Abstract:
A fuel system for a gas turbine engine that utilizes a centrifugal pump is presented. The system includes a fuel metering valve that is adapted to set a metered flow of fuel, and a throttle valve that is adapted to accurately control pressure drop across the fuel metering valve. The throttle valve has at least two variable orifices and a compensation chamber between the variable orifices. The throttle valve includes a differential valve piston slidable in a valve body. The differential valve piston comprises working surfaces of at least two different diameters such that changes in chamber pressures effect different axial forces upon the piston.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This patent application is a divisional of copending U.S. patent Ser. No. 10/463,701, filed Jun. 16, 2003. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention pertains to gas turbine fuel systems and more particularly to gas turbine fuel systems that use high pressure centrifugal pumps.  
       BACKGROUND OF THE INVENTION  
       [0003]     There are two types of high pressure fuel pumping systems for gas turbine engines. The first type utilizes a positive displacement pump (typically a gear pump). The other type utilizes a centrifugal pump. The fuel metering units for these types of fuel systems are substantially different in design, application and practice due to the fact that positive displacement pumps provide a predetermined flow rate based on pump speed (a flow generation source), whereas a centrifugal system generates pressure (a pressure generation source) proportional to pump speed squared.  
         [0004]     Examples of positive displacement pump fuel metering systems are disclosed in U.S. Pat. No. 4,458,713 to Wernberg, U.S. Pat. No. 5,433,237 to Kao et al., and U.S. Pat. No. 6,381,946 to Wernberg et al. In these systems, the speed of the pump determines the fuel flow supplied to the fuel metering unit. For positive displacement systems, it is necessary for the fuel metering unit to recirculate (e.g. bypass and return) a portion of the pumped fuel flow back to the inlet of the high pressure pump. This is due to the fact that the pump is sized large enough to provide enough fuel flow to meet the maximum demanded fuel flow rates for the gas turbine engine.  
         [0005]     Centrifugal pumps, by contrast do not provide a predetermined flow rate based upon speed. The fuel metering unit for centrifugal pumping systems throttles (restricts) pump flow rather than bypasses flow.  
         [0006]     Referring to a prior art centrifugal system schematically shown in  FIG. 1 , which generally depicts the relevant portions of a typical centrifugal pump type engine fuel system, the engine fuel system includes a fuel tank and a low pressure centrifugal boost pump. The boost pump supplies fuel to a variable displacement starting pump and to two high speed centrifugal pumps, one for the core engine and the other for the afterburner. The fuel for the high speed centrifugal pump for the core engine is controlled with a fuel metering valve that is positioned by an electrohydraulic servovalve (EHSV), which is in turn controlled by the FADEC (full authority digital electronic controller). A position sensor (such as a LVDT or linear variable displacement transducer) provides metering valve position feedback to the FADEC. A throttle valve is arranged in series with the metering valve. The throttle valve provides a variable restriction orifice in the fuel flow path that controls the pressure drop across the fuel metering valve (at 50 PSI for example). The throttle valve opens and closes the variable restriction orifice to maintain the pressure drop constant. To keep the metering valve pressure drop constant with excellent accuracy as is typically desired, the system of  FIG. 1  employs a pressure sensor which typically contains a bellows or diaphragm that senses pressure drop across the fuel metering valve. Typically, this pressure sensor positions a low friction, low flow first stage valve which in turn positions the larger throttle valve. This is mathematically an integrating type system as flow from the first stage valve is integrated by the second stage throttle valve piston until the error in the predetermined pressure drop is zero.  
         [0007]     Unfortunately, incorporating the plumbing, multiple stages, valves and sensors to provide accurate control over metering valve pressure drop accuracy such as that schematically illustrated in  FIG. 1  has added substantial weight, size, and expense. It has also reduced dynamic performance, stability and the reliability of centrifugal pump metering systems. These are all disadvantages, particularly in aircraft applications where there is always a constant desire to reduce weight while maintaining or increasing performance and reliability.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     It is a general objective of the present invention to provide an improved centrifugal pump metering system for gas turbine engine fuel metering units utilizing high pressure centrifugal pumps.  
         [0009]     In accordance with this general objective, one aspect of the present invention is directed toward an improved fuel system for a gas turbine engine that pumps fuel utilizing a high pressure centrifugal pump that pumps fuel from a fuel supply. The system includes a fuel metering valve that is adapted to set a metered flow of fuel, and a throttle valve that is adapted to control pressure drop across the fuel metering valve. The high pressure centrifugal pump, the fuel metering valve, and the throttle valve are arranged in fluidic series with the nozzle outlet passage, which is adapted to convey fuel to the gas turbine engine for discharge and combustion. The throttle valve has at least two variable orifices and a compensation chamber which senses the pressure between the variable orifices. The throttle valve is movable to simultaneously change degrees of opening of the variable orifices. When the gas turbine engine and fuel system are operating, fuel pressure in the compensation chamber acts upon the throttle valve to control the position of the throttle valve.  
         [0010]     It is a further aspect of the present invention that the throttle valve includes a multiple diameter valve piston slidable in a valve body. The valve piston comprises working surfaces of at least two different diameters such that changes in chamber pressures effect different axial forces upon the piston. The valve piston may comprise first and second lands in spaced axial relation such that the throttle valve defines at least three chambers, including a first chamber subjected to fluid pressure upstream of the fuel metering valve, a second chamber subjected to fluid pressure downstream of the fuel metering valve, and the compensation chamber intermediate of the two variable orifices. The compensation chamber is arranged in fluidic series with the fuel metering valve and the nozzle outlet passage whereby fuel flows through the compensation chamber to the nozzle outlet passage.  
         [0011]     It is an advantage that the compensation chamber may be used to counteract variances in forces that can occur due to changes in valve position. For example the valve experiences different amounts of fluid flow forces (namely, Bernoulli forces) and spring forces at different valve positions. Changes in fluid pressure in the compensation chamber can be designed to compensate for changes in spring forces and/or naturally occurring fluid forces such as Bernoulli forces that may be generated by fluid flowing through the throttle valve.  
         [0012]     Another aspect of the present invention is directed toward a fuel metering unit that can be used in a fuel system for regulating fuel flow in a gas turbine engine pressurized by a high pressure centrifugal pump and delivered to a nozzle outlet passage. The fuel metering unit comprises a fuel metering valve and a throttle valve arranged in fluid series. The throttle valve comprises a valve body, a valve member and at least two variable orifices. The valve member is movable in the valve body to vary the size of the variable orifices. The variable orifices are arranged in fluidic series with a compensation chamber defined therebetween such that a fluid control pressure develops in the compensation chamber when fuel flows through the throttle valve. Fluid control pressure which is developed in the compensation chamber acts upon the valve member to control position of the valve member.  
         [0013]     Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a schematic representation of the relevant portions of a centrifugal pump engine fuel system as may be found in a military afterburning fighter engine;  
         [0015]      FIG. 2  is a schematic representation of a centrifugal pump fuel system incorporating different fuel metering units for both the afterburner and core sections of a gas turbine engine according to an embodiment of the present invention;  
         [0016]      FIG. 3  is a cross sectional view shown partly in schematic form of the fuel metering unit schematically illustrated in  FIG. 2  for the core section of a gas turbine engine, with the throttle valve and metering valve in the closed position;  
         [0017]      FIG. 4  is a similar view to that of  FIG. 3 , but with the throttle valve and metering valve in the open position;  
         [0018]      FIG. 5  is an exploded isometric representation of a throttle valve used in the fuel metering unit of FIS.  2 - 4 ; and  
         [0019]      FIG. 6  is a cross sectional view shown partly in schematic form of the fuel metering unit schematically illustrated in  FIG. 2  used for a vapor core centrifugal pump and the afterburner of a gas turbine engine. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     Referring to  FIG. 2 , two different fuel metering units  10 ,  12  according to two different embodiments of the present invention are illustrated for a fuel delivery system  14  for a gas turbine engine  16  according to an embodiment of the present invention. One of the fuel metering units  10  meters core engine fuel flow and is arranged to control primary fuel flow along a nozzle outlet passage  18  leading to nozzles that discharge into the core of the gas turbine engine  16 . The other fuel metering unit  12  meters afterburner fuel flow and is arranged to control fuel flow through a nozzle passage  20  leading to afterburner nozzles. As illustrated, the main fuel metering unit  10  and the after burner unit  12  are arranged in a parallel fluid circuit. It will be appreciated that many aircraft and gas turbine engines do not include afterburner systems, and the invention is applicable and covers these fuel delivery systems as well.  
         [0021]     The fuel delivery system  14  includes a fuel supply  22  comprising a fuel tank  24  and a low pressure centrifugal boost pump  26 . The boost pump  26  supplies fuel to each of the afterburner fueling system and the core turbine fueling system. The booster pump  26  generates a low pressure source Pb in a first conduit network section  27 .  
         [0022]     The core turbine fueling system includes a high pressure centrifugal pump  28  that pumps fuel toward the nozzle outlet passage  18 . A starting pump system is provided to pump fuel flow at low engine speeds when starting the gas turbine engine. The starting pump system includes a small variable displacement starting pump  30  in parallel circuit with the centrifugal pump  28 . The variable displacement start pump  30  pumps the fuel during initial engine startup when the engine speed is slow and the high pressure centrifugal pump  28  is unable to generate sufficient pressure and/or flow. A control valve  32  is also provided to sense pressure or flow to control operation of the start pump  30 , such that the start pump  30  may disengage or otherwise may stop pumping fuel upon a predetermined pressure representing adequate engine speed. A check valve  34  arranged downstream of the high pressure centrifugal pump  28  prevents fuel from backflowing through the centrifugal pump  28  at engine start up when the speed is slow.  
         [0023]     In either event, the centrifugal pump  28  and/or the start pump  30  generate a high pressure source Ps/P 1  in a second conduit section  36  that leads toward the core of the gas turbine engine.  
         [0024]     A fuel metering valve  40  is arranged in fluid series with the centrifugal pump  28  to meter fuel flow through the nozzle outlet passage  18 . In this embodiment the fuel metering valve  40  is arranged downstream of the high pressure centrifugal pump  28 . The position of the fuel metering valve  40  is set with a suitable servo-controller. For example, as shown in  FIG. 1 , an electro-hydraulic servo-valve (EHSV)  42 , which is in turn controlled by the full authority digital electronic controller (FADEC)  44  as schematically indicated. Also, preferably, closed loop control is provided over the fuel metering valve  40  with a position sensor indicated as a linear variable displacement transducer (LVDT)  46  providing electronic position feedback to the FADEC  44 . The position of the fuel metering valve  40  sets the fuel flow rate flowing through the fuel metering valve  40  to the nozzle outlet passage  18 . A small but significant pressure drop is also developed across the fuel metering valve  40  during operation (typically in a range of about 30-70 psi, but it could be significantly higher) which results in a reduced pressure P 2  in a third conduit section  48 .  
         [0025]     A throttle valve  50  is arranged in fluid series with the fuel metering valve  40  and the centrifugal pump  28  to regulate pressure drop across the fuel metering valve  40 . In this embodiment the throttle valve  50  is arranged downstream of the high pressure centrifugal pump  28  and the fuel metering valve  40 . The throttle valve  50  includes a valve body  52  and a movable valve member shown as a multiple diameter piston  54  (as shown in  FIG. 3 ).  
         [0026]     The valve body  52  may be comprised of an assembly of valve body components including an outer housing  52   a  (which may include one or more splits) and sleeve inserts  52   b ,  52   c  to provide for ready valve assembly. The valve body  52  defines a larger diameter bore  56  and a smaller diameter bore  58  to provide a valve body chamber  59  in which the piston  56  slides. The valve body  52  defines an inlet port  60 , an outlet port  62 , and a intermediate passage  64  through the valve body  52  connecting inlet and outlet ports  60 ,  62 . The piston  52  includes a larger cylindrical land  66  and a smaller cylindrical land  68 , which may be integrally connected by a shank portion  70  that provides for a fluid chamber therebetween. The combination of the piston  54  and the valve body  52  define three separate chambers including an inlet chamber  69 , a compensation chamber  71 , and an actuation chamber  73 .  
         [0027]     The inlet chamber  69  is fluidically connected to the third conduit section  48  and therefore receives metered fuel flow through the inlet port  60 . No restriction is provided at the inlet port  60  and therefore the inlet chamber  69  is considered to be at pressure (P 2 ). A spring  72  in the inlet chamber  69  acts upon the piston  54  and biases the piston  54  toward a closed position (as shown in  FIG. 3 ) against a valve seat  74 . The valve seat  74  may include an O-ring gasket  78  as shown to provide a seal and prevent leakage. Fluid pressure (P 2 ) in the inlet chamber also acts upon the larger diameter land  66  also provides an axial force that urges the differential piston  54  toward the closed position.  
         [0028]     At the other axial end, fluid pressure in the actuation chamber  73  acts in opposition to the force of the spring  72  and the fluid pressure (P 2 ) in the inlet chamber  69 . The fluid pressure in the actuation chamber  73  acts upon the smaller cylindrical land  68  to urge the differential piston  54  axially toward an open position (as shown in  FIG. 4 ). A solenoid valve  80  acts as a switch to connect the actuation chamber  73  to a higher pressure conduit section  36  (at pressure P 1 ) at the metering valve inlet or a lower sump pressure conduit section  27  (at pressure Pb). The solenoid valve  80  includes an electrical coil  82  that drives a spool valve  84 . The electrical coil  82  is electrically connected to the FADEC  44  for control thereby. In operation, the FADEC  44  selectively sends signals to the solenoid valve  80  to pressurize the actuation chamber with pressure P 1  or vent the actuation chamber to sump pressure Pb.  
         [0029]     The relative diameters of the lands  66 ,  68  of the differential piston  54  are sized and the spring force sufficient such that the throttle valve  50  closes when the actuation chamber  73  is vented to the lower sump pressure conduit section  27  (at pressure Pb). This is shown in  FIG. 3  where the solenoid valve  80  is positioned by the FADEC  44  to vent the actuation chamber to sump pressure Pb. In the closed position, the force of the spring  72  (and/or fluid pressure at P 1 ) seats the differential piston  54  against the seat  74  and the O-ring gasket  78  and o-ring gasket  96  and thereby prevents fuel flow to the nozzle outlet passage  18 . Thus, an advantage of the present invention is that the throttle valve  50  is biased to a closed position and thereby may be used to provide automatic shut-off upon engine shut down or when otherwise desired. An additional large shut-off valve does not need to be provided in series with the throttle valve  50 , thereby providing for weight and size advantages.  
         [0030]     In the embodiment of  FIGS. 3-4 , and during operation, the larger cylindrical land  66  partially covers an intermediate port  87  disposed along the intermediate passage  64  to control size of and provide for a first variable restriction  88 . As shown in  FIG. 5 , the intermediate port  87  may comprise several holes  89  formed into the larger valve body sleeve  52   b . The smaller cylindrical land  68  partially covers the outlet port  62  to control size of and provide for a second variable restriction  90 . The outlet port  62  may comprise several holes  91  formed into the smaller valve body sleeve  52   c.    
         [0031]     The first and second variable restrictions  88 ,  90  are arranged in fluid series in the throttle valve  50  between the nozzle outlet passage  18  (at pressure Pn) and the fuel metering valve. At selected positions of the throttle valve  50  a pressure drop is developed across the first restriction  88 . This generates a reduced pressure Pcomp (that is less than P 1  and P 2 ) inside the throttle valve in the compensation chamber  71 . This compensation pressure (Pcomp) is utilized to generate an axial compensation force on the valve piston  54 . In this embodiment, the compensation pressure (Pcomp) is between the larger cylindrical land  66  and the smaller cylindrical land  68 . Since the larger cylindrical land  66  has a larger working surface, more fluid pressure works upon the larger cylindrical land  66  as opposed to the smaller cylindrical land  68 . The differential in working surface areas determines how much axial compensation force is provided by the compensation pressure (Pcomp). The forces on the valve can be represented by the following equation: 
 
 F=P   comp *( IIR   lg   2   −IIR   sm   2 )+ P   actuation   *IIR   sm   2   P   2   *IIR   lg   2 −Spring Force±Fluid Force(s) 
 
 where: 
    F=Axial Force on Valve Piston (which is zero when the valve is balanced)     P comp =Compensation Pressure;     P actuation =Actuation Pressure (either P 1  or Pb depending upon state of solenoid valve);     P 2 =Pressure in Inlet Chamber;     R lg =Radius of larger diameter differential Piston Land;     R sm =Radius of smaller diameter differential Piston Land;    
 
         [0038]     The second variable restriction  90  provides a pressure drop to reduce the pressure from Pcomp inside the compensation chamber  71  to Pn, the pressure in the nozzle outlet passage  18 . Thus, two different pressure drops occur across the throttle valve  50 .  
         [0039]     The present invention achieves a substantially constant pressure drop across the metering valve  40 . To achieve this, the ports  62 ,  87  that restrict flow and form restrictions  88 ,  90  in the throttle valve  50  are configured to control pressure Pcomp in the compensation chamber  71  and thereby generate a controlled axial compensation force upon the piston  54 . The shape, size and configuration of these ports  62 ,  87  (and more specifically porting holes  89 ,  91  as shown in  FIG. 5 ) are selected to provide changes in compensation pressure that offset variances in spring forces generated by the spring  72  that occur as the throttle valve piston  54  moves axially and fluid flow forces that act upon the valve. Preferably the size of the restrictions  88 ,  90  (and thereby the variable porting orifices) change in flow area at different rates when the throttle valve moves between positions. Generally for most operational positions, the upstream variable orifice has a larger flow area than the downstream variable orifice during operation of the throttle valve (although there may be instances where this is not true). Spring force changes occur naturally since spring force is a function of position. The equation (Hooke&#39;s law) for determining changes in spring force is: 
 
ΔF=K*ΔX 
        where:     ΔF=the change in spring force;     K=the spring constant; and     ΔX=the change in spring/valve axial position 
 
 Likewise, naturally occurring fluid flow forces such as Bernoulli forces can change based upon changes in valve position. The present invention may be used to counteract changes in fluid flow forces in addition to counteracting changes in spring forces. 
       
 
         [0044]     For engine starting, after sufficient pressure is available at the fuel control, the FADEC  44  issues a signal that moves the shutoff solenoid valve  80  to the run position. This connects the actuation chamber  73  to the second conduit section and therefore pressure P 1 . At the same time, the FADEC  44  issues a signal to the EHSV  42  that holds fuel metering valve  40  in the closed position. This temporarily connects the inlet chamber  69  to the sump pressure Pb through an annulus  92  formed into the fuel metering valve  40  that communicates with the supply conduit network section  27  (at pressure Pb). The annulus  92  connects pressure Pb with the inlet chamber  69  when the fuel metering valve  40  is in the closed position (see e.g.  FIG. 3 ). This causes a substantial pressure drop to develop across the throttle valve  50  (for example P 1 -Pb may be about 250 PSI). In turn, this causes the differential piston  54  of the throttle valve  50  to move far to the right with the given orientation shown in the Figures effectively closing the outlet port  62  with the smaller diameter land  68  of the differential valve piston  54  (the throttle valve  50  wants to lower the metering valve pressure differential by throttling).  
         [0045]     Shortly thereafter, the FADEC  44  then issues a signal to the EHSV  42  to drive the fuel metering valve  40  to a low metered flow position for engine starting. The opening of the fuel metering valve  40  connects the inlet chamber  69  to pressure P 1  through the fuel metering valve  40  (which is quickly reduced somewhat to pressure P 2  by the pressure drop across the fuel metering valve). Since the fluid pressure drop across the throttle valve  50  is now near zero psi, the spring force of spring  72  forces the differential piston  54  to the left (with the given orientation of the Figures) and thereby opens the throttle valve  50 . This allows pressure drop across the fuel metering valve  40  to increase to a predetermined set point. For example, a pressure drop (P 1 -P 2 ) across the fuel metering valve  40  of about 50 p.s.i. is typical for many applications.  
         [0046]     The throttle valve  50  is now in regulation and is allowing metered fuel flow to flow therethrough to the nozzle outlet passage  18  and the core of the gas turbine engine  16 . In performing its regulating function, the throttle valve will control the pressure drop (P 1 -P 2 ) across the fuel metering valve  40  and maintain it substantially constant at the predetermined set point. In particular, if pressure P 2  is too high as compared with pressure P 1 , that excess pressure is communicated to inlet chamber  69  tends to urge the throttle valve  50  further open which in turn relieves the pressure in the inlet chamber  69  and thereby reduces pressure P 2 . Similarly, if pressure P 1  is too high as compared with P 2 , the excess pressure is sensed or communicated to the actuation chamber  73  which in turn restricts flow through the throttle valve  50  which increases pressure P 2  to correct the variance in pressure drop from the predetermined set point.  
         [0047]     As the throttle valve  50  repositions itself to maintain a constant pressure drop across the fuel metering valve  40 , the force of the spring  72  changes due to axial movement. The pressure Pcomp generated in the compensation chamber  71  is configured to offset those spring force changes. Pressure Pcomp is designed through configuration of the variable restriction outlet port  62  and intermediate port  87  to counteract changes in the spring force of spring  72  due to axial valve repositioning. As such changes in the compensation pressure Pcomp is a function of change in axial position ΔX. As noted previously, compensation pressure Pcomp may also be designed to counteract the fluid flow forces that may be experienced that would otherwise tend to create some error in addition to spring forces. The intent of the throttling valve is to maintain metering valve pressure drop (P 1 -P 2 ) as constant as possible for the entire engine fuel flow operating envelope. If the engine speed versus fuel flow requirements are known for both engine acceleration and deceleration conditions, the possible combinations of inlet pressure P 1 , outlet pressure Pn, and P 2  pressure can be mathematically determined by one of ordinary skill in the art. Substituting these values into the force balance and flow equations for the valve the compensating pressure in chamber  71  and port area  87  can be calculated to give nearly zero error in metering valve pressure drop (P 1 -P 2 ).  
         [0048]     For engine shutdown, the FADEC  44  issues a signal to move the shutoff solenoid valve  80  to the off position in which the actuation chamber  73  is vented to the supply conduit network section  27  at sump pressure Pb. At approximately the same time, the FADEC  44  issues a signal to the EHSV  42  to drive the fuel metering valve  40  to the closed position exposing the inlet chamber  69  to sump pressure Pb as well. Since fluid forces are now generally balanced across the throttle valve  50 , the spring  72  takes over and drives the differential valve piston  54  closed against valve seat  74 . The gasket  78  at the valve seat  74  prevents leakage to the outlet passage (as well as other strategically located seals  94 ,  96  which may be needed depending upon how many components make up an assembly for the valve body).  
         [0049]     It is an advantage in viewing the embodiment of the invention for the core engine of  FIGS. 2-5  that there is no need for a pressure sensor and integrating valve which are arranged in parallel with the fuel metering valve (schematically shown in  FIG. 1 ). The present invention achieves good to excellent accuracy for maintaining a constant pressure drop across the fuel metering valve while also reducing weight and expense, and at the same time increasing dynamic performance, stability and reliability of the fuel system Some or all of these advantages can be obtained with the present invention.  
         [0050]     Another embodiment of the invention is shown in  FIGS. 2 and 6 , incorporated in an afterburner system for a gas turbine engine  16 . The same general principles that apply to the first embodiment likewise generally apply to this embodiment. However, this embodiment demonstrates that many design alterations and different valve arrangements can be made without departing from the present invention.  
         [0051]     In this embodiment, a fuel metering valve  110  and a throttle valve  112  are arranged in fluid series upstream of a high speed, high pressure, vapor core centrifugal pump  114 . A check valve  116  and an overboard drain valve  118  may be positioned downstream of the pump  114 . The metering and shutoff/throttling valves are located at the inlet of the vapor core centrifugal pump rather than the discharge of the pump so the pump can be drained of fuel when the afterburner is not being used. This saves energy and prevents high fuel temperatures in the non-flowing pump. The drained pump is kept rotating at high speed whenever the engine is in operation and it can be brought on-line very quickly by opening the metering valve and throttling/shutoff valve on the pump inlet.  
         [0052]     Like the first embodiment, the position of the fuel metering valve  110  is set with an electro-hydraulic servo-valve (EHSV)  120 , which is in turn controlled by the full authority digital electronic controller (FADEC)  44  as schematically indicated. Also like the first embodiment, closed loop control is preferably provided over the fuel metering valve  110  with a position sensor indicated as a linear variable displacement transducer (LVDT)  122  providing electronic position feedback to the FADEC  44 . The position of the fuel metering valve  110  sets the fuel flow rate flowing through the fuel metering valve  110  to the nozzle outlet passage  20 . A pressure drop is also developed across the fuel metering valve  40  during operation (typically in a range of about 30-70 psi but could be much higher) which results a lower pressure Pbl in a second conduit section  124  of the afterburner system.  
         [0053]     The throttle valve  112  is arranged in fluid series with the fuel metering valve  110  and the centrifugal pump  114  to regulate pressure drop across the fuel metering valve  110 . In this embodiment, the throttle valve  112  is arranged downstream of the fuel metering valve  110 , but upstream of the high pressure centrifugal pump  114 . With two different arrangements being shown in different embodiments, it will be appreciated that the throttle valve, pump and fuel metering valve may be arranged in any number of different arrangements in fluid series with one another.  
         [0054]     The throttle valve  112  includes a valve body  126  and a movable differential valve piston  128 . Like the first embodiment, the valve body  126  may be comprised of an assembly of valve body components. With the valve piston  128  slidably mounted in the valve body  128 , the combination defines three chambers  130 ,  132 ,  134  which may be in communication with or subjected to different pressures Pb 1 , Pb and Pcomp 2  during operation. In this embodiment (like the first embodiment), a spring  136  is arranged in the Pb 1  chamber  130  to urge the valve piston  128  to open the throttling port  148 (Andy—The difference has to do with the shutoff function. In the first case the throttling valve provides the shutoff function and the spring first pushes the valve to open the throttling port ( 62 ) and then continues to drive the piston to a stop where it then provides shutoff. In the second case the metering valve provides the shutoff function so the throttling valve does not require the shutoff seals.) Fuel shutoff in this embodiment is achieved with the fuel metering valve  110 . Specifically, the fuel metering valve  110  is movable against a valve seat  138  which may include an annular seal  140  for shutoff. Also the check valve  116  is arranged prevent fuel leakage to the nozzles when fuel is shut off. There is also no need for a solenoid valve or fluid switch in this embodiment.  
         [0055]     Selected porting of the chambers in the throttle valve  112  is provided to control how the throttle valve regulates pressure. The spring chamber  130  is connected by a port and passage  131  to the pressure Pb 1  generated between the throttle valve  112  and fuel metering valve  110 . The differential chamber  132  is connected by a port and passage  133  to the Pressure Pb experienced upstream of the fuel metering valve  110  in conduit section  27 . In this embodiment, the compensation chamber  134  is connected by an inlet port  135  to the main fuel flow passage at pressure Pb 1  from the fuel metering valve  110  and an outlet port  137  connected to the core inlet chamber  139  of the centrifugal pump  114 . The vapor core centrifugal pump  114  includes a rotor  142  that impels fuel from the core inlet chamber  139  to a radial outlet  144  at pressure P 1 ′, which is then communicated through nozzle outlet passage  20  to the afterburner nozzles of the gas turbine engine  16 .  
         [0056]     As shown in  FIG. 6 , movement of the differential valve piston  128  opens and closes the inlet and outlet ports  135 ,  137  for the compensation chamber creating variable restrictions  146 ,  148  that control the compensation pressure Pcomp 2 . The differential valve piston  128  includes through-ports  150 ,  152  that align with the inlet and outlet ports  135 ,  137  to communicate fuel through the valve piston  128  into and out of the compensation chamber  134 . The inlet and outlet ports  135 ,  137  may be selectively configured in size and shape to control the size of the variable restrictions  146 ,  148  over the stroke of the valve piston  128 . Alternatively (or in addition) the through-ports  150 ,  152  of the differential valve piston  128  may be selectively configured to control the size of the variable restrictions  146 ,  148 . In either event, and when the throttle valve  112  moves/repositions, the restrictions  144 ,  146  cause the compensation pressure Pcomp 2  to change in a manner that counteracts forces and/or fluid forces to maintain substantially constant the pressure drop across the fuel metering valve  10 .  
         [0057]     In this embodiment, there is no need for a separate shutoff solenoid, since the metering valve provides the shutoff function. The throttle valve  112  automatically moves to a pressure regulating position when the metering valve opens and sufficient pressure and fuel flow is available from the boost pump.  
         [0058]     All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.  
         [0059]     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.  
         [0060]     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.