Abstract:
A fuel flow system for a gas turbine engine includes a first pump, a second pump, a bypass loop, an integrating bypass valve and a pilot valve. The first pump connects to an actuator and a metering valve. The second pump connects to the metering valve and is arranged in parallel with the first pump. The bypass loop recycles fuel flow from the first pump and the second pump to inlets of the first pump and second pump integrating bypass valve includes first and second windows. The first window regulates fuel from the first pump through the bypass loop and the second window that regulates fuel from the second pump through the bypass loop. The pilot valve controls the size of the first and second windows.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    Reference is made to application Ser. No. ______ entitled “DUAL PUMP FUEL FLOW SYSTEM FOR A GAS TURBINE ENGINE” which is filed on even date and is assigned to the same assignee as this application. 
     
    
     BACKGROUND 
       [0002]    In a gas turbine engine, oil is distributed to various components, such as bearings, for cooling and lubrication. The oil is heated as it circulates around or through these various components. The oil can be cooled by cooling air or fuel flowing to the combustion chamber. Cooling air is typically taken from the fan, which reduces thrust of the engine. Fuel flowing to the combustion chamber can also be used to cool the hot circulating oil. Rejecting heat from the oil into the fuel incurs few of the penalties of air cooling. However, the amount of rejected heat is limited by the maximum temperature tolerable by the fuel. 
         [0003]    The fuel system of a gas turbine engine includes a fuel pump for pressurizing and transporting the fuel through the system to the combustion chamber. The fuel pump is generally a boost stage and single positive displacement main stage which is attached to the gearbox such that the speed of the main fuel pump is proportional to the engine speed. At certain conditions, such as cruise, the engine operates at a relatively high speed while a relatively low fuel flow is required. Further, the main fuel pump stage is typically sized by high power or start conditions, resulting in extra flow capacity at all other engine operation conditions. In this way, the main fuel pump stage results in excess fuel flow. The excess fuel is recycled through a bypass loop to the low pressure side of the main pump. At low fuel requirements, the fuel may be recycled several times before being sent to the combustion chamber. The combination of recycling excess fuel and pump inefficiencies increases the temperature of the fuel. This additional heat limits the amount of heat that can be rejected into the fuel from the circulating oil. Reducing the amount of heat rejected into the fuel by the fuel pump would improve engine performance. Further, a large amount of time spent is in the cruise condition during a flight, and reducing the amount of heat rejected into the fuel by the main pump during the cruise condition may have a larger impact on engine performance than similar reductions during other flight conditions. 
       SUMMARY 
       [0004]    A fuel flow system for a gas turbine engine includes a first pump, a second pump, a bypass loop, an integrating bypass valve and a pilot valve. The first pump connects to an actuator. The second pump connects to the actuator and is arranged in parallel with the first pump. The bypass loop recycles fuel flow from the first pump and the second pump to inlets of the first pump and second pump. The integrating bypass valve includes first and second windows. The first window regulates fuel from the first pump through the bypass loop and the second window regulates fuel from the second pump through the bypass loop. The pilot valve controls the size of the first and second windows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The FIGURE is a schematic representation of a fuel flow system. 
       
    
    
     DETAILED DESCRIPTION 
       [0006]    The FIGURE is a schematic representation of fuel flow system  10  for a gas turbine engine onboard an aircraft. Fuel flow system  10  receives fuel through conduit  12   a  from the air frame (A/F). In one example, fuel flow system  10  receives fuel from a fuel tank onboard the aircraft. Boost pump  14  increases the pressure of the fuel and supplies the fuel to fuel-oil heat exchanger (FOHE)  16  and filter  18 . The fuel is then supplied to the system comprising cruise pump  20 , wash filter  22 , metering valve (MV)  24 , minimum pressure and shut off valve (MPSOV)  26 , integrating bypass valve  28  (including cruise bypass window  28   a  and idling bypass window  28   b ), pressure control or pilot valve  30  (including upstream pressure signal line  32   a,  downstream pressure signal line  32   b,  high pressure line  34   a,  low pressure line  34   b,  and control signal  36 ), idling pump  38 , check valve  40 . Conduits  12   a,    12   b,    12   c,    12   e,    12   f,    12   g,    12   h , and  12   i  connect the components so that fuel flows from boost pump  14  to nozzles  46  of a combustion chamber. 
         [0007]    Boost pump  14  receives and pressurizes fuel from the air frame (A/F). Boost pump  14  can be a typical centrifugal pump designed to operate at an essentially constant pressure for a given engine speed. The fuel flows from boost pump  14  to FOHE  16 . 
         [0008]    Lubricating and cooling oil for engine components, such as the main engine bearings, circulates through an oil loop represented by inlet conduit  42   a  and outlet conduit  42   b . Hot oil from the engine enters FOHE  16  through inlet conduit  42   a.  The oil rejects heat into the fuel flowing through FOHE  16 . The cooled oil exits FOHE  16  through outlet conduit  42   b  and is directed back to the engine components. FOHE  16  transfers heat from the oil to the fuel. The amount of heat transferred to the fuel is limited by the maximum temperature tolerable by the downstream components. Decreasing heat rejection by components downstream of FOHE  16  enables more heat to be rejected into the fuel by the oil in FOHE  16 . 
         [0009]    After FOHE  16 , the fuel flows through filter  18 . Filter  18  protects contaminate sensitive components of fuel flow system  10 . Filter  18  filters contaminants that might enter fuel flow system  10  through the fuel. 
         [0010]    After filter  18 , the fuel is divided between cruise pump  20  and idling pump  38 , which operate in parallel. A portion of the fuel flows through conduit  12   b  to cruise pump  20 . Cruise pump  20  can be a positive displacement pump that is sized, as a minimum, to meet the maximum burn flow requirements at cruise condition plus parasitic internal leakage losses in the fuel system. Flow from cruise pump  20  is divided between actuators  48  and engine burner nozzles  46 . Cruise pump  20  increases the pressure of the fuel sufficiently to satisfy the load requirements of the actuators  48  while also providing fuel to the engine burner nozzles  46 . 
         [0011]    The actuators  48  can be high pressure fluid actuators which operate engine components, such as but not limited to, inlet guide vanes, bleed valves, turbine cooling valves and nozzle actuators. The minimum pressure and shutoff valve (MPSOV)  26 , regulates the discharge pressure of cruise pump  20  above the inlet pressure of cruise pump  20  to assure the positive operation of the actuators  48  against their design loads. In one example, cruise pump  20  is operated at about 1724 kilopascals difference (254 psid). High pressure fuel (Pt) is provided to the actuators  48  from the discharge of cruise pump  20  through conduit  12   h.  Low pressure fuel (Pd) from the actuators  48  is returned through conduit  12   i  to a location upstream of the inlets of cruise pump  20  and idling pump  38 . 
         [0012]    Metered fuel flow path  44  is defined between cruise pump  20  and MV  24 . MV  24  measures the flow of fuel to the nozzles  46 . The pressure drop across the known area of MV  24  is measured by pressure control valve  30  using upstream pressure signal line  32   a  and downstream pressure signal line  32   b.  The fuel flow to the nozzles  46  is too high when the pressure drop or differential across MV  24  is higher than a specified value. Conversely, the fuel flow to the nozzles  46  is too low when pressure drop across MV  24  is lower than a specified value. As described further below, the fuel flow from cruise pump  20  and idling pump  38  through metered fuel flow path  44  is controlled based on feedback from pressure control valve  30  in order to adjust the flow of fuel to the nozzles  46 . 
         [0013]    Fuel flow from cruise pump  20  in excess of actuator and engine burn flow needs is directed through cruise bypass window  28   a  of integrating bypass valve  28  and through a bypass loop comprised of conduit  12   e.  The recycled or bypassed fuel is reintroduced into the fuel flowing to the inlets of cruise pump  20  and idling pump  38 . In the FIGURE, the bypassed fuel is introduced at a location downstream of the outlet of FOHE  16 . However, the bypassed fuel can be introduced at any location upstream of the inlets of cruise pump  20  and idling pump  38 . 
         [0014]    Cruise bypass window  28   a  is a variable restriction through which bypass fuel from cruise pump  20  flows. The area or size of cruise bypass window  28   a  is varied to adjust the flow of fuel from cruise pump  20  through metered fuel flow path  44  to the nozzles  46  and through the bypass loop formed by conduit  12   e.    
         [0015]    As described above, a portion of fuel from the air frame (A/F) is directed through cruise pump  20 . The remaining fuel is fed to idling pump  38  through conduit  12   f.  Similar to cruise pump  20 , idling pump  38  can be a positive displacement pump. Idling pump  38  operates in parallel with cruise pump  20  and typically has a larger capacity than cruise pump  20 . The combined capacity of cruise and idling pumps  20 ,  38  is sized to satisfy engine burn flow, actuator transient flow and parasitic leakage flow under all engine operating conditions including starting and high power conditions. In one example, the capacity of idling pump  38  is approximately two-thirds of the total capacity of cruise pump  20  and idling pump  38 . Idling pump  38  supplements the fuel flow when flow requirements exceed the capacity of cruise pump  20 . 
         [0016]    Check valve  40  and idling bypass window  28   b  of integrated bypass valve  28  are in fluid communication with idling pump  38 . Idling bypass window  28   b  is a variable restriction and operates in a fashion similar to cruise bypass window  28   a.  Check valve  40  is designed to default to a closed position so that the fuel flow from idling pump  38  is directed through idling bypass window  28   b  and the bypass loop formed by conduit  12   e.  Check valve  40  opens when the pressure in conduit  12   g  is equal to or greater than the pressure in conduit  12   c.  When check valve  40  opens, fuel from idling pump  38  flows through metered fuel flow path  44  to the nozzles  46  and supplements the fuel flow from cruise pump  20 . 
         [0017]    When check valve  40  is closed, all fuel flow from idling pump  38  bypasses the nozzles  46  and flows through idling bypass window  28   b.  The bypass fuel from cruise pump  20  and idling pump  38  mix in conduit  12   e.  The bypass fuel is directed to a location upstream of cruise pump  20  and idling pump  38  and downstream of boost pump  14 . 
         [0018]    Integrating bypass valve  28  and pressure control valve  30  schedule the flow of fuel to the engine burner nozzles  46 . Pressure control valve  30  senses pressure upstream and downstream of MV  24  through upstream and downstream pressure signal lines  32   a  and  32   b , respectively. Pressure control valve  30  sends control signal  36  to integrating bypass valve  28  in order to vary the area or size of cruise bypass window  28   a  to maintain a constant pressure differential between the pressures upstream and downstream of MV  24 . Pressure control valve  30  is a pilot valve that uses high and low pressure fuel (Pf and Pd, and signal lines  34   a  and  34   b , respectively) to form control signal  36  and translate integrating bypass valve  28 . High pressure fuel Pf is taken from a location downstream of cruise pump  20  and upstream of the metering valve  24 . Low pressure fuel Pd is taken from a location downstream of the boost pump  14  and upstream of the inlets of cruise pump  20  and idling pump  38 . 
         [0019]    As the integrating bypass valve  28  moves or translates to maintain a constant pressure drop across MV  24 , the areas or sizes of idling bypass window  28   b  and cruise bypass window  28   a  are varied together with a predetermined relationship. That is, idling bypass window  28   b  and cruise bypass window  28   a  are mechanically linked such that they vary together with a predetermined relationship. Flow perturbations are minimized because cruise bypass window  28   a  and idling bypass window  28   b  are mechanically linked. Further, integrated bypass valve  28  is formed so that cruise bypass window  28   a  fully closes before idling bypass window  28   b  (i.e., fuel from idling pump  20  cannot flow through cruise bypass window  28   a ). This enables all the fuel flow from cruise pump  20  to be supplemented by the fuel flow of idling pump  38  when necessary. 
         [0020]    In fuel flow system  10 , cruise pump  20  is the sole provider of the pressure and flow for both the burn path to the nozzles  46  and the flow path to the actuators  48  for some engine operating conditions, such as cruise. As discussed above, cruise pump  20  is smaller than idling pump  30  such that the majority of the displacement is on idling pump  38 . Cruise pump  20  is sized to satisfy the requirements of the maximum cruise condition plus the actuator slew requirements. At thermally critical conditions such as cruise, idling pump  38  is only recirculating fuel to the outlet of FOHE  16 . The minimum pressure drop of idling pump  38  can be as low as the pressure drop required for recirculating the fuel. The minimum pressure rise of idling pump  38  is not limited by the burn flow path (i.e., the requirements of the flow path to the nozzles  46  and actuator requirements). In fuel flow system  10 , at thermally critical operating conditions such as cruise, cruise pump  20  is operated at a higher pressure than idling pump  38 . However, the higher pressure is across a small portion of the total displacement of cruise pump  20  plus idling pump  38 . The larger portion of the displacement is across idling pump  38  which is operating at a relatively low pressure. For example, cruise pump  20  can be operated at about 1724 kilopascals difference (about 254 psid) and idling pump  38  can be operated at about 517 kilopascals difference (about 75 psid). Relative to a conventional single pump system where the higher pressure rise (254 psid) would be required to be across the total pump displacement, the design of fuel flow system  10  reduces the horsepower required at cruise conditions and the amount of heat rejected by the pumps into the fuel. 
         [0021]    Cruise bypass window  28   a  and idling bypass window  28   b  are variable restrictions. Closing cruise bypass window  28   a  increases the restriction and reduces bypass flow from cruise pump  20 , thus making more fuel flow available to satisfy engine burn and actuator requirements. Idling bypass window  28   b  operates in a similar manner. At cruise condition when the fuel flow demand is very low, the fuel flow demand is satisfied entirely by the fuel flow from cruise pump  20 . As described above, cruise pump  20  is sized to meet the requirements of the maximum cruise condition and the actuator slew requirements. Further, at the cruise condition, idling bypass window  28   b  has a low restriction such that the opening in idling bypass window  28   b  is large and approximately all of the fuel from idling pump  38  passes through idling bypass window  28   b  to the bypass loop comprised of conduit  12   e.  The low restriction of idling bypass window  28   b  minimizes the pressure rise across idling pump  38  at the cruise condition and reduces the amount of heat rejected into the fuel by idling pump  38 . 
         [0022]    When the demand for fuel flow increases, cruise bypass window  28   a  is variably restricted based on control signal  36  from pressure control valve  30 . Increasing the restriction of cruise bypass window  28   a  reduces fuel flow from cruise pump  20  through conduit  12   e  of the bypass loop and increases the flow of fuel from cruise pump  20  through main fuel flow path  44  to the nozzles  46 . The maximum capacity of cruise pump  20  is reached when cruise bypass window  28   a  is completely restricted such that all fuel from cruise pump  20  flows to the nozzles  46 . Because cruise bypass window  28   a  and idling bypass window  28   b  are mechanically linked, the timing of when idling pump  38  is brought on line can be controlled. 
         [0023]    Integrating bypass valve  28  is translated or adjusted based on control signal  36  from pressure regulating valve  30 . Pressure regulating valve  30  is a pilot valve which measures the pressure differential across metering valve  24  using upstream and downstream pressure signal lines  32   a  and  32   b.  Pressure regulating valve imports high pressure fuel Pf or low pressure fuel Pd to integrating bypass valve  28  through control signal  36 . Pressure signal  36  causes integrating bypass valve  28  to move or translate and adjusts the area or restriction of cruise bypass window  28   a  and idling bypass window  28   b.    
         [0024]    Integrating bypass valve  28  can be a half area servo which has a stepped diameter such that integrating bypass valve  28  is in force balance when control signal  36  is half way between high pressure Pf and low pressure Pd by the area ratio of integrating bypass valve  28 . In a specific example, integrating bypass valve  28  is in force balance and does not move when the pressure differential across MV  24  is, for example, 50 psid, although it will be understood that other values can be used within the scope of the invention. If the pressure differential across MV  24  is greater than 50 psid, pressure regulating valve  30  will move such that more high pressure fuel Pf and less low pressure fuel Pd is provided such that control signal  36  has a high pressure. This causes integrating bypass valve  28  to translate or change position. When the pressure differential across MV  24  again equal 50 psid, pressure regulating valve  30  returns to null position where control signal  36  to integrating bypass valve  28  equals half way between high pressure Pf and low pressure Pd. 
         [0025]    Pressure regulating valve  30  in conjunction with integrating bypass valve  28  is capable of regulating to a pressure setting with a significantly improved accuracy compared to a dual window regulating valve with a spring setting a regulating pressure. For a valve with a spring, the spring may cause the regulated pressure to deviate from a nominal regulated pressure setting. Other effects, such as seal friction, leakages, and unexpected flow force effects which can also cause deviations from the nominal regulated pressure setting are also eliminated by using pressure regulating valve  30  and integrating bypass valve  28 . 
         [0026]    In the current pressure regulating system, the regulated pressure is set by the preload on pressure regulating valve  30 . Because pressure regulating valve  30  operates around null at any bypass condition, deviations in regulated pressure due to spring compression are almost non-existent. Pressure regulating valve  30  regulates the modulated pressure on one side of integrating bypass valve  28  using control signal  36  until the regulated pressure setting is accurately achieved. 
         [0027]    In a conventional spring-loaded regulating valve, the regulated pressure accuracy and valve stability are two conflicting requirements. A high window gain (dA/dx), or width, tends to destabilize the valve. In order to lower the window gain, the valve stroke needs to be increased to maintain a maximum window area value. When the valve window stroke is increased, the deviation in regulated pressure tends to increase as well due to spring compression. Because of their conflicting nature, it is difficult to achieve both requirements at the same time. In the current concept, the valve window and stroke can be adjusted without considering the deviation in regulated pressure. Hence, the stability problem is made more flexible. 
         [0028]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.