Ignition Systems - Aircraft Gas Turbine Engines #17 — Transcript

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1. 0:00all gas turbine engines use high-energy
2. 0:02igniter units or h e-- I use for engine
3. 0:05starting will investigate the
4. 0:08intricacies of the high energy igniter
5. 0:09unit later
6. 0:12all gas turbine engines have a dual
7. 0:14ignition system fitted which means that
8. 0:16they have two high-energy igniter units
9. 0:18the high-energy igniter units each feed
10. 0:21a separate igniter plug
11. 0:25systems have an output of approximately
12. 0:2712 joules
13. 0:31it may sometimes be necessary to help
14. 0:33prevent engine flameout to have the
15. 0:36ignition system selected in
16. 0:37circumstances other than engine starting
17. 0:39for instance during takeoff from
18. 0:42contaminated runways or flight through
19. 0:44heavy precipitation
20. 0:47the use of the high-energy ignition
21. 0:49system on these occasions will cause the
22. 0:51igniter plug to erode so quickly that it
23. 0:54will shorten its working life
24. 0:55dramatically
25. 0:58to minimize erosion of the ignitor plugs
26. 1:00some aircraft engines are fitted with a
27. 1:02combination ignition system which
28. 1:04includes a low energy three to six
29. 1:07joules continuous selection as well as
30. 1:10the high energy 6 to 12 joules starting
31. 1:13selection
32. 1:16continuous ignition which energizes the
33. 1:18low-energy mode of the igniters is
34. 1:20usually activated by selection of the
35. 1:23appropriate switch on the engine start
36. 1:25panel
37. 1:28the starting ignition system is
38. 1:29activated when the engine starts
39. 1:31sequence is initiated either
40. 1:33automatically or by the operation of the
41. 1:35high-pressure fuel start lever or
42. 1:37fuel and ignition switch
43. 1:42the igniters are automatically
44. 1:44deactivated at some point after
45. 1:46self-sustaining speed usually by the
46. 1:49operation of a speed switch incorporated
47. 1:51in the high-pressure compressor RPM
48. 1:52indicator system
49. 1:56some aircraft have an automatic ignition
50. 1:58system it
51. 2:00all is detected the aircraft stall
52. 2:02warning system will automatically select
53. 2:04the continuous ignition system
54. 2:10the high energy ignition unit works on
55. 2:13the principle of charging up a very
56. 2:14large capacitor and then discharging it
57. 2:17across the face of an igniter plug
58. 2:21the size of the capacitor makes it a
59. 2:23potentially lethal device and several
60. 2:25safety factors have to be built into the
61. 2:28high energy ignition unit so that it
62. 2:30conforms to safety regulations
63. 2:33this diagram shows the high energy
64. 2:35ignition unit mounted on the side of an
65. 2:38engine and also the position of the
66. 2:40igniter within the combustion chamber
67. 2:45the circuit shown here illustrates all
68. 2:48of the components within a high-energy
69. 2:50ignition unit which is supplied by 28
70. 2:52volts direct current
71. 2:55with the supply connected the primary
72. 2:58coil in the trembler mechanism are fed
73. 3:00with 28 volts direct current
74. 3:04the trembler mechanism works on the same
75. 3:06principle as that of an electric Bell by
76. 3:09doing so it causes the primary chord
77. 3:11input to become a sawtooth waveform this
78. 3:15sawtooth waveform is a very crude form
79. 3:17of alternating current
80. 3:20because we've generated alternating
81. 3:22current we can use a transformer to
82. 3:24boost the voltage in the primary coil to
83. 3:2725,000 volts in its secondary coil
84. 3:31the 25,000 volts alternating current is
85. 3:34changed back to direct current in the
86. 3:36rectifiers the direct current commences
87. 3:39charging the reservoir capacitor
88. 3:43as the value of the charge in the
89. 3:45reservoir capacitor builds up it
90. 3:47eventually reaches a level that causes a
91. 3:49spark to jump at the discharge cap
92. 3:53the discharge gap exists within an
93. 3:56evacuated tube the fact that the tube is
94. 3:59evacuated means that changing conditions
95. 4:01of humidity and altitude will have no
96. 4:04effect on the voltage required to jump
97. 4:06the gap within the tube consequently the
98. 4:09power of the spark of the igniter plug
99. 4:11will be constant regardless of ambient
100. 4:13conditions
101. 4:16the electrical energy which has crossed
102. 4:18the discharge gap as then to flow
103. 4:20through the choke the choke acts as an
104. 4:23inductance and slows down the current
105. 4:25flow slowing the rate of current flow
106. 4:28makes the duration of the spark longer
107. 4:32the energy then passes to the igniter in
108. 4:35the combustion chamber
109. 4:38if the unit has to be removed from the
110. 4:40engine for servicing any charge which
111. 4:42may remain in the capacitor even after
112. 4:45the electrical part of the unit has been
113. 4:46disconnected could be lethal to anyone
114. 4:49touching the casing of the high-energy
115. 4:50igniter unit
116. 4:53the discharge resistors act as a safety
117. 4:55device by allowing energy trapped in the
118. 4:58capacitor to leak away to earth once the
119. 5:00supply has been removed
120. 5:04the safety resistors act as a kind of
121. 5:06safety valve if the igniter plug becomes
122. 5:09disconnected
123. 5:11if the ignitor plug did become
124. 5:13disconnected and she would continue to
125. 5:15build up in the capacitor the buildup of
126. 5:17energy in the capacitor would eventually
127. 5:19cause it to explode
128. 5:22to safeguard against a potentially
129. 5:24engine disabling explosion the safety
130. 5:27resistors allow energy in excess of the
131. 5:29normal level to flow through them in an
132. 5:31attempt to balance the charge on the
133. 5:33plates of the capacitor
134. 5:36the normal spark output rate of the
135. 5:38high-energy ignition unit is between 60
136. 5:41to 100 sparks per minute
137. 5:44however the production of Sparks is
138. 5:46completely random and if relight is
139. 5:49selected with the aircraft on the ground
140. 5:50anyone listening at the jet bike before
141. 5:52engine start should hear an
142. 5:54unsynchronized beat if both units on the
143. 5:57engine are working correctly as well as
144. 6:01the direct current high-energy ignition
145. 6:03unit there are transistorized
146. 6:05high-energy ignition devices the
147. 6:08aircraft which have an alternating
148. 6:09current electrical system there are
149. 6:11units which will work on that supply
150. 6:20there are two types of igniter plug
151. 6:24the older of these two types works in a
152. 6:26similar manner to that of the piston
153. 6:27engine sparkplug but it has a much
154. 6:30bigger spark gap
155. 6:34it's approximately 25,000 volts this
156. 6:38extremely high potential requires very
157. 6:40good insulation standards both within
158. 6:42the unit and in the cabling to the
159. 6:44igniter
160. 6:51a more modern version of the igniter
161. 6:54plug is the surface discharge igniter
162. 6:56plug shown here
163. 6:58the end of the insulator of the surface
164. 7:01discharge igniter plug is formed from a
165. 7:03semiconductor material
166. 7:07the semiconductor material allows an
167. 7:09electrical leakage to occur between the
168. 7:12hot electrode to the body of the igniter
169. 7:14the electrical dica CH ionizes the
170. 7:17surface of the semiconductor material
171. 7:19which provides a relatively low
172. 7:21resistance path for the energy stored in
173. 7:23the capacitor
174. 7:26the discharge takes the form of a
175. 7:28high-intensity flashover from the hot
176. 7:31electrode to the body of the igniter
177. 7:33which only requires approximately 2,000
178. 7:36volts this concludes the lesson on
179. 7:39ignition systems
180. 7:47you