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HYDRAULIC
GOVERNORS.-For its speed-sensitive elements, the hydraulic governor, as
shown in figure 9-27, depends on a similar flyweight arrangement to that of the
mechanical governor. However, the power supply that moves the fuel mechanism is
operated hydraulically rather than through direct mechanical linkage with the
flyweights. The flyweights of the hydraulic governor are linked directly to a
small pilot valve that opens and closes ported passages, admitting oil under
pressure to either side of a power piston that is linked to the fuel control
mechanism. Since the flyweights move only a lightweight pilot valve, the
inherent design of the hydraulic governor is more sensitive to small speed changes
than the design of the mechanical type of governor, which derives all of its
working power from the flyweights. The larger and heavier the fuel control
mechanism, the more important it is to employ a hydraulic governor. There is always a lag between a change in fuel setting and the time the engine reaches the new desired speed. Even when the fuel controls are set as required during a speed change, hunting caused by overshooting will occur. As long as engine speed is above or below the desired new speed, the simple hydraulic governor will continuously adjust (overcorrect) the fuel setting to decrease or increase the delivery of fuel. For this reason, a hydraulic governor must have a mechanism that will discontinue changing the fuel control setting slightly before the new setting has actually been reached. This mechanism, used in all modern hydraulic governors, is called a COMPENSATING DEVICE. One type of compensating device is illustrated in figure 9-27. The buffer piston, buffer springs, and needle valve in the hydraulic circuit between the control land of the pilot valve plunger and the power piston comprise the BUFFER COMPEN-SATING SYSTEM of the governor. Lowering the pilot valve plunger permits a flow of pressurized oil into the buffer cylinder and power cylinder. This flow of oil moves the power piston up to increase fuel. As the pilot valve plunger moves up, oil is permitted to flow from the buffer cylinder and power cylinder to the governor sump, and the power piston spring moves the power piston down to decrease fuel. The rate of compensation is adjusted by regulating the oil leakage through the compensating needle valve. If the compensating needle valve is adjusted correctly, only a slight amount of hunting will occur after a load change. This hunting will quickly be dampened out, resulting in stable operation through the operating range of the governor. |
 
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