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
圖12-5 發動機轉速指示器及發電機
26．除了提供轉子轉速的指示之外，轉速探頭中感應的電流可以用來電亮儀表板上的警告燈，向駕駛員指示轉子組件正在轉動。這在發動機起動時特別重要，因為它通知駕駛員何時打開燃油開關，來向發動機供油。該燈與起動電路相連，因而僅僅在起動過程中才亮。
25.轉速探頭位于壓氣機機匣上，與音輪對齊。音輪是壓氣機軸上經機械加工的部分。每轉一圈音輪外圓上的齒通過探頭一次，通過改變探頭中線圈的磁通量而誘導出一股電流。電流的大小由磁通量的變化率控制，因而與發動機轉速直接相關。
23.發動機轉速發電機供應三相交流電，其頻率取決于發動機轉速。發電機的輸出頻率控制指示器中的同步馬達的轉速，位于鼓筒或阻力杯中的磁鐵組件的旋轉導致鼓筒的運動，進而轉動指示器的指針。
24.在沒有其它設備來驅動發電機的場合，可以采用可變磁阻式轉速探頭，它與一個音輪相接，產生感應電流，經放大后送入指示器(圖12-6)。本方法可以用于提供發動機的轉速指示，而不需有單獨驅動的發電機以及其相應的傳動機構，因而減少了發動機的部件和運動零件的數量。
渦輪燃氣溫度
27.始終要指示排氣溫度，以保證在任一特定的工作狀態下，能夠檢查渦輪組件的溫度。而且，通常還備有一個自動燃氣溫度控制系統，保證最大燃氣溫度不致被超過(第10章)。
Controls and instrumentation
28．渦輪燃氣溫度(T.G.T.)有時用排氣燃氣溫度（E.G.T.)或噴管溫度(J.P.T)來表示。它是發動機工作中的關鍵參數，因而提供該溫度的指示極為重要。按照理想，應當測定渦輪進口溫度(T.E.T.)。但是涉及到高溫，這是不現實的。但是渦輪中的溫度降是按已知的方式變化的，所以渦輪出口溫度通常由適當安排的熱電偶測量。作為代替方式，也可以測量渦輪組件中間級的溫度，如圖12-7所示。
29.用于將溫度信號傳到指示器的熱電偶探頭由兩種不同的金屬絲構成，在一個金屬套管內部連接在一起。套管上的一些傳導孔允許排氣流流過接點。用于制造熱電偶絲的材料通常是鎳鉻和鎳鋁合金。
30.這些探頭在燃氣流中的配置要保證能獲得良好的平均溫度讀數，通常將探頭連在一起構成一個并聯電路。在電路(圖12-8)上連接一個指示器，它基本上是一個毫伏計，其讀數按攝氏度作了標定。
Fig. 12-6 Variable-reluctance speed probe圖12-6 可變磁阻式轉速探頭和音輪
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.
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