44.
The control of servo pressure by the hydro-mechanical governor is very similar to that of the pressure control governor, except that the governor pressure is obtained from pump delivery fuel passing through a restrictor and the restricted pressure is controlled by a rotating spill valve; this type of governor is unaffected by changes in fuel specific gravity.
45.
At low engine speeds, the rotating spill valve is held open; however, as engine speed increases, centrifugal loading moves the valve towards the closed position against the diaphragm loads. This restricts the bleed of H.P. fuel to the L.P. side of the drum until, at governed speed, the governor pressure deflects the diaphragm and opens the fuel pump servo pressure spill valve to control the maximum fuel flow and engine speed.
46.
If the engine gas temperature exceeds its maximum limitation, the solenoid on the proportion-ing valve unit is progressively energized. This causes a movement of the rocker arm to increase the effective flow area of one restrictor, thus increasing the proportional flow and opening the A.S.U. spill valve to reduce servo pressure. The fuel flow is thus reduced and any further increase of gas temperature is prevented.
47.
To prevent the L.P. compressor from over-speeding, some twin-spool engines have an L.P. shaft r.p.m. governor. A signal of L.P. shaft speed is fed to an amplifier and solenoid valve, which limits
48.
An idling speed governor is often fitted to ensure that the idling r.p.m. does not vary with changing engine loads. A variation of idling r.p.m. causes the rocker arm to move and alter the propor-tional flow, and the A.S.U. adjusts the pump delivery until the correct idling r.p.m. is restored.
49.
On some engines, a power limiter is used to prevent overstressing of the engine. To achieve this, compressor delivery pressure acts on the power limiter capsule. Excess pressure opens the power limiter atmospheric bleed to limit the pressure on the
A.C.U. capsule and this controls the fuel flow through the metering plunger.
50. To enable the engine to be relit and to prevent flame-out at altitude, the engine idling r.p.m. is made to increase with altitude. To achieve this, some engines incorporate a minimum flow valve that adds a constant minimum fuel flow to that passing through the throttle valve.
Combined acceleration and speed control
51.
The combined acceleration and speed control system (fig. 10-9) is a mechanical system without small restrictors or spill valves. It is also an all-speed governor system and therefore needs no separate governor unit for controlling the maximum r.p.m. The controlling mechanism is contained in one unit, usually referred to as the fuel flow regulator (F.F.R.). An H.P. fuel pump (para. 85) is used and the fuel pump servo piston is operated by H.P. fuel on one side and main spray nozzle (servo) pressure on the spring side.
52.
The F.F.R. is driven by the engine through a gear train and has two centrifugal governors, known as the speed control governor and the pressure drop control governor. Two sliding valves are also rotated by the gear train. One valve, known as the variable metering sleeve, has a triangular orifice, known as the variable metering orifice (V.M.O.), and this sleeve is given axial movement by a capsule assembly. The
V.M.O. sleeve moves inside a non-rotating governor sleeve that is moved axially by the speed control governor. The other valve, known as the pressure drop control valve, is provided with axial movement by the pressure drop control governor and has a triangular orifice, known as the pressure drop control orifice, and a fixed-area rectangular orifice. The speed control governor is set by the throttle lever through a cam, a spring and a stirrup arm inside the regulator.
46．如果发动机的燃气温度超过丁它的最大极限值，比例活门装置上的线圈逐渐充电。这样便导致摇臂移动，增大一个限流器的有效通流面积，以增大比例流量并打开高度传感装置溢流活门，以减小伺服压力。因此，燃油流量随之减小，防止了燃气温度的进一步增高。
45．在低的发动机转速下，旋转溢流活门保持打开位置；但是，当发动机转速增加时,离心载荷在抵消膜片载荷后将活门向关闭位置移动。这便限制了高压燃油向鼓筒的低压侧回油，直到在调节转速下，调节器压力使膜片挠曲，打开燃油泵伺服压力溢流活门，以控制最大燃油流量及发动机转速为止。
变化，引起摇臂移动并改变比倒流量，此时高度传感装置调节油泵供油，直到恢复正确的慢车转速为止。
49．在某些发动机中，采用了功率限制器来防止发动机过应力。为了实现这项功能，压气机供气压力作用在功率限制器膜盒上。过高的压力打开功率限制器的大气放气口，以限制加速控制装置膜盒上的压力，进而通过节流柱塞控制燃油流量。
47．为防止低压压气机超转，某些双转子发动机上装有一个低压轴转速调节器。低压轴的转速信号被送入一个放大器和电磁活门，它按燃气温度控制相同的方法来限制燃油的输出。
48．通常装有慢车转速调节器，以保证慢车转速不随发动机载荷的变化而改变。慢车转速的
Fuel system
50．为了保证发动机能再点火，防止高空熄火，发动机慢车转速设汁得随高度增加而增加。为了实现这一点，某些发动机上装一个最小油量活门。给流过油门活门的油量增加一个恒定的最小燃油流量。
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