Patent Description:
Fuel supplied to the combustor is provided by a mechanical pump driven by a rotating shaft of the engine. The mechanical pump is reliable and supplies fuel in proportion to engine speed. The minimum capacity of the mechanical pump is sized such that sufficient fuel is provided for high power conditions. Excess fuel not needed is recirculated back to the fuel tank. The fuel is further utilized as a coolant for other systems of the engine. Recirculation of fuel increases the temperature of the fuel and thereby reduces the available capacity to absorb heat from other systems. The capacity of the fuel to absorb heat from other systems is further limited by the characteristics of the fuel. At a certain temperature the fuel begins to degrade and can reduce engine efficiency. Reducing the amount of fuel that is recirculated during engine operation may improve the capacity of the fuel to absorb heat from other systems.

Turbine engine manufacturers continuously seek improvements to engine performance including improvements to thermal, transfer and propulsive efficiencies.

An example of such an improvement is shown in <CIT> which describes a fluid pumping system in which a fluid is pumped from a fluid supply to a load, and two continuously driven fixed displacement pumps are operated in parallel to provide the pumping action. The two pumps are arranged with a common inlet, and with a common outlet, and the flow delivered to the load from one of the two pumps passes through a check valve upstream of the common outlet. The fluid flow is regulated to provide a constant pressure by sequentially bypassing fluid around each pump through separate pump bypass ducts. This sequential fluid bypassing or pump unloading is effected by a single control responsive to the flow requirements of the load necessary to maintain that constant pressure. The pump whose delivered flow outlet contains the check valve is the pump whose flow is bypassed initially when less than full pumping capacity is desired. During maximum flow demand operation, both pumps are delivering against full delivery pressure. During low flow demand operation only the pump without the check valve delivers against full pressure, the other pump being operated with essentially no pressure load.

<CIT> describes a fuel supply which includes a plurality of fuel Pumps connected to said supply and a fuel system delivery communication for receiving fuel under pressure individually discharged by said pumps. Fuel control means operable for regulating flow of fuel under pressure from said delivery communication in the form of biased relief valve devices are provided and interposed between the respective pumps and said delivery communication. The biasing means tends to close each of said relief valve devices. Fluid pressure means cooperative with said biasing means renders said devices selectively operative to effect bypassing of fuel discharged from each pump to the fuel supply, and fluid interlocked control means cooperative with said fluid pressure means for each of said devices for controlling successive operation thereof in accordance with variations in back pressure of fuel in said fuel system delivery communication established during regulation of said fuel control means.

<CIT> describes a structural unit for an aircraft engine having at least one fuel pump of a fuel circuit and at least one hydraulic fluid pump of a hydraulic fluid circuit, where the structural unit can be coupled to an accessory gearbox shaft of an accessory gearbox of the engine.

<CIT> describes a fuel pumping unit which has a low pressure centrifugal pump and a high pressure centrifugal pump. In use, the low pressure pump supplies fuel at a boosted pressure to the high pressure pump for onward supply to a fuel metering unit. The pumping unit further has a drive input which drives the low and high pressure pumps. A gear arrangement is operatively located between the drive input and the low and high pressure pumps such that the low and high pressure pumps are driven at different speeds by the drive input.

<CIT> describes a fuel feed circuit for an aeroengine, the circuit including a high-pressure pumping system including first and second positive displacement pumps, a hydraulic actuator, and a fuel metering unit. As a function of a position of a slide of the actuator, a feed orifice of the actuator may be connected to a high-pressure delivery orifice connected to an outlet of the second pump, or to a low-pressure delivery orifice connected to a low-pressure feed line. The fuel metering unit includes through sections, one of these through sections being connected to an outlet of the high-pressure pumping system and the other through section being connected to an outlet of the high-pressure pumping system and leading to a high-pressure pilot chamber of the hydraulic actuator.

A fuel system for a gas turbine engine according to an aspect of the present invention is defined in claim <NUM> and comprises, an accessory gearbox driven by a mechanical link to the gas turbine engine, a primary fuel pump providing a first fuel flow during engine operation, and a secondary fuel pump providing a second fuel flow. The primary fuel pump and the secondary fuel pump are driven by an output of the accessory gearbox. A first control valve is upstream of the secondary fuel pump and a second control valve is downstream of the secondary fuel pump. The first control valve and the second control valve controlling communication of fuel to and from the secondary fuel pump. The first fuel pump and the second fuel pump both receive fuel flow from a common inlet passage. Both the first fuel pump and the second fuel pump communicate the corresponding one of the first fuel flow and the second fuel flow to a common outlet passage.

In an embodiment not forming part of the invention the fuel system comprises a pump drive gearbox which is selectively coupled to drive the secondary fuel pump by a clutch means.

In an embodiment not forming part of the invention the fuel system comprises, a first pressure relief valve in which the first pressure relief valve is for switching the primary fuel pump and the secondary fuel pump between a series arrangement, where the first fuel flow is provided by both the primary and secondary fuel pumps. A parallel arrangement is included where the first fuel flow is provided by the primary fuel pump and the secondary fuel flow is provided by the secondary fuel pump.

In an embodiment not forming part of the invention the the first pressure relief valve is disposed between an outlet of the primary fuel pump and an inlet of the secondary fuel pump. The first pressure relief valve opens to communicate fuel from the primary fuel pump to the secondary fuel pump to provide the first fuel flow in a first operating condition. The first pressure relief valve closes such that the secondary fuel pump provides the second fuel flow in parallel with the first fuel flow provided by the primary mechanical fuel pump to a common fuel passage in a second operating condition.

In an embodiment not forming part of the invention the fuel system comprises, a first check valve, in which the first check valve is in a first passage downstream of the primary mechanical fuel pump to control fuel flow from the first passage into the common fuel passage. A second check valve is in a second passage communicating fuel to an inlet of the secondary fuel pump.

In an embodiment not forming part of the invention the fuel system comprises, a second pressure relief valve in which the second pressure relief valve is downstream of both the primary fuel pump and the secondary fuel pump for directing fuel flow away from the common fuel passage in response to a pressure within the common fuel passage above a predefined pressure.

In an embodiment of any of the above embodiments, a flow capacity of the primary fuel pump and the secondary fuel pump are different.

In an embodiment of any of the above embodiments, a flow capacity of the primary fuel pump and the secondary fuel pump are the same.

A gas turbine engine according to a further aspect of the present invention comprises, a fan rotatable within a fan nacelle, and a core engine which includes a compressor communicating compressed air to a combustor where compressed air is mixed with fuel and ignited to generate a high-energy gas flow expanded through a turbine and a fuel system as claimed in claim <NUM>.

A method of supplying fuel to a combustor of a gas turbine engine according to an aspect of the present invention is defined in claim <NUM> and comprises, operating a primary fuel pump to provide a first fuel flow, and operating a secondary fuel pump to provide a second fuel flow. Operating the primary fuel pump and the secondary fuel pump comprises driving the primary fuel pump and the secondary fuel pump with an output from an accessory gearbox. The first fuel flow is communicated to a combustor of the gas turbine engine in a first operating condition and communicating the first fuel flow and the second fuel flow to the combustor in a second operating condition. The operation of a first control valve (<NUM>) upstream of the secondary fuel pump (<NUM>) and a second control valve (<NUM>) downstream of the secondary fuel pump (<NUM>) is used to determine first or second operating condition for the supply of fuel to the combustor (<NUM>) of the gas turbine engine (<NUM>).

In a method not forming part of the invention the first fuel flow communicated to the combustor comprises directing fuel from an outlet of the primary fuel pump to an inlet of the secondary fuel pump in the first operating condition. Communicating both the first fuel flow and the second fuel flow comprises blocking fuel flow from the outlet of the primary fuel pump to the inlet of the secondary fuel pump. Fuel from a fuel source is communicated to the inlet of the secondary fuel pump and both the first fuel flow from the primary pump and the secondary fuel flow from the secondary pump is routed to a common fuel outlet passage.

In an embodiment not forming part of the invention of the above method of supplying fuel to a combustor of a gas turbine engine comprises communicating the first fuel flow to the combustor, and further comprises flowing fuel from primary fuel pump to a common fuel outlet passage and blocking flow from the secondary fuel pump during the first engine operating condition. Communicating the first fuel flow and the second fuel flow to the combustor in the second operating condition comprises communicating both the first fuel flow from the primary fuel pump and the second fuel flow from the secondary fuel pump to the common fuel outlet passage.

Although depicted as a two-spool turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with two-spool turbofans as the teachings may be applied to other types of turbine engines including but not limited to three-spool architectures.

It should be understood that the various bearing systems <NUM> may alternatively or additionally be provided at different locations, and the location of bearing systems <NUM> may be varied as appropriate to the application.

The inner shaft <NUM> is connected to a fan section <NUM> through a speed change mechanism, which in exemplary gas turbine engine <NUM> is illustrated as a geared architecture <NUM> to drive fan blades <NUM> at a lower speed than the low speed spool <NUM>.

For example, gear system <NUM> may be located aft of the low pressure compressor <NUM> and the fan blades <NUM> may be positioned forward or aft of the location of the geared architecture <NUM> or even aft of turbine section <NUM>.

The low pressure turbine <NUM> pressure ratio is pressure measured prior to the inlet of low pressure turbine <NUM> as related to the pressure at the outlet of the low pressure turbine <NUM> prior to an exhaust nozzle. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present invention is applicable to other gas turbine engines including but not limited to direct drive turbofans.

"Low corrected fan tip speed" is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram °R) / (<NUM> °R)] <NUM> (where °R = K x <NUM>/<NUM>).

The example gas turbine engine includes the fan section <NUM> that comprises in one non-limiting embodiment less than about <NUM> fan blades <NUM>. In another non-limiting embodiment, the fan section <NUM> includes less than about <NUM> fan blades <NUM>. Moreover, in one disclosed embodiment the low pressure turbine <NUM> includes no more than about <NUM> turbine rotors schematically indicated at <NUM>. In another non-limiting example embodiment, the low pressure turbine <NUM> includes about <NUM> turbine rotors. A ratio between the number of fan blades <NUM> and the number of low pressure turbine rotors is between about <NUM> and about <NUM>. The example low pressure turbine <NUM> provides the driving power to rotate the fan section <NUM> and therefore the relationship between the number of turbine rotors <NUM> in the low pressure turbine <NUM> and the number of blades <NUM> in the fan section <NUM> disclose an example gas turbine engine <NUM> with increased power transfer efficiency.

Fuel is delivered to the combustor <NUM> by a fuel system <NUM>. The example fuel system <NUM> includes a primary system <NUM> and a secondary system <NUM>. Fuel from a fuel tank <NUM> is pumped to a desired pressure and provided to the combustor <NUM>. The disclosed fuel system <NUM> tailors a flow of fuel to the combustor <NUM> based on engine operating conditions. Instead of simply providing a fuel flow that provides for extremes of operating demands, the disclosed fuel system <NUM> tailors the flow of fuel according to a demand for fuel. By tailoring the flow of fuel to engine operating demand, fuel directed through a fuel recirculation loop for excess fuel can be reduced and/or eliminated.

Fuel is utilized as a heat sink to cool other flows within the engine such as lubricant and air flows. In this example, a heat fuel/oil heat exchanger <NUM> cools a flow of lubricant generated by a lubricant system <NUM>. Recirculation of fuel results in an increased temperature of the fuel and thereby a reduced capability to accept heat from other engine systems, such as the example lubricant system <NUM>.

The disclosed fuel system <NUM> varies the flow of fuel based on demand to reduce and/or eliminate the recirculation of fuel and thereby increase the ability to accept heat from other engine systems. The disclosed fuel system <NUM> includes mechanically driven pumps to provide a reliable and robust fuel system <NUM>. The example fuel system <NUM> is driven by outputs from an accessory gearbox <NUM>. The accessory gearbox <NUM> is in turn driven by a shaft of the gas turbine engine <NUM>. In this example, the accessory gearbox <NUM> is driven though a tower shaft <NUM> coupled to the outer shaft <NUM>. Although the example gearbox <NUM> is driven by the tower shaft <NUM> coupled to the outer shaft <NUM> of the high speed spool <NUM> other couplings could be utilized to drive the accessory gearbox <NUM> and are within the scope and contemplation of this disclosure.

Referring to <FIG> with continued reference to <FIG>, the fuel system includes a primary fuel pump <NUM> that provides a first fuel flow <NUM> during engine operation. The system <NUM> includes a secondary fuel pump <NUM> that provides a second fuel flow <NUM>. Both the primary fuel pump <NUM> and the secondary fuel pump <NUM> are driven by outputs of the accessory gearbox <NUM>. In the disclosed example, a first shaft shown schematically at <NUM> drives the primary fuel pump <NUM> and a second shaft schematically shown at <NUM> drives the secondary fuel pump <NUM>.

The first fuel pump <NUM> and the second fuel pump <NUM> both receive fuel flow from a common inlet passage <NUM> and both the first fuel pump <NUM> and the second fuel pump <NUM> communicate fuel flow to a common outlet passage <NUM>. The first fuel pump <NUM> communicates fuel flow through a first passage <NUM>. The second fuel pump <NUM> communicates fuel flow through a second passage <NUM>. A recirculation passage <NUM> communicates excess fuel from near the outlet <NUM> to a location upstream of both the first and second pumps <NUM>, <NUM>.

A first control valve <NUM> is disposed within the second passage <NUM> upstream of the secondary fuel pump <NUM>. A second control valve <NUM> is disposed within the second passage downstream of the secondary fuel pump <NUM>. A controller <NUM> governs operation of first control valve <NUM> and the second control valve <NUM> to control communication of fuel to and from the secondary fuel pump <NUM>.

Both the primary and secondary fuel pumps <NUM>, <NUM> are mechanical constant volume fuel pumps driven by the shafts <NUM>, <NUM> from the accessory gearbox <NUM>. The primary and secondary pumps <NUM>, <NUM> in one disclosed embodiment provide identical fuel flow volumes. In another disclosed embodiment, the primary and secondary pumps <NUM>, <NUM> provide different fuel flow volumes.

Because both the primary and secondary pumps <NUM>, <NUM> are mechanically linked to corresponding shafts <NUM>, <NUM>, the secondary fuel pump <NUM> runs even when fuel is not suppled through the second passage because the control valves <NUM>, <NUM> are closed. In the first operating condition with both the first and second control valves <NUM>, <NUM> closed, the primary fuel pump <NUM> generates a first fuel flow <NUM> through the first flow passage <NUM>. The secondary fuel pump <NUM> does not provide fuel flow because the control valves <NUM>, <NUM> are closed.

The first fuel flow <NUM> of a defined volume determined to provide sufficient fuel for engine operating conditions that are less then maximum. Accordingly, when the engine is operating in low fuel demand conditions such as during a cruise or descent condition, only the first fuel flow <NUM> is provided. The reduced fuel flow during the low demand conditions reduces the amount of fuel that may be recirculated through the recirculation passage <NUM>. The recirculation passage <NUM> includes a pressure relieve valve <NUM> that enables a uniform pressure of fuel flow to the combustor <NUM>.

Referring to <FIG>, with continued reference to <FIG>, in higher fuel demand conditions such as take-off and climb conditions, the controller <NUM> opens the control valves <NUM>, <NUM> to communicate fuel to the secondary fuel pump <NUM>. The secondary fuel pump <NUM> generates a second fuel flow <NUM> through the second passage <NUM> that combines with the first fuel flow <NUM> from the first passage <NUM>. The combined first and second fuel flows <NUM>, <NUM> are both communicated through the common outlet <NUM> to the combustor <NUM>. Once the engine transitions back to a low fuel demand operating condition, the control valves <NUM>, <NUM> are closed and the first fuel flow <NUM> continues to be communicated to the combustor <NUM>. The second fuel flow <NUM> is stopped and the reduced fuel flow continues at levels tailored to current engine operation.

Referring to <FIG>, with continued reference to <FIG>, another fuel system is schematically shown at <NUM>'. The fuel system <NUM>' includes a clutch <NUM> for selectively coupling the shaft <NUM> to the accessory gearbox <NUM>. The clutch <NUM> may be decoupled to deactivate the secondary pump <NUM>. Accordingly, rather than continually drive the secondary pump <NUM> when not needed, the example fuel system <NUM>' decouples the secondary pump <NUM>. Because the secondary pump <NUM> is decoupled and therefore not provide the secondary fuel flow <NUM>, the control valves <NUM>, <NUM> are not needed and are removed. The controller <NUM> selectively actuates the clutch <NUM> when the additional fuel flow is needed for engine operation.

The fuel systems <NUM>, <NUM>' thereby operate to combine fuel flows in parallel fuel passages to accommodate fuel demands according to engine operating conditions.

Referring to <FIG>, another fuel system is disclosed and schematically indicated at <NUM>. The fuel system <NUM> includes a primary fuel pump <NUM> and a secondary fuel pump <NUM>. The primary fuel pump <NUM> and the secondary fuel pump <NUM> are identical constant volume mechanical gear mesh pumps arranged to operate both in series and in parallel depending on fuel flow demand.

The primary fuel pump <NUM> includes an inlet <NUM> and an outlet <NUM> and is disposes upstream of the secondary fuel pump <NUM>. The secondary fuel pump <NUM> includes an inlet <NUM> and an outlet <NUM>. The fuel system <NUM> includes a first fuel passage <NUM> in parallel with a second fuel passage <NUM>. Both the first fuel passage <NUM> and the second fuel passage <NUM> are in communication with a common fuel outlet <NUM> and the fuel tank <NUM>.

A first pressure relief valve <NUM> is disposed within a crossover passage <NUM> that communicates fuel from the outlet <NUM> of the primary fuel pump <NUM> to the inlet <NUM> of the secondary fuel pump <NUM>. The first pressure relief valve <NUM> enables switching between a series arrangement where a first fuel flow <NUM> is provided through both the primary and secondary fuel pumps <NUM>, <NUM> and a parallel arrangement where the first fuel flow <NUM> is provided by the primary fuel pump <NUM> and a secondary fuel flow <NUM> (<FIG>) is provided by the secondary fuel pump <NUM>.

The first pressure relief valve <NUM> opens to communicate fuel from the primary fuel pump <NUM> to the secondary fuel pump <NUM> to provide the first fuel flow <NUM> in a first operating condition (<FIG>). The first operating condition corresponds with low fuel demand operation such as during decent or cruise conditions. In low fuel demand operating conditions, a fuel pressure at the common outlet <NUM> maintains a first check valve <NUM> in a closed position and the first pressure relief valve <NUM> is open. A second check valve <NUM> upstream of the secondary fuel pump <NUM> is also closed to prevent communication of fuel independent of the primary fuel pump <NUM>.

Upon an increase in fuel demand for engine operating conditions such as takeoff and climb operations, a pressure at the common outlet passage <NUM> will drop due to the increased fuel flow. The drop in fuel pressure opens the first check valve <NUM> and closes the first relief valve <NUM>. The second check valve <NUM> also opens. Fuel flow from the primary fuel pump <NUM> proceeds through passage <NUM> to the common outlet <NUM> independent of fuel flow in the second passage <NUM>. The second passage <NUM> is now open to receive fuel from the fuel tank <NUM> and provides a second fuel flow <NUM> to double the fuel flow through the common outlet <NUM>. Accordingly, once the first pressure relief valve <NUM> closes, the secondary fuel pump <NUM> provides the second fuel flow <NUM> in parallel with the first fuel flow <NUM> provided by the primary mechanical fuel pump <NUM> to the common fuel passage <NUM>.

A second pressure relief valve <NUM> is disposed downstream of both the primary fuel pump <NUM> and the secondary fuel pump <NUM> for directing excess fuel flow through a recirculation passage <NUM> in response to a pressure within the common fuel passage <NUM> above a predefined pressure. However, once pressure increase at the common fuel passage <NUM>, the fuel system will switch back to the series arrangement. In response to an increase in fuel pressure that would accompany a drop in fuel demand based on engine operating conditions, the first check valve <NUM> would close. Closing of the first check valve <NUM> is followed by opening of the first relief valve <NUM> such that fuel from the primary fuel pump <NUM> is directed through the cross-over passage <NUM> to the secondary fuel pump <NUM> as is shown in <FIG>. Accordingly, the disclosed fuel system <NUM> switches between series and parallel flow arrangements in response changes in fuel pressures caused by changes in fuel flow demands. The first relief valve <NUM> may be a controlled valve or may be a mechanical valve that opens in response to fuel pressure. In this example, the first relief valve <NUM> is configured to open at a lower differential pressure than the second pressure relief valve <NUM>.

Accordingly, the fuel systems <NUM>, <NUM>' and <NUM> tailor fuel flow to engine operating demands while maintaining the proven reliability and robust operation of mechanical constant volume pumps.

Claim 1:
A fuel system (<NUM>) for a gas turbine engine (<NUM>) comprising:
an accessory gearbox (<NUM>) driven by a mechanical link to the gas turbine engine (<NUM>);
a primary fuel pump (<NUM>) providing a first fuel flow (<NUM>) during engine operation;
a secondary fuel pump (<NUM>) providing a second fuel flow (<NUM>), wherein the primary fuel pump (<NUM>) and the secondary fuel pump (<NUM>) are driven by an output of the accessory gearbox (<NUM>):
characterised in that the fuel system (<NUM>) further comprises a first control valve (<NUM>) upstream of the secondary fuel pump (<NUM>);
a second control valve (<NUM>) downstream of the secondary fuel pump (<NUM>); the first control valve (<NUM>) and the second control valve (<NUM>) configured to control communication of fuel to and from the secondary fuel pump (<NUM>), wherein the first fuel pump (<NUM>) and the second fuel pump (<NUM>) both receive fuel flow from a common inlet passage (<NUM>) and both the first fuel pump (<NUM>) and the second fuel pump (<NUM>) communicate the corresponding one of the first fuel flow (<NUM>) and the second fuel flow (<NUM>) to a common outlet passage (<NUM>).