22.
The ignition unit shown in fig. 11-10 is atypical
D.C. trembler-operated unit. An induction coil, operated by the trembler mechanism, charges the reservoir capacitor (condenser) through a high voltage rectifier. When the voltage in the capacitor is equal to the breakdown value of a sealed discharge gap, the energy is discharged across the face of the igniter plug. A choke is fitted to extend the duration of the discharge and a discharge resistor is fitted to
23.晶體管化點火裝置的工作與直流斷續器控制的點火裝置的工作相似，但其中的斷續器裝置由晶體管斷續器線路所取代。一種典型的晶體管化點火裝置示于圖11-11。與斷續器控制的裝置相比，它的優點很多，因為這種裝置中沒有運動零件，因此其壽命長得多。而且晶體管化點火裝置的尺寸小，重量也比斷續器控制的裝置為輕。
儲能電容器
Starting and ignition
ensure that any residual stored energy in the capacitor is dissipated within one minute of the system being switched off. A safety resistor is fitted to enable the unit to operate safely, even when the high tension lead is disconnected and isolated.
23. Operation of the transistorized ignition unit is similar to that of the D.C. trembler-operated unit, except that the trembler-unit is replaced by a transistor chopper circuit. A typical transistorized unit is shown in fig. 11-11; such a unit has many advantages over the trembler-operated unit because it has no moving parts and gives a much longer operating life. The size of the transistorized unit is reduced and its weight is less than that of the trembler-operated unit.
圖1-10 一種直流電斷續器控制的點火裝置
去火花塞的高壓接線柱
Starting and ignition
24．交流電點火裝置示于圖11-12，它接受交流電，電流通過變壓器和整流器對電容器充電。當電容器中的電壓等于封嚴放電間隙的擊穿值時，該電容器從點火火花塞端面釋放能量。和用斷續器工作的裝置一樣，它也裝有安全和放電電阻器。
圖11-12 一種交流電點火裝置
25．火花塞有兩種基本型，即收縮或約束空氣間隙式，以及分路表面放電式。空氣間隙式與常規活塞發動機的火花塞相似，但其火花要擊穿的電極和本體之間的空氣間隙較大。火花產生之前為了使間隙電離需要大約25，000伏電位差。這樣高的電壓要求整個線路具有非常好的絕緣。表面放電式火花塞(圖11-13)有一個絕緣的端頭，它由半導體雷管構成，容許自中央的高壓電極向本體漏電，使得雷管表面電離，為儲存在電容器中的電能提供了一條低電阻通路。放電采取從電極到本體高電壓跳火的形式，它僅要求約為2000伏的電位差就能工作。
發動機能順利再點火的
高度和速度包線
Starting and ignition
圖11-13 一種火花塞
Fig. 11-13 An igniter plug.
24.
The A.C. ignition unit, shown in fig, 11-12, receives an alternating current which is passed through a transformer and rectifier to charge a capacitor. When the voltage in the capacitor is equal to the breakdown value of a sealed discharge gap, the capacitor discharges the energy across the face of the igniter plug. Safety and discharge resistors are fitted as in the trembler-operated unit.
25.
There are two basic types of igniter plug; the constricted or constrained air gap type and the shunted surface discharge type. The air gap type is similar in operation to the conventional reciprocating engine spark plug, but has a larger air gap between the electrode and body for the spark to cross. A potential difference of approximately 25,000 volts is required to ionize the gap before a spark will occur. This high voltage requires very good insulation throughout the circuit. The surface discharge igniter plug (fig. 11-13) has the end of the insulator formed by a semi-conducting pellet which permits an electrical leakage from the central high tension electrode to the body. This ionizes the surface of the
圖11-14 一種典型的空中再點火包線
Fig. 11-14 A typical flight relight envelope.
pellet to provide a low resistance path for the energy stored in the capacitor. The discharge takes the form of a high intensity flashover from the electrode to the body and only requires a potential difference of approximately 2000 volts for operation.
26.
The normal spark rate of a typical ignition system is between 60 and 100 sparks per minute. Periodic replacement of the igniter plug is necessary due to the progressive erosion of the igniter electrodes caused by each discharge.
27.
The igniter plug tip protrudes approximately 0.1 inch into the flame tube. During operation the spark penetrates a further 0.75 inch. The fuel mixture is ignited in the relatively stable boundary layer which then propagates throughout the combustion system.
RELIGHTING
28. The jet engine requires facilities for relighting should the flame in the combustion system be extin-guished during flight. However, the ability of the engine to relight will vary according to the altitude and forward speed of the aircraft. A typical relight envelope, showing the flight conditions under which an engine will obtain a satisfactory relight, is shown in fig. 11-14. Within the limits of the envelope, the airflow through the engine will rotate the compressor at a speed satisfactory for relighting; all that is required therefore, provided that a fuel supply is available, is the operation of the ignition system. This is provided for by a separate switch that operates only the ignition system.
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