Patent GB-811747-A (GB)
Title: Fuel control system for gas turbine engines
IPC: F02C9/16
Filed: 1956-07-30
Abstract: 811,747. Gas turbine plant. BENDIX AVIATION CORPORATION. July 30,1956 [Sept. 8, 1955], No. 23495/56. Class 110 (3). [Also in Group XXIX] In order to avoid compressor stall by overfeeding fuel should a gas turbine aircraft engine be re-accelerated towards maximum speed shortly after decelerating therefrom, i.e. before the engine temperatures are stabilized at the lower speed, the normal fuel metering unit such as 20a, Fig. 2, is associated with an automatic control unit 20b which reduces the normal fuel flow for a predetermined time-delay period commencing at the beginning of a deceleration from maximum speed. In the unit 20a, fuel is supplied by a pump (not shown) at pressure P1 to a chamber 37, the pressure P1 being maintained constant by a by-pass valve 39. From chamber 37 fuel passes through an automatic regulating valve 29 to a chamber 51 whence it flows, at regulated pressure P2, into a pilot-actuated slidable metering valve 41 of the all-speed governor type. The valve 41 opens into a chamber 53, whence fuel at pressure P4 flows across a cut-off valve 55 to the engine burners. The valve 41 is controlled by a pair of governor weights 73 on an engine-driven shaft 75, the weights being opposed by a spring whose loading is determined by a lever 71 linked to the pilot's throttle control lever. The fuel flow is thus determined by the position of valve 41 and by the pressure P2 set by the regulating valve 29. The valve 29 is opened as the engine speed increases by a second pair of governor weights 77 linked to the valve and acting against the fuel pressure on a spring-loaded diaphragm 27. The resultant pressure differential across valve 41 is proportional to the square of engine speed and the fuel flow is proportional to engine speed directly. In order to correct the position of regulating valve 29 for variations in compressor air inlet pressure and temperature a needle 91 controlled by a density-responsive capsule 109 exposed to the compressor air inlet varies an orifice 93 interposed in passages between the chamber 53 at P4 pressure and a chamber 25 at P3 pressure, the chamber 25 communicating with the P2 pressure in chamber 51 through control jets in the diaphragm 27. Should the air density fall, the orifice 93 is opened and reduces the pressure drop P3-P4 across it, thus increasing the P2-P3 differential across the diaphragm 27 and, by unbalancing the weights 77, causing the valve 29 to reduce pressure P2 and hence reduce engine speed until equilibrium is restored. The control unit 20b (shown on an enlarged scale) comprises three chambers 117, 119, 129 connected respectively to chamber 25 at P3 pressure, chamber 51 at P2 pressure and, through a time-delay bleed 163, to chamber 53 at P4 pressure. A spring-closed valve 141 controls a removable bleed. 139 between chambers 119 and 117, the valve being held open by a spring-pressed piston 125 acting against the P2-P4 differential, which is small at stabilized low speeds. Operation.-When the accelerating engine reaches 70-80 per cent maximum speed the P2-P4 differential, increased by the opening of valves 41 and 29, becomes sufficiently great to shift piston 125 against its spring and allow the spring-loaded valve 141 to close. The closing of valve 141 causes a reduction of P3 in chamber 117 and thus also in chamber 25, and hence increases the P2-P3 differential across diaphragm 27, which in turn shifts valve 29 to reduce the P2 pressure. The fuel Row is thus reduced while the engine completes its acceleration to maximum speed. If the pilot now moves the throttle lever to idle position the valves 41, 29 move towards a closed position and the P2-P4 differential across valve 41 is reduced. Due, however, to the bleed 163 there is a time delay before piston 125 moves to open valve 141 and if during this time delay and at, say, 50 per cent maximum speed the pilot reopens valve 41 to reaccelerate, only the aforesaid reduced fuel flow is available and compressor stall is thus avoided. If the engine is allowed to stabilize at any speed in the critical stall area range the bleed 163 has sufficient time to move the piston 125 to open the valve 141; the metering unit 20a then operates on its normal flow conditions when next the engine is accelerated, the stabilized engine being able to tolerate the richer fuel without stalling. In a modification wherein the normal fuel flow is reduced during deceleration from maximum speed and is maintained at the reduced level for a time delay period, the unit 20b is replaced by a unit 20c, Fig. 3, the unit 20a being the same as in Fig. 2. The P2 pressure of chamber 51 is connected to a chamber 175 and acts on a spring-loaded diaphragm 179; the P4 pressure of chamber 53 is connected through an adjustable bleed 217 to a spring-opened valve 221 opening into a chamber 189; and the P3 pressure of chamber 25 is connected both to the chamber 189 and, through a bleed 203, to a chamber 187 in turn connected with a chamber 177 through openings in an adjustable stop 227. A spring-loaded diaphragm 191 separating the chambers 187 and 189 holds the valve 221 closed at low engine speeds. When the engine is accelerated towards maximum speed the increasing P2-P3 differential, caused by the restriction of the control jets in the diaphragm 27, shifts the diaphragm 179 to the right, a rapid fuel now from chambers 177 and 187 being permitted by a spring-loaded check valve 205 in parallel with the bleed 203. Since the P3 pressures on either side of diaphragm 191 are substantially equal the valve 221 remains closed and the unit 20a provides a normal fuel flow until the engine reaches maximum speed. If the engine is now decelerated the decreasing P2-P3 differential shifts diaphragm 179 to the left thereby, due to the bleed 203, reducing the P3 pressure in chambers 177 and 187. The diaphragm 191 is thus displaced to the left and allows valve 221 to open, whereupon fuel at P3 pressure in chamber 189 passes through bleed 217 to P4 chamber 53 and so reduces the P3-P4 differential across the density needle 91. The P2-P3 differential across diaphragm 27 is correspondingly increased and operates the regulating valve 29 (not shown) to reduce the P2 pressure acting on the throttle valve 41, and consequently reduce the fuel flow schedule. The reduced P2 pressure is maintained for a time-delay period until sufficient fuel at P3 pressure has returned through the bleed 203 into chambers 177 and 187 to allow the diaphragm 191 to close the valve 221. Should the pilot reaccelerate during this period only the reduced fuel flow schedule is available and consequently compressor stall is avoided. The reduction in fuel flow is controlled by adjustment of the bleed 217. In a fuel control of the constant head, variable area type, Fig. 4, fuel supplied by a pump 231 to a chamber 229 is fed to an inlet annulus of a hollow cylindrical metering valve 251. The valve 251 is axially and rotatably movable to vary the area of overlap of square ports, the said overlap forming the main metering port. The outlet ports of valve 251 are connected through an adjustable restriction 263 to supply fuel at P4 pressure to the engine manifold. The axial position of valve 251 is varied by a pilot-controlled lever 275 and an engine all-speed governor control unit 277, the unit 277 incorporating an acceleration fuel control responsive to engine speed and compressor inlet temperature. The rotational position of valve 251 is controlled, through rack and pinion gearing, by an evacuated bellows in a chamber 295 connected to the compressor discharge pressure Pc. A by-pass valve 267 controlling the P1 pressure in chamber 229 is actuated by a spring- loaded diaphragm in a chamber 270 supplied with metered fuel at P4 pressure, through a bleed and a passage 303. The valve 267 thus maintains a constant pressure differential across the metering valve 251. The normal fuel feed is reduced for a time-delay period following a deceleration by the opening of a spring-closed slide valve 342 whose stem is secured to three diaphragms separating chambers 311, 313, 315 and 317. Chambers 311 and 317 are connected in parallel to a passage 347 communicating with metered fuel pressure P4, the parallel connection avoiding pressure sensitivity of the valve. Chamber 313 is connected to compressor discharge pressure Pc and communicates with chamber 315 through a time-delay bleed 355. At any stabilized engine speed the pressures in chambers 313 and 315 are equal and valve 342 is held closed by its spring. If the engine accelerates the pressure Pc in chamber 313 increases and opens a check valve 357 in parallel with the bleed 355 so that a maximum Pc pressure develops in chamber 3] 5. During deceleration the check valve 357 closes and, due to bleed 355, the pressure in chamber 313 decreases faster than that in chamber 315, the consequent pressure differential causing diaphragm 321 to open valve 342. Fuel at P4 pressure from chamber 311 then escapes through an adjustable bleed 365 to the inlet of pump 231 and the reduction of P4 pressure in chamber 270 actuates valve 267 to reduce the P1-P4 differential across valve 251 and thus reduce the fuel flow to the engine. If the engine is reaccelerated before the time-delay bleed 355 has allowed valve 342 to close, only the reduced fuel flow is available. It is arranged that the increasing Pc pressure in chamber 313 during a re-acceleration closes valve 342 only when the engine has passed the stall region. In a fuel control, Fig. 5, of variable head, variable area type a metering valve 381 passing fuel from a pump 411 to a chamber 371 connected to the engine is actuated by a bellows in a chamber 385 having calibrated bleeds 401, 403 communicating respectively with compressor inlet pressure Pi and compressor discharge pressure Pc. With a selected bleed area ratio the pressure Pm in chamber 385 is a measure of a function of compressor