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(fā)動(dòng)機(jī)的燃?xì)鉁囟瘸^(guò)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(dǎo)致?lián)u臂移動(dòng)，增大一個(gè)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?xì)鉁囟鹊倪M(jìn)一步增高。
45．在低的發(fā)動(dòng)機(jī)轉(zhuǎn)速下，旋轉(zhuǎn)溢流活門保持打開位置；但是，當(dāng)發(fā)動(dòng)機(jī)轉(zhuǎn)速增加時(shí),離心載荷在抵消膜片載荷后將活門向關(guān)閉位置移動(dòng)。這便限制了高壓燃油向鼓筒的低壓側(cè)回油，直到在調(diào)節(jié)轉(zhuǎn)速下，調(diào)節(jié)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(fā)動(dòng)機(jī)轉(zhuǎn)速為止。
變化，引起搖臂移動(dòng)并改變比倒流量，此時(shí)高度傳感裝置調(diào)節(jié)油泵供油，直到恢復(fù)正確的慢車轉(zhuǎn)速為止。
49．在某些發(fā)動(dòng)機(jī)中，采用了功率限制器來(lái)防止發(fā)動(dòng)機(jī)過(guò)應(yīng)力。為了實(shí)現(xiàn)這項(xiàng)功能，壓氣機(jī)供氣壓力作用在功率限制器膜盒上。過(guò)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(jìn)而通過(guò)節(jié)流柱塞控制燃油流量。
47．為防止低壓壓氣機(jī)超轉(zhuǎn)，某些雙轉(zhuǎn)子發(fā)動(dòng)機(jī)上裝有一個(gè)低壓軸轉(zhuǎn)速調(diào)節(jié)器。低壓軸的轉(zhuǎn)速信號(hào)被送入一個(gè)放大器和電磁活門，它按燃?xì)鉁囟瓤刂葡嗤姆椒▉?lái)限制燃油的輸出。
48．通常裝有慢車轉(zhuǎn)速調(diào)節(jié)器，以保證慢車轉(zhuǎn)速不隨發(fā)動(dòng)機(jī)載荷的變化而改變。慢車轉(zhuǎn)速的
Fuel system
50．為了保證發(fā)動(dòng)機(jī)能再點(diǎn)火，防止高空熄火，發(fā)動(dòng)機(jī)慢車轉(zhuǎn)速設(shè)汁得隨高度增加而增加。為了實(shí)現(xiàn)這一點(diǎn)，某些發(fā)動(dòng)機(jī)上裝一個(gè)最小油量活門。給流過(guò)油門活門的油量增加一個(gè)恒定的最小燃油流量。
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