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．如果發動機的燃氣溫度超過丁它的最大極限值，比例活門裝置上的線圈逐漸充電。這樣便導致搖臂移動，增大一個限流器的有效通流面積，以增大比例流量并打開高度傳感裝置溢流活門，以減小伺服壓力。因此，燃油流量隨之減小，防止了燃氣溫度的進一步增高。
45．在低的發動機轉速下，旋轉溢流活門保持打開位置；但是，當發動機轉速增加時,離心載荷在抵消膜片載荷后將活門向關閉位置移動。這便限制了高壓燃油向鼓筒的低壓側回油，直到在調節轉速下，調節器壓力使膜片撓曲，打開燃油泵伺服壓力溢流活門，以控制最大燃油流量及發動機轉速為止。
變化，引起搖臂移動并改變比倒流量，此時高度傳感裝置調節油泵供油，直到恢復正確的慢車轉速為止。
 
49．在某些發動機中，采用了功率限制器來防止發動機過應力。為了實現這項功能，壓氣機供氣壓力作用在功率限制器膜盒上。過高的壓力打開功率限制器的大氣放氣口，以限制加速控制裝置膜盒上的壓力，進而通過節流柱塞控制燃油流量。
47．為防止低壓壓氣機超轉，某些雙轉子發動機上裝有一個低壓軸轉速調節器。低壓軸的轉速信號被送入一個放大器和電磁活門，它按燃氣溫度控制相同的方法來限制燃油的輸出。

48．通常裝有慢車轉速調節器，以保證慢車轉速不隨發動機載荷的變化而改變。慢車轉速的
Fuel system

50．為了保證發動機能再點火，防止高空熄火，發動機慢車轉速設汁得隨高度增加而增加。為了實現這一點，某些發動機上裝一個最小油量活門。給流過油門活門的油量增加一個恒定的最小燃油流量。
　
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