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ELECTRICAL SYSTEMS
IGNITION SYSTEM
10-57
The distributor is of the single contact type
with an 8-lobe cam. High speed operation is ob-
tained by an especially light contact breaker
arm and a high speed cam. Maximum operating
efficiency of the engine is obtained under all
speed and load conditions by the centrifugal
advance mechanism and the vacuum advance
mechanism built into the distributor.
d. Spark Plugs
AC type 48 spark plugs are used on all series.
These plugs have 14 M
/M
threads and a
10/16"
hex shell. These plugs have aluminum oxide
insulators and an extremely wide heat range.
e. Radio Suppressor and Capacitor
If
car is equipped with a
radio,
a suppressor
is installed in the center
high
tension terminal
of the distributor cap. A capacitor is mounted
on the side of ignition coil and connected to the
positive (battery) terminal of coil.
In most instances, factory approved radios
do not require suppressors on the spark plugs.
The 'use of radio spark plugs and splice type
suppressors is not recommended.
An
additional
capacitor must never be con-
ne
cted
to th
e distributor
terminal as this will
cause
exc
essive pitting
of breaker points or
engine
missing.
10-43 OPERATION OF IGNITION
SYSTEM
a. Ignition Operating Circuits
To clarify operating principles as well as to
simplify the process of tracing troubles, the
parts of the ignition system should be under-
stood to provide two separate and distinct cir-
cuits, as follows:
1.
The Primary Circuit carries the low
voltage current supplied by the battery or gen-
erator.
In addition to these sources of elec-
trical energy, the primary circuit contains the
ignition switch, primary winding of the igni-
tion coil, distributor contact points, condenser,
and all
connecting low
tension wiring.
2. The S
econdary Circu it
carries the high
voltage surges produced by the ignition coil,
which result in high voltage spark between the
electrodes of the spark plugs in engine cylin-
ders. This circuit contains the secondary wind-
ing of the ignition coil, coil to distributor high
tension lead, distributor rotor and cap, ignition
cables, and spark plugs.
b. Cycle of Operation
When the ignition switch is turned on and
the distributor contact points are closed, bat-
tery or generator current flows through the
primary winding of the coil and through the
contact points to ground. This flow of current
through the primary winding of the coil pro-
duces a magnetic field around the coil windings
and thereby stores electrical energy in the coil.
When the contact points are separated by
the revolving distributor cam, the primary cir-
cuit is broken. The condenser absorbs the cur-
rent which tends to surge across
the
gap as the
points separate, thereby producing a sharp
break in the flow of current.
If
the flow of cur-
rent were not sharply broken it would form an
arc which would burn the points badly and
would 'also drain away most of the energy
stored in the coil. There would be insufficient
energy left in the coil to produce the necessary
high voltage surge in the secondary circuit.
The very rapid change in strength of the
magnetic field when the primary circuit is
sharply broken causes a high voltage to be in-
duced in every turn of both the primary and
secondary windings. In the primary winding
the voltage may reach a value as great as 250
volts, resulting in further charging of the con-
denser. In the secondary winding the voltage
may reach a value of 25,000 volts, although the
va lue is usually 4,000 to
18,000
volts, depend-
ing on operating conditions.
The high voltage surge produced in the
secondary winding of the coil travels through
the cable to the center of distributor cap,
through the rotor to the adjacent distributor
cap segment from which it is conducted to the
proper spark plug by the ignition cable. The
high voltage surge jumps the gap between the
insulated center electrode and the grounded
side electrode of the spark plug, thus producing
the spark required to ignite the charge in the
selected combustion chamber of the engine.
As the spark appears at the spark plug gap
the energy in the coil begins to drain from the
coil through
the secondary circuit,
thus sus-
taining the
spark for a small fraction of a
second.
During this interval the condenser dis-
charges back through the primary circuit, pro-
ducing an oscillation of the current flow in the
primary circuit during the brief instant that
is required for the primary circuit to return to
a state of equilibrium. Note particularly that
the ignition condenser does not discharge until
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