Abstract:
In a fuel injector circuit for a gas turbine engine, a fuel staging valve assembly for distributing fuel into multiple zones in the combustor, the staging valve assembly, including a pilot valve operatively interconnected with at least one main valve, having high pressure and no leak capabilities, which are used to open, close and modulate the mass flow rate volume of fuel within the fuel injection circuit, with the position of the normally-closed valve being controlled by the pressure difference between the nozzle fuel supply circuit and a separately supplied signal circuit. As long as the desired pressure differential is maintained, fuel flow may be modulated without affecting the position of the valve, with the valve seats and valve seals being so configured as to prevent fuel leakage into the downstream nozzle circuit under these conditions.

Description:
CROSS-REFERENCE TO RELATED CASES 
   The present application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 60/462,797 filed Apr. 11, 2003, the disclosure of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention generally relates to gas turbine engines, and more particularly, to fuel injectors for supplying fuel to nozzles in such engine combustion chambers wherein, for each nozzle, a fuel staging valve assembly, including an operatively interconnected pilot valve and at least one main valve, distribute fuel in a predetermined manner. 
   BACKGROUND OF THE INVENTION 
   Fuel injector assemblies are useful for applications such as gas turbine combustion engines for directing pressurized fuel from a manifold to one or more combustion chambers. Such assemblies also function to prepare the fuel for mixing with air prior to combustion. Each injector assembly typically has an inlet fitting connected the manifold, a tubular extension or stem connected at one end to the fitting in a typically cantilevered fashion, and one or more spray nozzles connected to the other end of the stem or housing for directing the fuel into the combustion chamber. A single or multiple fuel feed circuit(s) extend through the housing to supply fuel from the inlet fitting to the nozzle or nozzle assembly. Appropriate valves and/or fuel dividers, such as fuel system staging valves are generally provided to direct and control the fuel flow through the nozzle. The fuel provided by the injector(s) is mixed with air and ignited so that the expanding gases of combustion can, for example, move rapidly across and rotate engine blades in the gas turbine engine to provide power, for example, to an aircraft in a manner well known to those skilled in this art. 
   In a known prior art check valve, namely the Parker Hannifin Corporation Microseal check valve, Part Number 372720, that is designed for high-pressure hydraulic applications, the load on the hard seat metal valve seat increases as the supply pressure increases and also requires precise lapping of the opposed mating surfaces. A plurality of dynamic seals is required and moving parts of the valve are located downstream of the valve seat, all of which can have a negative effect upon both performance and longevity. 
   Attempted prior art solutions have been numerous, with some being set forth in the following patents: U.S. Pat. No. 3,082,304 to Radway; U.S. Pat. No. 3,410,304 to Paul, Jr.; U.S. Pat. No. 4,172,469 to Boehringer; U.S. Pat. No. 4,570,668 to Burke, et al.; U.S. Pat. No. 4,738,282; U.S. Pat. No. 4,825,649 to Donnelly, et al.; U.S. Pat. No. 4,967,791 to Stemberger; U.S. Pat. No. 5,076,144 to Karakama et al; U.S. Pat. No. 5,558,129 to Mayeux; and U.S. Pat. No. 6,158,208 to Hommema. 
   However, no single one or combination of these references either discloses or suggests all of the claimed features of the present invention. 
   SUMMARY OF THE PRESENT INVENTION 
   Accordingly, in order to overcome the deficiencies of the prior art devices, the present invention provides a device or structure in the form of a fuel staging valve, having high pressure and no leak capabilities, which are used to open, close and modulate the flow size of gas turbine engine fuel nozzle injection circuits. 
   The position of the normally-closed staging valve assembly is controlled by the pressure difference between the nozzle fuel supply circuit and a separately supplied signal circuit. As long as the desired pressure differential is maintained, fuel flow may be modulated without affecting the position of the valve assembly. During engine operation, the closed valve assembly, and particularly the main valve can be exposed to a wide range of fuel supply pressures. The valve seal and seat features of the present invention are configured to prevent fuel leakage into the downstream nozzle circuit under these conditions. This “No Leak” feature eliminates a fuel source that can contribute to combustor emissions and nozzle coking. Furthermore, this “No Leak” feature also prevents drainage of upstream fuel into the combustor at engine shutdown. 
   Specifically, one embodiment of this invention pertains to a fuel staging valve assembly, for distributing fuel flow to multiple zones of a nozzle or combustor in a gas turbine engine, including a pilot valve operatively interconnected with at least one main valve, the at least one main valve comprising a dual diameter valve housing; a dual diameter cylindrical valve sleeve fixedly, sealingly and conformably received within the valve housing, the sleeve having a first diameter portion and a second diameter portion joined via an intermediate annular portion, with a peripheral land cavity being located between the sleeve intermediate portion and an adjacent portion of the valve housing; and the first diameter portion having a peripheral, recessed, annular gland area with axially spaced first and second pluralities of discrete radial passages therethrough; a dual diameter hollow cylindrical valve spool, having a central cavity in communication with a source of fuel, conformably and slidably received within the cylindrical valve sleeve, the spool having a first diameter portion and a second diameter portion joined via an intermediate annular portion, with predetermined diametral clearance spaces being provided between corresponding adjoining first and second diameter portions of the sleeve and spool, thereby permitting a predetermined amount of fluid leakage therebetween, during operation of the pilot poppet valve; and an annular pressure signal cavity, interconnected with the land cavity, being located between intermediate annular portions of the valve sleeve and the valve spool; a centrally apertured spring retainer fixedly received within the sleeve first diameter portion and closing one end thereof; a main valve spring, interposed between the spring retainer and the spool intermediate annular portion, for preloading the spool against the sleeve; a centrally-apertured dual diameter valve seat retainer member having a first diameter portion and a second diameter portion joined via an intermediate radial surface portion, the retainer member first diameter portion being fixedly and sealingly received within an open end of the sleeve second diameter portion, with the retainer member intermediate radial surface portion being provided with an inwardly-directed, raised, central contoured seal seat portion adjoining the retainer member second diameter portion; a generally cup-shaped closure member is fixedly and sealingly received on the retainer member second diameter portion and includes a central main valve discharge orifice adapted to be operatively interconnected with the nozzle; a multiple diameter poppet member having interconnected first, second and third diameter portions, the first diameter portion being yieldingly, slidably received within an open end of the valve spool second diameter portion, with the axial movement of the poppet member being restricted via a split retaining roll pin press-fitted relative to the valve spool second diameter portion but having a predetermined peripheral clearance relative to the poppet member; a poppet spring, operatively interposed between the poppet member and a peripheral internal wall portion in the spool member second diameter portion, for axially biasing the poppet member toward the contoured seal seat portion, with the biasing being limited via the predetermined axial clearance, relative to the roll pin; a stiff, elastic, annular seal member, contoured in cross-section, fixedly retained within a mating contoured recess within the poppet member second diameter portion, having an axial outer surface adapted to sealingly mate with the raised valve seat portion of the valve seat retainer member, with the third diameter portion of the poppet member, in an at-rest position, axially extending, beyond the seal member axial outer surface and in the vicinity of the seal seat portion, with at least one predetermined diametral clearance, into the retainer member second diameter portion; and a shim, fixedly abutting and acting on the retainer member intermediate radial surface portion, provides an initial, predetermined sealing force, relative to the seal member axial outer surface, against the retaining roll pin, thereby preventing fuel leaks at low fluid supply pressure, with the predetermined peripheral clearance, relative to the poppet member serving to limit the compression of the elastic seal member as well as allowing compensating for any seal compression set. 
   In variations thereof, the axial outer surface of the elastic, annular seal member is one of being generally flattened, contoured, stepped and relieved, via surface finishing, after being fixedly retained within the recess; the elastic, annular seal member is fixedly retained within a matingly contoured annular recess located in the poppet member second diameter portion, the seal member being one of bonded, molded-in-place and cast-in-place; the at least one of the contoured recess and seal member has, in cross-section, an at least partial dovetail shape; and the elastic seal member is comprised of a stiff rubber-based composition having an approximate 90 durometer hardness; with the contoured valve seat portion being one of gradually tapering, semicircular and of a double inwardly-tapering shape. 
   In another variation thereof, the predetermined diametral clearance spaces between the corresponding first and second diameter portions of the valve sleeve and valve spool are located in at least one of the mutually adjoining sleeve inner wall and spool outer wall surfaces. 
   In a further variation thereof, the diametral clearance spaces function as predetermined, controlled, leakage paths through which a high pressure signal fluid can flow, via a fluid signal pressure conduit connected with the peripheral land cavity, from the high pressure signal cavity to adjoining areas of lower pressure between the valve sleeve and the valve spool via at least one further intermediate aperture and the diametral clearance spaces; with the percentage of fluid leakage being below about two percent of the total fluid flow within the assembly. 
   In still further variations, all axial movements of the valve spool, relative to the valve sleeve, are devoid of any contact with a dynamic seal. 
   In yet differing variation, all axially movable components of the main valve are located upstream of the valve seat retainer member, in a direction opposite to the flow of fuel exiting from the discharge orifice, the axially movable components thereby being protected from combustion products produced during operation of the gas turbine engine; with the axially movable components including the valve spool, the main valve spring, the poppet member, the poppet spring and the seal member. 
   In an additional variation thereof, the main valve centrally apertured spring retainer includes an inner annular end portion, having a plurality of spaced radial passages, at a location generally radially inwardly of the annular gland area, the retainer annular end portion, during certain predetermined operating positions of the staging valve assembly, being axially spaced differing distances, relative to the main valve spool; and wherein during at least one of the certain predetermined operating positions of the staging valve assembly, one of the axially spaced first and second pluralities of discrete radial passages is blocked by the main valve spool first diametral portion. 
   A differing variation thereof further including a fluid inlet plate, having a central cylindrical portion extending through the centrally apertured spring retainer into the cylindrical cavity, the fluid inlet plate being interposed between the spring retainer and a retaining ring in the main valve sleeve first cylindrical portion, the inlet plate cylindrical portion serving as an inlet for the fuel into the main valve; and a fuel strainer having a closed end and an open end, the open end being affixed to the inlet plate cylindrical portion, the fuel strainer extending into the central interior cavity of the main valve. 
   In another embodiment of this invention, the pilot valve of the staging valve assembly comprises: a dual diameter valve housing; a dual diameter cylindrical valve sleeve fixedly, sealingly and conformably received within the valve housing, the valve sleeve having a first diameter portion and a second diameter portion joined via an intermediate annular portion, with a peripheral land cavity being located between the sleeve intermediate portion and an adjacent portion of the valve housing; and the first diameter portion having a peripheral, recessed, annular gland area with a plurality of discrete radial passages therethrough; a dual diameter hollow cylindrical valve spool, having a central cavity, conformably and slidably received within the cylindrical valve sleeve, the spool having a first diameter portion and a second diameter portion joined via an intermediate annular portion, with predetermined diametral clearance spaces being provided between corresponding adjoining first and second diameter portions of the sleeve and spool, thereby permitting a predetermined amount of fluid leakage therebetween, during operation of the pilot poppet valve; and an annular pressure signal cavity, interconnected with the land cavity, being located between annular portions of the valve sleeve and the valve spool; a spring retainer fixedly and sealingly received within the sleeve first diameter portion and closing one end thereof; a pilot valve spring, interposed between the spring retainer and the spool intermediate annular portion, for preloading the spool against the sleeve; a centrally-apertured dual diameter valve seat retainer member having a first diameter portion and a second diameter portion joined via an intermediate radial surface portion, the retainer member first diameter portion being fixedly and sealingly received within an open end of the sleeve second diameter portion, with the retainer member intermediate radial surface portion being provided with an inwardly-directed, raised, central contoured seal seat portion adjoining the retainer member second diameter portion; a generally cup-shaped closure member is fixedly and sealingly received on the retainer member second diameter portion and includes a central pilot valve discharge orifice adapted to be operatively interconnected with the nozzle; a dual diameter poppet member having a first diameter portion and a second diameter portion, the first diameter portion being yieldingly, slidably received within an open end of the valve spool second diameter portion, with the axial movement of the poppet member being restricted via a split retaining roll pin press-fitted relative to the valve spool second diameter portion but having a predetermined peripheral clearance relative to the poppet member; a poppet spring, operatively interposed between the poppet member and a peripheral internal wall portion in the spool member second diameter portion, for axially biasing the poppet member toward the contoured seal seat portion, with the biasing being limited via the predetermined axial clearance, relative to the roll pin; a stiff, elastic, annular seal member, contoured in cross-section, fixedly retained within a mating contoured recess within the poppet member second diameter portion, having an axial outer surface adapted to sealingly mate with the raised valve seat portion of the valve seat retainer member; and a shim, fixedly abutting and acting on the retainer member intermediate radial surface portion, provides an initial, predetermined sealing force, relative to the seal member axial outer surface, against the retaining roll pin, thereby preventing fuel leaks at low fluid supply pressure, with the predetermined peripheral clearance, relative to the poppet member serving to limit the compression of the elastic seal member as well as allowing compensating for any seal compression set. 
   A variation thereof further includes a fluid pilot supply conduit interconnecting the pilot and main valve spool central cavities at their respective valve gland areas; and a source of fluid signal pressure, connected with the pilot valve peripheral land cavity, and a fluid pressure signal conduit, interconnecting the pilot and main valves at their respective peripheral land cavities, for supplying the fluid signal pressure to the main valve. 
   A further embodiment of this invention pertains to a gas turbine engine having a fuel staging valve assembly, for distributing fuel flow to a multiple zone nozzle therein, including a pilot valve operatively interconnected with at least one main valve, each of the valves including: a dual diameter valve housing; a dual diameter cylindrical valve sleeve fixedly, sealingly and conformably received within the valve housing, the sleeve having a first diameter portion and a second diameter portion joined via an intermediate annular portion, with a peripheral land cavity being located between sleeve intermediate portion and an adjacent portion of the valve housing; and the first diameter portion having a peripheral, recessed, annular gland area with at least an axially spaced first plurality of discrete radial passages therethrough; a dual diameter hollow cylindrical valve spool, having a central cavity, conformably and slidably received within the cylindrical valve sleeve, the spool having a first diameter portion and a second diameter portion joined via an intermediate annular portion, with predetermined diametral clearance spaces being provided between corresponding adjoining first and second diameter portions of the sleeve and spool, thereby permitting a predetermined amount of fluid leakage therebetween, during operation of the pilot poppet valve; and an annular pressure signal cavity, interconnected with the land cavity, being located between intermediate annular portions of the valve sleeve and the valve spool; a centrally apertured spring retainer fixedly received within the sleeve first diameter portion and closing one end thereof; a valve spring, interposed between the spring retainer and the spool intermediate annular portion, for preloading the spool against the sleeve; a centrally-apertured dual diameter valve seat retainer member having a first diameter portion and a second diameter portion joined via an intermediate radial surface portion, the retainer member first diameter portion being fixedly and sealingly received within an open end of the sleeve second diameter portion, with the retainer member intermediate radial surface portion being provided with an inwardly-directed, raised, central contoured seal seat portion adjoining the retainer member second diameter portion; a generally cup-shaped closure member is fixedly and sealingly received on the retainer member second diameter portion and includes a central main valve discharge orifice; a multiple diameter poppet member having interconnected first and second diameter portions; a poppet spring, operatively interposed between the poppet member and a peripheral internal wall portion in the spool member second diameter portion; a stiff, elastic, annular seal member, contoured in cross-section, fixedly retained within a mating contoured recess within the poppet member second diameter portion, having an axial outer surface adapted to sealingly mate with the raised valve seat portion of the valve seat retainer member; a shim, fixedly abutting and acting on the retainer member intermediate radial surface portion; a fluid pilot supply conduit interconnecting the pilot and main valve central cavities at their respective gland areas; a source of fluid signal pressure connected with the pilot valve peripheral land cavity; and a fluid pressure signal conduit interconnecting the pilot and main valves at their respective peripheral land cavities, wherein the improvement comprises: the poppet member first diameter portion being yieldingly, slidably, received within an open end of the valve spool second diameter potion, with the axial movement of the poppet being restricted via a split retaining roll pin press-fitted relative to the valve spool second diameter portion but having a predetermined clearance relative to the poppet member; the poppet spring axially biasing the poppet member toward the contoured seal seat portion, with the biasing being limited via the predetermined axial clearance, relative to the pin; and the shim providing an initial, predetermined, sealing force, relative to the seal member axial outer surface, against the retaining roll pin, thereby preventing fuel leaks at low fluid supply pressure, with the predetermined peripheral clearance, relative to the poppet member serving to limit the compression of the elastic member as well as allowing compensation for any seal compression set. 
   In a variation thereof, the main valve poppet member further includes an integral third diameter portion, with the third diameter portion, in an at-rest position, axially extending beyond the main valve seal member axial outer surface and in the vicinity of the main valve seal portion, with at least one predetermined diametral clearance, into the second diameter portion of the valve seal retainer member of the main valve. 
   In another variation thereof, the elastic, annular, seal member is fixedly retained within a matingly contoured annular recess located in the poppet member second diameter portion, the seal member being one of bonded, molded-in-place and cast-in-place; and the axial outer surface of the elastic, annular, seal member is one of being generally flattened, contoured, stepped and relieved, via surface finishing, after being fixedly retained within the recess. 
   In a further variation thereof, the elastic, annular, seal member is comprised of a stiff rubber-based composition having an approximate 90 durometer hardness; at least one of the contoured recess and seal member has, in cross-section, an at least partial dovetail shape; and the contoured valve seat portion is one of gradually tapering, substantially semicircular and of a double-inwardly tapering shape. 
   In yet another variations thereof, the predetermined diametral clearance spaces between the corresponding first and second diameter portions of the valve sleeve are located in at least one of the mutually-adjoining sleeve inner wall and spool outer wall surfaces, with the diametral clearance spaces functioning as predetermined, controlled leakage paths through which the high pressure signal fluid can flow, from the high pressure signal cavity to adjoining areas of lower pressure between the valve sleeve and the valve spool via at least one further intermediate aperture and the diametral clearance spaces; and the percentage of fluid leakage being below about 2% of the total fluid flow within the assembly. 
   In yet a further variation thereof, all axially movable components of the valve spool, relative to the valve sleeve, are devoid of any contact with a dynamic seal. 
   In additional variations thereof, all axially movable components of the valves are located upstream of the valve seat retainer member, in a direction opposite to the flow of fuel exiting from the discharge orifices, the axially slidable components thereby being protected from combustion products produced during operation of the gas turbine engine; with the axially movable components including the valve spools, the main springs, the poppet members, the poppet springs and the seal members. 
   In a differing embodiment of this invention the pilot valve of the fuel staging valve assembly comprises: a multiple diameter valve housing having a central cavity and a fluid signal pressure input port extending into the cavity; a multiple diameter cylindrical valve sleeve, the sleeve having multiple differing diameter sleeve portions, with one of the differing diameter sleeve portions having a peripheral, recessed, annular gland area with a plurality of discrete radial passages extending therethrough; and an intersection of two adjacent ones of the sleeve differing diameter portions serving as a seal seat portion; a multiple diameter cylindrical spool conformably and slidably received within the cylindrical valve sleeve, the spool having multiple differing diameter spool portions; a spring retainer slidably fixedly received on one of the spool differing diameter portions; a pilot valve spring, interposed between the spring retainer and another of the valve spool differing diameter portions; a generally cup-shaped, centrally apertured, valve seal retention member fixedly retained on another one of the valve spool differing diameter portions; a generally cylindrical blocking member having one end attached to and movable with the another one of the sleeve differing diameter portions, with another end of the blocking member being sealingly received against a step portion of the valve housing under certain predetermined valve operating conditions while permitting communication between the housing central cavity and a peripheral land cavity located between the blocking member and the housing; a stiff, elastic, annular seal member fixedly retained within the valve seal retention member, with a peripheral longitudinal portion of the retention member limiting the degree of compression of the seal member; a shim, fixedly abutting and acting upon the valve seal retention member, provides an initial, predetermined, sealing force, relative to the seal member, against the valve seal seat portion; and a centrally apertured disc closure member, attached to an exit portion of one of the two adjacent ones of the sleeve differing diameter portions, and including a central pilot valve discharge orifice. 
   A variation thereof further includes a fluid pilot supply conduit interconnecting the pilot and main valve peripheral land cavities; and a source of fluid signal pressure, connected with the pilot valve central cavity, and a fluid pressure signal conduit, interconnecting the pilot and main valves at their respective valve gland areas, for supplying the fluid signal pressure to the main valve. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic, greatly simplified cross-sectional side view of a gas turbine engine combustion chamber having main and signal fuel manifolds connected to and utilizing pluralities of individual fuel staging valve assemblies constructed according to the principles of this invention. 
       FIG. 2  is a simplified, schematic showing of one the fuel staging valve assemblies of  FIG. 1  with reference to but one nozzle tip. 
       FIG. 3  is a vertical sectional view of one embodiment of the fuel staging valve assemblies of this invention, comprised of interconnected pilot and main valves. 
       FIG. 3   a  is an enlarged showing of the discharge portion of the pilot valve of  FIG. 3 , particularly of the valve seat and seal. 
       FIG. 4  is an enlarged showing of the discharge portion of the main valve of  FIG. 3 , particularly of the valve seat and seal. 
       FIG. 5  is a further enlarged showing of the valve seat and seal portion of  FIG. 4 . 
       FIGS. 6   a  and  6   b  are schematic, opposite half sections of a valve seal utilizing two differing valve seat configurations, respectively. 
       FIGS. 7   a  and  7   b  are sections, similar to  FIGS. 6   a  and  6   b , but showing yet a differing valve seat configuration in  FIG. 7   a , while showing a valve seal having a differing cross-section in  FIG. 7   b.    
       FIGS. 8 to 12  show the fuel staging valve assembly of  FIG. 3  in shut-down, positions  1 , position  2 , position  3  and position  4 , respectively. 
       FIGS. 13   a  and  13   b  show enlarged portions of the circled areas A and B, of the main valve of  FIG. 12 , which detail the main signal to supply fluid leakage paths. 
       FIGS. 13   c  and  13   d  show enlarged portions of the circled areas C and D, of the pilot valve of  FIG. 12 , which detail the pilot signal to supply fluid leakage paths. 
       FIG. 14  is a vertical sectional view of another embodiment of the fuel staging valve assemblies of this invention, comprised of interconnected pilot and main valves. 
       FIG. 15  is an enlargement of the pilot valve of  FIG. 14 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the several drawings, and initially to  FIG. 1 , there is shown a schematic and greatly simplified portion of a gas turbine engine, generally indicated at  20 . Mounted on an upstream, outer wall portion  24  of a combustion chamber  22  are a plurality fuel injector assemblies, for example as indicated generally at  26 . Each fuel injector assembly  26  includes a nozzle tip  28  and a fuel staging valve assembly  30 , which is constructed according to the present invention. The plurality of preferentially circumferentially-spaced fuel injector assemblies  26 , each including a fuel staging valve assembly  30 , are connected via circumferential main fuel manifold  32  and signal fuel manifold  34 . Combustion chamber  22  is a typical combustion chamber for aircraft engine applications, known to those skilled in the art, and will thus not be discussed further, for the sake of brevity. Fuel injector assemblies  26  atomize and direct fuel into combustion chamber  22  for ignition. A compressor (not shown) is mounted upstream of the fuel injectors and provides pressurized air at elevated temperatures in combustion chamber  22  to facilitate combustion. The air is typically provided at highly elevated temperatures which can reach over 1000 degrees F. in aircraft applications. 
   While fuel injector assemblies  26  and fuel staging valve assemblies  30  of the present invention are particularly useful in gas turbine engines for aircraft, these assemblies are also deemed to be useful in other types of applications, such as industrial power generating equipment as well as marine and overland propulsion applications. 
   Turning now to  FIG. 2 , there is illustrated a simplified schematic of one of the fuel staging valve assemblies  30  of  FIG. 1 , with reference to but a single nozzle tip  28 . Fuel staging valve assembly  30  (hereinafter “staging valve  30 ”) includes an operatively interconnected pilot or primary valve  38  and at least one main or secondary valve  40 , which in turn are also operatively interconnected with nozzle tip  28  via one or more known fuel feeds  42 . It should also be understood by those skilled in the art that staging valve  30  is operatively interconnected with both one or more main manifolds  32  and signal manifold  34 , which provide fuel at main and signal pressures, respectively to staging valve  30  in a manner to be explained in more detail hereinafter. 
   As better seen in  FIG. 3 , there is illustrated a first embodiment of staging valve  30 , generally comprised of a first embodiment of a pilot or primary valve  38  and one or more main or secondary valves  40 , linked via a fuel supply pressure plenum  44  (interconnected with main fuel manifold  32 —not shown here), by a fluid supply conduit  46 , and by fluid signal pressure conduit  48 , the latter initially extending into a peripheral cavity portion  57  of pilot valve  38 . For the sake of simplicity, the following description will be limited to an example arrangement that utilizes only one main valve  40 , although additional such valves could be utilized in engines that require more than two zones per nozzle or more complex fuel staging. 
   Turning first to pilot valve  38 , its dual diameter, hollow, cylindrical sleeve  52 , which is sealingly received within a conforming housing cavity portion  54  via multiple elastic sealing members  56  and  58 , has its larger diameter outer portion  52   a  in abutting but not operative contact with pressure plenum  44 , while its smaller diameter end portion  52   b , having a pilot discharge aperture  60 , is operatively connected to a fuel feed  42  ( FIG. 2 ). A dual diameter, hollow, cylindrical valve spool  62  is slidably and conformably received within sleeve  52 , with larger diameter spool portion  62   a  being slidably received within sleeve portion  52   a  while smaller diameter spool portion  62   b  is slidably received within sleeve portion  52   b . Joining spool portions  62   a  and  62   b  is a radial annular wall portion  64 , the inner end of which serves as an inner support portion for one end of a valve spring  66  whose other end abuts a washer  68 , received within an interior cylindrical cavity  72  of a basically cup-shaped spring retainer  70 , which is sealingly received, via an elastic sealing member  74 , within sleeve portion  52   a  and retained therein by a removable retaining ring  76 . It should be noted that cup-shaped retainer  70  blocks all direct communication between supply pressure plenum  44  and the interior cavity  78  of pilot valve  38 . 
   A perusal of the structure of pilot valve  38 , in  FIG. 3 , which depicts valve  38  in an at-rest or shutdown position, shows valve spring  66  at its full extension, i.e., the outer portion of valve spool radial wall portion  64  abuts an inner end ledge portion  80  of sleeve portion  52   b . At least one of valve sleeve  52   a  and spool portion  62   a  is peripherally undercut or relieved so as to produce an intermediate space or cavity  55  there between. Cavity  55  is in communication with an annular intermediate cavity  65 , between spool radial portion  64  and sleeve portion radial surface  82 , the noted communication, between cavities  55  and  65  taking place via at least one aperture  86  between wall portion  64  and ledge portion  80 , thus producing a basically cup-shaped cavity by the joining of cavities  55  and  65 . A tapered portion  52   c , joining pilot valve sleeve portions  52   a ,  52   b , is provided with at least one aperture  59  which serves to provide communication between cavities  55 ,  65  and peripheral cavity portion  57  in housing cavity  54  which in turn communicates with fluid signal pressure conduit  48 . 
   Near the outer end of valve sleeve portion  52   a , adjacent to one of the elastic sealing members  56 , there is provided a recessed annular area  88  having a plurality of radial apertures  90  which lead into an internal pilot valve cavity  78 , past discreet passages  92  in the annular end portion  93  of spring retainer  70 , the latter serving as an abutment for annular surface  94  of the open end of movable valve spool portion  62   a , as will be explained in more detail hereinafter. 
   As best seen in  FIG. 3   a , which is an enlarged showing of the discharge portion of the pilot valve  38  of  FIG. 3 , sealingly received, via seal member  104 , within a recessed cylindrical portion  98  of valve sleeve portion  52   b , is the larger diameter portion  102  of a generally cylindrical valve seat retainer member  100  also having a smaller diameter portion  106  with an inner cavity  156 , sealed via an elastic sealing member  108  against an inner surface of generally cup-shaped closure member  112 . Member  112 , which is provided with bottom, central discharge aperture  60 , has its inner end portion abutting against a radial surface portion  116  of valve seat retainer member  100  which is held in place via a removable retaining ring  118  and an intermediate washer or shim  114 . Radial surface portion  116  is provided with a raised, central contoured surface seal seat portion  120 . 
   Turning again to  FIG. 3   a , pilot valve spool portion  62   b , situated approximately radially inwardly of the location of elastic sealing member  58  is provided with an annular guide ring portion  122  which provides a guidance and support function for the lower part of hollow cylindrical valve spool portion  62   b  which is in turn provided with lateral cross ports  124 , thus permitting fluid communication between pilot valve cavity  78  and a cavity  128  formed between valve spool portion  62   b  and valve sleeve portion  52   b . Extending transversely across spool portion  52   b  is a lateral wall portion  130  that serves to separate valve cavity  78  from axially adjacent open ended cylindrical cavity  132  having a cylindrical first portion  136  of a poppet  134  axially slidingly located therein. The axial movement of poppet  134  is limited via a retaining split roll pin  138  extending through poppet  134  and the radially adjacent part of spool portion  62   b . While pin roll  138  is press-fitted relative to spool portion  62   b  via spring force applied by an intermediate gap or split  139  in pin  138 , the spool aperture portion  140 , within poppet cylindrical poppet portion  136 , is greater in diameter than the diameter of pin  138 , thus permitting a limited amount of floating or axial movement of poppet  134  within cavity  132 . A poppet spring  144 , interposed between wall portion  130  and poppet  134  normally biases the latter against pin  138 . 
   Poppet  134  also includes a larger diameter second portion  148  which has an annular elastic seal member  150 , contoured in cross section, fixedly received within a matingly contoured annular recess  154  located within poppet portion  148 . Elastic seal  150  is preferably comprised of a stiff elastomeric (e.g., 90 durometer hardness) composition that is bonded, molded-in or cast-in-place and has a contoured locked-in profile, relative to recess  154 , so as to prevent displacement thereof both at high static pressure conditions that can reach a magnitude of e.g. 800 psi., and during high dynamic pressure conditions which occur during flowing operation. The preferably flat axial outer surface  152  of seal member  150  is adapted to sealingly mate with control valve seat portion  120 , under certain operating positions which will be explained hereinafter, to block fluid communication between the regions upstream of gap  151 , located around the annuli of poppet portion  148 , and pilot discharge aperture  60 . It should be noted that since contoured seat portion  120  is raised, relative to seat retainer surface portion  116 , this results in the open area or gap  151  which in turn provides a space for the expansion of seal  150  and at the same time limiting the extent of the compression of seal  150 . 
   Turning now to main or secondary valve  40 , its construction, shown in  FIG. 3 , is very similar, though not identical to that of pilot or primary valve  38 . Therefore, like or similar parts are denominated with like, but primed, numerals. Thus, in main valve  40 , its dual diameter, hollow, cylindrical sleeve  52 ′, which is sealingly received within another conforming housing cavity portion  54 ′ via elastic multiple sealing members  56 ′ and  58 ′, has its larger diameter outer portion  52   a ′ in abutting and operative contact with pressure plenum  44 , while its smaller diameter end portion  52   b ′, having a main discharge aperture  60 ′, is operatively connected to a fuel feed  42  ( FIG. 2 ). A dual diameter, hollow, cylindrical valve spool  62 ′ is slidably and conformably received within sleeve  52 ′, with larger diameter spool portion  62   a ′ being slidably received within sleeve portion  52   a ′ while smaller diameter spool portion  62   b ′ is slidably received within sleeve portion  52   b ′. Joining spool portions  62   a ′ and  62   b ′ is a radial annular wall portion  64 ′, the inner end of which serves as an inner support portion for one end of an extensible higher rate valve spring  66 ′ whose other end abuts a washer  68 ′, received within an interior annular ring portion  67  of a basically centrally apertured cup-shaped spring retainer  70 ′, which is slidably received within sleeve portion  52   a ′ and retained therein by a removable retaining ring  76 ′. Interposed between ring  76 ′ and spring retainer portion  70 ′ is a fluid pressure inlet plate  61  having a central cylindrical portion  69  extending through ring portion  67 . The exterior surface of cylindrical portion  69  serves as a retainer for a fluid inlet strainer  71  that is open to supply pressure plenum  44 . It should be noted that aperture  75  in cup-shaped retainer  70 ′ allows direct communication between supply pressure plenum  44  and the interior cavity  78 ′ of pilot valve  40 . 
   A perusal of the structure of main valve  40 , in  FIG. 3 , which depicts valve  40  in an at-rest or shutdown position, shows valve spring  66 ′ at its full extension, i.e., the outer portion of valve spool radial wall portion  64 ′ abuts an inner end ledge portion  80 ′ of sleeve portion  52   b ′. At least one of valve sleeve  52   a ′ and spool portion  62   a ′ is peripherally undercut or relieved so as to produce an intermediate space or cavity  55 ′ therebetween. Cavity  55 ′ is in communication with an annular intermediate cavity  65 ′, between spool radial portion  64 ′ and sleeve portion radial surface  82 ′, the noted communication, between cavities  55 ′ and  65 ′ taking place via at least one aperture  86 ′ between wall portion  64 ′ and ledge portion  80 ′, thus producing a basically cup-shaped cavity by the joining of cavities  55 ′ and  65 ′. A tapered portion  52   c ′, joining main valve sleeve portions  52   a ′,  52   b ′, is provided with at least one aperture  59 ′ which serves to provide communication between cavities  55 ′,  65 ′ and peripheral cavity portion  57 ′ in housing cavity  54 ′ which in turn communicates with fluid signal pressure conduit  48 . 
   Near the outer end of valve portion  52   a ′, adjacent to one of the elastic sealing members  56 ′ and aligned with signal supply conduit  48 , there is provided a recessed annular gland area  88 ′ having a plurality of first and second spaced radial apertures  90 ′ and  91  which lead into an internal pilot valve cavity  78 ′, past discrete passages  92 ′ the annular end portions  93 ′ of spring retainer  70 ′, the latter serving as abutments for annular surface  94 ′ of the open end of movable valve spool portion  62   a ′, as will be explained in more detail hereinafter. 
   Turning now to  FIG. 4 , which is an enlarged showing of the discharge portion of main valve  40  of  FIG. 3 , sealingly received, via seal member  104 ′, within a recessed cylindrical portion  98 ′ of valve sleeve portion  52   b ′, is the larger diameter portion  102 ′ of a generally cylindrical valve seat retainer member  100 ′ also having a smaller diameter portion  106 ′, sealed via an elastic sealing member  108 ′ against an inner surface of generally cup-shaped closure member  112 ′. Member  112 ′, which is provided with bottom, central discharge aperture  60 ′, has its inner end portion abutting against a radial surface portion  116 ′ of valve seat retainer member  100 ′ which is held in place via a removable retaining ring  118 ′ and an intermediate washer or shim  114 ′. Radial surface portion  116 ′ is provided with a raised, central contoured surface seal seat portion  120 ′. 
   Turning again to  FIG. 4 , pilot valve spool portion  62   b ′, situated approximately radially inwardly of the location of elastic sealing member  58 ′ is provided with lateral cross ports  124 ′, thus permitting fluid communication between pilot valve cavity  78 ′ and a cavity  128 ′ formed between valve spool portion  62   b ′ and valve sleeve portion  52   b ′. The lower end of valve cavity  78 ′ terminates as a slightly larger diameter open cylindrical cavity  142 , a portion of which has a cylindrical first portion  136 ′ of a poppet  134 ′ axially slidingly located therein. The axial movement of poppet  134 ′ is limited via split roll retaining pin  138 ′ extending through poppet  134 ′ and the radially adjacent part of spool portion  62   b ′. While pin  138 ′ is press-fitted relative to spool portion  62   b ′ via spring force applied by an intermediate gap  139 ′, the aperture portion  140 ′ thereof, within poppet cylindrical poppet portion  136 ′, is greater in diameter than the diameter of pin  138 ′, thus permitting a limited amount of floating or axial movement of poppet  134 ′ within cavity  142 . A poppet spring  144 ′, interposed between a shoulder portion  144 , located at the intersection of cavities  78 ′,  142  and poppet  134 ′ normally biases the latter against pin  138 ′. 
   Poppet  134 ′ also includes a larger diameter second portion  148 ′ which has an annular elastic seal member  150 ′, contoured in cross section, fixedly received within a matingly contoured annular recess  154 ′ located within poppet portion  148 ′. Elastic seal member  150 ′ is preferably comprised of a stiff elastomeric (e.g., 90 durometer hardness) of, for example, a rubber-based composition that is bonded, molded-in or cast-in-place and has a contoured locked-in profile, relative to recess  154 ′, so as to prevent displacement thereof both at high static pressure conditions that can reach a magnitude of e.g. 800 psi., and during high dynamic pressure conditions during flowing operation. The preferably flat axial outer surface  152 ′ of seal member  150 ′ is adapted to sealingly mate with control valve seat portion  120 ′, under certain operating positions which will be explained hereinafter, to block fluid communication between the regions upstream of gap  151 ′ and main discharge aperture  60 ′. It should be noted that since contoured seat portion  120 ′ is raised, relative to seat retainer surface portion  116 ′, this results in the open area or gap  151 ′ which in turn provides a space for the expansion of seal  150 ′ and at the same time limiting the extent of the compression of seal  150 ′. 
   As best seen in  FIG. 5 , poppet larger diameter second portion  148 ′, at the plane of seal axial outer surface  152 ′, merges into_a smaller cylindrical third portion  160 , which in the at-rest or shut-down position of main valve  40 , as illustrated in  FIGS. 3 ,  4  and  5 , extends into cylindrical cavity  156 ′ of valve seat retainer portion  106 ′. Poppet cylindrical third portion  160  further includes a first or small diameter band  162 , abutting the plane of seal valve outer surface  152 ′ and a second or larger diameter band  166 , with bands  162 ,  166  being separated via a circular undercut or recess portion  164 . An angled relief band  170  separates a circular end face  168  of poppet third portion  160  from band  166 . It should be understood that the area between valve seat outer surface  182  and the diameter of cylindrical cavity  156 ′, which is also the inner diameter of contoured seat portion  120 , determines the force area acting on elastic seals  150  ( FIGS. 3 ,  3   a ) and  150 ′. In addition, there is a clearance space  172 , between the diameters of cavity  156 ′ and band  166 , for fuel flow, in a position of operation to be described in more detail hereinafter. 
   Returning now to seal  150 ′ in  FIG. 5 , it illustrates same as having a contour, such as the half-dovetail-conforming shape  174  of its inner annular surface, while retaining a generally cylindrical outer surface  176 , normal to generally flat outer surface  152 ′. The converse thereof can also be utilized in that a half-dovetail-conforming shape  174   a  can also be utilized as an outside surface while using a cylindrical annular inner surface, as schematically shown in  FIG. 6   b . Additional retention, for seal  150 ′, within poppet contoured recess  154 ′ can be achieved via the use of a full-dovetail-conforming shape  178 , as shown in  FIG. 6   a . If deemed necessary, the generally flat outer surface  152 ′ of seal  150 ′ can be contoured, stepped or relieved, e.g., via surface finishing such as machining after being cast-in-place, in the manner generally indicated at  180  in  FIG. 7   b  in order to limit the extrusion of seal  150 ′ during valve operation. The noted variations are equally applicable to pilot valve seal  150 . 
   Furthermore, while valve seat  120 ′ in  FIG. 5  utilizes a contoured surface emerging from the straight-sided inner surface of cylindrical cavity  156 , its outer surface  182  tapers more gradually toward poppet radial surface portion  116 ′. Other valve seat shapes include a semicircular shape  184 , in cross section, shown in  FIG. 6   b  and a double-sided inwardly-tapered shape  186 , shown in  FIG. 7   a . The exact shapes of seal  150  or  150 ′ and valve seat  120  or  120 ′ depend upon the material composition of seal  150  or  150 ′ as well as the degree of seal material deformation as well as the fluid pressures and fluctuations thereof encountered during actual operation of valves  38  and  40 . 
   One of the numerous advantages of the staging valve assembly  26  of this invention is that neither of valves  38  and  40  utilizes any dynamic seals, such as O-rings, to seal the movements of spools  62 ,  62 ′ relative to sleeves  52 ,  52 ′, respectively. The absence of such dynamic seals provides for much greater consistency of operation since, during operation, the use of dynamic seals results in a stick-slip effect that varies the coefficient of friction—an undesirable characteristic that contributes to unacceptable valve hysteresis. In addition, dynamic seals will deteriorate and harden as a result of usage and aging, thus again adversely affecting performance as well as shortening service life. In order to avoid the use of such dynamic seals, a predetermined, limited amount of controlled leakage or flow is therefore permitted between sleeves  52 ,  52 ′ and spools  62 ,  62 ′ in the areas of movement of the latter, relative to the former, in the general areas indicated schematically in  FIG. 12 , as circled areas A–D, and illustrated in detail in  FIGS. 13   a – 13   d , respectively. Specifically, at least one of the mutually adjoining sleeve inner wall and spool outer wall surfaces  53   a ′,  53   b ′ (or  53   a ,  53   b ) and  63   a ′,  63   b ′ (or  63   a ,  63   b ), respectively, is provided with a predetermined controlled leakage path, flow channel or clearance space  73  (or  73 ′) through which, as will be explained hereinafter, higher pressure signal fluid can leak or flow from higher pressure signal cavity  65 ′ (or  65 ) to areas of main fuel feed which is at a lower pressure. The actual amount or percentage of fluid leakage is quite small—on the order of 2% of the total flow. Although not negligible, this flow rate can be readily calculated and thus taken into account for control purposes. 
   In terms of the operation of fuel staging valve  26 , attention is now directed to an example,  FIGS. 8–12  which illustrate staging valve assembly  26 , comprised of operatively interconnected pilot valve  38  and main valve  40 , in sequential shut-down position ( FIG. 8 ); position  1  ( FIG. 9 ); position  2  ( FIG. 10 ); position  3  ( FIG. 11 ); and position  4  ( FIG. 12 ), respectively. In the  FIG. 8  shut-down position, both valve spool portions  62   a  and  62   a ′ abut their respective valve sleeve ledges  80 ,  80 ′ by being biased there against via their respective valve springs  66 ,  66 ′. In the shut-down position the main manifold  32 , pilot manifold  34 ,_supply plenum  44  and internal valve cavities  78 ,  78 ′,  128 ′  128 ′ and  132  are filled with_stagnant fuel under low pressure. 
   Cavities  78  and  78 ′ are also interconnected via pilot supply conduit  46 , with cavity  78 ′ also being interconnected with plenum  44 . Conduit  46  serves to supply fuel to pilot valve  38 . Specifically, fuel flows from plenum  44  to main cavity  78 ′, through filter/strainer  71 , and then proceeds through pilot supply conduit  46  into pilot valve  38 . At the same time, no signal pressure fluid enters into pilot valve peripheral cavity  57  from pressure signal conduit  48 . The pressures in all mentioned cavities during shut-down position are equal and no fluid flow is present. 
   Turning now to the position  1  operation, in  FIG. 9 , fuel at a predetermined pressure, for engine operative purposes, now completely fills plenum  44  as well as cavities  78 ,  78 ′,  128 ,  128 ′ and  132 . At the same time, pressurized signal fluid (also fuel), flows from peripheral land cavity  57 , via aperture  59 , into pilot valve cavity  65 , at another predetermined pressure sufficient to compress pilot valve internal spring  66 , via valve spool intermediate or annular radial wall portion  64 , for a first predetermined distance, but short of upper annular surface  94  of spool portion  62   a  abutting or bottoming out on spring retainer lower end portion  70 , (staging valve position  1 ). This first movement is sufficient to lift elastic seal surface  152 , via poppet  134  and spool portion  62   b , from its sealing engagement with valve seat  120 , through the upward displacement of spool  62  away from sleeve inner end surface  82 , thereby permitting the pressurized fuel within cavity  128  to exit therefrom via pilot valve discharge aperture or exit orifice  60 . It should be understood that during position  1  operation, although pressurized signal fluid also enters into main valve cavity  65 ′, this fluid pressure is insufficient to overcome the force of main valve spring  66 ′ and thus main valve  40  remains fully closed, with all of the fuel being delivered into the engine combustion chambers  20  being supplied by pilot valve  38 . 
   Continuing further to position  2  operation in  FIG. 10 , a further increase in the predetermined pressure of the signal fluid causes further lifting of seal member  150 , and due to the bottoming out of pilot spool  62  on spring retainer  70 , this second predetermined amount of signal fluid pressure, which also enters main valve cavity  65 ′ is now sufficient to initially compress main valve internal spring  66 ′ for a first predetermined distance. This first distance is sufficient to lift elastic main seal surface  152 ′, via poppet  134 ′ and spool portion  62   b ′, from its sealing engagement with valve seat  120 ′ through the upward displacement of spool  62 ′ away from sleeve inner end surface  82 ′. As a result, pressurized fuel is permitted to flow beyond valve seal  150 ′, poppet band  166 , and through clearance  172  into main valve cavity  156  and thereafter into combustion chamber  22  ( FIG. 2 ), via main valve discharge aperture  60 ′. It should be understood that in position  2  operation, poppet band  166  is still located within main valve cylindrical cavity  156 , with fuel exiting therefrom via clearance  172 . 
   Continuing further to position  3  operation in  FIG. 11 , while a further increase in the predetermined pressure of the signal fluid can no longer cause further lifting of seal member  150 , due to the previous bottoming out of pilot spool  62  on spring retainer  70 , this third predetermined amount of signal fluid pressure which also enters main valve cavity  65 ′ is now sufficient to further compress main valve internal spring  66 ′ for a second predetermined distance (staging valve position  3 ) so as to cause elastic seal  150  to be lifted yet further away from valve seat  120 ′, thereby fully extracting poppet plug portion third cylindrical portion  160  from cavity  156 , thus permitting greater fuel flow from cavity  156  into combustion chamber  22 , via aperture  60 ′. 
   Arriving at position  4  operation in  FIG. 12 , while a yet further increase in the predetermined pressure of the signal fluid has no additional influence on pilot spool  62  since it already bottomed out in position  2 , this fourth predetermined amount of signal fluid pressure which also enters main valve cavity  65 ′ now further compresses main valve spring  66 ′ for a third predetermined distance thereby causing further axial movement of main valve spool portion  62 ′ until upper annular surface  94 ′ of spool portion  62   a ′ abuts or bottoms out on spring retainer lower end portion  92 . This movement also passes over and covers or blocks main sleeve orifice  91  (with sleeve orifice  90 ′ still remaining open) thereby throttling the passage of pressurized fluid from main cavity  78 ′ pilot cavity  78  via pilot supply conduit  46 . 
   In position  4  operation, fuel flow also proceeds through aperture  90 ′ and continues via pilot supply conduit  46 , through pilot valve  38  and into combustion chamber  22 . The fuel leakage around pilot valve spool  62  flows into pilot valve inner cavity  78  and then through pilot orifice  60  into combustion chamber  22 , via the pilot injection circuit. The fuel leakage around main valve spool  62 ′ will flow either directly into the main fuel circuit, via clearance  63   b ′, or into the pilot and main fuel circuits, via clearance  63   a′.    
   While the progression of the movements of pilot valve seal  150  can readily be seen by a perusal of the changing positions of valve seal  150 , relative to valve seat  120 , in  FIGS. 8 to 10 , the progression of movements of main valve seal  150 ′ is best seen by a perusal of the changing positions of valve seal  150 ′, relative to valve seat  120 ′, in  FIGS. 9 to 12 . 
   Another way to look at the noted progression of the valving action of main valve  40  is to view, in  FIG. 5 , the progressive location of the line  171 , depicting the intersection of poppet relief band  170  and poppet second band  166 , wherein the points M 1 , M 2 , M 3  and M 4  represent the progressive locations of line  171  in main valve  40 , corresponding to positions  1 – 4  in  FIGS. 9–12 , respectively. 
   In the preceding description, the fuel flow division or fuel ratio delivery, within gas turbine engine  26 , through the recited positions  1  to  4 , for the one pilot valve  38  and the one main valve  40 , occurs as noted. However, as known by those skilled in the art, the amount of fuel mass flow is usually determined by a separate hydromechanical or electronic fuel metering unit that is not shown here. A typical, known, unit of this type uses engine revolutions per minute along with other parameters, such as throttle position, air temperature and pressure, etc., to determine the required fuel mass flow rate. Staging valves  30  then take this amount of fuel and determine the amount of fuel that will flow to the previously-described zones, for example a pilot and main zone. Since future engines may well require more than two such zones, additional zones, such as three or four zones per nozzle, may necessitate using more main valves, as the fuel mass flow rate increases, fuel pressure increases in the feed manifold, if operation were restricted solely to position  1  and the engine could only accelerate to part power. When position  2  is selected, the fluid pressure then experiences a stepped reduction and the engine continues to accelerate. The fuel mass flow rate then increases linearly from low to high on takeoff and the fuel pressure first ramps up and then rapidly decreases to an intermediate level, in position  2 , position  3 , etc. An approximate analogy can be made with reference to an automatic transmission in an automobile in that the power or torque, produced by the engine, remains the same but the transmission shifts sequentially to additional ratios in order to permit higher and higher road speeds. 
   Again, it will be understood by those skilled in the art that this invention is not limited to the fuel divisions or fuel ratio deliveries, previously described with reference to positions  1  to  4 . For example, the position  1 – 4  splits can be changed, in one manner, by varying the sizes and/or clearances of one or more of the recited pilot and main valve restrictions, such as, e.g.,  60 ,  60 ′,  90 ′  90 ′  91  and  172 . In addition, the spring rates of valve springs  66  and  66 ′ will determine the level of the signal pressure that is required to activate a given position. In this example, the position  4  split is controlled by the size of orifice  90 ′, in main valve  40 , e.g., if its diametral size is reduced, less fluid flow will be delivered to pilot valve  38  during position  4  operation. 
   Turning now to  FIG. 14 , it represents a simplified vertical sectional view of another embodiment  230  of the fuel staging valve assembly of this invention which is comprised of previously-described main or secondary valve  40 , operatively interconnected with a simplified pilot or prime valve  238 . As best seen in  FIG. 15 , which depicts an enlargement of valve  238 , it has a multiple diameter, hollow, generally cylindrical sleeve  252  sealingly received, via sealing members  256 , within an adjoining or conforming split housing portion  254   a ,  254   b  (not fully detailed here), with housing portion  254   a  including a fluid signal pressure input port  248  connected with a valve central cavity  278  and adapted to be connected to a source of such fluid signal pressure. A multiple diameter cylindrical valve spool  262 , having spaced and differing diameter land areas  262   a ,  262   b ,  262   c  and reduced diameter areas  262   d  and  262   e , is slidably and conformingly received within sleeve  252 , with areas  262   a ,  262   b , and  262   d  being slidingly adjacent to sleeve conforming areas  252   a ,  252   b , and  252   d , respectively. 
   The juncture of sleeve portions  252   a ,  252   b  occurs at a radial stepped annular intermediate portion  251  which serves as an inner support portion for one end of a valve spring  266  whose other end abuts an annular stepped portion  269  of an annular spring retainer  270  which in turn is slidably restrained via a removable retaining ring  276  located in an elongated longitudinal groove  267  in spool portion  262   d . The juncture of sleeve portions  252   b  and  252   c  occurs at a radial stepped intermediate land portion  253  which terminates at an inner edge portion  255  that serves as a seal seat for a pilot seal member  250 , such as an O-ring, interposed between spool portion  262   b  and a centrally apertured cup-shaped valve seat retention member  312 . Member  312 , in turn, is held in place via a stop retainer ring or washer  318  located in a groove  319  of spool portion  262   c . It should be evident that the longitudinal portion of member  312  also serves to limit the compression of pilot seal member  250  relative to land edge portion  255 . A closure member  322 , having a central pilot orifice  260 , closes off sleeve portion  252   c.    
   Between sleeve land portion  252   a  and its adjacent housing portion  254   b  there is provided a recessed, elongated annular gland area  288  which communicates with an annular pilot cavity  265  at sleeve recessed area  262   e , via a plurality of spaced lateral apertures or bores  244  in sleeve portion  252   a . As best seen in  FIG. 14 , pilot cavity  265  is operatively connected with main valve cavity  88 ′ via a fluid pilot supply conduit  246 . 
   Surrounding a lower portion of valve spring  266  and attached to intermediate portion  251  and movable therewith is one end of a generally cylindrical blocking member  241 . The annular other end  242  of blocking member  241  is sealingly received against an annular step portion  254   c  in housing  254   a , under certain operating conditions of valve  238 , but which under other operating conditions permits communication between an adjacent peripheral land cavity  257  and central valve cavity  278 . Valve cavity  257  is located in the vicinity of the junction of housing members  254   a ,  254   b , and bounded by blocking member  241 , with cavity  257  being operatively interconnected, via fluid signal pressure conduit  248 , with main valve peripheral land cavity  57 . 
   In terms of the operation of staging valve  230 , comprised of previously-described main valve  40  and simplified pilot valve  238 ,  FIG. 14  illustrates staging valve  230  in a shut-down position, similar to that shown in  FIG. 8  for previously-described staging valve  30 . The operation in positions  1  to  4  of staging valve  230  follows, in principle, the operation of noted staging valve  30 . 
   Even though the detailed operation of the structure of pilot valve  238  differs somewhat from that of pilot valve  38 , these differences are deemed to be readily ascertainable by those skilled in the art and will thus, in the interest of brevity, not be described in further detail. The functionality of pilot valve  238  is substantially similar to that of previously-described pilot valve  38 . However, pilot valve  238  has reduced pressure leakage and is lighter in weight. 
   Some of the advantages over the known prior art afforded by the present invention are as follows: 
   1. The dual diameter construction of valve spools  62 ,  62 ′ permits the desired spool stroke direction, relative to their respective sleeves  52 ,  52 ′ with signal high pressure, via the separate signal circuit that includes lines  48  and peripheral housing land cavities  57 ,  57 ′ as well as spool cavities  65 ,  65 ′, together with land cavities  57 ,  57 ′. If desired this construction could also be altered so as to utilize low signal pressure. For this, the ratio of the main fuel feed to signal pressure areas would be reversed. 
   2. All moving parts of valves  38 ,  40  are located upstream of valve seats  120 ,  120 ′ and thereby protected from combustion products during staging valve operation. 
   3. Low fuel volume, downstream of valve seats  120 ,  120 ′, reduces circuit fluid fill and drain times. 
   4. Spool valve springs  66 ,  66 ′ preload spools  62 ,  62 ′ against valve sleeves  52 ,  52 ′ at ledges  80 ,  80 ′. 
   5. Washers/shims  114 ,  114 ′ are sized to set up initial sealing loads on soft or resilient seals  150 ,  150 ′, relative to contoured valve seat portions  120 ,  120 ′, against the facing or downstream portions of retainer pins  138 ,  138 ′, thereby preventing leaks at low fluid supply pressure. 
   6. At higher supply pressures, the loading of poppet seals  150 ,  150 ′, against seal seats  120 ,  120 ′, increases due to the fluid pressure differential from supply pressure plenum  44  to discharge apertures  60 ,  60 ′. This loading, maintains “no leak” seals  150 ,  150 ′ at high fluid supply pressures. 
   7. Elastic seals  150 ,  150 ′ are preferably comprised of a stiff, elastomeric, composition that is bonded, molded or cast-in-place and have a contoured locked-in profile (in cross section) so as to prevent displacement thereof at high fluid pressure drops. 
   8. Gaps  139 ,  139 ′, between pins  138 ,  138 ′ and poppet apertures  140 ,  140 ′ and gaps  151 ,  151 ′, between seal outer surfaces  152 ,  152 ′ and valve retainer radial surface portions  16 ,  116 ′, serve to limit the compression of seals  150 ,  150 ′ and allow compensation for seal compression set. Poppet springs  144 ,  144 ′ maintain the contact of seals  150 ,  150 ′ with seal seats  120 ,  120 ′ even during seal compression set. Specifically, gaps  139 ,  139 ′ limit the compression forces acting on seals  150 ,  150 ′ since any excess forces, upon the closures of gaps  139 ,  139 ′, are taken up by the upstream-facing portions of pins  138 ,  138 ′. 
   9. Pilot valve  138  opens when the fluid signal pressure, entering valve cavity  65  from signal pressure conduit  48  and land cavity  57  via sleeve aperture  59 , overcomes the loading of valve spring  66  and poppet spring  144 , at poppet seal seat  120 , with the poppet seal force diameter being that of cavity  156 . 
   10. Actuation of main valve spool  62 ′, relative to sleeve aperture  91 , modulates pilot circuit flow, with valve spool  62 ′ blocking orifice  91  which thus reduces the flow area leading to pilot valve  38 . 
   11. During valve operation, by permitting a predetermined, controlled, amount of fluid leakage between valve sleeves  52 ,  52 ′ and valve spools  62 ,  62 ′, the need for dynamic seals, such as O-rings, with their attendant hysteresis and aging problems, is obviated. 
   12. Other advantages include reduced size and weight of the valve systems, both of which are particularly beneficial in aircraft applications. 
   While there are shown and described several presently preferred embodiments of the staging valve assemblies of this invention, it is to be distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims.