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Aviation History
1941
1941 - 2102.PDF
J5O FLIGHT SEPTEMBER IITH, 1941. PRESSURE INJECTION which meters the fuel, together with the mechanism ofthe manual mixture control. Let us first examine the airflow side of the diagram,Fig. 1. On the left we have the throttle body, which con- tains the Venturis and orthodox-type throttles. Thesethrottles do nothing except control the airflow, and there are no critical " throttle-edge " idle or progression devices,since the idle control is incorporated H the main carbu- rettor. Those familiar with Stromberg carburettors willrecognise the arrangement of double venturi, but here the Venturis meter air only. No fuel passes through the ven-turi or throttle body. The pressure of the incoming air through the airscoop is recorded by a number of smallimpact tubes, so arranged on the upstream end of the large venturi that they face the airstream. The suction in thesmall or "boost" Venturis is taken from an annular channel round the neck of the latter, the edge of thispointing downstream. This arrangement is, in effect, an airflow meter, recording mass airflow; and, since there isno discharge of fuel at this point to upset the venturi action, its calibration is extremely accurate. In the re-search work on the carburettor during development it was found that this layout permits more than twice as largea metering head per unit of resistance as could be obtained in previous carburettor systems. Turning again to the diagram, the suction from the smallventuri is taken through the passage shown to the right- hand side of the air diaphragm, known as chamber B.The airscoop pressure from the impact tubes is likewise carried (via the altitude control needle valve) to the left-hand chamber, called chamber A. The air diaphragm is thus subject to the pressure differential across the venturisystem, and so will be pulled to the right as the airflow is increased, and vice versa. Turning now to the fuel section of the regulator (on theright of the illustration) there is another diaphragm chamber, the fuel diaphragm being connected to the airdiaphragm by a rod, so that both must move together. At the extreme right-hand end of this coupling rod is oneof the most important items in the carburettor, the poppet valve which controls the main flow of fuel into the car-burettor. Let us now follow the passage of fuel through the car-burettor. Fuel enters the strainer chamber at a pressure of approximately 15 1b. per sq. in. from the fuel pump.It then passes the poppet valve (which is never quite closed, as will be explained later), and so into the chamberon the right, called chamber D. Before passing into the next chamber (chamber C) the fuel must first pass throughthe Fuel Control Body, where it is metered by the jet system as shown. Chamber C, it will be observed, is indirect communication with the discharge nozzle, and the fuel therein is metered fuel, as against the unmetered fuelin chamber D. We have seen that the pressure differential across theventuri, acting upon the air diaphragm, exerts a force to the right, and this is known as the air metering force. Thegreater the airflow through the carburettor the greater this TO CHAMBER A-SCOOP PRESSURE AIK FLOW ECONOMIZER DIAPHRAGM -1O CHAMBER B—BOOST VENTURI SUCTION METERED FUEL TO CHAMBER C FUEL CONTROL UNIT- AUTOMATIC MIXTURE CONTROL UNIT ECONOMIZER \ALVE SPRING METEPEO FUEL ID SPRAY NOZZLES AUTOMATIC LEAN- CONTROL POSITION (SPECIAL) CRUISE METERING JET' AUXILIARY TAKEOFF JET IDLE NEEDLE' VALUE METERED FUEL TO REGULATOR CHAMBER C FUEL FROM CHAMBER D OF RECU.ATO* UNIT TO IDLE NEEDLE MLVE, CRUISE JET AND ECONOMIZER VALVE MASTER TAJff-OFF JET -THROTTLE BODY UNIT -CONNECTION FOR RETURN LINE FROM VAPOUR SEPARATOR TO TANK -SECTION OF WPOUR SEPARATOR AND SCREEN - POPPET \»LVE f-FUEL STRAINER -.." .CHAMBER D OF "REGULATOR UNIT -ADAPTER -ACCELERATING PUMP SPRAY NOZZLE Fig. 2. In this model the enrichment diaphragm is controlled by the air differential. The illustration shows the practical form of the arrangement.
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