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.晶体管化点火装置的工作与直流断续器控制的点火装置的工作相似，但其中的断续器装置由晶体管断续器线路所取代。一种典型的晶体管化点火装置示于图11-11。与断续器控制的装置相比，它的优点很多，因为这种装置中没有运动零件，因此其寿命长得多。而且晶体管化点火装置的尺寸小，重量也比断续器控制的装置为轻。
储能电容器
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.
图1-10 一种直流电断续器控制的点火装置
去火花塞的高压接线柱
Starting and ignition
24．交流电点火装置示于图11-12，它接受交流电，电流通过变压器和整流器对电容器充电。当电容器中的电压等于封严放电间隙的击穿值时，该电容器从点火火花塞端面释放能量。和用断续器工作的装置一样，它也装有安全和放电电阻器。
图11-12 一种交流电点火装置
25．火花塞有两种基本型，即收缩或约束空气间隙式，以及分路表面放电式。空气间隙式与常规活塞发动机的火花塞相似，但其火花要击穿的电极和本体之间的空气间隙较大。火花产生之前为了使间隙电离需要大约25，000伏电位差。这样高的电压要求整个线路具有非常好的绝缘。表面放电式火花塞(图11-13)有一个绝缘的端头，它由半导体雷管构成，容许自中央的高压电极向本体漏电，使得雷管表面电离，为储存在电容器中的电能提供了一条低电阻通路。放电采取从电极到本体高电压跳火的形式，它仅要求约为2000伏的电位差就能工作。
发动机能顺利再点火的
高度和速度包线
Starting and ignition
图11-13 一种火花塞
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
图11-14 一种典型的空中再点火包线
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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