Patent Description:
Gas turbine engines may be employed to power various devices. For example, a gas turbine engine may be employed to power a mobile platform, such as an aircraft. Generally, gas turbine engines combust fuel supplied by a fuel supply system to drive a turbine to generate power to propel the aircraft. The fuel supply system typically includes a fuel pump to pressurize the fuel and a fuel metering valve to modulate the amount of fuel that is delivered to a combustor. The position of the fuel metering valve may be controlled by a full authority digital engine control (FADEC), acting through an electrohydraulic servo valve (EHSV). In many cases, an electronic position sensor is attached to the fuel metering valve to feedback valve position information to the FADEC, enabling closed-loop control of the fuel flow metering. The inclusion of the position sensor increases the weight, cost, and complexity associated with the fuel supply system.

Accordingly, it is desirable to provide a fuel metering system in which the electronic position sensor is eliminated, and the fuel metering valve operates in an open-loop system, with sufficient accuracy to satisfy the performance needs of the engine. By eliminating the position sensor, the weight associated with the fuel supply system, as well as the cost and complexity, are reduced. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.

<CIT> discloses a two-stage fuel metering system for metering the flow of fuel between a fuel supply and a combustion chamber of an engine the fuel in the supply being pressurized by a pump and delivered to the combustion chamber. The fuel metering system includes a first primary passageway for receiving fuel from a fuel supply and a second primary passageway fluidically connected to the first passageway for delivering fuel to a combustion chamber. A servo chamber is fluidically connected to both the first and second primary passageways and a servo valve is disposed between the first passageway and the servo chamber to selectively regulate the flow of fuel entering the chamber. A metering valve is disposed between the first and second passageways to regulate the flow of fuel from the first passageway to the second passageway the metering valve being biased to a closed position that obstructs the fuel flow. The metering valve further includes an actuating surface disposed in the chamber and responsive to the pressure in the chamber, wherein the metering valve is adapted to move from the closed position toward an open position in response to sufficient fuel pressure in the chamber. A conduit fluidically connects the chamber to the second passageway to restrict the flow of fuel therethrough to an upper limit.

Aspects and preferred embodiments of the invention are defined in the appended claims. According to various embodiments, provided is a fuel metering system for a gas turbine engine. The fuel metering system includes a metering valve having a first inlet fluidly coupled to a source of fluid at a first pressure, a second inlet, a second outlet to be coupled to the gas turbine engine and a first outlet. The metering valve includes a slot defined proximate to the first outlet, and a valve body movable to control an amount of fluid supplied to the first outlet and to the second outlet. The metering valve includes a biasing member that applies a force to the valve body. The fuel metering system includes a servo valve fluidly coupled to the second inlet of the metering valve and to a second source of fluid at a second pressure. The servo valve is in fluid communication with the second inlet and a body of the servo valve is movable relative to the second inlet to supply a fluid from the second source of fluid to apply a fluid pressure to move the valve body. The slot is configured to variably restrict the flow of the fluid through the first outlet of the metering valve to modify the fluid pressure exerted on the valve body to balance the force applied by the biasing member. The fuel metering system includes a fixed flow restriction downstream of the first outlet.

A position of the valve body relative to the second outlet is based on a current supplied to the servo valve by a controller associated with the gas turbine engine. The current supplied to the servo valve includes a first current range and a second current range, and a relationship between the current supplied to the servo valve and a position of the valve body is different in the first current range and the second current range. A correlation between an exposed area of the slot and the fixed flow restriction results in a first relationship between the position of the valve body and the current supplied to the servo valve in the first current range, and a second relationship between the position of the valve body and the current supplied to the servo valve in the second current range. The source of fluid is a fuel pump and the second source of fluid is a pressure regulator. The pressure regulator is fluidly coupled to the fuel pump to receive the fuel at the first pressure and to regulate the pressure of the fuel to the second pressure. The first outlet is fluidly coupled to the pressure regulator downstream of the flow restriction. The metering valve includes a third inlet that is fluidly coupled to the first outlet. The pressure regulator includes a regulator valve body movable against a force of a second biasing member by a first regulator control chamber and a second regulator control chamber. The first regulator control chamber is fluidly coupled to the fuel pump to receive the fuel at the first pressure and the second regulator control chamber fluidly coupled to the first outlet. The first outlet is fluidly coupled to the source of fluid downstream of the flow restriction.

Further provided is a fuel metering system for a gas turbine engine. The fuel metering system includes a metering valve having a first inlet fluidly coupled to a fuel pump to receive fuel at a first pressure, a second inlet, a second outlet to be coupled to the gas turbine engine and a first outlet. The metering valve includes a slot defined proximate to the first outlet, and a valve body movable to control an amount of fuel supplied to the first outlet and to the second outlet. The metering valve includes a biasing member that applies a force to the valve body. The fuel metering system includes a pressure regulator fluidly coupled to the fuel pump to receive the fuel at the first pressure, fluidly coupled to the servo valve and fluidly coupled to the first outlet. The pressure regulator includes a regulator valve body movable based on a pressure differential between the fuel at the first pressure and a pressure of a fuel at the second outlet to provide the servo valve with fuel at a second pressure. The fuel metering system includes a servo valve fluidly coupled to the second inlet of the metering valve and to the pressure regulator. The servo valve is in fluid communication with the second inlet and a body of the servo valve is movable relative to the second inlet to supply the fuel from the pressure regulator to apply a fluid pressure to move the valve body. The slot is configured to variably restrict the flow of the fuel through the first outlet of the metering valve to modify the fluid pressure exerted on the valve body to balance the force applied by the biasing member. The fuel metering system includes a fixed flow restriction downstream of the first outlet.

A position of the valve body relative to the second outlet is based on a current supplied to the servo valve by a controller associated with the gas turbine engine. The current supplied to the servo valve includes a first current range and a second current range, and a relationship between the current supplied to the servo valve and a position of the valve body is different in the first current range and the second current range. A correlation between an exposed area of the slot and the fixed flow restriction results in a first relationship between the position of the valve body and the current supplied to the servo valve in the first current range, and a second relationship between the position of the valve body and the current supplied to the servo valve in the second current range. The metering valve includes a third inlet and a third outlet, the third inlet is fluidly coupled to the first outlet. The pressure regulator includes a regulator valve body movable against a force of a second biasing member by a first regulator control chamber and a second regulator control chamber. The first regulator control chamber is fluidly coupled to the fuel pump to receive the fuel at the first pressure and the second regulator control chamber fluidly coupled to the first outlet. The first outlet is fluidly coupled to the fuel pump downstream of the flow restriction.

Also provided is a fuel metering system for a gas turbine engine. The fuel metering system includes a metering valve having a first inlet fluidly coupled to a fuel pump to receive fuel at a first pressure, a second inlet, a second outlet to be coupled to the gas turbine engine and a first outlet. The metering valve includes a slot defined proximate to the first outlet, and a valve body movable to control an amount of fuel supplied to the first outlet and to the second outlet. The metering valve includes a biasing member that applies a force to the valve body. The fuel metering system includes a pressure regulator fluidly coupled to the fuel pump to receive the fuel at the first pressure, fluidly coupled to the servo valve and fluidly coupled to the first outlet. The pressure regulator includes a regulator valve body movable based on a pressure differential between the fuel at the first pressure and a pressure of a fuel at the second outlet to provide the servo valve with fuel at a second pressure. The fuel metering system includes a servo valve fluidly coupled to the second inlet of the metering valve and to the pressure regulator. The servo valve is in fluid communication with the second inlet and a body of the servo valve is movable relative to the second inlet to supply the fuel from the pressure regulator to apply a fluid pressure to move the valve body. A position of the valve body relative to the second outlet is based on a current supplied to the servo valve by a controller associated with the gas turbine engine and the slot is configured to variably restrict the flow of the fuel through the first outlet of the metering valve to modify the fluid pressure exerted on the valve body to balance the force applied by the biasing member. The fuel metering system includes a fixed flow restriction downstream of the first outlet.

The current supplied to the servo valve includes a first current range and a second current range, and a relationship between the current supplied to the servo valve and a position of the valve body is different in the first current range and the second current range. A correlation between an exposed area of the slot and the fixed flow restriction results in a first relationship between the position of the valve body and the current supplied to the servo valve in the first current range, and a second relationship between the position of the valve body and the current supplied to the servo valve in the second current range.

In addition, those skilled in the art will appreciate that embodiments of the present disclosure may be practiced in conjunction with any type of engine that would benefit from having an open loop fuel metering system, and the gas turbine engine described herein for use onboard a vehicle is merely one exemplary embodiment according to the present disclosure. In addition, while the fuel metering system is described herein as being used with a gas turbine engine onboard a vehicle, such as a bus, motorcycle, train, motor vehicle, marine vessel, aircraft, rotorcraft and the like, the various teachings of the present disclosure can be used with a stationary platform. Further, it should be noted that many alternative or additional functional relationships or physical connections may be present in an embodiment of the present disclosure. In addition, while the figures shown herein depict an example with certain arrangements of elements, additional intervening elements, devices, features, or components may be present in an actual embodiment. It should also be understood that the drawings are merely illustrative and may not be drawn to scale.

With reference to <FIG> is a functional block diagram of a fuel metering system <NUM> for supplying combustible fuel to an engine, such as a gas turbine engine <NUM>. In this example, the gas turbine engine <NUM> is onboard a vehicle <NUM>, such as an aircraft. In one example, the fuel metering system <NUM> includes a fuel pump <NUM>, a pressure regulator <NUM>, an electrohydraulic servo valve or servo valve <NUM>, a fuel metering valve <NUM>, a bypass valve <NUM> and a combination pressurizing and shutoff valve <NUM>. Generally, as will be discussed, the fuel metering valve <NUM> controls an amount of fuel received by the gas turbine engine <NUM> from the fuel pump <NUM> based on inputs received from the fuel pump <NUM>, the pressure regulator <NUM> and the servo valve <NUM>. In this regard, a first predefined relationship between a current supplied to the servo valve <NUM> and a position of the fuel metering valve <NUM> ensures accuracy in a low-flow region or lower power levels of the gas turbine engine <NUM>, and a second predefined relationship between the current supplied to the servo valve <NUM> and position of the fuel metering valve <NUM> ensures sufficient fuel flow when the gas turbine engine <NUM> is operating at higher power levels where accuracy is generally less critical. Thus, the fuel metering system <NUM> and the fuel metering valve <NUM>, as discussed herein, provides sufficient accuracy to satisfy the performance requirements of the gas turbine engine <NUM> without requiring an external position sensor and while operating as an open loop system.

In one example, the gas turbine engine <NUM> is a non-propulsive engine, such as an Auxiliary Power Unit (APU) deployed onboard the vehicle <NUM>, although other arrangements and uses may be provided. For example, the gas turbine engine <NUM> may be in the form of a turboprop gas turbine engine within the vehicle <NUM>. In other embodiments, the gas turbine engine <NUM> may assume the form of an industrial power generator. As the gas turbine engine <NUM> may be any suitable gas turbine engine for use with the fuel metering system <NUM>, the gas turbine engine <NUM> will not be discussed in great detail herein.

Briefly, the gas turbine engine <NUM> includes an intake section <NUM>, a compressor section <NUM>, a combustor section <NUM>, a turbine section <NUM>, and an exhaust section <NUM>. The intake section <NUM> includes an inlet duct for receiving air from a source, such as a source external to the vehicle <NUM>. The compressor section <NUM> includes at least one compressor, which is coupled to a shaft. The rotation of the shaft drives the compressor, which draws in air from the inlet duct of the intake section <NUM>. The compressor raises the pressure of the air and directs majority of the high pressure air into the combustor section <NUM>. In one example, the combustor section <NUM> includes an annular combustor, which receives the compressed air from the compressor, and also receives a flow of fuel from a fuel source <NUM> via the fuel metering valve <NUM>. The fuel and compressed air are mixed within the combustor, and are combusted to produce relatively high-energy combustion gas. The combustor can be any suitable combustor, including, but not limited to can-type combustors, various reverse-flow combustors, various through-flow combustors, and various slinger combustors. The relatively high-energy combustion gas that is generated in the combustor is supplied to the turbine section <NUM>.

The turbine section <NUM> includes a turbine. However, it will be appreciated that the number of turbines, and/or the configurations thereof, may vary. The turbine can comprise one of numerous types of turbines including, but not limited to, a vaned radial turbine, a vaneless radial turbine, and a vaned axial turbine. In this embodiment, the high-temperature combusted air from the combustor section <NUM> expands through and rotates the turbine. The air is then exhausted through the exhaust section <NUM>. As the turbine rotates, it drives equipment in the gas turbine engine <NUM> via a shaft or spool.

The gas turbine engine <NUM> also includes a controller <NUM>, such as a full authority digital engine control (FADEC). The controller <NUM> includes at least one processor 40a and a computer readable storage device or media 40b. The processor 40a can be any custom made or commercially available processor, a central processing unit (CPU), a graphics processing unit (GPU), an auxiliary processor among several processors associated with the controller <NUM>, a semiconductor based microprocessor (in the form of a microchip or chip set), a macroprocessor, any combination thereof, or generally any device for executing instructions. The computer readable storage device or media 40b may include volatile and nonvolatile storage in read-only memory (ROM), random-access memory (RAM), and keep-alive memory (KAM), for example. KAM is a persistent or non-volatile memory that may be used to store various operating variables while the processor 40a is powered down. The computer-readable storage device or media 40b may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by the controller <NUM> in controlling components associated with the gas turbine engine <NUM>, including the fuel metering system <NUM>.

The instructions may include one or more separate programs, each of which comprises an ordered listing of executable instructions for implementing logical functions. The instructions, when executed by the processor 40a, receive and process input signals, perform logic, calculations, methods and/or algorithms for controlling the components of the gas turbine engine <NUM>, including the fuel metering system <NUM>, and generate signals to components of the gas turbine engine <NUM>, including the fuel metering system <NUM> to control power generated by the gas turbine engine <NUM>, as well as to control an amount of fuel supplied by the fuel metering system <NUM> based on the logic, calculations, methods, and/or algorithms. Although only one controller <NUM> is shown in <FIG>, embodiments of the vehicle <NUM> can include any number of controllers <NUM> that communicate over any suitable communication medium or a combination of communication mediums and that cooperate to process the signals received from other systems associated the vehicle <NUM>, perform logic, calculations, methods, and/or algorithms, and generate control signals to control features of the fuel metering system <NUM> and the gas turbine engine <NUM>.

In various embodiments, one or more instructions of the controller <NUM> are associated with the fuel metering system <NUM> and, when executed by the processor 40a, the instructions receive and process signals from a human-machine interface, for example, to determine an amount of fuel needed for the gas turbine engine <NUM>. For example, the instructions of the controller <NUM>, when executed by the processor 40a, determine an amount of current to provide the fuel metering system <NUM> to result in a predefined amount of fuel for the gas turbine engine <NUM>.

The fuel pump <NUM> is fluidly coupled to the fuel source <NUM>. In one example, the fuel source <NUM> is one or more onboard fuel tanks associated with the vehicle <NUM>. The fuel pump <NUM> draws fuel <NUM> through a fuel pump inlet 16a from the fuel source <NUM> and pressurizes the fuel <NUM> to provide a supply of a high pressure fuel <NUM>, which is fluid or fuel at a first pressure. In one example, the fuel pump <NUM> is a two-stage device, with a low-pressure centrifugal boost stage followed by a high-pressure positive displacement gear stage, which pressurizes the fuel <NUM> to more than a gauge pressure relative to the atmospheric pressure of <NUM> MPa (<NUM> pounds per square in gauge (psig)). The fuel pump <NUM> is fluidly coupled upstream from the fuel metering valve <NUM>. The fuel pump <NUM> provides the fuel metering valve <NUM> with the high pressure fuel <NUM> from a fuel pump outlet 16b to a first metering valve inlet <NUM> of the fuel metering valve <NUM>. The fuel pump <NUM> is also fluidly coupled upstream from the pressure regulator <NUM>. The fuel pump <NUM> provides the pressure regulator <NUM> with the high pressure fuel <NUM> from the fuel pump outlet 16b to a high pressure regulator inlet <NUM> of the pressure regulator <NUM>.

The fuel pump <NUM> is also fluidly coupled to the fuel metering valve <NUM> to receive a low pressure return or bleed fuel <NUM> from a first metering valve outlet <NUM> of the fuel metering valve <NUM>. The low pressure bleed fuel <NUM> is fluid or fuel at a second pressure, which is different and less than the fluid or fuel at the first pressure (i.e. the high pressure fuel <NUM>). In one example, the fuel pump <NUM> has a discharge 16c of the low-pressure centrifugal boost stage that is fluidly coupled downstream of the first metering valve outlet <NUM> of the fuel metering valve <NUM> to receive the low pressure bleed fuel <NUM>. The fuel pump <NUM> may mix the low pressure bleed fuel <NUM> with the discharge 16c of the low-pressure centrifugal boost stage of the fuel pump <NUM> for pressurizing into the high pressure fuel <NUM>.

The bypass valve <NUM> regulates the pressure differential across the fuel metering valve <NUM> and returns unmetered fuel to the low pressure fuel pump inlet 16c of the fuel. The bypass valve <NUM> comprises any suitable known bypass valve for use with the fuel metering system <NUM>. The bypass valve <NUM> is downstream of the fuel pump <NUM> and upstream from the fuel metering valve <NUM>. In one example, the bypass valve <NUM> is fluidly coupled between the fuel pump <NUM> and the fuel metering valve <NUM> to control an amount of the high pressure fuel <NUM> received at the fuel metering valve <NUM>. In one example, the bypass valve <NUM> is also fluidly coupled to the fuel pump <NUM> to return excess fuel to the fuel pump <NUM>. In this regard, in the instance where the fuel pump <NUM> provides more high pressure fuel <NUM> than needed at the fuel metering valve <NUM>, the bypass valve <NUM> reroutes the excess fuel back to the fuel pump <NUM>. Thus, the bypass valve <NUM> ensures that the fuel metering valve <NUM> receives the portion of the high pressure fuel <NUM> needed for the gas turbine engine <NUM>, while maintaining the pressure differential across the fuel metering valve <NUM>.

The combination pressurizing and shutoff valve <NUM> is downstream of the fuel metering valve <NUM> and upstream from the gas turbine engine <NUM>. The combination pressurizing and shutoff valve <NUM> allows or cuts off fuel flow to the combustor. The combination pressurizing and shutoff valve <NUM> comprises any suitable known combination pressurizing and shutoff valve for use with the fuel metering system <NUM>. In one example, the combination pressurizing and shutoff valve <NUM> includes a valve body that is spring biased, such that the valve body of the combination pressurizing and shutoff valve <NUM> is maintained by the spring in a closed position until the fuel pressure within the fuel metering system <NUM> reaches a minimum pressure. Once the fuel metering system <NUM> reaches the minimum pressure, the valve body of the combination pressurizing and shutoff valve <NUM> overcomes the force of the spring to move into an opened position to enable fuel flow to the gas turbine engine <NUM>. Thus, the combination pressurizing and shutoff valve <NUM> sets the minimum pressure for the fuel metering system <NUM> and enables the cut off of flow to the combustor based on the pressure within the fuel metering system <NUM>.

The pressure regulator <NUM> is fluidly coupled to the fuel pump <NUM> to receive the high pressure fuel <NUM> at the high pressure regulator inlet <NUM>. With reference to <FIG>, the pressure regulator <NUM> is shown in greater detail. In one example, the pressure regulator <NUM> includes a valve body <NUM>, a biasing member <NUM>, the high pressure regulator inlet <NUM>, a second regulator inlet <NUM> and a regulator outlet <NUM>. The valve body <NUM> is movably or slidably disposed in a mating sleeve <NUM> between a first regulator control chamber <NUM> and a second regulator control chamber <NUM>. The mating sleeve <NUM> is positioned within a stationary housing that is fluidly coupled to the fuel pump <NUM>, the fuel metering valve <NUM> and the servo valve <NUM>. In this example, the valve body <NUM> is a piston style, and includes a first head <NUM> opposite a second head <NUM>. The first head <NUM> is interconnected with the second head <NUM> via a valve stem <NUM>. The first head <NUM> may include a stop flange 62a, which has a diameter that is different and greater than a diameter of the mating sleeve <NUM> to provide a stop for the travel of the valve body <NUM> relative to the mating sleeve <NUM>. The first head <NUM> is in communication with the first regulator control chamber <NUM>. The first regulator control chamber <NUM> receives the low pressure bleed fuel <NUM> from a low pressure circuit <NUM>, which is at an outlet pressure P0, via the second regulator inlet <NUM>.

The second head <NUM> is fluidly coupled to the servo valve <NUM> and to the regulator outlet <NUM>. The second head <NUM> is in communication with the second regulator control chamber <NUM>. The second head <NUM> receives servo supply fuel <NUM>, which is at a servo supply pressure Pr. The second regulator control chamber <NUM> also receives the servo supply fuel <NUM>. The servo supply fuel <NUM> is a portion of the high pressure fuel <NUM> that flows through the pressure regulator <NUM> based on a position of the valve body <NUM>. The high pressure fuel <NUM> is at an inlet pressure P1.

The biasing member <NUM>, in this example, is a spring. The biasing member <NUM> is coupled between the first head <NUM> and a stationary ground point coincident with the stationary housing of the pressure regulator <NUM>. In one example, the biasing member <NUM> may be coupled between the first head <NUM> and a cover that is threadably attached to the stationary housing of the pressure regulator <NUM>. The biasing member <NUM> acts on or applies a force to the first head <NUM> and biases the pressure regulator <NUM> toward a second end 59b of the mating sleeve <NUM>. Generally, the pressure regulator <NUM> acts to restrict a flowpath through the fuel metering valve <NUM> to maintain a constant pressure differential for the servo valve <NUM>. The valve body <NUM> is movable within the mating sleeve <NUM> against a force of the biasing member <NUM> based on a magnitude of the pressure of the servo supply fuel <NUM> discharged from the pressure regulator <NUM> and the low pressure bleed fuel <NUM> that flows through the second regulator inlet <NUM>. Stated another way, the valve body <NUM> is movable against the biasing member <NUM> based on a pressure differential between the servo supply fuel <NUM> discharged from the pressure regulator <NUM> and the low pressure bleed fuel <NUM>, and maintains a substantially constant pressure differential between the two.

The servo valve <NUM> has a servo inlet <NUM>, a servo outlet <NUM> and a body or flapper <NUM>. In this example, the servo valve <NUM> is a single stage electrohydraulic servo valve, which is in communication with the controller <NUM> (<FIG>) over a suitable communication medium, such as a bus, to receive a control signal <NUM>. The servo inlet <NUM> is fluidly coupled to the regulator outlet <NUM> of the pressure regulator <NUM>, and receives the servo supply fuel <NUM> at the pressure Pr. The servo outlet <NUM> is in fluid communication with the fuel metering valve <NUM>. In one example, the servo outlet <NUM> includes a nozzle <NUM>. The servo outlet <NUM> provides the servo supply fuel <NUM> to the fuel metering valve <NUM> to provide a control fuel <NUM> at a control pressure Px to the fuel metering valve <NUM>.

The flapper <NUM> is positioned within a servo chamber <NUM> so as to obstruct or seal the servo outlet <NUM> in a first state, and is movable relative to the servo outlet <NUM> to a second state, in which the servo outlet <NUM> is substantially unobstructed or fully open. The flapper <NUM> is also movable to positions between the first state and the second state by the servo valve <NUM>. The flapper <NUM> is movable by the servo valve <NUM> relative to the servo outlet <NUM> based on the control signal <NUM> received from the controller <NUM> (<FIG>). In this example, the control signal <NUM> is a current that is supplied by the controller <NUM> (<FIG>) from a power source onboard the vehicle <NUM> to the servo valve <NUM>. The power source may comprise any suitable current source associated with the vehicle <NUM> that is capable of supplying a current to the servo valve <NUM>. As will be discussed, the servo valve <NUM> moves the flapper <NUM> relative to the servo outlet <NUM> based on an amount of a predefined total current. In this example, the flapper <NUM> obstructs the servo outlet <NUM> in the first state when the amount of current received is <NUM>% of the total current such that the fuel metering valve <NUM> is in a first, closed position; and the servo outlet <NUM> is substantially unobstructed or least obstructed by the flapper <NUM> in the second state when the amount of current received is at <NUM>% of the total current. In the second state of the flapper <NUM>, the fuel metering valve <NUM> is in a second, opened position. The flapper <NUM>, and thus, the fuel metering valve <NUM> are movable to positions between the first, closed position and the second, opened position based on the amount of current received by the servo valve <NUM>. Thus, as will be discussed, an amount of fuel going to the gas turbine engine <NUM> from the fuel metering valve <NUM> is also based on the amount of the current supplied to the servo valve <NUM> from the controller <NUM> (<FIG>).

With brief reference to <FIG>, the fuel metering valve <NUM> is fluidly coupled to the fuel pump <NUM> to receive the high pressure fuel <NUM>. The fuel metering valve <NUM> is also fluidly coupled to the gas turbine engine <NUM> to provide the gas turbine engine <NUM> with a metered portion of the high pressure fuel <NUM>. The fuel metering valve <NUM> is fluidly coupled to the servo valve <NUM> to receive the control fuel <NUM>, and is fluidly coupled to the low pressure circuit <NUM> to receive the low pressure bleed fuel <NUM>. With reference to <FIG>, the fuel metering valve <NUM> includes the first metering valve inlet <NUM>, the second metering valve inlet <NUM>, a third metering valve inlet <NUM>, a first metering valve outlet <NUM>, and a second metering valve outlet <NUM>. The first metering valve inlet <NUM> is fluidly coupled to the fuel pump <NUM> (<FIG>) to receive the high pressure fuel <NUM> at the pressure P1. The high pressure fuel <NUM> enters the fuel metering valve <NUM> via the first metering valve inlet <NUM>.

The second metering valve outlet <NUM> is the main metering flow window, which has a tightly controlled shape, that opens up a specific flow area as a function of an axial position of a metering valve body <NUM>. In one example, the second metering valve outlet <NUM> has a triangular or exponential (trumpet-like) shape. The first metering valve outlet <NUM> includes a conduit <NUM> having an inlet <NUM> defined at the first metering valve outlet <NUM>, a first conduit outlet <NUM> and a second conduit outlet <NUM>. In this example, a fixed flow restriction <NUM> is defined within the conduit <NUM> so as to be downstream of the first metering valve outlet <NUM> and upstream of each of the first conduit outlet <NUM> and the second conduit outlet <NUM>. In one example, the fixed flow restriction <NUM> is a narrowing of the conduit <NUM> such that the conduit <NUM> has a first diameter D3 upstream and downstream of the fixed flow restriction <NUM>, and a second diameter D4 at the fixed flow restriction <NUM>, with the first diameter D3 different, and in this example, greater than the second diameter D4. The fixed flow restriction <NUM> reduces a pressure P0' of the low pressure bleed fuel <NUM> that is received through the first metering valve outlet <NUM>. After passing through the fixed flow restriction <NUM>, the low pressure bleed fuel <NUM> at a pressure P0 flows through the first conduit outlet <NUM> and the second conduit outlet <NUM>. The first conduit outlet <NUM> is fluidly coupled to a first metering control chamber <NUM> to provide the low pressure bleed fuel <NUM> to the first metering control chamber <NUM> and is fluidly coupled to the first regulator control chamber <NUM> to provide the low pressure bleed fuel <NUM> to the first regulator control chamber <NUM>. The second conduit outlet <NUM> is fluidly coupled to the fuel pump <NUM> (<FIG>) to return the low pressure bleed fuel <NUM> to the discharge 16c of the low-pressure centrifugal boost stage of the fuel pump <NUM>. The first conduit outlet <NUM> and the second conduit outlet <NUM> define the low pressure circuit <NUM> that receives and supplies the low pressure bleed fuel <NUM> to the pressure regulator <NUM>, the fuel metering valve <NUM> and to return the low pressure bleed fuel <NUM> to the discharge 16c of the low-pressure centrifugal boost stage of the fuel pump <NUM>.

The second metering valve outlet <NUM> is downstream of the first metering valve inlet <NUM>. The high pressure fuel <NUM> flows through the fuel metering valve <NUM> from the first metering valve inlet <NUM> to the second metering valve outlet <NUM>. The second metering valve outlet <NUM> is fluidly coupled to the combination pressurizing and shutoff valve <NUM>, which in turn, is fluidly coupled to the gas turbine engine <NUM> to provide the gas turbine engine <NUM> with the high pressure fuel <NUM>. Generally, the high pressure fuel <NUM> at pressure P1 enters the fuel metering valve <NUM> via the first metering valve inlet <NUM> and flows through the fuel metering valve <NUM> to the second metering valve outlet <NUM> at pressure P2. The P1-P2 pressure differential is the differential across the metering window or the second metering valve outlet <NUM> and is maintained essentially constant by the bypass valve <NUM>.

The second metering valve inlet <NUM> is fluidly coupled to the servo chamber <NUM> to receive the control fuel <NUM> based on the position of the flapper <NUM>. The third metering valve inlet <NUM> is fluidly coupled to the first metering control chamber <NUM> and is fluidly coupled to the low pressure circuit <NUM> to receive the low pressure bleed fuel <NUM>. The third metering valve inlet <NUM> supplies the low pressure bleed fuel <NUM> to the first metering control chamber <NUM>. Thus, the first regulator control chamber <NUM> and the first metering control chamber <NUM> are supplied with the low pressure bleed fuel <NUM> from the low pressure circuit <NUM> in parallel.

In this example, the fuel metering valve <NUM> includes a housing <NUM>, which defines a bore <NUM> and also includes a valve sleeve <NUM> that receives a metering valve body <NUM>. The metering valve body <NUM> is responsive to the first metering control chamber <NUM> and a second metering control chamber <NUM> to move or slide within the valve sleeve <NUM> to control an amount of the high pressure fuel <NUM> that is supplied to the second metering valve outlet <NUM>; and to control an amount of the control fuel <NUM> that is supplied as the low pressure bleed fuel <NUM> to the first metering valve outlet <NUM>. In this example, the fuel metering valve <NUM> also includes a biasing member <NUM>, which acts on or applies a force to the metering valve body <NUM>. The valve sleeve <NUM> has a first sleeve end <NUM> opposite a second sleeve end <NUM>. The first metering valve inlet <NUM>, the second metering valve inlet <NUM>, a third metering valve inlet <NUM> and a second metering valve outlet <NUM> are each defined in the valve sleeve <NUM>. The housing <NUM> is stationary, and includes fluid conduits, for fluidly coupling the first metering valve inlet <NUM>, the second metering valve inlet <NUM>, the third metering valve inlet <NUM>, the low pressure circuit <NUM>, the first metering valve outlet <NUM> and the second metering valve outlet <NUM> to the respective one of the fuel pump <NUM>, the servo valve <NUM> and the combination pressurizing and shutoff valve <NUM>. The housing <NUM> may also define an annulus about the valve sleeve <NUM> at the first metering valve inlet <NUM> and the second metering valve outlet <NUM> to facilitate the flow of the high pressure fuel <NUM> through the valve sleeve <NUM> at the first metering valve inlet <NUM> and the second metering valve outlet <NUM>. The first metering valve inlet <NUM> and the second metering valve outlet <NUM> are each fluidly coupled to the valve sleeve <NUM> between the first sleeve end <NUM> and the second sleeve end <NUM>. The second metering valve inlet <NUM> is fluidly coupled to a second end 112b of the bore <NUM> to be in fluid communication with the valve sleeve <NUM>. The second end 112b is opposite a first end 112a. The third metering valve inlet <NUM> is fluidly coupled to the valve sleeve <NUM> at the first sleeve end <NUM>. The first metering valve outlet <NUM> is fluidly coupled to a slot <NUM> defined in the valve sleeve <NUM>.

In this regard, with reference to <FIG>, the slot <NUM> is defined in the valve sleeve <NUM> between the first sleeve end <NUM> and the second sleeve end <NUM>. In this example, the slot <NUM> is defined a distance Ds from the second sleeve end <NUM>. The distance Ds is predetermined such that the slot <NUM> is opened coincident with or just before the point at which second metering valve outlet <NUM> is opened by the movement of the metering valve body <NUM>. Opening the slot <NUM> before the second metering valve outlet <NUM> opens ensures that the second metering valve outlet <NUM> is closed when zero current is applied to the servo valve <NUM>. Thus, in this example, the slot <NUM> is defined proximate or adjacent to the second sleeve end <NUM>. The slot <NUM> is also defined proximate or adjacent to the first metering valve outlet <NUM>. The slot <NUM> extends for a length Sl that is predetermined to be greater than the total stroke of the metering valve body <NUM>, such that the flow area of the slot <NUM> is continuously modulated throughout the full stroke range of the metering valve body <NUM>. A width Sw of the slot <NUM> is predetermined as a function of the flow capacity of the servo valve <NUM> and the size of the fixed flow restriction <NUM> to produce the predetermined or predefined flow vs current relationship shown in <FIG>. To minimize flow through the flow path defined by the first metering valve outlet <NUM>, the width Sw of the slot <NUM> may be set to the minimum limit of manufacturing capability, on the order of <NUM> (<NUM> inches), if desired.

Generally, the slot <NUM> is defined so as to be in fluid communication with the second metering control chamber <NUM> and in fluid communication with the first metering valve outlet <NUM>. The slot <NUM> has a cross-sectional area A that is different and less than a cross-sectional area A1 of the second metering control chamber <NUM>. The cross-sectional area A of the slot <NUM> is also different, and greater than, a cross-sectional area A2 (<FIG>) of the fixed flow restriction <NUM>. The slot <NUM> is also fluidly coupled to an annulus defined in the housing <NUM>, and the annulus fluidly couples the slot <NUM> to the first metering valve outlet <NUM> and the conduit <NUM> upstream from the fixed flow restriction <NUM>. As will be discussed, a movement of the metering valve body <NUM> axially relative to the valve sleeve <NUM> against the biasing member <NUM> opens and closes the slot <NUM>, which cooperates with the metering valve body <NUM> to control an amount of high pressure fuel <NUM> flowing to the gas turbine engine <NUM> (<FIG>) when a low current is supplied to the servo valve <NUM> and an amount of the control fuel <NUM> that flows to the first metering valve outlet <NUM>. The valve sleeve <NUM> may include one or more sealing members 114a, such as elastomeric O-rings, etc., to inhibit the flow of fuel between the valve sleeve <NUM> and the bore <NUM>.

With reference back to <FIG>, the metering valve body <NUM> is movably or slidably disposed in the valve sleeve <NUM> between the first metering control chamber <NUM> and the second metering control chamber <NUM>. In this example, the metering valve body <NUM> is a piston style, and includes a first metering head <NUM> opposite a second metering head <NUM>. The first metering head <NUM> is interconnected with the second metering head <NUM> via a valve stem <NUM>. The first metering head <NUM> may include a stop flange 140a, which has a diameter that is different and greater than a diameter of the valve sleeve <NUM> to provide a stop for the travel of the metering valve body <NUM> relative to the valve sleeve <NUM>. Generally, when the stop flange 140a contacts the valve sleeve <NUM>, the slot <NUM> is closed by the metering valve body <NUM>. The low pressure bleed fuel <NUM> in the first metering control chamber <NUM> applies a fluid pressure that acts on the first metering head <NUM>; and the control fuel <NUM> in the second metering control chamber <NUM> applies a fluid pressure that acts on the second metering head <NUM>. The second metering head <NUM> is movable relative to the valve sleeve <NUM> against the force of the biasing member <NUM> to expose the slot <NUM> (<FIG>). The slot <NUM> variably restricts the flow of the control fuel <NUM> through the first metering valve outlet <NUM> to modify the fluid pressure exerted on the metering valve body <NUM> to balance the force applied by the biasing member <NUM> on the metering valve body <NUM>.

The biasing member <NUM>, in this example, is a spring. The biasing member <NUM> is coupled between the first metering head <NUM> and a stationary ground point coincident with the stationary housing <NUM> of the fuel metering valve <NUM>. In one example, the biasing member <NUM> may be coupled between the first metering head <NUM> and a cover that is threadably attached to the housing <NUM> of the fuel metering valve <NUM>. The biasing member <NUM> acts on or applies the force to the first metering head <NUM> and biases the fuel metering valve <NUM> toward the second end 112b of the bore <NUM>. Generally, the biasing member <NUM> and the biasing member <NUM> are referenced to the same pressure P0 or the low pressure bleed fuel <NUM> received from the low pressure circuit <NUM>.

Generally, the metering valve body <NUM> is movable against a force of the biasing member <NUM> to expose or open the slot <NUM> based on a current applied to the servo valve <NUM>. In this regard, the metering valve body <NUM> is movable against the biasing member <NUM> based on a fluid pressure differential between the control fuel <NUM> at the control pressure Px and the low pressure bleed fuel <NUM> at the pressure P0. As the amount of current supplied by the controller <NUM> (<FIG>) via the control signal <NUM> to the servo valve <NUM> increases, the servo valve <NUM> moves the flapper <NUM> to increase an amount of the control fuel <NUM> entering through the nozzle <NUM>. As the current is increased, the flow through the nozzle <NUM> of the flapper <NUM> is increased, which increases the control pressure Px. The higher control pressure Px causes the metering valve body <NUM> to move against the biasing member <NUM>, opening the slot <NUM> (<FIG>). The opening of the slot <NUM> dumps or reduces the control pressure Px, and the metering valve body <NUM> stops at a new, more open, position once equilibrium is achieved. Therefore, for every current applied to the servo valve <NUM> there is a corresponding position of the fuel metering valve <NUM>, and the combination of the slot <NUM> and the fixed flow restriction <NUM>, in concert with the variable flow through the nozzle <NUM> of the flapper <NUM>, creates the characteristic curve shown in <FIG>.

In one example, with reference to <FIG>, a graph of an applied current to the servo valve <NUM> versus a position of the metering valve body <NUM> of the fuel metering valve <NUM> is shown. In this example, the applied current is on an x-axis <NUM>, and the position of the metering valve body <NUM> of the fuel metering valve <NUM> is on a y-axis <NUM>. In this example, the position of the metering valve body <NUM> relative to the second metering valve outlet <NUM> is based on the current supplied to the servo valve <NUM> by the controller <NUM> (<FIG>). As shown, in a first current range <NUM>, from about <NUM>% to about <NUM>% of total current, the relationship between an exposed cross-sectional area A of the slot <NUM> to the cross-sectional area A2 of the fixed flow restriction <NUM> results in a first relationship <NUM> between the position of the metering valve body <NUM> and the current supplied to the servo valve <NUM>. In the first current range <NUM>, the slope of the curve of the position of the metering valve body <NUM> relative to the current supplied to the servo valve <NUM> is shallow, and the exposed cross-sectional area A of the slot <NUM> (<FIG>) dominates this position relationship. In a second current range <NUM>, from about <NUM>% to about <NUM>% of total current, the relationship between the exposed cross-sectional area A of the slot <NUM> to the cross-sectional area A2 of the fixed flow restriction <NUM> results in a second relationship <NUM> between the position of the metering valve body <NUM> and the current supplied to the servo valve <NUM>. In the second current range <NUM>, the slope of the curve of the position of the metering valve body <NUM> relative to the current supplied to the servo valve <NUM> is steeper, and the cross-sectional area A2 of the fixed flow restriction <NUM> (<FIG>) dominates this position relationship.

Thus, in the first current range <NUM>, the first relationship <NUM> ensures the accuracy of the high pressure fuel <NUM> supplied by the fuel metering valve <NUM> to the gas turbine engine <NUM> (<FIG>) when low fuel quantities are needed such as at start-up of the gas turbine engine <NUM> (<FIG>). In the second current range <NUM>, shown in FIG. 2A, the second relationship <NUM> provides a higher flow capacity for the high pressure fuel <NUM> through the fuel metering valve <NUM> when larger quantities of fuel are needed outside of start-up, during normal operation of the gas turbine engine <NUM> (<FIG>), for example. Generally, when it is desired to provide a higher volume of fuel to the gas turbine engine <NUM> (<FIG>) in the second current range <NUM>, the accuracy of the provided volume of fuel may not be as critical.

Thus, with reference to <FIG>, the fuel metering system <NUM> is an open-loop system, which eliminates the need for a position sensor as the slot <NUM> (<FIG>) defined in the valve sleeve <NUM> cooperates with the fixed flow restriction <NUM> to define a metering valve position relationship as shown in <FIG>. In this regard, as the flow area through the servo outlet <NUM> increases with current through the movement of the flapper <NUM>, the control fuel <NUM> supplied to the second metering control chamber <NUM> increases, which causes fluid pressure to increase and move the metering valve body <NUM> against the force of the biasing member <NUM> while also opening the slot <NUM> (FIG. The opening of the slot <NUM> reduces the pressure within the second metering control chamber <NUM>. The movement of the metering valve body <NUM> stops when the flow of the control fuel <NUM> and the force of the biasing member <NUM> balance or an equilibrium state is achieved. The result is that for every current applied to the servo valve <NUM> by the controller <NUM> (<FIG>) there is a corresponding position of the metering valve body <NUM>. In addition, the fixed flow restriction <NUM> positioned downstream of the slot <NUM> creates a "knee" in the relationship or curve of the applied current versus position of the metering valve body <NUM> shown in <FIG>. Without the fixed flow restriction <NUM>, this curve would be substantially linear. At a low percentage of the total current applied, the cross-sectional area A of the slot <NUM> is small in comparison to the cross-sectional area A2 of the fixed flow restriction <NUM> (<FIG>), creating a low slope region in the curve, dominated by the cross-sectional area A of the slot <NUM>. At a high percentage of the total current applied, the cross-sectional area A2 of the fixed flow restriction <NUM> is relatively smaller in comparison with the cross-sectional area A of the slot <NUM>, creating the higher slope region. In a typical embodiment, the low slope region would be such that <NUM>% of the maximum metered flow level provided by the fuel metering valve <NUM> would be reached at <NUM>% of the maximum current of the servo valve <NUM>, with the corresponding high flow region configured to reach <NUM>% metered flow provided by the fuel metering valve <NUM> at <NUM>% or maximum current of the servo valve <NUM>.

Claim 1:
A fuel metering system (<NUM>) for a gas turbine engine (<NUM>), comprising:
a metering valve (<NUM>) having a first inlet (<NUM>) fluidly coupled to a source of fluid (<NUM>) at a first pressure, a second inlet (<NUM>), a second outlet (<NUM>) to be coupled to the gas turbine engine and a first outlet (<NUM>), the metering valve including a slot (<NUM>) defined proximate to the first outlet (<NUM>), a valve body (<NUM>) movable to control an amount of fluid supplied to the first outlet and to the second outlet, and a biasing member (<NUM>) that applies a force to the valve body, the metering valve (<NUM>) including a valve sleeve (<NUM>) that receives the valve body (<NUM>), wherein the slot (<NUM>) is defined in the valve sleeve (<NUM>);
a servo valve (<NUM>) fluidly coupled to the second inlet of the metering valve and to a second source of fluid (<NUM>) at a second pressure, the servo valve in fluid communication with the second inlet and a body (<NUM>) of the servo valve is movable relative to the second inlet to supply a fluid from the second source of fluid to apply a fluid pressure to move the valve body, and the slot is configured to variably restrict the flow of the fluid through the first outlet (<NUM>) of the metering valve to modify the fluid pressure exerted on the valve body to balance the force applied by the biasing member; and
a fixed flow restriction (<NUM>) downstream of the first outlet (<NUM>).