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．在本系统中采用了称为动力活门的一种变型溢流活门。该活门中有2个对置的喷嘴，一个接受油泵的供油压力，另一个接受油泵的伺服压力。该活门中还有一个遮断叶片，它可以在两个喷嘴之间移动(图10-8)。当叶片离开此二喷嘴时，高压燃油射流的动力使伺服压力增高(溢流活门关闭)，油泵移到最大行程来增大燃油流量。当叶片降低到二喷嘴之间，压力射流被其偏转，伺服压力下降，从而减小油泵行程及燃油流量。当发动机稳定运转时，叶片处于中间位置，允许从伺服机构缓慢回油，使燃油泵输出保持平衡。
32．除发动机转速调节器之外，所有控制装置都包容在一个综合燃油控制装置之内。控制装置的主要组成部分是高度传感装置(A.S.U.)，加速控制装置(A.C.U.)，油门和增压活门装置，以及比例活门装置。
33．在低于调节转速下的任意稳定运转条件时，燃油泵供油由高度传感装置控制为一个固定值。在这个装置中的滥流活门通过各种力、弹簧力和活塞力的平衡保持在控制位置。活塞感测传感活门前后的压差，这个压差是由从比例活门流回燃油泵进口的燃油流产生的。
34．比例活门膜片在平衡状态下保持在打开位置，使燃油流过高度传感装置。这说明，限制器出口压力等于油门出口压力。而且当它们的进口压力相等时，限制器和油门前后的压差相等，因而燃油流量保持恒定。
35．当油门缓慢打开时，油门活门和比例式流量限制器前后的压差减小，同时比例式活门膜片调节其位置。它降低了比例流量，这比例流量又使高度传感装置溢流活门关闭，并增大伺服压力。燃油泵增加了它的供油量，这使得油门活门前后的压差得以恢复，并使各限制器前后的压差相等。比例流量恢复到其原先值。高度传感装置中力的平衡使溢流活门返回到控制位置。
36．由于飞机飞行速度或高度的改变导致的进气道空气压力的改变由高度传感装置中的膜片感测。压力的降低导致高度传感装置膜片膨胀，因而增大了滥流活门的回油。这降低了燃油泵的供油量，直到燃油流量与空气流量相匹配为止，供油量的降低导致油门活门及比例活门限制器前后的压差降低。降低后的比例流量使高度传感装置恢复平衡，这又使溢流活门返回到其控制位置。反之，飞机飞行速度增加，会使进气道空气压力增加，它减少溢流活门的回油，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.
图10-8 由动力活门驱动的伺服压力控制
伺服
高压燃油
活门中间位置
（油泵供油不变）
活门关
（油泵供油增加）
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