Abstract:
A fuel system for delivering fuel from a fuel source to combustor in a gas turbine engine has a pump receiving fuel flow from the fuel source and producing a pressurized fuel flow that flows to a flow meter that measures the fuel flow and generates a signal thereof. A bypass valve bypasses, to the input side of the pump, a portion of the pressurized fuel flow before it reaches the flow meter. An electronic control unit is included that receives the signal and in response thereto adjusts the bypass valve until the measured fuel flow equals a predetermined desired fuel flow stored in the electronic control unit. The fuel system is also self-calibrating.

Description:
This invention relates fuel system for jet engines or other types of gas turbine engines. 
     BACKGROUND OF THE INVENTION 
     FIG. 1 shows a typical fuel control system  10  for a gas turbine engine such as a jet propulsion engine. The system  10  includes in flow series arrangement a supply of fuel represented by arrow  12  which is fed to a boost pump  14  and then to a high pressure pump  16 . From the high pressure pump  16  the fuel flow splits with a first portion flowing to a variable area metering valve  18  and a second portion to a delta-p valve  20  which bypasses fuel back to the upstream side of the high pressure pump  16  to set a fixed differential pressure, ΔP, across the metering valve  18 . From the metering valve  18  the fuel flows through an ecology and pressurizing valve  22  which maintains a pressure level on the downstream side of the metering valve  18 . The fuel then flows to a flow meter  24  which measures the fuel flow and provides a signal to the aircraft. From the flow meter  24  the fuel flows through a flow divider  26  and then to the primary and secondary fuel nozzles represented by arrows  28  and  30  which spray the fuel into the combustor of the engine. 
     A Full Authority Digital Engine Control (FADEC)  32  controls the operation of the engine including the fuel control system  10 . In particular, the FADEC  32  adjusts the area of the metering valve  18  so that the fuel flow (Wf) exiting the metering valve is delivered in accordance with the following equation. 
     
       
         Wf=CA(ΔP) ½   
       
     
     where C is a flow constant and A is the area of the metering valve. Though not shown, a linear variable displacement transducer, (LVDT), is mounted to the metering valve to provide a signal to the FADEC indicative of the position of the metering valve. 
     Disadvantages to this prior art fuel system are (a) the mechanical complexities required to maintain a constant metering valve differential pressure with varying altitude, (b) precision components like the LVDT are required to control metering area with varying temperature, and (c) the inherent inaccuracy of controlling fuel flow through indirect parameters such as area and pressure. 
     Accordingly, there is a need for a simplified fuel system for a jet engine that controls fuel flow based on measured fuel flow. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a simplified fuel system that controls fuel flow based on measured fuel flow. 
     Another object of the present invention is to provide a fuel system that is self calibrating. 
     The present invention meets these objectives providing an a fuel system for delivering fuel from a fuel source to combustor in a gas turbine engine having a pump receiving fuel flow from the fuel source and producing a pressurized fuel flow that flows to a flow meter that measures the fuel flow and generates a signal Wfm thereof. A bypass valve bypasses, to the input side of the pump, a portion of the pressurized fuel flow before it reaches the flow meter. An electronic control unit is included that receives the Wfm signal and in response thereto adjusts the bypass valve until the measured fuel flow equals a predetermined desired fuel flow stored in the electronic control unit. 
     Thus, a fuel system is provided that control fuel flow based on measured fuel flow. Because it controls on measured fuel flow, a fuel system is simpler and hence more reliable than prior art fuel systems. The fuel system is also self-calibrating as is explained in the specification. 
     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 when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a prior art jet engine fuel system. 
     FIG. 2 is a schematic of the fuel system contemplated by the present invention. 
     FIG. 3 is a block diagram of the control logic used with the fuel system of FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 2, a simplified fuel control system is generally denoted by reference numeral  50 . The system  50  comprises in flow series arrangement a supply of fuel represented by arrow  52  which is fed to a boost pump  54  and then to a high pressure pump  56 . From the high pressure pump  56  the fuel flow splits with a first portion flowing to a flow meter  58  and a second portion to a bypass valve  60  which bypasses fuel back to the upstream or input side of the high pressure pump  56 . The bypass valve is driven by a torque motor  61 . From the flow meter  58  the fuel flows through an ecology and pressurizing valve  62  which maintains a pressure level on the downstream side of the flow meter  58 , and then to a flow divider  66 . From the flow divider  66  the fuel flows to the primary and secondary fuel nozzles represented by arrows  68  and  70  which spray the fuel into the combustor of the engine. In an alternative embodiment, where only a single fuel nozzle is used, the flow divider  66  can be eliminated. An electronic control unit  72  controls control the operation of the fuel system  50 . In the preferred embodiment, the unit  72  is part of the Full Authority Digital Engine Control (FADEC) which controls the operation of the entire engine. 
     Referring to FIGS. 2 and 3, the FADEC  72  receives a signal N indicative of the rotational speed of the engine, a signal P 3  indicative of the pressure in the combustor of the engine downstream of the fuel nozzles  68 ,  70 , a P 0  signal indicative of the pressure downstream of the bypass valve  60 . These signals are generated by properly mounted sensors in a manner familiar to those skilled in the art. The FADEC  72  also has control logic that generates a desired fuel flow signal Wfd as a function of the operating conditions of the engine. An example of this type of control logic that is used during the start of the engine can be found in LaCroix, U.S. Pat. No. 4,337,615 which is assigned to the Assignee of this application. Other types of fuel schedules are used by the FADEC during the different modes of the engine. Electronically integrated into the FADEC  72  is a control system generally denoted by reference numeral  100  that adjust the bypass flow through bypass valve  60  until the measured fuel flow from the flow meter  58 , Wfm, equals the desired fuel flow Wfd as determined by fuel schedules in the FADEC  72 . 
     With continued reference to FIG. 3, the control system  100  has a function block  102  that contains a table, curve, or algorithm that relates fuel flow to the pressure drop across the fuel nozzles  68 ,  70  and fuel divider  66 . This relationship is predetermined and programmed into the control system  100 . The function block  102  generates a ΔP signal indicative of this pressure drop. This ΔP signal is added to the P 3  signal in summer  104  to arrive at a signal indicative of the output pressure Pn of the ecology and pressurizing valve  62 . In a summer  106  the Pn signal is added to a ΔPother signal indicative of the pressure drops across the ecology and pressurizing valve  62  and the flow meter  58 . This ΔPother signal is programmed into the control system  100  and is determined through a calibration process familiar to those skilled in then art. The ΔPother signal is added to the Pn signal in summer  106  to arrive at a signal Ppd indicative of the high pressure pump  56  discharge pressure. Using the Ppd signal and the N signal, function block  108  generates a signal Wpd indicative of the fuel flow from the high pressure pump  56 . Function block  108  contains a table, curve or algorithm that relates these parameters to Wpd. This relationship is programmed into the system  100  and is available from the pump manufacturer or can be determined through a calibration test on the pump  56 . The Wpd signal from function block  108  is then added in summer  110  to the Wfd signal to arrive at a Wbp signal indicative of the bypass fuel flow fuel needed to obtain the desired fuel flow Wfd to the engine. 
     Function block  112  receives the Ppd signal, the Wbp signal and the P 0  signal and then solves the following equation to arrive at a signal Abp indicative of the area of the bypass valve that results in the bypass flow Wbp. 
     
       
         Abp=Wbp/(Ppd−P 0 ) ½   
       
     
     Function block  114  contains a curve, table, or algorithm that relates the area of the bypass valve  60  with the current to the torque motor  16 . Using the Abp signal function block  114  generates the necessary current I which is then sent to the torque motor  61  which then moves the bypass valve  60 . A multiplier  120  whose purpose is described later is disposed between the function block  112  and the function block  114 . The control system  100  as described so far is designed to operate during transient engine conditions when the measurements from the flow meter  58  are not reliable. Under these circumstances the control system generates the necessary current I based on desired fuel flow only thus avoiding the unreliable fuel flow measurements. 
     A unique advantage to the fuel system  50  and the corresponding fuel control system  100  is that the flow meter  58  can be used for self calibration. 
     FIG. 3 also depicts a trimming circuit by which the system  100  can be trimmed to account for wear of the pump, changes in nozzle flow characteristics and other variations in the fuel system  50  that change over time. The trimming circuit includes a summer  122  that subtracts measured fuel flow signal Wfm from the flow meter  58  from the value desired fuel flow signal Wfd to generate an error signal. A gain  124  multiplies the error signal by the value of 1/K where K is initially set at a predetermined value based on calibration testing of the bypass valve  60 . The gain takes the resulting product and stores it as a new 1/K. Disposed between the gain  124  and the multiplier  120  is an integrator  126  that integrates 1/K over time. The integrator would be “held” at its present value until other software represented by function block  128  in the controller  72  determined that “steady state” conditions were prevailing in the engine. This steady state logic could sample EGT or N over a period of time to determine that no significant changes are occurring. After this period of time is completed the integrator would be released to trim the value of 1/K as required by any new error between actual and desired fuel flow. The resulting 1/K is then multiplied with the Abp signal in the multiplier  120 . Once the engine returns to transient conditions, the integrator is turned off and it holds it last value of 1/K until steady state conditions return. 
     Thus an improved and simplified fuel system is provided. This system eliminates expensive metering valves and LVDTs found in prior art systems and thus is more reliable. By eliminating the metering valve, the leakage for the entire system is reduced. This means that the pump  56  can be smaller which means bypass flows can be smaller reducing pump heating and increasing the life of the pump. Further the system is self calibrating which will extend the service life of the system  50 . 
     Although the invention has been described in terms of a fuel system for a jet propulsion engine, it will be appreciated by those skilled in the art that the invention can be used with gas turbine engine fuel system. Accordingly, various changes and modifications may be made to the illustrative embodiment without departing from the spirit or scope of the invention. It is intended that the scope of the invention not be limited in any way to the illustrative embodiment shown and described but that the invention be limited only by the claims appended hereto.