Fuel controls for gas turbine engines

Apparatus for controlling the flow rate of fuel fed to a gas turbine engine, particularly but not exclusively to prevent overspeeding of the engine above a maximum safe level in the event of failure of a primary control system and associated components. A first control valve V3 is associated with means for sensing pressure drop .DELTA.P across a series metering valve V1 and arranged to control the opening or closing of a bypass valve V2 to keep .DELTA.P essentially constant and equal to a set point. A second control valve V4 is associated with a speed-sensing governor and arranged to control the opening or closing of that same bypass valve V2, so that engine fuel rate is controlled to maintain speed at an overspeed set point if abnormal conditions or failures tend to cause overspeeding. When the overspeed governor valve begins to be active as speed exceeds a first threshold and moves toward the overspeed set point, the governor valve V4 disables the first control valve to prevent the latter's action from closing the bypass valve and thus tending to increase engine fuel rate.

The present invention relates in general to the control of gas turbine 
engines and, in particular, to the control of fuel feed to such engines 
for governing or limiting rotational speed. Although not so limited in its 
various applications, the invention will find particular advantage with 
what are commonly called electrical or electronic control systems for gas 
turbine engines. 
OBJECTS AND ADVANTAGES OF THE INVENTION 
A primary aim of the invention is to provide overspeed safeguarding in the 
control of gas turbine engines in a fashion which acts successfully in 
response to failure or malfunction of essentially any component of a 
primary control system, including particularly failure of either a main 
fuel metering valve or a constant pressure drop control valve 
conventionally associated with the metering valve. 
A coordinate objective is to provide an overspeed governor in a fuel 
control system for gas turbine engines, wherein the governor is in series 
with a constant pressure drop control--with the result that the 
maintenance of constant pressure drop across a main metering valve is 
disabled when governing action begins, and speed is isochronously 
controlled at the overspeed set point. 
Another object of the invention is to provide a gas engine fuel control 
system in which constant pressure drop across a main metering valve is 
normally maintained by a .DELTA.P control valve which acts upon a variable 
fuel bypass valve, but in which an overspeed governor is coupled to the 
same bypass valve via the .DELTA.P control valve--such that overspeeding 
is prevented by governing action even if the .DELTA.P control valve sticks 
or fails in a fashion that tends to make the main valve pressure drop 
excessive. 
And it is also an object of the invention to provide such overspeed 
protection without shutting down the engine or drastically reducing power, 
but on the contrary by keeping the engine speed isochronously governed at 
the overspeed set point so long as the conditions tending to cause 
overspeeding continue to exist. This enables the pilot of an aircraft 
powered by the engine more readily to keep control of the aircraft without 
emergency glide or disturbing power surges.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
To explain the background environment in which the invention resides, the 
drawing in simplified fashion illustrates an aircraft gas turbine engine 
and a primary fuel control which normally determines and establishes the 
rate at which fuel is supplied to the engine. That is, an engine 10 is 
shown as including a housing 11 with its conventional tailpipe 12 and 
containing a journaled rotor shaft carrying a compressor 14 and a driving 
turbine 15. The turbine is disposed downstream of a plurality of 
circularly spaced burners 16 (or annular combustion chamber) which are fed 
with fuel via nozzles connected to a manifold ring (not shown). Those 
skilled in the art will, without more, understand the organization of the 
engine, the details of which are not critical to the practice of the 
invention. The engine may, of course, be of the so-called "fan" type and 
it may have two or more rotors with a so-called "core". 
Fuel is fed to the burner manifold via a main conduit 18 on the output side 
of a positive displacement pump 19 driven from the engine 10 and having 
its input leading via a boost pump (not shown) from a source 20 of liquid 
fuel, i.e., from a fuel tank. The rate of fuel flow Q.sub.p from the pump 
19 is proportional to engine speed. The pressure P1 at the output side of 
that pump is determined by the "hydraulic impedance" created by two 
parallel flow paths. The first flow path for fuel fed to the engine at a 
rate Q.sub.e (expressible as pounds per hour or gallons per minute) 
proceeds along the main conduit 18, through a main metering valve V1 and 
thence to the burner manifold and burners 16. The second flow path shunts 
liquid fuel at a flow rate Q.sub.b from the pump output (i.e., from the 
main conduit) via the variable opening of a bypass valve V2 back to a low 
pressure sump, here shown as the fuel tank or source 20. It is 
self-apparent that the pump flow rate Q.sub.p is equal to the sum of the 
flow rate Q.sub.e to the engine and the bypass flow rate Q.sub.b. As 
engine speed N (expressible in units of r.p.m.) increases, one sees that 
the bypass valve must open wider if the opening of the metering valve V1 
remains the same and if engine fuel rate Q.sub.e is to be kept unchanged. 
On the other hand, if the metering valve V1 (in series with the burner 
nozzles) is opened or closed, the engine fuel rate Q.sub.e will increase 
or decrease. Thus, engine fuel rate, engine speed, and engine thrust power 
are determined by a primary control system which acts to adjust the series 
metering valve V1 as a final controlled element. 
Although the primary control system may take any one of various known 
configurations to act on the series metering valve V1, the drawing 
illustrates, by way of example, a "full authority digital electronic 
control" (FADEC) 24 familiar to those skilled in the art. This includes 
one or more programmed digital computers associated with analog-to-digital 
converters (ADC's) and digital-to-analog converters (DAC's) so that the 
values of several sensed engine parameters are dynamically treated 
according to a pre-established algorithm to arrive at a dynamically 
changed output signal A representing the commanded position (opening) of 
the metering valve V1. That commanded position is, in effect, tantamount 
to a commanded engine input fuel rate Q.sub.e for reasons made clear 
below. 
Merely as typical, the FADEC 24 is here shown as receiving input signals 
representing various parameters, e.g., atmospheric pressure, engine speed 
N, engine inlet air temperature, compressor discharge pressure (CDP), and 
power (pilot's) lever position. These are used in "computing" the command 
signal A to establish and vary the engine fuel supply rate Q.sub.e which 
is required or safe in various combinations of conditions (air 
temperature, aircraft speed, altitude, CDP, engine speed--to name 
examples) with appropriate "scheduling" of acceleration and deceleration 
limits which avoid excessive burner temperatures, compressor stall, or 
flame-out. The command signal A is fed to a torque motor 25 to adjust the 
position of a spring-biased control member serving as the input to a 
hydromechanical amplifier 26 which rotates the valve V1 to a position 
agreeing with the A signal value. Agreement is assured by closed loop 
feedback from a position sensor 27--so that corrective action is continued 
until the effective value of the signal A equals that of the signal B. 
According to common practice, the FADEC is organized and programmed on the 
premise that engine fuel flow rate Q.sub.e is a known monotonic function 
of the metering valve position. To make that premise valid, the valve V1 
is constructed such that it behaves according to the well known hydraulic 
relationship: 
##EQU1## 
where Q.sub.e is the rate of flow through the valve, A.sub.1 is the area 
of the valve opening, .DELTA.P is the differential pressure or pressure 
drop across the valve, and k is simply a factor of proportionality. 
Obviously, 
EQU .DELTA.P=P1-P2 (2) 
Thus, when flow rate through the valve increases, .DELTA.P will increase if 
area A.sub.1 stays the same, and vice versa. Stated differently, if 
.DELTA.P is held constant, then one knows that flow rate Q.sub.e may be 
accurately increased or decreased--and established at a desired value--by 
adjusting the valve position to make the area A.sub.1 have a 
proportionally corresponding value. 
To achieve an essentially constant pressure drop .DELTA.P, a closed loop 
arrangement is associated with the bypass valve V2. As here shown, a 
.DELTA.P control valve V3 is made responsive to means for sensing the 
actual, existing value of the pressure drop .DELTA.P,--and that valve V3 
is hydraulically coupled to control the bypass valve V2 such that, under 
normal conditions, the value of .DELTA.P is maintained essentially 
constant and equal to a predetermined desired value (the .DELTA.P "set 
point") .DELTA.P.sub.d. 
For an understanding of how this constant .DELTA.P is achieved, it may be 
noted first that the bypass valve V2 in the present embodiment is 
associated with and in part formed by a piston actuator. That is, the 
valve V2 includes a piston 30 vertically slidable in a cylinder 31, and 
with the upper portion of that piston disposed to reduce or increase the 
variable, effective area of a valve passage through which the bypass flow 
Q.sub.b passes. A compression spring 32 creates an upward preload force on 
the piston 30 partially to remove the effect of the downward force arising 
from the pressure P1 acting on the upper surface area of that piston. In 
net effect, however, the vertical position of the piston 30 (and thus the 
opening or area of the valve V2) is determined by the volume of liquid 
fuel which is present in a "chamber" behind that piston. The "chamber" C 
is here collectively formed by the cylinder 31, a conduit 34, a cylinder 
35 in which a buffer piston 36 resides, and a conduit 38 leading back to 
the valve V3. 
Conduit 38 may be viewed as coupled to a controlled port or output line 38a 
of the valve V3. The latter is constituted by a land 40 formed on a valve 
rod 41 vertically movable in a stationary housing (and rotationally 
driven, if desired, to eliminate stiction), there being a pressure input 
line 42 above the land 40 and a return line 44 below the land 40. The 
return line leads to a low pressure P.sub.B at a sump, here shown as 
returning to the source or fuel tank 20. Sealing lands 40a and 40b are 
spaced above and below the valve land 40. The vertical position of the rod 
41--and thus of the valve land relative to the port 38a--is determined by 
the balance of forces exerted (i) in a downward direction by a compression 
spring 45, (ii) in an upward direction due to pressure acting on the 
underside of an actuator piston 46, and (iii) in a downward direction by 
pressure acting on the upper surface of the piston 46. The set point value 
.DELTA.P.sub.d is established by vertical adjustment of a stop screw 48 
against which the spring 45 bears. 
The actuator piston 46 constitutes a means for sensing the differential 
pressure drop .DELTA.P across the metering valve V1. As shown, hydraulic 
lines 49 and 50 lead from the upstream and downstream sides of that valve 
to the lower and upper portions of the housing cylinder in which the 
actuator piston is disposed. Thus, the upward and downward forces on the 
lowermost and uppermost equal surface areas of the piston 46 are 
proportional to the respective pressures P1 and P2. The net upward force 
is proportional to P1-P2 and thus to the pressure drop .DELTA.P. The 
position of the rod 41 will change (moving the land 40 relative to its 
port 38a) until the spring 45 is sufficiently relaxed or compressed to 
balance the net upward force on the piston 46. When the actual .DELTA.P is 
equal to the set point .DELTA.P.sub.d established by adjustment of the 
stop screw 48, the valve land 40 will be centered on the port 38a--thus 
connecting neither the pressure line 42 nor the sump return line 44 to the 
output line 38a and the chamber C associated with the bypass piston 30. 
In normal modes of operation, a source of high pressure fluid is coupled to 
the pressure input line 42. In accordance with the present invention, and 
for reasons to be described below, this coupling is established through a 
normally open overspeed governor valve V4. The overspeed governor will be 
treated later; for the present, it is sufficient to note that, in this 
example, fuel at the relatively high pressure P1 in the main conduit 18 is 
coupled via a filter 50 and a supply line 51 to a cylinder 52 in which a 
land 54 (forming the valve V4) is disposed. As shown, that land is 
normally beneath its associated output port and conduit 42a so the valve 
V4 normally makes the pressure P1.sub.fg equal to P1.sub.f --the latter 
being the pressure on the output side of the filter and essentially equal 
to P1. In summary, in normal modes of operation, fluid at high pressure is 
supplied to the input line 42 of the .DELTA.P valve V3 via a normally open 
valve V4 associated with an overspeed governor. 
The operation by which .DELTA.P is normally held constant may now be 
understood. If in steady state conditions the valve land 40 is centered 
(as shown) on its output port 38a, then fluid is for all intents and 
purposes trapped in the "chamber" 38, 35, 34, 31. The bypass piston 30 is 
stationary at a position which makes the opening of the valve V2 produce 
bypass flow at a rate Q.sub.b which causes the flow Q.sub.e to produce a 
pressure drop .DELTA.P which is equal to the set point .DELTA.P.sub.d. If 
now the FADEC 24 should cause the metering valve V1 to open or close, 
thereby increasing or decreasing the area A.sub.1 appearing in Eq. (1) 
above, the actual value of .DELTA.P will tend to decrease or increase. 
In the first case, .DELTA.P becomes less than .DELTA.P.sub.d and the spring 
45 will shift the valve land 40 downwardly-- so that the pressure P1.sub.f 
forces fluid to flow via conduit 51, the open valve V4, conduit 42, and 
into the output line 38a, 38. As such fluid enters the bypass chamber C, 
it shifts the buffer piston 36 upwardly against the tension of a 
bi-directional spring 55, and in part passes through an orifice 56, so 
that the piston 30 moves upwardly (and essentially as if the buffer piston 
were not present) to progressively close the bypass valve V2. This in turn 
reduces the bypass valve flow Q.sub.b and increases the engine flow 
Q.sub.e until the pressure drop .DELTA.P rises again to the set point 
value and the valve land 40 is restored to its illustrated neutral 
position--whereupon the admittance of fluid into the "chamber" ceases. The 
rate of fluid admittance into the "chamber" will be generally proportional 
to the opening of the valve V3, which is in turn proportional to the 
difference or error between .DELTA.P and .DELTA.P.sub.d. The piston 30 
moves upwardly at a velocity proportional to the V3 opening which connects 
the lines 42 and 38 and thus the position of the piston changes as a time 
integral function of the .DELTA.P error. 
In the second case, when the metering valve V1 is for any reason closed 
somewhat, the actual drop .DELTA.P will increase. The valve rod 41 will 
therefore move up to lift the land 40 above the illustrated neutral 
position, thereby establishing a flow path from the chamber C via the 
conduit 38, the port 38a and the return line 44 to the sump 20. The 
pressure P1 at the top of piston 30 keeps the fluid in the chamber C 
pressurized, although a value somewhat less than P1 due to the upward 
force of the spring 32. The presence of that pressure causes venting 
action, i.e., flow of fluid through the conduit 38, the port 38a, and the 
return line 44 to the low pressure P.sub.b at the sump. As fluid is 
vented, the piston 30 moves down (again at a rate proportional to the 
amount by which the valve V3 is open to the line 44) to progressively open 
the bypass valve V2. This increases the flow Q.sub.b, decreases the flow 
Q.sub.e, and causes the pressure drop .DELTA.P to fall until it is 
restored to the set point value .DELTA.P.sub.d --and the land 40 is 
restored to neutral. 
It may be seen, therefore, that closed loop action keeps the pressure drop 
.DELTA.P essentially constant and equal to the desired value 
.DELTA.P.sub.d (which is selected by setting the screw 48) when for any 
reason a difference arises between the two. If the FADEC 24 moves the 
valve V1 in a closing direction, or if the engine speed increases to 
increase the pumped flow Q.sub.p, the bypass valve V2 shifts to become 
more widely open until .DELTA.P is restored to the set point. If the FADEC 
24 moves the valve V1 in an opening direction, or if the engine speed 
decreases to decrease the pumped flow Q.sub.p, the bypass valve V2 shifts 
in a closing direction. Because .DELTA.P is thus kept essentially constant 
at a known value, the flow Q.sub.e is directly proportional to the area 
A.sub.1, and thus related by a known function to the position of the valve 
V1. 
In the operation of a gas turbine engine, it is imperative that speed never 
be permitted to exceed some designated safe value N.sub.os which is 
specified by the engine designer. Among other considerations, centrifugal 
forces on compressor or turbine components may cause them to literally fly 
apart at an excessive speed, or lubrication and bearing failures may be 
experienced. Normally the primary control system will not let the main 
metering valve V1 pass fuel to the engine at a rate Q.sub.e which lets the 
speed N exceed a maximum speed N.sub.m which is the highest value which 
the FADEC 24 may schedule or permit when there is no failure or 
malfunction. The sensed speed signal N fed to the FADEC 24 is processed by 
the latter so it causes the main valve V1 to move in a closing direction 
if engine speed attempts to exceed the value N.sub.m. But when the engine 
is accelerated during a "throttle burst", the fuel flow rate Q.sub.e may 
indeed be greater than that which ultimately (if the valve V1 were not 
reclosed) would run the engine speed up above the top speed N.sub.m, and 
indeed above the safe overspeed value N.sub.os. Or, when the engine is 
operating at a given speed at low altitude with a given setting of the 
valve V1, as altitude is increased, engine speed may well exceed that safe 
value if the valve V1 is not closed somewhat, and the primary control must 
then move the valve V1 in a closing direction. 
Wholly unexpected, unintended and low-odds failures may occur in the 
primary control system, however, for various reasons. For example, the 
sensor which supplies the signal N to the FADEC 24 might malfunction to 
produce a signal saying that engine speed is lower than it actually is; 
the FADEC may then in blithe ignorance call for the metering valve V1 to 
open so wide that the engine overspeeds beyond the value N.sub.os. It has 
been a common practice, therefore, to provide an overspeed governor 
separate from the primary control system to take over and limit fuel flow 
if an overspeed "trip point" is reached. In such arrangements, however, 
that overspeed governor acts with an overriding effect on the main 
metering valve; the overspeed governor is helpless if the metering valve 
itself sticks or fails when opened to a position, for example, during 
acceleration or at low altitude, which under different conditions will 
cause the overspeed ceiling to be exceeded. In another type of overspeed 
protection mechanism (exemplified by the Woodward Governor Company product 
X83556, which is a fuel control for the General Electric CFM 56 type of 
gas turbine engine), tachometer speed sensing flyweights are provided to 
furnish speed-representing rotational input to a three-dimensional cam 
which participates in acceleration/deceleration scheduling of fuel input 
rate to the engine by the action of a series metering valve; as the 
rotational input reaches a magnitude corresponding to an overspeed trip 
point, a camming arm affirmatively displaces a .DELTA.P control valve 
which then vents fluid behind a bypass valve piston. In this prior 
mechanism, overspeeding is prevented by opening of the bypass valve, and 
despite failure of the series metering valve, but such prevention (a) does 
not govern with stability at the overspeed trip value, (b) requires 
participation of the .DELTA.P control valve, and (c) cannot be obtained if 
the .DELTA.P control valve sticks or fails in a position which keeps fluid 
pressure behind the bypass valve piston. Thus, prior overspeed limiting 
arrangements have provided the desired insurance factor against certain 
types of unexpected malfunctions, but they have not provided protection 
against failure of either or both the main metering valve and a constant 
.DELTA. P control valve. 
In accordance with the present invention, a governor is arranged to act not 
upon the main metering valve but rather upon the bypass valve to control 
and hold engine speed at a maximum reference value N.sub.os if and when 
for any reason the engine speed tends to rise above that safe value. The 
governor acts upon the bypass valve through the .DELTA.P control valve in 
a fashion such that failure of either the metering valve or the .DELTA.P 
control valve will preclude overspeeding. 
In the embodiment here illustrated, the overspeed governor G comprises 
speed sensing means independent of that which supplies the signal N to the 
FADEC 24, and specifically a set of flyweights 58 driven from the engine 
and acting to transform centrifugal force into an upward force on a 
rotating governor rod 60. That rod is vertically slidable in an hydraulic 
casing and may shift the valve land 54 relative to the port or output line 
42a of the governor valve V4. When the governor is active, the position of 
the rod 60 (and the opening of the port 42a) is determined by balancing 
the vertically upward force of the flyweights 58 and the vertically 
downward force of a compression spring 61 disposed between an upper rod 
land 62 and a bearing 64 whose vertical position is adjustable by a stop 
screw 65. Adjustment of the screw 65 determines the speed "set point" 
N.sub.os of the governor. The set point value N.sub.os is that value of 
engine speed which results in the valve land 54 being centered relative to 
the output port 42a. 
Because the governor G here illustrated is an overspeed governor for 
back-up safety, the spring 61 and screw 65 are chosen and adjusted such 
that they normally predominate over the flyweight force to hold the rod 60 
down at a position determined by suitable mechanical stop means. Stops 66 
below the flyweight fingers establish the lower limit position of the rod 
60, and determine the "normal" position of the land 54 which, as shown, 
makes the valve V4 normally open to provide an unimpeded connection 
between the conduits 51 and 42. In this position, the land 54 and valve V4 
close off a connection between the controlled port 42a and a return line 
68 which leads back to the sump 20, here assumed to reside at a low 
pressure P.sub.B. Assuming merely for purposes of discussion that maximum 
engine speed N.sub.m is 7000 r.p.m. and that engine specifications 
designate that an overspeed value of 7500 r.p.m. is not to be exceeded, 
then the spring 61 is adjusted to make the valve land 54 rise to a 
"neutral" position (just covering the port 42a) at what may be called the 
overspeed set point N.sub.os equal to about 7420 r.p.m. With that as a 
reference set point, the valve rod will be held down with the flyweight 
arms against the stops 66 so long as engine speed is below or at 7000 
r.p.m. As speed progressively rises above 7000 r.p.m., the valve land 54 
will progressively close off the connection between conduits 51, 42--with 
maximum impedance to flow occuring at 7420 r.p.m. As speed increases still 
further, a progressively wider valve connection is created between the 
conduits 42 and 68--so that fluid may be returned to the sump 20 from the 
conduit 42. 
The governor rod 60 is formed to include a pressure sensing piston 69 
riding in a cylinder and bounded by sealing lands 70, 71. This piston 
responds to any pressure drop across the orifice 56 in the buffer piston 
to create a compensating force (supplementing the upward force of the 
flyweights 58 and the downward force of the spring 61), thereby making the 
governor stable and isochronous when conditions tending to cause 
overspeeding continue to exist. The compensating action--of a 
spring-centered buffer piston, associated with an orifice, to provide 
"temporary droop" feedback to a governor valve rod--is described in the 
prior art, for example, U.S. Pat. Nos. 2,478,752 and 2,478,753 and 
2,765,800. Without repeating that description in full, it may be noted 
here that when fluid is admitted to or vented from the conduit 38 at a 
high rate, it cannot pass through the small orifice 56 at that rate. The 
buffer piston 36 is thus displaced from the centered position established 
by the spring 55, making the pressure different on opposite sides of that 
piston. Then as flow through the conduit 38 falls off toward zero, the 
spring 55 forces the piston 36 back to its centered position, with the 
slow rate of return established by flow through the orifice 56. So long as 
there is flow through that orifice, and thus a differential pressure drop 
across it, there is a difference in pressures applied to upper and lower 
surfaces of the sensing piston 69--thereby creating a force which may be 
viewed as algebraically adding to or subtracting from the force of the 
spring 61. This compensating force gradually and smoothly returns to zero, 
however, as the buffer piston returns to its centered position. 
To consider the action of the governor G, assume first that the valve rod 
41 has been shifted downwardly from the illustrated neutral position so 
that the valve V3 essentially fully connects conduit 42 to port 38a and 
conduit 38. When actual engine speed N is equal to the overspeed set point 
N.sub.os and valve land 54 is centered on its port 42a, then for all 
intents and purposes fluid is neither fed to nor vented from the "chamber" 
42, 38, 35, 34, 32--and the bypass piston will remain in a steady-state 
position which holds engine speed at the overspeed set point N.sub.os. If 
engine speed rises above that value, the valve land 54 rises to make the 
valve V4 connect the conduit 42 to the sump return line 68--so that fluid 
is vented by flow through the open .DELTA.P control valve V3 from the 
"chamber", the piston 30 falls to move the valve V2 in an opening sense, 
the flow Q.sub.b increases, the flow Q.sub.e decreases (for any given, 
then-existing setting of the main valve V1), and the engine speed is 
correctively reduced back down to the set point N.sub.os. 
On the other hand, if the engine has been operating at the overspeed set 
point N.sub.os, and if engine speed begins to fall, the valve land 54 will 
move downwardly to progressively connect the conduit 51 to the conduit 
42--and now fluid is admitted via 51, 42, the open port 42a, and the open 
.DELTA.P valve V3 to the "chamber" C. The piston 30 will therefore rise to 
move valve V2 in a closing sense, the flow rate Q.sub.b will fall, the 
flow rate Q.sub.e will rise, and engine speed will tend to increase back 
up to the overspeed set point N.sub.os. Any speed greater than maximum 
speed N.sub.m is abnormal, however, and thus it is not important that the 
overspeed governor bring engine speed back up to the overspeed set point. 
Indeed, it is preferable if the conditions which initially tended to cause 
overspeeding cure themselves, that the speed then continue downwardly to 
that value called for by the FADEC 24. 
The cooperative interaction of the constant .DELTA.P valve V3 and the 
governor valve V4 may now be explained. From what has been said above, one 
sees that the governor valve V4 is a three-way valve in series with the 
pressure feed or input line 42 of the three-way valve V3. Valve V4 has 
high pressure (input) and low pressure (return) lines 51 and 68 
selectively and alternatively coupled to its output line 42 according to 
the position of the valve land 54. Valve V3 has high pressure (input) and 
low pressure (return) lines 42 and 44 selectively coupled to its output 
line 38 (and the "chamber" C) according to the position of its valve land 
40. When the valve land 54 is down and open, then valve V3 controls the 
positioning of the bypass valve piston 30 to keep the main valve pressure 
drop .DELTA.P essentially equal to the set point .DELTA.P.sub.d. When the 
valve land V3 is down and open, the valve V4 controls the positioning of 
the bypass valve piston 30 to adjust the flow rate Q.sub.e in a manner 
such that engine speed is held at or below the set point N.sub.os. 
When everything is "normal", speed N will be at or below rated maximum 
speed N.sub.m and the flyweight arms will bear against the stops 66 to 
locate the valve land 54 down and open (as the drawing shows). The valve 
land 40 will be essentially centered. If now some malfunction occurs 
(e.g., if the engine has been accelerating, the valve V1 has been very 
widely opened, and that valve linkage breaks or sticks) which causes 
engine speed to continue increasing above N.sub.m (e.g., 7000 r.p.m.), 
then the flyweights 58 will progressively raise the valve land 54 to 
progressively close the port 42a and create what amounts to a decreasing 
orifice or an increasing impedance to fluid flow. The valve V3 is not 
"ideal"; even when land 40 is centered, some fluid leaks from the high 
pressure Pl.sub.fg in conduit 42 to the lower pressure in conduit 38, and 
some fluid leaks from the higher pressure in conduit 38 to the lower 
pressure (P.sub.b) beneath the land 40 and in conduit 44. Normally, these 
leakages balance and the net flow into or from the conduit 38 is zero when 
the land 40 is centered. Thus, as the port 42a progressively closes when 
speed rises above N.sub.m, a pressure drop develops across the valve port 
54/42a and the pressure P1.sub.fg falls relative to the pressure P1.sub.f. 
Leakage rate from 42 into 38 is thus reduced as valve land 54 closes, but 
leakage rate from 38 into 44 remains the same. The net leakage is thus 
from the conduit 38 and the chamber C, so that the bypass valve V2 begins 
to open as speed begins to rise from N.sub.m and valve V4 begins to close. 
Such opening of the bypass valve increases the flow rate Q.sub.b and 
reduces the flow rate Q.sub.e --the effect of the latter being to (i) 
reduce the drop .DELTA.P and (ii) reduce the rate of rise of engine speed. 
Engine speed may continue to increase, despite this "anticipation" effect; 
as it does so the valve V4 closes more in an upward sense, the pressure 
P1.sub.fg falls more, the leakage imbalance increases, and net flow from 
the "chamber" increases to open valve V2 more, and further reduce both 
.DELTA.P and Q.sub.e. 
As the overspeed set point N.sub.os is approached, therefore, the "constant 
.DELTA.P" control valve V3 is disabled because .DELTA.P is forced to fall 
by the progressive closure of the governor valve V4. As .DELTA.P falls 
below .DELTA.P.sub.d, the sensing piston 46 permits the spring 45 to shift 
the valve land 40 to a somewhat "opened down" position--but no pressure 
fluid is in consequence forced into the "chamber" C because the valve V4 
is essentially closed and presents a large hydraulic impedance to flow 
from the conduit 51. 
If the conditions causing the increase in speed continue to exist, the 
valve V4 will rise to its centered position and essentially all flow into 
the "chamber" will cease, even with the valve V3 disabled, i.e., opened 
down from its neutral position. The pressure in the "chamber" will be 
essentially equal to P1.sub.fg. Indeed, the conduit 42 will be, in effect, 
a part of the chamber, and the governor valve land 54 vents fluid 
therefrom to the sump return line 68 if speed N rises above N.sub.os, or 
admits pressure fluid from the line 51 if speed N falls below N.sub.os. In 
the former case, the bypass valve V2 will open, the engine fuel rate 
Q.sub.e will decrease, and the engine speed brought back down to N.sub.os. 
In the latter case, the valve V2 will tend to close, the flow rate Q.sub.e 
will tend to increase, and speed will rise to N.sub.os. 
If the malfunction tending to cause overspeed should cure itself, the 
protective system will revert to a normal status. That is, if speed is 
being held at N.sub.os by the governor G, and the fuel flow rate Q.sub.e 
for some reason (e.g., the metering valve V1 recloses somewhat from a 
stuck wide open position), then (i) .DELTA.P will rise and (ii) speed N 
will fall. The valve land 40 will rise and the valve land 54 will 
fall--thereby converting valve V3 back to its neutral position and the 
valve V4 to its "open down" position. The governor will thus be restored 
to its normal, inactive condition; the valve V3 will aqain act to maintain 
.DELTA.P at the set point; and engine fuel rate will be controlled by the 
opening of the metering valve V1. 
Whenever the governor G is in control and the "constant .DELTA.P" valve V3 
has been disabled by the progressive action described above to make the 
valve land 40 "opened down", fluid is admitted from the pressure source 
51, or vented to the sump pressure P.sub.B at conduit 68, to or from the 
"chamber" by the control action of the governor valve land 54 connecting 
the conduit 42 either to 51 or 68. Such fluid flow into or out of the 
chamber C passes through the valve V3 essentially without restriction. 
Since the valve land 54 will be above or below its neutral position on the 
port 42a by an amount essentially proportional to the speed error 
(difference between the set point N.sub.os and actual speed), the rate of 
fluid flow into or out of the "chamber" will be generally proportional to 
the speed error, and the bypass piston velocity will be proportional to 
the speed error. This means that the position of the piston 30 will vary 
as the time integral of the speed error. Such integrating action would 
ordinarily cause hunting and instability absent any provision for speed 
droop. To remove that instability, and achieve return of any speed error 
transient to zero, so that isochronous control of speed can be realized, 
the integral action described above is modified by the buffer piston 36 
with its orifice 56 to provide a temporary, initially high, and gradually 
diminishing feedback force to the sensing piston 69 on the governor valve 
rod 60. This has the effect of giving the governor an integral plus 
proportional gain action, which is more conducive to stability than 
integral action alone. When a speed error is present, the inflow or 
outflow of fluid cannot all pass through the orifice 56 without creating a 
differential pressure drop thereacross. The difference in pressure on the 
upper and lower sides of the buffer piston makes the latter shift upwardly 
or downwardly from its centered position and loads the bi-directional 
spring 55 in tension or compression. The motion of the buffer piston 36 
will be proportional to speed error, and will displace a volume of fluid 
into or out of the cylinder 31 to cause a corresponding motion of the 
bypass piston 30 since the fluid is essentially incompressible. Thus the 
contribution to the bypass piston's motion due to displacement of the 
buffer piston 36 creates the proportional action of the governor, while 
the bypass piston's motion due to flow through the orifice 56 produces the 
integral action. The proportional action is temporary because as speed 
error begins to decrease, the spring 55 pulls or pushes the piston back to 
its original position where it comes to rest as the speed error reaches 
zero--flow through the orifice 56, and the pressure drop thereacross, and 
the displacement of the buffer piston 36 thus gradually returning to zero. 
So long as any pressure drop exists across that orifice, then a 
compensating force is exerted on the sensor piston 69 of the governor 
valve rod. For example, if the actual speed N rises suddenly above the set 
point N.sub.os, and fluid is vented at a high flow rate from the chamber C 
via the valves V3 and V4 and the conduit 68, the buffer piston will begin 
moving downwardly (compressing the spring 55). The pressure above the 
orifice 56 will be greater than that below the orifice; these two 
pressures--applied respectively to the upper and lower surfaces of the 
piston 69--thus create a net downward force on the rod 60 which aids the 
spring 61. This makes the speed error appear less than it really is, i.e., 
it prevents the valve V4 from opening between conduits 42, 68 as much as 
it otherwise would in the absence of the compensating feedback. Stated 
another way, the gain factor or transfer function (which makes the 
velocity of the piston 30 proportional to the speed error) is temporarily 
reduced that is, it contains a temporary proportionality effect. 
Therefore, the velocity of the piston 30 is less than it otherwise would 
be for a given speed error magnitude, due to the presence of the temporary 
proportionality, so that underspeeding below and hunting about the set 
point N.sub.os is prevented. The compensating force on the piston 69 falls 
off to zero as speed error approaches zero, however, so that the piston 30 
ends up at the new steady state position required to maintain a zero speed 
error. 
The compensation action of the buffer piston 36 is per se old and known in 
the art, but only in association with governors which act on a series 
metering valve or the like as a final control element which directly 
controls engine fuel input rate Q.sub.e. It is believed to be new in the 
art to employ the buffer piston 36 in association with a governor that 
acts on a bypass valve as the final control element--and thereby to 
achieve stable, isochronous control without droop and without instability. 
By such speed control as here described, surges and fluctuation in engine 
power are avoided when any malfunction, tending to cause overspeeding, 
arises. The fuel rate Q.sub.e is not severely reduced; engine flame-out is 
avoided; and the pilot is able to control his aircraft without guessing 
and correcting for decreases below and fluctuations about the engine power 
level produced when the speed is maintained at the set point N.sub.os. 
To review the operation of the interconnected pressure drop control means 
(the valve V3 and its .DELTA.P sensor 46) and the speed control means (the 
valve V4 and its speed sensor 58), one may first observe that regardless 
of the position of the governor valve V4, the bypass valve chamber C is 
vented by the valve V3 if .DELTA.P is greater than .DELTA.P.sub.d. On the 
other hand, if the valve land 40 is below center, and if the speed N is 
above N.sub.os, the bypass valve chamber C is vented through the open V3 
passage between 38 and 42, and through the open V4 passage between 42 and 
68, to the low pressure sump. Thus, it may be said that: 
The bypass valve is moved in an opening sense to reduce fuel rate Q.sub.e 
when 
EQU .DELTA.P&gt;.DELTA.P.sub.d (1) 
or 
EQU .DELTA.P.ltoreq..DELTA.P.sub.d and N&gt;N.sub.os. (2) 
When the valve V4 is in the illustrated position because N is less than 
N.sub.m, then pressure fluid will be admitted to the bypass valve chamber 
at a rate determined by the opening of valve V3 below its centered 
position. Similarly, if valve V3 is substantially below the centered 
position, pressure fluid will be admitted to the bypass valve chamber C at 
a rate determined by the V4 opening which connects conduits 51 and 42. 
Both such cases may be expressed: 
The bypass valve is moved in a closing sense to increase engine fuel rate 
Q.sub.e when 
EQU .DELTA.P&lt;.DELTA.P.sub.d and N&lt;N.sub.os 
If .DELTA.P=.DELTA.P.sub.d, and the valve V3 is centered, fluid is, in 
effect, trapped in the bypass valve chamber C if the valve V4 is open as 
shown in the drawing. The system is then at steady state with 
N.ltoreq.N.sub.m and .DELTA.P at the desired set point. If in these 
circumstances, however, speed N should rise above the value N.sub.m so 
that the valve land 54 begins to close the port 42a, then the pressure 
P1.sub.fg will begin to fall, and imbalanced leakage from 42 to 38 versus 
that from 38 to 44 will cause the "chamber" to be vented. This causes the 
bypass valve V2 to start opening, thereby causing .DELTA.P to start 
falling, and thereby forcing the valve rod 41 down to disable the .DELTA.P 
control loop. In other words, if the valve V3 is centered and normally in 
control of the bypass valve V2, the governor valve V4 "takes over"; it 
removes the ability of the valve V3 to increase .DELTA.P, and it forces 
.DELTA.P to fall after the speed rises above N.sub. m and even before it 
reaches N.sub.os. This tends to anticipate an overspeed and correct it 
even before the set point N.sub.os is reached; but if and by the time the 
set point N.sub.os is exceeded, the valve land 40 has been lowered 
sufficiently that the chamber C is vented via 42, 68 when the overspeed 
set point is exceeded. 
The overspeed protection afforded by the present invention is more nearly 
universal and complete, in safeguarding against all types of failures or 
malfunctions, compared to overspeed governors or limiters which act on a 
series metering valve. If the FADEC 24 malfunctions due to an electronic 
circuit failure, and increases the signal A to a value which opens the 
valve V1 so as to raise engine speed above the safe overspeed value 
N.sub.os, the known and conventional "constant .DELTA.P" apparatus of the 
prior art would tend to close the bypass valve and therefore aggravate or 
reinforce the overspeeding. But here, the overspeed governor G anticipates 
any overspeed by preventing the "constant .DELTA.P" apparatus from closing 
the bypass valve, and then affirmatively vents the bypass valve chamber if 
the overspeed set point is exceeded. If the FADEC circuits fail, if the 
torque motor 25 fails, if, the actuating amplifier 26 fails, if position 
sensor 27 fails, or if the valve V1 mechanically breaks or sticks in a 
wide open position--in any of these possibilities, the valve V3 will be 
prevented from increasing .DELTA.P when speed rises above some threshold 
value (here described as N.sub.m); and if speed should continue to rise to 
exceed the overspeed set point, the governor valve V4 will affirmatively 
open the bypass valve further to hold speed at the N.sub.os ceiling. If 
the .DELTA.P control valve V3 should stick in a position above neutral, or 
if the conduit 50 should rupture, or in the case that the conduit 49 
becomes blocked, overspeeding will not occur because the valve land 40 
will rise above the port 38a and continue to vent the chamber C until the 
valve V2 is essentially wide open and fuel flow rate Q.sub.e is reduced to 
a level that overspeeding is impossible even with valve V1 wide open. On 
the other hand, if the valve V3 should stick in a "down open" position, or 
if the conduit 49 should rupture, or if the conduit 50 becomes 
blocked,--the tendency would be for the bypass valve to go to a fully 
closed position, thereby to increase Q.sub.e and engine speed above a safe 
level unless the metering valve were closed down to a severe extent. But 
with the arrangement described, when overspeeding from that source tends 
to occur (with the valve V3 open down), the governor valve V4 will vent 
the bypass valve chamber C to restore and hold the speed at the set point 
N.sub.os. 
The cooperative synergism of (1) the means for maintaining constant 
.DELTA.P across the metering valve V1, and (2) the means for governing 
engine speed at an overspeed set point N.sub.os if speed exceeds a given 
threshold N.sub.m --both of which act on the bypass valve V2 as a common 
final element--is especially advantageous from the standpoint of economy 
and reliability. The .DELTA.P control valve V3 is required and utilized in 
any case so that the primary control system (e.g., the FADEC 24 here 
shown) can operate such that the engine fuel rate Q.sub.e is known for any 
given position of the metering valve V1. But with the governor G and its 
valve V4 coupled through the .DELTA.P control V3 as here described, 
overspeeding is prevented when its cause is failure in not only the 
primary control components or the metering valve V1 but also in the 
.DELTA.P control valve.