图10-7 一种比例式流量控制系统
Fig. 10-7 A proportional flow control system.
24．在低的高压轴转速下，旋转溢流活门保持打开，但是当发动机转速增加时，离心载荷使活门向关闭方向移动，抵消膜片载荷。这样便限制了向活门低压侧的回油，直到在调节转速下，调节器压力使伺服控制膜片挠曲，并打开伺服溢流活门，由此来控制燃油流量，进而控制高压轴转速。
102
燃油控制装置
空气开关
加速控制装置
油门及增压活门装置
功率限制器
油门开关及停车开关
最小油量活门
分布配重
燃油节流柱塞
慢车活门
燃油喷嘴
电磁线圈
压降控制
低压轴转速信号
限制器
温度控制
信号放大器
燃气温度
热电偶
进气道空气
压气机供气
慢车转速调节器
高度传感器
比例活门
敏感活门
比例活门装置
减少压气机供气
空气开关
控制空气压力
伺服活塞
燃油泵
加速控制装置伺服
调节器
比例流量
伺服控制
控制燃油压力
低压燃油
油泵供油(高压油)
油门进口
油门出口
初级燃油
主燃油
液压机械调节器
压力分布
25．如果发动机燃气温度要超过最大极限值，在低压转速限制器及温度控制器的线圈中的电流减小，使溢流活门打开，以减小作用在压力降控制膜片上的压力。然后，流量控制溢流活门打开，使油泵伺服压力和燃油泵输出减小。
26．为防止低压压气机超转，通常在多转子发动机上装有一个低压压气机轴转速调节器。低压轴转速及进气口温度信号被输入放大器和电磁活门，该活门以控制燃气温度(第25段)的同样方法来限制燃油流量。
27．在上述的系统中采用了由高压截止活门控制的主喷嘴和起动喷嘴。在燃烧室内装有2个起动喷嘴，每个喷嘴都位于点火电嘴之前。当发动机起动之后，向这些喷嘴供应的燃油由高压截止活门切断。
28．为了在高空条件下保证能维持供应喷嘴的燃油压力适当，位于油门活门下游的反压活门将压力提高，足以保证燃油泵伺服系统工作顺利。
流量控制
29．燃油流量控制系统通常比压力控制系统更为紧凑，它对于油门下游流量变化的影响反应不敏感。燃油泵供油压力与发动机转速相关；因此，在发动机低转速下，供油压力相当低。控制燃油泵的输出是为了在恒定的进气道条件下保持油门活门前后的压力差恒定。还采用其他各种装置，依据进气道空气
压力变化、慢车和加速控制，燃气温度和压气机出口压力控制来调节燃油流量。
Fuel system
30．比例式流量控制系统(图10-7)是流量控制系统的一种，它更适合用于发动机要求大燃油流量的场合，它还使燃油微调装置能更精确地调节燃油流量。这种系统能形成一股小的控制流量，它与主流量具有相同的特性，该控制流量(即比例流量)被用来调节主流量。
until the fuel flow matches the airflow. Conversely, an increase in air intake pressure closes the spill valve to increase the fuel flow.
23.
H.P. compressor shaft r.p.m. is governed by a hydro-mechanical governor which uses hydraulic pressure proportional to engine speed as its controlling parameter. A rotating spill valve senses the engine speed and the controlling pressure is used to limit the pump stroke and so prevent over-speeding of the H.P. shaft rotating assembly. The controlling pressure is unaffected by changes in fuel specific gravity.
24.
At low H.P. shaft speeds, the rotating spill valve is held open, but as engine speed increases, centrifugal loading moves the valve towards the closed position against the diaphragm loads. This restricts the bleed of fuel to the L.P. side of the valve until, at governed speed, the governor pressure deflects the servo control diaphragm and opens the servo spill valve to control the fuel flow and thereby the H.P. shaft speed.
25.
If the engine gas temperature attempts to exceed the maximum limitation, the current in the
L.P. speed limiter and temperature control solenoid is reduced. This opens the spill valve to reduce the pressure on the pressure drop control diaphragm. The flow control spill valve then opens to reduce the pump servo pressure and fuel pump output.
26.
To prevent the L.P. compressor from over-speeding, multi-spool engines usually have an L.P. compressor shaft speed governor. A signal of L.P. shaft speed and intake temperature is fed to an amplifier and solenoid valve, the valve limiting the fuel flow in the same way as the gas temperature control (para. 25).
27.
The system described uses main and starting spray nozzles under the control of an H.P. shut-off valve. Two starting nozzles are fitted in the combustion chamber, each being forward of an igniter plug. When the engine has started, the fuel flow to these nozzles is cut off by the H.P. shut-off valve.
28.
To ensure that a satisfactory fuel pressure to the spray nozzles is maintained at high altitudes, a back pressure valve, located downstream of the throttle valve, raises the pressure levels sufficiently to ensure satisfactory operation of the fuel pump servo system.
Flow control
29.
A flow control fuel system is generally more compact than a pressure control system and is not sensitive to flow effect of variations downstream of the throttle. The fuel pump delivery pressure is related to engine speed; thus, at low engine speeds pump delivery pressure is quite low. The fuel pump output is controlled to give a constant pressure difference across the throttle valve at a constant air intake condition. Various devices are also used to adjust the fuel flow for air intake pressure variations, idling and acceleration control, gas temperature and compressor delivery pressure control.
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