GB1591217A - Engine fuel control system - Google Patents

Description

(54) ENGINE FUEL CONTROL SYSTEM (71) We, THE GARRETT CORPO RATION, a Corporation organised under the Laws of the State of California, United States of America, of 9851-9951 Sepulveda Boulevard, P.O. Box 92248, Los Angeles, California 90009, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an engine fuel control system, and one object is to provide a system suitable for reliably permitting emergency operation of the engine.

For example, the invention is applicable to aircraft having one or more propulsion engines where a high thrust level may be required in an emergency possibly during take-off if one of a pair of engines fails, so that the other engine requires to have its thrust boosted.

According to the present invention, an engine fuel control system includes a fuel controller capable of allowing operation of the engine at an emergency rate of fuel supply, trim means arranged to trim the fuel supply to a normal, lower, rate, and a source of an input signal arranged to control the trim means to remove the trim and allow the fuel supply to be at the emergency, higher, rate.

The advantage is that the system is set for emergency performance, although as long as the trim is provided, operation is at a lower normal level. A sudden requirement in an emergency for emergency thrust can be arranged merely to remove the trim so that the emergency thrust will be immediately available, and that can be brought into action automatically in response to means detecting the emergency, or can be brought into action manually, for example by removing the power supply from the trim means.

Thus it is not necessary to terminate operation of an automatic performance reserve system if it is desired to stop the engines, and engine thrust can be diminished simply by operation of the normal power levers. Moreover the system is fail-safe because removal of the trim signals makes emergency thrust available.

The invention may be carried into practice in various ways, and one embodiment will now be described by way of example with reference to the accompanying drawings; in which FIGURE 1 is a generally schematic diagram illustrating a gas turbine engine fuel control system in accordance with this invention; FIGURE 2 is a generally schematic diagram of an automatic performance reserve system for use in the fuel control system of FIGURE 1; and FIGURES 3 and 4 are graphs illustrating operation of the automatic performance reserve system of FIGURE 2.

Referring now to FIGURE 1, a gas turbine engine 10 is operated with a suitable fuel which is provided by a fuel delivery system 12 which may include appropriate storage tanks and valves for controlling fuel flow from the tanks to the engines. Fuel delivery system 12 controls the flow of fuel to the gas turbine engine 10 in accordance with a fuel command signal which is received from a fuel control computer 14.

Appropriate inputs and outputs are provided to and by the fuel control computer 14 in accordance with the requirements of the particular engine control system. An example of such inputs and outputs is illustrated in U.S. Patent 3,971,208. Two particular inputs will be specifically utilized in describing the operation of this invention and comprise a speed feedback signal received through a path 16 and a temperature feedback signal received through a path for monitoring rotational velocity of an engine spool, preferably the high pressure spool in an engine having a plurality of spools, and engine exhaust temperature respectively.

The computer 14 controls the amount of thrust produced by the engine 10 in response to the commands dictated by position or angle of a power lever 20. The power level 20 is connected to the wiper of a rheostat 22. The rheostat 22 is connected to the fuel control computer 14 by conductors 24 and 26 which establish a voltage drop across rheostat 22 between voltages corresponding to maximum speed and minimum speed, respective. The wiper of the rheostat 22 is connected by a conductor 28 to carry a signal indicative of power lever angle and, accordingly, desired engine speed to the fuel control computer 14. In response to this signal, the fuel control computer transmits a fuel command to the fuel delivery system 12 which provides the appropriate amount of fuel to the engine 10.

It is necessary to programme the fuel control computer 14 to limit speed of the high pressure spool and limit turbine exhaust temperature to optimize airplane performance and engine life. The establishment of these limits determines the maximum amount of thrust which may be produced by the engine. This relationship of spool speed and exhaust temperature is shown in FIG URE 3 wherein an engine running line 'A' for a typical gas turbine engine is shown with turbine exhaust temperature plotted along the vertical axis and high pressure spool speed (corrected for temperature) plotted along the horizontal axis. The dashed line 'B' illustrates maximum permissible turbine exhaust temperature and the dashed line 'C' is indicative of maximum permissible spool speed (100% speed) characteristic of optimum performance levels.In emergency situations, it is desirable to increase available engine thrust.

Because prior art fuel systems are set to limit turbine temperature and spool speed, any command requesting higher thrust would be thwarted by action of the control system. For this reason, it has been necessary to provide the additional thrust through use of independently controlled solenoid actuated, control valves.

In accordance with this invention the computer 14 is preset to permit the engine to reach a higher level of exhaust temperature indicated by the line 'D' and a higher level of spool speed, indicated by line 'E'.

This adjustment permits the engine to enhance its available thrust from a level corresponding to engine performance at point 'F' on curve 'A' to the higher level corresponding to point 'G'.

However, it would not be desirable, as already discussed to permit the engine to run continuously within these high limits.

Thus, an automatic performance reserve (APR) system 30 is provided to control the fuel control computer 14 for selectively switching between a normal operating mode, wherein the engine is capable of producing thrust corresponding to point 'F' in FIGURE 3, and an APR mode, wherein the engine is capable of producing thrust corresponding to point 'G'.

The APR system 30 produces two outputs for each engine being controlled. A speed trim signal is transmitted through a conductor 32 to a summing junction 34 interposed in the speed command conductor 28. A temperature trim signal is transmitted through a conductor 36 to a summing junction 38 in the temperature feedback path 18.

The speed trim signal modifies the speed command received from the rheostat 22 such that, whenever the power lever is in the maximum speed position and the APR system 30 is providing a speed trim signal, the fuel control computer will receive a speed command calling for spool speed to be maintained at the level indicated as 'C' instead of the maximum level 'E'. Corresponding reductions in spool speed occur at all settings of the power lever.

At the same time, the temperature trim signal combines with the temperature feedback signal to indicate to the fuel control computer that the exhaust temperatute of the engine 10 is greater than it actually is by an amount AT equal to the temperature difference betwene levels 'B' and 'D' in FIG URE 3.

Accordingly, as long as the APR system 30 is producing its output trim signals, engine thrust will be limited such that the engine is only permitted to operate within normal limits. When the APR system 30 ceases to produce the trim signals, thrust is permitted to increase from the normal limit (point 'F') to the emergency limit (point 'G') on the engine running line 'A' by the increase in high pressure spool speed and permissible temperature otherwise allowed by the fuel control computer 14.

The APR system 30 is best illustrated in FIGURE 2, and comprises a pair of substantially identical units 30L and 30R for controlling the fuel control computers of the left and right engine, respectively. Identical components will be described using the same reference numerals but specific reference to one of a pair of so-numbered components will be made using the letter 'L' to designate the component in the left portion 30L and the letter 'R' to correspond to components of the right portion 30R. While this two channel system is designed to operate in conjuction with an airplane having two engines, it should be noted that any number of engines could be readily accom modated by an APR system in accordance with this invention.

Each of the left and right APR unit portions has a speed trim signal generator 40 which transmits the speed trim signal through its corresponding conductor 32, and a temperature trim signal generator 42, which transmits the temperature trim signal through its corresponding conductor 36.

The speed and temperature trim signal generators produce these signals whenever they are receiving power from their corresponding power supply 44 to which they are connected by suitable conductors.

Each power supply 44 also provides operating power for a AN detector 46. Each AN detector receives a signal which is functional of the difference between the speed of the high pressure spool of two engines.

While this signal may be produced by any convenient means, in the preferred embodiment it is received from an engine speed synchronizer 45 through conductor 48. The synchronizer 45 may be of any suitable type, well known to those skilled in the art.

Each AN detector 46 is connected to energise a relay 50 when the difference between engine speeds reaches a predetermined value. Each of the relays 50 is connected for operation of contacts within both of the APR unit portions 30L and 30R. The relay 50L has contact 50La in an APR portion 30L and 50Lb in APR portion 30R.

The relay 50R has contacts 50Ra in portion 30R and 50Rb in portion 30L.

Power for each power supply 44 is received from a voltage source 52 through a switch 54 and a junction 56. From the junction 56 energisation of the power supplies 44 is through independent parallel sources.

A conductor 58 connects the junction 56 through a switch 60 to a junction 62. A conductor 64 is connected between the power supply 44L through a diode 66, the junction 62 and a diode 68 to the power supply 44R.

A conductor 70 connects the junction 56 through contact 50La and contact 50Rb and a diode 72 to power supply 44L. A conductor 74 connects junction 56 through contact 50Lb, contact 50Ra and a diode 76 to power supply 44R.

An alternate terminal of contact 50La is connected to a suitable indicator such as a light 78 and an alternate terminal of contact 50Rb is connected to a source of voltage 80.

Operation of.the automatic performance reserve system of this invention will now be described. As has already been discussed, the fuel control computer 14 (FIGURE 1) is adjusted to permit operation of the engine 10 at the enhanced power levels to be provided by the APR system 30. However, as long as speed and temperature trim signals are received from the speed trim signal generators 40 and temperature trim generators 42 associated with each engine, the fuel control computer will be limited to operation within a normal limit. These sign als will be produced as long as the power supplies 44L and 44R are energising the signal generators.

Switches 54 and 60 are manually oper ated switches mounted on a flight deck panel in conjunction with the APR function light 78. Opening of the switch 54 removes all power from the APR unit and thus terminates generation of speed and temperature trim signals so as to act as manual APR operating switch. When the switch 54 is closed, automatic functioning of the APR system 30 is possible.

Whenever switch 54 is closed, switch 60 serves as an arm/disarm switch. As long as switch 60 is closed, the power supplies 44 will be energised so as to result in the generation of speed and temperature trim signals.

Thus, when the switch 60 is closed, the APR system is said to be "disarmed" in that automatic functioning is impossible. When the switch 60 is open, energisation of power supplies 44 is through the contacts of the relay systems 50 so that automatic functioning in a manner to be described is possible.

Thus when the switch 60 is open, the system is said to be "armed".

With the system armed, energisation of power supply 44L is from the voltage source 52 through switch 54, contact 50La and contact 50Rb. Energisation of power supply 44R is from voltage source 52 through switch 54, contact 50Lb and contact 50Ra.

When both engines are operating properly, such that any difference in spool speed is less than the predetermined level detected by the AN detectors 46, the relays 50 are in an energised state such that the contacts associated therewith close the circuits for energisation of the power supplies 44.

Should a difference in spool speed between engines occur which is of a magnitude greater than the predetermined level detectable by the AN detectors, the signal from the synchronizer 45 will cause each of the AN detectors 46L and 46R to de-energise its associated relay 50L and 50R, respectively.

De-energistation of relay 50L opens contact 50La and 50Lb. De-energisation of relay 50R open contacts 50Ra and 50Rb.

The opening of the contacts for either of the relays terminates the application of power to both power supplies 44L and 44R to terminate generation of speed and temperature trim signals. Thus, the use of two AN detectors 46L and 46R and two associated relays 50L and 50R provides redundancy in that operation of either speed detector and its associated relay will produce the desired boost in thrust to both engines.

When contacts 50La and 50Rb both open, they also close their alternate terminals to complete a circuit connecting the APR function light 78 to the voltage source 80. Illumination of light 78 indicates to the pilot that the APR unit has functioned. If either relay 50L or 50R does not properly operated, the light will not illuminate and will provide an indication of malfunction of one of the channels. Unless both channels have failed, the APR system will still operate properly in its automatic mode.

The APR function light 78 also provides a means for testing automatic operation of the APR system 30 prior to take-off. With the switch 54 in the closed or "automatic" position and the switch 60 in the open or "armed" position, the power lever 20 for each engine may be adjusted so that the difference between the spool speeds of the two engines or AN is greater than the predetermined value required for operation of the relays 50. This should trigger operation of the relays 50L and 50R to cause a change in condition of contacts 50La and 50Rb, respectrively, and energise the APR function light 78. If either relay 50L or 50R fails to operate, the light will fail to function because one of the two contacts will remain in its original position.If the APR function light has failed to illuminate during operation on test, the pilot can determine whether a single or dual channel failure has occurred by examination of his flight instruments.

FIGURE 4 illustrates the relationship of engine thrust polotted on the vertical axis to power lever angle plotted on the horizontal axis. Curve 'H' shows the relationship between engine thrust and the power lever angle when the engine is under normal control; this means that the speed and temperature trim signals are being applied to the fuel control computer 14. Curve I gives the same relationship for the same engine operating under emergency conditions wherein the speed and temperature trim system have been removed from the computer 14. As can be seen at the point indicated as TO on the horizontal axis, indicating the power lever position corresponding to engine take-off power, a significant increase in thrust is provided by the removal of trim signals by the APR unit 30. However, as the power lever is moved toward the idle position, the thrust increase is greatly diminished.As a result, not only can actual operation of the APR system 30 be tested while the engines are at or near idle speed but it is also unnecessary to manually terminate operation of the system to permit the engines to be cut to a thrust level permitting an airplane to be safely stopped.

To prepare the APR system 30 for operation prior to takeoff of an airplane, the switch 54 is placed in the closed (automatic) position. The switch 60 is then placed in the closed (disarmed) position so that the power supplies 44 will be energised. This will cause the speed trim signal generators 40 and temperature trim signal generators 42 to transmit their outputs to the fuel control computer 14 resulting in engine operation in a conventional mode. If there is no difference between spool speeds of the engines greater than the predetermined value, energisation of the power supply will also cause the relays 50 to activate their contacts causing the power supplies 44 to be energised therethrough. After this has occurred, the switch 60 may be changed to open (armed) configuration.

To effect take-off, the power levers are moved to the take-off position causing a corresponding increase in thrust along the curve designated 'H' in FIGURE 4. To avoid triggering of the APR system due to uneven movement of the power levers, it is preferable that switch 60 remain in the 'disarm' position until commencement of the take-off roll. As long as both engines 10 operate in a normal manner, the speed and temperature trim signals generators will continue to be energised by the power supplies 44 and no change in engine thrust will be caused by the APR system 30. However, should a malfunction occur in one of the engines, this will result in a change in its spool speed such that a AN signal will be transmitted by the synchronizer 45 to the AN detectors 46.As soon as this signal indicates a difference in spool speeds at least equal to be the predetermined value, the relays 50L and 50R will be energised, opening the contacts to remove power from the power supplies 44. As a result, the speed and the temperature trim signals will be removed from both engines resulting in an increase in high pressure spool speed and a permissivie increase in engine exhaust temperature which will change the thrust from curve 'H' in FIGURE 4 to the corresponding point on the curve "I". It should be understood, however, that this increase in thrust will only apply to a properly operating engine. Any increase in thrust or power level of a defective engine will depend upon the nature and the magnitude of the defect.

However, a significant increase in available thrust will be provided for the airplane.

The opening of the relay contacts completely disconnects the power supplies 44 from their source of power 52 so that inadvertent removal of the increased thrust by accidental automatic application of the trim signals is not possible. This could only occur by action of the flight crew in changing the configuration of the switch 60 from its open or armed mode to the closed or disarmed mode whereupon the trim signals would again be applied to the fuel control computer 14.

Should automatic functioning of the APR unit 30 fail for any reason so that the contacts of relays 50L and 50R remain closed, engine thrust may still be increased manually by the pilot simply by changing the position of the switch 54 from the closed (automatic) position to the open (manual) position wherein power is immediately removed from the power supplies 44 and application of the trim signals is terminated.

Accordingly a turbine engine thrust boosting system is disclosed wherein thrust is enhanced by modification of inputs to a fuel control computer to permit the engine to increase temperature and speed parameters to result in increased thrust. No additional fuel control valving is required and the system provides additional safety of operation by not requiring manual shut-off to stop the airplane and providing fail safe operation which results in increased thrust upon occurrence of a failure of power to the APR unit.

WHAT WE CLAIM IS: 1. An engine fuel control system including a fuel controller capable of allowing operation of the engine at an emergency rate of fuel supply, trim means arranged to trim the fuel supply to a normal, lower, rate, and a source of an input signal arranged to control the trim means to remove the trim and allow the fuel supply to be at the emergency, higher, rate.

2. A system as claimed in Claim 1 including a fuel control computer, and in which the trim means comprises a trim signal generator capable of transmitting trim signals to the computer, and means arranged in response to the input signal to remove the trim signals from the computer.

3. A system as claimed in Claim 1 or Claim 2 including an indicator arranged to indicate when the trim has been removed.

4. A system as claimed in any of the preceding claims including manual means for removing the trim.

5. A system as claimed in any of the preceding claims including a source of power for the trim means, and a manual switch capable of operation to remove the power supply from the trim means whereby the trim is removed.

6. A system as claimed in Claim 5 including switch means between the said manual switch, and the trim means arranged to disconnect the trim means from the manual switch in response to an input signal so as to remove the trim.

7. A system as claimed in any of the preceding claims in which the source of input signal is responsive to malfunction of the engine.

8. A system as claimed in Claim 6 or Claim 7 including an arming switch connected in parallel with the said control contacts.

9. A system as claimed in any of Claims 2-8 in which the computer receives inputs dependent upon engine exhaust temperature, and the speed of a component of the engine, and also receives a trim signal for causing the computer to maintain the rate of fuel supply such that the said temperature and speed are less than the maximum possible temperature and speed.

10. A system as claimed in any of the preceiding claims for controlling two engines, and in which the source of an input signal is responsive to the difference between the speeds of the respective engines.

11. A system as claimed in Claim 10, including a manually operated power lever for each engine.

12. A system as claimed in Claim 10 or Claim 11 in which the engines are gas turbine propulsion engines.

13. A system as claimed in any of the preceding claims for controlling the engine, or engines or an aircraft.

14. A system as claimed in any of Claims 10-13 including means for providing separate speed trim and temperature trim signals to each engine, and a common source of an input signal to both means responsive to a predetermined difference between the engine speeds.

15. An engine fuel control system arranged to operate substantially as herein specifically described with reference to the accompanying drawings.

16. A method of controlling transfer of an engine from a normal operating condition to an emergency operating condition, said method comprising the steps of powering the engine with a fuel control adjusted for operation under emergency operation conditions, applying trim signals to said fuel control for changing the engine to opertation under normal operating conditions; and terminating application of said trim signals upon occurrence of an emergency condition.

17. A system for applying emergency power to an aircraft having two or more propulsion engines, each having a fuel delivery control system including means for setting each fuel delivery control system to permit maximum engine speed and exhaust temperature for an emergency thrust level for the engine, means for applying speed and temperature trim signals to each fuel delivery control system to limit engine speed and exhaust temperature to a normal thrust level, and means responsive to a certain difference between speeds of the engines arranged to terminate application of the speed and temperature trim signals.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. Should automatic functioning of the APR unit 30 fail for any reason so that the contacts of relays 50L and 50R remain closed, engine thrust may still be increased manually by the pilot simply by changing the position of the switch 54 from the closed (automatic) position to the open (manual) position wherein power is immediately removed from the power supplies 44 and application of the trim signals is terminated. Accordingly a turbine engine thrust boosting system is disclosed wherein thrust is enhanced by modification of inputs to a fuel control computer to permit the engine to increase temperature and speed parameters to result in increased thrust. No additional fuel control valving is required and the system provides additional safety of operation by not requiring manual shut-off to stop the airplane and providing fail safe operation which results in increased thrust upon occurrence of a failure of power to the APR unit. WHAT WE CLAIM IS:

1. An engine fuel control system including a fuel controller capable of allowing operation of the engine at an emergency rate of fuel supply, trim means arranged to trim the fuel supply to a normal, lower, rate, and a source of an input signal arranged to control the trim means to remove the trim and allow the fuel supply to be at the emergency, higher, rate.

2. A system as claimed in Claim 1 including a fuel control computer, and in which the trim means comprises a trim signal generator capable of transmitting trim signals to the computer, and means arranged in response to the input signal to remove the trim signals from the computer.

3. A system as claimed in Claim 1 or Claim 2 including an indicator arranged to indicate when the trim has been removed.

4. A system as claimed in any of the preceding claims including manual means for removing the trim.

5. A system as claimed in any of the preceding claims including a source of power for the trim means, and a manual switch capable of operation to remove the power supply from the trim means whereby the trim is removed.

6. A system as claimed in Claim 5 including switch means between the said manual switch, and the trim means arranged to disconnect the trim means from the manual switch in response to an input signal so as to remove the trim.

7. A system as claimed in any of the preceding claims in which the source of input signal is responsive to malfunction of the engine.

8. A system as claimed in Claim 6 or Claim 7 including an arming switch connected in parallel with the said control contacts.

9. A system as claimed in any of Claims 2-8 in which the computer receives inputs dependent upon engine exhaust temperature, and the speed of a component of the engine, and also receives a trim signal for causing the computer to maintain the rate of fuel supply such that the said temperature and speed are less than the maximum possible temperature and speed.

10. A system as claimed in any of the preceiding claims for controlling two engines, and in which the source of an input signal is responsive to the difference between the speeds of the respective engines.

11. A system as claimed in Claim 10, including a manually operated power lever for each engine.

12. A system as claimed in Claim 10 or Claim 11 in which the engines are gas turbine propulsion engines.

13. A system as claimed in any of the preceding claims for controlling the engine, or engines or an aircraft.

14. A system as claimed in any of Claims 10-13 including means for providing separate speed trim and temperature trim signals to each engine, and a common source of an input signal to both means responsive to a predetermined difference between the engine speeds.

15. An engine fuel control system arranged to operate substantially as herein specifically described with reference to the accompanying drawings.

16. A method of controlling transfer of an engine from a normal operating condition to an emergency operating condition, said method comprising the steps of powering the engine with a fuel control adjusted for operation under emergency operation conditions, applying trim signals to said fuel control for changing the engine to opertation under normal operating conditions; and terminating application of said trim signals upon occurrence of an emergency condition.

17. A system for applying emergency power to an aircraft having two or more propulsion engines, each having a fuel delivery control system including means for setting each fuel delivery control system to permit maximum engine speed and exhaust temperature for an emergency thrust level for the engine, means for applying speed and temperature trim signals to each fuel delivery control system to limit engine speed and exhaust temperature to a normal thrust level, and means responsive to a certain difference between speeds of the engines arranged to terminate application of the speed and temperature trim signals.