Abstract:
A low hysterisis fuel control valve for proportioning fuel being supplied to the combustor of the turbine engine into a first portion for delivery to primary fuel nozzles and a second portion for delivery to secondary fuel nozzles is provided. The valve includes a piston mounted within a linear ball bearing bushing. In response to fuel pressure, the piston is continuously operable between a first position where said no fuel flow to the fuel nozzles occurs, a second position where fuel flows to the primary nozzles and a third position where fuel flows to the secondary nozzles.

Description:
TECHNICAL FIELD 
     This invention relates to fuel control systems for gas turbine engines, and in particular, to a fuel control valve for proportioning fuel being supplied to the combustor of the turbine engine into a first portion for delivery to primary fuel nozzles and a second portion for delivery to secondary fuel nozzles. 
     BACKGROUND OF THE INVENTION 
     FIG. 1 shows a prior art fuel control valve generally denoted by reference numeral  1 . The fuel control valve  1  comprises a generally cylindrical, axially extending hogged out sleeve  2 . The casing  2  has a first set of circumferentially spaced holes  3  and a second set of circumferentially spaced holes  4 . Disposed within the casing  2  is a plate seal  5  having holes  6 . The plate seal  5  has a hollow center and is coupled to valve  7 . Disposed within the plate seal  5  and valve  7  is a piston  8  mounted on a spring  9 . In operation fuel enters the inlet of  10  and flows through holes  3  and hole  6 , pushing piston  8 , then flowing out hole  4 . As the fuel pressure builds within the interior, the piston moves and compresses the spring. Due to frictional engagement of the various parts, as the piston moves so does the valve  7  and the plate seal  5 . A disadvantage to this prior art fuel control valve is that because of the frictional contact, hysterisis develops which makes the fuel flow from the valve unpredictable. 
     Accordingly, there is a need for a fuel control valve that overcomes this hysterisis problem. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a fuel control valve that is not as susceptible to hysterisis as prior art valve. 
     The present invention achieves the above-stated objective by providing a fuel control valve for proportioning fuel being supplied to the combustor of the turbine engine into a first portion for delivery to primary fuel nozzles and a second portion for delivery to secondary fuel nozzles comprising: 
     an axially extending hollow casing having a first hole and a second hole axially spaced apart from said first hole, said first hole on fluid communication with said primary nozzles and said second hole in fluid communication with said secondary nozzles; 
     an annular metering block disposed within said casing, said metering block having a third hole in fluid communication with said first hole and a fourth hole in fluid communication with said second hole, said third and fourth hole being axially spaced from each other; 
     an annular bushing disposed in said casing adjacent said metering block to define a conduit extending through said casing from a first opening for receiving a flow of fuel and a second opening; and 
     a stop valve disposed in said second opening and having a piston mounted thereto, said piston slidingly engaging said bushing to be continuously operable between a first position where said piston covers said third and fourth holes and a second position where said third and fourth holes are not covered by said piston. 
     These and other objects, features and advantages of the present invention are specifically set forth in, or will become apparent from, the following detailed description of a preferred embodiment of the invention then read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-section of a prior art fuel control valve. 
     FIG. 2 is a schematic representation of a fuel control system for a gas turbine engine incorporating the fuel control valve contemplated by the present invention. 
     FIG. 3 is a cross-section of a fuel control valve contemplated by the present invention. 
     FIG. 4 is a profile view of a metering block of the fuel control valve of FIG.  3 . 
     FIG. 5 is a graph of fuel flow vs. fuel pressure and compares the performance of a prior art fuel control valve with a fuel control valve contemplated by the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2 shows a typical fuel delivery system for a gas turbine engine  10 . Low pressure fuel from a supply tank, (not shown), is pumped to a main fuel control  12 . The operation of the fuel control  12  is controlled by an electronic control unit  14  which receives commands from the cockpit  16 . When commanded to deliver fuel to the engine  10 , the fuel control  12  delivers fuel through a conduit  15  to the fuel control valve  20 . In a manner more fully described later in the specification, the fuel control valve  20  delivers to the combustor of the turbine engine  10  a first portion of fuel through a conduit  19  to the combustor&#39;s primary fuel nozzles and a second portion of fuel through conduit  17  to the combustor&#39;s secondary fuel nozzles. 
     Referring to FIG. 3, the fuel control valve  20  is mounted inside a valve housing, not shown. The valve  20  comprises a generally cylindrical, axially extending hogged out casing  24 . The casing  24  has a first set of circumferentially spaced holes  26  and a second set of circumferentially spaced holes  28 . The two sets of holes are axially spaced from each other. In the preferred embodiment, each set has four holes equally spaced in the circumferential direction. However, the number and spacing of the holes may vary in alternate embodiments depending on the operating conditions of the valve. 
     Viewing FIG. 3 from left-to-right, disposed within the casing  24  is a conventional annular spacer  30 . Adjacent the spacer  30  is an annular metering block  32 . The inner diameter of the spacer  30  is less than the inner diameter of the metering block  32  thereby defining an annular stop  31 , the function of which will be described later in the specification. The metering block  32 , which is shown in greater detail in FIG. 4, has a first rim portion  34  having a first diameter. Circumferentially disposed about the first rim  34  are six evenly spaced holes  36 . It should be appreciated that the spacing and number of such holes may vary in alternate embodiments. Adjacent the rim  34  is a conventional o-ring receiver member  38  having a diameter greater than that of the rim  34 . Axially spaced from the receiver member  38  is a second o-ring receiver member  40  at the same diameter as member  38 . Disposed between members  38  and  40  is a recessed portion  42  having a diameter less than that of the rim  34 . The recessed portion  42  has four circumferentially spaced holes  44 . The holes  44  are evenly spaced and are oblong in the axial direction. Again, in alternate embodiments of the present invention, the number, spacing, and shape of the holes  44  may vary. 
     Referring again to FIG.  3  and continuing from left-to-right, adjacent the metering block  32  is an annular, linear ball bearing bushing  46 . The bushing  46  is an antifriction device and is commercially available. In the preferred embodiment, the bushing  46  is procured, for example, from NB corporation, part number SM 16G. The spacer  30 , metering block  32 , and bushing  46  are held within the valve housing by a retainer spring  50  and together define a generally cylindrical conduit  52  extending all the way through the casing  24  from a first opening  54  to a second opening  56 . 
     A stop valve  60  is comprised of a disc  62  and a rod  64  extending from the center of the disc  62 . A helical spring  66  mounted over the rod  64  and a piston  70  is mounted over the helical spring. The spring and piston are held in place by a retainer spring  68 . This assembly is inserted into the second opening  56  with a portion of the outer surface of the piston  70  slidingly engaging a portion of the inner surface of the bushing  46 . The piston  70  is essentially a cylindrical member having a closed first end  72  and an opened second end  74  through which the spring  66  and rod  64  are received. The closed end  72  is dimensioned so that when the piston is extended all the way forward, (all the way left viewing FIG.  3 ), it abuts stop  31 . With the piston abutting the stop  31  the valve  20  is closed. 
     Starting from this closed position, in response from a command from the cockpit  16 , such as a command to start the engine  10 , the electronic control unit  14  causes the fuel control  12  to deliver pressurized fuel through conduit  15  to the fuel control valve  20 . This fuel flow is received through opening  54  into conduit  52  until it flows against the closed end  72  of the piston  70 . As the pressure of the fuel increases it starts to overcome the force of spring  66  which holds the piston  70  in the closed position. As the spring force is overcome, the piston  70  retracts, that is it moves to the right viewing FIG.  3 . As the piston  70  retracts holes  36  become opened and fuel flows through these holes, then through holes  26  and into conduit  17  to the primary fuel nozzles. This fuel flow is often referred to as the primary fuel flow. The fuel pressure in conduit  52  continues to build, the piston  70  continues to retract until oblong holes  44  open. Fuel then flows through these holes, through holes  28  into conduit  19  and then to the secondary fuel nozzles. This fuel flow is often referred to as the secondary fuel flow. When the engine is shut down the process is reversed as fuel pressure drops the piston  70  extends until it returns to the closed position. 
     The advantages of fuel control valve  20  compared to the prior art fuel control valve shown in FIG. 1 can be seen in FIG.  5 . FIG. 5 shows data from rig testing of both valves during a simulated engine start-up and shut-down. In FIG. 5 the y-axis is fuel flow and the x-axis is fuel pressure at the inlet to the valve. Data from a start-up sequence followed by shut-down sequence for the prior art valve, (“old”) is represented by the triangles for start-up and “X”s for shut-down. From this graph the deficiency in the prior art valve is readily apparent as the start-up curve defined by the triangles is significantly different from the shut-down, “X”s. This difference is referred to as hysterisis and not only makes it difficult to calibrate the valve but can also impact the performance of the engine as predictability of fuel flow is lost. In contrast, the diamonds represent data from a simulated start-up using the valve contemplated by the present invention, (“new”), while the squares represent a shut-down. It is clear from FIG. 5 that the hysterisis found in the prior art valve has been eliminated. Another advantage to the present invention over the prior art is fewer parts and consequently lower cost. 
     Though the preferred embodiment has described the subject invention with reference to a gas turbine engine, the invention is equally applicable to other types of devices requiring metered flow. Accordingly, various modifications and alterations to the above described embodiments will be apparent to those skilled in the art and therefore this description of the invention should be considered exemplary in nature and not as limiting to the scope and spirit of the invention as set forth in the following claims.