Patent Description:
Helicopters are often provided with at least two turboshaft engines. Both engines are connected to drive the main rotor via a common reduction gearbox. Each of the engines is sized to account for the worst-case scenario of the other engine failing at takeoff. Accordingly, the power of each engine is significantly greater than what is required for cruising.

In cruising conditions, operating a single engine at a relatively high regime instead of both at a lower regime can allow significantly better fuel efficiency. However, once a turboshaft engine is stopped, there is a significant delay in starting it back up again. This delay is associated with the required amount of time to get the engine running at a sufficient RPM (and draw in a sufficient amount of air) for engine operation to begin. For safety purposes, the typical approach is not to shut down the second engine completely, but to keep it idling, which limits the gain in fuel efficiency.

A prior art method of operating a gas turbine engine having the features of the preamble to claim <NUM> is disclosed in <CIT>. Other prior art methods of purging fuel from a manifold in a gas turbine engine are disclosed in <CIT>, <CIT>, and <CIT>.

In one aspect, the disclosure describes a method of operating a gas turbine engine as disclosed in claim <NUM>.

Further details of the subject matter of this application will be apparent from the detailed description included below and the drawings.

The following disclosure relates to multi-engine power plants for rotary-wing aircraft (e.g., helicopters) applications and associated methods of operation. In some embodiments, the disclosed multi-engine power plants may allow one engine of the power plant to operate in a low-power mode of operation while another engine is operated at a high-power mode of operation in some situations.

Turning now to <FIG>, illustrated is an exemplary multi-engine system <NUM> (e.g., twin-pack), showing axial cross-section views of two exemplary gas turbine engines 12A and 12B, that may be used as a power plant for an aircraft, including but not limited to a rotorcraft such as a helicopter. The multi-engine system <NUM> may include two or more gas turbine engines 12A, 12B. In the case of a helicopter application, these gas turbine engines 12A, 12B will be turboshaft engines. Control of the multi-engine system <NUM> is effected by one or more controller(s) <NUM>, which may be Full Authority Digital Engine Control(s) ("FADEC(s)"), electronic engine controller(s) (EEC(s)), or the like, that are programmed to manage, as described herein below, the operation of the engines 12A, 12B to reduce an overall fuel burn, particularly during sustained cruise operating regimes, wherein the aircraft is operated at a sustained (steady-state) cruising speed and altitude. The cruise operating regime is typically associated with the operation of prior art engines at equivalent part-power, such that each engine contributes approximately equally to the output power of the system <NUM>. Other phases of a typical helicopter mission would include transient phases like take-off, climb, stationary flight (hovering), approach and landing. Cruise may occur at higher altitudes and higher speeds, or at lower altitudes and speeds, such as during a search phase of a search-and-rescue mission.

In the present description, while the aircraft conditions (cruise speed and altitude) are substantially stable, the engines 12A, 12B of the system <NUM> may be operated asymmetrically, with one engine operated in a high-power "active" mode and the other engine operated in a lower-power (which could be no power, in some cases) "standby" mode. Doing so may provide fuel saving opportunities to the aircraft, however there may be other suitable reasons why the engines are desired to be operated asymmetrically. This operation management may therefore be referred to as an "asymmetric mode" or an "asymmetric operating regime", wherein one of the two engines is operated in a lower power (which could be no power, in some cases) "standby mode" while the other engine is operated in a high-power "active" mode. In such an asymmetric operation, which is engaged for a cruise phase of flight (continuous, steady-state flight which is typically at a given commanded constant aircraft cruising speed and altitude). The multi-engine system <NUM> may be used in an aircraft, such as a helicopter, but also has applications in suitable marine and/or industrial applications or other ground operations.

Referring still to <FIG>, according to the present description the multi-engine system <NUM> driving a helicopter may be operated in this asymmetric manner, in which a first of the turboshaft engines (say, 12A) may be operated at high power in an active mode and the second of the turboshaft engines (12B in this example) may be operated in a lower power (which could be no power in some cases) standby mode. In one example, the first turboshaft engine 12A may be controlled by the controller(s) <NUM> to run at full (or near-full) power conditions in the active mode, to supply substantially all or all of a required power and/or speed demand of the common load <NUM>. The second turboshaft engine 12B may be controlled by the controller(s) <NUM> to operate at lower power or no-output-power conditions to supply substantially none or none of a required power and/or speed demand of the common load <NUM>. Optionally, a clutch may be provided to declutch the low-power engine. Controller(s) <NUM> may control the engine's governing on power according to an appropriate schedule or control regime. The controller(s) <NUM> may comprise a first controller for controlling the first engine 12A and a second controller for controlling the second engine 12B. The first controller and the second controller may be in communication with each other in order to implement the operations described herein. In some embodiments, a single controller <NUM> may be used for controlling the first engine 12A and the second engine 12B.

In another example, an asymmetric operating regime of the engines may be achieved through the one or more controller's <NUM> differential control of fuel flow to the engines, as described in pending <CIT>. Low fuel flow may also include zero fuel flow in some examples.

Although various differential control between the engines of the engine system <NUM> are possible, in one particular embodiment the controller(s)<NUM> may correspondingly control fuel flow rate to each engine 12A, 12B accordingly. In the case of the standby engine, a fuel flow (and/or a fuel flow rate) provided to the standby engine may be controlled to be between <NUM>% and <NUM>% less than the fuel flow (and/or the fuel flow rate) provided to the active engine. In the asymmetric mode, the standby engine may be maintained between <NUM>% and <NUM>% less than the fuel flow to the active engine. In some embodiments of the method <NUM>, the fuel flow rate difference between the active and standby engines, e.g. via the first and second manifolds respectfully, may be controlled to be in a range of <NUM>% and <NUM>% of each other, with fuel flow to the standby engine being <NUM>% to <NUM>% less than the active engine. In some embodiments, the fuel flow rate difference may be controlled to be in a range of <NUM>% and <NUM>%, with fuel flow to the standby engine being <NUM>% to <NUM>% less than the active engine.

In another embodiment, the controller <NUM> may operate one engine (say 12B) of the multi-engine system <NUM> in a standby mode at a power substantially lower than a rated cruise power level of the engine, and in some embodiments at substantially zero output power and in other embodiments less than <NUM>% output power relative to a reference power (provided at a reference fuel flow). Alternately still, in some embodiments, the controller(s) <NUM> may control the standby engine to operate at a power in a range of <NUM>% to <NUM> % of a rated full-power of the standby engine (i.e. the power output of the second engine to the common gearbox remains between <NUM>% to <NUM>% of a rated full-power of the second engine when the second engine is operating in the standby mode).

In another example, the engine system <NUM> of <FIG> may be operated in an asymmetric operating regime by control of the relative speed of the engines using controller(s) <NUM>, that is, the standby engine is controlled to a target low speed and the active engine is controlled to a target high speed. Such a low speed operation of the standby engine may include, for example, a rotational speed that is less than a typical ground idle speed of the engine (i.e. a "sub-idle" engine speed). Still other control regimes may be available for operating the engines in the asymmetric operating regime, such as control based on a target pressure ratio, or other suitable control parameters.

Although the examples described herein illustrate two engines, asymmetric mode is applicable to more than two engines, whereby at least one of the multiple engines is operated in a low-power standby mode while the remaining engines are operated in the active mode to supply all or substantially all of a required power and/or speed demand of a common load.

In use, the first turboshaft engine (say 12A) may operate in the active mode while the other turboshaft engine (say 12B) may operate in the standby mode, as described above. During this asymmetric operation, if the helicopter needs a power increase (expected or otherwise), the second turboshaft engine 12B may be required to provide more power relative to the low power conditions of the standby mode, and possibly return immediately to a high- or full-power condition. This may occur, for example, in an emergency condition of the multi-engine system <NUM> powering the helicopter, wherein the "active" engine loses power the power recovery from the lower power to the high power may take some time. Even absent an emergency, it will be desirable to repower the standby engine to exit the asymmetric mode.

In the embodiments described herein gas turbine engines 12A, 12B may be referred to as turboshaft engines used in a power plant of a helicopter. However, it is understood that aspects of the present disclosure are not limited to engines of the turboshaft type and may be applicable to other types of gas turbine engines. Regarding to <FIG>, each of engines 12A, 12B may comprise, in serial flow communication, air intake 14A, 14B through which ambient air is received, multistage compressor 16A, 16B for pressurizing the air, combustion chamber 18A, 18B in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section for extracting energy from the combustion gases. Engines 12A, 12B may comprise respective exhaust ducts 19A, 19B via which the combustion gases exit engines 12A, 12B.

The turbine section may comprise one or more high-pressure turbines 20A, 20B and one or more low-pressure power turbines 22A, 22B. High-pressure turbine(s) 20A, 20B may be drivingly coupled to compressor 16A, 16B via high-pressure shaft 24A, 24B to form a high-pressure spool. Power turbine(s) 22A, 22B may be coupled to low-pressure power shaft 26A, 26B to form a low-pressure spool. Accordingly, each of engines 12A, 12B may have a dual-spool configuration.

In some embodiments, first engine 12A and second engine 12B may be of substantially identical construction and may have substantially identical power output ratings. Alternatively, in some embodiments, first engine 12A and second engine 12B may be of different constructions and may have different power output ratings.

First engine 12A and second engine 12B may be configured to drive common load <NUM>. In some embodiments, load <NUM> may include a rotary wing of a rotary-wing aircraft. For example, load <NUM> may be a main rotor of a helicopter. Engines 12A, 12B may be drivingly coupled to load <NUM> via gear box <NUM>, which may be of a speed-changing (e.g., reducing) type. For example, gear box <NUM> may have a plurality of inputs to receive mechanical energy from respective power shafts 26A, 26B of respective engines 12A, 12B. Gear box <NUM> may be configured to direct at least some of the combined mechanical energy from the plurality of engines 12A, 12B toward a common output shaft <NUM> for driving load <NUM> at a suitable operating (e.g., rotational) speed.

In some situations, it may be desirable (e.g., for improved fuel economy) to drive load <NUM> using mainly first engine 12A at a relatively high output power level, which may be a more fuel efficient operating regime while, second engine 12B is operated at a low power standby mode at which no useful power output to load <NUM> is produced but is enough to keep engine 12B running. In some embodiments, such standby mode may be below a "flight idle" mode of operation. Such situations may include a cruise phase of flight of the aircraft for example. Having second engine 12B operating in a low-power standby mode instead of being shut down may permit second engine 12B to remain ready to power-up in an emergency or other situation for example. Such other situations may include a climb or other manoeuver(s) performed by the aircraft where second engine 12B may be required to supplement first engine 12A.

<FIG> is a schematic representation illustrating an exemplary combustion chamber 18A or 18B of engines 12A or 12B in conjunction with two fuel manifolds with associated respective fuel nozzles. During operation of both engines 12A, 12B at a power level above the low-power condition, fuel may be provided to two or more manifolds supplying fuel to respective one or more fuel nozzles delivering fuel to the combustion chambers 18A, 18B of both engines 12A, 12B. However, when it is desirable to operate with one engine 12B in a low-power standby mode during flight while the other engine 12A drives load <NUM>, the fuel flow through at least some of the nozzles may be slow enough to present a risk of coking, which may be undesirable. Accordingly, it may be desirable to supply a higher fuel flow to fewer manifolds in order to maintain the low-power standby mode of operation of engine 12B while fuel flow to other manifold(s) is/are interrupted in order to reduce the potential for coking. In addition, it may also be desirable to purge the inactive fuel manifold(s) and associated nozzles of fuel to also reduce the risk of coking.

<FIG> are schematic representations of an exemplary fuel system <NUM> of a gas turbine engine 12A or 12B. Fuel system <NUM> may include combustion chamber 18A or 18B, and a flow divider assembly (FDA) <NUM>. Flow divider assembly (FDA) <NUM> may include one or more valves, e.g. spool-type flow divider valves, that may be suitable for controlling fuel flow to one or more fuel manifolds of one engine. FDA <NUM> may also include a fuel reservoir tank to receive fuel from a fuel manifold when the manifold is purged. FDA <NUM> may also direct fuel flow to one or more fuel manifolds. In the illustrated examples of <FIG>, fuel system <NUM> has two manifolds, i.e. first manifold <NUM> and second manifold <NUM>. The one or more valves of FDA <NUM> may be configured to direct the fuel pressurized by the fuel pump to the first manifold while stopping the flow of the fuel pressurized by a fuel pump (not shown) to other fuel manifolds, such as second manifold <NUM> in the example of <FIG>. The one or more valves of FDA <NUM> may be configured to receive compressed gas from the combustor 18A, 18B to purge fuel in the manifold(s) <NUM> or <NUM>, e.g. second manifold <NUM> in which fuel flow has stopped.

Fuel system <NUM>, including first manifold <NUM> and second manifold <NUM>, may operate in a flight idle condition illustrated in <FIG>. Fuel may be supplied to Flow Divider Assembly (FDA) <NUM> by a fuel pump via fuel supply line <NUM> at a flow rate Wftot which is equal to Wfpri + Wfsec. Fuel supply line <NUM> may receive fuel from a fuel pump (not shown) which pressurizes fuel to a desired pressure. FDA <NUM> may distribute flow to first manifold <NUM> and second manifold <NUM>, via first manifold supply line <NUM> and second manifold supply line <NUM> respectively. Fuel to the first manifold <NUM> and second manifold <NUM> is illustrated as flow rates Wfpri and Wfsec respectively.

In flight idle mode, shown in <FIG>, the engine may not be operating efficiently with respect to fuel consumption due to flow rates to each fuel manifold being below an optimal condition. Accordingly, fuel flow Wfpri may be increased so that the engine fed by first manifold <NUM> may operate more efficiently. Fuel system <NUM> may also be transitioned into a standby mode which is illustrated in <FIG>. As shown in <FIG>, fuel flow in fuel supply line <NUM> is sent to first manifold <NUM> while fuel flow to second manifold <NUM> is interrupted and stopped. When fuel flow to second manifold <NUM> is interrupted, Wfpri may equal Wftot. Accordingly, in the example shown in <FIG>, fuel entering fuel system <NUM> is equal to fuel flow to first manifold <NUM>. Diverting fuel from second manifold <NUM> to first manifold <NUM> may increase fuel flow in the first manifold <NUM> and optimize performance and fuel consumption in the gas turbine engine.

In reference to the example shown in <FIG>, residual fuel in second manifold <NUM> may be purged by introducing compressed gas, e.g. compressed gas such as pressurized air from a combustor 18A or 18B of the gas turbine engine 12A or 12B, into the second manifold <NUM> and associated fuel nozzles, which may pressure the residual fuel into a reservoir which may be separate and distinct from the fuel tank of the aircraft. FDA <NUM> may be configured to permit flow to first manifold <NUM> while receiving residual fuel purged from second manifold <NUM>. Because residual fuel is purged from the second manifold <NUM> and associated fuel nozzle, the risk of coking at the fuel nozzles may be reduced or substantially eliminated.

During flight idle, or active operation, the one or more valves of FDA <NUM> may be configured to allow fuel to selectively flow through the fuel manifolds <NUM>, <NUM>, e.g. a first manifold and a second manifold, under a pressure produced by the fuel pump in a first mode of operation. The pressure may be the pump head pressure required to pump fuel through the fuel manifold and associated nozzles to the combustor. In a standby mode, i.e. a second mode of operation, the one or more valves of FDA <NUM> may be configured to allow fuel to flow back from the shut down manifold, e.g. second manifold <NUM> in <FIG>, through the one or more valves to the reservoir due to a pressure in the combustor, while continuing to flow fuel to the first manifold. The pressure from the combustor 18A or 18B may be created by compressed air from the compressor stage 16A or 16B to overcome residual pressure of the fuel in the deactivated manifold, e.g. second manifold <NUM> in the example illustrated in <FIG>, causing fuel to reverse flow from the nozzles back through the fuel manifolds and into a reservoir.

Transition to a standby mode may be explained with reference to the method <NUM> illustrated in the flow chart of <FIG>, some embodiments may provide for a method of operating a gas turbine engine. Method <NUM> or part(s) thereof may be performed using FDA <NUM> described herein or using other system(s).

At <NUM>, fuel is supplied to a combustor 18A or 18B of the gas turbine engine 12A or 12B, via a first manifold <NUM> and a second manifold <NUM> during a first mode of operation.

At <NUM>, fuel supply to the combustor 18A or 18B via the second manifold <NUM> is stopped, while continuing fuel supply to the combustor 18A or 18B via the first manifold <NUM> during a second mode of operation.

At <NUM>, after stopping fuel supply to the combustor 18A or 18B via the second manifold <NUM>, fuel from the second manifold <NUM> is purged using a pressure inside the combustor 18A or 18B to drive the fuel in the second manifold <NUM> in the upstream direction and away from the combustor 18A or 18B in order to empty the second manifold of fuel.

<FIG> is a schematic representation of flow divider valve assembly (FDA) <NUM> that may include one or more spool-type flow divider valves <NUM> that may be suitable for controlling fuel flow to one or more fuel manifolds <NUM>, <NUM> of one engine 12A or 12B, and/or purging fuel from one or more fuel manifolds <NUM>, <NUM> of that engine 12A or 12B when that engine 12A or 12B enters the low-power standby mode of operation during flight, as an ecology system for example. The fuel supplied to the combustion chamber 18A or 18B of the engine 12A or 12B may be supplied via two or more manifolds <NUM>, <NUM> by way of FDA <NUM>. Outlets <NUM> and <NUM> of the flow divider valve <NUM> illustrated in <FIG> may be connected to the first manifold <NUM> and second manifold <NUM> respectively. FDA <NUM> may include two or more separate (e.g., spool-type) flow divider valves <NUM> in a common housing (or in separate housings) that are positively isolated from each other when one or more of the manifolds <NUM>, <NUM> are shut off and purged empty from fuel by way of gas (e.g., pressurized air) during engine operation. As shown in <FIG>, the compressed gas that is introduced via the nozzles of the one or more fuel manifolds to be purged may cause the fuel in the manifold(s) <NUM>, <NUM> and associated nozzles to be (e.g., substantially completely) flushed out of the nozzles and associated manifold(s) <NUM>, <NUM> into a reservoir of the gas turbine engine. In an embodiment, flow divider valve <NUM> has one or more pressure-actuator valves, e.g. first valve <NUM> and second valve <NUM> shown in <FIG>, that may be configured to (<NUM>) prevent fuel <NUM> from flowing through the flow divider valve <NUM> to the second manifold <NUM> through outlet <NUM> when the fuel pressure created by the fuel pump varies to become lower than a first value (e.g., threshold); and (<NUM>) to allow fuel to flow only through the first section <NUM> of the flow divider valve to the first manifold <NUM>; and (<NUM>) to allow fuel <NUM> to flow (illustrated by the arrowed lines in <FIG>) through both the first section <NUM> and second section <NUM> of the flow divided valve <NUM> to the respective first and second manifolds <NUM>, <NUM>, and associated nozzles, when the fuel pressure varies to become higher than the first value or a second value (threshold) that is different from the first value.

The first value may be set to optimize the fuel efficiency of the turbine engine(s) 12A, 12B in a standby mode of operation by directing more fuel to an engine 12A or 12B allowing it to operate efficiency while shutting off or reducing flow to another engine 12A or 12B that is operating relatively inefficiently. The second value may be set based on a pressure or flow rates indicative of a flight idle or active mode of operation where the engine 12A or 12B may require more fuel to provide more power. In an embodiment, a reservoir valve <NUM> may restrict flow to reservoir outlet <NUM> to prevent fuel from filling the reservoir during the first mode of operation (e.g. flight idle or active operation).

<FIG> illustrates an embodiment of FDA <NUM> in transition to a stanbdy mode, i.e. a second mode of operation. Second flow valve <NUM> is closed to isolate second section <NUM> of flow divider valve <NUM> from first section <NUM>. As such, second manifold <NUM> is isolated from first manifold <NUM> and from the fuel source. Reservoir valve <NUM> is in an open position to permit fuel flow to the reservoir purged form second manifold <NUM>. Fuel flow is indicated by the arrows which illustrate fuel <NUM> from the fuel pump (not shown) pumped to the first manifold via outlet <NUM>, and fuel purged from the second manifold flowing from outlet <NUM> to reservoir outlet <NUM>. In an embodiment, FDA <NUM> does not have a reservoir valve and fuel may flow without restriction to the reservoir.

<FIG> illustrates the embodiment of FDA <NUM> according to the invention having a reservoir <NUM>. Similar to <FIG>, FDA <NUM> is illustrated in transition to a standby mode. Reservoir <NUM> has a variable volume that is manipulated by a piston <NUM>. Piston <NUM> is biased toward an expanded position in which the variable volume of the reservoir <NUM> is maximized (shown in <FIG>). A biasing member <NUM>, a spring, is configured to bias the piston <NUM> toward an expanded position. As shown in <FIG>, compressed gas pressurizes fuel in the second manifold <NUM> into the reservoir <NUM>. As the compressed gas pressures the residual fuel in the second manifold <NUM> into the reservoir <NUM>, biasing member <NUM> may permit the piston <NUM> to move toward the expanded position. As reservoir <NUM> fills with fuel, a corresponding amount of fuel is displaced from the second manifold <NUM> and associated fuel nozzles, which may reduce the risk of coking in the fuel nozzles. In an embodiment, piston <NUM> may initially be in the contracted position but is be biased toward the expanded position to expand the variable volume of reservoir <NUM>. As the variable volume of reservoir <NUM> expands, a suction force may be created to withdraw fuel from secondary manifold <NUM> into the reservoir <NUM>.

<FIG> illustrates an embodiment of FDA <NUM> not falling under the scope of the present invention which is in transition from a standby mode to flight idle or active power mode. Compressed gas flow to the second manifold <NUM> is stopped which may permit biasing member <NUM> to bias piston <NUM> into a closed position. In the closed position, the variable volume of the reservoir <NUM> is minimized forcing fuel in the reservoir <NUM> into the second manifold <NUM> and to the combustion chamber 18A or 18B. In this illustrated embodiment, when the compressed gas flow to the second manifold is stopped, the pressure of the fuel pump will be greater than the pressure in the second manifold <NUM>, causing the piston <NUM> to minimize the variable volume of the reservoir and force fuel into the second manifold <NUM>. To re-establish flow to the second manifold, in a flight idle, or active power mode illustrated in <FIG>, flow valve <NUM> may be opened and reservoir valve <NUM> may be closed.

<FIG> illustrate other embodiments of FDA <NUM>. In an embodiment, flow divider valve <NUM> has one or more pressure-actuator valves, e.g. first valve <NUM> and second valve <NUM> shown in <FIG>, that may be configured to (<NUM>) prevent fuel <NUM> from flowing through the flow divider valve <NUM> to the second manifold <NUM> through outlet <NUM> when the fuel pressure created by the fuel pump varies to become lower than a first value (e.g., threshold); (<NUM>) to allow fuel to flow only through the first section <NUM> of the flow divider valve to the first manifold <NUM>; and (<NUM>) to allow fuel <NUM> to flow (illustrated by the arrowed lines in <FIG>) through both the first section <NUM> and second section <NUM> of the flow divided valve <NUM> to the respective first and second manifolds <NUM>, <NUM>, and associated nozzles, when the fuel pressure varies to become higher than the first value or a second value (threshold) that is different from the first value. Shuttle valve <NUM> may also be provided to selectively allow flow to return to tank line <NUM> which is in communication with a fuel tank (not shown). Shuttle valve <NUM> may be biased to an open position and configured to close at a third value (e.g. threshold). In an embodiment, the third value may be higher than the first value but the same as second value. In another embodiment, the third value may be higher than both the first and second values.

At start-up of engines 12A, 12B, fuel <NUM> may be directed to flow divider valve <NUM> via the fuel-in port <NUM>. Initially, at start-up, valves <NUM>, <NUM> may be closed and pistons <NUM>, <NUM> may be biased into an expanded position. Reservoirs <NUM>, <NUM> have a variable volume that may be manipulated by their respective pistons <NUM>, <NUM>. Pistons <NUM>, <NUM> may be biased by a biasing member <NUM>, <NUM>, respectively, toward an expanded position in which the variable volume of each reservoir <NUM>, <NUM> is maximized. As fuel <NUM> moves into flow divider valve <NUM>, pressure from the fuel may move piston <NUM> to a contracted position directing fuel in reservoir <NUM> to the first manifold <NUM> through outlet <NUM>. As fuel pressure increases, valve <NUM> may open such that fuel flows through valve <NUM> to the primary manifold through outlet <NUM> while valve <NUM> remains closed (shown in <FIG>). Shuttle valve <NUM>, may be initially biased in an open position such that a portion of fuel flowing through valve <NUM> may pass through outlet <NUM> to a return to fuel tank line <NUM>.

<FIG> illustrates an embodiment of FDA <NUM> in transition to a flight idle or active mode, i.e. a first mode of operation, pressure increase of fuel <NUM> may cause valve <NUM> to open and piston <NUM> to move to a contracted position pushing fuel in reservoir <NUM> into the second manifold <NUM> through outlet <NUM>. Shuttle valve <NUM> may also close to stop flow to return to tank line <NUM>.

<FIG> illustrates an embodiment of FDA <NUM> in transition to a standby mode, i.e. a second mode of operation. As pressure decreases of fuel <NUM>, second flow valve <NUM> is closed to isolate second section <NUM> of flow divider valve <NUM> from first section <NUM>. As such, second manifold <NUM> is isolated from first manifold <NUM> and from the fuel source. Shuttle valve <NUM> may be biased to an open position to permit fuel flow to the return to fuel tank line <NUM>. Piston <NUM> may be biased to an extended position to draw fuel from second manifold <NUM> and associated fuel nozzles. In an embodiment, piston <NUM> is configured to move to an extended position at the same or a pressure lower than the pressure that valve <NUM> closes. As such, when valve <NUM> closes, piston <NUM> moves to an extended position drawing fuel through outlet <NUM> from second manifold <NUM> and its associated nozzles to reservoir <NUM>. As the fuel is drawn away from the nozzles associates with second manifold <NUM>, coking of the nozzles may be prevented or hindered. Compressed gas may also force fuel from second manifold <NUM> into reservoir <NUM> due to the back pressure in combustor 18A or 18B (i.e., the pressure of the compressed gas being higher than the pressure of the fuel in second manifold <NUM>). The compressed gas may be prevented from reaching the return to fuel tank line <NUM> by piston <NUM> and valve <NUM>. Piston <NUM> and valve <NUM> may each be provided with seals <NUM> to mitigate against reverse flow of fuel and compressed gas from the second manifold <NUM> to first section <NUM> or the return to fuel tank line <NUM>.

Transition from the stanbdy mode back to flight idle or active power, may occur following the sequence described above with respect to <FIG>.

In some embodiments, the flow divider valve <NUM> may be capable of positively sealing the manifolds <NUM>, <NUM> from one another to avoid or significantly limit fuel leakages from a fuel flowing (i.e., active) manifold <NUM> or <NUM> to an emptied (i.e., inactive) manifold <NUM> or <NUM> and suit the need of keeping one or more of the manifolds <NUM>, <NUM> empty of fuel during a specific engine operating mode(s). In an embodiment, seals <NUM> are provided on at least one of valves <NUM>, <NUM>, pistons <NUM>, <NUM>, <NUM>, and shutter valve <NUM>.

Purging residual fuel from one or more manifolds <NUM>, <NUM> may be done on a continuous basis or intermittently by opening the purge over a short period of time and reclosing it until the next purge sequence.

Claim 1:
A method of operating a gas turbine engine (12A, 12B), the method comprising:
driving a compressor (16A, 16B) to supply pressurized gas to a combustor (18A, 18B) of the gas turbine engine (12A, 12B);
supplying fuel (<NUM>) to the combustor (18A, 18B) of the gas turbine engine (12A, 12B) via a first manifold (<NUM>) and a second manifold (<NUM>) during a first mode of operation;
stopping fuel supply to the combustor (18A, 18B) via the second manifold (<NUM>) and continuing to supply fuel (<NUM>) to the combustor (18A, 18B) via the first manifold (<NUM>) during a second mode of operation; and
after stopping fuel supply to the combustor (18A, 18B) via the second manifold (<NUM>), using the pressurized gas inside the combustor (18A, 18B) to drive fuel (<NUM>) in the second manifold (<NUM>) upstream and purge the second manifold (<NUM>) of fuel (<NUM>);
supplying fuel (<NUM>) to the first manifold (<NUM>) while receiving fuel (<NUM>) purged from the second manifold (<NUM>) into a reservoir (<NUM>),
wherein:
purging the second manifold comprises suctioning fuel (<NUM>) from the second manifold (<NUM>) to purge the second manifold (<NUM>) of fuel (<NUM>) by a piston (<NUM>) configured to move between a first position and a second position;
movement of the piston (<NUM>) toward the first position suctions fuel (<NUM>) from the second manifold (<NUM>) and movement of the piston (<NUM>) toward the second position returns fuel (<NUM>) to the second manifold (<NUM>); and
the piston is biased toward the first position by a spring.