Patent GB-1238010-A (GB)
IPC: A47B3/12
Filed: 1969-06-23
Abstract: 1,238,010. Gas turbine engine fuel systems. BENDIX CORP. June 23, 1969 [June 27, 1968], No.31625/69. Heading F1G. A gas turbine engine fuel system comprises a fuel metering valve actuated in accordance with the position of one or more manual control levers, atmospheric pressure and a variable, e.g. compressor pressure rise, related to engine power output; and a fuel by-pass valve controlling the pressure drop across the metering valve and operatively connected to a governor responsive both to the position of the manual control lever or levers and the engine speed so that the pressure drop across the metering valve is a constant maximum or minimum value during transition from one engine speed to another and an intermediate value, variable in accordance with engine speed, during steady state operation. Fuel is supplied from an engine driven pump, preferably of gear type, via an inlet 48 to a line 68 containing a throttle valve 74 and a metering valve 76 in series and thence via a line 64 to the burners. A by-pass valve 82 returns fuel from a point intermediate the valves 74, 76 to the suction side of the pump. The throttle valve 74 and the bypass valve 82 are operated by diaphragms 112, 116 and 88, 90 respectively, each pair of diaphragms being subjected to the difference between the pressure P 3  in the outlet line 64 and the pressure p<SP>1</SP>, developed in a line 104 downstream of a restrictor 102. The line 104 is connected at its upstream end to a point on the line 68 upstream of the valve 74, and at its downstream end is connected via a governor-controlled orifice 110 and a line 111 to a point on the line 68 downstream of the valve 74. The governor controlling the orifice 110 is driven by the free turbine of the engine and comprises a sleeve 268 movable by weight 262 against the action of a spring 270, the spring abutment 276 being movable by means of the manual control lever (not shown) of the engine. The manual control lever (or another one) also acts via a bell crank 200 to control the position of rollers interposed between levers 164, 186 whereby to vary the leverage of the lever 164. The metering valve 76 is actuated by a servopiston 128 subjected on one side via a passage 132 to the pressure P 2  in the line 68 upstream of the valve 76 and is subjected on the other side to a servopressure P s  developed downstream of a restrictor 142 in a line 136 connected between a point on the line upstream of the throttle valve 74 and a flapper valve 144 discharging via a line 140 to the pressure P 2 . The flapper valve 144 is acted on, via the levers 186, 164 and a spring-loaded tappet 160, by an evacuated bellows 192 subjected to the atmospheric pressure P a  It may also be acted on during acceleration or deceleration by one or other of the tines 214, 216 of a lever 210 acted on by a diaphragm 222 subjected to the difference between the compressor discharge pressure P c  and the atmospheric pressure P a . Feedback is provided by a lever 232 abutting the metering valve 76 and connected through springs 250, 254 to the levers 164, 210. The throttle valve 74 and the by-pass valve 82 act to maintain a constant pressure drop P 2 -P 3  across the metering valve 76. When the manual control lever is advance to accelerate the free turbine to a higher speed, it moves the spring abutment 276 downward to compress the spring 270 and thereby increase the opening of the orifice 110 to a maximum, and also moves the rollers between the levers 164, 186 away from the pivot axis of the lever 164 to increase the torque exerted by lever 186 on lever 164. The lever 164 moves clockwise to allow the flapper valve 144 to reduce its opening, thereby raising the pressure P s  acting on servopiston 128 to increase the opening of the metering valve 76. To prevent escessive acceleration, the tine 214 of lever 210 acts as a stop to prevent excessive opening of the flapper valve 114. So the compressor speeds up, the resultant increase of the compressor discharge pressure P c  acting on the bellows 222 causes the tine 214 to move to the left to permit greater opening of the flapper valve 144. So the desired free turbine speed is reached, the governor weights 262 reduce the opening of the orifice 110, the resultant increase of the pressure P<SP>1</SP>, acting on a diaphragm 88, 116 to reduce the opening of the throttle valve 74 and increase the opening of the by-pass valve 82. This reduced the pressure drop P 2 -P 3  maintained across the metering valve 76 and thereby reduces the fuel-flow to the value appropriate to the new speed. A time lag in the response of the governor to speed changes is afforded by the application of the pressure P 1 <SP>1</SP> via a restrictor 310, a volume 296 and a passage 314 to act at 316 on the governor sleeve 268 in the same sense as the governor weights 262. The time constant afforded by the restrictor 310 and the volume 296, together with the effect of a resilient spring- loaded wall 294, produces a delayed response of the pressure in the volume to changes of the pressure P1<SP>1</SP> produced by speed changes. A similar sequence of events occurs on deceleration, excessive deceleration being prevented by the tine 216. To prevent overspeeding of the turbine driving the compressor, a governor 318 driven by this turbine may be provided. The orifice controlled by this governor is connected upstream of the orifice 110 of the main governor.