Abstract:
A fuel control system having a combustive energy value evaluator determining a combustive energy value of the fuel, and a controller calculating a desired flow rate based at least on the combustive energy value and controlling a fuel metering device such that the fuel flow rate corresponds to the desired fuel flow rate.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to gas turbine engines and, more particularly, to an improved fuel control system thereof. 
       BACKGROUND OF THE ART 
       [0002]    Gas turbine engines are generally adapted to be used with a single type of jet fuel, for example JP4 jet fuel. As such, use of a different fuel, for example ethanol, in these engines can be detrimental to the engines&#39; performances, as the fuel flow in the engine is usually controlled through a series of fuel schedules established for a specific type of fuel and as such not adapted for other types of fuel of mixtures thereof. 
         [0003]    With the rise of fuel costs, some areas of the world may choose to use Ethanol or mixtures of Ethanol in Jet fuel, accepting a reduced flight range for the cost savings. However, refuelling with a fuel that may be different from the fuel already contained in the tank can cause the precise equivalent content of the fuel tanks to be unknown. A pilot who is confused as to the exact type of fuel contained in the fuel tanks can be mistaken upon calculation of the range of the aircraft. This can be hazardous, especially in cases where the range is overestimated. 
         [0004]    Accordingly, there is a need to provide an improved fuel control system for a gas turbine engine. 
       SUMMARY OF THE INVENTION 
       [0005]    It is therefore an object of this invention to provide an improved fuel control system for a gas turbine engine. 
         [0006]    In one aspect, the present invention provides a method of controlling a flow of a fuel in a gas turbine engine comprising sensing at least one characteristic of the flow, determining a combustive energy value of the fuel based on the at least one characteristic, determining a desired fuel flow rate at least based on the combustive energy value, and controlling a fuel metering device of the engine to obtain the desired fuel flow rate. 
         [0007]    In another aspect, the present invention provides a method of monitoring a flow of a fuel in a gas turbine engine comprising sensing at least one characteristic of the flow, determining a combustive energy value of the fuel based on the at least one characteristic, determining at least one equivalent characteristic of a reference fuel corresponding to the combustive energy value, and displaying the at least one equivalent characteristic. 
         [0008]    In a further aspect, the present invention provides a fuel control system for a gas turbine engine comprising at least one sensor determining at least one characteristic of a fuel flow in the engine, a combustive energy value evaluator determining a combustive energy value of the fuel from the at least one characteristic, a fuel metering device metering a fuel flow rate in the engine, and a controller calculating a desired flow rate based at least on the combustive energy value and controlling the fuel metering device such that the fuel flow rate corresponds to the desired flow rate. 
         [0009]    Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]    Reference is now made to the accompanying figures depicting aspects of the present invention, in which: 
           [0011]      FIG. 1  is a schematic, cross-sectional view of a gas turbine engine; and 
           [0012]      FIG. 2  is a schematic representation of a fuel control system according to a particular embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]      FIG. 1  illustrates a gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a compressor section  14  for pressurizing the air, a combustion section  16  in which the compressed air is mixed with fuel atomized into a combustion chamber  17  by a fuel injection system  20 , the mixture being subsequently ignited for generating hot combustion gases before passing through a turbine section  18  for extracting energy from the combustion gases. 
         [0014]    Referring to  FIG. 2 , the flow of fuel to the fuel injection system  20  is controlled by a fuel control system  22 . The fuel control system  22  includes a fuel metering device  24  metering the fuel reaching the fuel injection system  20 . The fuel metering device  24  is electrically controlled in a precise and predictable manner by a controller  26 . In a particular embodiment, the controller  26  is part of the electrical and electronic engine control (EEC) (not shown) of the engine  10 . 
         [0015]    The controller  26  generally receives data from various engine sensors  28 . This data includes pressure and temperatures at various points of a flow path of the engine  10 , as well as fuel mass flow. Based on this data, the controller  26  refers to fuel schedules  30  both upon start-up of the engine  10  and once the engine  10  is lit, to determine a desired fuel flow and control the fuel metering device  24  accordingly. The fuel schedules  30  generally determine ranges for the desired fuel flow for one type of reference fuel, for example JP4 jet fuel. The controller  26  also sends data to other systems for calculations and display on a display unit  32  which displays the received data, for example in the cabin of the aircraft (not shown). Such data may include, for example, the fuel mass flow, quantity of fuel burned and remaining quantity of fuel. The controller  26  further controls an ignition system  34  of the engine  10  upon start-up. 
         [0016]    In order for the engine  10  to be able to function with fuel such as ethanol or an unknown mixture of ethanol and jet fuel, the fuel control system  22  includes a fuel mixture sensor  36  which is in line with the fuel injection system  20  (shown in  FIG. 1 ) and which determines the general mixture between ethanol and jet fuel contained in the fuel supplied to the engine  10 . The fuel mixture sensor  36  thus sends data on the composition of the mixture to the controller  26 . The fuel control system  22  also includes an energy value evaluator  38  which determines an energy value of the fuel mixture, either directly by measurement or by reference to tables based on mixture ratios. In a particular embodiment, the energy value evaluator  38  determines the lower heating value (LHV) of the fuel mixture. The fuel control system  22  further includes a reference fuel table  40 , which contains characteristics of the reference fuel in relation to the corresponding energy value. Such characteristics may include for example fuel mass flow required for various mixtures such that a specific LHV flow rate can be provided to the combustion system. 
         [0017]    In use, at start-up of the engine  10 , the controller  26  actuates the fuel mixture sensor  36  through a signal  50  and the fuel mixture sensor  36  determines the composition of the fuel supplied to the engine  10 , i.e. the proportion of ethanol and jet fuel in the fuel. Alternately, the LHV of the fuel may be measured directly with an in line LHV sensor, providing directly the information desired. The fuel mixture sensor  36  then sends corresponding proportion data  52  to the controller  26 . The controller  26  also actuates engine sensors  28  through a signal  54  and receives sensor data  56  therefrom, which includes for example the air mass flow and the pressure and temperature in the flow path at the end of the compressor section  14 . In the case where a direct LHV sensor is implemented, the fuel mass flow required for a given engine inlet air mass flow can be directly calculated by the engine control system 
         [0018]    The controller  26  then sends data  58  to the energy value evaluator  38 , the data  58  including the proportion of ethanol and jet fuel in the fuel and relevant sensor data. The energy value evaluator  38  determines the instantaneous LHV of the fuel mixture, for example from a database correlating the relevant sensor data and fuel proportion to the LHV. A given fuel mass flow of a particular fuel corresponds to a given amount of fuel energy (LHV*mass flow) that this fuel provides to the combustor. For example, the LHV of JP4 jet fuel is roughly about 18,000 BTU (British Thermal Units) per pound mass of the fuel, and thus at an exemplary fuel consumption rate of about 200 pounds of fuel flow per hour, the LHV input value to the combustor would be about 3.6 million BTU per hour. For the same engine condition using ethanol, for example, 3.6 million BTU per hour would still be required, however since ethanol has only about 60% of the energy content compared to JP4, the fuel mass flow rate of ethanol would need to be about 1.666 times as high as that for JP4, which in this example corresponds to about 366 pound of ethanol per hour. Once the instantaneous LHV of the fuel mixture has been determined by the energy value evaluator  38 , the energy value evaluator  38  then sends the LHV  60  to the controller  26 . 
         [0019]    Alternately, the fuel mixture sensor  36  can directly determine the LHV as a means to determine the proportion of ethanol and jet fuel in the fuel, and in this case the data  52  sent by the fuel mixture sensor  36  to the controller  26  includes the LHV, and the energy value evaluator  38  is used during start-up. However, the proportion of ethanol to jet fuel need not be known if the LHV is measured direction as described above. In one embodiment, the LHV is determined specifically for the start up flow settings, as once the engine is running the LHV of the fuel can be determined using the measured temperatures and air mass flow rates of the engine. Thus, the specifically measured LHV values may not necessarily be needed once the engine has been started. By using the engine air mass flow, the air inlet and outlet temperatures, and the fuel mass flow rate, the instantaneous LVH values in the fuel can be determined. The plurality of instantaneous calculated LVH values are then held in a temporary memory register, and used by the control system to allow smooth control of the engine fuel flow for both steady state running and transient running. The value of LHV may be calculated once the engine is running every second or two, or alternately more or less often. Regardless, the general approach of establishing the LHV of the fuel before start is nonetheless desirable such that correct start-up flow rates can be set without fear of overheating during start or failing to start properly. The LHV values can then be periodically determined, from the engine parameters as described. The LHV before start is determined by establishing the mixture and then using a table or calculation method to determine LHV and then set the start flow based on this. Alternately, however, the LHV may be sensed directly, and the start fuel flow can then be set based on a direct knowledge of the LHV. 
         [0020]    Regardless of how the LHV is determined, the controller  26  accesses the fuel schedules  30 , as shown at  62 , based on the sensor data, and retrieves corresponding fuel schedule data  64  related to start-up. The controller  26  then uses the LHV or the equivalent LHV and fuel proportions as described above, to adapt the fuel schedule data to the actual fuel used in the engine in order to determine a desired fuel flow and ignition settings. 
         [0021]    The BTU input requirement to the engine is always predetermined by engine designers. The fuel flow rate is measured and controlled, and given that the LHV specifications of aircraft fuel must by regulations be within a specific narrow range, the need for LHV measurement or determination by the engine system never existed in the past. 
         [0022]    The controller  26  then actuates the fuel metering device  24  through a signal  66  corresponding to the desired fuel flow, optionally receiving feedback  68  from the device  24 . The controller  26  also actuates the ignition system  34  through a signal  70  corresponding to the ignition settings, optionally receiving a feedback  72  from the ignition system  34 . 
         [0023]    Once the engine  10  is lit, the controller  26  still receives the sensor data  56  from the engine sensors  28 , which includes for example, fuel mass flow, air mass flow, temperature in the flow path at the end of the combustion section  16 , etc. The controller  26  sends this data to the energy value evaluator  38 , as shown at  58 , which determines the corresponding instantaneous LHV of the fuel mixture. The LHV of the fuel input can be calculated by knowing the air mass flow and the temperature rise of that air mass when combustion occurs. This then provides the total BTU/hour of energy released by the fuel, and thus dividing by the fuel flow rate (in pounds per hour, for example) will provide the energy in the fuel in BTU per pound. 
         [0024]    The energy value evaluator  38  sends the LHV  60  to the controller  26 , which again accesses the fuel schedules  30 , as shown at  62 , based on the sensor data, and retrieves corresponding fuel schedule data  64  related to lit operation. The controller  26  uses the LHV to adapt the fuel schedule data to the actual fuel used in the engine in order to determine a desired fuel flow. As during start-up, the controller  26  then actuates the fuel metering device  24  through the signal  66  corresponding to the desired fuel flow. 
         [0025]    The controller  26  thus regulates the operation of the fuel metering device  24  based on the LHV determined by the energy value evaluator  38 . As the energy value evaluator  38  constantly calculates the LHV during the operation of the engine  10 , the controller  26  reacts to variations in composition of the fuel and adjusts the fuel metering device  24  accordingly to optimize performance of the engine  10 . 
         [0026]    While the engine  10  is running, the controller  26  also accesses the reference fuel table  40 , as shown at  74 , and retrieves at least one reference characteristic  76  of the reference fuel corresponding to the LHV of the actual fuel used. For example, the controller  26  determines the corresponding mass flow of the reference fuel which would have been required to obtain the LHV obtained with the actual fuel and determined by the energy value evaluator  38 . Based on the history of the operation of the engine  10 , the controller  26  can also calculate an equivalent quantity of reference fuel burned. The controller  26  then sends normalized data  78 , i.e. the equivalent characteristics of the reference fuel corresponding to the actual LHV of the fuel used, to the display unit  32 , which displays it for the pilot. As such, the pilot can see the fuel data normalized to a reference fuel, i.e. as if the reference fuel was being used in the engine  10 . The display unit  32  also provides the pilot with an indication of the real fuel quantity being used versus how much real fuel quantity is present in the fuel tanks. Accordingly, the actual fuel mass flow is measured and used for any information provided to the pilot. For example the mass flow and quantity of reference fuel corresponding to the actual operation of the engine can be displayed. Using this normalized data, standard calculations for range of the aircraft can be done by the pilot regardless if he/she is aware of the real composition of the fuel being used in the engine  10 , thus eliminating errors due to confusion of fuel type used. This includes corrections of SG (specific gravity) which is also one of the parameters sensed, as the fuel level in the fuel tank may not be representative in terms of available range from one fuel to another, even if the BTU per Lb is known (i.e. the volume of fuel is what is provided by a level measurement device not the mass of fuel in the tank). 
         [0027]    As such, the fuel control system  22  provides for control of the fuel metering device  24  adapted to a fuel which is a mixture of two different fuels with variable proportions, as well as normalisation of the operation of the engine to a known, reference fuel to facilitate operation of the engine. As such, the pilot can use the normalized data displayed to perform standard calculations such as range calculation without the need to verify the exact composition of the fuel. 
         [0028]    Although in the preferred embodiment, the energy value used is the lower heating value or LHV, it is understood that other energy values can similarly be used. 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 department from the scope of the invention disclosed. For example, the fuel control system  22  can be used with other configurations of gas turbine engines and with other types of engine. The reference fuel used can be any appropriate type of fuel. Still other modifications which fall within the scope of the present invention 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.