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Chapter ignition
• Each engine cylinder has two sparking plugs fed by two
separate magnetos.
• Magnetos are self-contained engine-driven electrical
generators. They produce a
series of extra high tension (EHT) electrical sparks at the
sparking plugs in thecorrect firing se!uence for ignition of the
petrol and air mixture.
• "e!uires step up transformer #s the primary magnetic field
collapses the lines of
magnetic force cut thousands of turns of $ery thin wire which
comprise the
secondary coil and this induces within it an EHT $oltage.
• The induced EHT $oltage is taken to a rotary switch called the
%distributor& which
distributes it to the sparking plugs in the correct firing
se!uence. The cam'operated
contact breaker points and the distributor rotor are geared
together so that the spark
will appear at the sparking plug as the contact breaker points
ust open. The contact breaker cam and distributor rotor rotate
at half engine speed.
• The prime function of the capacitor or condenser is
to pre$ent burning or arcing
across the contact breaker points and to assist in creating the
extra high $oltage in the
secondary coil by causing a rapid change of flux (magnetic
field) in the primary coil.
The capacitor is fitted in parallel with the contact breaker
points
•
The ignition switch pro$ides complete control of the
engine&s magneto circuit themagneto being made inoperati$e by
earthing the primary circuit.
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The spark appears at the plug electrodes when the contact
breaker opens.
• rounding'wire is used to switch'off the Magneto *f the
roundingwire breaks in
when the engine is running there will be no apparent changes in
the engines
performance. *f the grounding'wire breaks and touches the
engine'body or airframethen this is the e!ui$alent of grounding the
+rimary'circuit and the magneto is
switched off.
The %Dead Cut Check & is carried out at slow running,
RPM M!" DR#P $"
%&'(&% M!" " !"#P )*(+% !)("C*(&' #&% M'&%"#
# "
"(M%.
• The %Magneto rpm Drop Check & is carried out at
approximately ,- of the
maximum engine speed. This checks that the magneto and sparking
plugs are
functioning correctly.
(+(R/ !"R"(&' D%0(C%!
The *igh "ension $ooster Coil- supplies a succession of
high voltage electrical impulses.
The +ow "ension $ooster Coil supplies a low voltage to the
magneto primary during the
starting se!uence this augmentation of the primary permitting
normal operation of the
magneto.
The (mpulse Coupling. This is a mechanical de$ice which uses a
spring to temporarily
increase the speed of rotation of the magneto gi$ing a large
retarded spark during the
starting cycle.
#ther points1
• The high'$oltage current that normally arcs across the air
gaps of the distributor can
flash across a wet insulating surface to ground or the
high'$oltage current can be
misdirected to some spark plug other than the one that should be
firing. This condition
is called %flashover& and usually results in cylinder
misfiring.
• /iesel engines does not re!uire sparkplug as it works on
compression ignition
process. 0or cold starting diesel engines usually employ a
system of %glow'plugs& or
%pre'heaters& which pro$ide initial localised heating to the
combustion'chamber area.
