ker88839_ch11.qxd
1/9/06
11:31 AM
Page 229
The Ignition: Primary and Secondary Circuits and Components
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Figure 11-1. The ignition coil produces high-voltage current in the secondary winding when current is cut off in the primary winding.
cuit, the magnetic flux lines cross over into the secondary windings of the coil and induce an even greater voltage charge. The amount of voltage in the primary circuit builds up to 240 volts. The secondary windings meanwhile are building 200 times the primary voltage, or 48,000 volts. This is enough voltage to ignite the air-fuel mixture under most operating conditions. This collapsing magnetic field can be observed on an oscilloscope as a primary waveform pattern and is often referred to as the firing line. The voltage fluctuations vary by manufacturer, but most ignition systems range in amplitude between 250 to 400 volts. The secondary windings within the ignition coil are much thinner and have between 100 to 200 times the amount of windings. The voltage induced by the collapsing magnetic field is in the kilovolt (1000 X) range. This kind of ignition system, based on the induction of a high voltage in a coil, is called an inductive-discharge ignition system. An inductive-discharge system, using a battery as the source of low-voltage current, has been the standard automotive ignition system for about eighty years.
BASIC CIRCUITS AND CURRENT The ignition system consists of two interconnected circuits: • The low-voltage primary circuit • The high-voltage secondary circuit
When the ignition switch is turned on, battery current travels: • Through the ignition switch or the primary
resistor if present • To and through the coil primary winding • Through a switching device • To ground at the negative and the grounded
terminal of the battery Low-voltage current in the coil primary winding creates a magnetic field. When the switching device interrupts this current: • A high-voltage surge is induced in the coil
secondary winding. • Secondary current is routed from the coil to
the distributor.
