In addition to the physics lesson, we got to examine parts such as the fuel and air diaphragms, which are made of Dacron/fluorosilicone material. The same thing happens in reverse when the power is reduced, Rivera said. This change in MSD causes the forces in the regulator to go out of equilibrium as increases in the valve-opening force generated by the air side of the regulator drive the regulator ball to move off the seat, allowing more fuel to flow to the engine.Īs fuel flow increases, the fuel MH pressure increases, driving the fuel diaphragm to close the ball regulator valve until the MSD across the air diaphragm and the MH across the fuel diaphragm again achieve a state of equilibrium. Remember that the forces generating the MSD are from the venturi, which generates signals that react almost instantly to throttle-valve changes, and impact air, which varies with changes in atmospheric pressure (altitude and to some degree, temperature). In ActionĪs Rivera explained, throttle movement causes a change in MSD, shifting the air diaphragm, which is almost instantly transferred through the shaft to the fuel diaphragm and the regulator ball, resulting in a change in fuel flow.įor instance, when the pilot opens the throttle valve, airflow increases, causing a change to the MSD. API stocks a range of fittings that work for most common installations. These fuel inlet and outlet fittings fit a particular project. The fixed seat can be shimmed to fine-tune fuel flows. During normal cruise power settings the regulator ball moves from 10 thousands (0.010 inch) to 20 thousands (0.020 inch) off the seat. Changes in MSD and MH cause changes in the position of the respective diaphragms, prompting movement of the regulator ball in relation to a fixed seat. The two diaphragms in the Airflow unit are connected through a small-diameter steel shaft with a regulator ball on one end the other end extends through and is secured to the center of both the fuel and air diaphragms. The two areas are proportioned so that when the engine is operating at a fixed throttle-valve setting, the forces generated by the two diaphragms are balanced, and the fuel flow is steady. Fuel pressures are greater than the air pressures, so the dynamic area of the air diaphragm is considerably larger than the dynamic area of the fuel diaphragm. Venturi theory states that doubling the airflow creates four times the MSD. Rivera tied together the importance of MSD and MH when we learned jet theory, which states that doubling the fuel flow through the fuel jet creates four times the MH. The MSD is always trying to open the fuel regulator ball valve, which will increase fuel flow. These pressures are applied to opposite sides of the air diaphragm in the regulator. Metering suction differential (MSD) is the differential between ram air pressure and venturi pressure. The before-jet pressures are applied to one side of the fuel diaphragm in the regulator section of the fuel control the after-jet pressures are applied to the opposite side of the diaphragm. During the classroom session, Rivera defined terms such as metering head (MH), which is the differential pressure, or pressure drop across the fuel-metering jet. A perfect fuel-metering system would measure mass airflow, but velocities-which are much easier to measure-are related closely enough to mass airflows to be usable for fuel-metering purposes, Rivera explained.īy constant referencing to the airflow velocities and balancing them against fuel pressures, the regulator section of APIs fuel control maintains the fuel-air ratios required to support combustion for all power settings. The bullet-type venturi in the throat of Airflows fuel control increases the velocity of the airflow through the control. The key to understanding fuel-injection theory is Bernoullis principle, which states that with an increase in the velocity of a fluid there is a resulting decrease in pressure. The parts on the right are the main components of the regulator section. The throttle body and the mounting pad for the regulator section of the API fuel control are on the left.
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