Abstract:
An aircraft, controller, and method for simultaneously using a liquid fuel and a gaseous fuel. The use of natural gas and other similar fuels in gas turbines engines can enable an aircraft to operate less expensively. However, aircraft often use liquid fuels en route to the gas turbine engine burners for secondary purposes, such as oil cooling and hydraulic pressure. The aircraft, controllers, and methods described herein feed a minimal quantity of liquid fuel to an engine to satisfy the secondary purposes while simultaneously feeding a quantity of gaseous fuel to the engine to satisfy a thrust command for the engine.

Description:
BACKGROUND 
     Field of the Invention 
       [0001]    The present invention relates to providing fuel to a gas turbine engine, and more specifically, to providing two fuels to a gas turbine engine simultaneously. 
       SUMMARY 
       [0002]    According to various embodiments, an aircraft can include a first fuel storage container that stores a liquid fuel and a second fuel storage container that stores a gaseous fuel. The aircraft can include at least one gas turbine engine that has a fuel nozzles configured to inject the liquid fuel and the gaseous fuel into a burner of the gas turbine engine. The aircraft can include a controller configured to simultaneously meter flow of both the liquid fuel and the gaseous fuel to the fuel nozzles from the first fuel storage container and the second fuel storage container. The controller can be configured to meter the flow of the liquid fuel to satisfy an operating limit of the gas turbine engine. 
         [0003]    According to various embodiments, a fuel flow controller can include a first input configured to receive a throttle command, a second input configured to receive a dual fuel indication, and a third input configured to receive an engine sensor signal from an engine. The controller can also include a first output configured to output a liquid fuel flow signal to a liquid fuel metering valve and a second output configured to output a gaseous fuel flow signal to a gaseous fuel metering valve. The controller can also include computer logic configured to achieve a concurrent gaseous fuel flow and liquid fuel flow to the engine by outputting a liquid fuel flow signal based on at least a received throttle command and a received engine sensor signal and by outputting a gaseous fuel flow signal based on at least a received dual fuel indication and received throttle command. 
         [0004]    According to various embodiments, a method for operating a gas turbine engine using a liquid fuel and a gaseous fuel simultaneously can include measuring an engine parameter that depends on a fuel flow of the liquid fuel. The method can also include receiving an engine thrust request. The method can also include metering fuel flow of the liquid fuel to satisfy a predetermined constraint on the measured engine parameter and simultaneously metering fuel flow of the gaseous fuel to satisfy the received engine thrust request. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0005]      FIG. 1  is a depiction of an airplane with two fuel tanks, a first fuel tank for a first fuel and a second fuel tank for a second fuel; 
           [0006]      FIG. 2A  is a side view of a dual-fuel fuel nozzle for use in a gas turbine engine; 
           [0007]      FIG. 2B  is a cross-sectional view of the dual-fuel fuel nozzle of  FIG. 2A ; 
           [0008]      FIG. 3A  is a block diagram of a dual fuel system for use in an aircraft 
           [0009]      FIG. 3B  is a block diagram of ancillary systems in communication with the liquid fuel source; 
           [0010]      FIG. 3C  is a block diagram of an ancillary system in communication with the gaseous fuel source that includes a vaporizing heat exchanger; 
           [0011]      FIG. 4  is a graphical representation of a flight profile; and 
           [0012]      FIG. 5  is a method for operating a gas turbine engine using two fuels simultaneously. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    An aircraft typically operates on a single fuel source. For example, an aircraft powered by gas turbine engines may use jet A fuel, JP-8 fuel, diesel, or gasoline as fuel. In addition to using such fuels for combustion in the engine, the fuels may be used for ancillary purposes such as oil cooling or providing hydraulic pressure in the engine. For example, the fuel may pass through an oil cooler and/or through a hydraulic system before going to fuel nozzles in the gas turbine engine. Less expensive fuels (e.g., cryogenic fuels such as liquefied natural gas and liquid methane or non-cryogenic fuels such as natural gas, methane, butane, propane, or heptanes) may provide better operating costs, but may not be compatible with the ancillary systems that also use fuel. For example, such fuels may not be able to provide oil cooling or hydraulic pressure. 
         [0014]    Referring to  FIG. 1 , an embodiment of an aircraft  100  described herein can include a first fuel storage container  104  (e.g., a fuel tank) that stores a liquid fuel, such as jet A, JP-8, diesel, or gasoline. As used herein, the term “liquid fuel” refers to a fuel that is injected into a burner of a gas turbine engine in a liquid state. The liquid fuel can be distributed amongst several fuel storage containers  104  distributed about the aircraft (e.g., in the wings and fuselage). The aircraft  100  also includes a second fuel storage container  106  that stores a gaseous fuel (e.g., as a cryogenic fuel such as liquefied natural gas, liquid methane, and liquid hydrogen or as a non-cryogenic fuel such as compressed natural gas, compressed methane, butane, propane, or heptanes). As used herein, the term “gaseous fuel” refers to a fuel that is injected into a burner of a gas turbine engine in a gaseous state. Such gaseous fuel may be stored in the second fuel storage container  106  in a gaseous state or in a liquid state, for example. The gaseous fuel can be distributed amongst several fuel storage containers  106  distributed about the aircraft. The aircraft can include one or more gas turbine engines. As described in greater detail below, embodiments of gas turbine engines  102  described herein for the aircraft can simultaneously operate on liquid fuel received from the first fuel storage container  104  and gaseous fuel received from the second fuel storage container  106 . The engines gas turbine can consume sufficient quantities of the liquid fuel to satisfy the ancillary systems, such as oil cooling and/or engine hydraulic pressure while simultaneously consuming sufficient quantities of the gaseous fuel to satisfy the thrust command from the aircraft  100 . Put differently, the aircraft  100  could be commanding a certain level of engine thrust, and the combined liquid fuel and gaseous fuel injected into the burner of the gas turbine engine  102  provides the commanded level of thrust. 
         [0015]    In various embodiments, the second fuel storage container  106  and other systems, described below, for supplying a gaseous fuel to the gas turbine engines  102  can be retrofit to an existing aircraft. For example, the second fuel storage container  106  may be arranged in a cargo area of the aircraft  100 . For example, tanker aircraft used by the military (e.g., the KC-10 or KC-135) may include an unutilized cargo area or other interior space in which the second fuel storage container  106  may be arranged. As another example, the second fuel storage container  106  may be arranged as a conformal tank on an exterior surface of the aircraft  100 . The second fuel storage container  106  may require insulation and/or pressurization capability to store the gaseous fuel. 
         [0016]    Referring now to  FIGS. 2A and 2B , a dual-fuel fuel nozzle  200  can include a body  202  with a liquid fuel passage  204  that terminates in a liquid fuel orifice  206 . The body  202  can also include a gaseous fuel passage  208  that leads to the manifold  210 . The manifold  210  terminates in a plurality of gaseous fuel orifices  212 . The gaseous fuel orifices  212  can surround the liquid fuel orifice  206 .  FIG. 2B  illustrates four gaseous fuel orifices  212  arranged around the liquid fuel orifice  206 . In various embodiments, different numbers of gaseous fuel orifices  212  can be arranged relative to different numbers of liquid fuel orifices  206 . The dual-fuel fuel nozzle  200  can also include a mixing body  214  that includes an inlet  216  and an outlet  218 . Compressed air from a compressor of the gas turbine engine can enter the mixing body  214  through the inlet  216  to mix with the liquid fuel from the liquid fuel orifice  206  and with the gaseous fuel from the gaseous fuel orifices  212 . The mixture of air and fuel can exit the mixing body  214  through the outlet  218  to be burned in the burner of the gas turbine engine. 
         [0017]    In various embodiments, a gas turbine engine could include a plurality of single-fuel fuel nozzles, wherein some of the single-fuel fuel nozzles output liquid fuel into the burner of the gas turbine engine and the remainder of the single-fuel fuel nozzles output gaseous fuel into the burner of the gas turbine engine. 
         [0018]      FIGS. 3A ,  3 B, and  3 C illustrate a block diagram of a system  300  for simultaneously providing two fuels to a gas turbine engine. The system  300  can include a liquid fuel source  302  and a gaseous fuel source  304 . The liquid fuel from the liquid fuel source  302  can pass through a shutoff valve  303 , a pump  305  to increase pressure, and a liquid fuel metering valve  314  in or in line with conduits to reach the engine fuel nozzles  318  of a gas turbine engine. The liquid fuel can also pass through one or more ancillary systems  330  (see  FIG. 3B ) for the aircraft or for the engine before reaching the engine fuel nozzles  318 . For example, the liquid fuel may pass to or through a fuel/oil cooler  315   a  that cools oil for the gas turbine engine. As another example, the pressurized liquid fuel may pass to hydraulic actuators  315   b  for the gas turbine engine. The liquid fuel metering valve  314  can vary between a fully-closed position and a fully-open position (and partially-open positions therebetween). The liquid fuel metering valve  314  can be operated to meter or set the flow rate of liquid fuel that reaches the engine fuel nozzles  318  and the ancillary systems. The liquid fuel metering valve  314  can be connected to a liquid fuel metering valve controller  316  that controls the position of the liquid fuel metering valve  314 . The conduits can be any type of tubing, pipes, or the like for transporting the liquid fuel. The conduits are indicated in  FIGS. 3A and 3B  by the arrows connecting the liquid fuel source  302 , the engine fuel nozzles  318 , and the various components therebetween. 
         [0019]    In various embodiments, the gaseous fuel from the gaseous fuel source  304  can pass through a shut off valve  306  and a gaseous fuel metering valve  310  in or in line with conduits to reach the engine fuel nozzles  318 . In instances in which the gaseous fuel is a cryogenic fuel, additional systems may be used to convert the fuel from its cryogenic state to a gaseous state. For example, the gaseous fuel source  304  may comprise a cryogenic storage tank. The gaseous fuel can be pumped from the cryogenic storage tank by a cryopump  307  to increase the pressure of the gaseous fuel to a pressure above the pressure in the burner of the gas turbine engine. The gaseous fuel can then pass through ancillary systems  332  (see  FIG. 3C ), such as a vaporizing heat exchanger  311 , which converts the gaseous fuel from a cryogenic state to a gaseous state. The shut off valve  306  can be fully opened or fully closed to enable or prevent, respectively, flow of the gaseous fuel from the gaseous fuel source  304  to the engine fuel nozzles  318 . The shut off valve  306  can be controlled by a gaseous fuel shut off valve controller  308 , which can be in communication with a mode switch  324 . The mode switch  324  can have a first state and the second state. When the mode switch  324  is in the first state, the gas turbine engine runs solely on the liquid fuel from the liquid fuel source  302 . When the mode switch  324  is in the second state, the gas turbine engine runs on the liquid fuel from the liquid fuel source  302  and the gaseous fuel from the gaseous fuel source  304  simultaneously. In various embodiments, the mode switch  324  can be a pilot-selectable switch arranged in the cockpit of an aircraft. In various embodiments, the mode switch  324  can be computer-controlled such that the position of the mode switch  324  is automatically selected based on the various flight criteria, discussed in greater detail below. The mode switch  324  could output or trigger a dual fuel indication signal when the mode switch  324  is in the second state. The conduits can be any type of tubing, pipes, or the like for transporting the liquid fuel. The conduits are indicated in  FIGS. 3A and 3C  by the arrows connecting the gaseous fuel source  304 , the engine fuel nozzles  318 , and the various components therebetween. 
         [0020]    After passing through the shut off valve  306 , the gaseous fuel can pass through a gaseous fuel metering valve  310 , which can vary between a fully-closed position and a fully-open position (and partially-open positions therebetween). The gaseous fuel metering valve  310  can thereby meter or set the gaseous fuel flow rate to the engine fuel nozzles  318 . The gaseous fuel metering valve  310  can be connected to a gaseous fuel metering valve controller  312  that controls the position of the gaseous fuel metering valve  310 . In various embodiments, the system  300  may omit the shut off valve  306  and the gaseous fuel shut off valve controller  308 , and the gaseous fuel metering valve  310  can serve to cut off the gaseous fuel supply in its fully-closed position. 
         [0021]    The liquid fuel metering valve controller  316  and the gaseous fuel metering valve controller  312  can be connected to each other and to various sensors and systems onboard the aircraft to provide adequate liquid fuel flow to support ancillary systems and to provide a total fuel flow that results in the amount of thrust commanded by the aircraft. For example, the liquid fuel metering valve controller  316  can be connected to one or more sensors that indicate conditions of the ancillary systems that use the liquid fuel. For example, as described above, the liquid fuel may pass through an oil/fuel intercooler to cool engine oil. The liquid fuel metering valve controller  316  can be in communication with an engine oil temperature sensor  326 , and the liquid fuel metering valve controller  316  can incrementally open or close the liquid fuel metering valve  314  (e.g., by sending a liquid fuel flow signal) to maintain the engine oil temperature (detected by the engine oil temperature sensor  326 ) below a predetermined limit value (e.g., a maximum oil temperature). As another example, the liquid fuel may provide hydraulic pressure for hydraulically actuated systems on the engine (e.g., actuation of variable stator vanes). The liquid fuel metering valve controller  316  can be in communication with an engine hydraulic pressure sensor  328 , and the liquid fuel metering valve controller  316  can incrementally open or close the liquid fuel metering valve  314  to maintain the engine hydraulic pressure (detected by the engine hydraulic pressure sensor  328 ) above a predetermined limit value (e.g., a minimum engine hydraulic pressure). 
         [0022]    The liquid fuel metering valve controller  316  and the gaseous fuel metering valve controller  312  can both be connected to a throttle command input  322  (i.e., throttle control) that can output a throttle command signal. The throttle command input  322  can be a throttle command unit, such as a throttle quadrant on the flight deck, and/or a flight management computer (FMC) in the cockpit of the aircraft. The throttle command signal can indicate a thrust request. As described above, in various embodiments, the system  300  can meter the flow of the liquid fuel and the gaseous fuel so that a minimal amount of liquid fuel is used (e.g., to satisfy various ancillary systems) and gaseous fuel is added to the liquid fuel to satisfy the thrust command of the aircraft. The liquid fuel metering valve controller  316  and the gaseous fuel metering valve controller  312  can also be in communication with a gas turbine speed sensor  320 . Engine power output (i.e., thrust) is a function of engine speed. Consequently, receiving an engine speed indication from the gas turbine speed sensor  320  can provide feedback to the liquid fuel metering valve controller  316  and the gaseous fuel metering valve controller  312  for control of gaseous fuel metering valve  310  and liquid fuel metering valve  314 , respectively, based on the throttle command input  322 . 
         [0023]    In various embodiments, the throttle unit and/or FMC can communicate with a full authority digital engine control (FADEC) that schedules fuel flow to the gas turbine engine to meet a thrust command and that operates the engine within limits. The FADEC can use one or more computer processors to compute a fuel flow (i.e., a rate of fuel flow). In such embodiments, the throttle unit and/or FMC is typically in communication with the FADEC via a “fly by wire” system. In various embodiments, the throttle unit and/or FMC can be connected to a hydromechanical fuel control for the gas turbine engine via a mechanical linkage (e.g., a control cable). In such embodiments, the hydromechanical fuel control schedules fuel flow to the gas turbine engine based on the position of the mechanical linkage and various analog signals from the engine. For example, the hydromechanical fuel control may include one or more pressure inputs from various points in the engine, engine speed in revolutions per minute (RPM), or combinations thereof. The hydromechanical fuel control can include an analog computer that schedules fuel flow based on the position of the mechanical linkage and the various pressure inputs, RPM, or the like to provide commanded engine thrust while staying within the limits of the engine. For example, the pressure inputs may include a pressure at the inlet of the compressor for the gas turbine engine and a pressure at the outlet of the compressor. The RPM may be the speed of a fan rotor or the speed of a high-speed rotor in a turbofan engine. A commanded increase in fuel flow (e.g., by the pilots pushing throttle lovers forward) may be performed at a slower rate than commanded to avoid a compressor stall, for example. The FADEC and the hydromechanical fuel control can each control a metering valve that controls fuel flow of the liquid fuel to the engine fuel nozzles  318  of the engine. 
         [0024]    In various embodiments, the liquid fuel metering valve controller  316  can be installed downstream of the FADEC or hydromechanical fuel control so that the existing engine systems do not have to be changed. Put differently, the FADEC or hydromechanical fuel control can operate as if the liquid fuel is the sole fuel source for the engine. The liquid fuel metering valve controller  316  can then divert or block the liquid fuel that is not needed because the engine is also operating on gaseous fuel. For example, the liquid fuel metering valve  314  can divert a portion of the liquid fuel back to the liquid fuel source  302 . As another example, the liquid fuel metering valve  314  can “deadhead” the flow of the liquid fuel from the liquid fuel source  302  to the engine fuel nozzles  318 . Put differently, the liquid fuel metering valve  314  can block or restrict flow of the liquid fuel to the engine fuel nozzles  318 . 
         [0025]    In various instances in which the engine is controlled by a FADEC, the liquid fuel metering valve controller  316  can be incorporated into the FADEC. For example, in some instances, computer code could be added to the FADEC to operate the FADEC-controlled fuel valve in combination with the gaseous fuel metering valve controller  312 . In such instances, the liquid fuel metering valve controller  316  can be omitted since the FADEC performs the operations of the liquid fuel metering valve controller  316  and the FADEC-controlled fuel valve operates the liquid fuel metering valve  314 . Similarly, in some instances, computer code could be added to the FADEC to operate the gaseous fuel metering valve  310 . In such instances, the gaseous fuel metering valve controller  312  could be omitted since the FADEC performs the operations of the gaseous fuel metering valve controller  312 . 
         [0026]      FIG. 4  illustrates a chart  400  of an exemplary altitude profile for a flight of an aircraft, showing altitude over time for the flight. In this scenario, the aircraft takes off at time  402  and climbs to a first altitude at time  404 . The aircraft cruises at the first altitude from time  404  to time  406 . The aircraft then climbs again from time  406  to time  408  to a second altitude. The aircraft then cruises at a constant altitude from time  408  to time  410 . At times  410 , the aircraft begins to descend and includes two steps (between times  412  and  414  and times  416  and  418 ) during the descent before landing at time  420 . Various other altitude profiles are possible. For example, in some instances, air traffic control allows for a continuous climb to cruise altitude without any holds at intermediate altitudes. As another example, the cruise portion may include step climbs as the aircraft burns fuels, becomes lighter, and can fly higher. 
         [0027]    The sizing of the components of the gaseous fuel delivery system may affect when the engine may be fed the liquid fuel and the gaseous fuel simultaneously. For example, burner pressure in a gas turbine engine is highest at high power settings. For example, during the take off and climb periods between times  402  and  404  and times  406  and  408 , the burner pressure in a gas turbine engine of the aircraft could be high. To overcome such a large burner pressure, the various fuel lines, pumps, and valve that deliver the gaseous fuel from the gaseous fuel source (e.g. gaseous fuel source  304 ) to the engine fuel nozzles (e.g., engine fuel nozzles  318 ) would have to be larger and (likely) heavier than if the maximum pressure required was lower. The components of the gaseous fuel delivery system could be smaller if the gaseous fuel system is sized to only operate at lower burner pressures, such as a cruise power setting for the gas turbine engine. Further, for a dual fuel burner to maintain an acceptable combustion pattern factor (i.e., turbine inlet temperature profile during maximum thrust operation), limiting maximum thrust for only liquid fuel operation may allow for easier modification of the burner design (to add the gaseous injectors). Alternatively, a more highly-modified or new engine could use a cryogenic gas as a heat sink for engine internal cooling to allow a high power output design. For the embodiment described above in which the maximum gaseous fuel pressure is limited, the aircraft flying the profile shown in  FIG. 4  may only operate in dual fuel mode from times  404  to  406 , times  408  to  410 , times  412  to  414 , and times  416  to  418  when the aircraft is cruising at level flight. In various embodiments, the engines may be able to operate on liquid fuel and gaseous fuel simultaneously during descent periods. However, descents are typically performed at an idle thrust or minimal thrust setting. At such low thrust settings, the amount of liquid fuel required to support ancillary systems, such as oil cooling and hydraulic pressure, may also be sufficient to satisfy the thrust command from the engine such that gaseous fuel would not be introduced. 
         [0028]      FIG. 5  is a flow chart of a process  500  that a dual fuel system, such as the system  300  shown in  FIG. 3 , may perform to implement dual fuel operation in a gas turbine engine. In block  502  of the process  500 , the gas turbine engine starts operation using liquid fuel only. As discussed above, in various instances, a gaseous fuel system may require a vaporizer and other equipment to convert a cryogenically stored fuel into a gaseous form. Such a vaporizer may require engine heat to operate. In such instances, the engine may need to start using only liquid fuel and run for a period of time using only liquid fuel before gaseous fuel can be introduced. Additionally, in various instances, gaseous fuel may not have satisfactory throttle response qualities. Put differently, use of gaseous fuel when the throttle command is being changed (e.g., when the pilots command take off power) may result in abrupt or unexpected thrust changes. In such instances, gaseous fuel may only be used during relatively steady-state operations, such as cruise flight. In block  504 , the system determines whether dual fuel use has been commanded. If dual fuel use has not been commanded, then the process  500  can return to block  502 . If dual fuel use has been commanded in block  504 , then the process  500  can move to block  506 , in which the engine operates using liquid fuel and gaseous fuel. At block  506 , the process  500  can simultaneously (or in sequential order) monitor different parameters to ensure safe and proper operation of the engine. The process  500  can continually check to ensure that dual fuel usage has been commanded (as indicated by the arrow referring back to block  504 ). Also, the process  500  can monitor engine performance criteria and/or ancillary systems criteria to determine proper fuel flow rates of liquid fuel and gaseous fuel. The flowchart illustrated in  FIG. 5  shows several exemplary monitoring processes. For example, in block  508 , the process  500  can determine whether the engine oil temperature is exceeding a limit. As discussed above with reference to  FIG. 3 , the liquid fuel metering valve controller  316  can monitor the engine oil temperature from an engine oil temperature sensor  326 . In the event the engine oil temperature is exceeding the limit, then in block  510 , the liquid fuel flow rate can be increased (e.g., by the liquid fuel metering valve controller). If the engine oil temperature is not exceeding a limit in block  508 , then the process  500  can return to block  506 . Also, if the engine oil temperature is below the limit, then the liquid fuel flow rate can be decreased. 
         [0029]    As another example, in block  512 , the process  500  can determine whether the engine speed is below a minimum engine speed. A gas turbine engine must maintain a minimum speed to sustain combustion, assure adequate rapid power up capability, and/or provide sufficient secondary power output (e.g., to an electrical generator, to a pneumatic bleed, and a power takeoff shaft). If the engine speed drops below the minimum speed, then, in block  514 , the process  500  can check whether the gaseous fuel flow rate is at a maximum fuel flow rate. If the gaseous fuel flow rate is not at a maximum rate, then, in block  518 , the process can increase the gaseous fuel flow rate to increase the engine speed. For example, the gaseous fuel metering valve controller  312  referenced in  FIG. 3  can open the gaseous fuel metering valve  310  to increase the gaseous fuel flow rate. Returning to block  514 , if the gaseous fuel flow rate is at a maximum, then, in block  516 , the process  500  can increase the liquid fuel flow rate. For example, referencing  FIG. 3 , the liquid fuel metering valve controller  316  could open the liquid fuel metering valve  314  to increase the liquid fuel flow rate until the engine speed is at or above the minimum speed. 
         [0030]    As another example, in block  520 , the process  500  can determine whether the engine speed is different from a set point speed. As discussed above, the thrust provided by gas turbine engine is a function of engine speed. Generally, the faster the rotation speed of the engine, the more thrust it is providing. If the engine speed is different from a set point speed, then, in block  520 , the process  500  can adjust at least one of the liquid fuel flow and gaseous fuel flow to adjust the engine speed. For example, if the engine speed is below a set point speed, then the process  500  may increase the gaseous fuel flow to increase the engine speed. Similarly, if the engine speed is above a set point speed, then the process  500  may decrease the gaseous fuel flow to decrease the engine speed. 
         [0031]    As described above the various monitoring processes (e.g. processes in blocks  508 ,  512 ,  514 , and  520  can operate simultaneously. The various monitoring processes may also operate parametrically, meaning that there could be interrelationships between the different monitoring processes. For example, in block  510 , the process  500  may increase the liquid fuel flow rate to satisfy the engine oil temperature exceeding a limit. However, such an increase in liquid fuel flow rate may cause the engine speed to deviate from the set point speed (block  520 ). In such instances, the increase in liquid fuel flow in block  510  could require a commensurate decrease in gaseous fuel flow in block  522  to maintain the engine speed at the set point speed in block  522 . 
         [0032]    In the embodiments described above, aircraft gas turbine engines can operate on two fuels simultaneously. The two fuels can be used at any ratio that satisfies the requirements of ancillary systems and thrust requirements. For example, in various instances, the ratio of gaseous fuel to liquid fuel may be 90% to 80%. In various other instances, the ratio of gaseous fuel to liquid fuel may be 80% to 70%. In various other instances, the ratio of gaseous fuel to liquid fuel may be 70% to 60%. In various other instances, the ratio of gaseous fuel to liquid fuel may be 60% to 50%. In various other instances, the ratio of gaseous fuel to liquid fuel may be 50% to 40%. In various other instances, the ratio of gaseous fuel to liquid fuel may be 40% to 30%. In various other instances, the ratio of gaseous fuel to liquid fuel may be 30% to 20%. In various other instances, the ratio of gaseous fuel to liquid fuel may be 20% to 10%. In various other instances, the ratio of gaseous fuel to liquid fuel may be above 90%. In various other instances, the ratio of gaseous fuel to liquid fuel may be below 10%. 
         [0033]    The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein. 
         [0034]    In the following, reference is made to embodiments presented in this disclosure. However, the scope of the present disclosure is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice contemplated embodiments. Furthermore, although embodiments disclosed herein may achieve advantages over other possible solutions or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the scope of the present disclosure. Thus, the following aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to “the invention” shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s). 
         [0035]    Aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” 
         [0036]    The present invention may be a system, a method, and/or a computer program product. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention. 
         [0037]    The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire. 
         [0038]    Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device. 
         [0039]    Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++ or the like, and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention. 
         [0040]    Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions. 
         [0041]    These computer readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks. 
         [0042]    The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks. 
         [0043]    The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions. 
         [0044]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.