Page 138 - The Jet Engine

Starting and ignition

give outputs which may vary according to require-
ments. A high value output (e.g. twelve joule) is
necessary to ensure that the engine will obtain a sat-
isfactory relight at high altitudes and is sometimes
necessary for starting. However, under certain flight
conditions, such as icing or take-off in heavy rain or
snow, it may be necessary to have the ignition
system continuously operating to give an automatic
relight should flame extinction occur. For this
condition, a low value output (e.g. three to six joule)
is preferred because it results in a longer life of the
igniter plug and ignition unit. Consequently, to suit all
engine operating conditions, a combined system
giving a high and low value output is favoured. Such
a system would consist of one unit emitting a high
output to one igniter plug, and a second unit giving a
low output to a second igniter plug. However, some
ignition units are capable o! supplying both high and
low outputs, the value being pre-selected as
required.
Fig. 11-8 Air impingement starting.

Fig. 11-9 A gas turbine starter.

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