19.
A torquemeter system is shown in fig. 12-4. In this system, the axial thrust produced by the helical gears is opposed by oil pressure acting on a number of pistons; the pressure required to resist the axial thrust is transmitted to the indicator.
20.
In addition to providing an indication of engine power; the torquemeter system may also be used to automatically operate the propeller feathering system if the torquemeter oil pressure falls due to a power failure. It is also used, on some installations, to assist in the automatic operation of the water injection system to restore or boost the take-off power at high ambient temperatures or at high altitude airports (Part 17).
Engine speed
21. All engines have their rotational speed (r.p.m.) indicated. On a twin or triple-spool engine, the high pressure assembly speed is always indicated; in most instances, additional indicators show the speed of the low pressure and intermediate pressure assemblies.
22. Engine speed indication is electrically transmitted from a small generator, driven by the engine, to an indicator that shows the actual revolutions per minute (r.p.m.), or a percentage of the maximum engine speed (fig. 12-5). The engine speed is often used to assess engine thrust, but it
图12-5 发动机转速指示器及发电机
26．除了提供转子转速的指示之外，转速探头中感应的电流可以用来电亮仪表板上的警告灯，向驾驶员指示转子组件正在转动。这在发动机起动时特别重要，因为它通知驾驶员何时打开燃油开关，来向发动机供油。该灯与起动电路相连，因而仅仅在起动过程中才亮。
25.转速探头位于压气机机匣上，与音轮对齐。音轮是压气机轴上经机械加工的部分。每转一圈音轮外圆上的齿通过探头一次，通过改变探头中线圈的磁通量而诱导出一股电流。电流的大小由磁通量的变化率控制，因而与发动机转速直接相关。
23.发动机转速发电机供应三相交流电，其频率取决于发动机转速。发电机的输出频率控制指示器中的同步马达的转速，位于鼓筒或阻力杯中的磁铁组件的旋转导致鼓筒的运动，进而转动指示器的指针。
24.在没有其它设备来驱动发电机的场合，可以采用可变磁阻式转速探头，它与一个音轮相接，产生感应电流，经放大后送入指示器(图12-6)。本方法可以用于提供发动机的转速指示，而不需有单独驱动的发电机以及其相应的传动机构，因而减少了发动机的部件和运动零件的数量。
涡轮燃气温度
27.始终要指示排气温度，以保证在任一特定的工作状态下，能够检查涡轮组件的温度。而且，通常还备有一个自动燃气温度控制系统，保证最大燃气温度不致被超过(第10章)。
Controls and instrumentation
28．涡轮燃气温度(T.G.T.)有时用排气燃气温度（E.G.T.)或喷管温度(J.P.T)来表示。它是发动机工作中的关键参数，因而提供该温度的指示极为重要。按照理想，应当测定涡轮进口温度(T.E.T.)。但是涉及到高温，这是不现实的。但是涡轮中的温度降是按已知的方式变化的，所以涡轮出口温度通常由适当安排的热电偶测量。作为代替方式，也可以测量涡轮组件中间级的温度，如图12-7所示。
29.用于将温度信号传到指示器的热电偶探头由两种不同的金属丝构成，在一个金属套管内部连接在一起。套管上的一些传导孔允许排气流流过接点。用于制造热电偶丝的材料通常是镍铬和镍铝合金。
30.这些探头在燃气流中的配置要保证能获得良好的平均温度读数，通常将探头连在一起构成一个并联电路。在电路(图12-8)上连接一个指示器，它基本上是一个毫伏计，其读数按摄氏度作了标定。
Fig. 12-6 Variable-reluctance speed probe图12-6 可变磁阻式转速探头和音轮
and phonic wheel.
does not give an absolute indication of the thrust being produced because inlet temperature and pressure conditions affect the thrust at a given engine speed.
23.
The engine speed generator supplies a three-phase alternating current, the frequency of which is dependent upon engine speed. The generator output frequency controls the speed of a synchronous motor in the indicator, and rotation of a magnet assembly housed in a drum or drag cup induces movement of the drum and consequent movement of the indicator pointer,
24.
Where there is no provision for driving a generator, a variable-reluctance speed probe, in conjunction with a phonic wheel, may be used to induce an electric current that is amplified and then transmitted to an indicator (fig. 12-6). This method can be used to provide an indication of r.p.m. without the need for a separately driven generator, with its associated drives, thus reducing the number of components and moving parts in the engine.
25. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：劳斯莱斯喷气引擎－中英(71)
