. 9  depicts a fuel pressure situation that may result when the engine is turned off. 
     Turning now to  FIG. 9 , the co-axial pressure relief mechanism  100  depicts a scenario in which the engine  12  is not operating. Further, the fuel  136  in the fuel line  14 , may be subject to heat such that the fuel pressure may increase on the engine side  76  of the co-axial pressure relief mechanism  100 . To begin, when the engine  12  is turned off, the fuel pump  20  stops pumping fuel to the engine  12  and the force of flowing fuel that normally biases the outer spring  122  and the outer carriage  104  against the interior end  124  of the casing  102  no longer exists. As a result, the outer valve spring  122  may bias the end face  110  of the outer carriage  104  against the interior surface  137  of the casing  102  to preserve the fuel pressure in the engine side  76  of the fuel line  14 . By preserving the fuel pressure on the engine side  76  of the fuel line  14 , the fuel pump  20  does not have to re-generate or re-create such pressure before re-starting the engine. Desirably, as a result, the engine  12  will re-start faster. However, by essentially making the engine side  76  of the fuel line  14  a closed vessel when the outer valve  54  closes (the face  110  against interior surface  137 ), the fuel pressure may further increase on the engine side  76  and at a particular pressure, damage to fuel injectors  46 , a common rail  45 , the fuel line  14 , or other connections, such as the connections that secure the co-axial pressure relief mechanism  100  to the fuel line  14  may result. 
     Continuing with  FIG. 9 , to relieve pressure in the engine side  76  fuel line with the outer valve  54  in a closed position, as depicted in  FIG. 9 , the inner valve  52  will open. The inner valve  52 , in part, utilizes an inner valve carriage  60 , an inner valve spring  58 , and an inner valve body  56  that work in conjunction with the outer valve carriage  104 . More specifically, when the resulting force of the fuel pressure in the engine side  76  of the fuel line  14  against the head  138  is greater than the opposing force of the inner valve spring  58 , which counters the force due to the fuel pressure, the head  138  lifts or separates from the interior surface of the outer carriage  104  to permit the relief of pressure and fuel flow  136  through the inner valve  52 . As a result, the fuel flow passes from the engine side  76  of the co-axial pressure relief mechanism  100  to the fuel pump side  78  of the co-axial pressure relief mechanism  100 , thus maintaining pressure in the engine side  76  that is a desirable re-starting pressure, yet relieving pressure over such desirable re-starting pressure. Such relieved pressure passes through the fuel line  14  and into the fuel tank  16 , where one of more vapor vent valves may process such vapor pressure. 
     There are multiple advantages to the present disclosure, such as maintaining some degree of fuel pressure in the fuel line  14  on the engine side  76  of the co-axial pressure relief mechanism  42  to more quickly start the engine  12  when a user desires. If fuel pressure was not maintained in the engine side  76  of the fuel line  14 , time to fill and/or pressurize the line would be necessary. Such time would be undesirable for a vehicle operator. Thus use of the co-axial pressure relief mechanism  42  permits pressure to remain on the engine side  76  of the fuel line  14  yet relieve excess pressure that may otherwise damage fuel system components on the engine side  76  of the co-axial pressure relief mechanism  42 , such as fuel line  14  itself, the fuel rail  45  and/or fuel injectors  46 . 
     There are additional advantages of the co-axial pressure relief mechanism  42  as described in the present disclosure. First, the co-axial pressure relief mechanism  42  will permit the use of one, two or more fuel pumps without requiring the fuel pumps to have an internal check valve. This will result in a cost savings to each fuel pump since an internal check valve is not necessary. Additionally, by not requiring an internal check valve, there is a reduced possibility of part failure. Second, the co-axial pressure relief mechanism  42  will permit the jet pumps on the fuel pump module to operate using high pressure filtered fuel for better fuel delivery module performance. That is, the jet pumps will not interfere with the pressurized fuel flowing to the engine during engine on or engine off conditions. Third, the co-axial pressure relief mechanism  42  is optimizes fuel pressure losses throughout the fuel system thereby promoting longevity of the fuel pump(s) and the fuel pump module. Pressure losses are optimized because the direction of fuel through the co-axial design is permitted to flow largely in a straight line flow, as opposed to changing directions, as in non-co-axial valves. 
     Continuing with advantages of the teachings of the present disclosure, fourth, the co-axial pressure relief mechanism  42  can be easily implemented into an MRFS fuel system with minimal modifications to meet demands of customers with ERFS requirements. Fifth, the co-axial pressure relief mechanism  42  may be implemented into vehicles with ERFS and MRFS because the co-axial pressure relief mechanism  42  is an inline device, is relatively small in cross-section, and does not need to be installed within a fuel pump module itself, thereby eliminating fuel pump module modifications related to such a valve  42 . Sixth, the co-axial pressure relief mechanism  42  permits the elimination of a traditional pressure regulator, normally associated with an MRFS, because the co-axial pressure relief mechanism  42  has the ability to relieve fuel pressure at a specific set pressure. The elimination of a pressure regulator in the fuel pump module also results in a cost reduction and elimination of a potential failure point within a fuel pump module. Seventh, with the use of the co-axial pressure relief mechanism  42  the overall complexity of a fuel pump module in an MRFS (elimination of check valve on the pump and elimination of a pressure regulator) and in an ERFS (elimination of a check valve on the fuel pump) is reduced. Eighth, the co-axial pressure relief mechanism  42  maintains high pressure fuel in the fuel line during fuel pump and engine operation, while allowing the high pressure fuel to be relieved when the engine is off. Such relief may be from a secondary valve installed between the co-axial pressure relief mechanism  42  and the fuel pump module  18 , and in one example, actually in the co-axial pressure relief mechanism  42  casing. Furthermore, because the co-axial pressure relief mechanism is of a co-axial design, as opposed to parallel or side-by-side designs, the pressure drop caused by the mechanism is minimal. 
     So, further to what is disclosed above, a pressure relief mechanism  42  may employ or be comprised of an outer valve  54  having an outer valve body  62 . The outer valve  54  relieves pressure in a first direction, and an inner valve  52 , having an inner valve body  56 , relieves pressure in a second direction. The inner valve  52  may be completely or fully contained within the outer valve body  62 . The pressure relief mechanism  42  may further comprise an outer valve centerline  63 , and an inner valve centerline  63 . That is, the outer valve centerline  63  and the inner valve centerline  63  coincide or are coincident. The pressure relief mechanism  42  may further comprise a pressure relief valve inlet  65 , and a pressure relief valve outlet  67 . The outer valve centerline  63  and the inner valve centerline  63  are concentric with the pressure relief valve inlet  65  and the pressure relief valve outlet  67 . The pressure relief mechanism  42  may further comprise a casing  44 , which may be one or more pieces, and the first valve  54  and the second valve  52  reside within the casing  44 . The pressure relief mechanism  42  may further operate such that wherein fuel entering from the pressure relief valve inlet  65  is directed to force open the outer valve  54  and force closed the inner valve  52  closed and fuel entering from the pressure relief valve outlet  67  is directed to force open the inner valve  52  and force closed the outer valve  54 . 
     In another example, a pressure relief mechanism  42  may employ an inner valve  52  and an outer valve  54 . The outer valve  54  may comprise an outer valve carriage  66 , an outer valve body  62 , and an outer valve spring  64  that biases the outer valve body  62  against the casing  44 . The outer valve  54  permit fuel flow in a first direction ( FIG. 5 ) when fluid or fuel pressure is great enough to bias the outer valve spring  64  and mover the outer valve head  70  from the inside wall  74 . The inner valve  52  may comprise an inner valve carriage  60 , an inner valve body  56 , and an inner valve spring  58  that biases the inner valve body  56  against the outer valve body  62 . In accordance with  FIG. 6 , the inner valve  52  permits fluid or fuel flow in a direction opposite to the first direction. More specifically, when the fuel pressure in the fuel line  14  on the engine side  76  of the valve  42  rises after the engine  12  is turned off, the inner valve  52  will open by compressing the inner valve spring  58  and relieving pressure above which the inner valve spring  58  compresses. 
     Continuing with  FIG. 6 , the outer valve body  62  defines an outer valve body cavity  71  and the inner valve  52  is contained within the outer valve body cavity  71 . The pressure relief mechanism  42  may further comprise a casing  44  and the inner valve  52  and the outer valve  54  are contained within the casing  44 . The pressure relief mechanism  42  may further possess or have an outer valve centerline  63  and an inner valve centerline  63  such that the outer valve centerline  63  and the inner valve centerline  63  coincide. 
     The pressure relief mechanism  42  may further comprise a pressure relief valve inlet  65  and a pressure relief valve outlet  67 . The outer valve centerline  63  and the inner valve centerline  63  coincide with the pressure relief valve inlet  65  and the pressure relief valve outlet  67 . The pressure relief mechanism  42  may further possess a side relief valve  84  resident in the casing  44 . The side relief valve  84  contains a valve body  88 , a valve spring  90 , and a valve carriage  92 . Fuel flow  94  may exit from the side relief valve  84  when the force of the fuel pressure in the fuel line  14  on the fuel pump side of the pressure relief mechanism  42  causes the valve spring  90  to compress. 
       FIGS. 8 and 9  depict a pressure relief mechanism  100  that may further employ a casing  102  that defines an internal cavity  103  with the first valve  54  residing within the internal cavity  103 . The second valve  52  may also reside within the internal cavity  103 . The first valve  54  may further comprise a first valve carriage  104  and a first valve spring  122  that biases the first valve carriage  104  against a first end of the cavity  103 . The first valve spring  122  may reside against a second end  124  of the cavity  103 . A fluid inlet  114  into the cavity  103  may exist at the first end of the cavity  103  and a fluid outlet  120  from the cavity  103  may exist at a second end of the cavity  103 . The fluid entering from the fluid inlet  114  causes the first valve carriage  104  to move and compress the first valve spring  122  and permit passage of fluid  68  through the internal cavity  103 . 
     Continuing with  FIGS. 8 and 9 , the pressure relief mechanism  100  may further be arranged or designed so that the first valve carriage  104  defines a first valve internal cavity  105  with a fluid inlet  128 . Additionally, the pressure relief mechanism  100  may further employ a second valve  52  residing within the first valve internal cavity  105 . The second valve  52  may employ a second valve carriage  60 . The second valve body  56  may be disposed in, or pass through, the second valve carriage  60 . A second valve spring  58  biases the second valve body  56  against the fluid inlet  130  of the first valve internal cavity  105 . 
     The pressure relief mechanism  100  may further be designed such that the first valve carriage  104  further defines a fluid outlet  128  to disperse fluid pressure from the first valve internal cavity  105 . The first valve carriage  104  slides within the internal cavity  103  of the casing  102 . The first valve carriage  104  may further employ an inner circumferential structure  118 , and an outer circumferential structure  106 . The inner circumferential structure  118  is covered by the first valve spring  122  and the outer circumferential structure  106  slidably contacts a wall  112  of the internal cavity  103  of the casing  102 . The inner circumferential structure  118  and the outer circumferential structure  106  define a gap  140  therebetween through which fluid  68  may flow from a mechanism inlet  114  to a mechanism outlet  120 . 
     With continued reference to  FIG. 8 , a pressure relief mechanism  100  may employ a casing  102  that defines a casing internal cavity  103 , an internal cavity fluid inlet  114 , and an internal cavity fluid outlet  120 . Additionally, a first valve  54  may reside within the casing internal cavity  103 . The first valve  54  may further comprise a first valve carriage  104 , and a first valve spring  122  that forces the first valve carriage  104  against a first end of the casing internal cavity  103 . The first valve spring  122  may reside against a second end  124  of the casing cavity  103 . The first valve  54  may permit fuel flow  68  in a first direction, that is, to the right when viewing  FIG. 8 . 
     A second valve  52  may reside within the internal cavity  103  and employ a second valve carriage  60 , a second valve body  56 , and a second valve spring  58  that biases the second valve body  56  against the first valve body  104 . The second valve  52  permits fuel flow  136  opposite to the first direction. The first fuel flow  68  is to the right in  FIG. 8  while the opposite fuel flow  136  is to the left in  FIG. 9 . The pressure relief mechanism  100  may be designed so that the casing  102  defines a casing centerline  140 . Additionally, the first valve  54  defines a first valve centerline  140  while the second valve  52  defines a second valve centerline  140 . The casing centerline  140 , the first valve centerline  140  and the second valve centerline  140  are coincident. 
     The pressure relief mechanism  100  may be designed in a way that the first valve carriage  104  defines a first valve carriage cavity  105  within which the second valve is disposed. The pressure relief mechanism  100  may be designed such that the first valve carriage  54  may further employ an inner circumferential structure  118  and an outer circumferential structure  106 . The inner circumferential structure  118  is covered or wrapped by the first valve spring  122 . The outer circumferential structure  106  slidably contacts a wall or a wall surface  108 ,  112  of the internal cavity  103  of the casing  102 . The pressure relief mechanism  100  may be designed such that the inner circumferential structure and the outer circumferential structure  106  define a gap therebetween through which fluid  68  may flow from the internal cavity fluid inlet  114  to the internal cavity fluid outlet  120 .