Capacitive-Discharge Ignition

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Figure 15-17. Solid State Ignition System.

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TM-9-8000 Principles of Automotive Vehicles Manual

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Figure 15-19. Vacuum Timing Controls.

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

15-8.

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

The

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

15-15