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TM 9-8000
can be made to handle higher voltages without harmful
battery life.
effects, whereas ignition points cannot. The quicker
switching time of this system allows longer coil primary
Ignition Timing Devices.
circuit buildup and longer induction time when the
primary circuit collapses. This increased time allows the
a. General. Spark advances are required so that
primary circuit to build up more current and the
the spark will occur in the combustion chamber at the
secondary circuit to discharge more current. The higher
proper time in the compression stroke. At high speed,
secondary current produces a hotter spark at the spark
the spark must appear earlier so the fuel-air mixture will
plug, which allows the engine to operate more efficiently.
have ample time to burn and give up its energy to the
piston. At part throttle, when the fuel-air mixture is less
15-7.  Capacitive-Discharge Ignition (Fig.  15-18).
highly compressed, a spark advance is required to ignite
capacitive-discharge  system  offers  several
the slower-burning mixture in ample time for it to burn
advantages over the other systems.  This system is
and deliver its power. Centrifugal and vacuum advance
similar to full transistor-magnetic control, except that
mechanisms produce these advances.
certain components have been added to the primary
circuit. These are the power converter, capacitor, and
b. Vacuum Control (Fig.  15-19).  The vacuum
the resistor or silicon controlled rectifier (SCR).  The
advance mechanism makes use of a vacuum chamber
power converter has an output voltage of 250 to 300
connected to the intake manifold and a vacuum
volts. This voltage is used to charge the capacitor with
diaphragm linked to the breaker plate assembly. The
the the resistor In its off condition. When the the resistor
breaker plate is supported so it can turn back and forth a
gate (1) receives a signal from the pulse generator, the
few degrees. When there is a wide-open throttle and
circuit from the anode (2) to the cathode (3) is closed,
little or no vacuum in the intake manifold, a full measure
and the capacitor immediately discharges at a high rate
of fuel-air mixture is entering the cylinders and no spark
through the primary. A high secondary voltage can be
advance beyond the centrifugal advance is needed. But
reached  about  100 times faster with the capacitive-
when the throttle is closed partially, part of the fuel-air
discharge system than with the inductive systems. This
mixture is throttled off and the mixture entering the
decreases spark plug fouling, materially Increasing
cylinders is less highly compressed.  For satisfactory
potential spark plug life.  Another advantage is that it
combustion, the spark must be advanced further beyond
uses less current than either the conventional system or
the advance produced by the centrifugal mechanism. To
the other solid-state systems. This means less demand
secure this  additional advance, the vacuum  line
on the battery during starts and a potentially longer
Figure 15-18. Capacitive-Discharge Ignition System


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