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 TM 55-1905-219-14-8
4-7.5. MAGNETO ASSEMBLY - MAINTENANCE INSTRUCTIONS.
a. Operation.
(1) The magneto is essentially a generator. It is mounted on the cylinder and crankcase assembly and
consists of an armature plate upon which are mounted the ignition coil and lamination assemblies, condensers,
and breaker assemblies. A magnet is cast into the rim of the flywheel.
(2) In operation, as the permanent magnet poles in the flywheel pass over the pole shoes of the coil
laminations, the magnetic field causes a current to flow through the primary winding of the coil. This current
flow induces a voltage in the secondary winding. The current flow through the primary winding is normally
grounded through the closed breaker points. When the pistons near their top dead-center positions, a cam
arrangement opens the breaker points, breaking the ground connection on the primary windings. This causes
the magnetic field around the primary coil to collapse. The collapse is hastened by the action of the condenser,
which tends to store up the current induced by the collapse of the primary field and suddenly discharges it in
the reverse direction. The condenser also prevents sparking across the breaker points to minimize
deterioration of the contact surfaces.
(3) At the instant the primary field collapses, a high voltage is induced into the secondary winding.
This voltage is applied to the spark plugs through the high tension spark plug wire. It arcs across the spark plug
gap, and ignites the fuel charge which has just been compressed by the piston. This cycle is repeated for each
cylinder, each revolution of the crankshaft.
b. Description.
(1) The ignition spark is supplied to the spark plugs by a magneto, which uses a permanent magnet
built into the engine flywheel as a means of generating the necessary voltage. As the piston nears its top dead-
center position on the compression stroke, a cam arrangement on the crankshaft opens the magneto breaker
points, which, through the magneto circuitry causes voltage to be applied to the spark plug. This ignites the
compressed vapor within the cylinder, initiating another power stroke. The power thus generated is used to
drive the fire pump.
(2) The magneto and throttle are synchronized, functioning through a system of linkages so that a
proportionate volume of fuel-vapor charge is admitted to the compression chamber for any given degree of
spark advance throughout the speed range of the engine. The degree of spark advance and throttle valve
opening is controlled by the operating cylinder.
4-158
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