Patent Description:
Gas turbine engines typically burn liquid fuel (e.g. kerosene) and use on/off staging for turn-down and startup, for example using two manifolds, which does not easily translate to gaseous fuels. Therefore there is always a need in the aerospace industry for more precise control of gaseous fuel flow in gas turbine engines.

A prior art fuel system having the features of the preamble of claim <NUM> is disclosed in <CIT>. Another prior art fuel system is known from <CIT>. A method for operating a proportional flow valve is disclosed in <CIT>.

In accordance with an aspect of the present invention, there is provided a fuel system for a gas turbine engine of an aircraft in accordance with claim <NUM>.

In embodiments, the controller is operable to control the secondary fuel flow valve to pulse a flow of fuel passing therethrough in pulse width modulation (PWM) pulses at a low end of a fuel flow range for the secondary manifold. In certain embodiments, the controller can be an electronic engine control (EEC) of a gas turbine engine.

In certain embodiments, a fuel pressure sensor is operatively connected to an inlet of the of the fuel metering mechanism and operable to generate a signal indicative of a fuel pressure at the inlet of the fuel metering mechanism. In certain such embodiments, the controller is operatively connected to the fuel pressure sensor and operable to receive the signal from the fuel pressure sensor. In certain embodiments, the controller is operable to validate whether an inlet pressure is sufficient for operation at the intended operating condition and/or adjust a position of the fuel metering mechanism relative to a gaseous fuel compressibility at a rated pressure.

In certain embodiments, a position feedback sensor is operatively connected to the fuel metering mechanism and operable to generate a signal indicative of a position of the fuel metering mechanism. In certain such embodiments, the controller is operatively connected to the position sensor. In certain embodiments, the controller is operable to control the position of the fuel metering mechanism based on the signal indicative of the position of the fuel metering mechanism.

In certain embodiments, a delta pressure sensor is operatively connected to the inlet of the fuel metering mechanism and outlet of the fuel metering mechanism and operable to generate a signal indicative of a pressure drop across the fuel metering mechanism. In certain such embodiments, the controller is operatively connected to the delta pressure sensor and operable to receive the signal from the delta pressure sensor. In certain embodiments, the controller is operable to measure a gaseous fuel flow rate through the fuel metering mechanism based on the pressure drop and an adjusted position of the fuel metering mechanism.

In certain embodiments, a temperature sensor is operatively connected to the main inlet feed conduit at the outlet of the fuel metering mechanism and operable to generate a signal indicative of a temperature of the gaseous fuel at the outlet of the fuel metering mechanism. In certain such embodiments, the controller is operatively connected to the temperature sensor and is operable to output a temperature correction factor and control the position of the fuel metering mechanism based on the signal indicative of the temperature of the gaseous fuel flow at the outlet of the fuel metering mechanism.

In certain embodiments, a downstream pressure sensor is disposed in the main inlet feed conduit at an inlet of a flow divider assembly downstream of the fuel metering mechanism operable to generate a signal indicative of a fuel pressure at the inlet of the flow divider assembly. In certain such embodiments, the controller is operatively connected to the downstream pressure sensor and operable to receive the signal from the downstream pressure sensor. In certain embodiments, the controller is operable to control the fuel metering mechanism to act as a pressure regulator in low flow operating conditions.

In embodiments, the controller includes machine readable instructions to cause the controller to place the primary fuel control valve and the secondary fuel flow valve in respective closed positions, pressurize the main inlet feed conduit feeding the primary and secondary fuel flow valve valves with gaseous fuel, place the primary fuel flow valve into its open condition to supply the gaseous fuel to fuel injectors of the primary manifold downstream from the primary fuel flow valve, and modulate the primary fuel flow valve between its open and closed position to pulse a flow of gaseous fuel passing therethrough in pulse width modulation (PWM) pulses at a low end of a fuel flow range for the primary manifold.

In certain embodiments, the controller includes machine readable instructions to cause the controller to schedule a PWM dwell time relative to sensed engine conditions, and schedule a position of the fuel metering mechanism relative to a sensed pressure at an inlet of a flow divider assembly. In certain such embodiments, the sensed engine conditions derive from signals indicative of at least one of: the pressure at the inlet of the fuel metering mechanism, the pressure drop across the fuel metering mechanism, the temperature of the gaseous fuel at the outlet of the fuel metering mechanism, and/or the position of the fuel metering mechanism. In certain embodiments, the sensed pressure at the inlet of the flow divider assembly is derived from a signal indicative of the pressure at the inlet of the flow divider assembly.

In certain embodiments, the controller includes machine readable instructions to cause the controller to modulate the secondary fuel flow valve between its open and closed position to pulse a flow of gaseous fuel passing therethrough in pulse width modulation (PWM) pulses at a low end of a fuel flow range for the secondary manifold to prevent surge or notch when placing the secondary fuel flow valve its opened position.

In accordance with another aspect of the present invention, there is provided a gas turbine engine for an aircraft in accordance with claim <NUM>.

In accordance with yet another aspect of the present invention, there is provided a fuel control method for a gas turbine engine in accordance with claim <NUM>.

In certain embodiments, the method includes modulating the secondary fuel flow valve between its open and closed position to pulse a flow of gaseous fuel passing therethrough in pulse width modulation (PWM) pulses at a low end of a fuel flow range for the secondary manifold to prevent surge or notch when placing the secondary fuel flow valve its opened position.

In certain embodiments, the sensed engine conditions derive from signals indicative of at least one of: a pressure at an inlet of the fuel metering mechanism, a pressure drop across the fuel metering mechanism, a temperature of the gaseous fuel at an outlet of the fuel metering mechanism, and/or a position of the fuel metering mechanism. In certain such embodiments, the method includes, controlling a position of the fuel metering mechanism and the PWM pulses of the first and second flow valves based on the plurality of sensed inputs.

In certain embodiments, the sensed pressure at the inlet of the flow divider assembly is derived from a signal indicative of the pressure at the inlet of the flow divider assembly. In certain such embodiments, the method further includes, controlling the fuel metering mechanism to act as a pressure regulator in low flow operating conditions based on the sensed input.

These and other features of the embodiments of the subject disclosure will become more readily apparent to those skilled in the art from the following detailed description taken in con-junction with the drawings.

Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject disclosure. For purposes of explanation and illustration, and not limitation, an illustrative view of an embodiment of a system in accordance with the disclosure is shown in <FIG> and is designated generally by reference character <NUM>. Other embodiments and/or aspects of this disclosure are shown in <FIG>. Certain embodiments described herein can be used to improve control of gaseous fuel flow in a gas turbine engine.

The present disclosure relates generally to fuel control for gas turbine engines, and more particularly to control of gaseous fuel flow. A gas turbine engine may be fueled with gaseous fuel such as hydrogen gas. It is possible to gasify liquid hydrogen from an aircraft supply through an appropriate fuel pump, heat exchangers, pressure regulator, and metering valves. It is desired to control gaseous fuel delivery to the engine such that stable and responsive control over the wide range of flow conditions would be maintained. However, the typical fuel control for aircraft engines is designed for purely liquid fuel flow. Liquid fuel is an incompressible fluid, whereas gaseous fuel is compressible. For hydrogen, the full fuel system can include a combination of liquid and gaseous hydrogen, meaning the fuel control system needs to control both compressible and incompressible flows.

Gas turbine engines typically burn liquid fuel (e.g. kerosene) and use on/off staging for turn-down and startup, for example using two manifolds, which does not easily translate to gaseous fuels. However, when using gaseous fuel, high turn down ratio can render control of the gaseous flow in a low flow regime particularly challenging, for example exposing the engine to a risk of flame out or surging as a result of inaccurate fuel flow supply. Because the gaseous hydrogen is pressurized from the fuel delivering system, the pressure supplied to the fuel line P1 is regulated at a much higher pressure of the burner pressure P3 (e.g. at least double). Therefore, variation of fuel metering valve output sets a variable throat area to the valve which is maintained choked (e.g. in a sonic state) at all system flow conditions.

The fuel control systems and methods as provided herein utilizes pulse width control of fuel flow valves to control pressure for the fuel manifolds at startup, for example, or at any other engine condition in which fine tuning control of fuel pressure is desired. In certain embodiments, the flow valves pulse during start up, first for a primary manifold and later for the secondary manifold. Pulsing allows for stable control of start up to bring both manifolds up to power without loss of control of combustion during start up. Additional metering devices, if included, provides additional pressure control, but may not be as fine tuned as the fuel flow valves.

Accordingly, as will be described herein with greater detail, the aircraft fuel system includes one or more controllable flow valves which can be used to manage and maintain the distribution of the gas flow into the engine (e.g. the fuel manifolds) during starting and large transient maneuvers. For example, the system introduces the ability to actively control the gaseous flow distribution between the primary and secondary manifolds. In the gaseous state, the fuel used for combustion is a compressible media.

In certain embodiments, referring to <FIG>, an aircraft <NUM> can include an engine <NUM>, where the engine <NUM> can be a propulsive energy engine (e.g. creating thrust for the aircraft <NUM>), or a non-propulsive energy engine, and a fuel system. As described herein, the engine <NUM> is a turbofan engine, although the present disclosure may likewise be used with other engine types. The engine <NUM> includes a compressor section <NUM> having a compressor <NUM> in a primary gas path <NUM> to supply compressed air to a combustor <NUM> of the aircraft engine <NUM>. The primary gas path <NUM> includes a nozzle manifold <NUM> for issuing fluid to the combustor <NUM>.

The primary gas path <NUM> includes, in fluid communication in a series: the compressor <NUM>, the combustor <NUM> fluidly connected to an outlet <NUM> of the compressor <NUM>, and a turbine section <NUM> fluidly connected to an outlet <NUM> of the combustor <NUM>. The turbine section <NUM> is mechanically connected to the compressor <NUM> to drive the compressor <NUM>.

The combustor <NUM> includes a plurality of fuel nozzles <NUM> (e.g. including a primary set 120a and a secondary set 120b) fluidly connected to the fuel manifold <NUM>, where the primary set 120a is fluidly connected to a primary manifold <NUM>, and the secondary set 120b is fluidly connected to a secondary fuel manifold <NUM>. A main inlet feed conduit <NUM> fluidly connects a gaseous fuel supply <NUM> to feed a primary manifold feed conduit 110a and to feed a secondary manifold feed conduit 111a. The main inlet feed conduit <NUM> includes an inlet end <NUM> and an outlet end <NUM> to fluidly connect the gaseous fuel supply <NUM> to the combustor <NUM> through the plurality of fuel nozzles <NUM>. In embodiments, the gaseous fuel supply <NUM> can be any suitable gaseous fuel, such as a gaseous pressure and/or temperature regulated fuel supply, which may be or include hydrogen gas.

Certain additional components may also be included in fluid communication between the combustor and the gaseous fuel supply in any suitable order or combination, such as a fuel shut off valve <NUM>, a fuel pump <NUM>, a liquid/gaseous fuel evaporator <NUM>, a turbine air cooling heat exchanger <NUM>, a gaseous fuel accumulator <NUM>, a gaseous fuel metering unit <NUM>, a fuel manifold shut off valve <NUM>, and/or additional pressure regulating devices. In certain embodiments, the pre-pressurized gaseous fuel accumulator <NUM> can be used as backup supply pressure source.

Turning now to <FIG>, a fuel control system <NUM> for controlling the flow of fuel to the aircraft engine <NUM> through the main inlet feed conduit <NUM> and the plurality of fuel nozzles <NUM> includes a means for regulating flow through the main inlet feed conduit <NUM>, a means for generating a signal indicative of an engine state or condition, and a controller <NUM>. The controller <NUM> is operatively connected to the means for regulating flow and to the means for generating a signal to control the means for regulating a signal based on the signal such that the controller is operable to control a state of the means for regulating to achieve a desired power output (e.g. a command from a pilot, autopilot, or drone software for acceleration of the engine <NUM>).

As described herein, the means for regulating flow through the main inlet feed conduit <NUM> includes at least one metering mechanism <NUM>. The means for generating a signal indicative of an engine state or condition can include any suitable means, for example any number and/or combination of pressure sensors, temperature sensors, position sensors, or the like, disposed in the engine <NUM> and/or main inlet feed conduit <NUM>, and operatively connected as disclosed herein.

As shown in <FIG>, the fuel metering mechanism <NUM> is disposed in the main inlet feed conduit <NUM> between the inlet end <NUM> and the outlet end <NUM> and is operable to regulate flow through both the main inlet feed conduit <NUM> and in certain embodiments the primary fuel manifold <NUM>. The fuel metering mechanism <NUM> can include all required instrumentation to validate gas fuel flow therethrough relative to a commanded flow.

In embodiments, a flow divider assembly <NUM> is fluidly connected to the outlet <NUM> end of the main inlet feed conduit <NUM> to receive fuel from the fuel metering mechanism <NUM> and divide and issue flow from the main inlet feed conduit <NUM> into the combustor <NUM> and to the plurality of fuel nozzles <NUM> through the first fuel manifold <NUM> and the second fuel manifold <NUM>. In embodiments, the inlet of the flow divider assembly is the outlet <NUM> of the main inlet conduit <NUM> and is a branch point for the primary manifold <NUM> and the secondary manifold <NUM> to divide the gaseous fuel between the manifolds <NUM>, <NUM>. The first fuel manifold <NUM> can be a primary fuel manifold configured to provide sufficient fuel during low fuel consumption such as during start up, and the second fuel manifold <NUM> can be a secondary fuel manifold configured to supplement the primary fuel manifold during high fuel consumption. For example, the engine <NUM> can use minimum of two fuel manifolds to regulate the gaseous fuel flow and corresponding back pressure over the full operating range of the engine <NUM>. The primary manifold <NUM> can be used in starting, to provide a reduced nozzle count which is spread evenly around the engine <NUM>. In applications, the reduced nozzle count can allow for a greater control of the gas flow being introduced through an increased restriction/back pressure, and can reduce the risk of over fueling on start. As engine power increases, the secondary manifold <NUM> is added to provide additional flow at the same gaseous supply pressure if needed or desired.

Fuel flow to the first and second fuel manifolds <NUM>, <NUM> is controlled by a primary controlled flow valve <NUM> disposed in the first fuel manifold feed conduit 110a, and a secondary controlled flow valve <NUM> disposed in the second fuel manifold feed conduit 111a. The primary and secondary fuel flow valves <NUM>, <NUM> each have an open condition operative to permit flow of fuel through the primary manifold feed conduit <NUM> and a closed condition operative to inhibit (e.g. reduce or completely block) flow of fuel through the primary manifold feed conduit <NUM>. The first and second controlled flow valves <NUM>, <NUM> can be any suitable controllable flow valve, such as solenoid valves operatively connected to a controller to selectively energize and de-energize the first and second flow valves <NUM>, <NUM> to selectively allow flow through the first and second manifolds <NUM>, <NUM> to the combustor <NUM>. While two active valves <NUM>, <NUM> are shown, it is contemplated that only the primary flow valve <NUM> can be an actively controlled valve and the secondary valve <NUM> can be passive. In certain embodiments, valves <NUM>, <NUM> can be or include a globe valve, or a variable opening, electrically actuated valve. In certain other embodiments, the valves <NUM>, <NUM> can be a calibrated mechanical regulating valve, and once calibrated, provide a similar response and effect. In certain embodiments, the first and second controlled flow valves <NUM>, <NUM> can be electrohydraulic servo valves operatively connected to the controller <NUM> to operate in a similar manner to provide a similar response and effect.

The controller <NUM> is operatively connected to control at least the primary fuel flow valve <NUM> to pulse a flow of gaseous fuel passing therethrough in pulse width modulation (PWM) pulses at a low end of a fuel flow range for the primary manifold <NUM>. PWM control of the flow valve <NUM> can allow for high precision flow control even at low flow engine states. The same or a similar technique can still be beneficial to the secondary flow valve <NUM> in other engine states, even if the secondary fuel flow valve <NUM> remains closed during startup.

The controller <NUM> can include any suitable controller, for example an electronic engine controller (EEC). The controller <NUM> can be or include both hard wired circuits that cause a logic to be executed, and/or software-based components, for example, simple electric circuits employing analog sensors and/or components, or the controller <NUM> can include a CPU, a memory, machine readable instructions in the memory that when executed cause the CPU to perform a method. In certain embodiments, the controller <NUM> automatically controls the fuel flow valves <NUM>, <NUM> and a fuel metering mechanism <NUM> (e.g. without user input). The controller <NUM> is operable to collect and process signals from a plurality of inputs, including for example, a downstream temperature, an upstream and downstream pressure, and a delta pressure across the fuel metering mechanism to use and calculate a fuel flow schedule.

In certain embodiments, a fuel pressure sensor <NUM> (e.g. an absolute pressure) is operatively connected to an inlet of the fuel metering mechanism <NUM> and operable to generate a signal <NUM> indicative of the upstream fuel pressure at the inlet of the fuel metering mechanism <NUM>. The controller <NUM> is operatively connected to the fuel pressure sensor and operable to receive the signal <NUM> from the fuel pressure sensor <NUM>. The upstream pressure, and its respective signal <NUM>, can have two functions for the controller <NUM> and fuel metering mechanism <NUM>. First, the controller <NUM> can use the upstream pressure for validating that the inlet pressure of the fuel metering mechanism <NUM> is sufficient for operation at the intended operating condition (e.g. start-up or based on the commanded power). Second, the controller <NUM> can use the upstream pressure for correcting a position of the opening of the fuel metering mechanism <NUM> relative to the gaseous fuel compressibility at the rated pressure.

In certain embodiments, a position feedback sensor <NUM> is operatively connected to the fuel metering mechanism <NUM> and operable to generate a signal <NUM> indicative of an actual position of the fuel metering mechanism. The position feedback sensor <NUM> can be or include any suitable position sensor, for example a linear variable differential transformer (LVDT). Because the accuracy of a valve actuator system (e.g. a torque motor driver) may vary with operating conditions (e.g. fluid temperature, pressure, ambient conditions), a direct measurement of actual position (or rate of change of position) of the fuel metering mechanism <NUM> allows for a more precise control and calculation of metered flow.

In embodiments, the controller <NUM> is operatively connected to the position sensor <NUM>, and is therefore is operable to control the position of the electronic metering valve <NUM> based on the each of the signal indicative of an upstream pressure <NUM> and the signal <NUM> of the position of the fuel metering mechanism <NUM>, and in certain embodiments, a command power for a desired power output of the aircraft engine <NUM> to achieve the desired power output (e.g. as simultaneous inputs). Upon receipt of a command power to the controller <NUM>, the position of the fuel metering mechanism <NUM> is ultimately driven by a driver operatively connected to the fuel metering mechanism <NUM>.

In certain embodiments, a delta pressure sensor <NUM> is operatively connected to the inlet of the fuel metering mechanism <NUM> and outlet of the of the fuel metering mechanism <NUM> and is operable to generate a signal <NUM> indicative of a pressure drop across the fuel metering mechanism <NUM>. The delta pressure sensor <NUM> provides the differential pressure across the metering mechanism <NUM> itself for a given opening position (e.g. in conjunction with the correction factors) and can be used to measure the gas flow rate through the fuel metering mechanism <NUM>, for example. In certain embodiments, the delta pressure sensor <NUM> can be a differential pressure sensor connected to a pressure tap the main inlet feed conduit <NUM> at the inlet and outlet of the fuel metering mechanism <NUM>, the delta pressure sensor <NUM> itself determining the pressure differential. It is also contemplated that the delta pressure sensor <NUM> can be an electronic device connecting to separate absolute pressure sensors located at each of the inlet and outlet of the fuel metering mechanism <NUM>, where the electronic device includes a module of the controller <NUM> that simply takes the difference between the signals for each individual sensor.

In certain such embodiments, the controller <NUM> is operatively connected to the delta pressure sensor <NUM> and operable to receive the signal <NUM> from the delta pressure sensor <NUM>, and is operable to measure a gaseous fuel flow rate through the fuel metering mechanism <NUM> based on the pressure drop and an adjusted position of the fuel metering mechanism (e.g. the corrected position as described above).

In certain embodiments, a temperature sensor <NUM> is operatively connected to the outlet of the fuel metering mechanism <NUM> and operable to generate a signal indicative <NUM> of a temperature of the gaseous fuel at the outlet of the fuel metering mechanism <NUM>. The temperature sensor <NUM> measures the gas flow temperature at the outlet of the fuel metering mechanism <NUM> and can also be used to provide a correction factor for the position of the fuel metering mechanism <NUM>. Because the gas temperature will increase as it passes through the fuel metering mechanism <NUM> due to the Joule Thomson effect, the temperature correction factor can then be used by the controller <NUM> to control the position of the fuel metering mechanism <NUM> as a function of the correction factor. Therefore, in certain such embodiments, the controller <NUM> is operatively connected to the temperature sensor <NUM> and is operable to output a temperature correction factor to control the position of the fuel metering mechanism <NUM> based on the signal <NUM> indicative of the temperature of the gaseous fuel flow at the outlet of the fuel metering mechanism <NUM>.

In certain embodiments, a downstream pressure sensor <NUM> is disposed in the main inlet feed conduit <NUM> at an inlet (e.g. outlet <NUM> of main inlet feed conduit <NUM>) of the flow divider assembly <NUM> downstream of the fuel metering mechanism <NUM> and upstream of the combustor <NUM>, the sensor <NUM> being operable to generate a signal <NUM> indicative of a fuel pressure at the inlet of the flow divider assembly <NUM>. The downstream pressure sensor <NUM> can be an absolute pressure transducer and can be used in 'open loop' operation, when compressibility of the gaseous fuel may be transient in nature as the supply lines are filled/primed with the gaseous fuel. In conjunction with the flow divider <NUM> architecture, the downstream pressure sensor <NUM> allows the controller <NUM> (e.g. via signal <NUM>) to control the metering mechanism <NUM> to be used as secondary pressure regulator to improve the metering accuracy at certain engine conditions (e.g. low flow operating conditions). Additionally, by locating the sensor <NUM> downstream of the fuel metering mechanism <NUM>, the commanded flow can be trimmed to accommodate for any line losses in between the metering valve and the fuel manifold inlet.

Each of the signals, including the signal <NUM> indicative of a position of the fuel metering mechanism <NUM>, the signal <NUM> indicative of a fuel pressure at the inlet of the fuel metering mechanism <NUM>, the signal <NUM> indicative of the pressure drop across the fuel metering mechanism <NUM>, the signal <NUM> indicative of a fuel temperature at the outlet of the fuel metering mechanism <NUM>, and the signal <NUM> indicative of the pressure at the inlet of the flow divider assembly <NUM>, can be input into a control algorithm executable at least in part by the controller <NUM> to generate a control signal as an output based on the plurality of inputs. Accordingly, the controller <NUM> is operable to control both the fuel metering mechanism <NUM> and each of the controllable flow valves <NUM>, <NUM> by sending the control signal to the metering mechanism <NUM> and to each of the flow valves <NUM>, <NUM>. In embodiments, the algorithm could be constructed using the functionality as described above in addition to known general engineering principles as applied to the specific characteristics of each particular fuel system to which the technology of the present disclosure is applied.

In certain embodiments, the controller <NUM> includes machine readable instructions to cause the controller to perform a method, for example, a fuel control method for the gas turbine engine. The method includes placing a primary fuel flow valve (e.g. primary flow valve <NUM>) and a secondary fuel flow valve (e.g. secondary flow valve <NUM>) in respective closed positions, pressurizing a main inlet feed conduit (conduit <NUM>) feeding the primary and secondary fuel flow valves with a flow of gaseous fuel (e.g. from fuel supply <NUM>), placing the primary fuel flow valve into an open condition to supply the flow of gaseous fuel to a first plurality of fuel injectors (e.g. injectors 120a) of a primary fuel manifold (e.g. primary manifold 110a) downstream of the primary fuel flow valve, and modulating the primary fuel flow valve between its open and closed position to pulse a flow of gaseous fuel passing therethrough in PWM pulses at a low end of a fuel flow range for the primary manifold.

In certain embodiments, the method includes, scheduling a PWM dwell time relative to sensed engine conditions (e.g. downstream temperature or pressure). As used herein, dwell time refers to the amount of time the flow valve is in the open position, for example, increasing the PWM dwell time will increase the opening of the fuel flow valve. In embodiments, the method further includes scheduling a position of the fuel metering mechanism relative to a sensed pressure at an inlet of a flow divider assembly (e.g. flow divider assembly <NUM>). In certain embodiments, the method includes modulating the secondary fuel flow valve between its open and closed position to pulse a flow of gaseous fuel passing therethrough in pulse width modulation (PWM) pulses at a low end of a fuel flow range for the secondary manifold to prevent surge or notch when placing the secondary fuel flow valve its opened position.

In certain embodiments, on engine start-up for example, both fuel flow valves <NUM>, <NUM> can be held in the closed position while the fuel metering mechanism <NUM> is in an open position to allow the priming and pressurization of the main inlet feed conduit <NUM> to pressurize the conduit <NUM>. Once pressurized and primed, the primary flow valve <NUM> can be moved to the open position to allow gas flow into the primary manifold <NUM>. Priming and/or pre-pressurizing the conduit <NUM> can ensure the fuel velocity when entering the manifold are sufficiently high to pre-vent flashback of the gaseous fuel upon ignition. With the fuel flow valve <NUM> open for the primary manifold <NUM>, the fuel metering mechanism <NUM> can be adjusted to maintain the required pressure at the inlet of the flow divider assembly <NUM>. As engine speed/power increases, the secondary manifold flow valve <NUM> can be opened to allow gas flow into the secondary manifold <NUM>. Similar to the primary manifold <NUM>, flow into the secondary manifold <NUM> will occur at a flow divider inlet pressure where the gas pressure will result in a sufficiently high velocity gaseous flow into the secondary manifold <NUM> to prevent flash back.

With both fuel flow valves <NUM>, <NUM> open, fuel scheduling can then be solely controlled by the fuel metering mechanism based off of the inputs to the controller <NUM> (e.g. inputs as described above in addition to pilot command, engine power, engine speed, operating parameters for temperature, pressure, altitude).

Since the gaseous supply <NUM> is maintained at a sufficiently high pressure to obtain the required gas flow at high power, the low flow operating conditions (i.e. engine start) require a significant pressure drop to be imposed on the fuel to accurately meter the flow. With the fuel in a compressible, gaseous state, this can lead to a loss of fidelity in the metering due to the fuel reaching trans sonic and sonic speeds within the metering valve. With the active control of fuel flow valves <NUM>, <NUM> in the fuel manifolds <NUM>, <NUM>, it is possible to use the flow divider <NUM> as a second metering mechanism, allowing the gas pressure to be reduced in steps, restoring some of the fidelity in metering control.

To improve the metering fidelity and control in low flow conditions, in certain embodiments, the fuel metering mechanism <NUM> can be scheduled to provide a targeted flow divider inlet pressure which may be higher than the required pressure for the given operating condition. In that case, once the pressure is stabilized, the primary fuel flow valve <NUM> current can then be pulse width modulated (PWM) by the controller <NUM>, which controls the flow valve's <NUM>, <NUM> opening time per second, effectively simulating a smaller restriction than if the valves were just set to the wide open position. Therefore, while operating the engine <NUM>, the PWM dwell time can be scheduled relative to the appropriate engine parameters (i.e. downstream temperature or pressure) while the position of the fuel metering mechanism <NUM> can be scheduled relative to the pressure at the inlet of the flow divider <NUM>. A similar technique can be used when engaging the secondary manifold to prevent any 'surge' or 'notch' felt when adding the additional nozzle flows.

Claim 1:
A fuel system (<NUM>) for a gas turbine engine (<NUM>) of an aircraft (<NUM>) comprising:
a main inlet feed conduit (<NUM>) fluidly connected to a primary manifold feed conduit (110a) and to feed a secondary manifold feed conduit (111a);
a primary manifold (<NUM>) fluidly connecting the primary manifold feed conduit (110a) to a plurality of primary fuel injectors (120a);
a secondary manifold (<NUM>) fluidly connecting the secondary manifold feed conduit (111a) to a plurality of secondary fuel injectors (120b);
a primary fuel flow valve (<NUM>) disposed in the primary manifold feed conduit (110a) wherein the primary fuel flow valve (<NUM>) has an open condition operative to permit flow of fuel through the primary manifold feed conduit (110a) and a closed condition operative to inhibit flow of fuel through the primary manifold feed conduit (110a);
a secondary fuel flow valve (<NUM>) disposed in the secondary manifold feed conduit (111a) wherein the secondary fuel flow valve (<NUM>) has an open condition operative to permit flow of fuel through the secondary manifold feed conduit (111a) and a closed condition operative to inhibit flow of fuel through the secondary manifold feed conduit (111a); and
a controller (<NUM>) operatively connected to control the primary fuel flow valve (<NUM>),
wherein the controller (<NUM>) is operable to control the primary fuel flow valve (<NUM>) to pulse a flow of fuel passing therethrough in pulse width modulation (PWM) pulses at a low end of a fuel flow range for the primary manifold (<NUM>); and
characterised in that
the fuel system (<NUM>) further comprises a gaseous fuel source (<NUM>) defining an inlet end (<NUM>) of the main inlet feed conduit (<NUM>) to supply gaseous fuel to the primary fuel manifold (<NUM>) via the main inlet feed conduit (<NUM>), and a fuel metering mechanism (<NUM>) disposed in the main inlet feed conduit (<NUM>) operable to regulate flow through both the main inlet feed conduit (<NUM>) and the primary fuel manifold (<NUM>).