GAS TURBINE ENGINE FUEL CONTROL SYSTEM

Abstract: Fuel from a supply line (l6) is passed to an outlet connection (5l) of an engine by way of a pressurising valve (30) and a throttle valve (50) in series. Fuel flow is controlled by a spill valve (34) responsive to a servo pressure which is regulated by an electrically operated valve (38). The pressurising valve (30) is responsive to the pressure in the line (l6) and the throttle valve (50) maintains the pressure difference across the pressurising valve (30) substantially constant. A selector valve (58) is operable to shut both pressurising valve (30) and a shut-off element (57) in the throttle valve.

Description: This invention relates to a fuel control system for a gas turbine engine. It is known, for example from U.S. Patent 39584l5, to provide a fuel control system in which flow to a gas turbine engine is regulated by a spill valve which is responsive to an electrical flow demand signal, the system including a pressure responsive valve in the flow path to the engine. The pressure responsive valve acts to increase flow in response to an increase in its own upstream pressure and has a profile such that the rate of change of flow is greater at higher pressures than at lower pressures. Such a pressure responsive valve is commonly referred to as a pressurising valve and has the effect of counteracting the square law characteristic of fuel flow through the orifices of the engine burners, whereby flow through the burners is more nearly proportional to the pressure of the fuel upstream of the pressurising valve, and hence to the magnitude of the electrical flow demand signal. It is a disadvantage of the above system that response of the pressurising valve is to pressure only, and this has the effect that flow to the engine burners may be affected by factors other than that of the magnitude of the electrical demand signal, and may vary by as much as 20% from the demanded flow. It is an object of the present invention to provide a fuel control system of the foregoing general type in which correspondence of metered and demanded flows is improved, this being effected by the inclusion, inter alia, of means for controlling the pressure difference across the pressurising valve. According to the invention there is provided a fuel control system for a gas turbine engine, comprising a passage through which fuel can flow to said engine, a spill valve responsive to the magnitude of an electrical flow demand signal, for spilling fuel from said passage, a pressurising valve for controlling flow through said passage in accordance with the pressure at an inlet of said spill valve, and a throttle valve in series with said pressurising valve, said throttle valve being responsive to an increase or decrease in pressure difference across said pressurising valve to decrease or increase respectively the flow through said passage, thereby to maintain said pressure difference substantially constant. An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which:- Figure l is a diagram showing a gas turbine engine and an associated fuel control system, and Figure 2 is a diagram of a flow control arrangement forming part of Figure l. As shown in Figure l a gas turbine engine l0 is supplied with fuel by a series arrangement of a centrifugal pump ll and a positive displacement pump l2, by way of a flow control arrangement l3. The flow control arrangement l3 is responsive to electrical signals from a control circuit l4 which is in turn responsive to input signals from the engine l0 and to an engine speed demand control l5. The circuit l4 is of the type shown and described in U.S. Patent 39584l5, but forms no part of the present invention. Fuel flows to the arrangement l3 through a supply line l6 and a low pressure return line l7 communicates with the upstream side of the pump l2. A relief valve l8 of a conventional type is connected between the lines l6, l7. A high pressure fuel line l9 communicates with the line l6 through a filter unit 20. The arrangement l3 is shown in more detail in Figure 2 and includes a pressurising valve 30 having a spool control element 3l one end of which is responsive to the pressure in the line l6. The element 3l is biased against this pressure by an adjustable spring 32 and the pressure in a chamber 33. The supply line l6 can communicate with the return line l7 through a spill valve 34 having a control element 35 which is urged against the pressure in line l6 by a spring 36 and by a servo pressure in a chamber 37. The pressure in chamber 37 is controlled by a valve device 38 having a fixed orifice 39 and a variable orifice 40 connected in series between the lines l6, l7, the pressure in chamber 36 being derived from that between the orifices 39, 40. The orifice 40 is controlled by a torque motor 4l responsive to fuel flow demand signals on lines 42 from the control circuit l4. A throttle valve 50 is connected in series with the valve 30 between the supply line l6 and an outlet connection 5l to the engine l0. The throttle valve 50 has an inlet port 52 and an outlet port 53, flow through the outlet port 53 being regulated by a control element 54 which is urged against the pressure in a chamber 55 by a spring 56 and the pressure in the port 52. Secured to the control element 54 for movement therewith is a closure 57 for the port 52. In normal operation the closure 57 does not affect flow through the ports 52, 53, flow through the port 53 being regulated only by the element 54. The pressure in the chamber 55 is controlled by a solenoid-operated selector valve 58 which, in the energised position shown, connects the high pressure line l9 to the chamber 55, maintaining the port 52 open. In this condition the control element 54 is responsive to the pressure immediately downstream of the valve 30 as well as to the pressure in the line l6, applied through the filter unit 20, line l9 and valve 58. The control element 54 is thus responsive to the pressure difference across the valve 30, an increase in that pressure difference moving the control element 54 in a direction to reduce fuel flow through the valve 30, thereby maintaining the aforesaid pressure difference substantially constant. The solenoid valve 58 is controlled by the circuit l4 and is de-energised to shut off fuel flow to the engine l0, in which circumstance the chamber 55 is isolated from the high pressure line l9, the pressure in chamber 55 bleeding through a flow restrictor 59 to the outlet connection 5l and allowing the control element 54 and closure 57, which may be an elastomeric element, to move upwardly to shut the port 52. In this condition the solenoid valve 58 applies high pressure through a line 60 to the chamber 33 of the valve 30, urging the latter to shut off flow from the supply line l6. A requirement to shut off fuel flow to the engine l0 will be accompanied by a minimum fuel demand signal on the lines 42 and the valve arrangement 38 will cause the pressure in chamber 37 of the spill valve 34 to fall to that in the return line l7. The spill valve 34 thus becomes substantially fully open and fuel delivered on the line l6 is returned to the upstream side of the pump l2 through the line l7. The high pressure line l9 can communicate with the line 60 through an adjustable orifice 6l, the connection between the orifice 6l and line l9 being shut by the control element 35 when the spill valve 34 is fully shut. In normal operation the orifice 6l is in series with the chamber 33 and a fixed orifice 62 between the lines l9 and l7. A servo pressure between that in the lines l9, l7 is thus maintained in the chamber 33 and assists the spring 32 in opposing the pressure in line l6. This servo pressure is adjustable by means of the orifice 6l to compensate for manufacturing tolerances in the valve 30. At low pump delivery flow during engine starting the spill valve 34 shuts and servo flow to the chamber 33 is arrested, flow to the engine l0 being thereby increased. The face of the control element 3l on which the pressure in line l6 acts also defines a metering edge of the valve 30. At higher pressures in the line l6 the valve 30 will be wider open, having a larger effective flow area. Thus, the flow area of valve 30 is a function of the pressure in line l6, which is proportional to the fuel flow demand signal on line 42 to valve 38. The pressure drop across this flow area is kept constant by the throttle valve 50 so that flow is also a similar function of the pressure in line l6, and is independent of such factors as pressureat the engine burners, pressure at the gear pump inlet, etc. Thus flow can be made any desired function of electrical signals on lines 42 and is independent of other factors.

Claims: 1. A fuel control system for a gas turbine engine (l0), comprising a passage (l6) through which fuel can flow to said engine (l0), a spill valve (34) responsive to the magnitude of an electrical flow demand signal, for spilling fuel from said passage (l6), a pressurising valve (30) for controlling flow through said passage (l6) in accordance with the pressure at the inlet of said spill valve (34), characterised in that there is provided a throttle valve (50) in series with said pressurising valve (30), said throttle valve (50) being responsive to an increase or decrease in pressure difference across said pressurising valve (30) to decrease or increase respectively the flow through said passage (l6), thereby to maintain said pressure difference substantially constant. 2. A system as claimed in Claim l which includes means (6l, 62) for applying a servo pressure signal to said pressurising valve (30) to urge the latter to decrease flow through said passage (l6), and a selector valve (58) operable in one of its states to raise said servo pressure signal to a level at which said pressurising valve (30) shuts off flow through said passage (l6). 3. A system as claimed in Claim 2 in which said selector valve (58) is operable in another of its states to apply to said throttle valve (50) an operating pressure derived from the pressure at the inlet of said pressurising valve (30). 4. A system as claimed in Claim 3 in which said throttle valve (50) comprises a first element (54) for regulating flow to maintain said pressure difference substantially constant, and a second element (57) movable with said first element (54) for shutting off flow through said throttle valve (50). 5. A system as claimed in Claim 4 which includes means for biasing said second element (57) towards its shut position, said pressure at the inlet of said pressurising valve (30) being operable to urge said second element (57) against said biasing means. 6. A system as claimed in Claim 5 which includes means (59) for venting said operating pressure to a low pressure when said selector valve (58) is in its one state. 7. A system as claimed in Claim 6 in which said venting means comprises a flow restrictor (59) interconnecting zones on opposite sides of said first element (54).

Inventors: SMITH, TREVOR STANLEY
Applicants: LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1986
Main classification: F02C9
IPC codes: F02C9