GB675368A - Improved device for controlling engine acceleration and/or speed - Google Patents

Abstract

675,368. Gas turbine plant. OFFNER, F. F. Nov. 15, 1949 (March 31, 1949], No. 29210/49. Class 110(iii) [Also in Group XXXV] In a system for controlling the acceleration and regulating the speed of an engine e.g. a turbc jet engine of an aircraft, the acceleration from an idling condition to a predetermined speed is controlled in accordance with engine temperature relatively to a selected maximum permissible temperature until when the predetermined speed is reached, control is transferred from temperature to engine speed for maintaining constant the predetermined speed. As shown in Fig. 2, the fuel valve 35 of a jet engine 9 is controlled by an electromagnetically operated fluid servomotor 24 over the output lead 21 of an amplifier T1, T2, T3 in response to exhaust temperature measured by a thermocouple 3 and speed as measured by a D.C. generator 9e driven by the engine. Switchover from acceleration control by the thermocouple 3 to speed control by the generator 9e, is effected at a predetermined generator speed by altering the bias on rectifiers D1, D2 which are arranged to pass the speed and temperature signals respectively. Over-riding control by the temperature is also provided in the event of excessive exhaust temperatures during operation at the predetermined speed. The speed control also may be modified in accordance with altitude of'the aircraft. Acceleration control. -Acceleration is controlled so that temperature of the exhaust is maintained below a predetermined maximum by a difference voltage derived by comparison of the thermocouple voltage (3) with a predetermined and adjustable datum voltage across resistor R31. The datum voltage is obtained from the inductive kick from the operating coil 4 of a vibrating switch S3B as rectified by D5 and stabilized by neon tube t5. The comparison is made over contacts 6, 7 of a vibrating switch. S3A and a -centre-tapped transformer L3, the difference signal being passed through amplifier T4 to output lead 13 connected to a differentiating network C17, R51 which in association with an integrated feed back signal in network C13, R41 from the main amplifier T1 T2 T3 feeds input over lead 16 to the main amplifier. Rectifier D4 limits the maximum output of the temperature signal and a resistance network R37, 38, 42, 43 connected in the thermocouple circuit, corrects for ambient temperature and for possible voltage variations of the stabliizer t3. Speed control.-The voltage of the generator 9e is proportional to engine speed and is compared with a reference voltage derived from resistance R102 set by the throttle T and connected in a bridge circuit including resistors R1, R2. The difference voltages which varies in amplitude and sign according to the deviations in speed from the set valves, is combined with the derivative components of speed developed in a circuit C2, R3 and adjustable in accordance with the speed setting of the throttle by circuits C101/102, R107/108 and the altitude by circuits C60, R60 variable by bellows B. Change-over from acceleration control to speed control.-The acceleration signal from lead 16 and the speed signal from the bridge at junction R1, R2 are respectively " chopped " at contacts 20, 19 of a continuously driven vibrating switch, the contact 18 of which is grounded. The two signals are applied respectively to oppositely connected rectifiers D2, D1 variably biased from the junction of R11, R12, the arrangement being such that the bridge is balanced at a speed of about 80 per cent of the speed pre-set by the throttle and in consequence rectifier D2 is operative below this speed and rectifier D1 is operative at greater speeds. Thus, temperature signals controlling engine acceleration are passed into the main amplifier through D2 until a predetermined speed is reached, after which the speed signals take over control through D1. The signals are passed through the amplifier T1, T2, T3 in the form of A.C. signals by reason of the action of the switch S2A and the output is re-rectified by a synchronously operated switch S2B to produce a D.C. output over transformer L2, lead 21 and switch S101A to the control coil 22a of the fuel valve. Over-riding control by temperature signals.-An over-riding control is effective when the engine is under " speed " control if the engine temperature rises above the datum level selected by the resistor R31. This control is provided by rectifier D3 which is biassed from network R55/56, C29 and is rendered conductive when the temperature rises above the predetermined datum notwithstanding that acceleration temperature control is cut off at the rectifier D2. Engine stall suppressor.-Surging of the engine compressor due to stalling of the compressor blading e.g. at high altitudes of an aircraft, may also be prevented by controlling the engine so that temperature is scheduled as a function of the temperature of air inlet to the compressor. This operation is effected by a negative-temperature resistor K responsive to air inlet temperature and associated with resistors R301 ... R308 and rectifiers D301/302 which modify the control signals derived from the thermocouple 3. Modifications.-(1) A modified temperature control and stall suppressor circuit, Fig. 5 (not shown), comprises two thermocouples, one on each side of a row of compressor blades and connected in opposition. associated with the differentiated output of an auxiliary generator which develops a voltage proportional to engine speed. The net output of this circuit is applied to the vibrator-amplifier S3A, T4, S3B for supplying control signals to rectifier D2. In this arrangement the temperature rise varies as the square of the engine speed. This circuit also may be employed in addition to the thermocouple control shown in Fig. 2. (2) To prevent possible leakages through the rectifiers D1, D2 when they are biassed to cut off, an additional rectifier may be connected in each channel.