Engine fuel control system

Abstract

An engine fuel control system is provided, including a supply line for the supply of fuel to a fuel metering valve which controls the flow of fuel to burners of an engine. Fuel is delivered at a first high pressure to the supply line by a pump arrangement. The engine fuel control system includes a restrictor located in the supply line for passage of the fuel delivered by the pump arrangement therethrough. The restrictor is configured such that fuel exiting the restrictor for onward supply to the fuel metering valve is at a second high pressure which is lower than the first high pressure. The engine fuel control system includes pressure limiting valves which actuate when the pressure difference between the first high and low pressure reaches a predetermined level to open a flow path for fuel on the supply line to by-pass the restrictor, thereby limiting the pressure difference.

Claims

1. An engine fuel control system including: a pump arrangement; a supply line for a supply of fuel to a fuel metering valve which controls a flow of fuel to burners of an engine, in use the fuel being delivered at a first high pressure to the supply line by the pump arrangement which receives fuel at a low pressure; a restrictor located in the supply line for passage of the fuel delivered by the pump arrangement therethrough, the restrictor being configured such that fuel exiting the restrictor for onward supply to the fuel metering valve is at a second high pressure which is lower than the first high pressure; and a pressure limiting valve which is actuated when a pressure difference between the first high pressure and the low pressure reaches a predetermined level to open a flow path for fuel on the supply line to by-pass the restrictor, thereby limiting the pressure difference such that the first high pressure and the low pressure act in opposite directions on the valve.

2. An engine fuel control system according to claim 1, wherein the restrictor is a fixed restrictor.

3. An engine fuel control system according to claim 1 further including an off-take line which extends from a junction with the supply line, the off-take line supplying fuel at the first high pressure to one or more fuel-pressure operated auxiliary engine devices via one or more control devices.

4. An engine fuel control system according to claim 3 further including a filter at said junction for filtering the fuel supplied through the off-take line.

5. An engine fuel control system according to claim 1, wherein the pump arrangement is a single pump.

6. An engine fuel control system according to claim 1, wherein the pump arrangement comprises a small displacement pump and a large displacement pump, the small displacement pump delivering the fuel at the first high pressure to the supply line.

7. An engine fuel control system according to claim 1 further including a low pressure source from which the pump arrangement receives the fuel at the low pressure.

8. An engine fuel control system according to claim 1 further including the fuel metering valve which controls the flow of the fuel to the burners of the engine, the fuel metering valve being supplied with the fuel by the supply line.

9. A gas turbine engine having the engine fuel control system according to claim 1.

Description

BRIEF DESCRIPTION THE DRAWINGS

(1) Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

(2) FIG. 1 shows a longitudinal cross-section through a gas turbine engine: and

(3) FIG. 2 illustrates schematically part of a fuel control system for the engine of FIG. 1.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES OF THE INVENTION

(4) With reference to FIG. 1, a ducted fan gas turbine engine incorporating the invention is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.

(5) During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed, into it before delivering that air to the high pressure compressor 14 where further compression takes place.

(6) The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.

(7) A fuel control system of the engine, a part of which is illustrated diagrammatically in FIG. 2, has a pump unit 31 and a hydro-mechanical unit (HMU) 35. The pump unit 31 comprises a low pressure pump 32 which draws fuel from a fuel tank of the aircraft at pressure PLPinlet and supplies the fuel at boosted pressure LP to the inlet of a high pressure pumping arrangement comprising a small displacement pump 33 and a parallel large displacement pump 34. The low pressure pump 32 typically comprises a centrifugal impeller pump while the high pressure pumping arrangement may comprise twin pinion gear pumps.

(8) The low pressure pump 32 and the high pressure pumps 33, 34 are typically connected to a common drive input, which is driven by the engine high-pressure or intermediate-pressure shaft via an engine accessory gearbox, The inter-stage flow between the low pressure and high pressure pumping stages is typically used to cool engine lubrication oil in a fuel/oil heat exchanger (FOHE—not shown).

(9) Not all the fuel exiting the large displacement high pressure pump 34 (at pressure HPL) may be burnt in the engine. A substantial proportion may be recirculated back to the pump unit 31 via a combining spill valve 37 and a spill return of the HMU 35, For example, when the engine is operating at altitude where the engine burns little fuel, the total large pump delivery flow may be recirculated, but any resulting fuel temperature use is low because the position of the combining spill valve is such that the spill port taking the large pump delivery flow is large so that the large pump pressure rise is small.

(10) Only a part of the HMU 35 of the fuel control system for the engine is illustrated in FIG. 2, The high pressure pumps 33, 34 feed fuel to a supply line 36 which extends to a fuel metering valve 45, the metering valve being operable to control the rate at which fuel is allowed to flow from the supply line 36 via a pressure raising and shut-off valve (PRSOV—not shown) of the HMU to a delivery line and thence to burners of the engine. A constant pressure differential is maintained across the metering valve by the combination of a pressure drop control valve 46 (PDCV) and the combining spill valve 37.

(11) The small displacement high pressure pump 33 has a relief valve which prevents excess pressure build up in the supply line 36 in the event of a downstream blockage e.g. caused by coking of fuel in the engine fuel manifold.

(12) A restrictor 38, in the form of an orifice, is located in the supply line 36. The restrictor sets a higher fuel pressure HPa at delivery of the small displacement high pressure pump 33 than the fuel pressure HP which is supplied by the supply line to the metering valve 45. The part of the fuel from the large displacement high pressure pump 34 which is not recirculated back to the pump unit 31 enters the supply line 36 via a non-return valve 39 and downstream of the restrictor 38.

(13) A pressure limiting valve 40 is provided in parallel to the restrictor 38 and is actuated when the pressure difference (HPa−LP) reaches a predetermined level to open a flow path for fuel on the supply line 36 to by-pass the restrictor 38, thereby limiting the pressure difference (HPa−LP).

(14) An off-take line 41 branches from the supply line 36 upstream of the restrictor 38, a flow-washed filter 42 being located at the junction of the off-take line and the supply line. The off-take line 41 supplies filtered fuel at pressure HPaf to a dual lane servo-valve 43 operating external actuators 44 for the engine variable inlet guide vanes (VIGVs) and variable stator vanes (VSVs).

(15) The restrictor 38 and the pressure limiting valve 40 arrangement together set a higher minimum pump pressure rise (HPa−LP), separate to the HMU 36 pressure rise (HP−LP), for actuation control at engine speeds above idle.

(16) The normal minimum system pressure rise (HP−LP) in the HMU is set by the PDCV 46 spring, the PRSOV spring and the PRSOV orifice potentiometer. However, as the HMU pressure rise is no longer required for operation of the external actuators 44, this can be set to a lower value, e.g. 250 psid (1.72 MPa) compared to a conventional 440 paid (3.03 MPa) discussed earlier.

(17) As mentioned above, the fixed restrictor 38 downstream of the small displacement pump 33 sets a higher pressure HPa at pump delivery. However, since the bulk of the small pump delivery flow passes through the fixed restrictor, the additional pressure rise (HPa−HP) is a function of the pump speed (and hence small pump flow) and the size of the restrictor. The restrictor may be sized so that at an idle type speed (60%), the flow through it provides about an additional 300 psid (2.07 MPa) for actuation control. Thus:
HPa−LP=(HPa−HP)+(HP−LP)=300+250=550 psid (3.79 MPa)
where (HP−LP) is set by the HMU,

(18) At start speeds, the pump delivery falls so HPa−HP falls, giving reduction in the pump pressure rise. For example:
HPa−LP=320 psid (2.21 MPa) at 25% speed, 254 psid (1.75 MPa) at 6% speed

(19) This is beneficial because (i) it reduces pump/HMU leakage at start so that more of the pump delivery flow is made available to the engine and (ii) it reduces the risk of mixed film bearing operation. That is, the pumps are no longer operating at low speed and high pressure.

(20) At idle and above, a minimum HPa−LP pressure rise of 550 paid (3.79 MPa) is available to drive the VIGVs and VSVs via the servo-valve 43. Benefits of this are greater slew capability/force margins of the external actuators 44 and a potential to reduce the sizes of the servo-valve 43 and the external actuators 44.

(21) The pressure limiting valve 40 limits the small displacement pump 33 pressure rise, For example, the piston area and spring of the pressure limiting valve can be sized so that the valve cracks open when the pressure drop across the fixed restrictor reaches a level where HPa−LP rises to 600 psid (4.17 MPa). Once open, flow through the valve port by-passes the fixed restrictor 38, passing from HPa to HP. Thus at cruise/idle conditions, the valve regulates HPa−LP to around 600 psid (4.17 MPa). The maximum size of the valve port can be set so that at 100% speed, the valve pressure drop HPa−HP is no more than e.g. 50 psid (0.34 MPa).

(22) The fuel control system can provide the following advantages: 1) It separates HMU and external actuator pressure rise requirements. The HMU 35 can operate at a constant low minimum system pressure rise (250 paid-1.72 MPa) providing benefits in terms of hydromechanical control stability (lower gains), constant metering valve 45 slew rates and reduced metering valve fail safe rates at low flow conditions. The pressure rise available to move the external actuators can be increased without adversely affecting the HMU, providing an increased actuator force margin/slew capability, and allowing smaller actuators and servo-valves to be used. 2) The reduced value of (HP−LP) at start conditions is a predictable function of restrictor size and pump speed. It alleviates pump bearing problems (the pumps no longer have to operate at high pressure and low speed) and reduces pump/HMU leakages (more of the pump flow at low speed conditions is available for lighting the engine). 3) It reduces the potential for cavitation in the HMU 35. For example, cavitation across the metering ports of the combining spill valve 37 is less likely when the port pressure drop is reduced. 4) Relative to the arrangement proposed in U.S. Pat. No. 6,176,076, the minimum system pressure rise (HPa−LP) available to move the actuators 44 can be much less sensitive to off-take flows. In particular, because the pressure drop across the fixed restrictor 38 is directly related to pump delivery flow (and hence pump speed) rather than spill flow, the reduction in (HPa−LP) during actuator transients is lower. If a restrictor were to be placed in a pump spill return line (as U.S. Pat. No. 6,176,076), at some low spill flow conditions, movement of the actuators would result in the spill flow reducing to almost zero with a consequent loss of the pressure rise available to drive the actuators. However, in the present invention, part of the small pump delivery flow always passes to the burners so even when part of the delivery flow is pulled off for actuator control, some of the delivery flow still passes through the restrictor 38 to maintain a high pump pressure rise. Thus there is always some pressure drop across the restrictor 38, even when the actuators 44 are slewed. 5) Additionally, placing a restrictor in a pump spill return line (as U.S. Pat. No. 6,176,076). would be particularly disadvantageous for a twin pump system, The restrictor would have to be placed in the small pump spill return line and be sufficiently small to ensure sufficient pump pressure rise to drive the external actuators at all conditions. At some conditions, this would result in an excessively high pump pressure rise (HP−LP), raising heat input to the fuel and also worsening pump bearing integrity issues.

(23) While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. For example, instead of dual high pressure pumps 33, 34, the system may have a single high pressure pump. As another example, some or all of the flow-washed filter 42, the restrictor 38, the non-return valve 39, and the pressure limiting valve 40 can be located in the HMU 35 rather than in the pump unit 31. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.