basically the same, but can be achieved by various methods. The type and power source for the starter varies in accordance with engine and aircraft require-ments. Some use electrical power, others use gas, air or hydraulic pressure, and each has its own merits. For example, a military aircraft requires the engine to be started in the minimum time and, when possible, to be completely independent of external equipment. A commercial aircraft, however, requires the engine to be started with the minimum disturbance to the passengers and by the most economical means. Whichever system is used, reliability is of prime importance.
4.
The starter motor must produce a high torque and transmit it to the engine rotating assembly in a manner that provides smooth acceleration from rest up to a speed at which the gas flow through the
Electric
5.
The electric starter is usually a direct current (D.C.) electric motor coupled to the engine through a reduction gear and ratchet mechanism, or clutch, which automatically disengages after the engine has reached a self-sustaining speed (fig. 11-2).
6.
The electrical supply may be of a high or low voltage and is passed through a system of relays and resistances to allow the full voltage to be progres-sively built up as the starter gains speed. It also provides the power for the operation of the ignition system. The electrical supply is automatically cancelled when the starter load is reduced after the engine has satisfactorily started or when the time cycle is completed. A typical electrical starting system is shown in fig. 11-3.
122
圖11-2 一種電動起動機
Starting and ignition
圖11-3 一種低壓電起動系統
Fig. 11-3 A low voltage electrical starting system.
注:繼電器示于‘起動’位置
減小電流
計時開關
火花塞
起動機馬達
起動電路
再點火電路
吹除電路
高能點火裝置
主繼電器
點火繼電器
全電流計時開
慢速起動電阻
點火開關
絕緣繼電器
火藥起動
7.火藥筒起動有時用于某些軍用發動機中，它是一種快速的獨立起動方式。起動機馬達基本上是一臺小型沖擊式渦輪，由火藥燃燒的高速燃氣驅動。渦輪的功率輸出通過減速齒輪和自動脫開機構帶動發動機轉動。一個電發火起爆器開始點燃火藥裝藥。當用無煙火藥裝藥為這種起動機提供功率時，所要求的裝藥尺寸會大大限制火藥筒起動機的使用。圖11-4所示為三筒式起動機。
起動開始
截止計時開關
點火開關絕緣繼電器
超轉繼電器
指示燈亮
起動
起動／再點火選擇開關
再點火
吹除
起動機主開關
28伏直流供電
異丙基硝酸酯起動
8.這種起動機可產生大的功率輸出，而且具有快速起動的特性。它有一個渦輪，通過減速齒輪將功率傳給發動機。在這種情況丅，渦輪由異丙基硝酸酯燃燒產生的高壓燃氣帶轉。這種燃料噴入作為起動機一部分的一個燃燒室中，由高能點火系統進行電點火。油泵將燃料從儲箱中供入燃燒室，在每次起動之前先由空氣泵將起動機燃燒室中的霧氣抽出去。燃料和空氣泵、點火系統，以及起動循環終止的整個工作由繼電器和定時開關進行電控制。一種異丙基硝酸酯起動系統示于圖11-5。
Starting and ignition
噴口導向器
Fig. 11-4 A triple-breech cartridge starter.
Cartridge
7. Cartridge starting is sometimes used on military engines and provides a quick independent method of starting. The starter motor is basically a small impulse-type turbine that is driven by high velocity gases from a burning cartridge. The power output of the turbine is passed through a reduction gear and an automatic disconnect mechanism to rotate the engine. An electrically fired detonator initiates the burning of the cartridge charge. As a cordite charge provides the power supply for this type of starter, the size of the charge required may well limit the use of the cartridge starters. A triple-breech starter is illustrated in fig. 11-4.
Iso-propyl-nitrate
8. This type of starter provides a high power output and gives rapid starting characteristics. It has a turbine that transmits power through a reduction gear to the engine. In this instance, the turbine is rotated by high pressure gases resulting from the combustion of iso-propyl-nitrate. This fuel is sprayed into a combustion chamber, which forms part of the starter, where it is electrically ignited by a high-energy ignition system. A pump supplies the fuel to the combustion chamber from a storage tank and an air pump scavenges the starter combustion chamber of fumes before each start. Operation of the fuel and air pumps, ignition systems, and cycle cancellation, is electrically controlled by relays and time switches. An iso-propyl-nitrate starting system is shown in fig. 11-5.
Air
9.
Air starting is used on most commercial and some military jet engines. It has many advantages over other starting systems, and is comparatively light, simple and economical to operate.
10.
An air starter motor transmits power through a reduction gear and clutch to the starter output shaft which is connected to the engine. A typical air starter motor is shown in fig. 11-6.
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