10.
The usual method of varying the fuel flow to the spray nozzles is by adjusting the output of the H.P. fuel pump. This is effected through a servo system in response to some or all of the following:
(1)
Throttle movement.
(2)
Air temperature and pressure.
(3)
Rapid acceleration and deceleration.
(4)
Signals of engine speed, engine gas
temperature and compressor delivery
pressure.
Pressure control (turbo-propeller engine)
11.
The pressure control system (fig. 10-4) is a typical system as fitted to a turbo-propeller engine where the rate of engine acceleration is restricted by a propeller speed controller. The fuel pump output is automatically controlled by spill valves in the flow control unit (F.C.U.) and the engine speed governor. These valves, by varying the fuel pump servo pressure, adjust the pump stroke to give the correct fuel flow to the engine.
12.
At steady running conditions, at a given air intake pressure and below governed speed, the spill valve in the F.C.U. is in a sensitive position, creating
13. When the throttle is slowly opened, the pressure to the throttle valve falls and allows the
F.C.U. spill valve to close, so increasing the servo pressure and pump delivery. As the pressure to the throttle is restored, the spill valve returns to its sensitive or controlling position, and the fuel pump stabilizes its output to give the engine speed for the selected throttle position. The reverse sequence occurs as the throttle is closed.
14.
A reduction of air intake pressure, due to a reduction of aircraft forward speed or increase in altitude, causes the F.C.U. capsule to expand, thus increasing the bleed from the F.C.U. spill valve. This reduces fuel pump delivery until the fuel flow matches the airflow and the reduced H.P. pump delivery (throttle inlet pressure), allows the spill valve to return to its sensitive position. Conversely, an increase in air intake pressure reduces the bleed from the spill valve and increases the fuel flow. The compensation for changes in air intake pressure is such that fuel flow cannot be increased beyond the pre-determined maximum permissible for static International Standard Atmosphere (I.S.A.) sea-level conditions.
15.
The engine speed governor prevents the engine from exceeding its maximum speed limitation. With increasing engine speed, the centrifugal pressure from the fuel pump rotor radial drillings increases and this is sensed by the engine speed governor diaphragm. When the engine reaches its speed limitation, the diaphragm is deflected to open the governor spill valve, thus overriding the F.C.U. and preventing any further increase in fuel flow. Some pressure control systems employ a hydro-mechanical governor (para. 23).
16.
The governor spill valve also acts as a safety relief valve. If the fuel pump delivery pressure exceeds its maximum controlling value, the servo pressure acting on the orifice area of the spill valve forces the valve open regardless of the engine speed, so preventing any further increase in fuel delivery pressure.
Pressure control (turbo-jet engine)
17. In the pressure control system illustrated in fig. 10-5, the rate of engine acceleration is controlled by a dashpot throttle unit. The unit forms part of the fuel control unit and consists of a servo-operated throttle, which moves in a ported sleeve, and a control valve.
压力控制（涡轮喷气发动机）
17．在图10-5所示的压力控制系统中，发动机的加速率由阻尼油门装置控制。该装置为燃油控制装置的一部分，由一个伺服机构驱动的油门活门和一个控制活门所组成。伺服机构驱动的油门活门在一个有开孔的套筒中移动，而控制活门则在油门活门的内孔中自由滑动，并通过一个齿条和小齿轮机构与驾驶员的油门杆相联接。油门杆的移动导致油门活门逐渐打开套筒上的孔，增加燃油流量。图10-6所示为油门活门及控制活门在它们各种控制位置下的情况。
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