Breaker-Controlled Magneto Ignition - Stihl 009 User Manual

STIHL 009, 010, 011
4.2.1 Breaker-Controlled
Magneto Ignition
Diagram of ignition system:
1 Flywheel
2 Permanent magnet with north and
south poles and pole shoes
3 Ignition armature with primary and
secondary windings
When the magnet poles of the
flywheel pass the iron core of the
armature coils a low-tension voltage
is induced in the coils as a result of
the magnetic flux.
Without any form of control the mag-
netic flux would rise and fall like a
sine-wave and finally change direc-
tion. The same applies for the
electric voltage. However, the
magnitude of a voltage pulse
generated in this way would not be
sufficient to produce a sparkover.
This means that the voltage curve
must be controlled. In this ignition
system the mechanical contact brea-
ker performs the control function. It
is opened by the cam lobe ground
on the hub of the flywheel and
closed by spring action. At the
moment of maximum flux the
contact breaker, and also the
primary circuit, is closed. The
induced voltage thus allows a
current to flow in the primary winding
which
4 Contact breaker
5 Condenser
6 Spark plug
7 Ignition stop switch
8 Air gap
9 Edge gap
builds up a magnetic field (armature
field) around the coil. This is opposed
to the inducing magnetic field (exciter
field) and counteracts its tendency to
change the flux direction. The further
the flywheel rotates, the greater the
tendency of the exciter field to change
the flux direction. The opposing
armature field and thus the primary
current must also increase accordingly.
When the current finally reaches its
maximum value the contact breaker
opens the primary circuit - this instant
is called "magnet breakdown". This
causes the magnetic field in the
armature core to suddenly change
direction and induce a high-tension
pulse in the armature's secondary
winding which is proportional to the
high number of turns in that winding.
This pulse is fed via the hightension
ignition lead to the spark plug and is
discharged as a sparkover from the
center to the ground electrode and
thus ignites the fuel-air mixture.
A condenser is wired in parallel with the
contact breaker in the primary circuit in
order to prevent excessive sparking
(arcing) between the breaker points
while they are opening and insure there
is no loss of energy or premature
corrosion of the points.
The primary circuit is permanently
closed by means of the ignition stop
switch. This suppresses the abrupt
change in direction of the magnetic flux
so that no further high-tension pulse can
be induced.
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