Patent Publication Number: US-2020300169-A1

Title: Mechanical demand fuel pumping system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 62/821,055 which was filed on Mar. 20, 2019. 
    
    
     BACKGROUND 
     A gas turbine engine typically includes a fan section, a compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high-energy exhaust gas flow. The high-energy exhaust gas flow expands through the turbine section to drive the compressor and the fan section. The compressor section typically includes low and high pressure compressors, and the turbine section includes low and high pressure turbines. 
     Fuel supplied to the combustor is provided by a mechanical pump driven by a rotating shaft of the engine. The mechanical pump is reliable and supplies fuel in proportion to engine speed. The minimum capacity of the mechanical pump is sized such that sufficient fuel is provided for high power conditions. Excess fuel not needed is recirculated back to the fuel tank. The fuel is further utilized as a coolant for other systems of the engine. Recirculation of fuel increases the temperature of the fuel and thereby reduces the available capacity to absorb heat from other systems. The capacity of the fuel to absorb heat from other systems is further limited by the characteristics of the fuel. At a certain temperature the fuel begins to degrade and can reduce engine efficiency. Reducing the amount of fuel that is recirculated during engine operation may improve the capacity of the fuel to absorb heat from other systems. 
     Turbine engine manufacturers continuously seek improvements to engine performance including improvements to thermal, transfer and propulsive efficiencies. 
     SUMMARY 
     A fuel system for a gas turbine engine according to an exemplary embodiment of this disclosure includes, among other possible things, an accessory gearbox driven by a mechanical link to the gas turbine engine, a primary fuel pump providing a first fuel flow during engine operation, and a secondary fuel pump providing a second fuel flow. The primary fuel pump and the secondary fuel pump are driven by an output of the accessory gearbox. 
     In a further embodiment of the foregoing fuel system for a gas turbine engine, the first fuel pump and the second fuel pump both receive fuel flow from a common inlet passage. Both the first fuel pump and the second fuel pump communicate the corresponding one of the first fuel flow and the second fuel flow to a common outlet passage. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, a first control valve is upstream of the secondary fuel pump and a second control valve is downstream of the secondary fuel pump. The first control valve and the second control valve controlling communication of fuel to and from the secondary fuel pump. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, a pump drive gearbox is selectively coupled to drive the secondary fuel pump by a clutch means. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, a first pressure relief valve is included for switching the primary fuel pump and the secondary fuel pump between a series arrangement, where the first fuel flow is provided by both the primary and secondary fuel pumps. A parallel arrangement is included where the first fuel flow is provided by the primary fuel pump and the secondary fuel flow is provided by the secondary fuel pump. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, the first pressure relief valve is disposed between an outlet of the primary fuel pump and an inlet of the secondary fuel pump. The first pressure relief valve opens to communicate fuel from the primary fuel pump to the secondary fuel pump to provide the first fuel flow in a first operating condition. The first pressure relief valve closes such that the secondary fuel pump provides the second fuel flow in parallel with the first fuel flow provided by the primary mechanical fuel pump to a common fuel passage in a second operating condition. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, a first check valve is in a first passage downstream of the primary mechanical fuel pump to control fuel flow from the first passage into the common fuel passage. A second check valve is in a second passage communicating fuel to an inlet of the secondary fuel pump. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, a second pressure relief valve is downstream of both the primary fuel pump and the secondary fuel pump for directing fuel flow away from the common fuel passage in response to a pressure within the common fuel passage above a predefined pressure. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, a flow capacity of the primary fuel pump and the secondary fuel pump are different. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, a flow capacity of the primary fuel pump and the secondary fuel pump are the same. 
     A gas turbine engine according to an exemplary embodiment of this disclosure includes, among other possible things, a fan rotatable within a fan nacelle, and a core engine which includes a compressor communicating compressed air to a combustor where compressed air is mixed with fuel and ignited to generate a high-energy gas flow expanded through a turbine. An accessory gearbox is driven by a mechanical link to the turbine. A primary fuel pump provides a first fuel flow to the combustor during engine operation, and a secondary fuel pump provides a second fuel flow to the combustor during engine operation in response to a predefined engine operating condition. The primary fuel pump and the secondary fuel pump are driven by an output of the accessory gearbox. 
     In a further embodiment of the foregoing gas turbine engine, the first fuel pump and the second fuel pump both receive fuel flow from a common inlet passage. Both the first fuel pump and the second fuel pump communicate the corresponding one of the first fuel flow and the second fuel flow to a common outlet passage. 
     In another embodiment of any of the foregoing gas turbine engines, a first control valve is upstream of the secondary fuel pump and a second control valve is downstream of the secondary fuel pump. The first control valve and the second control valve control communication of fuel to and from the secondary fuel pump. 
     In another embodiment of any of the foregoing gas turbine engines, a pump drive gearbox is selectively coupled to drive the secondary fuel pump by a clutch means. 
     In another embodiment of any of the foregoing gas turbine engines, a first pressure relief valve is for switching the primary fuel pump and the secondary fuel pump between a series arrangement. The first fuel flow is provided by both the primary and secondary fuel pumps and a parallel arrangement where the first fuel flow is provided by the primary fuel pump and the secondary fuel flow is provided by the secondary fuel pump. 
     In another embodiment of any of the foregoing gas turbine engines, the first pressure relief valve is disposed between an outlet of the primary fuel pump and an inlet of the secondary fuel pump. The first pressure relief valve opens to communicate fuel from the primary fuel pump to the secondary fuel pump to provide the first fuel flow in a first operating condition. The first pressure relief valve closes such that the secondary fuel pump provides the second fuel flow in parallel with the first fuel flow provided by the primary mechanical fuel pump to a common fuel passage in a second operating condition. 
     A method of supplying fuel to a combustor of a gas turbine engine according to an exemplary embodiment of this disclosure includes, among other possible things, operating a primary fuel pump to provide a first fuel flow, and operating a secondary fuel pump to provide a second fuel flow. Operating the primary fuel pump and the secondary fuel pump comprises driving the primary fuel pump and the secondary fuel pump with an output from an accessory gearbox. The first fuel flow is communicated to a combustor of the gas turbine engine in a first operating condition and communicating the first fuel flow and the second fuel flow to the combustor in a second operating condition. 
     In a further embodiment of the foregoing method of supplying fuel to a combustor of a gas turbine engine, the first fuel flow communicated to the combustor comprises directing fuel from an outlet of the primary fuel pump to an inlet of the secondary fuel pump in the first operating condition. Communicating both the first fuel flow and the second fuel flow comprises blocking fuel flow from the outlet of the primary fuel pump to the inlet of the secondary fuel pump. Fuel from a fuel source is communicated to the inlet of the secondary fuel pump and both the first fuel flow from the primary pump and the secondary fuel flow from the secondary pump is routed to a common fuel outlet passage. 
     In a further embodiment of the foregoing method of supplying fuel to a combustor of a gas turbine engine, communicating the first fuel flow to the combustor comprises flowing fuel from primary fuel pump to a common fuel outlet passage and blocking flow from the secondary fuel pump during the first engine operating condition. Communicating the first fuel flow and the second fuel flow to the combustor in the second operating condition comprises communicating both the first fuel flow from the primary fuel pump and the second fuel flow from the secondary fuel pump to the common fuel outlet passage. 
     Although the different examples have the specific components shown in the illustrations, embodiments of this invention are not limited to those particular combinations. It is possible to use some of the components or features from one of the examples in combination with features or components from another one of the examples. 
     These and other features disclosed herein can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an example gas turbine engine. 
         FIG. 2  is a schematic view of an example fuel system embodiment in a first operating condition. 
         FIG. 3  is another schematic view of the example fuel system embodiment in a second operating condition. 
         FIG. 4  is a schematic view of another example fuel system embodiment. 
         FIG. 5  is a schematic view of yet another example fuel system embodiment in a first operating condition. 
         FIG. 6  is a schematic view of the example fuel system embodiment in a second operating condition. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates a gas turbine engine  20 . The gas turbine engine  20  is disclosed herein as a two-spool turbofan that generally incorporates a fan section  22 , a compressor section  24 , a combustor section  26  and a turbine section  28 . The fan section  22  drives air along a bypass flow path B in a bypass duct defined within a nacelle  18 , and also drives air along a core flow path C for compression and communication into the combustor section  26  then expansion through the turbine section  28 . Although depicted as a two-spool turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with two-spool turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures. 
     The exemplary engine  20  generally includes a low speed spool  30  and a high speed spool  32  mounted for rotation about an engine central longitudinal axis A relative to an engine static structure  36  via several bearing systems  38 . It should be understood that the various bearing systems  38  may alternatively or additionally be provided at different locations, and the location of bearing systems  38  may be varied as appropriate to the application. 
     The low speed spool  30  generally includes an inner shaft  40  that interconnects, a first (or low) pressure compressor  44  and a first (or low) pressure turbine  46 . The inner shaft  40  is connected to a fan section  22  through a speed change mechanism, which in exemplary gas turbine engine  20  is illustrated as a geared architecture  48  to drive fan blades  42  at a lower speed than the low speed spool  30 . The high speed spool  32  includes an outer shaft  50  that interconnects a second (or high) pressure compressor  52  and a second (or high) pressure turbine  54 . A combustor  56  is arranged in exemplary gas turbine  20  between the high pressure compressor  52  and the high pressure turbine  54 . A mid-turbine frame  58  of the engine static structure  36  may be arranged generally between the high pressure turbine  54  and the low pressure turbine  46 . The mid-turbine frame  58  further supports bearing systems  38  in the turbine section  28 . The inner shaft  40  and the outer shaft  50  are concentric and rotate via bearing systems  38  about the engine central longitudinal axis A which is collinear with their longitudinal axes. 
     The core airflow is compressed by the low pressure compressor  44  then the high pressure compressor  52 , mixed and burned with fuel in the combustor  56 , then expanded over the high pressure turbine  54  and low pressure turbine  46 . The mid-turbine frame  58  includes airfoils  60  which are in the core airflow path C. The turbines  46 ,  54  rotationally drive the respective low speed spool  30  and high speed spool  32  in response to the expansion. It will be appreciated that each of the positions of the fan section  22 , compressor section  24 , combustor section  26 , turbine section  28 , and fan drive gear system  48  may be varied. For example, gear system  48  may be located aft of the low pressure compressor  44  and the fan blades  42  may be positioned forward or aft of the location of the geared architecture  48  or even aft of turbine section  28 . 
     The engine  20  in one example is a high-bypass geared aircraft engine. In a further example, the engine  20  bypass ratio is greater than about six ( 6 ), with an example embodiment being greater than about ten ( 10 ), the geared architecture  48  is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine  46  has a pressure ratio that is greater than about five. In one disclosed embodiment, the engine  20  bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor  44 , and the low pressure turbine  46  has a pressure ratio that is greater than about five 5:1. Low pressure turbine  46  pressure ratio is pressure measured prior to inlet of low pressure turbine  46  as related to the pressure at the outlet of the low pressure turbine  46  prior to an exhaust nozzle. The geared architecture  48  may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3:1 and less than about 5:1.  5 It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present invention is applicable to other gas turbine engines including direct drive turbofans. 
     A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section  22  of the engine  20  is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet (10,668 meters). The flight condition of 0.8 Mach and 35,000 ft (10,668 meters), with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (“TSFC”)”—is the industry standard parameter of 1 bm of fuel being burned divided by 1 bf of thrust the engine produces at that minimum point. “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram ° R)/(518.7° R)] 0.5 . The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second (350.5 meters/second). 
     The example gas turbine engine includes the fan section  22  that comprises in one non-limiting embodiment less than about  26  fan blades  42 . In another non-limiting embodiment, the fan section  22  includes less than about  20  fan blades  42 . Moreover, in one disclosed embodiment the low pressure turbine  46  includes no more than about 6 turbine rotors schematically indicated at  34 . In another non-limiting example embodiment, the low pressure turbine  46  includes about 3 turbine rotors. A ratio between the number of fan blades  42  and the number of low pressure turbine rotors is between about 3.3 and about 8.6. The example low pressure turbine  46  provides the driving power to rotate the fan section  22  and therefore the relationship between the number of turbine rotors  34  in the low pressure turbine  46  and the number of blades  42  in the fan section  22  disclose an example gas turbine engine  20  with increased power transfer efficiency. 
     Fuel is delivered to the combustor  56  by a fuel system  62 . The example fuel system  62  includes a primary system  74  and a secondary system  76 . Fuel from a fuel tank  68  is pumped to a desired pressure and provided to the combustor  56 . The disclosed fuel system  62  tailors a flow of fuel to the combustor  56  based on engine operating conditions. Instead of simply providing a fuel flow that provides for extremes of operating demands, the disclosed fuel system  62  tailors the flow of fuel according to a demand for fuel. By tailoring the flow of fuel to engine operating demand, fuel directed through a fuel recirculation loop for excess fuel can be reduced and/or eliminated. 
     Fuel is utilized as a heat sink to cool other flows within the engine such as lubricant and air flows. In this example, a heat fuel/oil heat exchanger  70  cools a flow of lubricant generated by a lubricant system  72 . Recirculation of fuel results in an increased temperature of the fuel and thereby a reduced capability to accept heat from other engine systems, such as the example lubricant system  72 . 
     The disclosed fuel system  62  varies the flow of fuel based on demand to reduce and/or eliminate the recirculation of fuel and thereby increase the ability to accept heat from other engine systems. The disclosed fuel system  62  includes mechanically driven pumps to provide a reliable and robust fuel system  62 . The example fuel system  62  is driven by outputs from an accessory gearbox  64 . The accessory gearbox  64  is in turn driven by a shaft of the gas turbine engine  20 . In this example, the accessory gearbox  64  is driven though a tower shaft  66  coupled to the outer shaft  50 . Although the example gearbox  64  is driven by the tower shaft  66  coupled to the outer shaft  50  of the high speed spool  32  other couplings could be utilized to drive the accessory gearbox  64  and are within the scope and contemplation of this disclosure. 
     Referring to  FIG. 2  with continued reference to  FIG. 1 , the example fuel system includes a primary fuel pump  78  that provides a first fuel flow  96  during engine operation. The system  62  includes a secondary fuel pump  80  that provides a second fuel flow  98 . Both the primary fuel pump  78  and the secondary fuel pump  80  are driven by outputs of the accessory gearbox  64 . In the disclosed example, a first shaft shown schematically at  102  drives the primary fuel pump  78  and a second shaft schematically shown at  104  drives the secondary fuel pump  80 . 
     The first fuel pump  78  and the second fuel pump  80  both receive fuel flow from a common inlet passage  105  and both the first fuel pump  78  and the second fuel pump  80  communicate fuel flow to a common outlet passage  100 . The first fuel pump  78  communicates fuel flow through a first passage  82 . The second fuel pump  80  communicates fuel flow through a second passage  84 . A recirculation passage  86  communicates excess fuel from near the outlet  100  to a location upstream of both the first and second pumps  78 ,  80 . 
     A first control valve  88  is disposed within the second passage  84  upstream of the secondary fuel pump  80 . A second control valve  90  is disposed within the second passage downstream of the secondary fuel pump  80 . A controller  92  governs operation of first control valve  88  and the second control valve  90  to controlling communication of fuel to and from the secondary fuel pump  80 . 
     Both the primary and secondary fuel pumps  78 ,  80  are mechanical constant volume fuel pumps driven by the shafts  102 ,  104  from the accessory gearbox  64 . The primary and secondary pumps  78 ,  80  in one disclosed embodiment provide identical fuel flow volumes. In another disclosed embodiment, the primary and secondary pumps  78 ,  80  provide different fuel flow volumes. 
     Because both the primary and secondary pumps  78 ,  80  are mechanically linked to corresponding shafts  102 ,  104 , the secondary fuel pump  80  runs even when fuel is not supplied through the second passage because the control valves  88 ,  90  are closed. In the first operating condition with both the first and second control valves  88 ,  90  closed, the primary fuel pump  78  generates a first fuel flow  96  through the first flow passage  82 . The secondary fuel pump  80  does not provide fuel flow because the control valves  88 ,  90  are closed. It should be understood, that the control valves  88 ,  90  are provided in a disclosed example embodiment and in some systems may not be needed or may be located in alternate locations. 
     The first fuel flow  96  of a defined volume determined to provide sufficient fuel for engine operating conditions that are less then maximum. Accordingly, when the engine is operating in low fuel demand conditions such as during a cruise or descent condition, only the first fuel flow  96  is provided. The reduced fuel flow during the low demand conditions reduces the amount of fuel that may be recirculated through the recirculation passage  86 . The recirculation passage  86  includes a pressure relieve valve  94  that enables a uniform pressure of fuel flow to the combustor  56 . 
     Referring to  FIG. 3 , with continued reference to  FIG. 1 , in higher fuel demand conditions such as take-off and climb conditions, the controller  92  opens the control valves  88 ,  90  to communicate fuel to the secondary fuel pump  80 . The secondary fuel pump  80  generates a second fuel flow  98  through the second passage  84  that combines with the first fuel flow  96  from the first passage  82 . The combined first and second fuel flows  96 ,  98  are both communicated through the common outlet  100  to the combustor  56 . Once the engine transitions back to a low fuel demand operating condition, the control valves  88 ,  90  are closed and the first fuel flow  96  continues to be communicated to the combustor  56 . The second fuel flow  98  is stopped and the reduced fuel flow continues at levels tailored to current engine operation. 
     Referring to  FIG. 4 , with continued reference to  FIG. 1 , another fuel system is schematically shown at  62 ′. The fuel system  62 ′ includes a clutch  106  for selectively coupling the shaft  106  to the accessory gearbox  64 . The clutch  106  may be decoupled to deactivate the secondary pump  80 . Accordingly, rather than continually drive the secondary pump  80  when not needed, the example fuel system  62 ′ decouples the secondary pump  80 . Because the secondary pump  80  is decoupled and therefore not provide the secondary fuel flow  96 , the control valves  88 ,  90  are not needed and are removed. The controller  92  selectively actuates the clutch  106  when the additional fuel flow is needed for engine operation. 
     The example fuel systems  62 ,  62 ′ thereby operate to combine fuel flows in parallel fuel passages to accommodate fuel demands according to engine operating conditions. 
     Referring to  FIGS. 5 and 6 , another fuel system embodiment is disclosed and schematically indicated at  110 . The fuel system  110  includes a primary fuel pump  112  and a secondary fuel pump  114 . The primary fuel pump  112  and the secondary fuel pump  114  are identical constant volume mechanical gear mesh pumps arranged to operate both in series and in parallel depending on fuel flow demand. 
     The primary fuel pump  112  includes an inlet  124  and an outlet  126  and is disposes upstream of the secondary fuel pump  114 . The secondary fuel pump  114  includes an inlet  130  and an outlet  128 . The fuel system  110  includes a first fuel passage  116  in parallel with a second fuel passage  118 . Both the first fuel passage  116  and the second fuel passage  118  are in communication with a common fuel outlet  120  and the fuel tank  68 . 
     A first pressure relief valve  132  is disposed within a crossover passage  144  that communicates fuel from the outlet  126  of the primary fuel pump  112  to the inlet  130  of the secondary fuel pump  130 . The first pressure relief valve  132  enables switching between a series arrangement where a first fuel flow  140  is provided through both the primary and secondary fuel pumps  112 ,  114  and a parallel arrangement where the first fuel flow  140  is provided by the primary fuel pump  112  and a secondary fuel flow  142  ( FIG. 6 ) is provided by the secondary fuel pump  114 . 
     The first pressure relief valve  132  opens to communicate fuel from the primary fuel pump  112  to the secondary fuel pump  114  to provide the first fuel flow  140  in a first operating condition ( FIG. 5 ). The first operating condition corresponds with low fuel demand operation such as during decent or cruise conditions. In low fuel demand operating conditions, a fuel pressure at the common outlet  120  maintains a first check valve  136  in a closed position and the first pressure relief valve  132  is open. A second check valve  138  upstream of the secondary fuel pump  114  is also closed to prevent communication of fuel independent of the primary fuel pump  112 . 
     Upon an increase in fuel demand for engine operating conditions such as takeoff and climb operations, a pressure at the common outlet passage  120  will drop due to the increased fuel flow. The drop in fuel pressure opens the first check valve  136  and closes the first relief valve  132 . The second check valve  138  also opens. Fuel flow from the primary fuel pump  112  proceeds through passage  116  to the common outlet  120  independent of fuel flow in the second passage  118 . The second passage  118  is now open to receive fuel from the fuel tank  68  and provides a second fuel flow  142  to double the fuel flow through the common outlet  120 . Accordingly, once the first pressure relief valve  132  closes, the secondary fuel pump  114  provides the second fuel flow  142  in parallel with the first fuel flow  140  provided by the primary mechanical fuel pump  112  to the common fuel passage  120 . 
     A second pressure relief valve  134  is disposed downstream of both the primary fuel pump  112  and the secondary fuel pump  114  for directing excess fuel flow through a recirculation passage  122  in response to a pressure within the common fuel passage  120  above a predefined pressure. However, once pressure increase at the common fuel passage  120 , the fuel system will switch back to the series arrangement. In response to an increase in fuel pressure that would accompany a drop in fuel demand based on engine operating conditions, the first check valve  136  would close. Closing of the first check valve  136  is followed by opening of the first relief valve  132  such that fuel from the primary fuel pump  112  is directed through the cross-over passage  144  to the secondary fuel pump  114  as is shown in  FIG. 5 . Accordingly, the disclosed fuel system  110  switches between series and parallel flow arrangements in response changes in fuel pressures caused by changes in fuel flow demands. The first relief valve  132  may be a controlled valve or may be a mechanical valve that opens in response to fuel pressure. In this example, the first relief valve  132  is configured to open at a lower differential pressure than the second pressure relief valve  134 . 
     Accordingly, the example fuel systems  62 ,  62 ′ and  110  tailor fuel flow to engine operating demands while maintaining the proven reliability and robust operation of mechanical constant volume pumps. 
     Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the scope and content of this disclosure.