Abstract:
A method and apparatus for measuring a start fuel flow to a pilot nozzle of a fuel system of a gas turbine engine using pressure differential between fuel passages leading to fuel nozzles.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to gas turbine engines and more particularly, to an improved fuel system for gas turbine engines. 
       BACKGROUND OF THE ART 
       [0002]    During a gas turbine engine starting, fuel is first provided to primary fuel nozzles which function as pilot nozzles, in order to deliver a very small amount of fuel near to the igniter system in the combustor for ignition at these nozzles to produce pilot torches in the combustor. Fuel is then provided to a set of main fuel nozzles, often through a manifold, to deliver the fuel at a relatively higher fuel pressure and high flow rate to start and maintain the continuous combustion in the combustor. The start flow needs to be accurately delivered and is normally metered by a metering valve/pump in demand fuel pumping systems. Controlling the start fuel flow with the metering valve/pump may lead to flow accuracy problems, given the factor that the fuel flow is in a very small amount in contrast to the maximum fuel flow to be pumped by the demand fuel pump, and that pump wear over the life of the pump could lead to problems with metering the start fuel flows. It is also desirable to eliminate the fuel metering valve/pump as a cost and weight savings. 
         [0003]    Accordingly, there is a need to provide an improved low fuel flow metering control of a fuel system of gas turbine engines. 
       SUMMARY 
       [0004]    In one aspect, provided is a method for measuring a start fuel flow to a pilot nozzle in a fuel system of a gas turbine engine for ignition in a combustion chamber during an engine start procedure, the fuel system including a first fuel passage leading to the pilot nozzle and a second fuel passage leading to a main manifold, both the pilot nozzle and main manifold being in fluid communication with the combustion chamber, the method comprising: a) measuring a pressure differential between the first fuel passage and the second fuel passage while the start fuel flow is being directed through the first fuel passage to the pilot nozzle, until a light-up condition of the pilot nozzle is detected; and b) calculating the start fuel flow using a flow number of the pilot nozzle and a measured value of the pressure differential. 
         [0005]    In another aspect, provided is an apparatus for determining a start fuel flow to a pilot nozzle of a fuel system for ignition in a combustion chamber of a gas turbine engine, the apparatus comprising: a differential pressure transducer connected between first and second fuel passages of the fuel system, the first fuel passage leading to the pilot nozzle and the second fuel passage leading to a main manifold, both the pilot nozzle and the main manifold being in fluid communication with the combustion chamber; and means for calculating the start fuel flow using a known flow number of the pilot nozzle and a measured value of the differential pressure transducer. 
         [0006]    In another aspect, provided is fuel system of a gas turbine engine which comprises a fuel pump for pressurizing fuel from a fuel source; at least a first nozzle in fluidic communication with a combustion chamber of the engine; at least a second nozzle in fluidic communication with the combustion chamber of the engine; a fluidic connection extending from the fuel pump and dividing into at least first and second passages leading to the respective first and second nozzles; a differential pressure transducer connected between the first and second passages of the fluidic connection for measuring a pressure differential between the first and second passages; and a control unit in contact with the fluidic connection for controllably operating the fuel system, the control unit including a device for using a measured value of the differential pressure transducer to calculate a start fuel flow through the first passage before a light-up condition of the first nozzle is detected. 
         [0007]    Further details of these and other aspects will be apparent from the detailed description and figures included below. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0008]    Reference is now made to the accompanying figures in which: 
           [0009]      FIG. 1  is a schematic cross-sectional view of a turbofan gas turbine engine; 
           [0010]      FIG. 2  is a schematic illustration of a fuel system used for the engine of  FIG. 1 , showing one embodiment of the present technique; 
           [0011]      FIG. 3  is a schematic illustration of the fuel system of  FIG. 2 , showing a step of the fuel system operation for supplying a start flow to a pilot nozzle while a main manifold is in a dry condition; and 
           [0012]      FIG. 4  is a schematic illustration of the fuel system of  FIG. 2 , showing a further step of the fuel system operation for supplying both the start flow and main manifold flow under a high fuel pressure to the respective pilot torch nozzle and the main manifold of the combustor. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    Referring to  FIG. 1 , a turbofan gas turbine engine incorporating an embodiment of the present approach includes a bypass duct  10 , a core casing  13 , a low pressure spool assembly seen generally at  12  which includes a fan assembly  14 , a low pressure compressor assembly  16  and a low pressure turbine assembly  18 , and a high pressure spool assembly seen generally at  20  which includes a high pressure compressor assembly  22  and a high pressure turbine assembly  24 . The core casing  13  surrounds the low and high pressure spool assemblies  12  and  20  in order to define a main fluid path (not indicated) therethrough. In the main fluid path there are provided a combustor seen generally at  25  and a fuel system  28 , including fuel nozzles (not depicted in  FIG. 1 ) for delivery of fuel to the combustor  25  for combustion. The compressor assemblies  16  and  22  provide a compressed airflow (not indicated) through the main fluid path and in communication with the combustor  25  for combustion therein. 
         [0014]    Referring to  FIGS. 1 and 2 , the fuel system  28  according to one embodiment, comprises a fuel pump  30  (a vane type of fuel pump is preferred, which is driven independent of the high pressure spool) for pressurizing the fuel to establish a fuel pressure under which fuel is delivered from a fuel source  32  through a fluidic connection of the fuel system  28  preferably to at least one pilot nozzle  34  such as a torch nozzle or some other form of primary nozzle, which is used to initialize ignition of combustion in a combustion chamber defined by the combustor  25 , and a main manifold  36  of the combustor  25  which distributes fuel to a plurality of main fuel nozzles  37  (only one shown) of the combustor  25  in order to supply fuel for combustion within the combustor  25 . Both the pilot nozzle  34  and the main fuel nozzles  37  of the main manifold  36  are in fluid communication with the combustion chamber which, in turn, is in a fluidic communication with an outlet stage of the compressor  22 . The fluidic connection of the fuel system  28  further includes, for example, a minimum pressure/flow divider valve  38  having an inlet  40  and outlets  42 ,  44 , which are normally closed under a spring force of the minimum pressure/flow divider valve  38 . The minimum pressure/flow divider valve  38  is adapted to open the outlet  42  only when inlet  40  is exposed to a low pressure which is equal to or above a predetermined minimum pressure threshold, but is lower than a predetermined high pressure threshold, or to open both outlets  42  and  44  when inlet  40  is exposed to a high pressure, which is equal to or above the predetermined high pressure threshold. This will be further discussed with reference to the system operation process. 
         [0015]    A fuel flow passage  46  interconnects the fuel pump  30  and the inlet  40  of the minimum pressure/flow divider valve  38 , and a fuel flow passage  48  is connected between the outlet  42  and the pilot nozzle  34 . There is a fuel flow passage  50  extending between the outlet  44  of the minimum pressure/flow divider valve  38  and the main manifold  36  in a parallel relationship with the fuel flow passage  48 . It should be noted that due to the flow rate difference between the required fuel flow to the pilot nozzle  34  (the igniter flow) and the fuel flow to the main manifold  36  (the manifold flow), the fuel flow passage  48  is sized in cross-section smaller than the fuel flow passage  50 , thereby resulting in a high flow resistance of the fuel flow passage  48  relative to the fuel flow passage  50 . 
         [0016]    A differential pressure transducer  52  is preferably connected between the fuel flow passage  48  and the fuel flow passage  50  such that a pressure differential between fuel flow passages  48  and  50  can be monitored from time to time and particularly during engine start up while no fuel flow is delivered to the main manifold  36 . The differential pressure transducer  52  is electrically connected to an electrical engine control (EEC)  60  such that the pressure differential between the fuel flow passages  48  and  50  monitored by the differential pressure transducer  52 , can be used by EEC  60  as a reference signal for controlling the operation process of the fuel system  28 . 
         [0017]    A flow equalization solenoid valve  58  is preferably connected by fuel flow passages  54 ,  56  to the respective fuel flow passages  48  and  50 , in a parallel relationship with the differential pressure transducer  52 . The flow equalization solenoid valve  58  is a normally open valve to allow a fluidic communication between the fuel flow passages  48  and  50  when the minimum pressure/flow divider valve  38  closes outlets  42  and  44  thereof. The flow equalization solenoid valve  58  is electrically connected to and controlled by EEC  60  and is adapted to close the fuel flow passages  54 ,  56  when a control signal is received from the EEC  60 . 
         [0018]    The differential pressure transducer  52  is in fluidic connection with the respective pilot nozzle  34  and the main fuel nozzles  37  via the main manifold  36  which are, in turn, in fluid communication with the combustion chamber, which is supplied with air pressure from the compressor, for example, P3 compressor air. However, the pressure measured in the combustion chamber is somewhat lower than the P3 compressor air pressure due to a pressure drop across the combustor liner, and is indicated as P4 combustion chamber air pressure. Therefore, the P4 combustion chamber air pressure is automatically provided to the differential pressure transducer  52  as a reference pressure via fuel flow passage  50 , when the flow equalization solenoid valve  58  is in the closed position and outlet  44  of the minimum pressure/flow divider valve  38  is closed (when the compressor  22  is rotated either by the turbine  24  or by a starter) for monitoring the pressure differential between the fuel flow passages  48  and  50 . For example, the pressure differential between the fuel flow passages  48  and  50  monitored by the differential pressure transducer  52 , can be used for monitoring a fuel flow through the fuel flow passage  48  to the pilot nozzle  34  during the engine start-up process, and to determine when to deactivate the flow equalization solenoid valve  58  to open the fuel flow passages  54 ,  56  in order to allow the fuel flow to pass through the fuel flow passage  50  to the main manifold  36 . This will be further described hereinafter. 
         [0019]    An ecology solenoid valve  62  is preferably provided to control fuel flow passages  64 ,  66  which are connected to the respective fuel flow passages  46  and  48  to form a bypass over the minimum pressure/flow divider valve  38 . The ecology solenoid valve  62  is normally closed and is electrically connected to EEC  60 . The ecology solenoid valve  62  can be controlled by EEC  60  to selectively open for establishing the fluidic connection of the fuel system  28  between the fuel source  32 ′ and the main fuel nozzles  37  of the main manifold  36 , as well as the pilot nozzle  34  when required. 
         [0020]    A check valve  68  is optionally provided within the fuel flow passage  66 . Should the ecology valve  62  be opened in malfunction, the check valve  68  ensures that the bypass connection over the minimum pressure/flow divider valve  38  should be used only for fuel flowing therethrough back to the fuel pump  30  and the fuel source  32 , but not for fuel supply therethrough from the fuel pump  30 . 
         [0021]      FIGS. 3-4  illustrate the steps of operation of the fuel system  28 . For convenience of description, different numerals in those Figures are used in connection with arrows to indicate fluid flows under pressure differentials having different values. A single head arrow indicates the direction of the fluid flow and a double head arrow indicates the fluid flow is blocked. 
         [0022]    Referring to  FIG. 3 , EEC  60  controls the fuel pump  30  to operate at a speed to establish the low fuel pressure during engine start conditions. The low fuel pressure forces the minimum pressure/flow divider valve  38  to open the inlet  40  and outlet  42 , allowing a fuel flow indicated by arrow  70  to pass through the fuel passages  46 ,  48  to the pilot nozzle  34 . The ecology solenoid valve  62  is normally closed such that there is no fuel flow through the bypass formed by the fuel flow passages  64 ,  66 . The flow equalization solenoid valve  58  is activated by EEC  60  to be closed during the initial engine start condition such that there is no fuel flow passing through fuel flow passage  50  to the main manifold, either via the minimum pressure/flow divider valve  38  or via the fuel flow passages  54 ,  56 . The fuel flow passage  50  and the main manifold  36  may remain in a dry condition (empty of fuel), having a pressure therein equal to the air pressure in the combustor  25  of  FIG. 1 , i.e. the P4 combustion chamber air pressure. The air inside of the fuel flow passage  50  and the main manifold  36  under such air pressure conditions, is indicated by the hollow double-head arrows  72 . The low fuel pressure in the fuel flow passages  46 ,  48  is higher than the pressure in the fuel flow passage  50 , thereby forming a pressure differential therebetween. The pressure differential is monitored by the differential pressure transducer  52  which sends corresponding signals to EEC  60 . A measured value (which may be a varying value) of the differential pressure transducer  52  is indicated as ΔP. 
         [0023]    It should be noted that a relatively low range (i.e. sensitive) pressure transducer may be preferred for the purpose of monitoring flow during start and fuel pulses on manifold filling. It is preferable to use a sensitive or low range pressure transducer in practical terms, because the transducer never has a high pressure differential applied to it. The differential pressure is shunted out via fuel passages  54  and  56  in conjunction with flow equalization valve  58 , limiting the maximum differential pressure to which the transducer is exposed. For example, the differential pressure during start may be of the order of 120 PSI maximum, however the fuel system pressure may be over 1000 PSI during take off conditions. A transducer used for applications involving 1000 PSI is very poor at resolving small pressure differentials needed to control flow at low flow conditions. Therefore, it is optional to have a transducer having a maximum pressure indication for example, not greater than 150 PSI. 
         [0024]    During the engine start procedure, the low start fuel flow to the pilot nozzle  34  is accurately controlled by adjustment of fuel pump  30  which in turn is controlled by EEC  60 . Nevertheless, such accurate control of the low start fuel flow is based on the accurate metering of the low start fuel flow, which is achieved by a start fuel flow calculating software  61  which may be included in EEC  60  using the measured values of pressure differential by the differential pressure transducer  52 , in this embodiment 
         [0025]    If Pp is used to indicate the low fuel pressure established by the fuel pump  30  during the engine start procedure as shown in  FIG. 3 , the start fuel flow  70  can be calculated as F=PN(Pp−P4) 1/2  wherein F represents the calculated amount of start fuel flow  70  and PN represents the flow number of the pilot nozzle  34 . It is understood that Pp−P4 represents the pressure differential which causes the start fuel flow  70  because the start fuel flow  70  is driven by the established low fuel pressure Pp against the combustion chamber air pressure P 4  to which the pilot nozzle  34  is exposed. It is further noted that the air pressures inside the empty passage  50  and the main manifold  36  are substantially equal to the combustion chamber air pressure P 4  because the main manifold  36  is in fluid communication, through the main fuel nozzles attached thereto, with the combustion chamber air pressure P 4 , while the fluid communication between passage  50  and passage  48  is closed. Therefore, a measured value ΔP of the differential pressure transducer  52  is equal to Pp−P4. The measured value ΔP can replace (Pp−P4) and can therefore be used to calculate the start fuel flow amount F, that is F=FN(ΔP) 1/2 . The software  61  for calculating the start fuel flow, includes the formulation F=FN(ΔP) 1/2 . The flow number of pilot nozzle  34  is determined by the configuration of the pilot nozzle  34  and the fuel system  28 , which is known and is stored in the software. 
         [0026]    During the engine start procedure, the fuel flow passage  50  and the main manifold  36  are generally in a dry condition, because in a previous operation of the engine the residue fuel existing the fuel system  28  has been purged back to the fuel source  32  by the residual air pressure remaining in the combustion chamber upon engine shutdown—however, this ecology function is not part of this concept and will not be further discussed in this application. Nevertheless, when the fuel from the previous engine operation remains in the fuel system  28 , the fuel remaining in the fuel flow passage  50  and the main manifold  36  is substantially stationary and the stationary fuel pressure within the fuel flow passage  50  and the main manifold  36  is generally equal to the combustion chamber air pressure P 4  or may be slightly different from P4 affected by the height of the fuel in the fuel flow passage  50  above the differential pressure transducer  52 . Considering the value ΔP measured by the differential pressure transducer  52  being of in the order of 120 PSI maximum, the minor difference relative to the combustion chamber air pressure P 4  caused by the fuel remaining in the fuel flow passage  50 , is ignorable with respect to the accuracy of the start fuel flow calculation. 
         [0027]    The combustion chamber air pressure P 4  may vary during the engine start procedure and therefore the measured value ΔP of the pressure differential may also be a varying value. The start fuel flow calculation process is conducted at least until the light-off condition of the pilot nozzle  34  is detected. The instant result of the start fuel flow calculation is continuously used as an input of a controlling process of the rotational speed of the fuel pump  30  in order to provide an adequate amount of fuel to the pilot nozzle  34  for ignition. 
         [0028]    In  FIG. 4 , during the engine start-up procedure the flow from the pilot nozzle  34  is lit up, upon which EEC  60  commands the fuel pump to increase the pump drive to establish a higher fuel pressure in order to force the minimum pressure/flow divider valve  38  to open both outlets  42  and  44  which results in a gradual and controlled increase in the fuel flow, as the compressor speed increases. Meanwhile, EEC  60  commands the flow equalization solenoid valve  58  to open the fuel flow passages  54 ,  56 , thereby allowing fuel flow via both outlets  42 ,  44  through the fuel flow passage  50  to the main manifold  36  for establishing a properly distributed fuel flow between all nozzles and a stable combustion process in the combustor  25  of  FIG. 1 . At the same time, fuel flow  76  moves via outlet  42  of the minimum pressure/flow divider valve  38  through the fuel flow passage  48  to the pilot nozzle  34  to maintain the pilot flame. This process begins upon the light-up of the pilot nozzle  34  during the engine start procedure and will be maintained during engine operation for a stable combustion in the engine combustor  25 . 
         [0029]    The check valve  68  in fuel flow passage  66  does not allow fuel flow from the fuel pump  30  to pass the bypass formed by the fuel flow passages  64 ,  66 , to the fuel flow passage  48 . EEC  60  also commands the ecology solenoid valve  62  to close the bypass. Therefore, during the entire engine operation process, fuel is supplied from the fuel source  32  to the pilot fuel nozzle  34  and the main nozzles  37  of the main manifold  36  through the fluidic connection of the fuel system  28  via the minimum pressure/flow divider valve  38 , but not via the closed bypass of fuel flow passages  64 ,  66 . 
         [0030]    The minimum pressure/flow divider valve  38  includes a leakage drain tube or duct  80  to collect any fuel that may leak along the length of the valve  38  to the location where the spring is located (not indicated). The leakage drain tube  80  is connected to the inlet side of the pump  32 . The leakage drain tube  80  preferably serves to both (i) collect fuel that may leak past the valve  38  piston, and (ii) provide a reference pressure to the rear of the valve  38  piston, such that, if fuel is delivered under pressure to the inlet of the pump  32 , the fuel pressure will not be capable of opening the minimum pressure/flow divider valve  38  to inadvertently cause a fuel flow before the pump  32  is deliberately rotated. It will be understood that the supply or boost pressure of the fuel delivered to the inlet of the main fuel pump will also appear at the outlet of the pump, and will therefore be applied to the minimum pressure/flow divider valve  38 . However, since the leakage tube  80  permits this supply or boost pressure to also be applied to the other side of the minimum pressure/flow divider valve  38 , pressure across the valve  38  piston is equalized, thus preventing the valve from inadvertently opening. Once the pump begins to rotate and generate pressure at its outlet, the minimum pressure/flow divider valve  38  will open, since the reference pressure provided by the leakage tube  80  does not increase when the pump is rotated, and thus a differential pressure across the valve  38  results. 
         [0031]    The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departure from the scope of the invention disclosed. For example, the present teachings can be applied to various types of gas turbine engines other than a turbofan gas engine which is used as an example to illustrate one application hereof. Any suitable fuel nozzle(s) arrangement may be employed, and any suitable fuel system architecture may be employed—the invention is not limited to the nozzle or manifold arrangements described in the example. Any suitable manner of determining pressure differential may be used. A fuel system may include more or less components therein for various types of gas turbine engines without departing from the spirit of the invention disclosed, and may include but is not limited to fuel reheating devices. Still other modifications which fall within the scope of the invention disclosed will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.