Trade of Motor Mechanic - eCollege

Module 4 - Unit 1
Trade of Motor Mechanic - Phase 2 Course Notes
6
Basic Ignition Systems
The energy required can be obtained from the vehicle’s electrical
system, but the nominal battery voltage of 12 volts must be increased
or “stepped up” to provide a firing voltage of many thousands of
volts, typically 18000 volts. This high firing voltage causes the spark
gap to become electrically conductive, enabling an ignition spark to
occur.
It must have sufficient heat energy to ignite the mixture so that it
can continue to burn by itself. Exactly how much energy is required
varies according to the condition of the mixture and the pressure
in the cylinder at the end of the compression stroke.
With an engine at normal operating temperature and under a light
load, a mixture with a ratio close to the ideal ratio of 14.7: 1 ignites
readily. However, suddenly depressing the accelerator to increase
speed or to maintain speed when hill-climbing, causes cylinder
pressures to rise. This increases the firing voltage needed.
The ignition system is designed to have reserve energy available
in excess of its normal requirements so that it is able to produce
ignition, even when conditions are unfavourable.
Most light vehicle ignition systems are of the inductive type: they use
an induction coil with primary and secondary windings to “step-up”
the nominal 12 volts at the battery to the required firing voltage.
The automotive ignition system is divided into two electrical circuits
-- the primary and secondary circuits. The primary circuit has low
voltage. This circuit operates only on battery voltage and is controlled
by the breaker points and the ignition switch. The secondary circuit
has high voltage it consists of the secondary windings in the coil, the
high tension lead between the distributor and the coil (commonly
called the coil wire) on external coil distributors, the distributor cap,
the distributor rotor, the spark plug leads and the spark plugs.
Revision 2.0 July 2007