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Aviation History
1941
1941 - 2103.PDF
SEPTEMBER IITH, 1941. FLIGHT 15* • PRESSURE INJECTION UgMETERED FUEL TO CHAMBER D ENRICHMENT VALVE VENT RESTRICTION METERED FUEL TO- SPRAY NOZZLES RECULATOR VENT RESTRICTION AUTOMATIC RICH POSITION MANUAL MIXTURE SELECTOR VALVE AUTOMATIC LEAN METERING JET METERED FUEL FROM REGULATOR CHAMBER C AUTOMATIC MIXTURE CONTROL UNIT REGULATOR UNIT REGULATOR 'FILL VALVE (cuoseD oNur IN IDLE CUT-OFF POSITION) AUTOMATIC RICH METERING JFT- POWER ENRICHMENT METERING JET POWER ENRICHMENT METERING VALVE » METERED FUEL TO REGULATOR CHAMBER C FUEL FROM CHAMBER D OF REGULATOR UNIT • FILLING VENTS FOR DIAPHRAGM CHAMBERS THERE 5 NO FUEL CIRCULATION THROUGH THESE -THROTTLE BODY UNIT CONNECTION FOB RETURN LINE FROM VAPOUR SEPARATOR TO TANK SECTION OF VAPOUR SEPARATOR AND SCREEN POPPET VALVE FUEL 5TRAINER CHAMBER D OF REGULATOR UNIT FUEL SUPPLy INLET ADAPTER ACCELERATING PUMP SPRAY NOZZLE Fig. 3. A different model from that shown in Fig. 2. The enrichment diaphragm is controlled by the fuel differential. The valve which gives the manual mixture positions is of the rotary disc type. force becomes, and the more the air diaphragm will move to the right, so opening the fuel poppet valve still farther and admitting more fuel. The system is kept in balance by the fuel diaphragm, which exerts a force in the oppo- site direction, known as the fuel metering force. This force is the difference between the unmetered and the metered fuel. The pressure of the metered fuel being kept constant at 5 1b. per sq. in., the pressure differential across the air and fuel diaphragms is always the same, but acting in opposite directions, since the air metering force will* always be balanced by the fuel metering force. Additional small diaphragms are used in both chambers for sealing and balancing., Owing to the possibility of inertia in the diaphragm assembly at very small airflow, a light spring is arranged to hold the poppet valve from seating on idle. This intro- duces a rich bias when idling, which is corrected as requiredb y an external idle mixture adjustment. The constant-pressure discharge nozzle is a simple arrangement, and its operation will be readily grasped from ™e illustration. The spring permits the valve to open ata Pproximately 5 lb. per sq. in., and when the pressure tends0 increase as the flow becomes greater, the diaphragm com- presses the spring and opens the valve farther, maintaininglb e pressure constant. lne fuel-control body and jet system assembly are shown ^grammatically at the bottom right-hand corner of Fig. 1. ere are actually several arrangements of jet system which ner only slightly, but for the purposes of this descriptionwe will deal only with that illustrated in the diagram. From chamber D the fuel passes through the master take- off jet, past the idle needle and so through the cruise jet into chamber C. The idle needle is of large diameter and is linked with the throttle spindle in such a manner that from the nearly closed or idling position it is withdrawn completely in the first ten degrees of'throttle travel. Mixture regulation on idle is effected by moving the idle needle in or out from its seat by an external adjustment. Since the needle is located directly in the main fuel circuit in the carburettor there can be no flat spot on opening the throttle. When the throttle is opened to enter the cruise range the needle is withdrawn, and the fuel flows past it and so to the cruise and economiser jets. On this model (known as the "Airflow Economiser") the manual control takes the form of a movable needle which can be pushed forward so as to restrict slightly the orifice of the cruise jet. This gives the auto-lean cruising mixture posi- tion. Withdrawing the needle completely gives the auto- rich mixture position, while moving the needle right for ward so that the flange seats on the jet itself gives the idle cut-off position. The operation of the full rich position will be described in connection with the Altitude Control. Enrichment for take-off and high power is effected by the economiser needle, which by its contour and move- ment permits the required amount of fuel to pass through into chamber C. In this model the economiser is governed by another and smaller diaphragm, one side of which is in communication with chamber A, the other side with chamber B. When the pressure differential reaches a cer- tain predetermined figure the diaphragm will move suffi-
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