JT8D Ignition System

>> Tuesday, March 2, 2010

JT8D Ignition System
1)                 The ignition supplies a fast sequence of electrical sparks to ignite the fuel / air mixture in combustion chambers 4 and 7.
2)                 It is used when the engine is started (on the ground) or restarted in flight. It is also used as precautions to prevent flameout during take off, landing and bad weather. It's controlled via the flight deck when turned on electrical current flows from the aircraft supply to the exciter.
3)                 The exciter sends this high energy sparks via the high tension leads to the igniters.
4)                 Each electrical pulse cause the igniter to produce a strong spark at it's tip in the combustion chamber.
5)                 The exciter increases the voltage of the input electrically and builds a charge on a storage capacitor.
6)                 After the charge has built up sufficiently on the capacitor, a part of the charge is output to the igniter.
7)                 The igniter makes a trigger spark which lionises the air at it's tip.
8)                 Then the capacitor discharges fully, and the remaining charge sent to the igniter.
9)                 The heat that is transmitted from the very hot spark to the fuel / air mixture causes combustion.
Note: The electrical output pulse from the exciter has two parts which occur in a very fast sequence. The first part causes the Trigger Spark and the second part cause the Hot Spark.
10)             Seven combustion chambers that do not have igniters have there mixtures ignited by a flame that propagates through the cross over tubes between adjacent cans.
11)             The are two types of system on the 727.
a) The standard 20-4 Joule system that has a single exciter box.
It's supplied by 115V AC and 28 V DC. The 20-4 joule exciter can operate in the intermittent or continuous mode. In the intermittent mode, it fires both igniters. In the continuous mode it only fires the igniter in combustion chamber 7.

b) The Dual 20 joule system has two exciter boxes bolted together (Sometimes called a Twin Pack).
Both are supplied by 115V AC. One fires the igniter in chamber 4 the other chamber 7. Two exciters can be operated individually or at the same time. Earlier models were for intermittent use only. Later models are continuous. Remember 10min on LH the 10min RH for the earlier systems.
Notes: All of the ignition system components are LRU's


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