BOSCH HIGH TENSION MAGNETO
OPERATION, ADJUSTMENT AND CARE
Like all other types of high tension magnetos, the Bosch Type ZR. Ed.
16 explained in this chapter, generates its own current and is usually
employed as sole ignition on an engine.
The timer and distributor are integral; and the rotation of the
armature, between the poles of strong permanent field magnets, sets up
or induces a current in the armature primary circuit, which is farther
augmented at every one hundred and eighty degrees of revolution of the
armature shaft, by the abrupt interruption of the primary circuit by
means of the magneto interruptor. At the opening of the primary circuit
the resulting discharge of current from this circuit induces a current
of high voltage in the armature secondary circuit. The high tension
current thus created is collected by a slip ring on the armature and
passes to the slip ring brush then to the various magneto distributor
terminals each of which is connected to a spark plug in its respective
cylinder.
The operation of the instrument will be more clearly understood from a
study of the complete circuits, primary and secondary, which follows.
=The Primary or Low Tension Circuit.=--The beginning of the armature
primary circuit is in metallic contact with the armature core, and the
end of the primary circuit is connected by means of the interruptor
fastening screw to the insulated contact block supporting the long
platinum contact on the magneto interruptor. The interruptor lever
carrying a short platinum contact, shown in Fig. 60 at C is mounted
on the interruptor disc, which in turn, is connected to the armature
core. The primary circuit is completed whenever the two platinum
contacts of the interruptor are brought together, and separated
whenever these contacts are separated.
From the latter point the high tension current passes to the
distributor brush (shown at D) which is held in a brush holder on
the distributor gear, and consequently rotates with the distributor
gear. Metal segments are imbedded in the distributor plate and as
the distributor brush rotates it makes successive contacts with the
segments, passing the current onto the spark plug gaps through the high
tension cables which are attached to the segment terminal posts.
[Illustration: Fig. 60. Bosch M Distributor and Interruptor--Housing
Removed]
Fig. 61 shows a circuit diagram of the Type ZR. Ed. 16. Bosch Magneto.
Note that the spark plugs must be connected up in accordance with
the firing order of the engine. The metal segments imbedded in the
distributor plate are connected with the terminal studs on the face of
the plate, and the latter are connected by cable to the spark plugs
in the various cylinders. In the cylinders the high tension current
produces a spark which produces ignition, and then returns through the
ground and the engine to the magneto armature, thus completing the
circuit.
=Timing the Magneto.=--With the average four cycle engine the proper
operating results are obtained by timing the magneto as follows:
The crank shaft is rotated to bring the piston in No. 1 cylinder (in
automobile practice this is the cylinder nearest the radiator) exactly
on top dead center of the compression stroke. The timing control lever
on the housing is then placed in the fully retarded position. With this
done, the magneto distributor plate should be removed by withdrawing
the two holding screws, or by releasing the two holding springs as the
case may be.
[Illustration:
DISTRIBUTOR
BRUSH
HOLDER
SAFETY
SPARK GAP
SLIPRING
CONDENSER
ARMATURE
INTERRUPTER
GROUND
GROUND
Fig. 61. Wiring Diagram Bosch Magneto, Type ZR-4]
The operation of the platinum contact points is controlled by the
action of the interruptor lever as it bears against the two steel
segments secured to the inner surface of the interruptor housing.
In Fig. 60, A shows the distributor with the face plate removed to
show the position of the distributor segments which are connected to
the terminal posts on the back of the plate. B shows the interruptor
housing and cover removed from its position on the magneto. C shows the
complete assembly of the distributor and interruptor. Note that the
face plate of the distributor is fastened on with a set of screws while
the interruptor cover is held in position with a latch.
=The Secondary or High Tension Current.=--The high tension current is
generated in the secondary circuit only when there is an interruption
of the primary circuit, the spark being produced at the instant the
platinum interruptor contact points separate. The armature secondary
circuit is a continuation of the armature primary circuit, the
beginning of the secondary circuit being connected to the primary,
while the end of the secondary is connected to the insulated current
collector ring mounted on the armature just inside the driving shaft
end plate of the magneto. The slip ring brush is held in contact
with the slip ring by a brush holder at the shaft end of the magneto
which receives the high tension current collected by the slip ring by
means of a connecting bar which passes under the arch of the magnets,
and passes the current to the center of the distributor plate, thus
exposing the distributor brush and gear. The cover of the interruptor
housing is also to be removed to permit observation of the interruptor
points.
The armature should then be rotated by means of the exposed distributor
gear in the direction in which it is driven until the platinum contact
points are about to separate, which occurs when the interruptor lever
begins to bear against one of the steel segments of the interruptor
housing. Timing or installation is completed by replacing the
interruptor housing cover and distributor plate, and connecting the
cables between the magneto and the spark plugs.
=Exact Magneto Timing.=--The foregoing will establish the desired
relationship between the magneto armature shaft and the engine crank
shaft. It should be noted, however, that while these instructions cover
the average engine, the exact magneto timing for individual engines is
best determined by trial.
When specific instructions for magneto timing are given by the engine
manufacturer, it is recommended that such instructions be followed in
preference to those herein given.
It must always be borne in mind that while making connections the
distributor brush travels in the opposite direction to the rotation of
the armature shaft.
=The Condenser.=--The condenser consists of a set of metal discs,
insulated from one another with tin foil. It is carried at the
interruptor end of the magneto. It is connected in the primary current
and forms a shunt connection with the interruptor contact points, and
through resistance to the returning ground current prevents excessive
sparking at the interruptor contact points which would soon burn the
points and ruin the coils.
=The Safety Spark Cap.=--A safety spark cap is provided to protect the
armature and other current carrying parts. Under normal conditions the
current will follow its path to the spark plug, but if for any reason
the resistance in the secondary wire is increased to a high point, as
when a cable becomes disconnected, or a spark gap too wide, the high
tension current will discharge across the safety spark gap.
=Caution.=--The current should never be allowed to pass over the safety
spark gap for any length of time, and if the engine is operated on a
second or auxiliary ignition system, the magneto must be grounded in
order to prevent the production of high tension current. The snapping
sound by which the passage of current across the safety spark gap may
be noted should always lead to an immediate search for the cause of the
difficulty.
=The Safety Spark Gap.=--The safety spark gap consists of a pointed
metal electrode projecting from the mounting flange of the slip ring
holder, inside the shaft end hood. The tip of the electrode extends to
within a short distance of the connecting bar, extending from the brush
holder to a magneto distributor plate center post.
=Timing Range.=--The magneto interruptor housing is arranged so that it
may be rotated through an angle of thirty-four to thirty-seven degrees
with respect to the armature shaft. The movement of this housing in one
direction or another causes the interruptor lever to strike the steel
segments earlier or later in the revolution of the armature, the spark
occurring correspondingly earlier or later in the cylinder. The spark
can be advanced by means of moving the interruptor housing which is
connected to the spark lever on the steering gear, in the direction
opposite the rotation of the armature. The armature rotation is usually
indicated by an arrow on the cover at the driving end of the magneto.
=Cutting Out Ignition.=--Since a high tension current is generated only
on the interruption of the primary circuit, it is evident that in order
to cut out the ignition, it is merely necessary to divert the primary
current to a path that is not affected by the action of the magneto
interruptor. This is accomplished as follows: An insulated grounding
terminal is provided on the cover of the magneto interruptor housing
with its inner end consisting of a spring with carbon contact pressing
against the head of an interruptor fastening screw. The outer end of
the grounding screw is connected by low tension cable to one side of
the switch, and the other side of the switch is grounded by connecting
a cable between it and the engine or frame. When the switch is open
the primary current follows its normal path across the interruptor
points, and is interrupted at each separation of these contact points.
However, when the switch is closed, the primary current passes from the
head of the interruptor fastening screw to the carbon contacts of the
grounding terminal, thence through the switch to the engine and back
to the magneto, and as the primary current remains uninterrupted when
following this path, no ignition current is produced.
=Care and Maintenance.=--Aside from keeping the magneto clean
externally, practically the only care required is the oiling of the
bearings. Of these there are two sets supporting the armature, and a
single plain bearing supporting the shaft of the distributor gear. Any
good light oil may be used for this purpose (never cylinder oil), and
each of the bearings should receive not more than two or three drops
about every thousand miles. Apply the oil through the oil ducts at
each end of the armature shaft. The interruptor is intended to operate
without oil, as oil on the interruptor platinum points prevents good
contact, and causes sparking, burning, and misfiring. Care should be
taken to prevent oil entering these parts.