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劳斯莱斯喷气引擎-中[1].英文96-190页(共286页)(中)

6.An air cooled high pressure nozzle guide vane and turbine blade arrangement illustrating the cooling airflow is shown in fig. 9-2. Turbine vane and turbine blade life depends not only on their form but also on the method of cooling, therefore the flow design of the internal passages is important. There have been numerous methods of turbine vane and turbine blade cooling which have been used throughout the history of gas turbines. Generally,single pass internal (convection) cooling was of great practical benefit but development has lead to multi-pass internal cooling of blades, impingement cooling of vanes with external air film cooling of both vanes and blades, these are shown in fig. 9-3. and fig. 9-4.

7.The 'pre-swirl nozzles' (fig. 9-2) reduce the temperature and pressure of the cooling air fed to the disc for blade cooling. The nozzles also impart a

substantial whirl velocity to assist efficient entry of the air into the rotating cooling passages.

8.Cooling air for the turbine discs enters the annular spaces between the discs and flows outwards over the disc faces. Flow is controlled by interstage seals and, on completion of the cooling function, the air is expelled into the main gas stream (fig. 9-5); see para. 23., Hot gas ingestion.

Bearing chamber cooling

9.Air cooling of the engine bearing chambers is not normally necessary since the lubrication system (Part 8) is adequate for cooling purposes.Additionally, bearing chambers are located, where possible, in the cooler regions of the engine. In instances where additional cooling is required, it is good practice to have a double skinned bearing housing with cooling air fed into the intermediate space.

Internal air system

88

Fig. 9-3Development of high pressure turbine blade cooling.

7.“预旋喷嘴”(图9-2)降低了供往轮盘用于叶片冷却的空气的压力和温度。这些喷嘴还使空气得到很大的周向速度,以帮助空气有效地进入旋转的冷却通道。

8.冷却涡轮盘的冷却空气进入轮盘之间的空腔。并往外流过轮盘的表面。气流由级间封严件控制,在完成冷却功能之后,排入主燃气流(图9-5),参见第23段热燃气吸入部分。

低压冷却空气

高压冷却空气

单通道,内部冷却

(60年代)

单通道,多路内部冷却及气膜冷却(70年代)

五通道,多路内部冷却,

广泛使用气膜冷却

图9-3 高压涡轮叶片冷却的发展

轴承腔冷却

9.在正常情况下,不需要用空气来冷却发动机的轴承腔,因为润滑系统(第8章)对于冷却目的来说是足够的。而且,只要有可能,轴承腔总是会安排在发动机较冷的部位。在需要额外冷却的情况下,好的做法是设一个双层壁的轴承座,让冷却空气通入其中间的空腔。

Accessory cooling

10.A considerable amount of heat is produced by some of the engine accessories, of which the electrical generator is an example, and these may often require their own cooling circuit. When air is used for cooling, the source may be the compressor or atmospheric air ducted from intake louvres in the engine cowlings.

11.When an accessory unit is cooled during flight by atmospheric air it is usually necessary to provide an induced circuit for use during static ground running when there would be no external airflow. This is achieved by allowing compressor delivery air to pass through nozzles situated in the cooling air outlet duct of the accessory. The air velocity through the nozzles create a low pressure area which forms an ejector, so inducing a flow of atmospheric air through the intake louvres. To ensure that the ejector system only operates during ground running, the flow of air from the compressor is controlled by a valve. A generator cooling system with an ejector is shown in fig. 9-6.SEALING

12.Seals are used to prevent oil leakage from the engine bearing chambers, to control cooling airflows

and to prevent ingress of the mainstream gas into the turbine disc cavities.

13.Various sealing methods are used on gas turbine engines. The choice of which method is dependent upon the surrounding temperature and pressure, wearability, heat generation, weight, space available, ease of manufacture and ease of installa-tion and removal. Some of the sealing methods are described in the following paragraphs. A hypothetical turbine showing the usage of these seals is shown in fig. 9-5.

Labyrinth seals

14.This type of seal is widely used to retain oil in bearing chambers and as a metering device to control internal airflows. Several variations of labyrinth seal design are shown in fig. 9-7.

15.A labyrinth seal comprises a finned rotating member with a static bore which is lined with a soft abradable material, or a high temperature honeycomb structure. On initial running of the engine the fins lightly rub against the lining, cutting into it to give a minimum clearance. The clearance varies throughout the flight cycle, dependent upon the thermal growth of the parts and the natural flexing of the rotating members. Across each seal fin there is a pressure drop which results in a restricted flow of

Internal air system

89

Fig. 9-4

High pressure nozzle guide vane construction and cooling.

附件冷却

10.发动机的一些附件会产生大量的热,其中发电机即是一例。这些附件常常需要有它们自己的冷却通路。当用空气进行冷却时,气源可以是压气机,或者是从发动机整流罩中进气道的引气口处引入的外界空气。

11.当一个附件装置在飞行中由外界空气冷却的时候,通常需要配备一条诱导通路,以便在地面静态运转没有外部空气流的时候使用。这是将压气机输出的空气流经位于附件冷却空气出口导管处的几个喷口而实现的。流经这些喷口的空气速度造成一个低压区,这个低压区形成了一个引射器,由此来引射一股大气空气流流经进气道中的引气口。为了保证该引射器系统仅仅在地面工作,来自压气机的空气流由一个活门控制。带引射器的发电机冷却系统示于图9-6。

封严

12.封严件用于防止滑油从发动机轴承腔漏出,控制冷却空气流和防止主气流的燃气进入涡轮盘空腔。

13.在燃气涡轮发动机上使用了多种封严方法。选择何种方法取决于周围的温度和压力、可磨蚀性、发热量、重量、可用的空间,易于制造及易于安装和拆卸。下列各段叙述了某些封严方法。图9-5所示为一台假想的涡轮,用来表明这些封严的使用方法。

冲击冷却管

叶根平台气膜冷却孔

冷却空气

图9-4 高压涡轮导向叶片的结构和冷却

Fig. 9-5A hypothetical turbine cooling and sealing arrangement.

预旋喷嘴

低压空气 排出机外

导向器时片

涡轮叶片

刷式 封严件高压冷却空气进入燃气流

极间篦齿式封严件

浮动环 封严件

涡轮轴

涡轮盘

级间蜂窝封严件

涡轮盘

涡轮盘

低压空气 高压空气

低压冷却空气

16.两根旋转轴之间的封严件,由于两根轴同时发生弯曲,所以更易导致篦齿与摩擦材料之间的摩擦。这会产生过量的热,使轴损坏。为了防止这一点,使用了一种不产生热的封严件,这种封严件中的可磨蚀衬带由旋转中的滑油环带来取代。当轴弯曲时,篦齿浸入滑油并保持封严件不产生热(图9-7中左侧第1图)。

sealing air from one side of the seal to the other.91

Fig. 9-6A generator cooling system.刷式封严件

22.刷式封严件(图9-7中左侧第4图)有一个由很多细钢丝制成的刷组成的静止环。它们不断地与旋转轴相接触,与硬的陶瓷涂层相摩擦。这种封严件的优点是可以承受径向摩擦而不增加渗漏。

(图9-7中左侧第3图)由一个封严齿从压气机

放出的空气来自进气道放气口

图9-6 一种发电机冷却系统引射器

出口导管

Internal air system

92

Fig. 9-7Typical seals.

热燃气吸入

23.防止高温主燃气流吸人涡轮盘的空腔是非常重要的,因为这会导致过热和引起有害的热膨胀和疲劳。涡轮环腔内的压力迫使旋转的轮盘和相邻的静止零件之间的高温燃气进入涡轮盘轮缘的空间。而且,靠近旋转轮盘表面的空气由于摩擦而加速,使它被向外抽走。这就会诱发一股向里填补的高温燃气流。

24.通过不断地向轮盘空腔供入足量的冷却和封严空气,来阻挡高温燃气的向里流动,达到防止燃气吸入的目的。冷却和封严空气的流量和压力由级间封严件(图9-5)控制。

摩擦衬环

滑油旋转腔道

液体和摩擦衬环篦齿式封严件

级间连续槽 (篦齿式) 空气封严件

螺纹式(篦齿式)滑油封严件浮动环式滑油封严件

轴间液压封严件石墨件

弹簧

石墨封严件

陶瓷涂层

刷式封严件

图9-7 几种典型的封严件

封严空气 滑油 旋转组件

upon a fixed diameter pressure balance seal to ensure the location bearing is adequately loaded throughout the engine thrust range.

AIRCRAFT SERVICES

26.To provide cabin pressurization, airframe anti-icing and cabin heat, substantial quantities of air are bled from the compressor. It is desirable to bleed the air as early as possible from the compressor to minimize the effect on engine performance. However, during some phases of the flight cycle it may be necessary to switch the bleed source to a later compressor stage to maintain adequate pressure and temperature.

Internal air system

93

Fig. 9-8Control of axial bearing load.

轴承载荷控制

25.发动机轴承受交变的轴向燃气载荷(第20章),在压气机中是向前的,在涡轮上是向后的。压气机与涡轮之间的轴便经常处于拉伸应力之下,载荷之间的差额则由装在静止机匣上的定位轴承(又称止推轴承)承受(图9-8)。内部空气的压力作用在一个固定直径的压力平衡封严件上,以保证在整个发动机推力范围内,定位轴承承受的载荷是适当的。飞机服务

26.为了提供座舱增压、飞机机体防冰和座舱供热,从压气机中引出了大量的空气。最好是尽可能从压气机前几级引气,以减小对发动机性能的影响。但是,在飞行循环的某些阶段,可能需要将引气部位变换到压气机的后面级,以维持足够的压力和温度。

压气机向前的载荷涡轮向后的载荷

增大面积导致 增大向前的载荷

封严件向前的载荷

压力平衡封严件定位轴承

内部空气

图9-8 轴承轴向载荷的控制

Rolls-Royce Gem 60

Rolls-Royce AJ65 Avon

Work commenced early in 1945 on the AJ65axial flow turbo-jet with a design thrust of 6500lb. This figure was reached in 1951 with the 100 series RA3. In 1953 the considerably redesigned 200 series RA14 was type tested at 9500 lb thrust. Development culminated in the 300 series RB146 which produced 17.110lb thrust with afterburning.

罗尔斯-罗伊斯公司 “宝石”(Gem)60发动机

设计推力为6500磅的轴流式涡轮喷气发动机AJ65的设计工作于1945年初开始进行,指标于1951年在100系RA3发动机达到。1953年,基本重新设计的200系RA14进行了定型试验,推力为9500磅。最终制成300系RB146,带加力的推力为17,110磅。

罗尔斯-罗伊斯公司 AJ65“埃汶”(Avon))发动机

10: Fuel system

Contents

Page

Introduction 95Manual and automatic control 96Fuel control systems 99

Pressure control (turbo-propeller engine)

Pressure control (turbo-jet engine)Flow control Combined acceleration and speed control

Pressure ratio control

Electronic engine control 111Speed and temperature control amplifiers Engine supervisory control

Low pressure fuel system 112Fuel pumps

112Plunger-type fuel pump Gear-type fuel pump

Fuel spray nozzles 114Fuel heating

116Effect of a change of fuel 116Gas turbine fuels

117

Fuel requirements

Vapour locking and boiling Fuel contamination control

INTRODUCTION

1.The functions of the fuel system are to provide the engine with fuel in a form suitable for combustion and to control the flow to the required quantity necessary for easy starting, acceleration and stable running, at all engine operating conditions. To do this, one or more fuel pumps are used to deliver the fuel to the fuel spray nozzles, which inject it into the combustion system (Part 4) in the form of an atomized spray. Because the flow rate must vary according to the amount of air passing through the engine to maintain a constant selected engine speed or pressure ratio, the controlling devices are fully automatic with the exception of engine power selection, which is achieved by a manual throttle or

95

燃油的技术条件 蒸气堵塞及沸腾 燃油污染控制

第十章 燃油系统

1.燃油系统的功能是以适合于燃烧的形式向发动机供应

燃油,并按易于起动、加速和在发动机所有工作状态下稳定运转所需要的油量控制燃油流量。为此,用一台或几台油泵向燃油喷嘴供油,喷嘴将油雾化并注入燃烧系统(第4章)。为了保持选定的发动机转速或压比恒定,燃油流量必须随流过发动机的空气流量而变化,除了发动机通过手动油门或功率杆实现功率选择之外,所有控制装置是完全自动的。燃油截止活门(开关)操纵杆也可用于使发动机停车,虽然在某些情况下,这两种手动控制由一根操纵杆的动作完成。

目 录绪言绪言

手动及自动控制 燃油控制系统

压力控制(涡轮螺桨发动机) 压力控制(涡轮喷气发动机) 流量控制 组台式加速及转速控制

压力比控制

发动机电子控制

转速及温度控制放大器 发动机管理控制低压燃油系统 燃油泵

柱塞式燃油泵 齿轮式燃油泵

燃油喷嘴 燃油加温

燃油改变的影响 燃气涡轮发动机燃油

2.为了防止发动机燃气温度、压气机出口压力、及旋转组件的转速超出它们的最大极限值,一些自动安全控制器也是必要的。

7.Described in this Part are five representative Fuel system

96Fig. 10-1 Airflow changing with altitude.

Fig. 10-2 Fuel flow changing with altitude.4.通常燃油系统还有一些辅助功能,如滑油冷却 (第8章)和发动机的各种控制系统的液压控制;例 如,压气机空气流量控制(第3章)。

手动和自动控制

5.燃气涡轮发动机的功率或推力的控制受注入燃 烧系统的燃油量调节的影响。当需要增大推力时,油门开大,由于增大燃油流量,燃油雾化喷

嘴的压力增大。这便产生了增高燃气温度的作用,它进而又增加了通过涡轮的

燃气加速度,提高发动机转速,并相应增加空气流量,从而增大发动机的推力。

6.而且,高度、

空气温度和飞机速度的变化使流过发动机的空气流量和供应的燃

油之间的关系更为复杂。这些变量改变着发动机进口的空气密度,从而改变流过发动机的空气

质量。图10-1所示为典型的空气流量随高度的变化曲线。为了适应空气流量的这

一变化,燃油流量(图10-2)应当

发生相似的变化。否则空气流量与燃油流量之

比将会变化,使 发动机转速增大或减小而偏离油门杆位置选定的原定值。3.对于涡轮-螺桨发动机,由于螺旋桨转速及桨距的变化对发动机功率输出有影响,应将它们纳入考虑。因此,通常将油门杆和螺旋桨控制器相互连接起来。这样,就可以在发动机所有转速下在燃油流量和空气流量之间保持正确的关系,驾驶员控制发动机只要一根油门杆就行。虽然,通常发动机的最大转速是由螺旋桨转速控制器决定的,但其超转最终是用燃油系统中的调节器来防止。

图10-1 空气流量随高度的变化图10-2 燃油流量随高度的变化

高度x1000英尺高度x1000英尺

7.本章介绍5种典型的自动燃油控制系统,即液压机械式的压力控制和

流量控制系统,和机械式的加速及转速控制和压比控制系统。除了压比

控制系统采用了齿轮泵之外,其他各个系统均采用可调行程、多柱塞式

燃油泵向喷嘴供油。

空气耗量

(海平面静止状态最大值的%数)

500海里/小时 发动机恒定转速

97

Fig. 10-3 Simplified fuel systems for turbo-propeller and turbo-jet engines.

高压轴调节器

调节器

油门杆

高压燃油泵

油门装置高压停车开关

进气道空气温度

燃油控制装置

压力活门

燃油喷嘴

螺旋桨控制装置

虚线表示从发动机来的传感信号

图10-3 涡轮螺桨和涡轮喷气发动机的两种燃油系统简图

高压燃油泵

燃油流量调节器

低压轴调节器

高压停车开关高压压气机出口压力限制器

温度控制作动筒

排气温度放大器

排气温度高压 压气机进口

高压轴调节器

进气道空气温度

燃油喷嘴

油门杆高压压气机出口

节直升机旋翼的转速。

桨发动机和涡轮喷气发动机的高压燃油控制系统的简图示于图10-3,基本上每一个系统中都有一台高压油泵,一个油门控制器,嘴。另外,为了对发动机的要求作出反应,其中还有一置,以便对燃油流量提供自动控制。在涡轮螺桨发动机上,对燃油和螺旋桨系统作了协调,使燃油/转速配合10.改变通向喷嘴的燃油流量的常规方法是调节高压燃油泵的输出。它经由伺服系统对下列几项或所有因素的影响作出反应:

(1)油门移动。 (2)空气的温度和压力。 (3)快速加速和减速。 (4)发动机转速、发动机燃气温度和压气机出口压力信号。

Fuel system

98

Fig. 10-4 A pressure control system (turbo-propeller engine).流量控制装置

低压开关

空气进口压力

溢流活门

低压油滤

控制活塞

油门开关

油门旁路调节器

高压开关

伺服活塞

反压活门

至压差开关

燃油泵

溢流活门

燃油总管

发动机转速调节器

图10-4 压力控制系统(涡轮螺桨发动机)

燃油喷嘴

低压燃油

油泵供油(高压油)油门出口压力

喷嘴压力

伺服压力调节器压力

燃烧室压力

空气进口压力

压力控制(涡轮螺桨发动机)

11.压力控制系统(图10-4)是装在涡轮螺浆发动机上的一种典型的系统,发动机的加速率受螺旋桨转速控制器的限制。燃油泵的输出由在流量控制装置(F.C.U.)中的溢流活门及发动机转速调节器自动控制。这些活门通过改变燃油泵的伺服压力调节油泵的行程,以向发动机供应合适的燃油流量。

12.在稳定运转状态,在给定的空气进口压力和低于调节转速时,流量控制装置中的溢流活门处于感测位置,在燃油泵伺服活塞前后造成力的平衡,保证油门活门压力稳定。

13.当油门缓缓打开时,油门活门的压力降低,使流量控制装置的溢流活门关闭,导致伺服压力增加,燃油泵供油增加。当油门的压力恢复之后,溢流活门回到感测位置,即控制位置,油泵使其输出保持稳定,以获得选定的油门位置下的发动机转速。油门关小时。过程与此相反。

hydro-mechanical, and the acceleration and speed control and pressure ratio control systems, which are mechanical. With the exception of the pressure ratio control system, which uses a gear-type pump, all the systems use a variable-stroke, multi-plunger type fuel pump to supply the fuel to the spray nozzles.8.Some engines are fitted with an electronic system of control and this generally involves the use of electronic circuits to measure and translate changing engine conditions to automatically adjust the fuel pump output. On helicopters powered by gas turbine engines using the free-power turbine principle (Part 5), additional manual and automatic controls on the engine govern the free-power turbine and, consequently, aircraft rotor speed.FUEL CONTROL SYSTEMS

9.Typical high pressure (H.P .) fuel control systems for a turbo-propeller engine and a turbo-jet engine are shown in simplified form in fig. 10-3, each basically consisting of an H.P . pump, a throttle control and a number of fuel spray nozzles. In addition, certain sensing devices are incorporated to provide automatic control of the fuel flow in response to engine requirements. On the turbo-propeller engine, the fuel and propeller systems are co-ordinated to produce the appropriate fuel/https://www.wendangku.net/doc/1d16653071.html,bination.

10.The usual method of varying the fuel flow to the spray nozzles is by adjusting the output of the H.P .fuel pump. This is effected through a servo system in response to some or all of the following:(1) Throttle movement.

(2) Air temperature and pressure.

(3) Rapid acceleration and deceleration.(4) Signals of engine speed, engine gas

temperature and compressor delivery pressure.

Pressure control (turbo-propeller engine)

11.The pressure control system (fig. 10-4) is a typical system as fitted to a turbo-propeller engine where the rate of engine acceleration is restricted by a propeller speed controller. The fuel pump output is automatically controlled by spill valves in the flow control unit (F.C.U.) and the engine speed governor.These valves, by varying the fuel pump servo pressure, adjust the pump stroke to give the correct fuel flow to the engine.

12.At steady running conditions, at a given air intake pressure and below governed speed, the spill valve in the F.C.U. is in a sensitive position, creating

a balance of forces across the fuel pump servo piston and ensuring a steady pressure to the throttle valve.

13.When the throttle is slowly opened, the pressure to the throttle valve falls and allows the F.C.U. spill valve to close, so increasing the servo pressure and pump delivery. As the pressure to the throttle is restored, the spill valve returns to its sensitive or controlling position, and the fuel pump stabilizes its output to give the engine speed for the selected throttle position. The reverse sequence occurs as the throttle is closed.

14.A reduction of air intake pressure, due to a reduction of aircraft forward speed or increase in altitude, causes the F.C.U. capsule to expand, thus increasing the bleed from the F.C.U. spill valve. This reduces fuel pump delivery until the fuel flow matches the airflow and the reduced H.P . pump delivery (throttle inlet pressure), allows the spill valve to return to its sensitive position. Conversely, an increase in air intake pressure reduces the bleed from the spill valve and increases the fuel flow. The compensation for changes in air intake pressure is such that fuel flow cannot be increased beyond the pre-determined maximum permissible for static International Standard Atmosphere (I.S.A.) sea-level conditions.

15.The engine speed governor prevents the engine from exceeding its maximum speed limitation. With increasing engine speed, the centrifugal pressure from the fuel pump rotor radial drillings increases and this is sensed by the engine speed governor diaphragm. When the engine reaches its speed limitation, the diaphragm is deflected to open the governor spill valve, thus overriding the F.C.U. and preventing any further increase in fuel flow. Some pressure control systems employ a hydro-mechanical governor (para. 23).

16.The governor spill valve also acts as a safety relief valve. If the fuel pump delivery pressure exceeds its maximum controlling value, the servo pressure acting on the orifice area of the spill valve forces the valve open regardless of the engine speed, so preventing any further increase in fuel delivery pressure.

Pressure control (turbo-jet engine)

17.In the pressure control system illustrated in fig.10-5, the rate of engine acceleration is controlled by a dashpot throttle unit. The unit forms part of the fuel control unit and consists of a servo-operated throttle,which moves in a ported sleeve, and a control valve.

Fuel system

99

15.发动机转速调节器防止发动机超过其最大转速极限值。随着发动机转速的增加,燃油泵转子径向孔中的离心压力增加。该压力由发动机转速调节器的膜片感测。当发动机到达其转速极限时。该膜片挠曲,打开调节器的溢流活门,从而取代了流量控制装置,并防止燃油流量进一步增加。有些压力控制系统使用了液压机械调节器(23段)。

16.调节器的溢流活门还起着安全回油活门的作

用。当燃油泵供油压力超过其最大控制值时,不论发动机转速的大小,作用在溢流活门小孔面积上的伺服压力迫使活门打开,从而防止燃油供油压力进一步增加。

14.由于飞机飞行速度降低或高度增加,进气道空气压力减小,导致流量控制装置膜片膨胀,因而流量控制装置溢流活门的回油增加。这样减少了燃油泵的供油,直至燃油流量与空气流量和减少了的高压油泵的输出(油门进口压力)相适应,使溢流活门返同其感测位置。反之,进气道空气压力的增加就减少溢流活门的回油,增加燃油流量。如此,对进口空气压力改变所作的的补偿是这样的,也即燃油流量的增加不得超出在国际标准大气(ISA)海平面静止状态下所预先规定的最大容许值。

所示为油门活门及控制活门在它们各种控制位置下的情况。

真空膜盒

伺服滥流活门

溢流活门

旋转式

溢流活门

流量控制

压降控制膜片

电磁线圈

来自放大器

低压转速限制器和燃气温度控制

燃油泵

油门开关

油门杆伺服弹簧

控制活门

带孔套筒起动燃油喷嘴

反压活门主燃油

阻尼油门

起动

高压停车活门

油门出口压力

调节器压力

温度微调信号 进气道空气压力燃油控制装置

伺报控制膜片

高压轴调节器

(液压机械式)

低压燃油

油泵供油(高压燃油)

油门控制压力

油门伺服压力

伺服压力

18.在稳定状态,阻尼油门活门保持平衡,因为油门控

制压力加上弹簧力与油门伺服压力互相抵消。压力降掩

制膜片前后的压力处于平衡,油泵伺服压力调节燃油

Fig. 10-5 A pressure control system (turbo-jet engine).

The control valve slides freely within the bore of the throttle valve and is linked to the pilot's throttle by a rack and pinion mechanism. Movement of the throttle lever causes the throttle valve to progressively uncover ports in the sleeve and thus increase the fuel flow. Fig. 10-6 shows the throttle valve and control valve in their various controlling positions.18.At steady running conditions, the dashpot throttle valve is held in equilibrium by throttle servo pressure opposed by throttle control pressure plus spring force. The pressures across the pressure drop control diaphragm are in balance and the pump servo pressure adjusts the fuel pump to give a constant fuel flow.

19.When the throttle is opened, the control valve closes the low pressure (L.P .) fuel port in the sleeve and the throttle servo pressure increases. The throttle valve moves towards the selected throttle position until the L.P . port opens and the pressure balance across the throttle valve is restored. The decreasing fuel pressure difference across the throttle valve is sensed by the pressure drop control diaphragm, which closes the spill valve to increase the pump servo pressure and therefore the pump output. The spill valve moves into the sensitive position, controlling the pump servo mechanism so that the correct fuel flow is maintained for the selected throttle position.

20.During initial acceleration, fuel control is as described in para. 19; however, at a predetermined throttle position the engine can accept more fuel and at this point the throttle valve uncovers an annulus,so introducing extra fuel at a higher pressure (pump delivery through one restrictor). This extra fuel further increases the throttle servo pressure, which increases the speed of throttle valve travel and the rate of fuel supply to the spray nozzle.

21.On deceleration, movement of the control valve acts directly on the throttle valve through the servo spring. Control valve movement opens the flow ports through the control valve and throttle valve, to bleed servo fuel through the L.P . port. Throttle control pressure then moves the throttle valve towards the closed position, thus reducing the fuel flow to the spray nozzles.

22.Changes in air intake pressure, due to a change in aircraft altitude or forward speed, are sensed by the capsule assembly in the fuel control unit. With increased altitude and a corresponding decrease in air intake pressure, the evacuated capsule opens the spill valve, so causing a reduction in pump stroke

Fuel system

101

Fig. 10-6 Acceleration control by dashpot

throttle.

19.当油门打开,控制活门关闭套筒上的低压燃油孔,油门伺服压力增加。油门活门向选定的油门位置方向移动,直到低压孔打开,油门活门前后的压力恢复平衡为止。由压力降控制膜片感测油门活门前后降低着的炼油压差,关闭溢流活门,以增大油泵伺服压力,进而增加油泵输出。溢流活门移到感测位置,控制油泵伺服机构,使选定的油门位置下的正确的燃油流量得以保持。

20.在加速开始时,燃油控制如第l9段所述;但是,在预定的油门位置,发动机可得到更多的燃油,而且 在这一点,油门活门打开一个环形通道,引入额外的较高压力的燃油(油泵通过一个限制器供油)。这部分额外的燃油进一步增加了油门的伺服压力,这压力增加了油门活门的移动速度和向喷嘴的供油率。

21.在减速时,控制活门的移动通过伺服弹簧直接作用在油门活门上。控制活门的移动通过控制活门和油门活门,打开燃油的出口,通过低压孔放出伺服燃油。因此,油门控制压力使油门活门向关闭位置移动,因此,油门减少向喷嘴的供油量。

22.进气到空气压力由于飞机高度或飞行速度的变化而引起的变化,由燃油控制装置中的膜片组件感测。随着高度的增加,进气道空气压力相应降低,真空膜片打开溢流活门,导致油泵行程减小,直到燃油流量与空气流量匹配为止。反之,进气道空气压力增加,关闭溢流活门,增加供油量。

图10-6 由阻尼油门控制的加速性控制

关闭位置

油门杆

油门开关

控制活门

开始加速

最终加速

环腔

燃油压力

油泵供油 油门出口

低压 油门伺服 油门控制

Fuel system

Fig. 10-7 A proportional flow control system.燃油节流柱塞

液压机械调节器

燃油泵

伺服活塞

敏感活门

比例活门

高度传感比例活门装置

压降控制

限制器

加速控制装置

功率限制空气开关

燃油控制装置

油门及增压活门装置

油门开关及停车开关分布配重

燃油喷嘴

慢车活门

最小油量活门

电磁线圈

低压轴转速信号

温度控制 信号放大器

慢车转速调节器燃气温度 热电偶

压力分布

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

控制燃油压力

比例流量 伺服控制

加速控制装置伺服 调节器

减少压气机供气 空气开关

控制空气压力

进气道空气

压气机供气

23.高压压气机轴的转速用液压机械调节器调节,它采用与发动机的转速成正比的液压油压力作为其控制参数。旋转式溢流活门感测发动机的转速,然后用控制压力来限制油泵的行程,借以防止高压轴旋转组件超转。控制压力不受燃油比重变化的影响。

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

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.

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.Fuel system

10325.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。

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。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。

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.36.A variation of air intake pressure, due to a change of aircraft forward speed or altitude, is sensed by the capsule in the A.S.U. A pressure reduction causes the A.S.U. capsule to expand, thus increasing the bleed from the spill valve. This reduces fuel pump delivery until the fuel flow matches the airflow and results in a lower pressure difference across the throttle valve and the propor-tioning valve restrictors. The reduced proportional flow restores the balance in the A.S.U. which returns the spill valve to its controlling position. Conversely,an increase in aircraft forward speed increases the air Intake pressure, which reduces the bleed from the spill valve and increases the fuel flow.

37.During a rapid acceleration, the sudden decrease in throttle pressure difference is sensed by the A.S.U., causing the spill valve to close, Such a rapid increase in fuel supply would, however, create an excessive gas temperature and also cause the compressor to surge (Part 3). This occurs because the inertia of the rotating assembly results in an appreciable time lag in the rate of airflow increase. It is essential therefore, to have an acceleration control to override the A.S.U. to give a corresponding lag in the rate of fuel flow increase.

38.The rapid initial increase of fuel flow causes a rise in the pressure difference across the fuel metering plunger and this is sensed by a diaphragm in the pressure drop control section. At a fixed value of over fuelling, the pressure drop control diaphragm opens its servo spill valve to override the A.S.U, and maintains a constant pressure difference across the metering plunger.

39.The increased fuel supply causes the engine to accelerate and the fuel metering plunger gives the maximum permissible fuel flow to match the increasing compressor delivery pressure. This it achieves through the A.C.U. servo system, which is under the control of a spill valve operated by compressor delivery air pressure acting on a capsule.

40.As the compressor delivery pressure continues to rise, the capsule is compressed to open the spill valve and to bleed pressure from above the metering plunger. Pump delivery pressure acting underneath the plunger causes it to lift, this increases the area of the main fuel flow passage.

Fuel system

104

Fig. 10-8 Servo pressure control by kinetic

valve.

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。

37.当快速加速时,高度传感装置感测到油门压差突然降低,导致溢流活门关闭。然而,燃油供应如此急速的增加会产生过高的燃气温度,并导致压气机喘振(第3章)。这种情况发生的原因是,旋转组件的惯性使得空气流量增加的速率有相当大的时间滞后。因而必须有一套加速控制器来超控高度传感装置,使燃油流量增加的速率也有相应的滞后。

活门开

(油泵供油减少)

活门关

(油泵供油增加)

活门中间位置

(油泵供油不变)

高压燃油图10-8 由动力活门驱动的伺服压力控制

伺服

41.The pressure drop control spill valve closes to increase the fuel pump delivery and maintains the controlling pressure difference across the plunger. The fuel flow, therefore, progressively rises as airflow through the compressor increases. The degree of overfuelling can be automatically changed by the air switch, which increases the pressure signal on to the capsule. The full value of compressor delivery pressure is now passed on to the A.C.U. capsule assembly, thus increasing the opening rate of the metering plunger.

42.As the controlled overfuelling continues, the pressure difference across the throttle valve increases. When it reaches the controlling value, the A.S.U. takes over due to the increasing proportional flow and again gives a steady fuel flow to the spray nozzles.

43.The engine speed governor can be of the pressure control type described in para. 15, or a hydro-mechanical governor as described in para. 23.

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 the fuel output in the same way as the gas temperature control.

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.

Fuel system

105

38.燃油流量快速地开始增加导致燃油节流柱塞前后的压差增加,这由压降控制部分的膜片感测。在过量供油的某个定值下,压降控制膜片打开它的伺服溢流活门,超控高度传感装置的控制,并使节流柱塞前后的压差保持恒定。

39.增大了的燃油供应量导致发动机加速,同时,燃油节流柱塞供应允许的最大燃油流量,使之与增加着的压气机出口压力相匹配。这一功能是通过加速控制装置的伺服系统而实现的。该伺服系统由溢流活门控制,而该活门由作用在膜片上的压气机出口空气压力来操作。

40.当压气机供气压力继续提高时,膜片受压缩,将溢流活门打开,从节流柱塞上方释放压力。作用在柱塞下方的油泵的供油压力将其升起,这样便增大了主燃油流动通道的面积。

41.压降控制溢流活门关闭,使燃油泵供油增加,并保持柱塞前后的控制压差。因此,燃油流量随着压气机空气流量的增加逐渐增加。通过负责增大膜片上压力信号的空气开关能自动改变过量供油的程 度。现在,压气机供气压力的全部数值已传递给加速控制装置的膜片组件,从而增加了节流柱塞的开启率。

42.当受控的过量供油继续进行时,油门活门前后的压差增大。当其到达控制值时,高度传感装置由于增加了比例流量而起作用,重新为喷嘴提供稳定的燃油流量。

43.发动机转速调节器可以是第15段所述的压力控制式,也可以是第23段所述的液压机械式的。

44.液压机械调节器的伺服压力控制,与压力控制调节器非常相似,但调节器压力由油泵供油流过一个限制器之后取得,限制压力受旋转式溢流活门的控制;这种调节器不受燃油比重改变的影响。

45.在低的发动机转速下,旋转溢流活门保持打开位置;但是,当发动机转速增加时,离心载荷在抵消膜片载荷后将活门向关闭位置移动。这便限制了高压燃油向鼓筒的低压侧回油,直到在调节转速下,调节器压力使膜片挠曲,打开燃油泵伺服压力溢流活门,以控制最大燃油流量及发动机转速为止。46.如果发动机的燃气温度超过丁它的最大极限值,比例活门装置上的线圈逐渐充电。这样便导致摇臂移动,增大一个限流器的有效通流面积,以增大比例流量并打开高度传感装置溢流活门,以减小伺服压力。因此,燃油流量随之减小,防止了燃气温度的进一步增高。

Fig. 10-9 A combined acceleration and speed control system.

的大气放气口,以限制加速控制装置膜盒上的压力,进而通过节流柱塞控制燃油流量。

车转速设汁得随高度增加而增加。为了实现这一点,某些发动机上装一个最小油量活门。给流过油门活门的油量增加一个恒定的最小燃油流量。

组合式加速及转速控制

51.组合式加速和转速控制系统(图10-9)是一种机械系统,没有小型限制器或溢流活门。它也是一种全转速调节器系统,因而不需要单独的控制最大转速的调节器装置。控制机构装入一个装置之内,通常称为燃油流量调节器。采用了一个高压燃油泵(第85段),燃油泵伺服活塞的一侧由高压燃油驱动,在弹簧那一侧由主喷嘴(随动)压力驱动。

功率限制器

伺服活塞

燃油泵

高压停车开关

分布配重

燃油喷嘴

低压轴 调节器

膜盒组件

托架臂

可调节流油孔

(V.M.O.)

燃油流量 调节器图10-9一种组合式加速和转速控制系统

通向油门

湿度控制

作动筒燃气温度

热电偶

转速控制调节器

压降控制孔压降控制调节器

温度控制 信号放大器控制燃油压力 初级燃油 主燃油

高压压气机进口

高压压气机供气

减少压气机供气

控制空气压力进气口

分布压力 低压燃油

油泵供油

53.At any steady running condition, the engine speed is governed by the regulator controlling the fuel flow. The fuel pump delivery is fixed at a constant value by applying the system pressure difference to the fuel pump servo piston. This is arranged to balance the servo piston spring forces.

54.When the air intake pressure is at a constant value, the rotating V.M.O. sleeve is held in a fixed axial position by the capsule loading. The fixed throttle setting maintains a set load on the speed control governor and, as the r.p.m. is constant, the governor sleeve is held in a fixed position.

55.The fuel pump delivery is passed to the annulus surrounding the V.M.O.; the annulus area is controlled by the governor sleeve, and the exposed area of the orifice is set by the axial position of the V.M.O. sleeve. Consequently, fuel passes to the inside of the sleeve at a constant flow and therefore at a constant pressure difference.

56.The pressure drop control valve, which also forms a piston, senses the pressure difference across the V.M.O. and maintains the fuel flow at a fixed value in relation to a function of engine speed, by controlling the exposed area of the pressure drop control orifice.

57.When the throttle is slowly opened, the load on the speed control governor is increased, so moving the governor sleeve to increase the V.M.O. annulus area. The effect of opening the V.M.O. is to reduce the pressure difference and this is sensed by the pressure drop control governor, which opens the pressure drop valve. The reduced system pressure difference is immediately sensed by the fuel pump servo piston, which increases the pump stroke and consequently the fuel output. The increased compressor delivery pressure acts on the capsule assembly, which gradually opens the V.M.O. so that the fuel flow and engine speed continue to increase. At the speed selected, centrifugal forces acting on the speed control governor move the governor sleeve to reduce the V.M.O. annulus area. The resultant increased pressure difference is sensed by the pressure drop control governor, which adjusts the pressure drop valve to a point at which the pump servo system gives an output to match the engine requirements. The function of the governors and the control of the fuel flow is shown diagrammatically in fig. 10-10.

58.During a rapid acceleration, the initial degree of overselling is mechanically controlled by a stop that limits the opening movement of the speed control governor sleeve. A similar stop also prevents the fuel supply from being completely cut off by the governor sleeve during a rapid deceleration.

59.Changes in altitude or forward speed of the aircraft vary the fuel flow required to maintain a constant engine speed. To provide this control, the capsule assembly senses changes in H.P. compressor inlet and delivery pressures and adjusts the V.M.O. accordingly. For instance, as the aircraft altitude increases, the compressor delivery pressure falls and the capsule assembly expands to reduce the V.M.O. The increased system pressure drop is sensed by the fuel pump servo piston, which adjusts the pump output to match the reduced airflow and so maintain a constant engine speed. Conversely, an increase in aircraft forward speed causes the capsule assembly to be compressed and increase the V.M.O. The reduced system pressure drop causes the fuel pump to increase its output to match the increased airflow.

60.To prevent the maximum gas temperature from being exceeded, fuel flow is reduced in response to signals from thermocouples sensing the temperature (Part 12). When the maximum temperature is reached, the signals are amplified and passed to a rotary actuator which adjusts the throttle mechanism. This movement has the same effect on fuel flow as manual operation of the throttle.

61.To ensure that the engine is not overstressed, the H.P. compressor delivery pressure is controlled to a predetermined value. At this value, a pressure limiting device, known as a power limiter, reduces the pressure in the capsule chamber, thus allowing the capsule assembly to expand and reduce the V.M.O. so preventing any further increase in fuel flow.

62.A governor prevents the L.P. compressor shaft from exceeding its operating limitations and also acts as a maximum speed governor in an event of a failure of the F.F.R. The governor provides a variable restrictor between the regulator and the main fuel spray nozzle manifold. Should the L.P. compressor reach its speed limitation, flyweights in the governor move a sleeve valve to reduce the flow area, The increased system pressure drop is sensed by the fuel pump servo piston, which reduces the fuel flow to the spray nozzles.

63.This fuel system has no pressurizing valve to divide the flow from the fuel pump into main and primary fuel flows. Primary fuel pressure is taken from the fixed-area orifice of the pressure drop control valve. This pressure is always higher than the

Fuel system

107

53.在任何稳定运转状态,发动机转速由控制燃油流量的调节器控制。靠油泵伺服活塞上施加的系统压力差,使燃油泵的供油被固定在一个恒定值。该压力差用于平衡伺服活塞的弹簧力。54.当进气道空气压力为一恒定值时,膜盒载荷使旋转的可调节流孔套筒保持在一个固定的轴向位置。由于油门位置是固定的,它在转速控制调节器上保持一个固定的载荷,因而只要转速恒定,调节器套简便处于一个固定的位置。

55.燃油泵将油输入围绕可调节流孔的环腔;环腔面积由调节器套筒控制,孔露出的面积由可调节流孔套筒的轴向位置来设定。同时,燃油以恒定的流量流入套筒里面,因而压力差是恒定的。 56.压力降控制活门,它也是一个活塞,感测可调节流孔前后的压差,并通过控制压力降控制孔的暴露面积来保持燃油流量为一个定值,并与发动机的转速成涵数关系。

57. 当油门缓缓打开时,作用在转速控制调节器上的载荷增加,因此移动调节器套筒,增大可调节流孔环腔面积。打开可调节流孔的效果是减少压差,它由压力降控制调节器感测,并打开压力降活门。系统压力差的降低立即由燃油泵伺服活塞感测,它增大了油泵行程,从而增加燃油输出。增加了的压气机供气压力作用在膜盒组件上,该组件逐渐打开可调节流孔。这样,燃油流量和发动机转速继续增加。在选定的转速下,作用在转速控制调节器上的离心 力移动调节器套

筒,来减小可调节

流孔环腔的面积。所增加的压差由压力降控制调节器感测。它

将压力降活门调整到油泵伺服系统的输出与

发动机的要求相符合的程度。调节器的功能

及其对燃油流量的控制示于图10-10。58.当快速加速时,开始时的过量供油量由一个止动销作机械控制,它限制转速控制调节器套筒上孔口的开启移动。另一个相似的止动销也用来防止在快速减速时燃油供应被调节器套筒完全切断。

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