Patent GB-1331418-A (GB)
Title: Fuel control system for a gas turbine engine
IPC: F02C7/26
Filed: 1972-02-03
Abstract: 1331418 Gas turbine engine fuel systems WESTINGHOUSE ELECTRIC CORP 3 Feb 1972 [9 March 1971] 5027/72 Heading F1G [Also in Division G1 ] A fuel system for a gas turbine engine rotatable during a low speed cranking period, an intermediate ignition period and a high speed operating period, comprises a valve to admit fuel during the ignition and operating periods; means for continuously determining the fuel flow required and for providing a signal thereof; and means for applying the signal to the valve to control the fuel flow, the latter means including two alternative paths for the signal, the first of the paths being operative during the ignition period; transfer means responsive to the level of the signal for transferring the signal to the second path when the signal reaches a predetermined level; and means associated with the second path for maintaining a minimum signal level corresponding to the minimum fuel flow required to maintain combustion during high speed operation. The fuel flow is determined by a signal comparator 26, Fig.1, in which the input signals and/or the output signal are electrical, pneumatic or hydraulic. The input signals represent shaft speed, power output, compressor inlet temperature, combustion chamber pressure and the average temperature at the turbine exhaust 24a and in the blade path 24b. If the output signal is electrical, it can be converted by a transducer 41 into a pneumatic or hydraulic signal. During the ignition period, this signal passes via a line 44, a diverter 48 and a line 47 to operate the fuel valve 21. When the signal reaches a predetermined pressure, a sensor 57, comprising a diaphragm actuated switch, sends an electrical signal via a line 51 to the diverter 48, where it energizes electromagnetically operated valves to close the connection of line 44 to line 47 and open the connection of an alternative line 46 to line 47. Line 46 contains a limiter 49, see also Fig.3, which acts to prevent the signal passed from line 46 to line 47 from falling below the minimum value necessary to maintain combustion. For this purpose, a pressure source 50 is connected via a pressure regulator 53 and a non-return valve 55 to the line 46 downstream of a non-return valve 56. Normally, the signal in the line 46 is at a pressure above the pressure exerted by the pressure source 50 via the regulator 53, and the non-return valve 55 is closed and the non-return valve 56 is open. If the signal pressure falls below the pressure exerted by the source 50, the non- return valve 56 closes and the non-return valve 55 opens, so that the source 50 supplies a signal to the line 46 in place of the normal signal so as to hold the fuel valve 21 open to the minimum extent necessary to ensure combustion. As shown in Fig.2, the comparator 26 may comprise a digital or analogue computer 37 in which the compressor inlet temperature signal and the combustion chamber pressure signal jointly provide a surge function 29a, the turbine temperature signal provides a temperature limit function 25a and the power signal provides a power limit or control function 32a. The lowest of the three resulted signals is selected at 31 and passed to a circuit 36, where it is compared with a speed error signal supplied by an error detector 33a which compares the shaft speed signal with a reference signal from a speed reference source 33, the lower of the two inputs to the circuit 36 being passed as an output signal to the transducer 41.