MAGNETOS
The perfection of the magneto and its application to cars of all
classes and sizes has marked the most important step in gasoline motor
ignition since the introduction of the electric spark. The magneto is
now considered one of the most vital parts of the car, and while it is
possible for the motor to be run for many miles on the batteries that
form the auxiliary ignition sources, the mechanical current generator
has left the field of the desirable accessories and has become an
actual, physical portion of the engine.
The operation of the magneto is simple, its whys and wherefores are
logical, and if one investigates the subject, even superficially, he
will discover that the much-maligned machine seldom gives trouble,
and that when it does, such action, or failure to act, is due to
neglect, abuse, or some other perfectly legitimate reason, rather than
"pure cussedness" on the part of the instrument itself. If the mere
mechanical aspect is considered; if it is realized that the magneto
consists mainly of a bundle of wires which, when revolved near the ends
of a magnet, collects that magnetism and sends it through the circuit
in the form of the electric current, and that consequently the magneto
is a converter that changes part of the mechanical energy of the motor
into the spark-forming fluid, the chief idea of magneto principles may
be more easily grasped.
To be sure, the magneto is delicate, and for that reason it should
never be dissected by the amateur, but inasmuch as what few adjustments
it has are readily accessible, it is seldom that the machine need to
be taken apart. The platinum points of the contact breaker, usually
located in the small box on the end of the armature shaft, may need to
be smoothed with emery paper occasionally if they have become pitted
from excessive sparking, but this is a simple operation and is not
greatly different from the care given to the vibrator of the dashboard
spark coil, as described in the preceding chapter.
A few drops of oil should be fed to the lubricating cups or holes of
the armature shaft as often as the directions call for--usually about
once every five hundred miles--but aside from this, the owner can
generally forget that he has a magneto, and will only be reminded of
the fact by the pleasing absence of ignition trouble. If ignition
trouble does occur, it is more than probable that the fault lies with
the plugs, timer, or wires, rather than with the magneto.
The man who drives a magneto-equipped car knows that the current
producer is run by a gear connected, either directly or through the
medium of other gears with the crank shaft of the motor. He knows,
then, that the magneto is driven positively and that there is a
constant relation between its speed and the number of revolutions of
the motor.
But does he know that it is absolutely necessary that a certain
position of the armature shall always correspond with a similar
position of the crank shaft of the motor, and that consequently
the same teeth of the driving gears must always mesh? He will most
assuredly be made aware of this if he disconnects his magneto and
then fails to replace the gears so that exactly the same teeth are in
mesh, for even the difference of a single tooth between the normal
positions of the armature and crank shaft will prevent the magneto from
delivering a sufficient spark to enable the motor to run.
The reason for this is simple. All of these direct-driven magnetos
are of the alternating current type, as this form allows of the
simplest construction of armature and windings. The alternating
current generator obtains its name from the fact that there are no
regularly-defined north and south poles at any part of the circuit, as
these keep changing continuously, or alternating.
During each revolution of the armature of the alternating current
magneto, there are but two positions at which a current will be formed.
Now the spark in any cylinder of a motor is required at about the top
of the compression stroke of the piston in that cylinder. Consequently
when the piston is at the top of its compression stroke, ready for
the spark that will ignite the charge, the armature of the magneto
must be in one of its two current-generating positions, and there must
therefore be a constant relation between the position of the crank
shaft, to which each piston is connected, and that of the revolving
part of the magneto.
If, now, the driving gear of the magneto is returned to its place
without regard to the teeth of the next gear with which it meshes, it
will be seen that the proper relation between the position of the
armature and that of the crank shaft will not be maintained. Under
these conditions, when the piston is at the top of the compression
stroke, ready for the spark, the armature will not be in a position
at which a current can be generated, and there can consequently be
no spark formed at the plug. Conversely, when the armature has been
revolved to the position at which a current will be formed, none of the
pistons will be requiring the spark, and this consequent lack of "team
work" will prevent the operation of the motor.
In order to maintain this team work between the armature of the magneto
and the crank shaft of the motor, the intermeshing teeth of the gears
should be marked with a prick punch before they are removed, so that
they may be returned to their proper place without trouble. Only in
this manner can accurate results be obtained, if it is at any time
necessary to remove all or part of the magneto driving gear.
The magnets forming the "fields" of the magneto in which the armature
revolves are of the permanent kind; that is, they do not depend upon
windings and a separate electric current for their excitation, as
is the case with some of the larger generators. These magnets may
be considered to be the most faithful part of the machine, for they
generally retain their strength under all conditions of rest or work,
and it is upon them that the proper operation of the magneto largely
depends.
A magneto in which the magnets have become weakened is useless for
ignition purposes until the fields can be remagnetized, and as this
can only be done at the factory, the machine in its entirety must be
removed from the motor. It is a comparatively easy matter to determine
whether or not the fields have lost their magnetism by placing a
piece of iron or steel within close range of the base or sides of the
magneto. An appreciable pull will be exerted by the magnets if they
still retain their strength, although it is not to be supposed that the
force thus exhibited will be very vigorous from such a small machine.
If the magneto has been disconnected from its driving gear for any
reason, the amount of magnetism remaining in the fields will be best
determined by turning the armature shaft with the hand. A resistance
should be offered to the turning at first until a certain point is
reached, after which the armature should exhibit a strong tendency to
fly forward to a new position, one hundred and eighty degrees beyond
its former normal position of rest. This activity of the armature is
one of the best guides to the amount of magnetism remaining in the
fields.
Many magnetos that have been installed on old motor cars not
previously so equipped are of the friction-driven, direct-current type
that produces a uniform spark at any point throughout the armature
revolution. Current from these may be used to charge a storage battery
for the operation of electric lights or to supply auxiliary ignition
current for starting. The positively-driven, alternating-current
magneto may also be used to operate electric lights on the car, but
this type of current cannot be stored in a battery, and consequently
the lights are available only when the motor is running. The magneto,
however, is not primarily an electric-lighting outfit, and unless it is
especially designed for the double purpose, a separate machine should
generally be used for supplying illuminating current.