Abstract:
An example fuel system includes a fuel sensor configured to sense at least one characteristic of a fuel provided to an engine. The fuel is selected from a plurality of different fuel types. The fuel system also includes a controller that is configured to meter the fuel in response to the at least one characteristic of the fuel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This disclosure claims priority to U.S. Provisional Application No. 61/323,022, which was filed on 12 Apr. 2010 and is incorporated herein by reference. 
    
    
     BACKGROUND 
     This disclosure relates to a gas turbine engine and, more particularly, to a flexible fuel system for a gas turbine engine. 
     Aerospace gas turbine engines must operate at high efficiencies to compete in today&#39;s environment. Also, high fuel prices and regulations drive engine makers and aircraft manufacturers to improve gas turbine engine efficiency and reduce fuel burn. 
     SUMMARY 
     An example fuel system includes a fuel sensor configured to sense at least one characteristic of a fuel provided to an engine. The fuel is selected from a plurality of different fuel types. The fuel system also includes a controller that is configured to meter the fuel in response to the at least one characteristic of the fuel. 
     An example gas turbine engine fuel system includes a conduit configured to deliver a flow of fuel from a fuel supply to a combustor of a gas turbine engine. The flow of fuel is selected from different fuel types. A fuel sensor is configured to determine the energy content of the fuel. A controller is configured to meter the flow of fuel in response to the energy content of the fuel. 
     An example fuel delivery method includes delivering a flow of fuel to an engine. The fuel is selected from different fuel types. The method senses an energy density of the fuel. The method adjusts the flow of fuel based on the energy density of the fuel. 
     The various features and advantages of the disclosed examples will become apparent to those skilled in the art from the detailed description. The figures that accompany the detailed description can be briefly described as follows. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic view of a flexible fuel system. 
         FIG. 2  is a schematic view of a FADEC interaction with an energy density compensator within a fuel flow path. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a flexible fuel system  20  is used within an aircraft, vehicle, or other system. The example flexible fuel system  20  includes a fuel tank  24 , a boost pump  26 , a fuel properties sensor  28 , a main fuel pump  30 , one or more heat exchangers  32   a - c , and a main fuel throttle valve (MFTV)  40 . The flexible fuel system  20  communicates a flow of fuel from the fuel tank  24  to a combustor  36  of a gas turbine engine  34  along a conduit or path  37 . A Full Authority Digital Engine Control (FADEC)  38 , or other control module, communicates with at least the fuel properties sensor  28  and the MFTV  40  to control the delivery rate of the different types of fuels along the path  37  to the combustor  36 . 
     The flexible fuel system  20  contains other systems and components, such as a gear pump  41 , hydraulics  42  and seals, that facilitate delivery of the fuel to the combustor  36 . 
     The flexible fuel system designed to accommodate different types of fuels that have differing viscous and lubricating properties. That is, in this example, the systems and components of the flexible fuel system  20  will accommodate, pump, and meter many different types of fuels. 
     In this example, the main fuel pump  30  is an electric fuel pump that allows for pumping and metering as a function of engine demand, as well as a function of fuel characteristics. The FADEC  38  controls the main fuel pump  30  based on the fuel characteristics. The main fuel pump  30  compensates for variable fuel properties and is resistant to changes in viscosity or lubricity. 
     A thermal management portion of the example flexible fuel system  20  includes the one or more heat exchangers  32   a - 32   c  that heat the fuel to the maximum temperature allowable without the formation of coke. Some other examples may also include a fuel stabilization unit that is used to inhibit the formation of coke. In this example, the heat exchangers  32   a - 32   c  allow for the introduction of heat into the fuel, based on the allowable temperature limit of the particular fuel. The delivery of heated fuel to the combustor  36  thereby extracts the most energy from the fuel, yet maintains the cleanest burn and lowest emissions. One or more of the heat exchangers  32   a - 32   c  may be a buffer air cooler. 
     The heat exchangers  32   a - 32   c  adjust the temperature of the fuel depending on the type of fuel that is being delivered. Adjusting the fuel temperature based on the fuel enables delivering the fuel at that type of fuel&#39;s optimum temperature so that the fuel burns with, for example, minimal release of CO2 or NOx emissions, optimum performance, or a desired combination thereof. The FADEC  38  controls the temperature adjustments made by the heat exchangers  32  based on characteristics of the fuel that is being delivered to the combustor  36 . 
     The flexible fuel system  20  senses characteristics of the fuel during operation of the engine  34 . The sensed characteristics help the FADEC  38  determine an appropriate adjustment to the temperature of the fuel. The example FADEC  38 , or another module, also meters the fuel delivered to the combustor  36  to accommodate different energy densities associated with each of the different types of fuel. 
     In this example, the fuel properties sensor  28  is operable to identify at least the energy content in the fuel being delivered to the combustor  36 . The fuel properties sensor  28  provides the FADEC  38  with sufficient information to meter the flow of the fuel to the combustor  36  by adjusting the position of the main fuel throttle MFTV  40  in response to the energy content sensed in the fuel. 
     In one specific example, the type of fuel being delivered to the combustor  36  is a blend of fuels that is not known when the aircraft is fueled. The FADEC  38  adjusts the MFTV  40  in real time in response to information about the blend of fuels provided by the sensor  28 . The particular blend may change over time, and the FADEC  38  responds to these changes, for example. The different fuel types may include an aviation fuel, a jet fuel, a bio-based fuel, or some blend of these. In general, reciprocating piston engines use aviation fuel, and turbine engines use jet fuel. One example aviation fuel is AVGAS100/130. Example jet fuels include J-4, Jet A, Jet-1, and Jet B. 
     The example sensor  28  is an energy density compensator that includes a fuel density meter. The fuel density meter measures fuel density properties to determine the energy characteristics of the type of fuel being communicated to the combustor  36 . The energy characteristics are communicated to the FADEC  38  in real time. 
     The example sensor  28  generally determines the energy characteristics by using some combination of the coriolis vibration effect, fuel density measurement using electrical capacitance, carbon dioxide detection and speed of sound sensing, optical BTU measurement, or other technologies. The FADEC  38  readily adjusts fuel metering in response to the measurements to accommodate changes to the type of fuel in real time. 
     It should be understood that various other operations may also be provided by the sensor  28 . Other examples of sensing technologies that may be scaled and ruggedized for aerospace applications include chromatography, acoustic resonance, calorimetry, and catalytic reaction monitoring. 
     Density measurements may also use temperature compensation to correct for volume-based calculations. Alternatively, the sensor  28  or other elements of the flexible fuel system  20  may incorporate a mass-flow meter for use in conjunction with real-time estimates of energy content of the fuel on a mass basis as calculated by the FADEC  38 . 
     Also, the sensor  28  can be located elsewhere within the flexible fuel system  20  and can be combined with other elements. For example, the sensor  28  can be located in a common housing or integrated with the boost pump  26 , the main fuel pump  30 , or the MFTV  40 . 
     The example sensor  28  includes power and data interfaces with the FADEC  38 . On command, the fuel properties sensor  28  measures fuel properties and communicates the data back to the FADEC  38 . The FADEC  38  compares the fuel energy density data against baseline fuel properties (e.g., JP-8). The difference is calculated and passed to the MFTV  40  as a command to open or close as required to maintain baseline engine performance regardless of the fuel characteristics. 
     As can be appreciated, periodic monitoring of the fuel density ensures that performance is maintained even as fuel characteristics change over time due to mixing in the tank or refueling in flight or on the ground. 
     Referring to  FIG. 2  with continuing reference to  FIG. 1 , an example fuel delivery method  100  includes a step  104  of commanding a flow of fuel using the FADEC  38 . The MFTV  40  then moves to a first position that provides flow at a step  108 . At a step  112 , the sensor  28  measures the energy content within the flow of fuel. The sensor provides the energy content to the FADEC  38  at a step  116 . In response, the FADEC  38 , at a step  120 , determines an appropriate offset of the MFTV  40  to compensate for the energy content. The FADEC  38  commands the MFTV  40  to offset at a step  124 . The MFTV  40  moves in response to the command at a step  128 . 
     Although particular step sequences are shown, described, and claimed, it should be understood that steps may be performed in any order, separated or combined. 
     Further, as understood by those having skill in the art, and the benefit of this disclosure, the functions described in the method  100  may be enacted in either dedicated hardware circuitry or programmed software routines capable of execution in a microprocessor-based electronics control embodiment. For example, the FADEC  38  may be a portion of a flight control computer, a portion of a central vehicle control, an interactive vehicle dynamics module, a stand-alone line replaceable unit or other such control module. 
     Features of the disclosed examples include a flexible fuel system that is configured to deliver the different types of fuels to a combustor while providing precise fuel metering and thermal management of the engine and of components within the engine. The flexible fuel system provides safe and reliable engine control for different types of fuels, such as standard jet fuels, alternative fuels, and blends of fuels. 
     The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. Thus, the scope of legal protection given to this disclosure can only be determined by studying the following claims.