30.
A variation of the flow control system is the pro-portional flow control system (fig 10-7), which is more suitable for engines requiring large fuel flows and which also enables the fuel trimming devices to adjust the fuel flow more accurately. A small controlling flow is created that has the same charac-teristics as the main flow, and this controlling or pro-portional flow is used to adjust the main flow.

31.
A different type of spill valve, referred to as a kinetic valve, is used in this system. This valve consists of two opposing jets, one subjected to pump delivery pressure and the other to pump servo pressure, and an interrupter blade that can be moved between the jets (fig. 10-8). When the blade is clear of the jets, the kinetic force of the H.P. fuel jet causes the servo pressure to rise (spill valve closed) and the fuel pump moves to maximum stroke to increase the fuel flow. When the blade is lowered between the jets, the pressure jet is deflected and the servo pressure falls, so reducing the pump stroke and the fuel flow, When the engine is steadily running, the blade is in an intermediate position allowing a slow bleed from servo and thus balancing the fuel pump output.

32.
All the controlling devices, except for the engine speed governor, are contained in one combined fuel control unit. The main parts of the control unit are the altitude sensing unit (A.S.U.), the acceleration control unit (A.C.U.), the throttle and pressurizing valve unit, and the proportioning valve unit.

33.
At any steady running condition below governed speed, the fuel pump delivery is controlled to a fixed value by the A.S.U. The spill valve in this unit is held in the controlling position by a balance of forces, spring force and the piston force. The piston is sensitive to the pressure difference across the sensing valve, the pressure difference being created by fuel flowing from the proportioning valve back to the fuel pump inlet.

31．在本系統中采用了稱為動力活門的一種變型溢流活門。該活門中有2個對置的噴嘴，一個接受油泵的供油壓力，另一個接受油泵的伺服壓力。該活門中還有一個遮斷葉片，它可以在兩個噴嘴之間移動(圖10-8)。當葉片離開此二噴嘴時，高壓燃油射流的動力使伺服壓力增高(溢流活門關閉)，油泵移到最大行程來增大燃油流量。當葉片降低到二噴嘴之間，壓力射流被其偏轉，伺服壓力下降，從而減小油泵行程及燃油流量。當發動機穩定運轉時，葉片處于中間位置，允許從伺服機構緩慢回油，使燃油泵輸出保持平衡。
32．除發動機轉速調節器之外，所有控制裝置都包容在一個綜合燃油控制裝置之內。控制裝置的主要組成部分是高度傳感裝置(A.S.U.)，加速控制裝置(A.C.U.)，油門和增壓活門裝置，以及比例活門裝置。

33．在低于調節轉速下的任意穩定運轉條件時，燃油泵供油由高度傳感裝置控制為一個固定值。在這個裝置中的濫流活門通過各種力、彈簧力和活塞力的平衡保持在控制位置。活塞感測傳感活門前后的壓差，這個壓差是由從比例活門流回燃油泵進口的燃油流產生的。
34．比例活門膜片在平衡狀態下保持在打開位置，使燃油流過高度傳感裝置。這說明，限制器出口壓力等于油門出口壓力。而且當它們的進口壓力相等時，限制器和油門前后的壓差相等，因而燃油流量保持恒定。
 
35．當油門緩慢打開時，油門活門和比例式流量限制器前后的壓差減小，同時比例式活門膜片調節其位置。它降低了比例流量，這比例流量又使高度傳感裝置溢流活門關閉，并增大伺服壓力。燃油泵增加了它的供油量，這使得油門活門前后的壓差得以恢復，并使各限制器前后的壓差相等。比例流量恢復到其原先值。高度傳感裝置中力的平衡使溢流活門返回到控制位置。
 
36．由于飛機飛行速度或高度的改變導致的進氣道空氣壓力的改變由高度傳感裝置中的膜片感測。壓力的降低導致高度傳感裝置膜片膨脹，因而增大了濫流活門的回油。這降低了燃油泵的供油量，直到燃油流量與空氣流量相匹配為止，供油量的降低導致油門活門及比例活門限制器前后的壓差降低。降低后的比例流量使高度傳感裝置恢復平衡，這又使溢流活門返回到其控制位置。反之，飛機飛行速度增加，會使進氣道空氣壓力增加，它減少溢流活門的回油，并增大燃油流量。
Fuel system

34. The proportioning valve diaphragm is held open in a balanced condition allowing fuel to pass to the
A.S.U. This means that the restrictor outlet pressure is equal to the throttle outlet pressure and, as their inlet pressures are equal, it follows that the pressure difference across the restrictors and the throttle are equal; therefore, a constant fuel flow is obtained.
35. When the throttle is slowly opened, the pressure difference across the throttle valve and the proportioning flow restrictors decreases and the pro-portioning valve diaphragm adjusts its position. This reduces the proportional flow, which closes the
A.S.U. spill valve and increases the servo pressure. The fuel pump increases its delivery and this restores the pressure difference across the throttle valve and equalizes the pressure difference across the restrictors. The proportional flow is restored to its original value and the balance of forces in the A.S.U. returns the spill valve to the controlling position.
圖10-8 由動力活門驅動的伺服壓力控制
伺服
高壓燃油
活門中間位置
（油泵供油不變）
活門關
（油泵供油增加）
　
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