GB2256729A

GB2256729A - Fuel control system

Info

Publication number: GB2256729A
Application number: GB9212107A
Authority: GB
Inventors: Trevor Stanley Smith
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1991-06-11
Filing date: 1992-06-08
Publication date: 1992-12-16
Anticipated expiration: 2012-06-08
Also published as: GB2256729B; GB9212107D0

Abstract

In a fuel control system for a gas turbine of an aero-engine, a pressure raising and shut off valve 15 is operated by a piston and cylinder arrangement. A shut off motor-operated pilot valve 120 controls the fuel pressure in a control line 137 of a rapid shut down valve 21 which is operable to connect the cylinder of the valve 15 to a high pressure fuel supply (HPf) via relatively large diameter pipes and so augment the closing force of a spring 27 which acts on the piston 26 of the valve 15. The valve 120 may be operated in response to engine overspeed to close an orifice 142 and thus pressurize the control line 137. Normal shutdown of the valve 15 is effected by closing a second orifice 163 of the valve 120 to pressurize a small bore pipe 165 which connects with the cylinder of the valve 15 and thus provide for a slower closing action. <IMAGE>

Description

FUEL CONTROL SYSTEM.

The present invention relates to a fuel control system.

Such a system may be used to control the supply of fuel to the burners of a gas turbine, for instance for aerospace applications.

According to the invention, there is provided a fuel control system comprising: a first cylinder and a first piston; a first valve body movable between a first position for preventing fuel flow and a second position for permitting fuel flow, the first valve body being connected to the first piston; biasing means for biasing the first valve body towards the first position; and valve means for selectively connecting the first cylinder to a first source of fuel arranged to supply fuel at a first pressure and at a first rate of flow.

Advantageously the valve means may be arranged to selectively connect the first cylinder to a second source of fuel arranged to supply fuel at a second pressure and at a second rate of flow which is lower than the first rate, or to a fuel sink.

The first and second pressures may be equal to each other.

The biasing means may comprise resilient biasing means, such as a spring. Alternatively or additionally, the biasing means may comprise supplying means for supplying fuel at a third predetermined pressure, which is less than the first pressure, to the first cylinder.

The supplying means may comprise a high pressure fuel connection and a low pressure fuel connection connected to the first cylinder via first and second flow restrictors, respectively.

The valve means may comprise a second valve body movable between a first position for connecting the first cylinder to the first fuel source and a second position for disconnecting the first cylinder from the first fuel source, a second cylinder connected to a controllable pressure fuel source, and a second piston located within the second cylinder and connected to the second valve body for movement therewith. The controllable pressure fuel source may comprise a variable orifice connected to the second cylinder and via a third flow restrictor to a high pressure connection. The variable orifice may comprise a fixed orifice co-operating with a closure element for providing variable venting of fuel from the fixed orifice.

The second fuel source may comprise a further variable orifice connected to the first cylinder and via a fourth flow restrictor to a high pressure connection. The further variable orifice may comprise a further fixed orifice co-operating with the closure element for providing variable venting of fuel from the further fixed orifice. The closure element may be connected to an actuator, such as a torque motor. The actuator may have a first stable state in which the fixed orifice and the further fixed orifice are vented, a second stable state in which the fixed orifice is closed and the further fixed orifice is vented, and a third stable state in which the fixed orifice is vented and the further fixed orifice is closed.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic cross-sectional diagram of a fuel control system constituting a first embodiment of the invention; Figure 2 is a schematic cross-sectional diagram of part of the system of Figure 1; and Figure 3 is a schematic cross sectional diagram of a part of a fuel control system constituting a second embodiment of the invention in which the part shown in Figure 3 is substituted for the part shown in Figure 2.

The fuel control system receives high pressure fuel at an inlet 1 from a fuel pump (not shown) and returns low pressure fuel to the inlet of the pump from an outlet 2.

The high pressure fuel passes directly through a flow wash filter 3 to the inlet of a metering valve 4, which controls the rate of flow of fuel. Filtered fuel (HPf) from the filter 3 is supplied to a servo-pressure regulator 5, which provides at its outlet 6 fuel at a constant regulated pressure (Pr). Excess fuel is vented from the regulator 5 via a filter 7 to the low pressure fuel outlet 2, the filter 7 serving to remove contaminants when the fuel system is initially charged and fuel may flow in the reverse direction through the filter 7. The regulator 5 establishes the constant regulated pressure with respect to the low pressure fuel (LP).

A fuel flow control actuator 8 comprising a valve arrangement controlled by a main torque motor receives the constant regulated pressure fuel and controls the position of a metering body with respect to a metering orifice in the metering valve 4. The torque motor comprises a plurality of windings which control the torque applied to an armature 9. The armature 9 acts between two orifices so as to control their degree of venting to a low pressure outlet 10 and hence the differential fuel pressure in outlets 11 and 12. The outlets 11 and 12 are connected to control inlets of the metering valve 4 so as to control the position of the metering valve body with respect to the metering orifice.

In particular, the differential pressure causes the metering body to move towards the side of lower pressure, thus altering the uncovered area of the metering orifice.

The position of the metering body is converted into a corresponding electrical signal by a transducer 13 comprising a linear variable differential transformer, whose output is used by an electronic controller (not shown) to form a feedback servo-control loop for the rate of fuel supply. The main outlet 14 of the metering valve 4 is connected to a pressure raising and shut-off valve 15.

Filtered high pressure fuel is supplied to a low flow adjuster 16, which is connected to the metering valve 4 and to a pressure drop and spill valve 17. The low flow adjuster 16 permits calibration of low fuel flow rates of the metering valve 4.

The high pressure filtered fuel is supplied via a filter 18 and a restrictor 19 to an inlet of the pressure drop and spill valve 17, which is connected to the low pressure fuel outlet 2. The pressure drop and spill valve 17 provides a constant pressure drop across the metering orifice of the metering valve 4, so that fuel flow rate is controlled by the position of the metering valve body and is substantially independent of fluctuations in the flow rate of the high pressure fuel at the inlet 1. Excess fuel is spilled by the valve 17 to the inlet of the fuel pump via the outlet 2.

The pressure raising and shut-off valve 15 is associated with a shut-off torque motor 20, a rapid shut-down valve 21, and a dump valve 22, all of which are shown in more detail in Figures 2 and 3 for the first and second embodiments, respectively. As shown in Figures 2 and 3, the pressure raising and shut-off valve 15 has an inlet 23 for receiving a metered fuel flow from the metering valve 4. The valve 15 has an outlet 24 for supplying the metered fuel flow to an engine manifold for the burners of a gas turbine engine, for instance for use in an aircraft. A valve 25 is arranged between the inlet 23 and the outlet 24 and comprises a movable valve body 26 urged in a closing direction by a compression helical spring 27. The valve body 26 acts as a piston within a cylinder having an inlet/outlet 28.The pressure raising and shut-off valve 15 is provided at one end with microswitches 49 for signalling to the electronic controller the operating state of the valve 15. At the other end, the dump valve 22 is actuated by the valve body 26 and has an inlet 50 connected to the outlet 24 and an outlet 51 connected to a drain tank. When the pressure raising and shut-off valve 15 is closed to prevent the flow of fuel to the engine manifold, the dump valve 22 is opened so as to vent fuel from the manifold to the drain tank.

When the valve 15 is open, the dump valve 22 is closed.

In the first embodiment the inlet/outlet 28 is connected via a filter 29 and a flow restrictor 30 to receive the constant regulated pressure fuel from the regulator 5.

The inlet/outlet 28 is also connected via a filter 31, a flow restrictor 32, and a filter 33 to the low pressure fuel outlet 2. The restrictors 30 and 32 thus serve to divide the pressure difference between the constant regulated pressure fuel and the low pressure fuel so that a predetermined pressure acts on the valve body 26, thus adding to the bias provided by the compression spring 27 which tends to close the pressure raising and shut-off valve 15. Thus, until the pressure of fuel in the inlet 23 exceeds this preloading, the valve 15 prevents the supply of fuel to the engine manifold. Once the pressure of the high pressure fuel exceeds this preloading, so that all of the devices shown in Figure 1 are operational, the valve 25 opens to permit the flow of fuel to the engine manifold.

The inlet/outlet 28 is connected to the outlet of the rapid shut-down valve 21. The valve 21 has a valve body 34 which is movable between first and second positions for connecting and disconnecting, respectively, the inlet/outlet 28 to and from the high pressure filtered fuel supplied from the filter 3 via an inlet 35. The valve body 34 is biased by a compression helical spring 36 so as to prevent the flow of high pressure filtered fuel through the valve 21 to the inlet/outlet 28.

The valve body 34 acts as a piston within a cylinder which is connected via a pipe 37 to the outlet of a valve arrangement 38 controlled by the torque motor 20. The valve arrangement comprises an inlet 39 which receives high pressure filtered fuel and supplies this via a filter 40 and a restrictor 41 to an orifice 42 and, via a filter 43, to an outlet connected to the pipe 37. The motor 20 comprises a magnetically latched torque motor having four coils for controlling the position of an armature 44. The armature is movable to control the flow of fuel through the orifice 42 to an outlet 45 and thence to the low pressure fuel outlet 2. Thus, the position of the armature 44 controls the pressure within the pipe 37.

The pipe 37 is further connected via an outlet 48 to-a spill or unloading valve within the fuel pump.

During operation, for instance of an aero-engine, the magnetically latched motor 20 is latched in its open position such that the armature 44 substantially closes the orifice 42 and high pressure filtered fuel is supplied via the pipe 37 to the rapid shut-down valve 21 and via the pipe 48 to the pump unloading valve. This causes the pump unloading valve to remain closed so as not to affect operation of the fuel pump. The high pressure fuel acting on the valve body 34 causes it to prevent the supply of high pressure fuel from the inlet 35 to the inlet/outlet 28. Thus, the fuel pressure acting on the valve body 26 is determined by the flow restrictors 30 and 32 and, assuming that the pressure of the high pressure fuel is greater than the minimum permissible value, the pressure raising and shut-off valve 15 is open.

When it is desired to effect a rapid shut-down of the engine, a pulse of a suitable polarity is supplied to one or more of the coils of the torque motor 20. This causes the armature 44 to latch in its other position such that the orifice 42 is opened and fuel is vented via the pipe 45 to the low pressure fuel outlet. The pressure in the pipes 37 and 48 thus rapidly falls causing the valve body 34 of the rapid shut-down valve 21 to be moved against the action of the spring 36 to connect the inlet/outlet 28 to the high pressure filtered fuel. The pump unloading valve is likewise opened and causes excess fuel from the pump outlet to spill to the pump inlet.

The inlet/outlet 28 and the ducts now connecting it to the high pressure filtered fuel are of relatively large bore so that high pressure fuel is supplied at a high rate to the inside of the valve body 26. The valve 25 thus closes rapidly to prevent the flow of fuel to the engine manifold. The dump valve 22 opens so as to vent fuel in the outlet 24 and the manifold to the drain tank.

It is therefore possible to provide a very rapid shutdown of the engine, for instance in response to an engine overspeed detection signal from a suitable transducer.

In order to open the pressure raising and shut-off valve 15, the torque motor 20 is actuated such that the armature 45 becomes magnetically latched in a position closing the orifice 42. Pressure in the pipes 37 and 48 thus rises so that the pump unloading valve is closed and the rapid shut-down valve 21 disconnects the inlet/outlet from the high pressure filtered fuel. Fuel within the valve body 26 can thus be vented via the filters 31 and 33 and the restrictor 32 until the fuel pressure inside the valve body 26 returns to the predetermined value between the constant regulated pressure and the low pressure. The pressure of fuel from the metering valve 4 thus opens the valve 25 to allow fuel to be supplied to the engine manifold and the dump valve 22 is closed.

In the second embodiment, as illustrated in Figure 3, the pressure raising and shut off valve 15 is urged in the closing direction by the preloading provided by the helical compression spring 27, and once the pressure of the high pressure fuel exceeds this preloading, the valve 25 opens to permit the flow of fuel to the engine manifold.

The inlet/outlet 28 is connected via a pipe 129 to the outlet of the rapid shut-down valve 21 and via a flow restrictor 130 to a pump unloading valve. The valve 21 has a valve body 134 which is movable between first and second positions for connecting and disconnecting, respectively, the inlet/outlet 28 to and from the high pressure filtered fuel supplied from the filter 3 via an inlet 135. The valve body 134 is biased by a compression helical spring 136 so as to prevent the flow of high pressure filtered fuel through the valve 21.

The valve body 134 acts as a piston within a cylinder which is connected via a pipe 137 to the output of a valve arrangement 138 controlled by the torque motor 120.

The valve arrangement comprises an inlet 139 which receives high pressure filtered fuel and this is supplied via a filter 140 and a restrictor 141 to an orifice 142 and, via a filter 143, to an outlet connected to the pipe 137. The inlet 139 also supplies high pressure fuel via a line 160 and a restrictor 162 to an orifice 163 and, via a filter 164, to an outlet which is connected via a pipe 165 to the inlet/outlet 28.

The motor 120 comprises a centre stable torque motor having four coils for controlling movement of an armature 144 between three positions. In the first position, the armature 144 allows fuel flow through the orifices 142 and 163 to an outlet 145 and thence to the low pressure fuel outlet 2. In the second position, the armature 144 closes the orifice 142 and allows fuel flow through the orifice 163 to the outlet 145. In the third position, the armature 144 allows fuel flow through the orifice 142 to the outlet 145 and closes the orifice 163. Thus the position of the armature 144 controls the pressure within the pipes 137 and 165.

A start up solenoid valve 166 is connected between a line 169, the outlet 24 of the valve 15, and the inlet/outlet 28 of valve 15 via a line 170. An armature 180 is biased by a spring 181 to hold a valve member 182 such that the connection to the outlet 24 from the start up solenoid valve 166 is closed and lines 169 and 170 are in fluid communication.

Coils 183 of the start up solenoid valve 166 can be energised to move the armature 180 against the action of the spring 181 closing the path between the line 169 and the line 170. The ports 168 are uncovered when the valve 15 is in the shut position, thereby allowing fluid pressure communication between a high pressure fuel line 167 and the line 169. The ports 168 are covered when the valve 15 is in the normal operating position, ie when the valve 15 is open.

During normal operation, for instance of an aero-engine, the motor 120 is in its first position so that both the orifices 142 and 163 are open. Fuel in the pipe 137 is vented via the orifice 142 to the outlet 145 so that the valve body 134 is urged by the spring 136 to the left in Figure 3. The rapid shut down valve 21 is thus closed and prevents the supply of high pressure filtered fuel to the pipe 129. Fuel in the pipe 129 is vented via the pipe 165 and the orifice 163 to the outlet 145. The pressure raising and shut off valve 15 is open for as long as the pressure of the high pressure fuel supplied by the metering valve 4 is greater than the minimal permissible value defined by the preloading provided by the spring 27. The low pressure applied via the restrictor 130 keeps the pump unloading valve closed so as not to affect operation of the fuel pump.

When it is desired to effect a rapid shut-down of the engine, the motor 120 is actuated so that the armature 144 moves to its second position. The orifice 163 remains open but the orifice 142 is closed, thus applying high pressure to the rapid shut-down valve- 21 via the pipe 137. The valve body 134 moves rightwardly in Figure 3 against the action of the spring 136 to supply a rapid flow of high pressure fuel from the inlet 135 via the pipe 129 to the inlet/outlet. The high pressure is supplied via the restrictor 130 so as to open the pump unloading valve which causes excess fuel from the pump outlet to spill to the pump inlet.

The inlet/outlet 28 and the ducts now connecting it to the high pressure filtered fuel are of relatively large bore whereas the pipe 165 is of relatively narrow bore, so that high pressure fuel is supplied at a high rate to the inside of the valve body 26. The valve 25 thus closes rapidly to prevent the flow of fuel to the engine manifold. The dump valve 22 opens so as to vent fuel in the outlet 24 and the manifold to the drain tank. It is therefore possible to provide a very rapid shut-down of the engine, for instance in response to an engine overspeed detection signal from a suitable transducer.

When the pressure raising and shut off valve 15 is shut, high pressure fuel is fed via the ports 168, the line 167, the start up solenoid valve 166 and the line 170 to the inlet/outlet 28, thereby holding valve 15 shut. The torque motor 120 can be returned to the first position, the armature 144 moving so that fuel flows through the orifices 142 and 163 to the outlet 145. Pressure in pipe 137 thus falls and the valve body 134 moves under the action of spring 136 to remove the supply of high pressure filtered fuel to the pipe 129 via the rapid shut down valve 21. The pressure raising and shut off valve 15 remains shut as high pressure fuel is still supplied to the inlet/outlet 28 via the start up solenoid valve 166.

In order to open valve 15, the start up solenoid valve 166 is energised, thereby removing the supply of high pressure fuel to the inlet/outlet 28. The reduced pressure via the restrictor 130 closes the pump unloading valve. The valve 25 opens once the pressure at the inlet 23 is sufficiently great to move the valve body 26.

When it is desired to effect a normal shut-down of the engine, the torque motor 120 moves the armature to the third position so that the orifice 163 is closed. The inlet/outlet 28 is now connected via the relatively small bore pipe 165 to the high pressure filtered fuel so that the pressure raising and shut-off valve 15 closes relatively slowly. Thus, potential engine damage and potential pressure surge with subsequent damage to low pressure fuel pipes etc., caused by rapid reduction in fuel flow when the rapid shut-down valve 21 operates can be prevented during normal engine operation. Subsequent movement of the armature 144 to the first position uncovers the orifice 163 to allow the pressure to be vented via the pipe 165 and the orifice 163 to the outlet 45. However, valve 15 remains closed due to the latching action via ports 168 and solenoid valve 166 as previously described.

A further advantage is that, in the event of a dry-lift start, air/vapour in the pipes can be pumped out of the system by a gear pump via lines 167, ports 168 and the solenoid valve, at low pressure and without the need to open the valve 25. Once all the air/vapour is vented and fuel starts to flow valve 25 will open, shutting off the ports 168 and hence the leakage path from line 167.

Claims

CLAIMS.

1. A fuel control system, comprising: a first cylinder and a first piston; a first valve body movable between a first position for preventing fuel flow and a second position for permitting fuel flow, the first valve body being connected to the first piston; biasing means for biasing the first valve body towards the first position; and valve means for selectively connecting the first cylinder to a first source of fuel arranged to supply fuel at a first pressure and a first rate of flow.

2. A fuel control system as claimed in Claim 1, in which the valve means is further arranged to selectively connect the first cylinder to a second source of fuel arranged to supply fuel at a second pressure and a second rate of flow which is less than the first rate, or to a fuel sink.

3. A fuel control system as claimed in Claim 1 or 2, in which the valve means comprises a second valve body movable between a first position for connecting the first cylinder to the first fuel source and a second position for disconnecting the first cylinder from the first fuel source, a second cylinder connected to a controllable pressure fuel source, and a second piston located within the second cylinder and connected to the second valve body for movement therewith.

4. A fuel control system as claimed in Claim 3, in which the controllable pressure fuel source comprises a variable orifice connected to the second cylinder and via a first flow restrictor to a high pressure connection.

5. A fuel control system as claimed in Claim 4, in which the variable orifice comprises a fixed orifice co operating with a closure element for providing variable venting of fuel from the fixed orifice.

6. A fuel control system as claimed in Claim 4 or 5 when dependent on Claim 2, in which the second fuel source comprises a further variable orifice connected to the first cylinder and via a second flow restrictor to the high pressure connection.

7. A fuel control system as claimed in Claim 6, in which the further variable orifice comprises a further fixed orifice co-operating with the closure element for providing variable venting of fuel from the further fixed orifice.

8. A fuel control system as claimed in Claim 5 or Claims 6 and 7 when dependent on Claim 5, in which the closure element is connected to an actuator.

9. A fuel control system as claimed in Claim 8, in which the actuator is a torque motor.

10. A fuel control system as claimed in Claim 8 or 9, in which the actuator has a first stable state in which the fixed orifice is open and a second stable state in which the fixed orifice is closed.

11. A control system as claimed in Claim 10 when dependent on Claim 2, in which the further fixed orifice is open when the actuator is in the first and second stable states, and in which the actuator has a third stable state in which the fixed orifice is open and the further fixed orifice is closed.

12. A fuel control system as claimed in Claim 2 in which the first and second pressures are equal.

13. A fuel control system as claimed in any one of the preceding claims in which the biasing means comprises a spring.

14. A fuel control system as claimed in any one of the preceding claims in which the biasing means comprises supplying means for supplying fuel at a third predetermined pressure, which is less than the first pressure, to the first cylinder.

15. A fuel control system as claimed in Claim 14, in which the supplying means comprises a high pressure fuel connection and a low pressure fuel connection connected to the first cylinder via third and fourth flow restrictors respectively.

16. A fuel control system as claimed in any one of the previous claims, further comprising means for supplying fuel at high pressure to the first cylinder when the first valve body is at the first position and means for selectively removing that supply of fuel at high pressure.

17. A fuel control system comprising: a first cylinder and a first piston; a first valve body movable between a first position for preventing fuel flow and a second position for permitting fuel flow, the first valve body being connected to the first piston; biasing means for biasing the first valve body towards the first position; and a valve for selectively connecting the first cylinder to a source of fuel at a first pressure.

18. A fuel control system substantially as hereinbefore described with reference to and as shown in Figure 2 of the accompanying drawings.

19. A fuel control system substantially as hereinbefore described with reference to and as shown in Figure 3 of the accompanying drawings.