Fuel control

The present invention concerns a fuel control for a gas turbine aircraft engine. The control de-riches fuel-air mixture during ground start-ups in order to reduce a temperature within the engine.

The present invention relates to fuel controls for gas turbine engines and, 
more particularly, to apparatus for de-riching the fuel flow in gas 
turbine aircraft engines during ground starts. 
BACKGROUND OF THE INVENTION 
The rate of fuel delivery in a gas turbine aircraft engine is scheduled 
according to selected engine parameters, such as selected pressures, 
temperatures, and speeds of engine components. However, the engine 
performance changes with time: the engine "deteriorates." As a result, the 
original fuel schedule may become non-optimal as time progresses. 
Further, in some instances, it is impossible to predict in detail the exact 
performance which an engine will exhibit under a given fuel schedule. 
As an example, the original fuel schedule for a particular gas turbine 
engine can cause an unexpectedly high temperature in the low pressure 
turbine stage. It is desirable to alleviate this temperature problem, and 
to do so without alteration of the original fuel schedule. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide a new and improved fuel 
control. 
SUMMARY OF THE INVENTION 
In one form of the present invention, a source of pressure is connected to 
a line which carries a pressure signal to which a fuel scheduling control 
responds. This connection raises the pressure in the line, thus inducing 
the control to decrease fuel-air ratio.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a gas turbine engine 3 and an associated main engine 
control (MEC) 6. The MEC receives signals indicative of the pressure, 
temperature, or rotational speed of one or more engine components, and two 
of these signals are indicated as traveling along lines 12 and 15. One of 
the signals is a compressor discharge pressure (CDP) signal in line 12 and 
the other is a compressor bleed pressure (CBP) signal in line 15 which 
indicates a compressor bleed level. A check valve 16 in the CBP line 15 
prevents flow toward the engine. 
A T-connection or tap 18 is made in the CBP line 15 downstream of the check 
valve 16. The tap 18 connects the CBP line to a line 21 which leads 
through a check valve 24, a relief valve 27, and through a solenoid 30, 
thereby allowing a connection to be made between the CBP line 15 and a 
source of starter air pressure SAP. The solenoid 30 is connected to a 
sensor (not shown) which senses the fact that the aircraft's landing gear 
is down and, in such a case, opens the solenoid 30 so that a connection is 
made between T-connnection 18 and SAP. 
The operation of the present invention is as follows. Starter air pressure 
is applied to the starter (not specifically shown) of the engine 3 by 
ducts (not shown), as known in the art, to thereby set both the starter 
and one of the turbines into rotation. Several sources of SAP are commonly 
used, including bottled air, an auxiliary power unit which is carried 
aboard the aircraft, or a mobile ground power unit. If the landing gear is 
down, solenoid 30 provides a fluidic connection between SAP and the CBP 
line 15, thereby adding the SAP pressure to the CBP pressure, thus 
increasing the pressure at point 35. 
The MEC is programmed, as known in the art, such that when the ratio of 
CBP/CDP is less than unity, the MEC enriches the fuel flow and, 
conversely, when this ratio is greater than unity, the MEC de-riches fuel 
flow. The present invention artificially increases CBP at point 35 by 
adding SAP at the T-junction 18 to thereby make the CBP/CDP ratio greater 
than unity at a selected engine speed. Thus, the MEC is induced to de-rich 
(or lean out) the fuel-air mixture. The de-riching will, in some types of 
engines, reduce low pressure turbine temperature, thus reducing the 
possibility of high temperature damage. This artificial increase of CBP is 
only allowed to occur during ground starts because solenoid 30 disconnects 
SAP from the CBP line when the landing gear of the aircraft is up. The 
check valve 24 serves to prevent the pressure in line 21 from dropping 
below CBP when the SAP is below CBP; it prevents flow out of the CBP line 
toward the solenoid 30. Thus, when starter air pressure is terminated, the 
present invention has no influence on the pressure in line 21. 
The present invention was installed on a gas turbine of the CFM56 family 
and test results are shown in FIG. 2. In that figure, fuel flow P.sub.s3 
is plotted on the vertical axis and engine core speed is plotted on the 
horizontal axis. P.sub.s3 is the pressure at the ninth compressor stage 
and is customarily referred to as CDP. Line 45 illustrates the fuel flow 
as originally programmed into the MEC 6 of FIG. 1. Line 50A illustrates 
actual, empirically measured fuel flow, but without the present invention. 
Lines 50B-E illustrate the changed fuel flows induced by the present 
invention, for different settings of relief valve 27 in FIG. 1. Lines 
50B-E in FIG. 2 indicate respectively relief valve settings of 10, 13, 15, 
and 17 psig. These settings refer to the pressure which the relief valve 
27 in FiG. 1 ports to the CBP line. The relief valve 27 is constructed 
that so that the relief valve pressure (10, 13, 15 or 17 psig) is added to 
ambient, reference pressure. That is, the pressure difference between 
points 55 and 57 (as shown by the symbol .DELTA.P in FIG. 1) is one of the 
10, 13, 15, 17 psig values just mentioned. The pressure at point 55 is 
ambient pressure. CDP is raised in pressure accordingly. 
The reader will note that the influence of the present invention upon fuel 
flow decreases at approximately 5-6,000 rpm as shown by the approximate 
merger of lines 50B-E with the empirical line 50A at those speeds. One 
reason for this is that CDP increases with speed, thus decreasing the 
ratio CBP/CDP as a function of speed. Also, SAP is reduced and terminated 
when speed becomes sufficiently great, thus reducing .DELTA.P to zero. 
Restrictions 60 and 62 inhibit pressure loss in the event of a line 
failure. 
An invention has been described wherein one of the pressure signals which 
is fed to the fuel control in a gas turbine engine is artificially 
increased during ground startup of the engine. The increase causes the 
fuel control to de-rich the fuel-air ratio of the engine, thereby reducing 
the temperature of the low pressure turbine. The artificial increase of 
the pressure is induced by connecting the line containing the pressure to 
the source of starter air pressure which is used to start the engine. A 
solenoid allows this connection only when landing gear is down, as when 
the aircraft is on the ground. A check valve 24 prevents this connection 
when starter air pressure is turned off, as well as when the starter air 
pressure is below the pressure in the line. The amount of pressure added 
to the control line (i.e., .DELTA.P) is determined by a relief valve. 
.DELTA.Ps of 10, 13, 15, and 17 psig have been used. 
Numerous substitutions and modifications can be undertaken without 
departing from the true spirit and scope of the present invention. What is 
desired to be secured by Letters Patent is the invention as defined in the 
following claims.