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Aviation History
1946
1946 - 0071.PDF
JANUARY IOTH, 1946 FLIGHT 43 CONTROL' RING THE JET-ENCINE FUEL SYSTEM contact between it and the rocker is broken andthe plate valve is re-seated ; pressure above the control piston is restored and the cam plate ismoved to increase the stroke. It is essential for correct functioning of the pump that no air isallowed to accumulate on either side of the dia- phragm. A ball-type bleed valve is provided toevacuate air from both upper and lower spaces, and after use is sealed off by a blank nipple to pre-vent leakage. In contradistinction to a barostat, which regu-lates the delivery of a constant-output pump by spilling excess fuel at altitude back to the tank,-the barometric pressure control unit varies the delivery pressure of the pump in accordance withchange of altitude. As the pressure-control piston of the pump is connected to the cam plate, the stroke is variedand the delivery flow of fuel adjusted to specific require- ments. Barometric Control Unit Constructionally, the unit comprises a body divided intoa capsule chamber and a valve chamber by a diaphragm which isolates the two chambers andserves as a pivot plate to which is secured a rocker lever extending intoboth. The barometric capsule is mounted with its lower pivot seatedin one end of the rocker lever and its upper pivot located in an adjustablescrew . mounted in the chamber cover. A stop screw in the base ofthe chamber limits movement of the rocker lever. The other end of the rocker leveris loaded by a spring and carries a half-ball seating on an orifice sup-plied with fuel at servo pressure from above the pressure-control piston ofthe pump. Control of the rocker lever is by fuel at delivery pressurefrom below the pressure-control pis- ton Pressure is applied to a rubberdiaphragm and transmitted by a small piston and an operating rod,the upper end of which abuts the end of an adjuster screw in the rockerarm. These components are mounted in the base of the chamber in aneccentric sleeve to enable the end of the rod to be moved across the face of the adjuster screwto alter the operative distance from the fulcrum point of the lever. The valve chamber is in open communicationwith the inlet side of the pump. Connections are made clear in the diagram of the pump and barometric controlunit. Should the force applied by the pump delivery^pressure I DELIVERY FROM PUMP BAROMEJIC Diagram of pump and barometric pressure control unit showinginterconnections. exceed the spring loading the rocker lever will pivot, liftthe half-ball from the orifice and allow servo pressure fuel to spill into the valve chamber. This unbalances thepressure-control piston, which moves to reduce the stroke of the pump. Consequently the pump output decreases,and the delivery pressuie is lowered until it just suffices to balance the spring load. Balance is maintained by asn^all spill past the half-ball. A fall in pump delivery pres- sure allows the spring to seat thehalf-ball, closing the orifice and, in turn increasing the pump stroke.. The barometric capsule exerts aforce which assists the pump delivery pressure to oppose the spring load.At ground level the force is deter- mined by adjustment of the upperpivot screw which compresses the capsule. As altitude is increased andbarometric pressure falls, the capsule tends to lengthen, and the pressuieexerted on the rocker lever becomes greater. The spring load remainsconstant, and consequently the pump delivery pressure reduces until abalance is reached. The reverse action takes place as altitude is re-duced and barometric pressure rises. Accumulator, trip valve and high-pressure cock. I, Connection to pump inlet ; z, Connections to manifold ; 3, Relief valve ; 4, High-pressure cock ; 5, Trip valve ; 6, Spill to atmosphere. Flow diagrams of accumulator, trip valve and high-pressure cock. A. Cock open. Trip valve closed. Accumulator charging. HB. Cock open. Trip valve open. Accumulator discharging to manifold. C. Cock dosed. Trip valve connected to pump inlet. Manifold spilling to atmosphere. Throttle Valve Located between the pump and theaccumulator unit, the pilot's throttle valve, is a tapered plunger valve inwhich the cross-sectional area of the flow space between plunger and seat-ing is varied by axial movement of the plunger. Operation is positive, as a pinion on theactuation lever spindle engages a rack cut on the plunger. When the throttle is closed the plunger is fully seated butthe engine is not shut down. A supply of fuel to. maintain the engine running in the idling condition is by-passed fromthe inlet to the outlet by way of a spring-loaded valve opening automatically under fuel pressure. An adjustment screw limits the lift of thisvalve to determine the CONNECTION TO „ , , PUMP INLET flow for slow running To shut down the en gine a high-pressure cock is provided to stop the fuel flow to the burners. This is embodied in the accumulator 'unit. At starting and whilst run- ning the cock remains in the fully opened position. When starting-up the en gine, the cranking speed is insufficient to produce a delivery of fuel yielding a SPILL TO ATMOSPHERE
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