Fuel control system for a gas turbine engine

A fuel control for a gas turbine engine comprises a metering device and a pressure relief valve for spilling fuel from an upstream side of the metering device. The relief valve is also responsive to a servo pressure which is regulated by a valve which is operated when a speed of the engine rises above a predetermined level, so that in these circumstances an increased amount of fuel is spilled to reduce flow to the engine. A further valve is operable to vent the servo pressure completely, and to effect spill of substantially all of the fuel, when engine shut-down is required.

BACKGROUND OF THE INVENTION 
This invention relates to a fuel control system for a gas turbine engine. 
It is known, for example, from UK patent application 2,088,962A, to provide 
a gas turbine engine fuel control system which includes a flow limiting 
valve in series with a metering device. The flow limiting valve operates 
when selected engine conditions exceed predetermined levels, and may also 
be operated to shut off flow completely, in response to a signal which 
requires the engine to be stopped. It is also known, for example from the 
above patent application, to provide a pressure relief valve which is 
connected between the outlet and inlet of a positive displacement pump 
which supplies fuel to the engine. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a fuel control system 
in which the aforesaid series flow limiting valve is dispensed with and in 
which flow limitation is effected by a modified form of pressure relief 
valve. The modified pressure relief valve may also be maintained in a 
fully open condition to spill fuel sufficiently to reduce pressure within 
the system to a level at which flow to the engine ceases. 
According to the invention, there is provided a fuel control system for a 
gas turbine engine, comprising a metering device, means for maintaining a 
substantially constant pressure difference across said metering device and 
a first valve, responsive to fuel pressure upstream of said metering 
device and to values of an engine speed in excess of a predetermined 
level, for spilling fuel from said upstream side of said metering device. 
In a preferred embodiment, said first valve is responsive to a servo 
pressure, and there is provided a second valve responsive to said engine 
speed for regulating said servo pressure, and a third valve for setting 
said servo pressure to a level at which said first valve is fully open. 
In a particular embodiment there is provided a fourth valve in series with 
said metering device, and means for biassing said fourth valve shut, said 
fourth valve being operable against said biassing means by the fuel 
pressure downstream of said metering device, and a fifth valve actuable by 
said fourth valve, said fifth valve being operable when said fourth valve 
is shut to maintain said servo pressure at the level at which said first 
valve is fully open.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1, a gas turbine engine 10 is supplied with fuel from a 
positive displacement pump 11, fuel flow being regulated by a variable 
metering device 12 which is under control of a high integrity digital 
computer 13 which is responsive to desired and sensed operating conditions 
of the engine 10. The metering device 12 is shown in more detail in FIG. 
3, and fuel flow therefrom passes to the engine 10 by way of an 
arrangement 14 shown in more detail in FIG. 4. Fuel flow to the engine 10 
may be limited by a combined pressure relief and spill valve arrangement 
15 which is shown in more detail in FIG. 2, and to which fuel is delivered 
by the pump 11 through a line 16. A filter connection 17 to the line 16 
provides high pressure filtered fuel to a delivery line 18. 
As shown in FIG. 2, the arrangement 15 includes a relief valve 20 having a 
control element 21 which has a lesser area responsive to the pressure in 
line 16 and a larger area responsive to a servo pressure in a chamber 22. 
The element 21 is also biassed against the pressure in line 16 by a spring 
23. The valve 20 is operable to spill fuel from the line 16 to a low 
pressure return line 24 which communicates with the upstream side of the 
pump 11. The pressure in chamber 22 is derived from that in line 18 in a 
manner to be described and is applied through a line 68. The pressure in 
chamber 22 can also be regulated through a line 25 and a port 26, the port 
being shut off by the control element 21 when the valve 20 is half open. 
Reduction of pressure in the line 16 in response to a reduction of the 
servo pressure in line 25 is thereby limited. The chamber 22 can be 
connected to the low pressure return line 24 by means of an 
electromagnetic valve 27 which is opened by an electrical signal on a line 
28 in response to a requirement to shut down fuel flow to the engine 10. 
Energisation of the valve 27 allows the valve 20 to become fully open and 
substantially all of the fuel in line 16 to be spilled to the return line 
24. The consequent reduction of pressure in line 16 is used, in a manner 
to be described, to maintain a low pressure in the chamber 22, and hence a 
low pressure in line 16, even after the valve 27 has subsequently been 
shut. 
The metering device 12 shown in FIG. 3 includes a metering valve 30 which 
receives fuel on the line 16 from the pump 11. The valve 30 includes a 
control element 31 which is responsive to the pressure in a chamber 32. A 
valve 33 and a flow restrictor 34 are arranged in series between a chamber 
35 and the high pressure line 18. The chamber 35 communicates by way of a 
passage 36 with the low pressure return line 24. The pressure in chamber 
32 is that intermediate the valve 33 and restrictor 34. The valve 33 is 
pivotally movable by a dual-coil torque motor 37, each separate coil of 
the motor 37 being effective to operate the valve 33. Current is supplied 
to respective coils of the motor 37 from drive circuits (not shown) 
associated with the digital computer 13. The control element 31 regulates 
fuel flow from the line 16 to a line 38 and the operating position of the 
element 31 is detected by a synchro-resolver 39 from which the computer 13 
derives position signals. A feedback spring 40 coacts with the control 
element 31 and with the valve 33 so that movement of the element 31 alters 
the bias on the valve 33 in such a way as to vary the pressure in chamber 
32 to oppose the aforesaid movement of the control element 31. An 
adjustable orifice 41 allows a small quantity of fuel to flow from the 
high pressure line 18 to the line 38, providing an additional fuel flow to 
the engine, this additional fuel flow having a maximum effect during 
engine starting, when metered fuel flow would otherwise be low. 
The metering device 12 also includes a valve arrangement 50 for spilling 
fuel from the line 16 back to the low pressure return line 24. The spill 
valve 50 has a control element 51 which is slidable in a bore 52 to 
control flow between the lines 16, 24. The control element 51 is 
responsive to a servo pressure in a chamber 53 which is defined between a 
housing 54 and one end of the control element 51. A piston 55 is slidable 
within the control element 51 and includes a stem 56 which extends through 
the end of the control element 51 and defines, in conjunction with that 
end, a valve 57 for regulating servo pressure in the chamber 53. The valve 
57 is in series with a flow restrictor 58 between the line 18 and a 
passage 59 which communicates with the line 38 downstream of the metering 
valve 30. The piston 55 is thus responsive to a difference between the 
pressures upstream and downstream of the metering valve 30 and is biassed 
against movement in response to this pressure difference by a spring 60 
which is supported by means of a collar 61 on a stack 62 of bi-metal discs 
which are responsive to fuel temperature. The bias applied by the spring 
60, for a given position of the piston 55, is thus adjusted in accordance 
with fuel temperature. A further spring 63 acts to shut the valve 50 on 
the absence of a pressure in the line 16. 
An increase in the pressure difference across the metering valve 30 causes 
the piston 55 to move downwardly, opening the valve 57 and reducing the 
servo pressure in the chamber 53. The control element 51 then moves 
downwardly to increase spill flow until the aforesiad pressure difference 
returns to its original value. 
As shown in FIG. 4, the arrangement 14 includes a pressure raising valve 70 
to which fuel can flow through the line 38 and has a control element 71 
biassed by a spring 72 towards a shut position, the element 71 being 
movable against the spring 72 by the pressure in line 38, to allow fuel 
flow to the engine 10 through a delivery line 73. A function of the valve 
70 is to prevent fuel flow to the engine 10 until the fuel pressure within 
the system has reached a predetermined level which is sufficient to 
provide the necessary operating servo pressures. The control element 71 
includes an annular groove 69 which, when the valve 70 is shut, 
interconnects ports 74 and 75. The port 74 communicates through a line 68 
with the servo pressure chamber 22 of the spill valve 20 (FIG. 2). The 
arrangement 14 also includes a servo pressure control device 76 which has 
a valve 77 which is biassed shut by an adjustable spring 78 and which can 
be moved against this spring by a torque motor 79 when the latter is 
rotated clockwise, as viewed in the drawing. The valve 77 is operable by 
the torque motor 79 to connect the chamber 22 (FIG. 2) to the low pressure 
return line 24, by way of the port 26 and line 25. 
The device 76 includes a further valve 80 which is also controlled by 
torque motor 79, the valve 80 being open in the de-energised condition of 
the motor 79. The valve 80 connects the low pressure return line 24 to the 
port 75 of valve 70. The port 75 also communicates with the line 18 
through flow restrictor 81. The line 68 communicates with the line 18 
through a restrictor 82. With the valve 77 and port 75 shut the pump 
delivery pressure in line 18 is thus applied to the larger area of the 
control element 21 (FIG. 2) to urge the latter shut against the pressure 
in line 16. 
The torque motor 79 is operated by signals from a computer 83 (FIG. 1) 
which is responsive to the speeds NL and NI of low and intermediate 
pressure shafts respectively of the engine 10. An apparatus for 
controlling a valve in response to overspeeding by either of two shafts of 
a gas turbine engine is described in UK patent application No. 8220913, 
and such an apparatus may be used to energise the torque motor 79 in the 
present invention. If either speed exceeds a predetermined level the 
computer 83 energises the torque motor 79 clockwise to open the valve 77 
and thereby to reduce the servo pressure in chamber 22 by way of the line 
25. The relief valve 20 thus opens to reduce fuel pressure in the line 16, 
and hence flow to the engine 10. The amount by which the valve 20 can open 
is limited by the position of the port 26, which shuts when the valve 20 
is approximately half open. With the valve 77 open the control element 21 
of the valve 20 adopts an equilibrium position in which flow through the 
restrictor 82, line 68, port 26, line 25 and valve 77 results in an 
intermediate pressure in chamber 22, this intermediate pressure acting on 
the larger area of the element 21 to balance the pump delivery pressure 
acting on the smaller area of that element. When the engine speed has 
fallen to an acceptable level the torque motor 79 allows the valve 77 to 
be shut by the spring 78, the servo pressure in line 25 rises towards that 
in line 18, and the valve 20 shuts. 
The valve 77 is normally shut and, as described above, if the valve 27 
(FIG. 2) is energised the servo pressure in chamber 22 falls to a low 
value and the valve 20 can become fully open to spill substantially all of 
the fuel from line 16. The resulting drop in system pressure allows the 
valve 70 (FIG. 4) to shut under the influence of the spring 72, thereby 
preventing fuel from reaching the engine 10 through the line 73. In this 
condition of the valve 70 the ports 74, 75 are interconnected by the 
groove 69 so that the servo pressure in the chamber 22 of the valve 20 is 
connected to the low pressure return line 24 through the line 68, the 
ports 74, 75 and the valve 80. 
When the pressure raising valve 70 is shut an indicator device 84 (FIG. 1) 
is energised through a microswitch 85, and a plate valve 86 is opened to 
connect a drain line 87 from the engine fuel manifold to a dump connection 
88. The microswitch 85 also causes the electromagnetic valve 27 to be 
de-energised, and therefore shut. Pressure in chamber 22 is nevertheless 
maintained low by the interconnection between the ports 74, 75 as 
aforesaid. 
When the engine is to be started the torque motor 79 (FIG. 4) is energised 
anticlockwise, shutting the valve 80. The pressure raising valve 70 will 
at this stage be shut and the ports 74, 75 interconnected. Shutting the 
valve 80 effectively over-rides the valve provided by the ports 74, 75 and 
the pressure in line 68 can rise. 
As the system pressure increases, the rising pressure in line 68 shuts the 
valve 20 and further raises the pressure in lines 16, 18. The pressure in 
line 18 is applied through restrictors 81, 82 and the port 75 to open a 
start valve 90 against a spring 92. Though the valve 70 is shut fuel can 
nevertheless pass from the line 38 through a passage 95 to a start supply 
line 91. When the system fuel pressure reaches a predetermined value the 
valve 70 opens, supplying fuel to the main burners of the engine 10 
through the line 73. 
After the engine 10 has been started the torque motor 79 is de-energised, 
opening the valve 80 and removing pressure from the start valve 90 which 
is then shut by its spring 92. The valve 90 is maintained shut by the 
pressure in line 38, applied through a line 95. An orifice 96 (FIG. 1) 
interconnects the lines 73 and 91 and permits fuel to flow continuously to 
the starter jets of the engine 10, through the line 91, and thereby 
prevents charred fuel residues from accumulating in the starter jets. 
The fuel pressure in line 16 is applied to the valve 77 through the line 
18, restrictor 82, line 68 and line 25. If this pressure exceeds a value 
which is set by the spring 78, and which is a predetermined multiple, for 
example 1.1, of normal system pressure in line 16, the valve 77 lifts and 
thereby prevents the pressure in the chamber 22 of valve 20 (FIG. 1) from 
rising in response to any further increase in the system pressure. Any 
such further increases will cause the valve 20 to open and spill fuel from 
the outlet of the pump. The valves 77 and 20 thereby co-operate to provide 
a system pressure relief device.